Why is there a thin lip or rounded flange left around the bottom edge of my 3D or double-sided part?

Why it happens

A ball nose bit cannot make a sharp inside corner at the bottom of a slope. When the toolpath stops exactly at the bottom of the shape, the rounded tip leaves a small fillet of material there. On a single-sided part that shows as a flange along the bottom edge. On a double-sided part, where each side stops at the middle of the board, the two leftovers meet and form a thin lip or a flat band around the seam.

Fix for sloped edges and molding profiles (one side)

1. Extend the side profile or the vectors downward by at least the radius of the bit. For a 0.25 inch ball nose that is 0.125 inch.
2. Put a sacrificial board under the work, so the extra depth cuts into waste and not into your machine bed.
3. Recalculate. The rounded part of the cut now lands below the part, and the edge comes out sharp.

Fix for double-sided 3D parts

1. Add a zero plane (educators also call it a limit plane) on its own level.
2. Lower it slightly below the midpoint of the material, by a little more than the radius of the ball nose.
3. Do the same on the other side, so the two sides overlap slightly instead of stopping on the same line.
4. If the edges still look rough or unfinished, adjust the boundary offset of the 3D toolpaths as well.

A ridge on only one side of a bowl

That is usually a raster stepover problem, not a depth problem. When the stepover does not divide evenly across the component, the last pass can stop short on one side. Increasing the stepover percentage slightly, or using a larger boundary offset, removes the ridge.

Pro tips

• In Aspire, check both sides with a model cross section before cutting. It shows wall thickness and warns you before the two sides carve through each other.
• Put tabs in the same places on both sides so the part stays supported once the sides meet.
• See the entries on the zero plane floor trick and on side two not lining up after a flip.