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Question about the small arc method

constadin
Hello all!

Please let me know if I am doing something wrong following the small arc method.

Btw, I am using vector points and auto circles with NO CAD always in DCC mode. Let's also assume that I am measuring in Z plane.

1) I am measuring the small arc with a fixed radius auto circle.
2) I am setting my origin XY to this circle.
3) Just to be sure I am repeating with a new auto circle fixed radius and realign to it.
4) I am taking 3 vector points in polar coordinates to be precise.
5) I am reporting the PR of the points.

This morning I had a 80deg on an arc R2.8mm to measure. I used this method and I was getting repeatable results at 2.80. The issue is that the radius has been measured to be 2.75mm (verified with 2 other methods also).

I also tried to change the fixed rad value based on the R measurement I had performed before with least squares (around 2.75 non repeatable value) and I was getting results close to that value (the least square value).

With this method, does the fixed Radius value provided alter the centre that is being found? Doesn't this beat the purpose of using this method?

Thanks in advance!
Parents
  • Small arcs are susceptible to form error just as any other shape. Make sure your vector points are not near a breakedge, chamfer, gate location or injector pin location (if injection molded)...
    --I would suggest taking more hits, just to be sure that form isn't contributing to your delta.
    --If you need to report ONE radius for the feature, it would be beneficial to average all the vector hits with an assignment and a generic feature.

    Assign/PRaverage= (PRHIT1.PR + PRHIT2.PR + PRHIT3.PR)/3

    GEN _AVG_PR generic feature/Polar coords
    theo Angle 0+0/PR you can set to the nominal+0 /Z 0+0 ijk 0+0 or 1+0 in one of the vectors
    act same as above but use PRAVERAGE in the PR
    Report GEN_AVG_PR polar radius and you get one average value of PR from the 3 (or more) hits that you made.
Reply
  • Small arcs are susceptible to form error just as any other shape. Make sure your vector points are not near a breakedge, chamfer, gate location or injector pin location (if injection molded)...
    --I would suggest taking more hits, just to be sure that form isn't contributing to your delta.
    --If you need to report ONE radius for the feature, it would be beneficial to average all the vector hits with an assignment and a generic feature.

    Assign/PRaverage= (PRHIT1.PR + PRHIT2.PR + PRHIT3.PR)/3

    GEN _AVG_PR generic feature/Polar coords
    theo Angle 0+0/PR you can set to the nominal+0 /Z 0+0 ijk 0+0 or 1+0 in one of the vectors
    act same as above but use PRAVERAGE in the PR
    Report GEN_AVG_PR polar radius and you get one average value of PR from the 3 (or more) hits that you made.
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