Question about the small arc method

theoretical center is more estimation than measurement the smaller arc and radius are

you can take 10 pieces from different days of production and you will see they are all different you will find differences with your eye no need any profile or microscope for this :)  

i spend lot of time with alignments on ~~ R1  arcs, with tip 1mm calibrated on ~~0.003 mm it was not accurate enough 

When you take any 3 points on some arc drawn in CAD you always have same results, but make one point of arc moved 0.01mm and see your results, then move it along the arc few times

While see small arcs on profile testers  you find out they have roundness, waviness, roughness, they are always rotated around its TED center, more if are smaller 

While measuring R0.2mm sometimes you can say "its not even a part of circle" its just some parabolic shape or what ... 

I saw measurement of R0.7 but it was never few points 

in my opinion need accurate machine to measure small radius with small angular coverage, use always smallest probe you can, take as much points like profile tester - not 10 but 1000 5000 because your machine needs some data for least squares method calculation and for filtering surfaces and measurement errors , and do it like on profile tester - scan not only arc but surfaces close to it so you can choose and decide (even with every single measurement)  witch points you consider as circle and what are rather part of surrounding planes. 

Or just do it on profile tester its usually better, because some things need just too much luck do be done 

theoretical center is more estimation than measurement the smaller arc and radius are

you can take 10 pieces from different days of production and you will see they are all different you will find differences with your eye no need any profile or microscope for this :)  

i spend lot of time with alignments on ~~ R1  arcs, with tip 1mm calibrated on ~~0.003 mm it was not accurate enough 

When you take any 3 points on some arc drawn in CAD you always have same results, but make one point of arc moved 0.01mm and see your results, then move it along the arc few times

While see small arcs on profile testers  you find out they have roundness, waviness, roughness, they are always rotated around its TED center, more if are smaller 

While measuring R0.2mm sometimes you can say "its not even a part of circle" its just some parabolic shape or what ... 

I saw measurement of R0.7 but it was never few points 

in my opinion need accurate machine to measure small radius with small angular coverage, use always smallest probe you can, take as much points like profile tester - not 10 but 1000 5000 because your machine needs some data for least squares method calculation and for filtering surfaces and measurement errors , and do it like on profile tester - scan not only arc but surfaces close to it so you can choose and decide (even with every single measurement)  witch points you consider as circle and what are rather part of surrounding planes. 

Or just do it on profile tester its usually better, because some things need just too much luck do be done