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Thread: GD&T Angular Datum Question

  1. #1
    Associate Engineer
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    Aug 2018
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    GD&T Angular Datum Question

    Hi,

    I'm fairly new to GD&T and have a question about creating an angular datum. I have a wheel with pockets along the outer edge. I would like the pockets to be toleranced based on their distance from a center hub and their angle from the imaginary line formed between the center hub and a pin hole. I've attached an image of how I think this should be done. I'm not sure of using a centerline as a datum is appropriate or if their would be a better method from tolerancing this. Any help is appreciated.

    Thanks,
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  2. #2
    Administrator Kelly Bramble's Avatar
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    Feb 2011
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    Bold Springs, GA
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    Assuming your dimensioning and tolerancing to the ASME Y14.5-2009 standard.

    1) Datums are never attached or defined as an arbitrary center line. Datums are features that are measured from and must be physically touched by the measuring equipment to be created or established. It is unknown which physical feature this center line is created from.

    2) To establish rotatational reference of the cutout features of size one feature in the pattern should defined as you datum "B". Then a center plane is established and the rest of the cutout features area then referenced to it.

    You're asking a very fundamental question - you should seek training or acquire a book.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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