Stepover Calculator
Calculate the cusp height (scallop) left by a ball nose end mill at a given stepover, or find the stepover needed for a target cusp height.
Why Stepover Matters
When a ball nose end mill makes parallel passes, it leaves small ridges (scallops) between each pass. Stepover controls how much material is left behind between passes, so it's a trade-off between machine time and finish quality — or machine time and manual finishing time. A tighter stepover means a smoother surface off the machine but more passes and longer cycle times. A wider stepover cuts faster but leaves taller ridges that may need sanding or polishing.
Rules of Thumb
By finish goal
- 5–10% — Fine/mold finish, minimal hand work
- 10–20% — Good finish, light sanding
- 20–35% — Semi-finish, plan to sand
- 35%+ — Roughing only
By application
- Mold cavities — 5–8%, cusps create witness marks
- Visible surfaces — 8–12%
- Functional surfaces — 15–25%
- Will be painted/coated — 10–15%, filler hides cusps
Ball Nose Cusp Height Calculator
Common Stepover Quick Reference
Cusp heights for common stepover percentages. Click a row to load it into the calculator.
| Stepover % | Stepover | Cusp Height | Finish Quality |
|---|
Understanding Ball Nose Stepover
What is Cusp Height?
When a ball nose end mill makes parallel passes across a surface, it leaves small ridges (scallops) between each pass. The height of these ridges is the cusp height. A smaller stepover produces a smaller cusp height and a smoother finish, but takes more passes and more machining time.
The Formula
Cusp Height = R − √(R² − (S/2)²)
R = ball radius, S = stepover distance
Choosing a Stepover
- 5–10% — Fine finish. Minimal hand finishing needed. Use for visible surfaces, mold cavities, and parts that need to look good off the machine.
- 10–20% — Good finish. Acceptable for most parts. Light sanding removes cusps.
- 20–35% — Roughing or semi-finish. Visible scallops. Use when another finishing pass will follow or surface quality doesn't matter.
- 35–50%+ — Aggressive roughing only. Large scallops. Never use as a final pass.
Tips
- A larger ball nose produces a smaller cusp height at the same stepover percentage — use the biggest ball that fits your geometry
- Cusp height grows rapidly above 30% stepover — the relationship is not linear
- For 3D contour finishing, the effective stepover varies with surface angle — steeper surfaces produce larger cusps at the same XY stepover
- Some CAM software lets you specify a target cusp height directly and calculates the stepover for you
Preview Your Finishing Passes
Use CutViewer to visualize your ball nose toolpaths in 3D and confirm stepover before cutting.
Launch CutViewerFrequently Asked Questions
How do you calculate cusp height for a ball nose end mill?
Cusp height = R − √(R² − (s/2)²), where R is the tool radius and s is the stepover. A 1/4-inch ball nose (R = 0.125) at 0.010-inch stepover leaves a cusp of about 0.0001 inches.
What stepover should I use for a ball nose finishing pass?
Most finishing passes use 5–15% of the tool diameter. Smaller stepovers leave a smoother surface but take longer; the cusp height shrinks roughly with the square of the stepover, so halving the stepover cuts the cusp height to about a quarter.
What is the difference between cusp height and scallop height?
They mean the same thing: the height of the ridge left between two neighboring passes of a ball nose or bull nose cutter.
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