Hi skycastle - although it looks simple, it's more difficult for the fillet engine to process than it first appears. The way fillets (and also chamfers) works is by calculating an arc of a specific radius, but that arc will be a longer or shorter segment depending on what angle different surfaces meet each other. So that means that when you have surfaces that meet each other at different angles, the fillet or chamfer surfaces between them will be of different sizes and the fillet engine seems to be having problems trimming the different sized pieces with each other in your case here.

It tends to be easier to fillet things if you fillet everything that meets at a juncture point instead of only some edges - that gives the fillet or chamfer some space to put in a corner juncture patch where things meet up.

For now I'd recommend using the method Barry shows above.

- Michael