CIR datum, does it use diameter, location, or both?

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  • CIR datum, does it use diameter, location, or both?

    I have a program, no CAD available, that uses an oddly angled circle as the tertiary datum. My question is, when using a circle datum, does it use diameter, location, or both? I have a nearby circle that I can get the correct diameter on. Will that work or is the location important too? I know how to create the dimensions, but I don't understand how they all work. Thanks in advance!

  • #2
    The tertiary datum can only lock down the degrees of freedom that are not locked down by the primary and secondary datums. The answer to your question will vary depending on what the primary and secondary datums constrain. That being said, the location AND/OR orientation of this circle will drive how it constrains your part, the diameter has no effect on the Datum Reference Frame. And if that "nearby circle" is not a datum on the print, you CANNOT use it as one

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    • #3
      Hi WhiteRabbit...
      might be nice to see your print callout, but..... a tertiary datum is only one of the 6 DoF (degrees of freedom) It could be X. It could be Y or it could clock (rotate). It's not going to use the size as anything unless it has a modifier. A 'nearby' circle? That is a bit of nonsense. Again, if you have no CAD, then your designer must mean the print is master. For datums, you should lose the thought of size until you align.

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      • #4
        So using the datum circle that I have, even if it is not the correct diameter, is the proper way to go?

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        • #5
          If size is wrong (you checked it another method?), that isn't good, but the size of your tertiary is irrelevant. The circle you state is difficult to check for size, so the Demon will report with most likely a funky location as well. IMHO I'd ask the designer to refine the Datum structure. OR... throw darts at the tertiary board.

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          • WhiteRabbit
            WhiteRabbit commented
            Editing a comment
            Yes. As I do not have a CAD model, I can't use an auto circle, which would be preferable in this case. The circle measures well within spec with a caliper, but getting the hits at even points around the 60 degree circle is proving challenging. As a result, the circle is tilted slightly forward or back depending on the location of the hits.

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