CMM will not correlate with a Dyer Bore Gage

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  • CMM will not correlate with a Dyer Bore Gage

    We have a Final Inspection area that measures a 1.500 ±.001 ID with a Dyer Bore Gage. We had some parts show .0003 out of tol. I went to check the production CMM (Shop floor) data for that part & it showed almost perfect. I then went into our lap& manually checked the same part & the results were close to the Dyer bore Gage results. Both CMM's were just calibrated. There is a few tenths of form in the ID but still doesnt explain why the CMM's & Gage do not correlate. Both CMM's measured the same points in the same place.

    I was wondering if someone can tell me if using the "LEAST SQR, MIN_SEP, MAX_INSC, MIN_CIRCSC" options would help at all?

    Lab CMM is a 7-10-7 global
    SF CMM is a 4-5-4 Hexagon SF
    MIKE OXLONG

  • #2
    What's the form of the one measured on the shop floor? Did the shop floor CMM measure big or small? Small could be dirt on probe.

    What type of probes are they? i.e. scanning or touch trigger? Are they both the same? Least sq etc will make a difference, but for a fair test they should both be the same.

    Have you checked a ring gauge on all three pieces of equipment?



    Applications Engineer
    Hexagon UK

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    • #3
      There is a few tenths of form in the ID but still doesnt explain why the CMM's & Gage do not correlate
      A few tenths of form error is everything.
      Is the dyer gage a 2 of 3 point?

      What's the form of the one measured on the shop floor? Did the shop floor CMM measure big or small? Small could be dirt on probe.

      What type of probes are they? i.e. scanning or touch trigger? Are they both the same? Least sq etc will make a difference, but for a fair test they should both be the same.

      Have you checked a ring gauge on all three pieces of equipment?
      As NigaBadger said above, there are a ton of reasons.

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      • #4
        In addition to what NinjaBadger and BKulpa have already posted, there is the issue of # of points to be considered. For a Ø1.500 bore with a tolerance of ±.001 I would use more than 20 points per level, with the number of levels being determined mostly by the length (depth) of the bore and to a lesser extent the method of manufacture. All of the fitting algorithms work better with more points. Best case is you have a scanning probe and are collecting thousands of points.

        Beyond that, comparing a 2 or 3 point bore gage measurement to a 360° CMM measurement is not exactly apples to apples. Precision ring gages are marked and checked at one (or two- the second being 90° from the first) places as a 2 point check only for the purposes of calibration and determining exact size even when those measurements are being made with a CMM. One thing you might do is measure the bore as say 8 sets of opposed points with the CMM, then compare those results to the 2-point bore gage results.


        When you are trying scrutinize down to the level of a few tenths anything and everything can make a difference. Such as temperature. In the case of all your checks be they shop floor CMM, lab CMM, or bore gage, if they are not all made with the part and the gages fully soaked to the same temperature (20° C [68°F]), that alone could explain your tenths of difference.

        HTH
        sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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        • #5
          We had an instance where our calibration ball was switched out with one of a (very) slightly different diameter. If the CMM thinks it is calibrating to a 1.0000" DIA ball, but it is really only 0.9998", it will throw everything off that is measured after that. Are you sure the DIA of your calibration sphere is correct and accurate? Are you sure the DIA of your probe tip is accurate, as well?

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          • #6
            slight form deviations will quickly give discrepancies between 2 point and 3 point measure... it is more unusual to not find this

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            • #7
              Also, aren't Shop Floor CMM's not as accurate an inspection lab grade CMM? Thought that that was the compromise so that a CMM could work in all the muck...

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              • #8
                Yeah, as a couple folks have mentioned about the number of hits you're taking with the CMM as compared to a two or three point bore mic, another thing you might want to keep in mind is roundness. The CMM will take the average of all the hits and give an average. Have you checked the roundness, as long as you're touching more spots with the CMM than you are with the bore mic?

                That could easily be .0002.

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                • #9
                  Take a lot of points and use Max Inscribed for a hole (Minimum Circumscribed for a shaft). Filter out anything over 3 sigma. You can take a ring gage with a known diameter, calibrate your tip and play with the diameter of the tip until it measures the ring gage perfect.
                  Just don't tell your boss. LOL

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                  • #10
                    Originally posted by Jason Rose View Post
                    Take a lot of points and use Max Inscribed for a hole (Minimum Circumscribed for a shaft). Filter out anything over 3 sigma. You can take a ring gage with a known diameter, calibrate your tip and play with the diameter of the tip until it measures the ring gage perfect.
                    Just don't tell your boss. LOL
                    That is an invalid approach, without even touching the ethics of the deception you propose, as the size on the ring gage comes from a 2 point measurement, as I mentioned previously.

                    sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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                    • #11
                      I once adjusted the filtering down to 3 sigma checking a bearing bore. It removed over 800 scanpoints... and left only the 3 that created a perfect size circle.
                      Then I laughed and put the filter settings back to what they should be because I'm not a complete idiot like Jason Rose

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                      • JacobCheverie
                        JacobCheverie commented
                        Editing a comment
                        It's interesting that it did that. Shouldn't 3 sigma only throw away about .3% of your normalized data?

                    • #12
                      Originally posted by fatso666 View Post
                      We had an instance where our calibration ball was switched out with one of a (very) slightly different diameter. If the CMM thinks it is calibrating to a 1.0000" DIA ball, but it is really only 0.9998", it will throw everything off that is measured after that. Are you sure the DIA of your calibration sphere is correct and accurate? Are you sure the DIA of your probe tip is accurate, as well?
                      Thank you. Thank you. Thank you. Pulling my hair out trying to figure out why I was getting a different number from the CMM and a digital mic. New CMM had wrong number for sphere diameter from installation. Never thought to check until reading your post.

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