measuring small arc

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  • measuring small arc

    Can the attached arc (colored green) be measured on the cmm? The radius is 1mm.
    Attached Files

  • #2

    Since the radius is so small I would make an alignment local to where the radius is to help me find it. Level to that plane, make a high point on the large top sweeping radius, then measure that ID, then make a line from the ID to the high point...then lvl plane, rotate line, make circle and plane origins.

    Then, measure the radius using fixed rad, then origin to fixed rad, then remeasure the radius you'll use for reporting using least square.

    Be mindful of workplanes when doing this.


    • #3
      I just scan them and use profile and dimension form only..... Yeah, I know some people don't approve of this.....


      • DAN_M
        DAN_M commented
        Editing a comment
        If you have a surface profile callout back to specific datums..& you're aligned to those datums..then yeah sounds legit.

        I think he's focused more on finding the arc and is concerned with it being so small and not having much surface to probe.

    • #4
      I can't get any repeatability measuring that arc. The surface area of the arc is 0.5mm so one hit takes up the entire surface using a 0.5mm stylus. I like the profile method but not acceptable here. We have a surfcom that can measure it but it must be done on the cmm.


      • DAN_M
        DAN_M commented
        Editing a comment
        If that whole surface area is only 0.5mm...and MUST be on CMM (with all due respect this WOULD NOT be a CMM check at any company I have ever worked at but regardless IF I was giving it my best attempt...), I would use the fixed rad method I said above and then use a 0.3mm probe to measure the actual surface if you have one.

        Sounds like you're in a rough spot.

    • #5
      I would try this (even if I believe that it's not "measurable" accurately on a cmm !) :
      1- Scan with a high density from the left start of the large circle to the end of the plane
      2- Measure min circ circle on the large circle (or create a circle from the scan hits)
      3- Measure the tagent plane (or create a line from the scan hits)
      4- Level on the plane, origin on the circle
      5- Calculate PR of each hit : ASSIGN/V1=SQRT(DOT(SCN1.HIT[1..SCN1.NUMHITS].XYZ,SCN1.HIT[1..SCN1.NUMHITS].XYZ)..................(here, coordinates = vector components because the origin is on the circle)
      6- Search the hits with a PR > circle radius ASSIGN/V2=((V1-(CIRC1.R+0.001))+ABS(V1-(CIRC1.R+0.001)))/2..................should give an array with 0 for hits on the circle. I added 0.001 mm to avoid hits of the min circ !
      7- Search the hits higher than the plane ASSIGN/V3=((SCN1.HIT[1..SCN1.NUMHITS].Z-(PL1.Z+0.001))-ABS(SCN1.HIT[1..SCN1.NUMHITS].Z-(PL1.Z+0.001)))/((SCN1.HIT[1..SCN1.NUMHITS].Z-(PL1.Z+0.001))-ABS(SCN1.HIT[1..SCN1.NUMHITS].Z-(PL1.Z+0.001))).......................should give an array of 0 and 1 for all the hits > PL1.Z
      8- ASSIGN/V4=(V2/V2)*V3...........................should be an array of 0 and 1
      9- ASSIGN/V5=MININDEX(V2).........................should be the first hit of the 1 mm circle
      10- ASSIGN/V6=SUM(V4)........................................ .should be the number of hits of the 1 mm circle
      Then construct a circle from SCN1.HIT[V5..V6].............................(maybe a synthax like ASSIGN/V7="SCN1.HIT["+V5+".."+V6+"]" and use V7 in the circle)


      • #6
        cough, cough, profile !, cough....


        • #7
          The requirement is to inspect on a CMM.
          If the customer adheres to their own requirement, they will have the same trouble measuring it... and get the same sketchy results.
          One screenshot is worth a thousand Need more info
          Never attribute to malice that which is adequately explained by stupidity...


          • #8
            Hexagon has a pdf you can download that explains how to do it. It's pretty much exactly what DAN_M described, but you can keep a copy of it for yourself. The link should be in the first comment of this old thread.


            NOTE: You will need to have an account with Hexagon and be signed in for the download to be available!


            • #9
              That's look like an interesting PDF but it appears that the link is broken. Does anyone have it to hand.



              • #10
                Originally posted by panelbeater1 View Post
                That's look like an interesting PDF but it appears that the link is broken. Does anyone have it to hand.

                try here :


                • #11
                  Any kind of feature can be measured on the a CMM bu the question is will it be accurate and repeatable. Some things are not to be measured on the CMM even many in engineering and/or management seem think otherwise, this is one that is not unless measured as part of a profile callout as stated above
                  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.


                  • #12
                    An alternative is to use the fixed rad method as Dan described, but then measure vector points and report the polar radius (in effect you're doing profile but is shows as Rad on the report.
                    Applications Engineer
                    Hexagon UK


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