I've had a bear of a time with 303 stainless. I had a collection of large SS metric bolts which I could rework and get precise, near mirror finishes. They were marked A2-70 which should be 304.
I have an assortment of 303 from Speedy Metals (who I can't praise enough otherwise) that has a somewhat coarse ground finish and machines like crap.
Just picked up some 304 elsewhere to test and it machines beautifully, almost a fine ground appearance and able to hold tight tolerances.
Why is the 303 giving me such heartburn? Could it have been heat treated or otherwise pre-hardened (or toughened)? Den
303 is free machining. Easy on tools, not as easy to get polished finish.
What kind of surface speed are you running? It won't shine like 304 will when turned, but if you cut it with adequate speed it gives a smooth matte finish thats typically better than the finish you'll get on 304. Surface finish and shine aren't the same thing in turned surfaces, and 304 doesn't really shine because of producing a great finish. It shines because its tough to cut, and the shine is more of a burnishing of the surface. Similar to the way 4140 typically comes out looking lots better than 1018, even though the 1018 is easier to cut.
I've run at recommended speeds dry and at lower speeds with cutting oil (Mobil Omicron). With sharp, ground all over TPG inserts, the finish almost appears ground. I can get the fine matte finish on the 303 but maybe I need to be looking at the actual surface finish in micro-inches instead of the visual (and thumbnail drag).
I think I get the burnishing part. It's the cutting wedge riding over and burnishing as the material flows where the free machining stuff is sort of tearing more. I guess what I have a hard time understanding is why the free machining material doesn't burnish down even better. Is that sort of it?
added - I think it also just dawned on me that the 304 is not going to chip break as well as the 303 ... now where is that light bulb icon
303 is nice when you have to drill and tap holes. Other then that I don't like it and I'll take 304 or 316 over it anyday.
I've seen some 303 hex stock that was split in half from the center line. But that was probably from a bad manufacturer.
The problem with buying small lots of steel is that you do not know the manufacturer or the machinability ratings that are recommended. My present employer uses Ugine for the 303 stainless and the material machines great. As stated before 304 usually is not much worse to machine than 303, but we had a 304 come through for a job in the last year that was impossible to machine and sent it back!
How aboot a pipe tap in 304? That sounds scary. (1/4" dryseal)
Think Snow Eh!
Ox, Ream it first and then use a heavy cutting oil or Moly-Dee. We do 'em in cast 316 (CF8M) all the time.