Download - Tool wear, tool life & machinability
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Tool Wear, Tool life & Machinability
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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.
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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
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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
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Tool wear
Tool wear causes the tool to lose its original
shape- ineffective cutting
Tool needs to be resharpened
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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
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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
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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.
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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
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Electrochemical wear
When ions are passed b/w tool & w/p
Oxidation of the tool surface
Break down of tool material @ chip tool interface
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Chemical wear
Interaction b/w tool and work material
Plastics with carbide tools
Cutting fluid
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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
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Thermal cracking
Due to cyclic thermal stresses at cutting edge
Comb cracks
Transverse cracks
Chipping of tool
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Geometry of tool wear
Flank wear (edge wear)
Crater wear (face wear)
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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.
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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.
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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
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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
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Measurement of tool life
Diamond indentor technique
Radioactive techniques
Test at elevated cutting speeds
Facing tests
Test with low wear criterion
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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 w\p
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Cutting fluids
Decreasing power requirement
Increasing heat dissipation
Neat oils+ extreme pressure additives
Water emulsions