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CNC routing

Surface finish and stepover

Turns a stepover into the ridge height it leaves, and a wanted finish into the stepover that produces it — with the pass count beside each, because that is the price.

The question this answers: “How fine a stepover do I actually need, and what is it costing me in run time?”

The cutter itself, not the shank.

Sideways distance between adjacent passes.

Scallop height. Below about 0.01mm you cannot feel it.

Used only to turn stepover into a pass count.

Result

Scallop height at this stepover
0.042 mm
The ridge left standing between adjacent passes. Sand or accept it.
Passes across the surface
200
Across 200mm at 1mm per pass. Run time scales with this.
Stepover for a 0.02mm scallop
0.692 mm
Which would take 290 passes — 90 more than above.

Worth knowing about this answer

  • This is geometry and nothing else. It says how tall the ridges between passes will be; it says nothing about feed rate, spindle speed or whether your machine can hold the cut — those depend on the machine, the cutter and the material, and this tool deliberately does not guess at them.
Show the working
  1. Cutter radius6 ÷ 23 mm
  2. Scallop height3 − √(3² − (1 ÷ 2)²)0.042 mm
  3. Stepover for the target2 × √(3² − (3 − 0.02)²)0.692 mm
  4. Passes at that stepover200 ÷ 0.692, rounded up290

Scallop height at this stepover: 0.042 mm. Passes across the surface: 200. Stepover for a 0.02mm scallop: 0.692 mm

How it works

Pure geometry. A ball of radius R stepping over by s leaves a cusp of height R minus the square root of R squared less s-over-two squared, and the inverse solves the same equation for s. The pass count is the surface width divided by the stepover, rounded up.

What it assumes

  • The surface is flat and the passes are parallel. On a curved surface the effective stepover varies with the slope and the real scallop is larger on the steep parts.
  • The cutter is a true ball nose. A bull nose or a flat cutter with a corner radius leaves a different and generally smaller cusp.
  • Nothing here depends on the machine, the material or the spindle, which is why it can be calculated at all.

Where it stops being reliable

  • It does not tell you what finish to choose, because that depends on what happens next — a surface that will be sanded wants a different scallop from one that will be oiled off the machine.
  • It says nothing about feed rate, spindle speed or depth of cut, and deliberately so. Those depend on the machine, the cutter and the material, and a universal figure for them is the commonest cause of broken cutters in this hobby.
  • Real run time depends on feed rate and acceleration as well as pass count. The pass count is a proportional guide, not a duration.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on CNC routing.