Fcf datum out of spec

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  • Fcf datum out of spec

    any one can explain about this case, datum B have MMC and the result get out of spec , where i can change tolerance or how to calculation the bonus datum B and C
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  • #2
    First, if you are adhering to ASME Y14.5 you need to change your standard to reflect that. FIT TO DATUMS shouldn't be off. I'd like to see the result when you turn it on.

    STND.jpg


    Second, your datum reference frame is [A|B(M)|C(M)]. What kind of features are A, B, and C?
    Third, per ASME Y14.5, B needs some sort of geometric tolerance defined back to A, and that dimension needs to be put above this dimension in your code.
    Fourth, C needs some sort of geometric tolerance defined back to [A|B(M)], and that dimension needs to be put between the two dimensions (the one listed above and yours).

    The reason that these datums need a geometric tolerance defined back to datums of higher precedence is to properly compute the MMB of the datum feature and allow you the proper amount of DRF mobility to compute this positional tolerance. Please refer to ASME Y14.5-2009 4.11.6/4.11.6.1.

    You should have some sort of value in the +TOL column for your two datum features. Once you add the proper geometric controls as stated above (they should be on your blueprint) there will be a corresponding value in the +TOL column.
    Last edited by JacobCheverie; 04-29-2020, 07:42 AM.

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