What is the difference between spindle speed and cutting speed?

Spindle speed is the rotational velocity of the machine spindle and cutting tool, measured directly in revolutions per minute (RPM). It indicates how many complete turns the tool makes every sixty seconds regardless of its size.

Cutting speed, also referred to as surface speed, is the linear rate at which the cutting edge travels across the workpiece material, measured in surface feet per minute (SFM) or meters per minute (m/min). Cutting speed is determined by the specific material machinability, cutting tool substrate, and coating to manage thermal load and tool wear. Spindle speed is the calculated machine output derived from that recommended cutting speed and the tool diameter. For a given cutting speed, a smaller tool diameter requires a higher spindle speed (RPM) to maintain the required linear surface speed at the cutting edge.