Metrology, the science of measurement, should and does play a significant role in manufacturing. A number of unrelated factors can significantly affect the accuracy or quality of manufactured product. Limitations in machine precision, tool wear, setup errors, temperature changes, contaminants (such as dirt or metal shavings), and material variations all introduce flaws in the resulting product. Some level of flaws are simply unavoidable in the real world. The essential questions are how much does the work product deviate from the design ideal, and is that deviation within acceptable limits? Metrology, then, is the process of performing measurements of the actual state of the work product, comparing it to a nominal design definition, and evaluating whether the deviations are within an allowable tolerance limit. Hand tools for measurement include hole gauges (size pins), feeler gauges, calipers, and runout dial indicators. Of course, the mathematical operations of calculating and tolerancing are ideally suited for computer software, and even the measurements can be more repeatably performed with a computer-controlled machine; and so, for decades various CMMs (Coordinate Measurement Machines) have been sold for manufacturing inspection. Most CMMs operate by taking discrete point measurements and constructing a mathematical "substitute" feature from the measurement point data. The elements (location, orientation, size, etc) of these substitute features are then compared to the nominal "blueprint" features, and the deviations are compared to tolerance limits. This can provide information for pass/fail decisions, as well as an indication of specific errors in the manufacturing process. CMMs typically can be used to measure holes and pins, planes, lines, circular fillets, slots, pockets, and other machined entities. Some CMMs also supports two dimensional curves and three dimensional surfaces. The DMIS standard, a widely used CMM file format and programming language, defines a number of "prismatic" features (those with a well defined geometry), including arcs, circles, and ellipses, cylinders and cones, planes, lines, points, plus two and three dimensional slots, spheres and toruses. DMIS also supports Bezier curves and NURBS and/or raw surfaces. A representative sampling of tolerances includes cylindricity, circularity, runout, flatness, position, angularity, and form. These capabilities allow detection of many possible defects, for example deformations of surfaces and bores, errors in size or depth, and misalignments of mating features.
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