So another problem area that's probably causing some of the difficulty with filleting the top edges is this area here:






The thing that's bad about that structure is that it makes for a pretty small sized edge fragment up here at the top:




The filleter is going to try to build a separate little fillet segment for that edge and try to figure out how to splice it in with the others, and it's going to be compounded by the 2 surfaces that meet at this edge here:



Those surfaces are another area where they are not quite smooth or sharp to one another - they are close to being smooth to each other, but they're about 1 degree off from being tangent with one another - that's enough to possibly make the filleter have to try and build a corner juncture patch between fillets that meet there.

You want to make sure that things like that in your original curves are either smooth to one another or sharp to each other, you don't want to just make things look like they are smooth to one another without actually using snaps or fillets to do it, because that ends up with this type of shallow angle situation which will then make things difficult for the filleter later on.

Ideally you'd want to get the curve that was used to cut or extrude those pieces to be all one single segment so that the extrusion was all one single smooth face instead of 2 different face pieces with one small slice at the end like that.

That's kind of difficult to fix in the already finished model, you might need to go back to some of your original curve structure to fix that.


Then the other potential thing that may also be stressful on the filleter is having surfaces come to a "needle thin" type end like this shape on the inner hub:



I would not be too surprised if that shape there would be causing some other problems for the filleter as well.



The filleter has a complex sequence of tasks that it needs to do, like generate surface offsets, intersect the offset surfaces, generate fillet rails, generate fillet surfaces, extend and intersect fillet surfaces between each other on sharp corners, trim them back, and put in corner patches between areas where there are holes between fillet surfaces.

It doesn't take too much to make things complicated for any one of these stages, a lot of times things that involve little slivery pieces will make things hard to intersect and figure out.

Sometimes it may be that you've got some slivery pieces in your model, but also having things come close to being smooth to one another but not quite will implicitly generate slivery corner patch pieces because 2 fillet pieces will only exactly meet up to one another without any corner patch needed when 2 edges are smooth to each other where they meet up.

- Michael