Files are special cutting tools worked by hand. They present flat or rounded surfaces of steel, uniformly covered with small teeth. These have been produced by repeatedly striking the steel with a special chisel so as to raise regular burrs of sharp metal. File-cutting by machinery is attended by many difficulties that have not yet been completely overcome. The temper of the file must be accurately adjusted; if too hard the teeth are brittle, and tear off; if too soft the teeth flatten down, and the file is quickly rendered useless.
Files are classified according to their shape, size, and cut. The ordinary shapes are too familiar to need description. Single-cut files have their teeth cut in rows of one direction only; double-cut files have in addition another series of parallel rows intersecting the first series, and made of somewhat smaller teeth. The fineness of cut ranges from 20 to 250 teeth to the inch. The teeth are sharpened by the process of sand-blasting (q.v.)