Tool wear, tool life & machinability

1. Tool Wear, Tool life & Machinability

2. Tool Life
 Useful cutting life of tool expressed in time
 Time period measured from start of cut to failure
of the tool
 Time period b/w two consecutive resharpenings
or replacements.

3. Ways of measuring tool life
 No. of pieces of work machined
 Total volume of material removed
 Total length of cut.
 Limiting value of surface finish
 Increase in cutting forces
 Dimensional accuracy
 Overheating and fuming
 Presence of chatter

4. Modes of tool failure
1. Temperature failure
a. Plastic deformation of CE due to high temp
b. Cracking at the CE due to thermal stresses.
2. Rupture of the tool point
a. Chipping of tool edge due to mechanical impact
b. Crumbling of CE due to BUE
3. Gradual wear at tool point
a. Flank wear
b. Crater wear

5. Tool wear
 Tool wear causes the tool to lose its original
shape- ineffective cutting
 Tool needs to be resharpened

6. Causes of Tool Wear
1. Attrition wear
2. Diffusion wear
3. Abrasive wear
4. Electrochemical wear
5. Chemical wear
6. Plastic deformation
7. Thermal cracking

7. Attrition wear
 At low cutting speeds
 Flow of material past cutting edge is irregular and
less stream lined
 BUE formed and discontinuous contact with the
tool
 Fragments of tool are torn from the tool surface
intermittently
 High
 Slow and interrupted cutting
 Presence of vibrations
 Found in carbide tools at low cutting speeds

8. Diffusion wear
 Diffusion of metal & carbon atoms from the tool
surface into the w/p & chip.
 Due to
 High temp
 High pressure
 Rapid flow of chip & w/p past the tool
 Diffusion rate depends on the metallurgical
relationship
 Significant in carbide tools.

9. Abrasive wear
 Due to
 Presence of hard materials in w/p material.
 Strain hardening induced in the chip & w/p due to
plastic deformation.
 Contributes to flank wear
 Effect can be reduced by fine grain size of the
tool & lower percentage of cobalt

10. Electrochemical wear
 When ions are passed b/w tool & w/p
 Oxidation of the tool surface
 Break down of tool material @ chip tool interface

11. Chemical wear
 Interaction b/w tool and work material
 Plastics with carbide tools
 Cutting fluid

12. Plastic Deformation
 When high compressive stresses acts on tool
rake face- tool deformed downways – reduces
relief angle
 Modifies tool geometry and accelerates other
wear processes

13. Thermal cracking
 Due to cyclic thermal stresses at cutting edge
 Comb cracks
 Transverse cracks
 Chipping of tool

14. Geometry of tool wear
 Flank wear (edge wear)
 Crater wear (face wear)

15. Flank Wear
 Tool slides over the surface of the work piece and
friction is developed
 Due to Friction and abrasion.
 Adhesion b/w work piece & tool- BUE
 Starts at CE and starts widening along the
clearance face
 Independent of cutting conditions and tool / work
piece materials
 Brittle and discontinuous chip
 Increases as speed is increased.

16.  Primary stage rapid
wear due to very high
stress at tool point
 Wear rate is more or
less linear in the
secondary stage
 Tertiary stage wear
rate increases rapidly
resulting in
catastrophic failure.

17. Crater wear
 Direct contact of tool and w/p
 Forms cavity
 Ductile materials – continuous chips
 Initiates rapid rupture near to nose
 Leads to
 weakening of the tool
 Increase in cutting temp
 Cutting forces & friction

18. Measurement of tool life
 Time for Total destruction in case of HSS or time
to produce 0.75 mm wear for carbide tools
 Tool life expressed by Taylor’s eqn
 VTb = C
 V = cutting speed in cm/min
 T= tool life in min
 b= const= 0.1 for HSS
 C= 50 for HSS
 Cemeted carbide : b=0.125, C=100
 Tool life expressed in volume of metal removed
 L = TVfd

19. Measurement of tool life
 Diamond indentor technique
 Radioactive techniques
 Test at elevated cutting speeds
 Facing tests
 Test with low wear criterion

20. Factors affecting tool life
1. Cutting speed
2. Physical properties of w/p
3. Area of cut
4. Ratio of feed to depth of cut
5. Shape and angles of tool
6. Tool material and its heat treatment
7. Nature and quantity of coolants
8. Rigidity of tool and wp

21. Machinability
 Machinability is defined in terms of:
1. Surface finish and surface integrity
2. Tool life
3. Force and power required
4. The level of difficulty in chip control
 Good machinability indicates good surface finish and
surface integrity, a long tool life, and low force and power
requirements
 Machinability ratings (indexes) are available for each type
of material and its condition

22. Factors affecting machinability of
metals
1. Material of w/p- hardness, tensile properties,
strain hardenability
2. Tool material.
3. Size and shape of the tool.
4. Type of machining operation.
5. Size, shape and velocity of cut.
6. Type and quality of machine used
7. Quality of lubricant used in machining
8. Friction b/w chip & tool
9. Shearing strength of w/p material

23. Evaluation of machinability- factors
 Tool life
 Form and size of chip and shear angle.
 Cutting forces and power consumption
 Surface finish
 Cutting temperature
 MRR per tool grind
 Rate of cutting under standard force
 Dimensional accuracy

24. Evaluation of machinability
 Machinability decreases with increase in tensile
strength and hardness
 Machinability of a material is assessed by any of
the following.
 Tool life
 Limiting MRR at which the material can be
machined for standard short tool life.
 Cutting force
 Surface finish
 Chip shape

25. Relative machinability Mg alloys
 Bearing bronze
 Al alloys
 Zn alloys
 Free cutting sheet brass
 Gun metal
 Silicon bronze, Mn bronze
 S.G Cast iron
 Malleable cast iron
 Gray CI
 Free cutting steel
 Sulphur bearing steel
 Cu-Al alloys
 Low carbon steels
 Nickel
 Low alloy steels
 Wrought iron
 HSS
 18-8 SS
 Monel
 White CI
 Stellite
 Sintered carbides

26. Machinability index
 Machinability index= Vt/Vs x100
 Vt – cutting speed of metal for 1 min tool life
 Vs – cutting speed of standard free cutting steel
for 1 min tool life.
Material MI
SS 25
Low carbon steel 55-65
Cu 70
Red brass 180
Al alloys 300-1500
Mg alloys 500-2000

27. Machinability:
Machinability of Ferrous Metals
Steels
 If a carbon steel is too ductile, chip formation can produce built-up
edge, leading to poor surface finish
 If too hard, it can cause abrasive wear of the tool because of the
presence of carbides in the steel
 In leaded steels, a high percentage of lead solidifies at the tips of
manganese sulfide inclusions
 Calcium-deoxidized steels contain oxide flakes of calcium
silicates (CaSO) that reduce the strength of the secondary shear
zone and decrease tool–chip interface friction and wear

28. Machinability:
Machinability of Ferrous Metals
Effects of Various Elements in Steels
 Presence of aluminum and silicon is harmful, as it combine with
oxygen to form aluminum oxide and silicates, which are hard and
abrasive
 Thus tool wear increases and machinability reduce
Stainless Steels
 Austenitic (300 series) steels are difficult to machine
 Ferritic stainless steels (also 300 series) have good machinability
 Martensitic (400 series) steels are abrasive

29. Machinability:
Machinability of Nonferrous Metals
 Aluminum is very easy to machine
 Beryllium requires machining in a controlled environment
 Cobalt-based alloys require sharp, abrasion-resistant tool
materials and low feeds and speeds
 Copper can be difficult to machine because of builtup edge
formation
 Magnesium is very easy to machine, with good surface finish and
prolonged tool life
 Titanium and its alloys have very poor thermal conductivity
 Tungsten is brittle, strong, and very abrasive

30. Cutting fluids
 Decreasing power requirement
 Increasing heat dissipation
 Neat oils+ extreme pressure additives
 Water emulsions