Composite position, threaded hole pattern

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  • Composite position, threaded hole pattern

    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
    ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

  • #2
    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.

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    • #3
      Originally posted by sealevel View Post
      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).
      ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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      • #4
        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.

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        • #5
          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?
          Last edited by d.evans; 02-25-2013, 10:46 AM.
          ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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          • #6
            Originally posted by d.evans View Post
            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.

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            • #7
              Originally posted by DJAMS View Post
              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?
              Last edited by d.evans; 02-25-2013, 01:25 PM.
              ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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              • #8
                Datum A must remain "leveled" when evaluating lower tier. 3D best fit allows datum A to become "un-leveled".

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                • #9
                  Capture_sterling.jpg
                  Originally posted by DJAMS View Post
                  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.
                  Last edited by d.evans; 02-25-2013, 01:44 PM. Reason: attached drawing
                  ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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                  • #10
                    Originally posted by d.evans View Post
                    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.

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                    • #11
                      Originally posted by DJAMS View Post
                      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.
                      ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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                      • #12
                        For the lower tier, the holes are not checked from datum B in the z axis direction. Some sort of fitting must be done in Z axis to allow the pattern to translate "to itself".

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                        • #13
                          Originally posted by DJAMS View Post
                          For the lower tier, the holes are not checked from datum B in the z axis direction. 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."
                          ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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                          • #14
                            Originally posted by DJAMS View Post
                            For the lower tier, the holes are not checked from datum B in the z axis direction. Some sort of fitting must be done in Z axis to allow the pattern to translate "to itself".
                            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?

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                            • #15
                              Originally posted by Bruce Domer View Post
                              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 interpreted?
                              My dilemma is that just looking at the basic dimensions, I've added into the program (for analysis): distance along the z axis between one level of holes and the B datum (basic is 4.642"), distance from the first set of 4 holes to the next set (3.707" basic from the first level), angle between holes ( I constructed 2D lines from the axis of Datum A out to each hole, then dimensioned the angles between them). All this looks good, reasonable and held very close to the basic dimensions. But I'm befuddled as to how to use Position, and Alignment to agree with those numbers. When I used a Best Fit 2D alignment, using just the holes, and then dimensioning the position within that alignment @ .018 TP, 7 out of 8 holes fails, and badly. It makes no sense. 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.

                              But it just doesn't seem to work.

                              I don't think the part is bad, based on the analysis dimensioning I did. But I need to report it accurately, and I have about a dozen more of these to measure, so, I need to make the correct adjustments to the program. I'm stumped.

                              Running version 3.7MR3
                              ** "Well, ain't this place a geographical oddity. Two weeks from everywhere!"~ Ulysses Everett McGill **

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