This paper describes the stiffness characteristics of a 3-PUU translational parallel kinematic machine (PKM). An alternative method of generating the stiffness matrix is used when considering actuators and limitations, as well as the compliances of both actuators and legs. Extreme stiffness values and their design consequences are used to evaluate rigidity manipulator performance. Using the stiffness of a 3-PUU PKM in the design of its architecture is a good idea. Using an eigenscrew decomposition of the PKM's stiffness matrix, it is possible to identify the PKM's stiffness centre and the PKM's compliant axis, which provides a physical interpretation of PKM stiffness. Stiffness; workspace; parallel manipulator mechanics and design