A tool setting apparatus (40;200), comprising; a measurement probe (44;206) having a stylus (46; 208) and a deflection sensor for sensing deflection of the stylus, the distal end of the stylus comprising a tool-contacting tip , and a cover (48;214) having an aperture through which the distal end of the stylus can pass, the cover (48;214) being moveable relative to the measurement probe (44;206) between a protective configuration in which the cover (48;214) substantially surrounds the stylus and a tool measurement configuration in which the tool-contacting tip of the stylus protrudes through the aperture of the cover to allow tool measurement, characterised in that the apparatus comprises; at least one gas supply pathway for supplying gas into the region (58;231) enclosed by the cover (48;214) when the cover (48;214) is in the protective configuration and at least one gas inlet via which gas can be introduced into the at least one gas supply pathway , the apparatus being arranged such that when the cover (48;214) is in the protective configuration gas can be supplied into the region (58;231) enclosed by the cover (48;214) via the least one gas inlet and the at least one gas supply pathway , at least some of the supplied gas being expelled from the cover (48;214) through the aperture thereby providing a gas bleed. a measurement probe (44;206) having a stylus (46; 208) and a deflection sensor for sensing deflection of the stylus, the distal end of the stylus comprising a tool-contacting tip , and a cover (48;214) having an aperture through which the distal end of the stylus can pass, the cover (48;214) being moveable relative to the measurement probe (44;206) between a protective configuration in which the cover (48;214) substantially surrounds the stylus and a tool measurement configuration in which the tool-contacting tip of the stylus protrudes through the aperture of the cover to allow tool measurement, characterised in that the apparatus comprises; at least one gas supply pathway for supplying gas into the region (58;231) enclosed by the cover (48;214) when the cover (48;214) is in the protective configuration and at least one gas inlet via which gas can be introduced into the at least one gas supply pathway , the apparatus being arranged such that when the cover (48;214) is in the protective configuration gas can be supplied into the region (58;231) enclosed by the cover (48;214) via the least one gas inlet and the at least one gas supply pathway , at least some of the supplied gas being expelled from the cover (48;214) through the aperture thereby providing a gas bleed. at least one gas supply pathway for supplying gas into the region (58;231) enclosed by the cover (48;214) when the cover (48;214) is in the protective configuration and at least one gas inlet via which gas can be introduced into the at least one gas supply pathway , the apparatus being arranged such that when the cover (48;214) is in the protective configuration gas can be supplied into the region (58;231) enclosed by the cover (48;214) via the least one gas inlet and the at least one gas supply pathway , at least some of the supplied gas being expelled from the cover (48;214) through the aperture thereby providing a gas bleed. <EOS>