Free tool
Stepover and scallop calculator
A ball nose leaves small ridges between passes, called scallops. Their height depends only on the ball's radius and the stepover. Enter your bit and panel to see the ridge height, the number of passes, the finishing time, and the coarsest stepover whose ridges a light sanding removes (under 0.05 mm).
| Stepover | Distance | Ridge height | Passes | Finishing time |
|---|
Time is cutting distance divided by feed, for a raster finishing pass across the whole panel. Real jobs add acceleration and moves between passes, so allow more.
The formula
Scallop height h = r − √(r² − (s/2)²), where r is the ball radius and s the stepover distance. Halve the stepover and the ridge drops to about a quarter, while the finishing time doubles. That is why there is a stepover past which going finer buys hours, not quality.
Common questions
What stepover should I use with a 1/8 inch ball nose?
At 10 percent stepover a 1/8 inch (3.175 mm) ball nose leaves ridges 0.008 mm high, too small to see. The ridges stay under 0.05 mm, which fine sanding removes, up to about 24 percent stepover (0.762 mm). Going finer than that mostly adds hours, not detail.
What stepover should I use with a 1/4 inch ball nose?
At 10 percent stepover a 1/4 inch (6.35 mm) ball nose leaves ridges 0.016 mm high, too small to see. The ridges stay under 0.05 mm, which fine sanding removes, up to about 17 percent stepover (1.079 mm). Going finer than that mostly adds hours, not detail.
What stepover should I use with a 1/16 inch ball nose?
At 10 percent stepover a 1/16 inch (1.5875 mm) ball nose leaves ridges 0.004 mm high, too small to see. The ridges stay under 0.05 mm, which fine sanding removes, up to about 34 percent stepover (0.540 mm). Going finer than that mostly adds hours, not detail.
How do I calculate scallop height?
Scallop height h = r minus the square root of (r squared minus (s/2) squared), where r is the ball radius and s is the stepover distance. Halving the stepover cuts the scallop to about a quarter and doubles the finishing time.
Does a smaller stepover give more detail?
No. Stepover only changes the ridges between passes. The finest detail a ball nose can reach is set by its radius; for that, use a smaller bit or a rest-machining pass with one.
What about a tapered ball nose?
Enter the tip diameter. The scallop is cut by the round tip, so the same formula applies as long as the stepover stays smaller than the tip.
More on the trade-off, with surfaces you can see: What stepover buys you, and what it costs. What a smaller bit adds that a finer stepover cannot: 3 mm vs 1.5 mm ball nose. Check a whole file at once: Will it cut?