Scallop Height Calculator

What Is Scallop Height?

Scallop height, also called cusp height, is the small ridge left between adjacent tool paths during CNC finishing. When a ball nose cutter moves side-to-side with a fixed stepover, each pass removes material in an arc. The material left between those arcs forms a repeating profile. The vertical height of that residual profile is the scallop height.

In practical terms, scallop height is one of the most useful predictors of visual and tactile surface quality in 3D machining. A lower scallop height generally means a smoother surface and less polishing or hand finishing. A higher scallop height usually means faster cycle time but more post-processing.

Because machining cost is tightly linked to spindle time and secondary finishing, scallop height becomes a decision variable, not just a geometric output. A dedicated scallop height calculator helps you choose an informed stepover before you run expensive finishing operations.

Scallop Height Formula and What It Tells You

For ball nose finishing, the standard geometric relationship is:

h = R − √(R² − (s/2)²)

Where:

• h = scallop height (cusp height)
• R = tool radius (half the tool diameter)
• s = stepover distance between neighboring passes

If you already know your target finish and want to find the largest possible stepover, use the inverse formula:

s = 2√(2Rh − h²)

This is why the calculator offers two modes: one to find scallop from stepover, and another to find stepover from scallop target. In real shop use, the inverse mode is extremely valuable because engineers and mold makers often work backward from a finish requirement.

Why Scallop Height Matters for Surface Finish and Cost

Surface finish is never only about tool marks, but scallop height is one of the biggest geometric contributors in 3D contouring. If cusp marks are too tall, parts need sanding, stoning, bead blasting, polishing, or recut finishing passes. If cusp marks are controlled tightly at the machine, labor after machining drops sharply.

The tradeoff is time. Reducing stepover lowers scallop height, but it also increases toolpath count and cycle time. Moving from 12% stepover to 6% on a complex cavity can significantly increase machine hours. A proper scallop strategy therefore aligns with part function:

• Cosmetic consumer surfaces: lower scallop targets
• Mold and die optical zones: very low scallop targets
• Functional internal surfaces: moderate scallop may be acceptable
• Rough pre-finish operations: higher scallop acceptable before semi-finish

Using a calculator early in process planning makes quoting more accurate and prevents avoidable time loss on the machine or at the bench.

How to Use a Scallop Height Calculator Correctly

1) Enter tool diameter and confirm units

Start by selecting mm or inch and entering the actual finishing cutter diameter. The calculator internally uses tool radius, so any diameter error directly affects the result.

2) Choose your mode

Use Find Scallop Height when you already have stepover from CAM. Use Find Max Stepover when you have a target finish and need a stepover limit.

3) Validate stepover format

Many programmers think in percentage of diameter, while CAM often displays absolute distance. The calculator supports both. This prevents unit confusion and incorrect assumptions.

4) Evaluate result against process needs

Don’t look at the number in isolation. Compare it with polishing allowance, part visibility, coating thickness, and expected handwork. A mathematically low scallop may still require refinement in critical optical regions.

Typical Scallop Height Targets by Application

The ranges below are practical starting points. Exact targets depend on material, cutter geometry, spindle stability, and final finishing method.

• General 3D finishing: approximately 0.005 to 0.02 mm (0.0002 to 0.0008 in)
• Mold cavity cosmetic regions: approximately 0.002 to 0.01 mm (0.00008 to 0.0004 in)
• High polish / optical surfaces: often below 0.005 mm (0.0002 in), plus post-process
• Functional non-cosmetic surfaces: can be higher when texture is acceptable

These values are not universal standards; they are planning references. Always confirm with your customer’s surface requirement and your internal finishing workflow.

Stepover Strategy: Balance Quality with Cycle Time

A strong strategy combines geometry, machine capability, and economics:

• Use larger stepovers in low-visibility regions and tighter stepovers in focal surfaces
• Apply rest machining so tiny tools only cut where large tools cannot reach
• Use equal-scallop or constant cusp toolpaths when available in CAM
• Control tool deflection and vibration; chatter can dominate finish beyond theoretical cusp
• Match radial stepover to stable feed and spindle conditions, not just geometric formulas

In many shops, the biggest gains come from segmenting the surface into zones with different finishing objectives. A single global stepover is easy, but often inefficient.

Common Mistakes in Scallop Height Planning

• Mixing units: entering inch values in mm mode (or vice versa)
• Using tool diameter in place of radius: formula requires radius internally
• Ignoring practical finish factors: runout, tool wear, and vibration can exceed theoretical cusp
• Applying one stepover everywhere: wastes time or harms quality on critical features
• Over-optimizing for low cusp only: cycle time can become unacceptable without meaningful visual gain

The calculator gives a reliable geometric baseline. Best results come from combining it with stable machining parameters and a process-aware CAM strategy.

Frequently Asked Questions

Final Takeaway

A scallop height calculator is one of the fastest ways to improve CNC finishing decisions. It turns stepover from guesswork into measurable planning, helping you achieve the right balance between part quality and machining efficiency. Use geometric results as a baseline, then refine with real process data from your machine, tooling, and material.