Tool wear

tool wear

1. Tool Wear & Tool Life

2. •If cutting force becomes too high, the tool fractures
•If cutting temperature becomes too high, the tool material softens
and fails
•These conditions causes the tool to fail, continual wear of the
cutting edge ultimately leads to failure.
•Cutting tool technology has two principal aspects:
1.tool material-Developing materials to withstand the forces,
temperatures, wearing action
2. tool geometry -optimizing the geometry of the cutting tool
for the tool material and for a given operation

3. •Three possible modes by which a cutting tool can fail in
machining:
•1. Fracture failure -cutting force at the tool point
becomes excessive, fail suddenly by brittle fracture.
2. Temperature failure-Cutting temperature is too high
for the tool material, the tool point to soften, which
leads to plastic deformation and loss of the sharp edge.
3. Gradual wear -Gradual wearing of the cutting edge causes loss of
tool shape,
reduction in cutting efficiency,
acceleration of wearing as the tool becomes heavily worn,
finally tool failure in a manner similar to a temperature failure.

4. •Gradual wear is preferred because it leads to the longest possible
use of the tool,
•Gradual wear occurs at two principal locations on a cutting tool:
1. the top rake face
2. the flank.
Two main types of tool wear can be distinguished:
1. crater wear
2. flank wear

6. •Crater wear,
consists of acavityin the rake face of the tool that forms
and grows from the action of the chip sliding against the
surface.
High stresses and temperatures characterize the tool–
chip contact interface, contributing to the wearing action.
Crater can be measured either by its depth or its area.

7. •Flank wear
occurs on the flank, or relief face, of the tool.
It results from rubbing between the newly generated work surface
and the flank face adjacent to the cutting edge.
Flank wear is measured by the width of the wear band;
This wear band is sometimes called the flank wear land.
An extreme condition of flank wear often appears on the cutting
edge at the location corresponding to the original surface of the
work part. This is called notch wear.

8. •It occurs because the original work surface is harder and/or
more abrasive than the internal material, which could be
caused by work hardening from cold drawing or previous
machining, sand particles in the surface from casting, or other
reasons.
•As a consequence of the harder surface, wear is accelerated
at this location.
•A second region of flank wear that can be identified is nose
radius wear; this occurs on the nose radius leading into the
end cutting edge.

9. The mechanisms that cause wear at the tool–chip and tool
work interfaces in machining
•Abrasion-Mechanical wearing action caused by hard particles in
the work material gouge and removing small portions of the tool.
abrasive action occurs in both flank wear and crater wear
it is a significant cause of flank wear.
•Adhesion-When two metals are forced into contact under high
pressure and temperature, adhesion or welding occur between
them.
These conditions are present between the chip and the rake face
of the tool.
As the chip flows across the tool, small particles of the tool are
broken away from the surface, resulting in attrition of the surface.

10. •Diffusion
This is a process in which an exchange of atoms Occurs at the tool–
chip boundary,
Tool surface to become depleted of the atoms responsible for its
hardness.
As this process continues, the tool surface becomes more
susceptible to abrasion and adhesion.
Diffusion is believed to be a principal mechanism of crater wear.
•Chemical reactions
The high temperatures and clean surfaces at the tool–chip interface
in machining at high speeds can result in chemical reactions
oxidation, on the rake face of the tool.
The oxidized layer, being softer than the parent tool material, is
sheared away, exposing new material to sustain the reaction
process.

11. •Plastic deformation
The cutting forces acting on the cutting edge at high temperature
cause the edge to deform plastically, making it more vulnerable to
abrasion of the tool surface.
Plastic deformation contributes mainly to flank wear.
Most of these tool-wear mechanisms are accelerated at higher
cutting speeds and temperatures.
Diffusion and chemical reaction are especially sensitive to
elevated temperature
The various wear mechanisms result in increasing levels of wear on the
cutting tool

12. Three regions can usually be identified in the typical wear growth
curve.
1.break-in period -sharp cutting edge wears rapidly at the
beginning of its use.
occurs within the first few minutes of cutting.
2. steady-state wear region -wear that occurs at a fairly uniform
rate.,
3. failure region -the wear rate begins to accelerate. In which
Cutting temperatures are higher
the general efficiency of the machining process is reduced.
If allowed to continue, the tool finally fails by temperature failure.

14. •The slope of the tool wear curve in the steady-state region is
affected by work material and cutting conditions.
•Harder work materials cause the wear rate (slope of the tool
wear curve) to increase.
•Increased speed, feed, and depth of cut have a similar effect,
•speed being the most important of the three.
•As cutting speed is increased, wear rate increases so the same
level of wear is reached in less time.

15. Tool Life
•Tool life is defined as the length of cutting time that the tool can be
used.
•Operating the tool until final catastrophic failure is one way of
defining tool life.
•A convenient tool life criterion is a certain flank wear value, such as
0.5 mm
•When each of the three wear curves intersects that line, the life of
the corresponding tool is defined as ended

16. •Taylor Tool Life Equationif the tool life values for the three wear
curves are plotted on a natural log–log graph of cutting speed
versus tool life, the resulting relationship is a straight line

17. •The discovery of this relationship around 1900 is credited to F.W. Taylor.
•It can be expressed in equation form and is called the Taylor tool life
equation:
vT
n
= C
•where v = cutting speed, m/min; T = tool life, min; n and C are
parameters .
•The value of n is relative constant for a given tool material
•The value of C depends on tool material, work material, and cutting
conditions.
•Basically, Eq. states that higher cutting speeds result in shorter tool lives.
•Relating the parameters n and C to Figure n is the slope of the plot
C is the intercept on the speed axis.
•C represents the cutting speed that results in a 1-min tool life.