Sources and scope for this page: ISO 229:1973, covering the definition of surface speed (sfm / m/min). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Surface speed is the constant that governs tool life. Spindle speed is what you actually set on the machine, and the relationship between them depends on diameter: RPM = SFM × 3.82 ÷ D for D in inches.
This is why a 0.5 in end mill runs at 2,300 RPM where a 4 in face mill runs at 290 RPM — both at the same surface speed. A speed quoted as a bare RPM figure without a diameter is meaningless.
Common misuse
Treating a catalogue's spindle speed as transferable between tools of different size. It is not — only the surface speed carries across.