How does theoretical scallop height relate to surface roughness Ra?

In ideal rigid machining conditions with zero tool runout, average surface roughness Ra is approximately equal to one-quarter of the peak-to-valley scallop height: Ra ≈ h / 4.

Why should I use a larger diameter ball nose for 3D finishing?

A larger ball nose has a flatter curvature radius. A 1/2" ball mill at 0.020" stepover produces a 4x smaller scallop ridge than a 1/8" ball mill at the exact same stepover, cutting faster while leaving a smoother finish!

What is a bull-nose (toroidal) cutter and how does it compare to ball nose?

A bull-nose end mill has a flat bottom with radiused corners. On flat or shallow sloping contours (under 30 degrees), a bull-nose tool eliminates scallops completely, running 10 times faster than a ball nose.

Free CNC Stepover & Scallop Height Calculator - 3D Ball Nose Cusp Surface Tool

⚙️ Ball Nose Tool & Stepover

Standard Ball End Mill Size

Cutter Radius (R in Inches) Half of cutter diameter

Stepover Input Mode

Stepover Value (% or Inches) 10% stepover = 0.025"

Workpiece Material

📊 Scallop Height & Surface Quality

Scallop Cusp Height (h) 1.25 µm 49.2 micro-inches (0.00005")

Theoretical Roughness (Ra) 0.31 µm 12.3 µin Ra (Near Mirror)

Physical Stepover Distance (ae): 0.0250" (0.635 mm)

Stepover Percentage of Diameter: 10.0% of 1/4" Tool

Required Post-Machining Hand Sanding: Minimal / Quick Scotch-Brite buff

Cycle Time Tradeoff: Standard finishing pass

✅ Mirror Finishing Range: 10% stepover with 1/4" ball nose yields 1.25 µm cusp height. Ideal balance between machining cycle time and flawless surface smoothness.

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The Trigonometry of 3D Ball Nose Scallop Height

In 3D contour CNC machining, a spherical ball end mill creates parallel concave troughs across the surface. The overlapping ridge left between two adjacent tool passes is called the scallop or cusp.

Scallop height ((h)) is calculated by Pythagorean theorem from the cutter radius ((R)) and linear stepover distance ((ae)):

h = R - √[ R² - (ae / 2)² ] ≈ ae² / (8 × R)

The Exponential Stepover vs Cycle Time Tradeoff

Because scallop height is proportional to stepover squared ((ae^2)), cutting stepover in half (e.g. from 20% to 10%) slashes surface ridges by 75%, but doubles your machine run time!

15% to 20% Stepover: Fast semi-finishing pass. Ridges are visible and require substantial manual sanding.
8% to 10% Stepover (Golden Zone): True finishing pass. Scallops are microscopic (<0.0001" / 2.5 µm), requiring only a light touch of 320-grit sandpaper or a buffing wheel.
<5% Stepover: Optical mirror finishing. Run times become very long.

Free CNC Stepover & Scallop Height Calculator