Abstract: A communication passageway is formed to extend from the end surface of a cover member opposed to the axial end surface of the shoulder portion of a main shaft and through the main shaft so that in only one of the two particular phase conditions obtained per revolution of an oil cylinder, it establishes the communication between high and low pressure chambers to thereby equalize their pressures.The communication passageway establishes the communication between high and low pressure chambers in only one of the two particular phase conditions obtained per revolution of the oil cylinder and makes it possible to produce percussion torque in the other particular phase condition.

Abstract: A grinding and polishing apparatus wherein an air grinder is incorporated in the tool attaching spindle of a machining center main body in exactly the same manner as in the other exchange tools so that grinding and polishing operations can be performed. The tool attaching spindle is centrally formed with a tool mounting bore and a compressed air feed passage. The air grinder is attached to the tool mounting bore and driven by fluid being fed from the compressed air feed passage. The apparatus further includes air feed control means, whereby supply of fluid to the compressed air feed passage is started after the base portion of the air grinder is mounted in the tool mounting bore and supply of fluid is stopped immediately before the air grinder is removed from the tool mounting bore.