Zhongyi Cai

Multi-point forming technology for sheet metal

Abstract Multi-point forming (MPF) is an advanced manufacturing technology for three-dimensional sheet metal parts. In this paper, an MPF integrated system is described that can form a variety of part shapes without the need for solid dies, and given only geometry and material information about the desired part. The central component of this system is a pair of matrices of punches, and the desired discrete die surface is constructed by changing the positions of the punches though CAD and a control system. Typical examples show the applicability of the MPF technology. Wrinkles and dimples are the major forming defects in the MPF process, but numerical simulation is a feasible way to predict forming defects in MPF. In conventional stamping, the method to form sheet metal with a blankholder is an effective way to suppress wrinkling; and the same is true in MPF. An MPF press with a flexible blankholder was developed, and the forming results indicated the forming stability of this technique. Based on the flexibility of MPF, varying deformation path MPF and sectional MPF were explored that cannot be realized in conventional stamping. By controlling each punch in real-time, a sheet part can be manufactured along a specific forming path. When the path of deformation in MPF is designed properly, forming defects will be avoided completely and large deformation achieved. A workpiece can be formed section by section though the sectional MPF, and this technique makes it possible to manufacture large size parts in a small MPF press. Some critical experiments were performed that confirmed the validity of the two special MPF techniques.

3.06 – Multipoint Forming

The double curved sheet-metal parts with large size and thin-walled have been used in the field of aviation, spaceflight, shipping, vehicle, and architecture widely. Generally, the double curved sheet-metal parts are formed by solid die, but the cost for manufacturing of solid die is very high and the manufacturing cycle of solid die is very long. In order to solve the problems of solid die, Jilin University have done lots of works about flexible sheet-metal forming technology and digital manufacturing, such as multipoint forming (MPF) and flexible stretching forming (FSF). MPF is using a group of punch elements to shape the surface of the tool in place of a solid die firstly. Then, each element can be controlled by computer so that the curved tool surface can be changed at any time. MPF have been used for the manufacture of steel structure of Birds Nest Stadium (Beijing Olympic Game). FSF can increase the rate of materials utilization and close-fitting dies sharply than conventional stretch forming, by replace integrally gripping jaws to discrete multi-gripping jaws. Flexible control of multi-gripping jaws also can be achieved in a simple way based on new principle of stretching forming. FSF have been used in the cab of high-speed train and landmark building successfully. Furthermore, flexible roll forming, spin forming, continuous rolling, and stamping with flexible bank-drawer also pioneered at Jilin University, based on lots of numerical simulation and experiments.

An investigation on roll adjusting radius in three-dimensional rolling process for three-dimensional surface parts

Three-dimensional rolling is a novel forming process for three-dimensional surface parts, which combines the rolling process with multi-point forming technology. This process employs a pair of forming rolls as a forming tool; the residual stress of sheet metal makes the sheet metal generate three-dimensional deformation by controlling the nonuniform distribution of roll gap of the forming rolls. In this article, two types of forming processes are investigated for three-dimensional surface parts with the same target shape in different roll adjusting radius. The roll adjusting radius of a forming process is much larger than the target transverse curvature radius of the forming part, and the roll adjusting radius of another forming process is equal to the target transverse curvature radius of the forming part. Finite element analysis models are established; spherical and saddle surfaces are simulated. The corresponding experimental results are obtained. The dimensional accuracy of the forming parts using the two types of forming processes is compared, and the difference between the two types of forming processes for forming parts is analyzed through simulated results. The bendable roll rotates around its bent axis easily if its bending deformation is small; therefore, the forming process that the roll adjusting radius is much larger than the target transverse curvature radius of the surface part has relatively extensive application prospect. The comparison and analysis of the forming results may provide useful guidance on optimizing the three-dimensional rolling process for three-dimensional surface parts.