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Question about the small arc method

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!
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  • I can't say that I report the PR of individual points. I do think that having a fixed_rad of alternate size would cause some disruption in PR reporting for individual points.
    Typically I measure with fixed_rad and reduced arc span for initial location, align, repeat fixed_rad with larger (generally full) arc span, align, measure with least_sq(match last arc span) and report R.
    I found this practice does not match perfectly to mold/comp but it is pretty close. (subjective based on tolerancing)
Reply
  • I can't say that I report the PR of individual points. I do think that having a fixed_rad of alternate size would cause some disruption in PR reporting for individual points.
    Typically I measure with fixed_rad and reduced arc span for initial location, align, repeat fixed_rad with larger (generally full) arc span, align, measure with least_sq(match last arc span) and report R.
    I found this practice does not match perfectly to mold/comp but it is pretty close. (subjective based on tolerancing)
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