Fillet puzzle

I’m back using Rhino after a few months and I may have forgotten something vital here. I’m trying to fillet all the edges on the attached polysurface with a radius of 0.24. The central opening just will not fillet. I’ve tried Fillet Surface and Fillet Edge. I’ve rebuilt the curves that I extruded to get the surface. No luck.
Any ideas?
Rhino 6, SR18
(6.18.19266.14201, 2019-09-23)

fillet problem.3dm (163.6 KB)


Hello Nick,

Did you start by trying to fillet the central opening first ?

Thank you,


Hi Andy,
It doesn’t work, no matter which order I do the fillets. In this image you can see that Fillet Edge fails to trim the original surfaces + there is a weird anomaly where an extra seam is projected back into the model. When I explode and try Fillet Surface I get other strange problems. This is not a complicated model and I would have thought that filleting would be pretty easy.


I see no problem. Here is the file with all the corners having a 0,24 radius fillet.

Fillet works just fine.3dm (1.2 MB)

Hello B Design Bg2,

But what about the outside frame edges?

Thank you,


Yes, the outside fails when I try Rhino_Bulgaria’s file. There is something strange here.

if you split the extrusion at the theoretical kinks, it works

Also Run DivideAlongCreases with SplitAtTangents=Yes

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Sorry, I forgot to make fillets to the outside edges as I had the impression that the problem was within the holes. :slight_smile:
As Diego Krause mentioned, splitting the vertical surface around the tangent kinks (where the 4 corner blend curves were joined to the rest curves prior the extrusion) will do the trick. Rhino is getting confused exactly there. This is the first time I see the “Divide surface along creases and tangents” command fail. :slight_smile: Maybe the reason for that is because I always extrude grouped separate curves.

Fillet works just fine.3dm (2.0 MB)

Hello - the curves are G2 so DivideAlongCreases on the surface should do nothing here…
V6 filleting fails here, (outer) and V7 makes a mess. I’ll send this along to the developer for a look.


Thanks everyone.
So I would have been better off to have not joined the curves before extruding? What would have been the best way to have created this model so as not to cause Rhino’s filleting eccentricities to mess things up.

Hi Nick - so far, I could not say - it really ought to just work here as far as I can see so it’s hard to say what would have been better practice. The curves are G2 but have fairly rapid changes in curvature at the joins - I am speculating - I’ll fool around with it but separating the surfaces there might be better as @DiegoKrause pointed out - treat it as if it were tangent. But that should not be needed, from what I see. Hopefully the developer will be able to enlighten us…


If you extract the vertical extruded surface and re-position its seam with the “Adjust closed surface seam” tool to one of the kinks, then joint everything, trying to make a fillet will result in somewhat strange shape at the opposite end. If you move the seam at another area, the result after fillet is always different.

Funny fillet.3dm (151.8 KB)

The fillet edge did nothing but exploding and using fillet surface worked fine here. Rhino 6.18
fillet problemOK.3dm (509.5 KB)

I thought I read somewhere in the forum that at this stage of Rhino’s development there’s a lot of things that don’t work with extrusions, but if you explode them into polysurfaces a lot more can be done.

The answer is pretty simple:
Use rolling ball fillets and
use arcs and lines to make your curves

You should be able to do something as simple as this in a minute or two instead of spending a whole day.

fillet problemx.3dm (174.5 KB)

@Nick, try FilletEdgeOld :slight_smile:

Hello Gijs,

Thank you. The command worked great.


Hi Gijs. FilletEdgeOld worked perfectly!

Jim, it seems sad that in software created originally as “NURBS modeling for Windows” that I would have to convert the curves back to arcs to make filleting work.
My work process and aesthetic, such as it is, involves using NURBS curves, for better or worse.

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Hmm. I was still having problems when using FilletSurface.