Cutting speed (Vc or SFM) and spindle RPM are related by tool diameter — get this wrong and tool life collapses or surface finish suffers.
Core Formulas
RPM from Vc (metric): RPM = (Vc × 1000) / (π × D), where Vc is m/min and D is mm.
RPM from SFM (imperial): RPM = (SFM × 12) / (π × D), where SFM is surface feet per minute and D is inches.
Convert SFM ↔ Vc: 1 SFM = 0.3048 m/min. So 100 SFM ≈ 30.5 m/min, 1000 SFM ≈ 305 m/min.
Worked Example
Turning a 50mm 1045 steel shaft with carbide insert at 220 m/min:
RPM = (220 × 1000) / (π × 50) = 1,401 RPM.
Feed at 0.30 mm/rev = 1,401 × 0.30 = 420 mm/min table feed.
Vc Quick Reference (Carbide)
| Material | Vc m/min | SFM |
|---|---|---|
| Aluminum 6061 | 300-600 | 985-1970 |
| Mild steel 1018 | 180-280 | 590-920 |
| Carbon 1045 | 180-240 | 590-790 |
| Alloy 4140 (annealed) | 140-200 | 460-660 |
| Stainless 304/316 | 120-180 | 395-590 |
| Cast iron (gray) | 100-200 | 330-660 |
| Inconel 718 | 30-60 | 100-200 |
| Titanium Ti-6Al-4V | 60-90 | 200-295 |
| Hardened steel 50 HRC | 150-250 (CBN) | 490-820 |
Adjustment Factors
- Interrupted cut: ×0.7
- Roughing: feed ×1.2, speed ×0.9
- Finishing: feed ×0.7, speed ×1.1
- Worn tool: speed ×0.85
- Coolant on: speed ×1.1
For full feed-rate calc see our cutting parameter guide.
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