Coordinate positioning machine

A coordinate positioning machine has an operating module such as a tool or probe (10) mounted for translational motion relative to a table (16) with three degrees of freedom by means of three articulated linkages (18). Each of the linkages is made up of a torsion box (22) which is pivotally mounted to a vertical pillar (20) with one degree of freedom, and a pair of struts (26), each of which is pivotally mounted with two degrees of freedom to the torsion box at one end, and to the operating module at the other. The linkages cooperate to prevent all rotational motion of the operating module, while allowing translation. Motion of the operating module is actuated manually, and the mass thereof is counterbalanced by an articulated counterbalance arm (90) which guides a cable (96) that is in turn connected to a fusee. Displacement of the operating module is measured using scale and readhead encoders (40,50). Motion of the operating module can be restricted to a single plane by means of locking devices (132) operating between the struts (26), thereby to enable scanning of a surface in a series of parallel planes.