How to Compensate for Press Brake Deflection

Estimated reading time: 12 minute

When the sheet metal is bent, because the die has been compensated for convexity, when the bending is unloaded, the spring back will produce new longitudinal deflection. After the two effects are superimposed, the final deflection of the part is obtained. After analyzing the influence of the two types of compensation devices on the accuracy of the parts, the article compares the respective characteristics of the commonly used deformation compensation devices and points out that the upper die plus convex compensation is more advantageous. Therefore, a new type of compensation structure that can quickly adjust the upper die plus convex amount is developed. , Is one of the development directions of bending machines in the future.

When the bending machine is bending the workpiece, under the action of the bending force, the sliding block and the worktable will be deformed. At this time, the depth of the upper mold entering the lower mold opening is inconsistent with the full length of the workpiece, which will seriously affect the workpiece Accuracy. To this end, people have developed a variety of structural deformation compensation devices. It can be roughly divided into two categories: one type is a set of symmetrical curves that increase the center of the worktable to bulge upwards, which is called lower die plus convex compensation; the other type is to increase the upper die or slider to bulge the center downwards Symmetrical curve is called upper die plus convex compensation device.

The Influence of Two Types of Different Compensation Devices

• After the compensation device is convex, the deformation of the slider and the worktable during bending

For the convenience of description and expression, the slider and the workbench are simplified into slender rectangles. Without any compensation, the slider and worktable deform under the bending force as shown in Figure 1-1(a). At this time, the compensation and convexity f=0, the deformation of the slider is f₁, the deformation of the worktable is f₂, and the compensation device is activated to make the compensation convexity f=f₁+f₂.

In this ideal situation, the deformation curve of the press brake with lower die compensation is shown in Figure 1-1(b), and the deformation curve of the press brake with upper die compensation is shown in Figure 1-1(c). At this time, the depth of the upper mold entering the lower mold opening remains the same over the full length of the mold, and the bending angle of the sheet material after bending will also be the same over the full length. Of course, such an ideal situation is difficult to achieve, and people always try to get close to it.

From the above analysis, it can be seen that the two types of different compensation devices can effectively reduce the angle error of the bending part. But there is a difference in the impact on straightness.

• Natural deflection of bending parts

After the sheet is bent, the edges of the bent part will naturally bend, which is usually expressed by its maximum deflection δ. As shown in Figure 1-2.

When the sheet metal is bent, the metal in the bending deformation zone is in a highly plastic deformation state, and the longitudinal compressive stress parallel to the OX direction is generated in the inner arc of the deformation zone, and the outer layer is longitudinal tensile stress. These two opposing tension and compression stresses form a longitudinal moment M_y that rotates around the OY axis. It is the moment required to keep the longitudinal direction (OX direction) of the bent part consistent with the longitudinal line corresponding to the mold when the sheet is bent. After bending, the slider returns, the bending force and the longitudinal moment disappear at the same time, and the metal layers in the deformation zone rebound immediately, forming a bend in the longitudinal direction opposite to the longitudinal moment, that is, natural deflection. For the convenience of presentation, the bending deformation zone is expanded into a plane here. Under the action of bending force, the upper layer (inner arc layer) is compressed longitudinally, and the lower layer (outer arc layer) is under tension. The process and state of the force and rebound deflection of the sheet are shown in Figures 1-3.

The deflection of the bent part is generated during the unloading process, which is equivalent to adding an elastic moment M_y of equal value in the opposite direction of the longitudinal moment. According to the mechanics of materials, the calculation formula of the maximum deflection can be obtained:

Where: M_y—longitudinal moment;

L—the length of the bending piece;

E—material elastic modulus;

J_y—the moment of inertia of the mandrel Y of the cross-section of the bending piece.

When you notice that the mold has been compensated for convexity when the sheet is bent, the longitudinal generatrix of the deformation zone is not a straight line immediately before the bending part is unloaded, but a curve consistent with the compensation convexity. Its deflection value f=f₁+f₂, as shown in Figure 1-4. The rebound of the bent part after unloading is the second deflection that occurs on this basis. Obviously, the final deflection of the bent part must be considered to compensate for the influence of the convex curve.

• The influence of two types of different convex compensation methods on the straightness of bending parts

When the convex compensation of the lower die is affected, the curve of the compensation and the convexity bulges upward, and when the convex compensation of the upper die is applied, the curve of the convexity bends downward. Their change curves are shown in Figure 1-5 (a) and (b). The natural deflection curve of the bending part is an upward bulge, as shown in Figure 1-5(c).

Compensation plus convex amount f depends on the deformation amount of the slider and the worktable when bending, and its value is small. Compensation for the convexity increases the convexity and deflection formed by the bending parts, which will be somewhat reduced when the spring back is unloaded. This makes the deflection formed by the convexity generally lower than the natural deflection δ of the bent part.

It can be seen from Figure 1-5 that when the lower die is used for convex compensation since the convexity curve f is in the same direction as the natural deflection curve δ, the comprehensive deflection of the bending part increases. If the upper die plus convex compensation is used, the direction off and δ is opposite, and the comprehensive deflection of the bending part is reduced. Obviously, the use of the upper die plus convex compensation is beneficial to improve the straightness accuracy of the part. This conclusion is also proved by the following examples.

1. During the inspection of the working accuracy of the bending machine, it was found that the upper die plus convex compensation and the lower die plus convex compensation of the bending machine, when their specifications are the same, the rigidity is approximately equal, after the test piece is bent, the test piece is measured at the full length The upper deflection value, the former is generally smaller than the latter. This phenomenon is especially obvious when the bending machine is fully loaded. Because the amount of deformation is the largest at full load, and the amount of compensation plus convexity is also large, the resulting compensation effect is more significant.
2. In order to improve the accuracy of the parts, a set of compensation device for the upper mold wedge and the convexity is installed on the bending machine with the lower mold and convex. This configuration method has been widely adopted on medium and small CNC bending machines with hydraulic cylinders on the bottom beam with convex compensation. This helpless move also reflects the advantages of the upper die plus convex compensation in improving the accuracy of the parts.

Comparison And Analysis of Common Deformation Compensation Devices

• Hydraulic cylinder of lower beam plus convex compensation

The compensation device is composed of a crossbeam, a supporting plate, and an oil cylinder. A set of oil cylinders is set in the lower cross beam, as shown in Figure 1-6.

After the oil cylinder is filled with pressure oil, the beam will bulge upward to form a set of controllable convex curves. It is now widely used in CNC bending machines. Its characteristics:

1. The cylinders are evenly distributed in the beam, and the convex curve is close to the deformation curve of the slider and the table over the full length of the worktable.
2. Use hydraulic system pressure to control the protrusion amount, which is convenient and quick to operate.
3. It can improve the accuracy of the bending angle of the workpiece.
4. The structure is more complicated and the cost is high.

• Wedge in the worktable plus convex compensation

Multiple sets of wedges are installed under the worktable, and the bevel angle of each set of wedges is designed according to certain requirements. The upper wedge of each group of wedges has a fixed horizontal position. When the lower wedges move to the left at the same time, the work surface will bulge upwards according to the design requirements, as shown in Figure 1-7. It has been widely used in various types of bending machines. Its characteristics are:

1. The wedges are evenly distributed in the worktable, and the convex curve is designed as a deformation curve between the slider and the worktable, and the convex compensation is more accurate.
2. The movement length of the lower wedge is used to control the convexity, which can be manual or motorized, which is convenient for operation.
3. It can improve the accuracy of the bending angle of the workpiece.

• Upper die wedge plus convex compensation

There are multiple sets of wedges between the slider and the upper mold, and the specifications of each set of wedges are the same, as shown in Figure 1-8. The connecting plate and the lower edge of the wedge are fixed as a whole, and the upper wedge can be moved to obtain a downwardly swelling and convex curve, and finally, they are fixed between the slider and the upper mold with a pressing plate. Its characteristics: the wedges are evenly distributed under the sliding block, and the best convex curve can be obtained by adjusting each wedge; the structure is simple, the cost is low, but the operation is inconvenient, and it is commonly used on medium and small ordinary bending machines. Application: After precise adjustment, the accuracy of the bending angle of the part can be effectively improved, and the straightness of the part is better.

• Slider hydraulic cylinder plus convex compensation

Set a group of oil cylinders in the middle of the slider, as shown in Figure 1-9. After the oil cylinder is filled with pressure oil, the middle part of the slider bulges downward to form a controllable local convex curve. Due to structural limitations, effective convexity cannot be obtained on both sides of the slider, and this compensation method has not yet been widely adopted. Its characteristics are: the convexity compensation is concentrated in the middle of the slider, and a reasonable convexity curve cannot be formed on the full length of the upper die; the convexity is controlled by the hydraulic system pressure, which is convenient and quick to operate; it can appropriately increase the angle and straight line of the part. Degree accuracy.

When the bending machine is working, the convexity of the deformation compensation should be equal to the deformation of the slider and the worktable. This requires that the convexity of the compensation can be over the full length of the mold, which is convenient and quick to adjust. However, the currently used upper die plus convex compensation structure is difficult to achieve this requirement, which limits its scope of use.

In order to improve the working accuracy of the press brake and give full play to the advantages of the upper die plus convex compensation, the development of a new structure that can quickly adjust the upper die plus the convex amount is one of the future development directions of the press brake. At present, some departments have made useful attempts in this regard and achieved good results.