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Rule #1 of the
Standard tells us that if size is measured according to
the Rule, the form of the feature is automatically
controlled. In practical terms the feature is checked
with a full form element, such as a plug gage for a
hole, to make certain MMC is not violated. In addition,
two point measurements are required to be certain the
feature does not violate size at any cross section.
Therefore, the form controls of straightness, flatness,
circularity and cylindricity are usually not needed
since their deviation must be less than the total size
tolerance.
Position may then
be used to locate these features of size. If the feature
is cylindrical and shown coaxial to a datum axis, like
the Ø10 hole on this drawing, circular or total runout
may be used instead of position. Position and runout
both control the orientation of the feature. Therefore,
the orientation tolerances are usually not necessary but
may be used as a refinement if necessary. This drawing
illustrates how features may be located. Datum feature A
does not need a locating tolerance since the other
features are located and oriented relative to the datum
axis it establishes. Datum feature C establishes the
origin of all axial measurements and therefore does not
need to be located.
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