Get spindle speed and feed rate for turning from workpiece diameter, target surface speed, and feed per revolution.
RPM = (SFM x 3.82) / D. Feed rate = RPM x feed per revolution. On a lathe, diameter and surface speed change together as the part is turned down, so recheck RPM as the diameter drops.
How it works
Turning holds a roughly constant surface speed by raising RPM as the workpiece diameter gets smaller. A single pass on a part with a large diameter change should be split into a few RPM steps, or run under constant surface speed control if the control supports it, to avoid being too slow at the start or too fast at the end.
FAQ
Do I need to recalculate after every pass? Only when the diameter changes enough to meaningfully shift the surface speed, finish passes on a near final diameter are usually close enough to leave as is.
Related Guides
CNC Calculators · SFM Calculator · Taper Calculator