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Measuring the same part on 2 CMMs, Different Results

acgarcia
We have 2 4.5.4 SFs. I ran a part 5 times on CMM A and then ran the exact same part on CMM B. Got different and same results.

CMM A runs 2016 and CMM B runs 2017. Both use the same star probe build. Both checked the same features with the same parameter... number hits, prehit retract, etc..

One CMM seems to read a little larger than the other. CMM B measures one feature 0.002" smaller than CMM A and CMM A measure a feature 0.0006" smaller than CMM B. One feature was exact on both CMMS, exact nominal and deviation, 0.000.

Essentially I copied the program from CMM A to CMM B and just adjusted some movespeeds.

Is this something normal? Operators at night sometimes will fail parts on one cmm and then take them to the other cmm and it pass so they pass the part through. I know they CMM will not match perfectly but they should be closer than 0.002". I'm trying to build confidence in the CMMs for the operators as its only be a few years of having CMMS.

Any advice or ideas on this?
  • Movespeed can significantly affect diameter sizes.
    Try getting both machines to move as close as you can to exactly the same speed.
    You may see closer results.

    Also, make sure you are running programs at exactly the same speeds that you are calibrating at.

    Check for size accuracy by measuring standards, ring gages, jo-blocks, etc.
  • Well the are different controllers also and yes they the features are diameters. The newer CMM running 2017 is much faster than the other one. I had a big probe deflection problem in the beginning when the tech was calibrating it. He had to slow down the de acceleration a bit to keep the probe from deflecting just from moving around the CMM.

    I believe the touchspeed should be the same but I know the move speed varies.

    I checked a master ring to both CMMs. CMM A was measuring -0.0003 under nominal and CMM B was measuring 0.0001" over nominal. This this be a cause for concern on a individual cmm basis?
  • I know how to adjust move speed us using code but shouldn't the machine settings match as well? Like should both be using absolute speed or have the same acceleration speed as well? Can that be adjusted with code instead of messing with the machine settings?
  • Hi Garcia,
    • Are both CMM's in a climate controlled room (68º) or on the shop floor?
    • Is the part "normalized" prior to inspection?
  • : Whenever I write a new program, I never go into "file/new". Instead I have a blank program with some pre-set parameters. I open it up & save it as my new part number and then begin programming. Once of the settings I have built in is:

    I always have "Display Absolute Speeds" checked (you get this box by hitting F5). The reason for this is because if this is unchecked, pc dmis will read move speeds and touch speeds as a percentage, not as an absolute value. With this checked, you can control exactly how fast you qualify as well as move at, regardless of how fast one of your CMM controllers can be vs the other one. Make sense?
  • in reply to DAN_M
    Same here. Blank template program. Even though that doesn't work for everything.
  • in reply to acgarcia
    I'm not to sure I agree with the touchspeed thing. I myself have a 7-10-7 and a 5-4-5. The 7-10-7 clearly runs faster than the other, so I have to run it slower than the small one, but to get them to read the same, my 7-10-7 touchspeed is set at 1% and my 5-4-5 is at 2%. I still set mine up in % not mm.
  • We have 2 4.5.4 SFs. I ran a part 5 times on CMM A and then ran the exact same part on CMM B. Got different and same results.

    CMM A runs 2016 and CMM B runs 2017. Both use the same star probe build. Both checked the same features with the same parameter... number hits, prehit retract, etc..

    One CMM seems to read a little larger than the other. CMM B measures one feature 0.002" smaller than CMM A and CMM A measure a feature 0.0006" smaller than CMM B. One feature was exact on both CMMS, exact nominal and deviation, 0.000.

    Essentially I copied the program from CMM A to CMM B and just adjusted some movespeeds.

    Is this something normal? Operators at night sometimes will fail parts on one cmm and then take them to the other cmm and it pass so they pass the part through. I know they CMM will not match perfectly but they should be closer than 0.002". I'm trying to build confidence in the CMMs for the operators as its only be a few years of having CMMS.

    Any advice or ideas on this?

    Not an answer to your question.... which one measures closer to what the part is? In other words which is right? Not hijacking this thread with my question just wanna know how you go about choosing the "right" CMM output when results are in question like this?
  • in reply to Kp61dude!
    Ring gauges should be sent out for calibration. When you get them back with a cert. It will tell you the results of the size. Compare it to that
  • in reply to Roberto
    Yes they are both on the shop floor right next to each other. Part is normalized.
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