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Thread: Symmetricity and using hole as Datum

  1. #1
    Associate Engineer
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    Symmetricity and using hole as Datum

    Hi,

    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
    Administrator Kelly Bramble's Avatar
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    Assuming an ASME standard then yes the center line of symmetry symbol is used correctly.

    See:
    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
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    In addition, the rotational degree of freedom about datum B is not constrained. Consider adding a datum C.

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

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