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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
  • going from thread title... if you're using composite position in XactMeasure, you might wish to verify another way. XactMeas composite dim has issue, at least using it for profile. I would not trust totally for position either.
  • going from thread title... if you're using composite position in XactMeasure, you might wish to verify another way. XactMeas composite dim has issue, at least using it for profile. I would not trust totally for position either.

    I'm still using version 3.7MR3, so no XactMeasure is in play here. This is the only one of the parts measured so far (out of about 10 units) that have failed in this manner (all 8 hole-to-hole features).
  • If one set of four holes was closer to "end A" and the second set of four holes was closer to " B end " the max spacing between them could be 0.076. All eight holes as a pattern to themselves would fail to the 0.018 tol.
  • 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?

  • 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?

    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.
  • 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?
  • Datum A must remain "leveled" when evaluating lower tier. 3D best fit allows datum A to become "un-leveled".
  • Datum A must remain "leveled" when evaluating lower tier. 3D best fit allows datum A to become "un-leveled".

    OK, I get that. So, knowing that the holes appear to fit into the basic 90 deg X3 callout, and simultaneously into the basic 3.707" distance between levels, why would these fail when evaluated within a 2D best fit alignment, using the holes themselves (equally weighted)? It just seems to be falsely telling me that they are out of spec, hole-to-hole wise.

    Is there some other type of 2D BF I should use? I chose Z-plus (looking down onto the top of the cylinder that is Dat A) and rotate&translate. Is there something else that makes more sense? I attached a section of the drawing for clarity.
  • OK, I get that. So, knowing that the holes appear to fit into the basic 90 deg X3 callout, and simultaneously into the basic 3.707" distance between levels, why would these fail when evaluated within a 2D best fit alignment, using the holes themselves (equally weighted)? It just seems to be falsely telling me that they are out of spec, hole-to-hole wise.

    Is there some other type of 2D BF I should use? I chose Z-plus (looking down onto the top of the cylinder that is Dat A) and rotate&translate. Is there something else that makes more sense?

    You can try switching from least squares to min/max and see if the results improve.

    I am quite interested to hear how you are handling the fit along the Z axis for the lower tier.
  • I am quite interested to hear how you are handling the fit along the Z axis for the lower tier.

    Can you clarify? I'm not sure what you mean.
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