Turning speeds, feeds and surface finish

Lathe RPM, metal removal and the theoretical finish from feed and nose radius.

Theoretical roughness comes from the feed marks alone: peak-to-valley Rt = f² ÷ 8r and Ra = f² ÷ (18√3 × r). Real finishes are worse when the setup chatters or the insert wears, but the formula shows how much feed or nose radius you can trade for finish.