US Patent 7174652 Performing measurement or calibration on positioning machines

A machine having a pair of spaced structures, a plurality of ball and socket joints spaced apart on each structure, and six struts interconnecting the structures. Each strut engages with a respective ball and socket joint on each of the structures, and defines a variable distance between the centers of the ball and socket joints to which the strut is connected. At least one of the ball and socket joints to which the strut is connected has a plurality of the struts connected thereto, the ball and socket joint having a socket on each respective strut, co-operating with a sphere, the sockets having three engagement elements biased into engagement with the sphere.