How the Size of the Lower Die Opening in Air Bending Affects the Forming Dimensions?

Bending is a method of processing metal blanks into certain angles and shapes by means of bending, drawing etc.

It has high flexibility, wide applicability and low cost.

It is widely used and accounts for a large share in the sheet metal processing industry.

In the production of steel structures for modern electric locomotives, important structural parts are mostly made of medium thick plates with large R-angle design.

The bending process of such workpieces is generally completed by CNC press brakes.

Through the simple up and down movement of the ram, it cooperates with the bending tool to complete the forming process of more complex workpieces.

In production, it is found that under the same processing equipment, materials, and upper dies, workpieces formed by air bending are often slightly different in size due to different production batches.

After ruling out differences in material thickness and internal stress relief due to different furnace numbers, it was found that the problem could be attributed to the machine operator’s use of different opening sizes of the lower die for bending operations.

In this article, the effect of the selection of the lower die opening size on the forming dimensions in plate bending will be briefly discussed for the purpose of production guidance.

Two common bending methods and comparison

Air bending

Air bending is also called gap bending, air bending, etc.

In the bending process, the upper die and the lower die are not pressed together, and the desired bending angle is obtained by adjusting the depth of the opening of the upper die into the lower die.

The deeper the upper die enters the lower die, the less angle the bend gets, and vice versa.

And due to the influence of the bending rebound, the bending needs to be over-bent to ensure that the bending angle after the rebound is the same as the design angle, the bending state can be seen in Figure 1.

Free bending diagram

Figure 1 Air bending diagram (simplified lower die radius)

Currently, widely used CNC bending machine can automatically calculate the depth of bending through its CNC system.

With its feedback correction system control equipment hydraulic unit, it can achieve bending angle automatic control, the operator is not allowed to participate too much.

However, in actual production, it is difficult to reach the programmed angle through one bending due to factors such as deviations in the calculation model, errors in sheet thickness, differences in material types, and stress release within the material, so trial bending is still required before mass production.

The process method discussed in this paper is air bending.


With coining, the sheet is pressed between the upper and lower dies and the material is bent freely at the beginning.

As the upper die is pressed down, the material and the surface of the lower die are gradually brought closer together, and the bending zone of the material decreases until the lowest point of the stroke, when the material is completely close to the upper die.

The required bending angle and bending radius are obtained by bending force, and the bending process is shown in Figure 2.

Coining process

Fig. 2 Coining process (simplified lower die radius)

Air bending vs coining

Due to its high flexibility, wide applicability, low cost and other characteristics, air bending gradually exceeded the coining mode, becoming the preferred process method of sheet metal processing enterprises.

Compared with coining, the bending pressure of air bending is generally only one-third of the size of coining, which greatly reduces the tonnage requirement of the bending machine and is highly effective in cost control.

On the other hand, the lower die angle of coining determines the bending angle of the final formed product.

It is not adaptable to the current sheet metal market that pursues individual customization and flexible production, so is more suitable for medium and large-scale production.

And because of its excessive bending pressure, it can generally only be used to process thin sheets.

Although air bending has some shortcomings in terms of product accuracy, with the continuous development of bending equipment, the gap has been gradually reduced to an acceptable deviation for most products.

Influence of air bending die opening size on forming dimensions

In order to compare the effect of the selection of the die opening size on the bending shape size, a simple verification experiment is designed.

Experimental conditions

In order to ensure the reliability of the verification experiment, try to avoid the influence of the expected external variables on the experimental results, combined with the actual conditions of the experimental site and facilities, more comprehensive consideration of the type of materials used in the experiment, the direction of discharge, and the type of dies are carried out.

The conditions are shown in Table 1.

Table 1 Basic conditions of verification experiment

NO. Project name Content Note
1 Specimen material t16-S355 Same with the furnace number
2 Blanking CNC Fine Plasma Cutting Post-cut shot blasting
3 Workpiece machining Horizontal milling of both ends
4 Workpiece configuration The bending line is perpendicular to the direction of rolling of the sheet.
5 Workpiece specifications 300mm*B Actual measurement after B numerical milling
6 Experiment equipment 500T CNC press brake Amada
7 Upper die R40 overall upper die
8 Lower die Adjustable lower die for openings
9 Backgauge Test-fold and fix to ensure identical positioning dimensions.
10 Detection tools 500mm vernier caliper, wide seating square 50 graduation

Experimental process

The target measurement values of the verification experiment are the dimensions L1 and L2 of the workpiece after bending, and the sum L (L=L1+L2) is taken as the experimental comparison value, and the experimental variable is set as the size of the lower die opening.

At the same time, the adjustable opening size of the lower die is used to avoid the influence of other structural factors of the die on the experimental results, the structure of the specimen is shown in Figure 3.

Specimen structure

Figure 3 Specimen structure

During the experiment, the specimen was first measured by a 500mm vernier caliper after machining, and the linear dimension of the two processing surfaces at its end was measured to be 557.50mm.

Then the size of the opening of the lower die is gradually increased and a multiple trial folding is performed.

From the test pieces that are bent to form under each opening size, it should use the wide seat square to compare and select the test piece with the best bending angle.

Then the selected specimen size L1, L2 values are measured, and the experimental comparison value L is calculated.

Experimental results

It should use six different sizes of die opening from 160mm to 400mm, select the six best specimens from the folded sample and measure the dimensions L1 and L2 to obtain the calculated value L according to L=L1+L2.

The workpiece size L (opening size 160mm) folded by the 160mm lower die opening size is used as the reference size.

The deviation is compared with the L value of other test pieces and the results are shown in Table 2.

Table 2 The effect of the opening size of the lower die on the bending forming size

NO. The opening size of the lower die Calculated value L


 Deviation value
1 160 596.12 0
2 180 596.14 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

From the experimental results, it can be seen that there is a positive trend of correlation between the bending shape size and the opening size of the lower die.

It is calculated that the theoretical L-value of this specimen after bending should be 596mm.

If using the measured 596.12mm of the workpiece after folding a  lower die opening with 160mm die as a benchmark, when the opening size is 10 to 12.5 times the sheet thickness, the size is within the allowable tolerance for sheet metal parts.

Deviations from the normal workpiece tolerances are exceeded for lower die openings up to 300 mm.

The deviation is 10.02mm when 400mm is chosen as the opening size of the lower die, which is obviously a serious deviation from the size of the workpiece.

It can be seen, in the air bending of the plate, the selection of the size of the lower die opening has an important impact on the size of the workpiece forming.

Considering the selection of the lower die opening value, it should be about 10 times the thickness of the board, which makes it easy to guarantee the bending and forming dimensions.

Of course, the size of the lower die opening should also be chosen in consideration of the R-angle of the bend, otherwise, if a lower die with too small an opening is used, the ram on the machine will not be able to move down to a sufficient depth, which will lead to the bend incomplete, or even risk damage to the tooling.

Analysis of the causes of the influence of the opening size of the air bending bottom die on the forming dimension of the medium-thick plate

From the experimental validation results, it can be seen that the bend-formed size is positively correlated with the lower die opening size.

In this experiment, the L-shaped specimen is 557.50 mm long and all specimens are the same size.

It can be concluded that due to changes in the size of the opening, there is a tendency for the dimensions L1 and L2 to increase when the plate is formed by air bending of the workpiece.

Obviously, this change is caused by the change of the inner R angle after actual forming.

Since there are no accurate means to measure the inner R-angle after forming, it can be inferred that the size of the inner R-angle of the formed part is also positively correlated with the size of the opening of the lower die.

To ensure the accuracy of the formed dimensions of the workpiece, the smallest possible opening size of the lower die should be selected for bending.


This article mainly discusses the effect of the size of the die opening on the size of the formed workpiece when air bending is selected for thick plates.

A simple verification experiment shows that when other process conditions of air bending are the same, the opening size of the lower die and the bending forming size show a positive correlation trend.

Therefore, when the size of the workpiece is demanding, especially when the inner R-angle forming size is strict, the coining method and corresponding tooling should be used for forming, which can achieve twice the effect with half the effort.

In addition, due to the limitations of equipment, personnel, measuring tools and other factors, the verification experiments involved in this article are not rigorous and inaccurate.

But from the experimental results, it can play a good role in explaining the problem and guiding the production, which has some practical significance.

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