Scallop Height & Stepover Calculator (Ball Nose)
The 3D-milling trade-off in both directions: stepover → the cusp height it leaves, or a target surface finish → the stepover that achieves it. Drawn to scale, with the cycle-time consequence made explicit — because halving the scallop doubles the passes.
Roughly comparable to the Ra values on a drawing, though not identical.
Scallop height, stepover and the cost of smooth
A ball nose cutter finishing a surface in parallel passes leaves a row of cusps — scallops — whose height is pure geometry: h = R − √(R² − (s/2)²), radius and stepover. A Ø10 ball at 1 mm stepover leaves 25 µm cusps; tighten the stepover to 0.5 mm and the scallop drops to 6 µm. For small scallops the relationship is effectively h ≈ s² ÷ 8R, which carries the important news: halving the stepover quarters the scallop but doubles the passes. Smooth is bought with time, quadratically.
The reverse direction is the planning one: the drawing wants a finish, what stepover earns it? s = 2√(h(2R − h)). Aim the scallop a little below the drawing figure rather than at it — scallop is a peak-to-valley number while drawings usually quote Ra, and real surfaces carry machine texture on top of the geometry. A common working compromise on general 3D work is a scallop of 5–10 µm; genuine optical or sealing surfaces move the conversation from stepover to polishing.
Two levers people forget. A bigger ball drops the scallop linearly for free — if the geometry’s smallest concave radius allows it, the largest cutter that fits is the cheapest finish improvement available. And on sloped walls the effective radius the surface sees is reduced by the contact angle, so a stepover chosen for flat areas leaves taller cusps on steep ones — which is why constant-scallop toolpaths exist in CAM, and why we machine complex 5-axis surfaces with the machine, not the calculator, deciding pass spacing.
Scallop height — FAQ
How do you calculate scallop height?
h = R − √(R² − (s/2)²), where R is the ball nose radius and s the stepover. A Ø10 mm ball at 1 mm stepover leaves 0.025 mm (25 µm) cusps. For small scallops, h ≈ s²/8R.
What stepover do I need for a given finish?
s = 2√(h(2R − h)). For a 5 µm scallop with a Ø10 ball: s = 2√(0.005 × 9.995) ≈ 0.45 mm. Aim below the drawing Ra figure, since scallop is peak-to-valley.
Is scallop height the same as Ra?
No. Scallop is the geometric peak-to-valley cusp; Ra is an average roughness over the whole profile. Ra of a pure scallop pattern is roughly a quarter to a third of the scallop height, but real machine texture adds on top — treat them as related, not interchangeable.
Does a bigger ball nose give a better finish?
Yes, linearly — doubling the radius halves the scallop at the same stepover, with no cycle-time cost. The limit is the smallest concave radius the cutter must reach into.
Why is the finish worse on steep walls?
On a slope, the surface contacts the ball away from its tip, reducing the effective radius, so the same stepover leaves a taller cusp. Constant-scallop (adaptive) toolpaths compensate by tightening pass spacing on steep regions.
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Related: Feeds & speeds · Turned finish (Ra) · 5-Axis Machining · All tools