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Question on Datum selection

Question on Datum selection

Question on Datum selection

I have an Assembly that has both a ridged, relatively small part and a larger, flexible part each bolted and riveted together thru a series of other parts. In regards to selection of a datum scheme, the primary datum could be a feature on either of the parts as they both equally represent functioning surfaces. There is about a 10 to 1 ration of the are of the larger surface to the area of the smaller. So with all other datum considerations the same, the larger surface typically are chosen as primary. However, the larger surface area in this case is flexible and the smaller surface is rigid. Without a check fixture, the “flexible datum candidate” is unrepeatable.
So the choice are:
Option A: The smaller datum is primary and accept the measurements include variation of the larger surface not seen in the next higher assembly.
Option B: The larger surface is primary, recognizing its unrepeatable nature due the flexibility of the feature and error of measuring the smaller feature (and any other feature that required to be inspected).
These are not the ideal situations as opposed to having a check fixture, but option A is the better choice, or the lesser of two evils, because it gives more meaningful inspection data in the end.
Sound reasoning? Are there other considerations? Any thoughts are appreciated.

RE: Question on Datum selection

Any chance at attaching a sketch or drawing?

Certified Sr. GD&T Professional

RE: Question on Datum selection

Tough call here. The is some what of a cop-out, but whichever of the two provides LESS reputability error would be my choice.

Certified Sr. GD&T Professional

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