Cutting Speed Calculator for Milling
The cutting speed of a milling operation is the surface speed at which the outside edge of the end mill passes through the workpiece. It is measured in SFM (surface feet per minute) or m/min, and it is the single number that connects a material to a spindle RPM. Pick the wrong SFM and your end mill wears out in minutes or rubs without cutting at all. This calculator looks up the right SFM for your material and tool, then converts it to spindle RPM.
The cutting speed equation
The cutting speed calculator milling formula is unchanged since the Machinery's Handbook first published it:
RPM = (SFM x 12) / (pi x D)
SFM is the surface speed for your material and tool. D is the end mill diameter in inches. The 12 converts feet to inches so the units cancel cleanly. For a 1/2 inch carbide end mill in 6061 aluminum at 1400 SFM: RPM = (1400 x 12) / (pi x 0.5) = 10,695 RPM.
SFM values by material and tool
| Material | HSS SFM | Carbide SFM |
|---|---|---|
| Aluminum 6061 | 600 | 1400 |
| Mild steel (1018) | 100 | 400 |
| Alloy steel (4140) | 70 | 280 |
| Stainless 304 | 60 | 220 |
| Stainless 316 | 50 | 200 |
| Cast iron | 80 | 260 |
| Titanium Ti-6Al-4V | 40 | 120 |
| Inconel 718 | 20 | 80 |
| Plastic (Delrin) | 800 | 2000 |
Carbide consistently runs 2.5x to 4x the SFM of HSS because carbide stays hard past 1800 F while HSS softens at 1000 F. Every cutting speed calculator for milling should show both columns so you can see the difference.
Metric milling speed (Vc)
Outside North America, milling cutting speed is measured in meters per minute (m/min or Vc). The conversion is Vc (m/min) = SFM x 0.3048. The RPM equation in metric form is RPM = (Vc x 1000) / (pi x D) with diameter in millimeters. A 12 mm carbide end mill in mild steel at 120 m/min gives RPM = (120 x 1000) / (pi x 12) = 3,183 RPM. This calculator returns both SFM and m/min so you can work in whichever system your control uses.
When to adjust the default SFM
The defaults are safe midpoints. Use the SFM override when a tool manufacturer publishes a speed for their specific geometry (Harvey Tool, Destiny Tool, YG-1, Seco). High-feed mills and trochoidal toolpaths can push SFM 30-50 percent above the table. Old machines with loose spindles, long stickout end mills, or heavy radial engagement should run 20-30 percent below the table. Chip color tells you whether you are in the zone: light straw in steel, silver in aluminum.