COMMONLY BROACHED MATERIALS
Broaches have been used on almost every material at one time or another - most of the known metals and alloys, some plastics, hard rubber, wood, composites, graphite, and so on. Metals and alloys are, by far, the most commonly broached materials. The products made from the other materials are not usually made to the stringent
dimensional tolerances, or in the quantities, that make broaching economical.
In general, any material that can be machined can be broached. And the higher the machinability of the material, the easier it is to broach. In steels, machinability correlates closely with hardness. That is why work pieces with a high surface hardness, such as produced by previous work-hardening or scale, require that the first broach tooth cut beneath the scale or hard surface is possible.
The hardness of the work piece material also influences the allowable cut per tooth. On harder metals, it is customary to take a relatively fine finishing cut; on softer nonferrous metals, a fine surface finish can be achieved with a heavier finishing cut. Too heavy a cut, however, will tend to overload the broach tool - no matter what material is being broached. Too fine a cut, on the other hand, tends to interfere with free-cutting action and increases the tendency of the material to glaze, gall, or tear. Smaller steps can be used for finishing than for roughing.
Stainless steels
Stainless steels with hardness above Rockwell C 35 can be broached. Stainless harder than this, however, tends to dull broach teeth fairly fast, reducing the number of pieces produced between grinds. The approximate rise per tooth (round broaches) runs from 0.001 to 0.005 in. This range will cover practically all types of stainless steel. Broaches with hook angles between 12 and 18 usually give the best results. Backoff should be held to a minimum; a 2° angle is preferable, but in no case should it exceed 5°. Chip breakers should be used.
Free-cutting steel
Free-cutting steel will allow a greater cut per tooth, or step, than will hard or tough steel. However, a step of 0.0005 in. on a broach diameter is practical minimum. Hook angles also vary with the material being cut as was mentioned previously. They range between 15° and 20° for the soft steels and between 8° and 12° for the hard steels. Backoff angles of 2° to 3° on the roughing teeth, 1° on the semi-finishing teeth, and 1/2° on the finishing teeth give good results when broaching steel. Chip breakers should be used.
Cast and malleable irons
Cast and malleable irons permit a greater rise per tooth than even the free-machining steel. Brittle materials such as cast iron call for small hook angles, usually around 6° to 8°. Backoff angles are the same as for the general run of steels. Usually, a shorter pitch is permissible in broaching cast irons than in broaching steels because less chip room is required for the irons.
Brasses and bronzes
Brasses and bronzes allow a slightly heavier step, or rise per tooth, than steel. Too heavy a rise, however, will tend to overload the broach. Hook angles usually range from 0° up to 10° and even higher, increasing with ductility of the metal being broached. Brittle brasses call for smaller angles, from +5° to -5°. Backoff angles are usually 2° on the roughing teeth, 1° on the semi-finishing teeth, and 1/2° degrees on the finishing teeth. Some form of chip breaker is required.
Aluminum and magnesium
Aluminum and magnesium can be broached with standard tool design, although special broaches give even better results. A hook angle of 10° to 15° and a backoff angle between 1° and 3° are recommended. Heavier cuts can be taken; even the finishing teeth can remove as much as 0.002 in. each. If trouble is experienced in maintaining proper tolerances, the size of the finishing cut can be increased, rather than decreased, to correct the situation.
Ductility of a metal
The ductility of a metal has a considerable influence on the selection of an optimum hook angle for the broach teeth. In general, this angles decreases with decreasing ductility. Brittle materials, therefore, call for very small hook angles.