Short notes on Cutting Tool materials for GATE Mechanical Engineering
Below are useful short notes on Cutting Tool materials for GATE Mechanical Engineering.
An ideal tool material is one which will remove largest volume of work material at all speeds. Properties of an ideal tool are:
- Red or Hot hardness: It is the ability of the tool material to withstand high temperature without losing its cutting edge. The red hardness of the tool can be increased by adding Cr, Molybdenum, Tungsten etc. All these materials make carbides.
- Wear Resistance: A good cutting tool material should have enough hardness to resist wear due to adhesion and abrasion.
- Toughness: The tool material should be tough enough to withstand impact loading and vibrations during the cutting operation.
- Lower friction: A good tool material should have low friction between the tool and chip interface.
- Low coefficient of thermal expansion: This ensures that the tool geometry is not altered.
- High specific heat
C = 0.6 to 1.5%
V = 10 m/min
Can resist temperatures above 200oC to 250oC.
High Speed Steel (HSS):
- HSS is used for making cutting tools which are to be used at high speed. These are M series (Molybdenum type 10% Mo) or T series (Tungsten type 12%-18% W). 18-4-1 is an extensively used HSS in which Tungsten is 18%, Chromium is 4% and Vanadium is 1%.
- Tungsten series: Consists of 12-18% W, Cr, V and Co
- Molybdenum series: Consists of 10% Mo, W, Cr, V and Co
- M Series, has high abrasion resistance as compared to T series. It undergoes less distortion and is economical.
- Stellite: It is a non-ferrous alloy containing Co 40-55%, Cr 30-35%, W 10-20%, C 0-5% along with Tentalum, Molybdenum and Boron.
- These are produced by casting and are especially useful for form tools.
- They have intermediate hardness between HSS and carbides.
- The toughness is low as compared to HSS and are susceptible to impact forces.
- They retain hardness upto 900 degrees and are useful upto 50 m/min.
Cemented Carbides (Cermets):
- Cermets are produced by using Powder Metallurgy by using a mixture of W, Titanium, Tantalum, and Carbon to produce carbides.
- The carbides are in powder form and are mixed with Co which acts as a binder. After this high pressure is applied at a high temperatures of 15000 degrees.
- These carbides are brazed or mechanically fastened to the tool shank.
- These tools retain their hardness upto 1000 degrees and can be used for a speed range of 60 m/min - 300 m/min.
Standardization of Carbide tools:
P - for material of large chip like steel
M - between P and K
K - short chipping materials like CI
- 01 - finishing operation
- 10, 20, 30....toughness increases
The smaller values are used for finishing operations whereas the larger values are used for rough machining because they have high toughness.
2. Coated Carbides:
Coating such as titanium carbide, Al oxide, Hafnium nitride are used to increase the tool life. These coatings are in the range of 2-15 μm but the increase in tool life is 5-10 times.
Ceramics or Cemented Oxides
- The composition consists of Aluminium Oxide added with Silicon Carbide, Mg oxide, Chromium oxide, titanium carbide etc. and they are cold pressed into insert shapes under high pressure and sintered at high temperatures. Addition of Titanium carbide and Zirconium oxide helps improve properties of toughness and shock resistance.
- Ceramic tools cannot be used to machine certain materials like Aluminium and Titanium since they have a strong affinity towards them and due to that certain chemical reactions might take place.
- It is the highest substance among all known material.
- It has high hardness, good thermal conductivity, low friction, good wear resistance.
- Diamond is very costly and cannot be used to machine steels as it goes into allotropic transformation into graphite on interaction with iron at 730 degrees.
Cubic Boron Nitride (CBN)
- Boron has hexagonal structure but when it is heated under constant pressure it converts into cubic structure which is very hard.
- It is used to machine stainless steel at 600 m/min to 700 m/min.
- It is used as abrasives in grinding wheels for grinding very hard material.
SiAlON (Silicon Nitride based Ceramics)
- SiAlON stands for Silicon Nitride based materials with Al and oxygen.
- SiN - Silicon Nitride based ceramic tools consists of Silicon Nitirde with aluminium oxide, Yitrium oxide, Titanium carbide.
- It has chemical affinity towards iron, so it is not used for machining steels.
- It is produced by rolling.
- It is used for machining at high speeds and feeds.
- Cutting speed is 60% more and 3.5 times more edge life compared to conventional carbides.