Section 13. qxd

whisker-reinforced 
cutting
tools, with enhanced toughness, cutting-edge strength, and thermal-
shock resistance. A common whisker material is silicon carbide. Ceramic
tools have very high abrasion resistance, are harder than carbides, and
have less tendency to weld to metals during cutting. However, they gen-
erally lack impact toughness, and premature tool failure can result by
chipping or general breakage. Ceramic tools have been found to be
Insert
Lockpin
Seat
Shank
(b)
Fig. 13.4.8
(
a
) Insert clamped to shank of a toolholder; (
b
) insert clamped with
wing lockpins.
Section_13.qxd 10/05/06 10:32 Page 13-53

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MACHINING PROCESSES AND MACHINE TOOLS
effective for high-speed, uninterrupted turning operations. Tool and
setup geometry is important. Tool failures can be reduced by the use of
rigid tool mountings and rigid machine tools. Included in oxide cutting-
tool materials are 
cermets
(such as 70 percent aluminum oxide and 30
percent titanium carbide), combining the advantages of ceramics and
metals.
Polycrystalline diamond
is used where good surface finish and dimen-
sional accuracy are desired, particularly on soft nonferrous materials that
are difficult to machine. The general properties of diamonds are extreme
hardness, low thermal expansion, high heat conductivity, and a very low
coefficient of friction. The polycrystalline diamond is bonded to a car-
bide substrate. 
Single-crystal diamond
is also used as a cutting tool to pro-
duce extremely fine surface finish on nonferrous alloys, such as
copper-base mirrors.
Next to diamond, 
cubic boron nitride (cBN)
is the hardest material
presently available. Polycrystalline cBN is bonded to a carbide sub-
strate and used as a cutting tool. The cBN layer provides very high wear
resistance and edge strength. It is chemically inert to iron and nickel at
elevated temperatures; thus it is particularly suitable for machining
high-temperature alloys and various ferrous alloys. Both diamond and
cBN are also used as abrasives in grinding operations.