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You Do Not Need to Tolerance Datum Targets
(In accordance with Y14.5M-1994 standard)

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Many folks are confused because the Y14.5 standard states that datum targets may be defined by basic or directly toleranced dimensions. Where basic dimensions are used, established tooling and gaging tolerances apply. If you think about it, it really doesn’t make a lot of sense to tolerance datum targets on the part drawing. Datum targets are defining the setup or fixture that will align the part to the datum reference frame. Datum targets are doing the same thing as a surface plate, angle plate, gage pin or a stack of gage blocks does. They are all used to establish the datum points, axes and planes. You wouldn’t tolerance the surface plate used to establish a planar datum so why tolerance datum targets that are doing the same thing. It would be like tolerancing the datum reference framework. Sure, if a fixture is built to establish the datum targets, there will be a drawing of the fixture and this fixture drawing will be toleranced. But, rule of thumb (Y14.43) says the tolerances will usually be a factor of 10 better than the part tolerances.

On this drawing the datum targets require that a fixture or equivalent be used to establish the datum reference frame when inspecting the part. If a fixture is made, it would be made to tolerances tighter than the 2 mm general profile tolerance.

Back to Tips Tip added Jun 2007