MACHINABILITY
UNIT II
J Ranganayakulu
Assistant Professor
- MS (By Research) IIT Madras
- Master of Human Resource Management
- PG. Dip., TQM
• Machinability denotes the relative ease with which a material
(usually a metal) can be machined using appropriate tooling and
cutting conditions.
• The term machinability refers to the ease with which a metal can
be machined to an acceptable surface finish.
• Materials with good machinability require little power to remove
material, achieve cutting at high speed, easily obtain a good finish,
and do not cause tool wear.
Definitions of Machinability:
• The first based on material properties,
• The second based on tool life, and
• The third based on cutting speed
1. Machinability is defined by the ease or difficulty with which the
metal can be machined.
• In this light, specific energy, specific horsepower, and shear stress
are used as measures, and, in general, the larger the shear stress
or specific power values, the more difficult the material is to
machine, requiring greater forces and lower speeds.
• In this definition, the material is the key.
2. Machinability is defined by the relative cutting speed for
a given tool life while cutting some material, compared to
a standard material cut with the same tool material.
3. Cutting speed is measured by the maximum speed at
which a tool can provide satisfactory performance for a
specified time under specified conditions.
4. Other definitions of machinability are based on the
ease of removal of the chips (chip disposal), the quality
of the surface finish of the part itself, the dimensional
stability of the process, or the cost to remove a given
volume of metal.
Criteria used to evaluate machinability
1. Surface finish and surface integrity of the machined part.
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.
• As for chip control, and as stated earlier regarding continuous chips,
long, thin, stringy, and curled chips can interfere severely with the
cutting operation by becoming entangled in the cutting zone.
CRITERIA TO EVALUATE MACHINABILITY
• The relative performance is expressed as an index number,
called the machinability rating (MR).
• The base material used as the standard is given a machinability
rating of 1.00.
• B1112 (Free cutting) steel is often used as the base material in
machinability comparisons.
• Materials that are easier to machine than the base have ratings
greater than 1.00, and
• Materials that are more difficult to machine have ratings less
than 1.00.
• Machinability ratings are often expressed as percentages rather
than index numbers.
Various conditions of work material
that affect machinability
Physical properties of work material
that affect machinability
Cutting parameters affecting machinability
• A series of tool life tests are conducted on two work materials under
identical cutting conditions, varying only speed in the test
procedure. The first material, defined as the base material, yields a
Taylor Tool Life equation VT0.28 = 350, and the other material (test
material) yields a Taylor equation VT0.27 = 440, where speed is in
m/min and tool life is in min. Determine the machinability rating of
the test material using the cutting speed that provides a 60 min tool
life as the basis of comparison. This speed is denoted by V60.
