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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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  • OK, I checked the distance along that axis, (Cyl1 is Datum A, and its axis is the one on which the most linear deviation can occur). So they were pretty much spot on. The basic distance between the holes at level 1 and level 2 is 3.707". They all measured within +/- .003" of that value. So, I don't think that is an issue. 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. So, hole-to-hole seems to be good just looking at those numbers.

    Since this is a pattern of holes equally spaced around a cylindrical feature, would it be better to use for the lower half of the composite callout a 3D best-fit alignment (using the 8 holes to rotate/translate) instead of the 2D-best fit alignment that I used originally? I tried both, and with the 3D best-fit, all the holes show as being within the .018" positional tolerance. But going back to the 2D, they fail. It just seems to my feeble brain that this is a 3D datum, and makes sense to me to use the 3D BF instead of a 2D.

    Your thoughts?
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  • OK, I checked the distance along that axis, (Cyl1 is Datum A, and its axis is the one on which the most linear deviation can occur). So they were pretty much spot on. The basic distance between the holes at level 1 and level 2 is 3.707". They all measured within +/- .003" of that value. So, I don't think that is an issue. 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. So, hole-to-hole seems to be good just looking at those numbers.

    Since this is a pattern of holes equally spaced around a cylindrical feature, would it be better to use for the lower half of the composite callout a 3D best-fit alignment (using the 8 holes to rotate/translate) instead of the 2D-best fit alignment that I used originally? I tried both, and with the 3D best-fit, all the holes show as being within the .018" positional tolerance. But going back to the 2D, they fail. It just seems to my feeble brain that this is a 3D datum, and makes sense to me to use the 3D BF instead of a 2D.

    Your thoughts?
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