Stiffness performance is of importance for the use of parallel manipulators in the industrial applications. For this consideration, this paper proposes to realize the desired stiffness properties of parallel mechanism by adding redundantly-actuated limbs. Based on the stiffness mapping models of both the full-DOF and limited-DOF parallel mechanisms, the stiffness variation rules when redundant limbs is introduced into the mechanism are discussed. Moreover, an algorithm for designing the types and configurations of redundant limbs is studied. Two cases are investigated to validate the presented approach. One is about the stiffness decoupling of the Stewart platform, the other is focused on the enhancement of normal stiffness of a Tricept supporting mechanism used in a mirror milling machine designed by us. The result shows that the stiffness performance of Stewart platform can be decoupled when adding six redundantly-actuated limbs that are symmetric with the original active limbs. Besides, the normal stiffness of Tricept mechanism can be enhanced significantly by transforming the passive UP chain to be a redundant actuated chain.

Rear casing is a key part of the aeroplane engine. Its dimensional precision is significant to the quality of the aeroplane engine. In the rear casing manufacturing process, the assembly variation of its corresponding wax dramatically affects the final dimensions. In this paper, a polar-coordinate based model is proposed to calculate the assembly variation of ring-shaped rear casing wax part. It avoids the variation caused by the coupling relationship between Cartesian coordinate systems and locating position. We also compare the polar-coordinate based model with the ordinary one in practical application. The results show that the polar-coordinate based model can simplify the calculating process and improve the computational accuracy for the assembly variation analysis of the ring-shaped part.

For conventional metal forming, finite element (FE) method becomes a powerful tool in process design. However, conventional material models cannot describe material behaviors precisely in micro/meso scale due to the size/scale effects. As a result, know-how obtained in traditional macro-forming is not suitable for the micro-forming process. In micro/meso scale forming process, the reaction force, localized stress concentration and formability are not only dependent on the strain distribution and strain path but also the strain gradient and strain gradient path caused by decreased scale. This study presented a model based on conventional mechanism based strain gradient (CMSG) plasticity. A user-defined material (UMAT) subroutine incorporating the CMSG plasticity in the ABAQUS finite element (FE) program was established. Based on the subroutine, FE simulations were performed to analyze the effects of the channel width in sheet forming process of micro-channel features. Die sets were fabricated according to the scale law and Micro/meso scale sheet forming experiments were conducted to study the effect of channel width in the die. Experimental results indicated that the CMSG plastic theory achieved better agreements compared to the conventional plastic theory. It found that the influence of the strain gradient to the forming process increased with the decrease of the geometrical parameters of the tools. Furthermore, various tool geometrical parameters were designed by Taguchi Method to explore the feature size effects caused by the decrease of tool geometrical dimensions. According the scale law, similarity difference and the similarity accuracy were calculated to evaluate the size effects. Greater equivalent strain gradient was found with the decrease of the tool geometric dimension, which lead larger maximum reaction force error due to increasing size effects. The main effect plots for equivalent strain gradient and reaction force indicated that the influence of the tools clearance was larger than punch radius, die radius and die width.