The speed at which the cutting edge passes through the material, independent of tool diameter.
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.
Treating a catalogue's spindle speed as transferable between tools of different size. It is not — only the surface speed carries across.
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