Abstract: When the main reducer housing is machined on the vertical multi-tool semi-automatic lathe, the standard hydraulic three-jaw chuck can not effectively locate the clamping workpiece. According to the structural characteristics of the workpiece, the design can be completed in one setup. The special turning tooling fixture for rough and two-step machining completely solves the safety problem that the workpiece clamping is unreliable due to the simultaneous rough machining of multiple knives, large turning resistance. Through the high and low pressure conversion of the clamping force, the workpiece is effectively prevented from being clamped and deformed. The production practice proves that the fixture is simple and reliable, easy to operate, improves production efficiency and ensures machining accuracy.