away from the bottom when a well is being bored the rock breaking tool 4 can be mounted in the front part of the casing for axial movement so that the hammer 2 will simultaneously strike against the end face of the tool 4 and the front part 3 of the casing 1 the rock breaking tool shown in fig2 and denoted in the general view on fig1 as item 4 has a casing 7 formed with a chamber 8 open at the end face of the casing 7 and oriented toward the bottom of a well being bored a part 9 of the casing 7 intended for breaking up the well bottom is pointed and forms an annular cutting edge 10 fig3 with an internal conical surface 11 fig2 in addition the casing 7 is provided with at least two ducts 12 for a forced displacement of fragmented rock from cavity 8 into the space surrounding the casing 7 the chamber 8 is formed as a cone with a rounded off top whose base is presented to the bottom of a well being drilled a top part of the casing 7 has a seating cone 14 which is inserted into the front part 3 of the self propelled percussive action machine fig1 when the tool 4 is put into operation the above rock breaking tool 4 operates in conjunction with the self propelled percussive action machine in the manner below compressed air supplied to the self propelled percussive action machine causes the hammer 2 fig1 to reciprocate in the casing 1 and strike against the front part 3 of the casing 1.