The Tooth and the Kerf: Why a Saw Cuts a Slot Wider Than Itself

Hold a hand saw with the teeth pointing away from you and sight straight down the line of them, like looking down a rifle. The teeth are not in a single line — they zigzag, one leaning left, the next right. Now look at the blade from the side. The body of the blade is thinner than the width that zigzag spans. That difference is the whole reason a saw works: the teeth cut a slot wider than the steel behind them, so the steel runs free. A blade with no set at all will cut for a centimetre and then seize solid, because the wood closes on it. The exceptions prove the point: a Japanese pull saw has very little set and gets away with it because it cuts on the PULL, so the blade is in tension and cannot buckle.

Lay a rule on the blade and count the teeth in one inch. A coarse rip saw has four or five; a tenon saw has fourteen; a hacksaw blade for thin tube has thirty-two. Fewer, bigger teeth each take a bigger bite, so the saw cuts fast and leaves a rough edge. More, smaller teeth cut slowly and leave a clean one. There is no setting that is simply better — the choice is the same speed-against-finish trade the sharpening grit progression makes, and for the same reason. The other thing TPI decides is what the saw can cut at all, which the hacksaw rung takes up: too few teeth for the material's thickness and the teeth straddle it, catch, and snap off.

Because the kerf has width, the blade must run entirely on the WASTE side of your line. Cut down the middle of the line and the piece ends up half a kerf short. Decide which side is waste before the saw touches the wood, and mark it — a scribble of pencil on the offcut side takes a second and is unambiguous. On a cutting list where several pieces come from one board, the waste side alternates, and this is exactly where people lose a piece. The disciplined version is to leave the line visible on the piece you are keeping. That is the same rule as filing to a line, and for the same reason: material can come off afterwards, and it cannot go back on.