Tool Materials

Tool Materials

A Cutting tool should have the following properties
·         High Hardness
·         High Toughness
·         High Heat Resistance
·         High Wear Resistance
·         Edge Holding Properties
Materials for Turning Tools

Cutting Tool Working

1.     Carbon Tool Steel (CTS)

This is a plain carbon steel containing 0.8 to 1% of Carbon. This is the first material used for making telescope. Cutting Tools made out of this material acquire good hardness (60-63 HRc) on hardening, but they quickly lose their hardness at temperatures above 250^○C. They have very low toughness and wear resistance. Their chief advantages are their good machinability and low cost.
Now-a-days, carbon tool steel is not much used for turning tools, because they have to be operated at very low cutting speeds. (Below 10 m/min) and therefore, their productivity is low. However, due to their machinability, low cost and easy hardening, it is sometimes used for making special turning tools, for limited operation, for specially turning soft materials. Plenty of coolant must be used while turning with these tools.

2.     High Speed Steel (HSS)

This is general purpose cutting tool material commonly available as tool bits in the form of square, Round, Rectangular or blocks. This is highly alloyed steel containing besides Iron and carbon, Alloying elements like tungsten (W), Chromium (Cr), Molybdenum (Mo), Vanadium (V), Cobalt (Co) etc. Carbon is from 0.7 to 0.8%. Due to these alloying elements HSS has much higher heat resistance (Up to 650^○C), Higher Hardness (62-65 HRc), Toughness, Wear resistance and good edge holding property. Turning tools made out of HSS can be used at higher cutting speeds. (3 to 4 Times)
There are different types and grades of HSS, which are to be selected according to the material to be turned, and the type of operation for economic machining.

3.     Carbide Steel

Carbide is much harder, heat resistant and wear resistant than HSS. Therefore, it is extensively used these days for High Speed Machining. Carbide tools can retain their cutting properties even up to 950^○C. They can be used at cutting speeds 5 to 6 times higher than HSS. They are manufactured by a special process called power technology.
The basic ingredient of most carbide is tungsten carbide, Titanium and Tantalum Carbides are also added along with cobalt which acts as a binder for these carbides.
The main drawback of carbide tools is their high brittleness; their cutting edges will get chipped off if they are subjected to shocks or vibration. Therefore, they must be rigidly supported during operation and M/C should be free of vibrations.
Carbide is costly material; therefore it is made into form of tips of various shapes and sizes, which are either brazed or mechanically clamped to shanks. The shanks may be made out of medium carbon steel. There are different grades of carbide. They are selected according to the material to be cut and the type of operation. Carbides are also sensitive to thermal shocks. Therefore, either they should be used dry or with continuous supply of coolant.
Intermittent or inadequate supply of coolant will harm the tip.

4.     Ceramics

Ceramic tool materials are made out of certain oxides of which aluminum oxide is the major constituent. It is also made in the form of tips of various shapes and sizes and is generally used by mechanically clamping then to the tool holders.
The hardness of ceramic cutting tools is only next to diamond. They can retain their cutting properties up to 1200^○C. Therefore, they can be used at extremely high cutting speeds, nearly 6 times that of carbide and 40 times that of HSS, but they are very brittle and have very; low resistance to impact. They can only be used efficiently under conditions of extreme rigidity of machine and setup, due to this; they have still limited use only.

5.     Diamond

Diamond is the hardest of all known materials. Its heat resistance is up to 1400^○C. As a result diamond tools have exceptional high tool life even when operated at highest possible cutting speeds. Diamonds are available in nature but they are manufactured also. Manufactured diamonds are called as Synthetic Diamonds. At present diamond turning and boring tools employed for finish turning of aluminum and its alloys, Magnesium alloys, bronze, babbits, gold, silver, platinum, rubber and such non-ferrous and non-metallic materials.