What is a Coordinate Measuring Machine?

A coordinate measuring machine is a computer-controlled digitizer of three-dimensional geometric data. The machine can operate grid-like search patterns in a free-style mode wherein simple contours 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 upon which the specimen is placed. A stylus or pointer is connected 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 may move on supports across the examination table or via a goose-neck mechanism. The size of the specimens and the degree of accuracy required determines the machine choice.

To record the geometry of the sample, the stylus is positioned in a chosen spot. The contours of the piece are determined by the stylus tracing the altitude while 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 within the machine’s memory.

Upon completion of the data accumulation step, the computer within the coordinate measuring machine maps out a three-dimensional model of the object. The model may be vector-based if sufficient data were gathered. Otherwise, a connect-the-dots map of the points sampled can be produced. The data can typically be exported for other applications that use geometrical data, such as engineering design programs or certain surgical planning software. The data can be fed into graphics processing programs that will create visual models that can be displayed 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 the manufacture of objects, such as tooling, semiconductor patterns, and intricate mechanical parts. In the reverse of the above procedure, the specifications of the part to be examined are programmed into the machine. The data is entered in such a way that the optical system of the coordinate measuring machine can find a home location, such as the intersection of two edges or the center of a circular cutout.

This usage requires knowledge of how parts are specified and tolerances are established. If every possible dimension is specified as an exact measurement, there is no room for manufacturing variances. The designer of the part must take into account the abilities of the machines making that part and the additive effects of stacked tolerances. Having done so, a properly prepared blueprint, will specify dimensions, tolerances, and datums for the programmer of a coordinate measuring machine to follow.