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Process method for realizing large arc bending of work-piece

The large arc-shaped work-piece is a common type of locomotive steel structure sheet metal products. Its structure is changeable and the processing difficulty is larger than ordinary work-pieces. This article introduces a process method for realizing large arc bending of work-pieces. The use method is described in detail in conjunction with a specific work-piece. In the production practice of large arc work-pieces in the material preparation workshop, there are generally three forming methods:

First one is a single-step molding die forming, which is suitable for work-pieces with complex shapes, good forming effect, smoothly surface, no indentation and high efficiency.  but the cost of the mold is high and the versatility is poor. It is usually used in the case that the general process cannot be formed or the surface of the work-piece with high requirement;

Second one is the customized mold for press brake large arc . The radius of the work-piece arc is large, but it does not exceed the limit of the bending machine table. The cost is lower than that of the single-step pressing die, and the mold has certain versatility.

The third one is the bumping forming method introduced in this article. The basic principle is the arc is converted into a polygonal line segment, and the existing mold and equipment can realize forming. Generally the non-exposed parts can be used, the application range is wide, the cost is low, and the customized mold is not needed, and the forming quality is good.

Analysis of bumping forming process for large arc work-piece

The basic principle of the bumping forming process is to roughly divide the arc segment into polyline. As shown in Fig. 1, to achieve the purpose of processing a large arc work-piece with a small arc tire. Compared with the circular arc segment, the forming effect of the polyline is positively related to the number of polyline. With this method, it is difficult to avoid the appearance of a prismatic structure on the surface of the work-piece. But for the non-exposed work-piece, the production cycle and cost are comprehensively considered. This process methods can be used.

Figure 1 approximately simplified of the arc poly-line

 

Process method for realizing large arc bending of work-piece (2)

Figure 2 Example of work-piece

Large arc bending process application example

Poly-line segmentation and bending angle determination of arc segments

As shown in Fig. 2, the radius of the bending arc of the work-piece is R350, the bending angle is 120°, and the thickness is 5 mm. Since the work-piece working environment satisfies the use condition of the above-mentioned large arc bumping forming process, The bumping forming method is used for processing. According to the previous experiences, combined with the workshop existing mold, the upper mold adopts the R120 circular arc mold. Analyze the large arc segment of the work-piece in Fig. 2, and divide the R350 arc segment into 6 poly-line . At this time, it should be noted that generally the arc segment of the work-piece is tangent to the straight segment (two end positions of the arc segment ).

The Tangent place between arc segment and straight segment, the segmentation angle should be the half of the other segments. As we can see from Figure 3, the work-piece in Figure 2 is formed by 7 bends in electro-hydraulic servo press brake. It need three basic parameters, metal sheet thickness, bending angle and bending line position size. The first and end division angles are calculated as 6°, the others are 12°, and the bending angle is utilized after division. The CAD software directly measures(the specific bending angle as shown in Figure 3).

Figure 3 Segmentation of the bend segment

Bend line position size and expansion size confirmation

To ensure the accuracy of the bending line dimension , there are generally two methods. The neutral layer expansion calculation method and the software-assisted expansion method. To obtain the bending data in a simple, fast and accurate manner, the software-assisted expansion method is used for calculation. Directly put the segmented part profile into Catia, use the software Genetative Sheetmetal Design module to generate the work-piece model, and then derive the bend line and the expansion picture, as shown in Figure 4; the work-piece isometric view is shown in Figure 5.

Process method for realizing large arc bending of work-piece (5)

Figure 4 Expanded view and bend line position

Process method for realizing large arc bending of work-piece (4)

Figure 5 work-piece isometric drawing

Several problems in the bumping forming process should be paid attention to

Thin Metal Sheet work-piece processing

In the production, it is found that some work-pieces with a small thickness (about 2mm) will have serious deformation during processing. It will seriously reduce the processing accuracy and fail to meet the design requirements. The reason for the analysis is that the stress release in the plate is insufficient. When the laser cutting is performed, the batch of sheets has warpage due to internal stress. At the another hand, it also provides a method. For preliminary determination of whether or not the bumping process can be directly used to process a thin plate large arc-shaped work-piece. To observe the deformation of the laser cutting material. If the deformation is serious, measures must be taken to release the internal stress, otherwise it is difficult to obtain a qualified work-piece.

If the production cycle permits, natural aging is an economical and effective method. However, after the separated work-piece has been treated for a long time, it is inevitable to produce floating rust on the surface. It must be wiped or shot blasted with rust remover. It will increase manpower and Equipment costs, so the direct use the fully natural failure steel plate is the best choice.

If the production cycle is not allowed, after the separation of the plates, the annealing process can also eliminate the internal stress, but the hardness of the material will down after annealing decreases. It is necessary to comprehensively consider whether to anneal according to the design requirements of the work-piece.

Calculation of work-piece expansion image when using multi-turn forming process

It must be noted that in the bumping forming process. When process a large arc work-piece,the expansion image must be calculated according to the actual work-piece state. Using the arc segment after the approximated poly-line segment. and the work-piece expansion image calculated cannot be directly used. Otherwise the work-piece will be inferior after processing.

Shape inspection of work-pieces using multi-turn forming process

It is worth noting that during using a multi-turn forming process. And general comparison template for arc size detection. the comparison template must be designed as an outer card type, and the outside of the work-piece arc is used as the detection surface. If the inside is used as the detection surface to design the comparison template, the comparison between the template and the broken line segment will inevitably result in the template card not being in place, resulting in the detection being impossible.

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