TOOL FOR DRILLING BOREHOLES

TOOL FOR DRILLING BOREHOLES A tool for cutting percussive rotary drilling of boreholes, comprising a hollow casing having a bit. A hammer piston is installed in the casing for reciprocations and for cooperation with the bit for imparting impact impulses thereto. The hammer piston divides a working space of the casing into a workstroke chamber and a return stroke chamber which are alternately connected through distribution means provided in the casing with a source of a fluid under pressure for reciprocations of the hammer piston. There are provided controlled throttling means in permanent communication with distribution means which can be set to two limit positions so that in the first position they provide for a partial supply of a fluid for reciprocations of the hammer piston with minimum kinetic energy and in the second position the throttling means are completely closed so as to provide for the full supply of a fluid to the workstroke and return stroke chambers through the distribution means for reciprocations of the hammer piston at maximum kinetic energy. There are provided control means which are coupled to the bit and to the controlled throttling means for setting them to the first position when the bit encounters a soft rock and to the second position when the bit encounters a hard rock.