What a File Actually Is: Rows of Tiny Chisels

Put a file under a loupe with the light raking along it. Each tooth is a tiny wedge with a cutting edge across the top and an open space behind it — the gullet — where the chip goes. The teeth all lean the same way: forward, toward the tip. That is why a file cuts on the PUSH and does nothing on the pull, and why lifting it on the return stroke costs nothing but saves the teeth. Dragging a loaded file backwards under pressure drags trapped chips across the work and scores it. The other thing visible under the loupe is that the teeth are in rows running diagonally, not straight across. A straight row would cut a groove its own width; the diagonal makes each tooth land on fresh metal.

SINGLE CUT has one set of parallel diagonal rows. Each tooth is a long straight edge, it takes a wide shallow chip and leaves a smooth surface — this is what a finishing file and a saw file use. DOUBLE CUT adds a second set of rows crossing the first, which breaks the long edges into a grid of small pointed teeth. Each takes a small deep chip, so it removes metal far faster and leaves a rougher surface. Most general-purpose hand files are double cut. A RASP is neither: its teeth are individually raised punched points, widely spaced, with huge gaps between them. That makes it useless on metal and ideal on wood, where the chip is bulky and would instantly clog a file.

File teeth are hardened to around 65 HRC and left untempered, which makes them harder than nearly all working steel and also extremely brittle. Two consequences. First, a file will cut mild steel, brass, aluminium, cast iron and most tool steel in the annealed state, but it skates uselessly on anything hardened — a hardened blade, a bearing race, a file. That is in fact the classic workshop hardness test: if a file bites, the part is soft. Second, never use a file as a lever or a pry bar and never strike it. It will snap without bending, and it does so suddenly. Fit a handle before using one: the bare tang is a spike aimed at your palm, and it is the single commonest workshop injury with this tool.