Iterative Alignment

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  • Iterative Alignment

    Hi.I am new to PC DMIS but have experience on other CNC CMM's. Just trying to measure a part using an IGES model and a drawing which details the datum settings..and having problems!!!

    The drg. shows 03 points(A1 to A3)which are not coplaner(most probably jig location pts.) to level on, a face in the second axis for rotation and another target pt. for the third axis. After I have done my manual alignment(to roughly show where the part is), I go over the points in DCC mode and create my alignment(iterative). After doing this, it comes up with a dialog box detailing errors in X,Y or Z.

    What value should I set in the Fixture tol column (based on drg tol. maybe??). and if I get errors in the alingnment, what does that mean? Should I report/ log these values as well?Typically, the tol. are wide(+/-0.15mm) but there are two bores to a positional tol of Dia. 0.025.

    Any help would be appreciated.

  • #2
    Are you using six points for the iterative alignment? You have to use specific feature combinations of features, check the help file it explains the features that can be used for iterative alignments.
    Since you have CAD data you could create an iterative alignment using features other than the datums specified on the print then go back and pick up the datums then align to them if that would make it easier to create the iterative alignment.


    • #3
      I am using vector points - 04 off in 1st axis, 12off in 2nd and 05off in the third. I made sure that the points for each axis were in the same direction.



      • #4
        Not sure what the "04 off in 1st axis, 12off in 2nd and 05off in the third" means but I've seen errors in iterative alignments. This has to do with using more features than 3 to level too, or more than 2 to rotate too and so on.

        In other words a plane that you measure with 4 points will never be perfectly to their nominal. There will be some error, if this error is too large it will report it to you in some dialog boxes right after the alignment. A way around this is to use 3 points only for the plane, 2 points to rotate too, and finally only one point for your tertiary. kmcmm's 6 points

        Of course if your print has 6 points to define your A datum or whatever you're pretty much nailed to what the print says. I'd attack that by changing the process, or clamp the thing down flat on a jig of some form during inspection. Assuming there is flex in your product
        David Monroe
        Is there any way to stop the infernal beeping!?

        3.5 MR2
        Dual Arm Vento



        • #5
          The Iterative alignment will Locate the CAD model within the measuring envelope of the CMM. Try doing the iterative as a simple 3-2-1 using Points. This is done in manual mode first to teach the CMM where the Job is. It then locates the CAD to the Job, then you should measure the apecified points from the drawing and create an alignment (NOT ITERATIVE!!!!!) to these new points... Job done!!
          I am a Mushroom................Kept in the dark and fed on sh*t!!!!!


          • #6
            Remember, it is not just a matter of measuring points, you must be pulling the points from the cad data. While still in manual mode, use DCC Features (auto features) by selecting the cad model and then measuring. The measured points will not appear on the cad data UNTIL you do the iterative alignment. BUT, you can not just start measuring points, Pcdmis must have the nominal location of the points you are measuring from the cad data.
            Originally posted by AndersI
            I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.


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