What Is Crowning and Why Is It Important?
In the process of bending, the oil cylinder at both ends of the press brake drives the ram to apply pressure to the workpiece for bending.
The servo valve above the left and right oil cylinders and the positioning device below the oil cylinders can control the lifting position of the oil cylinder.
During each bending, the servo-hydraulic system controls the position of the oil cylinder through preset parameters.
Make sure the ram reaches the preset position of the controller, so as to achieve a more accurate bending angle.
However, when bending the metal plate, the two ends of the ram of the press brake are subjected to the maximum force, and the reaction force generated when the plate is bent deforms the lower surface of the ram.
As a result, both ends of the sheet metal are bent at the correct angle, but the deformation in the middle of the ram is the largest, especially when bending long metal plates.
As shown in the figure, when bending the metal plate, the ram of the press brake will be upward and the bed of the machine will be downward.
In order to improve the bending accuracy and eliminate the bending angle error caused by the deformation of the ram and workbench, it is necessary to compensate for the deflection deformation of the ram and workbench.
A compensation or crowning system can be installed to generate an opposite force to balance the deflection of the ram and the workbench.
This crowning system is usually installed for large sheet metal and high-strength bending because the deflection error of large sheet metal bending will be relatively large.
Before the invention of the compensation system, some press brake manufacturers used convex worktables to control crowning. But the compensation is less accurate.
Now, with the improvement of the CNC system, the crowning system is controlled by the CNC system.
Therefore, the operator only needs to input the length, thickness, bending angle, and other information about the bending.
The crowning system will automatically calculate the compensation values.
The system can store this information and directly reuse the data during the next repeated bending procedure.
The crowning system improves the bending accuracy and efficiency of small-batch workpieces.
For large tonnage and high-strength bending, the accuracy of the bending angle can be guaranteed.
Choices of Crowning System
Crowning is usually used for large-tonnage, high-strength press brakes.
Because the tonnage of the machine is directly proportional to the demand for deflection compensation.
Compensation can also be used for press brakes with lower tonnage.
The longer the length of the workbench and the ram of the press brake, the greater the deflection of the machine.
The commonly used crowning methods include hydraulic crowning and mechanical crowning.
Both of them will produce upward elastic deformation in the middle of the worktable to offset the deformation of the ram and the worktable.
Mechanical crowning can be an optional device for the press brake, while hydraulic crowning is a standard device for the press brake.
Hydraulic crowning requires a set of oil cylinders to be installed under the workbench of the press brake.
When the ram and the workbench deform during bending, the hydraulic crowning system transmits pressure to the compensation cylinder through hydraulic pressure.
Then the ram and the workbench will produce compensation deformation, so as to reduce the influence of machine deformation on the plate.
However, this hydraulic crowning system can not compensate for the whole workbench.
The local compensation depends on the position of the compensation cylinder.
Some hydraulic crowning systems only set one compensation cylinder in the center of the workbench, although there are also multi-cylinder systems.
Numerical control hydraulic crowning is to set the parameters according to the length, thickness, and other data of the bending workpiece in the numerical control system.
Then calculate the required compensation values through the machine computer system and transmit the command to the compensation amplifier.
The hydraulic crowning proportional valve receives the signal sent by the compensation amplifier to control the oil cylinders under the workbench.
Compensation is carried out to reduce the deflection deformation of the plate.
The hydraulic crowning system is complex, and there may be problems such as oil leakage, loose sealing ring, and so on.
These will affect the amount of hydraulic compensation each time, resulting in inconsistent plate processing each time.
The advantage of hydraulic crowning is that with the increase in service time, the machine will not have wear problems.
While mechanical crowning will have wear problems.
Mechanical crowning uses a wedge to compensate for the worktable.
Wedge blocks can be divided into single wedge blocks and multiple groups of wedge blocks.
The mechanical crowning system composed of multiple groups of wedge blocks has more compensation points than the three compensation points of ordinary hydraulic crowning, and there is no compensation blind area.
The mechanical crowning system can realize compensation along with the whole workbench.
The deflection can be compensated by convex operation at any position on the length of the whole worktable.
The folded workpiece has high precision and a small difference in plate processing each time, which is suitable for mass production.
Compared with the hydraulic crowning system, the mechanical crowning system has some advantages. Such as free of oil leakage, low failure rate, and environmental protection.
However, with the passage of service time, the wedge will be worn, which will affect the compensation effect.
High requirements for mechanical crowning operation. If the same workpiece needs to be bent several times, the worker needs to make real-time adjustments.
Even if workers have sufficient experience, unpredictable errors will occur. High requirements for mechanical compensation operation are needed.
If the workpiece does not need to be bent many times and the worker is skillful, mechanical crowning can be considered in terms of workpiece accuracy.