You Do Not Need
To Tolerance Datum Targets.
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 2mm general
profile tolerance.
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