How to use this calculator
- Pick what to solve for. Choose "Find scallop (from stepover)" or "Find stepover (from scallop)".
- Enter the tool diameter. Enter the ball-nose end-mill diameter D. The ball radius is r = D/2.
- Enter the stepover or target scallop. Enter the stepover s, or the target scallop height h, depending on the mode.
- Read the results. Read the scallop height and the stepover used or required.
How it works
A ball-nose end mill has a round tip of radius r = D/2. As it walks
sideways by the stepover s between parallel passes,
the curved nose leaves a small ridge — the scallop (or cusp) —
where neither pass reached the floor. Its peak-to-valley height is
h = r − √(r² − (s/2)²)
Tightening the stepover shrinks the scallop quickly: because s enters
squared, halving the stepover roughly quarters the cusp.
Working backwards, the stepover that produces a target scallop h is
s = 2·√(r² − (r − h)²). Switch the mode to Find stepover,
enter the diameter and the cusp you can tolerate, and the calculator returns the
largest stepover that meets it.
Worked example
Verified against the live calculator
A 6 mm ball nose (r = 3 mm) with a 0.5 mm stepover leaves
h = 3 − √(9 − 0.0625) = 3 − √8.9375 ≈ 0.0104 mm — about a ten-micron
cusp. To hit a 0.005 mm scallop instead, switch to Find stepover:
s = 2·√(3² − (3 − 0.005)²) ≈ 0.346 mm. The calculator returns exactly
these numbers.
Frequently asked questions
How do I calculate the scallop height of a ball-nose end mill?
With ball radius r = D/2 and stepover s, the cusp left between parallel passes is h = r − √(r² − (s/2)²). For a 6 mm ball nose stepping over 0.5 mm, h = 3 − √(9 − 0.0625) ≈ 0.0104 mm.
What stepover gives a target scallop height?
Invert the formula: s = 2·√(r² − (r − h)²), where r = D/2 and h is the scallop you want. Switch the calculator to "Find stepover (from scallop)", enter the tool diameter and target cusp, and it returns the stepover.
What is stepover in CNC milling?
Stepover is the sideways distance between adjacent parallel toolpaths — the radial step. A smaller stepover leaves a smaller scallop and a smoother surface, but takes more passes and more time.
Why does a ball nose leave a scallop?
A round (ball) nose can only touch the floor directly under its centreline. Between two parallel passes a thin ridge of uncut material — the scallop or cusp — is left where neither pass reached. Tightening the stepover shrinks it.
Is the calculated scallop the real surface finish?
No — it is the ideal geometric cusp on a flat, horizontal surface with a perfect tool. On a slope the effective cusp is larger, and tool deflection, runout, vibration and wear all add roughness. Treat it as a best-case starting point.
Does it work in metric and imperial?
Yes — enter the tool diameter, stepover and scallop in mm or inches and read the results back in the same system. Toggle SI/Imperial in the header.
Method & assumptions
- Ideal geometry on a flat, horizontal surface — the perfect-tool cusp. Real Ra is always higher.
- On a sloped surface the effective scallop is larger; this flat-floor figure is the best case.
- Ignores tool deflection, runout, vibration and wear, all of which add to the cusp.
- If the stepover exceeds the tool diameter the passes no longer overlap and the floor between them is left uncut.