Why is my router burning and scorching the wood?
Five causes of a scorched router cut — pausing, feeding too slowly, a resinous or blunt cutter, too high a speed for the bit diameter, and a seized bearing — plus how to remove burn marks that are already there.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
Short answer
Keep the router moving. Burning is a heat problem, and heat comes from a cutter rubbing rather than cutting: pausing mid-cut, creeping along, a blunt or resin-coated bit, or too high a speed for a large-diameter cutter. Clean the bit first — it is the most common cause — then speed up your feed and slow the router down if the bit is big.
Every burn mark in a routed edge records exactly where the cutter spent too long in one place. The mark is not superficial: scorched timber is case-hardened, it will not take stain or glue evenly, and removing it means removing real material. Fixing the cause is far cheaper than fixing the symptom.
Step by step
- Check where the burns are.Burns at one spot mean the router paused there. Burns along the whole cut mean a general feed, bit or speed problem. Burns at the start and end of a cut mean hesitation entering and leaving. A single burnt stripe at a fixed distance from the edge usually means a seized bearing.
- Clean the bit.Resin builds up behind the cutting edge on every bit used in softwood or MDF and it makes a sharp bit behave exactly like a blunt one. Soak in blade or resin cleaner, scrub with a brass brush — never steel, which damages carbide — dry, and wipe with a light oil.
- Check the bearing spins freely.A bearing that has seized, or that is packed with dust and resin, stops rolling and starts rubbing. It then burns a polished stripe down the workpiece edge. Spin it with a finger before every job; replace it if it drags, and never oil it with anything that will attract dust.
- Feed faster and keep moving.The instinct to slow down for a better finish produces exactly the opposite result. Move at a rate that produces chips rather than fine dust, and never stop with the cutter in the work. If you need to reposition your hands, take the router right off the end of the cut first.
- Slow the router down for a big bit.Rim speed rises with diameter. A large panel or moulding bit at full speed generates far more heat than the timber can carry away, and burns however fast you feed. Use the router's speed dial: large diameter, lower speed.
- Take lighter passes.A deep pass leaves the chips nowhere to go, they get recut, and recut chips are pure heat. Several light passes with a final skim burn far less.
- Check the bit is sharp.Blunt carbide has a rounded edge that compresses the fibres instead of shearing them. If a cleaned bit still burns and needs pushing, it has had its life — man-made board wears bits out fast.
- Watch for burns in specific timbers.Cherry, maple, beech and birch scorch at much lower temperatures than pine. In those timbers, plan on a very light final pass and accept that a sharp bit and a brisk feed are not optional.
- Remove existing burns properly.Scorching penetrates further than it looks. A card scraper or a very light final router pass removes it cleanly. Sanding works but takes a lot of material and rounds over the crisp arris the router just produced.
Tips
- A card scraper is the ideal tool for taking a burn out of a routed profile, because it follows the shape and removes very little material.
- Do not sand a burn out of a joint face you are about to glue — the scorched layer is sealed and the glue will not key to it. Cut or scrape it away.
- If the router leaves fine dust rather than chips, the feed is too slow. Chips are the sign of a cutter that is cutting.
If it doesn't work
Burn in one place, in an otherwise clean cut
Cause: The router stopped there — usually a hand repositioned, or a clamp reached — Fix: Plan the whole pass before starting so it can be made without stopping. Move clamps out of the router's path first.
A polished, burnt stripe along the workpiece edge
Cause: The bit's guide bearing has seized and is rubbing rather than rolling — Fix: Clean or replace the bearing. Check it spins freely before every session.
Burning that starts after several metres of clean cutting
Cause: Resin building up on the cutter as work proceeds, especially in pine and MDF — Fix: Stop and clean the bit mid-job. On a long run, expect to clean once or twice.
Burning with a large moulding bit at any feed rate
Cause: Rim speed too high for the diameter — Fix: Turn the router's speed down. Large bits are designed to run considerably slower than small ones.
Burning in the corners of a profiled edge only
Cause: The router slowing or pivoting as it turns the corner — Fix: Keep the base flat and the speed constant through the corner, or profile end grain first and long grain last so any damage is removed by the later passes.