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Stretchers, Positioning And Special Bending Sets In Sheet Metal Bending

When sheet metal bending is used in the production of locomotive parts, many bending workpieces cannot be produced directly according to the design drawings, so we need to add some necessary stretchers, positioning or special bending sets during blanking and remove them after bending to realize the production of the above products.

This post mainly discusses the process problems and solutions in bending processing. It is divided into three parts.

The first part is an overview of bending processing methods, the second part is the possible process problems and solutions in bending processing, and the third part is the conclusion.

Overview of bending processing methods

In essence, bending is a cold deformation process of bending the plate into a certain shape. It is a pressure processing method at room temperature.

In terms of processing materials, metal materials, non-metallic materials and composite materials can be bent.

In this processing method, the workpiece is processed by bending die and hydraulic press brake, and the dimensional accuracy of bending parts is guaranteed by upper and lower dies and bending equipment.

Its action process is that at the sheet metal, the upper die applies bending force and generates reaction force at the support point of the lower die to form bending moment and force the sheet metal to deform.

The final bending angle of sheet metal is determined by the depth of the upper die into the lower die.

Fig. 1 shows the main working part of the NC press brake, which is composed of the rear stop, upper die and lower die.

working parts of bending machine

Fig. 1 Main working parts of bending machine

The rear stop is a working part that controls the position of the bending line.

The upper die and lower die are used to control the bending angle and inner radius of the workpiece.

Whether the workpiece is qualified or not is mainly determined by these three parts.

Possible process problems in bending and their solutions

In the production of locomotive parts, many bending workpieces cannot be produced directly according to the design drawings, so we need to add some necessary stretchers, positioning or special molds during blanking and remove them after bending to realize the production of the above products.

The typical situations that need to set process stretchers are summarized as follows.

Generation and prevention of edge collapse

As shown in Fig. 2, due to the lack of material on one side of the workpiece, the material on one side can not be put on the lower die, resulting in the collapse of some structures to be bent during bending, which obviously can not meet the design requirements.

Collapsible edge structure

Fig. 2 Collapsible edge structure

As shown in the structural detail in Fig. 3, generally, when this angle α is less than 50°, there will be a problem that can not be folded. Process measures must be taken to avoid it and ensure that the workpiece is qualified.

Workpiece with easy collapse edge

Fig. 3 Workpiece with easy collapse edge

Process scheme for solving edge collapse

Fig. 4 shows the general process lacing scheme, and the double dotted line part is the set process lacing.

Process reinforcement scheme

Fig. 4 Process reinforcement scheme

Fig. 5 shows the detailed drawing of process lacing.

Process structure detail of workpieces easy to collapse

Fig. 5 Process structure detail of workpieces easy to collapse

If the length of the workpiece is enough for the material retaining and positioning after the bending machine, it is generally set as L≥ 0.6mm (V is the opening size of the lower die).

It must be ensured that the sheet can be laid on the lower die and exceed the edge of the lower die by more than 3mm.

If the length of the workpiece is short, the tie bar is generally added to the same width as the original structure of the workpiece.

In this way, on the one hand, it can solve the problem of folding, on the other hand, it can also ensure accurate positioning.

Solution to the difficulty of bending line positioning

As shown in Figure 6, a section of this kind of workpiece used for positioning has sharp corners or the straight edge that can be used for positioning is too short, so it is difficult to locate the bending line of the workpiece in the bending process.

The process structure must be added to ensure the accurate positioning of the bending line.

Fig. 6 Bending line positioning

Solution: Fig. 7 and fig. 8 are two process stretching schemes to solve the difficult positioning of the bending line, in which the double dotted line is the process stretching.

Scheme I for solving the difficulty of bending line positioning

Fig. 7 Scheme I for solving the difficulty of bending line positioning

Solution to the difficult positioning of bending line figure 2

Fig. 8 Solution to the difficult positioning of bending line figure 2

Fig. 9 shows the processing diagram of scheme 1. By adopting this scheme, the workpiece can be well-positioned.

Processing diagram of scheme I

Fig. 9 Processing diagram of scheme I

Fig. 10 shows the processing schematic diagram of scheme 2. In this scheme, wing plates are added on both sides of the workpiece, which can ensure that the processed workpiece meets the design requirements.

Processing diagram of scheme II

Fig. 10 Processing diagram of scheme II

Solution for easy deformation of the workpiece

As shown in Fig. 11, when the thickness of the workpiece is small, the workpiece is blanked by the laser cutting machine.

Laser cutting belongs to melting cutting, which will produce quite a high temperature.

When the workpiece has the structure shown in the figure, it may be deformed. The deformed workpiece leads to inaccurate bending positioning, which must be avoided by taking process measures.

Easily deformed structure

Fig. 11 Easily deformed structure

Solution: as shown in Fig. 12, for the large gap shown in the figure, the workpiece is easy to deform during handling or positioning, resulting in inaccurate bending positioning.

Generally, the process stretching shown in the figure will be added.

Adding process stretchers

Fig. 12 Adding process stretchers

Adding stretchers at the notch can improve the overall stiffness of the workpiece and ensure no deformation.

After the bending process is completed, use manual plasma to cut the stretcher.

If the workpiece is large and long, in order to prevent the workpiece from deformation during lifting or installation in the next process, the stretcher shall not be removed, and it shall be removed after the welding in the next process.

Solution that the workpiece groove width is too narrow to bend

As shown in Fig. 13, the groove width of the workpiece is too narrow, and the workpiece interferes with the upper die during bending, resulting in workpiece deformation, processing cannot be completed, and workpiece size cannot meet the requirements of the drawing.

Solution: as shown in Fig. 14, before bending the workpiece, reverse bend a certain angle in the middle of the workpiece, and then bend according to the requirements of the drawing.

Interference between the workpiece and upper die

Fig. 13 Interference between the workpiece and upper die

After pre-bending, the upper die no longer interferes with the workpiece

Fig. 14 After pre-bending, the upper die no longer interferes with the workpiece

After the workpiece is folded, press the pre-folded bend with a flattening die (Fig. 15), and ensure the flatness and size requirements of the workpiece.

Point pressing and flattening with flattening die at pre bend

Fig. 15 Point pressing and flattening with flattening die at pre bend

Solution to the problem that the workpiece is too long and the groove width is too narrow to bend

As shown in Fig. 16, the developed length is 2500mm, the groove width is 100mm, and the leg height is 64mm.

The parameters are input according to the requirements of the process card. R8 V60 ° upper die with height of 134mm is selected as required, and the lower die with No. V07 with an opening of 35mm and a depth of 65mm is selected for the lower mold.

Enter various bending deflection parameters. Lift the workpiece onto the worktable and bend it as required.

When the upper die of R8 gooseneck is bent, because the mold is thick and the gooseneck is too small, when the unilateral bending of the second side is fast to reach 90 °, the first bending edge interferes with the mold, resulting in the inability to guarantee the bending angle.

If forced bending, the workpiece will be deformed, the mold will be damaged, and the workpiece size does not conform to the drawing.

Workpiece groove width is too narrow to be bent

Fig. 16 Workpiece groove width is too narrow to be bent

Solution for too long workpiece and too narrow groove width

Through on-site research and through the study of the parameters of the machine tool itself and the observation of the existing mold, it is found that the existing mold of upper mold R6 has a large gooseneck opening, which meets the process requirements after the test.

The synchronous operation is carried out according to the above workflow, and there is no interference between the mold and the workpiece, which meets the requirements.

After repeatedly adjusting the parameters of the Y-axis and X-axis, the dimensions of each point of the workpiece meet the requirements of the drawing.

After identification, it meets the process requirements.

Conclusion

Process stretching and crack stop groove are indispensable in sheet metal bending and are the key to ensuring workpiece quality and processing efficiency.

It is worth noting that on the premise of ensuring product quality, the flexible application of process stretching and crack stop groove and reducing plate waste rate are also worth discussing.

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