Cutting speed is the speed at which the cutting edge moves over the work surface, quoted in surface feet per minute (sfm) in imperial practice or metres per minute (Vc, m/min) in metric. It is a property of the tool-and-material pairing, not the machine: the insert or end-mill manufacturer’s recommendation for your workpiece material is the authoritative number, with the Machinery’s Handbook speeds-and-feeds tables as the standard reference when you have none. The source must match the tool in the spindle — uncoated HSS runs far slower than coated carbide in the same material, and insert grade and coating can change the number several-fold.
The machine setting falls out of the standard relation RPM = 12 × V / (π × D) with V in sfm and D in inches, or RPM = 1000 × Vc / (π × D) with mm. At 100 sfm a 1-inch tool turns 382 rpm; halve the diameter and the RPM doubles to 764. For milling and drilling D is the rotating tool’s diameter, but for turning it is the workpiece diameter at the cut, so RPM rises as you turn closer to center at constant surface speed — the reason CNC lathes offer a constant-surface-speed mode. Small diameters at carbide speeds produce huge answers: 500 sfm on a 1/50-inch tool is over 95,000 rpm, beyond most spindles, in which case the practice is to run at maximum RPM and accept the reduced surface speed.