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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?
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  • You have quite a challenge in front of you.
    We use an "artifact" for comparing our machines.
    It also serves as a sort of verification device whenever we have a major change; IE; software upgrade, after a crash, after calibration. We have documented that we have a +/-.001" tolerance on any of the data results from the artifact inspection.
    In reality, if I see anything greater than .0003" then we get to work on figuring out why and correct whatever is needed to get back within my happy zone.
    The most common problem is we will sometimes get a few points that vary greatly, maybe .002". We usually just re-run the artifact. We have attributed the anomalies to vibration..forklifts, trains, etc.
    We made the artifact ourselves.
    It is just a circle-diamond-square shape that we use for the various 5-axis machines we have.
    We had the artifact anodized and then sent it to an independent lab for "certification".
    That lab reported back every point measured (about 200 vector points), XYZIJK.
    I created a program that duplicated all of those points.
    This works well for us. We had 3 machines and they all repeated within the +/-.0003" zone. (It is interesting that the oldest machine, which was all steel and cast iron, always repeated within .0002". In other words, it was better than the brand new machines.)
    Good luck. You are on the right path. Just keep going.
    You will get them repeating better if you try.
    That's the trick.
    You have to try.
Reply
  • You have quite a challenge in front of you.
    We use an "artifact" for comparing our machines.
    It also serves as a sort of verification device whenever we have a major change; IE; software upgrade, after a crash, after calibration. We have documented that we have a +/-.001" tolerance on any of the data results from the artifact inspection.
    In reality, if I see anything greater than .0003" then we get to work on figuring out why and correct whatever is needed to get back within my happy zone.
    The most common problem is we will sometimes get a few points that vary greatly, maybe .002". We usually just re-run the artifact. We have attributed the anomalies to vibration..forklifts, trains, etc.
    We made the artifact ourselves.
    It is just a circle-diamond-square shape that we use for the various 5-axis machines we have.
    We had the artifact anodized and then sent it to an independent lab for "certification".
    That lab reported back every point measured (about 200 vector points), XYZIJK.
    I created a program that duplicated all of those points.
    This works well for us. We had 3 machines and they all repeated within the +/-.0003" zone. (It is interesting that the oldest machine, which was all steel and cast iron, always repeated within .0002". In other words, it was better than the brand new machines.)
    Good luck. You are on the right path. Just keep going.
    You will get them repeating better if you try.
    That's the trick.
    You have to try.
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