Lathe Cutting Speed (RPM) Calculator
Calculate the correct lathe RPM for any material and cutting tool diameter.
Avoid burning tools or damaging workpieces with the wrong spindle speed.
Why cutting speed matters:
Running a lathe at the wrong RPM damages tools and workpieces. Too fast: the tool tip overheats, loses hardness, and fails — or the workpiece chatters and produces a rough surface. Too slow: rubbing instead of cutting, poor surface finish, wasted time.
The fundamental formula:
RPM = (Cutting Speed × 12) ÷ (π × Diameter)
Or using the simplified machinist’s formula:
RPM = (CS × 3.82) ÷ Diameter (inches)
Where:
- CS = Cutting Speed in Surface Feet per Minute (SFM)
- Diameter = workpiece diameter in inches at the cut
- 3.82 = 12 ÷ π (a machinist’s shortcut)
For metric (m/min and mm):
RPM = (CS × 1,000) ÷ (π × Diameter in mm)
Recommended cutting speeds (SFM) by material:
| Material | HSS Tool | Carbide Tool |
|---|---|---|
| Aluminum (6061) | 200–300 SFM | 600–1,000 SFM |
| Mild steel (1018) | 80–100 SFM | 300–500 SFM |
| Stainless steel (304) | 50–80 SFM | 200–350 SFM |
| Cast iron | 60–80 SFM | 200–300 SFM |
| Brass / bronze | 150–200 SFM | 400–600 SFM |
| Copper | 100–150 SFM | 400–600 SFM |
| Titanium | 25–50 SFM | 100–200 SFM |
| Nylon / plastics | 200–400 SFM | 600–1,000 SFM |
Worked example:
Turning 1-inch diameter mild steel with an HSS tool at 90 SFM:
- RPM = (90 × 3.82) ÷ 1.0 = 344 RPM
- Select the nearest lower lathe speed (e.g., 315 or 340 RPM depending on your lathe’s speed steps)
Tool life and feeds:
RPM gives you spindle speed; combine it with feed rate (inches per revolution) for surface finish:
- Rough cuts: 0.010–0.020" per revolution
- Finishing cuts: 0.002–0.005" per revolution
Coolant:
Steel and stainless: use cutting fluid (sulfurized oil or water-soluble coolant). Aluminum: use kerosene or WD-40 for better finish. Brass and cast iron: typically run dry.