a hammer drill having a motor mounted within a body and an output spindle ; a tool holder mounted on the body capable of holding a cutting tool ; a hammer mechanism having a piston ; a reciprocating drive for converting rotary movement of the motor into reciprocating movement of the piston ; a ram reciprocatingly driven by the piston via an air spring to strike a cutting tool held in the tool holder , the hammer mechanism performing one hammer cycle each time the ram strikes a cutting tool during normal use ; an air replenishment mechanism capable of refreshing the air spring during certain time periods during normal use ; the air replenishment mechanism capable of being adjusted to refresh the air spring during time periods within the hammer cycle and / or the system allows different volumes of air into or out of the air spring during the refreshment time periods .