file, in hardware and metalworking, tool of hardened steel in the form of a bar or rod with many small cutting edges raised on its longitudinal surfaces; it is used for smoothing or forming objects, especially of metal. The cutting or abrading action of the file results from rubbing it, usually by hand, against the workpiece.
Files are classified according to their cross-sectional shapes, the form of the cutting edges, and the coarseness of the cut (i.e., the number of teeth per inch or centimetre). There are at least 20 different cross-sectional shapes; the most common are rectangular with various width-to-thickness ratios, square, triangular, round or rattail, and half round. There are three general classifications of tooth form: single-cut, double-cut, and rasp. The single-cut file has rows of parallel teeth cut diagonally across the working surfaces. The double-cut file has rows of teeth crossing each other. Rasp teeth are disconnected and round on top; they are formed by raising small pieces of material from the surface of the file with a punch. Rasp files, or rasps, are usually very coarse and are used primarily on wood and soft materials.
Classification according to coarseness or spacing of the teeth is confined to single- and double-cut files. There are six main classes: rough, coarse, bastard, second-cut, smooth, and dead smooth. The number of teeth per inch varies considerably for different shapes and sizes.