A broach is a series of chisel points mounted on one piece of steel. A broach is typically used to enlarge a circular hole into a larger noncircular shape such as a square or other desired shape. Another typical use of a broach is to cut a square keyway on objects such as gears, pulleys etc. The amount of material removed by each broach tooth (or chisel) varies with the material being cut. A broach designed to cut steel might remove 0.0025 (2.5 thousandths) of an inch, while a broach designed to cut brass might remove as much as 0.004 (4 thousandths) of an inch. The succession of chisels removes the total amount of material required. A broach might be designed to be pushed through an existing hole, or, alternatively to be pulled through the hole. Broaching machines are designed to either push or pull a broach.
However, the broaches obtained from tool and material dealers are not normally fitted with handles – instead they have a tang similar to that found on a file.
Traditionally, broaches were always fitted with specially shaped wooden handles, designed to give both purchase for removing large amounts of metal, and also a sensitive feel when taking out a final delicate scraping.
Although they were readily available at minimal cost, the making of a set of broach handles was often regarded as a first class exercise for apprentices learning to turn with the graver – after sufficient practice had been obtained in turning the wood, the brass ferrule provided the next part of the exercise.
For internal broaching, the hole should be drilled 0.005"-0.015" larger than smallest diameter of the broach, and countersink it at 90 degrees to slightly larger than largest dimension of broach.
For external broaching, turn the O.D. to be broached 0.005"-0.015" smaller than the largest dimension of the broach and form a 45 degree chamfer on the front end down to at least the smallest dimension of the broach.
Since the broach is driven by the leading edge of the hole (ID) against the nearest surface of the broach (BB), the space between the broach and the hole caused by the back clearance allows the broach to rotate slightly and cut a spiral as shown in Figure 4.
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