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Don’t Forget the Tangent Plane
(In accordance with Y14.5M-1994 standard)

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When we establish datums, we use the high points of the feature to simulate how the part will be contacted in its assembly. In many assemblies the considered features, those being controlled with geometric tolerances, also contact mating features on the high points of mating surfaces. Sometimes it makes sense to apply the geometric control to a plane established by the high points of the feature. That is why the tangent plane modifier was added to the ASME Y14.5M-1994 standard. Here the Disk contacts the high points of the flat surface on the Drive Shaft. The tangent plane modifier, the letter T in a circle, means that the tolerance applies to a plane established by the high points of the surface. The flatness deviation in the surface would be ignored when the surface profile tolerance is inspected. A flat plate placed against the surface would have to be in the profile tolerance centered at the 14 mm basic dimension relative to the datum reference frame.

This modifier works well where it is expected that the process will create a surface that is concave.

Back to Tips Tip added Sep 2006