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Tool wear Mechanism

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Major wear mechanisms

1. Abrasive wear
2. Adhesion wear
3. Diffusion
4. Oxidation
5. Fatigue
6. Chemical decomposing

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1.Abrasive wear 1.Abrasive wear cont.

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• Softer material sliding over the face of hard material may

contain appreciable concentration of hard particles
• Hard particles act as small cutting edge like grinding wheel
• Hard particles result worn out of tool material
• Particles of hard material are intermittently turn out from the
surface and dragged along the surface

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2. Adhesive Wear Adhesive Wear cont.

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•When softer metal slide over hard metal , parts of soft metal •Finally the built up edges will torn from the surface result uneven
adhere high spots on the metal due to: structure on the tool surface
9Friction
9High temperature
9Pressure
•The spots result irregular flow of chip over the face and build
up of more particles on the tool

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3.Diffusion Diffusion cont.

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•When a metal is in sliding contact with another metal the

•hence the strength and abrasiveness of the softer material
temperature at the interface is high
Increase
•The high temperature allows the atoms of hard material to
•Atoms of the softer metal may also diffuse into harder
diffuse into softer material matrix
medium, thus weakening the surface of harder material
medium
di
•Diffusion phenomenon is strongly dependent on temperature

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4.Oxidation 5. Fatigue Wear

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Oxidation is the result of reaction between tool face and oxygen •Will occur when two surfaces slides in contact with other under
Ex. When machining steel work piece with HSS or cemented high pressure
carbide tool , groove formation is greatly accelerated if the •Roughness of one surface interlocks with those of other.
cutting zone is subjected to a jet oxygen. •Due to friction , compressive force will be produced in one side
and
d ttensile
il on other
th side
id ,These
Th phenomenon
h cause surface
f crackk

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Fatigue Wear cont. Types of Wear

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•Flank wear(wear land)

•Crater wear
•Chipping

• The cracks ultimately combines with one another and lead

surface crumble
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Flank Wear Flank wear

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•Primary region
•It produces wear land on the sides and breakage of sharp cutting edges
end of a flank •Secondary zone
•Tool-work piece interface The predominant zone caused by abrasion
•Predominant at low speed •Tertiary zone
Very sensitive to temperature ,caused
by diffusion wear
Increment of MRR increases flank wear
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Crater wear Crater Wear

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•It occurs on the rake face of the tool in the •Diffusion ,

form of pit called crater the lose of hardened atoms at the tool chip inter face is the major

•Predominant at high speed cause of creator wear

•Tool-chip interface

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Chipping (premature wear ) Tool life

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•It refers the breaking

g awayy of small chips
p from the cutting
g Criteria for tool life
edge of tool or an insert on account of impact or excessive
¾Change of quality of machined surface
plastid deformation
¾Change in magnitude of cutting force
•Large chipping can be caused by interrupted cutting and
¾Change in cutting temperature
sudden shock.

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Tool Life Prediction Tool Life Prediction cont.

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Modified Taylor’s tool life equation

• Taylor’s tool life equation
VT n d n1 f n2
=C
VT = C n
where V is the cutting speed,
where V is the cutting speed, T f is the feed
is the tool life, n is Taylor
d is depth of cut
exponent.
n and C are constants depends on T is the tool life,
work material, feed, depth of n, n1 and n2 are Taylor exponent.
cut and cutting speed.
n, n1 and n2 and C are constants depends on
work material, feed, depth of cut and cutting
speed.
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Tool life cont. Surface Quality

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Example
A steel bar of 40mm diameter is to be turned over a length of
200mm with a depth of cut 1.2mm and feed of 0.2mm/rev at
200rpm. If the tool life equation is given by … Ideal transverse roughness
† Rmax = f 2 /8R
VT 0 .2
d 0 . 13 f 0 . 31
= 40 † where R is the tool nose radius and f is the feed.

Determine the number of components that may be turned before

regrinding the tool.

a b
Tool life and Tool Wear 1/4/2013
Surface produced on steel during cutting by SEM, a) turning and b) shaping

Temperature in metal cutting Heat generation in metal cutting

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High temperature
a)) RRegion
i AB is i PPrimary
i shear
h zone-
• Reduces strength of the tool and formation of plastic deformation ( Ps )
create wear. b) Region BC is Secondary shear
• Shortens tool life zone friction ( Pf )
c) Region BD Between tool &
• Causes thermal distortion work piece plowing force (negligible)
• Caused dimensional change in work piece , Pm Power consumed
making of control dimensional accuracy difficult Pm = FcV
Fc Cutting force

Pm = Ps + Pf Pf = Ff V
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Temperature distribution in metal Temperature distribution in metal

25
cutting 26
cutting

Isotherms in temperature distribution

Temperature distribution at V = 25m s , HSS
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Cutting temperature control Temperature and Cutting Fluid

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… Function of cutting fluids

Temperature in metal cutting can be controlled by
† Lubrication
L bi i
• Application of cutting fluid(coolant)
† Cooling
• Changing the cutting condition by reduction of
† Chip removal
cutting speed and/or feed.
• Selection of proper cutting Geometry
ex Positive tool orthogonal rake angle
ex.

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Cutting temperature as a function of

cutting parameters
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Cutting temperature as a function of cutting speed V, feed f and depth of cut d

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