CNC RPM & Cutting Speed Calculator
Convert between RPM and SFM, or find optimal spindle speed for any material. 3-in-1 calculator with real-time conversion and material-specific recommendations.
Calculate RPM or SFM
📚 How to Use
Mode 1: RPM → SFM
Enter your spindle RPM and tool diameter to find the surface speed (SFM/m/min).
Mode 2: SFM → RPM
Enter desired surface speed and tool diameter to calculate required RPM.
Mode 3: Optimal RPM
Select material, tool type, and operation to get recommended RPM range.
💡 Pro Tip: Start with optimal RPM mode if you are unsure, then fine-tune using SFM ↔ RPM conversion.
Understanding RPM and Cutting Speed
The Relationship Between RPM and SFM
RPM (Revolutions per Minute) and SFM (Surface Feet per Minute) are intrinsically linked through tool diameter. Understanding this relationship is crucial for optimizing CNC machining operations.
Formula: SFM → RPM
RPM = (SFM × 12) / (π × Diameter in inches)Or in metric: RPM = (m/min × 1000) / (π × Diameter in mm)
Formula: RPM → SFM
SFM = (π × Diameter in inches × RPM) / 12This calculates the linear speed at which the cutting edge travels across the workpiece.
Why Surface Speed (SFM) Matters
- •Material compatibility: Different materials require different cutting speeds for optimal tool life and finish
- •Heat generation: SFM directly affects cutting temperature - too high causes burning, too low causes rubbing
- •Tool diameter independence: Recommended SFM stays constant regardless of tool size
- •Surface finish: Higher SFM typically produces better finish (within limits)
Common SFM Values by Material
| Material | Roughing SFM | Finishing SFM | Notes |
|---|---|---|---|
| Aluminum 6061 | 1000 | 1500 | Soft, easy to machine |
| Mild Steel (1018) | 130 | 180 | Good machinability |
| Stainless 304 | 70 | 100 | Work hardens quickly |
| Titanium Ti-6Al-4V | 60 | 90 | Low thermal conductivity |
| Brass | 400 | 600 | Excellent machinability |
Tool Diameter Impact on RPM
For the same surface speed (SFM), smaller tools require proportionally higher RPM:
Example: 800 SFM Target
- • 1-inch (25.4mm) tool: 3,056 RPM
- • 0.5-inch (12.7mm) tool: 6,112 RPM
- • 0.25-inch (6.35mm) tool: 12,224 RPM
- • 0.1-inch (2.54mm) tool: 30,560 RPM
This is why micro-milling requires high-speed spindles (40,000+ RPM) to achieve proper cutting speeds with tiny tools.
Frequently Asked Questions
SFM (Surface Feet per Minute) is the speed at which the cutting edge moves across the workpiece surface, while RPM (Revolutions per Minute) is how fast the tool spins. They are related through tool diameter. A larger tool at lower RPM can have the same SFM as a smaller tool at higher RPM. For example, a 1-inch tool at 3,000 RPM and a 0.5-inch tool at 6,000 RPM both run at approximately 785 SFM. Cutting recommendations are given in SFM because the cutting edge experience is what affects tool wear and finish, regardless of tool size.