Statistical Engineering Division, ITL
Precision Engineering Division, MEL
Mathematical and Computational Sciences Division, ITL
Department of Mechanical Engineering, George Washington University
The accurate determination of the dimensions of manufactured parts is fundamental to the production of quality products. A coordinate measuring machine (CMM) offers an effective and flexible solution to the problem. U.S. industry alone uses more than 20,000 CMMs. However, there is currently no rigorous methodology to determine the accuracy of the measurements from a CMM. Consequently, CMMs are considered untraceable to the SI according to ISO 9000 definitions. Developing such traceability methodology for CMMs would (1) promote improvement in quality and efficiency through better determination of part dimensions and (2) facilitate international trade that requires ISO 9000 compliance. As part of a NIST competency project, SED scientists play an active role in a cross-disciplinary group developing traceability methods.
Basically, a CMM is a robotic machine that positions a sensing probe in its working volume. The probe contacts a sample of locations on the part surface and the CMM records corresponding three-dimensional point coordinates. The measurement process contains many sources of uncertainty. Some of the largest sources are the geometric distortions of the machine frame, the systematic effect of the probe, and thermal and mechanical effects of the operating environment. In the first two years of the project, our group developed a reliable model for real-time correction of the systematic effect of the probe. The result is an improved system without significant added costs. The paper ``Error Compensation for CMM Touch Trigger Probes'' by Estler et al, published in Precision Engineering in 1996, summarizes the results.
Currently, our group is working on the next large source of uncertainty, the geometric distortions of the machine frame. In order to fully describe these distortions, a high-dimensional (R3 R6) function is required. Such a function would require an inordinate amount of data to estimate. Based on established results, our group is attempting to account for the bulk of the distortions with a mathematical model of the rigid-body mechanics of the CMM. We are currently exploring designs to efficiently estimate these models. The accompanying figure displays 28 different positions of a calibrated artifact in the working volume of the CMM. The shaded positions are a minimum set required to estimate the model. Additional positions provide information to assess the validity of the model. Eventually the model will be used to predict the uncertainty of CMM measurements and will form part of a traceability methodology.
Figure 5: Calibrated artifact positions.
Date created: 7/20/2001