The type of broach cutting tool required for a given job is the single most important factor in determining the type of broaching machine to be used. Second in importance is the production requirement. Taken together, these factors usually determine the specific type of machine for the job.

The type of broach tool (internal or surface) immediately narrows down the kinds of machines that could be used. The number of pieces required per hour, or over the entire production run, will further narrow the field.

For example, a dual-ram machine with one operator may be chosen over two single-ram machines requiring two operators, to provide higher output per man-hour. The single operator can load one table of a dual-ram unit while the other ram is cutting. Even higher production requirements may dictate a continuous chain horizontal machine. The machine size in a particular model is a function of the tool size, workpiece size, broaching power requirements, and available production space.

For internal broaching, the length of a broach in relation to its diameter may determine whether it must be pulled rather than pushed through the workpiece, for a broach tool is stronger in tension than in compression. This in turn, helps determine the type of machine for the job. A short push broach often is handled in a press instead of expensive ram-type broaching machine. Presses, of course, can be converted to pull short broaches by the addition of a pull-down adapter that converts push strokes to pull strokes.

Lubrication, workpiece size, chiphandling characteristics, and surface finish help determine whether a pull-up or a pull-down broach should be used. The trend is strongly to pull-down machines because gravity helps feed lubricant to the cutting teeth. Large workpieces are more easily handled in a pull-down than in a pull-up machine.

The type of drive - hydraulic or electro-mechanical - is another important factor in machine selection. So are convertibility and automation. Some machine designs allow for conversion from internal to surface work, for example. Some designs are fully automated: others are limited in scope and operate only with close operator supervision.

Here is a rundown of the major types of broaching machines:

VERTICAL BROACHING MACHINES

About 60% of the total number of broaching machines in existence are verticals, almost equally divided between vertical internals and vertical surface or combination machines. Vertical broaching machines, used in every major area of metalworking, are almost all hydraulicallly driven. One of the essential features that promoted their development, however, is beginning to turn into a limitation. Cutting strokes now in use often exceed existing factory ceiling clearances. When machines reach heights of 20 feet or more, expensive pits must be dug for the machine so that the operator can work at factory floor level unless they are the table-up type.

Vertical Internal Broaching Machines
These are either table-up, pull-up, pull-down, or push-down, depending upon their mode of operation.

Vertical Internal Pull-Up Type
The pull-up type, in which the workpiece is placed below the work table, was the first to be introduced. Its principal use is in broaching round and irregular-shaped holes. Pull-up machines are now furnished with pulling capacities of 6 to 50 tons, strokes up to 72 in., and broaching speeds of 30 fpm. Larger machines are available; some have electro-mechanical drives for greater broaching speed and higher productivity.

Vertical Internal Pull-Down Type
The more sophisticated pull-down machines, in which the work is placed on top of the table, were developed later than the pull-up type. These pull-down machines are capable of holding internal shapes to closer tolerances by means of locating fixtures on top of the work table. Machines come with pulling capacities of 2 to 75 tons, 30 to 110 in. strokes, and speeds up to 80 fpm.

Vertical Push Down Type
Vertical push down machines are often nothing more than general-purpose hydraulic presses with special fixtures. They are available with capacities of 2 to 25 tons, strokes up to 36 in., speeds as high as 40 fpm. In some cases, universal machines have been designed which combine as many as three different broaching operations - such as push, pull, and surface - simply through the addition of special fixtures.

Vertical Surface or Combination Broaching Machines
This type is found mainly in the automotive and hand tool industry. These machines produced in single-and double-ram versions (and even more rams occasionally), are hydraulically powered, with a few notable exceptions. Capacities range from 3 to 50 tons, with up to 130-in. strokes, and speeds of up to 120 fpm.

Electro-Mechanically Driven Vertical Surface Broaching Machines
These are available with either single or double rams and with strokes up to 120 in., capacities of 25 tons, and speeds of 60 fpm.

HORIZONTAL BROACHING MACHINES

The favorite configuration for broaching machines seems now to have come full circle. The original gear-or screw-driven machines were designed as horizontal units. Gradually, the vertical machines evolved as it became apparent that floor space could be much more efficiently used with vertical units. Now the horizontal machine, both hydraulically and mechanically driven is again finding increasing favor among users because of its very long strokes and the limitation that ceiling height places on vertical machines. About 40% of all broaching machines are now horizontals. For some types of work, such as roughing and finishing automotive engine blocks, they are used exclusively.

Horizontal Internal or Combination Machines
This type was among the first used after the advent of powered broaching, have been driven hydraulically for many years. Hydraulic drives, developed during the early twenties, offered such pronounced advantages over the various early mechanical driving methods that only within recent years have any other methods been used.

By far the greatest amount of horizontal internal broaching is done on hydraulic pull-type machines, for which configurations have become somewhat standardized over the years. Fully one-third of the broaching machines in existence are of this type, and of these, nearly one-fourth are over twenty years old. They find their heaviest application in the production of general industrial equipment but can be found in nearly every type of industry.

Hydraulically driven horizontal internal machines are built with pulling capacities ranging from 2l/2 to 75 tons, the former representing machines only about 8ft. long, the latter machines over 35 ft. long. Strokes up to 120 in. are available, with cutting speeds generally limited to less than 40 fpm.

Horizontal Surface Broaching Machines
This type accounts for only about 10% of existing broaching machines, but this isn't indicative of the percentage of the total investment they represent or of the volume of work they produce. Horizontal surface broaching machines belong in a class by themselves in terms of size and productivity. Only the large continuous horizontal units can match or exceed them in productivity. Horizontal surface units are manufactured in both hydraulically and electro-mechanically driven models, with the latter now becoming dominant.

The older hydraulically driven horizontal surface machines now are produced with capacities up to 40 tons, strokes up to 180 in., and normal cutting speeds of 100 fpm. These machines, a major factor in the automotive industry for nearly 30 years, turn out a great variety of cast-iron parts. They use standard carbide cutting tools and some of the highest cutting speeds used in broaching.

But electro-mechanically driven horizontal surface machines are taking over at an ever-increasing rate for certain applications, despite their generally higher cost. Because of their smooth ram motion and the resultant improvements in surface finish and part tolerances, these machines have become the largest class of horizontal surface broaching units built. They are available with pulling capacities in excess of 100 tons, strokes up to 30 ft., and cutting speeds, in some instances of over 300 feet per minute.

Larger machines have fully stress-relieved welde4d steel frames, rather than gray-iron castings. Frequently two sets of cutting tools are attached to the ram so parts can be broached on both the forward and return strokes. A common operation on automobile engine blocks is broaching head surfaces on one stroke of the ram, and pan rail and bearing surfaces on the return stroke.

These machines can also be equipped with dual-speed controls, whereby the ram is driven at one pre-selected speed during one portion of the stroke and changed to a second pre-selected speed during another portion of the stroke. A typical application is the use of the high speed for the initial roughing cut on pinetree slots in turbine wheels, and the slower speed while the finishing teeth are cutting.

Continuous Surface Horizontal Chain Broaching Machines
These have been the most popular type of machine produced for high-production surface broaching. has the distinction of having built, the largest and smallest service units ever produced.

The large continuous machine was a 42.5-ton giant with a 29-ft.-long bed, a 220-in. stroke, and a 40-ton broaching capacity. It performs nine separate operations on 71/2-lb., 121/2 in.-long connecting rod-and-cap sets for farm-machinery engines. The smallest was an 8.5-ton chain broach with a 2.5-ton capacity and a 20-in. stroke it broaches 5-oz., 2.5-in.-long manual transmission shaft shifters in four different configurations for automobiles.

The key to the productivity of a continuous horizontal broaching machine is elimination of the return stroke by mounting the workpieces, or the tools, on a continuous chain. Most frequently, the tools remain stationary, mounted in a tunnel in the top half of the machine, and the chain-mounted workpieces pass underneath them.

SPECIAL BROACHING MACHINES

Special broaching machines also fall under the general categories of internal or surface use, but beyond that it is difficult to classify the wide and often unique variety of special machines. Nevertheless, here is a sampling.

Sometimes it is impossible to bring the workpiece to the machine. This is particularly true in the marine, power-generation construction, and air-frame industries. Therefore broaching machine builders have designed portable machines that can be bought to the work.

A form of internal broaching called strip broaching is used occasionally to effect large gains in productivity per machine and man-hour through reduced broaching time cycles. In strip broaching, the broach is returned directly through the hole just broached, immediately after the cutting stroke, eliminating the necessity for disengaging the broach tool from its pulling or pushing head. Broach life is reduced because the cutting edges rub against the work on the return stroke, but not to the extent where the overall saving derived from this technique is lost.

Internal broaching of helicopter rotor spar sections is an unusual special broaching application. In one instance, a 24-ft. long workpiece had about 1/8 in. of 4153 aluminum removed around the periphery of the irregularly-shaped internal form by 35 progressively stepped broach sections. These were pulled through the workpiece one at a time by a special electro-mechanical horizontal machine with a 64-ft. long bed. Broach sections were semi-automatically loaded and unloaded from the pulling bar at the beginning and end of each stroke. One operator handled the entire job, riding from loading to unloading stations in an electric cart.