Large arc-shaped workpieces are a common type of locomotive steel structural sheet metal products, with variable structures and greater processing difficulty compared to general workpieces. This article introduces a method for bending large arc-shaped workpieces and provides a detailed description of its application using specific workpieces.
In the production practices of the blanking workshop, there are generally three forming methods for large arc-shaped workpieces. The first is single-process die forming, suitable for complex-shaped workpieces, offering advantages such as good forming effects, smooth and flat workpiece surfaces, no pressure marks, and high processing efficiency. However, the mold cost is high, and it is less versatile, usually used when conventional methods cannot form the workpiece or when a high surface finish is required. The second method involves using customized molds for large arc bending machines, which are generally suitable for workpieces with a large arc radius that does not exceed the limit of the bending machine's worktable. These molds have lower costs compared to single-process die forming and offer a certain degree of versatility. The third method, which is detailed in this article, is the multi-bend forming method, where the basic principle involves converting the arc into line segments, allowing for forming using existing molds and equipment. This method is generally applicable for non-exposed parts, has a wide range of applications, lower costs, and does not require customized molds, resulting in good forming quality.
Analysis of the multi-bend forming process for large arc-shaped workpieces
The basic principle of the multi-bend forming process involves approximating the arc segment into line segments, as shown in Figure 1, to utilize small arc tooling for processing large arc-shaped workpieces. Compared to the arc segment, the forming effect of the line segment is positively correlated with the number of line segments. When using this method, the workpiece surface is likely to exhibit angular structures. However, for non-exposed parts, considering production cycle and cost, this method can be adopted.
Application example of large arc bending process
Segmentation of the arc and determination of the bending angle
For the workpiece shown in Figure 2, with a bending arc inner radius of R350, a bending angle of 120°, and a thickness of 5mm, the conditions for using the multi-bend forming method for large arcs are met. Following past experience and considering the existing mold situation in the workshop, a R120 arc mold is used for the upper die.
The R350 arc segment in the workpiece in Figure 2 is divided into 6 line segments. It is important to ensure a smooth transition at the tangent points between the arc segment and the straight segment. The segmentation angle at the tangent points should be half of the angle used for other segments. As shown in Figure 3, the workpiece in Figure 2 undergoes 7 bending processes. When processing sheet metal parts on an electro-hydraulic servo bending machine, three basic parameters are required: sheet thickness, bending angle, and the position size of the bending line. The calculated segmentation angles for the first and last bends are 6°, while the others are 12°. The bending angle after segmentation is directly measured using CAD software.
Confirmation of bending line position size and unfolded dimensions
To ensure the accuracy of the unfolded dimensions of the bending line, two methods are generally used: the neutral layer unfolding calculation method and the software-assisted unfolding method. To obtain bending data in a concise, fast, and accurate manner, the software-assisted unfolding method is used. The sectional drawing of the segmented part is directly imported into Catia, and the GenetativeSheetmetalDesign module is used to generate the part model, followed by exporting the bending line and unfolded drawing.
Points to Note in the Application of Multi-Bend Forming Process
Processing Thin Sheet Metal Workpieces
In production, it has been observed that for some workpieces with relatively small thickness (around 2mm), severe deformation occurs during processing, leading to a significant decrease in processing accuracy, which fails to meet design requirements. The analysis indicates that this is due to insufficient release of internal stress within the sheet metal. Upon observation during laser cutting, it was noted that this batch of sheet metal had already exhibited workpiece warping due to internal stress. From another perspective, this provides a preliminary judgment as to whether the multi-bend process can be directly used for processing thin sheet metal large arc-shaped workpieces. That is, by observing the deformation during laser cutting, if severe deformation occurs, measures must be taken to release the internal stress; otherwise, it is difficult to obtain qualified workpieces.
If the production cycle permits, natural aging is an economical and effective method. However, workpieces that have been aged for a long time are prone to surface rust, requiring the use of rust removers or shot blasting, which increases labor and equipment costs. Therefore, directly using steel plates that have undergone sufficient natural aging is the best choice.
If the production cycle does not allow for natural aging, annealing can be used after the separation of the sheet metal to eliminate internal stress. However, the material hardness decreases after annealing, so it is necessary to comprehensively consider whether annealing is required according to the design requirements of the workpiece.
Calculation of the Unfolded Diagram for Workpieces using the Multi-Bend Forming Process
It is essential to note that when using the multi-bend forming process to process large arc-shaped workpieces, the unfolded diagram must be calculated based on the actual state of the workpiece, i.e., using the arc segment approximated by line segments for the calculation, rather than directly using the unfolded diagram calculated based on the arc. Otherwise, the workpiece will inevitably deviate after processing.
Dimensional Inspection of Workpieces Processed using the Multi-Bend Forming Process
It is worth noting that when processing workpieces using the multi-bend forming process and conducting dimensional inspection using general comparison templates for arc size, the comparison template must be designed as an external card type, with the outer side of the workpiece arc serving as the inspection surface. If the inspection template is designed with the inner side as the inspection surface, interference between the template and the line segments will inevitably lead to situations where the template cannot fit properly, rendering the inspection impossible.
Conclusion
As a clever process method, multi-bend forming of large arcs has its flexible and efficient characteristics, but it also has its limitations. It is necessary to select the processing method based on the specific requirements and usage scenarios of the workpiece, conducting a comprehensive analysis from various aspects such as production cost and workpiece appearance quality.
