Thanks for the update! An electric actuator is probably more convenient than a pneumatic actuator. How are you going to do the internal portions of the spindle? With my own spindle concept, I have no fear of turning the exterior, and then boring, but to really get the accuracy I want, I'm afraid I'd have to job it out to a grinding shop.

Metallurgists following this thread... any advice on a tough steel which can be turned and bored to a decent finish w/o grinding, and still deliver acceptable hardness and wear resistance without resorting to heat treatment and subsequent finish grinding? All I am truly experienced with are mild steels (too soft), tool steels (expensive and perhaps not suitable w/o HT), and 4000 series carbon steels.

Hendrix, didn't mean to push you into posting, I just wanted you to be aware that there are lots of thread lurkers interested in your project. Please post when you can, and when you are ready. Thanks!