About 303|The free-machining stainless. Sulfur is added to break chips into short, manageable pieces instead of the bird's-nest razor wire you get with
304. Machines at roughly 78% machinability rating versus 304's 45%. You can run it faster, drills and taps go in clean, and tool life is significantly better. The tradeoff is reduced corrosion resistance and terrible weldability due to the sulfur content. If the part needs welding, don't use 303. If it doesn't, and you're doing any significant amount of machining, there's almost no reason to use 304 instead. Screw machine shops run this all day long for fittings, shafts, bushings, and valve components. Compare to 304 (better corrosion resistance and weldability but a bear to machine) and
416 (even easier to machine but less corrosion resistant).