Hi Tree, in fact I think the problem is that the fillets themselves do not even actually cross each other at all in that spot, it needs some kind of special logic to build a special connecting piece between the different fillets. The geometry engine that MoI uses just does not handle that particular kind of corner case right now.

Here's a bit more of an illustration - here I have manually constructed the fillets for that spot by breaking your object into individual surfaces and doing the fillet 2 surfaces at a time, which performs a surface/surface fillet which is different kind of calculation than an edge-based fillet:




You can see the fillets don't actually fully run into each other, they kind of touch just at a single point and pass by each other. Also see the attached 3DM file where you can zoom in and rotate around for a better look.

The ACIS modeling kernel knows how to handle this particular situation by trimming the fillets where they barely touch and putting in this connector piece here:


The part that is difficult there is that connector is done with special logic that is dedicated to just this particular corner configuration.

That's what makes fillets so difficult to calculate, many cases such as this one are more complex than just constructing the fillet surfaces themselves, it's handling the logic to connect the pieces together in different ways and handling a lot of different special cases.

It's a lot easier if you are filleting things that have a smooth outline in them rather than sharp or anti-tangent, etc.. type things, because smooth outlines will naturally make the fillet surfaces themselves touch each other and won't need complex corner connection logic.

- Michael