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Composite position, threaded hole pattern

d.evans
I have a cylindrical part with a set of holes drilled 90 deg apart (3 o'clock, 6 o'clock, 9 o'clock and noon) around the circumference. Total of 8 holes- 4 at one level, 4 at another. The callout is very common to this type part, a PLTZF callout of .038" RFS to A|B, then a FRTZF callout of .018" RFS to A.

My question, if I have done this properly in my PC-DMIS program, is there any way that all 8 holes could pass for the upper PLTZF callout, and simultaneously all 8 holes fail the FRTZF callout? I can't see how they could, but when I executed a program recently that I've used on many of these same parts, this is exactly what the results show.

On a side-note, the drawing doesn't restrain all 6 degrees of freedom, so I chose a feature on the part that made sense to me to use to close that gap.

Thanks in advance Confused
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  • Are you allowing the features to "float" from datum B when evaluating the lower tier?

    Yes, I am allowing the features to float from datum B when evaluating the lower segment.

    Some sort of fitting must be done in Z axis to allow the pattern to translate "to itself".

    Do you have any suggestions? I'm still not sure what you mean by "fitting."

    So, I questioned the alignment I used,,,,first an alignment level to A, translated to B. Then within that, a 2D best fit alignment, least squares, Z axis, rotate & translate (this is the way I usually handle composite position call-outs.

    I am using the terms float and fit interchangeably - sorry for switching terminology on you. The best-fit alignment you are using is doing rotation about Z axis, and translation in X and Y. You are not allowing the holes to "float" from datum B, because your best fit is not allowing translation in Z axis. To deal with this, add a 2nd best-fit alignment(recalling the first best fit alignment) and choose "Specified Constraints", then use the radio buttons to lock down all degrees of freedom except translation in Z axis. Thing is, I'm not sure this method is available in 3.7MR3. If it is not, then I would deal with it this way (after the first best fit alignment):
    - construct midpoint between the 2 holes at 3 o'clock
    - construct midpoint between the 2 holes at 6 o'clock
    - construct midpoint between the 2 holes at 9 o'clock
    - construct midpoint between the 2 holes at 12 o'clock
    - construct plane from the 4 midpoints
    - set Z origin on plane

    I also checked the angular measurement of the holes in the pattern. Going with a generic +/- 1degree tol, I checked the angluar distance between the holes. They were all within .4 degrees from the basic 90.

    Not sure why you feel that +/-1 degree is a valid tolerance for comparison. 0.4 degrees over the 1.634 radius of the datum A diameter is ~.011", so that is some very significant variation considering that you have a .018 positional tolerance. It is possible that the part is just bad.
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  • Are you allowing the features to "float" from datum B when evaluating the lower tier?

    Yes, I am allowing the features to float from datum B when evaluating the lower segment.

    Some sort of fitting must be done in Z axis to allow the pattern to translate "to itself".

    Do you have any suggestions? I'm still not sure what you mean by "fitting."

    So, I questioned the alignment I used,,,,first an alignment level to A, translated to B. Then within that, a 2D best fit alignment, least squares, Z axis, rotate & translate (this is the way I usually handle composite position call-outs.

    I am using the terms float and fit interchangeably - sorry for switching terminology on you. The best-fit alignment you are using is doing rotation about Z axis, and translation in X and Y. You are not allowing the holes to "float" from datum B, because your best fit is not allowing translation in Z axis. To deal with this, add a 2nd best-fit alignment(recalling the first best fit alignment) and choose "Specified Constraints", then use the radio buttons to lock down all degrees of freedom except translation in Z axis. Thing is, I'm not sure this method is available in 3.7MR3. If it is not, then I would deal with it this way (after the first best fit alignment):
    - construct midpoint between the 2 holes at 3 o'clock
    - construct midpoint between the 2 holes at 6 o'clock
    - construct midpoint between the 2 holes at 9 o'clock
    - construct midpoint between the 2 holes at 12 o'clock
    - construct plane from the 4 midpoints
    - set Z origin on plane

    I also checked the angular measurement of the holes in the pattern. Going with a generic +/- 1degree tol, I checked the angluar distance between the holes. They were all within .4 degrees from the basic 90.

    Not sure why you feel that +/-1 degree is a valid tolerance for comparison. 0.4 degrees over the 1.634 radius of the datum A diameter is ~.011", so that is some very significant variation considering that you have a .018 positional tolerance. It is possible that the part is just bad.
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