Investigation On Lightweight Design Of A 3-DOf Parallel Spindle Head

In order to meet the demand for high-speed machining of large aircraft components, this dissertation deals with some key issues in the design of a novel 3-DOF spindle head----A3 Head in terms of inverse displacement analysis and kinematic performance evaluation, semi-analysis stiffness modeling, fine evaluation of components compliances as well as lightweight design of the machine as a whole. The following contributions have been made.Given the specified translational and rotational movement capabilities of the A3 Head, a method to determine the ranges of the actuated and passive joint variables is proposed by examining the effects of the dimensional parameters on the variables throughtout the entire task workspace. In addition, a method to evaluate the effects of dimensional parameters on the kinematic performance is also presented using a set of homogeneous and dimensionless indices. The both provide the reasonable geometric and kinematic constraints in the components lightweight design of the A3 Head.By taken into account gravity of all movable components, the linear map between forces/torques in the joint space and the externally applied equivalent wrenches imposed upon the platform in the operation space is formulated. On this basis, the linear map between the interface deflections in the directions of the permitted and restricted theoretically instantaneous motions and the deflections of the platform twist is developed by considering the limb deflections caused by the gravity. These considerations lead to the development of a semi-analytical stiffness model that enables the rigidities of the system over the entire workspace and the deflection twist of the platform caused by the payload and gravity to be predicted in a very effective manner.A hierarchical strategy for computing the interface compliance of the limb-body and an approach for FEA refined modeling of 1-DOF joints are presented, allowing the compliance of the interface to be effectively and accurately evaluated. Meanwhile, a process which needs only one implementation of FEA modeling is developed for evaluating rigidities of all the sub-assemblies within a limb, and its effectiveness is verified via static and dynamic tests at typical configurations. Then, the weak links within a limb and the deformations caused by the gravity of different components are investigated based on the proposed method.In order to achieve high speed and high dynamic required for the 5-axis CNC high-speed machining of large aircraft structural components, a hierarchical methodology for lightweight design of the A3 Head is proposed by minimizing the total weight of the movable components subject to the specified geometric, kinematic as well as rigidity constraints. The criteria to choose the proper stiffness ratio between the directions of actuations and constraints at the limb level, and that between the joints and the limb-body at the component level are investigated because the overall stiffness of a serially connected spring is heavily dominated by the weakest component within the system. Meanwhile, the response surface method and the multi-island genetic algorithm are employed to achieve the optimal solutions of a bunch of design variables involved. The computer simulations show that stiffness of the modified A3 Head has been increased over 100% and its lower order natural frequencies have been increased over 36% with little change of total weight of the movable components, thus verifying the feasibility and effectiveness of the proposed approach.The outcomes of this dissertation have been employed for the re-design of the A3 Head, a significant step to promote its application in high-speed 5-axis machining of large structural components in aircraft industry.