| 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 14mm 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.
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