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Thread: Correct Tolerance callout

  1. #1
    Associate Engineer
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    Correct Tolerance callout

    I work for a company that makes these parts that were designed in the early 50's. So when looking at the prints There is not a datum structure defined nor any GD&T established. I have attached a picture of a mock part that we make. Its a casting that gets the I.D. turned on that lathe and then the part is taken to the mill where its sat on a fixture that locates the part on the I.D. then two holes are drilled and bored through the center of the part. Now in a perfect world the centerline of the two holes drilled and bored would intersect the centerline of the I.D. of the part. What kind of GD&T callout would that be. In my opinion its a true position call out(centerline of the two holes in relation with the centerline of the I.D. I work with a guy that keeps telling me its TIR. Well in my opinion TIR would be is you installed a shaft through the holes and checked the runout on that shaft. That would be TIR. But centerline position should be called out by a true position. Please let me know what you think. Thanksexample small.jpg

  2. #2
    Administrator Kelly Bramble's Avatar
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    Specify the turned ID as Datum A, identify to one of the holes 2X size +/- tol. then a feature control frame | position | dia. tol_h | A |

    This will control the axis of the two holes to tol_h accuracy. tol_h is equal to a diameter thus tol_h /2 = +/- some specified tolerance.

    TIR is a surface displacement to axis control
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Kelly, Thanks for the reply! Ok so TIR has nothing to do with the centerline of the two holes intersecting the centerline of the i.d. of the part?

  4. #4
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by mromano1988 View Post
    Kelly, Thanks for the reply! Ok so TIR has nothing to do with the centerline of the two holes intersecting the centerline of the i.d. of the part?
    For clarity - not useful for hole center lines perpendicular to each other.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    Thanks!

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