Measuring a Large spherical Radius over a small surface

[[Ri...]]

I'm trying to measure a 20.0" spherical radius (+0.0/-0.25) on the end of a 1.0" diameter shaft with a .06" corner fillet.
I've tried different strategies, helical and concentric circles. These give multiple results.

I've also cut two sections at 90 degrees thru the SR center and scanned the created arcs, this seems to give more consistent results, but if there is any variance in the surface (like a tit at center) everything gets thrown way off.

Any suggestions??

[[Cl...]]

How many degrees is the segment from the start point of the measurement to the finish point?
(How much surface angle do you have to measure)? That could be part of the problem. Is it 180
degrees, or less. Got a pic?

[[Ke...]]

I had pretty good luck using circle segments that pass over the "top" of the diameter, and pulling those into a sphere. Due to the small area of the sphere that was available for measurement, the "L1 Feature" evaluation worked best, and I was able to correlate my results with linear profiles, measured with a TalySurf and MarSurf.

Small Area Sphere.png

[[Cl...]]

Keith, were you also using any constraints?

[[Ke...]]

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No constraints were used.
This was a sample part for "benchmarking", so I had no known size/locations to use to constrain the feature.
I have to wait until I can get my hands on "known" parts, before I can compare the "L1 Feature" with no constraints vs LSQ & Outer Tangential *with* constraints (and correlating with other equipment)

The data that I attached are averages & delta values from a 5 part sample.

Small Area Sphere Comparison.png

[[Ro...]]

12 or more points is how i did it.

12 points-2.png