Machine tool metorology

1. Machine tool metrology

2. Machine tools for the work shop must be able to produce work-piece of
given accuracy within prescribed limits consistently and without require in
artistic skill on the part of the operator and that the quality of work
pieces depends upon:
1. Stiffness and rigidity of the machine tool and its components
parts.
2. The alignment of various machine parts in relation to one
another.
3. The quality and accuracy of the control devices and the driving
mechanism.

3. Machine tools are tested at various stages, during assembly, after
assembly, installation, repairs or overhauls as per accuracy test chart in
order to determine whether it meets requirement of specification or not.

4. Acceptance test for a machine tool
1. Alignment test or Geometrical test
2. Performance test or Practical test

5. Alignment test is carried out to check the grade of manufacturing
accuracy of the machine tool.
It consists of checking the relationship between various machine
elements (such as bed, table, spindle etc) when the machine tool is idle
and unloaded.

6. Performance test consisting of checking the accuracy of the finished
components and is known as practical test.
Performance test, therefore, consists of preparing the actual test jobs on
machine and checking the accuracy of the jobs produced.
Performance test is carried out to know whether the machine tool is
capable of producing the parts within the specified limits or not

7. In addition to the manufacturing accuracy the working accuracy of the
machine is influenced by the following factors.
1. Geometry of the cutting tool (rake angle, clearance angle etc,)
2. Material of the cutting tool, shape and rigidity;
3. Material of the work piece, its size, shape and rigidity
4. Cutting speed, feed and depth of cut,
5. Work holding and clamping equipment,
6. Skill of operator,
7. Working conditions etc.

8. Various geometrical checks generally made on machine tool are:
(Before conducting these tests, it is essential that the machine is set up and principle
horizontal and practical planes and axes are checked with sprit levels, etc.)
a. Straightness
i. Straightness of a line in two planes
ii. Straightness of sideways of machine tools
iii. Straight line motion
b. Flatness
c. Parallelism, equidistance and coincidence.
Parallelism includes
i. Parallelism of lines and planes
ii. Parallel motion
d. Rectangular movements or squareness of straight lines and planes. Quality of the guiding
and bearing surface of beads, upright and base plates are also tested.
e. Rotation
i. Out of round
ii. Eccentricity
iii. Radial throw of an axis at a given point
iv. Out-of-true running (running)
v. Periodical axial slip
vi. Camming
f. Movement of all the working components
Main spindle is the fundamental element of the machine and is tested for concentricity, axial
slip, accuracy of axis and position, relative to other axes and surfaces.

9. Machine Tool Tests
The various test applied to any machine tool could be grouped as below
a. Tests for the level of installation of machine in horizontal and vertical
planes
b. Tests for flatness of machine bed and for straightness and parallelism of
bed ways or bearing surfaces.
c. Tests for perpendicularly of guide ways to other guide ways or bearing
surfaces.
d. Tests for true running of the main spindle and its axial movements.
e. Tests for parallelism of spindle axis to guide ways or bearing surfaces.
f. Test for the line of movement of various members e.g., saddle and table
cross-slides etc., along their ways
g. Practical tests in which some test pieces are machined and their
accuracy and finish is cheeked.

10. Instruments used
1. Dial gauges,
2. Tests mandrills
3. Straight edges and squares
4. Spirit levels
5. autocollimators., etc.