Hi Burr, yeah actually believe it or not filleting just 1 edge like that where it has sharp corners is a lot more difficult than the whole boundary.

It's because the fillet itself won't naturally connect to other fillet pieces, it will want to open up a kind of hole in the model sort of like this:



A couple of different things get difficult there - the fillet does not all by itself cut the top piece, so some additional connections need to be traced on the top to cut the top surface, and then the "side wall" piece has to try to be extended to fill in the gap that is opened up after that.

It's a pretty different kind of scenario than when everything is being filleted - when everything is filleted the fillets and corners all by themselves define a complete trimming boundary.

- Michael

I had seen that if I selected the "Straight corners" option, then it actually would produce that "Fillet-but-leave-the-original-surface-alone" result, which would allow me to do the lower level surface work if I really needed just that edge filleted... :o

Thanks to Michael, Danny and BurrMan for their help with which I was able to resolve the issues. I have realised that some edges just can't be filleted but I managed to get a useful result nonetheless. Cheers, Dan