fz describes the load on one cutting edge. vf is the axis speed commanded at the machine. The difference determines whether changing RPM preserves a similar chip thickness.
fz, fn and vf are different quantities
fz is feed per tooth. fn is feed per revolution, used in drilling and turning. vf is linear machine feed per minute. In a basic milling program, vf is normally the linear F value.
Convert fz into machine feed
Milling
vf = fz × z × nWith fz = 0.06 mm, z = 3 and n = 8000 RPM, machine feed is 1440 mm/min. If RPM falls to 6000, preserving the same fz requires 1080 mm/min.
Drilling and turning
vf = fn × nWhy the distinction matters
Too little chip load can produce rubbing instead of a clean chip, especially when runout is significant. Too much chip load increases edge load and deflection. Machine feed alone says little about one edge unless RPM and active edge count are also known.
A short pre-run check
- confirm whether the catalogue value is fz, fn or vf,
- count active rather than merely visible edges,
- check whether the control uses feed per minute or feed per revolution,
- recalculate feed whenever RPM changes.