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Thread: Basic or not?

  1. #1
    Associate Engineer
    Join Date
    Sep 2019
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    4

    Basic or not?

    Hello,

    This seems like a rudimentary question, but it has me questioning myself. I have a part with odd hole spacing. The part also has a notch. The relation to the holes and notch is not critical as the notch is only clearance for wires to pass through. In the attached drawing, should the angular dimension for the 1st hole from the centerline be basic or not?

    I am thinking not, as I do not need critical alignment with the notch. My general notes allow 1 for angles and this tolerance would be fine for the first hole location. The true position callout would locate the 1st hole radially and perpendicular to Datums C and A. (Datum A is the ID and Datum C is the top surface of the part). Then the other 11 holes would be held in proper angular relation to the 1st hole by the true position callout and basic angular dimensions.

    My thoughts are that the 1st angular dimension should not be basic since the vertical centerline is not a datum.

    Thanks
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  2. #2
    Administrator Kelly Bramble's Avatar
    Join Date
    Feb 2011
    Location
    Bold Springs, GA
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    2,259
    Unless there is a geometric tolerance to the feature defined by that center line the Basic dimension is not needed. However, a limit dimension is required since all features should be specified one way or another.

    A test for the requirement of a basic dimension is whether or not there is a datum associated or a geometric tolerance.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
    Join Date
    Sep 2019
    Posts
    4
    Thanks Kelly. I'll put a limit on that 1st angle dimension.

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