Feed and Speed Calculator for Milling
A feed and speed calculator milling setup resolves to two numbers the machine control actually reads: spindle RPM and feed rate. This calculator takes end mill geometry (diameter, flutes), workpiece material, and the depth and width of cut, then returns RPM, IPM, chip load per tooth, SFM, metric feed rate, and material removal rate. A chip-thinning factor is applied automatically when the radial engagement is less than half the tool diameter, the single biggest source of feed-rate error in a quick hand calculation.
The two milling equations
A feed and speed calculator milling result is always these two equations:
- RPM = (SFM x 12) / (pi x D) where D is end mill diameter in inches.
- Feed rate (IPM) = RPM x chip load x flutes.
For a 1/2 inch 3-flute carbide end mill in 6061 aluminum at 1400 SFM and 0.004 IPT: RPM = (1400 x 12) / (pi x 0.5) = 10,695 RPM. Feed rate = 10,695 x 0.004 x 3 = 128.3 IPM. At 0.1 inch axial depth and full slot, MRR = 0.1 x 0.5 x 128.3 = 6.4 in^3/min.
SFM reference for milling
| Material | HSS SFM | Carbide SFM | Carbide chip load (0.5 in D) |
|---|---|---|---|
| Aluminum 6061 | 600 | 1400 | 0.004 IPT |
| Mild steel (1018) | 100 | 400 | 0.002 IPT |
| Alloy steel (4140) | 70 | 280 | 0.0015 IPT |
| Stainless 304 | 60 | 220 | 0.0015 IPT |
| Titanium Ti-6Al-4V | 40 | 120 | 0.001 IPT |
| Inconel 718 | 20 | 80 | 0.0008 IPT |
| Plastic (Delrin) | 800 | 2000 | 0.005 IPT |
Chip thinning and radial engagement
When radial width of cut (Ae) drops below half the tool diameter, each flute sees a thinner chip than the programmed IPT because it only contacts the work across a shallow arc. The Sandvik correction scales chip load up by D / (2 x sqrt(Ae x (D - Ae))). At Ae = 10 percent of diameter the factor is about 1.67x. This calculator applies the correction so the programmed feed rate produces the target chip thickness at the tooth.
Imperial, metric, and mm/min
A feed and speed calculator milling setup has to work in both unit systems. The RPM equation is unit-independent when metric diameters use RPM = (Vc x 1000) / (pi x D_mm) with Vc in m/min. Feed rate in mm/min = IPM x 25.4. Every output in this calculator is returned in both imperial and metric so controls programmed in either system can use the result directly.