We have tooled this operation several times for many different manufacturers and have learned that the optimum operation for holding the tolerances to military standards requires a two pass broaching arrangement. A relatively simple starting hole that is milled through the pre-machined forged aluminum lower receiver allows the first pass broach tool to start through the part. The fixture clamps the part in place while fixture supports provide critical backup preventing part distortion during the cut. Guided broach tools cut the sides, top, bottom, and corners of the milled slot out to the finish form. After broaching, the fixture is unclamped and the part is removed with all critical dimension through the magazine well held within tolerance.
To accomplish this broaching operation we use an “Ohio” HF1572 horizontal broach machine with 15 tons of cutting force and 72 inches of cutting stroke. The machine is equipped with a retriever unit for broach handling. The machine and retriever are configured with a fixture hydraulically clamped and automatically actuated within the cycle of the machine. The whole operation is automatic after the operator loads a part and depresses the cycle start buttons. The operator removes a lower receiver with a completely broached magazine well when the machine has finished its cycle, which requires approximately (1) minute from start to finish.