A CMM can benefit any shop

Introduced in 1959, coordinate measuring machines have become increasingly common over the last 2 decades. The complexity and cost once meant CMMs were only suitable for large manufacturers. Advances in CMM technology and manufacturing techniques have reduced the cost of ownership, and the machines are now found at all types and sizes of shops.

In operation, a CMM collects a series of 3D points by touching workpiece surfaces with a probe. Collection is done predominately through tactile scanning or touch-trigger operation, but some CMMs gather points through noncontact methods, such as laser scanning.

Courtesy of C. Tate

Joel Johnson, continuous improvement engineer for Mitsubishi Hitachi Power Systems Americas, sets up a CMM.

Touch-trigger operation is the more traditional and familiar technique. This method positions the probe above a part and, through a known vector, moves the probe until it contacts the surface. When contact is made, the probe is triggered and the CMM saves the coordinates of the touched point.

Tactile scanning places the probe in contact with the surface and follows a programmed path while maintaining a constant force against the surface. As the probe is driven along the path, a series of points is gathered and stored.

Determining the best measurement method depends mostly on the part configuration and the analysis that needs to be performed. In general, the scanning probe is a better choice when measuring features for flatness, roundness, profile and similar forms because it quickly gathers large numbers of data points. More data points give greater detail, making analysis easier. Touch-trigger probing can gather the same number of points and is as accurate, but takes longer.

After the points are collected, algorithms compare the condition of the surface to a programmed nominal value. Deviations from the nominal surface are compared to the allowable tolerance, and a report tells the operator which dimensions are acceptable and which are not.

A CMM can be more desirable than measuring with manual instruments such as micrometers, calipers and height gages because the CMM automates the process. Automating part measurement with a CMM can reduce the need for skilled personnel in the same way a CNC machine tool can. It is common for experienced personnel to program and set up CMMs and then have others operate them.