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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
  • Coupe of observations/questions:

    You say the rad has been verified at 2.75 by two other methods - what methods are these? Any system that calculates the radius from points would be susceptible to the same issue (this is not a PC-Dmis issue, it's a math issue).

    The point of using the fixed rad method is that, by calculating a circle from points over an insufficient degree of arc, can give an inaccurate and non-repeatable centre point, giving bogus radius values.



    So 1rst method is giving it to an external metrology laboratory. It came up at 2.75 something. Second method is a mixture of results such as: Isomat projector (2d visual measurement), CMM simple measurement with least squares and finally testing fixed dia pins and seeing how they best fit on the rad cutoff.

    To tell you the truth, I am disappointed of this method because it is dependent on the value the fxd rad we initially give. As said, I tried 2.8 (nominal) and got repeatable close to 2.8 results and then I tried using...

    CYL_2_8.D (a least square circle that was taken before that gives 2.75..mm) as the nominal D and the results I was getting were repeatable 2.75 something.

    The repeatability is there but not the accuracy.
Reply
  • Coupe of observations/questions:

    You say the rad has been verified at 2.75 by two other methods - what methods are these? Any system that calculates the radius from points would be susceptible to the same issue (this is not a PC-Dmis issue, it's a math issue).

    The point of using the fixed rad method is that, by calculating a circle from points over an insufficient degree of arc, can give an inaccurate and non-repeatable centre point, giving bogus radius values.



    So 1rst method is giving it to an external metrology laboratory. It came up at 2.75 something. Second method is a mixture of results such as: Isomat projector (2d visual measurement), CMM simple measurement with least squares and finally testing fixed dia pins and seeing how they best fit on the rad cutoff.

    To tell you the truth, I am disappointed of this method because it is dependent on the value the fxd rad we initially give. As said, I tried 2.8 (nominal) and got repeatable close to 2.8 results and then I tried using...

    CYL_2_8.D (a least square circle that was taken before that gives 2.75..mm) as the nominal D and the results I was getting were repeatable 2.75 something.

    The repeatability is there but not the accuracy.
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