Basic properties that cutting must posses are:
- Tool material must be at least 30 to 50% harder than the work piece material.
- Tool material must have high hot hardness temperature.
- High toughness
- High wear resistance
- High thermal conductivity
- Lower coefficient of friction
- Easiness in fabrication and cheap
Different elements used in cutting tool materials and their properties are
|Tungsten||Increases hot hardness
Hard carbides formed
|Molybdenum||Increases hot hardness
Hard carbides formed
|Chromium||Depth hardenability during heat treat hard carbides are formed
improving abrasion resistance
some corrosion resistance
|Vanadium||combines with carbon for wear resistance
retards grain growth for better toughness
|Cobalt||Increases hot hardness, toughness|
|Carbon||Hardening element forms carbides|
Different cutting tool materials used for cutting operations in practice are high carbon steel, high speed steel, non -ferrous cast alloys, cemented carbides, ceramics and sintered oxides, ceremets, diamond, cubic boron nitride, UCON and sialon.
1. High Carbon Steel tools
- Its composition is C = 0.8 to 1.3%, Si = 0.1 to 0.4% and Mn = 0.1 to 0.4%.
- It is used for machining soft metals like free cutting steels and brass and used as chisels etc.
- These tool loose hardness above 250°C.
- Hardness of tool is about Rc = 65.
- Used at cutting speed of 5m/min.
2. High speed steel (H.S.S)
General use of HSS is 18-4-1.
18- Tungsten is used to increase hot hardness and stability.
4 – Chromium is used to increase strength.
1- Vanadium is used to maintain keenness of cutting edge.
In addition to these 2.5% to 10% cobalt is used to increase red hot hardness.
- H.S.S is used for drills, milling cutters, single point cutting tools, dies, reamers etc.
- It looses hardness above 600°C.
- Some times tungsten is completely replaced by Molybdenum.
- Molybdenum based H.S.S is cheaper than Tungsten based H.S.S and also slightly greater toughness but less water resistance.
3. Non – ferrous cast alloys
It is an alloy of
Cobalt – 40 to 50%,
Chromium – 27 to 32%,
Tungsten – 14 to 29%,
Carbon – 2 to 4%
- It can not heat treated and are used as cast form.
- It looses its hardness above 800°C
- It will give better tool life than H.S.S and can be used at slightly higher cutting speeds.
- They are weak in tension and like all cast materials tend to shatter when subjected to shock load or when not properly supported.
4. Cemented carbides
- Produced by powder metallurgy technique with sintering at 1000°C.
- Speed can be used 6 to 8 times that of H.S.S.
- Can withstand up to 1000°C.
- High compressive strength is more than tensile strength.
- They are very stiff and their young’s modulus is about 3 times that of the steel.
- High wear resistance.
- High modulus of elasticity.
- Low coefficient of thermal expansion.
- High thermal conductivity, low specific heat, low thermal expansion.
According to ISO the various grades of carbide tool materials grouped as
- For cutting CI and non ferrous metals are designated as K10 to K50
- For cutting steel are designated as p10 to p50
- For general purpose application are designated as M10 to M50.
The advantages of carbide tools are
- They have high productivity capacity.
- They produce surface finish of high quality.
- They can machine hardened steel.
- Their use leads to reduction in machining costs.
5. Ceramics and sintered oxides
- Ceramics and sintered oxides are basically made of Al2O3, These are made by powder metallurgy technique.
- Used for very high speed (500m/min).
- Used for continuous cutting only.
- Can withstand upto 1200°C.
- Have very abrasion resistance.
- Used for machining CI and plastics.
- Has less tendency to weld metals during machining.
- Generally used ceramic is sintered carbides.
- Another ceramic tool material is silicon nitride which is mainly used for CI.
- Cermets is the combination of ceramics and metals and produced by Powder Metallurgy process.
- When they combine ceramics will give high refractoriness and metals will give high toughness and thermal shock resistance.
- For cutting tools usual combination as Al2O3 + W + Mo + boron + Ti etc.
- Usual combination 90% ceramic, 10% metals.
- Increase in % of metals reduces brittleness some extent and also reduces wear resistance.
- Diamond has
- Extreme hardness
- Low thermal expansion.
- High thermal conductivity.
- Very low coefficient of friction.
- Cutting tool material made of diamond can withstand speeds ranging from 1500 to 2000m/min.
- On ferrous metals diamond are not suitable because of the diffusion of carbon atoms from diamond to work-piece.
- Can withstand above 1500°C.
- A synthetic (man made) diamond with polycrystalline structure is recently introduced and made by powder metallurgy process.
8. Cubic Boron Nitride (CBN)
- The trade name is Borozone.
- Consists of atoms of Nitrogen and Boron and produced by power metallurgy process.
- Used as a substitute for diamond during machining of steel.
- Used as a grinding wheel on H.S.S tools.
- Excellent surface finish is obtained.
- UCON is developed by union carbide in USA.
- It consists of Columbium 50%, Titanium 30 % and Tungsten 20%.
- This is refractory metal alloy which is cast, rolled into sheets and slit into blanks. though its hardness is only 200 BHN, it bis hardened by diffusing nitrogen into surface producing very hard surface with soft core. It is not used because of its higher costs.
10. Sialon (Si-Al-O-N)
- Sialon is made by powder metallurgy with milled powders of Silicon, Nitrogen, Aluminium and oxygen by sintering at 1800°C.
- This is tougher than ceramics and so it can be successfully used in interrupted cuts. Cutting speeds are 2 to 3 times compared to ceramics.
- At present this is used for machining of aerospace alloys, nickel based gas turbine blades with a cutting speed of 3 to 5 m/sec.