Calibration of Three Co-ordinate Measuring Machine (CMM)

The optical setup for the X-axis calibration is shown in fig below.

In this fig the laser head is mounted on the tripod stand and its height is adjusted corresponding to the working table of CMM. The interferometer contains a polarized beam splitter which reflects F₁ component of the laser beam.

Now, let the F₂ component passes through it. The retro reflector reflects the laser beam back along a line parallel to the original beam by twice the distance. It is made up of polished trihedral glass prism. For distance measurement, the F₁ and F₂ beams that leave the laser head are aimed at the interferometer which splits F₁ and F₂ via polarizing beam splitter.

In two beams, the component F₁ becomes the fixed distance path, and F₂ is sent to a target which reflects it back to the interferometer. Relative motion between the interferometer and the remote retro reflector causes a Doppler shift in the returned frequency. So, the measurement receiver photo detector in the laser head reads a frequency difference given by F₁ – F₂ ± ΔF₂.

Now, the difference F₁ – F₂ ± ΔF₂ signal that is returned from the external interferometer is compared in the measurement display unit to the F₁ – F₂ reference signal.

The difference, ΔF₂ is related to the velocity and hence to the distance. The longitudinal micrometer microscope of CMM is set at zero and the laser display unit is also set at zero. The CMM microscope is then set at the following points and readings are taken.

For to 10mm, the measurement is taken for every mm.
For 10 to 200mm, the measurement is taken in steps of 10mm.

The accuracy of linear displacement measurements is affected by change in air temperature, pressure and humidity. The automatic compensation for wavelength change is provided by the material sensor attached to the machine. Similar readings for Y and Z axes are taken. The optical arrangements for Y and Z axes are shown in fig.

Performance of CMM

Geometrical accuracies such as positioning accuracy, straightness and squareness.
Total measuring accuracy in terms of axial length measuring accuracy.
Volumetric length measuring accuracy and length measuring repeatability i.e., CMM has to be tested as complete system.
Environmental effects have great influence for the accuracy testing, including parameters, vibrations and relative humidity and required.

Applications of CMM:

The applications of CMM are,

CMM finds applications in automatic, machine tool, electronics, space and many other large companies.
CMM is ideally suited for development of new products and construction of prototype.
It is very much useful in checking NC produced work piece in various steps of production.
For aircraft and space vehicle, hundred percent inspections are carried out by using CMM.
CMM can be used for determining the dimensional accuracy of the components.
CMM is ideal for determination of shape and position, maximum metal condition, linkage of results etc.
CMM is best suited for the test and inspection of test equipment – gauges and tools.
CMM can also be used for sorting tasks to achieve optimum pairing of components with tolerance limits.
A CMM can replace several single purpose equipment with a low degree of utilization like gear tester, gauge tester, length measuring machine-measuring microscope etc.
CMMs are also best suited for ensuring economic viability of NC machines by reducing their downtime for inspection results. They also help in reading cost, rework cost at the appropriate time with a suitable CMM.