WOOD VENEER: LOG SELECTION, CUTTING, AND DRYING

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WOOD VENEER: LOG SELECTION, CUTTING, AND DRYING

Forest Service U.S. Department of Agriculture Technical Bulletin No. 1577

Lutz, John F.

1977. Wood veneer : log selection, cutting, and drying. U.S. Dep. Agrie, Tech. Bull. No. 1577, p. 137

Summarizes current information on cutting and drying veneer from many species of wood. Particular emphasis is placed on wood and log characteristics that affect veneer production; techniques for peeling, slicing, and drying veneer; and species involved.

KEYWORDS: Peeling, slicing, lathe, slicer, veneer quality, wood species, plywood, decorative panels, containers, thickness, physical properties, mechanical properties, grades. Oxford No. 832.20

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Stock No. 001-O0O-03723-4.


by

John F. Lutz, Technologist, Forest Products Laboratory, Forest Service,

U.S. Department of Agriculture

The Laboratory is maintained at Madison, Wis. in cooperation with the University of Wisconsin.

Forest Service

U.S. Department of Agriculture

Technical Bulletin No. 1577

January 1978

PREFACE

The broad spectrum of veneer cutting and handling for a multitude of uses obviously covers a wide range of operations by many special- ists, and involves hard-learned secrets. No one individual can be an expert in all areas—yet his efforts must be in line with those of others in research and industry. In these days of material shortages and pressure on energy sources, it seems doubly important to summarize some of the principles and coordinate the terminology.

This bulletin is a view of the art of veneer manufacture as seen by a specialist who spent the last 25 years in research and industry contacts. It represents an attempt to tie together the experiences of many for the benefit of all.

Contributions to this web of information have come from literally hundreds of people throughout the United States. The references listed here represent noteworthy contributions, but only a few of them. Harder to document are the thoughts and philosophies that have been shared with the author over the last quarter century.

Outstanding among these have been the contributions of other members of the Forest Products Laboratory staff. The research efforts and considered judgment of H. 0. Fleischer,

Curtis Peters, Harry Panzer, Joe Clark, and John McMillen stand out.

Other members of the Forest Service have been particularly helpful with information on wood species, especially John Putnam and those involved with surveys of the forest resources.

From representatives of the wood industry have come advice, assistance, and encourage- ment. The contributors are legion, with particular help from Tom Batey of the American Plywood Association and Bill Groah of Hard- wood Plywood Manufacturing Association on many phases.

In preparing this bulletin, the author relied heavily on three research publications he had written earlier. These three were published as U.S. Department of Agriculture Forest Service Research Papers, by the Forest Products Laboratory. These were:

''Wood and Log Characteristics Affecting Veneer Production,'' by John F. Lutz, USDA Forest Service Research Paper FPL 150, 1971.

"Veneer Species That Grow in the United States,'' by John F. Lutz, USDA Forest Service Research Paper FPL 167,1972.

"Techniques for Peeling, Slicing, and Dry- ing Veneer," by John F. Lutz, USDA Forest Service Research Paper FPL 228, 1974.

Use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval of any product or service by the U.S. Department of Agriculture to the exclusion of others that may be suitable.

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CONTENTS Page

Introduction

Wood and log characteristics affecting veneer production 1 Veneer quality as related to end uses ^ Hardwoods or softwoods for veneer 2 Physical properties of wood ^ Mechanical properties of wood -'■^ Properties of veneer logs ^^

Veneer from wood species that grow in the United States 21

Techniques for peeling, slicing, and drying veneer 29 Log storage ^^ Bark removal ^^ Sawing into bolts or flitches ^^ Conditioning wood prior to cutting veneer 34 Veneer cutting equipment 45 Knife and pressure bar on lathe and slicer ^^ Conveying and clipping veneer ^^ Veneer drying '^^ Quality control '^^

Veneer yields and volume needed for a plant 87 Veneer yields (rotary cutting) 87 Veneer yields (sliced) 87 Volume of timber needed to set up a veneer plant 88

Literature cited 89

Appendix I—Nomenclature of wood species and veneer 91

Appendix II—Physical properties of U.S. woods for veneer 95

Appendix III—Mechanical properties of U.S. woods for veneer Ill

Appendix IV—Some processing variables of U.S. woods for veneer 116

Appendix V—Effects of log storage and processing on veneer characteristics 121

Appendix VI—Appearance and suitability of individual U.S. species for various uses of veneer 125

Glossary 133

Index 135

Requests for copies of illustrations contained in this publication should he directed to the Forest Products Laboratory y U.S. Department of Agriculture, Forest Service, P.O. Box 5130, Madison, Wis. 53705.

iii


INTRODUCTION

The wood veneer industry uses over a thou- sand different wood species to make products as diversified as rotary-cut box shook VL inch (6.35 mm) thick to sliced decorative face veneer Vioo inch (0.25 mm) thick. In the United States, the major veneer uses are for structural and industrial plywood components % to 1 inch (9.25 to 25.40 mm) thick and decorative wall panels and furniture parts ?l6 to 1 inch (4.76 to 25.40 mm) thick.

With such a wide array of raw materials and final end uses, the field may at first seem overly complex. In part, this may be due to the scar- city of written information summarizing the technical aspects of wood veneer manufacture.

This bulletin describes the basic information known about the processes used in manufacture of wood veneer. Wherever possible, the log selection, log heating, veneer cutting, and drying processes are generalized and described as a continuum. To be sure, many individual processing problems are related to specific wood species. However, whenever possible the under- lying cause is described and a generalized approach to the problem is suggested.

Still, it is impossible to avoid some effects of individual species. In the past, when only a comparatively few species were used for veneer, this was not a great problem. It began to increase, however, as the favored species could not continue to meet increased demands

for veneer. Other U.S. species received closer looks for this product, and species from other countries are being imported into the country in an increasing swell of species, qualities, and quantities.

All of this has required more information— information that has been pieced together painstakingly. Material on individual species is compiled for the benefit of the reader in the tables of the Appendix. But, whenever possible, the text of this bulletin tries to present the generalized approach, and for native U.S. species.

Common names of wood species are generally given in this publication. But experienced users are well aware of the pitfalls of common names. Therefore, the corresponding official name of the tree from which the wood comes is shown in Appendix I, along with the specific botanical name.

The information contained herein comes from Forest Products Laboratory publications, from other research organizations, and from contacts with the veneer and plywood industry.

The bulletin is written primarily for people responsible for some part of the veneer manufacturing process. It may also be of interest to others, including those growing trees for use as veneer, for log buyers, users of veneer, and wood technology students.

WOOD AND LOG CHARACTERISTICS AFFECTING VENEER PRODUCTION

A successful veneer operation depends on three items: A supply of suitable logs, good processing techniques, and a good sales organi- zation. Most important is an adequate supply of suitable logs. Then to produce suitable

veneer, the logs must have the appropriate wood and log characteristics. The desired wood and log characteristics, in turn, depend on the end uses of the veneer.

VENEER QUALITY AS

In this bulletin, veneer is defined as wood cut Vioo to % inch (0.26 to 6.35 mm) in thickness by a knife, whether by rotary or slicing methods. Three characteristics of veneer that are desirable for all end uses are uniformity of thickness, minimum surface roughness, and minimum buckle. For decorative face veneer, control of figure, color, and depth of checks into the veneer are important. Other veneer containing natural defects, such as knots, knot- holes, splits, and discoloration, can be used as inner plies in many products and as faces of some products like sheathing and container plywood.

Four broad categories and typical end uses of veneer are given in table 1, as well as some wood qualities as they relate to uses of veneer.

The classification of species of veneer specified in Product Standard PS 1-74, Construction and Industrial Plywood, is listed in table 2. The classification is based primarily on the stiffness and strength of the species. Group 1 woods are

RELATED TO END USES

the stiffest and strongest and group 5 the least stiff and strong. Properties that are considered include bending (modulus of elasticity and modulus of rupture), compression parallel and perpendicular to the grain, and shear.

Classification of species of veneer specified in Product Standard PS 51-71 for Hardwood and Decorative Plywood is given in table 3. As indicated in the table, the classification is based on specific gravity. Face veneer for decorative plywood is graded primarily by appearance.

Species for use in wirebound boxes as specified in Federal Specification PPP-B-585b are listed in table 4. The four groups are based on specific gravity and other properties of importance in containers such as strength as a beam, resistance to nail withdrawal, shock resistance, and tendency to split when nailed or stapled.

An indication of the importance, for specific end uses, of all of the wood and log properties that are discussed in this paper is shown in table 5.

HARDWOODS OR SOFTWOODS FOR VENEER

Most species can be successfully cut into veneer. However, some are much easier to process than others. Hardwoods, as a class, are easier to cut into veneer than softwoods. This probably is because hardwoods can be bent more readily than softwoods (65) ^ All veneer bends severely as it passes over the knife that separates it from a bolt or flitch. Hardwoods, having better bending properties, bend with less damage as checks in the veneer than do softwoods.

The reasons for the better bending properties of hardwoods are not definitely known. Two possible explanations are that the hardwoods have less lignin than the softwoods, and that lignin in hardwoods is more thermoplastic than the lignin in softwoods.

While construction and industrial plywood is generally made from softwoods, hardwoods are preferred for most other uses listed in table 1. Good bending properties are particularly useful for some types of furniture.

1 Italicized numbers in parentheses refer to Litera- ture Cited.

PHYSICAL PROPERTIES OF WOOD

Generally, the first information about a species is obtained by a wood taxonomist or wood anatomist. Working with herbarium material and small wood samples, he classifies the species and describes its structure. This information is valuable for screening species to be considered for use as veneer. Such information is often available from libraries or by contacting Federal and State wood research laboratories or wood technology departments of forestry schools throughout the world.

Physical properties of wood of interest to potential veneer producers include specific gravity, moisture content, permeability, shrinkage, extraneous cell contents, figure, odor, and cell size, type, and distribution. (Values for individual species are given in Appendix II, 'Thysi- cal Properties of U.S. Woods for Veneer.")

Specific Gravity

Specific gravity or density is easily obtained and is often one of the first properties known about a species. As indicated in table 1, it can be used as a general guide in screening woods for use as veneer. For example, a wood with moderately low specific gravity is preferred for use as core and crossbands of decorative plywood.

Detailed information is available about the variation in specific gravity of many species, and additional data are being collected for other species. Information on the specific gravity of wood species can prove commercially valuable. For one example, knowledge of specific gravity for the various pines proved important in founding the southern pine plywood industry. When this industry started, the question was asked if all species of southern pine could be used and still make a product that could be marketed in the same strength category as Douglas-fir for structural softwood plywood. (Species are placed in various groups for use as structural plywood primarily on the basis of stiffness and strength; in general, the strength of wood is related to specific gravity.)

Based on the recorded strength values and specific gravity records, the major southern pines—loblolly, longleaf, shortleaf, and slash pine—were permitted to be marketed in the

same category as West Coast Douglas-fir. The minor southern pines, which have lower specific gravities, did not meet these requirements. Thus, while not foolproof, specific gravity can be used to quickly screen new species for ten- tative classification.

While most species can be cut into veneer by suitable manipulation of the cutting conditions, it is more difficult to cut wood at the two extremes of the range of specific gravity. Very lightweight species tend to cut with a fuzzy surface. Dense species require more power to cut and tend to develop deep cracks in the veneer as it passes over the knife. Basswood, with a specific gravity (based on green volume and ovendry weight) of about 0.32, is toward the low end of the range for species that are successfully cut into veneer. Hickory, with about 0.65, is near the high end. Still, a valuable species like rosewood, specific gravity of 0.75, can be successfully sliced into face veneer, but this requires suitable heating and limiting the cutting to thin veneer.

In gluing, also, the denser the wood the more difficult it generally is to glue (62),

Typical specific gravities of woods used for construction plywood are 0.41 to 0.55 ; for hardwood face veneer 0.43 to 0.65; for core and crossband veneer of decorative panels from 0.32 to 0.45; and for container veneer from 0.36 to 0.65 (table 1). Obviously, there are exceptions to these general guidelines. For example, butternut, with a specific gravity of 0.36, is a high-value face veneer. It is suitable for wall paneling but less suitable where hardness is a factor, such as the top of a desk.

Green Moisture Content

Veneer is often cut from logs soon after the trees are felled. Such bolts or flitches have essentially the moisture content found in the living tree. This moisture content in the wood has a distinct effect on cutting. In general, wood with a moisture content above fiber saturation but not excessively high is best suited for cutting into veneer; this makes the wood more pliable than drier wood. In a number of studies we found that species with a natural uniform moisture content of about 50 to 60 percent cut well.

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Some of the free water is forced out during cutting. This water apparently acts as a lubri- cant between the wood and the knife and pressure bar and aids the cutting process.

The driest wood that we have cut successfully into veneer at the Forest Products Laboratory was a flitch of teak with a moisture content of 25 percent. Like all teak, this flitch had a waxy extractive that probably aided the cutting.

We tried cutting even drier wood, but were not successful. This came about because a manufacturer wanted to slice air-dried planks of ponderosa pine into veneer VLG inch (1.50 mm) thick. The wood, which was at about 15 percent moisture content, was heated to about 200° F in water. Continuous sheets of veneer were produced from the flitches but the veneer had pronounced checks on the side that was next to the knife during cutting. After cutting, the veneer sheets immediately curled into tight rolls like window shades, so they were unsatisfactory.

Because slicing of the wood at 15 percent moisture content was unsuccessful, we took sapwood air-dried planks from the same ship- ment, and pressure-treated them with water to a moisture content of over 100 percent. Veneer Vie inch (1.59 mm) thick was then successfully sliced from these planks. In other words, when water is put back into relatively dry wood, the wood can be cut into veneer.

Some species have a higher moisture content in one part of the tree than another. For example, the sapwood of Douglas-fir has approximately three times as much water as the heartwood. Butt logs of redwood often have much higher moisture content than upper logs. In addition to requiring long drying times, wood having a very high moisture content is more difficult to cut into veneer than wood of the same species but with a lower moisture content. Examples are some western hemlock (as high as 215 pet), redwood (as high as 245 pet), and Douglas-fir (as high as 130 pet).

In normal veneer cutting, the wood is compressed just ahead of the knife. Wood with a very high moisture content can not compress until some water is forced out. As water is relatively noncompressible, it is forced from the wood structure so fast that it ruptures the wood (fig. 1). Commercial experience indicates

M 88966 Figure 1.—"Shelling" or shattering of redwood veneer that was rotary-cut from a "sinker" log. The wood shattered because water was forced out of the wood

too fast during cutting.

that high moisture content in "sinker" logs of species like redwood makes them undesirable for veneer because of cutting and drying problems. Likewise, for a long time sapwood veneer of Douglas-fir was not considered A-grade; part of the difficulty was in cutting it into smooth veneer as easily as the heartwood, which has a lower moisture content.

Wood may be damaged by freezing if it is stored in a cold climate. For instance, southern pine sapwood was damaged when logs were stored outdoors during the winter in Madison, Wis. Even worse damage was observed in a sweetgum log stored through a winter at Madi- son when the temperature went from above freezing to as low as -20° F. The end of a bolt cut from this log is shown in figure 2. Ice was found in many of the cracks seen on this end section. Industry reports that walnut logs grown in California and shipped by rail to the East froze when crossing the Rocky Mountains. Veneer cut from those logs was nearly useless due to splits caused by freezing.

Moisture content in the tree, then, is generally not a decisive factor in determining

M 84166 F

Figure 2.—Splits and shake in this sweetgrum log were caused by alternate freezing and thawing.

whether wood is suitable for use as veneer. Wood with a very high moisture content is usually more difficult to process than wood having a moderate moisture content such as 50 to 60 percent. On the other hand, it is very difficult or impossible to cut good veneer from wood below the fiber saturation point, approximately 30 percent for all species.

Permeability

Permeability has a distinct effect on veneer cutting, drying, and gluing characteristics. Sap- wood is often more permeable than heartwood of the same species. Bacterial attack in log storage may increase the permeability of wood, thereby changing its cutting characteristics. Wood that is permeable is easier to cut because water is readily forced from the wood; forces that could rupture the wood do not develop. Furthermore, plywood made from veneer that is naturally permeable, such as yellow-poplar, is less subject to "blowout" in the hot press than plywood made from such relatively imper- vious veneer as spruce. Extremely permeable veneer, such as the sapwood of pine that has been attacked by bacteria, may require a heavy glue spread or changes in gluing techniques to obtain satisfactory bonds.

Shrinkage

A small degree of shrinkage is desirable for all wood that is to be cut into veneer. In general, low shrinkage is related to low specific gravity. The low shrinkage of teak and mahogany is one reason these are preferred woods for face veneer. However, even within species having the same specific gravity, a considerable range of shrinkage exists.

High shrinkage is undesirable because it; Puts more stress on plywood gluelines with changes in moisture content; may cause cracks in face veneer of crossbanded panels during service; and causes warping unless the crossbanded panels are perfectly balanced.

Radial shrinkage is generally less than tangential shrinkage. Consequently, quarter-sliced veneer will often perform better as face veneer or cross band veneer than flat-sliced or rotary- cut veneer of the same species.

Longitudinal shrinkage may also be a factor in use of veneer. On several occasions we have seen thin decorative plywood panels bow seriously because of the different longitudinal shrinkage characteristics of face and back veneer. Excessive longitudinal shrinkage may be due to short grain, to compression wood in softwoods, or tension wood in hardwoods.

Shrinkage is a factor in all veneer uses but perhaps is most important for crossband veneer.

Drying conditions may affect the total shrinkage of refractory species like some eucalypts.

Wood Structure and Growth Rate

In general, it is desirable to have uniform wood structure for ease of cutting, drying, and processing of wood into veneer. The relatively uniform structure, regardless of growth rate, is one reason why diffuse porous hardwoods like yellow-poplar, sweetgum, and yellow birch are such good veneer species. Similarly, softwoods like white pine and Klinki pine are good veneer species. Uniform structure is particularly desirable for crossbands of decorative panels to minimize "telegraphing" of the grain to the face.

Such species as Douglas-fir, southern pine, and the oaks have a pronounced difference in density between springwood and summerwood. Assuming other factors are equal, veneer pro-

ducers generally prefer slow-grown wood of such species. In practice this is not always possible; for example, most construction plywood is made from Douglas-fir and southern pine, much of it fast grown. However, veneer from slow-grown logs of these species cuts better, dries with less buckle, and is generally preferred by production personnel. For ease in cutting and drying, veneer logs of such species should have a minimum of six rings per inch. Ponderosa pine growing in the Southeastern United States often has 30 rings or more per radial inch of growth. In tests at the Labora- tory, we found this to be excellent wood for cutting into veneer.

One of the problems that sometimes occurs with fast-grown softwoods is '^shelling,'' a local separation of the annual rings at the springwood-summer wood boundary (fig. 1). The first few layers of springwood cells are apparently weaker in resistance to shear than cells formed later in the year. Shelling may also occur with slow-grown wood that has soft, weak springwood and high moisture content. Examples are western redcedar and redwood. Shelling is ag- gravated by use of high compression by the nosebar and by excessive heating of the wood prior to cutting.

Fast-grown wood of species such as Douglas- fir and southern pine may cause problems in drying, gluing, and finishing {W).

The same relationship holds for ring-porous hardwoods like oak. In such woods, it is desirable that the springwood portion of the annual ring be narrow and the summerwood be of moderate density. In other words, the desirable thing is to get as uniform wood structure as possible. Such oak wood cuts well, does not shell readily between rings, and performs well as furniture, paneling, or flooring.

Texture

Open-grained or coarse-textured woods such as oak and ash have large pores. This is relatively unimportant in veneer cutting and drying but may be important in finishing. A furniture wood with pores larger than those in birch must have the pores filled to get a continuous film of finish. Large pores also affect the appearance of the wood. The size of the pores and the color of the filler used to fill them will

affect the appearance of the finished wood surface. If desired, the filler can be used to accent the figure of the wood.

Straight vs. Irregular Grain

For ease of veneer processing and for most end uses, straight grain is desirable.

Straight-grained wood is easier to cut than irregular grain and the veneer is more likely to remain flat. On the other hand, the market value of certain finished items of irregular grain may be high enough to pay for the extra care needed in handling it. Examples are the curly grain in species like walnut and maple and interlocked grain in mahogany. The curly grain often shows on a flat-cut or tangential surface. Interlocked grain shows as a stripe on quarter-cut or radial surfaces. Identifying irregular grain in logs is discussed further under ''Log Properties."

Geneticists are studying the inheritance of interlocked grain in species like red gum. Such information would help in selecting straight- grained trees to breed for lumber and veneer production.

Parenchyma

Parenchyma cells occur most frequently in wood rays and as concentric bands at the edge of growth rings. These cells are comparatively thin-walled and function primarily for storage of food. They are generally weaker than most other wood cells and so may form zones of weakness when they occur in large bands.

Terminal bands of parenchyma in angelique make it difficult to rotary-cut that species without getting a "shelling*' type of failure at the bands of parenchyma. To a lesser extent this same problem occurred when rotary-cutting veneer from Brazil nut (fig. 3).

Parenchyma in wood rays may be trouble- some when quarter-slicing veneer. The cut will be smooth when the knife moves across the wood in the direction in which the rays run out at the surface being cut. Conversely, when the rays run out at the surface in the direction opposite to the movement of the knife, the cut is rough. In the first instance, the rays are compressed by the cutting action and so cut smoothly. In the second case, the rays are stressed in tension perpendicular to the grain

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Figure 3.—Separation of a parenchyma band in rotary- cut Brazil nut veneer. The scale is in inches.

by the cutting action. As they are weak in tension, they split ahead of the knife into the wood and cause a rough surface. This phenomenon of differing roughness of the surface also applies to the orientation of annual rings and fibers (39).

Extraneous Cell Contents and Some Effects

Cellulose, hemicellulose, and lignin are the primary structural elements of the cell wall. Being polymeric in nature they are essentially insoluble in water and neutral organic solvents.

Many other materials may also be present in the wood. They are not part of the wood structure, but they contribute to the wood such properties as color, odor, and resistance to decay. They are grouped under the general heading of extraneous resins, waxes, hard deposits, and the like. Gluing problems have sometimes been attributed to resinous and waxy deposits in the wood. Extraneous materials can generally be removed from the wood by neutral

solvents such as water, alcohol, acetone, benzene, and ether.

The range and mixture of extraneous compounds found in wood is very large (28). Many of them have not been fully identified. Further, the amount of extractives varies widely from tree to tree and often within a tree. Therefore, only a few of the extraneous materials that may affect the use of wood as veneer will be discussed here.

In general, the extractives constitute only a small percent of the dry weight of the wood. In exceptional cases, however, such as the resin in longleaf pine stumps, the total may be as high as 20 percent. Often the high concentration of extraneous materials that cause diflS- culties in processing veneer results from a tree's response to injury. Heavy oleoresin concentrations are often found in southern pine trees that have been tapped for resin. Pitch pockets and blisters are generally considered to be caused by injury to the cambium of trees that secrete oleoresin. The wood contains pockets of oleoresin, which flows readily when the defect is cut open. Fires are reported to stimu- late gum production in several species. Insect attack is considered a principal cause of gum spots in black cherry. Wounding of hickories or pecans by cambium-boring insects often results in deposits of calcium carbonate or magnesium carbonate that are hard and large enough to nick a sharp knife.

These examples suggest that the percentage of veneer logs free of objectionable concentrations of extraneous materials can be increased in two ways: By selection of tree breeding stock that is resistant to insect attack, and by silvicultural practices that minimize injury to the trees.

The terminology concerning extractives is sometimes confusing to nonspecialists in this field. This problem is complicated because most extractives consist of more than one compound.

Giim

The word "gum" has been used in the past to describe any plant exúdate that feels gummy when fresh and that hardens on exposure to air. In recent years chemists have used the word "gum" specifically for certain types of polysaccarides. True gum is more or less sola-

ble in water and insoluble in nonpolar organic solvents. Arabinogalactan, which may be present in amounts sufficient to interfere with the gluing of veneer cut from butt logs of western larch, is a true gum. Gum spots in black cherry probably consist of true gum and polyphenols, with the polyphenols causing their dark brown color. While a slight amount of gum is permitted in cherry face veneer (7), moderate or heavy concentrations of gum lower the grade. Figure 4 shows the gum that limits use of Brazil nut for veneer.

Resin and Oleoresin In contrast to gum, resin denotes materials

that are insoluble in water but soluble in neutral organic solvents. Resins occur in ray parenchyma cells of both hardwoods and softwoods. Oleoresin is a mixture of resin and essential oils; it is insoluble in water but soluble in alcohol, alkalies, and most organic solvents. Oleoresin is secreted by vertical and horizontal resin canals in such softwood groups as pine, spruce, Douglas-fir, and tamarack. In hemlock, fir, and redwood, resin canals are nor- mally absent but may be produced by injury to the tree.

In veneer cutting, resin is a handicap. It may collect on the pressure bar and encourage chips to jam between the pressure bar and the wood bolt, causing depressions in the veneer. Frozen or solidified resin in knots is very hard and will quickly blunt a sharp knife.

Ether-soluble resin occurs in small amounts in many U.S. hardwoods, but generally has little effect on their use for veneer. The relatively large amounts of ether-soluble components found in basswood may explain why this species is more difficult to glue than would be expected from its specific gravity. Resin in core and crossply veneers, such as may occur in the heartwood of cativo and southern pine, is objectionable because it may bleed through the face veneer. Similarly resin in face veneer species like white pine can interfere with furniture finishes. This is particularly true if the end pi'oduct is a TV cabinet, which becomes warm during use.

Among the imported hardwoods, vertical and horizontal resin canals are found only in certain species of Dipterocarpaceae. The contents of these canals usually appear white or yellow.

M 136 441

Figure 4.—Gum in a sheet of rotary-cut Brazil nut veneer.

These extractives may be part of the problem in gluing kapur and keruing.

Polyphenols

Polyphenols can be broadly grouped into tannins and nontannins. Most tannins are of a molecular size generally soluble in water. Poly- phenols that are not soluble in water can be removed from wood with polar organic solvents like alcohol or alcohol-benzene. Polyphenols occur in most species and are generally more common in the heartwood than in the sapwood.

Color

One reason polyphenols are important is because they give wood its typical color. Colored heartwood of decorative face veneer of species like rosewood is much more valuable than light- colored sapwood.

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Almost all sapwood is white. This light color is preferred for some face veneer of species like maple. Light-colored wood may also be pref- erable for containers as it makes a good background for stenciling or other markings. Color is of little importance for construction plywood or for core and crossband veneers.

Metal Stain

Many polyphenols react with iron and steel in the presence of water to form a blue-black stain. This becomes very obvious and objectionable on face veneer of species like oak and redwood if the wet wood is in contact with iron or steel for even a brief time. Hot wet wood will stain more readily than cold wet wood.

Dimensional Stability

Nearn (49) showed that many heartwood extractives will partially stabilize the wood dimensionally. One result is that dry, rotary- cut heartwood veneer of species like yellow- poplar and Douglas-fir has less end wrinkling and buckle than sapwood veneer cut from the same logs. Flat veneer is easier to handle in plant processing than buckled or wavy veneer.

Checks in Veneer

Checks in the heartwood veneer of rotary- cut types are measurably deeper than checks in the sapwood veneer cut under the same conditions. Similarly, high-speed photographs have shown that breaks into the heartwood veneer of yellow birch were more conspicuous than breaks into sapwood veneer cut in the same revolution of the bolt. One possible explanation of these phenomena is that the polyphenols in the heartwood make it less plastic than the sapwood.

Wax A few species of wood have waxy extractives

that seem to be an advantage when cutting veneer. Pencil manufacturers recognize this advantage and add wax to incense-cedar pencil blanks to improve the whittling properties of the wood. Conversely, waxy extractives make wood more difficult to glue and finish. Examples of wood that feel waxy to the touch include teak, determa, and cypress.

Hard Deposits

The ash content of wood is usually less than 1 percent but in small areas in the wood it can be much greater. The principal inorganic deposits contain calcium, magnesium, or silica. Concentrated minerals have a distinct blunting effect on sharp tools. However, scattered individual crystals of calcium oxalate, which are common in the longitudinal parenchyma and ray cells of many hardwoods, do not obviously affect veneer cutting.

Hard deposits that do cause rapid dulling of knives are limited to a few native species such as maple, pecan, and hickory. The ash content in mineral streaks of hard maple is reported to average 5.2 percent and to be high in man- ganese. Calcium deposits, concentrated in hickory and pecan that is injured by cambium- mining insects, will nick a sharp knife.

In contrast to continental U.S. species, many tropical hardwoods contain silica. If the silica content exceeds 0.5 percent, it causes rapid blunting of cutting tools.

Figure

Figure is defined as the pattern produced in a wood surface by annual growth rings, rays, knots, deviations from regular grain such as interlocked and wavy grain, and irregular coloration. Figure is one of the most important characteristics of decorative face veneer. How- ever, for uses of veneer other than decorative face stock, highly figured wood is generally not desired.

Odor

Most woods have little odor when dry. Some species, such as cedars, have a pleasant odor that is used to promote the use of the wood. Other woods have a sour or unpleasant odor, particularly if they become damp. Logs stored in a warm climate may develop objectionable odors due to the action of bacteria. The problem is most likely to occur with species that have wide bands of sapwood containing large deposits of starch. Such odors are particularly objectionable in veneer that is to be used for products like food containers or paneling for walls of homes.

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MECHANICAL PROPERTIES OF WOOD

Besides physical properties, the information most generally available about a species is its mechanical properties. The most likely sources of information on mechanical properties of wood are libraries, Federal and State wood research laboratories, and wood technology departments of forestry schools.

Mechanical properties of particular interest for veneer are strength in tension perpendicular to the grain, hardness, modulus of elasticity, modulus of rupture, shear, and compression parallel and perpendicular to the grain. (Values for individual woods are given in Appendix III, ''Mechanical Properties of U.S. Woods for Veneer.'')

A wood strong in tension perpendicular to the grain is desirable for veneer because it is less likely to split in log form, when cutting into veneer, or in subsequent handling of the veneer.

Hardness is of interest in veneer used for furniture and flooring, or other places where it will receive abrasion and impacts during service.

Modulus of elasticity, or stiffness, is important to veneer because stiffness of the plywood

is generally the critical factor for such structural uses as subflooring and roofing.

Modulus of rupture is a measure of the ultimate bending strength of the wood. It is of interest for containers and for construction plywood.

Shear is important in structural applications such as the use of plywood as the web in a box beam.

When plywood is used as a stressed skin, strength in compression parallel to the grain is important.

Compression perpendicular to the grain is an important property when a bearing load is involved, such as a refrigerator on a plywood subfloor.

Referring to end uses listed in table 1, construction plywood is generally made from softwoods. A major reason is that, for a given specific gravity, softwoods generally have a higher modulus of elasticity than hardwoods. The longer cells and higher lignin content of the softwoods may account for the higher stiffness.

Softwood logs are also more readily available in large quantity and are less expensive than veneer-grade hardwood logs.

PROPERTIES OF VENEER LOGS

Selection of species for decorative face veneer is based primarily on the appearance of the wood. Other physical and mechanical properties are important for construction plywood, core and crossband veneer, and container veneer.

In addition to the wood properties of various species, their tree and log properties must be taken into account. The average diameter and form of the trees are of obvious interest to any timber user. At one time it was thought that only prime logs, large in diameter and clear of defects, could be used for veneer. While only partially true, this popular concept of an ''ideaF' veneer log nicely introduces the subject of log grades.

"Ideal" Veneer Log An "ideaF' veneer log is cylindrical in form

with the pith in the geometric center of the log end sections. The bark surface of the log and the end sections are entirely free from blemishes. The annual rings on the end sections indicate uniform slow growth so the specific gravity and texture of the wood varies a minimum amount. The grain of the log is straight. The minimum diameter of this ideal log is 14 inches if it is to be rotary-cut, 18 inches if it is to be flat-sliced, and 24 inches if it is to be quarter-sliced.

Very few logs meet the criteria of an ideal veneer log. But logs having other characteristics may still be eminently suited and valuable

12

for veneer. For example, the most obvious exception to this concept is if fancy face veneer is planned ; here irregular grain of a particular type is desired.

Function of Log Grades

Wood is a natural product and has many variable characteristics. Such characteristics as sweep, log end splits, and knots are among the many factors evaluated when grading a log. Based on all these considerations, the log grader estimates the quality and quantity of veneer that can be produced from the logs. For example, one criterion for a No. 1 Douglas-fir peeler, as given by the official Log Scaling and Grading Rules for five western softwood grading bureaus (58), is that it be suitable for manufacture of clear uniform-colored veneer, to an amount not less than 50 percent of the net scaled content. Log quality used in softwood plants today go from No. 1 peelers to almost any log that can be held by the lathe chucks and turned into veneer.

Changing Requirements for Veneer Logs

While plant managers and production fore- men would rather work with high-grade peeler logs, the availability of raw material and the changing end uses of veneer and plywood have forced the veneer industry to handle lower grade logs. Improved methods for handling small logs have made it practicable to manufacture veneer from species like aspen, birch, and southern pine with log diameters of 12 inches or less. Equipment developments such as retractable chucks, backup rolls, driven roller bars, and lathe chargers have permitted eco- nomic handling of lower grade lots (2),

One reason for this switch has been the change in the end use of the veneer. At one time the main end products of the softwood plywood industry were such items as wall paneling and faces for doors. Now the major use is structural C-D grade plywood. Knots as large as 3 inches in diameter and splits as wide as 1 inch can be tolerated in this end product. As a result, the raw material requirements have shifted from peeler grade logs to No. 1 and No. 2 grade sawlogs.

The same sort of change has occurred in the requirements for hardwood face veneer. At one time such veneer had to be perfectly clear. In recent years such characteristics as small pin knots, insect tracings, and slight stain have been well accepted by the public for prefinished wall paneling, the major use for hardwood plywood. As a result lower grade logs are suitable for manufacture into hardwood face veneer.

Veneer Log Grades While there are some formal veneer log

grades, many mills have their own local rules for acceptable logs. In their simplest form they specify minimum diameter and length of logs, and the size and number of permissible surface defects, like knots.

Harrar (27) has described the frequency and importance of defects in southern hardwood and veneer logs. Grading rules for northern hardwood and softwood veneer logs are published by the Northern Hardwood and Pine Manufacturers Association (52). A guide to Hardwood Log Grading (51) describes a veneer log class. Veneer log scaling and grading of western softwoods have also been consolidated into one set of rules (58).

Specific Characteristics of Interest for Veneer Logs

The relative importance of any one characteristic in a veneer log depends on the end use of the veneer. For example, figured wood may be desirable for hardwood face veneer but undesirable for core and crossband veneer. A summary of some log characteristics and their relative importance according to the end use is given in table 5.

Diameter and Length

While it is true that logs as small as 10 inches (25.4 cm) or less are rotary-cut into veneer, this is not the preferred diameter. Other factors being equal, large-diameter logs are preferred for all veneer cutting. Large-diameter logs mean less handling for a given volume of veneer. Furthermore, better quality veneer can be rotary-cut from large-diameter logs than those of small diameter. This is particularly true for thick veneer such as % inch (4.23 mm).

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Log diameter is even more important for sliced veneer where the width of the veneer is limited to the width of the flitch. The minimum diameter of logs that are used for flat-slicing is about 15 inches (38.1 cm) and for quarter- slicing, 21 to 22 inches (53.3 to 55.9 cm).

In terms of log length, a species that does not have a bole 8 feet (2.44 m) or longer is of limited value for veneer. Most bolts that are rotary-cut are 8 feet (2.44 m) long, even though shorter bolts are cut for core plies, for furniture, and for containers. Most face veneer slicers are 12 to 16 feet (3.66 or 4.88 m) long. While much of the sliced veneer is used in 8-foot (2.44 m) and shorter lengths, a premium is paid for 12- and 16-foot (3.66 and 4.88 m) lengths.

Log Form

For rotary-cutting, it is important that veneer logs have a cylindrical form with the pith in the geometrical center of the log ends. Laboratory and industry tests show that 5 to 6 percent of a typical veneer bolt is lost in rounding it to obtain usable widths of veneer.

Taper and Eccentricity

Taper is more of a problem than slight eccentricity. Narrow widths of veneer are usable, but short length or fishtails generally are not. Taper also causes short grain in rotary-cut veneer. Such short grain is weak in bending and shrinks excessively in length. It may also lead to bleed-through of the glue in thin face veneer.

Logs with pronounced eccentricity result in many narrow pieces of rotary-cut veneer. This veneer tends to be rougher than veneer cut from cylindrical logs because a part of each revolution of veneer is cut against the grain of the annual rings. Eccentric logs are also undesirable because they frequently have abnormal wood (55,57)—tension wood in hardwoods or compression wood in softwoods.

Taper and eccentricity may also increase the amount of thick and thin veneer produced.

Sweep

Sweep or lengthwise curvature of a log is a defect for both rotary and sliced veneer. For one thing such logs often have tension wood

or compression wood. Sweep limits the number of full-length sheets that can be produced from the log. Sometimes sweep can be minimized by judicious bucking of the logs into bolts for rotary-cutting, but individual bolts must be straight. Slight sweep can be tolerated in logs that are to be sliced, but the flitches should be so sawn that the sweep in the log is perpendicular to the plane of the knife used in slicing. This will permit production of full-length veneers from the start of slicing.

Abnormal Wood

Logs with the pith off center often have tension wood or compression wood. Both of these forms of abnormal wood shrink more in length than normal wood and so cause buckling of the veneer during drying.

Tension Wood

Tension wood (57) is often found in leaning hardwood trees. It is most pronounced in low- density species such as cottonwood and aspen. Identifying characteristics in log form include an eccentric pith and silvery, crescent-shaped bands on the log cross section. When tension wood is pronounced, the bands are fuzzy or stringy, because the saw did not cut them cleanly (fig. 5). Tension wood is characterized

M 7.-. 160

Figure 5.—Tension wood in a cottonwood log is indicated by the arrow.

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M 28426 F

Figure 6.—Compression wood in a southern pine log is indicated in the outlined area.

by having little of the lignin that stiffens normal fibers. As a result, the wood tends to bend and cling to the knife rather than sever cleanly in veneer cutting. The cutting of tension wood can be improved by using an extra hard knife (such as a 62 to 64 on the Rockwell C-scale) and by keeping the knife very sharp. The wood is sometimes cooled to about 40° F with low- density species like basswood to improve the cutting of the softer wood.

Compression Wood

Compression wood is typically found in softwood logs that have a pronounced eccentric pith. The crescent-shaped bands are most often found on the wide radius (fig. 6). They are dull, hornlike in appearance, and sometimes have a reddish cast. Compression wood is dense and superficially appears like extra-wide bands of summerwood. Because it is lignified, compression wood cuts well to form a smooth wood surface. However, the stresses in severe compression wood will often cause the green veneer to buckle. The buckle becomes worse in drying and may cause warping in plywood. Pillow (55) gives further information.

Growth Stresses

Most species of wood have growth stresses. However, the severity of these stresses varies

widely. Kubier (36) and others have demonstrated that the wood near the surface of the log is in tension in the longitudinal direction, while the wood near the center of the log is in compression in the longitudinal direction. In the transverse plane or cross section of the log, the wood is in compression near the outside of the log and in tension near the center of the log. In some cases these stresses cause the log ends to split as soon as the log is cut to length. Such an observation should serve as a caution sign when considering a species for veneer.

Log End Splits Due to Growth Stresses

Splits that are in the log typically radiate from the pith like spokes of a wheel. When green wood is heated, it expands tangentially and shrinks radially, enlarging these splits (fig. 7). Splits are particularly bad in logs that are to be rotary-cut, because either the bolt is lost completely from splitting during cutting or from the corresponding splits in the veneer. Veneer splits are limiting defects as defined by the product standards for plywood (table 1 and U.S. Department of Commerce {63,64)).

M 136 337-1

Figure 7.—Splits in the end of a Brazil nut bolt. The splits came from growth stresses in the tree and were

greatly enlarged by heating the bolt to 200° F.

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Log end splits are not quite so serious when the wood is to be sliced. The log can often be sawed to eliminate the major split by making the first saw cut through the split. It is sometimes possible to eliminate other splits if the log is to be quarter- or rift-sliced.

Even with careful cutting, some of the stresses in the tree are retained in the flitches. Consequently the flitches tend to bow toward the bark side, particularly during heating. Sometimes flitches are strapped together during heating to reduce this bow. The bow in the flitch that is to be flat-sliced can often be forced out when the flitch is mounted on the flitch- table before slicing. On the other hand, the bow in a quartered flitch is not changed when the flitch is mounted and sliced. Bowed veneer results in considerable loss when the edges of veneer are made parallel by clipping.

All in all, a species known to have marked growth stresses will generally yield more veneer by flat-slicing than by quarter-slicing.

Ring Shake

Ring shake is another undesirable characteristic in logs to be used for veneer. Shake is accentuated by heating in water or steam, and there is no way of eliminating it. To prevent additional damage, plastic clips are sometimes driven across the ring shake to help hold the bolt together during rotary cutting. The plastic can be cut without damaging the knife edge. Use of a roller bar rather than a fixed nosebar is reported to permit an operator to come closer to shake without having the bolt break out. The roller bar exerts less drag on the bolt, so there is less shear force to cause the wood to break at the ring shake. Shake is much more common in old growth than in young trees.

Knots

Knots are one of the most common and important imperfections in veneer logs. Knots may be sound and intergrown, encased, or decayed. Most encased or decayed knots fall out during the drying of veneer. Knot holes are more limiting defects in standard veneer grades than intergrown knots of the same diameter.

In general, there are fewer knots on logs of large diameter, on logs from trees grown in fully stocked stands, and on butt logs. Fre-

M 87667 F Figure 8.—Knot sequence from the indicator on the bark of Douglas-fir (1) to bolt diameters of 38 inches (2) ; 35 inches (3) ; 30 inches (4) ; 21 inches (5) ; and

17 inches (6).

quency of knots is also related to species. Logs of white fir and eastern hemlock, for instance, have many more knots than species like noble fir, longleaf pine, and yellow-poplar.

Some species have many knots because the limbs persist for many years. For example, limbs persist on Douglas-fir logs for up to 150 years. In contrast, the limbs of southern pine frequently fall off a few years after they die. One implication is when all logs come from second growth, 100 years or less in age, the southern pines will furnish more knot-free veneer than Douglas-fir.

Knot indicators are retained in the bark many years after the limbs have been overgrown. The ability to recognize these indicators is a key factor in accurate grading of logs (37). How an indicator on the surface of a Douglas- fir log signaled a serious defect is illustrated in figure 8.

The one exception to the degrading effect of knots is decorative veneer of species like western redcedar and white pine. These specialized products call for flitches having sound intergrown knots 1 inch or smaller in diameter. A limited number of knots are permitted and are

16

desirable in some but not all decorative veneers used as faces of paneling.

Epicormic branches and adventitious buds are relatively minor defects that occur on most hardwoods, particularly elm, oak, maple, and sweetgum (37). They are not permitted in clear veneer for some furniture grades but are accepted in many grades of wall paneling.

* • 'M

if**r

.î^. M 91476 F

Figure 9.—A bolt of black gum with many burls on the surface. Veneer cut from this bolt is shown in

figure 10.

Straight and Irregular Grain

Straight grain is generally considered desirable for veneer logs. A typical commercial veneer log grade will state that deviation from straight grain shall not exceed so many inches per foot of length of log. Spiral grain in the wood is often indicated by spiral grain in the bark. As described under physical properties of wood, straight-grained wood is easier to cut and dry and generally performs better in plywood panels than veneer having irregular grain.

The one exception to this rule is for logs suitable for decorative face veneer. In some cases irregular grain is desired because it enhances figure in the veneer cut from the log.

The detection of figured wood in standing trees and logs is described in Pillow (56). Essentially the method is based on examining the bark and log end sections for inclination and waviness of the cellular structure of the wood. In some instances this can be detected from the rough outer bark. For example, yellow birch with a smooth bark is generally straight- grained, while that with rough irregular bark often contains curly grain or other grain irregularities. Curly grain may not be apparent in the outer bark but if the outer bark is removed with a draw shave to reveal the soft layers of the inner bark, then the grain pattern can be seen. Figured wood may also be indicated by the shape of splits in the log end surface. If the splits have alternate zig-zag patterns, the wood will almost certainly have a pronounced figure.

Another technique is to cut a small section from the log end in a radial direction and then split this piece. The split will follow the grain direction and indicate if it is wavy or curly.

Burls on the surface of the log may indicate that the entire log has irregular grain (figs. 9 and 10). Veneer that is figured throughout from small or large burls is often valued for its decorative effects. Other face veneers (table 1) are limited in the size and number of burls that are permitted.

Color

In general a uniform color is desirable in veneer logs, but the color desired varies with the end use. Light-colored wood is appropriate for containers as it makes a good background for marking and is psychologically pleasing to

17

M 92128 F Figure 10.—Burls in the sapwood (1) and heartwood (2) of rotary veneer cut from the bolt shown in figure

9. The burls persisted to the 8-inch core of the bolt.

the consumer. Maple logs with wide sapwood zones are currently in demand because of the popularity of white face stock. In contrast, the heartwood color is in demand for species like cherry and walnut.

Nonuniformity in color between logs can cause problems. For example, the preferred color of walnut is a light gray-brown. Green and purple tinges that sometimes occur in walnut are not wanted because they cause special problems in finishing.

Studies at the Forest Products Laboratory have shown that the color of walnut varies with the geographic area in which the trees grow. There is some evidence that the color of walnut heartwood is related to the type of soil in which the tree has grown. In addition, color in veneer can be regulated to some extent by the heating and drying process.

When a mixture of species, such as the lauans, is used, the material typically available for faces will display a variety of heartwood colors. Recently some veneer and plywood manufacturers have been using electronic devices to separate such veneer into several groups according to color. This simplifies the finishing process, and aids in marketing the products.

Gum Streaks and Pockets

Gum streaks and pockets in hardwood logs can sometimes be seen on log ends. Large gum pockets may be detected as bulges on the log. While a small amount of gum can be permitted in some products, gum is generally regarded as undesirable.

Pitch and Pitch Pockets

Pitch is found in one hardwood genus and in many softwoods like Douglas-fir, ponderosa pine, and southern pine. Massed pitch and pitch pockets are limiting defects in veneer (table 1).

Bark Pockets

Bark pockets occur in some softwoods and in hardwoods like the oaks and hickories. They may show on the log ends or as overgrowths on the bark (37). Bark pockets are limiting defects for most veneer uses.

Holes

Large holes such as those resulting from a rotted branch stub or a woodpecker nest are major defects in veneer logs.

Medium holes up to V2 inch (12.7 mm) in diameter—if extensive—may seriously degrade the log for use as veneer. Such holes may have been made by grubs that tunnel in living trees like oak, or result from tapping sugar maple trees, from bullets, or from an increment borer. Medium-size holes are generally accompanied by severe stain.

Pin worm holes made by ambrosia beetles occur in hardwoods like oak and ash and also in various softwoods. This defect can be particularly serious with tropical hardwoods. A few scattered pin worm holes can be tolerated in most veneer uses, but heavy attack seriously degrades the veneer.

Decay

Decay is a severe defect in veneer logs, especially for rotary cutting. If the log center is decayed and soft, the chucks may not be able to hold the veneer bolts securely enough to permit rotary cutting. Slightly or moderately decayed logs can sometimes be cut into veneer, providing the wood is still reasonably firm. The best example of this is Douglas-fir

18

that has been attacked by F ornes pini (white- pocket). Millions of square feet of softwood plywood have been made from rotary-cut Douglas-fir veneer containing white-pocket. Sound ñitches for slicing can sometimes be sawn from logs with considerable decay.

Fire Scars

Fire scars are generally obvious on the cross section of a log, and often indicate associated decay. Extensive fire scars make logs of doubt- ful value for use in veneer.

Seams

Seams are radial cracks that may or may not be overgrown. They may be caused by wind, lightning, or frost. Seams generally originate at the surface of the log and occur in the standing tree. In contrast, splits due to growth stresses start at the pith and generally do not extend to the surface of the log. As a result, seams are visible on the standing tree but splits are not. As seams occur through the cambium layer they may be overgrown by callus tissue. Splits never have such overgrown tissue. Spe- cies that may have seams include oak, ash, maple, beech, and birch. The seriousness of this defect depends on how deeply it penetrates the log and whether it is parallel to the log length or spiralled. A straight seam can be clipped from the veneer with less waste than that caused by a spiralled seam.

Bird Peck

Bird peck and associated stain is a common characteristic on such species as yellow-poplar and hickory. Bird peck can be detected by characteristic holes in the bark and by strain on the log ends. Logs with this characteristic are generally suitable for core and crossband veneer but may be limited for use as face veneer.

Stain

The term "stain'' has been used to describe several different phenomena. Causes of some stains are known, such as fungal or contact with iron, while others are still being studied. Further, the severity of some stains is directly related to the amount of sapwood and the environment in which the log is held. One way

of separating stains for practical purposes is to consider those in the standing tree as op- posed to those that may develop after the tree is felled.

Stain in Standing Trees

The terminology concerning stain in standing trees is not well accepted. For example, some authors limit the term mineral stain to small olive or greenish-black discolorations in the heartwood and sapwood of the maples and the gums. Others use the same term to describe brown stains in species like aspen and oak. Still other authors attribute these and other stains in oak and aspen and other hardwoods to oxidation of cell materials and call them oxidation stains. Bacteria have also been reported as associated with various stains in living trees.

For purposes of this publication, stains in standing trees will not be separated. Stains are found in both heartwood and sapwood of the living tree and are often associated with injury to the tree such as insect attack or broken branches.

In addition to the discoloration, intense areas of stain are more likely to collapse and check during drying. Higher ash content has been found in dark green or black stained maple than in normal bright wood. Some plant personnel report more rapid dulling of tools when cutting such stained wood.

Brown stain is common in oak trees growing on upper slopes or ridge tops. Because of the poor growth site, these trees are generally also of poor form and do not supply many potential veneer logs. Oak trees growing on moist soils that may be water-saturated for extended periods are also subject to stain. Logs from such trees may in other respects appear to be of quality suitable for veneer.

Stain in standing trees may be sporadic and localized to small streaks or it may occur over broad areas. Consequently, the stain may or may not be visible on freshly cut log ends.

Stain that May Develap in Stored Logs or Green Veneer

Four types of stains that may develop in stored logs or during veneer processing are sap stain, mold, oxidative stain, and iron stain.

Sap stain is fungal in origin and is most commonly blue in color. It is particularly trouble- some in the sapwood of species like sweetgum

19

and southern pine if the logs are stored during periods of warm, humid weather. The color is caused by a concentration of hyphae. For many veneer uses, blue stain is objectionable. It should be controlled by keeping log storage to a minimum and, if necessary, by use of chemical sprays or water sprays. Veneer can be protected by drying the stock as rapidly as possible or by dipping or spraying with an antistain solution if drying is to be delayed.

Molds are also fungal in origin, but the color (yellow, brown, red, purple, green, blue, or black) comes mainly from spores of the fungi. Mold is characterized by a downy growth on the surface of the wood. Mild temperature, still air, and abundant moisture promote growth of mold. Under these conditions mold may be a problem in green sapwood veneer that is stored 3 or more days before drying. Control methods are similar to those suggested for blue stain.

Oxidative stain is a chemical stain that is thought to be the result of oxidation, sometimes promoted by enzymatic action on certain materials stored in the wood cells. Like blue stain and mold, it develops in the sapwood of logs and green veneer when favorable moisture and temperature exist. It has caused objectionable discoloration of light-colored face veneers of species like birch and maple. In logs, the stain progresses gradually in from the ends during warm-weather storage, so cold-weather storage or reducing storage time is the best method for preventing this stain. Use of a white lead paste end coating, or especially of a waterspray, during storage may materially reduce this stain but will not stop it. Drying the veneer as soon as possible after cutting also reduces the chance of oxidation stain. Concentrated oxalic acid will generally bleach oxidation stain but not fungal- caused blue stain.

Tannin and other polyphenols react with iron and steel in the presence of water to form a blue-black stain. This becomes very obvious and objectionable on face veneer of species like oak and redwood if the wet wood is in contact with iron or steel for even a brief time. Concentrated

oxalic acid or hydrofluoric acid will bleach out iron stain. These acids must then be flushed from the wood or the stain may reappear.

Some references on stains originating during processing logs into veneer include Scheffer (60) and Scheffer and Lindgren (61).

Man-Made Defects Other than Holes

Man-made defects include stump pull, felling splits, log handling damage, and embedded metal.

Stump Pull and Felling Splits

Both these defects cause splits in the veneer cut from the logs. Stump pull is generally obvious as a jagged hole on the long end. Butt logs should be carefully examined as felling splits may close and be difficult to detect.

Log Handling Damage

Handling logs with tongs is a needless source of defect. Not only may the tongs put holes in otherwise clear veneer, but they also frequently embed sand or grit that damages the knife used to cut the veneer. Similar problems occur with logs that have dirt or gravel embedded in the outer sapwood when the logs were dropped or damaged on a gravel or cinder surface.

Embedded Metal

Buried metal is a serious problem in logs cut from street trees and fence rows. Because barbed wire and nails will generally damage a veneer lathe or slicer knife, many veneer log buyers will not purchase logs that come from along fences or streets. Buried metal may be detected because it has formed a bump on the log. Many veneer mills have magnetic metal detectors for screening all logs and flitches.

Soft lead from buck-shot and small arms can be cut without damaging the lathe or slicer knife. However, steel-jacketed bullets or shrap- nel such as may be found in timber from a battle zone are very serious defects. Aside from the damage to the knives used to cut the veneer, buried metal often causes extensive stain in the wood.

20

VENEER FROM WOOD SPECIES THAT GROW IN THE UNITED STATES

This chapter covers tree species that grow large enough and in sufficient volume in the United States so that they can be considered for veneer.

While the use of veneer and plywood is increasing, the timber available in such well- known veneer species as yellow birch and Douglas-fir has declined. As a result, it is be- coming more important to know the potential for making useful veneer from all species that grow in the United States.

A number of species have been studied for use as veneer at the U.S. Forest Products Lab- oratory. In addition, other Government laboratories and universities have published information on veneer species. Still further information is available from the veneer industry. From such sources scattered information has been collected and condensed for this publication. If no published information was available on a species for veneer, the species has been evaluated on the basis of the known physical and mechanical properties of the wood.

Information about the use of veneer from various species is given in tables 6 and 7. The specific gravity figures given there help classify the species as in tables 2 to 4.

The specific gravity figures for hardwoods can be used with the information in the next section to select a favorable range for heating bolts or flitches prior to cutting veneer.

The last four columns of table 6 and 7 rate the species for use in various products. Detailed information on log characteristics, wood characteristics, processing into veneer, and use of the veneer as related to wood species that grow in the United States is given in Appendixes II to VI.

Similar but abbreviated information on wood species from around the world is given in the report, "Veneer Species of the World," published in 1976 for the International Union of Forestry Research Organizations. Copies can be purchased from the National Technical Infor- mation Service.^

2 The National Technical Information Service of the U.S. Department of Commerce is located at 5285 Port Royal Road, Springfield, Va. 22161.

21

Table 2.—Classification of species for construction and industrial plywood, PS 1-714.

Group 1 Group 2 Group 3

Apitong !• 2 Beech, American Birch

Sweet Yellow

Douglas-fir ^ Kapur 1 Keruing i« 2 Larch, western Maple, sugar Pine

Caribbean Ocote

Pine, southern Loblolly Longleaf Shortleaf Slash

Tanoak

Cedar, Port Orford Cypress Douglas-fir ^ Fir

California red Grand Noble Pacific silver White

Hemlock, western Lauan

Almon Bagtikan Mayapis Red lauan Tangile White lauan

Maple, black Mengkulang 1 Meranti, red ^' * Mersawa ^ Pine

Pond Red Virginia Western white

Spruce Red Sitka

Sweetgum Tamarack Yellow poplar

Alder, red Birch, paper Cedar, Alaska Fir, subalpine Hemlock, eastern Maple, bigleaf Pine

Jack Lodgepole Ponderosa Spruce

Redwood Spruce

Black Engelmann White

Group 4 Group 5

Aspen Bigtooth Quaking

Cativo Cedar

Incense Western red

Cottonwood Eastern Black (western poplar)

Pine Eastern white Sugar

Basswood Fir, balsam Poplar, balsam

^ Each of these names represents a trade group of woods consisting of a number of closely related species. 2 Species from the genus Dipterocarpus are marketed collectively: Apitong if originating in the Philippines; Keruing if originating in Malaysia or Indonesia. ' Douglas-fir from trees grown in the states of Washington, Oregon, California, Idaho, Montana, Wyoming, and the Canadian Provinces of Alberta and

British Columbia is classed as Douglas-fir No. 1. Douglas-fir from trees grown in the states of Nevada, Utah, Colorado, Arizona and New Mexico is Douglas- fir No. 2.

* Red meranti is limited to species having a specific gravity of 0.41 or more based on green volume and ovendry weight.

Table 3.—Density categories of the most commonly used species based on specific gravity ranges for hardwood and decorative plywood, PS 51-71

Category A High-density species (0.56 or more specific

gravity)

Category B Medium-density species

(0.43 through 0.55 specific gravity)

Category C Low-density species

(0.42 or less specific gravity)

Ash, commercial white Beech, American Birch, yellow, sweet Bubinga Elm, rock

Madrone, Pacific Maple, black (hard) Maple, sugar (hard) Oak, commercial red Oak, commercial white Oak, Oregon

Paldao Pecan, commercial Rosewood Sapele Teak

Ash, black Avodire Bay Cedar, Eastern red ^ Cherry, black

Chestnut, American Cypress ^ Elm, American (white, red, or gray) Fir, Douglas ^

Gum, black Gum sweet Hackberry Lauan (Philippine mahogany) Limba

Magnolia Mahogany, African Mahogany, Honduras Maple, red (soft) Maple, silver (soft) Primavera Sycamore Tupelo, water Walnut, American

Alder, red Aspen Basswood, American Box elder

Cativo Cedar, Western red ^ Ceiba Cottonwood, black Cottonwood, Eastern

Pine, white and ponderosa ^

Poplar, yellow Redwood ^ Willow, black

1 Softwood.

22

Table 4.—Species^ for wirebound boxes as listed in Federal Specification PPP-B-585b

Group I Group II Group III Group IV

Aspen (popple) Douglas-ñr Ash (except white ash) Ash, white Basswood Hemlock Elm, soft Beech Buckeye Larch, western Maple, soft Birch Cedar Pine, southern yellow Sweetgum Elm, rock Chestnut Tamarack Sycamore Hackberry Cottonwood Tupelo Hickory Cypress Maple, hard Fir (true firs) Oak Magnolia Pecan Pine (except southern yellow) Redwood Spruce Yellow-poplar Willow

1 Groupings are based on specific gravity and other properties of importance in container construction. When a group is specified, any species in the group can be used.

Table 5.—Importance of physical and mechanical wood properties and log characteristics as related to manufacture and use of the veneer

Construction Decorative Core and Container Property and indus- face crossband veneer Comments

trial veneer veneer for and plywood decorative

panels plywood

Physical property Specific gravity A B A B Green moisture content B B B B-C Permeability B C B B-C Shrinkage B B A B Close grain B A-B A B-C Fine texture C B B C Straight grain A A-B A B Parenchyma B B B B-C Wax B B B B Polyphenols B B B B Color of heartwood C A C A-B Dimensional stability B B A B Resin B A A B Gum B A A B Hard deposits B A-B B B Figure C A C C Figure is desirable for face

„j 1 ; _l-i_ f^-

Odor

Mechanical property Strength in tension

perpendicular to grain Hardness Modulus of elasticity Modulus of rupture Shear Compression perpendicular

to grain Compression parallel to grain

A

veneer and other uses Odor is important for containers used with food.

B B B B B B C B A C C B A C C A A C C C

A B C B A C C B

23

Table 5.—Importance of physical and mechanical wood properties and log characteristics as related to manufacture and use of the veneer—continued

Construction Decorative Core and Container Property and indus- face crossband veneer Comments

trial veneer veneer for and plywood decorative

panels plywood

Log characteristic Cylindrical form A B A B Taper A B A B Eccentricity B B B B Tension wood B A A B Compassion wood A B A B Sweep A B A B Growth stress B B B B Log end splits A B B B Ring shake A A A A Knots B A A B Epicormic branches and

adventitious buds C B B C Burls B B B B Color C A C B Pitch pockets B A A B Pitch in crossbands may

bleed through face veneer Bark pockets B A A B Grub holes B A A B Pinworm holes B B C B Heavy pinhole damage will

degrade all veneer Decay A A A A Some types of decay are per-

mitted in Construction grade plywood

Fire scars B A A B Frost cracks B A A B Veneer from other parts of

the log may be top grade Mineral streak C A C C Other stains C A C B Bird peck C A B B Stump pull A A A A Felling splits A A A A Handling damage A A A A Embedded metal A A A A Growth rate B A B B

A—Of major importance Í B—Of moderate importance < These ratings are not hard and fast but are indicative of relative importance of various characteristics. C—Of little importance (

24

Table 6.—Specific gravity and suitability of some U.S. species for various veneer uses ^

Common name

Relative suitability Specific gravity 2 Construction Decorative Inner plies of Container

plywood face decorative veneer and veneer panels plywood

HARDWOODS

0.34 C B A-B A .37 C B B B

.45 B A B A

.53 B A B A

.53 B A B A

.50 C A B A

.48 C B C B

.47 B A B A

.55 B A B A

.35 C B A A

.35 C B A A

.32 C C A A — C C A A

.56 B B C A

.49 B A-B B B

.45 C B B B

.48 B A-B B B

.49 B B B B

.60 B A B B

.55 B A B B

_ C C A B .33 C C A B .36 C A C C .47 B A B A

0.30 C B B A .31 C C B A .37 C C B A — C B B A

.46 B A B A

.59 B A C A

.57 B A C A

.48 B A B A

.60 B A C A

.60 B A-B C B

.49 B A-B C A

.60 B A C B

.56 B A C B

.60 B A C B

.61 B A C B

.64 B A C B

.66 B A C B

.64 B A C B

.62 B A C B

.50 C A C C

.60 C A C B

.53 B-C A B B-C

.51 C A C C

.66 C B C B

.58 C A C B

Alder Nepal Red

Ash Black Blue Green Oregon Pumpkin Shamel White

Aspen Bigtooth Quaking

Basswood American White

Beech, American Birch

Alaskan paper Gray Paper River Sweet Yellow

Buckeye Ohio Yellow

Butternut Cherry, Black Cottonwood

Balsam poplar Black Eastern Swamp

Elm American Cedar Rock Slippery Winged

Eucalyptus Hackberry Hickory, pecan

Bitternut Nutmeg Pecan Water

Hickory, true Mockernut Pignut Shagbark Shellbark

Holly, American Honeylocust Koa Laurel, California Locust, Black Madrone, Pacific

25

Table 6.—Specific gravity and suitability of some U,S, species for various veneer uses ^—continued

Common name

Specific gravity 2 Construction

plywood

Relative suitability

Decorative face

veneer

Inner plies of decorative

panels

Container veneer and

plywood

HARDWOODS—continued Magnolia

Cucumbertree Southern

Maple Bigleaf Black Boxelder Red Silver Sugar

Oak, red Black California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet Shumard Southern red Water Willow

Oak, white Bur Chinkapin Delta post Durand Live Oregon white Overcup Post Swamp chestnut Swamp white White

Ohia Persimmon, common Sassafras Silk-oak Sugarberry Sweetgum Sweetbay Sycamore, American Tanoak Tupelo

Black Swamp Water

Walnut, Black Willow, Black Yagrumo hembra Yellow-poplar

0.44 B C A A .46 B C A A

.44 C A B A

.52 B A B A

.41 C B C B

.49 B B A A

.44 C B A A

.56 B A B A

.56 B A B B

.51 B A B B

.61 B A B B

.57 B A B B

.56 B B C B

.56 B A B B — B A B B

.58 B A C B

.60 B A B B — B A B B

.52 B A B B

.56 B B C B

.56 B B C B

.58 B B B B — B B C B

.60 B A B B — B A B B

.81 C B C B

.64 C B C B

.57 B B C B

.60 B B C B

.60 B A B B

.64 B A B B

.60 B A B B 0.70 B B C B

.64 C A-B C B

.42 C B C B

.51 B A B B

.47 B B C A

.46 B B B A

.42 B C A A

.46 B A B A

.58 B B C B

.46 B B B A

.45 B B B A

.46 B B B A

.51 B A B B

.34 C B-C B B

.26 C C B-C B

.40 B B A A

26

Table 6.—Specific gravity and suitability of some U.S, species for various veneer uses ^—continued

Common name

Specific gravity 2

Relative suitability-

Construction plywood

Decorative face

veneer

Inner plies of decorative

panels

Container veneer and

plywood

Cedar Alaska- Atlantic white- Eastern redcedar Incense- Northern white- Port-Orford- Western redcedar

Cypress Baldcypress Pondcypress

Douglas-ñr Coast Interior north Interior west

Fir Balsam California red Grand Noble Pacific silver Shasta red Subalpine White

Hemlock Eastern Mountain Western

Juniper Alligator Rocky Mountain Western

Larch, Western Pine

Digger Eastern white Jack Jeffrey Knobcone Limber Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Sand Shortleaf Slash Spruce Sugar Table-Mountain Virginia Western white Whitebark

Redwood

.42

.31

.44

.35

.29

.40

.37

.42

.45

.45

.46

.34

.36

.35

.37

.40

.36

.31

.37

.38

.43

.38

.50 .51 .51

.48

.34

.39

.37

.37

.47

.38

.54

.45

.50

.38

.44

.36

.46

.56

.41

.35

.49

.45

.36

.37

.38

SOFTWOODS

B C C

B-C B-C

B A-B

A-B B

A A A

B-C A-B A-B A-B A-B A-B B-C A-B

B-C B

A-B

C C C

A

B-C B-C B-C

B B-C B-C

A B A

B-C B B B

B-C A A

B-C B-C B-C B-C

B B-C A-B

B B A B B B A

A A

B-C B-C B-C

C C C C C C C C

C C C

C C C B

C A-B

C A C C C B C C C A B C C C C A C C A C A

A A B B B A

B-C

B B

B B B

C B-C B-C B-C B-C B-C

C B-C

B-C B B

C C C C

C B C B C C C C C C C B C C C C C B C C B C C

A A C B B A B

A A

A-B A-B A-B

A A A A A A A A

A-B A A

C C C B

B A B A A A B A B B B A A B B B B A B B A A A

27

Table 6.—Specific gravity and suitability of some U.S. species for various veneer uses ^—continued

Common name

Relative suitability Specific gravity 2 Construction Decorative Inner plies of Container


SOFTWOODS—continued

.38 B-C C C A — B-C C C A

.33 B c c A

.38 B c c A

.37 A-B B B A

.37 B-C C C A

.49 A-B B C B

.60 C A C B

Spruce Black Blue Engelmann Red Sitka White

Tamarack Yew, Pacific

1 Rating of A indicates species is well suited for end product; B, intermediate; and C, generally not well suited for this product. 2 Based on weight when ovendry and volume when green.

Table 7.—Specific gravity and suitability of some imported species for various veneer uses

Common name Relative suitability ^ for

Specific gravity 2 Construction Decorative Inner plies of Container


0.60 B B-C C B-C .59 A B B B .51 B A B B .56 B B B-C B

.65-.76 B A B-C B .68 A-B C B-C B .40 B B A-B A .25 C C A-B C .51 C C C B-C .64 A-B B-C C A-B

.46-.70 A-B B-C C A-B .38 B B A A

.40-.46 B A B A-B .49 B A-B B B .45 B A A B .56 A-B B-C B-C B

.36-.51 B A A B .51 B B B B

.45-.60 B B-C B B .55 A C B-C B .37 B B A A .60 B A B-C B .39 B-C A B B .80 B-C A B-C B-C .60 B A B A .57 B A B B

Angélique Apitong Avodire Brazil nut Bubinga Caribbean pine Cativo Ceiba Determa Kapur Keruing Klinki pine Lauan Limba Mahogany Mengkulang Meranti Mersawa Muritinga Ocote pine Okoume Paldao Primavera Rosewood Sapele Teak

1 Information primarily from "Properties of Imported Tropical Woods," by B. F. Kukachka, USDA Forest Serv. Res. Pap. FPL 125, 1970 and from "Veneer Species of the World," lUFRO Interim Report, 1976.

2 Specific gravity based on volume when green and weight when ovendry. 3 Rating of A indicates species is well suited for end product; B, intermediate; and C, generally not well suited for this product.

28

TECHNIQUES FOR PEELING, SLICING, AND DRYING VENEER

From a given supply of logs, the processor can improve the quality of veneer in two general ways: Handle the wood so that variability is minimized, and carefully select and adequately maintain processing equipment.

Handling the wood to minimize variability involves such things as log storage, breaking logs into bolts or flitches, and heating or cooling the wood prior to cutting.

Processing equipment involves the basic de-

sign of debarkers, lathes, slicers, veneer conveyors, clippers, and dryers. All of this equipment must be properly maintained, set up, and operated to consistently produce good-quality veneer.

The veneer processing techniques described in this bulletin follow the chronological steps in which they occur from log to dry veneer. This is followed by a section on quality control and trouble shooting to minimize veneer defects.

LOG STORAGE Veneer logs can be kept in good condition for

some time providing the storage conditions are suitable. With poor storage conditions, logs can deteriorate by drying and cracking of the log ends and other exposed wood; development of blue stain, decay, and oxidation stain ; attack by insects; cracks and grain separation due to freezing and thawing; development of undesirable odor ; and increased porosity due to attack by bacteria.

End drying and splits in logs can occur with susceptible species like dense hardwoods in one hot, dry, windy day when the sunlight falls directly on the log end. End drying is less of a problem with a species like Douglas-fir stored in winter in the damp Northwest. Blue stain and mold can occur in a week to 10 days on the sapwood of species like sweetgum and southern pine stored in humid summer weather in the South. Decay generally requires weeks or months to develop. Oxidation stain, which low- ers the value of white sapwood of species like birch and maple, may occur through the ends of unprotected logs stored several weeks during summer.

Insects like lyctus beetles may attack a log within hours after felling. To minimize insect attack, logs stored in warm weather should be used within 2 weeks after felling, treated with an approved chemical,^ or stored under water.

Freezing and thawing of logs of species such as sweetgum and claro walnut may fracture the wood so that it is useless for veneer. This is less of a problem with species grown in northern climates.

3 Check with the local County Agricultural Agent or State Agricultural Experinment Station for approved recommendations.

The sapwood of many species is subject to attack by anaerobic bacteria even though the wood is kept wet. This has caused objectionable odor, particularly in tropical hardwoods like muritinga, ceiba, and cativo. Bacteria may also cause excessive porosity in pines like ponderosa and the southern species. The best way to control bacterial action is processing felled trees within 1 month or by storing the wood below 40° F. (5° C). Spraying with chemicals may help, providing the bacteria has not already en- tered the wood.

Given these many possible problems, what is the best procedure for log storage? In general, veneer log storage should be kept to a minimum. The first logs into storage should be the first ones out of storage for processing. Ideal storage conditions would be to end coat and keep the bark intact on tree-length logs that are either held at high humidity and a temperature just above freezing (34° F or 1° C) or completely submerged in cold water (34° to 40° or 1° to 5° C). The next best system would be to keep the logs under a roof and all surfaces constantly wet by a water spray. This would be just as good as the first method, providing the temperature was between 34° and 40° F (1° to 5° C).

A common storage method that is generally satisfactory is to keep all log surfaces wet with a water spray but without using a roof. When water spray is not feasible, then a chemical spray and end coating may permit satisfactory storage. Less desirable methods which are sometimes suitable include floating the logs in a pond and cold-decking the logs. A much more complete discussion of log storage is given by Scheffer (60).

29

BARK REMOVAL

The subject of bark removal is one in which two people, both knowledgeable in the field, may disagree. The reason is the wide variability in difficulty of removing bark. Three factors that must be considered are: (1) variability of bark adhesion within a species; (2) variability of bark adhesion between species; and (3) type of equipment used for debarking.

Variability Within a Given Species

Spring-cut logs are easier to debark than fall- cut logs of the same species. This general state- ment is true for all species. Actual measurements of the wood-to-bark bond on several species indicate that this increase of bond strength from spring to fall may be 100 to 200 percent.

A second factor is the temperature of the wood and bark at the time of bark removal. Heated wood is much easier to debark. When veneer logs were commonly debarked by hand, a main reason for heating the logs was to make bark removal easier. Frozen logs are particularly difficult to debark. A plant may even install a hot pond to get logs above freezing so they can be more readily debarked with a mechanical debarker.

Another factor in debarking is whether or not the bark has been allowed to dry on the log. Assuming no bacterial action has taken place, the bark generally adheres more tightly after it has partially dried.

A fourth factor is the action of bacteria. Logs stored in a warm pond or under a sprinkler during summer may be subject to attack by bacteria. Bacteria seem to prefer the inner bark as a food source. Consequently, logs stored in a pond and attacked by bacteria may have the bark loosened so that it will come off in one big sheet. Such a big piece may jam the bark conveyor. Conversely, bacteria attack may make peeling of bark much easier when using hand tools.

Species Differences

Individual wood species differ in strength of the bond between the bark and the wood. In one study of fall-cut logs, the bark-to-wood bond of quaking aspen was more than 40 percent stronger than that of red spruce.

Some species like basswood and elm have stringy bark. This becomes a problem in con-

veying the material from a mechanical debarker, as bark may come off in large sheets.

In general, softwoods like pine are easier to debark than hardwoods like hickory, but there are many exceptions. For example, fall-cut eastern hemlock is reported to be more difficult to debark than northern hardwoods like maple and birch. Other examples of softwoods that are difficult to debark are cypress with a fluted base, western redcedar with stringy bark, and redwood with very thick bark. The difficulty of bark removal of species that grow in the United States is shown in Appendix IV.

Types of Equipment Used

Different systems have been used for debarking veneer logs, including hand tools, bark saws, water under high pressure, flailing chains, and drum debarkers. Some mills have used an old lathe to debark and round bolts. At present, however, two methods are by far the most common for debarking veneer logs—^the cambio- shear or ring debarker, and the rosser-head debarker. Combination machines may use either cambio-shear or rosser-head or both.

Some factors to consider in choosing a debarker, besides the original and operational costs, include species to be debarked, volume of wood to be debarked, maximum and minimum diameter of logs to be debarked, importance of fiber loss, pollution, ease of operation, and ease of maintenance.

In general, the rosser-head debarker has a lower initial cost, lower maintenance cost, is easier to adjust, and is more adaptable for logs of a wide range of diameters. The rosser-head is generally preferred for debarking rough logs of species like hickory, logs that vary widely in diameter, and logs that may be frozen.

The cambio-shear or ring debarkers are generally preferred by plants processing logs with relatively uniform diameters and where high production and low fiber loss are important. A typical installation would be in a large southern pine plywood plant.

Several manufacturers of cambio-shear debarkers state that, by proper adjustment of tool pressure and feed, their equipment can debark any species under any conditions, including frozen logs. Similarly, manufacturers of rosser- head debarkers state their equipment can be used to debark any species under any conditions.

30

SAWING INTO BOLTS OR FLITCHES

It is generally desirable to harvest logs in as long lengths as possible and to saw into bolts or flitches at the veneer-cutting plant. The reasons for doing this include less waste from end drying of the logs, a better opportunity to observe all sides of the log before cutting, availability of skilled labor trained to buck and saw flitches from the logs for the best use, and better mechanical equipment for handling and sawing the logs.

The sequences of debarking, bucking into bolts, and heating depends on type of logs, debarking and sawing equipment at the plant, and whether log end splitting is a factor during heating. In general, debarking reduces heating time, as bark is a good insulator. Heating in long lengths reduces waste due to log end splits. On the other hand, bark indicators of hidden defects in the logs may help the sawyer decide where to break the logs for best grade. The bark may also protect the logs during handling.

A method sometimes used with hardwoods that tend to end split is to debark in long log lengths, heat in long log lengths, and then buck into bolts just prior to cutting veneer. This method reduces the time required to heat the bolt by eliminating insulation by the bark. Log end splits are confined largely to the ends of the long log and minimized at bolt ends exposed by crosscutting after heating. The process requires a continuous debarker, long heating vats, and equipment to handle long logs. Other disadvantages are that the bark indicators of defects are lost before bucking, and care must be used to prevent the debarked logs from picking up grit during handling.

A method used with softwoods like southern pine is to debark in long log lengths, crosscut bolts, and then heat prior to peeling. This requires a continuous debarker but permits heating vats and handling equipment which work with 8-foot and shorter blocks. It is a satisfactory method if end splitting is not a serious problem and the handling equipment is kept clean so the debarked logs do not pick up grit.

Large-diameter logs such as old-growth Douglas-fir are sometimes cut to bolt length in a pond, debarked in a machine designed for 8-foot lengths, and then heated or cut at room temperature.

The debarking-sawing-heating sequence used

for flitches is generally to buck to length, then saw the flitches, and finally heat the flitches. As flitches are generally a step in producing face veneer, bark indicators are important for cutting logs to length and for producing the flitches. Most or all of the bark is removed in sawing and so does not significantly retard heating. The heated flitches are cleaned and any remaining bark removed with a flitch planer just prior to slicing.

Saws Used in Processing Logs to Bolts and Flitches

Logs are cut to length of bolts or flitches primarily with large circular saws or with chain saws. In both cases it is important that the log and saw be positioned so the cut is at a right angle to the axis of the log.

Logs are generally sawn into flitches with a handsaw or a circular saw. The vertically movable circular saw that is mounted over the log carriage permits sawing logs into thirds as well as halves and quarters. In all cases it is important that the log can be accurately positioned with respect to the sawline and that the sawyer can see both ends of the log. If both lumber and veneer flitches are to be produced, the band- saw may be advantageous, as generally a smaller saw kerf is produced.

What Does the Sawyer Look For?

Bolts

Factors to be considered in bolts are sweep in the log, end trim, presence of large defects like knots, and the length of the bolts required. If possible, sweep in the log should be minimized as it results in excessive roundup and short grain in the veneer. Thus, even though long bolts are generally more valuable than short bolts, a log with excessive sweep would probably be more valuable if cut into two or more bolts to minimize the sweep. Logs that have been end coated or that have dried and checked should be end trimmed. The cut should be at a right angle to the longitudinal axis of the bolt. Crosscutting with a hand-held saw can result in irregular bolt ends, which in turn can reduce the surface engaged by the lathe chucks and also cause the veneer to vary in length or require excess spurring at the lathe.

31

Flitches

A log with sweep should be sawn into flitches so the sweep is perpendicular to the plane of the knife used in slicing. This permits full- length veneers from the start of slicing. A large split or frost crack in a log may be minimized by dividing a log along this longitudinal plane. If possible, knots or other defects indicated in the bark should be trimmed out or be put at one edge or end of the flitch so the defect will occur at the edge or end of the veneer. In general, it is desirable to saw the flitch parallel to the bark and take the taper from the center of the log. This makes for straighter grain and a balanced design in the face veneer. The side of the flitch that is to be the exit side for the knife at the end of the cut should be sloped, with the wide side next to the flitch table to minimize tear-off during slicing. The top and bottom of the back of the flitch should be squared so the slicer dogs can obtain a good grip. The recent developments of remotely controlled extension dogs and a fixture for holding the flitch by vacuum make this precaution less important.

Frequently the sawyer preparing flitches for face veneer has the option of sawing the log for lumber. This judgment is generally made after he has sawn through the pith and can see the quality and figure in the wood. If the log has some limitation for slicing, such as ring shake, it may still be possible to recover high-quality lumber.

Choice of Cutting Direction

Some of the ways bolts or flitches are pre- pared and cut into veneer on a lathe or a slicer are illustrated in figure 11.

There are two main directions in which veneer can be cut—parallel to the annual rings (rotary-cut) or parallel to the wood rays (quarter sliced). The other methods fall between these two extremes. Half-round, flat-slicing, and back-cutting all result in cutting parallel to the rings in the center of the veneer and at angles to the rings at the two edges of the veneer sheets. Rift-slicing is a deliberate attempt to cut midway between parallel to the rays and perpendicular to them.

The lathe is used to cut practically all veneer used in construction plywood, some decorative face veneer, and most container, core, and cross-

band veneer. Slicing and stay-log cutting is done primarily to produce decorative face veneer. A stay-log is an attachment for a veneer lathe on which flitches may be mounted for cutting into half-round, back-cut, or rift veneer. Very high- quality core and crossband veneer is occasionally produced by quarter-slicing. Small, fast slicers have been used to produce container veneer.

Rotary Eighty to 90 percent of all veneer is cut

by the rotary method (fig. 11-A). The rotary method gives the maximum yield; it results in the widest sheets; knots are cut to show the smallest cross-section; and most juvenile wood and splits are left in the core. Some rotary-cut veneer is used for the decorative eflfect of annual rings or irregular grain, such as that causing "blister'' figure.

Flat-Slicing and Half-Round Cutting Flat slicing (fig. 11-F) is done on a slicer,

and half-round cutting (fig. 11-B, C) is done on a lathe. Half-round cutting may be done with flitches mounted on a stay-log (fig. 11-C), or by chucking a bolt at one edge rather than at the center, and by having the lathe chucks mounted eccentrically (fig. 11-B). Veneers produced by the flat-slicing and by half-round cutting are similar in appearance. The centers of the sheets are essentially flat-grain while the edges are rift or even quartered material. The half-round method gives slightly wider sheets and a bigger area of flat cutting in the center of the sheet than the flat-slicing method. These two cutting methods show growth rings to advantage. When the grain dips in and out of the sheet, the figure is broadly termed ''crossfire.'* Burls are generally cut by the half-round method and crotches by the flat-sliced method.

Rift-Cut A quarter section of a log is cut and mounted

so that the knife cuts about a 45° angle to the wood rays. This can be done with a stay-log on a lathe (fig. 11-E) or on the slicer (fig. 11-H). The method is used primarily with white oak to produce a figure caused by the wood rays. When the veneer is coarse-textured and the annual rings are not exactly parallel to the edge of the veneer, the figure is called rift-cut. A form of rift-cut that is particularly desirable is comb

32

LATHE S LI CE R

A, ROTARY (YELLOW BIRCH) F. FLAT SLICED (WALNUT)

B, ONE'HALF ROUND (RED OAK) 6. OUARTER SLICED (PRIMAVERA)

C. ONE-HALF ROUND (BLACK CHERRY) H. RIFT SLICED (WHITE OAK)

D, BACK CUT (ROSEWOOD) I. WHOLE LOG (FLAT SLICED) (ASPEN)

E, RIFT CUT (WHITE OAK) J. 1. FLAT SLICED 2. BACK CUT 3, OUARTER SLICED

M 140 660

Figure 11.—Some of the cutting directions used to obtain different grain patterns in veneer. The species in parentheses are typical of those cut by the method diagramed. The wide dark lines under ''slicer" represent the back-

board left at the end of slicing.

33

grain. By contrast with the more familiar form, comb grain has fine texture, straight grain, and no broad flakes.

Quarter-Sliced

Quarter-slicing (fig. 11-G) produces straight, narrow stripes in straight-grained softwoods like Douglas-fir, redwood, and western redcedar or straight-grained hardwoods like oak and walnut. Quarter-slicing is also done with species having interlocked grain such as mahogany and primavera. This produces a plain stripe or ribbon-grain which reflects light in different directions depending upon the position of the viewer. Plain-stripe is a comparatively broad stripe and not too pronounced. A ribbon stripe has narrower bands and is more highly reflec- tive. When the grain in the wood dips in and out of the sheet, the figure is called a broken stripe.

Back-Cut

Back-cutting (fig. 11-D) is done on a lathe with a stay-log, much like half-round cutting, However, instead of cutting from the sapwood side, the cut is from pith side of the flitch. Back-cutting is uncommon and is done where the heartwood is narrow and much more valu-

able than the sapwood. Rosewood is an example of this.

Sawn

At one time sawing was a common method of producing veneer, but it is almost obsolete because of the large volume of material lost as sawdust. Sawing does have the advantage that it is not necessary to heat the log or flitch prior to cutting, the two sides of the veneer are essentially the same in quality, and thicker veneers can be produced without developing cracks into the veneer. An example where these advantages are important would be the top or back of a musical instrument, such as the guitar. Species like spruce, oak, cypress, and eastern redcedar are occasionally sawn. Sawn material can be flat-cut, quarter-cut, or rift-cut much the same as when slicing with a knife.

Figure in Veneer

As briefly described under the different cutting directions, the appearance of veneer can be greatly affected by whether the veneer is cut tangential to the annual rings, at a right angle to the annual rings, or somewhere in between. Figures 12 to 15 are examples of the appearance of face veneer.

CONDITIONING WOOD PRIOR TO CUTTING VENEER

The moisture content, permeability, and the temperature of wood can have a marked effect on veneer cutting.

Wood Moisture Content

Poor cutting results if nearly all cell cavities in the wood are filled with water or if the moisture content is below the fiber saturation point (about 30 percent for all species). Unfortu- nately, there is little the plant manager can do to drastically change the moisture content in a bolt or flitch. Rapid processing, storage under water, or a sprinkler system will prevent green logs from drying. Logs having very high moisture content cannot be partially dried quickly without developing degrade at the outer portions of the log. Steaming may slightly reduce the time required to dry the veneer.

Wood Permeability

The more permeable wood is to water, the easier it is to cut. But permeability is also largely inherent in the species. Sapwood of some species can be made more permeable by storing in a warm, wet condition so bacteria will attack it. This may make it easier to cut into veneer but it may also affect the odor of the wood and its gluing properties. These disadvantages make it unlikely industry will purposely induce bacterial attack to improve cutting.

Wood Temperature

The major factor under control of the plant manager is the temperature of the wood when it is cut. This is an area where strong differences of opinion exist among veneer plant managers. For example, a hardwood plant manager

34

M 139 946

Figure 12.—Rotary-cut yellow birch with the figure caused by annual rings.

in Wisconsin stated that the entire quality control in his plant hinged on proper heating of bolts prior to cutting veneer. He stated that many things depend on whether or not the bolts are properly heated: Smoothness, tightness, and thickness control when cutting the veneer; buckle, splits, and uniform moisture content after drying; and quality of glue bonds.

In contrast, a softwood plant manager in Oregon stated that heating of veneer bolts was not worth the cost and he did not want log heating equipment in a plant that was to be built.

M 139 948

Figure 13.—Flat- or plain-sliced black walnut with figure from the annual rings and also a dip in the grain. The dip in the grain is sometimes called cross figures.

Before commenting on these statements, let's examine some of the known effects of heating on green wood.

Some Effects of Heating on Green Wood

Plasticity

Heating green wood makes it more plastic. This fact is easily demonstrated with mechanical tests and is the basis of steam bending of wood. Within the limits used in veneer production, plasticity is not time-dependent; as soon

35

! '

il 1. f.

Figure 14.—Rift-sliced white oali. The pencil stripe figure is caused by cutting the wood rays at an angle of

about 45°.

M 139 945

Figure 15.—Quarter-sliced primavera. The broken stripe figure is caused by interlocked grain which dips in and

out of the sheet.

as green wood reaches a given temperature, it is as plastic as it will get at that temperature. Veneer cut from heated bolts or flitches can be bent with fewer fractures than veneer cut from unheated wood. This effect is more noticeable with dense species and when cutting thick veneer. If a plant is interested in cutting tight, thick veneer from dense species, then heating of the bolts or flitches is an important part of the process.

Hardness Heating wet wood makes it softer. Hard

knots, which if unheated may nick a sharp knife, will often be softened by heating so they can be cut. Heat also softens pitch but does not soften mineral deposits like calcium carbonate and silica.

While heating generally aids cutting of dense species, it may oversoften less dense species and result in tearing of fibers and a fuzzy surface

36

on the veneer. This phenomenon occurs at different temperatures for different species. In general, if the wood cuts with a fuzzy surface, it is too hot.

Dimensional Changes

When green wood is heated, it expands tangentially and shrinks radially. This fact has been verified for both softwoods and hardwoods by a number of researchers. The amount of shrinking and swelling varies with the species. This thermal movement increases with temperature but the rate of increase is slow up to about 150° F (66° C) and then increases more rapidly. Consequently, if a species tends to develop splits through the pith and shake due to heating, a general recommendation is to not heat above 150° F.

Tangential expansion and radial shrinkage can occur in flitches without causing end checks or shake. It is, therefore, often possible to use higher heating temperatures with flitches that do not contain the pith than with bolts that do contain the pith.

Growth Stresses

When bolts with large growth stresses are heated, the wood at the bolt ends is temporarily weakened in tension perpendicular to the grain ; the growth stresses, together with dimensional changes discussed earlier, may cause star-shaped cracks radiating from the pith at the end of the bolts.

The longitudinal growth stresses act primarily at the two log ends. When the wood is heated to a temperature of 180° F (82° C), or higher, 90 percent or more of the growth stresses are relieved. If the wood is heated in long log lengths and then crosscut to bolt lengths, the newly formed bolt ends will have less end splits than would develop if the bolts were cut to length prior to heating.

Longitudinal growth stresses tend to cause flitches to bow toward the bark side. This bow may become worse during heating. Bowing can be reduced by strapping the flitches together with the bark side out and allowing the heat to relieve the growth stresses while the flitches are mechanically held flat. A wide, strong strap or heavy chains and bolts must be used as the forces involved are large.

Experimental strapping has also been tried to reduce end splits in bolts during heating. A

steel strap 1 inch (25.4 mm) wide applied by a tool commonly used to strap containers was in- effective in preventing splits at the bolt ends during heating of Brazil nut about 2 feet (0.6 m) in diameter. End splits were reduced in another bolt of Brazil nut heated to 160° F (71° C) by wrapping the ends with i/i>-inch (12.7 mm cable and applying a tensile force of 40,000 pounds (18,000 kg) to the cable. Similar results have been obtained experimentally with red oak.

Steel strapping is used on the ends of flitches by some face veneer plants. Plant managers report this reduces splitting. As indicated earlier, the forces that tend to cause end splits during heating are less in flitches than in bolts of the same species.

Color Changes

Heating green logs may darken or lighten the wood. Heating in steam is reported to change color more than heating in water. The color changes may be desirable or undesirable. In general, heating tends to darken sapwood of all species. Similarly, the heartwood of oak, beech, and Port-Orf ord-cedar are darkened by heating. To keep the wood as light in color as possible, minimum heating times and temperatures are recommended for ash, oak, maple, and beech.

Heating may also affect later color changes. Wet sapwood of yellow birch may develop orange streaks, thought to be an oxidation stain promoted by enzymes. Adequate log heating tends to inactivate the enzymes and reduce the likelihood of this undesirable orange stain occurring. Similarly, adequate heating of oak sapwood reduces the development of gray-brown oxidation stain.

Warm, wet walnut veneer is often held in storage until the sapwood becomes darker from oxidation stain and the heartwood reaches a desirable light gray-brown color. It is then dried to minimize further color changes.

Strength of Wood

Occasionally a question is raised about whether heating bolts or flitches prior to cutting veneer weakens the dry veneer. As discussed earlier, heating wet wood plasticizes and softens it while it is hot. After drying, the wood cut from heated veneer bolts or flitches has much the same strength as wood cut from unheated controls.

37

However, excessively long heating* and high temperatures can reduce the strength perma- nently. For example, Douglas-fir and sitka spruce heated for 50 days at 150° F (66° C) in water lost 10 percent of the modulus of rupture of unheated controls. When the same species were heated at 200° F (93° C), the modulus of rupture was reduced about 10 percent in approximately 10 to 12 days. Modulus of elasticity (stiffness) values were affected even less by heating.

Heating has a greater effect on hardwoods than on softwoods. In contrast to the 10 to 12 days of heating required for the softwoods, Douglas-fir and sitka spruce, to lose 10 percent in modulus of rupture, it took only 6 to 7 days of heating for a comparable loss in yellow birch.

Torque to Turn Bolts

The torque required to turn a bolt into veneer depends on wood density, veneer thickness, bolt diameter, setting of the knife and pressure bar, and wood temperature. In one test, we found that 1/4-inch (6.35 mm) basswood veneer cut at 200° F (93° C) required 42 percent less torque than matched material cut at 35° F (2° C). However, the torque that the bolt end would accept at 200° F (93° C) was 44 percent less than matched material at 35° F (2° C). In other words, heating reduced the cutting force about as much as it reduced the wood's ability to resist spin-out.

Bolt heating is sometimes blamed for spinout, or turning of the chucks in the bolt ends. This can happen if bolts are heated at a high temperature for a short time. The bolt ends are then hot and soft, while the inner part of the bolt is cooler and requires a relatively higher force for cutting. The better procedure is to heat the bolts at a lower temperature long enough so each bolt is uniformly hot. This procedure also minimizes splits at the bolt ends that may contribute to break-out of the bolts during rotary cutting.

Shrinkage

Heating of softwood veneer bolts or flitches has no detectable eflfect on the shrinkage of veneer cut from them. In contrast, heating bolts or flitches of some collapse-susceptible hardwoods may result in noticeably higher shrinkage of veneer cut from the preheated wood. In one trial, alpine ash veneer peeled at

60° F (16° C) shrank 13.3 percent, while matched veneer peeled at 135° F (57° C) shrank 15.1 percent. The effect was greater the higher the conditioning temperature and the longer the heating time.

Drying Time

When sound, green logs with a high moisture content are heated in hot water or steam to 150° F (66° C) or higher, they generally lose 1 to 10 percent of the moisture in the log. This is believed to be caused by air in the cell cavities expanding and pushing out free water. Decayed logs may pick up water during heating in water.

It is sometimes thought that warm veneer cut from heated wood will dry faster than veneer cut from bolts or flitches that are not heated. Grantham and Atherton (25) report that Douglas-fir sapwood cut from bolts at about 140° F (60° C) dried 10 percent faster than sapwood from unheated bolts. They found the drying time for Douglas-fir heartwood was the same for veneer cut from heated and unheated bolts. Thin veneer cut from hardwoods generally requires the same drying time whether the bolts or flitches are heated or not. These results are to be expected from the relatively small amount of energy required to heat wood compared to the large amount of energy required to dry it.

Warp

Veneer cut from heated wood is generally tighter than veneer cut from unheated wood. Tight-cut rotary veneer may tend to reassume the curvature of the bolt more than loosely cut veneer. The tendency of the veneer to curl is also related to the setting of the pressure bar during cutting.

If the logs or flitches are heated and then cooled in water, the end grain will pick up water. If the veneer is not spurred at the lathe, this extra water at the ends may affect drying at the ends of the sheets.

Decay Resistance of Naturally Durable Woods

The heartwood of green Douglas-fir, Alaska- cedar, white oak, and true mahogany was heated at 212° F (100° C) for various times from 1 to 48 hours. After 12 hours of heating, the white oak and mahogany were slightly less resistant to decay than similar but unheated

38

logs. The Douglas-fir and Alaska-cedar were not noticeably affected. After 48 hours at 212° F (100° C), all of the woods were slightly less decay resistant than the unheated controls of the same species. The results indicate the heating of wood in normal veneer processing does not degrade decay resistance from a practical point of view.

Conclusions on Some Effects of Heating

Some Benefits

The most obvious effect of heating is that it makes it possible to cut tighter veneer than can be cut from unheated wood. Tighter cutting means greater strength in tension perpendicular to the grain of the veneer and so less splitting of the veneer in handling and less checking of face veneer in service. A second effect of heating is that it softens knots, thereby reducing nicks in the lathe or slicer knife. The sharper knife in turn helps produce smooth veneer surfaces. Other possible benefits of heating include less power to cut equally tight veneer, improvement of color by decreasing oxidation stain in the sapwood, and reduced veneer drying time.

Frozen wood cannot be cut satisfactorily into veneer with a knife. Wood bolts or flitches yield veneer of varying quality with varying temperature. The changing temperature may ad- versely affect the lathe or slicer settings.

In general, heating is beneficial when slicing figured face veneer from dense species. Heating is also important if tight veneer is to be produced in thicknesses of % inch or greater.

Some Disadvantages

Most disadvantages of heating can be attributed to using too high a heating temperature or too long a heating time. Overheating may cause excessive end splits in bolts of species like oak, fuzzy surfaces on springwood and glossy surfaces on summerwood, shelling or separation on springwood and summerwood during cutting, unwanted darkening of the veneer, increased spinout of bolts by softening the end grain, or increased shrinkage. But the heating temperature must be very high and the heating time very long to affect the strength and dura- bility of the wood.

Conclusions Related to Wood Temperature

The improved veneer tightness, smoothness, and color are sufficiently beneficial so that almost all producers of hardwood face veneer heat bolts or flitches prior to cutting veneer. Whether heating has a significant effect on veneer thickness, moisture content after drying, and quality of glue bonds is not well docu- mented. The Wisconsin plant manager was, nevertheless, correct when he stated that heating must be done properly in order to produce high-quality hardwood face veneer.

The softwood plant manager in Oregon who did not want heating equipment was producing a very different product. The panels from his plant were to be used mainly for construction such as sheathing. Here, properties of veneer tightness, smoothness, and color are less important. Many western softwood plywood plants make satisfactory construction plywood from unheated veneer bolts. However, research and plant experience indicates that heating pays even for manufacture of construction plywood. This is particularly true if there is any danger that the logs will freeze. Both researchers and industrial veneer producers have found that it is not possible to cut veneer from frozen logs. If the logs do freeze, then the plant without heating facilities will be shut down.

Another advantage of heating for this kind of product is that the veneer is tighter cut and as a result a higher percentage of 4-foot-wide and 2-foot-wide sheets is produced. That is, less splitting occurs during handling of the green, tightly cut veneer than when handling loosely cut veneer. In addition, heated knots are softer and result in less knife wear than cutting similar wood from unheated blocks. Grantham and Atherton (25) conclude that heating does pay when cutting veneer for plywood to be used in construction.

Time Required to Heat Veneer Bolts and Flitches

Most investigators agree upon some points about the time required to heat veneer bolts and flitches, but other points are controversial. First, let us examine the points that are generally accepted.

39

Generally Accepted Points Uniform Final Temperature

The bolt or flitch should be heated long enough so that temperature of the wood from the start of cutting to the end of cutting varies no more than 10° F (6° C). To achieve this goal, the heating time must be sufficiently long and the heating medium (steam or hot water) must circulate freely to all surfaces of bolts and flitches.

Effect of Diameter The time required to heat a large-diameter

bolt or flitch is much longer than the time required to heat one of small diameter and generally increases with the square of the diameter. For example, while a bolt 1 foot (0.3 m) in diameter might be heated in 14 hours, a bolt of the same species 2 feet (0.6 m) in diameter would require about 60 hours. This example is from the report by Fleischer (20) for wood having a specific gravity of 0.50, an initial wood temperature of 60° F (16° C), a temperature of the water used to heat the bolt of 150° F (66° C), and the final temperature at a 6-inch (15 cm) core of 140° F (60° C). The time required to heat nonfrozen logs increases approximately as the square of the log diameter (11).

Effect of Temperature Gradient The greater the difference in temperature be-

tween the wood and the heating medium, the faster the heating rate. As the wood approaches the temperature of the heating medium, the rate of heating becomes very slow. As a result, when selecting heating schedules, it is generally practical to aim for a core temperature 10° F (6° C) lower than the temperature of the heating medium. Some veneer plants use an equaliz- ing period at the end of the heating cycle to take advantage of the faster heating with a large temperature gradient and still end up with relatively uniform temperature throughout the block (25). A limitation to this practice is the bolt end splitting that may occur with the high initial temperature.

Total Temperature Change Required The colder the wood, the longer the heating

time required to bring it to the desired cutting temperature. In other words, the heating capacity of a plant should be figured for the worst winter conditions rather than the average am-

bient temperature. This is particularly true if logs of high moisture content may be frozen at some part of the year. While ice conducts heat faster than water, the heat required to melt the ice can result in longer heating time (11). When heating frozen wood of species with a low moisture content like Douglas-fir heartwood, the heating times are shorter than for frozen logs with very high moisture content like western hemlock.

Effect of Grain Direction

End grain heats about 2V2 times as fast as side grain. The rate of heating in the tangential and radial directions is about the same. Because most flitches and bolts are long compared to their cross sections, heating through side grain generally is the controlling factor. Faster end-grain heating probably means knots heat faster than surrounding clear wood. This is fortunate as one of the reasons for heating is to make the knots soft enough so they will not turn the edge of the lathe or slicer knife.

Variability of Heating

The rate of heating the flitches, even of the same species, is somewhat variable due to irregular shapes, differences in specific gravity, and defects like cracks. Therefore, heating schedules cannot be precise. In general, the schedules should be developed for the largest bolts or flitches that are to be heated, starting from the lowest ambient temperatures in the log storage area. The most common problem in heating veneer logs is insufficient vat capacity to adequately heat the logs or flitches under all operating conditions.

Controversial Points Effect of Heating Medium

MacLean (i6) reported that water heats wood 5 to 10 percent more slowly than steam. He found the slowest rate of heating in air at low humidities, but the rate was increased as the humidity was increased. In contrast, Feihl (11) found that hot water heats as fast or faster than steam. Feihl points out that this apparent conflict may be due to the experimental conditions. MacLean was using steam at 212^ F (100° C) and higher, while Feihl used a steam-air mix at a temperature generally below 200° F (93° C).

40

Some commercial plants inject steam into a vat and at the same time spray hot water over the bolts or flitches. In addition to adding heat to the wood, the hot water spray prevents drying and checking.

A commercial modification of the steaming hot-water spray method is to blow steam through an alkaline water solution. Salts added to water will raise the boiling point slightly. These few degrees change in temperature would not seem to be important for conditioning veneer logs. This would appear to be mildly alkaline because.we know strong alkali solu- tions will break down wood structure. However, in typical heating cycles, the alkali would not penetrate more than a fraction of an inch in most wood species.

Effect of Differences in Moisture Content and Specific Gravity

MacLean H6) found wood that is well below 30 percent moisture content heats more slowly than green wood, but differences in moisture content above about 30 percent had no important effect on the rate of heating. All veneer cutting, of course, is done with wood at a moisture content of 30 percent or higher. For practical purposes, MacLean is suggesting that green wood of any given species will heat at about the same rate at any moisture content above 30 percent.

In contrast, he found that the rate of heating of wood varied inversely with the specific gravity {J^6), Although the heat conductivity of wood increases with the increase in specific gravity, the diffusivity (a measure of the rate of temperature change) decreases as specific gravity increases. In other words, the lighter woods will heat to a given temperature more rapidly than the heavier woods, although the heavier ones are better conductors of heat.

Feihl {11) found that the rate of heating is related to the specific gravity of the total log (that is, the wood and the water). He reported that sinker logs require longer heating time than logs that float one-third out of water, and that logs that float one-half out of water require less time to heat than logs that float one-third out of water.

In general, Feihl and MacLean agree heavier logs require longer heating.

MacLean attributes the longer heating time to higher specific gravity of the wood; Feihl

attributes it to either higher specific gravity of the wood or higher moisture content in the log.

Log End Splits Many veneer plant operators believe rapid

heating increases end splits. Meriluoto {Jf.8) reports that heating frozen birch at less than 5° C (41° F) until the wood was above 0° C (32° F) resulted in less end splits than when heating frozen bolts in water at 14° C (57° F).

A few trials at the U.S. Forest Products Laboratory with nonfrozen bolts showed little difference between end splits in bolts heated slowly and matched bolts put directly into water that was at the desired final temperature. While slow heating may slightly reduce end splits, the maximum heating temperature seems to be more important. The higher the heating temperature, the larger the end splits.

Duration of Heating at Constant Temperature

Some researchers have reported that long- time heating using a low temperature has the same effect in conditioning wood for cutting veneer as shortime heating at a high temperature. Experiments at the Forest Products Lab- oratory in Madison indicate this is question- able. Duration of heating up to several days does not affect the plasticity and hardness of the wood. This in turn means excessively long heating periods do not improve the tightness or smoothness of the veneer compared to short- term heating to the same final temperature.

Heating longer than necessary to bring the wood to the desired cutting temperature may darken the wood and increase shrinkage.

Conclusions on Time Required The most common difficulty in heating veneer

bolts and flitches is insufficient vat capacity. The single largest factor in the required heat-

ing time is the diameter of the bolt or flitch to be heated, with required heating time increasing directly with the square of the log diameter. Good estimates of the required heating time for unfrozen logs can be made from Forest Products Laboratory Report No. 2149 (20). FeihFs report (11) can be used to estimate the heating time for frozen and unfrozen wood.

A special heating cycle may be appropriate if the color of the wood is important.

41

220

200

180

160

140

120

100-

80-

60-

40V

LEGEND:

/—BASSWOOD

2-ASPEN, QUAKING

3-COTTONWOOD, WESTERN

4-YELLOW - P'OPLAR

5- SWEETGUM 3 TUPELO

6-WALNUT, BLACK

- 7-BIRCH, YELLOW

8-MAPLE, SUGAR

9- OAK, NORTHERN RED

10-BEECH, AMERICAN

~ II-OAK, WHITE 12 - HICKOR Y, SHA GBA RK

0.30 0.35 O 40 0.45 0.50 0.55 0.60 0.65

SPECIFIC GRAVITY M 145 394

Figure 16.—Favorable temperature range (area between heavy lines) for cutting veneer of hardwood species of various specific gravities. Points show favorable temperatures for the individual hardwood species indicated. The data apply to the rotary cutting of veneer Vs inch thick, of straight-grained wood, free of defects such as knots

or tension wood (''soft streaks").

42

Hot Water Versus Steam Heating

Producers of hardwood face veneer generally prefer hot water vats while many softwood construction plywood operations use steam chambers. Recently heating- by hot water has become the preferred method for all types of veneer plants.

The rate of heating in the two systems is about the same, assuming that both are properly operating. The actual temperature throughout the vat can be controlled more accurately with water than with a steam-air mix. End drying is never a problem when heating in water vats, while it can be a problem in steam chambers if the relative humidity is not kept high. For workers, steam chambers are safer, as a fall into a hot water vat is generally fatal. In terms of manpower, one man with a lift truck can load and unload steam chambers for a large plant while two or more men are generally needed to operate hot water vats.

Heating Suggestions with Hot Water or Steam

Debark logs prior to heating. Heat in tree lengths or the maximum length

possible. Segregate logs by diameter so the larger di-

ameter logs can be given the needed longer heating time.

Heat U.S. species at the temperature suggested in Appendix IV.

For unfamiliar hardwoods, use the heating temperature indicated for the specific gravity of the wood (fig. 16).

The heating tanks should be arranged for circulation of steam or hot water so heat can flow easily to all sides of the bolts or flitches. Steam should not impinge directly on the ends of logs, bolts, or flitches.

The temperature in the vats should be recorded at half-hour or shorter intervals. Heat- ing should preferably be controlled by automatic valves on the steam lines, regulated by heat sensors in the heating chamber.

Temperature-sensing devices should be placed in several locations in the vats or steam chambers. These in turn should automatically control the heating of the water vats or steam

chambers. With a good system it is possible to keep the temperature in the vat to within 2° to 4° (l"" to 2° C) of the desired temperature.

A system that works well with hot water vats is to pump the water from one vat to another. After heating one vat, the hot water is pumped to a second vat which has just been loaded with unheated logs. The process is re- peated and the hot water goes from tank to tank. Only enough heat is added to maintain the temperature of the water.

Some mills strap a number of bolts or flitches together so they can be handled as a bundle. This practice is all right provided the flitches are separated by stickers to allow for circulation of the hot water to all wood surfaces.

Another method that has been used commercially is to move the bolts or flitches progressively through a hot water or steam tunnel. This practice has the merit of straight-line production. To be successful, the bolts or flitches should be about the same diameter, the heating time should be long enough to ensure heating to the core of the bolts or center of the flitch, and the heating medium should circulate so that all surfaces of the bolts or flitches are heated the same amount.

The temperature at the core of larger bolts should be checked by drilling a hole in the middle of the core as it comes from the lathe. The hole should be 1 to 2 inches deep and just large enough to accept the thermometer. The thermometer should be inserted immediately and the temperature recorded.

If the cores of the large-diameter bolts are within 10° F (5° C) of the temperature in the vat, the smaller bolts will also be adequately heated.

Proper heating will aid in producing tightly cut veneer of uniform thickness. Underheating will result in less tight veneer and may result in excessive handling splits and variation of veneer thicknesses. Overheating may cause large end splits in the bolts, spin-out, fuzzy veneer, and shelling of the grain.

An example of the heating times required for bolts of different diameters is given below. More detailed information and tables are given in (11,20),

43

Examples : Heating time as related to bolt diameter.—

(Green 8-ft-long bolts with a specific gravity of 0.50, an initial temperature of 70° F (21° C), water or air-steam vat temperature of 150° F (66° C) and a final temperature at a 6-in. (15 cm) core of 140° F (60° C).)

Bolt diameter Required heating time

(in.) (cm) (h) 12 31.5 14 24 63 60

Heating time as related to final core temperature,— (Green 8-ft-long bolts with a specific gravity of 0.50, an initial temperature of 70° F (21° C), water or air-steam vat temperature at 150° F (66° C), and various final temperatures at a 6-in. (15 cm) core.)

Final Required Bolt core heating

diameter temperature time (in.) (cm) (°F) (°C) (h) 24 63 140 60 60 24 63 120 49 34 24 63 100 38 22

Heating time as related to initial ivood temperature.— (Eight-ft-long bolts with a specific gravity of 0.56, a green moisture content of 80 pet, various initial temperatures, water or air-steam vat temperature of 150° F (55° C), and final temperatures at 4-in. (10 cm) core of 140° F (60° C).)

Initial Required Bolt wood heating

diameter temperature time (in.) (cm) (°F) (°C) (h) 12 31.5 0-18 27 12 31.5 40 4 21 12 31.5 70 21 16

Construction of Steam and Hot Water Vats

Most heating vats or stegm chambers are made from reinforced concrete. The vats should be constructed so that good circulation of the heating medium can be attained. The steam pipes should not be placed so live steam will impinge directly on the logs ends. Steam blow- ing directly on the log ends overheats them and accentuates log end splits. If logs that float are to be heated in hot water, the tanks should

have hold-downs that will keep the logs under water during heating. The doors on steam chests or covers on water vats should be tight and preferably be insulated. In many commercial operations as much as heat is lost to the atmosphere as is used to heat the wood.

Some Other Methods of Heating Veneer Logs and Flitches

Some methods other than hot water or steam, or steam-air mixtures below 212° F, have been used on a small scale commercially or tried in the laboratory. These include heating in steam under pressure, electrically heating the wood, and forcing hot water or steam longitudinally through the wood.

Heating in Steam Under Pressure A few veneer mills heat veneer bolts in steam

under pressure. This shortens the heating time as there is a bigger differential between the starting temperature of the wood and the temperature of the heating medium. However, there is no special change at a temperature of 212° F (100° C). In going from 210° to 220° F (99° to 104° C) the reduction in heating time is comparable to the time in going from 200° to 210° F (94° to 99° C). The disadvantages of a short heating cycle in steam under pressure include the very large temperature gradient from the surface to the core of the bolts and excessive bolt end splits.

Electric Heating

Electrical methods have been used experimentally to heat bolts or flitches in an attempt to reduce the heating time required with water or steam.

In one set of experiments, electrodes were placed at each end of a bolt or flitch and an electrical current sent through the wood by as high as 5,000 volts. Because the wood acted as a resistor, it was heated. This method is fast but has not been accepted commercially due to nonuniform heating. The electrical current follows the path of the least resistance, which may be wet streaks, cracks, or mineral streaks in the wood. These areas overheat and the other parts of the bolt or flitch are underheated.

High frequency has also been used experimentally to heat veneer bolts. High frequency

44

tends to overheat the wetter parts of the wood, and is much more expensive than heating in steam or water.

Forcing Hot Water or Steam Longitudinally Through Wood

Short beech veneer bolts have been heated experimentally by forcing hot water longitud-

inally through the wood structure (35). The experimenters report that heating time was reduced to minutes and that satisfactory veneer was cut from bolts heated this way. The method requires that the wood be permeable and that a cap be attached to each bolt.

VENEER CUTTING EQUIPMENT

In selecting veneer cutting equipment, it is important to remember the forces involved in cutting. In one rotary-cutting study (^5), calculated loads were as high as 200 pounds per inch of knife and 500 pounds per inch of pressure bar. Pictures comparing early lathes and modern lathes indicate that experience has dic- tated the desirability of more rigid lathes. A lathe or slicer operator never has trouble because the equipment is too rigid, but excessive movement of machine parts is a common problem. If smooth, tight veneer of uniform thickness is to be produced, it is better to have a lathe or slicer that is stronger than necessary rather than to have one that is underdesigned.

Some face veneer slicers are made so excessive pressure cannot be applied to the flitch. The knife and bar carriage is not fastened on the ways of some horizontal slicers. Thus, if the total force against the flitch exceeds the weight of the knife and pressure bar carriage, the carriage is lifted from the ways. Similarly, on vertical slicers, the knife and bar assembly is not held on the half-bearings that allow the knife to be offset on the upstroke of the flitch table. If there is excessive nosebar pressure, thin veneer sheets are produced and eventually the flitch will not clear on the upstroke. At this time, the knife and bar carriage will be lifted slightly from the half-bearing. When the carriage falls back, the noise alerts the slicer operator that he has too much nosebar pressure.

There is no mechanism such as this for lathes. Excessive nosebar pressure can progressively build up until the bolt spins out of the chucks, the motor stalls, or some part of the lathe breaks.

Any moving part on a lathe or slicer is subject to wear. Consequently, preloaded antifric- tion bearings are a good investment as well as

wear plates and mechanisms for taking up slack or play when it occurs.

Similarly, it is desirable to have hydraulically operated dogs on slicers and hydraulically operated chucks on a lathe. Any tendency of the wood work piece to come loose in cutting is automatically corrected as hydraulic pressure resets the dogs or chucks.

Another source of unwanted movement of the lathe or slicer is heat distortion. The use of A-frames with screw takeups on the nosebar casting is one method of correcting for this.

Another desirable feature is a means of keeping the lathe or slicer at a uniform temperature during setup of the knife and pressure bar and during cutting. An added benefit is the reduction of blue stain caused by the reaction of iron or steel with wet wood. Keeping the knife and pressure bar warm reduces condensation and so reduces the staining.

The heart of any lathe or slicer is the knife and pressure bar. The machine should permit rapid change of the knife and bar and easy adjustment of the clearance angle of the knife and the lead and gap between the knife and nosebar. If these adjustments are diflScult to make, the operator will make as few adjustments as practical. Consequently, the machine will produce poorer quality veneer than would be produced on easily adjustable equipment.

Retractable dogs on slicers and retractable chucks on lathes permit secure holding of large wood flitches and bolts; when the dogs or chucks are retracted they permit continuous cutting to thin backboards or small-diameter cores.

Recent development of the vacuum table permits fast loading of flitches and cutting to a thin backboard. However, the flitch back should

45

be wide, flat, and smooth to maximize the holding power of the vacuum table.

The lathe drive is often a separate item allowing the purchaser to specify the type desired. Some options include a steam engine, a.c. motor with a speed changer, d.c. motor with a motor-generator set, and hydraulic motor. In all cases it is desirable to be able to increase spindle speed as the block diameter decreases to keep the cutting speed constant. Hancock and Hailey (26) describe lathe drives in some detail.

Cutting Action on Lathe and Slicer

Similarities of Lathe and Slicer

The knife and pressure bar are very similar on both the lathe and slicer and perform the same function. Cross sections of a lathe (fig. 17) and slicer (fig. 18) illustrate the position of the knife and bar in the two machines. Ter- minology used to describe the knife and pressure bar on lathes and slicer is shown in figure 19.

The knife severs the veneer from the bolt or flitch. The knife bevel angle is about the same for knives on a lathe or slicer. The knife used on a lathe may be slightly more hollow ground.

BOLT

CHUCK

KNIFE ANGLE

Figure 17.—Cross section of a veneer lathe having a fixed pressure bar.

46

M 140 657

M 140 656 Figure 18.—Cross section of a vertically operating veneer slicer.

The pressure bar on both the lathe and slicer compresses the wood, with maximum compression ideally occurring just ahead of the knife edge. This compression reduces splitting of the wood ahead of the knife, reduces breaks into the veneer from the knife side, and forces the knife bar assembly against the feed mechanism, thereby helping control veneer thickness. For both the lathe and slicer, the pressure bar is, therefore, important in controlling the roughness, depth of checks, and thickness of the veneer. The slicer has a fixed nosebar while the lathe may have a fixed nosebar or a rotating roller bar.

Advantages of Lathe

Logs to be cut into veneer on a lathe need to be crosscut to the desired bolt length, but they do not need to be processed through a sawmill prior to cutting veneer. After roundup of the bolt, the lathe cuts a continuous strip of veneer.

Continuous cutting is advantageous because it means more production with a given cutting velocity, wider sheets of veneer, and a more uniform cutting condition. Full rotary cutting is approximately tangential to the annual rings and knots are exposed at their smallest cross section. In full rotary cutting, there is no impact at the start of cutting or tearofF at the end of cutting as may occur when slicing or cutting with a stay-log.

Advantages of Slicer

A main advantage of the slicer is that it permits sawing the log into flitches to present the most decorative grain pattern. As the veneer sheets are kept in consecutive order, figured veneer can be readily matched. Flitches can be heated with less danger of end splits developing than in comparable bolts being heated for rotary cutting. Sliced veneer is always cut from a flat surface, and most veneer is used on a flat

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KNIFE AND FIXED BAR KNIFE AND ROLLER BAR

Symbol

A B C D E F G H I J K

M 144 168 Figure 19.—Knife and pressure-bar terminology.

Preferred Term

Knife angle Knife bevel angle Clearance angle Lead Pressure bar bevel Gap Exit gap Nosebar compression angle Knife surface next to wood work piece ** Knife surface next to wood veneer ** Length of knife bevel

Alternate Term

Knife pitch Knife sharpness angle

Vertical opening * Pressure bar sharpness angle Horizontal opening * Restraint Bar angle

* Satisfactory for vertically operating lathe or slicer but is misleading for horizontally operating slicers. ** The term knife face is sometimes applied to J by knife manufacturers and to I by lathe operators. To reduce ambiguity, this terminology is suggested.

48

surface. By contrast, rotary veneer cut from a curved surface must be flattened for most uses. The disadvantage of cutting from a curved surface becomes more pronounced with thicker veneers cut from small-diameter bolts.

Sliced veneer is cut with a draw motion across the knife, while rotary veneer is cut with no draw motion. Theoretically, the draw cut should aid cutting. However, recent experiments at the U.S. Forest Products Laboratory indicate that the effect of the draw cut on smoothness, tightness, and veneer thickness is relatively unimportant.

Veneer as long as 16 feet is produced on a slicer while most rotary-cut veneer is 10 feet or shorter. The flitch on a slicer is backed by the flitch table while support for a veneer bolt may be provided by a backup roll.

Advantages of Cutting with Stay-Log on Lathes

The stay-log makes it possible to produce veneer on a lathe, similar in appearance to sliced face veneer (fig. 11-C). The advantages of stay-log cutting on the lathe are very similar to the advantages of slicing. The flitches can be selected for appearance of the grain and consecutive sheets can be matched for decorative faces. Sheets cut with the stay-log are generally wider than sheets cut on the slicer. For example, half-round veneer cut with a stay-log would probably be slightly wider than flat-sliced veneer cut from the same log. Veneer cut with a stay-log is taken from a curved surface in comparison with veneer that is sliced from a flat surface. Veneer cut with stay-log may be up to 10 feet in length.

Back-Roll Lathe

A modification of the rotary lathe is the back-roll lathe (fig. 20). It cuts the veneer ribbon to preset widths and so replaces a clipping operation. This special type of lathe has ways that carry the knife-bar head-blocks extended out on the log side of the lathe. On the extended ways, a frame is mounted to carry the back-roll. The entire mounting is fed toward the log by feed screws at the same rate at which the knife is fed. Knives mounted radially in the back-roll make an impression into the veneer bolt slightly deeper than the thickness of the veneer being

cut. Then as the veneer is cut, it separates into pieces the same width as the spacing of the knives on the back-roll.

Since the scoring knives cut slightly deeper than the veneer thickness, they generally leave a light score mark on the tight side of the next piece of veneer. The back-roll lathe is, therefore, better suited for cutting thick container veneer than thin decorative veneer.

All lathes are generally equipped with spur knives so veneer can be cut to one or more lengths while it is being peeled.

Some General Comparisons of Veneer Cut on the Lathe and Slicer

In general, the greatest yield is obtained by rotary cutting. Half-round, flat-slicing, or back cutting provide intermediate yields; and the least yield is obtained by quarter- or rift-slicing.

The smoothest and tightest veneer can be produced by quarter- or rift-slicing, followed by rotary cutting ; the roughest and loosest veneer is produced by flat slicing, half-round, or back- cutting. Differences in roughness are due to the effect of wood structure orientation (S9),

While slicing and rotary cutting involve some differences and inherent advantages, good-quality veneer can generally be produced by either method. The quality of the end product is determined more by the log quality, the heating of the bolts or flitches, and the setting of the knife and pressure bar than by differences in the cutting method.

Undesirable Movement of Wood and Machine Parts

Knife and pressure bar settings are meaning- ful only if the wood is held securely in the lathe or slicer and if the machine parts have a minimum of play.

Undesirable Movement of Wood on Lathe

Bolts are held by chucks in a lathe. In general, the larger the chucks the more securely the bolt is held. The chucks transmit the torque needed to cut the veneer and also must resist the tendency of the bolts to ride up on the knife. The spurs on the chucks should, therefore, be designed not only to transmit power to turn the bolt but also to keep it from shifting from the spindle center. The best spur configuration is not well established. Some mills prefer half

49

SIZED VENEER

M 140 658 Figure 20.—Back-roll lathe.

circles; others, star-shaped spurs and a ring around the circumference of the chuck. In practice, the spurs sometimes become battered and bent and may collect wood debris. For best per- formance, they should be in their original shape and clean. The chucks and spindle ends should be tapered for a positive secure fit.

The pressure used to set the chucks in the bolt ends depends on the wood species, heating, and chuck size. Generally, enough pressure is used to indent the spurs at least three-fourths their length into the bolt ends. Square-cut bolt ends allow a more uniform grip than bolts that are end trimmed on a bias.

The wood in contact with the spurs receives fluctuating loads during cutting, which may cause the bolt to become loose in the chucks. On older lathes, the operator must watch for this and further indent the spurs if any looseness of the bolt is observed. Newer lathes have hydraulic chucking. A relatively high pressure is used to set the chucks and then a lower pressure is maintained hydraulically to insure the spurs remain seated during cutting. If too

high hydraulic end pressure is used during cutting, the wood bolt may bend when it reaches a small diameter.

Another modern solution to holding the bolts more securely is the use of retractable chucks. Larger chucks and spindles hold the bolt at the start of peeling ; they are retracted during peeling, allowing smaller inner chucks and spindles to hold and drive the bolt until the final core diameter is reached. A modification of this is sequentially retractable chucks such as 5-inch (13 cm) inner chucks, with one 8-inch (20 cm) outer chuck on one end and one 12-inch (30 cm) outer chuck at the other end. The bolt is first driven with the 12-and 8-inch chucks. At a bolt diameter of about 14 inches (35 cm), the 12- inch chuck is withdrawn and the bolt is then driven with one 8- and one 5-inch chuck. At a diameter of about 10 inches (25 cm), the 8- inch chuck is withdrawn. Cutting is continued with the two 5-inch chucks driving the bolt to the final core diameter.

To obtain maximum recovery, bolts are turned to as small a diameter as practical. The

50

bolt is loaded as a beam by the knife and pressure bar. Its resistance to bending is directly related to the cube of the radius of the bolt. At small bolt diameters, an unsupported bolt bends in the middle away from the knife. The bolt becomes barrel-shaped and the veneer ribbon wrinkles in the middle. To overcome this problem, backup rolls have been built to support the bolt during cutting.

Some early backup rolls operated with a fixed pressure against the bolt. But this caused problems. The cutting force fluctuates during peeling, and a fixed pressure against the bolt surface sometimes increased rather than reduced bowing of the bolt.

Improved backup rolls fix their position geo- metrically to keep the bolt cylindrical. One method of doing this is a servo-system with a follower at the end of the block that signals adjustments of pressure on the backup roll.

Another method (22) is to have this backup roll positioned mechanically by the feed mechanism so the bolt remains a cylinder.

When properly made and operated, backup rolls permit cutting bolts 8 feet long (2.44 m) to a final core diameter of about 4 inches (10 cm).

Undesirable Movement or Play in Lathe Machine Parts

All movable parts must have some clearance, and wear increases this clearance. Many lathes have built-in methods of taking up slack as wear progresses. However, it is not uncommon to find that production lathes have developed excessive wear and looseness or play in the mechanism. Some specific areas to check are spindle sleeves and bearings, feed screws, head- block or knife-angle trunnions, nosebar eccentric, and blocks under screws used to change the lead (vertical adjustment) of the pressure bar. The greatest wear is likely to be in the spindle sleeves and bearing, with the next largest amount in the feed screws and movable parts of the nosebar assembly. Some modern lathes minimize these problems by using preloaded roller bearings for the spindles and an air cylinder to keep the knife bar always against one side of the feed screw. In addition, some lathes have replacable wear surfaces for the ways.

The wear problem with feed screws is greatly reduced by a ball feed screw drive. Motion of the carriage for the pressure bar and knife is obtained by ball bearings turning a ball screw. This movement by rolling friction means less wear than for sliding friction with an acme screw and nut.

Most production lathes develop some play between the knife frame and the bar frame. The amount of movement depends on the looseness in the lathe and the amount of pressure exerted against the bar during cutting. To detect and correct for this play, dial gages should be mounted at each end of the lathe with the gage on the knife frame and the sensing tip against a bracket on the bar frame. These gages should be zeroed after setting the gap or horizontal opening. Any play will show on the gages as a reading other than zero and the original gap or horizontal opening restored by adjusting the nosebar until the gages read zero.

Walser (67) describes a method to preload the pressure bar assembly to improve accuracy when setting the veneer lathe.

Play can also affect the lead or vertical opening. This is less common than play in the gap or horizontal opening. Again, dial gages can be mounted to detect and guide correction of the play.

Spindle Overhang

Other things being equal, the greater the overhang of the spindles the more spring in the cutting system. This is most noticeable when short bolts are cut on a long lathe. If both short and long bolts are to be cut on the same lathe, the lathe should be equipped with spindle steady rests.

Heat Distortion of Lathe

Bolts that have been heat-conditioned prior to cutting may cause the knife and pressure bar to distort. It is generally agreed that heating causes the knife to rise in the middle, decreasing the lead. Heat may cause the pressure bar to drop or move in a horizontal plane, depending on the lathe. On some lathes, one method of correcting for these changes is to adjust the pull screws on the A-frame built over the pressure bars for this purpose. A better solution is to heat the knife and pressure bar to the expected operating condition prior to

51

the final fitting (setting) of the knife and bar. Some lathes have had heating elements built in them to prevent heat distortion.

Another good practice is to store sharpened knives in a warm area so they are at the same temperature they attain during cutting. Feihl and Godin {H) suggest heat distortion can also be controlled by continuous cooling of the knife bed and the pressure bar bed. However, they and others indicate heating the knife and bar works better than cooling, particularly for long lathes.

Undesirable Movement of Wood on Slicer

The wood flitch is generally held against the bed on a vertical or horizontal slicer with dogs. In some vertical and all horizontal slicers, gravity helps hold the back of the flitch against the flitch bed. However, in the most common vertically operating face veneer slicers, the flitch is cantilevered from the bed and dogging is very important.

Heated flitches may be bowed or twisted. Very often this bow or twist can be removed by forcing the flitch flat against the flitch table and dogging it securely. Here oversized dogs are useful at the start of the cutting. A recent development has been retractable dogs, which are extended for maximum holding power at the start of slicing and then automatically retracted when the slicing cut approaches the dogs.

Older slicers had the dogs set by screws. After intermittent cutting, the flitch would often become loose, so the slicer would have to be stopped and the dogs reset in the wood. Modern slicers have hydraulic dogs which maintain good contact with the flitch throughout cutting. The hydraulic cylinders actuating the dogs have check valves to prevent the flitch from shifting during slicing.

A recent practice is to glue valuable flitches such as walnut to an inexpensive backboard and then slice to the glueline. Special glues and gluing techniques are used to bond the hot wet flitches to the backboards. Another innovation is to hold the flitch against the table with a pattern of vacuum cups. The flitch back should be wide, smooth, and flat or the flitch may break loose from the table during cutting.

Undesirable Movement or Play in Slicer Parts

Play can develop in all moving parts such as feed screws, offset mechanism, flitch table ways, and knife-bar carriage ways. Most modern slicers have means of taking up slack in these parts. A regular maintenance schedule should be followed.

Feed by Pawl and Ratchet

Some slicers advance the knife by a pawl and ratchet for each stroke. This is highly accurate providing the same number of teeth are advanced each stroke, there is little play in the feed mechanism, and there is no overtravel of the carriage. The number of teeth advanced each stroke should be checked several times before and during actual cutting. The brake on the shaft which advanced the knife each stroke should be adjusted so there is no overtravel.

Feed to a Stop Plate

Some slicers feed by moving the previously cut surface against a stop plate. The surface of the flitch and of the stop plate must be free of splinters or other debris and the flitch must be advanced flush to the stop to produce veneer of uniform thickness.

Offset on Vertical Face Veneer Slicers

The offset mechanism on modern slicers is hydraulically operated and does not generally require attention once the cam is set to retract the knife at the bottom of the stroke. The amount of offset is adjustable and should be large enough to insure clearance of the flitch on the upstroke. Excess offset should not be used as it may induce slight vibration to the knife. The knife and bar carriage pivot on half bearings for the offset. Since the half bearings are not held at the top, if the flitch fails to clear on the upstroke, the knife bar carriage may be lifted from the half bearings. Similarly, high nosebar pressure cannot be used without danger of unwanted movement of the knife carriage on the half bearings.

As with the lathe, it is desirable to have dial gages mounted at each end of the slicer with the gage on the knife frame and the sensing tip against a bracket on the bar frame. The gages are particularly useful for returning to the

52

previous setting after the bar has been retracted to hone the knife.

Heat Distortion of Slicer

Since face veneer slicers are generally longer than lathes, heat distortion of the knife and bar may be more of a problem. As on the lathe, the heated knife rises in the middle and the pressure bar drops. The pull screws on the A-frame on the casting holding the bar can compensate for movement due to heat. A better solution, and one that is built into modern slicers, is a means of heating the knife and bar prior to fitting them, and then keeping these parts continually warm. This not only greatly reduces any changes in the knife-bar setting due to cutting hot flitches, but also reduces con- densate and the iron-tannate stain that results when iron or steel particles come in contact with wet wood.

Dynamic Equilibrium on Lathe and Slicer

Many have observed that the first sheets from a flitch on the slicer and the first few revolutions of veneer from a bolt on the lathe are thinner than the nominal knife feed. Hoad- ley (29) studied this phenomenon with a knife and pressure bar mounted on a pendulum dyna- mometer. He attributed the thin first cuts primarily to compression of the wood beyond the thickness of cut, followed by springback after the cut. With the same advance, both the compression and springback became progressively larger until a full thickness chip was produced. Hoadley called this dynamic equilibrium.

Later studies on both an experimental and commercial lathe at the Forest Products Labor- atory Hi) indicated that the thin cuts were due mainly to takeup of slackness in the lathe. Veneer cut from a small, more rigid experimental lathe reached full thickness quicker than veneer cut on a 4-foot-long commercial lathe. When the pressure bar was against the wood, it tended to force the bolt and knife in opposite directions. When the bar was retracted and the knife alone engaged the bolt, the knife and bolt were drawn together. As a result, opening the bar (for example, to clear a splinter) during cutting results in large changes of veneer thickness on a lathe that has slackness. In contrast, if the pressure bar is kept closed from the start of cutting, then much of

the slackness in the lathe will be taken out by the time the veneer is wide enough to use. This veneer will be more uniform in thickness than veneer cut just after the pressure bar has been closed.

Some slicer operators set to cut tight veneer and run into a gradual buildup of the flitch face with respect to the knife due to cutting veneer thinner than the feed. Eventually, the knife carriage will vibrate due to excessive pressure against the knife and pressure bar. The operator will then throw off the feed for one stroke, cutting a thick shim and continue to cut. This is poor practice as consecutive sheets cut after each shim are gradually changing in thickness. Better practice is to change the pressure bar setting (larger lead or gap) so that a constant full thickness veneer will be cut.

Eflfect of Speed of Cutting on Veneer Quality

When Knospe (SS) reviewed some of the veneer cutting literature in 1964, he concluded that cutting speed has a minimal influence on the quality of veneer. Recent unpublished work by A. 0. Feihl indicates that for practical purposes this is true within the speeds of about 100 to 500 feet (30 to 150 m) per minute. However, at least two studies (6,JfS) have shown that the strength of the veneer in tension perpendicular to the grain decreases with an increase in cutting speed. Lower strength in tension perpendicular to the grain is generally caused by deeper checks into the veneer. In addition, high cutting speed with wood species having a very high m.oisture content may increase the incidence of mashed grain and shelling.

In summary then, the cutting speed does not seem to be a critical controlling factor for most veneer production. However, if optimum veneer tightness and smoothness are important, it may pay to use a moderately slow cutting speed.

When slicing %-inch and thicker veneer, there may be a slight vibration of the slicer due to the impact at the start of the cut. Inclining the length of the flitch 3° to 5° from the long direction of the knife lessens this impact as the cutting starts at one corner of the flitch. A slower speed also reduces the impact at the start of each cut.

53

KNIFE AND PRESSURE BAR ON LATHE AND SLICER

Type of Knife

Selecting the Knife

The knife represents the largest maintenance cost in cutting veneer and consequently it is worthwhile to use good purchasing specifications and take care in grinding and setting the knives in the lathe or slicer.

What should be specified when ordering a knife for the lathe or slicer? The length of the knife and presence or absence of slots and their spacing will be determined by the equipment on which the knife will be used. Other factors such as depth, thickness, hardness, insert or solid, and the grind can be specified. In addition, the percent carbon and other components of the steel could be specified. However, the exact components of the knife steel are generally not published by knife manufacturers. As a result, most veneer plant managers deal with a reputable knife manufacturer and specify only the size, shape, hardness, and whether they want an insert or solid blade. An ideal knife should have maximum stiflfness, tough- ness, corrosion resistance, and wear resistance.

The most common knife thickness for lathes is % inch (16 mm), and for face veneer slicers, % inch (19 mm). Thinner knives such as V2 inch (13 mm) are sometimes used on the lathe; they are less expensive but also less stifli. The Euro- pean horizontal slicers may use a knife ^%2 inch (15 mm) in thickness, supported with a blade holder. In general, the veneer knife should be thicker when cutting thick veneer. When cutting thin veneer, thinner knives can be used if they are properly supported.

The choice of an inlaid knife or one hardened throughout may depend on the end product. Hardwood face veneer is generally cut with an inlaid knife. The mild steel used for backing is stable and easy to grind. It can be readily drilled so that the knife can be held firmly when back grinding. The highly refined hardened tool steel insert is generally of highest quality for cutting wood.

Knives that are hardened throughout reportedly may stand up better when cutting hard knots. They are sometimes, but not always, used in plants producing construction plywood.

Most veneer knives are supplied the full length of the lathe or slicer. However, two- and three-piece knives are sometimes used with a special clamping arrangement so they can be ground and set as a unit. If one section is damaged, it can be replaced without replacing the entire knife.

The hardness of the knife should be specified and can readily be tested. A soft knife can be easily honed and is tough but also wears rapidly. A hard knife is diflftcult to hone, is more likely to chip if it hits something hard, but holds a sharp edge much better. Most rotary veneer plants prefer a knife with a Rockwell hardness on the C scale of 56 to 58. Knives for face veneer slicers are often 58 to 60 on the Rockwell C scale. To keep as sharp an edge as possible when cutting low-density woods like basswood, a knife with a Rockwell hardness of 60 to 62 may even be used.

Bevel angle, wedge angle, and sharpness angle all refer to the angle that results from the intersection of the two surfaces which form the knife edge. This and other terminology used with the knife and pressure bar are shown in figure 19. The knife bevel angle may vary from about 18° to 23°. The smaller the angle, the less the veneer is bent as it is cut and hence the tighter the veneer. In contrast, the larger the bevel angle the stiffer the blade and the better the edge can withstand impact. More care must be taken when grinding the smaller bevel angles as the knife tip is more likely to heat than when grinding a knife to a large bevel angle.

An 18° bevel angle may be used to slice properly heated flitches of eastern redcedar while a 23° bevel angle is often used to rotary cut bolts of unheated softwoods. Many veneer knives are ground to a bevel angle of 20°or 21°.

Some lathe and slicer operators prefer to measure the length of the knife bevel rather than the knife bevel angle (fig. 19). Some rela- tions of knife thickness, knife bevel angle, and knife bevel length follow :

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Knife Thickness Knife Bevel Knife Bevel Angle Length

Inch Degrees Inch

1/2 (0.500) 18 1.618 19 1.536 20 1.462 21 1.395 22 1.335 23 1.280

5/8 (0.625) 18 2.023 19 1.920 20 1.827 21 1.744 22 1.668 23 1.600

3/4 (0.750) 18 2.427 19 2.304 20 2.193 21 2.093 22 2.002 23 1.919

The ground surface is generally slightly con- cave to make the knife easier to hone. For the lathe, the recommended hollow grind is 0.002 to 0.004 inch (0.05 to 0.10 mm) while slicer knives generally have a hollow of 0.001 to 0.002 inch (0.025 to 0.05 mm). The flatter grind for a slicer knife means less chance for the flitch to rub against the heel of the knife and stain the wood. More hollow can be used on a lathe knife as the bolt surface curves away from the ground surface of the knife. However, the hollow should not exceed 0.004 inch (0.10 mm) as this weakens the knife edge. While the details of the knife bevel can be changed by grinding at the veneer producing plant, the knife should be ordered as it will be used to eliminate an extra grinding.

Knife Wear Knife wear apparently takes place by three

methods: Impact, abrasion, and corrosion. Im- pact and abrasion are mechanical phenomena while corrosion is chemical in nature.

Mechanical impact is most obvious when a hard object, such as a small piece of gravel, chips the knife edge. Damage due to mechanical impact may also occur when the knife hits hard, unheated knots. Such knots may turn the extreme edge of the knife. Woods containing 1 percent or more of silica or calcium carbonate are abrasive and rapidly wear a rough edge on a veneer knife. Use of a tough tool steel rather than a brittle steel may help reduce the damage due to mechanical impact. Use of a microbevel (10) or back bevel reduces the chance of damage due to impact and may make

it possible to cut abrasive wood longer between honings. A microbevel about 0.015 inch wide is often applied at the edge of the knife to make the included angle about 30° (10). If a tough knife could be made from tungsten car- bide ground to a 20° included angle, this should be a good material for cutting wood containing silica or calcium carbonate crystals.

The third method of knife wear is corrosion as described by Kivimaa (SI ) and by McKen- zie and McCombe (^7). Acetic acid and polyphenols in some woods react with the steel knife and corrode it. This reaction makes the common blue iron stain that is so objectionable on face veneer as well as causing wear of the knife. Kivimaa (SI) found that knife wear was greatly retarded by putting a positive potential on the wood work piece and a negative potential of 1,500 volts on a planer knife.

Later at Madison we put a positive charge of 300 volts on a rigid pressure bar on a lathe 4 feet (1.2 m) long and a negative charge on the knife. The charge greatly retarded blue stain from the knife as compared to the stain that developed on oak veneer when the lathe was stopped momentarily without a charge to the pressure bar. However, a shallow brown stain occurred on the veneer next to the knife. In addition, blue stain from the tool steel pressure bar became worse. When a stainless steel pressure bar was used, the blue stain was nearly stopped next to the bar but the shallow brown stain again occurred on the wood next to the tool steel knife. Ralph Scott, a research chemist at the U.S. Forest Products Labora- tory, checked the wood next to the knife (negative terminal) and found it to be a strong base (pH 10 to 12). Apparently hydroxyl ions were released at the negative terminal and formed a base that turned the wood brown.

Another difficulty with running 300 volts direct current from the pressure bar to the knife was that sap forced from the bolt ends made a short and the arc caused a big crater in the knife at this point. A third problem was that the stain was spotty over the 4-foot (1.2 m) length of veneer, indicating the electric current took the path of least resistance and so was not acting uniformly to reduce stain and knife wear.

McKenzie and McCombe (Í7) successfully rotary-cut bolts 4% inches (12 cm) long with

55

the knife held at a negative potential of 60 volts with respect to the nosebar. They report that knife wear was reduced 60 percent.

In spite of the difficulties in applying a positive electrical potential to the bolt or flitch and a negative potential to possibly both the knife and pressure bar, the method does look tech- nically interesting. An alternative would be development of stainless knives than can hold an edge sharp enough for good veneer cutting.

Grinding Veneer Knives The purpose of grinding is to restore a

straight, sharp, tough edge. If these three requirements are kept in mind, they may help guide good grinding practice.

In order to grind a straight edge, it is necessary to start with a rigid level grinder. The most satisfactory veneer knife grinders have a fixed bed for mounting the knife and a travel- ing grinding wheel.

The abrasive may be a solid cup wheel or a segmented wheel. Some operators prefer the segmented wheel because it requires less dress- ing and replacement segments are less expensive than a new cup wheel.

A magnetic chuck makes it faster to set the knife for grinding. A V-belt drive in place of gears reportedly reduces chatter marks on the knife.

The knife bed as well as the ways on which the grinder moves must be rigid, straight, and parallel to one another. The ways are generally hand scraped for accuracy when the grinder is made. The ways should have wipers to keep them clean in use. The accuracy of the ways can be measured in the veneer plant by travers- ing them with a dolly holding a gage. A special telescope with a measuring crosshair is leveled like a transit and then sighted on the gage on the dolly. The dolly is moved along the ways and any deviation from a straight line can be recorded. If the ways are not straight, they must be straightened at the factory. After the ways have been determined to be straight, they are used as a reference to determine if the knife bed is straight and parallel to the ways. This can be readily done in the veneer plant by indexing with a surface gage, such as a dial indicator, from the grinding wheel carrier which moves on the ways.

If the knifebed is not parallel to the ways on

which the grinding wheel traverses, the knife bed should be adjusted until it is parallel to the ways.

To maintain even wear of the ways, the grinding wheel should traverse the entire length of the grinder even when grinding short knives.

The surface of the knife that goes against the grinder bed must be checked for bumps or other rough spots that will prevent the knife from lying perfectly flat. If necessary, the back of the knife should also be ground to restore a plane surface. (See "Back Grinding.")

Heat can cause metal to expand and deform. The grinder and knife should therefore be kept at as uniform a temperature as possible during grinding. An example of poor practice was a grinder set near a radiator. During summer the knife bed was straight. However, in winter with the radiator on, the grinder bed was heated on one side and warped enough to result in unsatisfactory grinding. Similarly, the water used to cool the grinding wheel and knife should be at room temperature and be recirculated. A stream of water with synthetic coolant should be directed against the grinding stone '¥1 inch ahead of where the stone contacts the knife edge during grinding.

Godin (2i) considers overheating of the knife tip the most serious problem in grinding and lists four main causes: (1) Too heavy a cut; (2) inadequate cooling; (3) clogged grinding wheel; and (4) too hard a grade of grinding wheel. Heating is less likely to occur if the knife edge is pointed up and engages the grinding wheel first during grinding. A feed of 0.0003 to 0.0005 inch (0.0008 to 0.012 mm) is suggested for each complete traverse of the wheel. At the FPL we like to dress the wheel and use a very fine feed for the last one or two traverses of the sharpening. This helps give a fine surface. Some manufacturers polish the knife by multiple passes without feeding. The smooth edge reportedly aids good veneer cutting. Care must be used with this technique or the grinding wheel may rub, heat, and weaken the knife tip.

Another cause of an irregular edge is dubbing at the two ends of the knife. The most likely causes are looseness in the grinding wheel spindle bearings, excessive end play, and slack in the feeding mechanism. However, even a

56

grinder in good mechanical condition may slightly round the ends of the knife. This may not be a problem as the end inch or two of the knife generally does not engage the wood when cutting veneer on a commercial lathe or slicer. If it is important to have the knife straight to the extreme ends, then dummy knife sections 4 to 6 inches (10 to 15 cm) can be attached to the knife bed at the two ends and in line with the knife being ground. Sections of a dis- carded knife can be used for this. The dummy sections absorb the heavier cut at the start of each traverse of the wheel and the main knife is not dubbed at the ends.

Back Grinding

After a knife is used, it may wear unevenly on the side where the veneer passes through the throat between the pressure bar and the knife. It may also be bent by excessive local pressure as from a knot or chip buildup. This can be detected by placing a straightedge at a right angle to the cutting edge. If this surface is not flat, then grinding the side of the knife that goes next to the bolt or flitch will not result in a straight edge. The solution is to grind a flat surface on the veneer side of the knife. The grinder bed is tilted ^/2° to 3° toward the knife and the knife is ground to produce a bevel % to 1-% inches long. A magnetic chuck on the grinder facilitates this grinding. Other- wise, the knife body must be drilled and tapped not more than 12 inches apart so the knife can be mounted securely for back grinding.

Some modern grinders are equipped with two grinding wheels so the face and back of the knife can be ground at the same time.

Haning Knife The knife should be ground only enough to

obtain a thin wire edge the length of the knife. The wire edge is removed by careful honing with a stone on one side of the knife, then the other. The stone should be medium grain and medium to soft in hardness. The stone should be saturated with kerosene. Some operators use one stone and others use two stones, one on each side of the knife simultaneously. In either case, each pass of the stone cuts at the base of the wire edge and bends it away from the stone. After several passes, most of the wire edge will fall off. Honing is continued until all of the wire edge is removed. If a badly nicked

knife develops a heavy wire edge, the grinding wheel can be stopped and the wire edge removed while the knife is still clamped in the grinder. A few more passes of the wheel will create a new fine wire edge that can be easily removed by honing. After the wire edge is removed, the edge is finished by lightly honing with a fine- textured stone that has been stored in kerosene.

More detailed suggestions for grinding and honing veneer knives are contained in Cana- dian Forestry Service Publication No. 1236 {2Í).

Secondary Knife Bevels

When a sharp knife ground to a bevel angle of about 21° is first put in the lathe or slicer, it is easily nicked by a knot or other hard substance. These nicks are removed by honing the knife in place on the lathe or slicer. After several bolts or flitches are cut, the knife edge wears slightly and this, plus the honing, makes the extreme edge more resistant to damage. This condition is sometimes called a work-sharp knife. When examined under a microscope, the edge is seen to be slightly rounded so it is probably closer to 30° to 35° than to 21° at the extreme tip. Such a knife will remain sharp and do a good job of cutting for several hours if no very hard material is hit.

For other steel knives used to cut wood, such as planer knives, the smaller the bevel or sharpness angle, the faster the knife wears. The rate of wear goes up much faster if the bevel angle or sharpness angle is less than 30° to 35°. This wear phenomenon is apparently the same for veneer knives. Realizing this, the veneer industry has long had a practice of putting a back bevel on the knife. This strengthens the knife edge and is commonly used with knives installed on core lathes for peeling unheated softwoods.

Kivimaa and Kovanen {32), Feihl {10), and others have studied the use of a precision microbevel put on either side of the knife. They report that a second bevel can be honed on either or both sides of the knife, and that the final included angle of 30° or 35° with a microbevel 0.010 to 0.020 inch in width greatly improves the strength of the knife edge. At least one commercial grinder has a separate grinding wheel that can grind a microbevel at the same time the main bevel is being ground.

Some slicer operators use a two-bevel knife.

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The main bevel is 19° and the second bevel is 21°. Grinding of the second bevel is continued until the length of the second bevel is about V2 inch. When cutting, this is the only part of the knife that rubs against the flitch, and so the two-bevel knife reduces stain. Some operators like the two-bevel knife and others do not.

Setting Knife

Information on setting the knife and bar in a lathe assumes that the knife frame and bar frame of the machine are in proper alinement with the center of rotation of the spindles. Similarly, it is assumed that the knife and bar ways on the slicer are level and perpendicular to the flitch ways. It is further assumed that there is a minimum of play in the moving parts of the lathe or slicer and that the machine parts are at the same temperature they attain in use. If these conditions are not met, the careful setting of the knife and bar on the static machine may be changed so much in the dynamic cutting condition that poor quality veneer will be produced. Feihl and Godin (15) describe methods of checking the basic alinement of lathes.

Setting the Knife in the Lathe and Slicer

A correctly ground flat knife with a straight cutting edge is the first requirement. If a knife holder is used, it must also be clean and flat. A clean, flat bed on the lathe or slicer is the second requirement. (If these conditions are not met, it is difficult or impossible to correctly set the knife.) The knife or knife and knife holder is then set on the two end adjusting screws. The clamping screws are tightened by hand so that the knife is flat against the bed but free to move. To this point, the procedure is the same for the lathe and the slicer.

Setting the Lathe Knife

After the knife is resting on the two end adjusting screws on the lathe, the knife edge is raised until it is level with the center of the spindles. This can be facilitated by using a template consisting of an accurately machined wood block cut out at one end to one-half the diameter of the spindle. The cutout end rests on the spindle and the other end on the knife edge. The height of the knife is then adjusted until a spirit level on the back of

the template indicates level. The same adjustment is then made at the other end of the knife. If the span is short and the knife deep and stiflf, the knife height should be the same across the lathe. However, with longer knives, particularly those that have been ground so they are not so deep, the knife may sag in the middle. One way of checking this is to level a transit with a telescope about 20 feet (6 m) from the lathe and swing it from one end of the knife to the other. The knife edge should be in line with the crosshairs along its length. If the knife sags in the middle, it should be raised with the leveling screws near the center of the knife. Once the knife edge is true, some operators make scribe marks on the lathe so they can reposition knives with precision. Another method is to measure the extension of the knife from the top of the knife bed.

To speed up knife changes, some lathes have knife holders. After grinding, the knife is preset to the desired height in the holder, and the holder quickly bolted in place in the lathe. Some plants in effect preset the knife by pouring babbit metal at the bottom edge of the knife after each grind. The depth of the knife is thus kept constant and the knife can then be placed on the height-adjusting screws without changing them.

Sag in the knife can also be checked with a tautly stretched fine wire.

If there is wear in the spindle bearings, the bolt will ride up during cutting, taking up the play. To compensate for this, the knife edge is sometimes set above the spindle centers the same amount as the play in the spindles. This results in the knife edge being at the spindle centers during cutting.

After the knife is set to the spindle centers, the knife angle is adjusted. In general, the side of the knife that contacts the bolt is approximately vertical (tangent to the surface of the bolt). Such a knife is said to have an angle of 90°. If the knife leads into the bolt 2°, the knife angle is 92° and the clearance angle 2°. A lathe knife can also be set with a negative clearance. A knife angle of 89° means the knife has 1° negative clearance.

Most lathes are built so the knife angle can be made to change automatically with the bolt diameter. The objective is to keep the width of the knife surface that rubs against the bolt

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about the same when cutting a bolt of a large diameter as at a small diameter. For example, when cutting at a bolt diameter of 3 feet (91 cm), the knife angle may be 91° ; at a diameter of 6 inches (15 cm) the angle may be 89° 30'. The means of changing the knife pitch varies with different lathes. Feihl and Godin (15) describe several methods that can be used to properly set the pitch ways. The lathe manufacturers should be consulted for recommended procedure for use with their lathes.

In general, lathe operators use less lead into the bolt (lower knife angles) when cutting low- density woods than when cutting thick veneer. For example, Fleischer (17) suggests a knife setting of 90° 30' when cutting Viö-inch (0.8 mm) yellow-poplar (low-density wood) and 90° 45' when cutting Mw-inch (0.8 mm) yellow birch (high-density wood). Fleischer shows a pronounced eifect of veneer thickness on the best knife setting. For Vmo-inch (0.25 mm) birch, he recommends a knife setting of 92°, for y32-inch (0.8 mm) 90° 45', for Mn-inch (1.6 mm) 90° 15', and for Vs-inch (3.2 mm) veneer 90°. These settings are for log diameter from 20 to 12 inches (50 to 30 cm).

When the correct knife angle is being used, the knife side next to the bolt will show Vic to VH inch (1.6 to 3.2 mm) of bright rub below the knife edge.

If the correct knife angle is not used, the veneer may show this. Too high an angle causes the knife or bolt to chatter and results in a corrugation on the veneer and the bolt surfaces. The waves are closely spaced with three or more waves per inch of veneer width. Too low a knife angle results in too much bearing on the knife, forcing it out of the ideal spiral cutting line. When the force on the knife builds up, it may then plunge into the bolt, resulting in thick and thin veneer with waves a foot or more apart.

Some lathe operators use low knife angles, as the heavy bearing of the knife against the bolt tends to smooth the surface of the veneer. Lathe and knife manufacturers do not like this practice because the pressures on the face of the knife may become so great that the knife will be bent and the knife failure blamed on the knife manufacturer. Low knife angles also require more power for turning the bolt and cause more stain and wear to the lathe.

To prevent these problems, some lathe operators increase the angle of the knife until a corrugated veneer surface results. They then reduce the knife angle gradually until the cor- rugations disappear and use this knife angle for cutting.

For best results, we recommend determining and recording the knife angles that are satisfactory and using an instrument for measuring this angle when the knife is set.

Instruments for measuring the knife angle are described by Fleischer (19), Feihl and Godin (15), Fondronnier and Guillerm (21), and Dokken and Godin (9). While all are suitable, the French design (21) (fig. 21) and the Canadian design (9) are easily read.

If the knives are all ground the same, they can be interchanged on a lathe or slicer without changing the knife angle or clearance angle. However, if the knives are ground so the bevel or sharpness angle is as little at 1-2° different, the cutting can be altered significantly. Conse- quently, we recommend the knife angle be

M 130 i)3il

Figure 21.—Instrument of French design for measuring the knife angle. It is held by magnets to the face of the knife, the bubble is centered, and the knife angle

is read on the vernier.

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checked with an instrument after each knife change.

Setting the SHcer Knife Setting the knife in the sheer is similar to

setting the knife in a lathe except that the position of the knife edge in a sheer is set by the extension of the knife from the bed. The sheer knife edge should extend above the knife bed just enough so the ground face of the knife clears the bolts that hold it against the knife bed. In other words, the knife should extend as little as possible and still make certain the flitch will clear. On vertical face veneer slicers, this distance is about 2 inches.

Like the lathe knife, the slicer knife should rest on the two end adjusting screws. The knife is then brought against the bed and any sag in the middle is removed with the height-adjusting screws near the middle of the slicer. Since slicer knives are often longer than lathe knives, this adjustment is more critical on the slicer. A taut ñne wire can be used as a guide to determine sag in the knife or, if the pressure bar bed is known to be straight, it can be used as a guide. A pressure bar that has been ground uniform in thickness is brought up against the pressure bar bed. The bottom of the pressure bar can then be used as a reference to determine if there is a sag in the slicer knife.

Once the knife edge is determined to be straight, the knife is bolted firmly in place and all of the adjusting screws are brought in contact with the bottom of the knife.

The knife angle of the slicer is relatively easy to set compared to the lathe knife. Since all cutting is from a flat surface, the knife angle does not change with flitch diameter. Further, the knife must lead into the flitch so the heel of the knife does not rub hard against the flitch. Experimentally, we have found that a sheer knife angle from 90° 20' to 90° 30' (about V2° clearance angle) can be used to slice wood from Vioo to % inch (0.25 to 6.3 mm) in thickness from both low-density and high- density woods.

Like the lathe knife, the angle of the slicer knife should be checked with an instrument each time a knife is replaced.

Pressure Bar The pressure bar is important for control-

ling thickness, smoothness, and depth of checks

into the veneer. It compresses the wood just ahead of the knife and so allows the knife to cut rather than split the veneer from the bolt or flitch. This helps control rough surfaces and checks into the veneer. By keeping a force between the knife carriage and the flitch or bolt, the pressure bar takes up slack in the machinery always in the same direction and so aids control of the veneer thickness.

There are two common types of pressure bars —the fixed pressure bar and the roller pressure bar.

Fixed Pressure Bar Two factors to consider when selecting a

fixed pressure bar are its stability and wear resistance. The most common metals are tool steel, steinte, and stainless steel. The tool steel bar is relatively stable, machines easily, and is relatively inexpensive. A stellite bar is more expensive, harder to grind, and less stable. However, the stellite bar will wear many times longer than the tool steel. Stainless steel is easier to grind than stellite and, like stellite, does not stain the veneer.

The fixed bar is generally ground to a bevel angle of about 74° to 78°. As the wood bolt or flitch approaches the fixed bar in the lathe or slicer, the wood is compressed along a plane 12° to 16° from the motion of the wood. When cutting Âs inch (0.9 mm) or thinner veneer from dense hardwoods, the bar should be ground to a sharp edge. The edge of the bar is generally slightly eased or rounded when cutting thicker veneer from low-density woods or woods subject to rupture on the tight side of the veneer from rubbing against the bar. Vari- ous researchers recommend an edge radius of about 0.015 inch (0.3 mm). But Fleischer (17) reports rounding the bar to Vs-inch (3.2 mm) radius did not improve the smoothness of western hemlock veneer and may be disadvan- tageous.

Roller Pressure Bar The roller bar is the second major type of

pressure bar. In U.S. practice, the bar is commonly of bronze, generally % inch (15.9 mm) in diameter if it is a single bar and %> inch (12.7 mm) in diameter if it is a double roller bar. The single roller bar is driven directly while the double roller bar is driven with a

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backup roll. Two advantages of the double roller type stand out: (1) The drive roller can be larger so there is less breakage of the rollers, and (2) the knife and pressure bar can advance very close to the chucks, permitting peeling to smaller diameter cores than with a single roller bar. The drive chain for a single roller bar may protrude up to 1 inch beyond the surface of the roller bar. Roller bars are generally lubricated with 1 percent vegetable oil mixed in water and introduced through holes in the cap that holds the bar.

Comparison of Fixed Bar and Roller Bar The fixed bar is the simplest and most com-

monly used pressure bar. It is used exclusively on slicers and is by far the most common bar used to cut hardwoods on a lathe. The roller bar is more common in the United States for cutting West Coast softwoods and has occasionally been used to cut eastern softwoods and hardwoods. The fixed bar can be used to cut veneer of any thickness. The %-inch (15.9 mm) diameter roller bar cannot be set to cut veneer much thinner than Vio inch (1.6 mm). Most veneer peeled with the aid of a roller bar is used in construction plywood and is yi2 inch (2.1 mm) or thicker. In general, it is easier to set a fixed bar precisely than a roller bar.

A major advantage of the driven roller bar is that it requires less torque to turn a bolt; this in turn means less spinout of the bolts at the chucks and less breakage at shake and splits in these bolts. Another advantage of the roller bar is that it pushes through small splinters that otherwise may jam between a fixed bar and the bolt and degrade the veneer.

Setting Pressure Bar The information on setting the pressure bar,

like the information on setting the knife, assumes the lathe or slicer is in good mechanical condition with a minimum of looseness in moving parts. The knife, pressure bar, and surrounding metal parts on the lathe or slicer should be at the approximate temperature they will attain during cutting.

Cross sections of the knife with a conventional fixed bar and a roller bar are shown in figure 19. Three openings between the knife and the bar are indicated—the lead, gap, and exit gap. With any knife-bar combination, the

position of the bar with respect to the knife is fixed if any two of the three openings are fixed. For example, if the lead and gap are set, this also automatically sets the exit gap. Which two are chosen for setting the knife and bar should depend on the ease with which the openings can be measured and on how the knife and bar can be adjusted on a specific lathe or slicer. Examples of how these three openings are interrelated for different veneer thicknesses and different settings are given in tables 8 through 11.

Setting Fixed Pressure Bar on Lathe (by Lead and Gap)

When the knife edge and the pressure bar edge are ground straight, it is much easier to set the bar. These two edges must be straight and as perfectly alined as posible for precision veneer cutting. All the precautions suggested under knife grinding should also be used when grinding a new edge on a fixed pressure bar.

The bed for the bar and the nosebar cap should be clean and straight. The bar is inserted between the bed and the cap and the nosebar locking screw tightened just enough to hold the bar against the bed but loose enough so the bar can be moved without bending it. The bar should extend from the supporting casting only a minimum amount so it is a rigid as practical.

After the knife is set, the bar is moved toward the knife with adjusting screws at the two ends of the bar until the bar is about %2 inch (0.8 mm) behind the knife edge.

Setting Lead The nosebar bed on most lathes has adjust-

ing screws at the two ends that allow the entire bed to be raised or lowered, increasing or decreasing the lead of the nosebar edge with respect to the knife edge. The amount of lead (vertical opening) is adjusted primarily for the thickness of veneer being cut. Some lathe operators set the lead one-third of the thickness of veneer being cut. Fleischer (17) suggests there is a straight-line relationship with a lead of 0.0005 inch (0.12 mm) when cutting Moo inch (0.25 mm) and a lead of 0.030 inch (0.8 mm) when cutting Vs-inch (3.2 mm) veneer. Some settings using a variable lead that depend on veneer thickness are shown in table 9. Certain lathes made in Germany do not

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Table 8.—Lathe settings with a fixed bar and a constant lead

Feed (veneer thickness)

Mm

Lead Gap Exit gap In. In. Mm In. Mm In. Mm

0.010 0.25 0.030 0.76 0.009 0.23 0.019 0.48 .032 .81 .030 .76 .029 .74 .038 .97 .042 1.07 .030 .76 .038 .97 .046 1.17 .0625 1.59 .030 .76 .056 1.42 .063 1.60 .100 2.54 .030 .76 .090 2.29 .095 2.41 .125 3.17 .030 .76 .112 2.84 .115 2.92 .1875 4.76 .030 .76 .169 4.29 .168 4.27 .250 6.35 .030 .76 .225 5.71 .221 5.61

I Fixed bar, knife bevel 20°, knife angle 90° (0° clearance), lead 0.030 in. (0.76 mm), and gap 10 pet less than feed.

Table 9.—Lathe settings with a fixed bar and a variable lead ^

Feed (veneer thickness) Lead Gap

Mm

Exit

In.

gap In. Mm In. Mm In. Mm

0.010 0.25 0.005 0.13 0.009 0.23 0.010 0.25 .032 .81 .010 .25 .029 .74 .031 .79 .042 1.07 .012 .30 .038 .97 .040 1.02 .0625 1.59 .017 .43 .056 1.42 .058 1.47 .100 2.54 .024 .51 .090 2.29 .093 2.36 .125 3.17 .030 .76 .112 2.84 .115 2.92 .1875 4.76 .043 1.09 .169 4.29 .173 4.39 .250 6.35 .056 1.42 .225 5.71 .230 5.84

1 Fixed bar, knife bevel 21°, knife angle 90° (0° clearance), 1 ead changing with veneer thickness (13), , and gap 10 pet less than feed.

Table 10.—Lathe settings with a roller bar and a fixed lead ^

Feed (veneer thickness) Lead Gap Exit gap

In. Mm In. Mm In. Mm In. Mm 0.0625 1.59 0.085 2.16 0.056 1.42 0.062 1.57

.100 2.54 .085 2.16 .090 2.29 .094 2.39

.125 3.17 .085 2.16 .112 2.84 .114 2.90

.1875 4.76 .085 2.16 .169 4.29 .167 4.24

.250 6.35 .085 2.16 .225 5.71 .220 5.59 1 5/8-in.-diameter roller bar, knife bevel 20°, knife angle 90° (0° clearance), lead 0.085 in. (2.16 mm), and gap 10 pet less than feed.

Table 11.—Lathe settings with a roller bar and a variable lead ^

Feed (veneer thickness) Lead Gap Exit gap > In. Mm In. Mm In. Mm In. Mm

0.0625 1.59 0.068 1.73 0.056 1.42 0.056 1.42 .100 2.54 .075 1.90 .090 2.29 .090 2.29 .125 3.17 .079 2.01 .112 2.84 .112 2.84 .1875 4.76 .089 2.26 .169 4.29 .169 4.29 .250 6.35 .100 2.54 .225 5.71 .225 5.71

' 5/8-in.-diameter roller bar, knife bevel 2V, knife angle 90° (0° clearance), gap equal exit gap equal 10 pet less than feed.

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have a lead or vertical-opening adjustment. This distance is built in the lathe to be about 0.020 inch (0.5 mm). It coincides with the lead suggested by Fleischer for cutting veneer about Vu inch (2 mm) thick.

All agree that the bar edge should be set above rather than at or below the knife edge. It is also generally agreed that the distance the bar is set to lead the knife must be the same at all points along the knife edge.

The common method of checking this opening is to insert a feeler gage of the proper thickness in the lead (fig. 22) between the knife edge and the bar. When the feeler gage is perpendicular to the ground face of the knife, the opening is the same as the thickness of the gage. After the bar is set this way at both ends, it should also be checked at other intervals along the knife. Some lathes have push-pulls so the bar can be warped locally to make the lead or vertical opening uniform across the lathe. However, if the knife and bar are ground straight and the knife bed and bar bed are also straight, any local adjustment of the lead should be minimal. Use of a feeler gage may

slightly nick the blade. It is, therefore, good practice to lightly hone the knife after setting the lead.

Setting the Gap

The second bar adjustment is the gap or horizontal opening. This is the distance from the leading edge of the pressure bar to a plane extended from the ground surface of the knife. Some experienced operators like to bring the edge of the bar to the same plane as the knife edge. Then by feeling with the thumb, they can tell if there are any spots where the bar is ahead or behind the knife edge. These local spots are brought in line with the push-pull screws at the back of the bar. Once the bar is "fit" to the knife, it is retracted to give the desired opening or gap and clamped.

We prefer to use instruments to help make this critical setting. Two such instruments are described by Fleischer (19) and Feihl and Godin (15). Both are essentially dial-micrometer depth gages that use the ground surface of the knife as a reference and measure to the edge of the bar. To automatically position the measuring pin, Fleischer (,19) suggests that

M 139 942

Figure 22.—Adjusting the lead of the pressure bar with a feeler gage. The lead of the bar is moved until a feeler gage of the desired thickness is at a right angle to the face of the knife when the gage is inserted in the opening

between the knife and the bar.

63

M 139 940

Figure 23.—Measuring the gap between the knife and pressure bar edge. Measurements are chalked on the nosebar casting and any deviations greater than 0.001 inch removed with the push-pull adjustment of the bar.

the instrument rest on the top of the pressure bar and on the ground face of the knife (fig. 23).

While one man holds the instrument in contact with the knife and the movable sensing pin against the leading nosebar edge, a second man advances the bar until the correct gap or horizontal opening is indicated. When advanc- ing the bar, the adjustment should always be made to take the play out of the adjusting screws. First the two ends are checked. If they do not indicate the same opening, then they must be brought to the same position with the adjusting screws at each end of the pressure bar bed. Assuming the knife and bar were ground straight and were not warped when mounted on the lathe, the gap should now be the same across the lathe. However, since this is one of the critical lathe settings, we routinely check the opening or gap at 4-inch intervals along the bar. The value of each reading is chalked on the casting holding the pressure bar. Any gradual bends or humps in the bar are then plainly visible. Local deviations are cor-

rected by the push-pull screws at the back of the bar. For accurate cutting, the gap should be within ±0.001 inch (0.025 mm) at all posi- tions.

The actual value of the gap will depend on the thickness of veneer and somewhat on the species being cut. A figure commonly quoted is for the gap to be 20 percent smaller than the thickness of veneer being cut. Experiments at the U.S. Forest Products Laboratory indicate this results in high compression of the wood by the nosebar. It would only be used when cutting thin veneer from an easily compressible species that is resistant to damage by scraping the nosebar over the wood surface.

It is possible the 20 percent figure may have been derived from measurements on lathes that had some looseness or play and not correcting for the looseness.

When the pressure bar is set as described earlier we have found a compression of 10 to 15 percent to be good for cutting veneer from i/io to Vs inch (1.6 to 3.2 mm) thick. Twenty percent compression may be satisfactory when

64

cutting thinner veneer. Higher compression (smaller gap or horizontal opening) may result in tighter veneer; it may also cause the veneer to be thinner than the knife feed and cause damage to the tight side, such as shelling of the grain on susceptible species like western redcedar and redwood.

The advantage of using instruments to measure the knife angle and pressure bar settings is that the setup can be readily duplicated. When experience shows that a certain setting is good for cutting a given thickness of veneer from a given species at a given temperature, then the information should be recorded and the exact processing conditions duplicated when this item is produced again.

Setting Fixed Pressure Bar on Slicer (by Lead and Gap)

The slicer bar is ground and set by the same method as described for setting the fixed bar on the lathe. The difference comes in the actual value of the settings. On the lathe, the lead or vertical opening may be set at various openings such as 0.010 inch (0.25 mm) for %o-inch (0.5 mm) veneer to 0.030 (0.75 mm) for Vs-inch (3.2 mm) veneer. On the slicer, the lead or vertical opening is generally set at about 0.030 inch (0.75 mm). We have cut veneer of satisfactory quality from Vioo to % inch (0.25 to 6.3 mm) in thickness with this lead. A smaller lead such as 0.020 inch (0.5 mm) can be used when cutting i/is-inch (0.9 mm) and thinner veneer. However, this smaller lead may result in more splinters breaking off at the end of the cut and more chance that splinters will become jammed between the knife and bar, causing rub marks on the veneer.

Not as much pressure can be applied with the nosebar on a vertical operating face veneer slicer as can be applied on a lathe. The knife and pressure bar rest on half bearings, permitting the knife and bar to be offset to clear the ñitch on the upstroke. If the pressure bar is set for excessive pressure against the flitch, it will cause the knife and bar carriage to rock on the half bearing; the result is poor veneer and possibly damage to the slicer.

When slicing Âs-inch (0.036-in.) (0.9 mm) veneer, we have found the range of satisfactory

gap or horizontal openings between the knife and bar to be between 0.029 and 0.032 inch (0.725 and 0.800 mm). In effect, the bar is then compressing the wood just ahead of the knife edge 0.004 to 0.007 inch (0.1 to 0.175 mm). Face veneer producers sometimes set the bar to compress the wood only 0.001 or 0.002 inch. When slicing thicker veneer such as Vs (0.125) inch (3.25 mm), the bar may be set to leave a gap of 0.115 inch (2.95 mm), or 0.010 inch (0.25 mm) less than the feed.

As with the lathe, more compression (slightly smaller openings) can be used when cutting low-density woods than when cutting high- density woods.

Setting Roller Pressure Bar on Lathe (by Lead and Gap)

The roller bar is most commonly used when rotary-cutting western softwoods ¥12 to ÂG inch (2.1 to 4.8 mm) in thickness. It is not suitable for cutting veneer thinner than Mc inch (1.6 mm). The reason is that the pressure should be applied against the bolt just ahead of the knife edge. When cutting veneer thinner than i/iß inch (1.6 mm), a roller bar set at a fixed bar lead would over-compress the veneer after it is cut by restricting the throat between the roller bar and the knife. This restraint may cause the veneer to jam and break.

In industry practice, %-inch- (15.9 mm) diameter roller bars are generally set with a lead of V16 (0.062) inch (1.6 mm) or more. From theoretical considerations and laboratory experiments, Feihl, Colbeck, and Godin (13) recommended a roller bar lead or vertical gap of 0.085 inch (2.16 mm) when cutting Douglas- fir Vio to 1/4 inch (2.54 to 6.35 mm) in thickness. They also describe an instrument for measuring the lead of a roller bar.

Lathe settings for several veneer thicknesses using a fixed lead are shown in table 10.

The gap is set much the same as with a fixed bar. That is, good results are obtained by compressing the wood ahead of the knife about 10 to 15 percent of the veneer thickness. This varies with species, wood density, and veneer thickness as discussed under the fixed pressure bar. The gap or horizontal opening can be set and checked with a depth gage reading to 0.001 inch (0.025 mm).

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Setting Roller Pressure Bar (By Gap and Exit Gap)

Collett, Brackley, and Gumming (7) suggest that lathes having a roller bar be set by gap and exit gap. They comment that, for veneer thicknesses from Mo to % inch (2.54 to 6.35 mm), the literature indicates that the gap and exit gap can be the same. This simplifies the recordkeeping as only one value needs to be recorded for each veneer thickness of each species. They recommend use of a depth gage to measure the gap and a feeler gage to measure the exit gap. The amount of compression they suggest at both the gap and exit gap is 10 to 20 percent of the veneer thickness. Table 11 shows some settings where the gap and exit gap are the same.

Setting Fixed Pressure Bar (By Lead and Exit Gap)

Lead and exit gap are suggested by Fondron- nier and Guillerm (21) as the openings to be measured when setting a lathe with a fixed bar. They list the lead changing in a regular man- ner with veneer thickness as follows :

Veneer Thickness Lead or Vertical Opening

(in.) (mm) (in.) (mm) 0.039 1 0.020 0.5

.078 2 .024 .6

.118 3 .028 .7

.157 4 .031 .8

.197 5 .035 .9

.236 6 .039 1.0

They suggest the exit gap should be 10 to 20 percent less than the veneer thickness. Further they recommend that feeler gages be used to measure both the lead and exit gap.

Setting Gap by Pressure Rather Than to Fixed Stops

During rotary cutting of veneer, the force against the pressure bar may vary as much as from 10 to 500 pounds per lineal inch (178 to 8,900 kg/m) of contact with the wood (45), Feihl and Carroll (12) adapted a research lathe to allow the bar to float and maintain the gap by pressure delivered by a cylinder and piston acting against the bar frame. In other words, they set the lead to stops but allowed the gap to be determined by the force against

the bar. They report that the method elimi- nates play in the horizontal mechanism; pro- vides a direct measure of pressure against the bar and so gives the operator good control of the setting; and finally that the veneer produced was equal in quality to veneer produced with a bar set to fixed stops. The method is being tried commercially.

Possible Ways to Generalize Setting of Lathe and Slicer

Optimization of veneer peeling or slicing may require different knife and pressure bar settings for each specific cutting situation. However, it would be convenient to have one knife setting that could be used to cut veneer of any species into any thickness from ¥32 to % inch (0.8 to 6.3 mm). Similarly, it would simplify pressure bar settings if one lead could be used for cutting all veneer. From an examination of the literature and our own experience, it is possible to do this.

Generalized Knife Settings

The knife settings specified in figure 24 are broadly applicable, and may be particularly valuable as a starting point for cutting unfamiliar species.

The knife should be ground to a 21° bevel with 0.002-inch (0.05 mm) hollow grind. The knife angle can be set to 90° 30' or, stated another way, with %° clearance angle. For lathes having an automatic change of knife angle with change in bolt diameter, the knife can be set at 90° 30' when it is 12 inches (30 mm) from the spindle center. This knife setting can be used to cut veneer V¿2 to % inch (0.8 to 6.3 mm) in thickness from any species on the slicer or on the lathe from bolt diameters of 24 inches (60 cm) to a 6-inch (15 cm) core.

Generalized Setting of a Fixed Pressure Bar

The pressure bar should be ground to have an included angle to 75°. This results in the woodwork piece being compressed along a plane approximately 15° from the cutting direction. The edge of the bar that contacts the wood should be rounded to an edge having a radius of about 0.015 inch (0.3 mm).

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KNIFE AND FIXED BAR KNIFE AND ROLLER BAR M 144 168

Figure 24.—Knife and pressure bar settings of general applicability are specified in terms of the diagram. These settings might be used to cut veneer from 1/32 to V^. inch in thickness.

Symbol

A B

C D

Generalized Settings

Knife angle = 90° 30' Knife bevel = 21° with 0.002-inch hollow grind Clearance angle = 30' (i/2°) Lead = 0.030 inch for fixed bar or 0.085 for %-inch-diameter roller bar

Symbol

E F

G

H

Generalized Settings

Pressure bar bevel = 75° Gap = 90 percent of veneer thickness (10 pet compression) Exit gap = Gap = 90 percent of veneer thickness (roller bar) Nosebar compression angle = 15° (fixed bar)

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The lead of the fixed pressure bar ahead of the knife edge can be 0.03 inch (0.75 mm) for both the lathe and the slicer.

The gap from the edge of the pressure bar to the plane of the ground face of the knife can be 90 percent of the thickness of the veneer being cut. Veneer Vs2 to V^ inch (0.8 to 6.3 mm) in thickness and of various species can be cut with these fixed pressure bar settings (fig. 24).

Generalized Setting of Roller Pressure Bar

The generalized settings for lathes with a roller pressure bar are for cutting veneer Vir, to % inch (1.6 to 6.3 mm) in thickness. The lead of the roller bar should be 0.085 inch (2.16 mm). That is, the center of the 5.8-inch- (15.9 mm) diameter roller bar should lead the knife edge by 0.085 inch (2.16 mm). The comparable figure for the fixed bar is 0.030 inch (0.75 mm) (fig. 24 and tables 8 and 10).

An Alternate Generalized Setting of Roller Pressure Bar

Collett, Brackley, and Gumming (6) describe setting a roller bar with the gap and exit gap equal. As with the rigid bar, a generalized setting would be to have the gap and exit gap both 90 percent of the thickness of the veneer being cut (fig. 24 and table 11).

Generalized Setting of the Gap by Pressure

Feihl and Carroll (12) report that pine veneer that is Vio to VG inch (2.5 to 4.2 mm) in thickness can be cut satisfactorily with the pressure on a floating roller bar of about 60 pounds per linear inch (1.070 kg/m) of bar contacting the wood bolt. They further conclude : ''It is not impossible that in some mills (when all species are fairly similar and veneer thicknesses are in the same range) it would be practical to use one pressure setting."

Summary of Generalized Lathe and Slicer Settings

Suggested ''universal" lathe and slicer settings—listed in figure 24—are not optimum settings, but they should permit cutting veneer of moderate quality from any species into any thickness from V32 to % inch (0.8 to 6.3 mm). (The roller bar is not satisfactory for use when cutting veneer thinner than Vw in. (1.6 mm).)

In general, excluding the extreme ranges of specific gravity, one species of wood acts much like another and the veneer cutting process does not change abruptly within the range of thickness from y32 to % inch (0.8 to 6.3 mm).

The settings listed with figure 24 will generally result in a moderately tight cut. If tighter and smoother veneer is desired, smaller openings between the knife and pressure bar may be used. Lathes having automatic pitch adjustment could be set to have a knife angle of 91° at a bolt diameter of 36 inches (91 cm) and a knife angle of 89° 30' at a bolt diameter of 6 inches (15 cm). Ideally, the rate of change of the knife pitch should be greater at the smaller diameters. A smaller fixed pressure bar lead such as 0.020 or 0.015 inch (0.5 to 0.4 mm) can be used for cutting Vio-inch (1.6 mm) and thinner veneer.

Positioning Bolts and Flitches

For maximum yield of rotary veneer, it is essential that bolts be chucked in the geometric center. If the bolts are chucked eccentrically as little as ¥2 inch, the recovery of veneer can be reduced significantly. H. C. Mason, an industry consultant, stated in 1972 that use of bolt- diameter-measuring instruments and a mini- computer controlling a lathe charger to precisely center the bolt in the chucks, will result in at least a 7-percent increase in recovery of veneer for a typical Douglas-fir veneer plant.

The way a flitch is mounted on the slicer table has little effect on yield, but it can aflfect the smoothness of the veneer (S9), An eccentric flat-cut flitch should be dogged with the pith toward the start of the knife cut. A quartered flitch should be turned 180° when the cut approaches the true quarter. These and related phenomena are discussed in detail in (39).

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CONVEYING AND CLIPPING VENEER

Conveying Veneer from Lathe

As veneer comes from the lathe, it may be manually pulled out on a table, but more generally it is moved to long trays in line with the clippers or is reeled.

The tray system is most common in both softwood and hardwood plants. As the veneer comes from the lathe, a short tipple directs unusable veneer to a waste conveyor. Usable veneer is directed into one of the trays with belts synchronized to the lathe speed. After one tray is full, the veneer is broken or cut, and the veneer directed to another tray. This must be done carefully to prevent the veneer ribbon from being folded and split.

The second mechanical means of conveying veneer from the lathe is with a reel. The reel system works best with Vs-inch (3.2 mm) and thinner hardwods cut from sound bolts. Like the tray system, the first unusable veneer is directed to a waste conveyor. Then the usable roundup is collected on a short tray or table. Finally, when a sound ribbon veneer comes from the lathe, it is tacked to a reel and the veneer reeled up as it is peeled. The speed of the reel is synchronized with the lathe. The veneer is reeled with the loose side out.

Combination tray and reeling is popular with some plants peeling species like lauan. The better grades are cut into thin face stock and reeled. Lower grades are cut into thicker core stock and conveyed on trays.

Conveying Veneer from Slicer

It is important to keep the sliced veneer sheets in consecutive order. In many plants, two men turn the veneer over as the sheets come from the slicer and stack them consecutively with the loose side up. In some cases, a short conveyor takes the veneer from the slicer to a position where it is more convenient to stack it. Some European plants automatically convey the sliced veneer to a veneer dryer. Dryer capacity should be sized for the wood veneer species, thickness, and production rate of the slicer.

A German machinery manufacturer recently announced a system to reel sliced veneer by first applying string to the ends of the veneer sheets as they come from the slicer. The string then "leads'' the veneer onto the reel where it can then be stored before unreeling into a dryer.

Clipping Green Veneer

Veneer stored on trays is fed to one or more clippers. In a typical installation, with six trays from a lathe, three trays would feed to one clipper and the other three to a second clipper. A modern clipper has some sensing and measuring device so veneer can be clipped to nominal 4-foot (1.2 m), 2-foot (0.6 m), or random widths. Random widths may be generated when defects such as knots and splits are clipped from the veneer ribbon. An accurate sensing device coupled with the clipper soon pays for itself by greater yields of usable veneer. The green veneer is then sorted by widths, grades, and possibly by sapwood and heartwood in preparation for drying.

Reeled veneer is stored in racks and unreeled just ahead of the clipper. The clipping operation is much the same as that described for veneer stored on trays. One limitation of reeled veneer is that, if it is cut from hot bolts, it should be clipped before the veneer cools and sets in a curved shape.

Flitches of green sliced veneer sometimes have defects clipped out or are trimmed before drying. Packs about Vi-inch (6.3 mm) deep are clipped together as a book. The green clipping saves drying of material that will not be used.

Clipping Dry Veneer

Veneer on trays or on reels is sometimes fed to the dryer in a continuous ribbon. As the veneer comes from the dryer, it is clipped to size. This system reportedly results in less waste and split veneer. One dryer manufacturer states that drying of a continuous ribbon will result in at least a 4-percent increase in recovery of dry veneer.

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VENEER DRYING

An essential part of the veneer-producing process is to dry the veneer. The amount of this drying varies widely. Products that require a minimum of drying—such as bushel baskets and fruit containers—may bring the veneer below a moisture content at which it will mold (about 20 pet). On the upper extreme is drying of softwood veneers that are to be glued with a phenolic hot-press glue, in which case the veneer must be 5 percent or lower in moisture content. In between are such products as decorative face veneer, generally dried to 8 to 10 percent moisture content, and commercial hardwood veneers that are to be glued with a urea glue, in which case 6 to 8 percent moisture content is desirable in the veneer. In all cases, a major criterion is to dry the veneer at the lowest total cost.

Because most veneer operations are set up in a straight-line production system and the production from the lathe and slicer is very high, it is generally necesary to have a fast drying system. Dried veneer should: (1) Have a uniform moisture content; (2) be dried without buckle or end waviness; (3) be free of splits; (4) be in good condition for gluing; (5) have a desirable color; (6) have a minimum of shrinkage; (7) avoid collapse and honeycomb; and (8) have a minimum of casehardening. (Veneer is casehardened when the outer layers are in compression and the center or core is in tension.)

Some Veneer Properties That Aflfect Drying

Factors that affect drying of veneer include both the wood itself and the drying conditions.

An obvious factor is the thickness of the veneer. Thicker veneers dry more slowly than thin veneers. A modification of this is variation in veneer thickness from the nominal thickness. Commercial %-inch (3.2 mm) veneer will often vary ±0.008 inch (0.2 mm) or more in thickness. The thicker portions of the veneer take longer to dry than the thinner portions and contribute to a nonuniform final moisture content.

A second factor is the grain direction on the surface of the veneer. End grain dries several times faster than tangential (fiat) grain. End- grain drying is significant at the ends of all

veneer sheets, which tend to dry faster than the bulk of the sheet. It may also be a factor in curly-grained or other figured veneer where at least partial end grain is exposed on the broad surface of the veneer. As these areas dry faster than surrounding straight-grain areas, they can cause stresses and buckling in the veneer sheet. The difference in drying rates between radial and tangential surfaces is small but may show up. Quarter-sliced veneer will take slightly longer to dry than rotary-cut veneer of the same thickness, and flat-sliced veneer may dry slower on the near-quarter edges than in the flat-grain area at the center of the sheet.

The moisture in the veneer naturally affects the total drying time, as expressed in several ways. Veneer from butt logs may have higher moisture content than top logs. For example, the difference in moisture content of the heartwood of redwood from different logs may be as much as 2 to 1. Furthermore, the wetter heartwood veneer requires significantly longer drying time than drier heartwood of the same species.

Comstock (8) indicates that density of the veneer may be another factor in total drying time. The denser wood heats more slowly than less dense wood and requires more total calories to heat and dry.

The differences between the sapwood and heartwood may be factors with some species and not with others. Bethel and Hader (3) report that the sapwood of sweetgum will dry 25 to 30 percent faster than the heartwood of sweetgum. The difference is attributed to the difference in permeability of the sapwood and the heartwood. This same phenomenon has been observed at the U.S. Forest Products Laboratory when drying veneer of túpelo and other hardwoods like overcup oak. In contrast, Comstock (8) reports that drying time in a jet dryer does not depend on whether the veneer is heartwood or sapwood.

Similarly, there is a lack of agreement on the effect of species on veneer drying. Fleischer (18) found that redwood and sweetgum heartwood dried at a slower rate than yellow-poplar heartwood. Bethel and Hader (3) also found differences in the drying of different species. Comstock (8) and Fleischer (18) indicate that veneer drying is controlled to a large extent by

70

the rate of heat transfer to the veneer. Fleischer qualifies this by saying that this controlling factor is a function of veneer thickness and also to some degree of veneer species. Comstock (8) states that differences between species and between hardwood and sapwood are not important independent variables aside from their effect on the veneer density and moisture content. He developed a general equation for the time required to dry veneer in a jet dryer. He was, therefore, interested in generalities that could be used for any given species. Bethel and Hader (3) concluded that the drying rate of veneer may be controlled by moisture diffusion.

From the literature then, it appears that the rate of heat transfer to veneer is an important factor in the rate of veneer drying. However, diffusion, at least in part, controls rate of drying in %-inch (3.2 mm) and thicker veneer of the impermeable species such as sweetgum heartwood.

Reaction wood—tension wood in hardwoods and compression wood in softwoods—shrinks more longitudinally than typical wood of the the same species. As a result, sheets of veneer containing streaks of tension wood or compression wood tend to buckle during drying.

Do breaks (knife checks) in the veneer during cutting have any effect on drying? Experi- ments at the Forest Products Laboratory do not show any difference in the drying rate of i/iß- or Vs-inch (1.6 or 3.2 mm) loosely cut and tightly cut sapwood veneer of sweetgum and yellow birch dried at 200° to 350° F (93° to 177° C) with an air velocity of 600 feet (180 m) per minute. The loosely cut veneer was easier to flatten after drying.

Some Dryer Conditions That Can Affect Veneer Drying

In general, dryers are operated to hold the veneer flat and transfer as much heat as possible to the veneer during drying.

The importance of holding the veneer flat can be judged by comparing matched sheets of veneer dried with various amounts of restraint. In general, buckle will be greatest in the veneer hung from the ends and allowed to dry at ambient room conditions. Next will be veneer restrained by stickers and dried in a kiln. Veneer dried in a mechanical dryer with a roller or wire-mesh conveyor will buckle less than

matched material dried in kiln. The least buckled will be veneer dried between flat hot- plates.

Temperature and drying time are factors that can affect the rate of drying. For example, Vg-inch (3.2 mm) heartwood of Douglas-fir dried at 250° F (121° C) may require 20 minutes in the dryer. The same kind of veneer dried at 320° F (160° C) may dry in 10 minutes. In- creasing the drying temperature to 400° F (204° C) may reduce this drying time to about 6 minutes. Douglas-fir heartwood veneer has been dried in 2-I/2 minutes by using a drying temperature of 550° F (288° C). Such a high drying temperature may, however, lead to problems in gluing the veneer.

Another factor which is universally agreed to affect the drying rate is the air velocity across the veneer surface. In loft drying, air movement is very slow from convection currents. Veneer dried in a kiln might be subject to air velocities of several hundred feet per minute. This higher air velocity, together with the higher temperatures used in the kiln, greatly accelerates the drying.

Prior to 1960, most mechanical veneer dryers had air circulation either in the longitudinal direction of the dryer or across the width of the dryer. Typical air velocities in such dryers were about 600 feet (180 m ) per minute. Most mechanical dryers made after 1960 have the air impinging directly onto the face of the veneer through slots or orifices. The air velocity is in the range of 2,000 to 10,000 feet (600 to 3,000 m) per minute. This very high air velocity tends to break up any boundary layer at the veneer surface and greatly improves heat transfer. As a result, with a given dryer temperature, thin veneer will dry about one-third faster in a jet dryer than in a mechanical dryer having longitudinal or cross circulation air movement.

The fastest heat transfer is by conduction. In general, with a given dryer temperature, veneer dried between heated platens requires less drying time than veneers in a dryer that depends on air circulation to transfer the heat. The drying occurs fastest when the metal cauls are perforated to allow moisture to escape while main- taining high heat transfer from the hot plates.

The roller conveyor or wire-mesh conveyor in conventional mechanical veneer dryer aids in the drying by transferring heat by conduction

71

to the veneer surface. Some investigators have reported that the heat transfer from the rolls may be as much as 20 percent of the total heat transferred to the veneer.

This heat transfer from the rolls is very- obvious when comparing the drying rates of veneer through an essentially empty dryer and one in which the conveyor is full of veneer. In the full dryer, the rolls are cooled by the wet veneer and the required drying time for a given final moisture content increases. This means the first veneer through an empty dryer will emerge much drier than veneer coming from a full dryer. If the drying time is set according to the first veneer through the dryer, the time will be too short, and veneer coming from a full dryer will be much higher in moisture content.

The relative humidity in a kiln can be used to control the final moisture content of the veneer. The relationship of wet-bulb and dry-bulb temperatures to the final equilibrium moisture content of the wood is shown in figure 25. The ability to control the final moisture content of the veneer is one of the main advantages of the dry kiln.

Most veneer is dried in mechanical dryers at temperatures above 250° F. At these higher temperatures, Fleischer reports that relative humidity has no effect on the drying rate (IS). As a matter of interest, the calculated equilibrium moisture content of wood in saturated steam at 220° F (104° C) is about 11 percent. At 240° F (116° C) it is about 5 percent. Re- cent experiments show that veneer steamed at 220° to 240° F in a kiln or in a hot press will come to the desired final moisture content. Dry- ing veneer to a controlled final moisture content should reduce degrade, reduce shrinkage, and provide a superior surface for gluing.

Types of Veneer Dryers

By far the most common veneer dryer is the direct-fired or steam or hot water-heated progressive conveyor type. The roller conveyor is used most commonly with rotary-cut veneer. A wire-mesh conveyor is used for drying continuous ribbons of rotary-cut veneer and for sliced and half-round veneer. It permits feeding the veneer sidewise so that the sheets can be kept in sequence for matching, in contrast to the roller dryer where the sheets are fed endwise. The wire-mesh conveyor is reported to work

most satisfactorily with a restraint weight of about 5 pounds per square foot (24 kg/m-) when drying thin face veneer. In a roller dryer the rollers are generally hollow tubes which rest directly on the veneer. Both the roller conveyor and the wire-mesh conveyor can contribute to drying by conducting heat directly to the surface of the veneer. Longitudinal, cross- circulation, and impingement air movement are used in these progressive dryers. The method most commonly used in new veneer plants today is the jet dryer with the air impinging on the veneer surface at velocities of 2,000 to 10,000 feet (600 to 3,000 m) per minute.

Some veneer is dried in progressive kilns. These kilns are operated at temperatures below 212° F (100° C) and, consequently, the relative humidity and equilibrium moisture content of the veneer can be controlled. Control of the final moisture content and production of veneer that is easily glued are two of the main advantages of the progressive kiln.

Some products, like baskets, are assembled from green veneer and then dried. Usually heated tunnels with conveyors are used to dry veneer to about 20 percent moisture content to prevent mold.

A few veneer plants use progressive platen dryers. Many users of face veneer redry their veneer in a platen dryer.

A rather unique face dryer made in Germany consists of perforated drums, with a partial vacuum inside the drums. The vacuum holds the veneer against the heated drum and reportedly works satisfactorily with relatively thin veneer. The dryer does not seem well adapted for veneer thicker than y2s-inch (0.9 mm).

An all-infrared dryer has been used commercially on the West Coast, but its use was discontinued because of high drying costs.

Recently banks of gas-fired infrared heaters have been placed at the green end of a few dryers used with softwood veneer for construction plywood. They boost the temperature to reduce the drying time of thick sapwood veneer.

Similarly, high-frequency and microwave energy have been used as a part of drying systems to equalize moisture content at the end of the drying cycle. These methods have not been generally used because of high equipment and power costs (59),

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Figure 25.—Lines of constant equilibrium moisture content.

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Drying veneer between perforated cauls in a hot press has been shown experimentally (30) to be a fast way to dry flat veneer.

Veneer Drying Emissions A factor of current interest is veneer dryer

emissions and whether they contribute to air pollution. Recent studies indicate the opacity of the plume from veneer dryers ranged up to 82 percent with an average of 21 percent (1),

Opacity is judged visually by qualified raters. Rating is in 20-percent increments similar to the Ringelmann Smoke Scale.

The State of Oregon passed a law in 1972 limiting opacity of plumes from existing veneer dryers to 20 percent and from new dryers to 10 percent.

The opacity of the plume can be reduced by using stack velocities over 2,000 feet (600 m) a minute. While this may pass the opacity limitation, it is costly because it results in a large heat loss. Also, it does not cut down on pollution.

Another approach is to filter the stack gases at high velocity through a fiberglass mat. This system can reportedly reduce the average opacity to 5 percent or less (5).

Still another approach is to recirculate the air in direct-fired dryers through a heated duct at 1,200° F. In one-half second the hydrocarbons are incinerated and visibility of stack emissions reduced accordingly (5). Heat of combustion of the hydrocarbons is recovered by a heat exchanger to lower the total fuel needed to operate the system.

Applied Drying Suggestions for Mechanical Dryers

Dry the veneer as soon as practical after cutting to minimize end splits, oxidation stain, mold, and blue stain. This is particularly important for light-colored wood.

To minimize drying time, operate the dryer at the maximum temperature consistent with good glue bonds and wood color. In general, this will be about 400° F (204° C) at the green end and 360° F (182° C) at the dry end of the dryer. If gluing or veneer color are problems, lower the dryer temperature. Decreasing the dryer temperature by 100° F (38° C) (for example, from 350° to 250° F (177° to 121° C) ) will approximately double the drying time.

Keep the dryer vents as nearly closed as practical. This will reduce the energy consumed and reduce veneer dryer emissions. If condensation and haze in the building become trouble- some, open the vents the minimum amount needed to correct the problem.

In general, operate the dryer with the maximum air circulation possible. It may sometimes be necessary to reduce the air velocity to prevent overdrying and splitting of very thin veneer.

Keep the dryer as full of veneer as possible. Dryer schedules should be based on a full dryer operating at a steady temperature and air movement.

Segregate green veneer by required drying time. The green veneer sorts should be by veneer thickness, species, and—for many softwoods—by sapwood and heartwood. Doubling the veneer thickness will more than double the drying time. Sapwood of species like Douglas- fir requires about twice as much drying time as heartwood veneer. Heartwood and sapwood of many hardwoods dry in about the same time. Veneer containing both sapwood and heartwood or wet streaks in the heartwood should be dried on the sapwood schedule.

The veneer drying time should be regulated by the kind of veneer being fed in the green end. It is tempting for the dryer operator to change the drying time from the dry end, depending on whether the emerging veneer seems too wet or too dry. If he does, there may be a constant shifting of drying times and a corresponding shifting in the average moisture content of the veneer out of the dryer. A better method is to carefully determine the proper time to dry veneer of a given thickness, species, and sapwood or heartwood and use this schedule when similar veneer is dried again.

Even when the best dryer schedules are maintained, there will be a range of moisture content in the emerging veneer. Consequently, it is very desirable to have a constant electronic check of the moisture content in the veneer. Veneer having wet spots can be pulled separately. After standing overnight or longer, the veneer can be rechecked for high moisture content and wet pieces redried.

If automatic moisture-detection equipment is not available, then the veneer out of the dryer should be checked regularly with a hand-oper-

74

ated moisture meter. When such meters are calibrated for a given species and make firm contact on cool veneer, they are quite accurate from about 6 to 15 percent moisture content.

An experienced dryer operator can sometimes tell in general the veneer is drying by subjective methods. When veneer is being overdried, static electricity makes the dryer snap and pop. Overdried veneer may be hotter to touch and in extreme cases may be darkened. Underdried veneer will be cool to touch, there

will be less noise from static electricity, and the veneer may be more free of end waviness and buckle.

All veneer should be cooled and held flat as it comes from the dryer. Cool veneer is less likely to buckle and will not contribute to pre- cure of gluelines.

The dried veneer should be neatly stacked on flat skids and the top of the pile weighted. Flitches of sliced veneer should be promptly strapped in flat crates.

QUALITY CONTROL

Undried Veneer The quality of veneer is affected by log qual-

ity, by the care used in storing the logs or flitches, by heating the wood prior to cutting, and by the mechanical condition, setup, and operation of the lathe or slicer.

Quantitatively five factors should be checked at regular intervals : Stain, uniformity of thickness, roughness of the veneer surface, breaks in the veneer, and buckle or other distortions of the veneer.

Control of Stain Stain on veneer may be due to fungus, oxida-

tion, or contact of the wet wood with iron or steel.

Blue stain is the most common fungus stain that occurs readily in the sapwood of most species if unprotected logs are stored during warm weather. The best control is rapid processing of the logs or storage of the logs under water or under a water spray. If water or water spray is not available, end coating the logs is beneficial.

Oxidation stain is generally a yellow or tan stain that may penetrate from the ends of unprotected logs during summer storage. Like fungus stain, it can be prevented by rapid processing of the logs or by storing logs under water or under a water spray. End coatings are also helpful.

Oxidation stain may also occur on the surface of veneer sheets between the time they are cut and dried. A common example is the yellow stain that may develop on birch or maple sapwood. The stain is sometimes compared to the

browning of a freshly cut surface of an apple. Enzymes, moisture, favorable temperatures, and air are factors in this color change.

Probably the best way to control this stain is to dry the veneer promptly after cutting so the surface is dried before oxidation takes place. Holding wet veneer over a weekend is likely to cause stain on susceptible wood species.

Another control method is to heat the logs sufficiently to inactivate the enzymes present in the wood. This generally means heating the logs for 2 days at 160° F or higher rather than limiting heating to overnight. We have been told that running the veneer through boiling water as soon as it is cut may prevent the stain.

When wet wood comes in contact with iron or steel, it reacts to form a blue-black stain. The stain becomes worse the longer the contact and the hotter the wood. It may be particularly prevalent on woods like oak that have a high tannin content, and is very noticeable on light- colored wood like the sapwood of maple. Such stain is not particularly important for uses like construction plywood but is very objectionable on decorative face veneer.

Control methods include keeping the knife and pressure bar as clean as possible; heating the knife and pressure bar to reduce condensation; lacquering the knife and pressure bar so that only the extreme edges have exposed steel that can stain the wood ; using stainless metals for the pressure bar and knife ; using a double bevel on the slicer knife so the heel of the slicer knife cannot rub against the flitch; using a greater knife angle (more clearance) so the heel of the slicer knife does not contact the flitch ; and using less nosebar pressure.

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Figure 26.—Micrometer for measuring veneer thickness to 0.001 inch. M 139 943

Control of Veneer Thickness Uniform veneer thickness is desirable for

production of high-quality glue bonds in plywood, for minimizing show-through of the core, and for producing panels to a specified thickness.

Since uniform veneer thickness is so important, it should be checked on a regular basis. As a minimum, at the green end, the foreman and the lathe or sheer operators should have hand micrometers that read to 0.001 inch (0.025 mm) (fig. 26). They should be encouraged to check veneer thickness at the start of each shift, at each knife change, after any change in thickness being cut, and randomly at other times.

For quality-control purposes, it would probably pay to have a comparator such as described by Bryant, Peters, and Hoerber (4). The size of the anvil or contacting surface should be about 1/2 inch (12.7 mm) in diameter and the weight on the top anvil about 0.66 pound (300 g). When checking thickness of heavy veneer, we have found an air-operated cylinder with adjustable contact pressure and anvils about 2 inches (5 cm) in diameter to be fast and accurate (fig. 27).

The tolerance permitted in green veneer will depend in part on the end use. For exacting end uses, this tabulation may be a guide:

The lathe or slicer will need to be in very good condition and set up and operated with

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Veneer Thickness Tolerance

(in.) (mm) (in.) (mm) 1/4 (0.250) 6.3 ±0.00 ±0.127 % ( .125) 3.2 ±.004 ±.102 Vv, ( .062) 1.6 ±.003 ±.076 %:> ( .031) .8 ±.002 ±.051 Mi4 ( .016) .4 ±.001 ±.025

care to produce veneer that will consistently meet these specifications. Many commercial operations run with tolerances approximately double those listed.

Control of Thickness of Veneer Cut on Lathe

The most common fault in veneer thickness is thin veneer for the first few revolutions of veneer cut on the lathe. The major cause of this thin veneer is looseness in the moving parts of the lathe. A secondary cause is deflection of the wood by the pressure bar beyond the knife edge (29). Further, when the knife alone is contacting the wood, the knife carriage and the wood work piece are pulled together. In contrast,

when the pressure bar is contacting the wood, the knife carriage and the wood work piece are forced apart. To minimize the production of thin veneer at the start of cutting, the lathe should have tight-fitting parts; the pressure bar should be closed from the start of cutting and throughout the cutting; and moderate nosebar pressure should be used. This is discussed in more detail by Lutz, Mergen, and Panzer (44).

Another cause of variable veneer thickness is an improper setting of the knife angle or knife pitch. If the pitch is too low, the veneer is thick and thin in waves, the crest of which may be 1 or more feet apart. Feihl and Godin {16) report, "This defect is particularly pronounced in winter when veneer is cut from logs that are not adequately heated and contain some frozen wood. When such logs are peeled with a low knife angle, the frozen parts tend to produce thin veneer and the thawed parts thick veneer." The corrective measures are to heat the logs to a uniform temperature and to change to a higher knife angle (greater clearance angle),

M 139 941

Figure 27.—An air-operated device for measuring veneer thickness. The pressure on the anvils can be easily changed to suit the species and thickness being measured.

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A number of investigators (4) have found that wood having high moisture content is more susceptible than drier wood to being cut thinner than the knife feed. An example is the tendency of Douglas-fir sapwood veneer to be thinner than heartwood veneer when cut with the same lathe settings. One solution is to use less nosebar pressure when cutting sapwood of conifers than when cutting heartwood.

Wood having high moisture content, such as southern pine sapwood, tends to be thinner than would be expected from the knife feed when cut at fast speed and with high nosebar pressure (ÍS), Slower cutting speed or less nosebar pressure should result in better thickness control.

Shake, heart checks, or splits in the log, and soft centers that allow the bolt to move in the chucks can cause irregular veneer thickness. These unwanted thickness variations are related to specific bolts and do not occur on sound bolts. Larger chucks and continuous end pressure help when cutting bolts with soft centers or with large end splits.

Misalinement of the pressure bar and knife may cause a thickness variation from one end to the other end of the veneer sheet. If the bar moves back at one end of the lathe, the gap or horizontal opening is wedge-shaped. As a result, the emerging sheet of veneer is thick and short at the edge cut with the large gap, and thin and long at the edge cut at the smaller gap. The veneer coming from the lathe runs in the direction of the thicker veneer and the bolt takes a conical shape. The corrective measure is to aline the bar parallel to the knife. Then check for play in the nosebar assembly. Movement of the pressure bar during cutting may be greater at one end than the other and so cause misalinement (16).

Misalinement of the lead of the pressure bar with respect to the knife may also cause this phenomenon but it is less likely to occur and relatively less important than misalinement of the gap.

A conical-shaped bolt may also be caused by a much larger overhang of one spindle than the other. The remedy is to center the bolt endwise with respect to the knife.

Similarly, if the knife edge is not parallel to the axis of the spindle, a conical bolt will be generated. The correction is to adjust the nut

of one of the feed screws of the lathe carriage until the knife frame is parallel to the axis of the spindles (15).

Misalinement of the knife and bar may cause barrel-shaped bolts and veneer that is thicker at the edges than in the middle. This may be caused by closing of the bar lead and gap at the center of the lathe due to heat expansion when cutting hot bolts. It can best be corrected by heating the knife and bar prior to setting up the lathe. Alternately, the lathe can be equipped with a cooling system or the nosebar frame may have a yoke and pull screw.

A barrel-shaped bolt may also be caused by bending of the bolt in the lathe. This is most likely to occur when cutting long bolts to a small diameter. Use of a backup roll can prevent bending of the bolt during peeling.

Control of Thickness of Veneer Cut on the Sheer

The pressure bar is generally bolted into position on the slicer and the flitch is backed up with a steel table. Consequently, the veneer cut on the slicer may be more uniform in thickness than veneer cut on the lathe. Since most veneer cut on a slicer is Míj-inch (1.6 mm) or thinner, this also makes thickness control less of a problem than with thicker rotary-cut veneer.

Even so, the first few sheets cut on a slicer may be thinner than nominal thickness. The cause is primarily play in the feed mechanism and the flitch table. As with the lathe, it may also be due to compression of the wood beyond the knife edge by the pressure bar (29). A warped flitch that is not held securely against the flitch table by the dogs may also result in thin veneer. Having all slicer parts closefitting, the flitch securely held against the flitch table, and using moderate nosebar pressure should minimize these sources of nonuniform sliced veneer.

Less common reasons for nonuniform veneer include heat distortion of the knife and pressure bar that results in veneer cut from near the center of the slicer to be thin. Heating the knife and pressure bar prior to setting up the slicer is the best way to overcome this problem. Yokes and pull screws on the pressure bar holder can also be used to help correct the alinement of the pressure bar to the knife edge. A nonuniformly heated flitch may also result in nonuniform veneer thickness.

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M 141 666 Figure 28.—An instrument for measuring roughness of wood surfaces by moving a stylus across the rough sur-

face. The insert shows the type of trace the instrument records.

A slicer that indexes the previously cut surface against a stop plate may produce uneven veneer if splinters or other debris come between the flitch and the stop plate.

Slicers having a pawl and ratchet feed must have the same number of teeth advanced every stroke. If the mechanism is not set carefully, an incorrect thickness may be produced. Simi- larly, if the feed index train is not braked, momentum may carry the knife carriage beyond the desired index.

Splits or shake in flitches can cause uneven veneer thickness. These thickness variations do not occur with sound flitches.

Control of Veneer Roughness

Like nonuniform veneer thickness, veneer roughness is undesirable for all end uses. Rough

veneer can cause gluing problems, require excessive sanding, and cause ñnishing problems.

Measuring the roughness of wood surfaces is a complex problem. Peters and Mergen (5^) described a stylus trace method they developed for measuring wood surfaces (fig. 28). Earlier Lutz (38) described a light-sectioning method for measuring roughness of rotary-cut veneer (fig. 29). Northcott and Walser (50) have published a visual veneer roughness scale which in turn was obtained by measuring depressions on the surface of the veneer samples with a dial micrometer. For research, the stylus trace method, the light-sectioning method, and the dial micrometer give values for comparative purposes. For mill use, a visual veneer roughness scale is probably more useful. Actual veneer samples that have been measured for

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surface roughness in the laboratory could be kept near the lathe or slicer for visual comparison with the veneer being produced.

The orientation of the wood structure (39) and the growth rate of softwood trees (iO) affect the smoothness of knife-cut veneers. When cutting against the grain of the wood fibers, annual rings, or wood rays, the wood tends to split ahead of the knife and into the wood work piece, causing depressions on the tight side of the veneer. The annual ring effect is most pronounced when rotary-cutting fast- grown softwoods at small core diameters. The ray effect is pronounced when quarter-slicing goes beyond the true quarter. Cutting against the fibers occurs around knots, with curly grain and with interlocked grain. The thicker the veneer, the more likely the veneer will be rough. It is sometimes possible to mount the flitch or

bolt to minimize cutting against the grain (39). Probably the best control is to adjust the nosebar to increase the pressure just ahead of the knife tip and so reduce splitting ahead of the knife. Proper heating of the wood and use of a sharp knife also help reduce this roughness.

Another type of roughness is a fuzzy surface. It is most common on low-density hardwoods like cottonwood that contain tension wood. Over- heating of any species may also cause fuzzy surfaces. Control may include log selection to avoid tension wood, cutting the wood at as low a temperature as is practical, and keeping the knife sharp. An extra hard knife will keep a sharp edge longer than a soft knife and can be used with low-density woods. Use of a slightly eased fixed nosebar edge and continuous flushing of the surface between the wood and the nosebar with cold water may also help.

M 141 667

Figure 29.—An instrument for measuring veneer surfaces by light sectioning. The insert shows what is seen through the magnifying glass of the instrument.

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Shelling or separation of the springwood from the summerwood may occur when rotary- cutting or flat-slicing both softwoods and hardwoods that have a relatively weak zone between the springwood and summerwood. Hemlock, true firs, western redcedar, and angelique are species that may develop shelling. Overheating of the wood, too much nosebar pressure, too sharp a nosebar, or a dull knife may contribute to shelling.

Shattering of the veneer surface is somewhat like shelling and may occur with wood having a high moisture content and low permeability. For example, Douglas-fir sapwood and sinker redwood bolts may develop shattered veneer surfaces if cut at high speed and with high nosebar pressure. Apparently water in the wood is compressed so fast that it ruptures the wood structure to escape. Lower nosebar pressure and slower cutting speed reduce the occurrence of shattered veneer surfaces.

Nicks on the knife edge or pressure-bar edge may cause scratches on the veneer. Scratches from the knife occur on both the tight and loose side of the veneer while scratches from the pressure bar occur only on the tight side of the veneer. These scratch marks are so common that they can often be used to distinguish one- half-round from flat-sliced veneer. The scratches on the half-round veneer are at a right angle to the length of the sheet while those on flat-sliced veneer are at some acute angle corresponding to the draw of the slicer. Careful examination of the veneer, followed by honing the knife and pressure bar when necessary, will minimize these scratch marks. This is particularly important for decorative face veneer. The scratches may take more stain than surrounding wood even if the sanded wood appears to be free of scratches.

Grain raising is occasionally seen on softwood veneer cut from wood having a dense summerwood and much less dense springwood. Excessive pressure from the nosebar overcom- presses the springwood. After the veneer is cut, the springwood recovers, resulting in raised grain. The corrective measure is to reduce the nosebar pressure. Feihl and Godin (16) report that bulging of knots in the core is related to raised grain and they suggest increasing the knife angle as well as decreasing the nosebar pressure as means of correcting this fault.

Corrugated veneer with three or four waves per inch of veneer is generally associated with too high a knife angle. Feihl and Godin (16) report corrugated veneer can also be caused by cold or dry wood and by setting the knife edge too low. Other causes are too much overhang on the spindles, cutting to a small core without adequate support for the core, and wood bolts that become loose in the chucks. Corrective measures are obvious from the stated causes.

Control of Cracks or Breaks into the Veneer

Breaks into the veneer may be on the side of the veneer that is next to the knife or on the side next to the pressure bar during cutting. By far the most common are small cracks that develop on the side of the veneer next to the knife. They may be caused by splitting ahead of the knife edge or by bending the veneer as it passes the knife after it is cut. The terms tight and loose side of the veneer refer to this phenomenon, with the loose side being the side that has the checks. These small breaks are also known as knife checks, lathe checks, or slicer checks.

Less prevalent but perhaps more serious are breaks on the bar side or tight side of the veneer. Three samples are grain separation, lifted grain, and cracks approximately perpendicular to the veneer surface.

Loosely cut veneer is weak in tension perpendicular to the grain. As a result, it may develop splits or break readily during handling, thus lowering the grade of the veneer. Deep checks in face veneer may also contribute to surface checks in furniture or other finished panels. On the other hand, loosely cut veneer may develop more wood failure than tightly cut veneer. As a result, veneer is sometimes cut loosely on purpose to increase the wood failure when the plywood is evaluated by the standard plywood shear test.

Three methods have been used to measure looseness of veneer. One method is to pull 1-inch- (2.54 cm) long veneer samples apart in tension perpendicular to the grain on a suitable test machine (fig. 30). Because of variability, a minimum of about 30 samples should be tested to obtain a value for a given cutting condition. The values obtained can be compared with values for matched sawn and planed pieces of the same size.

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A second method of evaluating veneer checks is to apply an alcohol-soluble dye to the checks by brushing it on the dry veneer surfaces or by dipping the end of the dry veneer in the dye. The dye penetrates into the checks. The depth of checks as a percentage of the veneer thickness can be estimated from scarfed sections of the samples (fig. 31). The method works very well with relatively impermeable veneer such as Douglas-fir heartwood where the dye is generally confined to the checks; it is less satisfactory with permeable veneer such as southern pine sapwood due to overall penetration of the dye into the wood.

A third method is to flex the veneer across the grain. Tightly cut veneer is suffer than loosely cut veneer.

Two factors are most important in minimizing depth of checks on the loose side of the veneer. They are adequate heating of the wood and use of adequate nosebar pressure. Factors that may increase checking are logs that have partially dried and use of a knife bevel much greater than is commonly used.

/

<] ^1

■■a \

§

Figure 30.-

V V M 108 074

-A veneer specimen in the grips of a tension testing machine.

Assuming proper heating schedules are being used as described earlier, the temperature through the flitch or bolts should be relatively uniform. One way to check the bolt temperature is to drill a %-inch- (6.3 mm diameter hole radially an inch or two (2.5 to 5 cm) deep at the center of the cores remaining after cutting veneer from large- and small-diameter bolts. A thermometer should immediately be inserted in the hole and the temperature recorded. This temperature should be within 10° F (5° C) of the desired temperature for good cutting. This method is recommended over measuring the temperature at the surface of the bolt, as the surface temperature of a heated block changes very fast when it is exposed to air.

If the measured temperature is not satisfactory, the heating schedules should be rechecked and the actual temperatures in various posi- tions in the heating vat should be monitored with thermocouples throughout the heating cycle.

Nosebar pressure was described in detail earlier. For quality control, perhaps the most useful procedure is to be certain that the lathe or slicer settings are made with instruments, and that gages are mounted on the equipment to show any unwanted movement of the nosebar with respect to the knife edge during cutting.

With good veneer species like yellow birch and yellow-poplar, it is possible to cut veneer as thick as Vs-inch (3.2 mm) with no visible checks on the knife side of the veneer.

Grain separation is similar to shelling and is a failure of wood between annual rings. The defect may not be noticed in the green veneer but later causes trouble when the plywood made from the veneer is bent as for a boat hull. Two species that have developed the defect are okoume and lauan. The cause is related to relatively weak zones in the wood and is generally considered to be due to setting the bar with too much lead and too small a gap. If suspected, it may be detected in dry veneer or plywood by tapping with a coin or stroking with a stiff brush. The void causes a different noise than the noise that comes when tapping or brushing sound veneer.

Lifted grain is a separation of large groups of fibers in figured veneer like curly birch (16). It is serious because such areas cannot be

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M 107 770 Figure 31.—A scarfed sample of birch veneer to show checks about one-third of the thickness of the veneer. A dye

was applied prior to scarfing to make the checks stand out.

sanded to a smooth surface. Careful setting of the knife and pressure bar may minimize this defect in thin face veneer such as %4-inch (1 mm). Extreme curly grain should not be cut into thicker veneer if lifted grain is to be avoided.

The last type of cracks to be discussed involves breaks perpendicular to the tight side. They may occur if excessive nosebar pressure is used, or if the nosebar lead puts excessive restraint on the veneer as it passes between the knife and the pressure bar. Breaks on the tight side of the veneer can be detected by the tension test and by the alcohol-soluble dye test the same as breaks into the loose side of the veneer. Careful setting of the pressure bar will eliminate this problem.

Control of Buckle in Green Veneer

Buckle is undesirable as it interferes with edge gluing, glue spreading, and panel layup. When it is severe it may cause overlaps or splits in the plywood. Buckled veneer caused by reaction wood may also cause warped panels in service.

Buckle, like end waviness, may be measured by deviation from a plane surface by placing the buckled veneer between two flat parallel

surfaces and recording the spacing. Commonly, buckle is rated visually as mild, moderate, or severe.

Buckle in green veneer may be caused by reaction wood or by uneven pressure against the bolt or flitch during cutting.

Compression wood in softwoods and tension wood in hardwoods have different longitudinal stresses than normal wood. When sheets of veneer containing both reaction wood and normal wood are cut, the veneer may buckle as it comes from the lathe or sheer. Drying accentuates this buckle. Logs from species known to be prone to develop reaction wood should be examined prior to cutting and not be cut into veneer if the reaction wood is pronounced.

Uneven pressure against the bolt or flitch may be due to heat distortion of the knife and pressure bar setting on the lathe or slicer ; bowing of small-diameter bolts on the lathe; jamming of a chip or splinter between the pressure bar and the bolt or flitch; or a tight spot due to a local deviation of the knife or pressure bar edges from a straight line.

As discussed earlier, heat distortion can be minimized by heating the knife and pressure bar prior to setting them. Bowing of the bolt

83

may be minimized by reducing the nosebar pressure and by using a backup roll. Some lathe operators judge the correct nosebar pressure by whether the veneer buckles in the center of the sheet. If the center of the veneer ribbon is buckled, the pressure is too high and the nosebar gap is widened.

A splinter or chip jammed between the knife and bar in effect puts very high local pressure on the wood and causes the veneer to be thin. A bump builds on the bolt or flitch. If it is pronounced, the veneer may develop a hole at this area and the knife may be bent. The correction is to stop cutting, open the pressure bar, remove the chip or splinter, close the bar, and resume cutting. Use of a roller bar helps reduce this defect as the chips are more readily pushed past the opening between the knife and pressure bar. Setting a fixed bar with more lead may help reduce this problem. Having the bolt or flitches clear of bark and loose splinters is good practice and will reduce jamming of particles between the surface of the bolt or flitch and the pressure bar.

Finally, if the knife and pressure bar are not ground straight, there may be a local tight spot that will result in buckled veneer. The correction is to grind the knife and bar straight. Both surfaces of the knife edge should be examined and if necessary both should be ground to straighten the edge (2i).

Dry Veneer Most veneer readily dries satisfactorily for

the intended end use. But since veneer is easy to dry, potential problems are sometimes over- looked.

Some veneer drying problems are nonuniform moisture content in the veneer as it emerges from the dryer, buckle and end waviness of veneer sheets, splits and checks in the veneer, a veneer surface that is difficult to glue, scorched veneer surfaces, veneer that shows signs of collapse, honeycomb, or casehardening, excessive veneer shrinkage, and undesirable color.

Control of Final Moisture Content Probably the most universal problem in dry-

ing veneer in a progressive mechanical veneer- type dryer operating above 220° F (104° C) is the nonuniform moisture content in the ve-

neer as it comes from the dryer. This is true of a dryer having longitudinal circulation, cross circulation, or jet impingement circulation. It is similarly true for a progressive platen-type dryer.

For example, veneer dried to an average moisture content of 8 percent will generally have a range of moisture content from about 2 to 20 percent. This is because the equilibrium moisture conditions in the dryer are for all practical purposes 2 percent or less. When drying to an average moisture content of 8 percent, the faster drying veneer may come to 2 percent and the slower drying to 20 percent moisture content. In other words, any difference in the drying rates of different areas of the same sheet of veneer then results in a wide range in final moisture content in the veneer as it comes from the dryer.

To keep this problem to a minimum, the green veneer should be sorted for thickness, moisture content, and density. Better control will probably result if the green veneer is also sorted for sapwood and heartwood and by species. Assuming the veneer is being sorted as well as possible to have veneer of one type being dried at a time, the next point to check is the uniformity of drying conditions in different parts of the dryer.

Modern veneer dryers are generally designed to have uniform temperature and air movement throughout the dryer. However, it may be worthwhile to check these factors. Is the temperature at the top conveyor the same as it is at the bottom conveyor? Is the air speed approximately the same in all parts of the dryer? One method of checking this is to run matched samples of veneer through different portions of the dryer. For example, one sample can be run through the left side of the upper conveyor, another through the right side of the upper conveyor, another through the left side of a lower conveyor, and so on. Then carefully check these samples for moisture content immediately out of the dryer. If this test shows that one portion of the dryer is consistently drying veneer faster than another, drying rates can sometimes be equalized by adding steam coils, baffles, or fans where needed in the dryer.

Another way of controlling the final moisture content is to dry all of the veneer to 5 percent moisture content or less. This may result

84

in overdrying of some of the veneer, but it will result in a narrower range of veneer moisture.

A very common method of reducing the spread of moisture in the veneer is to electron- ically measure the moisture content in each piece of veneer as it comes from the dryer. Veneer that has a moisture content higher than the desired maximum is marked and pulled separately for further drying. Leaving this wet veneer in a solid stack overnight will help to equalize the moisture content. A re-sort through the moisture detector the next day will reduce the number of pieces that need to be redried.

Some moisture meters are sensitive to wood temperature as well as moisture content. They should be calibrated under the conditions in which they will be used.

Another method that is sometimes used when nonuniform moisture content is a serious problem is to dry in two stages. In the first pass, the veneer is brought to an average moisture content of about 20 percent. It is then stacked overnight to allow some equalization and rerun the next day to the average moisture content desired.

High-frequency or microwave units have been used experimentally at the dry end of the dryer to equalize the moisture content of the veneer. Both these methods work on the principle that the higher moisture areas in the veneer absorb more energy. Heating and drying are propor- tional to this absorption of energy. Both of these methods do equalize moisture content in the veneer, but they have not been generally adopted because of cost (59).

It is possible to dry veneer to controlled moisture contents in superheated steam at atmos- pheric pressure. To date this method has not been used commercially.

Control of Buckle Buckle in veneer may be caused by stresses

in the wood, by reaction wood, by irregular grain with resulting irregular drying rates and irregular grain with resulting shrinkage, and possibly also by improper setting of the lathe or sheer. Use of the maximum restraint that will hold the veneer flat without causing it to split due to shrinkage stresses will help to minimize buckle. Similarly, anything that can be done to dry the veneer to as uniform a moisture content as possible will reduce buckling.

In most cases, buckling can be minimized by redrying in a plate dryer. The redrying temperature and time will depend on the moisture content of the veneer (4^1),

Control of Splits

Splits in veneer that has been dried in a progressive mechanical dryer are generally related to splits that were in the green veneer or result from rough handling. If stacks of green veneer must be held before drying, the ends should be protected from end drying by covering them with a plastic sheet (such as polyethylene) or if necessary by spraying them with water.

A recent development for controlling handling splits is green veneer taping. Tape is applied at the lathe primarily to veneer thinner than 1/26 inch (1 mm). Taping reportedly improves the veneer grade, and reduces the need to splice and repair veneer. Forest Products Laboratory experiments showed that i/2-inch- (12.7 mm) wide flexible tape applied to the spurred ends of the green veneer reduces end waviness.

Another method of reducing handling splits is to dry rotary-cut veneer in a continuous ribbon using a wire-mesh conveyor in a mechanical dryer. The method was used as early as 1950 with birch veneer which was reeled as it came from the lathe and then unreeled into the dryer. The dryer veneer was then clipped for grade.

More recently a system has been developed where softwood veneer is stored on long trays and then fed in line to the dryer. In addition to reducing splits, recovery is reportedly improved because the veneer is clipped dry and it is not necessary to oversize to compensate for variability in shrinkage.

Control of Veneer Surfaces for Gluability

Poor glue bonds have been reported with veneer dried in direct oil-fired dryers operating at temperatures as high as 550° F (288° C). This is less of a problem with direct gas-fired dryers and less yet with steam-heated dryers. Dropping the temperature to 400° F (208° C) or lower improved the gluability of the veneer. Causes of glue interference may be weakening of the surfaces and extractives brought to the wood surface during high-temperature drying.

85

At any rate, use of a lower drying temperature and prevention of overdrying the veneer are the common means of overcoming veneer gluing problems.

Control of Dryer Fires and Scorched Veneer

High drying temperatures may cause scorched veneer and possibly fires in the dryer. At temperatures from 200° to 300° F (93° to 149° C), extraneous materials volatilize from wood. From 300° to 400° F (149° to 204° C), there is scorching and slow evolution of ñam- mable gases from the wood. This progressively becomes more rapid until at about 600° to 650° (316° to 346° C) the wood can ignite spontaneously.

Even if wood does not ignite spontaneously until the temperature at its surface reaches about 650° F (346° C), if the surface becomes charred, charcoal gases may ignite at a temperature as low as 450° F (232° C). Extraneous materials such as turpentine also ignite at a temperature of about 450° F (232° C).

Veneer being dried in dryers operating at 400° F (204° C) or less sometimes ignites in the dryer. These fires may be caused by a static spark that ignites flammable gases of volatile extraneous materials.

Avoiding overdrying and use of controlled lower drying temperatures are the primary means of preventing dryer fires and scorched veneer.

Control of Collapse^ Honeycomb^ and Casehardening

Collapse and honeycomb may occur in species that are relatively nonporous. Typical examples would be Vs-inch (3.2 mm) and thicker heartwood of sweetgum and overcup oak. Collapse in sweetgum heartwood is likely to occur in

early stages of the drying. Sweetgum dried at 350° F (177° C) had much more honeycomb than sweetgum heartwood dried at 150° F (66° C). Experiments at Madison showed that Vs- inch (3.2 mm) overcup oak dried at 320° F (160° C) might shrink as much as 20 percent in thickness. The solution to these drying problems in all cases appears to be to use a lower drying temperature.

Casehardening was at a maximum in Vs-inch (3.2 mm) heartwood of sweetgum when dried at temperatures of 120° to 160° F (49° to 71° C). Casehardening can be removed by use of high temperature, particularly if the veneer has a high moisture content.

Control of Shrinkage Widthwise shrinkage of flat-grain veneer

generally decreases with increasing drying temperature. For example, Vs-inch (3.2 mm) yellow-poplar dried at 150° F (66° C) shrank 6 percent; when dried at 250° F (121° C) it shrank bVi percent; and when dried at 350° F (177° C) it shrank 4^2 percent. In contrast, the shrinkage in thickness tends to increase with an increase in drying temperature.

Control of Color Color in face veneer can often be controlled

to some degree by varying the time that the green veneer is held in a stack prior to drying. In general, the wet veneer tends to oxidize and darken in storage. Consequently, if a light color is desired, as with the sapwood of hard maple, the veneer should be dried as quickly as possible after cutting. In other cases, it may be desirable to have some color change take place in the green veneer stack. An example is black walnut. The color of the sapwood and heartwood changes gradually in the warm green stack. When the desired color is reached, the veneer is sent through the veneer dryer.

86

VENEER YIELDS AND VOLUME NEEDED FOR A PLANT

VENEER YIELDS With knife-cut veneer one might assume that

veneer recovery could equal the volume of the log minus the volume of the core. Unfortunately this is not the typical case. For example, during peeling of Douglas-fir in commercial plants, Woodfin (68) found losses due to: spurring, 2 percent; roundup, 5V2 percent; green end clipper loss, 22 percent; below-grade veneer, 6 percent; core, 91/2 percent; and veneer shrinkage 3 percent. Thus the actual recovery of dry veneer was only 52 percent of the total green block cubic volume. This is typical of yield studies in industrial plants.

The losses at different stages vary with the quality and diameter of veneer blocks. Cylin- drical logs have less loss from roundup than logs with pronounced taper or crook. Assuming the core diameter is constant, large-diameter logs have a smaller percentage loss as core than small-diameter logs.

Sharp increases in log costs in 1973 stim- ulated interest in means of improving veneer yields. Baldwin's book, *Tlywood Manufactur- ing Practices'' (2), describes good industry practice in 1975 to maximize recovery of ve-

(ROTARY CUTTING) neer. Some techniques described include backup rolls and retractable chucks to aid cutting to smaller cores, use of a moving knife to separate the veneer ribbon going to different trays, veneer clippers having devices to sense open defects and clip automatically for maximum yield, and veneer sheet composers.

A technique for increasing yield that has been described (23) but not adapted is to precisely measure block diameters, feed the information to a computer which in turn directs the charging device to precisely chuck the block in the geometric center. Estimated increased yields are up to 7 to 8 percent.

Drying veneer in a ribbon and clipping after drying has been reported to increase yields as much as 4 percent. However, extra energy is used to dry some veneer that is then clipped out and not used to make plywood.

If all conditions are favorable, it is possible to obtain high veneer recovery in a commercial plant. For example, Knutson (34) reported 87 percent yield of Vio-inch Douglas-fir from sound logs 20 to 23 inches in diameter.

VENEER YIELDS (SLICED) In general veneer recovery is highest by ro-

tary cutting, less by flat-slicing, and least by quarter-slicing. Yields are less for slicing because of losses when sawing the flitches and when clipping straight edges on the relatively narrow sliced veneer.

Some commercial slicing operators have reported that, for logs 15 inches and larger in diameter, the yield of flat-sliced veneer is about equal in equivalent thicknesses to the board foot value by the Scribner Decimal C log rule.

87

VOLUME OF TIMBER NEEDED TO SET UP A VENEER PLANT

A typical plant in the United States making construction and industrial plywood uses approximately 40 million board feet of logs per year. The smallest economically suitable construction plywood plant uses about 15 million board feet of logs a year. If the volume of wood available at a site is less than this, there is little point in considering it for structural plywood.

Hardwood and decorative plywood plants are generally smaller than structural plywood plants. In addition, they frequently use a variety of species. Therefore, while 12 to 15 million board feet of logs may be used in a year, a hardwood species that could be supplied at the rate of 5 million board feet a year could probably be used satisfactorily.

An even greater diversity of species is cut by mills making face veneers. Manufacturers of face veneers state that it is imperative that a continuing supply of a new face veneer must be available. Otherwise the cost of advertising and other promotion needed to get a new species accepted is not warranted.

Core and crossband veneer is generally not specified by the ultimate customer. Hence, introducing a new species is not as difficult as with face veneers. The technical properties of the wood and the volume availability at a rea- sonable cost are important for core and crossband veneers.

Container veneer often is made from a vari-

ety of species. Typical plants are small and use less volume of logs than plywood plants. The end-product is generally an expendable low-cost container. Cheap stumpage is essential. Lower quality logs than those acceptable for plywood panels are successfully used for container veneer.

Two examples of the importance of available timber are the development of southern pine softwood plywood and hickory- or pecan-faced hardwood plywood during the 1960's. Both of these groups of species are relatively difficult to process into veneer and plywood. Yet, because of the large available timber supply of each, they became realities. Southern pine is challeng- ing the western softwood plywood industry, and hickory and pecan are a major group used for decorative face veneer.

In some mixed forests of the tropics, the total stumpage is large, but no one species occurs in large volume. In these areas it is often difficult to exploit new species for veneer. This is true even for a species that has good technical properties for use as veneer.

The cost of developing information on a new species, determining how it should be handled in production, introducing it, and promoting it in a product line is very costly. If a species is available only on a sporadic basis, it is generally not economical for a manufacturer to uti- lize the species.

LITERATURE CITED

1. American Plywood Association 1973. How to control veneer dryer emissions.

APA sem., reprinted in Wood and Wood Prod. Nov. 1973, p. 94 B,C,D.

2. Baldwin, Richard F. 1975. Plywood manufacturing practices. Miller

Freeman Pub., Inc. San Francisco, p. 260. 3. Bethel, James S., and Robert J. Hader

1952. Hardwood veneer drying. J. For. Prod. Res. Soc. 2(5):205-215.

4. Bryant, B., T. Peters, and G. Hoerber 1965. Veneer thickness variation: its measure-

ment and significance in plywood manufacture. For. Prod. J. 15(6) :233-237.

5. Burrell, J. F. 1973. Plywood plants of the future. Plywood

and Panel Mag. 14(6) :28-30. Nov. 6. Cade, J. C, and E. T. Choong

1969. Influence of cutting velocity and log diameter on tensile strength of veneer across the grain. For. Prod. J. 19(7) :52-53.

7. Collett, B. M., A. Brackley, and J. D. Gumming 1971. Simplified, highly accurate method of pro-

ducing high-quality veneer. For. Ind. 98(1): 62-65.

8. Gomstock, G. L. 1971. The kinetics of veneer jet drying. For.

Prod. J. 21(9):104-111. 9. Dokken, H. M., and V. Godin

1975. Instrument for measuring knife pitch angle on veneer lathes. For. Prod. J. 25(6): 44-45. June.

10. Feihl, A. 0. 1959. Improved profiles for veneer knives. Gan.

Woodworker. Aug. 11. Feihl, A. 0.

1972. Heating frozen and nonfrozen veneer logs. For. Prod. J. 22(10) :41-50.

12. Feihl, A. 0., and M. N. Garroll 1969. Rotary cutting veneer with a floating bar.

For. Prod. J. 19(10) :28-32. 13. Feihl, A. 0., H. G. M. Golbeck, and V. Godin

1965. The rotary cutting of Douglas-fir. Gan. Dep. For., For. Prod. Res. Br., Pub. No. 1004.

14. Feihl, A. O., and V. Godin 1967. Wear, play, and heat distortion in veneer

lathes. Gan. Dep. For., For. Prod. Res. Br., Pub. No. 1188.

15. Feihl, A. 0, and V Godin 1970. Setting veneer lathes with aid of instru-

ments. Gan. Dep. For., For. Prod. Res. Br., Pub. No. 1206.

16. Feihl, A. 0., and V. Godin 1970. Peeling defects in veneer, their causes

and control. Gan. Dep. For., For. Prod. Res. Br., Tech. Note 25.

17. Fleischer, H. 0. 1949. Experiments in rotary veneer cutting. J.

For. Prod. Res. Soc. 3:137-155. 18. Fleischer, H. 0.

1953. Veneer drying rates and factors affecting them. J. For. Prod. Res. Soc. 3(3):27-32.

19. Fleischer, H. O. 1956. Instruments of alining the knife and nose-

bar on the veneer lathe and slicer. For. Prod. J. 6(l):l-5.

20. Fleischer, H. O. 1959. Heating rates for logs, bolts, and flitches

to be cut into veneer. U.S. For. Prod. Lab. Rep. No. 2149.

21. Fondronnier, J., and J. Guillerm 1967. Guide pratique de la dérouleuse (Fr.).

Cent. Tech. du Bois, 10 Ave. de St. Mande, Paris 12e, Fr.

22. Fondronnier, J., and J. Guillerm 1975. Le Flambage du bois lors de son déroulage.

Cent. Tech, du Bois, 10 Ave. de St. Mande, Paris 12e, Fr.

23. Foschi, R. 0. 1976. Log centering errors and veneer yield.

For. Prod. J. 26(2) :52-56. Feb. 24. Godin, V.

1968. The grinding of veneer knives. Gan. Dep. For., For. Prod. Res. Br., Pub. No. 1236.

25. Grantham, John and George Atherton 1959. Heating Douglas-fir blocks—does it pay?

Greg. For. Prod. Res. Center. Bull. No. 9. 26. Hancock, W. V., and H. Hailey

1975. Lathe operators^ manual VP-X-130. Can. West. For. Prod. Lab., Vancouver, B.C. Jan.

27. Harrar, E. S. 1954. Defects in hardwood veneer logs: their

frequency and importance. USDA For. Serv. Southeast. For. Exp. Stn. Pap. No. 39. Ashe- ville, N.C.

28. Hillis, W. E. 1962. Wood extractives. Academic Press, N.Y.

513 p. 29. Hoadley, R. B.

1962. Dynamic equilibrium in veneer cutting. For. Prod. J. 12(3) : 116-123.

30. Hann, R. A., R. W. Jokerst., R. S. Kurtenacker, C. C. Peters, and J. L. Tschernitz.

1971. Rapid production of pallet deckboards from low-grade logs. USDA For. Ser. Res. Pap. 154. For. Prod. Lab., Madison, Wis.

31. Kivimaa, E. 1952. Was ist die Abstumpfung der Holzbear-

beitungswerkzeuge? Holz als Roh- und Werkst. 10:425-428.

32. Kivimaa, E., and M. Kovanen 1953. Microsharpening of veneer lathe knives.

State Institute for Tech. Res., Helsinki, Fini., Rep. No. 126, 24p.

33. Knospe, Lothar 1964. The influence of the cutting process in

slicing and peeling on the quality of veneers. Holztechnologie (Wood Tech.) 5(1):8-14. (in Ger.)

34. Knudson, R. M., R. W. G. Scharpff, R. J. Mastin, and D. Barnes

1975. Effect of lathe settings on veneer yield. For. Prod. J. 25(10) :52-56.

35. Kubinsky, Eugen and Milan Sochor 1968. New softening treatment for beech logs

before rotary peeling to veneers. For. Prod. J. 18(3): 19-21.

36. Kubier, Hans 1959. Studies of growth stresses in trees. Holz

als Roh- und Werkst. 17(1) :l-9; 17(2) :44-54; and 17(3) :77-86.

89

37. Lockard, C. R., J. A. Putnam, and R. D. Carpenter 1963. Grade defects in hardwood timber and

logs. USDA Agrie. Handb. 244. 38. Lutz, John F.

1952. Measuring roughness of rotary-cut veneer. The Timberman 53(5) :97,98,100.

39. Lutz, John F. 1956. Effect of wood-structure orientation on

smoothness of knife-cut veneers. For. Prod. J. 6(ll):464-468.

40. Lutz, John F. 1964. How growth rate aifects properties of

softwood veneer. For. Prod. J. 14(3) :97-102. 41. Lutz, John F.

1970. Buckle in veneer. USDA For. Serv. Res. Note FPL^0207. For. Prod. Lab., Madison, Wis.

42. Lutz, John F. 1972. Veneer species that grow in the United

States. USDA For. Serv. Res. Pap. FPL 167. For. Prod. Lab., Madison, Wis.

43. Lutz, John F., A. Mergen, and H. Panzer 1967. Effect of moisture content and speed of

cut on quality of rotary-cut veneer. USDA For. Serv. Res. Note FPL-0176. For. Prod. Lab., Madison, Wis.

44. Lutz, John F., A. F. Mergen, and H. Panzer 1969. Control of veneer thickness during rotary

cutting. For. Prod. J. 19(12) :21-27. 45. Lutz, John F., and R. A. Patzer

1966. Effects of horizontal roller-bar openings on quality of roller-cut southern pine and yellow-poplar veneer. For. Prod. J. 16(10) : 15-25.

46. MacLean, J. D. 1946. Rate of temperature change in short-

length round timbers. Trans. Amer. Soc. Mech. Eng. 68(1:1): 1-16.

47. McKenzie, W. M., and B. M. McCombe 1968. Corrosive wear of veneer knives. For.

Prod. J. 18(3):45,46. 48. Meriluoto, Jaakko

1971. Melting of birch bolts. Paperi ja Puu 53(9):493-497.

49. Nearn, W. T. 1955. Effect of water soluble extractives on the

volumetric shrinkage and equilibrium moisture content of eleven tropical and domestic woods. Bull. 598, Pa. State Univ., Coll. of Agrie, Agrie. Exp. Stn., University Park, Pa.

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man. July. 51. Northeastern Forest Experiment Station

1965. A guide to hardwood log grading. USDA For. Serv., Northeast For. Exp. Stn. Handb., Rev., Upper Darby, Pa.

52. Northern Hardwood and Pine Manufacturers Association

1968. Offícial grading rules for northern hardwood and softwood, logs and tie cuts. Green Bay, Wis.

53. Palka, L. C. 1974. Veneer cutting review. VP-X-135. Can.

West. For. Prod. Lab., Vancouver, Canada. 54. Peters, C. C, and A. Mergen

1971. Measuring wood surface smoothness: a proposed method. For. Prod. J. 21(7) :28-30.

55. Pillow, Maxon Y. 1943. Compression wood: importance and detec-

tion in aircraft veneer and plywood. U.S. For. Prod. Lab. Rep. No. 1586. Madison, Wis.

56. Pillow, Maxon Y. 1955. Detection of figured wood in standing

trees. U.S. For. Prod. Lab. Rep. No. 2034. Madison, Wis.

57. Pillow, Maxon Y. 1962. Effects of tension wood in hardwood lum-

ber and veneer. U.S. For. Prod. Lab. Rep. No. 1943. Madison, Wis.

58. Puget Sound Log Scaling and Grading Bureau Columbia River Log Scaling and Grading Bureau Grays Harbor Log Scaling and Grading Bureau Southern Oregon Log Scaling and Grading Bureau Northern California Log Scaling and Grading Bureau

1969. Official log scaling and grading rules. Portland, Oreg.

59. Resch, H., C. A. Lofdahl, F. J. Smith, and C. Erb 1970. Moisture leveling in veneer by microwaves

and hot air. For. Prod. J. 20(10) :50-58. 60. Scheffer, T. C.

1969. Protecting stored logs and pulpwood in North America. Sonderdruck aus: Mater, und Organismen 4 Heft 3, 167-199. Verlag: Dunc- ker and Humblot, Berl. 41.

61. Scheffer, T. C, and R. M. Lindgren 1940. Stains of sapwood and sapwood products

and their control. USDA Tech. Bull. No. 714. 62. Seibo, M. L.

1975. Adhesive bonding of wood. U.S. Dep. Agrie, Tech. Bull. No. 1512.

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PS 51-71. 64. U.S. Department of Commerce

Construction and industrial plywood. Prod. Stand. PS 1-74.

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Boxes, wood, wirebound. Fed. Specif. PPP-B- 585b.

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improved veneer-lathe setting accuracy. For. Prod. J. 25(7) :44-45. July.

68. Woodfin, Richard O., Jr. 1973. Wood losses in plywood production. For.

Prod. J. 23(9), Sept.

90

APPENDIX I—NOMENCLATURE OF WOOD SPECIES AND VENEER

Accurate identification is the key to efliicient utilization of individual wood species. Wood is made up of a vast number of species, each with its own properties, and known by a variety of common names. Only the precise name properly

identifies an individual species. Included here are the oflflcial common name of a species and the corresponding botanical name. In turn, these wood names are tied to the names for veneer.

NOMENCLATURE OF WOOD SPECIES AND VENEER

Commercial name of veneer

General

Official common tree name

Botanical name

Specific

Alder

American ash

Aspen

Basswood

Beech Birch

Box elder Buckeye

Butternut Cherry Cottonwood

Nepal alder Red alder Black ash Oregon ash Pumpkin ash White ash

Shamel ash Popple

Elm

Eucalyptus Gum Hackberry

Hickory

Holly Koa Locust

Madrone Magnolia

Rock elm

Soft elm

UNITED STATES HARDWOODS

Nepal alder Red alder Black ash Oregon ash Pumpkin ash Blue ash Green ash White ash Shamel ash Bigtooth aspen Quaking aspen American basswood White basswood American beech Yellow birch Sweet birch Paper birch Alaskan paper birch Gray birch River birch Box elder Ohio buckeye Yellow buckeye Butternut Black cherry Balsam poplar Black cottonwood Eastern cottonwood Swamp cottonwood Cedar elm Rock elm Winged elm American elm (gray elm) Slippery elm (red elm) Robusta eucalyptus Sweetgum Hackberry Sugarberry Mockernut hickory Pignut hickory Shagbark hickory Shellbark hickory American holly Koa Black locust Honeylocust Pacific madrone Cucumbertree Southern magnolia Sweetbay

Alnus nepalensis A. rubra Fraxinus nigra F. latifolia F. profunda F. quadrangulata F. pennsylvanica F. americana F. uhdei Populus grandidentata P. tremuloides Tilia americana T. heterophylla Fagus grandifolia Betula alleghaniensis B. lenta B. papyrifera B. papyrifera var. humilis B. populifolia B. nigra Acer negundo Aesculus glabra A. octandra Juglans cinérea Prunus serótina Populus balsamifera P. trichocarpa P. deltoides P. heterophylla Ulmus crassifolia U. thomasii U. alata U. americana U. rubra Eucalyptus robusta Liquidambar styraciflua Celtis occidentalis C. laevigata Carya tomentosa C. glabra C. ovata C. laciniosa Ilex opaca Acacia koa Robinia pseudoacacia Gleditsia triacanthos Arbutus menziesii Magnolia acuminata M. grandiflora M. virginiana

91

NOMENCLATURE OF WOOD SPECIES AND VENEER—continued

Commercial name of veneer Official common Botanical name _ trPP TinTTTIP

General Specific — Lice IldHlc

UNITED STATES HARDWOOD—continued

Maple Hard maple Black maple Acer nigrum Sugar maple A. saccharum

Oregon maple Bigleaf maple A. macrophyllum Soft maple Red maple A. ruhrum

Silver maple A. saccharinum Oak Red oak Black oak Quercus velutina

California black oak Q. kelloggii Cherrybark oak Q. fálcala var. pagodaefolia Laurel oak Q. laurifolia Northern red oak Q. rubra Nuttall oak Q. nuttallii Pin oak Q. palustris Scarlet oak Q. coccínea Shumard oak Q. shuntardii Southern red oak Q. fálcala Water oak Q. nigra Willow oak Q. phellos

White oak Bur oak Q. macrocarpa Chestnut oak Q. prinus Chinkapin oak Q. muehlenhergii Delta post oak Q. stellata var. mississippiensis Durand oak Q. durandii Live oak Q. virginiana Oregon white oak Q. garryana Overcup oak Q. lyrata Post oak Q. stellata Swamp chestnut oak Q. michauxii Swamp white oak Q. bicolor White oak Q. alba

Ohia Ohia Metrosideros polymorpha

Oregon myrtle California laurel Umbellularia californica

Pecan Bitternut hickory Carya cordiformis Nutmeg hickory C. myristicaeformis Water hickory C. aquatica Pecan C. illinoensis

Persimmon Common persimmon Diospyros virginiana

Poplar Yellow-poplar Liriodendron tulipifera Sassafras Sassafras Sassafras albidum Silk-oak Lacewood Grevillea robusta Sycamore American sycamore Platanus occidentalis Tanoak Tanoak Lithocarpus densiflorus Teak Teak Tectona grandis Tupelo Black túpelo Nyssa sylvatica

Swamp túpelo N. sylvatica var. biflora Water túpelo N. aquatica

Walnut Black walnut Juglans nigra Willow Black willow Salix nigra Yagrumo hembra Yagrumo hembra Cecropia peltata

UNITED STATES SOFTWOODS

Cedar Alaska cedar Alaska-cedar Chamaecyparis nootkatensis Incense cedar Incense-cedar Libocedrus decurrens Port Orford cedar Port-Orford-cedar Chamaecyparis lawsoniana Eastern red cedar Eastern redcedar Juniperus virginiana Western red cedar Western redcedar Thuja plicata Northern white cedar Northern white-cedar T. occidentalis Southern white cedar Atlantic white-cedar Chamaecyparis thyoides

Cypress Baldcypress Taxodium distichum Pond cypress T. distichum var. nutans

92


Commercial name of veneer Official common tree name

Botanical name

General

Fir

Hemlock

Juniper

Western larch Pine

Redwood

Spruce

Tamarack Pacific yew

Specific

UNITED STATES SOFTWOODS—continued

Balsam fir Balsam fir Douglas-fir Coast Douglas-fir

Interior west Douglas-fir Interior north Douglas-fir Interior south Douglas-fir

Noble fir Noble fir White fir Subalpine fir

California red fir Shasta red fir Grand fir Pacific silver fir White fir Eastern hemlock Mountain hemlock Western hemlock Alligator juniper Rocky Mountain juniper Western juniper Western larch Digger pine Jack pine Jeffrey pine Knobcone pine Limber pine Lodgepole pine Red pine Ponderosa pine Sugar pine Western white pine Eastern white pine White bark pine Loblolly pine Shortleaf pine Longleaf pine Slash pine Spruce pine Pond pine Virginia pine Pitch pine Sand pine Table-Mountain pine Big tree Redwood Black spruce Red spruce White spruce Blue spruce Engelmann spruce Sitka spruce Tamarack Pacific yew

Eastern hemlock Mountain hemlock West Coast hemlock Western juniper

Digger pine Jack pine Jeffrey pine Knobcone pine Limber pine Lodgepole pine Norway pine Ponderosa pine Sugar pine Idaho white pine Northern white pine White bark pine Southern pine

Eastern spruce

Engelmann spruce

Sitka spruce

Abies balsamea Pseudotsuga menziesii P. menziesii P. menziesii var. glauca P. menziesii var. glauca Abies 'procera A. lasiocarpa Abies magnifica A, magnifica var. shastensis A, grandis A. amabilis A. concolor T. canadensis T. mertensiana T. heterophylla Juniperus deppeana J. scopulorum J. occidentalis Larix occidentalis Pinus sabiniana P. banksiana P. jeffreyi P. attenuata P. flexilis P. contorta P. resinosa P. ponderosa P. lambertiana P. monticola P. strobus P. albicaulis Pinus taeda P. echinata P. palustris P. elliottii P. glabra P. serótina P. virginiana P. rigida P. clausa P. pungens Sequoia gigantea S. sempervirens Picea mariana P. rubens P. glauca P. pungens P. engelmannii P. sitchensis Larix laricina Taxus brevifolia

Alpine ash Angélique Apitong Avodire Brazil nut Bubinga Cativo

OTHER SPECIES IMPORTANT TO U.S. VENEER

Alpine ash Angélique Keruing Avodire Brazil nut Bubinga Cativo

Eucalyptus gigantea Dicorynia guianensis Dipterocarpus spp. Turracanthus africanus Bertholletia excelsa Guibourtia spp. Prioria copaifera

93


Commercial name of veneer

General Specific

Official common tree name

Botanical name

Ceiba Determa Kapur Keruing Klinki Lauan

Limba Mahogany

Mengkulang Meranti Mersawa Muritinga Okoume Paldao Primavera Rosewood Sapele Teak Caribbean pine Ocote pine

Dark red Light red Light red Light red

OTHER SPECIES—continued

Ceiba Determa Keladan Apitong Klinki pine Philippine mahogany Tangile Almon Bagtikan Mayapis Limba Honduras mahogany African mahogany Mengkulang Meranti Palosapis Muritinga Okoume Paldao Primavera Rosewood Sapele Teak Caribbean pine Ocote pine

Ceiba pentandra and samauma Ocotea rubra Dryobalanops spp. Dipterocarpus spp. Araucaria klinkii

Shorea polysperma S. almon Parashorea plicata S. squamata Terminalia superba Swietenia macrophylla Khaya spp. Tarrietia spp. Shorea spp. Anisoptera spp. Maquira spp. Aucoumea klaineana Dracontomelon spp. Cybistax donnell-smithii Dalbergia spp. Entandrophragma cylindricum Tectona grandis Pinus caribaea Pinus oocarpa

94

APPENDIX II- -PHYSICAL PROPERTIES OF U.S. WOODS FOR VENEER

The column on specific gravity of the wood gives a quick comparison between species. In general, the higher the specific gravity, the higher the strength properties such as hardness and stiffness and the greater the shrinkage.

The green moisture content is given to the closest 10 percent for both sapwood and heartwood. If the moisture content of the sapwood and heartwood is very different, it may pay to separate sapwood and heartwood veneer for drying. Very high moisture contents, such as over 100 percent, may indicate problems in cutting and drying veneer from this species.

Permeability is listed as P, permeable; M, moderately permeable; or jR, refractory.

Shrinkage is given under three subheads: Tangential, radial, and volumetric. Tangential shrinkage indicates the widthwise shrinkage of

rotary-cut and flat-sliced veneer, while radial shrinkage is an estimate of the widthwise shrinkage of quarter-sliced veneer. Since these figures are given from green to ovendry, they can be interpolated for other moisture conditions. In general, shrinkage is considered to be a straight-line relationship from a moisture content of 30 percent (green) to 0 percent.

The volumetric shrinkage, together with specific gravity, can be used to describe the wood on the basis of weight at any moisture content.

The columns describing arrangement and size of vessels in hardwood veneer contribute to an understanding of the figure of this veneer. Small pores are under 100 microns in diameter; medium pores 100 to 150 microns; and large pores over 150 microns.

The grain direction and color of the sapwood and heartwood are self-explanatory.

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110

APPENDIX III- -MECHAÎVICAL PROPERTIES OF U.S. FOR VENEER

WOODS

Seven mechanical properties—tension perpendicular to the grain, hardness, modulus of elasticity, modulus of rupture, compression parallel to the grain, compression perpendicular to the grain, and shear—are given in this Ap- pendix. The figures for tension perpendicular are taken from green material while the others are for wood at 12 percent moisture content. Tension perpendicular is important during cutting when the wood is green while the other

mechanical properties are most important for use of veneer in the dry conditions.

Most of the mechanical properties listed here came from the Wood Handbook. In some cases, the information is from universities or from foreign laboratories. For up-to-date Canadian and U.S. values, it is suggested the reader check American Standards for Testing Materials D 2555.

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER

Common name Tension perpen-

12 percent moisture content

dicular Hardness Modulus of Modulus Compres- Compres- Shear to (side) elasticity of sion sion parallel

grain rupture parallel perpen- to (green) to the

grain— maximum crushing strength

dicular to the

grain— fiber stress

at pro- portional

limit

grain— maximum shearing strength

Lh/in.^ Lb 1,000 Lh/in.^

Lh/in.^ L6A'w.2 Lh/in.^ Lh/in.^

UNITED STATES HARDWOODS Alder

Nepal — 510 1,020 8,500 — — Red 390 590 1,380 9,800 5,820 440 1,080

Ash Black 490 850 1,600 12,600 5,970 760 1,570 Blue — 1,290 1,400 13,790 6,980 1,420 2,030 Green 590 1,200 1,660 14,100 7,080 1,310 1,910 Oregon 590 1,160 1,360 12,700 6,040 1,250 1,790 Pumpkin 770 990 1,260 11,060 5,690 1,460 1,720 Shamel — 860 1,660 12,800 — White 590 1,320 1,770 15,400 7,410 1,160 1,950

Aspen Bigtooth 310 420 1,430 9,100 5,300 560 1,080 Quaking 230 350 1,180 8,400 4,250 370 850

Basswood American 280 410 1,460 8,700 4,730 370 990 White — — — —

Beech, American 720 1,300 1,720 14,900 7,300 1,010 2,010 Birch

Alaskan paper 200 840 1,900 13,800 7,510 830 1,420 Gray — 760 1,150 9,800 4,870 750 1,340 Paper 380 910 1,590 12,300 5,690 600 1,210 River — — .—. — Sweet 430 1,470 2,170 16,900 8,540 1,080 2,240 Yellow 430 1,260 2,010 16,600 8,170 970 1,880

Buckeye Ohio — — — — — Yellow — — 1,170 7,490 4,170 360 960

Butternut 430 490 1,180 8,100 5,110 460 1,170 Cherry, Black 570 950 1,490 12,300 7,110 690 1,700

111

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER—continued

Common name Tension 12 percent moisture content perpendicular Hardness Modulus of Modulus Compres- Compres- Shear

to (side) elasticity of sion sion parallel grain rupture parallel perpen- to

(green) to the dicular grain— grain— to the maximum

maximum grain— shearing crushing fiber strength strength stress

at pro- portional

limit

Lh/in:^ Lh 1,000 Lh/in,' Lh/in.' Lh/in.^ Lb/in.^

Cottonwood Balsam poplar

(Balm of Gilead) Black Eastern Swamp

Elm American Cedar Rock Slippery Winged

Eucalyptus Hackberry Hickory, pecan

Bitternut Nutmeg Pecan Water


Holly, American Honeylocust Koa Laurel, California Locust, Black Madrone, Pacific Magnolia

Cucumbertree Southern

Maple Bigleaf Black Boxelder Red Silver Sugar

Oak, red Black California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet

160

UNITED STATEIS HARDWOODS—continued

300 1,100 6,800 4,020 370 790

270 350 1,270 8,500 4,500 300 1,040 410 430 1,370 8,500 4,910 380 930

590 830 1,340 11,800 5,520 690 1,510 690 1,320 1,480 13,500 6,020 950 2,240 — 1,320 1,540 14,800 7,050 1,520 1,920 640 860 1,490 13,000 6,360 820 1,630 850 1,540 1,650 14,800 6,780 1,020 2,370 — 1,330 2,200 15,600 8,200 — — 630 880 1,190 11,000 5,440 890 1,590

__ 1,580 1,790 17,100 9,040 1,680 1,960 — 1,810 1,700 16,600 6,910 1,570 1,850 680 1,820 1,730 13,700 7,850 1,720 2,800 — — 2,020 17,800 8,600 1,550 —

1,970 2,220 19,200 8,940 1,730 1,740 — 2,140 2,260 20,100 9,190 1,980 2,150 — 1,880 2,160 20,200 9,210 1,760 2,430 — 1,890 18,100 8,000 1,800 2,110 680 1,020 1,110 10,260 5,540 920 1,710 930 1,580 1,630 14,700 7,500 1,840 2,250 — 850 1,570 13,300 7,300 — — 780 1,270 940 8,000 5,640 1,130 1,860 770 1,700 2,050 19,400 10,180 1,830 2,480 — — 1,230 10,450 6,880 1,310 1,810

440 700 1,820 12,300 6,310 570 1,340 610 1,020 1,400 11,200 5,460 860 1,530

600 850 1,450 10,700 5,950 750 1,730 720 1,180 1,620 13,300 6,680 1,020 1,820

950 1,640 13,400 6,540 1,000 1,850 560 700 1,140 8,900 5,220 740 1,480 — 1,450 1,830 15,800 7,830 1,470 2,330

1,210 1,640 13,900 6,520 930 1,910 700 1,100 990 8,700 5,640 1,160 1,470 800 1,480 2,280 18,100 8,740 1,250 2,000 690 1,130 1,590 13,300 6,830 840 1,490 770 1,210 1,690 12,600 6,980 1,060 1,830 750 1,290 1,820 14,300 6,760 1,010 1,780

800 1,510 1,730 14,000 6,820 1,020 2,080 700 1,400 1,910 17,400 8,330 1,120 1,890

112


Common name Tension perpendicular

12 percent moisture content

Hardness Modulus of Modulus Compres- Compres- Shear to (side) elasticity of sion sion parallel

gram rupture parallel perpen- to (green) to the dicular grain—

gram— to the maximum maximum gram— shearing crushing fiber strength strength stress

at pro- portional

limit

Lh/in,' Lh 1,000 Lh/in,'^

Lh/in.' Lh/in} Lh/in,^ Lh/in.''

UNITED STATES HRDWOODS—continued

Oak (cont.) Shumard Southern red Water Willow

Oak, white Bur Chinkapin Delta post Durand Live Oregon white Overcup Post Swamp chestnut Swamp white White

Ohia Persimmon, common Sassafras Silk-oak Sugarberry Sweetgum Sweetbay Sycamore, American Tanoak Teak Tupelo

Blackgum Swamp Water

Walnut, Black Willow, Black Yagrumo hembra Yellow-poplar

Cedar Alaska- Atlantic white- Eastern redcedar Incense- Northern white- Port-Orford- Western redcedar

Cypress Baldcypress Pondcypress

480 1,060 1,490 10,900 6,090 870 1,390 820 1,190 2,020 15,400 6,770 1,020 2,020 760 1,460 1,900 14,500 7,040 1,130 1,650

800 1,370 1,030 10,300 6,060 1,200 1,820 730 1,190 1,420 12,600 — — —

1,040 2,680 1,970 18,400 8,900 2,840 2,660 940 1,660 1,100 10,320 6,530 1,710 2,020 730 1,190 1,420 12,600 6,200 810 2,000 790 1,360 1,510 13,200 6,600 1,430 1,840 670 1,240 1,770 13,900 7,270 1,110 1,990 860 1,620 2,050 17,700 8,600 1,190 2,000 770 1,360 1,780 15,200 7,440 1,070 2,000 950 2,090 2,370 18,300 8,900 1,400 2,360

1,200 2,300 2,010 17,660 9,170 1,990 2,160 520 630

930 960

1,120 9,030 4,760 850 1,240

— 1,140 9,900 5,620 1,000 1,280 540 850 1,640 12,500 6,320 620 1,600 — — 1,640 10,920 5,680 560 1,680 630 770 1,420 10,000 5,380 700 1,470

960 1,130 1,820 13,900 7,900 1,410 1,320

570 810 1,200 9,600 5,520 930 1,340

600 880 1,260 9,600 5,920 870 1,590 570 1,010 1,680 14,600 7,580 1,010 1,370 430 450 1,010 7,830 4,100 430 1,250 — 320 1,090 6,490 3,490 270 — 510 540 1,580 10,100 5,540 500 1,190


330 580 1,420 11,100 6,310 620 1,130 180 350 930 6,800 4,700 410 800 330 900 880 8,800 6,020 920 — 280 470 1,040 8,000 5,200 590 880 240 320 800 6,500 3,960 310 850 180 560 1,730 11,300 6,470 620 1,080 230 350 1,120 7,700 5,020 490 860

300 510 1,440 10,600 6,360 730 1,000

113


Common name Tension 12 percent moisture content perpendicular Hardness Modulus of Modulus Compres- Compres- Shear

to (side) elasticity of sion sion parallel grain rupture parallel perpen- to

(green) to the dicular gram— gram- to the maximum

maximum grain— shearing crushing fiber strength strength stress

at pro- portional

limit

Lh/in.'^ L6 1,000 Lh/in.^ Lh/in.^ Ld/in.^

Lh/in.^ Lh/in.^


Douglas-fir Coast Interior north Interior south Interior west

Fir Balsam California red Grand Noble Pacific silver Shasta red Subalpine White

Hemlock Eastern Mountain Western

Juniper Alligator Rocky Mountain Western

Larch, Western Pine

Digger Eastern white Jack Jeffrey Knobcone Limber Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Sand Shortleaf Slash Spruce Sugar Table-Mountain Virginia Western white Whitebark

Redwood Big tree

300 340 250 290

180 380 240 230 240

300

230 330 290

330

710 600 510 660

1,950 1,790 1,490 1,820

12,400 13,100 11,900 12,600

7,240 6,900 6,220 7,440

800 770 740 760

1,130 1,400 1,510 1,290

400 500 490 410 430

1,230 1,490 1,570 1,720 1,720

7,600 10,400 8,800

10,700 10,600

4,530 5,470 5,290 6,100 6,530

300 610 500 520 450

710 1,050

910 1,050 1,180

400 480

900 1,490

7,100 9,800

4,330 5,810

490 530

1,020 1,100

500 740 540

1,200 1,320 1,640

8,900 11,200 11,300

5,410 6,840 7,110

650 1,030

550

1,060 1,230 1,250

,160 650 720

6,700 8,310

4,120 5,340

1,380 890

1,042 1,065

830 1,870 13,100 7,640 930 1,360

250 380 1,240 8,600 4,800 440 900 360 570 1,350 9,900 5,660 580 1,170 260 500 1,240 9,300 5,530 790 1,210

270 430 1,170 9,100 5,290 580 800 260 690 1,800 12,800 7,080 800 1,370 220 480 1,340 9,400 5,370 610 880 330 870 1,990 14,700 8,440 960 1,500 280 620 1,430 10,800 5,940 1,010 1,360 280 740 1,750 11,600 7,540 1,120 1,380 310 460 1,290 9,400 5,320 580 1,130 300 560 1,630 11,000 6,070 600 1,210 380 730 1,410 11,600 6,920 1,030 1,100 320 690 1,760 12,800 7,070 810 1,310 400 1,010 2,060 15,900 9,100 1,020 1,730 — 660 1,230 10,400 5,650 730 1,490 270 380 1,200 8,000 4,770 480 1,050 320 660 1,550 11,600 6,830 980 1,200 400 740 1,520 13,000 6,710 910 1,350 260 370 1,510 9,500 5,620 440 850

260 480 1,340 10,000 6,150 700 940

114


Common name Tension 12 percent moisture content

dicular Hardness Modulus of Modulus Compres- Compres- Shear to (side) elasticity of sion sion parallel

gram rupture parallel perpen- to (green) to the dicular gram—

gram— to the maximum maximum grain- shearing crushing fiber strength strength stress

at pro- portional

limit

L6/tn.î Lb 1,000 Lh/in.^ Lh/in.^


Lh/in.' Lb/in.' Lb/in.^

Spruce Black Blue Engelmann Red Sitka White

Tamarack Yew, Pacific

100 520 1,530 10,300 5,320 530 1,030

240 390 1,300 9,300 4,480 410 1,200 220 490 1,520 10,200 5,890 470 1,080 250 510 1,570 10,200 5,610 580 1,150 220 480 1,340 9,800 5,470 460 1,080 260 590 1,640 11,600 7,160 800 1,280 450 1,600 1,350 15,200 8,100 2,110 2,230

115

APPENDIX IV—SOME PROCESSING VARIABLES OF U.S. FOR VENEER

WOODS

Ease of bark removal is based on fall-cut wood debarked by machine.

The conditioning temperatures are those suggested for cutting veneer about h inch thick. The recommended temperatures for rotary cutting take into account the tendency of the species to develop splits at the ends of the bolts during heating. For slicing, the recommended temperature will often be 10° to 20° F higher than for peeling because splitting is less of a problem when heating flitches for slicing.

The last columns are rated on an A, B, and C scale. A indicates that the speciñc property is basically favorable for use as veneer and C indicates that the particular property may be a problem in utilizing the species for veneer. For example, an A rating for log splitting due to heating indicates the species is little affected by heating while a C rating indicates that log end splits are a major problem with this species.

The A, B, and C ratings for drying times are comparative. The time required to dry veneer varies widely with species and with the type of dryer being used. For this reason, rather than give specific times for a specific dryer, drying times are given in comparison with other species—yellow birch for hardwood veneer and Douglas-fir for softwood veneer.

Yellow birch was selected as "typical" for hardwood veneer because this is a well-known veneer species and one on which FPL had much drying data. Besides, the sapwood and heartwood of yellow birch take about the same time to dry. Our data show that no other hardwoods dry much faster than yellow birch. In contrast, several hardwood species require considerably longer drying time than yellow birch. So drying time ratings for hardwoods are either B or C

For softwoods, the comparison is based on the drying of sapwood or heartwood of Douglas- fir. The sapwood of Douglas-fir takes significantly longer drying time than the heartwood.

The quality and recovery of veneer from all species is sensitive to the setting of the knife and pressure bar. However, acceptable veneer can be cut from some species with a wider range of settings than can be tolerated by other species. An A rating for sensitivity to settings of the knife and pressure bar indicates the species tolerates a wide latitude in machine setting; a C rating indicates the species cuts well only within a narrow range of machine settings.

Under defects in drying, an A rating means a species is relatively free of the characteristics listed, while a C rating means the veneer from the species is subject to this particular drying defect.

116

SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER

Common name Ease of

bark removal -

by machine 2

Suggested conditioning temperature

Rotary Sliced

Aggra- vation of log split- - ting

due to heating

Sensitivity to setting

of—

Knife Pres- sure bar

Drying time

Sap- wood

Defects in drying

Heart- Buckle Splits wood

Col- lapse


Nepal 1 100-140 140-160 A A A B B A A A Red 2 80-140 120-160 B A A B B A A A

Ash Black 2 120-140 140-160 B B B B B B A A Blue 2 140-160 160-180 — — — — — — — — Green 2 140-160 160-180 — — — — — — — — Oregon 2 140-160 160-180 — — — — — — — — Pumpkin 2 140-160 160-180 — — — — — — — — Shamel 2 140-160 170-180 B A B B B B B A White 2 140-160 160-180 B B B B B B B A

Aspen Bigtooth 1 40-70 40-70 A B A C C B A B Quaking 1 40-70 40-70 A B A C C B A B

Basswood American 3 40-70 40-70 A C B B B A A A White 3 40-70 40-70 A C B C C A A A

Beech, American 1 160-180 180-190 B B B B B B A A-B

Birch Alaskan

paper 2 140-160 160-180 B A A B B B B A Gray 2 120-140 140-160 — — — — — — — — Paper 2 120-140 140-160 B B B B B A B A River 2 120-140 140-160 B B B — — A A A Sweet 2 140-160 160-180 B B B B B B A A-B Yellow 2 140-160 160-180 B B B B B B A A-B

Buckeye Ohio 1 40-70 40-70 A — — — — — — — Yellow 1 40-70 40-70 A — — — — — — —

Butternut 2 70-90 100-200 A C C B B C B A Cherry, Black 2 120-140 150-170 B B B B B B A A Cottonwood

Balsam poplar 2 40-70 40-70 A B B C C C B C Black 2 40-70 40-70 A B B C C C B C Eastern 2 40-70 40-70 A B B C C C B C Swamp 2 40-70 40-70 A B B C C C B C

Elm American 2 120-140 150-170 B B B C C C B A Cedar 2 160-170 190-200 B B B C C — — — Rock 2 160-170 190-200 B B B C C c B A Slippery 2 120-140 180 then

150 190-200

B B B C C c B A

Winged 2 160-170 B B B C C — — — Eucalyptus 2 140-160 180-200 C B B C C B B B Hackberry 1 120-140 140-160 A A A B B A A A Hickory, pecan

Bitternut 3 160-180 190-200 C B B B C B B A Nutmeg 3 160-180 190-200 C B B B C B B A Pecan 3 160-180 170-180 C B B B C C B A Water 3 160-180 190-200 C C B C C B B A

Hickory, true Mockernut 3 160-180 190-200 C B B B C B B A Pignut 3 160-180 190-200 C B B B C C B A Shagbark 3 160-180 190-200 C B B B C B B A Shellbark 3 160-180 190-200 C B B B C B B A

117

SOME PROc: ESSING VARIABI .ES OF U.S. w- OODS . FOR VENEÎ ÎR 1—continued

Common name Ease Suggested Aggra- Sensitivity Drying time Defects in drying of conditioning vation to setting

bark temperature of log of- - Sap- Heart- Buckle Splits Col- removal - - split- - wood wood lapse

by Rotary Sliced ting Knife Pres- machine ^ due to sure

heating bar

op op

UNITED STATES HARDWOODS —continued

Holly, American 2 150-160 170-180 Honeylocust 3 140-160 180-190 B B B B B A B A Koa — 140-160 160-180 B A B B B B B A Jüaiirei,

California 150-160 190-200 B B B C C C B A Locust, Black 3 160-180 180-190 B B B — — B B A Madrone, Pacific 3 150-160 180-190 B B B C C B B A Magnolia

Cucumbertree 1 70-120 120-140 A A A — — A A A Southern 1 70-120 120-140 A A A — — A A A Sweetbay 1 70-120 120-140 A A A — — A A A

Maple Bigleaf 2 80-120 120-140 B A A B B B B A Black 2 160-180 170-190 B B B B B B B B Boxelder 2 80-120 120-140 — — — — — — — — Red 2 100-140 130-150 B A A C C A A A Silver 2 80-120 120-140 B A A C C B B A Sugar 2 160-190 170-190 A-B C C B B A-B B A-B

Oak, red Black 2 140-160 180-200 C B B C C A B A California

black 2 140-160 160-180 C B B C C B B A Cherrybark 2 140-160 180-200 C B B C C A B B Chestnut 2 140-160 180-200 C B B C C A B A Laurel 2 140-160 180-200 C B C C C B C C Northern red 2 140-160 180-200 C B B C C B B B Nuttall 2 140-160 180-200 C B B — — — — — Pin 2 140-160 180-200 C B B — — — — — Scarlet 2 140-160 180-200 C B B c c A B — Shumard 2 140-160 180-200 C B B — — — — — Southern red 2 140-160 180-200 C B B c c A B B Water 2 140-160 180-200 C B B c c A C C Willow 2 140-160 180-200 C B B c c A C C

Oak, white Bur 2 140-160 180-200 C B B c c — — — Chinkapin 2 140-160 180-200 C B B — — — — — Delta post 2 140-160 180-200 C B B — — — — — Durand 2 140-160 180-200 C B B c c A B — Live 2 160-170 200-210 C B B — c — C — Oregon white 2 140-160 180-200 C — — — — — — —

Overcup 2 140-160 180-200 C B B c c B C C Post 2 140-160 180-200 C B B c c — — — Swamp

chestnut 2 140-160 180-200 C B B — — — — — Swamp white 2 140-160 180-200 C B B c c A B B White 2 140-160 180-200 C B B c c A B B

Ohia 2 170-180 200-210 B B C B B B B A Persimmon,

common — 150-200 190-200 C C C B B B B B Sassafras 2 100-120 120-150 — — — — — — — — Silk-oak 2 150-160 170-180 B A A c c A A A Sugarberry 1 120-140 140-160 — — — — — — — — Sweetgum 1 120-140 140-160 A A B c c A B B Sycamore,

American 1 120-140 150-160 B A A c c C-B B B

118

SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER i—continued

Common name Ease Suggested of

bark removal —

by Rotary machine ^

conditioning temperature

Sliced

Aggra- vation of log split- - ting

due to heating


of—


Drying time

Sap- wood

Heart- wood

Defects in drying

Buckle Splits Col-

UNITED STATES HARDWOODS—continued

Tanoak 1 150-160 180-190 C B B C C B C C Teak 2 190-200 200-210 B A B C C A A A Tupelo

Blackgum 1 120-140 150-160 A A A C C B A B Swamp 1 120-140 150-160 A A A C C B A B Water 1 120-140 150-160 A A A C C B A B

Walnut, Black 2 180 then 150

180 then 150

B B B B B B A A

Willow, Black 3 40-70 40-70 B B B C C B B A Yagrumo hembra 2 50-80 70-80 A B A B — B B B Yellow-poplar 1 70-120 120-140 A A A B B A A A


Cedar Alaska- 3 120-140 140-160 B A B B B A A A Atlantic

white- 2 60-100 100-130 A A B B B A A A Eastern

redcedar 2 140-160 160-180 B C B B A B B A Incense- 3 70-120 70-120 A B B — C A A — Northern

white- 2 120-140 140-160 B C C — C A B B Port-Orford- 3 120-160 140-160 B A B B B A A A Western

redcedar 3 140-160 160-180 B C C B C A B B

Cypress Baldcypress 3 60-120 120-140 A B C C C A B A Pondcypress 3 60-120 120-140 A B C C C A B A

Douglas-fir Coast 1 60-140 140-180 A B B B B A B A Interior

north 1 60-140 140-180 A B B B B A B A Interior

south 1 60-140 140-180 A B B B B A B A Interior

west 1 60-140 140-180 A B B B B A B A

Fir Balsam 1 70-130 120-150 B B B B C B B A California red 1 70-150 130-160 B B B-C B C B B A Grand 1 70-150 130-160 B B B-C B C B B A Noble 1 70-150 130-160 B B B-C B B-C B B A Pacific silver 1 70-150 130-160 B B B B B-C B B A Shasta red 1 70-150 130-160 B B B-C B C B B A Subalpine 1 70-130 120-150 B B B B C B B A White 1 70-150 130-160 B B B-C C C B B A

Hemlock Eastern 2 120-160 160-180 B B C B C B B A Mountain 2 120-160 160-180 B B C B C B B A Western 2 120-160 160-180 B B C B C B B A

Juniper Alligator 3 140-160 160-180 B C B B A B C A Rocky

Mountain 3 140-160 160-180 B C B B A B C A Western 3 140-160 160-180 B C B B A B C A

Larch, Western 3 140-150 160-180 B B B B C A B A

119

SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER i—continued

Common name Ease of

bark removal -

by machine ^

Suggested conditioning temperature

Rotary Sliced

Aggra- vation of log splitting

due to heating


of—


Drying time

Sap- wood

Heart- wood

Defects in drying

Buckle Splits Col- lapse


Pine Digger ] L 60-140 140-180 A B B B B B B A Eastern white ] L 70-120 120-140 A B B B B B B A Jack ] L 70-120 120-140 A B B — Jeffrey ] L 60-140 140-180 A A A B B A B A Knobcone 1 L 60-140 140-180 A B B B — B B A Limber ] L 60-120 120-140 A C B B C B B A Loblolly ] L 120-160 160-180 A B B B B B-C B-C A Lodgepole 1 L 60-140 140-180 A A A B C B B A Longleaf 1 L 120-160 160-180 A B B B B B B A Pitch ] L 120-160 160-180 A B B B B B B A Pond ] L 120-160 160-180 A B C B B B B A Ponderosa ] L 60-140 140-180 A A A B B A B A Red 1 L 70-120 120-140 A B B B B B B A Sand ] L 120-160 140-180 A B B B B B B A Shortleaf ] L 120-160 160-180 A B B B B B B A Slash ] L 120-160 160-180 A B B B B B B A Spruce ] L 120-140 140-160 A B B B B B B A Sugar ] L 60-120 120-140 A B B B C A B A Table-

Mountain ] L 120-160 160-180 A B B B B B B A Virginia L 120-160 160-180 A B B B B B B A Western white 1 L 60-120 120-140 A B B B C A B A Whitebark ] L 60-120 120-140 A C B B B B B A

Redwood Í 5 70-160 160-180 B B C C C A C A Big tree Í I 70-160 160-180 B B C C C A C A

Spruce Black ] L 70-120 120-140 A C B B B B B-C A Blue ] L 70-120 120-140 A C B B B B B A Engelmann ] L 70-120 120-140 A C B B B B B A Red ] L 70-120 120-140 A C B B B B B-C A Sitka ] L 70-120 120-140 A C B B B B B A White : L 70-120 120-140 A C B B B B B-C A

Tamarack í I 140-160 150-160 B B B B C B B A Yew, Pacific 160-180 180-200 — B B — B C B A

1 A, species property very suitable for veneer; B, intermediate; and C, less desirable for veneer. 21, species relatively easy to debark; 2, intermediate to debark; and 3, difficult to debark.

120

APPENDIX V- -EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS

An A rating would indicate that the wood is résistent to development of a particular characteristic even under a wide range of processing conditions. A C rating indicates that the wood is highly susceptible to this particular characteristic and should indicate caution in processing to keep this specific characteristic to a minimum.

Most information in Appendix V is again based on the A, B, and C scale, and expresses relative ratings. Information in the columns head '^Relative freedom from veneer characteristics originating in log storage and processing'' involves a highly variable set of data. All these characteristics are at least to a degree under the control of the processor.

EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS '

Common name Relative freedom from veneer characteristics originating in log storage and in processing

Sap Mold Iron Oxida- Bacteria Surface I irregularities stains stain tive

stain Odor Extreme permeability

Fuzzy Shell- ing

Rough

UNITED STATES HARDWOODS Alder

Nepal B B B C B B B A A Red A B B C A A B A A

Ash Black B B B A A A A A B Blue B B B — — — A A B Green B B B — — — A A B Oregon B B B — — — A A B Pumpkin B B B — — — A A B Shamel B B A A A A A A A White B B B C A A A A B

Aspen Bigtooth B C A B C — C A B Quaking B C A B C — C A B

Basswood American B B A C A A C A A White B B A C A A C A A

Beech, American A B B B A A A A B Birch

Alaskan paper B B A B — — B A B Gray A B B C — — — — — Paper A B B C B A A B B River A B B C — — A A B Sweet A B B B A A A A B Yellow A B B B A A A B A

Buckeye Ohio — — — C — — — — — Yellow — — — C — — — — —

Butternut A B B B A A C A A Cherry, Black A A C B A A A A A Cottonwood

Balsam poplar B C A B C — C A B Black B C A B C B C A B Eastern B C A B C B C A B Swamp B C A B C — C A B

Elm American A A A B B A B B B Cedar B A A B B A — — — Rock A A A B B A B B B Slippery A A A B B A B B B Winged B A A B B A — — —

121

EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS i—con.


Sap Mold Iron Oxida- Bacteria Surface irregularities stains stain tive

stain Odor Extreme permeability

Fuzzy Shell- ing

Rough

UNITED STATES HARDWOODS—continued Eucalyptus B B C C A A A A B Hackberry C C B C A A B B B Hickory, pecan

Bitternut B B B A A A A A C Nutmeg B B B A A A A A C Pecan B A B B A A A A C Water B B B A A A A A C

Hickory, true Mockernut B B B A A A A A C Pignut B A B B A A A A C Shagbark B B B A A A A A C Shellbark B B B A A A A A C

Holly, American C — — — — — A A A Honeylocust A B B A A A A A B Koa A A B B — — A A B Laurel, California B — B C A A A A B Locust, Black A A C B A A A A B Madrone, Pacific A B B B A A A A A Magnolia

Cucumbertree B C A C C B A A A Southern B C A C C B A A A

Maple Bigleaf A B B C A A A A B Black A B B C A A A A B Boxelder A B B C — — — — — Red A B B C A A A A B Silver A B B C A A A A B Sugar C B B C A A B A B

Oak, red Black A A C C A A A A B-C California black A A C C A A A A B-C Cherrybark A A C C A A A A B-C Chestnut A A C C A A A A B-C Laurel A A C C A A A A B-C Northern red A A C C A A A A B-C Nuttall A A C C A A A A B-C Pin A A C C A A A A A-C Scarlet A A C C A A A A B-C Shumard A A C C A A A A B-C Southern red A A C C A A A A B-C Water A A C C A A A A B-C Willow A A C C A A A A B-C

Oak, white Bur A A C C A A A A B-C Chinkapin A A C C A A A A B-C Delta post A A C C A A A A B-C Durand A A C c A A A A B-C Live A A C c A A A A B-C Oregon white A A C c A A A A B-C Overcup A A C c A A A A B-C Post A A C c A A A A B-C Swamp chestnut A A C c A A A A B-C Swamp white A A C c A A A A B-C White A A C c A A A A B-C

Ohia A A B B A A A A B Persimmon, Common A A A C A A A A B Sassafras B B C — — — — — —

122



Sap stains

Mold Iron stain

Oxida- tive stain

Bacteria

Odor Extreme permeability

Surface irregularities

Fuzzy Shell- ing

Rough


Silk-oak A Sugarberry C Sweetgum C Sweetbay B Sycamore, American B Tanoak A Teak A Tupelo

Black B Swamp B Water B

Walnut, Black A Willow, Black C Yagrumo hembra C Yellow-poplar C

A C C C B A A

B B B B C B C

B C B A A C B

A A A C B B A

A C B C A C A

C C C B C B B

B C A A A

A B B C

A B A A A

A B B B

A A A A A

A A A B C B B

B A A A A

A A A A A A A

A A B C B

A B A A B A A


Cedar Alaska- A A B B A A A A A Atlantic white- C A B B A A B A B Eastern redcedar A A B A A A A A B Incense- A A C B A A B B B Northern white- A A B __ A A B C B Port-Orford- A A B B A A A A A Western redcedar A A C — A A B C B

Cypress Baldcypress B B B B B B B C B Pondcypress B B B B B B B C B

Douglas-fir Coast A A B A A A A B B Interior north A A B A A A A B B Interior south A A B A A A A B B Interior west A A B A A A A B B

Fir Balsam A A A A B B B B B California red A A A A B B B B B Grand A A A A B B B B B Noble A A A B B B B B B Pacific silver A A A A B B B B B Shasta red A A A A B B B B B Subalpine A A A A B B B B B White A A A A B B B B B

Hemlock Eastern B B B B B B B C B Mountain B B B B B B B C B Western B B B B B B B C B

Juniper Alligator A A B A A A A A B Rocky Mountain A A B A A A A A B Western A A B A A A A A B

Larch, Western A A B A A A A B B

123



Sap Mold Iron Oxida- Bacteria Surface irregularities stains íi^*^"i^^ 4-'ï'«'r^x Stain Live

stain Odor Extreme Fuzzy Shell- Rough perme- ing ability

UNITED STATES SOFTWOODS —continued Pine

Digger C B A B B C A B B Eastern white B B A B B B B B B Jack B B A B B B Jeffrey C B A C B C A A B Knobcone B B A B B B B B B Limber B B A B B B C B B Loblolly C C A A B C A B B Lodgepole B B A B B B B A A Longleaf C C A A B C A B B Pitch C C A A B C A B B Pond C C A A B C A B B Ponderosa C B A C B C A A B Red B B A A B B B B B Sand C C A A B C A B B Shortleaf C C A A B C A B B Slash C C A A B C A B B Spruce C C A A B C A B B Sugar B B A C B C B B B Table-Mountain C C A A B C A B B Virginia C C A A B C A B B Western white B B A C B C B B B Whitebark B-C B A B B B C B B

Redwood A A C B A A B C B Big tree A A C B A A B C B

Spruce Black B B A A A A C B B Blue B B A A A A C B B Engelmann B B A A A A C B B Red B B A A A A C B B Sitka B B A A A A C B B White B B A A A A C B B

Tamarack A A B A A A B B B Yew, Pacific — — — — A A A A B

1 A, good—species resists development of undesirable characteristics under a wide range of operating conditions; B, species intermediate in resistance and C, poor—species susceptible to this undesirable development.

124

APPENDIX VI—APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER

The last five columns of the Appendix VI table in a sense summarize all the data. An A rating indicates the species is well suited for the indicated product. A B rating indicates the species is moderately well suited for this prod-

uct, and a C rating indicates the species is generally not suited for the particular end product. In making these classifications, the following broad criteria were considered :

End Use

Construction plywood

Decorative face veneer

Inner plies for decorative panels

Container veneer and plywood

Typical Specific Uses

Building construction as subfloor, wall sheathing, roof sheathing, concrete forms, and overlaid panels.

Prefinished decorative wall panels, furniture, flush doors, kitchen cabinets, and case goods

Inner plies for prefinished wall panels, furniture, flush doors, kitchen cabinets, and case goods

Wirebound boxes, bushel baskets, paper-overlaid veneer, cleated panel boxes, and plywood-sheathed crates

Desirable Veneer Qualities

High stiffness and strength, moderate weight, and readily glued

Attractive figure and color, moderately hard, and readily glued

Low weight, low shrinkage, straight grain, fine uniform grain, and easily glued

High in stiffness, shock resistance, and resistance to splitting, light color, free from odor and taste, and moderate in weight

In some instances additional end uses and comments are listed under "other."

125

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER

Common name Clear ve-

neer 1

Figure of veneer Relative suitability for—2

Rotary- and flat-sliced Quarter- and rift-sliced Con- struction ply-

wood

Alder Nepal

Red Ash

Black

Blue Green Oregon Pumpkin Shamel

White

Aspen Bigtooth

Quaking Basswood

American White

Beech, American

Birch Alaskan paper

Gray

Paper River Sweet Yellow

Buckeye Ohio

Yellow Butternut


Faint growth ring. Large rays slightly darker than background do

Scattered large flakes from wood rays

Occasional large flakes C

B B B B A

A-B

B

A A B

Conspicuous growth ring, occasional burls and cross figure

.do.

.do.

.do.

.do. Pronounced parabolas

from the wide growth rings. Occa- sional pin knots

Conspicuous growth ring, occasional burls and cross figure

B Faint growth ring

.do

Distinct not conspicuous growth ring, occasional burl

do do do do Distinct stripe from

growth rings. Faint crossbar

Distinct not conspicuous growth ring, occasional burl

Occasional cross figure, silky luster

do

Faint growth ring do Faint g owth ring

C Faint growth ring pattern. Slow grown. Many knots and burls

C Distinct not conspicuous growth ring, occasionally wavy

B do — do A do A do

— Faint growth ring, close grain

— do C Faint to moderate

growth ring, very lustrous

Plain, fine texture do Numerous small flakes

up to 1/8 inch in height

Too small to quarter- slice

Generally plain. Occa- sionally wavy

do do do do

Plain

B

B B C C B

C C B

B B B B

do C Plain; the figure is due C

to color and luster

Decor- ative face

veneer

B

A

A A A B A

C C B

A-B

A-B B A A

C A

Inner plies

of decorative panels

A-B

B

B

B B B C B

A A C

B B B B

A C

Con- tainer veneer

and ply-

wood

Other

B

A

A A A B A

A Underlay- ment plywood

A ....do....

A A A Plywood

flooring

B B B B

B C

126

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued


neer ^

Figure of veneer Relative suitability for-

Rotary- and flat-sliced Quarter- and rift-sliced Con- Decor- Inner Con- struc- ative plies tainer tion face of veneer ply veneer decor- and

wood ative ply panels wood

Other

Cherry, Black

Cottonwood Balsam poplar Black Eastern Swamp

Elm American

Cedar Rock

Slippery

Winged

Eucalyptus

Hackberry

Hickory, pecan Bitternut

Nutmeg Pecan Water


Holly, American

Honeylocust

Koa

Laurel, California

B B B B

B

B B

B

B

UNITED STATES HARDWOODS—continued Faint growth ring, oc- Light colored small B

casional burl, pin ray flecks, satiny knots, and gum luster spots common

Faint growth ring do do do

Plain .do. .do. .do.

Distinct growth ring with fine wavy pattern within each ring do Conspicuous growth

ring with fine wavy pattern within each ring do

Distinct growth ring with fine wavy pattern within each ring

Faint growth patterns. Occasional crossbar. Many pin knots

Conspicuous growth ring

C Distinct not conspicuous growth ring, almost always straight grain

C do C do C do

C do C do C do C do C Very close grain, al-

most no visible pattern

A Conspicuous growth ring

Irregular grain, dark streaks

Faint growth ring, occasional burl or blisters

C C c c

B B

B

B

B

B

B

Faint growth ring stripe

do Faint growth ring

stripe

Distinct growth ring stripe


Ribbon grain. Occa- sional crossbar. Many pin knots

Distinct not conspicuous growth stripe, fine sparkle from small rays

Faint growth rings, fine rays, occasional dark stripes

do B do B do B

do B do B do B do B Very plain uniform C

texture

Distinct not conspic- C uous growth ring, occasional mild cross figure

Curly, wavy grain, B-C fiddle-back dark streaks

Mixture of plain and C highly figured due to mottle, stumps, and burls

B C C B

A A

A

A

A-B

A-B

A A A

A A A A A

B

B B B B

B

C C

B

C

C

C

C C C

C C C C C

B

A A A A

A A

A

A

B

A

B

B B B

B B B B C

B-C

127



neer 1


Rotary- and flat-sliced Quarter- and rift-sliced Con- Decor- Inner Con- Other struc- ative plies tainer tion face of veneer ply veneer decor- and

wood ative panels

ply wood


Locust, Black C Distinct growth ring, dark streaks associated with borer holes

Distinct not conspicuous growth ring

C B C B

Madrone, Pacific B Faint growth ring, close grain, figure due to pigment changes in heartwood

Bland figure is limited to color changes in the heartwood

C A C B

Magnolia Cucumbertree A Faint growth ring Plain B C A A Southern A do do B C A A

Maple Bigleaf B Faint growth ring, oc-

casional burls, blister, curly, and quilted

Most plain, occasion- nally curly and wavy

C A B A

Black A Faint growth ring, occasionally curly, wavy, birdseye

Most plain, occasionally curly and wavy, small dark rays

B A B A

Boxelder C Faint growth ring, close grain like the maples

Plain B B C B

Red B Faint growth ring, occasionally curly or wavy, often with pith flecks

Most plain, occasionally curly and wavy, small dark rays

B B A A

Silver B do do C B A A Sugar A Faint growth ring, oc-

casionally curly, fiddle-back, birdseye, wavy

do B A B A

Oak, red Black C Conspicuous growth

ring, rotary-cut veneer has a watery figure with great contrast

Pronounced flake on the true quarter and a narrow flake when rift cut; distinct not conspicuous growth ring stripe

B A B B

California black C do do B A B B Cherrybark B do do B A B B Chestnut B do do B A B B Laurel C do do B B c B Northern red B do do B A B B Nuttall B do do B A B B Pin C do do B A C B Scarlet B do do B A B B Shumard B do do B A B B Southern red B do do B A B B Water C do do B B c B Willow C do do B B C B

Oak, white Bur B do do B B B B Chinkapin B do do B B C B Delta post B do do B A B B Durand B do do B A B B

128



neer 1

Figure of veneer Relative suitability for—^


wood ative panels

ply wood

UNITED STATES HARDWOODS—continued Oak, white (cont.)

Live C Moderate growth ring Pronounced ray flakes C B C B Oregon white C Conspicuous growth

ring, rotary-cut veneer has a watery figure with great contrast

Pronounced flake on the true quarter and a narrow flake when rift cut; distinct not conspicuous growth ring stripe

c B C B

Overcup B do do B B c B Post C do do B B c B Swamp chestnut B do do B A B B Swamp white B do do B A B B White B do do B A B B

Ohia B Faint growth ring pattern. Occasional burls

Poorly defined ribbon grain

B B C B Face for plywood flooring

Persimmon, common C Distinct not conspic-

uous growth ring Occasional ribbon due

to interlocked grain C A-B C B Laminated

golf club heads

Sassafras — Pronounced growth ring

Distinct not conspicuous growth ring

C B C B

Silk-oak A Faint growth ring pattern

Moderate-sized ray flakes lead to the name "lacewood"

B A B B

Sugarberry Conspicuous growth ring

Distinct not conspicuous growth stripe, fine sparkle from small rays

B B C A

Sweetbay A Faint growth ring Plain B C A A Sweetgum A Faint growth ring, oc-

casionally irregular darker streaks

Distinct not pronounced ribbon occasionally irregular darker streaks

B B B A

Sycamore, American B Faint growth ring Pronounced reddish

flakes up to 1/4 inch in height

B A B A

Tanoak B Plain, occasional burls Inconspicuous wood rays and occasional burls

B B C B

Teak A Moderate growth Faint growth stripe. B A B B rings, dark irregular streaks, occasional burls

dark irregular streaks, sometimes mottled, fiddle- back or curly grain

Tupelo Black

Swamp Water

Walnut, Black

A A B

Faint growth ring Distinct not pronounced ribbon, low luster

do do do do Distinct not conspic- Inconspicuous growth

uous growth ring, stripe, occasional occasional wavy and burl, crotch, curly cross figure

B B

B B B A B B B A B A B B

129



neer 1



wood ative panels

ply wood


Willow, Black B Faint growth ring Plain, fine texture C B-C B B Yagrumo hembra A Plain, moderate-sized

vessels Plain C C B-C B Toy air-

planes Yellow-poplar A Faint growth ring Plain B B A A


Cedar Alaska- B Faint growth ring None B B A A Small boat

Atlantic white C Distinct, not conspicuous growth ring

None C B A A parts

Eastern redcedar B-C Distinct growth ring, many knots, streaks of white sapwood alternating with purple-red to dark red heartwood

Faint growth rings. Spike knots included sapwood

C A B C Cedar chests

Incense- C Faint growth ring Faint growth ring stripe

B-C B B B

Northern white C do do B-C B B B Port-Orford- A do do B B A A Western

redcedar B Distinct, not conspicuous growth ring

do A-B A B-C B Decorative knotty faces and etched veneer

Cypress Baldcypress B Conspicuous irregular

growth ring Distinct, not conspic-

uous growth ring stripe

A-B A B A

Pondcypress B do do B A B A Douglas-fir

Coast A-B Conspicuous growth ring

Distinct, not conspicuous growth ring stripe

A B-C B A-B

Interior north B do do A B-C B A-B interior south B do do B B-C B A-B Interior west B do do A B-C B A-B

Fir Balsam C Distinct, not conspic-

uous growth ring Faint growth ring

stripe B-C C C A

California red B-C Conspicuous growth ring


A-B C B-C A

Grand C do do A-B C B-C A Noble B do do A-B C B-C A Pacific silver C do Faint growth ring

stripe A-B C B-C A

Shasta red B-C do Distinct, not conspicuous growth ring stripe

A-B C B-C A

Subalpine C Conspicuous growth ring

do B-C C C A

130



neer 1

Figure of veneer Relative suitability for— _2


wood ative panels

ply wood


White C do do A-B C B-C A Hemlock

Eastern C Distinct, not conspicuous growth ring


B-C C B-C A-B

Mountain c do do B c B A Western B do do A-B c B A

Juniper Alligator C Distinct growth ring,

many knots, mixed white sapwood and light red-brown heartwood

Too small to quarter- slice

C c C C

Rocky Mountain C do do C c C C

Western C do do C c C C Larch, Western B Conspicuous growth

ring Distinct, not conspic-


A B C B

Pine Digger C Distinct, not conspic-

uous growth ring Faint growth ring

stripe B-C C C B

Eastern white B Faint growth ring None B-C A-B B A Decorative knotty faces

Jack C Distinct, not conspicuous growth ring


B-C C C B

Jeffrey B do do B A B A Knobcone C do do B-C C C A Limber C Faint growth ring None B-C C C A Loblolly B Conspicuous growth



A C C B

Lodgepole C Distinct, not conspicuous growth ring; faint "pocked" appearance


B B C A Decorative knotty faces

Longleaf B Conspicuous growth ring


A C C B

Pitch C do do B-C C C B Pond B do do B C C B Ponderosa B Distinct, not conspic-

uous growth ring Distinct, not conspic-


B A B A

Red B do Faint growth ring stripe

B B C A

Sand B Conspicuous growth ring


B-C C C B

Shortleaf B do do A C C B Slash B do do A C C B Spruce B do do B-C C C B Sugar A Faint growth ring None B-C A B A Table-Mountain C Conspicuous growth



B-C C C B

131



neer 1



wood ative panels

ply wood


Virginia C do do B-C C C B Western white A Faint growth ring None B A B A Whitebark C do do B-C C C A

Redwood A Distinct, not conspicuous growth ring; occasionally wavy and burl

Faint growth ring stripe; occasionally wavy and burl

A-B A C A Decorative etched veneer faces

Big tree A Distinct, not conspicuous growth ring


B A C A

Spruce Black C Faint growth ring None B-C C C A Blue C do do B-C C C A Engelmann C do do B C C A Red C do do B C C A Sitka B do do A-B B B A Aircraft

parts White C do do B-C C C A

Tamarack C Conspicuous growth ring


A-B B C B

Yew, Pacific C Mild growth ring figure

Not quarter-sliced C A C B

1 An A rating indicates veneer logs of the species tend to have a high percent of clear wood, a C rating indicates a low percent of clear wood, and a B is intermediate.

2 A, indicates species is well suited for end product; B, intermediate; and C, generally not well suited for this product.

132

GLOSSARY

Annual growth ring,—The layer of wood growth put on a tree during a single growing season. In the temperate zone the annual growth rings of many species (e.g., oaks and pines) are readily distinguished because of differences in the cells formed during the early and late parts of the season. In some temperate zone species (black gum and sweetgum) and many tropical species, annual growth rings are not easily recognized. Bird peck.—A small hole or patch of distorted grain resulting from birds pecking through the growing cells in the tree. In shape, bird peck usually resembles a carpet tack with the point towards the bark; bird peck is usually accompanied by discoloration extending for considerable distance along the grain and to a much lesser extent across the grain. Birdseye.—Small localized areas in wood with the fibers indented and otherwise contorted to form few to many circular or elliptical figures remotely resembling birds' eyes on the tangential surface. Sometimes found in sugar maple and used for decorative purposes; rare in other hardwood species. ßolt,— (l) A short section of a tree trunk; (2) in veneer production, a short log of a length suitable for peeling in a lathe. Burl,— (1) A hard, woody outgrowth on a tree, more or less rounded in form, usually resulting from the entwined growth of a cluster of adventitious buds. Such burls are the source of the highly figured burl veneers used for purely ornamental purposes. (2) In lumber or veneer, a localized severe distortion of the grain generally rounded in outline, usually resulting from over- growth of dead branch stubs, varying from 1/2 inch to several inches in diameter; frequently includes one or more clusters of several small contiguous conical proturberances, each usually having a core or pith but no appreciable amount of end grain (in tangential view) surrounding it. Cellulose,—ThQ carbohydrate that is the principal constituent of wood and forms the framework of the wood cells. Closed sicZe.—Side of veneer not touching knife as it is peeled from log (also called tight side of veneer). Com6i^ram.—Veneer cut at about a 45° angle to the wood rays. The rays show as narrow, straight stripes on the face of the veneer. White oak is commonly sliced to produce combgrain face veneer. Compression wood,—Wood formed on the lower side of branches and inclined trunks of softwood trees. Com- pression wood is identified by its relatively wide annual rings, usually eccentric, relatively large amount of summerwood, sometimes more than 50 percent of the width of the annual rings in which it occurs, and its lack of demarcation between springwood and summerwood in the same annual rings. Compression wood shrinks excessively lengthwise, as compared with normal wood. Crossband,—To place the grain of layers of wood at right angles in order to minimize shrinking and swelling; also, in plywood of three or more plies, a layer of veneer whose grain direction is at right angles to that of the face plies. Crossfire,—Figure in fancy face veneer caused by the grain of the wood dipping in and out of the face of the veneer sheet. Crotch veneer,—Veneer cut from fork of tree to provide pleasing grain, figure, and contrast.

Density,—As usually applied to wood of normal cellular form, density is the mass of wood substance en- closed with the boundary surfaces of a wood-plus-voids complex having unit volume. It is variously expressed as pounds per cubic foot, kilograms per cubic meter, or grams per cubic centimeter at a specified moisture content. Diffuse-porous wood.—Certain hardwoods in which the pores tend to be uniform in size and distribution throughout each annual ring or to decrease in size slightly and gradually toward the outer border of the ring. Dubbing.—The extra heavy cut that may occur at the ends of a lathe or slicer knife when it is ground. This rounds the ends of the knife and is undesirable. Taking up slack in the parts of the grinding machine or use of short dummy knife sections at the ends of the knife during grinding will reduce or eliminate dubbing. Earlywood.—The portion of the annual growth ring that is formed during the early part of the growing season. It is usually less dense and weaker mechanically than latewood. Equilibrium moisture content.—The moisture content at which wood neither gains nor loses moisture when surrounded by air at a given relative humidity and temperature. Extractive,—Substances in wood, not an integral part of the cellular structure, that can be removed by solution in hot or cold water, ether, benzene, or other solvents that do not react chemically with wood components. Fiber saturation point,—The stage in the drying or wetting of wood at which the cell walls are saturated and the cell cavities are free from water. It applies to an individual cell or group of cells, not to whole boards. It is usually taken as approximately 30 percent moisture content, based on ovendry weight. Figured veneer,—General term for decorative veneer such as from crotches, burls, and stumps. Flitch,—A portion of a log sawn on two or more faces —commonly on opposite faces, leaving two waney edges. When intended for resawing into lumber, it is resawn parallel to its original wide faces. Or, it may be sliced or sawn into veneer, in which case the resulting sheets of veneer laid together in the sequence of cutting are called a flitch. The term is loosely used. Gum.—A comprehensive term for nonvolatile viscous plant exudates, which either dissolve or swell up in contact with water. Many substances referred to as gums, such as pine and spruce gum, are actually oleo- resins. Hardwoods.—Generally one of the botanical groups of trees that have broad leaves in contrast to the conifers or softwoods. The term has no reference to the actual hardness of the wood. Heartwood.—The wood extending from the pith to the sapwood, the cells of which no longer participate in the life processes of the tree. Heartwood may contain phenolic compounds, gums, resins, and other materials that usually make it darker and more decay resistant than sapwood. Latewood.—The portion of the annual growth ring that is formed after the earlywood formation has ceased. It is usually denser and stronger mechanically than earlywood.

133

Lignin,—The second most abundant constituent of wood, located principally in the secondary wall and the middle lamella, which is the thin cementing layer between wood cells. Chemically it is an irregular polymer of substituted propylphenol groups, and thus no simple chemical formula can be written for it. Mineral streak,—An olive to greenish-black or brown discoloration of undetermined cause in hardwoods. Moisture content,—The amount of water contained in the wood, usually expressed as a percentage of the weight of the ovendry wood. Mold.—A fungus growth on wood products at or near the surface and, therefore, not typically resulting in deep discoloration. Mold discolorations are usually ash green to deep green, although black is common. Oleoresin.—A solution of resin in an essential oil that occurs in or exudes from many plants, especially softwoods. The oleoresin from pine is a solution of pine resin (rosin) in turpentine. Parenchyma,—Short cells having simple pits and func- tioning primarily in the metabolism and storage of plant food materials. They remain alive longer than the tracheids, fibers, and vessel segments, sometimes for many years. Two kinds of parenchyma cells are recognized—those in vertical strands, known more specifically as axial parenchyma, and those in horizontal series in the rays, known as ray parenchyma. Peel,—To convert a log into veneer by rotary cutting. Pitch streaks.—A well-defined accumulation of pitch in a more or less regular streak in the wood of certain conifers. Plywood.—A composite panel or board made up of crossbanded layers of veneer only, or veneer in combination with a core of lumber or of particleboard bonded with an adhesive. Generally the grain of one or more plies is roughly at right angles to the other plies. Pressure bar

Fixed.—A bar on a lathe or slicer set to compress the wood just ahead of the knife edge.

Roller,—Used on some lathes in place of a fixed pressure bar and performs the same function. Quarter-slicing,—A method of cutting face veneer nearly parallel to the wood rays. If the rays are large, as in oak, then they are prominent in the face veneer. Quarter-slicing also shows interlocked grain to advantage in species like mahogany. Reaction wood.—Wood with more or less distinctive anatomical characters, formed typically in parts of leaning or crooked stems and in branches. In hardwoods this consists of tension wood and in softwoods of compression wood. Resin,—Inflammable, water-soluble, vegetable substances secreted by certain plants or trees, and char-

acterizing the wood of many coniferous species. The term is also applied to synthetic organic products related to the natural resins. Resin ducts,—Intercellular passages that contain and transmit resinous materials. On a cut surface, they are usually inconspicuous. They may extend vertically parallel to the axis of the tree or at right angles to the axis and parallel to the rays. Short-grain,—Term used for cross grain as when end grain is exposed on face of veneer. Showthrough,—Term used when effects of defects within a panel can be seen on the face. Sliced veneer,—(See Veneer,) Softwoods,—Generally, one of the botanical groups of trees that in most cases have needlelike or scalelike leaves, the conifers; also the wood produced by such trees. The term has no reference to the actual hardness of the wood. Specific gravity,—As applied to wood, the ratio of the ovendry weight of a sample to the weight of a volume of water equal to the volume of the sample at a specified moisture content (green, air-dry, or ovendry). Stain.—A discoloration in wood that may be caused by such diverse agencies as micro-organisms, metal, or chemicals. The term also applies to materials used to impart color to wood. Straight-grained wood,—Wood in which the fibers run parallel to the axis of the piece. Tension wood,—A form of wood found in leaning trees of some hardwood species and characterized by the presence of gelatinous fibers and excessive longitudinal shrinkage. Tension wood fibers hold together tenaci- ously, so that sawed surfaces usually have projecting fibers, and planed surfaces often are torn or have raised grain. Tension wood may cause warping. Texture,—A term often used interchangeably with grain. Sometimes used to combine the concepts of density and degree of contrast between springwood and summer wood. Veneer,—A thin layer or sheet of wood.

Rotary-cut veneer,—Veneer cut in a lathe which rotates a log or bolt, chucked in the center, against a knife.

Sawed veneer,—Veneer produced by sawing. Sliced veneer,—Veneer that is sliced off a log, bolt,

or flitch with a knife. Veneer checks,—When wood is cut into veneer with a knife, checks often form on the side of the veneer next to the knife. In general, checks tend to be deeper in thick veneer of dense wood than in thin veneer of low- density wood. Also called knife checks, lathe checks, and slicer checks. Veneer clipper,—Machine for cutting veneers into desired sizes.

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INDEX

Abnormal wood, 15 Adventitious buds, 17, 24 Appearance, 125

Back grinding, 57 Back-roll lathe, 49 Bacterial action, 29,121 Bark pockets, 24 Bark removal, 30,117 Bird peck, 19, 24 Bolts for veneer, 31, 51, 68 Botanical names, 91 Box shook, 1,125 Bucking into bolts, 31 Buckle, 3, 83,117 Burls, 17, 24 Bushel baskets, 5,125

Case goods, 5,125 Checks in veneer, 11 Chucks, 49, 58 Cleated panel boxes, 5, 125 Clipping veneer, 69 Close grain, 23 Color, 10,17, 24, 95 Common names, 91, 95 Compression parallel, 23, 111 Compression perpendicular, 23, 111 Compression wood, 15, 24 Concrete form, 4,125 Conditioning wood, 34, 117 Construction plywood, 4,125 Container plywood, 22,125 Conveying veneer, 69 Core, 5,125 Cracks, quality control, 81 Crossband, 5, 125 Cutting :

back cut, 34, 49 direction, 32 equipment, 45 flat-slicing, 32, 49 half-round, 32, 49 quarter-sliced, 34, 49 rift-cut, 32, 49 rotary, 32, 45 sawn, 34 slicing, 45 speed, 53 stay-log, 49

Cylindrical form, 24

Debarking, 30,117 Decay, 24 Decorative plywood, 3, 125

Core, 4, 125 Crossband, 4, 125

Defects in drying, 117 Diameter effect, 40 Dimensional stability, 11, 23 Dryer :

emissions, 74 fires, 86 types, 72

Drying:

techniques, 29, 74 temperatures, 74 time, 38, 74,117 veneer, 70

Eccentricity, 14, 24 Electric heating, 44 Embedded metal, 20, 24 End uses, 4,125 Epicormic branches, 17, 23 Extractives, 9, 23 Extraneous cell content, 9

Faces, 4 Felling splits, 20 Figure, 11,17, 23, 34,129 Fine texture, 23 Fire scars, 19 Flat-slicing, 32, 49 Flitches for veneer, 32, 68 Flush doors, 5, 125 Function of log grades, 13 Furniture parts, 2, 5

Generalized settings, 66 Grain effects, 8,17, 40, 95 Grinding :

veneer knife, 56 back grinding, 57

Growth rate, 7, 24 Growth stresses, 15 Gum, 9, 23 Gum streaks and pockets, 19

Half-round cutting, 32, 49 Handling damage, 24 Hard deposits, 11, 23 Hardness, 23, 111 Hardwoods, 2 Heat distortion, 51 Heating :

benefits, 39 bolts and flitches, 31, 44 color changes, 37 decay resistance, 38 dimensional changes, 37 disadvantages, 39 drying time, 38 effects, 34 growth stresses, 37 hardness, 36 hot water, 40, 42 plasticity, 34 rate, 41 shrinkage, 38 steam, 40, 42, 44 strength, 37 time required, 40, 41 torque, 38 variability, 40 warp, 38

Hot water heating, 42

Ideal veneer log, 12 Individual species, 91, 95, 111, 116, 121, 125

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Industrial plywood, 4 Inner plies : 4

case goods, 4 flush doors, 4 furniture, 5 wall panels, 4

Irregular grain, 17, 23

Kitchen cabinets, 4 Knife:

angle, 48, 59 back grinding, 57 bevel, 48, 55 generalized settings, 66 grinding, 56 honing, 57 secondary bevels, 57 selection, 54 setting, 58,117 slicer, 60 terminology, 48 wear, 55 thickness, 55 type, 54

Knots, 16, 24

Lathe : advantages, 47, 49, 69 back-roll, 49 cutting action, 45 dynamic equilibrium, 53 operation, 45 stay-log, 49

Log: breakdown, 31 characteristics, 24 diameter eccentricity, 14 end splits, 15, 24 grades, 13 handling damage, 20 processing, 31 requirements, 13 splits, 29, 41 storage, 29, 121

Mechanical properties, 12, 23, 111 Metal stain, 11 Mineral streak, 24 Modulus of elasticity, 23, 111 Modulus of rupture, 23, 111 Moisture content, 3, 6, 23, 34, 41, 73, 84, 95, 111 Mold, 121 Movement, undesirable:

wood, 49, 51 machine parts, 49, 51

Names, 91, 95

Odor, 11, 23, 29 Oleoresin, 10 Overlaid panels, 5

Paper-overlaid veneer, 5 Parenchyma, 8, 23 Peeling techniques, 29 Permeability, 7, 23, 95 Physical properties of wood, 3, 23, 95 Pitch pockets, 24 Plywood :

block flooring, 4, 125

construction, 4, 22, 125 industrial, 5, 22, 125

Plywood-sheathed crates, 5, 125 Polyphenols, 10 Prefinished panels, 5 Properties of veneer logs, 11 Pressure bar:

flxed, 60, 65 generalized setting, 66 lead for lathe, 61 lead for slicer, 61 roller, 60, 65 setting, 61, 117 setting gap, 63 terminology, 48

Processing variables, 116, 121

Quality control: buckle, 83 casehardening, 86 checks or cracks, 81 collapse, 86 color, 86 honeycomb, 86 shrinkage, 86 stain, 75 veneer roughness, 79 veneer thickness, 75

Quarter-sliced, 34, 49

Requirements for veneer logs, 13 Resin, 10, 23 Resistance to splitting, 24 Retractable chucks, 50 Rift-cut, 32, 49 Ring shake, 16, 24 Roof sheathing, 4 Rotary cutting, 32, 47, 49, 87

Sawing into bolts, 31 Scars, 24 Seams, 19 Shake, 16 Shear, 23, 111 Shelling, 6, 8, 121 Shrinkage, 7, 23, 95 Slicer:

advantages, 47, 49 dynamic equilibrium, 53 heat distortion, 53 offset, vertical face, 52 mechanism, 45 parts movement, 52 pawl & rächet, 52 stop plate, 52 wood movement, 52 yields, 87

Slicing techniques, 29, 45, 49 Species :

appearance, 125 bark removal, 30 classiñcation for plywood, 22 density ranges, 22, 95 individual, 30, 91 log storage, 121 nomenclature, 91 processing variables, 116, 121 properties, 23 specific gravity, 25

136

suitability, 125 United States, 21, 25, 91, 95, 111, 116, 125

Species nomenclature, 91 Specific gravity, 3, 23, 25, 41, 42, 95 Specific uses, 125 Spindles, lathe, 50 Spinout, 38 Splits, 24, 117 Spur configuration, 50 Stains, 19, 24, 29, 121 Stay-log, 49 Steam heating, 42 Storage of logs, 29 Straight grain, 17, 23 Stresses, growth, 15 Stump pull, 20, 24 Subfloor, 4 Suitability for use, 125 Surface roughness, 121 Sweep, 24

Taper, 24 Temperature :

constant, 41 final, 34, 39, 40 gradient, 40 storage, 29 total change, 40

Tension perpendicular. 111 Tension wood, 15, 24 Terminology, 48, 67 Texture, 8 Thickness, 2, 76 Timber requirement, 88 Torque, 38 Tree names, 91

Undesirable movement, 49, 51 Uniformity of thickness, 2, 76

Veneer : appearance, 125 buckle, 3, 83 characteristics, 121 checks, 11 color, 17, 24, 95

conveying and clipping, 69 cutting, 1, 4, 45 decorative face, 4, 125 dryers, 74 drying, 70 figure, 11, 17, 23, 34, 129 flitches, 32, 68 gluability, 4 hardwoods, 22, 91, 121 lathe, 49, 69 properties, 70, 95, 111 quality, 2, 4, 75 roughness, 2, 121 slicer, 49, 69 softwoods, 22, 91, 121 species, 91 stiffness, 23, 111 strength, 23, 111 thickness,, 2, 76 uses, 4, 125 volume, 87

Veneer logs: characteristics, 13, 30 diameters, 13 form, 14 grades, 13 length, 13 properties, 13 sweep, 14 taper, 14

Veneer plant requirements, 88 Veneer yields, 87 Volume for plant, 87

Wall panels, 4, 125 Wall sheathing, 4, 125 Wax, 11, 23 Wirebound boxes, 5, 125 Wood:

conditioning, 34, 117 movement in cutting, 49 permeability, 7, 23, 34, 95 physical properties, 3, 95 species, 4, 22, 91 suitability for veneer, 125 temperature, 34

"l^ us GOVERNMENT PRINTING OFFICE: 1978 O-24S-770

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