A hammer drill

A hammer drill comprising: a body (2); a motor (24) mounted within the body (2) having an output spindle (30); a tool holder (14) mounted on the body, the tool holder being capable of holding a cutting tool (22); a hammer mechanism comprising: a piston (60; 502) slideably mounted within the body; a reciprocating drive mechanism mounted within the body which, when the motor is activated, converts the rotary movement of the spindle (30) of the motor (24) into a reciprocating movement of the piston (60) within the body; a ram (64; 508) slideably mounted within the body (2) which is reciprocatingly driven by the reciprocating movement of the piston (60; 502) via an air spring (62; 510) to repetitively strike a cutting tool when held by 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 (200, 202, 204; 526, 524; 600, 602) which is capable of refreshing the air spring (62; 510) during certain time periods within the hammer cycle during normal use; characterized in that the air replenishment mechanism is capable of being adjusted so that it refreshes the air spring during different time periods within the hammer cycle during normal use and/or it allows different the volumes of air allows into or out of the air spring during the refreshment time periods.