A fresh, sharp bit
A dull edge rubs and burns even at correct feeds. When the numbers are right and it still scorches, a sharp cutter is often the fix.
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Why is my CNC burning or scorching the wood?
Brown or black scorch marks along the cut, most often on the inside of curves or anywhere the machine slowed to change direction. The wood is not on fire - it is being cooked by a cutter that is rubbing instead of shearing, so the heat stays in the cut.
Quick answer. Burning means the chip load is too low: the bit is rubbing, generating heat it cannot carry away. The fix most beginners get exactly backwards is to feed faster, not slower - a thicker chip takes the heat out with it. Dropping the spindle RPM does the same thing by thickening the chip at the same feed.
Work down the list, most likely first
These are ordered by how often they are the answer, not by severity. Check each in turn and stop at the first one that confirms.
- 1
Chip load too low (the bit is rubbing, not cutting)
Confirm it's this
Burns concentrate where the feed dropped - inside curves and direction changes - and the instinct to slow down makes them worse. A too-thin chip cannot carry heat, so it stays in the wood.The fix
Feed faster, or lower the spindle RPM so the same feed produces a thicker chip. Both raise the chip load and pull heat out in the swarf. This is the single most common wood-cutting mistake and the fix is genuinely counter-intuitive. - 2
Spindle RPM too high for the feed
Confirm it's this
You are running the spindle near its top speed and the feed rate cannot keep up, so each tooth shaves a sliver.The fix
Lower the RPM to bring the chip load up at a feed your machine can actually hold. Use the calculator to see the chip load your RPM-and-feed combination produces. - 3
Dull bit
Confirm it's this
Burning has worsened over the life of the bit and the edge looks rounded. A dull edge rubs even at correct feeds.The fix
Replace the bit. A sharp cutter shears cleanly where a dull one burnishes and scorches. - 4
Machine can't hold the feed the chip load needs
Confirm it's this
The numbers say feed faster, but the machine loses steps or chatters when you do. The real limit is motion, not the wood.The fix
Address the motion limit (acceleration, rigidity) so you can run the feed the chip load requires - see losing steps and chatter. On a very light machine, a smaller cutter lets you reach the needed chip load at a lower feed.
Stopping it coming back
- Set feeds from a chip-load target, not from what sounds gentle.
- Keep bits sharp - a tired bit burns hardwood first.
- Avoid toolpaths that repeatedly slow at tight features; add lead-ins or increase corner speed where the machine allows.
Parts this fix may need
The consumables a fix like this usually calls for. Prices are live; we may earn a commission on a purchase, which never affects what we recommend.
In stock
Frequently asked
- My CNC is burning the wood - should I slow down?
- No, and this is the mistake almost everyone makes. Burning means the chip is too thin and the bit is rubbing; slowing down makes the chip thinner still and burns more. Feed faster, or drop the spindle RPM so the chip thickens at the same feed. The chip has to be thick enough to carry the heat out of the cut.
- Why does the burn appear only on curves?
- Because that is where the machine slows to change direction, dropping the feed - and with it the chip load - right at that spot. The cutter starts rubbing through the corner and scorches. Raising the overall chip load, or letting the machine carry more speed through corners, removes the local burn.
Keep going
Diagnostic patterns are distilled from manufacturer documentation and established tooling references; where the community converges on a real-world tell, we state it. Exact feeds and speeds always come from the calculator, never invented here.