Downhole drill bit and bit coupling

A downhole hammer impact bit having an enlarged front impact head and a rear elongated drive shank with a rear anvil end face adapted to be impacted for downhole impact drilling, the elongated drive shank having a set of external splines comprising a front section of primary spline segments for rotation of the bit during drilling with the bit in a retracted, coaxial, working position and a rear section of secondary spline segments, having a size less than the primary spline segments, for preventing rotation of the bit in an extended, non-working position thereof, the impact head having a leading, coaxial pilot impact bit and a trailing impact bit sector integral and coaxial with the pilot bit and largely on one side of the axis thereof, the pilot bit and impact bit sector providing, with the bit in its retracted, coaxial working position and upon rotation of the impact bit during drilling, for impact drilling guide and enlarged holes respectively, the impact bit having means, which includes the front section of primary spline segments, for coaxially positioning the bit in its retracted, working position and having means, which includes the rear section of secondary spline segments, for permitting lateral movement of the drive shank in the extended, non-working position of the bit, thereby to permit the impact bit sector to swing inwardly to reduce the diameter of the impact head envelope for installation and withdrawal of the impact bit.

For use in a downhole impact hammer, a subassembly comprising a drill bit chuck having an axial bore, with an annular arrangement of internal splines, and front and rear end faces at the opposite axial ends thereof, a drill bit having a front enlarged drill bit head and a rear elongated drive shank with a rear anvil end face adapted to be impacted for downhole drilling, the bit drive shank being slidably received in the axial bore of the chuck for axial movement of the drill bit between a retracted, working position with the enlarged drill head in engagement with the front end face of the chuck and an extended, non-working position with the drill bit supported on the rear end face of the chuck, and removable retaining ring means mounted on the rear end of the drive shank to engage the rear end face of the chuck to support the drill bit thereon, the drive shank having a front section of external splines which cooperate with the internal splines of the chuck to provide, with the drill bit in its retracted, working position, a drive coupling for rotation of the bit, the drill bit head having a drill bit sector coaxial with the drill bit and largely on one side of the axis thereof, and operable with the drill bit in its retracted working position and upon rotation of the bit, for drilling a hole, the drive shank and chuck having cooperating means for positioning the bit coaxially within the chuck in the retracted, working position of the bit and cooperating means for permitting lateral movement of the drive shank within the chuck in the extended, non-working position of the bit, thereby to permit the drill bit sector to swing inwardly to reduce the diameter of the drill bit head envelope.