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.

The heat sources on machine tools excite the thermal modes, and, as a result, the machine is always moving. The machine never gets warmed up. As an example, consider a machine tool with a spindle mounted on the front of a vertical column. When the spindle is first turned on, the heat is largely contained in the spindle, which grows out, away from the column. The spindle position moves in +Y. Later in the day, even with the spindle still running at the same speed and under the same load, the heat from the spindle begins to migrate into the column, but on the front side more than on the back side of the column. The column deforms and begins to lean back, moving the spindle in –Y. If, as is more common, the spindle starts and stops and speeds up and slows down, then thermally induced errors become difficult to predict.

CMMs do not have to “be” anywhere. Rather, they have to know accurately where they are when the probe “clicks.” As a result, the drives and guideways can be rather flexible. Machine tools have to be in particular locations and they have to hold those positions against high forces. The drives and guideways must be very stiff. Machine tool axes have to be preloaded to avoid backlash, and they exhibit friction. Reversal errors are a significant problem for machine tools, but far less so for CMMs. Unlike CMMs, machine tools must move axes at prescribed (and often high) speeds to coordinate with the cutting actions of the tool.

CMMs do not usually experience significant process loads. The contact force between the probe and the target may be millinewtons. As a result, the deformation of a CMM during measurement operations is usually small and generally limited to the effects of the weight of the part (which does not change on a CMM), and the changing center of mass of the moving components.

Machine tools experience significant process loads. The cutting force can easily be hundreds of pounds. Cutting produces large spindle torques, and the rapidly moving heavy axes produce inertial loads.

For these reasons, machine tools are resistant to volumetric error compensation. It does not mean that machine tools cannot be used for measurement. Rather, it means that CMM-base measurements are, at least for now, significantly more accurate. CTE

About the Author: Dr. Scott Smith is a professor at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at kssmith@uncc.edu.