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Thread: Profile tolerance Question

  1. #1
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    Profile tolerance Question

    I have the following callout that I have not seen before [profile of surface|A-B|C] where A is a datum surface , B and C are holes going thru Perpendicular to A. I have not seen the A-B Datum reference like this for a surface and a diameter. Can someone please explain this to me? Thanks

  2. #2
    Administrator Kelly Bramble's Avatar
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    Datum - Datum reference (A-B) simply means that one is creating a single datum reference from multiple separate features. If datum A and B where two separate plane surfaces - then the datum element would be a single datum plane derived from the true geometric counter part of both datum surfaces simultaneously.

    I'm not sure what the intent is for datum A = plane and datum B = cylinder in this application and would need to think about what the end datum reference plane(s) and axis might look like after simulation.

    My initial thought is that this is a drawing error. Can you post an image of the engineering drawing where this specification is show?

  3. #3
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    Quote Originally Posted by Kelly Bramble View Post
    Datum - Datum reference (A-B) simply means that one is creating a single datum reference from multiple separate features. If datum A and B where two separate plane surfaces - then the datum element would be a single datum plane derived from the true geometric counter part of both datum surfaces simultaneously.

    I'm not sure what the intent is for datum A = plane and datum B = cylinder in this application and would need to think about what the end datum reference plane(s) and axis might look like after simulation.

    My initial thought is that this is a drawing error. Can you post an image of the engineering drawing where this specification is show?

  4. #4
    Administrator Kelly Bramble's Avatar
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    So, this drawing is a hybrid between ISO and ASME 1994 standard or previous. "Gratseite" does in fact mean "Burr Side". Tolerances are probably ~ ISO 286 or a derivative of that standard.

    There might be a standard (default) tolerance reference to ISO-2768 stated on the engineering drawing as the datum B & C hole features Coaxiality is not stated on this view.

    The Profile of Surface (Profile any Surface - ISO) tolerance is unilateral outside (All of the Profile of Surface tolerance boundaries are from the nominal surface to the outside of the as-drawn surface).

    I'm not convinced that the datum order of precedence is specified “most correct” based on probable application for these Profile tolerances and associated surfaces.
    Never-the-less, I think the intent is that datum “A” sets up a plane, datum “B” is an axis located about the internal cylinder feature and includes the remaining two datum planes which I would suggest can rotate about the datum “B” cylinder feature. Datum “C” hole feature locks down the rotation of the two planes intersecting the axis about the datum B hole feature.

    So, your origin for measurement is located at the center of the datum B hole – top (where datum A intersects).


    Quote Originally Posted by marksmith View Post

  5. #5
    Administrator Kelly Bramble's Avatar
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  6. #6
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    I think you're being too kind, Kelly

    There seems to be no way to reconcile A and B into one theoretical datum, since they are fighting for different directions of space. Datum letters can only be hyphenated (in ASME and ISO too, if I recall) if they are establishing a common direction.

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