Lightweight Design Of A 3-(?)RS Parallel Spindle Head Subject To Stiffness Requirements

Driven by the needs from high-speed machining of large structural aluminum alloy parts,this thesis deals with the key issues on inverse kinematics,constrains of geometry and kinematic performance,semi-analytical stiffness modelling taking account of gravity and lightweight design of a 3-(?)RS parallel spindle head.The following contributions have been made.Based upon the inverse kinematics,the effects of the dimensional parameters on the actuated and passive joint variables are analyzed.With the aid of a set of dimensionless kinematic performance indices,the effects of dimensional parameters on the kinematic performance of the 3-(?)RS parallel mechanism is studied.Thereafter,the feasible region of dimensional parameters is determined subject to a set of specified geometric and kinematic constraints.By taking gravity and joint/link compliances into account,the semi-analytical stiffness model of the mechanism is formulated.The main contents includes:(1)establishing the mapping between the joint forces and the externally applied wrench including the cutting forces and moments and gravity;(2)formulating the deflection model by taking into account the deformations arising from the distributed gravity of limbs;(3)utilizing the semi-analytical stiffness model to derive the component compliance matrices at the interface.Thanks to the semi-analytical stiffness model,the stiffness distribution throughout the entire workspace and the deflection of the end-effector caused by the payload and gravity can be rapidly evaluated.In order to meet the requirements of high-speed milling of aluminum alloy structural compoments,a method for lightweight design of the 3-(?)RS parallel mechanism is proposed.First,the stiffness constraints is predicted to meet the requirement of high-speed milling of aluminum alloy “S” test-piece in the given machinining conditions;second,the effects of the design variables on the flexibility coefficients of all components in their local coordinate system are revealed by using sensitivity analysis method;third,the initial values of all design variables are obtained based on the above analysis,then the design variables are optimized in an iterative manner for achieving a lightweight design;finally,the static and dynamic behaviors of the optimized 3-(?)RS parallel mechanism are testified with the aid of the stiffness model and software SAMCEF.The outcomes of this thesis provide a useful method for the design of the 3-(?)RS spindle head.