Scallop height and stepover for 3D finishing
A ball nose leaves a ridge between passes because it is round. That ridge is the scallop. Halving the stepover roughly quarters the scallop height but doubles the carve time, which is the central trade in all 3D work.
Where the ridge comes from
A ball nose cuts a groove with a rounded bottom. Put two of those grooves side by side, slightly apart, and the material between where they touch is left standing. That standing material is the scallop, and its height depends only on the ball radius and the stepover.
R is the ball radius, s is the stepover. Both in the same units. This assumes a flat surface: on a slope the same stepover leaves a taller ridge, which is why steep areas always look worse.
Work this way round when you know the finish you want and need to find the stepover that produces it.
What each scallop height feels like
Rule of thumb These bands come from what hobby carvers report and what Kinetic Carving has found on its own work, not from a standard.
| Scallop height | How it reads | Suitable for |
|---|---|---|
| Under 0.0005 in | Cannot be felt. Sands out in moments. | Show pieces, faces, anything sold on its finish |
| 0.0005 to 0.0015 in | Very slight, sands out easily | The normal target for finished 3D work |
| 0.0015 to 0.004 in | Visible in raking light, felt with a thumb | Textured or painted pieces, rough previews |
| Over 0.004 in | Obvious corduroy | Preview cuts only, or a deliberate texture |
The time cost
The number of passes is inversely proportional to the stepover, so the time is too. Halving the stepover roughly doubles the time, and the scallop height drops by roughly a factor of four. That relationship is why:
- Going from a coarse to a good finish is usually worth it.
- Going from a good finish to a perfect one usually is not, because sandpaper does the same job in five minutes.
Diameter changes everything
For the same scallop height, a larger ball nose allows a much larger stepover, because its curve is shallower. That is why a 1/4 inch ball nose finishes a broad relief so much faster than a 1/16 inch one: not because it moves faster, but because it needs far fewer passes for the same surface quality.
The catch is that a larger ball cannot reach into small detail. Which brings you back to the same answer as everywhere else in CNC: use the largest tool that can do the job, and add a small tool only where the large one could not reach.
Why steep areas are always worse
The stepover is measured horizontally, but the surface on a steep slope is much longer than its horizontal projection. So the real distance between passes along the surface is larger, and the scallop is taller. A model with a vertical wall gets no useful finishing on that wall from a raster strategy running parallel to it: the passes either side of the wall never touch its face.
The fixes are a separate strategy for steep regions, a second raster pass at 90 degrees, or an offset strategy that follows the contours instead of ignoring them.