Cutting Speed Calculator Lathe
This cutting speed calculator lathe tool uses the workpiece diameter (not the tool diameter) because on a lathe the workpiece is what spins. Enter the OD you are currently turning, the material, the tool material (HSS or carbide insert), and whether you are roughing or finishing. The calculator returns spindle RPM from the standard SFM-to-RPM equation, plus a recommended feed per revolution in IPR.
Lathe turning equations
- RPM = (SFM x 12) / (pi x D_work) where D_work is the workpiece diameter in inches, not the tool diameter.
- Feed rate (IPM) = RPM x IPR where IPR is the feed per revolution.
- MRR (in^3/min) = 12 x SFM x depth of cut x feed (Sandvik turning handbook).
For a 2 inch diameter bar of mild steel turned with a carbide insert at 400 SFM: RPM = (400 x 12) / (pi x 2) = 764 RPM. Roughing feed = 0.012 IPR, so feed rate = 764 x 0.012 = 9.17 IPM. At 0.050 depth of cut, MRR = 12 x 400 x 0.050 x 0.012 = 2.88 in^3/min.
Turning SFM reference
| Material | HSS SFM | Carbide SFM |
|---|---|---|
| Aluminum | 500 | 1000 |
| Mild steel (1018) | 100 | 400 |
| Medium carbon (1045) | 80 | 300 |
| Alloy steel (4140) | 70 | 250 |
| Stainless (304, 316) | 60 | 200 |
| Cast iron | 80 | 280 |
| Titanium | 35 | 120 |
| Brass | 250 | 500 |
Workpiece diameter changes as you cut
This is the gotcha that bites every new lathe operator: the SFM formula uses the diameter right now, not the starting diameter. As you turn a 2 inch bar down to 1.5 inch, the cutting speed at 764 RPM drops from 400 SFM to 300 SFM. On a CNC lathe with G96 constant surface speed, the control increases RPM automatically. On a manual lathe, you step the RPM up as the diameter decreases.
Roughing vs finishing
Roughing prioritizes material removal rate: deeper depth of cut (0.050-0.150), heavier feed (0.010-0.020 IPR), normal SFM. Finishing prioritizes surface quality: shallow depth (0.005-0.020), lighter feed (0.003-0.006 IPR), 10-20 percent higher SFM to keep the chip moving. Switching between them in this calculator changes both feed and SFM.