How Cutting Speed and Feed Rate Work
Every CNC cut starts with two numbers: speed and feed. Speed is how fast the tool spins. Feed is how fast the tool moves through the material. Get them right and you make good parts fast. Get them wrong and you break tools or burn material.
Surface Speed (SFM)
SFM stands for surface feet per minute. It measures how fast the cutting edge moves across the workpiece. Each material has a safe SFM range. Aluminum cuts well at 800 SFM. Steel needs about 100 SFM. Titanium runs best at 50 SFM.
SFM depends on the material, not the tool size. A small tool and a big tool use the same SFM for the same material. The RPM changes to keep the surface speed constant.
Spindle Speed (RPM)
RPM is how fast the spindle turns. You calculate it from SFM and tool diameter. The formula is RPM = SFM x 12 / (pi x D). Smaller tools need higher RPM. A 0.25-inch end mill in aluminum runs at 12,223 RPM. A 1-inch end mill runs at 3,056 RPM.
Feed Rate (IPM)
IPM is inches per minute. It tells the machine how fast to move the tool. Feed rate depends on RPM, number of flutes, and chip load. The formula is IPM = RPM x flutes x chip load per tooth.
Pro tip: Start with the tool maker's chip load. Then adjust based on your setup. Too much chatter? Lower the chip load. Tool rubbing? Raise it.
Chip Load
Chip load is the thickness of material each tooth removes per revolution. Too small and the tool rubs instead of cuts. This creates heat and kills the tool. Too big and the tool can break. Typical chip loads range from 0.001 to 0.010 inches.
Material Removal Rate (MRR)
MRR tells you how fast you remove material. It equals width of cut times depth of cut times feed rate. Higher MRR means shorter cycle times. Shorter cycles mean lower cost per part. Optimize your speeds and feeds to get the best MRR your setup allows.
Worked Example: Milling Aluminum 6061
Here is a complete calculation from SFM to feed rate and MRR for a common milling operation.
| Parameter | Value |
|---|---|
| Material | Aluminum 6061-T6 |
| Operation | Face milling |
| Tool diameter | 0.500 in (1/2″ end mill) |
| Number of flutes | 4 |
| Recommended SFM | 800 |
| Chip load per tooth | 0.003 in |
| Calculated RPM | 800 × 12 / (π × 0.500) = 6,112 RPM |
| Calculated feed rate | 6,112 × 4 × 0.003 = 73.3 IPM |
| Depth of cut | 0.100 in |
| Width of cut | 0.400 in |
| Material removal rate | 0.400 × 0.100 × 73.3 = 2.93 in³/min |
SFM Reference by Material
Use these starting SFM values for carbide tooling. Adjust based on your specific grade, coating, and machine rigidity.
| Material | Starting SFM | Notes |
|---|---|---|
| Aluminum 6061 | 600–1000 | Up to 1200+ with carbide |
| Mild Steel 1018 | 80–120 | Use flood coolant |
| Stainless 304 | 50–100 | Low SFM prevents work hardening |
| Titanium Ti-6Al-4V | 40–70 | High pressure coolant required |
| Brass 360 | 200–400 | Excellent machinability |
| PEEK | 300–600 | Dry or air coolant |
| Delrin | 400–800 | No coolant needed |