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 Stabilizing
an Orientation Tolerance Zone
(in accordance with the ASME
Y14.5-2009 standard)
PDF of This Tip
 Adding
a second datum feature reference to the orientation
tolerances of perpendicularity or parallelism can be
confusing.
On this drawing there
is a perpendicular relationship of the surface being
controlled by perpendicularity and the datum plane
established by datum feature A. The problem is that
the tolerance zone is not stable. The part can spin
on an angle plate used to establish the datum plane
making the inspection difficult.
Adding a second datum
feature reference will solve this. However, the
feature control frame would read that “the surface
must be perpendicular to the da tums
established by datum feature A, primary and datum
feature B, secondary”. The problem with this is
that the feature being toleranced is not
perpendicular to the datum plane established by B.
It is parallel. The 1994 Standard said that this is
fine. The second datum feature reference is added to
stabilize the tolerance zone.
The 2009 revision of
the Standard gives us an Alternative Practice of
using angularity for any of the orientation
tolerances. Taking this approach eliminates this
issue with orientation tolerances.
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