Hi Alexander,

> I'm just used to selecting the line to radius.

So normally that is what you would do in MoI as well.

But that creates what is called an "edge based fillet", which creates the fillet by kind of tracking along those edges.

There is an alternate way you can do fillets in MoI which is called a "surface/surface fillet" - that method is enabled if you select 2 individual separate surfaces as the inputs to the Fillet command rather than selecting edges as the inputs.

The surface/surface fillet command is limited in some ways, like it doesn't know how to build fillets where multiple edges collide in to one another and need corner patches. But it works directly off of surfaces instead of edges and can sometimes construct a fillet in cases where the edge base filleter gets confused.

The edge-based filleter has to do some more complex kinds of calculations for the way it handles things and so it can get messed up more easily.

But the edge-based filleter tends to be more convenient so you'll usually want to try and use it first, but you can use the surface/surface filleting mechanism as a backup for cases where the edge-based one does not work.

If you had the top surface as an extruded piece that was trimmed rather than with those kind of single-surface-with-rounded-corners type thing then the edge based filleter would likely work fine on it.

- Michael