Cuting Tools for Machining

8/7/2019 Cuting Tools for Machining

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CUTTING TOOLS FOR MACHININGINTRODUCTION

Success in metal cutting depends upon the selection of the proper cutting tool (material and

geometry) for a given work material. A wide range of cutting tool materials is available with avariety of properties, performance capabilities, and cost. These include high carbon steels and

low/medium alloy steels, high-speed steels, cast cobalt alloys, cemented carbides, cast carbides,

coated carbides, coated high speed steels, ceramics, cermets, whisker-reinforced ceramics, sintered

polycrystalline cubic boron nitride (CBN), sintered polycrystalline diamond, and single-crystal

natural diamond. Figure 22-1 shows the common tool materials in a matrix of tool materials ranked

by the cutting speeds used to machine a unit volume of steel materials, assuming equal tool lives. As

the speed (feed rate and DOC) increases, so does the metal removal rate. The time required to

remove a given unit volume of material therefore decreases. Notice the fivefold increase in speed

that the TiC/ AL2O3 / TiN-coated carbide has over the WC/Co tool (77 ~ 370 m/ min).

The c utting too l materia l, c utting p a ramete rs, and too l geometry

selec ted d irec tly influenc e the p rod uc tivity of the ma c hining operation.

Figure 22-2 outlines the input variables that influence the tool material

selec tion d ec ision. The elements whic h influenc e the dec ision a re:

-Work material characteristics (chemical and metallurgical state,hardness)

-Part characteristics (geometry, accuracy, finish, and surface-integrity requirements)

-Machine tool characteristics, including the work-holders(adequate rigidity with high horsepower, and wide speed and

feed range s)

-Support systems (operator's ability, sensors, controls, method oflubric a tion a nd c hip remova l)

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Tool material technology is advancing rapidly, enabling many difficult-

to-machine materials to be machined at higher removal rates and/orcutting speeds with greater performance reliability. Higher speed

and/or removal rates usually improve productivity. Predictable tool

performance is essential when machine tools are computer controlled

and have minimal operator interaction. Long tool life is desirable when

machines are placed in cellular manufacturing systems.

The cutting tool is subjected to severe conditions. Tool temperatures

of 1000 C, severe friction and high local stresses require that the tool

have these characteristics.

1. High hardness (Figure 22-3)2. High hardness temperature, hot hardness(refer to Figure 22-3)3. Resistance to abrasion, wear, chipping of the cutting edge4. High toughness (impact strength) (refer to Figure 22-4)5. Strength to resist bulk deformation6. Good chemical stability (inertness or negligible affinity withthe work material)

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7. Adequate thermal properties8. High elastic modulus (stiffness)9. Consistent tool life10. Correct geometry and surface finish

Figure 22-3 compares various tool materials on the basis of hardness, the mostcritical characteristic, and hot hardness (hardness decreases slowly withtemperature). Figure 22-4 compares hot hardness with toughness or the abilityto take impacts during interrupted cutting.Naturally, it would be most wonderful if these materials were also easy to fabricate,

readily available, and inexpensive, since cutting tools are routinely replaced but this is

not usually the case. Obviously, many of the requirements conflict and therefore tool

selection will always require trade-offs.

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CUITING TOOL MATERIALS

In nearly all machining operations, cutting speed and feed are limited by

the capability of the tool material. Speeds and feeds must be kept lowenough to provide for an acceptable tool life.If not, the time lost changing

tools may outweigh the productivity gains from increased cutting speed.

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Coated and uncoated carbides are currently the most extensivelyused tool materials. Coated tools cost only about 15 to 20% morethan uncoated tools, so a modest improvement in performance canjustify the added cost. About 15 to 20% of all tool steels arecoated, mostly by the physical vapour deposition (PVD) processes.Diamond and CBN are used for applications in which, despite higher

cost, their use is justified. Cast cobalt alloys are being phasedout because of the high raw material cost and the increasingavailability of alternate tool materials. New ceramic materialscalled cermets (ceramic material in a metal binder) are beingintroduced that will have significant impact on futuremanufacturing productivity.

TOOL STEELSCarbon steels and low/medium alloy steels, called tool steels, wereonce the most common cutting-tool materials. Plain-carbon steels of 0.90 to 1.30% carbon when hardened and tem-pered have good hardness and strength and adequate toughness andcan be given a keen cutting edge.However, tool steels lose hardness at temperatures above 205Cbecause of tempering and have largely been replaced by othermaterials for metal cutting.

The most important properties for tool steels are hardness, hot hardness and tough-ness.Low/medium alloy steels have alloying elements such as Mo and Cr, which

improve harden ability, and Wand Mo, which improve wear resistance. These toolmaterials also lose their hardness rapidly when heated to about their tempering

temperature of 150 to 345 Cand they have limited abrasion resistance. Consequently, low/medium alloy steels are used in relatively

inexpensive cutting tools (e.g., drills, taps, dies, reamers, broaches,

and chasers) for certain low-speed cutting applications when the heat

generated is not high enough to reduce their hardness significantly.

High-speed steels, cemented carbides, and coated tools are also usedextensively to make these kinds of cutting tools. Although more

expensive, they have longer tool life and improved performance.

HIGH-SPEED STEELS

First introduced in 1900 by F. W. Taylor and White, high-

alloy steel was superior to tool steel in that it retains

its cutting ability at temperatures up to 594C,exhibiting good "red hardness." Compared with tool steel,

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it can operate at about double or triple cutting speeds

to about 30 m/min with equal life, resulting in its name

high-speed steel, often abbreviated HSS.

Today's high-speed steels contain significant amounts of W, Mo, Co, V, and Crbesides Fe and C. W, Mo, Cr, and Co in the ferrite as a solid solution providestrengthening of the matrix beyond the tempering temperature, thus increasingthe hot hardness. Vanadium (V), along with W, Mo, and Cr, improves hardness(RC 65-70) and wear resistance.

High-speed steel is still widely used for drills and many types of

general-purpose milling cutters and in single-point tools used in

general machining. For high-production machining it has been

replaced almost completely by carbides, coated carbides, and coatedHSS.

HSS main strengths are

. Grea t toug hness-superior transverse rupture streng th

. Easily fab ricated

. Best for seve r applic ations where c omplex tool geometry is needed

(gea r cutters, tap s, drills, rea mers, dies)

CAST COBALT ALLOYS

Cast cobalt alloys,popularly known as stellite tools,are cobalt-rich, chromium-tungsten-carbon cast alloys having properties and applications in the intermediaterange between high-speed steel and cemented carbides. Although comparable inroom-temperature hardness to high-speed steel tools, cast cobalt alloy tools retaintheir hardness to a much higher temperature. Consequently, they can be used athigher cutting speeds (25% higher) than HSS tools. Cast cobalt alloys are hard ascast, and cannot be softened or heat treated.Cast cobalt alloys contain a primary phase of Co-rich solid solution strengthened by

Cr and W and dispersion hardened by hard, refractory carbides of W and Cr.Tools of cast cobalt alloys are generally cast to shape and finished to size bygrinding. They are available only in simple shapes, such as single-point tools and saw

CARBIDE OR SINTERED CARBIDES

Carbide cutting tool inserts are traditionally divided into two primary groups

1.Straight W grades, which are used for machining cast irons, austenitic stainlesssteel, and non ferrous and non metallic materials.

2.Grades containing major amounts of titanium, tantalum, and/or columbiumcarbides, which are used for machining ferritic work pieces. There are also the

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titanium carbide grades, which are used for finishing and semifinishing ferrousalloys.

Carbides, which are nonferrous alloys, are also called sintered(o r

cemented) carbides because they are manufactured by powder

meta llurgy tec hniques.

These m ateria ls bec ame p opula r during World War II, as they a fforded

a four- or five fold inc rea se in c utting spee ds.

The ea rly versions had tung sten c a rb ide as the ma jor c onstituent, with

a cobalt binder in amounts of 3 to 13%. Most carbide tools in use

today are either straight WC or multi-carbides depending upon the

work material to be machined. Cobalt is the binder. These toolmaterials are much harder, and chemically more stable,

have better hot hardness, high stiffness, and lower fric-tion, and operate at higher cutting speeds than HSS. They

are more brittle and more expensive and use strategic metals (W, Ta,

Co ) more extensively.Cemented carbide tool materials based on TiC have been

developed primarily for auto industry applications using

predominantly Ni and Mo as a binder. These are used for

higher speed (> 305 m/min) finish machining of steels,

and some malleable cast irons.

Cemented carbide tools are available in insert form in

many different shapes: squares, triangles, diamonds, and

rounds. They can be either brazed or mechanically

clamped onto the tool shank. Mechanical clamping is more

popular because when one edge or corner becomes dull,

the insert is rotated or turned over to expose a new

cutting edge.

CERAMICSCeramicsare made of pure aluminium oxide, Al2O3 or Al2O3 used as

a metallic binder. Using P/M, very fine particles are formed into

cutting tips under a pressure of 267 to 386 MPa and sintered at

about l000C.Ceramics usually are in the form of disposable

tips. They can be operated at two to three timesthe cutting speeds of tungsten carbide. They

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almost completely resist cratering, run with nocoolant, and have about the same tool life at theirhigher speeds as tungsten carbide does at lowerspeeds.Ceramics are usually as hard as carbides but are

more brittle (lower bend strength) and thereforerequire more rigid tool holders and machine toolsin order to take advantage of their capabilities.

Their hardness and chemical inertness make ceramics a good

material for high-speed finishing and/or high-removal-rate

machining applications of superalloys, hard-chill cast iron, and

high strength steels. Because ceramics have poor thermal and

mechanical shock resistance, interrupted cuts and interruptedapplication of coolants can lead to premature tool failure. Ceramics

are not suitable for aluminium, titanium, and other materials that

react chemically with alumina-based ceramics.

CERMETSCermetsare a new class of tool materials best suited for finishing.

Cermets are ceramic TiC, nickel, cobalt, and tantalum nitrides. TiNand other carbides are used for binders. Cermets have superior

wear resistance, longer tool life, and can operate at higher cutting

speeds with superior wear resistance. Cermets have higher hot

hardness and oxidation resistance than cemented carbides. The

better finish imparted by a cermet is due to its low level of chemical

reaction with iron [less cratering and built-up-edge(BUE)].

Compared to carbide, the cermet has less toughness, lower thermalconductivity, and greater thermal expansion, so thermal cracking

can be a problem during interrupted cuts.

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DIAMONDSDiamondis the hardest material known. Industrialdiamonds are now available in the form ofpolycrystalline compacts, which are findingindustrial application in the machining ofaluminium, bronze, and plastics, greatly reducing

the cutting forces as compared to carbides. Diamond

machining is done at high speeds with fine feeds for finishing and

produces excellent finishes.

The salient features of diamond tools include high hardness, good

thermal conductivity, the ability to form a sharp edge of cleavage

(single-crystal, natural diamond), very low friction, non-adherence to

most materials, the ability to maintain a sharp edge for a long period

of time, especially in machining soft materials such as copper and

aluminium, and good wear resistance.

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To be weighed against these advantages are some

shortcomings, which include: diamond wears rapidly when

machining or grinding mild steel. It wears less rapidly

with high-carbon alloy steels than with low-carbon

steel, and has occasionally machined grey cast iron(which has high carbon content) with long life. Diamond

is very brittle and is difficult and costly to shape

into cutting tools.

TOOL GEOMETRY

Figure 22-12 shows the cutting tool geometry for a single point tool (HSS) used in

turning. The back rake angle affects the ability of the tool to shear the work

ma teria l and form c hip . It c an b e positive o r neg a tive. Positive rake angles red uce

the cutting forces, resulting in smaller deflections of the workpiece, tool holder,

and machine. In machining hard work materials, the back rake angle must be

small, even negative for carbide and diamond tools. Generally speaking, the

higher the hardness of the workpiece the smaller the back rake angle. For high-

speed steels, ba c k rake ang le is norma lly c hosen in the positive range .

True rake inclination of a cutting tool has a major effect in

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determining the amount of chip compression and the shear angle. Asmall rake angle causes high compression, tool forces, and friction,resulting in a thick, highly deformed, hot chip. Increased rake anglereduces the compression, the forces, and the friction, yielding athinner, less-deformed and cooler chip. Unfortunately, it is difficult to

take much advantage of these desirable effects of larger positive rakeangles, since they are offset by the reduced strength of the cuttingtool, due to the reduced tool section, and by its greatly reducedcapacity to conduct heat away from the cutting edge.In general, the power consumption is reduced by approximately 1 % for each 1 in

. is the end relief angle. The wedge angle determines the strength of the tool

and its capacity to conduct heat and depends on the values ofand . The reliefangles mainly affect the tool life and the surface quality of the workpiece. For

high-speed steel, relief angles in the range of 5 to 10 are normal, with smallervalues being for the harder work materials.

TOOL FAILURE AND TOOL LIFEIn metal cutting, the failure of the cutting tool can be classified into two broad

categories, according to the failure mechanisms that caused the tool to die (or fail):

1. Slow-death mechanisms:gradual tool wear on the flank (s) of the tool below the

cutting edge (called flank wear) or wear on the rake face of the tool (called crater wear)

or both.

2. Sudden-death mechanisms:rapid, usually unpredictable and often catastrophic fail-

ures resulting from abrupt, premature death of a tool

Figure 22-13 shows a sketch of a "worn" tool, showingcrater wearand flank wear,

along with wear of the tool nose radius and an outer diameter groove (the DCL

groove). As the tool wears, its geometry changes. This geometry change will influence

the cutting forces, the power being consumed, the surface finish obtained, the

dimensional accuracy, andeven the dynamic stability of the process. Worn tools are

duller creating greater cutting forces, often resulting in chatter in processes thatotherwise are usually relatively free of vibration.

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The sudden-death mechanisms are more straightforward but less predictable.These mechanisms are categorized as plastic deformation, brittle fracture,fatigue fracture, or edge chipping. Here again it is difficult to predict whichmechanism will dominate and result in a tool failure in a particularsituation. What can be said is that tools, like people, die (or fail) from agreat variety of causes under widely varying conditions. Therefore, tool lifeshould be treated as a random variable, or probabilistically, and not as adeterministic quantity.

FLANK WEAR

During machining, the tool is performing in a hostile environment wherein highcontact stresses and high temperatures are common place, and therefore tool wearis always an unavoidable consequence. At lower speeds and temperatures, the toolmost commonly wears on the flank. In Figure 22-14 four characteristic tool wearcurves (average values) are shown for four different cutting speeds, V1 through V4.

V4is the fastest cutting speed and therefore generates the fastest wear rates. Suchcurves often have three general regions, as shown in the figure. The central region isa steady-state region (or the region of secondary wear). This is the normal operatingregion for the tool. Such curves are typical for both flank wear and crater wear.

When the amount of wear reaches the value Wf,the permissible tool wear on theflank, the tool is said to be "worn out." Wfis typically set at 0.64 to 0.76 mm forflank wear. For crater wear, the depth of the crater is used to determine tool failure.

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Using the empirical tool wear data shown in Figure 22-14, which used

the values ofT(time in minutes) associated with V(cutting speed) for

a given amount of tool wear, Wf(see the dashed-line construction)

Figure 22-16 was developed. When V and T areplotted on log-log

scales, a linear relationship appears described by the equation

VT n= constant = K

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ECONOMICS OF MACHINING

MACHINABILITYMachinabilityis a much maligned term which has manydifferent meanings but generally refers to the ease withwhich a metal can be machined to an acceptable surfacefinish. The principal definitions of the term are entirelydifferent, the first based on material properties, thesecond based on tool life, and the third based on cuttingspeed.

1. Machinability is defined by the ease or difficulty with

which the metal can be machined. In this light, specificenergy, specific horsepower, and shear stress are used asmeasures, and in general, the larger the shear stress or

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specific power values, the more difficult the material is tomachine, requiring greater forces and lower speeds. Inthis definition, the material is the key.

2. Machinability is defined by the relative cutting speed for

a given tool life while cut ting some material, compared toa standard material cut with the same tool material.As shown in Figure 22-21, tool-life curves are used todevelop machinability ratings.In steels, the material chosen for the standard

material was B1112 steel, which has tool life of 60 min

at a cutting speed of 30 m/min.

Material X has a 70% rating, which implies that steel X

has a cutting speed of 70% of B1112 for equal toollife. Note that this definition assumes that the tool

fails when machining X by whatever mechanism dominated

the tool failure when machining the B1112.

3. Cutting speed is measured by the maximum speed at which

a tool can provide satisfactory performance for aspecified time under specified conditions.

4. Other definitions of machinability are based on the ease

of removal of the chips (chip disposal), the quality of thesurface finish of the part it self, the dimensional stabilityof the process, or the cost to remove a given volume ofmetal.

CUTTING FLUIDS

From the day that Frederick W. Taylor demonstrated that a heavy

stream of water flowing directly on the cutting process allowed the

cutting speeds to be doubled or tripled, cutting fluidshave flourished

in use and variety are employed in virtually every machining process.

The cutting fluid acts primarily as a coolant and secondly as a

lubricant, reducing the friction effects at the tool/chip interface and

the work/ flank regions. The cutting fluids also carry away the chipsand provide friction (and force) reductions in regions where the

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bodies of the tools rub against the workpiece.

Thus in processes such as drilling, sawing, tapping, and reaming,

portions of the tool apart from the cutting edges come in contact with

the work, and these (sliding friction) contacts greatly increase thepower needed to perform the process, unless properly lubricated. The

reduction in temperature greatly aids in retaining the hardness of the

tool, thereby extending the tool life or permitting increased cutting

speed with equal tool life.

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Cuting Tools for Machining