There continues to be the misconception that if a
feature is labeled as a datum feature, its use is somehow
implied. Take the drawing shown. The position tolerance on the
hole references datum features A and B but not C. Therefore
the hole must be perpendicular to the plane established by
datum feature A and 20 mm from the axis established by datum
feature B. Datum feature C has nothing to do with the position
of the hole. Yes, the hole is shown on a centerline that is
perpendicular to datum feature C. So, we know what the goal
is. But, since datum feature C was not referenced in the
position tolerance, there is no limit to how far the hole can
be off the 90° angle. A couple of possible, acceptable parts
are shown.
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| In order to avoid this, all
you have to do is add datum feature C to the position
tolerance. |
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