Broaching practice

For many years, broaching has been employed for cutting keyways and machining holes to a variety of shapes, but this method attracted comparatively little attention until extensive developments began to take place in the machine building field, and especially in connection with the manufacture of automobiles. While machines have been used for broaching for many years, they were not in common use until about 1900, when the automobile business developed rapidly. Then broaching began to replace other machining processes in the production of certain duplicate parts, because of the accuracy and efficiency of the broaching method, especially for finishing holes of irregular shapes. At the present time the broaching machine is used extensively, not only by automobile manufacturers but by companies building a large variety of other products.

This treatise covers the three important elements in broaching practice; namely, the broaching machines, the design of the broaches or cutting tools, and the application of the broaching process to commercial work. Examples of broaching were obtained from plants using broaching machines for conducting regular manufacturing operations. Since these examples include many classes of work and represent a great diversity of requirements in regard to size and shape, they not only indicate the possibilities of this process but show just how this efficient method of machining can be applied under different conditions. Some of these examples from practice were previously published in Machinery and the author is indebted to the contributors of these articles as the material thus obtained made it possible to prepare a more comprehensive treatise.

Prior to the period of intense activity in American industry, which was created by the war, broaching had demonstrated its practicability in many different manufacturing plants throughout this country and abroad; but the urgent demand for increased efficiency in the manufacture of munitions of war led to many improvements in broaching and to the application of this method in lines of work that had formerly been handled by slower and more expensive methods. As a typical example of this kind, the rifling of gun barrels on broaching machines may be mentioned. Formerly it required from twelve to sixteen hours to machine the grooves in a barrel, but when this work was put on broaching machines, the rate of production obtained was twelve barrels per hour. Another important factor in this connection was the fact that the operation of a broaching machine is so simple that unskilled labor could be utilized, which became a matter of great importance when the exigencies of war-time had greatly increased the demand for all classes of skilled factory employers.

The advantages of the broaching process are speed, interchangeability 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 hardened in the same way may not cut 2000. While broaching is chiefly applied to interior work, exterior work is also being successfully done, and one of the possibilities is the broaching of spur gears when the quantity of duplicate gears is large.

Principles of Process. Briefly defined, the process in internal broaching 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.

Pull and Push Broaching. 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. 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.

Example of Broaching. 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. 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 on 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.