My experience with milling stainless (of most grades!) is that cobalt tooling works slightly better than carbide.
Carbide tends to go from 'cutting OK' to 'broken' pretty quickly; cobalt is a little more forgiving.
I will typically use a fine diamond stone and put a .0005" (max) edge break on the flutes and corners for stainless. That helps to prevent a lot of the edge wear from becoming a problem, and tool life goes WAY up.
The down side is that you sacrifice surface finish a little, and it takes a little bit more horsepower to drive the tool And, since stainless is usually running at low rpm/higher feedrate, that can make a difference.
Just my $.02 |