The Y14.5 standard
tells us we can use a pattern of features to establish a datum
or datums (section 4.5.8). Unfortunately, the explanation that
follows in the Standard only covers a pattern of holes at MMC.
There is no support for using a pattern of features of size
RFS. Where two parallel holes are used as a pattern to help
establish a datum reference frame, it is pretty certain that
two datum planes will result - one plane through the two holes
and a second plane midway between the holes. Once more than
two holes are used as a pattern to establish a datum, the
resulting datum reference frame is unclear. This month's Tip
is to avoid using a pattern of more than two holes to
establish datums when referenced RFS.
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On this drawing the pattern of three holes
is identified as the datum feature(s). There is no
support for how to establish this datum reference
frame Regardless of Feature Size when inspecting
the profile and other position controls.
The next two drawings
illustrate ways to use two of the holes to help
clearly establish a datum reference
frame. |
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