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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 Kp61dude!
    Ring gages are calibrated in-house with a mic-trac
  • in reply to Kp61dude!
    , let me get my buddy to mic the part and compare the results.
  • in reply to DAN_M
    Blank template program, good idea. I'll look at the settings in the older CMM to see what it was set to and compare it to the newer CMM.

    When the tech was here setting up the new CMM, he said the machine needs to run at full speed for the best accuracy. is this contradicting that?
  • in reply to DAN_M
    If they aren't measuring at the same touch speed, this could definitely help play into the dimensional variation between the two machines. So good definitely look into that my friend.

    Running at full speed? I've never been concerned with how fast the CMM moves from feature to feature.....how fast it touches is what you need to control. You want your touch speed to be the same on both machines for when you qualify your tips as well as when you're measuring your part. Checking off this box ensures this is taking place. I have this box checked off, set my touch speed to 3mm/sec in every program, and I always qualify tips at 3mm/sec. Everything is the same speed on each of my machines...just some have a faster move speed than others.
  • Movespeed can significantly affect diameter sizes.


    Actually just touch speed, make sure both machines are in absolute with the same exact touch speed, make sure both machines have the same exact touch speed used when qualifying with your artifact as you do when you measure your parts, make sure both machines are using the exact same probing down to the module force and tip size, make sure both machines have been recently certified to be within spec by the manufacturer. Depending on who is doing your sticker and how long since the last cal between both machines, it could just as easily be a simple squareness error between the machines.


  • 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?

    Agreed, you need to KNOW something is correct to find what isn't Slight smile

    TK
  • 61The older CMM measures more like our mics do. So the newer CMM running 2017 seems to be off a little.

    the touchspeeds on the both cmm were set to 2mm per sec and absolute speed was checked on both as well. The difference was that the top speed on the older one is set to 400 and the newer one is set to 250. Is that correct?
  • in reply to acgarcia
    Sounds good. Move speed is just how fast your moving from point to point. Touch speed is what needs to be the same on both machines before you can begin to troubleshoot accuracy concerns
  • in reply to Peter Fuller
    I'm not familar with squareness error. I'm guessing that is something along the lines that's how the machine was built kind of thing
  • Unless you are dealing with wimpy parts, as in, parts that can and will move when you touch them to take point, then the touch speed from machine A to machine B should not matter. The ONLY thing that matters is that the touch speed is the same for calibration & measurement. You can use 2 on "A" and 6 on "B", if the part doesn't move when you touch, then they SHOULD both give the same result.
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