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Thread: Datum Letters

  1. #1
    Associate Engineer
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    Datum Letters

    Whatís the reason to have Datumís A, B, E, F, G, H? C and D is not referenced anywhere on mechanical drawing. Is the a concern? Bad practice? Good reason for it?

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    With respect to ANSI, ASME and ISO dimensioning and tolerancing standards one can use any letter designator as a datum other than I, O and Q. You can even use double letters like datum "AA" or "ZF" and even combine numbers with letters as typical with datum targets.
    e.g. A1, A2, A3, etc..

    Not using "C and D" would be a preference as there is not a rule on the order of letters chosen for datums. I prefer using the letters in the order given by the alphabet however I know this is not a requirement.
    Last edited by Kelly_Bramble; 09-07-2022 at 12:34 PM.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Thank you so much Kelley. You are a life saver.

  4. #4
    Technical Fellow jboggs's Avatar
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    The designer might have been using some software like Solidworks, in which datum designators are automatically assigned in sequence as they are placed. "A" followed by "B" followed by "C", etc. If one then later deletes one of those datums, the software does not automatically re-assign the others to keep a full sequence. So you could end up with A, B, C, and E. No D.

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