The Coordinate Measuring Machine (CMM) is a staple of industrial inspection, with the flexibility to inspect a multitude of different geometric features for conformance to specifications, or to collect "clouds of points" in space to capture unknown geometry for reverse engineering purposes. Geometric Dimensioning and Tolerancing (GD&T) is an established language - with syntax and semantics defined in national and international standards - with which a designer can specify the desired geometry of a part to be produced. Often, a CMM will be used to determine whether an actual part does or does not conform to this specification. On the face of it, there are two main issues with using discrete points from a CMM to assess conformance to a geometric specification. First, do the points we've measured on the part surface adequately represent the entire surface? Second, does the algorithm used by the CMM software to analyze these points give a correct answer? Fortunately, the answer to these questions is often "yes." However, as tolerance specifications become tighter, more and more parts are in the borderline area between conforming and non-conforming. This paper will discuss some cases where the interpretation of the tolerance specification will influence the outcome of the inspection (conforming vs. non-conforming).
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