Surface Finish Calculator

The formula squares the feed per revolution term, so a given percentage increase in feed rate (which raises feed per revolution proportionally) produces roughly double that percentage increase in predicted Ra. A 20% faster feed rate at the same spindle speed can noticeably worsen the theoretical finish, which is why finishing passes typically run at much lower feed rates than roughing passes.

In this geometric model, yes -- nose radius sits in the denominator of the Ra formula, so increasing it lowers the predicted roughness at a fixed feed per revolution. In practice, however, a larger nose radius also increases cutting forces and can promote chatter on less rigid setups, so the finish benefit has to be weighed against machine and workpiece rigidity.

The calculator's Ra is a geometry-only prediction that assumes a perfectly sharp tool and no vibration, tool deflection, or material tearing. Real measured roughness is usually equal to or worse than the theoretical value, since those additional factors only add roughness -- they don't improve on the geometric best case.

Rz (peak-to-valley height) is estimated here as roughly four times the calculated Ra, a common rule of thumb for typical machined surfaces where the roughness profile has a fairly regular, repeating pattern. Ra averages the profile while Rz captures the distance between the highest peak and lowest valley, so Rz is always the larger number.