CMM is not repeating measurements

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  • CMM is not repeating measurements


    I'm completely new in the CMM world and I'm working now with a CMM but the machine is not repeating the measurements when I measure the same part twice. Some of the dimensions vary from 30 to 50 microns between readings. The CMM programs was provided.
    1. We are using a fixture to hold the part. If the part is not located exactly in the same position of before, couple of microns different, could that affect the readings?
    2. I tried to check the alignments in the CMM programs and it seems like all the 6 degree of freedom are locked. Is there any way that all degree of freedom are locked and still having different readings?
    3. Anyone that recommend me a book, tutorial, YouTube channel to learn how to programming and alignments?
    Thanks in advance

  • #2
    30 to 50 microns is very small. However, did you calibrate the probes? If so, are they within acceptable standard deviation? Are the probes clean? Is the part clean? Did you plate the part to see if you got the same measurements the cmm got. Those are the things I would start with first.


    • hvelez55
      hvelez55 commented
      Editing a comment
      The probe was calibrated. I will be more aware about the cleaning of the part and probes.

      So it's normal have a variation of 30 to 50 microns between readings of the same part?

    • JEFMAN
      JEFMAN commented
      Editing a comment
      Darrollh : 30 to 50 µm are very small on plastic parts or sheet metal, not on machined parts !!! It's the same part measured twice, not two different parts...
      hvelez55 : I would say no, it's not normal...

    • hvelez55
      hvelez55 commented
      Editing a comment
      @jefman: Thanks for your input and you are correct, the parts are machined.

  • #3
    Tons of questions here:
    Type of measurements not repeating
    Probing system
    number of hits per feature
    Etc. Etc. Etc.


    • #4
      It would help if you could share some of the code, particularly your alignment and alignment features.


      • #5
        TONS of variables here. Some basic comments based off what you wrote....

        1) If you are creating an alignment ON the fixture THEN putting the part on the fixture and checking stuff on the part BACK to the fixture alignment...then YES.. it is a huge problem if the part doesn't locate/sit in the same spot every time.

        2) With all due respect to your knowledge/training I'd like to see your code before we feel comfortable saying that all 6 DOF are locked. The sequence of your alignment steps, your recall alignment options, and a few other factors all play big roles in how things get calculated out by the software. One "misunderstanding" on your behalf that doesn't get articulated to us because you think you're good might end up being your smoking gun!!

        3) Hexagon's Level 1 Training is very important. It provides you a PROPER foundation on which to build the rest of your CMM education. Aside from hex themselves, you're in the right place by being on this webpage. There are tons of great people who will go out of their way to help. Also check out cmmguysdotcom

        If you want to pursue this more, please post your code along with any technical info you're allowed to show us & we'll do our best.

        Code goes on the forum like this (remove spaces inside of brackets)-->

        [c o d e]

        copy & paste your program here

        [/c o d e]
        Last edited by DAN_M; 01-05-2021, 02:14 PM.
        SF7107(PCD), SF454(PCD), 152614(Quindos), 9159(Quindos), 7107(Quindos), B&S Manual, M&M Gear Checker


        • hvelez55
          hvelez55 commented
          Editing a comment
          Thank you for your respond!

          1. The alignment is on the part.

          2. Don't worry! I have 0 knowledge or training on CMM. What I learned until now is what I read on Google.

          3. I'm looking for recommendation where is the best place to start to learn programming. I'm finishing a master degree on data mining and I have knowledge in different programming languages. I know that it will be great if I can apply data mining concepts to the CMM readings.

          I don't think I can post any code here by company policy. I can try to do something similar.

        • DAN_M
          DAN_M commented
          Editing a comment
          No problem!

          1. Ok, so you should be fine. No problem. AS LONG as your code is correct (which we have yet to verify).
          2. You're in for a long and difficult road. I highly highly highly (did I say highly?) recommend you get some legit training from Hex if you want to be a competent programmer. I never would have been able to PROPERLY learn without it. Having other prg experience is very helpful but PCD is it's own quirky beast, not something you just pick up because you know other stuff (like other software).
          3. You're in a great place to start and we'll help as much as we can.

          I hope your policy **** near impossible to help people who can't post codel! Any problem you face could have a multitude of reasons for it popping up (especially as an untrained beginner). We can't diagnose what we can't see!

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