321 SS is a pain. Try using Cermit Uncoated Inserts from Sandvik .004 TNR. These are what I used and they worked like a charm.
Just try not to pull the burred edge back through the guide bushing because it will chip the carbide seats on the bushing.
I did end up using a G71 Rough Turning Cycle though, which is why I mentioned the bur chipping the guide bushing seats.
I hope you have better luck than I did.