Why Is My CNC Burning the Wood?

Why wood burns

Wood scorches somewhere around 200–300°C depending on species and resin content. A router bit reaches that easily when it stops shearing and starts sliding.

When each flute takes a proper bite, most of the heat generated leaves the cut inside the chip — the chip is the heat sink. When the bite is too thin, you produce dust instead of chips. Dust carries almost no heat away, so the energy stays at the cutting face and goes into the workpiece and the tool. The wall of the cut scorches, and once a bit is coated in scorched resin it rubs even more.

Two other causes produce the same mark for a different reason, and both are worth checking before you touch your feeds:

• Dwell. Any moment the bit is spinning but not advancing — a pause at a corner, a plunge before the feed starts, the machine decelerating into a tight arc — is pure friction. Burns concentrated at corners and the start of a cut are almost always dwell, not chipload.
• A dull or resin-loaded bit. A blunt edge cannot shear no matter how good the arithmetic is. Cherry, maple and resinous softwoods build up pitch on the flutes, which insulates the edge and makes it worse each pass.

The number that matters: chipload

Chipload = Feed rate (IPM) ÷ (RPM × number of flutes)

A worked example. 18,000 RPM, 80 IPM, 2-flute bit:

80 ÷ (18,000 × 2) = 0.0022″ per flute — under half what a 1/4″ bit in hardwood should be taking. That bit is polishing the wood, not cutting it.

Corrected, at the same 18,000 RPM with a 2-flute 1/4″ bit targeting 0.006″: 18,000 × 2 × 0.006 = 216 IPM. If your machine cannot feed that fast, lower the RPM instead — chipload depends on the ratio, so 12,000 RPM at 144 IPM gives the same 0.006″.

Target chiploads by material

Per flute, from LMT Onsrud 2025 catalog data for the 57-200 two-flute downcut wood rout series:

These are for one series. Manufacturers publish materially different figures for different geometries — a compression bit's chart is not an O-flute's chart, and Amana's numbers are not Onsrud's. Use the data for the bit you actually own; every bit we sell lists its own on its product page, and the Cut Doctor reads the series off your bit rather than applying a generic table.

Fixing it, in order

1. Check for dwell first. If the burns are at corners, at the entry point, or where the toolpath slows, no feed change will fix them. Add ramping or lead-ins, reduce sharp direction changes, and make sure the spindle is not sitting in the cut before motion starts.
2. Calculate your actual chipload with the formula above or the feed rate calculator.
3. Raise the feed rate into the band for your bit and material. This is the highest-impact single change.
4. If the machine cannot feed fast enough, drop the RPM. Most burning happens at high spindle speed with a timid feed.
5. Use fewer flutes. Each extra flute divides the chip. A 2-flute bit at the same feed and speed takes twice the bite of a 4-flute.
6. Clean the bit. Pitch and resin build-up insulates the edge. A resin-loaded bit burns even at correct settings.
7. Replace a dull bit. Hardwood, MDF and particleboard are abrasive; carbide dulls faster than people expect, and a rounded edge cannot shear.
8. Limit depth per pass to around one bit diameter so chips clear instead of recirculating and re-cutting.

Bits that resist burning

Browse hardwood router bits and composite wood bits, or use the Bit Finder to match a bit to your material and finish.

Common questions

Why does my CNC burn only on corners?

That is dwell, not chipload. The machine decelerates into the corner, so the bit keeps spinning while forward motion drops toward zero — pure friction. Add corner rounding or lead-in moves in CAM, or reduce the aggressiveness of the acceleration settings so the tool keeps moving.

Does a downcut bit burn more than an upcut?

It can, because a downcut pushes chips into the slot rather than lifting them out, and trapped chips get re-cut and generate heat. Downcut is still the right choice when the top face must stay crisp — just keep the depth per pass modest and clear the slot with air.

Which woods burn most easily?

Cherry, maple and hard white woods scorch readily and show it clearly. Resinous softwoods such as pine load the flutes with pitch, which then causes burning even when settings are correct. Dense hardwoods simply generate more heat per unit removed.

Will a sharper bit fix burning on its own?

Sometimes, if the bit was genuinely dull or gummed up. But if the chipload is far too low, a brand-new bit will burn too — it will just take slightly longer. Check the arithmetic before spending money on tooling.

Chipload ranges are published manufacturer application data for the series named, not a generic table. Manufacturers differ — sometimes substantially — for the same material and diameter, so use the figures for the bit you own. Start conservative, verify on scrap, and adjust for your machine's rigidity and hold-down.