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DatumsDatumsDatums are the reference surfaces or the starting points for the location and orientation of features. They are essential for appropriate and complete tolerancing of a part. Datum geometries can become very complicated when they are features of size, compound datums, or features of an unusual shape. Geometric dimensioning and tolerancing provides the framework necessary for dealing with these complex datums. The simpler plus or minus tolerancing system ignores these complexities, which means that plus or minus toleranced drawings cannot adequately tolerance size features. As a result, many plus or minus toleranced drawings are subject to more than one interpretation.…
Gene R. Cogorno: Geometric Dimensioning and Tolerancing for Mechanical Design. Datums, Chapter (McGraw-Hill Professional, 2006), AccessEngineering