Feed Rate Calculator Lathe
Lathes program feed in IPR (inches per revolution), not IPM. This feed rate calculator lathe tool takes RPM (entered directly or computed from workpiece diameter and SFM) and a material-specific feed per revolution to return feed rate in IPR, IPM, mm/min, and metal removal rate. Works for roughing and finishing cuts on any common lathe material.
Lathe feed rate equations
- Feed rate (IPM) = RPM x IPR where IPR is inches per revolution.
- RPM = (SFM x 12) / (pi x D) for the compute mode, where D is the workpiece diameter.
- MRR (in^3/min) = 12 x SFM x depth of cut x IPR (Sandvik turning math).
For a 2 inch mild steel bar turned with a carbide insert at 400 SFM: RPM = (400 x 12) / (pi x 2) = 764 RPM. Roughing IPR = 0.012, 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.
Typical lathe feed per revolution (IPR)
| Material | Roughing IPR | Finishing IPR |
|---|---|---|
| Aluminum | 0.015 | 0.006 |
| Mild steel (1018) | 0.012 | 0.005 |
| Alloy steel (4140) | 0.010 | 0.004 |
| Stainless steel | 0.010 | 0.004 |
| Cast iron | 0.015 | 0.006 |
| Titanium | 0.008 | 0.004 |
| Brass | 0.015 | 0.006 |
IPR vs IPM on a lathe
On a lathe, chip thickness and surface finish depend on feed per revolution, not feed per minute. That is why G95 (feed per revolution) is the normal mode on a CNC lathe, with the feed value in IPR or mm/rev. G94 (feed per minute) exists for special cases like threading with a live tool or rigid tapping, but day-to-day turning is G95. A feed rate calculator lathe should return IPR as the primary number, with IPM as a reference.
Roughing vs finishing feed
Roughing prioritizes material removal: heavy depth of cut (0.050-0.150) and heavy feed (0.010-0.020 IPR). Finishing prioritizes surface finish: light depth (0.005-0.020) and light feed (0.003-0.006 IPR). Finishing also bumps SFM up 10-20 percent to keep chips formed cleanly. The calculator handles both by switching the IPR default based on the cut type you pick.