| 摘要 |
PROBLEM TO BE SOLVED: To prevent a holder from being slipped off and jumped out from a recessed part when a reaction causing a swing in the direction of directing a roll is received by holding the holder fitted to the recessed part of a backup member so as to be embraced by the recessed part. SOLUTION: Since a holder 23 is pressed to a recessed part 37 of a backup member by the cutting resistance received by a bite 29, it receives the reaction from the recessed part 37. According to this backup member, the recessed part 37 is formed so that the angleαof rotating in the circumferential direction around its curvature center is larger than 180 deg.. Therefore, the holder 23 fitted to the recessed part 37 is held so as to be embraced, and even if a reaction as causes a swing in the direction of directing a roll is received, the holder 23 is never swinged so as to be slipped off and jumped out from the recessed part 37.
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