Files are planing implements for making plane surfaces upon pieces of work which have been roughly planed with a planing machine. The rough square file denoted by Fig. 324 is a quicker cutter when compared with the flat file shown by Fig. 325, because a smaller amount of cutting surface is applied with a square file than with a flat one, but a flat file is conveniently used after a square one, if the surface being filed is large enough, because the surface is easier planed with a large amount of cutting surface acting at one time than with a small amount. For some work, files are used upon a surface after it has been chipped with chisels; for such work, a square file is always the preferable one to commence with, after which, a flat one is effectual for taking off the ridges formed by the square file. The weight of any particular rough file selected for use should be according to the power of the operator, whether a boy or a man, whether a strong man or a weak one; and to make all files as light as possible, a side of a thick square file is made to possess about half the cutting surface of a thin flat file whose weight equals that of a square one. Any file which has one of its sides without teeth, is termed a safe-side file; such a file is used when it is necessary to put it into contact with a surface that must not be filed, or which has been filed and must not be further reduced. The safe-side of a file is that which is rubbed along in contact with a surface which requires protection, while another surface at right angles to the protected one is being reduced by the filing. In cases of emergency it is often necessary to make a safe-side to a file that does not possess one, and this is done by grinding off all the teeth from the intended safe-side by means of a grindstone.

To file slide-valve faces, cylinder faces, surface-tables, and other faces whose distance across is much greater than the length of a file, an appropriate method must be adopted for holding the files; for such work, holders and cranked files are used. Holders are of two principal shapes, that are indicated by Figs. 326 and 327 ; in Fig. 326 a dovetail groove is shown, into which the small end of the file named the tang is tightly fitted; this holder may be attached to the tang while straight, and the holder used with a wood handle of the ordinary shape. The holder shown in Fig. 327 is much simpler and more effectual, but requires the tang to be bent, to raise it a convenient distance from the surface which is to be filed. Another mode of raising the handle is shown by Fig. 328, and consists in cranking the tang, by which means no other handle is required than the ordinary wood handle.

Fig. 329 represents a file which is bent to a curve ; this bending may be performed on any flat file whether rough or smooth, and is effected by heating the file to redness throughout the entire length of the part which is to be curved; while thus heated, it is laid across a wood block having a hollow of proper width and depth, and a wood hammer is applied with a number of gentle blows which are given to the entire length of the portion to be bent. If a number of files are to be curved, a broad wood fuller should be made, and a corresponding bottom block, the fuller and block being curved to suit the files. Bent files are necessary for filing concave surfaces, and the curve to which the file is bent is always much less than that of the surface to be filed, to permit a comfortable handling of the tool. Bent files are consequently useful for the inner boundaries of steam-cylinders, air-pumps, packing-rings, lids having dishes, brasses, and guides.