Statistical Issues in Geometric Feature Inspection Using Coordinate Measuring Machines

A coordinate measuring machine (CMM) is a computer-controlled device that uses a probe to obtain measurements on a manufactured part's surface, usually one point at a time. Probe movements may be programmed or determined manually by operation of a joystick. CMM's have gained tremendous popularity over hard gauges for dimensional measurement due to their flexibility, accuracy, and ease of automation. Problems arise in their use, however, because tolerancing standards require knowledge of the entire surface but a CMM measures only a few points on the surface. This article describes current practices for assessing geometric feature conformance to tolerance using CMM data. Existing work from several engineering fields is reviewed, and pertinent statistical literature is identified. Statistical problems involved in the inspection process are discussed, including issues relevant to model fitting and evaluation, sampling design, and sources of measurement error.