Cotter files are always narrower than hand files of the same length and thickness; they are nearly flat on the sides and edges, so as to present almost the same section at every part of their length, in which respect they vary from 6 to 22 inches. Cotter files are mostly used in filing grooves, for the cotters, keys or wedges, used in fixing wheels on their shafts, whence their name. The taper cotter files, or as they are also called entering filet, are entirely different from the above, as they are taper both in width and thickness, and almost without any swell, or pyramidal, in which respect alone they differ from ordinary taper files that are usually much swelled or bellied.
Pillar files, also somewhat resemble the hand files, hut they. are much narrower, somewhat thinner, as in C, and are used for more slender purposes, or for completing works that have been commenced with the hand files. Pillar tiles have commonly one safe edge, and vary from 3 to 10 inches in length.
Half round files, are nearly of the section L, notwithstanding that the name implies the semicircular section; in general the curvature only equals the fourth to the twelfth part of the circle, the first being called full half round, the last flat half round files. The half round files, vary from about 2 to 18 inches in length, and are almost always taper. The convex side is essential for a variety of hollowed works, the flat side is used for general purposes.
Tringular files, commonly misnamed "three-square" files, are of the section R, and from 2 to 1 (J inches long; they are used for internal angles more acute than the rectangle, and also for clearing out square corners. One of the greatest uses of triangular files from 3 to 6 inches long, is the sharpening of saws, the greater number of which have teeth of the angle of 60 degrees; an angle doubtless selected, because it appertains to all the angles of the equilateral triangular file, the three edges of which are therefore alike serviceable in sharpening saws. In the southern parts of England, saw-files with single-cut teeth, are in more general use, from the idea that they "cut tweeter;" in the midland and northern counties, the double-cut tiles of the same dimensions are more in vogue, being esteemed more durable. Small saws for metal, which are harder than those for wood, are always sharpened with double cut files, the Lancashire being preferred.
Cross files, or crossing files, sometimes called double half-rounds, are of the section M, or circular on both faces, but of two different curvatures, they are used for concave or hollowed forms the same as the convex side of the half-round; but crossing files are on the whole shorter and less common than half-round files, and are probably named from the files being used in filing out the crosses of arms or small wheels, as in clockwork, in which case the opposite sides present a two-fold choice of curvature in the same instrument, which is convenient. Those cross files which are principally known as double half-rounds, are fuller or more convex on both faces than ordinary cross-files, and are employed by engineers.
Round files, of the section I, range from the length of 2 to 18 inches; they are in general taper, and much used for enlarging round holes. The round file is better adapted than the so-called half-round file, to works the internal angles of which are filled in or rounded, as the round file is much stronger than the half-round of the same curvature. Small taper round files, are often called rat-tail files, and the small parallel round files, are also called point files, as they are used in filing the hollows in the joints of snuff-boxes and similar objects, for the reception of the pieces of joint wire (vol. i. page 429), that are soldered in the hollow edges of the work for the joint pin or axis.
Square files, are used for small apertures, and those works to which the ordinary flat files are from their greater size less applicable. The square files measure in general from 2 to 18 inches long, and are mostly taper; they have occasionally the one side safe or uncut.
Equalling files, are files of the section D; in width, they are more frequently parallel than taper, in thickness they are always parallel. They are in general cut on all faces, sometimes, as in the warding files for locksmiths, the two broad surfaces are left uncut or safe, and they range from 2 to 10 inches long.
Knife files, are of the section T, and in general very acute on the edge, they are made from 2 to 7 inches long, and are as frequently parallel as taper. The knife files are used in cutting narrow notches, and in making the entry for saws, and for files with broader edges; knife files are also employed in bevilling or chamfering the sides of narrow grooves.
Rubbers, are strong heavy files generally made of an inferior kind of steel, they measure from 12 to 18 inches long, from 3/4 to 2 inches on every side, and are made very convex or fish-bellied; they are frequently designated by their weight alone, which varies from about 4 to 15lbs. Rubbers are nearly restricted to the square and triangular sections A and R. Some few rubbers are made nearly square in section, but with one side rounded, as if the sections K and B were united, these are called half thick. Rubbers are scarcely ever used by machinists and engineers, but only for coarse manufacturing purposes, where the object is rather to brighten the surface of the work, than to give it any specific form. Rubbers were formerly made only of bar or common steel, but are now also made of cast-steel, and in a more careful manner.
Many artizans, and more particularly the watchmakers, require other files than those described, and it is therefore proposed to add the names of some of the files to which the sections refer, premising that such names as are printed in Italics, designate small files especially used in watchmaking.