Hi Gent, yup basically this is a common problem. Fillets are a very difficult area of calculation, particularly in corner areas where more than two edges are coming together.

The fancy solid modelers do solve this in a kind of "brute force" method like you described - they have a lot of special case code that recognizes different specific configurations and does the right thing to handle each one on a case by case basis. They've spent a huge amount of time and effort adding more and more special cases to make them work better.

The geometry library that MoI uses just has not had as much time spent on all those different special cases.

Some general advice is to try and smooth out the areas to be filleted, because sharp corners are more difficult, and also to try and have pieces set up accurately. Things should be snapped together well, any kind of small mis-alignments cause further problems. Doing those kinds of procedures basically makes the filleters job easier and will make it more likely to work.

- Michael