Standard CMM Operations for Large Parts

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  • Standard CMM Operations for Large Parts

    I'm moving to another facility where we manufacture very large parts with critical dimensions to start looking for opportunities for a CMM or 2. Example of a part is 25"x48"x55" with internal dims ±0.001 and smaller. Looking at the cad and different loadmachine options, I think I would need a global extra 20.33.18. Even at that, I don't think I can check the whole part in one operation. A lot of the features are in between other features and I'd probably have a couple of programs with the parts facing different directions just to fully inspect the part.

    Is that something that is common on you all who use large CMMs? I would also like to use an indexing head if possible.

  • #2
    We've got a Global Performance 12.15.10, and while the table is large enough to accommodate your part, the travel in the Y and the Z just isn't up to it, I don't think. The scanning head adds 4 inches or so to the distance the probe is below the knuckle, and I think my max Z is like 43".

    Anyway, I think what you're looking for is a Romer Arm. That'll do what you're looking for, and they come in PC-Dmis too. Much of the knowledge is easily transferable.

    Hexagon Metrology U. has some free courses that would acclimate you.

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    • #3
      Originally posted by Quality ish View Post
      We've got a Global Performance 12.15.10, and while the table is large enough to accommodate your part, the travel in the Y and the Z just isn't up to it, I don't think. The scanning head adds 4 inches or so to the distance the probe is below the knuckle, and I think my max Z is like 43".

      Anyway, I think what you're looking for is a Romer Arm. That'll do what you're looking for, and they come in PC-Dmis too. Much of the knowledge is easily transferable.

      Hexagon Metrology U. has some free courses that would acclimate you.
      If it wasn't for the .001 and smaller tolerances mentioned you may be right about the Romer. If you need that accuracy, a portable CMM isn't the way to go.

      To answer the OP, yes it is common to have to run a part in more than one orientation to inspect the entire part.
      PC-DMIS 2016.0 SP8

      Jeff

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      • Quality ish
        Quality ish commented
        Editing a comment
        Huh. What tolerances would you say that Romers are reliable to? We've got a magnetic base that pretty well locks the unit in place, so movement at that point wouldn't be the issue, I don't think. I was under the impression that the Romers we have are every bit as accurate as the non portable units we run.

      • mckenzie
        mckenzie commented
        Editing a comment
        from the romer borchure

        best possible accuracy -
        6 axis probing specifications. 75 series model 7520 measuring range 6.6ft volumetric accuracy .0009 in.
        7 axis probing specifications. 75 series model 7520si/se measuring range 6.6ft volumetric accuracy .0023 in.

        it goes up from there

        https://www.google.com/url?sa=t&rct=...INMkTsLkjdj_xP

        i use a global imange 7.10.7 cmm. its accurate to less than .0001"
        Last edited by mckenzie; 06-12-2019, 08:39 AM.

      • acgarcia
        acgarcia commented
        Editing a comment
        On the other forum, members say ±0.005" is probably to smallest tolerance they trust for a diameter. to add to mckenzie, the longer the arm, the the least accurate they are. We have a faro arm edge with a laser line probe running polyworks. I'm not sure about how a romer works but with a Faro, the laser probe relies on the calibration of the hard probe making it less accurate than the hard probe itself. My boss insists on using the arm to measure this part even though I told him it isn't accurate enough. He's also going to insist on scanning it due to is size and the speed of scanning vs. probe hits. I even talked to my Faro sales guy and he confirmed that the arm isn't accurate enough for those tolerances.

    • #4
      I run a dual arm CMM with indexing heads. The indexing heads are wonderful when you have to use a lot of different probe angles because you don't to calibrate all the angles, and you can add or change angles to your program easily. I highly recommend them. It is common to have to change orientations of a part to check the whole thing, I do it myself. With tolerances that tight, you need to make sure your part fixturing is stable and repeatable for all of the orientations. Is that a tolerance that controls size and form only, like a bore diameter? Or does that also control and location as well?

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      • acgarcia
        acgarcia commented
        Editing a comment
        tolerances are for size, position not so much

    • #5
      We use Global CMM's with the HP-S-X3C fixed scanning probe.
      I doesn't take away nearly as much Z volume, but you need to get used to using star probe build ups all the time.

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      • acgarcia
        acgarcia commented
        Editing a comment
        Most of my CMMs have fixed heads and star builds but I have one that has a touch trigger indexing head and I can do so much with just one probe tip and multiple angles. I've run complete programs with just 1 probe tip and a few angles. Like most places, when the CMM arrives, "they" change their minds on what i'm going to run on it so if we were to go to a very big CMM, I would like that flexibility from the beginning if possible.

    • #6
      OK, measuring size isn't affected as much by alignment usually. If you were trying to control position to 0.001", you'd have a hard time maintaining accuracy if you are moving the part around and running separate programs.

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