It may be remarked, upon examining a file, that the first cut is always made more slantwise than the second. If this were riot done, the small teeth would all lie behind one another, in rows in the direc tion of the length of the file, which would make corresponding grooves in the face of any piece of work that might be to be filed, instead of leaving the workman at liberty to vary his strokes, as is necessary when a flat surface is to be produced. " When the file is cut and finished on both sides, and on one or both edges, as may be required, it is ready for harden ing, which is a chemical operation of some skill and ingenuity. The heat is given in a furnace, where the work can be regularly disposed, and for fine work a muffle is used. The file is first exposed to a low degree of ignition, which burns off any greasy or other matter that might adhere to its surface. It is then dipped, cold, in the grounds or thick sediment of beer, and while wet, into a powder made of burned or parched horn, or leather, or other coally animal matter, and of com mon salt, and in this state speedily dried by exposure to heat. Any other muci lage, which could be afforded at a mode rate price, would probably answer the same purpose as the beer grounds. The file being then put into the ignited muffle, smokes, and soon becomes red hot, being not only defended from oxydation, by the covering of fused salt and animal coal which envelopes it on all sides, but be ing even rendered more steely upon its surface by the absorption of carbon. As soon as it has acquired the low red heat called cherry-red, it is taken out and_ plunged into pure cold water, which in stantly cools it, and renders it very hard. There are several variations adopted in the hardening process by different workmen, by means of which they differ in their success. Some file-makers, as
well as gunsmiths and locksmiths, pro duce the intended effect so completely, that the whole surface of their work has a beautiful dull-grey aspect, every where alike ; whereas other operators produce coally spots, which are obliged to be cleaned off. The files, when quite dry and clean, are slightly oiled, and kept in oiled paper.
The simple operation of file-cutting seems to be of such easy performance, that it is not at all to be wondered at, that machines for this purpose should have been very early invented. Mathurin Jousse, in "La Fidelle Overture de l'Art de Serrurier," published at La Fleche, in Anjou, in 1627, gives a drawing and de scription of one, in which the file is drawn along by shifts by wheel-work, and the blow is given by a hammer, which is tripped by the machinery. There are several in the "Machines Approuvees par PAcademie Royale de Paris ;" and one in the "American Transactions ;" and a patent was granted a few years ago, for improvements in the art, to the editor of this work.
The principal requisites in a machine for file-cutting are, that the file should be steadily supported, and the chisel adapted to the face, without any unequal bearing. Files are, however, for the most part, cut by hand ; and the chief reasons are, 1. The cut by hand is, from its very nature, exactly of the depth the bur demands; whereas, in a machine, if the stroke be not nicely adapted to the shift, the file may be either shallow cut, or its bur may be thrown too close by an over heavy stroke ; and 2. In machine cut files, there must always be a piece left at the beginning, at each corner, which re quires to be cut off before hardening. This may be remedied the machinery, but it has not yet been done.