Milling Speed Calculator
This milling speed calculator is built around end mill geometry, not general machining. Enter the end mill diameter, material, flute count, and depth and width of cut, and it returns spindle RPM, cutting speed in SFM, feed rate, chip load per tooth, and material removal rate. A chip-thinning factor is applied automatically when radial engagement is less than half the tool diameter, the single biggest source of feed-rate error in a naive milling speed and feed calculator.
Milling speed equations
A cutting speed calculator for milling runs two core equations:
- RPM = (SFM x 12) / (pi x D) converts the target cutting speed (SFM) into spindle RPM.
- Feed rate (IPM) = RPM x chip load x flutes converts RPM into a linear feed rate.
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. With 3 flutes and 0.004 IPT, feed rate = 10,695 x 0.004 x 3 = 128.3 IPM at full slot.
End mill SFM reference
| Material | HSS SFM | Carbide SFM |
|---|---|---|
| Aluminum 6061 | 600 | 1400 |
| Mild steel (1018) | 100 | 400 |
| Alloy steel (4140) | 70 | 280 |
| Stainless 304 | 60 | 220 |
| Titanium Ti-6Al-4V | 40 | 120 |
| Inconel 718 | 20 | 80 |
| Plastic (Delrin) | 800 | 2000 |
Chip thinning
When radial engagement (width of cut, Ae) drops below half the tool diameter, the actual chip thickness the flute removes is less than the programmed IPT. The Sandvik chip-thinning factor compensates by scaling chip load up by D / (2 x sqrt(Ae x (D - Ae))). At Ae = 10 percent of diameter, the adjustment is roughly 1.67x, meaning feed rate can be that much higher without overloading the tool. This calculator applies the factor automatically.
When to trust the milling speed calculator and when to back off
The output is a safe midpoint. Dial feed up 10-20 percent on a rigid machine with high-pressure coolant; dial it down 20-30 percent on a benchtop mill, for long/thin end mills, or when the tool stickout is more than 4x diameter. Chip color tells you if you are in the zone: light straw to blue in steel, silver in aluminum.