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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 Rich P
    Touch speed for both machines is 2mm. Josh said to use 5mm in the ring gage test for both calibration and the ring gage measurement.
  • in reply to Rich P
    Should run touch speed @calibration same as touch speed for run is what you did? Maybe too fast for these

    Do machines look like they are bouncing when moving ? ie level
  • in reply to Rich P
    Yes, I have noticed that they tend to shake when it his a move point to move point.
  • 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

    Rich makes a good point here.
    Temperature can have a significant effect on results.
    I am not talking about part temp or air temp (though both of those DO matter), I'm referring to temperature effects on the machines themselves. Is there cool air blowing on one area of one machine and not on the other?
    Are these machines in the same room?
    Do you have vibration issues?
    There are many things which may be causing the problems you are seeing.
    If I were seeing those kind of discrepancies between our two machines, I would determine which one was the most accurate and put all the critical stuff on that machine.
    And I would keep working to find out what environmental issues are causing the discrepancies.

  • The CMMs are about 10-12' apart on the shop floor. There is a large cnc lathe in between them. Hex Tech said vibration is going to be an issue with our newer CMM as there are more CNC machines around it than the older 1. I haven't checked on any AC vents.

    My scenario is that these SF CMMs are integrated into cells. Both cells make the exact same part, our highest selling part. One is a little older and the other is newer. CMM A inspects cell A parts and CMM B inspect CMM B parts. Operators place the parts in a fixture and hit execute and walk away. They figured out that the CMM report slightly differently so they are using that to their advantage as in when 1 CMM fails a part, CMM B might pass it and vice versa.

    I'm just trying to get these too a little more close so the operators wouldn't have to move parts around. I've gotten a quote for 120K for a vibration system but I don't think management is going for it.
  • 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.
  • Sorry if some of this seems way too obvious, but when trying to get "tenths" out of comparisons, we should not assume anything.
    One thing I just thought of that I don't see specifically answered anywhere above: Are the two programs exactly the same?
    If not, then it could cause some of the problems.
    For instance, on a production part (not a ring gage) if the start points and end points on a circle are not in exactly the same place you could get very different results. Chatter, out-of-round, etc....
    Same goes for vector points.
    A production part is not like a ring gage or jo-block.

    Oh, one more thing...
    Could the two fixtures be different enough to cause issues?
  • 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


    Hmmm.... will get back to you on this...
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