The file is a strip or bar of steel, the surface of which is cut into fine points or teeth, that act by a species of cutting, closely allied to abrasion. When the file is rubbed over the material to be operated upon, it cuts or abrades little shavings or shreds, which from their minuteness are called file-dust, and in so doing, the file produces minute and irregular furrows of nearly equal depth, leaving the surface that has been filed more or less smooth according to the size of the teeth of the file, and more or less accurately shaped, according to the degree of skill used in the manipulation of the instrument. In treating this subject, it is proposed to divide the matter into the following sections: -

I.

General and descriptive view of files of usual kinds.

II.

General and descriptive view of files of less usual kinds.

III.

Preliminary remarks on using files, and on holding works that are to be filed.

IV.

Instructions for filing a flat surface, under the guidance of the straight-edge, and of the trial-plate, or planometer.

V.

Instructions for originating straight-edges and trial-plates, or planometers.

VI.

Instructions for filing rectilinear works, in which several or all the superficies have to be wrought.

VII.

Instructions for filing curvilinear works, according to the three ordinary modes.

VIII.

Comparative sketch of the applicatious of the file, and of the engineer's planing machine, etc.

The files employed in the mechanical arts are almost endless in variety, and which is to be accounted for by there being some four, five, or six features in even' file, that admit of choice, in order to adapt the instrument to the several kinds of work for which the file is used; and most of the names of files express these different features, for instance the three following files are in common use: -

6 inch,

9 inch,

12 inch, blunt, taper, parallel, single-cut, smooth, rough,

Sheffield,

Lancashire,

Sheffield, safe-edge, saw-file, half-round-file, cotter-file.

From the perusal of these compounded names it will be seen that six sources of variation have been noticed, and upon which several characters a few observations will be offered.

1. Length. - The length of files is always measured exclusively of the tang or spike, by which the file is fixed in its handle, and the length and general magnitude of the file require to be proportioned to the work to be performed. When the works are both large and coarse, the file should be long and strong, that the operator may be able to exert his entire muscular force in using the instrument; when the works are minute and delicate, the file should be proportionally short and slender, so that the individual may the more delicately feel the position of the file upon the work; as the vigorous employment of force, and the careful appreciation of position or contact, are at opposite extremes of the scale. Thus, it may be said, the watchmaker frequently uses files not exceeding three quarters of an inch in length, and seldom those above 4 or 5 inches long; artisans in works of medium size, such as mathematical instrument makers and gunmakers, employ files from about 4 to 14 inches long; and machinists and engineers commonly require files from about 8 to 20 inches long, and sometimes use those of 2, 3, feet and upwards in length.

The lengths of files do not bear any fixed proportion to their widths; but, speaking generally, it may be said the lengths of square, round, and triangular files, are from 20 to 30 times their widths, measured at the widest parts; and the lengths of broad files, such as flat files, half-round files, and many others, are from 10 to 12 times their greatest widths.

2. Taper, blunt, and parallel files. - Almost all files are required to be as straight as possible in their central line, and are distinguished as taper, blunt, and parallel files; a very insignificant number of files are made curvilinear in their central line, as in the rifflers used by sculptors and carvers, and some other files.

The great majority of files are made considerably taper in their length, and to terminate nearly in a point, such are called taper files; others arc made nearly parallel, and known as "blunt pointed," or simply as blunt files; but in each of these kinds the section of the file is the largest towards the middle, so that all the sides are somewhat arched or convex, and not absolutely straight. A very few files are made as nearly parallel as possible, and have, consequently, nearly straight sides, and an equal section throughout; such are designated as parallel files, and by some, as dead parallel files, just as we say "dead level" for a strictly level surface, but it is very far more general for the so-called parallel files to be slightly fuller in the middle.

8. Lancashire and Sheffield files. - In England the principal seats of the manufacture of files, are Sheffield and Warrington; those made at the latter place being more generally designated as Lancashire files. The Sheffield files are manufactured in very much the larger quantity, and for nearly every description of work, both large and small. The Lancashire files are less used for large than for small works, including watch and clock-work, some parts of mathematical instruments, and the finer parts of machinery.

Formerly all the Lancashire files bore a great pre-eminence over the Sheffield, in respect to the quality of the steel from which the files were made, their greater delicacy of form, the perfection and fineness of their teeth, and the success with which they were hardened; these circumstances rendered the Lanca-shire files more expensive, but also much more serviceable than the Sheffield. Of later years, this superiority is generally considered more particularly to apply to the smaller Lancashire files, not exceeding about 8 or 10 inches in length, as from tin-steady improvement amongst the best of the Sheffield file manufacturers, in respect both to the quality of the steel, and the workmanship, it now results, that the larger files made both in Lancashire and Sheffield, assimilate much more nearly in their respective qualities than formerly.

4. The teeth of files. - Many files that are in all other respects alike, differ in the forms and sizes of their teeth. Three forms of teeth are made, those of double-cut files, those of floats, or single-cut files, and those of rasps. The floats and rasps are scarcely used but for the woods and soft materials; the double-cut files are used for the metals and general purposes; and when the file is spoken of, a double-cut tile is always implied, unless a single-cut tile, or a rasp, is specifically named.