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If It Isn't Round, Use Profile of a
Surface.
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The very first Tip back
in 1997 dealt with concentricity and told you, "When in Doubt,
Use Runout." This is great advice where the feature is round
and you want it to stay that way. I have been asked what to do
with features that are shown coaxial on the drawing but are
not round. If these features are features of size, they can
certainly be controlled by position to the datum axis. If,
however, the features are not simple features of size, the
best choice is profile of a surface. Profile is the only
geometric tolerance that controls the location of surfaces.
Here the 4-lobed feature needs to be centered on the datum
axis.
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| There are a few other points
to note on this drawing. Datum feature B is referenced
in the profile of a surface control on the 4-lobed
feature. B only arrests 1 degree of freedom, the axial
movement. This degree of freedom does not affect the
profile of a surface tolerance zone on that feature; nor
does it do any harm to be referenced.
The 4-lobed feature is
defined by the R8, Ø28 and Ø36 basic dimensions. It is
also implied centered on datum axis A. The four 8mm
radii are implied to be 90° basic apart and oriented
relative to the third datum plane which is established
by datum feature C. These are all basic. There is no
tolerance on these dimensions and implied relationships.
They may all be considered the goals. The all around
profile of a surface tolerance of 0.12 is on the 4-lobed
feature, not the dimensions.
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