Various errors may occur when bending sheet metal with the press brake.
The causes of these errors include inaccurate bending radius, less bending force, and improper die clearance, etc.
The positioning error of the back gauge and the bending calculation error will also lead to inaccurate workpiece bending.
The bending angle of the workpiece is sometimes different from the calculated angle.
The main reason may be that the press brake is not leveled, or the crowning mechanism does not work.
Press Brake Parallelism
The ram and workbench will gradually deform in the long-term bending process.
This kind of deformation will lead to uneven force on the sheet metal, resulting in an uneven edge of the workpiece and reduced bending accuracy.
Therefore, the deformation of the ram and the workbench must be compensated by the crowning mechanism.
In this way, the force exerted by the ram on the plate is uniform, so as to ensure the bending accuracy of the workpiece.
The crowning mechanism of the press brake includes hydraulic crowning and mechanical crowning.
The electro-hydraulic press brake can be equipped with a hydraulic crowning mechanism.
The hydraulic cylinder on the beam and the auxiliary hydraulic cylinder under the workbench compensate for the ram and the workbench at the same time.
The hydraulic cylinder generates downward force, and the auxiliary hydraulic cylinder generates upward force so that the deformation of the ram and the workbench can be offset.
The whole hydraulic crowning mechanism is controlled by the numerical control system.
The compensation force can be set according to the sheet metal thickness, tensile strength, and die opening size.
Another compensation method is mechanical crowning, and the main working principle is through a triangular wedge structure.
The mechanical crowning mechanism needs to place two base plates above and below the workbench, which are composed of several wedges.
The upper and lower base plates are connected by disc springs and bolts, and the motor is used to drive the wedge to move relatively.
In this way, a curve can be formed to offset the original bulge.
Appropriate Sheet Metal Bending Method
Sheet metal bending method also affects the metal bending accuracy. There are three main bending methods.
The three methods are air bending, bottoming, and coining, which are distinguished according to the relationship between the end die position and the sheet metal thickness.
Using the air-bending method, the die and the workpiece do not need to be in full contact.
And the bending force required for air bending is relatively small.
The punch presses the sheet against the two points on the die shoulder into the U-shaped or V-shaped die.
The angle of air bending is generally determined by the shape and stroke of the punch and lower die.
The proper stroke depth can achieve more accurate bending.
The degree of springback after load release varies with the compressive strength of materials.
Different springback will lead to the change of bending angle. To modify the angle, you need to apply a certain amount of pressure to adjust it.
The bending angle error of air bending is about 0.5 degrees.
When using the bottoming method, the workpiece is placed at the opening of the punch and V-shaped die.
The opening size of the V-shaped die depends on the thickness of the sheet metal.
Generally speaking, the opening size of the V-shaped die is 6 to 10 times the thickness of the sheet metal.
The bottoming die changes according to the bending angle and material thickness.
After the load is released, the springback of the sheet is less, so the accuracy of the bottoming is higher.
When using the coining method for bending, the punch will completely press the material into the lower die.
The bending force required for coining is very large, which can make the material permanently shaped.
And the material springback after coining is very small, so the bending accuracy of this method is very high.
Sheet Metal Bending Parameters
In addition to selecting a suitable bending method, it is also necessary to calculate the bending parameters of the workpiece.
The process of metal bending makes the inner surface of the metal compressed, while the outer surface structure will be stretched.
Therefore, you need to know the tensile value of the material and calculate the minimum flange tolerance length.
These parameters include bending radius, K factor, bending deduction, bending allowance, setback, etc.
If the properties of the material are inconsistent, the bending angle of the workpiece will change when using air bending.
Moreover, if the plate thickness is the same and the die opening becomes narrower, the bending angle of the workpiece will change more.
Inconsistent material may still be within the thickness and strength tolerances of the mill.
Because the tensile strength of many materials is within a certain tensile strength tolerance.
In addition, pay attention to the outer surface of the sheet, for example, different natural texture directions require different bending pressures.
Generally speaking, these values are not the most accurate, so the angle and length need to be adjusted when bending.
Press Brake Balanced Operation
If it is allowed to run stably and bend the sheet more finely, then the press brake needs to be balanced.
Support the intermediate frame of the bending machine on a suitable bearing surface, clamp one end and support the other end.
During the formal operation, when the workpiece is rotating, first make the lower two supporting claws touch the supporting surface of the workpiece evenly and then lock it.
Then tighten the upper cover and adjust the position of the upper support claw until it is locked properly.
In the whole process, it must be ensured that all supporting claws of the intermediate frame of the press brake are evenly applied.
As far as possible, a layer of pure copper sheet or fine emery cloth should be placed between each supporting claw and the supporting surface of the intermediate frame.
This is to prevent the supporting claw from wearing the workpiece surface.
We can balance the press brake according to the above points to make it run smoothly.
This paper introduces some methods to improve the bending accuracy of the workpiece.
It includes the selection of the bending method, the leveling and compensation mechanism of the machine, etc.
It also includes the selection of bending materials and the accuracy of bending parameters.
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What Are the Possible Problems with Sheet Metal Bending?
The accuracy of metal forming can only be ensured by the correct positioning of the workpiece, matching die, reasonable die clearance, bending radius, etc.
If the parameter calculation error is too large, the die clearance is not appropriate, or the workpiece positioning is not accurate, the workpiece quality will be poor.
Various problems may occur to the workpiece, such as cracks at the bending, uneven edges of the bending workpiece, wrong bending size, convex surface of the workpiece, etc.