FILE, a bar of steel having sharp teeth on its surface, and used for abrading or smoothing hard substances. Some uncivilized tribes polish their weapons with such things as rough stones, pieces of shark skin, or fishes' teeth. The operation of filing is recorded in 1 Sam. xiii. 21; and, among other facts, the similarity of the name for the filing instrument among various European peoples points to an early practice of the art. A file differs from a rasp (which is chiefly used for working wood, horn, and the like) in having its teeth cut with a chisel whose straight edge extends across its surface, while the teeth of the rasp are formed by solitary indentations of a pointed chiseL According to the form of their teeth, files may be single-cut or double-cut; the former have only one set of parallel ridges (either at right angles or at some other angle with the length); the latter (and more common) have a second set cut at an angle with the first. The double-cut file pre-sents sharp angles to the filed surface, and is better suited for hard metals. Files are also classed according to fine-ness of teeth, being known (in order of increasing fineness) as rough, bastard, second-cut, smooth, and superfine or dead smooth. The shapes of files present almost endless varie-ties. Common forms arethe flat file, of parallelogram section, with uniform breadth and thickness, or tapering, or " bellied"; the four-square file, of square section, some-times with one side " safe," or left smooth; the so-called three-square file, having its cross section an equilateral triangle, the half-round file, a segment of a circle, the round or rat-tail file, a circle; these are generally tapered. The float file is like the flat, but single-cut. There are many others. Files vary in length from three-quarters of an inch (watchmakers') to two or three feet and upwards (engineers'). The length is reckoned exclusively of the spike or tang which enters the handle. Most files are tapered; the blunt are nearly parallel, with larger section near the middle; a few are parallel. The rifflers of sculptors and a few other files are curvilinear in their central line.
Cast-steel is the material chiefly used for files, though the larger and rougher varieties are sometimes made from blister-steel. In manufacture, the blanks are forged from bars that have been tilted or rolled as nearly as possible to the sections required. They are then annealed with great care, and when sufficiently softened are taken out, straight-ened, if necessary, with hand hammers, and then rendered clean and accurate in form by filing or grinding. They are now ready for cutting. In this process, as performed by hand, the cutter sits before a square stake or anvil, on which the blank, slightly greased, is held (having its tang towards him) by means of two leather straps passed round its ends, and held fast below, one by each foot. He holds in his left hand a short chisel (the edge of which always exceeds the width of the file), placing it on the blank with a slight inclination from him, and beginning near the further end. He strikes the chisel sharply with a hammer, an indentation is thus made, and the steel, slightly thrown up on the side next the tang, forms a ridge. The chisel is then transferred to the uncut surface, and slid from the operator till it reaches the ridge just made ; thus the posi-tion of the next cut is determined; the chisel is again struck, and so on. (The end part of the file is dealt with separately.) The workman seeks to give his blows as uniformly as possible. Sixty to eighty cuts are made in one minute. After finishing the first course of cuts, he proceeds, if the file is to be double-cut, to make the second course, the cuts of the latter being generally somewhat finer. Thus the surface is covered with teeth inclined towards the point of the file. If the file is flat and is to be cut on the other side, it is turned over, and a thin plate of pewter placed below it to protect the teeth. Triangular and other files are supported in grooves in lead. In cutting round and half-round files, a straight chisel is applied as tangent to the curve. The round face of a half-round file requires eight, ten, or more courses to complete it.
The file is next hardened. Being first covered with a paste to protect the teeth from the direct action of fire, <fcc. (e.g., passed through beer-grounds to make it sticky, then through a mixture of common salt with roasted and pounded cow's hoof), it is heated to an even red heat, and then suddenly cooled by plunging in cold water or brine. It is removed before cooling throughout, that it may be straightened if necessary (which is done by pressure). Then it is cooled in oil. The tang is next submitted to a soften-ing process, and the file, after being wiped, and the teeth brushed clean, is ready for fixing into the handle and for use.
In England files are chiefly made in Sheffield and Warrington, those of the latter place being generally known as Lancashire files. It is remarkable that while many other operations that appear more difficult than file-cut-ting are now effected by machinery, and while numerous file-cutting machines have been invented, the work con-tinues (in England) to be largely done by hand. This is perhaps partly due to strong opposition on the part of operatives to introduction of machinery, and also to a foolish prejudice in favour of hand-cut files (machine-cut files, indeed, are not unfrequently sold as hand-cut), but probably also to the problem of cutting files by machinery being really somewhat difficult. In most of the machines invented for that purpose, the idea has been to construct an iron arm and hand to hold the chisel, and a hammer to strike the blow, and so to imitate, as closely as possible, the manual process. Bernot's machine is an improvement on this. In it the blow is given by pressure of a flat steel spring pressing on the top of a vertical slide, at the lower end of which the chisel is firmly fixed. The slide is acted on by a cam making about one thousand revolutions a minute, and the chisel consequently strikes that number of blows per minute. The vibration is thus lessened. The history of file-making by machinery in America (where it has been extensively practised) seems to indicate that much of the failure experienced has been due to the fatal defect in the machinery used, of producing extreme regu-larity in the teeth. This gives rise to complaints by artisans about files "running in grooves," "chattering," &c. The grooves produced by the file (if double-cut especially) at the beginning of its movement are deepened as it is moved further. With irregular teeth, on the other hand, such as are found in all hand-made files, the grooves made in the first instance have their sides cut away as the file is advanced. The Nicholson File Company, in Provi-dence, Bhode Island, have used, with large and increasing success, a machine which imitates, to some extent, this irregular result of the hand process, cutting the file so that no two spaces are found exactly alike in the entire length.
The filing of a flat surface perfectly true is the test of a good filer; and this is no easy matter to the beginner. The piece to be operated upon is generally fixed about the level of the elbow, the operator standing, and, except in the case of small files, grasping the file with both hands, the handle with the right, the further end with the left. The great point is to be able to move the file forward with pressure in horizontal straight lines ; from the tendency of the hands to move in arcs of circles, the heel and point of the file are apt to be alternately raised. This is partially compensated by the bellied form given to many files (which also counter-acts the frequent warping effect of the hardening process, by which one side of a flat file may be rendered concave and useless). In bringing back the file for the next thrust it is nearly lifted off the work. Further, much delicacy and skill are required in adapting the pressure and velocity, ascertaining if foreign matters or filings remain interposed between the file and the work, &c. Files can be cleaned with a piece of the so-called cotton-card (used in combing cotton wool) nailed to a piece of wood. In draw-filing, which is sometimes resorted to to give a neat finish, the file is drawn sideways to and fro over the work. New files are generally used for a time on brass or cast-iron, and when partially worn they are still available for filing wrought iron and steel.
An interesting application of the sandblast to sharpen- ing the teeth of files has recently been made by Mr. Tilghman. (A. B. M.)