A file is a cutting tool for finishing surfaces to a desired smoothness and shape. Files are made in many forms, but must employ a harder material (usually steel) than the objects, or workpieces, they are designed to finish. They may be flat, round, half round, triangular with cutting surfaces on three sides, square with cutting surfaces on four sides, or tapered.
The working surfaces of files have numerous points or cutting edges. For extremely fine work the cutting points are shallow and close together. For coarse work they are much rougher, with deeper valleys and greater separation of the scraping elements. Files that are used as hand tools and are 10 inches (25 cm) long or more, exclusive of the tang, or handle end, are classified, in decreasing order of roughness, as rough, coarse, bastard, second cut, smooth, and dead smooth. File elements may also be used in certain machine tools.
For extremely coarse work, files known as rasps are used. These have raised points that cover the working surface, each point acting as a cutting element. Rasps are used for removing large quantities of material from relatively soft materials such as wood, leather, and lead.
For removal of large amounts of material with minimum scratching, files with milled curved teeth are manufactured. These files have teeth that are crescent-shaped and parallel to one another running from the blunt end of the file to the tang.
The object to be filed should be firmly held, as in a vise. The worker, holding the file by its tang or handle and pressing down with the other hand at the free end, pushes the file away for the cutting stroke. During the return stroke no harm is done by letting the file slide back over the workpiece without exerting pressure. The workpiece should be roughed down with a coarse file and then finished with a finer file.
In the manufacture of conventional steel files, steel stock is usually hot-rolled to the approximate width and thickness of the finished files and cut into appropriate lengths. The resulting blanks are reheated and forged or hammered into the desired form: flat, half round, or triangular. The tang is formed at the same time. After the forged blanks are annealed (heated to make them less brittle), they are cooled and cleaned and the surfaces ground to a fine finish. On the smooth working surface the teeth are machine-cut (milled) or ground to produce a specified surface such as single-cut, double-cut, or combinations. In multiple-cut files the cut lines intersect at angles. The file is then hardened.
In the hardening process, the file, while carefully protected, is brought to the proper quenching temperature for the carbon content of the steel. The file is then quenched, or rapidly cooled, usually in water or oil, to develop the hard working surface necessary. It is then cleaned.
Rasps are made by forcing a multiple-point gouge die under pressure into the annealed surface of a blank. This distorts the surface into a myriad of low spots. The corresponding number of raised points constitute the cutting edges of the rasp. Heating, quenching, and cleaning of the rasp complete the process.
Cheap mass-produced files can be made by merely pressing a superhard die into annealed steel stock to produce in one step the type of file tooth desired. With carefully made dies, thousands of files can be pressed before the die loses its sharp contours. Such files must be hardened to ensure a satisfactory working life.