Ballnose Milling Scallop Height from Stepover
The 3D-finish trade every mold toolpath makes.
Example
You enter
- Ballnose radius R (in = cutter dia / 2) 0.25
- Mode scallop-from-stepover
- Stepover s (in, for scallop mode) 0.03
You get
- Result 0.00045
Checked against published worked examples from: Mitsubishi Materials USA.
Details, formula, and sources
Scopea 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 or tablescallop mode: h = R - sqrt(R^2 - (s/2)^2); stepover mode: s = 2 sqrt(R^2 - (R - h)^2); approx h ~ s^2/(8R).
Source and editionThe ballnose scallop (cusp) height geometry and its inverse, with the small-scallop approximation, as compiled in the CAM and mold-machining references, by name.
Free accessThe ballnose scallop-height geometry is a public CAM/milling result in the standard machining references.
What governsEstimate. AHJ and licensed professional govern.
API field namesr_in, mode, s_in, out_in
- Scallop geometry h = R - sqrt(R^2 - (s/2)^2) between parallel passes of a ballnose radius RCAM geometry
- s^2 scaling near the bottom h ~ s^2/(8R), so doubling the stepover quadruples the scallopscallop approximation
- Flat, geometric theoretical cusp on a flat surface; slope, deflection, and runout change the real finishscope of this tile