Free CNC Stepover & Scallop Height Calculator
Calculate peak scallop cusp height, theoretical surface roughness, and optimal stepover percentage for ballnose 3D relief carving.
⚙️ Cutter & Stepover Inputs
📊 Scallop Height & Surface Quality
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The Geometry of CNC Ballnose Scallop Cusp
When a spherical ball nose endmill traverses across stock in parallel raster passes, its curved profile leaves a microscopic ridge of uncut material between passes known as the scallop height (or cusp height, $h$).
The 8% to 12% Golden Rule
In 3D relief carving (wood plaques, topographic models), a stepover of 8% to 12% produces scallop heights under 10 microns, which are nearly invisible to the naked eye and wipe away with light 320-grit hand sanding.
The Exponential Machining Time Tradeoff
Cutting stepover in half (e.g. from 10% to 5%) cuts the scallop height by nearly 75%, but doubles total machine run time. Understanding the math prevents wasting 8 extra hours milling an imperceptibly smoother part.
Frequently Asked Questions
What is an acceptable scallop height for metal mold making?
Injection molds typically demand scallop heights under 2 to 3 microns (0.0001" / 0.1 thou) to minimize manual polishing time. For decorative wood carving, 10 to 25 microns (0.4 to 1.0 thou) is completely acceptable.
Why does a larger diameter ball nose produce a smoother finish at the same stepover?
Because a larger tool has a wider radius of curvature ($R$). A 12mm ballnose cutting with a 1mm stepover produces a much shallower cusp than a 3mm ballnose cutting with that same 1mm stepover.
Can I use a flat endmill for 3D contoured surfaces?
Flat end mills create stepped 'staircase' artifacts on sloped surfaces because their cutting edge is perpendicular. Ballnose tools are essential for curved 3D surfaces.