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Stepover and Scallop Height

The ridge a ball-nose bit leaves between passes, from your stepover, or the stepover to hit a finish you want.

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A ball-nose bit cuts a round-bottomed groove, so between two passes it leaves a little ridge, the scallop. Space the passes far apart and the surface feels ribbed; crowd them together and it comes off smooth but slow. This gives you the ridge a stepover will leave, or the stepover to hit the finish you are after.

Where the ridge comes from

Picture the round tip of the bit dropping into the work twice, one stepover apart. The material left standing between the two scooped arcs is the scallop. Its height depends only on the tool radius and the stepover: a bigger bit or a tighter stepover leaves a shorter ridge. The exact height is the radius minus the square root of the radius squared minus half the stepover squared.

Choosing a stepover

It is a straight trade between finish and time. Halve the stepover and you roughly quarter the scallop, but you double the number of passes and the time. For most work a finishing stepover around 7 to 10% of the tool diameter looks clean to the eye and sands out in minutes. Go finer only where the surface really shows, like a sign face or a mold.

Bigger bit, fewer passes

A large ball nose is the quiet way to a smooth finish. Because the scallop shrinks with tool radius, a half-inch ball nose at a given stepover leaves a far shorter ridge than a quarter-inch one, and covers the surface in half the passes. Rough with whatever bit clears material fast, then finish with the biggest ball nose that still reaches into the detail.

Direction matters

The ridges all run the same way, along the path the bit traveled. On a flat panel that means raking light across them makes them pop, while light along them hides them. Point your finishing passes so the ridges run with the grain or away from where someone looks, and the same scallop height reads as a better finish.

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<p style="font:13px/1.4 system-ui,sans-serif;margin:6px 2px">Free calculator by <a href="https://makerkiln.com/stepover-scallop-height-calculator/" target="_blank" rel="noopener">MakerKiln</a></p>
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