Down hole percussion drill apparatus

Abstract

A downhole percussion drill or hammer (100) is disclosed having an outer casing (2) in which a reciprocating piston (26) which periodically impacts a drilling bit (16) disposed at a bottom end of the casing. A top chuck (1), attached to the top end of the casing, conducts pressurized air to the interior of the casing. A flow tube (5) includes a supply passage (27) and an exhaust passage (10). No cylinder sleeve exists between the inner diameter of the casing (2) and the outer diameter of the piston (26). Rather, the flow tube (5), piston (26) and interior structure of the casing cooperate to direct pressurized air alternately to the top and bottom ends of the piston while alternately opening and closing the exhaust passage. Accordingly, for a predetermined outer casing diameter, a relatively thicker casing wall and relatively larger top piston surface results in higher relative performance than prior hammers having a cylinder sleeve. The hammer design insures that all pressurized air through a choke (11) in the supply passage ( 27) of the flow tube (5) passes directly to the exhaust passage (29) of the drilling bit (6) and not to the space (6) above the piston (26). Such design also increases hammer performance because bypass pressure does not increase back pressure of the hammer (100). The piston (26) is constructed to move longitudinally downwardly with the bit as the hammer is lifted from the bottom of a bore hole. It is also cooperatively constructed and arranged such that when the apparatus is lifted, all pressurized fluid not bypassed to the bottom of the hammer is conducted to the top of the piston (26) and then exhausted via the drill bit 16, thereby preventing reciprocation and possible piston damage while exhausting such fluid through the bit.