Fillet approach


I was wondering what the way to go is solving this fillet? (aside of patch)

fillet approach.3dm (418.4 KB)

Hi Philippe - V6 gets this case if you do all the edges at once -

All at once.3dm (561.4 KB)


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This is a classic case that many users faced over the past decades.

I would use the following approach:

  1. Create “Blend curves” with the “Edge” option and snap them to the middle or quad of the surface edges. Then use each of them to split the other two so that you get 6 halves (I hide the original Blend curves in the scene).

  2. Create “Network surface” with “G2”. Repeat that on the remaining edges (or make copies with “Orient objects: 3 points”).

  3. Use “Rebuild surface” with Degree = 3 and Point count = 4 or 5 (in this example I used 4).

  4. Use “Match surface” with the “Multiple matches” option (or simply right-click to use “Match up to four surface edges”) and match each of these 6 surfaces’ edges to the two adjacent blend edges each one of them has. Use “Continuity: Curvature” and “Preserve other end: None”. Make sure that you also selected “Preserve isocurve direction” and “Match edges by closest points”, while the “Refine match” must be turned off.

  5. Use “Match surface” with the “Average surfaces” option and “Continuity: Tangency” to match each of the 6 surfaces to their adjacent twins at either side.

  6. Now use the “Insert knot” tool, set “Direction=Both” and click on the “Automatic” option in the command line. Follow this to all 6 surfaces.

  7. Repeat step number 4, but this time use the “Refine match option”.

  8. Repeat step number 5.

That’s it. The resulting surfaces could be further improved with manual adjusting of the control points around the middle of each of the 6 surfaces via the “Move UVN” tool. The 5th picture with the horizontal zebra clearly shows where the surfaces need some extra touch by human hand.

All at once in Bulgaria.3dm (6.2 MB)


Oh so weird, that didn’t happen when I first tried, now indeed it does… so easy :slight_smile: Thanks!

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Thanks so much for your time and thorough walkthrough… will investigate!!

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hi @Philippe1

patch is probably a bit underrated. no doubt its not one of the very best tools in Rhino, but it still may deliver some useful outcome depending on how demanding the need for quality is.

for this to work well you have to get rid of the low degree surrounding fillets first and replace them by blends. this would anyway be important to get a perfect middle blend later however you approach it. the fillets produced by rhino are never a good basis for good quality. the rest requires a bit of a feeling for the settings of patch and it will deliver an ok quality in a fast manner.

patch it up.3dm (140.9 KB)

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or xnurbs single patch + beers at the pub.

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no beer for mac users though :wink:

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They don’t need beer - they have life easy. Don’t they?


no, just “different” … :thinking:

Switch to pc, who needs mac anyway?

Ahhh. Ok. Still having these sorts of plug-ins available would/will be nice on Mac.


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As someone who doesn’t do much surfacing, is this really what a proper (Rhino only) workflow to maintain curvature continuity across a patch looks like!?

It just seems crazy too me how big the disconnect is between the fairly simple intent the user has versus how tedious the solution is.

This is easy enough to do in Rhino. Extract isocurve center. Quick blend curves Tangency. Shrink all surfaces. Then Surface Patch, select the inner curves and edges with these settings. You can get good results with Surface patch once you play around with the settings.

The quickest and fairly “acceptable” solution in Rhino (without any extra $$$ plug-ins) is to build 3 lines that connect the opposite farther edges, then use “Patch”. However, I always avoid using Rhino’s “Patch” command, because it’s not accurate enough and will not allow further control over the shape. But it’s the easiest way for non-professionals to close gaps like this one.


This is one of the easiest cases. Sometimes I end up devoting 30-60 minutes on a single surface with the “Move UVN” tool to get it right if the adjacent surfaces are too complex for Rhino’s automated tools.