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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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  • Wouldn't all holes be located to datum B as they are controlled with basic dimensions? They way I see the dimensioning position is two circles, one with a .038 diameter and the other with .018 diameter centers are the same. Your holes are out of tolerance if they fall in the area between the .018 and .038 diameters for the composite but are still within tolerance for the .038. Both direction from datum A and datum B are still used to locate holes for the .018 and any tilt of the hole also takes away from your location tolerance. Is this not the way the dimensioning should be interpretted?

    No - in a composite positional tolerance, the toleranced features must maintain basic orientation to the datums in the lower tier. The pattern is free to translate in all 3 axes, and rotate about axes not constrained by datums in the lower tier, as long as the tolerance zones remain within the boundary of the upper tier zones. (this is a very abbreviated description - there are other considerations and complexities. see ASME Y14.5)
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  • Wouldn't all holes be located to datum B as they are controlled with basic dimensions? They way I see the dimensioning position is two circles, one with a .038 diameter and the other with .018 diameter centers are the same. Your holes are out of tolerance if they fall in the area between the .018 and .038 diameters for the composite but are still within tolerance for the .038. Both direction from datum A and datum B are still used to locate holes for the .018 and any tilt of the hole also takes away from your location tolerance. Is this not the way the dimensioning should be interpretted?

    No - in a composite positional tolerance, the toleranced features must maintain basic orientation to the datums in the lower tier. The pattern is free to translate in all 3 axes, and rotate about axes not constrained by datums in the lower tier, as long as the tolerance zones remain within the boundary of the upper tier zones. (this is a very abbreviated description - there are other considerations and complexities. see ASME Y14.5)
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