I think you may actually have a case here where the fillet mechanism does not know how to handle this corner juncture with multiple fillets running into there.



It can do any one of those, like this:








But I don't think it knows how to handle all of them being filleted at once.

In fact, it's not very clear to me how that would actually work, I've combined those pieces together and it's something like this:






I guess it would ideally put a kind of rounded rail on the top or something like that... I'm pretty sure it does not understand this kind of corner condition, so this piece is probably just not going to get automatically filleted by MoI's fillet mechanism.

You probably will need a more "heavyweight" CAD program to do that one, something like SolidWorks or Pro/E maybe...

I think it looks like you would have to do some manual trimming and managing of the individual fillet pieces to make that happen in MoI, that's how I assembled the piece in the screenshot above. But then you've also got to figure out exactly how that corner should actually behave.

- Michael