Table of Contents
What Are Speeds and Feeds?
Speeds and feeds are the two fundamental parameters controlling CNC machining operations. "Speed" refers to the spindle rotational speed (RPM) or the surface cutting speed (SFM/m/min). "Feed" refers to how fast the tool moves through the material, measured in inches per minute (IPM) or millimeters per minute (mm/min). Getting these values right is critical for tool life, surface finish, and machining efficiency.
Setting the speed too high generates excessive heat, dulling the tool quickly. Too slow wastes time and can cause rubbing instead of cutting. Similarly, feed rate must balance between removing material efficiently and avoiding excessive cutting forces that could break the tool or damage the workpiece.
Formulas
Where SFM is surface feet per minute, D is tool diameter in inches. For metric: RPM = (Vc × 1000) / (π × D_mm). Material Removal Rate (MRR) = Width of Cut × Depth of Cut × Feed Rate.
Recommended Cutting Speeds
| Material | HSS (SFM) | Carbide (SFM) |
|---|---|---|
| Aluminum | 300-600 | 800-1500 |
| Mild Steel | 60-100 | 300-600 |
| Stainless Steel | 30-80 | 200-400 |
| Cast Iron | 50-80 | 250-400 |
| Titanium | 20-50 | 100-200 |
| Brass | 200-400 | 600-1000 |
| Plastics | 300-800 | 500-1500 |
Optimization Tips
- Start conservative: Begin at 70% of recommended speeds and increase gradually.
- Listen to the cut: A smooth, consistent sound indicates good parameters.
- Check chips: Ideal chips are small, curled, and slightly warm, not blue or dusty.
- Use coolant: Flood coolant or minimum quantity lubrication extends tool life dramatically.
- Tool engagement: Reduce speed when the tool is heavily engaged (slotting vs. profiling).
Frequently Asked Questions
What is chip load?
Chip load is the thickness of material each cutting edge removes per revolution. It is measured in inches (or mm) per tooth. Typical chip loads range from 0.001" for small end mills to 0.010" for large face mills. Proper chip load prevents rubbing (too thin) and tool breakage (too thick).
How does tool diameter affect RPM?
Smaller tools require higher RPM to maintain the same surface speed. A 1/4" end mill needs twice the RPM of a 1/2" end mill at the same SFM. Many CNC machines have maximum RPM limits that constrain the usable surface speed for small tools.
What is climb milling vs. conventional milling?
In climb milling, the cutter rotates in the same direction as the feed. This produces a better surface finish, generates less heat, and is preferred on rigid CNC machines. Conventional milling (cutter opposes feed) is safer on machines with backlash but produces more heat and a rougher finish.