Hi Martin, I played around some with your original ViaCAD file inside of ViaCAD and yup definitely it looks a lot better there, and I think it is probably due to it being handled as a special analytic blend surface at runtime in ViaCAD and then having an actual separate NURBS surface result generated for exports.

You can also see a NURBS surface generated if you turn on surface control points for that one surface in ViaCAD and move just one single control point (on some far off corner of the patch so as not to actually change much with the edit itself) - that converts the surface into a regular NURBS surface directly in ViaCAD and if you then paste back the original blend you can see that the control-point edited surface has those ripples in it and is slightly differnet in shape than the analytic blend.

The analytic blend seems to be handled by a lot of mechanisms though, intersections and offsets do not seem to degrade it so that means that they implement the special blend surface type deeply into the ACIS system and it's able to do a lot of processing on the kind of "ideal blend surface" and not need to use the NURBS surface version for too many places except of course for doing a file transfer out of the ACIS system to some other system that wants to deal just with plain NURBS surfaces, at which point the messier NURBS surface approximation of the blend gets used instead.

This does not bode all that well for trying to integrate ACIS into MoI just for some particular operations like fillets though, because in order to get the nice n-sided blend behavior the surfaces have to stay as those special ACIS blend surface types and not get converted into regular NURBS. So that means keeping all things as ACIS data all the time and not just using bits and pieces of ACIS to calculate NURBS surface results.

So anyway I think that's what's going on there, that there's a deep treatment of these n-sided blends as a special surface type in ACIS which seems to work nicely if you stay all inside the ACIS system for all the operations involved but the NURBS approximation result is not all that great...

- Michael