End Mill Cutting Speed Calculator
The cutting speed of an end mill is the linear speed of its outer cutting edge as it passes through the workpiece. Measured in SFM (surface feet per minute) in North America or Vc (m/min) elsewhere, it is the property that ties the workpiece material and tool to a spindle RPM. Pick the wrong SFM and the end mill burns up or rubs. This calculator looks up the SFM for your material-tool pair and converts it to spindle RPM.
The end mill cutting speed equation
The end mill cutting speed calculator formula comes straight from Machinery's Handbook:
RPM = (SFM x 12) / (pi x D)
SFM is surface feet per minute for the material and tool pair. D is end mill diameter in inches. The 12 converts feet to inches so units cancel. Metric form: RPM = (Vc x 1000) / (pi x D_mm) with Vc in m/min and D in millimeters.
Example: a 1/2 inch carbide end mill in 4140 alloy steel at 280 SFM has RPM = (280 x 12) / (pi x 0.5) = 2,139 RPM. Converted to metric: D = 12.7 mm, Vc = 85 m/min, RPM = (85 x 1000) / (pi x 12.7) = 2,130 RPM (matches within rounding).
SFM for end mill cutting speed
| Workpiece | HSS end mill (SFM) | Carbide end mill (SFM) |
|---|---|---|
| Aluminum 6061 | 600 | 1400 |
| Mild steel 1018 | 100 | 400 |
| Alloy steel 4140 | 70 | 280 |
| Stainless 304 | 60 | 220 |
| Stainless 316 | 50 | 200 |
| Tool steel (A2, D2) | 50 | 180 |
| Cast iron | 80 | 260 |
| Titanium Ti-6Al-4V | 40 | 120 |
| Inconel 718 | 20 | 80 |
| Plastic (Delrin) | 800 | 2000 |
HSS vs carbide end mills
HSS holds hardness to about 1000 F. Carbide holds hardness past 1800 F. That higher temperature capacity means carbide can run 2.5-4x the SFM of HSS in the same material. In aluminum, HSS tops out near 600 SFM; carbide cruises at 1400 SFM. In stainless, HSS makes 60 SFM; carbide hits 220 SFM. The calculator picks the right column automatically based on the tool material.
When to override the SFM
Tool manufacturers publish SFM recommendations for their specific geometries. Harvey Tool, Destiny, YG-1, Seco, and Iscar all have online catalogs listing speeds for aluminum, steel, stainless, and exotic alloys. Use the SFM override when the vendor publishes a number for your cutter. Trochoidal and high-feed toolpaths also run SFM 30-50 percent above the generic table because radial engagement is low and the cutting edge sees less heat per revolution.