Four operations, in a fixed order, and a set of angles that matter less than doing the same thing sixty times.
A saw does not cut with an edge in the way a chisel does. It cuts with a row of small, identical tools, each of which has to take the same bite as its neighbours. Where a saw cuts badly, the usual cause is not bluntness but inconsistency: a few tall teeth doing all the work while the rest ride along.
That is why the order below is fixed. Every step exists to make the teeth the same as each other before the last step makes them sharp.
The first decision is what the saw is for, because it sets every angle that follows.
Rip teeth cut along the grain. Each tooth is a tiny chisel, filed square across the plate, and it severs fibres by paring them end-on. Crosscut teeth cut across the grain. Each tooth is filed to an angle — the fleam — so that its leading edge becomes a knife that scores the fibres either side of the kerf before the waste is lifted out.
Tooth count is quoted in points per inch, counted including both ends, and is one more than teeth per inch. Coarse teeth cut fast and leave a rough kerf; fine teeth cut slowly and cleanly and are far easier to start.
| Saw | Points | Rake | Fleam |
|---|---|---|---|
| Rip, coarse | 4–6 ppi | 0–4° | 0° |
| Rip, fine / tenon | 10–14 ppi | 4–8° | 0° |
| Crosscut, panel | 8–12 ppi | 12–15° | 15–25° |
| Dovetail | 14–20 ppi | 8–15° | 0° |
Rake is how far the tooth's front face leans back from vertical. Zero rake is maximally aggressive and will grab on the first stroke; relaxing it a few degrees costs very little speed and makes a saw that starts where you put it. On a saw that is fighting you, rake is usually the thing to change.
Run a flat mill file along the tooth line, held square to the plate, until every tooth has been touched. Each one now carries a small bright flat on its tip, and those flats are the measurement for everything that follows: a big flat is a tall tooth, no flat is a short one. The tooth line should end up straight, or with a very slight convexity — breasting — which helps a long saw start.
With a triangular taper file, deepen the gullets so that every flat disappears at the same moment. This is where uneven teeth become even. On a badly abused saw it can take two or three cycles of jointing and shaping before the line is honest, and it is worth the patience: shaping is the step that is actually fixing the saw.
Bend the top third of each tooth outward, alternately left and right, so the kerf ends up wider than the plate and the saw does not bind. Only the top third — set applied down into the gullet cracks teeth off. A plier-type saw set does this in one squeeze per tooth.
Aim for a kerf about one and a half times plate thickness; on a typical hand saw plate of thirty to forty thousandths, that is a few thousandths of set per side. Less set than you think is almost always right. Too much set makes a saw that wanders, throws a ragged kerf and needs real effort to push.
Clamp the plate in a saw vice with the gullets just proud of the jaws, or the whole thing will chatter and the file will skate. Work along the saw filing alternate gullets, then turn the saw round and do the ones you skipped from the other side — that alternation is what produces the left-and-right knife edges on a crosscut saw.
Hold the file at the rake angle you chose, at the fleam angle you chose, and count strokes. Three strokes per gullet, the same three, all the way along. Counting is the whole technique: a saw filed consistently at the wrong angle cuts well, and a saw filed inconsistently at the right angle does not cut at all.
Saw files are cheap and they are consumables — a dull file burnishes rather than cuts and will polish a tooth into uselessness. Match the file to the tooth: the face of the file should be a little over twice the depth of the gullet, so a 6" slim taper for around 8 ppi, an extra-slim for finer work. One light chalking of the file stops it clogging.