U. S. industry uses more than 20,000 coordinate measuring machines (CMMs). A CMM helps determine if a part conforms to design specifications by measuring the coordinates of a sample of locations on the part surface. Currently, there is no rigorous methodology to determine the accuracy and the precision of the measurements from a CMM. Consequently, CMMs are considered uncalibrated and not traceable to the SI according to ISO definitions. Developing calibration methodology for CMMs is necessary for U. S. companies to trade internationally. Additionally, the methodology would promote improvement in quality and efficiency through better determination of part dimensions.
As part of a NIST competency project, CAML scientists play an active role in a cross-disciplinary group that is making significant progress towards solving this problem. So far, the group has concentrated on understanding and modeling the CMM probe, which is the largest source of error in the measuring process. The probe is the component of the CMM that senses the contact with the part being measured. The most common class of probes has a construction that leads to pronounced systematic effects in the CMM measurements. Through a large modeling effort, the group has produced a reliable model of probe errors.
This year, ACMD has participated in the development of a computer program that allows for real-time correction of CMM measurements. The result is an improved measurement system without significant added costs. The NIST software was installed and demonstrated successfully in May at Giddings & Lewis (Dayton, OH), a major manufacturer of CMMs.
The group continues to work on expanding the model to cover more complicated probe configurations. It is also beginning to look at modeling CMM errors resulting from other sources than the probe.