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If It Isn't Round, Use Profile of a Surface.

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.

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, 28 and 36 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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