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 (47, 212), 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 (47, 212) 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 (47, 212), 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 (47, 212) 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.