Cusp & Stepover

Imperial Metric

3D Surface Finish

In 3D contour milling with a ballnose end mill, the "scallops" or "cusps" left behind determine the surface roughness.

Fundamental Formula

$h = R - \sqrt{R^2 - (S/2)^2}$

h = Cusp Height, R = Tool Radius, S = Stepover

Rules of Thumb

Fine Finish: Target cusp heights between 0.0001" - 0.0003" (2.5 - 7.5µm).
Roughing: Larger stepovers (up to 25% of tool diameter) are used to clear material quickly.
Tool Size: Larger tools produce a flatter cusp for the same stepover distance.

Note: These calculations assume a flat surface. On steep walls or complex curves, the effective cusp height will vary.