Questions about diameters.

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  • Questions about diameters.

    Got something funny going on with my diameter measurements. I was wondering if someone could give me some insight. I am measuring a part thats outer diameter is coming in as 47.38. However checking it with calipers I cant find one single spot that measures that high. Looking at the min max diamters I see the max coming in as 47.48 which would be way higher than what I got with calipers. We have an old cmm that I also checked the parts with and the highest diameter I got in the parts was a 47.33 taking the same number of points. However when I check a ring gauge its pretty much spot on. However when I check a puck which would be in outer diameter it seems to be off by almost .05. So it almost seems like inner diameters are OK but outer diameters might be off. But the calibration sphere would be calibrating to an outer diameter so I dont know how that could be. To further complicate things I took individual points on the outer diameter to see thier distance from the center and I was getting a strange number like 22.88 when the radius should be more like a 23.69ish. Any help would be appreciated.

  • #2
    The part is probably lobed.

    calipers will not pick that up. They will measure a smaller diameter.

    B&S CHAMELEON/PCDMIS CAD++ V2011

    There are no bugs, only "UNDOCUMENTED ENHANCEMENTS!"

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    • #3
      I don't know your experience about cmm, so maybe my answer is stupid !
      Circle is a 2d feature, so you should have a plane perp to the cylinder defined (workplane). You can also "shank" if the cylinder is not // to the stylus.
      Can you post the code ?

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      • #4
        Thanks for the speedy responses. To clarify a little I am using a shank probe and have leveled out on the top of the cylinder. And the part is definitlely lobed, but the largest possible diameter I can get on it with calipers is a 47.35. Still doesnt make sense why it reports 47.38 on this CMM but on our older CMM using a similar probe and the same number of points I am getting a 47.33.

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        • #5
          Originally posted by justinthorne76 View Post
          Thanks for the speedy responses. To clarify a little I am using a shank probe and have leveled out on the top of the cylinder. And the part is definitlely lobed, but the largest possible diameter I can get on it with calipers is a 47.35. Still doesnt make sense why it reports 47.38 on this CMM but on our older CMM using a similar probe and the same number of points I am getting a 47.33.
          That's probably your issue, unless the axis of the diameter exactly matches the axis of the probe you'll be 'shanking' out.
          Automettech - Automated Metrology Technology

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          • #6
            I have worked with CMM's over 25 years ( mostly stampings) and have never had faith in the results for diameters using renishaw PH's. scanning heads yes, TP's no.
            sigpicSummer Time. Gotta Love it!

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            • #7
              You could use tri-mikes to measure the diameter. They will pick up the lobe effect.

              B&S CHAMELEON/PCDMIS CAD++ V2011

              There are no bugs, only "UNDOCUMENTED ENHANCEMENTS!"

              sigpic

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              • #8
                Check the circularity and turn on the CAD Graphical Analysis in the circularity dimension so you can see the high and low spots. You can also change the fit type on the feature itself to MAX_INSC or MIN_CIRC and compare the results of each to see just how lobed it is...

                Like dph.51 said, if its lobed (especially because of a 3-jaw chuck), calipers will not see it. Can the operator turn down the jaw pressure?

                Did you use the same probe tip on both CMM's? That could cause discrepancy as well if one shanks and the other doesn't

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                • #9
                  Well, first ditch the shank probe. Even with a similar probe the squareness to the table can be affecting the difference between the two CMMs.

                  Your machine could also be terrible out of square too. I've seen this when a machine takes a hard hit, what may be a perfect circle can measure lobed from squareness error.
                  Systems Integrator
                  Hexagon Manufacturing Intelligence

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                  • #10
                    +1 on #9. Keep in mind shank probes are best used with sheet metal.
                    sigpic3.7MR2

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