What’s a CMM?

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A coordinate measuring machine is a computer-controlled digitizer of three-dimensional geometric data. The machine can operate grid search patterns in a freestyle mode where the simple outlines of a shape are explored by free selection of data points. The machine is also the basis for some automated inspection systems.

The machine typically consists of a flat table on which the specimen is positioned. A stylus or pointer is attached to a head that can rotate in any direction. The weight of the head is counterbalanced so it is easily manipulated. In some tables, the head moves on an arm that travels the length of the table. In smaller applications, the head can move on supports across the examination table or via a gooseneck mechanism. The size of the specimens and the degree of accuracy required determine the choice of machine.

To record the geometry of the sample, the stylus is placed at a pre-selected point. The contours of the workpiece are determined by the stylus plotting altitude following a rectilinear grid pattern in the x and y directions. In a simpler version of the coordinate measuring machine, the operator touches a number of points on the object and the x,y,z data is recorded in the machine’s memory.

Upon completion of the data accumulation stage, the computer inside the coordinate measuring machine draws a three-dimensional model of the object. The model can be vector-based if enough data has been collected. Otherwise, a join the dots map of the sampled dots can be produced. The data can typically be exported for other applications that use geometric data, such as engineering design programs or certain surgical planning software. The data can be fed into graphics processing programs which will create visual models that can be viewed and rotated in space.

The second common use of a coordinate measuring machine, coupled with an optical system, is the inspection of parts with tight quality control tolerances and for manufacturing objects, such as tools, semiconductor patterns, and complex mechanical parts. In contrast to the procedure described above, the specifications of the part to be examined are programmed into the machine. The data is entered in such a way that the coordinate measuring machine’s optical system can find a starting position, such as the intersection of two edges or the center of a circular cut.

This usage requires knowledge of how parts are specified and tolerances established. If every possible dimension is specified as an exact fit, there is no room for manufacturing variations. The part designer must take into account the capabilities of the machines making that part and the additive effects of stacked tolerances. Once this is done, a properly prepared design will specify dimensions, tolerances, and origins for the programmer of a coordinate measuring machine to follow.

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