On Dec 27, 9:29*am, Chris Wilson wrote:
I need to machine a round billet of stainless steel on its flat face with a
square section groove to take an O ring.

I would probably have to buy or make a cutting tool for the lathe as I see
nothing obviously suitable amongst the stuff I have.

You'd best use HSS and low cutting speed, and the lathe is
definitely better than the rotary table and mill. The O-ring
has to seal against the bottom face, so the finish of that
part of the cut is critical. Sidewalls don't need to be
exactly at 90 degrees, usually they're low-tolerance.