| Datums
Must be of Sufficient Size!
When discussing parts like the one
shown below often people ask why datum A is referenced in the
position callout. After all, the hole locating dimensions are
from B and C. One reason to include A is to control the
perpendicularity of the four holes. Since the part is so thin
it is unlikely that the holes would ever fail the positional
requirement because they were tipped too much relative to
datum A. A more important reason to include A as a datum
reference is to assure a stable setup for inspection of the
part. A planar primary datum takes away 3 degrees of freedom.
B is too narrow to reproducibly take away more than 2 degrees
of freedom. It is not "sufficient in size to permit its use"
(section 4.3 of the Y14.5 standard), in this case as a primary
datum feature. A major purpose for selecting the correct datum
features is to assure reproducible inspection of parts. Datum
A is sufficient in size (area) to accomplish this. Using datum
features B or C as a primary datum would be like trying to
balance a credit card on its edge.
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