What Is Crowning and Why Is It Important?
Press brake crowning is crucial for correcting the bending accuracy of workpieces.
During the bending process, the oil cylinder at both ends of the press brake drives the ram to apply pressure to the workpiece.
The servo valve located above the left and right oil cylinders, as well as 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 to ensure that the ram reaches the preset position of the controller, resulting in more accurate bending angles.
However, when bending a metal plate, the two ends of the press brake's ram 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, during the bending of a metal plate, the ram of the press brake moves upward while the bed of the machine moves downward.
In order to improve bending accuracy and eliminate bending angle errors caused by ram and workbench deformation, it is necessary to compensate for their deflection.
A compensation or crowning system can be installed to generate an opposite force that balances 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 is relatively large.
Before the invention of the compensation system, some press brake manufacturers used convex worktables to control crowning, but the compensation was less accurate.
With the improvement of the CNC system, the crowning system is now 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.
Crowning includes compensating for natural deformation and bending that occur in the base and ram of the press brake during normal operation due to the bending force applied to the workpiece.
When a press brake bends the workpiece, both the upper and lower crossbeams may suffer deflection and deformation, which may have unwanted effects on the final product.
By effectively mastering press brake compensation, operators can ensure accurate bending, thereby reducing waste in the manufacturing process and improving productivity.
Choices of Crowning System
Crowning is typically used for large-tonnage, high-strength press brakes because the tonnage of the machine is directly proportional to the demand for deflection compensation.
However, 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.
There are two commonly used crowning methods: 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.
In modern press brakes, there are two types of crowning: hydraulic crowning and mechanical crowning.
We will introduce the working principle, advantages, and disadvantages of these two methods.
Hydraulic Crowning
Hydraulic crowning requires a set of oil cylinders to be installed under the workbench of the press brake.
The position and size of each crowning cylinder are designed according to the deflection crowning curve obtained from finite element analysis of the ram and workbench.
Hydraulic crowning is used to achieve the crowning of the neutral plate through relative displacement between the front, middle, and rear vertical plates.
Its principle is to achieve bulging through the elastic deformation of the steel plate itself, and its crowning can be adjusted within the elastic range of the workbench.
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 produce compensating deformation, thereby reducing the influence of machine deformation on the plate.
However, this hydraulic crowning system cannot compensate for the whole workbench.
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 sets 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 during bending.
The hydraulic crowning system is complex, and problems such as oil leakage, loose sealing rings, and other issues may arise.
These issues can affect the amount of hydraulic compensation each time, resulting in inconsistent plate processing.
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.
Compared to mechanical crowning systems, hydraulic crowning systems can provide precise control and timely adjustment of tonnage.
Mechanical Crowning
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 the whole workbench.
Deflection can be compensated by convex operation at any position on the length of the entire worktable.
The CNC system calculates the required crowning force according to the load force when the workpiece is bent.
This force will cause deflection deformation of the ram and the vertical plate of the workbench.
The system will automatically control the relative movement of the wedge blocks to effectively compensate for the deflection deformation caused by the ram and the vertical plate of the workbench.
The folded workpiece will have high precision and a small difference in plate processing each time, making it suitable for mass production.
Compared to the hydraulic crowning system, the mechanical crowning system has some advantages such as being free of oil leakage, having a low failure rate, and being environmentally friendly.
Mechanical crowning uses a linear scale to measure the position of the feedback signal. Digital control makes the crowning value more accurate.
However, with the passage of time, the wedge may wear, which can affect the compensation effect.
The mechanical crowning operation has high requirements. If the same workpiece needs to be bent several times, the worker needs to make real-time adjustments.
Even if the workers have sufficient experience, unpredictable errors can occur. Therefore, high requirements for mechanical compensation operations are needed.
If the workpiece does not need to be bent many times and the worker is skilled, mechanical crowning can be considered for its workpiece accuracy.
Conclusion
The crowning function of the press brake is to achieve consistent bending angles and improve part accuracy.
The crowning function is to adjust the natural deflection that occurs in the press brake workbench and ram during operation.
The press brake crowning system includes a hydraulic crowning system and a mechanical crowning system.
Due to the different structural designs of these two systems, factors such as bending accuracy and resolution vary.
The crowning system ensures that the angle of the whole workpiece on the press brake is consistent, reducing bending program settings and waste.
Press brake operators are freed from complicated calculations, and production efficiency is greatly improved.
At ADH, our machine experts can provide you with knowledge and experience in all aspects of the press brake.
Check our press brake here, or contact us for details of the press brake.
