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In this paper, the influence of two types of compensation devices on the accuracy of the parts is analyzed, and the respective characteristics of the commonly used deflection compensation devices are compared. It is pointed out that the compensation of the upper mold and the convex is more advantageous, so the development of a new compensation structure that can quickly regulate the upper mold and the convex amount is one of the development directions of the bending machine in the future.
Why Do Press Brakes Need Compensation Device?
When the bending machine bends the workpiece, the slider and table will be deformed under the action of the bending force. At this time, the depth of the upper die into the opening of the lower die is inconsistent in the total length of the workpiece, which will seriously affect the accuracy of the part.
To this end, a variety of structures of deformation compensation devices have been developed. It is roughly divided into two categories: one is a set of symmetrical curves that convex the table into a central upward bulge, called lower mold convex compensation; The other type is the symmetry curve of the upper mold or slider convex as a central downward bulge, called the upper mold plus convex compensation.
Two different types of compensators can effectively reduce the angular error of the bending parts. But there are differences in the impact on straightness.
Natural Deflection of Bent Parts
After the sheet is bent, the edges of the bent parts will naturally deflect. Usually, it is expressed by its maximum deflection.
When the sheet is bent, the metal in the bending deformation area is in a highly plastic deformation state, and the longitudinal compressive stress parallel to the OX direction is generated in the inner layer of the arc of the deformation area, and the outer layer is the longitudinal tensile stress.
These two tensile stresses opposite the stress form a longitudinal torque My that rotates around the OY axis. It is the torque required for the sheet to keep the longitudinal (direction) of the bent part consistent with the corresponding longitudinal line of the mold when bending.
After bending, the slider returns, the bending force and the longitudinal torque disappear at the same time, and the metal layers in the deformation area immediately rebound, forming a bend in the opposite direction of the longitudinal torque in the longitudinal direction, that is, natural deflection.
For the convenience of expression, the bending deformation area is expanded here into a plane. Under the action of bending force, the upper layer (the inner layer of the arc) metal is pressed longitudinally, and the lower layer (the outer layer of the arc) is stretched.
The Influence of Two Different Types of Convex Compensation Methods on The Straightness of The Bending Parts
When the lower mold is compensated with convexity, the curve of the compensation plus convexity is uplifted upwards. When the upper mold is compensated for convexity, the curve of the convex amount is bent downwards.
The natural deflection curve of the bent part is an upward bulge. The amount of compensation plus convexity depends on the amount of deformation when the slide and table are bent, and its value is small. The amount of compensation and convexity causes the convex deflection of the bent parts to be somewhat reduced when unloading the rebound. This makes the deflection of the convex amount usually lower than the natural deflection of the bent part.
Comparative Analysis of Four Commonly Used Compensation Device
Lower Beam Hydraulic Cylinder Convex Compensation
After the cylinder is pressed with pressure oil, the beam will be raised upwards to form a set of controllable convex curves, which are now widely used in CNC bending machines, and its characteristics:
- The cylinders are evenly distributed within the beam, and the convex curve is close to the deformation curve of the slider and the table at the full length of the table. The pressure control of the hydraulic system is used to increase the amount of convexity, which is convenient and fast to operate.
- It can improve the accuracy of the bending angle of the part. The structure is more complex and the cost is high.
Wedges in The Table With Convex Compensation
There are multiple sets of wedges under the workbench, and the bevel of each group of wedges is designed according to certain requirements. The upper wedge of each group of wedges is fixed horizontally, and the current wedge is moved to the left at the same time, and the work surface is raised upward according to the design requirements. It has been widely used in various types of bending machines.
Its features:
- The wedges are evenly distributed in the table, and the curve after the convex is designed as the deformation curve between the slider and the table, and the convex compensation is more accurate. Using the movement length of the lower wedge to control the amount of convexity, which can be manually or maneuvered, which is convenient to operate.
- It can improve the accuracy of the bending angle of the part.
Upper Die Wedge With Convex Compensation
There are multiple sets of wedges between the slider and the upper die, and each group of wedges has the same specifications. The lower wedge of the connecting plate and the wedge is fixed as one, and moving the upper wedge to obtain a downward bulge curve is obtained, and finally they are fixed between the slider and the upper die with the pressure plate.
Its features:
Wedges are evenly distributed under the slider, through the adjustment of each wedge, you can obtain the best convex curve structure is simple, the cost is low, but the operation is not convenient, in the small and medium-sized ordinary bending machine to get the general application of accurate adjustment, can effectively improve the accuracy of the bending angle of the parts, the straightness of the parts is better.
Slider Hydraulic Cylinder With Convex Compensation
Set up a set of cylinders in the middle of the slider. After the cylinder is filled with pressure oil, the middle of the slide rises downwards, forming 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 been widely adopted.
Its characteristics are: the convex compensation is concentrated in the middle of the slider, and a reasonable convex curve cannot be formed on the total length of the upper mold; With the hydraulic system pressure control and convexity, easy and fast operation; It can appropriately improve the accuracy of the angle and straightness of the part.
When the bending machine is working, the amount of deformation compensation should be equal to the deformation amount of the slider and the table, which requires the amount of compensation and convexity to be able to adjust the total length of the mold. The structure of the current application of upper die plus convex compensation is difficult to achieve this requirement, which limits its scope of use. In order to improve the working accuracy of the bending machine, give full play to the advantages of the upper mold plus convex compensation, and develop a new structure that quickly regulates the upper mold addition, which is one of the development directions of the future bending machine. At present, some departments have made useful attempts in this regard and have achieved good results.