[Aa...] Posted June 26 Share Posted June 26 We have a KOBA420 castellated stepper gage that we use for verifying the performance of our CMMs. Recently, we started taking measurements using off-axis styli. The crazy thing about it is that, this time around, the errors when measuring with an on-axis stylus were as high as 30 μm, while the errors with the off-axis styli (even as far off-axis as 150mm) were all sub-micron. Anyone have any thoughts as to why that would be? (BTW, the KOBA gauge was recalibrated March of 2023 by NIST.) Link to comment Share on other sites More sharing options...
[Ow...] Posted June 26 Share Posted June 26 For lack of replies, my guess is that it has something to do with the way it is aligned or the CMM is out of square. Link to comment Share on other sites More sharing options...
[Aa...] Posted June 26 Author Share Posted June 26 That would be a valid thought, except that the measurements are all in a base alignment from the gauge itself, not from machine coordinates. (I wouldn't even know how to do anything with machine coordinates.) Link to comment Share on other sites More sharing options...
[Aa...] Posted June 26 Author Share Posted June 26 Please sign in to view this quote. Owen, I just want to make sure I read what you wrote right. Do you think the gauge is out of square on the CMM or do you think the CMM itself is out of square? Link to comment Share on other sites More sharing options...
[Ow...] Posted June 26 Share Posted June 26 Please sign in to view this quote. The CMM. I've never used one of the KOBA gages and not sure how the deviations are calculated or represented so I could be completely wrong and maybe misinterpreted what you mean by off-axis . However, when I've seen linear differences between down and side probes show different locations measuring the same feature, the CMM was out of square. Just a small bump against the Z column (not the probe) when extended down can knock it out of square and standard probe qualifications will not detect it. Link to comment Share on other sites More sharing options...
[Aa...] Posted June 26 Author Share Posted June 26 Off-axis simply means the stylus tip does not lie on the ram-axis of the CMM. (Generally, using off-axis styli should increase the error of indiction; the further off-axis, the greater the increase.) https://koba.de/en/products/standards-f ... metrology/ shows what the gauge looks like (It's titled "Step Gauge Koba-Step"). If you've seen one of the Zeiss field techs set up a CMM, you've seen one of these. The measurements in question are all two-point bi-directional distance measurements (between faces of ceramic "lugs" along the axis of the gauge. I take what you said to mean your suspicion is that the ram axis is out of square with the granite, but I don't see how that would affect the measured two-point distance within a base alignment. Link to comment Share on other sites More sharing options...
[To...] Posted June 26 Share Posted June 26 Please sign in to view this quote. Help me understand this. This makes some sense to me if you were talking about dial indicator and cosine error. Am I missing the boat? Link to comment Share on other sites More sharing options...
[Cl...] Posted June 26 Share Posted June 26 Please sign in to view this quote. We had a similar problem with a CMM (not Zeiss). After some research we found that the machine was a demo prior to us buying it. There was unrepairable damage to the Z axis from a collision. There were no crashes at our end. If this machine is used or was a demo. Do some research on its history. Link to comment Share on other sites More sharing options...
[Aa...] Posted June 26 Author Share Posted June 26 Tom, I think you're onto it. The primary number for stating the accuracy (or non-inaccuracy) of a CMM is the E₀ value - A value from ISO 10360-2 for measurements on a step gauge that are taken with a stylus with a ram-axis offset of 0. There is another number, the E₁₅₀ value - a similar number from ISO 10360-2 for similar measurements with a stylus with a ram-axis offset of 150mm. Zeiss lists the E₀ and E₁₅₀ specs for all their CMMs as a function of measurement length. Link to comment Share on other sites More sharing options...
[Aa...] Posted June 26 Author Share Posted June 26 Please sign in to view this quote. Thanks! This particular CMM is one this company has had for over a decade and moved to our new location in 2023, including having a Zeiss Tech set it up. Link to comment Share on other sites More sharing options...
[Ow...] Posted June 27 Share Posted June 27 Sorry Aaron, I misinterpreted the off-axis meaning but it was a quick lunch reply and at least I got the discussion going. 😃 Link to comment Share on other sites More sharing options...
[Ch...] Posted June 27 Share Posted June 27 There could be many reasons why,... I just noticed the Zeiss techs indicate the VAST head itself to be square within a certain probe head been removed/replaced recently or any hard collisions, may want to check its 'squareness' Link to comment Share on other sites More sharing options...
[Aa...] Posted July 1 Author Share Posted July 1 Please sign in to view this quote. Yes, Owen, thanks for getting the discussion going... Link to comment Share on other sites More sharing options...
[Aa...] Posted July 1 Author Share Posted July 1 Please sign in to view this quote. I'm struggling to grasp how the head could be out of square, resulting in measurement error that would not be accounted for in the stylus qualification. Also, there is no mention of checking the squareness of the head in the 10360 Calibration certificate. My question still stands: why do I get results that are closer to the NIST-calibrated values when using off-axis styli than when using a stylus pointing straight down? (Not just closer...orders of magnitude closer.) Link to comment Share on other sites More sharing options...
[Aa...] Posted July 1 Author Share Posted July 1 Please sign in to view this quote. I suppose the head could be out of square, but If it were, I would think the software would account for that during qualification...and like I said, we have an "as left" certificate from a Zeiss field tech including the statement "Unless otherwise annotated in protocol results, machine condition is in good working order." Link to comment Share on other sites More sharing options...
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