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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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  • No - the features in the lower tier of a composite must maintain basic orientation to the datums listed. You can't use 3D fit for this.

    Are you allowing the features to "float" from datum B when evaluating the lower tier? Assuming that datum B is a stopping datum along the axis of datum A, you need to do 2 fits. One about the axis of A, the other along the axis of A.

    Yes, I am allowing the features to float from datum B when evaluating the lower segment. The callout is 8X holes|.038|A|B for upper segment. Then .018|A for lower segment. The B datum (a flat surface at one end of the part) is not used in the lower FRTZF, so I'm still not understanding why I couldn't use a 3D best fit for that. Maybe I'm not seeing the whole picture.

    A related question, if not in this scenario, what would be a good use/application of a 3D Best Fit alignment?
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  • No - the features in the lower tier of a composite must maintain basic orientation to the datums listed. You can't use 3D fit for this.

    Are you allowing the features to "float" from datum B when evaluating the lower tier? Assuming that datum B is a stopping datum along the axis of datum A, you need to do 2 fits. One about the axis of A, the other along the axis of A.

    Yes, I am allowing the features to float from datum B when evaluating the lower segment. The callout is 8X holes|.038|A|B for upper segment. Then .018|A for lower segment. The B datum (a flat surface at one end of the part) is not used in the lower FRTZF, so I'm still not understanding why I couldn't use a 3D best fit for that. Maybe I'm not seeing the whole picture.

    A related question, if not in this scenario, what would be a good use/application of a 3D Best Fit alignment?
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