How to stop tear-out when cutting plywood on a CNC
The short answer: tear-out comes from a dull or wrong bit, a feed rate that does not match the bit, and plywood that is not held down or not supported at the bottom. Switch to a sharp compression bit, check your feed, hold the sheet flat and cut slightly into the spoilboard.
What tear-out looks like
Splinters along the cut edge, chipped veneer on the top or the bottom face, and fuzzy fibres that stand up from the edge. It is most visible on the face veneer, where a bare edge or a clear finish will show it.
The usual causes
- A dull bit: it tears the fibres instead of cutting them. Burn marks on the edge are another sign.
- The wrong type of bit: an up-cut bit lifts the top veneer, and a down-cut bit chips the bottom. A compression bit cleans both.
- A feed that is too slow or too fast: too slow and the bit rubs and burns, too fast and it tears. Follow the maker's chart and adjust in small steps.
- Vibration: a sheet that lifts or flexes while it is cut chatters. Hold it down well. See how to hold plywood down.
- Thin or low-quality veneer: some plywood chips no matter what. Better plywood often fixes it.
How to fix it
- Put in a new, sharp 1/4" compression bit and run a short test cut. Read which bit to use.
- Set the final depth about 0.5 mm (0.02") into the spoilboard so the bottom veneer is cut through fully.
- Cut in two passes on the outside of each part: a roughing pass that leaves about 0.5 to 1 mm (0.02" to 0.04") and a light finishing pass.
- Cut small inside features first, then the outside of the part, so the sheet stays held while the details are cut.
- If a stubborn face still chips, apply painter's tape along the cut line before cutting, or score the line with a shallow first pass.
Cleaning up what is left
Light fuzz is normal and sands off in a few seconds with 120 to 180 grit paper. Do not sand parts that need to fit: sanding changes the thickness and the joint. Run a fit test first if you are not sure.
Frequently asked questions
Why does my plywood chip on the top face?
Usually an up-cut bit is lifting the veneer, or the bit is dull. Use a sharp compression or down-cut bit and check the feed rate.
How do I stop chipping on the bottom face?
Use a compression bit and cut about 0.5 mm (0.02") into the spoilboard so the bottom veneer is cut through fully.
Can I sand away the fuzz?
Yes, light fuzz sands off with 120 to 180 grit paper. Do not sand the faces of joints that have to fit.