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Datums Must Be of Sufficient Size
(In accordance with Y14.5M-1994 standard)

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When discussing parts like the one shown below often people ask why datum A is referenced in the position callout. After all, the hole locating dimensions are from B and C. One reason to include A is to control the perpendicularity of the four holes. Since the part is so thin it is unlikely that the holes would ever fail the positional requirement because they were tipped too much relative to datum A. A more important reason to include A as a datum reference is to assure a stable setup for inspection of the part. A planar primary datum takes away 3 degrees of freedom. B is too narrow to reproducibly take away more than 2 degrees of freedom. It is not "sufficient in size to permit its use" (section 4.3 of the Y14.5 standard), in this case as a primary datum feature. A major purpose for selecting the correct datum features is to assure reproducible inspection of parts. Datum A is sufficient in size (area) to accomplish this. Using datum features B or C as a primary datum would be like trying to balance a credit card on its edge.

Back to Tips Tip added Dec 1999