chap_1

Cutting

Tool

Applications

Cutting Tool Applications

By George Schneider, Jr. CMfgE

2 Tooling & Production/Chapter 1 www.toolingandproduction.com

1.1 IntroductionMany types of tool materials, ranging from high carbon steel to ceramics and dia-monds, are used as cutting tools in today’s metalworking industry. It is importantto be aware that differences do exist among tool materials, what these differencesare, and the correct application for each type of material.

The various tool manufacturers assign many names and numbers to their prod-ucts. While many of these names and numbers may appear to be similar, the appli-cations of these tool materials may be entirely different. In most cases the tool man-ufacturers will provide tools made of the proper material for each given application.In some particular applications, a premium or higher priced material will be justi-fied.

This does not mean that the most expensive tool is always the best tool. Cuttingtool users cannot afford to ignore the constant changes and advancements that arebeing made in the field of tool material technology. When a tool change is neededor anticipated, a performance comparison should be made before selecting the toolfor the job. The optimum tool is not necessarily the least expensive or the mostexpensive, and it is not always the same tool that was used for the job last time.The best tool is the one that has been carefully chosen to get the job done quickly,efficiently and economically.

Author’s NoteI wish to express my sincere appreciation to Prentice Hall and to Stephen Helba

in particular, for giving me permission to use some of the information, graphs andphotos recently published in Applied Manufacturing Process Planning authored byDonald H. Nelson and George Schneider, Jr.

The author also wishes to thank over 40 companies who have provided technicalinformation and photo exhibits...their contributions have made this reference textpossible.

And finally, I would like to express my appreciation to Tooling & Production’sStan Modic and Joe McKenna for giving me the opportunity to make this informa-tion available to the general public.

George Schneider, Jr.

Chapter 1

Cutting-Tool

Materials

George Schneider, Jr. CMfgEProfessor Emeritus

Engineering TechnologyLawrence Technological University

Former ChairmanDetroit Chapter ONE Society of Manufacturing Engineers

Former PresidentInternational Excutive BoardSociety of Carbide & Tool Engineers

Lawrence Tech. Univ.: www.ltu.edu

Prentice Hall: www.prenhall.com

Upcoming Chapters

Metal RemovalCutting-Tool Materials

Metal Removal MethodsMachinability of Metals

Single Point MachiningTurning Tools and Operations

Turning Methods and MachinesGrooving and Threading

Shaping and Planing

Hole Making ProcessesDrills and Drilling Operations

Drilling Methods and MachinesBoring Operations and Machines

Reaming and Tapping

Multi Point MachiningMilling Cutters and OperationsMilling Methods and Machines

Broaches and BroachingSaws and Sawing

Abrasive ProcessesGrinding Wheels and OperationsGrinding Methods and Machines

Lapping and Honing

A cutting tool must have the follow-ing characteristics in order to producegood quality and economical parts:

Hardness: Hardness and strength ofthe cutting tool must be maintained atelevated temperatures also called HotHardness

Toughness: Toughness of cuttingtools is needed so that tools don’t chipor fracture, especially during interrupt-ed cutting operations.

Wear Resistance: Wear resistancemeans the attainment of acceptable toollife before tools need to be replaced.

The materials from which cutting

tools are made are all characteristicallyhard and strong. There is a wide rangeof tool materials available for machin-ing operations, and the general classifi-cation and use of these materials are ofinterest here.

1.2 Tool Steels and Cast AlloysPlain carbon tool steel is the oldest ofthe tool materials dating back hundredsof years. In simple terms it is a highcarbon steel (steel which contains about1.05% carbon). This high carbon con-tent allows the steel to be hardened,offering greater resistance to abrasivewear. Plain high carbon steel served itspurpose well for many years. However,because it is quickly over tempered(softened) at relatively low cutting tem-peratures, (300 to 500 degrees F), it isnow rarely used as cutting tool materialexcept in files, saw blades, chisels, etc.The use of plain high carbon steel islimited to low heat applications.

High Speed Tool Steel: The need fortool materials which could withstandincreased cutting speeds and tempera-

Chap. 1: Cutting-Tool Materials

www.toolingandproduction.com Chapter 1/Tooling & Production 3

tures, led to the development of highspeed tool steels (HSS). The major dif-ference between high speed tool steeland plain high carbon steel is the addi-tion of alloying elements to harden andstrengthen the steel and make it moreresistant to heat (hot hardness).

Some of the most commonly usedalloying elements are: manganese,chromium, tungsten, vanadium, molyb-denum, cobalt, and niobium (columbi-um). While each of these elements willadd certain specific desirable character-istics, it can be generally stated that theyadd deep hardening capability, high hot

hardness, resistance to abrasive wear,and strength, to high speed tool steel.These characteristics allow relativelyhigher machining speeds and improvedperformance over plain high carbonsteel.

The most common high speed steelsused primarily as cutting tools are divid-ed into the M and T series. The M seriesrepresents tool steels of the molybde-num type and the T series representsthose of the tungsten type. Althoughthere seems to be a great deal of simi-larity among these high speed steels,each one serves a specific purpose andoffers significant benefits in its specialapplication.

An important point to remember isthat none of the alloying elements foreither series of high speed tool steels isin abundant supply and the cost of theseelements is skyrocketing. In addition,U.S. manufacturers must rely on foreigncountries for supply of these veryimportant elements.

Some of the high speed steels arenow available in a powdered metal

(PM) form. The difference betweenpowdered and conventional metals is inthe method by which they are made.The majority of conventional highspeed steel is poured into an ingot andthen, either hot or cold, worked to thedesired shape. Powdered metal isexactly as its name indicates. Basicallythe same elements that are used in con-ventional high speed steel are preparedin a very fine powdered form. Thesepowdered elements are carefully blend-ed together, pressed into a die underextremely high pressure, and then sin-tered in an atmospherically controlledfurnace. The PM method of manufac-turing cutting tools is explained inSection 1.3.1 Manufacture of CarbideProducts.

HSS Surface Treatment: Many sur-face treatments have been developed inan attempt to extend tool life, reducepower consumption, and to controlother factors which affect operatingconditions and costs. Some of thesetreatments have been used for manyyears and have proven to have somevalue. For example, the black oxidecoatings which commonly appear ondrills and taps are of value as a deterrentto build-up on the tool. The black oxideis basically a ‘dirty’ surface which dis-courages the build-up of work material.

One of the more recent developmentsin coatings for high speed steel is titani-um nitride by the physical vapor deposi-tion (PVD) method. Titanium nitride isdeposited on the tool surface in one ofseveral different types of furnace at rel-atively low temperature, which does notsignificantly affect the heat treatment(hardness) of the tool being coated.This coating is known to extend the lifeof a cutting tool significantly or to allowthe tool to be used at higher operatingspeeds. Tool life can be extended by asmuch as three times, or operatingspeeds can be increased up to fifty per-cent.

Cast Alloys: The alloying elementsin high speed steel, principally cobalt,chromium and tungsten, improve thecutting properties sufficiently, that met-allurgical researchers developed the castalloys, a family of these materials with-out iron.

A typical composition for this class oftool material was 45 percent cobalt, 32percent chromium, 21 percent tungsten,and 2 percent carbon. The purpose ofsuch alloying was to obtain a cuttingtool with hot hardness superior to high

Ceramics

Cast alloys

2000 400 600 800 1000 1200 1400

60

55

65

70

75

80

25

30

35

40

45

50

55

60

65

70

20

85

90

95100 300

Carbides

Carbontool

steels High-speedsteels

500 700

Temperature (˚F)

Temperature (C˚)

Har

dnes

s (H

-Ra)

Har

dnes

s (H

-Rc)

(a)

Diamond, CBN

Aluminum oxide (HIP)Silicon nitride

CermetsCoated carbides

Carbides

Strength and toughness

(b)

HSS

Hot

har

dnes

s an

d w

ear

resi

stan

ce

Figure 1.1. (a) Hardness of various cutting-tool materials as a function of temperature. (b)Ranges of properties of various groups of materials.


4 Tooling & Production/Chapter 1 2001 www.toolingandproduction.com

speed steel.When applying cast alloy tools, their

brittleness should be kept in mind andsufficient support should be provided atall times. Cast alloys provide high abra-sion resistance and are thus useful forcutting scaly materials or those withhard inclusions.

1.3 Cemented Tungsten CarbideTungsten carbide was discovered byHenri Moissan in 1893 during a searchfor a method of making artificial dia-monds. Charging sugar and tungstenoxide, he melted tungsten sub-carbidein an arc furnace. The carbonized sugarreduced the oxide and carburized thetungsten. Moissan recorded that thetungsten carbide was extremely hard,approaching the hardness of diamondand exceeding that of sapphire. It wasmore than 16 times as heavy as water.The material proved to be extremelybrittle and seriously limited its industri-al use.

Commercial tungsten carbide with 6percent cobalt binder was first producedand marketed in Germany in 1926.Production of the same carbide began inthe United States in 1928 and in Canadain 1930.

At this time, hard carbides consistedof the basic tungsten carbide systemwith cobalt binders. These carbidesexhibited superior performance in themachining of cast iron, nonferrous, andnon metallic materials, but were disap-pointing when used for the machiningof steel.

Most of the subsequent developmentsin the hard carbides have been modifi-

cations of the original patents, princi-pally involving replacement of part orall of the tungsten carbide with othercarbides, especially titanium carbideand/or tantalum carbide. This led to thedevelopment of the modern multi-car-bide cutting tool materials permittingthe high speed machining of steel.

A new phenomenon was introducedwith the development of the cementedcarbides, again making higher speedspossible. Previous cutting tool materi-als, products of molten metallurgy,depended largely upon heat treatmentfor their properties and these propertiescould, in turn, be destroyed by furtherheat treatment. At high speeds, andconsequently high temperatures, theseproducts of molten metallurgy failed.

A different set of conditions existswith the cemented carbides. The hard-ness of the carbide is greater than that ofmost other tool materials at room tem-perature and it has the ability to retain ithardness at elevated temperatures to agreater degree, so that greater speedscan be adequately supported.

1.3.1 Manufacture of Carbide Products

The term “tungsten carbide” describes acomprehensive family of hard carbidecompositions used for metal cuttingtools, dies of various types, and wearparts. In general, these materials arecomposed of the carbides of tungsten,titanium, tantalum or some combinationof these, sintered or cemented in amatrix binder, usually cobalt.

Blending: The first operation afterreduction of the tungsten compounds totungsten metal powder is the milling oftungsten and carbon prior to the carbur-izing operation. Here, 94 parts byweight of tungsten and 6 parts byweight of carbon, usually added in theform of lamp black, are blended togeth-er in a rotating mixer or ball mill. Thisoperation must be performed undercarefully controlled conditions in orderto insure optimum dispersion of the car-bon in the tungsten. Carbide BlendingEquipment, better known as a Ball Mill,is shown in Figure 1.2.

In order to provide the necessarystrength, a binding agent, usually cobalt(Co) is added to the tungsten (WC) inpowder form and these two are ballmilled together for a period of severaldays, to form a very intimate mixture.Careful control of conditions, including

time, must be exercised to obtain a uni-form, homogeneous product. BlendedTungsten Carbide Powder is shown inFigure 1.3.

Compacting: The most commoncompacting method for grade powdersinvolves the use of a die, made to theshape of the eventual product desired.The size of the die must be greater thanthe final product size to allow fordimensional shrinkage which takesplace in the final sintering operation.These dies are expensive, and usuallymade with tungsten carbide liners.Therefore sufficient number of the finalproduct (compacts) are required, to jus-tify the expense involved in manufac-turing a special die. Carbide

Figure 1.2. Carbide blending equipment,better known as ball mill is used to ensureoptimum dispersion of the carbon withinthe tungsten. (Courtesy American NationalCarbide Co)

Figure 1.3. Blended tungsten carbide pow-der is produced by mixing tungsten carbide(WC) with a cobalt (Co) binder in a ballmilling process. (Courtesy AmericanNational Carbide Co)

Figure 1.4. Carbide compacting equipment,better known as a pill press, is used to pro-duce carbide products in various shapes.(Courtesy American National Carbide Co)



Compacting Equipment, better knownas a Pill Press, is shown in Figure 1.4.Various pill pressed carbide parts areshown in Figure 1.5.

If the quantities are not high, a largerbriquette, or billet may be pressed. Thisbillet may then be cut up (usually afterpre-sintering) into smaller units andshaped or preformed to the requiredconfiguration, and again, allowancemust be made to provide for shrinkage.Ordinarily pressures used in these coldcompacting operations are in the neigh-borhood of 30,000 PSI. Various carbidepreformed parts are shown in Figure1.6.

A second compacting method is thehot pressing of grade powders ingraphite dies at the sintering tempera-ture. After cooling, the part has attained

full hardness. Because the graphite diesare expendable, this system is generallyused only when the part to be producedis too large for cold pressing and sinter-ing.

A third compacting method, usuallyused for large pieces, is isostatic press-ing. Powders are placed into a closed,flexible container which is then sus-pended in a liquid in a closed pressurevessel. Pressure in the liquid is built upto the point where the powders becomeproperly compacted. This system isadvantageous for pressing large pieces,because the pressure acting on the pow-ders operates equally from all direc-tions, resulting in a compact of uniformpressed density.

Sintering: Sintering of tungsten -cobalt (WC-Co) compacts is performedwith the cobalt binder in liquid phase.The compact is heated in hydrogenatmosphere or vacuum furnaces to tem-peratures ranging from 2500 to 2900degrees Fahrenheit, depending on thecomposition. Both time and tempera-ture must be carefully adjusted in com-bination, to effect optimum control overproperties and geometry. The compactwill shrink approximately 16 percent onlinear dimensions, or 40 percent in vol-ume. The exact amount of shrinkagedepends on several factors includingparticle size of the powders and thecomposition of the grade. Control ofsize and shape is most important and isleast predictable during the coolingcycle. This is particularly true with

those grades of cemented carbides withhigher cobalt contents.

With cobalt having a lesser densitythan tungsten, it occupies a greater partof the volume than would be indicatedby the rated cobalt content of the grade;and because cobalt contents are general-ly a much higher percentage of the massin liquid phase, extreme care is requiredto control and predict with accuracy themagnitude and direction of shrinkage.Figure 1.7 shows carbide parts beingloaded into a Sintering Furnace.

Figure 1.5. Various carbide compacts,which are produced with special diesmounted into pill presses. (CourtesyAmerican National Carbide Co)

Figure 1.6. If quantities are not high, presin-tered billets are shaped or preformed intorequired shapes. (Courtesy DurametCorporation)

Figure 1.7. Carbide parts are loaded into asintering furnace, where they are heated totemperatures ranging from 2500° to2900°F. (Courtesy American NationalCarbide Co)

Figure 1.8. Schematic diagram of the cemented tungsten carbide manufacturing process.



A more detailed schematic diagramof the cemented tungsten carbide manu-facturing process is shown in Figure1.8.

1.3.2 Classification of Carbide ToolsCemented carbide products are classi-fied into three major grades:

Wear Grades: Used primarily indies, machine and tool guides, and insuch everyday items as the line guideson fishing rods and reels; anywheregood wear resistance is required.

Impact Grades: Also used for dies,particularly for stamping and forming,and in tools such as mining drill heads.

Cutting Tool Grades: The cuttingtool grades of cemented carbides aredivided into two groups depending ontheir primary application. If the carbideis intended for use on cast iron which isa nonductile material, it is graded as acast iron carbide. If it is to be used tocut steel, a ductile material, it is gradedas a steel grade carbide.

Cast iron carbides must be moreresistant to abrasive wear. Steel car-bides require more resistance to crater-ing and heat. The tool wear characteris-tics of various metals are different,thereby requiring different tool proper-ties. The high abrasiveness of cast ironcauses mainly edge wear to the tool.The long chip of steel, which flowsacross the tool at normally higher cut-ting speeds, causes mainly cratering andheat deformation to the tool. Tool wearcharacteristics and chip formation willbe discussed in Chapter 2.

It is important to choose and use thecorrect carbide grade for each job appli-cation. There are several factors thatmake one carbide grade different fromanother and therefore more suitable fora specific application. The carbidegrades may appear to be similar, but thedifference between the right and wrongcarbide for the job, can mean the differ-ence between success and failure.

Figure 1.8 illustrates how carbide ismanufactured, using pure tungsten car-bide with a cobalt binder. The puretungsten carbide makes up the basic car-bide tool and is often used as such, par-ticularly when machining cast iron.This is because pure tungsten carbide isextremely hard and offers the best resis-tance to abrasive wear.

Large amounts of tungsten carbideare present in all of the grades in the twocutting groups and cobalt is always usedas the binder. The more common alloy-

ing additions to the basictungsten/cobalt material are: tantalumcarbide, and titanium carbide.

While some of these alloys may bepresent in cast iron grades of cuttingtools, they are primarily added to steelgrades. Pure tungsten carbide is themost abrasive-resistant and will workmost effectively with the abrasivenature of cast iron. The addition of thealloying materials such as tantalum car-bide and titanium carbide offers manybenefits:

• The most significant benefit of tita-nium carbide is that it reduces cra-tering of the tool by reducing the ten-dency of the long steel chips to erodethe surface of the tool.• The most significant contribution oftantalum carbide is that it increasesthe hot hardness of the tool which, inturn, reduces thermal deformation.Varying the amount of cobalt binder

in the tool material largely affects boththe cast iron and steel grades in threeways. Cobalt is far more sensitive toheat than the carbide around it. Cobaltis also more sensitive to abrasion andchip welding. Therefore, the morecobalt present, the softer the tool is,making it more sensitive to heat defor-mation, abrasive wear, and chip welding

and leaching which causes cratering.On the other hand, cobalt is strongerthan carbide. Therefore more cobaltimproves the tool strength and resis-tance to shock. The strength of a car-bide tool is expressed in terms of‘Transverse Rupture Strength’ (TRS).Figure 1.9 shows how TransverseRupture Strength is measured.

The third difference between the castiron and steel grade cutting tools, is car-bide grain size. The carbide grain sizeis controlled by the ball mill process.There are some exceptions, such asmicro-grain carbides, but generally thesmaller the carbide grains, the harderthe tool. Conversely, the larger the car-bide grain, the stronger the tool.Carbide grain sizes at 1500x magnifica-tion are shown in Exhibits 1.10 and1.11.

In the C- classification method(Figure 1.12), grades C-1 through C-4are for cast iron and grades C-5 throughC-8 for steel. The higher the C- numberin each group, the harder the grade, thelower the C- number, the stronger thegrade. The harder grades are used forfinish cut applications; the strongergrades are used for rough cut applica-tions.

Many manufacturers produce and

Figure 1.9. The method used to measure Transverse Rupture Strength (TRS) is shown aswell as the relationship of TRS to cobalt (Co) content.

Figure 1.10. Carbide grain size (0.8micron WC @ 1500×) consisting of 90%WC and 10% Co.

Figure 1.11. Carbide grain size (7 micronsWC @ 1500×) consisting of 90% WC and10% Co.



distribute charts showing a comparisonof their carbide grades with those ofother manufacturers. These are notequivalency charts, even though theymay imply that one manufacturer’s car-bide is equivalent to that of anothermanufacturer. Each manufacturerknows his carbide best and only themanufacturer of that specific carbidecan accurately place that carbide on theC- chart. Many manufacturers, espe-cially those outside the U. S., do not usethe C- classification system for car-bides. The placement of these carbideson a C- chart by a competing companyis based upon similarity of applicationand is, at best an ‘educated guess’.Tests have shown a marked differencein performance among carbide gradesthat manufacturers using the C- classifi-cation system have listed in the samecategory.

1.3.3 Coated Carbide ToolsWhile coated carbides have been inexistence since the late 1960’s, they didnot reach their full potential until themid 1970’s. The first coated carbideswere nothing more than standard car-bide grades which were subjected to acoating process. As the manufacturersgained experience in producing coatedcarbides, they began to realize that thecoating was only as good as the basecarbide under the coating (known as thesubstrate).

It is advisable to consider coated car-bides for most applications. When the

proper coated carbide, with the rightedge preparation is used in the rightapplication, it will generally outperformany uncoated grade. The microstructureof a coated carbide insert at 1500x mag-nification is shown in Figure 1.13.

Numerous types of coating materialsare used, each for a specific application.It is important to observe the do’s anddont’s in the application of coated car-bides. The most common coating mate-rials are:

• Titanium Carbide• Titanium Nitride• Ceramic Coating• Diamond Coating• Titanium Carbo-NitrideIn addition, multi-layered combina-

tions of these coating materials are

used. The microstructure of a multi-layered coated carbide insert at 1500xmagnification is shown in Figure 1.14.

In general the coating process isaccomplished by chemical vapordeposition (CVD). The substrate isplaced in an environmentally con-trolled chamber having an elevatedtemperature. The coating material isthen introduced into the chamber as achemical vapor. The coating materialis drawn to and deposited on the sub-strate by a magnetic field around thesubstrate. It takes many hours in thechamber to achieve a coating of0.0002 to 0.0003 inch on the substrate.Another process is Physical VaporDeposition (PVD).

Titanium Carbide Coating: Of allthe coatings, titanium carbide is themost widely used. Titanium carbide isused on many different substrate mate-rials for cutting various materialsunder varying conditions. Titanium

carbide coatings allow the use of highercutting speeds because of their greaterresistance to abrasive wear and crater-ing and higher heat resistance.

Titanium Nitride Coating - GoldColor: Titanium nitride is used onmany different substrate materials. Theprimary advantage of titanium nitride isits resistance to cratering. Titaniumnitride also offers some increased abra-sive wear resistance and a significantincrease in heat resistance permittinghigher cutting speeds. It is also said thattitanium nitride is more slippery, allow-ing chips to pass over it, at the cuttinginterface, with less friction.

Ceramic Coating - Black Color:Because aluminum oxide (ceramic) isextremely hard and brittle, it is not opti-

ClassificationNumber

Materials tobe Machined

Cast iron,nonferrousmetals, andnonmetallic

materialsrequiringabrasion

resistance

C-1

C-2

C-3

C-4

Steels and steel-alloys requiring

crater anddeformationresistance

C-5

C-6

C-7

C-8

MachiningOperation

Roughingcuts

Generalpurpose

Finishing

Precisionboring and

fine finishing

Roughingcuts

Generalpurpose

Finishing

Precisionboring and

fine finishing

Type ofCarbide

Wear-resistantgrades;

generallystraightWC–Co

with varyinggrain sizes

Crater-resistantgrades;various

WWC–Cocompositions

with TICand/or TaC

alloys

Cut

Increasingcutting speed

Increasingfeed rate

Characteristics Of

Carbide

Typical Properties

HardnessH-Ra

TransverseRuptureStrength(MPa)

89.0 2,400

92.0 1,725

92.5 1,400

93.5 1,200

91.0 2,070

92.0 1,725

93.0 1,380

94.0 1,035

Increasingcutting speed

Increasingfeed rate

Increasinghardness and

wear resistance

Increasingstrength and

binder content

Increasinghardness and

wear resistance

Increasingstrength and

binder content

Figure 1.12. Classification, application, characteristics, and typical properties of metal-cut-ting carbide grades.

Figure 1.13. Microstructure of a coatedcarbide insert at 1500× magnification.(Courtesy of Kennamental Inc.)

Figure 1.14. Microstructure of a multilay-ered coated carbide insert at 1500× magni-fication. (Courtesy of Kennamental Inc.)



mal for interrupted cuts, scaly cuts, andhard spots in the workpiece. This is notto say that it will never work underthese conditions, but it may be moresubject to failure by chipping. Evenwith these limitations, aluminum oxideis probably the greatest contributor tothe coated carbides. Aluminum oxideceramic allows much higher cuttingspeeds than other coated carbidesbecause of its outstanding resistance toabrasive wear and its resistance to heatand chemical interaction.

Diamond Coating: A recent devel-opment concerns the use of diamondpolycrystalline as coating for tungstencarbide cutting tools. Problems existregarding adherence of the diamondfilm to the substrate and the differencein thermal expansion between diamondand substrate materials. Thin-film dia-mond coated inserts are now availableusing either PVD (Physical VaporDeposition) or CVD (Chemical VaporDeposition) coating methods. Diamondcoated tools are effective in machiningabrasive materials, such as aluminumalloys containing silicon, fiber rein-forced materials, and graphite.Improvements in tool life of as much astenfold have been obtained over othercoated tools.

Titanium Carbo-Nitride - BlackColor Multilayered Coatings:Titanium carbo-nitride normallyappears as the intermediate layer of twoor three phase coatings. The role of tita-nium carbo-nitride is one of neutrality,helping the other coating layers to bondinto a sandwich-like structure (Figure 1-14). Other multi-layer coating combi-nations are being developed to effec-tively machine stainless steels and aero-space alloys. Chromium-based coatings such aschromium carbide have beenfound to be effective inmachining softer metals suchas aluminum, copper, andtitanium.

There are a few importantpoints to remember aboutusing coated carbides.Coated carbides will notalways out-perform uncoatedgrades but because of thebenefits offered by coatedcarbides, they should alwaysbe a first consideration whenselecting cutting tools.

When comparing the costbetween coated and uncoated

carbides there will be little differencewhen the benefits of coated carbides areconsidered. Because coated carbidesare more resistant to abrasive wear, cra-tering, and heat, and because they aremore resistant to work material build-upat lower cutting speeds, tool life isextended, reducing tool replacementcosts. Coated carbides permit operationat higher speeds, reducing productioncosts.

All coated carbides have an edgehone to prevent coating build-up duringthe coating process. This is because thecoating will generally seek sharp edges.The edge hone is usually very slight andactually extends tool life. However, acoated insert should never be regroundor honed. If a special edge preparationis required the coated carbides must beordered that way. The only time theedge hone may be of any disadvantageis when making a very light finishingcut. Carbide insert edge preparationswill be discussed in Chapter 2.

1.4 Ceramic and Cermet ToolsCeramic Aluminum Oxide (Al2O3)material for cutting tools was firstdeveloped in Germany sometimearound 1940. While ceramics wereslow to develop as tool materials,advancements made since the mid1970’s have greatly improved their use-fulness. Cermets are basically a combi-nation of ceramic and titanium carbide.The word cermet is derived from thewords ‘ceramic’ and ‘metal’.

Ceramic Cutting Tools: Ceramicsare non-metallic materials. This putsthem in an entirely different categorythan HSS and carbide tool materials.The use of ceramics as cutting tool

material has distinct advantages anddisadvantages. The application ofceramic cutting tools is limited becauseof their extreme brittleness. The trans-verse rupture strength (TRS) is verylow. This means that they will fracturemore easily when making heavy orinterrupted cuts. However, the strengthof ceramics under compression is muchhigher than HSS and carbide tools.

There are two basic types of ceramicmaterial; hot pressed and cold pressed.In hot pressed ceramics, usually blackor gray in color, the aluminum oxidegrains are pressed together underextremely high pressure and at a veryhigh temperature to form a billet. Thebillet is then cut to insert size. Withcold pressed ceramics, usually white incolor, the aluminum oxide grains arepressed together, again under extremelyhigh pressure but at a lower tempera-ture. The billets are then sintered toachieve bonding. This procedure issimilar to carbide manufacture, exceptno metallic binder material is used.While both hot and cold pressed ceram-ics are similar in hardness, the coldpressed ceramic is slightly harder. Thehot pressed ceramic has greater trans-verse rupture strength. Various shapesof both hot and cold pressed ceramicinserts are shown in Figure 1.15.

The brittleness, or relative strength,of ceramic materials is their greatestdisadvantage when they are comparedto HSS or carbide tools. Proper toolgeometry and edge preparation play animportant role in the application ofceramic tools and help to overcometheir weakness. Some of the advantagesof ceramic tools are:

• High strength for light cuts on veryhard work materials.• Extremely high resistanceto abrasive wear and cra-tering.• Capability of running atspeeds in excess of 2000SFPM.• Extremely high hot hard-ness.• Low thermal conductivi-ty.

While ceramics may notbe the all-around tool forthe average shop, they canbe useful in certain appli-cations. Ceramic toolshave been alloyed with zir-conium (about 15%) toincrease their strength.

Figure 1.15. Various sizes and shapes of hot- and cold-pressed ceramicinserts. (Courtesy Greenleaf Corp.)



Many ceramic tool manufacturers arerecommending the use of ceramic toolsfor both rough cutting and finishingoperations. Practical shop experienceindicates that these recommendationsare somewhat optimistic. To use ceram-ic tools successfully, insert shape, workmaterial condition, machine tool capa-bility, set-up, and general machiningconditions must all be correct. Highrigidity of the machine tool and set-up isalso important for the application ofceramic tools. Ceramics are beingdeveloped to have greater strength(higher TRS). Some manufacturers areoffering ceramic inserts with positivegeometry and even formed chip breakergrooves.

Cermet Cutting Tools: The manu-facturing process for cermets is similarto the process used for hot pressedceramics. The materials, approximately70 percent ceramic and 30 percent tita-nium carbide, are pressed into billetsunder extremely high pressure and tem-perature. After sintering, the billets aresliced to the desired tool shapes.Subsequent grinding operations forfinal size and edge preparation, com-plete the manufacturing process.

The strength of cermets is greaterthan that of hot pressed ceramics.Therefore, cermets perform better oninterrupted cuts. However, when com-pared to solid ceramics, the presence ofthe 30 percent titanium carbide in cer-mets decreases the hot hardness andresistance to abrasive wear. The hothardness and resistance to abrasive wearof cermets are high when compared toHSS and carbide tools. The greaterstrength of cermets allows them to beavailable in a significantly larger selec-tion of geometries, and to be used instandard insert holders for a greatervariety of applications. The geometriesinclude many positive/negative, andchip breaker configurations.

Silicon-Nitride Base Ceramics:Developed in the 1970’s, silicon-nitride(SIN) base ceramic tool materials con-sist of silicon nitride with various addi-

tions of aluminum oxide,yttrium oxide, and titaniumcarbide. These tools havehigh toughness, hot hard-ness and good thermalshock resistance. Sialonfor example is recommend-ed for machining cast ironsand nickel base superalloysat intermediate cuttingspeeds.

1.5 Diamond, CBN and Whisker-Reinforced Tools

The materials describedhere are not commonlyfound in a heavy metalworking environment.They are most commonlyused in high speed auto-matic production systemsfor light finishing of precision surfaces.To complete the inventory of tool mate-rials, it is important to note the charac-teristics and general applications ofthese specialty materials.

Diamond: The two types of dia-monds being used as cutting tools areindustrial grade natural diamonds, andsynthetic polycrystalline diamonds.Because diamonds are pure carbon, theyhave an affinity for the carbon of fer-rous metals. Therefore, they can onlybe used on non-ferrous metals.

Some diamond cutting tools are madeof a diamond crystal compaction (manysmall crystals pressed together) bondedto a carbide base (Fig. 1.16). These dia-mond cutting tools should only be usedfor light finishing cuts of precision sur-faces. Feeds should be very light andspeeds are usually in excess of 5000surface feet per minute (SFPM).Rigidity in the machine tool and the set-up is very critical because of theextreme hardness and brittleness of dia-mond.

Cubic Boron Nitride: Cubic boronnitride (CBN) is similar to diamond inits polycrystalline structure and is alsobonded to a carbide base. With the

exception of titanium, or titaniumalloyed materials, CBN will work effec-tively as a cutting tool on most commonwork materials. However, the use ofCBN should be reserved for very hardand difficult-to-machine materials.CBN will run at lower speeds, around600 SFPM, and will take heavier cutswith higher lead angles than diamond.Still, CBN should mainly be consideredas a finishing tool material because ofits extreme hardness and brittleness.Machine tool and set-up rigidity forCBN as with diamond, is critical.

Whisker-Reinforced Materials: Inorder to further improve the perfor-mance and wear resistance of cuttingtools to machine new work materialsand composites, whisker-reinforcedcomposite cutting tool materials havebeen developed. Whisker-reinforcedmaterials include silicon-nitride basetools and aluminum-oxide base tools,reinforced with silicon-carbide (SiC)whiskers. Such tools are effective inmachining composites and nonferrousmaterials, but are not suitable formachining irons and steels.

Figure 1.16. Polycrystalline diamond material bonded to acarbide base of various sizes and shapes. (Courtesy ofSandvik Coromant Co.)

chap_1

Documents

Transcript of chap_1