File Manufacture

FILE MANUFACTURE. Like every other steel tool of modern manufacture, the file has been improved greatly in quality by metallurgical and engineering science. The industry has been more especially developed in Sheffield and the United States, but it is also carried on in Switzerland, Germany, Sweden, France and Italy.

There is a great variety of patterns, sizes and cuts of files. They are made from bars whose cross-sectional area may be either round, square, flat, triangular (three-square), half-round--i.e., with one face flat, the back of the file being curved—and a large number of other shapes made for special purposes. Each of these shapes may be made in various lengths, from the tiny watch makers' and jewellers' file, to the heavy files used by the engineer -20 inches and upwards in length. The sawmaker uses a "three square" file, cut on the edges for sharpening hand-saws, but for the handsaw the edge of the file must be thicker and rounded, to minimize the risk of breaking the band saw tooth. The flat file with one or two rounded edges is used generally for sharp ening circular saw teeth. The locksmith uses a thin flat file varying in thickness, the jeweller has standard shapes drawn to a very fine point, while the dentist uses a very fine pointed, solid-handled, half-round file. The "rasp," with its large, sharp, single teeth is used for wood, hot iron and soft metal; different varieties are the shoe maker's rasp, the horse rasp for farriers and rasps for cabinet-makers, workers in bone and other similar materials. Each shape and size of file is made in a variety of cuts, the different cuts are known as:— rough, middle, common or bastard, second, smooth, dead-smooth and double-dead smooth.

Processes of Production.

The metal used for the manufacture of files must be hard enough to cut steel, and at the same time of a quality to give maximum durabil ity under rough treatment; the file teeth must be hard without being brittle. The steel used for the purpose is, therefore, of very good quality with high carbon content. Such a steel, however, in its normal con dition is a hard and difficult one to manipulate. The six chief processes in the production of the file consist of :—forging, an nealing, grinding, cutting, hardening and scouring.

Forging and Annealing.—The steel is first received from the rolling mill where it has been rolled to the desired section, and is cut into suitable lengths. These are then placed, a few at a time, in a furnace and heated to a bright red heat. They are then withdrawn separately and hammered under a machine hammer. The first operation of the hammer is to form the "tang" or sharp handle-end of the file, which is later to be inserted into the handle. The hammer is provided with shaping tools or dies to suit the special section of steel to be dealt with. The file is then returned to the furnace and the remaining part of the forging operation is completed. The forged file is known as a "blank." The power hammer employed consists of a top tool or "tup" and a solid base, into which is fixed the shaping die or tool. These hammers are designed to deliver from 25o to 500 blows per minute according to the size of the file required. The "blanks" from the hammer must now be brought to a ductile and uniform condition for the purpose of cutting the teeth. They are, therefore, put into an annealing furnace and heated gradually to the desired temperature. They are held at that heat, and then cooled. This treatment pro duces a change in the physical condition of the steel, removing all internal strains and rendering the material suitable for the subsequent operation of the cutting of the teeth.

Grinding.—The object of grinding is to produce a smooth, even surface on the face of the blank, and to remove the scale from the surface to be cut. This operation is largely carried out by machinery. A grinding wheel is mounted in a machine which is made to rotate at a suitable speed, and in addition an oscillating motion is imparted to the axle of the wheel to prevent grooves being worn into the face of the wheel. The files are fixed in a trough in the base of the machine, and are completely covered by water. The trough automatically moves to and fro under the grinding wheel, and provision is made for the grinding wheel to be lowered or raised to grind the files. Special arrangements are made for grinding half-round and round files.

Cutting.—The operation of forming the teeth in files is known as "file-cutting," and is now almost entirely a machine operation. Formerly the operation was performed by hand with flat chisels, and the process became an art in which the skilled file-cutter attained great efficiency. The file-cutting machine is designed to reproduce by mechanical means the processes adopted by the file-cutter, except that the number of cuts per minute made by the machine is enormously greater than is possible by hand. The file blank is placed on a lead bed made to the shape of the file to be cut. The chisel employed for cutting the grooves in the blank has a straight sharp edge and is sufficiently long to overlap the file blank. The edge is ground and shaped to the desired angles, whetted, and the sharp edge rubbed off. The grooves are cut across the face of the blank at an angle to the side of the file, which varies for different materials. Files are made either with a single series of cuts, known as "single-cut" files, or with a double series of cuts, crossing one another, known as "double cut" files. The single cut files are used for the softer materials such as brass, lead and wood, and the double cut file is the one commonly used in machine shops. The crossing of the face of the file by the two series of cuts placed at different angles with one another, as described, has the effect of breaking up the long rows of single teeth, stretching across the width of the file, into a large number of separate small pointed teeth each presenting its sharpened point to the surface of the work to be cut. (See figure for the single cut file, the double cut file and the rasp.) Hardening and Scouring.—On the completion of the cutting op eration the files undergo a process of hardening. The teeth of the file are first protected from injury by a special plastic covering, and the file is then heated to the desired temperature for high carbon steel files. It is then quenched by dipping into a brine bath out of which it is brought before the centre of the file is cold. Whilst in this condition, that is before complete cooling, the distorted files can be straightened. After hardening, the file is subjected to a process of passing it over a sand-blast jet, through which steam and sand are forced at about 7o lb. pressure. This treatment leaves a perfectly clean surface on the finished file. Files are tested in the works by passing, by hand, a "prover" which consists of a hard piece of steel, over the teeth of the new file, the effect of which must be to leave the teeth undamaged. Further tests of files are made in file-testing machines, which have been designed to measure the amount of material which the file is capable of removing from a standardized test-bar during a given number of strokes of the file, and at a standard pressure upon the file.

Care of the File.

Many files are impaired by being im properly used. A very sharp file, for example, should not be used on an oxidized surface of cast iron, a surface which is itself as hard as the file teeth. A partially worn file should first be used for the purpose, and the new file for finishing the work only. Care should also be taken to avoid excessive pressure when using a sharp new file. In using a file for cutting, just enough pressure should be put upon it during the forward stroke to make it cut freely, and the file should be slightly lifted from the work during the return stroke. Files should be cleaned after use and put away carefully, not thrown among other metal tools.

(W. R.)