Down-the-Hole Hammer Drill

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 keyed for reciprocating and driven rotation in the bore of a driver chuck secured to the lower end of the casing, at least one said key including a blind keyway in said bit to retain the bit in said chuck and being of a length selected to allow a selected reciprocal motion of said 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.