A straight bit chops. A spiral bit slices. The helical flutes shear the wood fibers instead of hitting them head-on, which is why spiral bits leave a cleaner edge, clear chips better, and last longer. That's the reason CNC woodshops have mostly moved to spiral tooling. But "spiral" only tells you half the story — the direction of the helix decides where the chips go and which face comes out clean.
Start with flute direction
Upcut bits pull chips upward and out of the cut. That means fast chip evacuation, a cooler edge, and a clean bottom surface. The trade-off: they lift the top fibers, so solid wood and thin sheet can show tear-out on the top face. Reach for an upcut when you're cutting deep slots, pockets, or solid wood where the top edge gets rounded over later.
Downcut bits do the opposite — they push chips down and press the surface fibers into the board. The top face comes out pristine. The downside is chip evacuation: everything packs into the bottom of the groove. Push too deep and heat builds up fast. Keep the depth per pass to about a quarter of the bit diameter, and use downcut bits for shallow cuts, engraving, and parts where the top is the show face.
Compression bits combine both geometries on one tool: an upcut section at the tip (roughly 3–5 mm) and a downcut section above it. The two spirals squeeze the fibers toward the middle, leaving both faces clean. For double-sided panels, melamine, and plywood — where both edges are visible — a compression bit is essentially the only answer.
Two rules are non-negotiable with compression bits. First, cut at full depth: if the first pass is shallower than the upcut section, you're only using the upcut half and the top will still tear out. Second, ramp into the cut instead of plunging straight down — a plunge packs chips at the transition point and burns the tool. Compression bits cost 2–3× a single-direction bit, but they pay for themselves on panel work.
Flute count
- One flute: the largest gullet, best chip clearance. For plastics, acrylic, and soft wood that tend to gum up.
- Two flutes: the default for woodwork — hardwood, softwood, plywood, MDF. Balanced.
- Three flutes: the finishing specialist. Smooth surface on hardwoods and laminates, and feed rates can run 30–50% faster than a two-flute. The small gullets clog on deep soft-material slots, so keep depth to about 1× the diameter.
Size and material
Shorter bits run stiffer — pick the shortest cutting length your job allows. A 1/2" shank resists vibration better than a 1/4" one. Solid carbide is the standard; for abrasive panel products, PCD (polycrystalline diamond) tools can outlast carbide by 200–300×.
Quick selection table
Your situation | Bit to use |
Deep slots, pockets, solid wood | Upcut |
Clean top face, shallow cuts, engraving | Downcut |
Full-depth cuts in plywood / melamine | Compression |
Plastics, acrylic, soft wood | Single-flute upcut |
Hardwood / laminate finishing | Three-flute |
The mistake I see most
Running the spindle too fast. Downcut and three-flute compression bits clear chips poorly, so drop the RPM (keep a three-flute compression bit around 16000 RPM or lower) and feed fast enough that the bit never rubs in place. You don't need a wall of bits — one upcut, one downcut, and one compression covers most CNC woodwork.