M06 KIBB5087 09 SE C06 - The Eye · Generally, a trade-off between maximum production and tool breakdown or loss is accepted in machining operations. Several factors cause or reduce

263

U N I T O N E Machinability and Chip Formation

U N I T T W O Speeds and Feeds for Machine Tools

U N I T T H R E E Cutting Fluids

U N I T F O U R Using Carbides and Other ToolMaterials

Preparation forMachining Operations

S E C T I O N F

M06_KIBB5087_09_SE_C06.QXD 6/3/09 4:38 AM Page 263

264 SECTION F PREPARATION FOR MACHINING OPERATIONS

Cutting tool materials tend to get hot during the processof machining, that is, in cutting metals and producing

chips. The heat rise on the cutting edge of a tool can be suffi-cient to burn the sharp edge and dull it (Figure F-1). Thetool must then be resharpened, or a new sharp edge must beindexed in place. A certain amount of wear and tool break-down is to be expected, but lost machining time while re-placing or sharpening the tool must be considered.Generally, a trade-off between maximum production andtool breakdown or loss is accepted in machining operations.

Several factors cause or reduce tool heating. Some toolmaterials can withstand higher temperatures than others.The hardness and toughness of workpiece materials are alsofactors in tool breakdown caused by heating. Tool breakagecan be caused by heavy cuts (excessive feed) and interruptedcuts or by accidentally reversing the machine movement orrotation while a cut is being made.

The speed of the cutting tool point or edge movingalong the workpiece or on the workpiece moving past thetool is a major factor in controlling tool breakdown andmust always be considered when cutting metal with a ma-chine tool. This is true of all machining operations—sawing,turning, drilling, milling, and grinding. In all these, cuttingspeed is of the utmost importance.

The feed on a machine is the movement that forces thetool into or along the workpiece. Increments of feed are usu-ally measured in thousandths of an inch. Excessive feed usu-ally results in broken tools (Figure F-2). Often, this breakageruins the tool, but in some cases the tool can be reground.

Cutting fluids are used in machining operations for twobasic purposes: to cool the cutting tool and workpiece andto provide lubrication for easy chip flow across the toolface.

Figure F-1 The end of this twist drill has been burned owing toexcessive speed.

Figure F-2 The sharp corners of this end mill have been brokenoff owing to excessive feed.

S H O P T I P

Three basic materials are used for twist drills: carbonsteel, high-speed steel, and tungsten carbide. High-speed twist drills can be burned by excessive speed, asshown in Figure F-1. However, carbon steel drills aremuch more sensitive to such damage.

Carbon steel is not much used for twist drills any-more except for inexpensive ones. Also, older carbonsteel drills may be mixed in with assortments of high-speed drills in a toolroom. If many spark bursts can beseen when grinding on a drill, it is made of carbon steel.High-speed steel shows red streaks with little or nospark bursts and often has globules of metal on theends of the streaks when it is ground.

Coolant must be used when drilling with carbonsteel drills so they can be kept from heating up andburning. Lower speeds must be used as compared withhigh-speed steel. Any heating that discolors the chip ortool end can soften the metal of the drill, which ruinsthe cutting edge. Unlike high-speed steel, which regainsits hardness when it cools down, carbon steel loses itshardness permanently and remains soft, because it isretempered when it is heated above the original tem-pering temperature. In such cases the softened end ofthe drill must be cut off or ground off and resharpened,taking care not to overheat it in that operation. Ofcourse, no such problems exist when using carbidedrills, but they are brittle and can be damaged byspeeds that are too low.

S A F E T Y F I R S T

Safety in Chip Handling Certain metals, when di-vided finely as a powder or even as coarse as machin-ing chips, can ignite with a spark or just by the heat ofmachining. Magnesium and zirconium are two suchmetals. Such fires are difficult to extinguish, and if wa-ter or a water or a water-based fire extinguisher isused, the fire will only increase in intensity. The great-est danger of fire occurs when a machine operatorfails to clean up zirconium or magnesium chips on amachine when the job is finished. The next operatormay then cut alloy steel, which can produce high tem-perature in the chip or even sparks that can ignite themagnesium chips. Such fires may destroy the entiremachine if not the shop. Chloride-based powder fireextinguishers are commercially available. These are ef-fective for such fires, because they prevent water ab-sorption and form an air-excluding crust over theburning metal. Sand is also used to smother fires inmagnesium.

Metal chips from machining operations are sharpand are a serious hazard. They should not be handledwith bare hands. Gloves may be worn only when themachine is not running.


SECTION F PREPARATION FOR MACHINING OPERATIONS 265

Some cutting fluids are basically coolants. These areusually water-based soluble oils or synthetics, and theirmain function is to cool the work–tool interface. Cuttingoils provide some cooling action but also act as lubricantsand are mainly used where cutting forces are high, as in diethreading or tapping operations.

Tool materials range from hardened carbon steel to dia-mond, each type having its proper use, whether in tradi-tional metal cutting in machine tools or for grindingoperations. This section will enable you to use correctly allthese cutting tools, to prevent undue damage to tools andworkpieces, and to use them to the greatest advantage.


U N I T O N E

Machinability and ChipFormation

266

Machinability is the relative difficulty of a machining op-eration with regard to tool life, surface finish, and

power consumption. In general, softer materials are easier tomachine than harder ones. Chip removal and control arealso major factors in the machining industry. These andother considerations concerning the use of machine toolsare covered in this unit.

O B J E C T I V E S

After completing this unit, you should be able to:

■ Determine how metal cutting affects the surfacestructures of metals.

■ Analyze chip formation, structures, and chipbreakers.

■ Explain machinability ratings and machiningbehavior of metals.

PRINCIPLES OF METAL CUTTING

In machining operations, either the tool material rotates ormoves in a linear motion or the workpiece rotates or moves(Table F-1). The moving or rotating tool must be made tomove into the work material to cut a chip. This procedure iscalled feed. The amount of machine feed controls the thick-ness of the chip. The depth of cut is often called infeed. Inaddition to single-point tools with one cutting edge, as theyare called in lathe, shaper, and planer operations, there aremultiple-point tools such as milling machine cutters, drills,and reamers. In one sense, grinding wheels could be consid-ered to be multiple-point cutters, because small chips areremoved by many tiny cutting points on grinding wheels.

One of the most important aspects of cutting toolgeometry is rake (Figure F-3). Tool rake ranging from nega-tive to positive has an effect on the formation of the chip

and on the surface finish. Zero- and negative-rake tools arestronger and have a longer working life than positive-raketools. Negative-rake tools produce poor finishes at low cut-ting speeds but give a good finish at high speeds. Positive-rake tools are freer cutting at low speeds and can producegood finishes when they are properly sharpened.

A common misconception is that the material splitsahead of the tool, as wood does when it is being split with anaxe. This is not true with metals; the metal is sheared off anddoes not split ahead of the chip (Figures F-4 to F-7). The metalis forced along in the direction of the cut, and the grains areelongated and distorted ahead of the tool and forced along ashear plane, as can be seen in the micrographs. The surfaceis disrupted more with the tool having the negative rakethan with the tool with the positive rake in this case, becauseit is moved at a slow speed. Negative-rake tools require morepower than positive-rake tools.

Higher speeds give better surface finishes and produceless disturbance of the grain structure, as can be seen inFigures F-8 and F-9. At a lower speed of 100 surface feet perminute (sfpm), the metal is disturbed to a depth of .005 to.006 in., and the grain flow is shown to be moving in the di-rection of the cut. The grains are distorted, and in someplaces the surface is torn. This condition can later producefatigue failures and a shorter working life of the part thanwould a better surface finish. Tool marks, rough surfaces,and an insufficient internal radius at shaft shoulders can alsocause early fatigue failure where there are high stress and fre-quent reversals of the stress. At 400 sfpm, the surface is lessdisrupted and the grain structure is altered to a depth ofonly about .001 in. When the cutting speeds are increased to600 sfpm and above, little additional improvement is noted.

There is a great difference between the surface finish ofmetals cut with coolant or lubricant and metals that are cutdry. This is because of the cooling effect of the cutting fluidand the lubricating action that reduces friction between tooland chip. The chip tends to curl away from the tool morequickly and is more uniform when a cutting fluid is used.Also, the chip becomes thinner and pressure welding is


UNIT ONE MACHINABILITY AND CHIP FORMATION 267

Operation Diagram Characteristics Type of Machines

Turning Work rotates; tool moves for feed Lathe and vertical boring mill

Milling (horizontal) Cutter rotates and cuts on periphery;work feeds into cutter and can be moved in three axes

Horizontal milling machine

Face milling Cutter rotates to cut on its end and periphery of vertical workpiece

Horizontal mill, profile mill, andmachining center

Vertical (end) milling Cutter rotates to cut on its end andperiphery; work moves on three axes for feed or position; spindle also moves up or down

Vertical milling machine, die sinker,machining center

Shaping Work is held stationary and tool reciprocates; work can move in two axes;toolhead can be moved up or down

Horizontal and vertical shapers

Planing Work reciprocates while tool is stationary;tool can be moved up, down, or crosswise;worktable cannot be moved up or down

Planer

Horizontal sawing (cutoff)

Work is held stationary while the saw cuts in one direction, as in band sawing, orreciprocates while being fed downward into the work

Horizontal band saw, reciprocatingcutoff saw

Table F-1 Machining Principles and Operations



Table F-1 Machining Principles and Operations (Continued)


Vertical band sawing(contour sawing)

Endless band moves downward,cutting a kerf in the workpiece,which can be fed into the saw on one plane at any direction

Vertical band saw

Broaching Workpiece is held stationary while amultitooth cutter is moved across thesurface; each tooth in the cutter cutsprogressively deeper

Vertical broaching machine, horizontalbroaching machine

Horizontal spindlesurface grinding

The rotating grinding wheel can be moved up or down to feed into theworkpiece; the table, which is made toreciprocate, holds the work and can also be moved crosswise

Surface grinders, specialized industrialgrinding machines

Vertical spindle surfacegrinding

The rotating grinding wheel can be moved up or down to feed into theworkpiece; the circular table rotates

Blanchard-type surface grinders

Cylindrical grinding The rotating grinding wheel contacts aturning workpiece that can reciprocate from end to end; the wheelhead can bemoved into the work or away from it

Cylindrical grinders, specializedindustrial grinding machines

Centerless grinding Work is supported by a workrest between a large grinding wheel and a smaller feed wheel

Centerless grinder



Table F-1 Machining Principles and Operations (Continued)


Drilling and reaming Drill or reamer rotates while work isstationary

Drill presses, vertical milling machines

Drilling and reaming Work turns while drill or reamer is stationary

Engine lathes, turret lathes, automaticscrew machines

Boring Work rotates; tool moves for feed on internal surfaces

Engine lathes, horizontal and verticalturret lathes, and vertical boring mills(On some horizontal and vertical boringmachines the tool rotates and the workdoes not.)

(a) Positive (b) Neutral (c) Negative

Figure F-3 Side view of rake angles.

Rake angle

Shearangle

Depth of cut

Chip

Figure F-4 Metal cutting diagram.

Tool

Figure F-5 A chip from a positive-rake tool magnified 100 diameters at the point of the tool. The grain distortion is notas evident as in Figures F-6 and F-7. This is a continuous chip.



reduced. When metals are cut dry, pressure welding is a defi-nite problem, especially in the softer metals such as 1100aluminum and low-carbon steels. Pressure welding pro-duces a built-up edge (BUE) that causes a rough finish and atearing of the surface of the workpiece. A built-up edge isalso caused by speeds that are too slow; this often results inbroken tools from excess pressure on the cutting edge.Figures F-10 to F-12 show chips formed with tools havingpositive, negative, and zero rakes. These chips were allformed at low surface speeds and consequently are thickerthan they would have been at higher speeds. There is alsomore distortion at low speeds. The material was cut dry.

Tool

Figure F-6 Point of a negative-rake tool magnified 100 diameters at the point of the tool.

Tool

Figure F-7 A zero rake tool at 100 diameters shows grain flowand distortion similar to that of the negative-rake tool.

Figure F-8 This micrograph shows the surface of the specimenturned at 100 sfm. The surface is irregular and torn, and the grainsare distorted to a depth of approximately .005 to .006 in. (250:).

Figure F-9 At 400 sfm, this micrograph reveals that the surfaceis fairly smooth and the grains are only slightly distorted to adepth of approximately .001 in. (250:).

At high speeds, cratering begins to form on the top sur-face of the tool because of chip wear against the tool; thiscauses the chip to begin to curl (Figure F-13). The cratermakes an airspace between the chip and the tool, which is anideal condition because it insulates the chip from the tooland allows the tool to remain cooler; the heat goes off withthe chip. The crater also allows the cutting fluid to get underthe chip.

Various metals are cut in different ways. Softer, moreductile metals produce a thicker chip, and harder metalsproduce a thinner chip. A thin chip indicates a clean cuttingaction with a better finish.

A machine operator often notices that using certaintools having certain rake angles at greater speeds (severalhundred surface feet per minute as compared with 50 to 100sfpm) produces a better finish on the surface. The operator



Figure F-10 A continuous-form chip is beginning to curl awayfrom this positive-rake tool.

Figure F-11 A thick, discontinuous chatter chip being formed atslow speed with a negative-rake tool.

Figure F-12 A discontinuous, thick chip is being formed with azero-rake tool.

Figure F-13 The crater on the cutting edge of this tool wascaused by chip wear at high speeds. The crater often helps tocool the chip.

may also notice that when speeds are too low, the surfacefinish becomes rougher. Not only do speeds, feeds, toolshapes, and depth of cut have an effect on finishes, thesurface structure of the metal itself is disturbed and alteredby these factors.

Tool materials are high-speed steel, carbide, ceramic,and diamond. Most manufacturing today is done with car-bide tools. Greater amounts of materials may be removedand tool life extended considerably when carbides, ratherthan high-speed steels, are used. Much higher speeds canalso be used with carbide tools than with high-speed tools.

ANALYSIS OF CHIP STRUCTURES

Machining operations performed on various machine toolsproduce chips of three basic types: the continuous chip, thecontinuous chip with a built-up edge on the tool, and thediscontinuous chip. The formation of the three basic typesof chips can be seen in Figure F-14. Various kinds of chipformations are shown in Figure F-15. High cutting speedsproduce thin chips, and tools with a large positive rake an-gle favor the formation of a continuous chip. Any circum-stances that lead to a reduction of friction between thechip–tool interface, such as the use of cutting fluid, tend toproduce a continuous chip. The continuous chip usuallyproduces the best surface finish and has the greatest

(a) (b) (c)

Figure F-14 The three types of chip formation: (a) continuous;(b) continuous with built-up edge; (c) discontinuous (segmented)(Neely, John E., Basic Machine Tool Practices, 1st, ©2000.Electronically reproduced by permission of Pearson Education,Inc., Upper Saddle River, New Jersey).



efficiency in terms of power consumption. Continuouschips create lower temperatures at the cutting edge, buthigh speeds lead to higher cutting forces and extremelyhigh tool pressures. Because there is less strength at thepoint of positive rake angle tools than with negative-raketools, tool failure is more likely with large positive rake an-gles at high cutting speeds or with intermittent cuts.

Negative-rake tools are most likely to produce a built-up edge with a rough continuous chip and a rough finish onthe work, especially at lower cutting speeds and with softmaterials. Positive rake angles and the use of cutting fluidplus higher speeds decrease the tendency for a built-up edgeon the tool. However, most carbide tools for turning ma-chines have a negative rake, for several reasons. Carbides arealways used at high cutting speeds, lessening the tendencyfor a built-up edge. Better finishes are obtained at highspeeds even with negative rake, and the tool can withstandgreater shock loads than positive-rake tools. Another advan-tage of negative-rake tools is that an indexable insert canhave 90-degree angles between its top surface and flank,making more cutting edges on both sides. A negative rake givesthe tool flank relief even though the tool has square edges.Negative-rake tools need more horsepower than positive-raketools do when all other factors are the same.

The discontinuous or segmented chip is producedwhen a brittle metal, such as cast iron or hard bronze, is cut.Some ductile metals can form a discontinuous chip whenthe machine tool is old or loose and a chattering condition is

present, or when the tool form is not correct. The discontin-uous chip is formed as the cutting tool contacts the metaland compresses it to some extent; the chip then begins toflow along the tool, and when more stress is applied to thebrittle metal, it tears loose and a rupture occurs. This causesthe chip to separate from the work material. Then, a new cy-cle of compression, the tearing away of the chip, and itsbreaking off begins.

Low cutting speeds and a zero or negative rake anglecan produce discontinuous chips. The discontinuous chip ismore easily handled on the machine, because it falls into thechip pan.

The continuous chip sometimes produces snarls or longstrings that are not only inconvenient but dangerous to han-dle. The optimum kind of chip for operator safety and pro-ducing a good surface finish is the 9-shaped chip, which isusually produced with a chip breaker.

Chip breakers take many forms, and most carbide toolholders have either an inserted chip breaker or one formedin the insert tool itself (Figure F-16). A chip breaker is de-signed to curl the chip against the work and then to break itoff to produce the proper type of chip.

Machine operators usually must form the tool shapethemselves on a grinder when using high-speed tools andmay or may not grind a chip breaker on the tool. If they donot, a continuous chip is formed that usually makes a wirytangle, but if they grind a chip breaker in the tool, depend-ing on the feed and speed, they produce a more acceptable

Discontinuouschip

Parting toolchip

(continuous)

Snarl

Millingchip

9-shapedchip

Chatterchip

Helix

Figure F-15 Chips formed in machining operations.



type of chip. A high rate of feed will produce a greater curl inthe chip, and often even without a chip breaker a curl can beproduced by adjusting the feed of the machine properly. Thedepth of cut also has an effect on chip curl and breaking. Alarger chip is produced when the depth of cut is greater. Thisheavier chip is less springy than light chips and tends tobreak up into small chips more readily. Figure F-17 showshow chip breakers may be formed in a high-speed tool. SeeUnit 4 for carbide chip breakers.

MACHINABILITY OF METALS

Various ratings are given to metals of different types on ascale based on properly annealed carbon steel containingabout .1 percent carbon (which is 100 on the scale). Muchinformation has been published on the relative machinabil-ity of the various grades of carbon steel. They have beencompared almost universally with B-1112, a free-machiningsteel that was once the most popular screw machining stock.Machinability ratings by some manufacturers are now basedon AISI 1212, which is rated at 100 percent and at 168 sfpm.Metals more easily machined have a number higher than100, and materials more difficult to machine are numbered

lower than 100. Table F-2 gives the machinability of variousalloys of steel based on AISI B-1112 as 100 percent. Ofcourse, the machinability is greatly affected by the cuttingtool material, tool geometry, and the use of cutting fluids.

In general, the machinist must select the type of tool,speeds, feeds, and type of cutting fluid for the material beingcut. The most important material property, however, is hard-ness. A machinist often shop tests the hardness of materialwith a file to determine relative machinability, because hard-ness is related to machinability. Hardness is also a factor inproducing good finishes. Soft metals such as copper, 1100 se-ries aluminum, and low-carbon hot-rolled steel tend to havepoor finishes when cut with ordinary tooling. Cutting toolstend to tear the metal away rather than cut these soft metals;however, sharp tools and larger rake angles plus the use ofcutting fluids can help produce better finishes. Harder,tougher alloys almost always produce better finishes evenwith negative-rake carbide tools. For example, an AISI 1020hot-rolled steel bar and an AISI 4140 steel bar of the same di-ameter cut at the same speed and feed with the same toolingwill have markedly different finishes. The low-carbon HRsteel will have a poor finish compared with the alloy steel.

lt should be noted that cutting speeds can be differentfor materials in the same classification.

Low-carbon steel, for example, can have a cutting speedof 70, 90, 100, or 120 depending on the hardness range ofthe material.

The operator must also understand the effects of heaton normally machinable metals such as alloy steel, tool steel,or gray cast iron. Welding on any of these metals mayharden the base metal near the weld and make it difficult to

(a) (b)

Figure F-16 The action of a chip breaker to curl a chip andcause it to break off: (a) plain tool; (b) tool with chip breaker.

Figure F-17 Types of chip breakers in high-speed tools.

Table F-2 Machinability Ratings for Some CommonlyUsed Steels

AISI Number

Machinability Index (% Relative Speed Based on AISI B1112 as 100%)

B1112 100C1120 81C1140 72C1008 66C1020 72C1030 70C1040 64C1060 (annealed) 51C1090 (annealed) 423140 (annealed) 664140 (annealed) 665140 (annealed) 706120 578620 66301 (stainless) 36302 (stainless) 36304 (stainless) 36420 (stainless) 36440A (stainless) 24440B (stainless) 24440C (stainless) 24



cut, even with carbide tools. To soften these hard areas, theentire workpiece must first be annealed.

Most alloy steel can be cut with high-speed tools but atrelatively low cutting speeds, which produce a poor finishunless some back rake is used. These steels are probably bestmachined by using carbide tools at higher cutting speedsranging from 200 to 350 sfpm, or even higher in some cases.Some nonferrous metals, such as aluminum and magnesium,can be machined with much higher speeds ranging from 500to 8000 sfpm when the correct cutting fluid is used. Some al-loy steels, however, are more difficult to machine becausethey tend to work harden. Examples of these are austeniticmanganese steels (those used for wear resistance), Inconel,stainless steels (SAE 301 and others), and some tool steels.

1. In what way do tool rakes, positive and negative, affect surfacefinish?

2. Soft materials tend to pressure weld on the top of the cuttingedge of the tool. What is this condition called and what is itsresult?

SELF-TEST

3. Which indicate a greater disruption of the surface material:thin uniform chips or thick segmented chips?

4. In metal cutting, does the material split ahead of the tool? Ifnot, what does it do?

5. Which tool form is stronger, negative or positive rake?

6. What effect does cutting speed have on surface finish? Onsurface disruption of grain structure?

7. How can surface irregularities caused by machining later af-fect the usefulness of the part?

8. Which property of metals is directly related to machinability?How do machinists usually determine this property?

9. Describe the type of chip produced on a machine tool that isthe safest and easiest to handle.

10. Define machinability.

INTERNET REFERENCESInformation on machinability of metals:

http://carbidedepot.com

http://en.wikpedia.org/wiki/Machinability


O B J E C T I V E S

U N I T T W O

Speeds and Feeds for Machine Tools

A small cylinder will have more revolutions per minute(rpm) than a large one with the same surface speed. For ex-ample, if a thin wire is pulled off a 1-in.-diameter spool atthe rate of 20 fpm, the spool will rotate three times fasterthan a 3-in.-indiameter spool with the wire being pulled offat the same rate of 20 fpm.

Because machine spindle speeds are given in revolu-tions per minute (rpm), they can be derived in the followingmanner:

If you use 3 to approximate the formulabecomes

This simplified formula is certainly the most common oneused in machine shop practice, and it applies to the fullrange of machine tool operations, which include the latheand the milling machine, as well as the drill press. The sim-plified formula

is used throughout this book. The formula is used, forexample, as follows, where or other rotating tool in inches and

for a particular material. For a -in. drill inlow-carbon steel the speed would be

Table F-3 gives cutting speeds and starting values forcommon materials. These may have to be varied up or downdepending on the specific machining task. Always observethe cutting action carefully and make appropriate speed cor-rections as needed. Until you gain some experience in

90 * 412

= 720 rpm

12cutting speed

CS = an assigned D = the diameter of the drill

rpm =

CS * 4

D

CS * 12

D * 3=

CS * 4

D

p 13.14162,

rpm =

cutting speed 1CS; in feet per minute2 * 12

diameter of cutter 1D; in inches2 * p

Modern machine tools are powerful, and with moderntooling they are designed for a high rate of produc-

tion. However, the operator who uses the incorrect feeds andspeeds for the machine operation will have a much lowerproduction rate and an inferior product. Sometimes, an op-erator will take a light roughing cut when it is possible totake a greater depth of cut. Often, cutting speeds are too lowto produce good surface finishes and for optimum metal re-moval. Labor costs make time important, and removal of alarger amount of metal can shorten the time needed to pro-duce a part and often improve its physical properties.

The importance of cutting speeds and machine feeds inmachining operations cannot be overemphasized. The rightspeed can mean the difference between burning the end of adrill or other tool, causing lost time, or having many hoursof cutting time between sharpenings or replacement. Thecorrect feed can also lengthen tool life. Excessive feeds oftenresult in tool breakage. This unit will prepare you to think interms of correct feeds and speeds.


■ Calculate correct cutting speeds for various machinetools and grinding machines.

■ Determine correct feeds for various machiningoperations.

275

CUTTING SPEEDS

Cutting speeds (CS) are normally given in tables for cuttingtools and are based on surface feet per minute (fpm orsfpm). Surface feet per minute means that either the toolmoves past the work or the work moves past the tool at arate based on the number of feet that pass the tool in oneminute. Cutting speed can be determined from a flat surfaceor on the periphery of a cylindrical tool or workpiece.



machining, use the lower values in the table when selectingcutting speeds. As you can see by the hardness values in thetable, there is a general relationship between hardness andcutting speed. This is not always true, however. Some stain-less steel is relatively soft, but it must be cut at a low speed.

Cutting tables for various materials areavailable in handbooks and on wall charts. The cuttingspeed for carbide tools is normally three to four times thatof high-speed cutting tools.

You will learn more about cutting tool materials in laterunits in this section, but here is a good rule to rememberconcerning cutting speeds: generally, cutting tools will wearout quickly at too-high cutting speeds. Carbide cutting toolswill chip or break up quickly at too-low cutting speeds.When steel chips become blue colored, it is a sign of a highertemperature caused by high cutting speed and/or dull tools.Blue chips are acceptable and in fact desirable when usingcarbides, but not when using high-speed steel tools. Thechips should not be discolored at all with high-speed tools,especially when cutting fluids are used.

Cutting speed constants are influenced by the cuttingtool material, workpiece material, rigidity of the machinesetup, and use of cutting fluids. As a rule, lower cutting speedsare used to machine hard or tough materials, or where heavycuts are taken and it is desirable to minimize tool wear andthus maximize tool life. Higher cutting speeds are used in ma-chining softer materials to achieve better surface finishes.

Cutting speeds for drills are the same for drilling opera-tions both where the drill rotates and where the work rotatesand the drill remains stationary, as in lathe work. For a stepdrill with two or more diameters, the largest diameter isused in the formula.

Where the workpiece rotates and the tool is stationary,as in lathes, the outer diameter (D) of the workpiece is usedin the speed formula. As the workpiece is reduced to asmaller diameter, the speed should be increased. However,there is some latitude in speed adjustments, and 5 to 10sfpm one way or another is usually not significant.Therefore, if a machine cannot be set at the calculated rpm,use the next-lower speed. If there is vibration or chatter, alower speed will often eliminate it.

speeds>rpm

For multiple-point cutting, as in milling machines, D isthe diameter of the milling cutter in inches. Cutting speed inmilling is the rate at which a point on the cutter passes by apoint on the workpiece in a given period of time.

Many grinding machines and portable grinders have afixed rpm, so it is relatively easy to select a grinding wheel forthat speed. However, tool and cutter grinders have variablespeeds, so a range of wheels from tiny, mounted wheels tolarge, straight wheels is available. The rpm for these machinesmust be correct. Every grinding wheel, wherever used, has asafe maximum speed, and this should never be exceeded.

Any grinding machine in the shop, properly handled, isa safe machine. It has been designed that way and should bemaintained to keep it safe. The negative image of grindingwheels and breakage comes primarily from portablegrinders, which often are not well maintained and some-times are operated by unskilled and careless persons.

You must always stay within the safe speeds, which areshown on the blotter or label on every wheel of any size(Figure F-18). Vitrified wheels generally have a maximumsafe speed of 6500 sfpm, and organic wheels (resinoid, rub-ber, or shellac), 16,000 sfpm. These speeds are generally setby the grinding wheel manufacturer.

Because of the importance of wheel speed in grindingwheel safety, it is important to know how it is calculated.This speed is expressed in terms of surface feet per minute(sfpm), which simply expresses the distance that a given spoton a wheel periphery travels in a minute. It is calculated bymultiplying the diameter (in inches) by 3.1416, dividing theresult by 12 to convert it to feet, and multiplying that resultby the number of revolutions per minute (rpm) of thewheel. Thus a 10-in.-diameter wheel traveling at 2400 rpmwould be rated at approximately 6283 sfpm, under the safespeed of 6500 sfpm of most vitrified wheels.

The formula for the safe speed in rpm of a 10-in.-diameter wheel is

Most machine shop–type flat surface or cylindricalgrinders are preset to operate at a safe speed for the largest

rpm =

CS * 4

D=

6500 * 4

10= 2600

Table F-3 Cutting Speeds and Starting Values for Some Commonly Used Materials

Tool Material

Work Material Hardness (BHN) High-Speed Steel Cemented Carbide

Aluminum 60–100 300–800 1000–2000Brass 120–220 200–400 500–800Bronze (hard drawn) 220 65–130 200–400Gray cast iron (ASTM 20) 110 50–80 250–350Low-carbon steel 220 60–100 300–600

Medium-carbon alloy steel (AISI 4140) 229a 50–80 225–400High-carbon steel 240a 40–70 150–250Carburizing-grade alloy steel (AISI 8620) 200–250a 40–70 150–350Stainless steel (type 410) 120–200a 30–80 100–300

aNormalized.


UNIT TWO SPEEDS AND FEEDS FOR MACHINE TOOLS 277

grinding wheel that the machine is designed to hold. As longas the machine is not tampered with and no one tries tomount a larger wheel on the machine than it is designed for,there should be no problem.

It should be clear that with a given spindle speed (rpm)the speed in sfpm increases as the wheel diameter is in-creased, and it decreases with a decrease in wheel diameter.Maximum safe speed may be expressed either way, but onthe wheel blotter it is usually expressed in rpm. However, ifthe cutting speed is sought and the rpm known, the follow-ing formula may be used:

FEEDS

The amount of feed is usually measured in thousandths ofan inch in metal cutting. Feeds are expressed in slightly dif-ferent ways on various types of machine tools.

Drills on drill presses rotate, causing the cutting edges onthe end of the drill to cut into the workpiece. When metal iscut, considerable force is required to feed the cutting edges ofthe drill into the workpiece. Drilling machines that havepower feeds are designed to advance the drill a given amountfor each revolution of the spindle. Therefore, a .006-in. feedmeans that the drill advances .006 in. for every revolution ofthe machine spindle. Thus feeds for drill presses are expressedin inches per revolution (ipr). The amount of feed varies withthe drill size and the kind of work material (Table F-4).

CS =

rpm * D

4

When drill presses have no power feed, the operatorprovides the feed with a handwheel or hand lever. Thesedrilling machines are called sensitive drill presses because theoperator can “sense” or feel the correct amount of feed. Also,the formation of the chip as a tightly rolled helix instead of astringy chip is an indicator of the correct feed. Excessive feedon a drill can cause the machine to jam or stop, or the drillto break, and can present a hazardous situation if the work-piece is not properly clamped.

Feeds on turning machines such as lathes are also ex-pressed in inches per revolution (ipr) of the lathe spindle. Thetool moves along the rotating workpiece to produce a chip. Thedepth of the cut is not related to feed. The quick-change gear-box on the lathe makes possible the selection of feeds in ipr in arange from approximately .001 in. to about .100 in., dependingon size and machine manufacturer. Feed rates for small (10-in.swing) lathe operations can be as high as .015 ipr for roughingoperations and usually .003 to .005 ipr for finishing. However,massive turning machines may use .030 to .050 ipr feed with a.750-in. depth of cut. Coarser feeds, depending on the machinesize, horsepower, and rigidity, necessary for rapid stock re-moval, are called roughing. As long as the roughing dimensionis kept well over the finish size, feeds should be set to the maxi-mum the machine will handle. In roughing operations, finish isnot important, and no attempt should be made to obtain agood finish; the only consideration is stock removal and, ofcourse, safety. It is in the finishing cuts with fine feeds that di-mensional accuracy and surface finish are of the greatest im-portance, and stock removal is of little consideration.

Feeds on milling machines refer to the rate at which theworkpiece material is advanced into the cutter by tablemovement or where the cutter is advanced into the work-piece in the manner of a drill cutting on its end. Becauseeach tooth on a multitooth milling cutter makes a chip, thechip thickness depends on the amount of table feed. Feedrate in milling is measured in inches per minute (ipm) andis calculated by the formula

where

rpm = revolutions per minute of the cutter

N = number of teeth in the cutter being used

F = feed per tooth

ipm = feed rate, in inches per minute

ipm = F * N * rpm

Table F-4 Drilling Feed Table

Drill Size Diameter (in.) Feeds per Revolution (in.)

Under 18 .001–.002

to 1418

.002–.004

to 1214

.004–.007

to 112

.007–.015

Over 1 .015–.025

V52WARNING!!! GRINDING WHEELS IMPROPERLY USED ARE DANGEROUS. CAUTION: Comply with American National Standards

Institute Safety Code B7 1 and Occupational

Safety and Health Act covering: SPEED

— SAFETY GUARDS— FLANGES— MOUNTING PROCEDURES— GENERAL OPERATING RULES— HANDLING, STORAGE AND INSPECTION

— GENERAL MACHINE

CONDITIONS

USE ASAFETY GUARD

ORDER # MAXIMUMSAFE R.P.M.830673

36009A-1O0-H8-V52

Figure F-18 The blotter on the wheel, besides serving as abuffer between the flange and the rough abrasive wheel,provides information as to the dimensions and the compositionof the wheel, plus its safe speeds in rpm. This wheel can be runsafely up to 3600 rpm.



Feeds for end mills used in vertical milling machines rangefrom .001 to .002 in. feed per tooth for small-diameter cut-ters in steel work material to .010 in. feed per tooth for largecutters in aluminum workpieces. Because the cutting speedfor mild steel is 90, the rpm for a -in. high-speed, two-fluteend mill is

To calculate the feed rate, we will select .002 in. feed pertooth. (See Section J, Table J-2, for a table for end mill feedrates.)

The cutter should rotate at 960 rpm and the table feed ad-justed to approximately 3.84 ipm.

Horizontal milling machines have a horizontal spindleinstead of a vertical one, but the feed rate calculations arethe same as for vertical mills. Horizontal milling cutters usu-ally have many more teeth than end mills and are muchlarger. Cutting speeds and rpm are also calculated in thesame way. (See Section K, Table K-2, for cutting speeds andfeed rates for horizontal milling.)

Feeds for surface grinding are set for the crossfeed sothat the workpiece moves at the end of each stroke of thetable. The crossfeed dial is graduated in thousandths, but thefeed is not usually calculated with precision. As a rule, thecrossfeed is set to move from one quarter to one third of thewheel width at each stroke. This coarse feed allows the wheelto break down (wear) more evenly than do fine feeds thatcause one edge of the wheel face to break down.

Plain cylindrical grinding, where the rotating workpieceis traversed back and forth against a large grinding wheel,uses table feeds, as in surface grinding. The feed or traversesetting must be adjusted for roughing or finishing to getoptimum results.

ipm = F * N * rpm = .002 * 2 * 960 = 3.84

rpm =

CS * 4

D=

90 * 4

3>8=

360

.375= 960 rpm

38

1. If the cutting speed (CS) for low-carbon steel is 90, and theformula for rpm is what should the rpm of the

spindle of a drill press be for a -in.-diameter high-speedtwist drill? For a .750-in. diameter drill?

2. If the two drills in question 1 are used in a lathe instead of adrill press, what should the rpm of the lathe spindle be forboth drills?

3. If, in question 2, the small drill could not be set at thecalculated rpm because of machine limitations, what couldyou do?

4. An alloy-steel 2-in.-diameter cylindrical bar having a cuttingspeed of 50 is to be turned on a lathe using a high-speed tool.At what rpm should the lathe be set?

5. A formula for setting the safe rpm for an 8-in. grindingwheel, when the cutting speed is known, is

If the safe surface speed of a vitrifiedwheel is 6000 sfpm, what should the rpm be?

6. Are feeds on a drill press based on inches per minute orinches per revolution of the spindle?

7. Which roughing feed on a small lathe would work best,.100 or .010 in. per revolution?

8. What kind of machine tool bases feed on inches per minuteinstead of inches per revolution?

9. Approximately what should the feed rate be on a -in.-widegrinding wheel on a surface grinder?

10. What is the recommended feed rate for drills over 1 inch indiameter?

INTERNET REFERENCESInformation on feeds and speeds for machining:

http://mrainey.freeservers.com

http://keyseaters.com

34

rpm = 1CS * 42>D.

18

1CS * 42>D,

SELF-TEST


279

O B J E C T I V E S

U N I T T H R E E

Cutting Fluids

cooling the tool and workpiece, and oil-based fluids are thebetter lubricants, there is a considerable overlap betweenthe two.

Cutting force and temperature rise are generated notso much by the friction of the chip sliding over the surfaceof the tool as by the shear flow of the workpiece metal justahead of the tool (Figure F-19). Some tool materials, suchas carbide and ceramic, withstand high temperatures.When these materials are used in high-speed cutting oper-ations, the use of cutting fluids may actually be counter-productive. The increased temperatures tend to promotean easier shear flow and thus reduce cutting force. High-speed, high-temperature cutting also produces better fin-ishes and disrupts the surface less than does lower-speedmachining. For this reason, machining with extremely highcutting speeds using carbide tools is often done dry. Theuse of coolant may cause a carbide tool to crack and breakup owing to thermal shock, because the flooding coolant

Since the beginning of the Industrial Revolution, whenmetals first began to be cut on machines, cutting fluids

have been an important aspect of machining operations.Lubricants in the form of animal fats were first used to re-duce friction and cool the workpiece. These straight fattyoils tended to become rancid, had a disagreeable odor, andoften caused skin rashes on machine operators. Althoughlard oil alone is no longer used to any great extent, it is stillused as an additive in cutting oils. Plain water was some-times used to cool workpieces, but water alone is a corrosiveliquid and tends to rust machine parts and workpieces.Water is now combined with oil in an emulsion that coolsbut does not corrode. Many new chemical and petroleum-based cutting fluids are in use today, making possible thehigh rate of production in machining and manufacturing ofmetal products we presently enjoy. Cutting fluids reducemachining time by allowing higher cutting speeds, and theyreduce tool breakdown and down time. This unit will pre-pare you to use cutting fluids properly when you begin tooperate machine tools.


■ List the various types of cutting fluids.■ Explain the correct uses and care in using several

cutting fluids.■ Describe several methods of cutting fluid

application.

EFFECTS OF CUTTING FLUIDS

Cutting fluids is a generic term that covers various productsused in cutting operations. Basically, two major effects arederived from cutting and grinding fluids: cooling and lubri-cation. Although water-based fluids are most effective for

Chip

Built-up edge

Friction zonesDeformation zone

Shearangle

Figure F-19 Metal-cutting diagram showing the shear flowahead of the tool.



can never reach the hottest point at the tip of the tool andso cannot maintain an even temperature on the tool.

It is at the lower cutting speeds at which high-speedtools and even carbides are used that cutting fluids are usedto the greatest advantage. Also, cutting fluids are a necessityin most precision grinding operations. Most milling ma-chine, drilling, and many lathe operations require the use ofcutting fluids.

Types of Cutting FluidsLubricants as well as coolants (which also provide lubrica-tion) reduce cutting forces at lower cutting speeds (allowingsomewhat higher speeds to be used). They also reduce or re-move the heat generation that tends to break down high-speed tools. Both lubricants and coolants extend tool lifeand improve workpiece finish. Another advantage in usingcutting fluids is that the workpiece dimensions can be accu-rately measured on the cool workpiece. A hot workpiecemust be cooled to room temperature to obtain a correctmeasurement.

Several types of cutting fluids are in use, and theirnomenclature is by no means universal, since a single fluid isoften called by many different names. Next we describe thenames used in this book.

Synthetic Fluids (Also sometimes called semichemicalfluids.) These can be true solutions, consisting of inorganicsubstances dissolved in water, which include nitrites, nitrates,borates, or phosphates. As their name implies, these chemicalfluids form true solutions in water, unlike the oils inemulsions. These chemical substances are water miscible andvary in color from milky to transparent when mixed withwater. This type is superior to others for machining andgrinding titanium and for grinding operations in general.The surface-active type is a water solution that containsadditives that lower the surface tension of the water. Sometypes have good lubricity and corrosion inhibitors.

Some advantages of these chemical fluids are their re-sistance to becoming rancid (especially those that containno fats), good detergent (cleansing) properties, rapid heatdissipation, and the fact that they are easy to mix, requiringlittle agitation. There are also disadvantages. A lack of lubri-cation (oiliness) in most types may cause sticking in ma-chine parts that depend on the cutting fluid for lubrication.Also, the high detergency has a tendency to remove the nor-mal skin oils and irritate workers’ hands with long continualexposure. Synthetic fluids generally provide less corrosionresistance than oilier types, and some tend to foam. Whenimproved lubricating qualities are needed for synthetic flu-ids, sulfur, chlorine, or phosphorus is added.

Semisynthetic Fluids (sometimes called semichemicalfluids). These cutting fluids are essentially a combination ofsynthetic fluids and emulsions. They have a lower oil contentthan the straight emulsion fluids, and the oil droplets aresmaller because of a higher content of emulsifying molecules,

thus combining the best qualities of both types. Because asmall amount of mineral oil is added to these semisyntheticfluids, they possess enhanced lubrication qualities. Inaddition, they may contain other additives such as chlorine,sulfur, or phosphorus.

Emulsions (also called water-miscible fluids or water-soluble oils). Ordinary soap is an emulsifying agent thatcauses oil to combine with water in a suspension of tinydroplets. Special soaps and other additives such as aminesoaps, rosin soaps, petroleum sulfonates, and naphthenicacids are blended with a naphtha-based or paraffin mineraloil to emulsify it. Other additions, such as bactericides, helpreduce bacteria, fungi, and mold and extend emulsion life.Without these additives, an emulsion tends to develop astrong, offensive odor because of bacterial action and mustbe replaced with a new mix.

These ordinary emulsified oils contain oil particles largeenough to reflect light, and therefore they appear opaque ormilky when combined with water. In contrast, many of thesynthetic and semisynthetic emulsion particles are so smallthat the water remains clear or translucent when mixed withthe chemical. The ratio of mixing oil and water varies withthe job requirement and can range from 5 to 70 parts waterto 1 part oil. However, for most machining and grinding op-erations, a mixture of 1 part oil to 20 parts water is generallyused. A mixture that is too rich for the job can be needlesslyexpensive, and a mixture that is too lean may cause rust toform on the workpiece and machine parts. When beingmixed with water, soluble oils should be poured into wa-ter. Failure to mix this way will cause rust, contact der-matitis, and deterioration of mechanical seals.

Some emulsions are designed for greater lubricatingvalue, with animal or vegetable fats or oils providing a “su-perfatted” condition. Sulfur, chlorine, and phosphorus pro-vide even greater lubricating value for metal cuttingoperations where extreme pressures are encountered in chipforming. These fluids are mixed in somewhat rich ratios: 1part oil with 5 to 15 parts water. All these water-soluble cut-ting fluids are considered coolants even though they do pro-vide some lubrication.

Cutting Oils These fluids may be animal, vegetable, petro-leum, or marine (fatty tissue of fish and other marine animals)oil, or a combination of two or more of these. The plain cuttingoils (naphthenic and paraffinic) are considered lubricants andare useful for light-duty cutting on metals of high machin-ability, such as free-machining steels, brass, aluminum, andmagnesium. Water-based cutting fluids should never be usedfor machining magnesium because of the fire hazard. Thecutting fluid recommended for magnesium is an anhydrous(without water) oil that has a low acid content. Fine magne-sium chips can burn if ignited, and water tends to make it burneven more fiercely. A mineral–lard oil combination is oftenused in automatic screw machine practice.

Where high cutting forces are encountered and extremepressure lubrication is needed, as in threading and tapping


UNIT THREE CUTTING FLUIDS 281

Figure F-21 Special wraparound nozzle used for surfacegrinding operations.

operations, certain oils, fats, waxes, and synthetic materialsare added. The addition of animal, marine, or vegetable oilto petroleum oil improves the lubricating quality and thewetting action. Chlorine, sulfur, or phosphorus additivesprovide better lubrication at high pressures and high tem-peratures. These chlorinated or sulfurized oils are dark incolor and are commonly used for thread-cutting operations.

Cutting oils also tend to become rancid and developdisagreeable odors unless germicides are added to them.Cutting oils tend to stain metals and, if they contain sulfur,may severely stain nonferrous metals such as brass and alu-minum. In contrast, soluble oils generally do not stain work-pieces or machines unless they are trapped for long periodsof time between two surfaces, such as the base of a millingvise bolted to a machine table.

Tanks containing soluble oil on lathes and milling andgrinding machines tend to collect tramp oil and dirt orgrinding particles. For this reason, the fluid should be re-moved periodically, the tank cleaned, and a clean solutionput in the tank. Water-based cutting fluid can be contami-nated quickly when a machinist uses a pump oilcan to applycutting oil to the workpiece instead of using the coolantpump and nozzle on the machine. The tramp oil goes intothe tank and settles on the surface of the coolant, creating anoil seal, where bacteria can grow. This causes an odorousscum to form that quickly contaminates the entire tank.

Special vegetable-based lubricants have been developedthat boost machining efficiency and tool life. Only smallamounts are used, and messy coolant flooding is unneces-sary. They are used in milling, sawing, reaming, and grind-ing operations.

Gaseous Fluids Fluids such as air are sometimes used toprevent contamination on some workpieces. For example,reactive metals such as zirconium may be contaminated bywater-based cutting fluids. The atmosphere is always, to someextent, a cutting fluid and is actually a coolant when cuttingdry. Air can provide better cooling when a jet of compressedair is directed at the point of cut. Other gases such as argon,helium, and nitrogen are sometimes used to preventoxidation of the chip and workpiece in special applications.

METHODS OF CUTTING FLUIDAPPLICATION

The simplest method of applying a cutting fluid is manuallywith a pump oilcan. This method is sometimes used onsmall drill presses that have no coolant pumping system.Another simple method is to dip a small brush into an openpan of cutting oil and apply it to the workpiece, as in lathethreading operations.

Most milling and grinding machines, large drill presses,and lathes have a built-in tank or reservoir containing a cuttingfluid and a pumping system to deliver it to the cutting area.The cutting fluid used in these machines is typically a water-soluble synthetic or soluble oil mix, with exceptions, as in ma-

chining magnesium or in certain grinding applications wherespecial cutting oils or fluids are used. The cutting fluid in thetank is picked up by a low-pressure (5 to 20 psi) pump and de-livered through a tube or hose to a nozzle where the machiningor grinding operation is taking place (Figure F-20).

When this pumping system is used, the most commonmethod of application is by flooding. This is done by simplyaiming the nozzle at the work–tool area and applying copi-ous amounts of cutting fluid. This system usually worksfairly well for most turning and milling operations, but notso well for grinding, since the rapid spinning of the grindingwheel tends to blow the fluid away from the work–tool in-terface. Special wraparound nozzles (Figure F-21) are oftenused on surface grinders to ensure complete flooding andcooling of the workpiece. On cylindrical grinders, a fan-shaped nozzle the width of the wheel is normally used, but aspecially designed nozzle (Figure F-22) is better.

In lathe operations, the nozzle is usually above the work-piece pointed downward at the tool. A better method of appli-cation would be above and below the tool, as shown in FigureF-23. Internal lathe work, such as boring operations and close

Floodcoolantcontrol

Coolant tank

Coolant pump

Coolant filter

Flood coolant spout

Figure F-20 Fluid recirculates through the tank, piping, nozzle,and drains in a flood grinding system (Courtesy of DoALLCompany).



Air flow

Airbubble

Standardnozzle

Work Work

Low-velocitynozzle

High-velocitynozzle

Air scraper

Figure F-22 This specially designed nozzle helps to keep thefanlike effect of the rapidly rotating wheel from blowing cuttingfluid away from the wheel–work interface (Courtesy of MAGIndustrial Automation Systems, LLC).

chuck work, cause the coolant flow to be thrown outward bythe spinning chuck. In this case, a chip guard is needed tocontain the spray. Although a single nozzle is normally usedto direct the coolant flow over the cutter and on the work-piece on milling machines, a better method is to use two noz-zles (Figure F-24) to make sure the cutting zone is completelyflooded. Some lathe toolholders are now provided withthrough-the-tool pressurized coolant systems. Also, some end

mills for vertical milling machines have coolant channels thatdirect the fluid at the cutting edge under pressure. Not only isthis an extremely efficient way to apply coolant, it also forcesthe chips away from the cutting area.

It should be noted that intermittent application of cut-ting fluids using a squirt bottle, pump, or brush should beavoided when using carbide cutting tools. The rapid and ir-regular cooling of the carbide tools may cause thermalcracking.

When a cutting fluid is properly applied, it can have atremendous effect on finishes and surfaces. A finish may berough when machined dry but may be dramatically im-proved with lubrication. When carbide tools are used,whether on turning or milling work, and coolant is used, thework–tool area must be flooded to avoid intermittent cool-ing and heating cycles that create thermal shock and conse-quent cracking of the tool. The operator should not shut offthe coolant while the cut is in progress to “see” the cuttingoperation.

There is no good way to cool and lubricate a twist drill,especially when drilling horizontally in a lathe. In deepholes, cutting forces and temperatures increase, often caus-ing the drill to expand and bind. Even in vertical drilling ona drill press, the helical flutes, designed to lift out the chips,also pump out the cutting fluid. Some drills are made withoil holes running the length of the drill to the cutting tips tohelp offset the problem. The fluid is pumped under rela-tively high pressure through a rotating gland. The cutting oilnot only lubricates and cools the drill and workpiece, it alsoflushes out the chips. Gun drills designed to make accurateand deep holes have a similar arrangement, but the fluidpressures are as high as 1000 psi. Shields must be providedfor operator safety when using these extremely high-pressurecoolant systems.

Air-carried mist systems (Figure F-25) are popular forsome drilling and end milling operations. Usually, shopcompressed air is used to make a spray of coolant drawnfrom a small tank on or near the machine. Because only asmall amount of liquid ever reaches the cutting area, little

Figure F-23 Although not always possible, this is the ideal wayof applying cutting fluid to a cutting tool.

Figure F-24 The use of two nozzles ensures flooding of thecutting zone.

Figure F-25 Mist grinding fluid application on a surface grinder(Courtesy of DoALL Company).


UNIT THREE CUTTING FLUIDS 283

SELF-TEST

lubrication is provided. These systems are chosen for theirability to cool rather than to lubricate. This factor may re-duce tool life. However, mist cooling has many advan-tages. No splash guards, chip pans, and return hoses areneeded, and only small amounts of liquid are used. Thehigh-velocity air stream also cools the spray further byevaporation. There are safety hazards in using mist sprayequipment from the standpoint of the operator’s health.Conventional coolants are not highly toxic, but whensprayed in a fine mist they can be inhaled; this could affectcertain people over time. However, many operators findthat breathing any of the mist is uncomfortable and offen-sive. Good ventilation systems or ordinary fans can re-move the mist from the operator’s area.

COOLANT SEPARATORS

Cutting chips and abrasive materials become intermingledwith the coolant in collection tanks. Tramp oil and dirt arealso contaminants. In manufacturing operations, simplydiscarding the old coolant and using new not only becomesexpensive but is an environmental disposal problem.Therefore, many different kinds of separators and filtrationmethods are used to reclaim the old coolant.

Magnetic separation has been in use for many years foroperations such as grinding, honing, broaching, and othermachining of ferrous metals. These devices can remove asmuch as 98 percent by volume of ferrous solids larger than20 or 30 However, nonferrous solids and other con-taminants must be removed by other processes, such as fil-tration. Oil skimmers are used to remove tramp oils thatcollect in the coolant tank. This process reduces the need forfrequent coolant maintenance. Portable mobile coolant re-conditioning systems are often used in place of large perma-nent stationary systems.

Wastewater treatment is another aspect of manufactur-ing today. Parts washers and other cleaning operations con-taminate the water with oily waste. New environmental

mm.

regulations are met in wastewater treatment by a filtrationsystem followed by a biotreatment operation before beingdischarged to the sewer system.

1. What are the two basic functions of cutting fluids?

2. Name the four types of cutting fluids (liquids).

3. When soluble oil coolants become odorous or cutting oil be-comes rancid in the reservoir, what can be done to correct theproblem?

4. When a machine such as a lathe has a coolant pump and tankthat contains a soluble oil–water mix, why should a machineoperator not use a pump can containing cutting oil on aworkpiece?

5. Some of the synthetic cutting fluids tend to irritate workers’hands. What causes this?

6. What method of applying cutting fluid is often used on asmall drill press?

7. What kinds of cutting fluid accessories are provided on mostmachine tools, lathes, milling machines, drill presses, andgrinding machines?

8. Describe the most common method of cutting fluid applica-tion from a nozzle.

9. Why is a single round nozzle rather inefficient when used ona grinding wheel?

10. Spray-mist cooling systems work well for cooling purposesbut do not lubricate the work area and tool very well. Why isthis?

INTERNET REFERENCESInformation on coolant and cutting fluids:

http://www.mfg.mtu.edu


U N I T F O U R

Using Carbides and Other Tool Materials

284

maintain a keen edge and can be used for metals that pro-duce low tool–chip interface temperatures—for example,aluminum, magnesium, copper, and brass. These tools, how-ever, tend to soften at machining speeds above 50 feet perminute (fpm) in mild steels.

HIGH-SPEED STEELS

High-speed steels (HSS) may be used at higher speeds (100fpm in mild steels) without losing their hardness. The rela-tionship of cutting speeds to the approximate temperatureof the tool–chip interface is as follows:

O B J E C T I V E S


■ List six different cutting tool materials and comparesome of their machining properties.

■ Select a carbide tool for a job by reference tooperating conditions, carbide grades, nose radii, toolstyle, rake angles, shank size, and insert size, shape,and thickness.

■ Identify carbide inserts and toolholders by numbersystems developed by the American StandardsAssociation.

The cutting tool materials such as carbon steels and high-speed steel that served the needs of machining in the

past years are not suitable in many applications today.Tougher and harder tools are required to machine the tough,hard, space-age metals and new alloys. The constant demandfor higher productivity has led to the need for faster stockremoval and quick-change tooling. You, as a machinist, mustlearn to achieve maximum productivity at minimum cost.Your knowledge of cutting tools and ability to select themfor specific machining tasks will affect your productivitydirectly.

The various tool materials used in today’s machiningoperations are high-carbon steel, high-speed steel, cementedcarbides, ceramics, diamond, and cubic boron nitride(CBN).

HIGH-CARBON STEELS

High-carbon tool steels are used for hand tools such as filesand chisels, and only to a limited extent for drilling andturning tools. They are oil- or water-hardened plain carbonsteels with .9 to 1.4 percent carbon content. These tools

100 fpm 1000°F (538°C)

200 fpm 1200°F (649°C)

300 fpm 1300°F (704°C)

400 fpm 1400°F (760°C)

High-speed steel is sometimes used for lathe tools when spe-cial tool shapes are needed, especially for boring tools.However, high-speed steel is extensively used for millingcutters for both vertical and horizontal milling machines.Because milling cutters are usually used at cutting speeds of100 sfpm in steels with cutting fluid, they never reach

and so have a relatively long working life.

CEMENTED CARBIDES

A carbide, generally, is a chemical compound of carbon anda metal. The term carbide is commonly used to refer to ce-mented carbides, the cutting tools composed of tungstencarbide, titanium carbide, or tantalum carbide and cobalt invarious combinations. A typical composition of cementedcarbide is 85 to 95 percent carbides of tungsten and the re-mainder a cobalt binder for the tungsten carbide powder.

Cemented carbides are made by compressing variousmetal powders (Figure F-26) and sintering (heating to weldparticles together without melting them) the briquettes.

1000°F (538°C)


UNIT FOUR USING CARBIDES AND OTHER TOOL MATERIALS 285

Figure F-26 The two basic materials needed to produce thestraight grades of carbide tools are tungsten carbide (WC) andcobalt (CO) (Kennametal, Inc.).

Cobalt powder is used as a binder for the carbide powderused—tungsten, titanium, or tantalum carbide powder, or acombination of these. Increasing the percentage of cobaltbinder increases the toughness of the tool material and atthe same time reduces its hardness or wear resistance.Carbides have greater hardness at both high and low tem-peratures than do high-speed steel or cast alloys. At temper-atures of and higher, carbides maintain thehardness required for efficient machining. This makes possi-ble machining speeds of approximately 400 fpm in steels.The addition of tantalum increases the red hardness of atool material. Cemented carbides are extremely hard toolmaterials (above RA 90), have a high compressive strength,and resist wear and rupture.

Coated carbide inserts are often used to cut hard ordifficult-to-machine workpieces. Titanium carbide (TiC)coating offers high wear resistance at moderate cuttingspeeds and temperatures. Aluminum oxide (Al2O3) coatingresists chemical reactions and maintains its hardness at hightemperatures. Titanium nitride (TiN) coating has highresistance to crater wear and reduces friction between thetool face and the chip, thereby reducing the tendency for abuilt-up edge.

Cemented carbides are the most widely used tool mate-rials in the machining industry. They are particularly usefulfor cutting tough alloy steels, which quickly break downhigh-speed tool steels. A large percentage of machining is onalloy steels for automotive and other industrial machineparts. Various carbide grades and insert shapes are available,and the correct selection should be made for machining aparticular material.

SELECTING CARBIDE TOOLS

The following steps may be used in selecting the correctcarbide tool for a job.

1. Establish the operating conditions.2. Select the cemented carbide grade.3. Select the nose radius.4. Select the insert shape.5. Select the insert size.6. Select the insert thickness.

1400°F (760°C)

7. Select the tool style.8. Select the rake angle.9. Select the shank size.

10. Select the chip breaker.

Step 1 Establish the Operating Conditions Thetool engineer or machinist must use three variables to estab-lish metal removal rate: speed, feed, and depth of cut. Cuttingspeed has the greatest effect on tool life. A 50 percent increasein cutting speed will decrease tool life by 80 percent. A 50 per-cent increase in feed will decrease tool life by 60 percent. Thecutting edge engagement or depth of cut (Figure F-27) is lim-ited by the size and thickness of the carbide insert and thehardness of the workpiece material. Hard workpiece materialsrequire decreased feed, speed, and depth of cut.

Figure F-27 shows how the lead angle affects both cut-ting edge engagement length and chip thickness by compar-ing a style D square insert tool, using a 45-degree leadangle, to a style A triangular insert tool, using a 0-degreelead angle. The two tools are shown making an identicaldepth of cut at an identical feed rate. The feed rate is thesame as the chip thickness with the style A tool or any toolwith a 90-degree lead angle. The chip would be wider butthinner using the style D with the same feed rate. Whenlarge lead angles are used, the style D tool has a muchgreater strength than the style A tool, because the cuttingforces are directed into the solid part of the holder. Largelead angles can cause chatter to develop, however, if thesetup is not rigid.

The depth of cut is limited by the strength and thick-ness of the carbide insert, the rigidity of the machine andsetup, the horsepower of the machine, and, of course, theamount of material to be removed. An example of a rela-tively large depth of cut with an insert that produced large 9-shaped chips is shown in Figure F-28.

Edge wear and cratering are the most frequent toolbreakdowns that occur. Edge wear (Figure F-29) is simplythe breaking down of the tool relief surface caused by fric-tion and abrasion and is considered normal wear. Edgebreakdown is also caused by the tearing away of minute

Style D Style A Uncutchip thickness

Uncutchip thickness

Feed – same forboth tools

Cutting edgeengagementlength

Depth of cut – same for both toolsCutting edgeengagement length

Figure F-27 The difference in style-A and style-D holders fordepth of cut and cutting edge engagement length (GeneralElectric Company).



Figure F-28 Large, well-formed chips were produced by thistool with built-in chip breaker (Kennametal, Inc.).

carbide particles by the built-up edge (Figure F-30). Thecutting edge is usually chipped or broken in this case. Lackof rigidity, too much feed, or too slow a speed results inchipped or broken inserts (Figure F-31).

Tiny projections on the cutting edges of new inserts of-ten break down and reduce tool life. Some machinists honethe edges of new inserts to increase tool life. However, if thispractice is not carried out correctly, it can damage the tooland actually reduce tool life. For this reason, some inserttool manufacturers provide prehoned carbide tools.Machine shop students should not hone the edges of theirtools until they have gained more experience in the field.

Thermal shock, caused by sudden heating and cooling, isthe cracking and checking of a tool that leads to breakage

Figure F-29 Normal edge wear.

Figure F-30 Tool point breakdown caused by a built-up edge.

(Figure F-32). This condition is most likely to occur whenan inadequate amount of coolant is used. It is better to ma-chine dry if the work and tool cannot be kept flooded withcoolant.

If edge wear occurs, try the following:

1. Decrease the machining speed.2. Increase the feed.3. Change to a harder, more wear-resistant carbide grade.

If the cutting edge is chipped or broken, do this:

1. Increase the speed.2. Decrease the feed and/or depth of cut.3. Change to a tougher grade carbide insert.4. Use a negative rake.5. Hone the cutting edge before use.6. Check the rigidity and tool overhang.

Figure F-31 Chipped or broken inserts. The triangular insertshows the typical breakage on straight tungsten carbide inserts.The square titanium-coated insert shows a stratified failurebecause of an edge impact.



material. The addition of titanium carbide to the mixture oftungsten carbide and cobalt provides an antiweld quality butalso loss in abrasive wear and strength in these tools.

Step 2 Select the Cemented Carbide Grade Thereare two main groups of cemented carbides from which toselect most grades: first, the straight carbide grades composedof tungsten carbide and cobalt binder, which are used for castiron, nonferrous metals, and nonmetallics where resistance toedge wear is the primary factor; and second, grades composedof tungsten carbide, titanium carbide, and tantalum carbideplus cobalt binder, which are usually used for machiningsteels. Resistance to cratering and deformation is the majorrequirement for these steel grades.

Cemented carbides have been organized into grades.Properties that determine grade include hardness, tough-ness, and resistance to chip welding or cratering. Theproperties of carbide tools may be varied by the percent-ages of cobalt and titanium or tantalum carbides.Increasing the cobalt content increases toughness but de-creases hardness. Properties may also be varied during theprocessing by the grain size of carbides, density, and othermodifications. Some tungsten carbide inserts are given atitanium carbide coating (about .0003 in. thick) to resistcratering and edge breakdown. Tantalum carbide is addedto sintered carbide principally to improve hardness char-acteristics. This increases the composition’s resistance todeformation at cutting temperatures.

The grades of carbides have been organized accordingto their suitable uses by the Cemented Carbide ProducersAssociation (CCPA). It is recommended that carbides be se-lected by using such a table rather than by their composi-tion. Cemented carbide grades with specific chip removalapplications are as follows:

The hardest of the nonferrous/cast iron grades is C-4, andthe hardest of the steel grades is C-8.

This system does not specify the particular materials oralloy, and the particular machining operations are not speci-fied. For example, for turning a chromium–molybdenumsteel, factors to be considered would include the difficulty ofmachining such an alloy because of its toughness. Given thisexample, a grade of C-5 or C-6 carbide would probably bebest suited to this operation. The proof of the selectionwould come only with the actual machining. Cemented car-bide tool manufacturers often supply catalogs designating

C-1 Roughing cuts (cast iron and nonferrous materials)

C-2 General purpose (cast iron and nonferrous materials)

C-3 Light finishing (cast iron and nonferrous materials)

C-4 Precision boring (cast iron and nonferrous materials)

C-5 Roughing cuts (steel)

C-6 General purpose (steel)

C-7 Finishing cuts (steel)

C-8 Precision boring (steel)

Mechanical Excess wear

Breakage

Thermal shock

Figure F-32 Three causes of tool breakage (Kennametal, Inc.).

Figure F-33 Cratering on a carbide tool.

For a buildup on the cutting edge:

1. Increase the speed.2. Change to a positive-rake tool.3. Change to a grade containing titanium.

For cutting edge notching (flank wear):

1. Increase the side cutting edge angle.2. Decrease the feed.

Cratering (Figure F-33) is the result of high tempera-tures and pressures that cause the steel chip to weld itself tothe tungsten carbide and tear out small particles of the tool



Table F-5 Grade and Machining Applications

Grade Hardness RA Typical Machining Applications

C06 Ceramica The hardest of this group; for finishing most ferrous and nonferrous alloys and nonmetals asin high-speed, light-chip-load precision machining, or for use at moderate speeds and chiploads where long tool life is desired

K165 93.5 Titanium carbide for finishing steels and cast irons at high to moderate speeds and light chip loads

K7H 93.5 For finishing steels at higher speeds and moderate chip loadsK5H 93.0 For finishing and light roughing steels at moderate speeds and chip loads through light

interruptions

K45 92.5 The hardest of this group. General-purpose grade for light roughing to semifinishing of steelsat moderate speeds and chip loads and for many low-speed, light-chip-load applications

K4H 92.0 For light roughing to semifinishing of steels at moderate speeds and chip loads and for formtools and tools that must dwell

K25 91.5 For light to moderate roughing of steels at moderate speeds and feeds through mediuminterruptions

KC75 For general-purpose use in machining of steels over a wide range of speeds in moderateroughing to semifinishing applications

K21 91.0 For moderate to heavy roughing of steels at moderate speeds and heavy chip loads throughmedium interruptions where mechanical and thermal shock are encountered

K42 91.3 For heavy roughing of steels at low to moderate speeds and heavy chip loads throughinterruptions where mechanical and severe thermal shocks are encountered

K11 93.0 The hardest of this group; for precision finishing of cast irons, nonferrous alloys, nonmetalsat high speeds and light chip loads, and for finishing many hard steels at low speeds andlight chip loads

K68 92.6 General-purpose grade for light roughing to finishing of most high-temperature alloys,refractory metals, cast irons, nonferrous alloys, and nonmetals at moderate speeds and chiploads through light interruptions

K6 92.0 For moderate roughing of most high-temperature alloys, cast irons, nonferrous alloys, andnonmetals at moderate to low speeds and moderate to heavy chip loads through lightinterruptions

K1 90.0 The most shock resistant of this group; for heavy roughing of most high-temperature alloys,cast irons, and nonferrous alloys at low speeds and heavy chip loads through heavyinterruptions

a The hardness of C06 is 91 Rockwell 45N (or about 94RA).

uses and machining characteristics of their various grades(see Table F-5).

Grade classification–comparison tables that converteach manufacturer’s carbide designations to CCPA “C”numbers are available (see Table F-6). There is, however, onemajor caution. The tables are intended to correlate gradeson the basis of composition, not according to tested per-formance. Grades from different manufacturers with thesame “C” number may vary in performance. General guide-lines to grade selection are as follows:

1. Select the grade with the highest hardness with suffi-cient strength to prevent breakage.

2. Select straight grades of tungsten carbide for the high-est resistance to abrasion.

Step 3 Select the Nose Radius Selecting the noseradius can be important because of tool strength, surfacefinish, or, perhaps, the forming of a fillet or radius on thework. To determine the nose radius according to strength

requirements, use the nomograph in Figure F-34. Considerthat the feed rate, depth of cut, and workpiece conditiondetermine strength requirements, since a larger nose radiusmakes a strong tool.

Large radii are strongest and can produce the best fin-ishes, but they also can cause chatter between tool and work-piece. For example, the dashed line on the figure indicatesthat a -in. radius would be required for turning with a feed rate of .015 in., and to obtain a - finish, a -in. radiuswould be required with a .020-in. feed rate.

Step 4 Select the Insert Shape Indexable inserts(Figure F-35), also called throwaway inserts, are clamped intoolholders of various design. These inserts provide a cut-ting tool with several cutting edges. After all edges have beenused, the insert is discarded.

The round inserts have the greatest strength and, aswith large-radius inserts, make possible higher feed rateswith equal finishes. Round inserts also have the greatestnumber of cutting edges possible but are limited to

14min.100

18

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289

Tab

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arbi

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rade

Cla

ssifi

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on-C

ompa

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wit

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CPA

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actu

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Figure F-34 Surface finish versus nose radius (General ElectricCompany).

Figure F-35 Insert shapes for various applications.

workpiece configurations and operations not affected by alarge radius. Round inserts would be ideally suited, for ex-ample, to straight turning operations.

Square inserts have lower strength and fewer possiblecutting edges than round tools but are much stronger thantriangular inserts. The included angle between cutting edges(90 degrees) is greater than that for triangular inserts (60 de-grees), and there are eight cutting edges possible, comparedwith six for the triangular inserts.

Triangular inserts have the greatest versatility. They canbe used, for example, for combination turning and facingoperations, whereas round or square inserts are often notadaptable to such combinations. Because the includedangle between cutting edges is less than 90 degrees, the tri-

angular inserts are also capable of tracing operations. Thedisadvantages include their reduced strength and fewer cut-ting edges per insert.

For tracing operations where triangular inserts can-not be applied, diamond-shaped inserts with smaller in-cluded angles between edges are available. The includedangles on these diamond-shaped inserts range from 35 to80 degrees. The smaller-angle inserts in particular may beplunged into the workpiece as required for tracing. A typ-ical setup for tracing is shown in Figure F-36. Note thatsmall clearance angles are used for each cutting edge topermit plunging.

Step 5 Select the Insert Size The insert selectedshould be the smallest insert capable of sustaining therequired depth of cut and feed rate. The depth of cut (DOC)should always be as great as possible. A rule of thumb is toselect an insert with cutting edges times the length of cut-ting edge engagement. The feed for roughing mild steelshould be approximately the depth of cut. Because theDOC has the least effect on tool life, a factor of 10 times thefeed rate will not cause more appreciable tool wear.

Step 6 Select the Insert Thickness Insert thickness isalso important to tool strength. The required depth of cutand feed rate are criteria that determine insert thickness. Thenomograph in Figure F-37 simplifies the relationship ofdepth of cut and feed rate to insert thickness. Lead angle isalso important in converting the depth of cut to a length ofcutting edge engagement.

The dashed line on the nomograph in Figure F-37 rep-resents an operation with a -in. length of engagement (notdepth of cut) and a feed rate of .020 ipr. Depending on the

34

110

112

Insertincludedangle

Toolholdernegative side cuttingedge angle

Workpiece

centerline

(maximum allowableplunge angle)

(clearanceangle)38º

50º

2º

35º3º

Figure F-36 A 38-degree triangular insert used for a tracingoperation (General Electric Company).



grade of carbide used (tough or hard), an insert of - or -in.thickness should be used.

Step 7 Select the Tool Style Tool style pertains to theconfiguration of toolholder for a carbide insert. To deter-mine style requires familiarity with the particular machinetool and the operations to be performed. Figure F-38 showssome styles available for toolholders.

Step 8 Select the Rake Angle When selecting the rakeangles, consider the machining conditions. Negative rakeshould be used where there is maximum rigidity of the tooland work and where high machining speeds can be main-tained. More horsepower is needed when using negative-rake tools. Under these conditions, negative-rake tools arestronger and produce satisfactory results (Figure F-39).

38

14

Negative-rake inserts may also be used on both sides,doubling the number of cutting edges per insert. This is pos-sible because end and side relief are provided by the angle ofthe toolholder rather than by the shape of the insert.

Positive-rake inserts should be used where rigidity ofthe tool and work is reduced and where high cutting speedsare not possible, for example, on a flexible shaft of smallerdiameter. Positive-rake tools cut with less force, so deflectionof the work and toolholder would be reduced. High cuttingspeeds (sfpm) are often not possible on small diameters be-cause of limitations in spindle speeds.

Step 9 Select the Shank Size As with insert thicknessand rake angles, the rate of feed and depth of cut are impor-tant in determining shank size. Overhang of the tool shank isextremely important for the same reason. Heavy cuts at highfeed rates create high downward forces on the tool. Thesedownward forces acting on a tool with excessive overhangwould cause tool deflection that would make it difficult orimpossible to maintain accuracy or surface finish quality.

Having established the feed rate, depth of cut, and tooloverhang, use the nomograph in Figure F-40 to determine the

Figure F-37 Insert thickness as determined by length of cuttingedge engagement and feed rate (General Electric Company).

Figure F-38 Several of the many tool styles available(Kennametal, Inc.).

(a) Positive (b) Neutral (c) Negative

Figure F-39 Side view of back rake angles.

Figure F-40 Determining shank size according to depth of cut,feed rate, and tool overhang (General Electric Company).



shank size. Begin by drawing a line from the depth of cut scaleto the feed rate scale. From the point where this line intersectsthe vertical construction line, draw a line to the correct pointon the tool overhang scale. Determine the shank size fromwhere this last line crosses through the shank size scale.

In the example shown on the graph, a line has beendrawn from the -in. depth of cut point to the .015 ipr feedrate point. Another line has been drawn from the point ofintersection on the vertical construction line to the amountof tool overhang. The second line drawn passes through theshank size scale at the -in.2 (cross-sectional area) point.Toolholders in. or in. would meet the require-ments. Throwaway carbide inserts are also used for boringbars (Figure F-41) of various shapes and sizes. Brazed car-bide tips are sometimes put on the end of a boring bar forspecial applications.

Step 10 Select the Chip Breaker Chip control is animportant aspect of machining operations, especially on thelathe. Cutting tools on the lathe that have no chip breakerstend to produce long, stringy, or coiled chips, which,because they are sharp on the edges and can cause severecuts, are a hazard to the operator. The ideal shape forchips is shown in Figure F-28, the compact 9-shaped chip.Such chip formation is a necessity in manufacturing oper-ations where the chips are removed by a conveyor. Mostcemented carbide inserts are provided with a chip breaker,whose object is to form the desirable chip shape (Figure F-42). However, other factors, such as feed rate, depth of cut,and workpiece material, affect the chip curl and its finalshape. A higher feed rate and deeper cut tend to curl the chipmore, whereas a light cut and small feed rate will often pro-duce a long stringy chip even if the tool has a chip breaker. Atough or hard workpiece will curl the chip more than a softmaterial. These factors are taken into account in Figure F-43and in Table F-7. These ranges for chip breaker applicationare for Kennametal inserts. However, every tool manufac-turer provides its own special shapes and specifications foruse with various materials and applications. Catalogs con-taining tool specifications are generally available from tooldistributors.

58 * 11

2 * 1

34

18

Figure F-41 A boring bar with various interchangeableadjustable heads (Kennametal, Inc.).

Figure F-42 Chip breakers used are the adjustable chip deflator(center) with a straight insert and the type with the built-in chipcontrol groove.

T Triangular insert shape

A 0-degree side cutting edge angle

N Negative rake

R Right-hand turning (from left to right)

8 Square shank, in. per side816

T Triangular shape

N 0-degree relief (relief provided by holder)

M Plus or minus .005-in. tolerance onthickness

G With hole and chip breaker

TOOLHOLDER IDENTIFICATION

The carbide manufacturers and the American StandardsAssociation (ASA) have adopted a system of identifying tool-holders for inserted carbides. This system is used to call outthe toolholder geometry and for ordering tools from manu-facturers or distributors. The system is shown in Figure F-44.

As an example, use this system to determine the geome-try of a toolholder called out as TANR-8:

CARBIDE INSERT IDENTIFICATION

As with toolholders, a system has been adopted by the car-bide manufacturers and the American Standards Associationfor identifying inserts (see Figure F-45).

For example, use this system to determine the specifica-tions for an insert called out as TNMG-323E:



0.050

MM

0.040

0.030

0.020

0.010

0.005

0.045

(c)Depth of cut (D.O.C.)

Fee

d ra

te (

ipr)

MM-M

*

0.500

MG

0.040

0.030

0.020

0.010

0.0050.500

0.015 0.125

(a)

Depth of cut (D.O.C.)

Fee

d ra

te (

ipr)

MG-K

*

Figure F-43a–c (a) Negative-rake two-sided Kenloc inserts; (b) negative/positive two-sided Kenloc inserts; (c) negative-rake one-sidedKenloc inserts.

*Maximum D.O.C. and feed rates (ipr) are limited by the insert thickness and cutting edge length. Application ranges are for AISI 1045steel at 180 to 220 BHN (Kennametal, Inc.).

0.500

MP

0.040

0.030

0.020

0.010

0.005

0.015 0.100

(b)Depth of cut (D.O.C.)

Fee

d ra

te (

ipr)

MP-K

*

seen in the tool. Avoid the following to prevent crazing whengrinding these tools:

1. Avoid the use of aluminum oxide wheels (except torough grind the steel shank for clearance). Use only sil-icon carbide wheels with a soft bond.

2. Avoid excessive grinding pressure in a small area.3. Avoid the poor cutting action of a low-concentration

diamond wheel.4. Avoid dry grinding.

Chip breakers may be ground on the edge of brazed carbidetools with diamond wheels on a surface grinder.

3 inscribed circle (inside square andtriangular insert)

38-in.

2 thickness216-in.

3 radius364-in.

E Unground honed

Brazed carbide tools have been used for many years andare still used on many machining jobs. These tools can besharpened by grinding many times, whereas the insert tool isthrown away after its cutting edges are dull. Grinding thetool often causes thermal shock by the sudden heating of thecarbide surface. This is evident when “crazing” or severaltiny checks appear on the edge or when a large crack can be



The side and end clearance angles are not to be con-fused with relief angles on carbide tools (Figure F-46).Clearance refers to the increased angle ground on the shankof a carbide-tipped tool. Clearance provides for a narrowflank on the carbide and for regrinding of the carbide with-out contacting steel.

Because heavy forces and high speeds are involved inlathe work when using carbides, safety considerations areessential. Chip forms such as the ideal 9- or C-shape areconvenient to handle, but shields, chip deflectors, orguards are required to direct chips away from workers inthe shop.

Make sure that the setup is secure, that centers are largeenough to support the work, and that work cannot slip inthe chuck. If a long, slender work is machined at high speedsand there is a possibility that it could be thrown out, guardsshould be provided.

Some milling cutters (Figure F-47), like lathe tools, aremade to use indexable throwaway inserts. Carbide cuttersused on vertical and horizontal milling machines can in-crease stock removal and production for certain applica-tions. The same considerations for use with certainworkpiece materials are given to the selection of insert car-bides as with lathe tools.

CERAMIC TOOLS

Ceramic or “cemented oxide” tools (Figure F-48) are madeprimarily from aluminum oxide. Some manufacturers addtitanium, magnesium, or chromium oxides in quantities of

10 percent or less. The tool materials are molded at pressuresover 4000 psi and sintered at temperatures of approximately3000 F (1649 C). This process partly accounts for the highdensity and hardness of cemented oxide tools.

°°

Table F-7 Guide to Effective Application of Kenloc Chip-Control Inserts

KenlocSeries Effective Rake

Number of Cutting Surfaces Application

MP-K Positive Two-sided For chip control when depths of cut are .100 or less and/or fine feed rates are required.+5° Nose radius (in.) Feed rate rangea (ipn)

.015 .003–.010

.031 .005–.015

.047 .005–.015

.062 .008–.020MG-K Negative

-5°

Two-sided For chip control in light to moderate cuts. The effective feed range extends from .005 to .025 ipr,depending on size of insert and nose radius. Excellent for work-hardening materials, using lightfeeds and light depths of cut.

MP Positive +5°

Two-sideda Used to minimize forces and/or workpiece deflection. Also, excellent for machining work-hardeningmaterials. Minimum depth of cut for effective chip control is .060 in.a

MG Negative -5°

Two-sided General-purpose machining with wide range of effective chip control. The feed range extends froma low of .010–.015 to a high of .030–.050 ipr, depending on size of insert and nose radius.a

MM-M Negative -5°

One-sided For maximum metal removal rates and additional strength in interrupted cutting. Effective positivecutting action reduces cutting forces and extends tool life.

MS High positive +15°

One-sided A supplemental geometry to minimize forces and workpiece deflection in light finishing cuts wherehorsepower is limited.

MM Negative - 5°

One-sided Supplemental geometry for chip control in applications that demand heavy depth of cut with light tomoderate feed rates.

aParameters based on AISI 1045 steel 180-220 BHN; feed rates and depth of cut will vary for harder and higher strength materials.

SOURCE: Courtesy of Kennametal, Inc.

N E W T E C H N O L O G Y

Recent innovations in surface-engineering technologygive common tool materials harder, more wear-resistantsurfaces. A titanium carbonitride layer is a surface appli-cation used to produce high-strength carbide cuttingtools. High-speed drills are coated with titanium nitrideby a sputter ion plating process, which gives themgreater wear resistance.

Diamond and cubic boron nitride (CBN) are both de-posited on tools. CBN deposited in this process is consid-ered as good as or better than polycrystalline diamonddeposited on tools.

Multilayering on carbide tools is a method used togive good wear resistance and toughness. Layers includealuminum oxide, titanium carbide, and carbonitride on acobalt-rich surface. These tools are excellent for machin-ing stainless steels, steel castings, and forgings.

A new process of producing a ceramic cutting in-sert uses the material TiB2 (titanium diboride). It is de-signed for milling or turning both ferrous and nonferrousmetals, hardened steels, and high-temperature alloys. Itis said to last five or six times longer than tungsten car-bide inserts.



Figu

re F

-44

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7° Relief10° Clearance

Figure F-46 Relief and clearance compared.

Figure F-47 This milling cutter uses carbide inserts that can beindexed. When one edge is dull, inserts are turned to a newsharp edge.

Figure F-48 Ceramic tool with carbide seat and chip deflectorshown assembled in toolholder behind the parts(Kennametal, Inc.).

Figure F-49 Turning at 725 sfm .010-in. stock is removed oneach of two passes at a rate of ipm (.0023 ipr) along the 29-in.length of the casting. A coolant is not required for this turning(Courtesy of Sandvik Coromant).

512

Cemented oxides are brittle and require that machinesand setups be rigid and free of vibration. Some machinescannot attain the spindle speeds required to use cementedoxides at their peak capacity in terms of sfpm. When practi-cable, however, cemented oxides can be used to machine rel-atively hard materials at high speeds.

Ceramic tools are either cold pressed or hot pressed,that is, formed to shape. Some hot-pressed, high-strengthceramics are termed cermets because they are a combinationof ceramics and metals. They possess the high shear

resistance of ceramics and the toughness and thermal shockresistance of metals. Cermets were originally designed foruse at high temperatures such as those found in jet engines.These materials were subsequently adapted for cutting tools.An insert tool composed of titanium carbide and titaniumnitride is an example. In machining of steels, these toolshave a low coefficient of friction and therefore less tendencyto form a built-up edge. They are used at high cuttingspeeds. A common combination of materials in a cermet isaluminum oxide and titanium carbide. Another method ofobtaining the high-speed cutting characteristics of ceramicsand the toughness of metal tools is to coat a carbide toolwith a ceramic composite. Ceramic tools should be used as areplacement for carbide tools that are wearing rapidly butnot to replace carbide tools that are breaking.

CUBIC BORON NITRIDE TOOLS

CBN is next to diamond in hardness and therefore can beused to machine plain carbon steels, alloy steels, and graycast irons with hardnesses of 45 Rc and above (Figure F-49).Formerly, steels over 60 Rc would have to be abrasive ma-chined, but with the use of CBN they can often be cut withsingle-point tools.

CBN inserts consist of a cemented carbide substratewith an outside layer of CBN formed as an integral part ofthe tool. Tool life, finishes, and resistance to cracking andabrasion make CBN a superior tool material to both car-bides and ceramics.



cobalt or nickel. Ferrous alloys chemically attack single- orpolycrystalline diamonds, causing rapid tool wear. Becauseof the high cost of diamond tool material, it is usually re-stricted to those applications where other tool materials cutpoorly or break down quickly.

Diamond or ceramic tools should never be used for in-terrupted cuts such as on splines or keyseats because theycould chip or break. They must never be used at low speedsor on machines not capable of attaining the higher speeds atwhich these tools should operate.

Each of the cutting tool materials varies in hardness.The differences in hardness tend to become more pro-nounced at high temperatures, as can be seen in Table F-8.Hardness is related to the wear resistance of a tool and itsability to machine materials that are softer than it is.Temperature change is important, because the increase oftemperature during machining results in the softening ofthe tool material.

Table F-8 Hardness of Cutting Materials at High and LowTemperatures

Tool MaterialHardness at RoomTemperature

Hardness at1400°F (760°C)

High-speed steel RA 85 RA 60Carbide RA 92 RA 82

Cemented oxide(ceramic)

RA 93-94 RA 84

Cubic boron nitride Near diamond hardnessDiamond Hardest known

substance

Figure F-52 To make abrasive fused silica tubes absolutelyround where they fit into tundish valves, FloCon turns the tubeswith a sintered diamond tool insert. After limited experience, theMegadiamond sintered diamond inserts appear to give about200 times as much wear as carbides in the same application(Courtesy Sii Megadiamond, Inc.).

DIAMOND TOOLS

Industrial diamonds are sometimes used to machine ex-tremely hard workpieces. Only relatively small removal ratesare possible with diamond tools (Figure F-50), but high speedsare used and good finishes are obtained. Nonferrous metals areturned at 2,000 to 2,500 fpm, for example. Sintered polycrys-talline diamond tools (Figure F-51), available in shapes similarto those of ceramic tools, are used for materials (Figure F-52)that are abrasive and difficult to machine. Polycrystalline toolsconsist of a layer of randomly oriented synthetic diamondcrystals brazed to a tungsten carbide insert.

Diamond tools are particularly effective for cuttingabrasive materials that quickly wear out other tool materials.Nonferrous metals, plastics, and some nonmetallic materialsare often cut with diamond tools. Diamond is not particu-larly effective on carbon steels or superalloys that contain

Figure F-50 Diamond tools are mounted in round or squareshank holders as replaceable inserts.

Figure F-51 Sintered diamond inserts are clamped intoolholders (Courtesy Sii Megadiamond, Inc.).



1. List the major materials used in “straight” cemented carbides.

2. What effect does increasing the cobalt content have on ce-mented carbides?

3. How can you identify normal wear on a carbide tool?

4. Is chip thickness the same as feed on a style-A tool?

5. Is the cutting edge engagement length the same as depth ofcut on style-B tools?

6. What effect does the addition of titanium carbides have ontool performance?

7. What effect does the addition of tantalum carbides have ontool performance?

8. What change in tool geometry can make possible an increased rate of feed with equal surface finish quality inturning operations?

9. How does increasing the nose radius affect tool strength?

10. What is the hazard in using too large a nose radius?

11. With respect to carbide turning tools, how is clearance differ-ent from relief?

12. What are ceramic tools made of?

13. When should ceramic tools be used?

SELF-TEST

14. According to the CCPA chart, what designation of carbidewould be used for finish-turning aluminum?

15. According to the CCPA chart, what designation of carbide isused for roughing cuts on steel?

16. According to the grade classification chart, what Carboloydesignation of carbide would be used for rough cuts in castiron?

17. According to the grade classification chart, what number onthe CCPA table would a Kennametal K5H have?

18. Extremely hard or abrasive materials are machined with dia-mond tools. Is the material removal rate high? What kind offinishes are produced?

19. Polycrystalline diamond tools are similar to ceramic inserts.What is their major advantage?

20. When sharpening brazed carbide tools should you use an alu-minum oxide wheel or a silicon carbide wheel?

INTERNET REFERENCESInformation on cutting tool materials:

http://kennametal.com

http://www.cutting-tool.americanmachinist.com



Documents

M06 KIBB5087 09 SE C06 - The Eye · Generally, a trade-off between maximum production and tool breakdown or loss is accepted in machining operations. Several factors cause or reduce