What Is the Influence of Press Brake Crowning Device on Accuracy?

1. Introduction

When the press brake bends the workpiece, under the action of bending force, the ram and the worktable will deform.

At this time, the depth of the upper die entering the lower die opening is not consistent with the whole length of the workpiece, which will seriously affect the precision of the workpiece.

For this reason, people have developed a variety of deformation compensation devices, which can be roughly divided into two categories.

  • One is that the worktable is convex as a group of symmetrical curves with the middle part rising upward, which is called the lower die convex compensation;
  • The other is that the upper die or ram is raised as the symmetrical curve of the middle downward uplift, which is called the upper die convex compensation.

2. The influence of two kinds of crowning devices on the accuracy of parts

2.1 Deformation of ram and worktable during bending after convex compensation device

For the convenience of description and expression, the ram and the table are simplified into slender rectangles.

Without any compensation, the ram and the table deform under the action of bending force.

At this time, the compensation convex amount f = 0, the ram deformation is f1, and the workbench deformation is f2.

Then it should start the compensation device to make the compensation convex value f = f1 + f2.

In this ideal case, the depth of the upper die entering the lower die opening remains the same on the whole die length, and the bending angle of the sheet metal after bending will be the same in the whole length.

Of course, such an ideal situation is difficult to achieve, but people always try to get close to it.

From the above analysis, it can be seen that the two kinds of compensation devices can effectively reduce the angle error of bending parts.

But the influence on straightness is different.

2.2 Natural deflection of bending parts

After bending, the edge of the bending part will be naturally flexed, which is usually described by its maximum deflection δ.

When the sheet metal is bent, the metal in the bending deformation area is in a state of high plastic deformation.

In the deformation zone, the inner layer of the circular arc produces longitudinal compressive stress parallel to OX direction, and the outer layer is longitudinal tensile stress.

These two opposite tensile and compressive stresses form a longitudinal moment My rotating around the OY axis.

It is the moment required to keep the longitudinal direction (OX direction) of the bending part consistent with the corresponding longitudinal line of the die during bending.

After bending, the ram returns, the bending force and longitudinal moment disappear at the same time, and the metal layers in the deformation area rebound immediately.

In the longitudinal direction, the bending opposite to the longitudinal moment is formed, i.e. natural bending.

For the convenience of expression, the bending deformation zone is expanded into a plane.

Under the action of bending force, the upper layer (circular inner) of metal is compressed longitudinally and the lower layer (circular outer) is tensile.

2.3 Influence of two different convex compensation methods on the straightness of bending parts

Fig. 6 Long parts bending and crowning

When the lower die is added with convex compensation, the curve of the compensating convex amount is raised upward.

When the upper die is added with convex compensation, the curve with convex amount is bent downward.

The curve of natural deflection of bending parts is upward bulge.

The amount of compensation convex is determined by the deformation of the ram and the table when bending, and its value is small.

The deflection caused by compensating for the amount of bulging will be reduced in unloading springback.

As a result, the deflection formed by the convex amount is usually lower than the natural deflection δ of the bending part

3. Comparative analysis of common deformation compensation devices

3.1 Convex compensation for hydraulic cylinder of the lower crossbeam

After the oil cylinder is filled with pressure oil, the crossbeam will rise upward and form a set of controllable convex curves, and it has been widely used in CNC press brakes.

Its characteristics are as follows:

1) The cylinder is evenly distributed in the crossbeam, and the convex curve is close to the deformation curve of the ram and the table on the whole length of the worktable.

2) It is easy to operate with hydraulic system pressure control.

3) It can improve the precision of the bending angle.

4) It has a complex structure and high cost.

3.2 Convex compensation of module in the worktable

There are many groups of wedges under the table, and the angle of each group is designed according to certain requirements.

The horizontal position of the upper wedge of each group of wedges is fixed, and the lower wedge moves to the left at the same time.

The working table is raised upward according to the design requirements, and it has been widely used in various types of press brakes.

Its characteristics are as follows:

1) The wedges are evenly distributed in the worktable, and the curve after convex is designed as the deformation curve of ram and worktable, and the convex compensation is relatively accurate.

2) It can be controlled by the moving length of the lower wedge, and it can be operated manually or flexibly.

3) It can improve the precision of the bending angle.

3.3 Upper die wedge convex compensation

A plurality of modules are arranged between the ram and the upper die, and the specifications of each group of wedges are the same.

The connecting plate and the lower wedge of the wedge are fixed as a whole.

Moving the upper wedge can obtain a downward convex curve.

Finally, they are fixed between the ram and the upper die by pressing plate.

Its characteristics:

The wedge blocks are evenly distributed under the ram.

By adjusting each wedge, the best convex curve can be obtained.

The structure is simple, the cost is low, but the operation is not convenient.

It is widely used in small and medium-sized ordinary press brakes.

After precise adjustment, the accuracy of the bending angle can be effectively improved, and the straightness of the parts is good.

3.4 Convex compensation of ram hydraulic cylinder

A group of oil cylinders is set in the middle of the ram.

After the oil cylinder is filled with pressure oil, the middle part of the ram rises downward, forming a controllable local convex curve.

Due to the structure limitation, the two sides of the ram can not be effectively convex, so the compensation method has not been widely used.

It has the following characteristics:

The convex compensation is concentrated in the middle of the ram, and a reasonable convex curve can not be formed on the whole length of the upper die;

The accuracy of the angle and straightness can be improved properly.

4. Conclusion

When the press brake is working, the convex amount of the deformation compensation should be equal to the deformation of the ram and the worktable.

This requires that the compensation convex amount can be adjusted conveniently and quickly on the whole die length.

However, the current structure of punch convex compensation is difficult to achieve this requirement, which limits its application.

In order to improve the working accuracy of the press brake and give full play to the advantages of the upper die convex compensation, the development of a new structure for rapid control of the upper die crown is one of the development directions of the future press brake.

At present, some departments have made beneficial attempts in this field and achieved good results.

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