I have this issue from time to time, but I am trying to add a R0.20" fillet Cyan fillet and the Dark Green surface. I have changed the isocurve density up to even try selecting in different places. What I have found is even if you try to create a curve based on intersection of two surfaces, it doesn’t find it. In the past, I have to recreate either surface to get it to work, but in this case, nothing is working.
Does anybody have any suggestions?
Sample.3dm (97.2 KB)
Reparameterize with the Automatic option sometimes helps with these mysterious won’t -intersect cases
Your tolerance settings are a bit wack, 0.1" for angle is enough, setting it to that produced a fillet between the blue and green. The continuity the orange fillet isn’t good enough, the results don’t match up going around.
Can you post a shaded view?
It’s hard to read the surfaces in this Tron-RetroArcade-BattleZone-wireframe mode. 
This is a long standing Rhino bug and Rhino8 is even worse that previous versions.
The surface you are looking for is a fillet that comes to a point where the Dark green and cyan surfaces become tangent to each other.
Rhino8 is completely unable to produce any result (you tried the right things).
Rhino6 is able to make the surface easily but to make it come to a point the user has to use setpt on the last 3 control points to make the point.
Samplex.3dm (80.4 KB)
Have you tried to force the radius at the point to be zero? Sometime this trick worked for me.
It seams that Rhino isn’t able to do the R0 by itself but if you do this by hand it handle the cutting much better
This is EXACTLY how I have done it in the past. Where the fillet comes to a point, or should come to a point, it seems to come short. So you use the SetPt command to fix it. Good, I thought that was just me.
I was able to get a fillet in there, but only as low as R0.40 which is too large. I ended up keeping the R0.40, deleting the R0.20, but further up used a R0.20 and then networksrf them together. Came out decent.
Thanks for the insight.
That’s the bug I was talking about in the first post. They made some changes to this type of fillet in Rhino8. I’ve never seen a case where its an improvement.
