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Thread: Symmetricity and using hole as Datum

  1. #1
    Associate Engineer
    Join Date
    Jan 2022

    Symmetricity and using hole as Datum


    New to this. May be very basic, sorry.

    Can anyone please help to understand whether the below used GD&T is right? Can symmetricity be defined without calling out to which datum? If the hole needs to be used as datum but have symmetricity, how do I define
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  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Assuming an ASME standard then yes the center line of symmetry symbol is used correctly.

    ASME Y14.5M - 1994, paragraph 1.8.8 and 1.7.1, fig. 1-33
    ASME Y14.5-2009, paragraph 1.8.9 and associated fig. 1-35
    ASME Y14.5-2018, paragraphs 4.5.9 and 4.4.1 and fig. 4-33

    The only real issues I see are:

    If this is an ASME specified engineering drawing and Rule #1 is invoked then the parallelism tolerance is greater than the limits of size rendering that specification meaningless.

    The two hole features ( 2X dia.9 H7) specified should most correctly be specified to a perpendicular and not a position tolerance. Though position gets the message across.

    Hole features should probably have a "thru" indication or show hidden lines in the top view.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
    Join Date
    Jul 2021
    In addition, the rotational degree of freedom about datum B is not constrained. Consider adding a datum C.

  4. #4
    Associate Engineer
    Join Date
    Jul 2021
    Ignore my previous post. Datum B is along two holes so rotation is constrained.

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