Mill Cutting Speed Calculator
Mill cutting speed is the surface speed of the end mill's outer flute as it passes through the workpiece. Measured in SFM (surface feet per minute) or Vc (m/min), it is the single parameter that decides how fast the spindle should turn for a given tool diameter and material. Too slow and the tool rubs; too fast and it wears out in minutes. This calculator returns RPM from the standard cutting speed equation for any end mill diameter and material combination.
The mill cutting speed equation
Every mill cutting speed calculator reduces to one equation from the Machinery's Handbook:
RPM = (SFM x 12) / (pi x D)
SFM is surface feet per minute for the material-tool pair. D is end mill diameter in inches. The 12 converts feet to inches so the units cancel. For a 3/8 inch carbide end mill in 4140 alloy steel at 280 SFM: RPM = (280 x 12) / (pi x 0.375) = 2,852 RPM. Metric form: RPM = (Vc x 1000) / (pi x D_mm) with Vc in m/min and D in millimeters.
SFM chart for common materials
| Workpiece | HSS SFM | Carbide SFM | Typical cut (0.5 in D) |
|---|---|---|---|
| Aluminum 6061 | 600 | 1400 | 10,695 RPM |
| Mild steel 1018 | 100 | 400 | 3,056 RPM |
| Alloy steel 4140 | 70 | 280 | 2,139 RPM |
| Stainless 304 | 60 | 220 | 1,680 RPM |
| Titanium Ti-6Al-4V | 40 | 120 | 917 RPM |
| Inconel 718 | 20 | 80 | 611 RPM |
| Plastic (Delrin) | 800 | 2000 | 15,279 RPM |
HSS vs carbide mill cutting speed
Tool material is the second biggest lever after workpiece choice. HSS stays hard up to about 1000 F; carbide holds its hardness past 1800 F. That higher temperature ceiling lets carbide run 2.5-4x the SFM of HSS in the same material. A carbide end mill in stainless runs at 220 SFM versus 60 SFM for HSS. The productivity gain is why every modern CNC shop uses carbide even for easy work.
When to override the default SFM
The built-in table is a safe mid-range starting point. Tool manufacturers (Harvey Tool, Destiny, YG-1, Seco, Kennametal) publish SFM recommendations for their specific geometries. A tough aluminum end mill like Destiny Diamondback runs 2000 SFM in 6061, well above the table. Use the SFM override when the manufacturer publishes a number for your tool. For trochoidal toolpaths, the SFM can go 30-50 percent higher because radial engagement is low and the cutting edge sees less heat per revolution.