The flat and square files are formed entirely by the ham mer. One man holds the hot bar, and strikes with a small hammer. Another stands before the anvil with a two-hand ed hammer. The latter is generally very heavy, with a broad face for the large files. They both strike with such truth as to make the surface smooth and flat, without what is called hand-hammering. This arises from their great experience in the same kind of work. The expedition ari sing from the same cause is not less remarkable.
The half-round files are made in a boss fastened into the groove above-mentioned. The steel being drawn out, is laid upon the rounded recess, and hammered till it fills the die.
The three-sided files are formed similarly in a boss, the recess of which consists of two sides, with the angle down wards. The steel is first drawn out square, and then placed in the boss with an angle downwards, so that the hammer forms one side, and the boss two. The round files are formed by a swage similar to those used by common smiths, but a little conical.
The whole of the working part of the file is formed and finished with the hammer before it is cut off from the rod. The finished part is then held in tongs, and heated a second time to form the tang of the file. • The very square shoulder formed by the tang of a file, does not seem easy to form by the hammer. This is effec ted by first placing the file upon a sharp-edged tool, stand ing with its edge upwards in the anvil ; a notch is now made on each side where the tang commences. It is then brought to the front edge of the anvil, and, by an acquired dexterity, the tang is drawn out without touching the shoul der with the hammer.
In order to prepare the files for cutting, they require to have.the surface perfectly metallic, smooth, and as even as possible. The state, however, in which the files leave the hammer, is too hard for the dressing and cutting. The first thing to be done, therefore, alter forging, is to soften the files by a process called annealing. This was formerly, and by many is still, performed by surrounding a close mass of the files with coals, keeping up the fire till the whole mass become red hot, and allowing them to cool gradually. In this process the files become softened, but the surface becomes so oxidated, that a stratum of considerable thick ness peals off. This scale, however, is very hard, and is
removed but with difficulty. This last is not the greatest evil attending this process ; the surface of the steel lying immediately under the oxide, must have partly lost its pro perty of steel. Indeed it is now known, that, by a similar process, steel, and even cast iron, can be converted into pure iron. It will be obvious, that, by the oxidation which takes place, the part which has to form the teeth of the file will be much impaired by the abstraction of its carbon. Hence it will forciby strike any one, that steel, particularly in this instance, should he annealed in close vessels, to ex clude the oxigen. This has been accomplished to a par tial extent by some manufacturers, but still requires more minute attention. The annealing should be performed in troughs of fire-stone or fire-brick, similar to the cavities in which steel is converted, having the flame of a furnace playing on every side, and over the top. The trough should be filled with alternate strata of the files to be annealed, and coal-ashes, or the dust of the coaks, formed in the forge-hearth. The upper stratum of files should be co vered with a thick stratum of the dust, and lastly with a mixture of clay and sand. The heat should be kept up no longer than till the mass will become red hot, quite through. The whole must now be suffered to cool. When the files are withdrawn, instead of being scaled as in the old me thod, they will exhibit a metallic surface, and the substance will be much softer than by the common annealing.
It should be here observed, that the mass to be heated should not be more than one foot in thickness, as it would he so long in heating and cooling, that the metal would put on the crystalline form, under which it is too brittle to form a cutting edge.
We have before observed, that the steel requires high conversion for files. This will evidently become unne cessary with this mode of annealing. The surface of the files, which is the principal part, will become converted in an extra degree, by using more carbon in the annealing, and thus make steel, of common conversion, sufficiently hard for files.