Compound Datum Advice

[[Ia...]]

This is more of a GD&T question as opposed to a Calypso question. Is this even remotely legal? I'd like to imagine the people that made this drawing are smarter than me, but this has left me a little flabbergasted.


These datum holes all reside on the face of datum A.

I think I understand what they are trying to communicate, but I don't think this is the right way to go about it.

[[Ri...]]

I would say no, but someone smarter will chime in hopefully.

Maybe the 29x holes make up the translational datum, and the 13 holes make up the rotational datum, but that is not how datum precedence works.

This is my gripe with engineers and drawings - no one follows the guidelines.

[[Da...]]

I was gonna say it doesn’t make any sense, but on second thought it seems plausible that the toleranced features are not evaluated to themselves but only to the common datum, which isn’t the same.

I don’t see an immediate reason why that shouldn’t work. The only tricky thing is to create the common datum correctly. The material boundary of the common datum must be the boundary of each hole at MMC simultaneously, nominal locations relative to each other and perpendicular to datum A, no datum shift allowed.

[[An...]]

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[[Ia...]]

Does it matter that B datum and C datum both contain groups of holes that have different nominal sizes and tolerances?

[[Ri...]]

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Not really.

[[Da...]]

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In your example, are all your holes the same size? As far as I know, the real complex part of the pattern as a common datum alignment both in ISO and ASME is that the datum simulator has to shrink/expand for each hole individually until it makes maximum contact to the actual diameters.

I know only one function in Calypso that can do that for more than one element, and that’s the step cylinder, but for a pattern? If it can be done at all, I don’t know how. My latest version of Calypso was 2014, don’t know what’s been cooking since then.