Patent ID: 8109348

Claim:
A down-hole hammer drill including: a hammer casing; a top sub at the upper end of said casing and adapted to connect the hammer to a pressurized drill string, the drill string and top sub forming a compressed air supply; a drill bit having a bit shank extending from a bit head to an anvil end, the bit shank being mounted for reciprocating and driven rotation in the bore of a driver chuck secured to the lower end of the casing, said mounting for reciprocating and driven rotation comprising a plurality of key and keyway pairs arranged in spaced relation circumferentially about the bit shank of the driver chuck bore, at least one said keyway being blind at its ends, and at least one other keyway opening to said anvil end, whereby the blind keyway and said key in it are each of a length selected to allow a selected reciprocal motion of said drill bit, the sidewall of said blind keyway being relieved toward a circumferentially adjacent keyway for at least a blind keyway length, said key being inserted into the adjacent keyway, and then indexed with the blind keyway by rotation of the bit in the chuck, insertion of keys into the keyways opening to said anvil end rotationally locking the driver chuck to the drill bit; a free piston motor powered by said air supply and dividing the casing into an upper working chamber defined between an upper piston face and the top sub and a lower working chamber between a piston hammer face and the anvil end of the bit shank and driver chuck; an air control assembly on which the free piston slides and including a ported tube extending axially from said air supply to an axial bore in the shank of the bit to guide exhaust air through discharge ports at the cutting face of the bit, the bit reciprocating on the end of the tube; and an upper check valve in the air control assembly and openable by supply pressure against a spring bias in response to reduced pressure in whichever of the upper and lower working chambers is at reduced pressure in the cycle.