How a spiral bit works
A spiral bit shears the wood rather than chopping it, and the direction of the spiral decides which way it moves the waste. That one choice trades a clean top edge against chip clearance.
Up-cut: clear chips, rougher top edge
An up-cut bit augers the chips up and out of the cut. That keeps a deep groove or mortise clear, stops the bit packing with waste and burning, and lets it run cooler. The cost is the top edge: the lifting action can fray the surface fibers, which shows on veneer and laminate.
Use it for: mortises, deep dadoes and grooves, slots, plunge cuts and anything where chip clearance matters more than the top face.
Down-cut: clean top edge, packed chips
A down-cut bit does the opposite. It pushes the surface fibers down for a crisp top edge and packs the waste into the cut. That is fine on a shallow pass and poor on anything deep, where the packed chips heat the bit and can scorch the wood.
Use it for: shallow dadoes and rabbets in plywood, veneered panels and melamine, inlay pockets and hinge recesses.
Compression bits for sheet goods
A compression bit combines both: up-cut at the tip, down-cut above it, so both faces of a sheet stay clean. It is the right tool for cutting double-sided laminate or veneered plywood all the way through, often on a CNC or with a template, and it costs accordingly.
Which to buy first
For most workshops, one up-cut and one down-cut in the same diameter covers almost everything. Buy the largest shank your router takes; our answer on 1/4-inch or 1/2-inch shanks explains why. Run them at the speed the maker recommends, as our guide to router bit speed sets out, and compare options in our router bits shelf. If you are building a kit from scratch, start with the first five router bits worth owning.
What comes up most often
Which spiral bit is best for cutting mortises?
An up-cut. A mortise is deep and closed, so the chips have nowhere to go unless the bit lifts them out. A down-cut packs them into the bottom of the cut, where they burn and push the bit off line. Cut in several shallow passes either way.
Which bit gives the cleanest edge on plywood or melamine?
A down-cut bit keeps the top face clean on a shallow cut; a compression bit keeps both faces clean when you cut right through a sheet, provided the cut is deep enough for its down-cut section to reach the top face.
Do up-cut bits pull the router into the work?
They pull the bit toward the material, which on a router table holds the workpiece down and in a handheld router tends to draw the bit into the cut. Hold the router firmly, take light passes and keep the work clamped.
Solid carbide or carbide-tipped spiral bits?
Spiral bits are almost always solid carbide, because the helix is ground into the whole cutting length. That makes them sharp and long-lasting but brittle: a dropped bit can chip, so store it in its case.
Last reviewed October 27, 2026