Patent attributes
The drilling tool includes a circular-cylindrical casing pipe; an annular ring bit disposed at a leading end of the casing pipe so as to be coaxial with the casing pipe and rotatable around a central axis of the casing pipe; and a pilot bit inserted into the casing pipe and has a leading end disposed at an inner peripheral part of the ring bit. The ring bit is engaged with the pilot bit in a rotational direction, and thereby is rotatable integrally with the pilot bit and is locked to the leading end of the pilot bit. Drilling tips are mounted on the leading ends of the pilot bit and the ring bit, and first drilling tips at an outermost periphery of the pilot bit and second drilling tips at an innermost periphery of the ring bit overlap each other in a rotational trajectory around the central axis.
