Broaches are cutting tools for enlarging holes by rotating either the broach or the piece of work to be broached. A broach is a straight piece of steel that cuts by edges which are formed along its sides, but does not cut at the end in the manner of a drill. A broach is used to enlarge a hole, or smooth it, after it has been formed by drilling; and it is rotated by either a wood handle, a spanner, or a brace. Both parallel holes and taper ones may be made by broaches, and for small holes, not exceeding an eighth of an inch in diameter, a broach may be fixed in an ordinary file handle shown in Fig. 351. When it is desired to make a hole smooth and parallel, a broach is used which is thickest at about a quarter of its length from the point, and it is rotated until the thickest portion has passed entirely throughout the length of the hole, by which means the hole is made parallel, and also straight, if the broach did not bend while in use. To avoid as much as possible the tendency to bend, it is necessary to make the broach only that length which is sufficient for the work ; and when a long one is requisite, it is rotated with only a gentle pressure while finishing a hole. Broaches for taper holes are denoted by Figs. 351, 352, 353, and 354. The three-cornered broach shown by Fig. 351 is a quick cutter for small holes ; and the five-cornered one shown by Fig. 352 is used for larger holes, and rotated with a spanner or brace. A superior cutter for taper holes is a half round taper tool shown by Fig. 353 ; and for smooth and straight taper holes, a broach having one flute is employed; this is denoted by Fig. 354. A good tool for parallel holes is shown by Fig. 355 ; this has three fluted grooves and also a short taper part at the point, to allow the tool to be easily entered into the work to be broached. Another excellent tool for making parallel straight holes is indicated by Fig. 356 ; this one possesses five grooves, and also a short taper portion for introducing the thick part of the tool to the work.

Fig. 357 represents a broach which may be used for both parallel holes and taper ones. The tool consists of a cylindrical piece of steel into which two grooves are planed, each groove being opposite the other; in one groove is tightly fitted a steel cutter, which is shown by C ; this cutter is forced out to any desired distance by means of the four small screws shown by S, and the cutter is made to produce either parallel holes or taper ones, according to the amount of inclination of the cutter to the cylindrical part of the tool. To ensure a good hole, it is necessary that the cutting edge of the cutter should not extend more than an eighth of an inch from the groove, and that the edges of the opposite groove in which are the adjusting screws should be smoothly curved.

The heads of broaches may be either square or round, and adapted to be used with a spanner having two handles, while the work to be broached is in a vice, or the heads of the broaches may be fitted to lathes and drilling-machines, and thereby rotated instead of being used by hand. When it is necessary to broach a hole so that it shall be larger in diameter than the broach, a smooth piece of sheet brass or copper is put to one side of the tool, and both the copper and broach are together rotated and passed through the hole.