TRUE Sphere

[[Al...]]

I am trying to measure the ball radius on this part, (5)

Should I measure top to bottom or measure the sphere itself? 

[[Cl...]]

I would do what you are doing here all the way around the part. Maybe 6/8 times (measuring the sphere.

Then create a secondary alignment (XYZ) from the measured sphere. Then measure a circle all the way

around the equator referencing that secondary alignment to ensure you are at Z zero. Then use that circle

in a radius characteristic.

[[Ch...]]

I would measure a sphere from above using 30 points or scan it.

Set this as zero, datum XYZ. Then measure again with several cross-sections.

As Clark suggests, 5-8 times. From this sphere, you can then evaluate the radius and also the shape to identify errors.

 

[[Jo...]]

One more aspect:

Do not spread you points (circles/scanning paths etc) regularly. Depending on the manufactoring process you could end up catching only specific parts (grooves for example).