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.
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
FAQ
Is this the same RPM formula used across all machining operations? The core relationship between surface speed, diameter, and RPM applies broadly to turning, milling, and drilling, though recommended surface speed values differ significantly by operation type and tool material.
Why would I need to recalculate RPM mid-operation? Whenever effective cutting diameter changes, such as a facing cut approaching center or a ball-nose cutter at a shallow depth of cut, RPM needs to be recalculated to maintain the same actual surface speed at the cutting edge.
Skip the arithmetic with the RPM Calculator in Feeds & Speeds.