Extendable bit

摘要

Extendable bit teeth are supported by the guide grooves formed on the end face at the point of a bit body in such a manner as to be slidable. The extendable bit teeth slide along the guide grooves in an excavating direction to thereby project from the end face at the point of the bit body. The extendable bit teeth slide in an opposite direction to the excavating direction to project from the outer periphery of the point of the bit body. Since the extendable bit teeth are pressed against the ground during excavation, the extendable bit teeth projects from the outer periphery of the point of the bit body so that the bit gauge can be extended, and most of the excavation hole is excavated by the bit body.