Hi cx - that definitely looks like a fillet bug, thanks for reporting it!

> Please explain why not generate the fillet?

You can actually see in your own screenshot why the fillet is not successful - it's because the fillet engine did not generate a long enough fillet surface, there is a gap between the end of the fillet here:



There's definitely some bug in the fillet surface generation step where it's not making that fillet long enough.

That's a somewhat unusual bug, I would have normally expected a pretty simple situation like this to work. Unfortunately the nature of geometry is that it comes in such infinite varieties that it's easy for there to be bugs that involve just some particular arrangements of shapes.

I'll put this on my list to investigate, but often times fillet bugs are quite difficult for me to fix up, the filleting in MoI comes from a library that I license called Solids++ and I'm mostly dependent on them to fix bugs in their library to improve filleting but sometimes I'm able to fix them up myself as well if I can figure out what exactly is going on. I won't be sure until I get a chance to look into it in more detail whether this one is something that I could have a chance at fixing up myself or whether it would need to be fixed by them.

In the meantime the easiest workaround is probably to do the fillet on the full cylinder before you cut the notch in it.

Thanks,
- Michael