Cutting tools materials:
Cutting tool is a device used to remove unwanted material from the given workpiece.
Characteristics of a cutting tool material:
- 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.
- The coefficient of friction at the tool chip interface should be low for better surface finish and less wear.
- The material should have wear resistance to prevent wear and tear of the cutting tool surface.
- 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.
- The material must have sufficient strength and toughness to withstand shocks and vibrations.
- 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:
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.
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.
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).
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.
Diamond: It is the hardest known material having cutting speed 15 times greater than that for high speed tools.
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.