Counterbore to Hole coaxial true position requirement

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  • Counterbore to Hole coaxial true position requirement

    I was reading thru the ASME Y14.5-2009 handbook and came across something I haven't really paid much attention to before. In Section 7.4.2, it says

    "Where different positional tolerances are used to locate holes and counterbores (relative to common datum features), two feature control frames are used. One feature control frame is placed under the note specifying hole requirements and the other under the note specifying counterbore requirements. See Fig. 7-25. Different diameter tolerance zones for hole and counterbore are coaxially located at true position relative to the specified datums."

    I normally dimension true positions for holes and counterbores as 2 seperate true position dimensions, and normally these DRFs are fully constrained so the coaxial requirement is fulfilled because the DRF is allowed to shift. But my question is this:

    In the example shown (Fig 7-25), there is no datum that clocks the holes. In PC-DMIS, how would I dimension the holes and counterbores with different true position tolerances while maintaining the coaxial requirement? Since the pattern is allowed to rotate about datum -B-, I'm not sure how to force PC-DMIS to calculate two different true position tolerances using the same rotational shift...


    Fig 7-25.jpg

  • #2
    It seems that the way it is toleranced, the coaxial requirement isn't taken into account.

    If you still would like to do a check, use concentricity (or position) for each counterbore using each counterbores Ø6.3 hole as datum? You should end up with 8 results.
    Last edited by vpt.se; 03-11-2020, 05:58 AM.
    PC-DMIS CAD++ 2o19 R1 SP11

    Comment


    • Mike Ruff
      Mike Ruff commented
      Editing a comment
      I wouldn't have thought that the coaxiality is a requirement, either. But the text in the handbook describing the picture says "Different diameter tolerance zones for hole and counterbore are coaxially located at true position relative to the specified datums." To me, this means that even though both FCFs are allowed to rotate about the axis of datum -B-, they must rotate together.

    • Matthew D. Hoedeman
      Matthew D. Hoedeman commented
      Editing a comment
      since BOTH are called out to the same datums, then the requirement of simultaneity comes into play. So yes, they would have to rotate the same, but they are not concentric.

    • Mike Ruff
      Mike Ruff commented
      Editing a comment
      So the datum shift (rotation about Z-axis) would have to be the same for the thru holes and counterbores, correct? If I dimension these in PC-DMIS as two separate callouts, won't they be allowed to rotate different amounts? But if I have them in the same callout, they will have the same positional tolerance? How can I satisfy both of these requirements in PC-DMIS? Or am I (hopefully) missing something obvious?

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