The compression router bit

The compression cutters are designed with both top and bottom cut grooves. The upper part of the cutting length closest to the shank has a left spiral forcing chips down.

When using the compression bit, you need to make sure that the first pass is under the cut out part of the bit, or you will get results similar to just cutting with a cut out bit; dip into the material with a ramp, immerse over a distance instead of all at once, and a guide that starts outside the design to reach the pass depth, ensuring that the entire piece is perfectly clean at the top and bottom. Compression bits are designed to cut in a single pass, but if your machine cannot handle this cutting depth, you can cut in multiple passes as long as that first pass is below the cut out part of the piece.