Upcut vs downcut vs compression router bits
Upcut lifts chips out, clears deep cuts well, and frays the top edge while pulling the workpiece up. Downcut pushes chips down, leaves a clean top edge and presses the work down, but packs chips in deep cuts. Compression does both: downcut at the top, upcut at the bottom, for clean faces on plywood.
The one thing to understand
A spiral bit is a ramp wrapped around a cylinder. As it spins, that ramp pushes material along its length. Which way it pushes decides almost everything else about how the bit behaves.
The comparison table
| Upcut | Downcut | Compression | |
|---|---|---|---|
| Chip evacuation | Excellent | Poor in deep cuts | Good at the bottom, poor at the top |
| Top edge quality | Fuzzy or chipped | Very clean | Very clean |
| Bottom edge quality | Clean | Can fray on breakthrough | Clean |
| Effect on workpiece | Lifts it | Presses it down | Roughly neutral |
| Heat in deep cuts | Lower | Higher, chips recut | Moderate |
| Good for | Deep pockets, slots, thick stock, anything where clearing matters | Shallow work, veneered material, tape and glue workholding, shallow pockets | Cutting through plywood and laminate in one pass |
| Bad for | Thin veneer tops, parts held by tape | Deep pockets, thick slots, chip packing | Anything shallower than the transition point |
Upcut, in detail
The default choice for general cutting, because chip evacuation prevents heat, and heat is what ruins tools and workpieces.
- What it does well. Deep pockets, through cuts in thick material, slots, anywhere a downcut would pack chips. It keeps the cut clear and the bit cool.
- The lifting force is real. On a thin part held down with tape, an upcut bit can and does pull it off the spoilboard mid-cut. This is one of the main causes of a ruined workpiece and a dangerous flying part.
- The top edge frays. It is physically pulling surface fibres upward as it cuts them. On solid wood that means a fuzzy edge you can sand. On veneered plywood it means splintered veneer you cannot fix.
Downcut, in detail
- What it does well. A genuinely crisp top edge, which matters for inlay, veneered sheet goods, lettering and any shallow pocket where the top surface is the visible surface.
- It holds the work down. Which makes it the right choice for parts held with tape and CA glue, or for small parts that an upcut would lift.
- It packs chips. In a deep pocket, the bit is actively pushing chips into a hole with only one exit, which it is blocking. The result is heat, burning and sometimes a seized bit. Keep downcut cuts shallow, or rough with an upcut and finish with a downcut.
- Breakthrough can fray. Cutting all the way through with a downcut pushes fibres out of the bottom face.
Compression, in detail
A compression bit has an upcut section at the very tip, usually somewhere between an eighth and a quarter of an inch long, and a downcut spiral above it. When the bit is cutting through material, the lower section lifts the bottom fibres up and the upper section pushes the top fibres down. Both veneer faces of a sheet get squeezed toward the middle and neither one splinters.
The rule that makes them work, and the one people miss:
Compression bits are a production tool for cutting parts out of plywood, MDF laminate and melamine. For that job they are excellent and nothing else matches them. For general hobby work where you pocket, carve and profile at varying depths, a good upcut and a good downcut cover more ground.
Choosing, in one paragraph
If the cut is deep or you are worried about heat, use an upcut. If the top surface is the visible one and the cut is shallow, use a downcut. If you are cutting parts out of a sheet with a veneer or laminate on both sides, use a compression bit and make the pass deeper than its upcut section. If you are not sure, an upcut with a shallower depth per pass and good dust collection is the safe default.
Straight bits, for context
A straight flute bit has no spiral at all, so it pushes chips in neither direction. It relies entirely on the flutes and airflow to clear, which is worse than either spiral. It also chops rather than shears, so it leaves a rougher edge and is noisier. See straight bits for where they still earn a place.