Types of CMM - Co-ordinate Measuring Machine Pedagogy Zone

The various types of CMM Co-ordinate measuring machine are,

Cantilever type.
Bridge type.
Horizontal boring type.
Vertical boring type.

Cantilever type

It is easy to load and unload, but it is most sensitive to mechanical error because of sag or deflection in y axis beam.

Bridge type

It is difficult to load but less sensitive to mechanical errors. A floating bridge type machine is also available in which the complete bridge can slide in y direction.

Horizontal boring mill type

It is best suited for large heavy work pieces.

Vertical boring mill type

Vertical boring mill is highly accurate but slower to operate.

Working principle

When the distance is measured between two holes using CMM, the work piece should be clamped to the workable and aligned for three measuring slides X,Y, and Z. Now, the tapered probe tip is seated in first the hole and probe position digital readout is set to zero.

The probe is then moved to successive holes, the read out represents the co-ordinate part print hole location with respect to the datum hole.

Automatic recoding and data processing units are provided to carryout complex geometric and statistical analysis. It is shown in above fig.

Special co-ordinate machines are provided both linear and rotary axes. These machines can measure various features of parts whose shapes are objects of revolutions like cones, cylinders and hemispheres.

Image shows spherical measuring machine having motions of their measuring head in R, Q, and Φ directions used for inspecting parts that are basically spherical.

Advantage of CMM:

The main advantage of co-ordinate measuring machine is the quicker inspection and accurate measurements.

Features of CMM

In faster machines with higher accuracies, the stiffness to weight ratio has to be high in order to reduce dynamic forces.
All the moving members, the bridge structure Z- axis carriage and Z-column are made of hollow box construction.
Errors in machine are built up and fed into the computer system so that error compensation is built up into the software.
All machines are provided with their own computers and the CMM can able to measure three-dimensional object from variable datums.
For compensation of temperature gradient, thermocouples are connected with the machine and interfaced with the computer. This will provide the CM in high accuracy and repeatability.
Rapid growth in software for three and four axes movements enable CMM to measure hole center distances and form measurements such as turbine blades, cam profiles.