The cutting of keyways and machining of holes in metal to shapes other than round by broaching is an old practice, but one that has attracted  comparatively little attention until within the past ten years. While machines were built and used for broaching they were not in common use until about 1900, when the automobile business developed rapidly. Now the broaching process is used very extensively, not only by automobile manufacturers but by many other concerns. While relatively large work is being broached at the present time, the trend of practice indicates that much larger and heavier parts will be machined in this manner when the quantity of work warrants the necessary investment in machines and tools. The broaching machine is also becoming a recognized means of cutting external shapes that are readily machined with standard tools. The reason for this practice is the high rate of production and low cost.

The advantages of the broaching process are speed, interchange ability of work, adaptability to irregular forms, employment of comparatively unskilled labor, and adaptability to a great variety of work. The chief disadvantage is the high cost of broaches and the uncertainty of their life. One broach may cut 20,000 holes while another made of the same steel and tempered in the same manner may fail before 2000 are cut. While chiefly applied now to interior work, exterior work is also being successfully done, and one of the possibilities is broaching spur gears when the quantity of duplicate gears is large.

In preparing this treatise on broaching, many examples from practice have been included, and we desire to acknowledge our indebtedness especially to the J. N. Lapointe Co., and the Lapointe Machine Tool Co. for much practical information relating to modern broaching methods.

The broaching process consists in machining holes in castings or forgings by drawing or pushing through the rough cored or drilled hole one or more broaches having a series of teeth which increase slightly in size from one end of the tool to the other, and successively cut the hole to the required form. Broaching is especially adapted to the finishing of square, rectangular or irregular-shaped holes. It is also applicable to a wide variety of miscellaneous work, such as the cutting of single or multiple keyways in hubs, forming splines, cutting teeth in small internal gears and ratchets, etc.

There are two general methods of broaching: One is by pushing comparatively short broaches through the work, usually by means of a hand press, a hydraulically operated press, or an ordinary punch press. With the other method, a special broaching machine is used, and the broach, which is usually much longer than a "push broach", is pulled through the work by means of a screw forming part of the machine. Push broaches must necessarily be quite short to prevent excessive deflection; consequently it is often necessary to force several broaches through the work. The longer broaches which are pulled through in regular broaching machines commonly finish parts in one passage, although a series of two or more broaches are often used for long holes, or when considerable stock must be removed. The number of broaches ordinarily used varies from one to four. Comparatively short broaches are sometimes used, because they are easier to make, are not warped excessively in hardening and are easier to handle. Two or more parts can frequently be finished simultaneously on a regular broaching machine, the pieces being placed one against the other, in tandem.

A simple example of broaching by drawing the broach through the work is illustrated by the diagrams, Fig. 1. A square hole is to be broached in the hub of a gear blank, this being a sliding gear (such as is used in automobile transmissions) that is to be mounted upon a square driving shaft. Prior to broaching, a hole is drilled slightly larger in diameter than the width of the square. The starting end of the broach, which at first is detached from the machine, is passed through the drilled hole in the blank, which rests against the end of the broaching machine. The end of the broach is then fastened to the "pull bushing" by a key A (which fits loosely to facilitate its removal), and the machine is started. By means of a powerful screw the broach is drawn through the hole in the gear blank and this hole is gradually cut to a square form by the successive action of the teeth which increases in size 0.002 or 0.003 inch per tooth. The process is illustrated by the enlarged diagrams at the top of the illustration.

The first few teeth take broad circular cuts which diminish in width so as to form a square-shaped hole. Of course, it will be understood that for cutting a hexagonal, round, or other form of hole, a broach of corresponding shape must be used. The blank to be broached does not need to be fastened to the machine, but is simply slipped onto the broach or a work bushing, in some cases, in a loose manner. As soon as the broaching operation begins, the work is held rigidly against the end of the machine or fixture when the latter is used.

From the preceding description of the broaching process, it will be seen that the function of the broaching machine is to draw the broach, through the work at the proper speed.