Bending is a machining process that involves shaping metal parts into desired angles and shapes through techniques such as bending and stretching. It offers high flexibility, wide usability and cost-effectiveness, making it a widely used method in the sheet metal processing industry.
In the production of steel frames for modern electric locomotives, the crucial structural parts are mainly made of medium-thickness plates with large R-angle designs. These workpieces are typically bent using CNC press brakes, which use simple up and down motion. using a battering ram and a bending tool to form complex shapes.
However, it has been observed that under the same superior processing equipment, materials and dies, parts formed by air bending may vary in size from batch to batch. After eliminating the effects of differences in material thickness and internal stress relief due to different numbers of furnaces, it was determined that the cause of the size variation was the use of different bottom die opening sizes by the machine operator during the bending operations.
This article aims to provide production guidance by briefly discussing the impact of bottom die opening size selection on forming dimensions in sheet metal bending.
Two common folding and comparing methods
airbending
Air bending, also known as gap bending, is a machining process in which the upper and lower dies are not pressed together. The desired bend angle is achieved by adjusting the depth of the upper die into the lower die. The deeper the upper die enters the lower die, the smaller the bending angle will be and vice versa.
To account for flexural bounce, the bending process must be over-bent to ensure that the final bending angle after rebound matches the design angle. The bending state can be visualized in Figure 1.
Figure 1 Air bending diagram (simplified lower die radius)
Today, widely used CNC bending machines can automatically calculate bending depth through their CNC systems. The machine is equipped with a feedback correction system and hydraulic unit that allows automatic control of the bend angle, minimizing operator involvement.
However, despite these advances, it is still a challenge to achieve the programmed angle in a single bending operation due to several factors, such as deviations in the calculation model, errors in sheet thickness, differences in material types and stress release within the material. As a result, experimental bending is still required before mass production.
The process method discussed in this article is air bending.
Minting
In coining, the sheet is placed between the upper and lower dies and folded freely at first. As the upper die is pushed downward, the material and the surface of the lower die gradually move closer together and the bending area of the material decreases until the lowest point of the stroke, at which point the material is fully pressed against the upper die . The desired angle and radius of curvature are achieved by applying bending force, as illustrated in Figure 2.
Fig. 2 Minting process (simplified lower die radius)
Airbending vs coining
Due to its high flexibility, wide range of applications, low cost and other advantageous features, air bending has overtaken coining as the preferred process method for sheet metal processing companies. Compared with coining, the pressure of air bending is typically only one-third, reducing the tonnage requirement for the bending machine and effectively controlling costs.
On the other hand, the angle of the lower die in coining determines the final bending angle of the product, making it less suitable for today's sheet metal market that prioritizes individual customization and flexible production. It is more suitable for medium and large-scale production. Furthermore, the excessive bending pressure of coining limits its use to processing thin sheets.
Although air bending has some limitations in terms of product accuracy, advances in bending equipment have gradually reduced this deviation to an acceptable level for most products.
Influence of air bending die opening size on forming dimensions
A simple verification experiment was designed to compare the impact of die aperture size selection on the bend shape size.
Experimental conditions
To ensure the reliability of the verification experiment, measures are taken to minimize the influence of potential external variables on the experimental results. The actual conditions of the site and experimental facilities, the type of materials used in the experiment, the direction of discharge and the type of matrices are comprehensively taken into consideration to minimize their impact on the results. The conditions are detailed in Table 1.
Table 1 Basic conditions of the verification experiment
| NO. | Project name | Happy | Observation |
| 1 | Sample material | t16-S355 | Same with oven number |
| two | Suppression | CNC fine plasma cutting | Post-cut blasting |
| 3 | Machining of parts | Horizontal milling of both ends | |
| 4 | Part configuration | The bend line is perpendicular to the rolling direction of the sheet. | |
| 5 | Part specifications | 300mm*B | Actual measurement after numerical milling B |
| 6 | Experimental equipment | 500T CNC press brake | Loved |
| 7 | Upper matrix | General upper matrix R40 | |
| 8 | Die inferior | Adjustable bottom die for openings | |
| 9 | Back gauge | Test bending and clamping to ensure identical placement dimensions. | |
| 10 | Detection Tools | 500mm vernier caliper, wide square seat | 50 graduation |
And experimental process
The purpose of the verification experiment is to measure the L1 and L2 dimensions of the part after bending and use the sum L (L=L1+L2) as a comparative value for the experiment. The experimental variable is the size of the lower opening of the die.
The adjustable aperture size of the lower die is used to eliminate the influence of other structural factors of the die on the experimental results. The sample structure is represented in Figure 3.
Figure 3 Sample structure
During the experiment, the sample was first measured using a 500 mm caliper after machining, and the linear dimension of the two processing surfaces at its end was recorded as 557.50 mm.
Then, the size of the lower opening of the die was gradually increased and multiple bending attempts were made. Of the specimens produced for each opening size, the one with the best bending angle was selected using a wide seat square.
The L1 and L2 values of the selected sample were then measured and the comparative L value was calculated.
Experimental results
Six different die opening sizes, ranging from 160 mm to 400 mm, were used in the experiment. From the folded samples, the six best specimens were selected and the L1 and L2 dimensions were measured to obtain the calculated L value (L=L1+L2).
The size L of the bent part using the bottom die opening size of 160 mm was used as the reference size. The deviation was compared with the L values of the other specimens and the results are presented in Table 2.
Table 2 The effect of lower die opening size on bending forming size
| NO. | The size of the lower die opening | Calculated value L (L=L 1 ＋L 2 ) |
Deviation value |
| 1 | 160 | 596.12 | 0 |
| two | 180 | 596.14 | 0.02 |
| 3 | 200 | 596.22 | 0.1 |
| 4 | 300 | 598.86 | 2.74 |
| 5 | 350 | 602.48 | 6.36 |
| 6 | 400 | 606.14 | 10.02 |
Experimental results indicate a positive correlation between the size of the bend shape and the size of the bottom die opening. The theoretical L value of the sample after bending was calculated to be 596 mm. Using the measured value of 596.12 mm for the bent part with a 160 mm lower die opening size as a reference, it is found that when the opening size is 10 to 12.5 times the sheet thickness, the size is within the acceptable tolerance for sheet metal parts. .
Deviations from normal part tolerances were observed for lower die openings of up to 300 mm. The deviation increased to 10.02 mm when a 400 mm lower die opening size was chosen, a significant deviation from the part size.
These results demonstrate the significant impact that selecting the lower die opening size has on the size of the part formed in air bending. To ensure the desired dimensions, it is recommended to choose a lower die opening size that is approximately 10 times the sheet thickness. However, it is important to also consider the R angle of the bend, as using a lower die with too small an opening may prevent the ram from reaching down far enough, leading to an incomplete bend or even damage to the tooling. .
Analysis of the causes of the influence of the opening size of the air-bent lower die on the forming dimension of the medium-thickness plate
The experimental results show a positive correlation between the size of the curvature formed and the size of the lower die opening. In this experiment, the L-shaped specimen was 557.50 mm long and all specimens were the same size.
It can be concluded that changes in the size of the lower opening of the die result in a tendency for L1 and L2 dimensions to increase when the part is air-bent. This change is likely due to a change in the internal R angle after formation.
Since there is no accurate means to measure the internal R angle after forming, it can be inferred that the size of the internal R angle is also positively correlated with the size of the bottom die opening.
To ensure the accuracy of the formed dimensions of the part, it is recommended to choose the smallest possible opening size for the lower die during bending.
Conclusion
This article focuses on the impact of die opening size on the size of the part formed during air bending of thick sheets. A simple verification experiment revealed that, under the same process conditions for air bending, there is a positive correlation between the size of the lower die opening and the size of the curvature formed.
In cases where the size of the workpiece is critical, especially if strict requirements are imposed on the forming size of the internal R angle, it is recommended to use the coining method and corresponding tools, which can produce twice the desired result with half of effort.
It should be noted that the verification experiments described in this article are not highly rigorous or accurate due to limitations in equipment, personnel, measurement tools, and other factors. However, experimental results can still provide useful explanations and guidance for production and have practical significance.
