Compound Datum??

[[Ca...]]

I have a really basic part, with an easy datum structure. It's a plane for orientation, a cylinder for origin, and a bolt circle for rotation. Easy. The thing stumping me is that there are 2 DRF's that list datum C as a compound datum. I'm not sure what this adds to the DRF that wouldn't otherwise be there.

Any ideas?

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[[Ri...]]

My assumption would be that you would create a best fit pattern of the 18x, and that would be your Datum C (rotational only), but they could also mean something else (maybe this is where the parallel cylinder option comes into play??).

Just because you put it onto your drawing doesn't make it correct. Lol.

[[Se...]]

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Hello Richard,
you´re right: according to drawing, datum C is the 18x bore pattern. That means best fit pattern is the right way. 😉

[[SH...]]

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Datum cc is not clear.

[[Ca...]]

Thanks for the input. I fully understand that datum C was a pattern, and a best fit pattern is the correct method. I'm not sure why the DRF lists datum C as compound, unless the engineer is reiterating that datum C is all 18 holes, which isn't necessary, because the "18x" already takes care of that.

Thanks again for the input!

[[Da...]]

Looks like the designer works with ISO system also and forgot to "switch" to ASME, if that is ASME at all. [s]In[/s] By ISO 5459:2011 such pattern should be marked as C-C.

[[To...]]

C-C would be totally be unnecessary in ASME.