| 摘要 |
PROBLEM TO BE SOLVED: To provide an outer bit which solve problems such as heavy abrasion in the outer bit and large torque in excavation caused by inferior discharge of cuttings when excavating a double-pipe in a boring work. SOLUTION: A plurality of cuttings collecting grooves 32 having the relation among the groove depth L2, the groove width L2, and the shank wall thickness t, L1=1t-2.5t and L2=1t-3t are disposed in the side of an excavation face 31a of a bit shank 31. The center line of the groove width is inclined by a prescribed angleαin the rotation direction A of the shank inner wall 31b and the groove bottom 32b is inclined by a prescribed angleβrearward of the axis O of the groove bottom 32b.
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