Troubleshooting · Marks on the workbeginner~3 min read

Burning: why CNC cuts scorch and how to stop it

Short answer

Burning is friction, and friction comes from chips that are too thin to cut. Feed faster, or lower the RPM. Slowing the feed down, which is the instinct, makes it worse.

Symptom

Brown or black marks on the cut surface or edge, sometimes with a smell of scorching or visible smoke. Often worse in corners, in tight detail, and in deep cuts.

Check
first

Work out your chipload. Feed rate divided by (RPM times flutes). Rule of thumb If it comes out under about 0.002 inches per tooth on anything bigger than a 1/8 inch bit, that is your answer and nothing else on this page matters until you fix it.

Most likely causes

Chipload too low: feed too slow, or RPM too highMost common
A thin chip cannot be cut, so the edge rubs and turns cutting energy into heat. This is the cause in the large majority of cases. See chipload.
Dull bitCommon
A dull edge needs more force and makes more friction. If settings that used to work now burn, suspect the bit before anything else.
Chips staying in the cutCommon
Recut chips add heat without removing material. Deep slots, downcut bits in deep pockets, and a blocked dust shoe all do this. See chip evacuation.
Too much tool engagementCommon
A full width slot or a heavy stepover means more of the tool is rubbing for longer. Reduce depth per pass first.
Material that burns easilyOccasional
Hard maple and cherry scorch at settings that pine tolerates. See hardwoods.
The machine slowing into cornersOccasional
Feed drops at every direction change while RPM stays constant, so the chipload collapses exactly there. This is why burning often appears in corners first.
Resin buildup on the bitOccasional
Pitch from softwood makes the bit effectively dull. Clean it with a resin remover.

Diagnostic steps

Look at the waste. Fine powder means the chipload is far too low. Real chips mean look elsewhere.
Touch the bit after stopping. Warm is fine. Too hot to hold means heat is the problem.
Look at where it burns. Only in corners and detail: the machine is slowing down there. Everywhere: settings. Only deep in the cut: chip evacuation.
Inspect the cutting edge under magnification, or just feel it carefully. A dull or chipped edge is visible once you know to look.
Run a test slot with the RPM dropped one step and nothing else changed. If the burning improves, you have confirmed the chipload.

Fixes, in the order to try them

Lower the RPM. Free, instant, and it raises the chipload without asking more of the machine.
Increase the feed rate, if the machine and the cut can take it.
Change to a sharp bit. If in doubt, put a known-good bit in and see.
Use fewer flutes. A single flute halves the feed needed for the same chip. See flute count.
Reduce the depth per pass so less of the tool is buried and chips clear more easily.
Improve chip clearing. Upcut bit, better extraction, air blast.
Add a light finishing pass so the final surface is cut fresh rather than rubbed.

Do not do the obvious thing

The reflex when something is burning is to slow the feed rate down. That makes each chip thinner, which increases rubbing, which increases heat. It is the single most common way people make a burning problem worse.

Why it happens

A cutting edge is a wedge with a tiny radius at its tip. To cut, it must bite into material thicker than that radius. Below that, it cannot get under the fibres, so it skids across them: pressing, compressing and heating rather than slicing. All the energy that should have gone into making a chip goes into friction instead, and wood scorches at a temperature a router reaches easily.

Prevention

Calculate the chipload before a job in an unfamiliar material rather than after it goes wrong.
Keep sharp bits for finishing and retire dull ones rather than persevering.
Empty the dust separator before long jobs so extraction does not quietly fade.
On light machines, prefer a single flute bit and a lower RPM over a high feed rate you cannot actually reach.