Work out surface finish estimate instantly with clear inputs, formula shown and shareable results.
The theoretical roughness left by a round-nosed turning tool is f² / (32 r): the scallops between successive feed marks. Because feed appears squared, halving the feed cuts roughness to a quarter, while doubling the nose radius only halves it, so feed is the more powerful lever.
Theoretical roughness
Ra = f² / (32 r)
Peak to valley
Rz is roughly 4 x Ra for a turned surface
Ra = f² / (32 r). The theoretical roughness left by a round-nosed turning tool is f² / (32 r): the scallops between successive feed marks.
The formula covers only the geometric feed marks. Built-up edge, vibration, tool wear and material tearing all add roughness that geometry cannot predict.
This calculator takes 2 inputs: Feed per revolution, Tool nose radius. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.