Surface changes after filleting

I model nurbs surfaces and join two surfaces together by aligning surface pointsso they need to have the same degree and number of points.
I have been finding that when I sometimes join these surafces then fillet an edge it physically changes the point and size of one or two of the surfaces?! very annoying!
when the points are then put together again these surfaces no longer join!
any help would be much appreciated

Post a .3dm file of the problem with the surfaces before and after filleting.

Also post exactly which command and what options you are using for the fillet.

Hi this is the surfaces, I have shown the before where all the points meet and it joins.
then with the fillet I have simply filled holes then filleted using a 50mm fillet
then what happens afterwards when you explode and untrim!.
I hope this makes it clearer?fillet surface problem.3dm (328.7 KB)

How did you fill the hole? Which command?

Hello - it does look like filleting is doing some shrinking of the inputs… I’ll see what I can find out.

-Pascal

The opposite. In this situation FilletEdge is extending the surface at the ends of the fillet, and then trimming that surface back to the original size. I was able to reproduce the behavior with the OP’s original geometry.

Yes, right, sorry, it extends the lower surface - I feel better already. It does not look like that should be needed though, in this case. I’ll ask…

-Pascal

cap planar holes, but I think it is the fillet that is doing the damage

great thanks,
yes it is extending the surface and moving all the points along the join edge,
the really strange thing is that if I move all the points back to the other surface it will not join?!

@pascal I was unaware that FilletEdge could alter geometry beyond trimming surfaces. At a minimum FilletEdge should warn the user when it is extending input surfaces. I can envision a scenario in which I copy an altered surface for use elsewhere and not realize it was previously extended and trimmed back by ExtendSrf.