Machine tools are not CMMs

To improve productivity, it seems logical to measure parts using the same machine tool that makes them. After all, the machine tool usually has the required axes, sufficient work volume and typically a touch-trigger probe. Yet, full on-machine measurement is often not good enough. Why are the most accurate measurements still made on special-purpose coordinate measuring machines?

CMMs, like metalcutting machine tools, are constructed to be as accurate as economically feasible. CMMs however, unlike machine tools, are made more accurate through computer-based error compensation throughout the work space—volumetric error compensation. After a CMM is built, its error map is carefully measured using laser interferometry, for example. The error functions for straightness, squareness, linear positioning accuracy and angular errors are measured and tabulated. Because the geometry of CMM components can be taken as constant, these errors can be combined and corrected using software. While volumetric error compensation is almost universal in CMMs, it is rare for machine tools to have correction for more than ballscrew errors.

So why is volumetric error compensation more difficult for machine tools? The geometry of machine tool components is not constant, making machine tools different than CMMs in at least four significant ways.

CMMs are usually housed in well-controlled thermal environments. Metrology laboratories are routinely maintained at 20° C, the temperature at which dimensional measurements are specified. These rooms have large airflows, temperature-lock doors and special thermal control systems to hold the temperature within a narrow range, routinely to a small fraction of 1° C. Before part measurements are made, the parts are required to “soak out”—that is, to come completely to the temperature of the metrology room.

On the other hand, machine tools are often operated in poorly controlled thermal environments. Windows may allow sunlight into the room, and doors to the outside may be open. While machine shops are routinely heated, only a minority are air-conditioned.

The changing thermal environment changes the geometry of the machine tool. The machine tool has thermal modes in the same way that it has vibrational modes, but the time constants of the thermal modes are long—on the order of hours or days.

CMMs do not have significant on-board heat sources. While there are servomotors to drive the CMM axes, the required power is low. The axes of CMMs have low friction, often riding on a cushion of air. Machine tools have drive motors, preloaded axes and spindles, and receive heat from the cutting process and the sprays of lower-temperature coolant. These heat sources turn on and off unpredictably, depending on the part.