Why is my circular saw burning and scorching the wood?
Six causes, in the order to check them: a blunt or resinous blade, too many teeth for the depth, a slow feed, a blade out of parallel with the guide, a bent plate, and a kerf closing on the blade.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- caution
Short answer
Clean the blade first — a coat of hardened resin makes a sharp blade behave exactly like a blunt one and it is the most common cause by a distance. Then check the tooth count is coarse enough for the thickness, that you are feeding steadily rather than creeping, and that the shoe is running parallel to your guide rather than skewing the blade in its own kerf.
A saw blade converts almost all the energy it uses into heat, and the wood carries that heat away in the chips. Anything that stops chips being produced — a dull edge, a packed gullet, a slow feed — leaves the heat in the timber instead, and the timber scorches. Every cause of burning is a version of that one mechanism.
Safety
Step by step
- Isolate the saw and look at the blade.Unplug it or remove the battery. A blade coated in brown or black lacquer-like deposit behind the teeth is resin-loaded. Pine, spruce and anything with knots do this quickly, and so does MDF.
- Clean the blade.Soak it in proprietary blade cleaner, or in a strong solution of washing-up liquid or oven cleaner, then scrub the deposit off with a brass brush. Dry it immediately and wipe with a light oil to stop flash rust. A cleaned blade very often cuts like new.
- Feel the teeth with a fingernail — carefully, blade off.Draw a nail lightly across the top of a tooth from behind. A sharp carbide tip catches; a blunt one feels rounded and lets the nail skate. Chipped or missing tips mean the blade is finished.
- Check the tooth count against the job.A 48- or 60-tooth blade ripping through 45mm construction timber will burn no matter what you do — there is not enough gullet space to clear that much waste. Fit a 24-tooth blade for thick timber and keep the fine blade for sheet.
- Feed faster, not slower.Creeping through a cut is a common instinct and it is exactly wrong. Each tooth needs to take a real bite; if it only rubs, all the energy becomes heat. Feed at a rate where the motor note drops slightly but does not labour.
- Check the shoe is parallel to your straightedge.If the saw is being steered rather than guided, the blade skews in its own kerf and the plate rubs on the kerf walls. Keep the shoe pressed flat against the guide through the whole cut and let the guide do the steering.
- Check the blade runs true.With the saw isolated, spin the blade slowly by hand and sight along it. A visible wobble means a bent plate, a chip of debris trapped behind the flange, or a bent arbor. Clean the flanges first, then replace the blade.
- Check the workpiece support.If the kerf is closing on the blade, the plate rubs on both sides at once. Support the work on both sides of the cut line and make sure the riving knife is present and correctly set.
- Check the depth setting.Blade set far deeper than the material means more of the blade is buried in the cut than needs to be, more heat and more friction. Set it so the teeth clear the underside by about one tooth.
Tips
- Burn marks are surprisingly deep. Sanding them off usually means going down at least a millimetre, which can spoil a fitted piece. It is cheaper to fix the cause and recut.
- A burnt edge will not take stain or glue evenly — the scorched surface is case-hardened and seals. Remove it before finishing or gluing.
- If you are cutting a lot of resinous softwood, clean the blade at the end of the session rather than when it starts misbehaving.
If it doesn't work
Burn marks only on one face of the cut
Cause: The blade is not parallel to the direction of travel — a skewed shoe, or a bent plate — Fix: Clamp a straightedge and keep the shoe hard against it. If it persists with a guide, spin the blade by hand and check for run-out.
Burning starts part way through a long rip
Cause: Stress in the board is closing the kerf behind the blade — Fix: Stop, wedge the kerf open behind the blade with a scrap of the same thickness, and continue.
Smoke and a very hot blade even in thin material
Cause: Blade fitted backwards, or resin-loaded — Fix: Isolate and check the arrow on the blade matches the guard, then clean the blade.
Motor bogs down and the cut burns
Cause: Too many teeth for the depth, or a blunt blade — Fix: Fit a coarse blade with large gullets for thick stock, or replace the worn blade.
Burning in hardwood only
Cause: Dense timber, especially cherry, maple and beech, scorches at a much lower threshold than softwood — Fix: Sharp blade, faster feed, and accept a light skim with a plane or a sander afterwards — some hardwoods will always mark slightly.