Hi Glenn yes, unfortunately Fillet is a very sensitive operation and there are a lot of things that can go wrong with it.

In your first screenshot for filleting the bottom part, the problem there is that it looks like you're asking it to use a fillet radius that is too large to fit in the available space. If you ask for a fillet radius that causes fillet pieces to run into each other that will cause problems.

In your case here it's this area here that is the limiting factor:




Note what happens when I start with a smaller radius like 0.1 and increase it gradually up to 0.9:







Notice how the space there is shrinking away as the fillet radius increases?

At radius = 0.9 the available space is almost entirely consumed. At radius 1.0 as you show in your screenshot, there isn't enough space for the fillets to fit without colliding into one another and that's when you will get problems.

So for that particular piece you will need to limit your radius to a small enough value that it can fit, like not greater than 0.9 .

You get that sort of strange inversion effect when things run into each other because the ordered sense of directions becomes confused when pieces overlap each other. Hope that helps to explain that particular type of thing.


For the top piece, that's a different problem it looks like there is some bug in trimming the fillets there. When fillet had some problem trimming things, it will still output the fillet surfaces so that you have a chance to still use them to finish it up with a manual trim or boolean operation, like OSTexo mentions above. It looks like it should be possible to salvage the fillet in this way, let me know what particular fillet parameters you want to use there and I'll see about getting the pieces into place to get you a finished result.

- Michael