Nothing showcases the mastery of a CNC operator like a deep 3D bas-relief architectural panel or calligraphy relief carved into solid timber. However, poor stepover planning leaves ugly scallop cusps that require dozens of hours of tedious hand sanding, destroying fine sharp details.
### The Scallop Height Geometry
When a ballnose tool steps over laterally by distance $w$, the curved profile leaves a ridge of unmachined stock called **Scallop Height ($h$)**:
$h = R - \sqrt{R^2 - \left(\frac{w}{2}\right)^2}$
Where $R$ is the tip radius of the ballnose.
### The 8% to 10% Golden Stepover Rule
For our 3D relief work:
- **Finishing Tool:** $0.5\text{ mm}$ or $0.25\text{ mm}$ tip radius tapered ballnose endmill with a $4.5^\circ$ taper angle for supreme rigidity.
- **Stepover:** Exactly **$8\% - 10\%$ of tool diameter** (e.g., $0.08\text{ mm}$ stepover on a $1.0\text{ mm}$ diameter ball tip).
- **Resulting Scallop Height:**
lt; 0.002\text{ mm}$ ($2\mu\text{m}$).
At this microscopic cusp height, the individual machining lines are invisible to the human eye and imperceptible to the touch. The carved wood emerges directly from the spindle ready for staining and lacquer, completely bypassing the sanding bench.