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Lathe speeds and feeds

On a lathe the part spins, so the cutting speed depends on the diameter being cut right now. That is why turning uses constant surface speed and feed per revolution.

Spindle speed from cutting speed

Use the diameter at the cutting edge. Facing toward center, the diameter drops to zero and the RPM needed for the same SFM climbs without limit.

Constant surface speed (G96) and the RPM clamp

G96 S500 holds 500 SFM (or m/min in metric) as the diameter changes. Always set a maximum spindle speed first, with G50 S… on Fanuc-style controls (G92 S… on some others), so the chuck doesn't overspeed near center. Use G97 (fixed RPM) for drilling and tapping on center and for threading.

Feed rate

Turning feed is programmed per revolution (G99 on Fanuc lathes, G95 on some controls). Finish and surface roughness depend on feed and the insert's nose radius. A common estimate for peak-to-valley roughness is Rt ≈ f² ÷ (8 × r).

Cutting time

Example: turning a 2.000" bar at 600 SFM and 0.008 IPR over 3" of length. RPM = 600 × 3.82 ÷ 2 = 1,146. Feed = 0.008 × 1,146 = 9.17 IPM. Time = 3 ÷ 9.17 = 0.33 min, about 20 seconds a pass.