Ballnose Milling Scallop Height from Stepover
The 3D-finish trade every mold toolpath makes, which turning-surface-finish never covers: a ballnose of radius R stepping over by s leaves a scallop h = R - sqrt(R^2 - (s/2)^2), or the inverse for the stepover that holds a target scallop. A 0.5 in ballnose at 0.030 in stepover leaves 0.45 mil; double the stepover and the scallop quadruples (the s^2 law) - a tighter stepover buys a finer finish at the cost of cycle time. Theoretical flat-surface cusp, not Ra. A shop aid; the real finish depends on the tool, deflection, and slope.
Formula and source
scallop mode: h = R - sqrt(R^2 - (s/2)^2); stepover mode: s = 2 sqrt(R^2 - (R - h)^2); approx h ~ s^2/(8R).
The ballnose scallop (cusp) height geometry and its inverse, with the small-scallop approximation, as compiled in the CAM and mold-machining references, by name.
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