File, a steel tool, having its surface covered with teeth, and used for smoothing and shaping metals, &c. Files are classified and named according to their shape, size, and fineness of cut, and the purpose for which they are made. Thus, in shape they may be flat, square, round or rat-tail, triangular, half-round, feather-edged, &c., besides being variously bent, in order to get at intricate work. Most files are made thicker in the middle, or 'bellied,' a form which best suits the hands in the act of filing. Files generally are made of the best crucible steel, the forged and shaped blanks when ready for cutting being held upon an anvil by means of a long loop of leather-strap, into which the cutter places his foot. The face of the anvil is covered with a flat piece of pewter, the softness of which preserves the teeth on one side of the file when those on the opposite face are being struck. The cutter forms the teeth by striking with a hammer a short, stout chisel, held obliquely at an angle of about 12° or 14° from the perpendicular. If the chisel were perpendicular, a furrow like the letter V would be indented, and an equal burr struck up on each side; but as a cutting tooth somewhat like that of a saw is required, this is effected by the oblique stroke of the chisel, by which a burr is thrown up on one side only—viz. towards the tang. The cutting is commenced at the point of the file; the chisel is then drawn backwards, laid upon the blank, and slid forwards till it reaches the burr raised by the last cut; the blow is now struck, and another tooth and burr produced, which serves as a guide for the next cut; and so on. The distance between the teeth thus depends on the force of the blow and the obliquity of the cut; for the heavier the blow, the greater the ridge or burr, and the obliquity determines the distance of the cut from the burr. The skill of the workman consists, therefore, in the precise regulation of the blows.
Files are either single cut—i.e. made with a single series of parallel cuts—or, more commonly, double cut—i.e. they have two series or courses of chisel-cuts, which are oppositely inclined at an angle of about 55° to the central line of the file. The second course is made in the same manner as the first, but with lighter blows, and is usually somewhat finer than the first. This angular crossing converts the ridges into pointed teeth. Files used for soft metals which are liable to clog the teeth are single cut—i.e. they have but one course of cuts. Taper files have the teeth finer towards the point. Rasps for wood or horn, &c. are cut with triangular punches, each tooth being an angular pit with a strong burr, instead of a long furrow. The rapidity with which the blows are struck varies with the fineness of the file: sixty or eighty cuts are commonly made per minute. Classified according to fineness of cut, files are known as rough, bastard, smooth, and superfine. A superfine file of small size may have as many as 216 cuts per inch, while a large rough file may have not more than twenty-one cuts to the inch. The sharpness and abrading effect of files is greatly improved by subjecting them after cutting to a sand-blast, whereby a slight recurvature of the burr, which is always present as the file leaves the cutter, is corrected, and the edge is rendered very keen.
Files have to be very carefully hardened and tempered. If heated too strongly, or made too hard, the steel is so brittle that the teeth tear off; if too soft, they wear down rapidly, and the file soon becomes useless. Great care is also required in keeping them straight, as the sudden cooling necessary for hardening is very apt to warp the steel.
Many attempts have been made to cut files by machinery, but with only partial success; the chief difficulty arises from the necessity of modifying the force of the blow to suit the hardness of the steel. It is practically impossible to supply a large number of blanks all of exactly the same hardness; and if the machine be adjusted to suit the hardness of one blank, it may strike too heavy or too light a blow for the next; whereas the workman feels at once the hardness of the steel he is working upon, and adjusts his blows accordingly.