Stepover Calculator

The geometry behind scallop height

A ball nose end mill leaves a rounded ridge, or scallop, between each finishing pass, and the size of that ridge depends on the ball's radius and how far apart the passes are stepped. Scallop height h = R − √(R² − (s/2)²), where R is the ball radius and s is the stepover — solved the other way, stepover s = 2√(2Rh − h²) gives the spacing needed to hit a target height.

Picking a target scallop height

A target scallop height well under what you'd need to hand-sand or polish out saves finishing time without over-cutting passes that add no visible improvement — 0.0002–0.0005 in is a common target for a mold-quality finish that still needs light polishing, while 0.00005–0.0001 in approaches a finish that needs no further work at all. Going tighter than necessary multiplies your total number of passes and machining time for a finish improvement that may not be visible or functionally meaningful.

Stepover isn't the only thing controlling finish

Surface angle matters as much as stepover on a 3D surface — a steep or vertical wall gets a much finer effective finish from the same stepover than a near-flat surface does, because the ball nose contacts the surface differently at different angles. CAM software that varies stepover by surface angle (constant scallop toolpaths) will typically get a better finish-to-time tradeoff than a fixed stepover calculated for the worst-case flat area alone.