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Bump Bending: To Achieve Large Bend Radius in Sheet Metal

Large arc workpieces are a common type of locomotive steel structure sheet metal products. Their structures are variable and the processing difficulty is greater than ordinary workpieces.

In this post, we introduce a process method to achieve large bend radius in sheet metal parts, and describe the use method in detail with the specific bend workpiece.

In the practical production of the workpieces with large radius, there are generally three forming methods:

  1. Integrally formed by single-step forming die

It is suitable for workpieces with complicated shapes, and has the advantages of good forming effect, smooth and flat surface of the workpiece, no indentation and high processing efficiency. However, the cost of the mold is high and the versatility is poor. It is usually used when the general process cannot be formed or the workpiece has high requirement for the surface.

  1. Customized punch with large radius for press brake machine

This type of mold is generally applicable to the case where the workpiece has a large radius but does not exceed the limit of the table of the press brake machine. The cost is lower than that of a single-step forming die, and the punch has certain versatility;

  1. Multi-bend forming method(step bending)

The basic principle is to convert the arc into a line segment, which can be formed by using the existing molds and equipment. Generally, non-exposed parts can be used. The application range is wide, the cost is low, no custom mold is required, and the forming quality is good.

Analysis of bending sheet metal parts with large radius

The basic principle of the multi-step forming process is to roughly divide the arc segment into polyline segments, as shown in Figure 1, so as to achieve the purpose of bending sheet metal with large radius with small radius punch.

Compared with the arc segment, the forming effect of the polyline segment is positively related to the number of polyline segments.

polyline segments

With this process method, it is difficult to avoid a prismatic structure on the surface of the workpiece, but for non-exposed workpieces, considering the production cycle and cost comprehensively, this process method can be used.

Application of bump bending

How to determine the polyline segmentation and bending angle of circular arc segments

As shown in Figure 2, the inner radius of the bending arc of this workpiece is R350, the bending angle is 120° and the plate thickness is 5mm.

Since the use environment of the workpiece satisfies the use conditions of the above-mentioned bump bending method, the multi-step bend forming method is used for processing.

According to the past experience and the existing mold conditions in the workshop, the upper mold adopts R120 radius punch.

Application of bump bending Application of bump bending

After analyzing the large arc segment of the workpiece in Figure 2, this R350 arc segment is divided into 6 polyline segments.

At this time, it should be noted that in order to ensure the smooth transition of the arc segment and the straight segment of the workpiece (ie, the two ends of the arc segment), the segmentation angle should be set to half of the other segments where the arc segment is tangent to the straight segment .

According to FIG.3, it can be seen that the workpiece of FIG.2 is formed by bending 7 times.

When the electro-hydraulic servo press brake machine bending the sheet metal parts, three basic parameters are required, which are the thickness of the sheet, the bending angle and the position of the bending line.

The first and second split angles are calculated as 6° and the others are 12°. The bending angle after the split can be directly measured by CAD software. The specific bending angle is shown in Figure 3.

segment of the bend

Bending line position size and unfolded size confirmation

To ensure the accuracy of the unfolded dimensions of the bend line, there are generally two methods, namely the neutral layer expansion calculation method and the software-assisted expansion method.

In order to obtain the bending data concisely, quickly and accurately, the software-assisted expansion method is used for calculation.

Directly import the divided part cross-section into Catia, use the software Genetative Sheetmetal Design module to generate the workpiece model, and then export the bend line and unfolded view, as shown in Figure 4; axonometric drawing of the workpiece is shown in Figure 5.

bend line and unfolded view

Tips in the application of bump bending process

Sheet metal processing

It was found in production that some workpieces with a small thickness (about 2mm) will have serious deformation during processing, which will cause the processing accuracy to drop severely and fail to meet the design requirements.

Analysis of the reason is due to insufficient stress release in the sheet.

We observe that during laser cutting and blanking, the workpiece in this batch has warped due to internal stress.

On the other hand, this also provides a preliminary method to judge whether it can directly use the multi-bend process to bend thin sheet metal with large radius, that is, observe the deformation of the workpiece during laser cutting. If the deformation is serious, measures must be taken to release its internal stress, otherwise it is difficult to obtain a qualified workpiece.

If the production cycle allows, natural aging is an economical and effective method.

However, the separated workpieces undergo aging treatment for a long time, which will inevitably produce floating rust on the surface, so it must be wiped or shot blasted with a rust remover, which increases labor and equipment costs.

Therefore, the direct use of steel plates that have undergone sufficient natural aging is the best choice.

If the production cycle is not allowed, after the sheet is separated, the annealing can also be used to eliminate the internal stress, but the hardness of the material after annealing is reduced.

Therefore, it is necessary to comprehensively consider whether annealing is performed according to the design requirements of the workpiece.

Calculation of workpiece expansion diagram in bump bending

It must be noted that when using the multi-step bending method to process sheet metal with large radius, the actual workpiece conditions must be followed.

That is, the expanded view is calculated using the arc segment after the polyline segment is approximated, and the expanded view of the workpiece calculated according to the arc cannot be directly used, otherwise the workpiece will inevitably be out of tolerance after processing.

Shape detection of workpiece using multi-step bending process

It is worth noting that when the workpiece processed by the bump bending, and to detect the arc size using a general comparison match sheet, the match sheet must be designed as an outer seizing type, and the outside of the workpiece arc is used as the detection surface.

If the comparison match sheet is designed with the inner side as the detection surface, and the match sheet interferes with the polyline segment, it will inevitably occur that the match sheet is not in place, resulting in detection failure.

Final Thoughts

As an ingenious process method, large arc bump bending has its characteristics of flexibility and efficiency, but also has its limitations.

The specific problems must be analyzed in accordance with the design requirements and application of the workpiece.

It is necessary to comprehensively consider the production cost and the appearance quality of the workpiece to select the processing method.

Step Bending Large Radi

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