Up Cut vs Down Cut vs Compression Router Bits: Which One Do You Need?

Three spiral bits, three different results, and the choice comes down to one question: which face of your material needs to be clean? Here is how each one works and when to use it. For the full tour of every flute type, read What the Flute.

Up cut: chips out, clean bottom

An up cut spiral works like a drill bit: the flutes carry chips up and out of the cut. That is what you want whenever chip clearance matters, deep slots, pockets, hardwood, plastics and aluminium. The bottom edge of a through-cut is clean because the cutting edge is pushing the fibres up against solid material.

The cost is the top surface. The same upward force lifts the top fibres and leaves fuzz or chipping on ply, melamine and veneered board, and it pulls small parts up off the bed, so your hold-down has to be good.

Use it for: hardwood profiling, deep pockets, plastics (as a single flute), aluminium, anything where the top face will be sanded or is the back of the job. Products: 2, 3 & 4 flute up cut, 1 flute for plastics, 1 flute DLC for aluminium.

Down cut: clean top, chips stay in

A down cut spiral is the mirror image: the flutes push chips down into the cut. The top edge is pressed down as it is cut, so it comes out crisp with no fuzz, which is exactly what you want for engraving, lettering, shallow pockets and the visible face of a sign or panel. It also pushes the work down onto the bed, so thin stock and small parts stay put.

The cost is chip evacuation. Chips are packed into the slot, which means more heat, a slower feed, and a slightly rougher bottom edge on through-cuts. Do not use a down cut for deep slots or for plastics and aluminium; the packed chips will melt or weld.

Use it for: engraving and lettering, shallow pockets, dados, ACM and sign board, veneered and melamine faces that are visible. Products: 2 flute down cut, 1 flute down cut for ACM.

Compression: clean both faces

A compression bit has an up cut section at the tip and a down cut section above it. Once the tip is below the surface, the two halves push chips toward the middle of the board and both faces are cut with the fibres pressed against solid material. That gives you a clean top and a clean bottom in one pass, which is why compression bits are the standard for nesting cabinet parts from ply, MDF and melamine.

Two rules. First, the first pass has to be deeper than the up cut section (usually 5 to 8mm) or the tip is doing up cut work on the top face and you get fuzz. Second, they are for through-cuts and full-depth profiles; in a shallow pocket only the up cut tip is engaged, so use a down cut for those.

Use it for: through-cutting ply, MDF, melamine, laminated board, flat-pack and cabinet parts. Products: 2 flute compression, 3 flute compression, DLC compression for abrasive boards, and the XB industrial range for production machines.

Quick picker

What about straight bits?

A straight flute bit has no helix, so it neither lifts nor pushes. The top and bottom finish are both acceptable rather than perfect, chips clear reasonably, and it is the cheapest of the four. For general work on a hobby machine, a 2 flute straight is a sensible first bit; the 3 flute chip breaker version cuts panels noticeably faster.

Still unsure? Call Adam on 0448 431 833 with your material and machine, or browse Router Bits for Timber, MDF & Ply.