The best CNC bit for plywood furniture
The short answer: a 1/4" (6.35 mm) carbide compression spiral bit is the best single bit for cutting plywood parts all the way through. It leaves a clean edge on the top and bottom faces. Keep a 1/8" bit for tight details, and remember that the bit you pick also decides your inside corners.
Up-cut, down-cut and compression
- Up-cut spiral: pulls chips up and out of the cut, so it clears dust well, but it lifts the top veneer and can leave a rough top edge.
- Down-cut spiral: pushes chips down, which leaves a clean top face, but chips pack into the cut and the bottom face can chip.
- Compression spiral: has an up-cut section at the tip and a down-cut section above it, so both faces are pushed toward the middle of the board. It is the usual choice for through-cuts in plywood.
Using a compression bit properly
- Choose a bit whose cutting length is longer than the thickness of the sheet.
- Set the depth so the point where the bit changes direction sits inside the board, not above it. The up-cut section at the tip should be shorter than the thickness of the sheet, so the change of direction stays inside the plywood.
- Cut slightly into your spoilboard on the last pass, about 0.5 mm (0.02"), so the bottom veneer is cut through fully.
- Follow the maker's feed and speed chart for your bit and your machine, and adjust from there. Fine dust instead of small chips usually means the feed is too slow and the bit is rubbing.
Bit size sets your corners
A round bit cannot cut a sharp inside corner. In plywood joints the corner radius equals the radius of the bit, and a square tenon will not fit into a rounded slot. The usual fix is a dog-bone, a small relief cut into the corner. See dog-bones for plywood furniture. A 1/4" bit makes a 1/8" (3.2 mm) corner radius; a 1/8" bit makes a much smaller one, but it is slower and breaks more easily.
When to use a smaller bit
- Small holes, narrow slots and lettering, where a 1/4" bit cannot fit.
- Fine details on a decorative panel.
- Pockets in very small parts, as a finishing pass after the main cut.
Bit care
A dull bit burns the edge and tears the veneer. If you see dark edges, smoke or a rough cut on a bit that used to cut clean, replace it before you blame the plywood. Clean resin off the flutes now and then.
Frequently asked questions
What is the best CNC bit for cutting plywood?
A 1/4" carbide compression spiral bit is the usual best choice for cutting parts all the way through, because it leaves a clean edge on both faces.
Why does my plywood burn when I cut it?
Burn marks usually mean a dull bit or a feed that is too slow, so the bit rubs instead of cutting. Check the bit and follow the maker's feed chart.
Does the bit size change my joints?
Yes. A round bit leaves a rounded inside corner whose radius is the bit radius, so square tenons need a dog-bone relief in the corner.