A steel instrument employed for shaping or giving a smooth surface to articles made of metal, bone, wood, etc. The varieties of files are very extensive, being expressly adapted to numerous different trades or branches of manufacture. There is, however, an immense variety of files which are applicable to general purposes; these are distinguished by the terms of flat, half-round, three-square, four-square, round, hand, pillar, cant, and other technical names, which denote their transverse sectional shape; then each of these shapes may be single or double cut; that is, the notches or incisions made upon them may consist of only one series of parallel lines, technically called floats, or of two series, in which the lines or incisions cross each other diagonally. Then again, either of these latter may be of different degrees of coarseness or fineness, denominated in the trade, rough, bastard, second-cut, and smooth; the latter term indicating teeth so fine and close as to produce upon metal a surface nearly smooth, and requiring only the aid of the burnisher to polish it.

It is then to be understood, that each of these classes and varieties, or most of them, are made of lengths differing from two or three inches, up to twenty inches long.

Even these are not all; there are also a numerous class of rasps, which have jagged teeth, and are chiefly used in working bone and hard woods; also rubbers, which are great, heavy square files, used by smiths and others; and a peculiar class of heavy files, chiefly used by millwrights and engineers, forming a medium between rubbers and files of ordinary thickness; besides a great variety of extremely delicate files of the best steel, used by watchmakers and others. When, therefore, it is considered that the file manufacture thus embraces several thousand distinctions, and that many thousands of families are constantly employed in their fabrication in the neighbourhood of Sheffield, Birmingham, and other places, an idea of the extent and importance of it may be formed. The steel employed for files is required to be very hard, and, in consequence, undergoes a longer process in the conversion, and is said to be double converted. The very heavy files are made of the inferior marks of blistered steel; those for sharpening the teeth of saws, and the more delicate kinds, are made of cast steel. The steel is previously drawn at the tilt hammer into rods of a suitable size. The flat and the square files are made wholly with the hammer and the plain anvil.

Two workmen, one called the maker the other the striker, are required in the forging of heavy files, the smaller being forged by one person only. The anvil is provided with a groove for the reception of bosses or dies, which are used for the purpose of forging the half-round and three-angled files. The half-round boss contains a hollow, which is the segment of a sphere, less than half a circle. That used for the three-angled files has a hollow consisting of two sides, terminating in an angle at the bottom. In forging the half-round file, the steel is drawn out, as if intended to make a flat file, it is then laid in the die and hammered till the underside becomes round. The steel for the triangular files are tilted into square rods. The part to form the file is first drawn out with the hammer, as if intended to form a square file; it is then placed in the die with one of the angles downwards, and by striking upon the opposite angle, two sides of the square are formed into one, and consequently a three-sided figure produced, which is perfected by successively presenting the three sides to the action of the hammer. In forming the tangs of most files, it is necessary to make the shoulders perfectly square and sharp.

This is performed by cutting into the file a little on both sides with a chisel, and afterwards drawing out the part so marked off to form the tang. After forging, and previous to being ground and cut, the files require to be annealed. This process is generally performed by piling up a great quantity together in a furnace for the purpose, and heating them red hot, suffering them afterwards to cool slowly. This method of annealing files, and indeed any other articles in which great hardness is requisite, is very objectionable, since the surface of steel, when heated red hot in the open air, is so liable to oxidation. A superior method of annealing is practised by some file-makers; and since hardness in a file is so essential a property, the process ought to be generally adopted. This method consists in placing the files in an oven or trough, having a close cover, and filling up the interstices with sand. The fire is made to play on every side of the vessel, as gradually and as uniformly as possible, till the whole mass becomes red hot. The fire is then discontinued, and the whole suffered to cool before the cover is removed from the trough.

Steel annealed in this way is perfectly free from that scaly surface acquired in the open air; and if each corticle be perfectly surrounded with the sand, and the cover not removed before the steel is cold, the surface will appear of a silvery white colour. If the steel be suspected to be too kind, from containing too little carbon, powdered charcoal may be employed instead of sand, or sand mixed with charcoal. In this case the files should be stratified alternately with the charcoal, in order that the extra conversion may be uniform. The next thing is to prepare the files for cutting, by making the sm-face to contain the teeth as level as possible. This was formerly effected by files, and the process is called striping. The same is still practised by the Lancashire file-makers (who excel in the manufacture), and by others not having the convenience for grinding. The greatest quantity of files are, however, ground, to prepare them for cutting. The stones employed for this purpose at Sheffield are of a compact and sharp texture, of great diameter, and about eight inches broad over the face. When used, the surface is kept immersed in water; the grinder sits in such a position as to lean over the stone, whilst its motion is directed from him.