Patent ID: 7681671

Claim:
A driving method of a borehole drilling apparatus comprising a guide device, wings for extending a borehole and a pilot bit, the driving method comprises the steps of: suppressing rotation of the pilot bit by frictional force produced when the extending wings rotate and descend integrally with the guide device in a state where the extending wings are received in the pilot bit, and then a bottom surface of the pilot bit begins to strike a bottom surface of the borehole; spreading the extending wings outwardly from a wing advancing and retracting window formed at a side of the pilot bit by movement of a plurality of spiral projections formed at a lower surface of the guide device along a plurality of guide grooves formed at a side of the extending wings in a direction of increasing radii of the spiral projections by rotational force of the guide device, when the guide device continues to rotate in a state where rotation of the pilot bit is suppressed; drilling the borehole with integral rotation of the guide device, the extending wings and the pilot bit, after the extending wings stop spreading when a retaining protrusion formed on an outer circumference of the guide device is caught by a retaining step formed at an inner circumferential surface of the pilot bit; suppressing rotation of the extending wings by frictional force produced when upper surfaces of the extending wings come into contact with a shoe of a casing to fit into the borehole, after the guide device rotates reversely and ascends upon spreading of the extending wings; returning the extending wings into the wing advancing and extracting window of the pilot bit by movement of the spiral projections of the guide device along the guide grooves of the extending wings in a direction of decreasing radii of the spiral projections by rotational force of the guide device, when the guide device continues to rotate reversely in a state where rotation of the extending wings is suppressed; and retracting the guide device, the extending wings and the pilot bit from the casing while integrally rotating the guide device, the extending wings, and the pilot bit after stopping the extending wings, when the retaining protrusion formed on the outer circumference of the guide device contacts with the retaining step formed at the inner circumferential surface of the pilot bit.