I have spent at least one hour trying to make water-tight joint between red surface and blue surface. I tried fillets, blends, sweeps, rebuilding edges - all in vain. The green fillet is one of many attempts to join these surfaces.
The edge trim curve of the red surface had 162 control points. I used Rebuild command to reduce the control points to 26. It did not help, so I rebuilt both surfaces with the same number of control points as the old surfaces. The rebuilding spoiled the surfaces. As you can see in the following screenshot, I now cannot make decent fillet with the FilletSrf command.
Yes, it does make the water-tight surace. Maybe the problem was that the trimming curve was too close to the edge of the surface?
Bad NURBS surfaces are as common as bad meshes. Rhino has many commands which repair bad meshes, so it is natural to expect equal number of Rhino commands that repair bad NURBS surfaces. In reality, there are few if any such commands. The Check command is nearly useless - it calls bad surfaces good surfaces. Worse yet, there are no diagnostic tools to help us figure out what is wrong with a bad NURBS surface.
Hello - I don’t think there was anything at all wrong with the surface, the problem was the existing gap between the two input surfaces did not leave enough surface for the rails in all locations… Untrimming removes that and lets the fillet find its rails on the surface.
You have not explained it well. If 0.1mm and 0.3mm fillets work, than 0.11mm fillet should work as well. Maybe the problem is that the edges of the 0.11mm fillet are too close to the edges of original surfaces?
The big question is why the fillets failed on the original surfaces. I still believe that the trimming curve was too close to the edge of the untrimmed NURBS surface. If I am correct, Rhino should check this problem every time it makes new NURBS surface. By the way, The original blue surface had double trimming curve on the bad edge and additional trimming curve near its middle.