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Cutting tool is a device used to remove unwanted material
from the given workpiece.
Characteristics of a cutting tool material
1.
The material should be harder than the workpiece
so that it is able to penetrate into the
workpiece and it should have hot hardness i.e.
the ability of material to retain hardness at
elevated temperatures.
2.
The coefficient of friction at the tool chip
interface should be low for better surface
finish and less wear.
3.
The material should have wear resistance to
prevent wear and tear of the cutting tool
surface.
4.
It should be chemically stable so that it does
not react with the workpiece and chemically
inert so that there is no oxidation and hence no
scales and pits are formed on the surface.
5.
The material must have sufficient strength and
toughness to withstand shocks and vibrations.
6.
The thermal conductivity should be high so that
there is heat dissipation which is generated
during the machining process thereby increasing
the life of the cutting tool.
Common cutting tool material used:
1.
Carbon steel:
Carbon steels having carbon percentage as high
as 1.5% are used as tool materials however they
are not able to with stand very high temperature
and hence are operational at low cutting speed.
2.
High speed steel (HSS): These are special alloy steel which are obtained by
alloying tungsten, Chromium, Vanadium, Cobalt
and molybdenum with steel. HSS has high hot
hardness, wear resistance and 3 to 4 times
higher cutting speed as compare to carbon steel.
Most commonly used HSS have following
compositions.
a)
18-4-1 HSS i.e. 18% tungsten, 4% chromium, 1%
vanadium with a carbon content of 0.6 - 0.7%. If
vanadium is 2% it becomes 18-4-2 HSS.
b)
Cobalt high speed steel: This is also referred
to as super high speed steel. Cobalt is added 2
– 15%. The most common composition is tungsten
20%, 4% chromium, 2% vanadium and 12% cobalt.
c)
Molybdenum high speed steel: It contains 6%
tungsten, 6% molybdenum, 4% chromium and 2%
vanadium.
3.
Cemented carbide: These are basically carbon cemented together by a binder. It is a
powder metallurgy product and the binder mostly
used is cobalt. The basic ingredient is tungsten
carbide-82%, titanium carbide-10% and cobalt-8%.
These materials possess high hardness and wear
resistance and it has cutting speed 6 times
higher than high speed steel (HSS).
4.
Ceramics: It
mainly consists of aluminum oxide (Al2O3)
and silicon nitride (Si3N4).
Ceramic cutting tools are hard with high hot
hardness and do not react with the workpiece.
They can be used at elevated temperature and
cutting speed 4 times that of cemented carbide.
These have low heat conductivity.
5.
Diamond: It is
the hardest known material having cutting speed
15 times greater than that for high speed tools.
6.
Cubic boron nitride (CBN): It is the second hardest material after diamond and a
economical alternative to the later. It is
manufactured through high temperature and
pressure to bond boron crystals in cubic form
with a ceramic or metal binder to form
polycrystalline structure with nitride particles
present. It is an excellent cutting tool
material because it combines extreme high hot
hardness up to high temperatures of 2000°C.
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