During the bending of sheet metal, due to the compensation made by the dies, a new longitudinal deformation occurs during the unloading of the bending, and the combined effect of both results in the final curvature of the workpiece. This article analyzes the impact of two types of compensation devices on the precision of the workpiece and compares the characteristics of commonly used deformation compensation devices. It points out that the upper die convex compensation has more advantages. Therefore, developing a new compensation structure that can quickly adjust the amount of convex compensation for the upper die is one of the directions for the future development of bending machines.
Introduction
When bending workpieces with a bending machine, the slider and the worktable deform under the action of bending force. At this time, the depth of the upper die entering the lower die opening is inconsistent along the length of the workpiece, which seriously affects the accuracy of the workpiece.
To address this issue, various structures of deformation compensation devices have been developed. They can be broadly divided into two categories: one is to add convexity to the worktable, forming a group of symmetrical curves that rise upward in the middle, known as the lower die convex compensation; the other is to add convexity to the upper die or slider, forming a group of symmetrical curves that rise downward in the middle, known as the upper die convex compensation.
Influence of the two types of compensation devices on the precision of the workpiece
2.1 Deformation of the slider and worktable under the compensation of the compensation device
For the sake of convenience in description and expression, here the slider and the worktable are simplified as elongated rectangles. In the absence of any compensation, the slider and the worktable deform under the action of bending force. At this time, with the compensation convexity amount set to 0, the deformation of the slider is represented by f1, and the deformation of the worktable is represented by f2. When the compensation device is activated, the compensation convexity amount is set to f: f1 + f2.
In this ideal situation, the depth of the upper die entering the lower die opening remains consistent along the entire length of the die, and the bending angle of the bent sheet metal will also be the same along its entire length. Of course, achieving such an ideal situation is difficult, but people always strive to approach it. From the above analysis, it can be seen that both types of compensation devices can effectively reduce the angular error of the bent workpiece. However, there are differences in their impact on straightness.
2.2 Natural curvature of the bent workpiece
After the sheet metal is bent, the edges of the bent workpiece will exhibit natural curvature, which is usually represented by its maximum deflection. During the bending of the sheet metal, the metal in the bending deformation zone undergoes highly plastic deformation. Inside the deformation zone, there is a longitudinal compressive stress parallel to the OX direction, while the outer layer experiences longitudinal tensile stress. These opposing tensile and compressive stresses generate a longitudinal moment My that rotates around the OY axis.
This moment is the force required for the sheet metal to maintain its longitudinal alignment with the corresponding longitudinal line of the die during the bending process. When the slider returns after bending, the bending force and the longitudinal moment disappear simultaneously, and the metal layers in the deformation zone rebound, resulting in a bending in the opposite direction of the longitudinal moment, which is the natural curvature. For convenience of expression, here the bending deformation zone is unfolded into a plane. Under the action of the bending force, the upper layer (inner layer of the arc) of the metal is subjected to longitudinal compression, while the lower layer (outer layer of the arc) is subjected to tension.
2.3 Influence of the two different convex compensation methods on the straightness of bent workpieces
When using lower die convex compensation, the curve of the compensation convexity rises upward, while with upper die convex compensation, the curve of the convexity bends downward. The curve of the natural curvature of the bent workpiece rises upward. The value of the compensation convexity depends on the deformation of the slider and the worktable during bending, and it is relatively small. The compensation convexity reduces the convex curvature formed by the bent workpiece during unloading and rebound. As a result, the curvature formed by the convexity is usually lower than the natural curvature of the workpiece.
Comparison and analysis of commonly used deformation compensation devices
3.1 Lower beam hydraulic cylinder convex compensation
After the hydraulic cylinder is filled with pressure oil, the beam will rise upward, forming a controllable convex curve. This method has been widely used in CNC bending machines. Its characteristics are as follows:
- The hydraulic cylinders are evenly distributed inside the beam, and the curve after convex compensation closely approximates the deformation curve of the slider and the worktable along the entire length of the worktable. The convex amount is controlled by the hydraulic system pressure, making the operation convenient and fast.
- It can improve the accuracy of the bending angle of the workpiece.
- The structure is more complex and costly.
3.2 Wedge block convex compensation in the worktable
Multiple sets of wedge blocks are installed below the worktable, and the inclination angles of each set of wedge blocks are designed according to specific requirements.
The upper wedge blocks of each set are fixed in the horizontal direction, and when the lower wedge blocks move to the left simultaneously, the worktable surface rises upward according to the design requirements. This method has been widely used in various types of bending machines.
Its characteristics are as follows:
- The wedge blocks are evenly distributed inside the worktable, and the curve after convex compensation is designed to match the deformation curve of the slider and the worktable. The convex compensation is more precise.
- The convex amount is controlled by the movement length of the lower wedge block, which can be done manually or automatically, making the operation convenient.
- It can improve the accuracy of the bending angle of the workpiece.
Introduction
When bending workpieces with a bending machine, the slider and the worktable deform under the action of bending force. At this time, the depth of the upper die entering the lower die opening is inconsistent along the length of the workpiece, which seriously affects the accuracy of the workpiece.
To address this issue, various structures of deformation compensation devices have been developed. They can be broadly divided into two categories: one is to add convexity to the worktable, forming a group of symmetrical curves that rise upward in the middle, known as the lower die convex compensation; the other is to add convexity to the upper die or slider, forming a group of symmetrical curves that rise downward in the middle, known as the upper die convex compensation.
Influence of the two types of compensation devices on the precision of the workpiece
2.1 Deformation of the slider and worktable under the compensation of the compensation device
For the sake of convenience in description and expression, here the slider and the worktable are simplified as elongated rectangles. In the absence of any compensation, the slider and the worktable deform under the action of bending force. At this time, with the compensation convexity amount set to 0, the deformation of the slider is represented by f1, and the deformation of the worktable is represented by f2. When the compensation device is activated, the compensation convexity amount is set to f: f1 + f2.
In this ideal situation, the depth of the upper die entering the lower die opening remains consistent along the entire length of the die, and the bending angle of the bent sheet metal will also be the same along its entire length. Of course, achieving such an ideal situation is difficult, but people always strive to approach it. From the above analysis, it can be seen that both types of compensation devices can effectively reduce the angular error of the bent workpiece. However, there are differences in their impact on straightness.
2.2 Natural curvature of the bent workpiece
After the sheet metal is bent, the edges of the bent workpiece will exhibit natural curvature, which is usually represented by its maximum deflection. During the bending of the sheet metal, the metal in the bending deformation zone undergoes highly plastic deformation. Inside the deformation zone, there is a longitudinal compressive stress parallel to the OX direction, while the outer layer experiences longitudinal tensile stress. These opposing tensile and compressive stresses generate a longitudinal moment My that rotates around the OY axis.
This moment is the force required for the sheet metal to maintain its longitudinal alignment with the corresponding longitudinal line of the die during the bending process. When the slider returns after bending, the bending force and the longitudinal moment disappear simultaneously, and the metal layers in the deformation zone rebound, resulting in a bending in the opposite direction of the longitudinal moment, which is the natural curvature. For convenience of expression, here the bending deformation zone is unfolded into a plane. Under the action of the bending force, the upper layer (inner layer of the arc) of the metal is subjected to longitudinal compression, while the lower layer (outer layer of the arc) is subjected to tension.
2.3 Influence of the two different convex compensation methods on the straightness of bent workpieces
When using lower die convex compensation, the curve of the compensation convexity rises upward, while with upper die convex compensation, the curve of the convexity bends downward. The curve of the natural curvature of the bent workpiece rises upward. The value of the compensation convexity depends on the deformation of the slider and the worktable during bending, and it is relatively small. The compensation convexity reduces the convex curvature formed by the bent workpiece during unloading and rebound. As a result, the curvature formed by the convexity is usually lower than the natural curvature of the workpiece.
Comparison and analysis of commonly used deformation compensation devices
3.1 Lower beam hydraulic cylinder convex compensation
After the hydraulic cylinder is filled with pressure oil, the beam will rise upward, forming a controllable convex curve. This method has been widely used in CNC bending machines. Its characteristics are as follows:
- The hydraulic cylinders are evenly distributed inside the beam, and the curve after convex compensation closely approximates the deformation curve of the slider and the worktable along the entire length of the worktable. The convex amount is controlled by the hydraulic system pressure, making the operation convenient and fast.
- It can improve the accuracy of the bending angle of the workpiece.
- The structure is more complex and costly.
3.2 Wedge block convex compensation in the worktable
Multiple sets of wedge blocks are installed below the worktable, and the inclination angles of each set of wedge blocks are designed according to specific requirements.
The upper wedge blocks of each set are fixed in the horizontal direction, and when the lower wedge blocks move to the left simultaneously, the worktable surface rises upward according to the design requirements. This method has been widely used in various types of bending machines.
Its characteristics are as follows:
- The wedge blocks are evenly distributed inside the worktable, and the curve after convex compensation is designed to match the deformation curve of the slider and the worktable. The convex compensation is more precise.
- The convex amount is controlled by the movement length of the lower wedge block, which can be done manually or automatically, making the operation convenient.
- It can improve the accuracy of the bending angle of the workpiece.
