Hi Jens, it's not so much that you're abusing it as that you're running into bugs and limitations in the filleting code.

But unfortunately it is not really an easy area to fix up - filleting is a tough area of calculation, and there are many cases that increase the difficulty of the calculation.

One in particular is when filleting would cause pieces and edges to be totally consumed by the fillet and kind of evaporate.

This kind of situation can easily happen in the process that you describe - particularly if you do some fillets of one radius, then do booleans, and then try to do more fillets of the exact same radius. The last fillets of the same radius tend to have this sort of issue.

One thing you can try is to use a smaller and smaller radius for each new "generation" of fillets. Often times if you use a smaller radius than the original, it will avoid this "parts being consumed" type situation and you might be able to get some more mileage from that kind of repeated process.

Basically the fillets work really pretty well when they have plenty of room to operate in, but as you cut more pieces, you introduce more complex corner situations (where multiple edges all come to a single point), and also cause the fillet surfaces to try and interact and run through more and more different surfaces and edges.. That's when you run into more of the bugs in the fillet processing.

It's going to be a limitation in MoI (and actually it is not uncommon for any solid modeler to have difficulties as well) for some time to come. I expect that it will gradually improve over time.

There is no hard-coded limit on the number of merges or booleans you can do, it is more dependent on how many little edges and little cramped areas in the model end up being created.

If you're working on a more final model, it is possible to create a fillet by more detailed "low level" operations, for example by separating your model into different surfaces and then doing some fillets between 2 surfaces at a time. That uses a slightly different surface/surface fillet calculation that does not involve as many corner matchups and can generate a fillet surface when the edge-based one will fail. But then you will generally have to trim it and form the corners where things meet up through more manual operations.

- Michael