Minimum Arc needed for Circle

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  • Minimum Arc needed for Circle

    Ahh, The age old question....

    What are the minimum degrees of an arc required for Pc-Dmis to construct a circle?
    How would be the repeatability and reproducibility be also?
    Where would I find the documentation?

    Reason:

    One of our suppliers is picking a fight with me about the ability of the CMM to inspect a 93.00 mm radius with 39 degrees of arc. The tolerance is +/-.13mm with a CAD Model. I laughed, "The CMM can measure that all day!" I said, "Within a 10% dimensional uncertainty too." I get my share of gunfights as a programmer.

    Thanks for your help!!
    sigpic

  • #2
    We have that argument internally and I have come up with a solution to please the bosses for short arc radii.

    Scan it and compare it to CAD nominal.
    Xcel & MicroVal Pfx & Global 37mr4 thru 2012mr1sp3
    Contura Calypso 5.4

    Lord, keep Your arm around my shoulder and Your hand over my mouth. Amen.

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    • #3
      I have had good repeatability and accuracy checking small arcs as cylinders. The more hits over a larger area the better. As circles, I have not done as well, but I am working with very small arcs of large diameters, usually. Once or twice when there has been a dispute I have put a certified ring on the table right next to the part. When you can measure the same arc on a ring gage that is , (or is almost), the same size as the feature it pretty well ends the debate one way or the other. HTH
      sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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      • #4
        Originally posted by Wes Cisco View Post
        Once or twice when there has been a dispute I have put a certified ring on the table right next to the part. When you can measure the same arc on a ring gage that is , (or is almost), the same size as the feature it pretty well ends the debate one way or the other. HTH
        I'll have to remember that.

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        • #5
          Originally posted by Wes Cisco View Post
          Once or twice when there has been a dispute I have put a certified ring on the table right next to the part. When you can measure the same arc on a ring gage that is , (or is almost), the same size as the feature it pretty well ends the debate one way or the other. HTH
          Sounds good, I guess that's my best shot. I heard a long time ago that Canada has a national standard for CMM measurement, does anybody have knowledge of that document?

          G
          sigpic

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          • #6
            I think you might be thinking of these "good practice" guides from the British NPL. Here is a link: http://www.npl.co.uk/publications/good_practice/
            Down the list you will find some specific to CMMs.

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

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            • #7
              THX Wes

              Originally posted by Wes Cisco View Post
              "good practice" guides from the British NPL : http://www.npl.co.uk/publications/good_practice/
              HTH

              Hmmm... very interesting, and free!!! Thanks again wes.
              sigpic

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              • #8
                I want to say that B&S recommends at least 1/3 to calculate.
                sigpic.....Its called golf because all the other 4 letter words were taken

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                • #9
                  I think that you might be able to calculate your maximum error by just doing the math.

                  Say that you have a 36 degree arc and let's say you measure it with 3 points. Then the worst case variation would be that the 2 points at the extremes (0 and 36) are both at you maximum high (based on the repeatability and uncertainty in your machine) and the point in the middle (18 degrees) the maximum minus. Then do the same for the other case (where the 2 extremes are maximum low and the middle is maximum high). That gives you 2 different diameters. It is likely that all measurements will come in somewhere in between. It helps to put in more points obviously.

                  And you may find that these 2 diameters may be very far apart. I have seen similar calcs where a seemingly very accurate machine could not even hold 0.125" because the arc was just too small. So care must be taken here. And your customer may be partially right.


                  Jan.
                  ***************************
                  PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
                  Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.

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                  • #10
                    I was told 180 deg was the min. But I have never tried what Wes said with cylinders. I will give that a try when I can. I have found that the larger the dia the more uncertainty that I have.
                    Time for the Trolls to leave.

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                    • #11
                      Suggest to search cmmtalk.com for "arc measurement" or "radius measurement".

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                      • #12
                        An option I often use when location is not critical is to measure as a 'Fixed Radius' circle. Then I turn on the min/max option in the dimension setup and report the radius. Of course it will show the actual radius as perfect but the min/max will be deviation of form, kinda like profile. If your customer is OK with this, your golden.

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                        • #13
                          Radial measurement uncertainty

                          Its a math thing, not a cmm issue. I will attempt to post an Excel sheet that shows you the amount of error to expect with the small chord of a circle. You enter in the arc size, along with the angle and tolerance. I have NOT lost a discussion with a client using this yet.

                          There are some methods of measuring small arcs, the cylinder as mentioned is a good one, but also with polar pts. There is some issue with that, but if your customer is accepting of the method, you are good to go.
                          Attached Files
                          Jim Jewell

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                          • #14
                            We'll,


                            I got my **** handed back to me again. I don't know what the secret is to winning a shooting match. I think I should try a more physical approach next time.
                            sigpic

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                            • #15
                              Originally posted by Cumofo View Post
                              We'll,


                              I got my **** handed back to me again. I don't know what the secret is to winning a shooting match. I think I should try a more physical approach next time.
                              Please elaborate. Maybe you can save one of us from the same pitfall. Did you show them your neck tattoo?
                              sigpic"Hated by Many, Loved by Few" _ A.B. - Stone brewery

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