Measurement of geometric tolerances in manufacturing pdf

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Measurement of Geometric Tolerances in Manufacturing - CRC Press Book

A datum reference or just datum is some important part of an object—such as a point , line , plane , hole, set of holes, or pair of surfaces —that serves as a reference in defining the geometry of the object and often in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value. For example, on a car's wheel, the lug nut holes define a bolt circle that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be concentric to within close limits or else the wheel will not roll smoothly. In carpentry, an alternative, more common name is "face side" and "face edge". The artisan nominates two straight edges on a workpiece as the "datum edges", and they are marked accordingly. For most work, the datum references of the workpiece need to be square. If necessary they may be cut, planed or filed to make them so.
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GD&T: The Basics of Geometric Dimensioning and Tolerancing

Stefan Menin, Ondrives. US Corp. May 10, Resources: Ondrives. Engineers who discount the importance of dimensional tolerance do so at their own risk.

Manufactured items differ in size and dimensions from the original CAD model due to variations in the manufacturing processes. For example, when drilling a mounting hole, the hole had to be within a specified X-Y area. An accurate tolerancing specification, however, would define the position of the hole in relation to the intended position, the accepted area being a circle. X-Y tolerancing leaves a zone in which inspection would have produced a false negative because while the hole is not within the X-Y square, it would fall within the circumscribed circle. Stanley Parker, an engineer who was developing naval weapons during World War II, noticed this failure in Driven by the need for cost-effective manufacturing and meeting deadlines, he worked out a new system through several publications.

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Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. The ability to produce quality products hinges on four key competencies: modeling of process form and precision levels, design tolerancing of parts and products, selecting production processes that match part specifications, and applying quantitative measurement methods for inspection and process control. The first two—process modeling and design tolerancing—are of primary importance and drive the second two; however, both are surprisingly ill-understood in a scientific sense. Mathematical models for predicting process precision, and quantitative precision and inspection data for actual processes, are scarce and often proprietary. Tolerancing today is based on informal definitions and on tolerance-assignment and inspection procedures of limited generality and validity. As a result, tolerancing; process selection and control; and, to some extent, metrology and nondestructive evaluation still rely largely on tradition.