Hi Tony, It really depends on your machine rigidity and toolholder selction, but 400SFM and .010"IPR is probably a better starting point for an average rough cut in H-13 (350SFM for 420SS). Depth of cut is crucial here also, as too shallow will cause stringers as you describe... Try at least .125" depth/side if your tooling is strong enough for that. I used to go up to about .250" per/side with some of the medium duty lathes I ran. There are a lot of good speed/feed calculators out there to give you a head start here also. If you call the tooling rep they can get you one or give you some starting values also... Good luck! Dave