[Cruiser]

I used to turn impellers in a large vertical lathe that were 15-5Ph castings. The rule is the same as what you have been doing. Slow or ceramic and no dull tools. We started roughing with ceramics, and it was a glow in the dark job ! Its the finishing tools we had to worry over, if they dulled and glowed then retract and try to re-cut the rub area, well, sometimes it took some doing to get back to where ya need to be. The problem we encountered most was that of the programmers point of view, he insisted on cutting the whole of the form in one continuous pass without regard to tool life, so it was the operators job to pick a good point to retract tool etc. !