Sunnen Honing Techniques - Sunnen | Sunnen

Sunnen® Honing Techniquesdata file: #104obtaining specified finishesby honing

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SUNNEN HONING TECHNIQUES

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Figure 1 - Finely ground surface. Looks good.Highly reflective surface finish.

PHOTO STORY OF HOW HONING IMPROVESSURFACE FINISH IN HARDENED STEEL

Figure 2 - The same surface, lightly oxidized andlightly honed, reveals grinding spirals.

Figure 3 - The same surface, rough honed. Thegrinding spirals have been removed, but faintlengthwise grinder chattermarks are exposed.These marks were caused by vibrations in thegrinding machine itslef or transmitted throughits foundation.

Figure 4 - The same surface, finish honed tospecifications..


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GENERALSince surface finish measurements becamerecognized and standardized in mechanicalengineering specifica-tions, more and moreblueprints specify surface finishes based on actualmeasurement. Likewise, inspection departments arenow equipped to check surface finishes by actualmeasurement.

There are several makes of surface analyzers in use.In general terms, such an instrument is a recordingoscillograph. The measurement is made by moving

the point of a pick-up stylus back and forth over aportion of the surface being measured (see Figure5).The stylus is connected to a recording mechanismthat produces a "microinch chart" (see Figure6).A microinch chart is a highly magnified surfaceprofile obtained by instruments and recorded on amoving, calibrated chart, with the peaks and valleysabove and below the centerline representingdeviations calibrated in millionths of an inch -"microinches." The less the height of the peaks or thedepth of the grooves, the finer the surface finish.

One method of assigning average microinch valuesto such a chart is to include any occasional extra highpeak or extra low valley into the normal deviation.This is done in establishing RMS (Root MeanSquare) values (see Figure7).The other and more recent method in computing anaverage value is to consider an occasional deeperscratch or extra-wide deviation as a flaw, and to useonly the normal ups and downs for computing theaverage value (see Figure8). This is the "AA"method (Arithmetical Average Height) in general usetoday by the American Standard Assn., ASA.

The microinch values arrived at when measuringhoned surfaces differ very little between the twomethods, and either is used in specifying honedfinishes. For instance, a 20-microinch "AA" valuewould figure about 22 microinch "RMS", adifference of about 10%.

The symbol 50 specifies a finish with roughness of

50 microinches AA or better. The symbol 50.

specifies a maximum roughness of 50 AA and a

minimum of 15.

Figure 6, Analyzer

HONING TECHNQUESOBTAINING SPECIFIED FINISHES BY HONING

Figure 5, Stylus

CLOSE-UP OF SURFACEANALYZER PICK-UP STYLUS.

SURFACE ANALYZER BEINGUSED TO CHECK FINISH INA BORE.

Figure 7, Stylus & Pin

Mechanical equivalent of stylus and ink pen. The longer the dimension 'Y'with respect to “X”, the greater the magnifications of the surface profile.

Figure 8, Typical Graph

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PLANNING TO MEET SURFACE SPECIFICATIONSToday, in planning a machining procedure for workwhere surface finish affects the function of the part,we have not only tolerances for size, roundness, andstraightness, but also tolerance on a "fourthdimension" that must be taken into consideration -the surface finish in microinches.

As a general rule, the specified finish is the top limitof allowable roughness, but there are applicationswhere the minimum as well as the maximum finishmust be met. An example is the finish of cylinderwalls in reciprocating engines and pumps whereneither too fine nor too coarse a finish is desired. Insome cases specific "roughness" is necessary to seatnew piston rings pro-perly, and too fine a surfacefinish is not desirable. A lubricant film often will notadhere to a highly polished surface. A crosshatchsurface, such as honing generates, has thousands ofmicroscopic pockets which are ideal for supporting auniform lubrication film.

HONING (THE SIMPLE WAY TO CONSISTENT RESULTS)Honing, long accepted as a dependable method formeeting close tolerances on roundness, straightness,size, and for duplicating piece to piece in cylindricalwork, is also the most dependable method formeeting this "fourth dimensional" tolerance - aspecified surface finish. Honing, as a rule, is asecondary operation, following primary machiningby either a cutting tool or a grinding operation.

Obtaining a specified finish by honing depends onjust a few factors, listed below in order of theirimportance (see Figure 9).1. GRIT SIZE of the honing stone has more to dowith determining the final finish than any othersingle factor. A very fine surface finish cannot begenerated with a coarse grit stone.

A fine finish calls for a fine grit stone because thegrit size determines the size of abrasive's cuttingteeth. One of the advantages of honing is that thesurface finish can be "predicted" and held to closelimits by selection of the correct honing stone.

2. NATURE OF WORK MATERIAL: The materialscomparative hardness, density and toughness arealso determining factors. The degree of penetrationmade by the abrasive grit determines the "depth" ofthe scratches.

3. TYPE OF GRIT: All three of the abrasive types incommon use (alu-minum oxide, silicon carbide, anddiamond) are of crystalline structure, each having itsown cleavage line. After being crushed and screened,each type grain differs from the other in generalshape. One type will crush into rugged burr shapeswhile another takes the shape of splinters.

The abrasive types also have distinct differences asto degree of hardness and brittleness, which have abearing on how long the abrasive will hold a cuttingpoint. These differences affect the surface finish.

4. TYPE OF BOND: The bulk of all honing is donewith vitrified bond stones. The reason for this is thatvitrified bond is friable (crumbly) to give the stoneself-dressing qualities so that new sharp cuttingpoints are presented to the work at the proper time.

Occasionally, when extremely fine finishes arerequired in soft materi-als, the bonding materialsused in the abrasive may cause minute scratches inthe surface being honed. For these applications acork bond stone is available. The bonding material inthis stone is pulverized cork, held together with aplastic-like binder. This "stone" will pro-vide anextremely high finish but will not correct geometricinaccuracies.

5. STRENGTH OF BOND or hardness of stone isnot extremely critical but can affect the finish insome cases. When the stone is so hard that it will"load" or "glaze" rather than breakdown, the"loaded" stone stops cutting but may cause scratchesin the surface. The correct stone will remain sharpand cut uniformly. A still softer stone will cut moredeeply but wear faster and, if too soft, will crumbleaway. Just a little "dulling" of the abrasive grits(bond slightly on the hard side) may improve surfacefinish under some conditions. In most cases a widerange of hardnesses will produce a satisfactoryfinish; but to achieve high production rates at lowcost, the correct stone hardness is all-important.6. HONING OIL: Many coolants do just what thename implies -- cool the tools and the work. Honing oilmust do much more, and Sunnen Honing Oil is specif-ically blended for use with Sunnen Honing Tools. Inthe case of fine surface finishes, the Honing Oil must

Figure 10, Stacking (Loose)

Fig. 12 - Two forces are required for cutting. A honing stone is like a file inmany respects. A downward force and a crosswise force are both necessaryto remove stock. The size of the grit is usually important, just as is the sizeof the file teeth. Coarse file teeth can be used for fast stock removal, but italways takes small teeth (or fine grit) to obtain a fine finish.

MOTION

PR

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lubricate the metal guide shoes and prevent galling. Itmust keep the stone clean to avoid loading. It mustwash away chips and loose abrasive grits to preventscratching. The use of the proper honing Oil is essentialto do all these jobs. Many surface finish problems arecaused by the use of improper oil or coolants.In seeking fine surface finishes the honing oil musthave a 'body'; that is, a viscosity that is just right tolimit grit point penetration under light hon-ingpressure - but not of a degree to interfere with stockremoval when sufficient pressure is used.

7. STONE CUTTING PRESSURE: The honingpressure used also has some effect on the surfacefinish produced. A given grit size, under high pres-sure, will bite more deeply into the work beinghoned, resulting in some-what rougher finish than ifa light or medium pressure is used.

The design of the honing pressure system in theSunnen machine provides for a "feathering out" ofcutting pressure as final size is reached. This occursas the indicator on the honing dial approaches "0".This "feathering out" feature is a great help inproducing fine finishes.

8. THE OPERATOR: There are areas, too, in whichthe operator can affect the final result throughvarious tech-niques. These will be discussed later.

SELECTING HONING STONESIn theory, a certain grain size should give the samefinish in all materials, considering the factorspreviously discussed.

In compiling specifications as to proper stones forobtaining certain finishes in various materials, thesecharacteristics of the work material must be takeninto account:

1. TYPE OF WORK MATERIAL (Steel, Bronze,Aluminum, etc.)

2. WORK MATERIALS DEGREE OF HARDNESS(Resistance to penetration by pressure)

In the table of finishes on the next page, thesecharacteristics have been taken into consideration.

When a stone of specified grit size and type isrecommended to obtain a certain measured finish ina given material, these figures have been obtainedfrom repeated experience with a free-cutting honingstone. The surface finish values are consideredtypical for the general classes of materials listed.

Three different types of abrasive grain are in generaluse - aluminum oxide, silicon carbide, and diamond.In many materials, all three abrasives will produceapproxi-mately the same finish; and other factorssuch as speed of stock removal and cost determinewhich abrasive should be used. An example would

be the honing of carbon steel of Rockwell C 45hardness. Diamond, aluminum oxide, or siliconcarbide could be made to cut this ma-terial and equalgrain sizes would give something like equal finish.However, in practice, aluminum oxide has beenfound to be the most economical for this particularmaterial.

Aluminum oxide crystals are "chunky"' in shape andtherefore resist impact and breakage where the holeis rough and has buffs, as is typical for steel.Aluminum oxide honing stones are therefore alwaysrecommended for deburring.

Silicon Carbide abrasive, in the finer grit sizes, isspecified for fine finishing because it is more brittleand the high points on the stone's surface will breakoff e with the surface. No occasional points willprotrude produce deeper random scratches- Incoarser grit sizes, silicon carbide's "needle -like",splintery shape is best for stock removal fromaluminum, brass, bronze, cast iron, and othermaterials (except hard steel) because even with thelow forces needed for cutting soft materials, thecrystals fracture more easily than aluminum oxideand therefore constantly present new cutting edges.

Diamond abrasive is economical only when either ofthe other two available grains will not cut the workmaterial, and in materials that are hard but of lowshear strength -cemented carbides, glass, andceramics. Diamond stones are costly but have verylong life, are fast cutting, and economical in workingthe materials mentioned.

As indicated in the approximate finish table - a 4-microinch finish can be generated in hard steel, a 7 finish in soft steel, a 6 finish in cast iron -- allwith a 500 grit stone. However, this same size gritwill generate only a 14 microinch finish in aluminumor soft bearing bronze because of deep grit point pen-etration into these soft materials. All the stones list-ed in the finish table are of vitrified bond, glass-likeand rigid. The bond will flake but not "give". Beingrigidly held, the individual abrasive grains cannotsink back into the bonding mass under honing pressure.

PROCEDURES INFLUENCING SURFACE FINISHApparent or visual finish can be changed by speed ofro-tation in relationship to speed of reciprocation,but actual measured finish will change very little. Asa rule, speed of rotation and rate of stroke arebalanced to leave as steep a crosshatch as ispractical. This steep cross-hatching allows a cuttingaction across the hills and valleys left by the oppositestroke, allows cutting with less pressure and,therefore, less heat generation. The averagecrosshatching in practice is something like 45 to 60degrees.


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As discussed earlier in connection with the Table ofSurface Finishes, when a stone is specified forobtain-ing a certain surface finish in a givenmaterial, this selection is based on repeatedexperience with that particular stone. It is assumedthat the stone will be used with full cutting pressure.However, finishes can be improved beyond thevalues obtained with a free-cutting honing stone, by"feathering out" the honing pressure as size isreached.

As an example, say we have a bore in a mild steelalloy for which we specify a 220 grit stone to obtaina 40-microinch finish. We expect the stone toproduce this finish under full cutting pressure, at itsmaximum stock removal rate. When this 220 gritstone is used with lighter cutting pressure forfinishing only; a finer finish (as low as 25microinches) can be obtained.

If the specifications call for 18 or better finish in thissame material, then a much finer grit stone must beused. We would specify 400 grit to obtain this finerfinish, and we would probably get around 12 AAunder production honing pressures. Again, theoperator could "feather out" his cutting pressure assize is reached and possibly get 10 microinches.

When still finer finishes are speci-fied - and if thereis also a considerable amount of stock to be removedor the starting finish is very rough - a third honingoperation will be necessary, using a 500 grit stone,which would produce a finish of about 5microinches. Each successive honing operation usesa pro-gressively finer grit stone and must go down tobase metal -- below the depth of cut of the previousoperation.

As a rule, a given grit size (honing pressureremaining constant) produces a finer finish in aharder material. As an example, the same stone justmentioned as producing a 12 AA or better finish inmild steel, would consistently produce 8 AA or betterin a harder material. The depth of grain penetration isless in the hard material.

Occasionally, parts made of soft material likealuminum or bearing bronze must have a mirror-likefinish. Such materials cannot be lapped with looseabrasive powder for they will take a "charge" ofembedded grit. To get such a finish the final finishhoning must be done with a fine grit, resilient bondtype of stone. For this type of finish Sunnen hasintroduced the 600 grit "cork" bond stonesmentioned earlier, which should properly be

NONFERROUS METALSGRIT HARD STEEL SOFT STEEL CAST IRON MED. SOFT SOFT MAT. CARBIDE CERAMIC GLASS

NUMBER (Rockwell C50 (Less than Forged Alum., Cast Alum.,or Harder) Rockwell C50) Hard Bronze, Brass, Bear-

Alum. Bronze, ing Bronze,etc. etc.

A B A B A B A B A B A B A B A B4 20 35 35 55 32 65 50 85 80 95 30* 35* 50* 65* 95* 10*

(150 Grit)5 18 32 25 40 20 28 33 50 55 70 20* 25* 40* 60* 70* 10*

(220 Grit)6 12 20 20 30 12 16 15 25 33 45

(280 Grit)7 10 15 15 24

(320 Grit)8 5 8 7 12 6 8 8 10 15 18 7* 10* 20* 25* 30* 40*

(400 Grit)9 3 4 4 7 5 6 6 8 12 14

(500 Grit)0 (1) (2) (2) (4) (3) (4) (2) (3) (2) (3) 3* 4* 15* 20* 15* 30*

(600 Grit)*Obtained with diamond stones in grit sizes shown.Obtained with cork-bond stones usable for polishing only.

SELECTION OF HONING STONE GRIT SIZEto obtain specified microinch finishes in the various materials.

A. Bold face figures show AA surface finish readings obtained when It oning with normal cutting pressure. These finishes can beimproved still further by "feathering out" cutting pressure at end of honing operation.

B. Figures in light italics give roughest AA finishes you can expect when using heavier cutting pressures for fastest stock removal.

NOTE: Surface readings are shown in microinches AA (arithmetic average). Where RMS (Root mean square) values are used,add 10% to convert figures in table to RMS readings.


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classified as polishing stones and are not suitable forstock removal.

To summarize, the finish obtained by honing is notaf-fected by the condition of the honing machine,since vibrations are not transferred to the work as incutting and grinding. The honing stones need not bediamond dressed to keep them trued and cutting.Finish, in honing, is primarily dependent on grit size;surface speeds and rate of stroking affect actualfinish only slightly.

With very few factors controlling honed finishes, it isnot difficult to specify a stone that will give a certainfinish in a given material and have the workconsistently come up to finish specifications.

In seeking super-fine finishes the mandrel used forroughing or stock removal should never be used forfinishing because in honing, the mandrel's shoe sur-faces take the same finish as the work. A separatemandrel should be reserved for such finishing opera-tions.

CONCLUSIONS - HOW TO MEET SPECIFIED FINISHES1. Heavy stock removal plus the need for a medium-fine or fine finish calls for more than one honingoperation, and requires separate tooling to get thebest results.

2. Inferior grade honing oils or other coolants willcause problems in stock removal as well as infinishing. Use only Sunnen Honing Oil to guaranteethe proper performance of Sunnen honing tools. Theoil must be kept clean and never diluted with sol-vents or other cutting agents. Oil should be changedand the machine cleaned out from time to time asneeded.

3. A new honing stone must be radiused to thediameter of the bore being honed; the same holdstrue for new mandrel shoes. Use a truing sleeve ofthe same diameter.

4. Finishing work with fast, slow or very slowstroking will change the appearance of the surface,due to light reflection, but will not change thesurface finish measurements.

5. Watch for "specks" of work material sticking tothe stone. Wipe the stone face and inspect itoccasionally between honing of parts - otherwise, 96scratching" (galling or welding) of the work surfacemight result. A stone that has a tendency to "load"with "specks" has too hard a bond; a stone's surfacemust wear away slowly to stay "clean" and sharp.Use a softer stone, if necessary, to avoid loading.

6. Stone cutting pressure will change the surfacemeasurements to some extent and as a rule should be"feathered out" on the final finishing stage of honing.

7. A change towards increased bore roughness in thelast machining operation prior to honing may call fora change in honing procedure or even necessitate anadded honing operation.

8. To "over-finish" is a waste of time - no need togenerate a 10 microinch when a 40 AA or better isspecified -a 30 or 35 AA finish will meet the spec'smore economically.

data files101 - Honing Bores With Keyways And Splines

102 - Honing Short Bores

103 - Honing Blind Holes

104 - Obtaining Specified Finishes By Honing

105 - Choosing the Right Stone

106 - Honing Tandem Holes

107 - Making Manual Honing Easier With Workholders

108 - Fixturing Parts For Power Stroking

109 - Vertical Hone Fixture

110 - Honing Small Bores

111 -

112 - External Honing

113 - Fixturing Design Considerations For Automatic Krossgrinding® Machines

SUNNEN CUSTOMER SERVICE AND TECHNICAL SERVICE CENTERSThe fully equipped Automotive and Industrial Technical Service Centers in St. Louis is available tohelp with any honing problem at any time without cost or obligation. Sunnen factory-trainedField Service Engineers cover the entire country and are always at your service - again, no cost orobligation. Call us whenever you have a bore-sizing problem.

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©2009 Sunnen Products Company Printed in USADF-104 9/09 - POD

Sunnen® reserves the right to change or revise specifications and product design in connection with any feature of ourproducts contained herein. Such changes do not entitle the buyer to corresponding changes, improvements, additions, orreplacements for equipment, supplies or accessories previously sold. Information contained herein is considered to beaccurate based on available information at the time of printing. Should any discrepancy of information arise, Sunnen recommends that user verify the discrepancy with Sunnen before proceeding.

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