I'm having difficulties with fillets

I am having some difficulties with fillets, and I can’t figure out what I am doing wrong.

I want to fillet the highlighted edges.

No matter what I try, I always end up with naked edges.

What am I doing wrong?

Nothing. You are doing well.
This kind of fillet were not properly handled.
The problem it’s in the small edge between the plane and the 2 ribs.

You can cut the remaining (pink) corner using an arc and creating the filling srf by a sweep2R or a Revolve.
Than joining everything together you get a solid back.


Thank you for the reply. I do not understand your explanation to resolve this though. Too much of a newbie I’m afraid.

Would you be willing to show a step by step explanation with images.

I continue to have issues with fillets that worked in the previous version of Rhino. I will post that one separately.

Do you mean by this that this is a bug in the current version?


I can assure you this not a bug or failing of Rhino’s fillet command. The geometric conditions for this fillet make it impossible. I would suggest that you change the geometry rather than try to trim and patch in the corners.

Can you please post a file with the part?



LB 02.3dm (1.5 MB)

Hi Abrasha- one thing I notice is that the vertical walls are very slightly less than 3.000 mm apart

Which means that the angled surface there will need to be completely consumed by the fillet operation (at R=1.5) and this is something that is not currently possible in FilletEdge. If we reduce the radius slightly to accommodate this, say to 1.45, the result is still not what you expect due to the four edges coming together where one is concave - this is yet another case that is difficult, but the results are maybe more manageable.

For what it is worth, here is one way to get at that corner situation using FilletSrf-

FilletCorner.3dm (595.6 KB)



You are absolutely right about the width being less than 3 mm. I made four 3mm curves (in step 3 in my file) that I used to make the polysurface to do the boolean difference in step 4, thinking that the resulting width of the two “arms” would be 3 mm wide. I completely missed the fact that they are diagonal and therefore narrower in width. Simple high school math really. A brain fart.

Given that the model now exists with those 2.99 mm sides I would be more than happy with fillets of 1.45 mm, if that would give me a clean watertight model. Besides, it seems that trying to make those sides exactly 3 mm, will not make the model watertight after filleting, because I assume the same thing will happen.

Thank you for uploading the file with your solution. Unfortunately I have not been able to reproduce it. I can’t get all the FilletSrf commands to work from the first to the second model. It’s the 5th one (in your step 2) I am having trouble with.

And even though your solution is a great deal better than anything I have been able to achieve, it is still not perfect. I am now wondering if Rhino is even able to do this perfectly and elegantlyly.

I am trying to duplicate a damaged plastic part that has fillets all around in the same areas, so I assumed that this would be possible.

How do you make those red text balloons that always face exactly perfectly forward in every viewport and even do the same when rotating the Perspective Viewport.

Hi Zews - I agree that the solution I posted is not very nice looking but I believe it is the correct ‘rolling ball’ fillet solution for a constant radius. I think it would look better if the short fillet at the top where the arm angles down were somewhat larger than the rest - say 2 units or even a bit more. That would also make it easier to complete using filletSrf and not have to deal with making the spheres.

Use the Dot command to make the text dots.



It’s always better to create the topology that allows for a smooth fillet.


After looking at these two images, I have no idea what you have done to get these results.

He changed your model, to show you how to plan to implement filleting smoothly.
Was your model a given from a customer, or is it your own idea, that you want to fulfill?


I downloaded the file FilletCorner.3dm that you created. I have now played with this issue for several days and for many hours. No mater what I try, I cannot get to the same result. Whenever I try to do FilletSurf on one side with one of the “arms”, a surface gets deleted on the other arm and a large hole opens up. So I never even get to any one of the other commands you have used.

Is it OK to post a video of my attempts so you can see what I tried?

I have also tried to do the example at http://wiki.mcneel.com/rhino/foursrf without success. I did not get any further than FilletSrf (5 times). And it never looks the same as in the example.

I was not able to figure out how to execute the Split commands. In the instructions it says “Using the Split command with the Isocurve option”. Whenever I check Isovurve it does not seem to work properly. When I leave it unchecked it seems to work. I was able to do a number of splits, but the result seemed to be wrong. The instructions seem vague to me, and geared to more experienced users.

BTW, I clicked on Download Rhino 3DM File on that page. The result was “Not Found”


I know he changed my model, however he did not show me how he got to those results. I also do not understand what he means with “It’s always better to create the topology that allows for a smooth fillet.” without further explanation. I am too much of a newbie to understand what he means to tell me.

What difference does it make if the model was “given from a customer”, or if I created it? FWIW, I created the model from scratch. It is a replacement part for a plastic table lamp clamp that recently broke.

Hi Zews- I set trim=No and Extend=No (usually, it depends) when using this command.(FilletSrf)



Sorry I thought it was pretty obvious what steps I used. It will require some persistence as Rhino fillets are prone to fail with these conditions.

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I’m done. I give up. :frowning:

the one you’re trying is doing this:

if you offset from the edges an equal distance, they don’t match up in that type of corner (it’s trying to match a straight cut to a mitered one… if that makes any sense)…

what @stratosfear is saying is to make the object as to allow this offset to match up all around…

I only asked in order to know if you could change it to your delight or had to stick to what was given. Like others since explained, the smooth fillet Stratosfear mentioned would be one that does look smooth. In order to work that way, it must be given enough room. I would now have a look at the original part that you need to replace and how much room you do have there, and see the suggestions, that you got so far, and see, what you could change…
Just try to grasp, what is the first thing, that you don’t understand, and ask from there on. First of course yourself…


I am not doing this part for a client. This is something I am doing for myself, and I have a lot of leeway as to what I can do with it as long as it works after it is done. This part is one of two parts of the desk lamp clamp that are broken on one of my cheap desk lamps in my studio. I’m sure you know the kind of lamps I am talking about.

You can’t buy replacements, and since I have an FDM 3D printer I can replace the clamp parts myself, and don’t have to throw out a perfectly good lamp. The fillets are not necessary for the part to function properly. The fillets are really not even that important for me to be able to print this part. However, I wanted to duplicate the part as true to the original part as possible, which I set as a goal. I can print the part completely without fillets, or only with fillets where I was able to create them successfully without creating naked edges.

I am fairly new at Rhino even though I have been using 3D modeling software for designing my work since 1997 using trueSpace on Windows OS. You can see some of my work at www.abrasha.com. The very old version I used for all those years could not do fillets or chamfers, so you can imagine how thrilled I have been to be able to do that now.

I recently took the first ever Level 1 training course for Rhino Mac offered by McNeel, which really only gave me the basics. I now realize that I have reached a plateau as far as my skills are concerned, and I will have to spend a lot of time getting better at this.

I am very grateful to all the people who have been so generous with their time and advice and have posted in response to this post, as well as several other question I have posted (booleans, STL export warning). I am taking notes of all the new commands I have learned so far.