How the SFM ↔ RPM conversion works
Cutting speed describes how fast the cutting edge travels past the material, while RPM is how fast the spindle turns. They are linked by the tool (or part) diameter:
- Imperial: RPM = (12 × SFM) / (π × D) ≈ 3.82 × SFM / D (D in inches)
- Imperial: SFM = (π × D × RPM) / 12 ≈ 0.262 × D × RPM
- Metric: RPM = (1000 × Vc) / (π × D) (Vc in m/min, D in mm)
- Metric: Vc = (π × D × RPM) / 1000
Feed rate & chip load
Once you know RPM, feed rate follows from how much each tooth removes per revolution (the chip load):
- Feed rate: IPM = RPM × chip load (IPT) × number of flutes
- Metric: mm/min = RPM × feed per tooth (mm) × number of flutes
- Chip load: per tooth = feed rate ÷ (RPM × number of flutes)
Recommended starting SFM by material
General starting points for HSS and carbide tooling. Always defer to your tool manufacturer's data and adjust for rigidity, coolant, and finish.
| Material | HSS (SFM) | Carbide (SFM) |
|---|---|---|
| Aluminum | 250–300 | 600–1200 |
| Brass | 150–250 | 400–700 |
| Mild steel (1018) | 80–110 | 300–500 |
| Alloy steel (4140) | 50–80 | 200–350 |
| Stainless (304) | 40–70 | 150–300 |
| Cast iron | 50–80 | 200–400 |
| Titanium | 30–50 | 100–200 |
Frequently asked questions
What is the formula to convert SFM to RPM?
RPM = (12 × SFM) / (π × D), where D is the diameter in inches. The shortcut RPM ≈ 3.82 × SFM / D gives the same result. In metric, RPM = (1000 × Vc) / (π × D).
What does SFM (surface feet per minute) mean?
SFM is the imperial unit of cutting speed — the distance the cutting edge travels past the workpiece each minute. The metric equivalent is meters per minute (m/min), sometimes written Vc.
How do I work out feed rate from RPM?
Feed rate (IPM) = RPM × chip load per tooth × number of flutes. Switch the unit toggle to metric for mm/min.
Does this work for drilling and turning too?
Yes. For drilling, use the drill diameter; for turning, use the part diameter. The SFM-to-RPM relationship is the same — only the diameter you enter changes.