| 摘要 |
PROBLEM TO BE SOLVED: 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. SOLUTION: 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.
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