Composite data control frame features

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  • Composite data control frame features

    Hello, I have a question about composite data control frames, in particular how PC-DMIS calculates perpendicularity.

    So I have a part with many 0.130 diameter holes going through the -A- plane. The company we are making the part for designated that they have to be perpendicular to -A- plane within 0.010.
    The composite frame is structured like the picture I included.


    So my issue lies in how exactly does PD-DMIS determine perpendicularity? My understanding is that when it takes hits in the holes it takes 4 hits near the bottom and 4 hits near the top then constructs a cylinder.
    Then it takes the worst deviated point on the bottom and the worst deviated point on the top and connects them with a line to compare to -A- plane. Our -A- plane is measured by taking about 30 hits under the part.

    We have issues where sometimes it says the holes are not perpendicular withing 0.010 of -A-. I find it really hard to believe that this is true since you can put a gauge pin in the hole and check it against -A- and it appears to have zero deviation. Also the holes pass for location but not perpendicularity.

    Maybe our constructed -A- is not flat enough and is throwing out our holes? Any help would be appreciated.


  • #2
    Its also checking the holes as "pattern" at the tighter tolerance. If I can see the report correctly some are deviating a little negative ( call it -x- ??? ) and some a re looking alot more + ?? Something in the manufacturing method ?


    • #3
      As Schlag said - it's also checking the location of the holes to each other,

      For simplicity say your Dat A plane is in Z. Then it's best fitting (rotating around Z and shifting in x and y) to minimise the average error, then reporting the positions.

      Think of it this way, this check is to check if the mating part will fit, the looser tolerance checks if the mating part is in the right location relative to everything else.

      You could also try this...

      1) Best fit as described above (assuming Dat A is in Z)
      2) Switch to legacy dimensions
      3) Report the positions of the holes using legacy (untick 'Use datums' and select all your holes in one go)
      4) you should see the same holes out as in the xact dimension.
      5) Insert > Report Command > Analysis, select all the dimensions you've just created, tick Graphical, enter a multiplier, hit apply, show all arrow then view window to see a graphical analysis of the hole pattern.

      Automettech - Automated Metrology Technology


      • #4
        If I understand you correctly you are asking why some of the holes are deviated negative and some positive? Well, the part is very long and has a curve in it that dictates the -B- plane where the "X" measurements are taken from. The holes are the last step in the machining process then the part goes to forming to match the curve check template. I know it is not the best way but its how it's done right now. So depending on how accurate the forming is done the -B- the plane will "shift". It is pretty hard to explain without showing the part which I think would be against company policy. Here is a drawing kind of explaining it.

        CURVE DWG.jpeg


        • #5
          Perfect candidate for an iterative alignment.using 2 points of the points on the curve. That's the only way you woulnd't be chasing the inspection based on the curve. We do curved parts like that here and for years the CMM guy here would be using the arc as a circle and manufacturing chased their tail for years and years on these types of parts. Every new part we do with our customers ( old and new ) we go over datum target points.


          • pmccabe@ashleymt
            pmccabe@ashleymt commented
            Editing a comment
            Schlag if I'm following correctly I believe we do use an iterative alignment using two points on the curve as your describing. We construct datum -B- from two holes in a wall that is normal to the drawing view. I just didn't draw it. There is a hole inboard and one outboard then the program constructs a line to rotate to. Our zero point is where -B- line intersects the inboard "larger end" of the part called -C- point. Then levels to -A-.

        • #6
          So does anyone know then how exactly PC-DMIS measures perpendicularity between a constructed datum plane made from points and a cylinder constructed from 8 points? Maybe it was answered already but I don't understand the answer. Does it construct a centroid for the bottom 4 hits and the a centroid for the top 4 hits and make a line and then compare that line to the constructed plane? I tried to explain what I mean in this drawing.



          • #7
            What you are showing is not a perpendicularity dimension, it's a position of all the holes at the same time (the effects of the position will constrain the allowed perpendicularity of the holes to ABC and each other) - i.e. a gage with seven pins should fit the pattern at the same time as it is locked to ABC.

            The lower segment is a tighter tolerance zone when the gage is only resting on A (and still has seven pins), so it will be the one constraining perpendicularity the most.

            All PC-DMIS knows is your hit points, and they should all lie inside the 'tolerance cylinders' for all of the holes, in both cases.

            If you verify the holes one by one, locally, you are doing a completely different evaluation...
            SW support - Hexagon Metrology Nordic AB


            • pmccabe@ashleymt
              pmccabe@ashleymt commented
              Editing a comment
              Thank you for the response Andersl, this definitely makes sense since late yesterday I did legacy dimensioning perpendicularity to -A- plane for all the holes and they were all within 0.001 so I knew I had to be interpreting the frame wrong. So since I checked the perpendicularity individually and they seem to be good then my only conclusion is that the holes need to be more accurate when they are drilled in relation to each other?

            • AndersI
              AndersI commented
              Editing a comment
              Yes, the "7 x Ø.1285" in the frame says "seven holes at the same time, with a single fitting of the datums", i.e. equivalent to a functional gage which fits the datums and the seven holes, all at once.

              How did you measure the plane -A-? Many points over the full surface and then creating a tangent plane from that? That's the ISO definition of a datum plane, and I guess ASME is quite similar.

          • #8
            Do what I suggested in post #3 above
            Automettech - Automated Metrology Technology


            • pmccabe@ashleymt
              pmccabe@ashleymt commented
              Editing a comment
              Okay ninja I will try what you are saying. I think it should help us a lot.

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