Hi Daniele, thanks, I'm glad you like MoI!

Looks like you have run into one of the tricky NURBS areas here though.

This looks like a bug in the fillet processor. But unfortunately this situation is more difficult to calculate than what it seems, it will probably be a while before this can get fixed up.

One thing you can do if the regular fillet fails is to use Edit / Separate to break everything down into individual surfaces, and then select 2 surfaces at a time and do fillet. This does a slightly different surface/surface fillet calculation which can succeed in places where the edge-based fillets fail. But it won't automatically clean up at corners like the edge-based one will.

But corner processing is the difficult part - after using Edit/Separate and doing some surface/surface fillets, you can see what it looks like here where 2 fillets meet:




It can be difficult to get a good result when different pieces are meeting at some kinds of sharp corners like this.


One thing that you can do that will work better is to kind of construct your base shape so that it is smooth along that top and bottom face. That makes the filleter's job easier, since fillet pieces on smooth edges touch up to each other nicely. Here's an example:




What I did for that one is I deleted the central part, and used Construct / Blend to put in smooth blend surfaces between the exposed edges. Then Edit/Join to join it together, and then that form will fillet nicely.


Unfortunately it is pretty easy for things to confuse the filleter. This area will get better over time but it will be kind of a slow process of improvement.

- Michael