Compressed air percussive mechanism for a down hole hammer and down hole hammer

Pressure-air driven percussion device for a down-the-hole drill ( 1 ) with a hammer-piston ( 86 ) which is axially reciprocally movable in a hammer-piston chamber ( 18 ) through a driving device ( 9 ), said hammer-piston ( 6 ) in operation acting with a hammer-end ( 16 ) against an upper end ( 17 ) of a drill bit ( 2 ) which is positioned inside a chuck ( 3 ), wherein an air-cushion for reducing percussive power is arranged to be formed at the hammer-end of the hammer-piston in positions where the drill bit ( 2 ) has been moved passed a predetermined distance in the percussion direction. The drill bit ( 2 ) is sealingly slidingly supported in a drill bit bushing ( 5 ), and the hammer-end ( 16 ) of the hammer-piston ( 6 ) is formed so that it sealingly cooperates with the drill bit bushing ( 5 ) in said positions in order to form said air-cushion ( 19 ). The invention also concerns a down-the-hole drill ( 1 ).