Abstract: 2R1T Parallel mechanism (PM) can undergo one translational degree of freedom (DOF) and two rotational DOFs. It contains two kinds of rotational axes: continuous rotational axis and instantaneous rotational axis. The property and geometrical distribution of rotational axes in space are important to trajectory planning and kinematic calibration of parallel mechanisms in which high orientation ability is required, such as five axes parallel kinematic machine (PKM). The 5-DOF Exechon hybrid machining center has been widely used in the advanced manufacturing. It consists of a 2-UPR-SPR PM which belongs to 2R1T PM and a RR serial mechanism. Using screw theory, limb twist system and limb constraint system were established when the 2-UPR-SPR PM is at the initial configuration and configurations after continuous rotation of 1 DOF and 2 DOFs. Based on the relation of rotation axis and constraint force of rigid body, two continuous rotational axes of the moving platform have been identified. These two continuous rotational axes is dependent on the configuration of the 2-UPR-SPR PM and pass through the origin of the reference frame and the center of the spherical joint.

Key words: Continuous rotational axes, Degree of freedom, Instantaneous rotational axes, Parallel mechanism