Hi Warren - MoI's filleter can have difficulty in several kinds of situations and that's one of them. The juncture area here makes this a more difficult thing to fillet than you might initially think:




It's able to generate fillet surfaces for that case but fails in figuring out how to intersect them with one another. Here I have used the raw fillets that were generated and manually trimmed them so you can see the kind of awkward hole area that needs to be filled. That's basically caused by all the surfaces surrounding that juncture being at different angles to one another:



The filleting engine in the geometry library that MoI uses is just not able to figure out how to process this kind of corner juncture very well, for cases like this you might try exporting your model to a different CAD program that uses the ACIS or Parasolid geometry kernels which have more sophisticated corner juncture processing in them, one program called ViaCAD is pretty inexpensive and can make a good companion program to be used for doing fillets which MoI has difficulty with.

- Michael