Alignment Features Not Worthy

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  • Alignment Features Not Worthy

    So I have the part that does not call out a datum scheme so usually experiment to what works best, repeatable in the program and matches hand tools.

    My superior is sitting with me telling which features to used as datums. Fine. I explain how alignments work and such. As we start, he starts finding something wrong with the features used in the alignment.

    For example a plane for leveling in Z and Z origin turns out not to be flat enough so he says it’s not useable as a datum. I have a cylinder that has a tight tol of +\- 0.0015” but he found it to be out of round by 0.0008” so we can’t use it either.

    I virtually have no features worthy enough to be used for the alignment. How and I suppose to measure a part?

    Parts are not perfect. Im the only CMM programmer and I usually don’t have to explain to people what I’m doing or how I’m doing it.

    Can someone share any advice on how to better explain what alignments are doing?

  • #2
    Usually the features with the most dimensions stemming from it are the Datums when there are no Datum callouts.

    How much is "not flat enough"? That seems pretty vague.
    Last edited by bfire85; 12-17-2019, 07:07 AM.


    • acgarcia
      acgarcia commented
      Editing a comment
      0.010 measured flatness, and yes it has the most dimensions stemmig from it. After we aligned to it of course.

  • #3
    For the plane, I would construct the LS plane, with a outlier filter =3.
    Then, I would construct a feature set from it, and construct a L2 constrained tangent plane, which should represent the granit if you measured with classical tools.
    The flatness is only a factor that increases the uncertainty, but the method above should ensure the repeatability...


    • bfire85
      bfire85 commented
      Editing a comment
      I agree with this!

  • #4
    The perfect part is on the drawing or the CAD, never on the surface plate.


    • #5
      i agree with paperback, do you have a cad model ?


      • #6
        Yes I have a model and a print.

        I just went back to basics and used the minimum number of points for an plane, cylinder, cylinder alignment based of features our machinist consider critical. Once I established my coordinate system, then I re probed the features to the print. There is no indications of gd&t on my print and my superior was over analyzing what the CMM does and deviation on the physical part. So we was asking my about flatness, concentricity, cylindricicty, and so on, it was just overkill.

        He asked me to probe unfinished machine surfaces and dimensions/callouts that were not on the print.


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