Small Arc, Big Datum

[[Pe...]]

I have about 30 degrees of a 30 inch diameter as my primary datum reference. It seems to me that accurately measuring the center axis of this cylindrical feature is relatively impossible so was wondering if anyone had any tricks for it. Unfortunately I have .005 true positions and .005 profiles from this datum.

Barring a calypso solution, is there a particular clause of Y14.5 that I can bash the engineer over the head with to make them see the error of their ways? The part is basically a clevis bracket that is mounted on this diameter so they are saying per 4.8 "To ensure proper assembly, corresponding interfacing features of mating parts should be selected as datum features."

Thanks

[[Ro...]]

Have them read the next sentence of 4.8.
However, a datum feature should be accessible on the part and of sufficient size to permit it's use.

30 degrees of an arc is not of sufficient size to use as a datum that references it's origin.

An alternate route could be to define the 30 degree segment as a profile and do a best fit of the surface to measure to, as opposed to trying to find the center of it.

[[Ke...]]

Please sign in to view this quote.

Using the Best Fit alignment method has worked well for me, in the past (on a metal stamping, no less)
Aside from that, you can try to find an acceptable datum simulator; you may want to consider this as a back-up method, for if (when) your results are out of tolerance.