Cutting tool materials
Selection of cutting tool materials is very important
What properties should cutting tools have
Hardness at elevated temperatures
Toughness so that impact forces on the tool can be taken
Wear resistance
Chemical stability
Types of tool materials
o Carbon + medium alloy steel
o High speed steel (HSS)
o Cast cobalt alloys
o Carbides
o Coated tools
o Ceramics
o Cubic boron nitride
o invented by GE in 1969
o Silicon nitride
o Diamond
High Speed Steel (HSS)
Early 1900s
Very highly used alloy steel
Can be hardened to various depths
Good wear resistance
High toughness
Good for positive rake angle tools.
Two basic types of HSS
Molybdenum: ( M Series)
Tungsten: (T Series)
T Series
12 18 % Tungsten
Chromium, vanadium etc.
M Series
10% Molybdenum
Chromium, Vanadium, Tungsten, Cobalt
Better abrasion resistance
Less expensive
Less distortion
95% of HSS used is M series
High Speed Steel
Manufacturing
Cast
Powder metallurgy
Applications
Taps
Gear cutters
drills
Cast Cobalt (Stellite tool)
38 53% Cobalt
30 33% Chromium
10 20 % Tungsten
High hardness (58-64 HRC)
Good wear resistance
High temperature hardness
No Toughness
not suitable for intermittent cutting
Good for deep boring, continuous turning ( better than HSS)
Carbides
Most HSS and Cast Alloy have very low high temperature hardness
Low life for high speed machining
Carbides
High temp Hardness
Low thermal expansion
High modulus of elasticity
Tungsten Carbide (W-C)
Used for cutting non ferrous abrasive and metal + cast iron
Tungsten Carbide particles are bonded in Cobalt matrix
Cermet (Titanium Carbide)
Invented in 1950
used since 1970
Coated Carbide
Made of WC Co TiC - TiN
3 4 coatings of Al2O3
Particles sized 1- 5 mm are pressed and sintered into desired shapes (% of
Co may vary)
W-C is also compounded, sometimes with Titanium and Tantalum to
improve hot hardness and crater wear
Titanium Carbide
Ti-C has Ni-Mb matrix
Good wear resistance and poor toughness
Good for machining steel
Higher speed than W-C
Stiffness of the machine is important
Low feed, low speed and chatter can cause failure
Carbide Inserts
Smaller angle has less strength
Coated tools
The coating is 5-10 mm in thickness
Titanium Nitride
Low friction
High hardness
Resistance to high temperature
Improves life of HSS, carbides
Ceramics
High temperature resistance
Chemical inertness
Wear resistance
Al203
Ceramic Cutting Tool
Brittle
Nowadays used extensively
Ceramic
Fine grained , high purity Al203 cold pressed at high temperature and
sintered at high temperature (white)
Cermets
70% Al203 30 % T-C
Very high temperature hardness
High abrasion resistance
More chemical stability
Less tendency for adhesion so less BUE
Good surface finish while machining steel and CI
Poor toughness for intermittent cutting
Cubic Boron Nitride
Next to diamond, the hardest material
0.5-1mm polycrystalline cubic boron nitride
High wear resistance
But brittle
Used for machining hardened steel and high temperature alloys ( Ni for
instance)
Silicon Nitride
Used for super alloys
Diamond
Low friction and high wear resistance
Good cutting edge
Single crystal diamond are used to machine copper to a high surface finish
Because they are brittle rake angle has to be low
Polycrystalline diamond tool
(Compacted) synthesized crystals
Fused at high temperatures and high pressures
