Thanks @jim, this is very helpful. I agree that this is not good enough and will keep at it. I’m going to either limit the use of revolved surfaces to trimmed spheres only, or see that similar quality checks are made that are applied in FilletEdge when fillet surfaces are simplified to revolved surfaces.
Some context for why it is what it currently is: I was going off of what you said earlier in the other topic:
A trimmed sphere object is what is used by every CAD program that does rolling ball (arc) fillets well. A nurbs surface with a singularity is more than likely to cause downstream problems for the user when it works and as far as I can tell it mostly does not work.
There is similar functionality in FilletEdge/BlendEdge/ChamferEdge, simplifying fillet surfaces that qualify with revolved surfaces. I’ve just tried your extend_error_1mm.3dm file with FilletEdge but it does not simplify the middle fillet to a revolved surface, so that is a clear indication that I should revisit how FilletEdge does this.