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Thread: True Position Callout with Basic off of Non-Datum?

  1. #1
    Associate Engineer
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    Question True Position Callout with Basic off of Non-Datum?

    Good afternoon everyone. I have a print I'm hoping to get some clarification on. There is a hole with a true position callout of [|.014(M)|A|B]. If we consider datum "A" to be the bottom of the part, one of the basic dimensions is referencing the top of the part (with the 2nd basic dimension referencing datum "B"). Is this correct, and if so how does the hole get inspected when the top of the part is just sheet tolerance to datum "A"?

    Thanks in advance for any and all replies!

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    Without an associated engineering drawing it's not clear how the datums are oriented or the basic dimensions referenced.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Hopefully this works. Datum "B" is irrelevant.
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  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    Well, we know where datum "A' is which specifies that the 2 holes tolerance boundaries are oriented and located parallel to that datum.

    The relationship of the two hole features to datum B is unknown.
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  5. #5
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    Wow, thank you for the quick response. Datum B is straightforward for me. The concern I am having is whether or not the .160" basic dimension is a valid way to call out the hole location, when the FCF is calling the hole out to datum A, and the surface the basic is referring to has a sheet tolerance of .010" to datum A.

  6. #6
    Technical Fellow Kelly_Bramble's Avatar
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    Sheet default tolerance is a separate requirement - normally "UNLESS OTHERWISE SPECIFIED".

    A datum is established from the physical feature regardless on how the datum feature is oriented, located or size specified whether by limit tolerance or geometric tolerances.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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