PURPOSE:To facilitate operation and improve the safety thereof by transmitting the operational arm of a handle through a notch mechanism in such a way that an operational device is moved in a perpendicular direction to a lock means provided in a master jaw. CONSTITUTION:An operational device 8 for operating a lock device 7 of a jaw 3 and of a master jaw 4 is moved in the perpendicular direction to the lock means 7. An angular hole 23 and a lateral hole 24 for inserting a handle 15 are provided on the operational part of the operational device 8, and a hard ball 9 is inserted into the lateral hole 24 in such a way that the ball is partially forced out to the angular hole 23. At the time of cancelling the lock means 7, the handle 15 is inserted into the angular hole 23, and thus comes into a recessed part 16 of the handle 15, and by further pressing it, the operational device 8 is pushed into the bottom part. The lock means 7 is moved by a spring to a cave 25 of the operational device 9 by the rotation of the handle 15, and the engagement between the jaw 3 and the master jaw 4 is released thereby. In the case of engagement, it is carried out by the reverse rotation of the handle 15. By merely verifying the handle 15, an erroneous operation is inhibited safely.
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