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Don’t Complicate Control of Your Datum Features
(In accordance with Y14.5M-1994 standard)

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Datum features are just that-features. Since they are features, they need to be controlled. It seems that some people do not apply controls to datum features because they think datums are perfect, which is true. However, the features that establish the theoretical datums must be controlled. Other folks tend to over control the datum features. Start with the primary datum feature. Since everything is located and oriented from the datum established by this datum feature, in this case A, the only control required is form. Flatness is the appropriate control here. The secondary datum feature B is nominally at right angles to the first datum plane so it should be controlled with perpendicularity. Datum feature C, in this example, must be located relative to the origin of the datum reference frame established by A and B. The only geometric tolerance that locates surfaces is profile of a surface, which is the required control on datum feature C. Once these datum features are controlled, other features must be located relative to the datum reference framework. Use profile for the surfaces and position for the features of size.

Back to Tips Tip added Jun 2005