10_Cutting Tool Materials

7/29/2019 10_Cutting Tool Materials

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K K iinn g g F F aahhdd U U nniivveer r ssiittyy oo f f PPeettr r oolleeuumm aanndd M M iinneer r aallss MET-276 Machining Technology

H H aa f f r r -- A All--BBaattiinn C C oommmmuunniittyy C C oollllee g gee CUTTING TOOL MATERIALS

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Introduction

The selection of cutting tool materials for a particular application is among the most

important factors in machining operations. As noted in the preceding lectures;

The cutting tool is subjected to high temperatures.

High contact stresses and rubbing along the tool-chip interface and along the

machined surface.

Consequently, the tool must possess the following characteristics:

a) Hot hardness.

b) Toughness and impact strength.

c) Thermal shock resistance.

d) Wear resistance.

e) Chemical stability and inertness.

Figure 1 Hot hardness of different tool materials

Cutting tool materials

Carbon Steels

It is the oldest of tool material. The carbon content is 0.6~1.5% with small quantities of

silicon, chromium, manganese, and vanadium to refine grain size. Maximum hardness is





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about HRC 62. This material has low wear resistance and low hot hardness. The use of

these materials now is very limited.

High-speed steel (HSS)They are highly alloyed with vanadium, cobalt, molybdenum, tungsten and chromium

added to increase hot hardness and wear resistance. They can be hardened to various

depths by appropriate heat treating up to cold hardness in the range of HRC 63-65. The

cobalt component gives the material a hot hardness value much greater than carbon

steels.

Figure 2 Thread tap and die made of high-speed steel

Cemented Carbides

These are the most important tool materials today because of their high hot hardness

and wear resistance. The main disadvantage of cemented carbides is their low

toughness. These materials are produced by powder metallurgy methods, sintering

grains of tungsten carbide (WC) in a cobalt (Co) matrix (it provides toughness). There

may be other carbides in the mixture, such as titanium carbide (TiC) and/or tantalum

carbide (TaC) in addition to WC.

Cemented carbides are available as inserts and are available in various shapes, and are

usually mechanically attached by means of clamps to the tool holder or brazed to the

tool holder. The clamping is preferred because after a cutting edge gets worn, the insert

is indexed (rotated in the holder) for another cutting edge. When all cutting edges areworn, the insert is thrown away. The indexable carbide inserts are never reground.





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Figure 3 Cemented carbides

(a) (b) (c)

Figure 4 Methods of attaching carbide inserts to tool holder. (a) Clamping, (b) wing

lockpin and (c) brazing.

35°

100° 90° 80° 60° 55°

Figure 5 Relative edge strength and tendency for chipping of inserts with various

shapes.

Increasing strength

Increasing chipping & breaking





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Ceramics

Ceramic materials are composed primarily of fine-grained, high-purity aluminum oxide

(Al2O3), pressed and sintered with no binder. Two types are available:

i. White, or cold-pressed ceramics, which consists of only Al2O3 cold pressed into

inserts and sintered at high temperature.

ii. Black, or hot-pressed ceramics, commonly known as cermet (from ceramics and

metal). This material consists of 70% Al2O3 and 30% TiC.

Both materials have very high wear resistance but low toughness; therefore they are

suitable only for continuous operations such as finish turning of cast iron and steel at

very high speeds. There is no occurrence of built-up edge, and coolants are not

required.

Cubic boron nitride (CBN) and synthetic diamonds

Diamond is the hardest substance ever known of all materials. It is used as a coating

material in its polycrystalline form, or as a single-crystal diamond tool for special

applications, such as mirror finishing of non-ferrous materials.

Next to diamond, CBN is the hardest tool material. CBN is used mainly as coating

material because it is very brittle. In spite of diamond, CBN is suitable for cutting ferrous

materials.

Figure 6 An insert of polycrystalline cubic boron nitride or a diamond layer on tungsten

carbide.

Tungsten-carbide

insert

Carbide substrate

Polycrystalline cubicboron nitride or

diamond layerBraze





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General operating characteristics of cutting tool materials

Tool materialsGeneral

characteristics

Models of tool

wear or failure

Limitations

High-speed steels

High toughness,

resistance, wide

range of roughing &

finishing cuts.

Flank wear, crater

wear.

Low hot hardness,

limited

hardenability and

limited wear

resistance.

Uncoated carbides

High hardness over

a wide range of

temperatures, wear

resistance, wide

range of applications.

Flank wear, crater

wear.

Cannot use at low

speed because of

cold welding of

chips andmicrochipping.

Coated carbides

Better wear

resistance over

uncoated carbides,

better frictional and

thermal properties.

Flank wear, crater

wear.

Cannot use at low

speed because of

cold welding of

chips and

microchipping.

Ceramics

High hardness at

elevated

temperatures, highabrasive wear

resistance.

Depth of cut line

notching,

microchipping,gross fracture.

Low strength, low

thermo-mechanicalfatigue strength.

Polycrystalling

cubic boron nitride

(cBN)

High hot hardness,

toughness, cutting

edge strength.

Depth of cut line

notching, chipping,

oxidation,

graphitization.

Low strength, low

chemical stability at

higher temperature.

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10_Cutting Tool Materials