Drilling tool

Abstract

A drilling tool of the present invention includes: a casing pipe; a ring bit having a larger diameter and disposed coaxially on a tip side of the casing pipe; and a pilot bit inserted into the ring bit through the inside of the casing pipe. An outer peripheral part at a tip of the pilot bit is provided with a bit head which is extended when rotated in a tool rotation direction during drilling. The ring bit is provided with a part to be engaged that is engaged with the extended bit head in the tool rotation direction, and a first abutting part capable of abutting against the extended bit head in a direction of an axis.

1 . A drilling tool, comprising:
 a cylindrical casing pipe centered on an axis;   an annular ring bit that is coaxially disposed on a tip side of the casing pipe and has a greater external diameter than the casing pipe; and   a pilot bit that is inserted into an inner peripheral part of the ring bit through an inside of the casing pipe,   wherein the pilot bit is rotatable around the axis and a bit head is provided at an outer peripheral part of a tip of the pilot bit,   wherein the bit head is rotatable around a centerline eccentric from the axis, and is configured that when the pilot bit is rotated in a tool rotation direction during drilling, the bit head is extended such that a radius of the bit head from the axis is enlarged and thereby is supported by the pilot bit, and   wherein the ring bit is provided with: a part to be engaged that is configured to be engaged with the extended bit head in the tool rotation direction during drilling; and a first abutting part capable of abutting against the tip side of the extended bit head in a direction of the axis.   
     
     
         2 . The drilling tool according to  claim 1 ,
 wherein a recessed portion that is recessed to an outer peripheral side is formed at the inner peripheral part of the ring bit, and the recessed portion serves as the part to be engaged.   
     
     
         3 . The drilling tool according to  claim 1 ,
 wherein a smaller-diameter part of which an internal diameter is one step smaller is formed at an inner peripheral part of a tip of the casing pipe; a second abutting part capable of abutting against the smaller-diameter part from a posterior end side in the direction of the axis is formed at an outer peripheral part of a posterior end of the pilot bit; and an internal diameter of the ring bit is equal to or greater than the internal diameter of the smaller-diameter part.   
     
     
         4 . The drilling tool according to  claim 1 ,
 wherein the bit head is provided with a third abutting part which is capable of abutting against a surface of the ring bit that faces a posterior end side in the direction of the axis in a state where the bit head is extended.   
     
     
         5 . The drilling tool according to  claim 4 ,
 wherein the greatest radius of the third abutting part of the extended bit head from the axis is greater than a radius of an outer peripheral part of the tip of the casing pipe from the axis.   
     
     
         6 . The drilling tool according to  claim 2 ,
 wherein a smaller-diameter part of which an internal diameter is one step smaller is formed at an inner peripheral part of a tip of the casing pipe; a second abutting part capable of abutting against the smaller-diameter part from a posterior end side in the direction of the axis is formed at an outer peripheral part of a posterior end of the pilot bit; and an internal diameter of the ring bit is equal to or greater than the internal diameter of the smaller-diameter part.   
     
     
         7 . The drilling tool according to  claim 2 ,
 wherein the bit head is provided with a third abutting part which is capable of abutting against a surface of the ring bit that faces a posterior end side in the direction of the axis in a state where the bit head is extended.   
     
     
         8 . The drilling tool according to  claim 3 ,
 wherein the bit head is provided with a third abutting part which is capable of abutting against a surface of the ring bit that faces a posterior end side in the direction of the axis in a state where the bit head is extended.   
     
     
         9 . The drilling tool according to  claim 6 ,
 wherein the bit head is provided with a third abutting part which is capable of abutting against a surface of the ring bit that faces a posterior end side in the direction of the axis in a state where the bit head is extended.   
     
     
         10 . The drilling tool according to  claim 7 ,
 wherein the greatest radius of the third abutting part of the extended bit head from the axis is greater than a radius of an outer peripheral part of the tip of the casing pipe from the axis.   
     
     
         11 . The drilling tool according to  claim 8 ,
 wherein the greatest radius of the third abutting part of the extended bit head from the axis is greater than a radius of an outer peripheral part of the tip of the casing pipe from the axis.   
     
     
         12 . The drilling tool according to  claim 9 ,
 wherein the greatest radius of the third abutting part of the extended bit head from the axis is greater than a radius of an outer peripheral part of the tip of the casing pipe from the axis.