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In order to improve the performance of back drawing type of flexible multi-gripper stretch forming machine used for double-curvature metal sheet forming, back and down drawing type of flexible multi-gripper stretch forming machine was designed by finite element method (FEM), which has simple structure and cheap cost. The FE model of flexible multi-gripper stretch forming was established, and extensive numerical simulations of spherical parts for two kinds of flexible stretch forming machines were carried out. The variations of stress, strain, thickness and springback value of spherical parts for two kinds of drawing modes were analyzed. The numerical results show that the quality of spherical parts formed by the back and down drawing type of stretch forming machine is better than that by the back drawing type of stretch forming machine. This work provides a machine for developing the technology of stretch forming.

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Multi-roll stretch forming process is a new flexible manufacturing technique that the general idea of discretizing is put in use in the design of stretch forming machine. In the new process, the metal sheet can be more easily formed, and the flexibility can be much higher, which the traditional process cannot compare with. In this paper, in through extensive numerical simulations of the MRSF stretching process of toroidal saddle parts, A series of finite element simulations have done for the process of forming toroidal saddle parts using different lubricant and two kinds of rollers named damped rollers and ordinary rollers. The results show that the smaller the friction coefficient is, the easier the center of toroidal saddle part is stretched. Damped rollers can increase the stretching force and the ordinary rollers can increase the mobility of sheet metal. Arranging the damped rollers and ordinary rollers at a reasonable position can make the workpiece a more uniform stretching.