Question
What is the correct way to document and control an elliptical feature of size (or other mathematically defined shape that isn’t covered by the ASME Y14.5 examples)?

In the past, I’ve used basic dimensions for the major and minor axis, a profile control, and a note that says, “TRUE ELLIPSE,” but some metrology labs don’t have the capability to measure ellipses with their CMM.

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Answer
Generally speaking, an ellipse is not a true feature of size because it does not have one size dimension that can apply to all opposed surface elements, like a pin, hole, or plate thickness.

You can have two different size dimensions for the major and minor axis, but these size dimensions will not control the surface elements between the major and minor axes. Basic dimensions applied to the opposed surface elements at the two axes and basic radii are best, because then you can specify a profile of surface tolerance applied all around to control the overall size and shape of the ellipse. Datum references can be added to the feature control frame to also control the location and orientation of the ellipse.

Most CMM’s are capable of measuring profile. You can reference the 3D CAD model in lieu of the 2D basic dimensions if the CMM has the capability to measure relative to the 3D model. If a metrology lab is telling you that they don’t have the capability to measure a profile tolerance, then they may be using an older CMM with outdated software or the CMM operator may not have been properly trained on how to correctly measure geometric tolerances.

Roy Cross
ASME GDTP Senior Level S-0488
Dimensional Engineering Mentor