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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?
  • in reply to acgarcia
    so you used the same Cal. sphere on both machines?
  • in reply to acgarcia
    If you did all that right, looks like there might be a slight issue with the CMM_A. Maybe its time for laser calibration. Even with that, you maybe looking at some wear on the probe head of CMM_A
  • in reply to DJAMS
    I also wonder if the machines are not in the same temp enviroment
  • in reply to DJAMS
    Earlier in the thread, he states that the machines are side by side on the shop floor
  • Good Morning All, My wife and I had our 3rd child last week. I wasn't in for obvious reasons. So to continue..........

    the results for CMM A are D. 3.989 Std Dev 0.002 and CMM B are D. 3.990 Std Dev 0.002

    , I used the cal sphere from CMM A. It has never been sent out to be measured. Just used the size that was marked on it.
  • in reply to DJAMS
    , you would have to elaborate a little more with temp comp. I've only used it in class.
  • one thing i have not seen mentioned here is have you verified your probe deflection pressure and compared it to the other cmm.
    i have seen where a probe will not trigger equally in all directions/
  • Hurricane went more North of my location, I'm considering myself lucky. Never lost power or any utilities. Others not so much.

    Ok, this is what I did.
    • Created Test Program in MM
    • Created Standard Force 4x20 (A0B0) (don't have a light force or 5x20)
    • Created Cal Sphere in mm units (29.0504, all 6 digits entered in menu)
    • Calibrated 25 hits at 4 levels at touchspeed of 5mm
    • Used a 117.5mm master ring for measurement with touchspeed of 5mm throughout
    • Used same probe, sphere, and ring for both machines.
    • Calibration results on both machines were 0.002
    • Created a model for autofeatures to be used during programming.
    • Manual and DCC alignment, Z+ plane, XY Orgin cylinder
    • Looped an auto circle with 23 pnts 5 times.
    RESULTS
    CMM_A
    117.513
    117.514
    117.515
    117.515
    117.514

    CMM_B
    117.501
    117.502
    117.502
    117.502
    117.502


    These results indicate a difference of about 5 tenths between the two machines when measuring a ring gauge.
    Considering the machines in question: Looks to me like this is about what you could expect. Certainly a lot better than the .002" you were reporting earlier.
    When you add the other variables that come with actual part measurement (as opposed to a controlled test on a master ring standard) I think you need to look at reducing those variables rather than assuming that one of the machines is significantly less accurate than the other.
  • Variables as in what Josh posted earlier about? Both machines use the exact same star probe build and there are no other probes on the machines. How do I explain to operators that when one CMM barely fails a part, run it on the other and it might pass it?

    I get it, these are not supper accurate machines and the tightest tolerance on this part is ±0.001". Where do I start to eliminate some of these variables? add more points to my features? more points on probe calibration?

    In other words, how do I explain to my boss who is not a CMM dude, why these 2 CMMS that are physically identical but report different output?
  • The 2 machines are repeating but there is as John stated .0005" variance between the 2
    try recalibrating with a lower touch speed, this looks like a calibration variable is cause
    CMM A is off 0.010mm ?? start there---that being said ring is 117.5 do you have a cert? what does that state. That would be comparison value.

    are these machines in an environmentally different area of shop? Had an issue with a correlation after a vent was altered for a machinist and changed to get cool air at him which also happened to be at the SF CMM.....caused a bit of a shift
    I level these machine when moved---no one said do it just seemed to make sense....just another thought
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