Cutting Speed
Cutting speed is the relative linear velocity between the cutting edge and the workpiece surface, typically expressed as surface feet per minute (SFM) or meters per minute (m/min). It is not the same as spindle speed (RPM), but RPM is derived from cutting speed and tool or workpiece diameter using the formula: SFM = π × D × RPM / 12 (imperial) or Vc = π × D × RPM / 1000 (metric).
On the shop floor, cutting speed is selected from tooling and material data to ensure the insert shears material at a stable chip formation rate, avoiding rubbing or overheating. In milling, tool diameter directly affects surface speed at a given RPM; larger cutters produce higher speeds, requiring recalculation when tools change. In turning, cutting speed depends on workpiece diameter and spindle RPM; constant-surface-speed control is often used to maintain consistent chip load and finish as diameter varies. Cutting speed is coordinated with feed rate, depth of cut, coolant, tool coating, and workholding to balance material removal rate, tool life, and surface finish. The standard relationship for converting cutting speed to RPM uses the tool or workpiece diameter, and this calculation is critical for every CNC setup sheet.
Is cutting speed the same as spindle speed?
No. Cutting speed is the linear surface velocity at the cutting zone, while spindle speed is rotational speed in RPM. They are related by the tool or workpiece diameter.
Why do toolmakers publish SFM or m/min instead of RPM?
Because cutting speed depends on diameter. The same RPM produces different surface speeds on different tool diameters, so surface speed is the transferable parameter for process setup.
What happens if cutting speed is too low?
The tool may rub instead of shear, increasing burrs, chip welding, poor finish, and uneven wear, especially in ductile metals.