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Thread: A question about datum reference mmb

  1. #1
    Associate Engineer
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    A question about datum reference mmb

    Hello Guys
    here is one question haunting in my head for months. please help see the attached image:
    so can I remove the MMC modifier in the feature control frame.how can I imagine two holes's simulator (if I remove the MMC modifier) adjusting in the same time or not?
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  2. #2
    Administrator Kelly Bramble's Avatar
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    Without the modifier specified for Datum B the Datum is established at RFS.. With a CMM, one would establish several circle starting at the top of the as-built hole features as well as several locations spaced along the length of each Datum B hole feature. From these circle measurements the CMM would establish a Datum axis.

    To establish Datum B at RFS in a fixture -- one would design the fixture with two expanding precision dowel pins or similar feature located at the basic dimension location and/or orientation. The as-built part is placed in the fixture with the expanding dowel pins collapsed and then the Datums are established and maintained in the order of precedence specified.


    Expanding Pin Example
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    Last edited by Kelly Bramble; 12-31-2014 at 07:44 AM. Reason: Clarify wording

  3. #3
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    " the Datums are established and maintained in the order of precedence specified." but for hole Dia17.2, two holes of B are its reference, which one will be the primary one , which one will be the secondary one, OR is it possible to change diameters for two references simultaneously?

  4. #4
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by lucien tian View Post
    " the Datums are established and maintained in the order of precedence specified." but for hole Dia17.2, two holes of B are its reference, which one will be the primary one , which one will be the secondary one, OR is it possible to change diameters for two references simultaneously?
    The primary datum is "A", which is established and maintained while establishing Datum "B". The Datum B holes features are established simultaneously.

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