To reduce rotational acceleration and to secure the accuracy of centering of a position of a piercing plug by being provided with a core bar shifter for restraining or releasing the end part of the core bar and a driving device for rotating a thrust block on the end face of the thrust block and executing prerotation of the thrust block and core bar before piercing.
By making the end part of the core bar 5 restrain by rotation restraining means which is provided on the end face of the thrust block 6 and executing the prerotation of the thrust block 6 and core bar 5 before piercing, the piercing is stood by. The rotational speed of the prerotation is defined as 10-100% of the rotational speed of the plug 4 and core bar 5 at the time of the piercing. Because the rotational acceleration of the plug 4, core bar 5 and thrust block is reduced by the prerotation, the fusing damage in the tip part of the plug 4 is reduced and its life is prolonged. The wear by slip between the plug 4 and the core bar 5 and also the deformation and wear in a contact part between the core bar 5 and the thrust block are prevented and the life is prolonged.
YOKOI YOICHI
OISHI TOSHIMITSU
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