Kinematic error model and non-uniform calibration method for heavy load multi-DOF envelope forming press

Multi-DOF envelope forming is a new continuous and local plastic forming process for manufacturing high performance complex components with thin wall and high rib used in the aerospace industry. To achieve the new process, this paper develops a new heavy load multi-DOF envelope forming press with parallel kinematic mechanism (PKM), which can achieve the multiDOF motion of envelope die. However, owing to the large scale and heavy load characteristics of the press, the kinematic error (fabrication and installation error) of the press is relatively large, resulting in the increased motion error of envelope die. Therefore, it is necessary to establish a reasonable kinematic error model and a calibration method to provide an essential basis for the precision design and control of the new press. Firstly, the kinematic error model, in which the 114 kinematic errors and multi-DOF motion error of envelope die are unitedly expressed and calculated based on the small displacement screw theory, and error Jacobian matrix for the press are established. Then the kinematic error mechanism for the press is disclosed. It is found that the motion error of envelope die is mainly attributed to 10 positional errors among 114 kinematic errors. Moreover, all the six error components of motion error of envelope die are time-varying with position changing of envelope die, and the position distributions of the maximum six error components are non-uniform during one motion period of envelope die. Based on the above results, a new non-uniform kinematic error calibration method for the press is proposed. Finally, the kinematic error calibration experiments are conducted. It is shown that the prediction accuracy of motion error of envelope die with the proposed non-uniform kinematic error calibration method is significantly improved from 89.06% to 99.67% compared with the traditional uniform kinematic error calibration method, which verifies that the established kinematic error model and non-uniform calibration method for heavy load multi-DOF envelope forming press are reliable.