Spindle RPM is set by the cutting speed your material and tool want (in surface feet per minute) and the diameter of the cutter. The same SFM needs a higher RPM on a small tool and a lower RPM on a large one.
RPM = (SFM × 12) / (π × D) ≈ SFM × 3.82 / D
What the variables mean
| Symbol | Meaning |
|---|---|
| RPM | Spindle speed (revolutions per minute) |
| SFM | Cutting speed (surface feet per minute) |
| D | Cutter or part diameter (inches) |
Worked example
A 1/2 in cutter in aluminum at 300 SFM: RPM = 300 × 3.82 / 0.5 = 2,292 RPM.
Skip the arithmetic with the RPM Calculator in Feeds & Speeds.