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Recently, during the fabrication of U-bent parts, increasing levels of precision have been required not only for the bend angle of the part but also for the bend radius and the flatness of its bottom surface. The conventional coined-bead technique is therefore unlikely to be applied to achieve these requirements. As an innovative variation of the coined-bead technique, the centered coined-bead technique is proposed and investigated in this study. The finite element method (FEM) was used to clearly identify the parameters of interest based on a stress distribution analysis. The results show that the centered coined-bead formation could be applied to produce a balance between the bending and reversed bending characteristics and to eliminate these characteristics in the bottom surface. Therefore, a precise bend angle and bend radius and a flat bottom surface could be achieved. The effects of the coined-bead geometry and its positioning are also examined and clearly identified, and laboratory experiments were carried out to validate the FEM simulation results. Each U-bent part could be fabricated with a precise bend angle and bend radius and a flat bottom surface using the centered coined-bead technique. The recommended coined-bead geometry and positioning are also reported.

The coined-bead technique is commonly applied in the V-bending process to eliminate the spring-back feature and to achieve the required bending angle. In the present research, which is based on a stress distribution analysis, the effects of a coined-bead geometry in the V-bending process on the increased compressive stress in the bending allowance zone and on the increased reversed bending zone in the leg of the workpiece were observed and characterized. The research also illustrated the variations in spring-back and spring-go characteristics for different coined-bead widths and heights. As the bead width increased, the spring-go decreased, increased, and decreased again for different simulated bead heights and workpiece materials. The research was able to disprove previously suggested theories for coined-beaded V-bending, which suggested that a smaller bead width should be applied to cancel out the spring-back characteristic. In addition, the spring-back characteristic returned when an oversized bead width was applied. Therefore, to achieve the required bending angle, the size of the bead width was optimized both to provide a balance between a favorable stress distribution in the bending zone and reversed bending zone and to reduce the bending load.

In recent years, the requirements for high dimensional precision on Z-bent shaped parts have become increasingly stringent. To attain these requirements, the suitable selection of the Z-die bending type has to be considered much more strictly. In this research, two types of Z-bending processes, offset Z-die bending and wiping Z-die bending, were investigated using the finite element method (FEM) to identify the spring-back characteristics and dimensions of Z-bent shaped parts. In the case of offset Z-die bending, the spring-back characteristics on both bend angles were similar. In contrast, in the case of wiping Z-bending, the spring-back characteristics on both bend angles were different. In addition, the dimensions of the Z-bent shaped parts were investigated. It was found, in the case of wiping Z-bending, that web thinning was generated and the outer bend radius was out of tolerance. To validate the FEM simulation results, experiments were carried out. The FEM simulation results showed good agreement with the experimental results in terms of the bend angles and the overall geometry of the Z-bent shaped parts. To achieve precise Z-bent shaped parts, the suitable selection of Z-die bending type in the Z-die bending process is very important.