Spindle speed (RPM)
- Imperial: RPM = (CS × 12) ÷ (π × D) ≈ CS × 4 ÷ D, with CS in surface feet per minute (SFM) and D in inches.
- Metric: RPM = CS × 1000 ÷ (π × D), with CS in metres per minute and D in mm.
- D is the drill or cutter diameter for drilling and milling, and the work diameter for turning.
- Carbide tools run roughly 2 to 4 times faster than HSS. Reduce speed for reaming (about half) and for poor rigidity or no coolant.
Typical HSS cutting speeds
Typical starting points for high-speed steel tools. Tool makers publish their own figures, and carbide or coated tools run faster. Start low and adjust for finish, chips and tool life.
| Material | SFM (ft/min) | m/min |
|---|---|---|
| Aluminium | 200 to 300 | 60 to 90 |
| Brass | 150 to 200 | 45 to 60 |
| Cast iron (grey) | 50 to 80 | 15 to 24 |
| Mild (low-carbon) steel | 80 to 100 | 25 to 30 |
| Alloy and tool steel (annealed) | 40 to 60 | 12 to 18 |
| Stainless steel | 30 to 50 | 9 to 15 |
Milling feed rate
- Feed rate (in/min or mm/min) = RPM × chip load per tooth × number of teeth
- Chip load (feed per tooth) comes from the cutter maker's chart for the material and cutter size.
- For drilling, feed rate = RPM × feed per revolution.
⚠️ Reference only. Always verify against the manufacturer's specs, your jurisdiction's code and your site's procedures.