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A-c datum alignment

Shane-o
Opinion needed:

Was wondering if it was possible for one of these holes (dat-c-) to give good t.p readings. Since the parts is symmetrical all 3 holes should be used with dat-a- for an alignment (a-c in feature control frame). So it would be possible for a different selected hole to be dat-c- and give out of tolerance true position readings. Correct?
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  • If the 3 holes are shifted off-center as a pattern, or if 1 hole is quite differently deviated than the others, I think that it is possible to receive differing results.

    That was my initial thoughts but I'm trying to think of it in terms of a hard gauge.

    I see no scenario with a hard gauge where you'd get a different pass/fail depending on which hole you put a mating part in first.
  • I think that depends on the tolerance of C to A|B, and the tolerance of the other holes to A|B|C. I can't make out the print at all, to determine much of anything.

    I think it might be possible only if only 1 hole is shifted from the pattern. I'm not 100% sure, but that is what my intuition is telling me.

    Also, since there seems to be no allowable datum shift.
  • The company who made this part has admitted that they didn't torque the parts down. if my good results repeat , as well as my out of tolerance results repeat. then I don't see why I shouldn't be 100% positive about this issue.
  • That was my initial thoughts but I'm trying to think of it in terms of a hard gauge.

    I see no scenario with a hard gauge where you'd get a different pass/fail depending on which hole you put a mating part in first.

    Your hard gage should have 3 fixed pins that you have to insert at one time, not one at the time
  • Your hard gage should have 3 fixed pins that you have to insert at one time, not one at the time

    One of them should be a 2-way locator pin.

    Part of the issue is that one of the holes is being used as C, and then being checked back to itself.
  • First the part is not symmetrical about the centre datum a small amount of rounding has been used in the order of 0.0003"-0.0004" this will have an effect on the final results.

    Depending on which hole you start with and set the ' C ' datum then the results can be out of tolerance.

    If you create a set of generic holes pitched perfect to the drawing dimensions then off set a centre point 0.0015" along the Y axis the position diameter will come out at 0.003" for all
    holes.

    If you now create a line between the offset centre point and one of the lower hole and rotate align to that then the position results now vary between 0.0021" 0.0045".

    Think you need to talk to your customer about using a best fit to the holes
  • That was my initial thoughts but I'm trying to think of it in terms of a hard gauge.

    I see no scenario with a hard gauge where you'd get a different pass/fail depending on which hole you put a mating part in first.

    Your hard gage should have 3 fixed pins that you have to insert at one time, not one at the time

    I know this is older, but I would like to get a little clarification on this. I would tend to be in the minority here I guess and would have thought that even though the datum is anchored to the 12 o'clock hole, that the "3X" would have implied that the bolt circle itself would have been Datum C. This is how I would have(and have in the past) handled this. If this is wrong, I guess I need to correct my process
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