Press brake crowning may not be a term that is familiar to most people, but it is a crucial process that ensures the accuracy and straightness of the bending workpiece in metal fabrication.
When a sheet of metal is bent using a press brake, the deformation force is concentrated in the middle, causing the ram and the worktable to deform along with the upper and lower die.
This can result in unevenness along the length of the die edge, affecting the quality of the bending workpiece. To counteract this deformation, deflection compensation devices are designed, and press brake crowning is one such method.
In this process, the amount of deformation is matched to the actual work, thus compensating for the deformation (crowning) and improving the bending quality of the sheet material.
The article explores three types of press brake crowning methods – geometric crowning, hydraulic crowning, and mechanical crowning – and their advantages and disadvantages.
While each method has its benefits, it’s essential to understand which method is best suited for different types of press brakes.
If you’re interested in learning more about press brake crowning and how it can improve the accuracy and quality of your metal fabrication work, read on.
1. What Is Press Brake Crowning?
Crowning is a system that compensates for the deformation of the press brake
during bending. In fact, the elastic structure of the machine can cause a 0.15mm
variation on the Y axis in the middle of the ram in a 3m press brake.
To compensate for this deformation, crowning creates an opposite force so that, during bending, the press brake applies the same force along the entire metal sheet. In this way, bending problems, such as curved profiles, are avoided.
The press brake is pressurized by two working cylinders located at both ends of the ram. As a result, the deformation force of the bending workpiece is concentrated in the middle. Therefore, the ram and the worktable deform together with the upper and lower die.
This causes the sheet material to be uneven along the length of the die edge, which directly affects the accuracy and straightness of the bending workpiece. Therefore, it is necessary to take corresponding measures to reduce or eliminate the deflection caused by the deformation.
The deflection compensation device is designed to counteract this deformation. It is preset to deform in the direction opposite to the force-deformation in the ram and the upper die, or the worktable and the lower die working table. The amount of deformation should match the actual work, thus compensating for the deformation.
Therefore, to realize the compensation of the relative deformation of the ram to the worktable, the pressure distribution between the dies is more uniform and the bending quality of the sheet material is improved.
Moderm press brakes are sometimes equipped with automatic crowning systems, such as tables with wedges driven by a geared motor or tables with hydraulic cylinders with sensors connected to the CNC （this is called active crowning）. In this instance, the sensors monitor pressure changes and immediately compensate for them to maintain the deformation.
2. Importance of Crowning in Press Brake Operations
The primary purpose of crowning in press brake operations is to provide flexibility in accommodating material variations and ensuring accurate and consistent bending. It helps to maintain a uniform force distribution across the bed and the ram, throughout the bending process. This minimizes the errors that can arise due to deflection and promotes better overall form.
In summary, crowning plays a crucial role in press brake operations by:
- Compensating for the deformation and deflection of the ram and bed during bending
- Ensuring uniform force distribution across the working parts
- Improving bending accuracy and consistency across different materials and lengths
- Minimizing errors and enhancing the overall form.
Crowning methods can vary from manual adjustments using an Allen key or digital readouts to motorized control systems, providing different levels of precision and automation in the process. Employing an effective crowning system is essential for press brake operators to achieve optimal bending accuracy, consistency, and efficiency in their work.
3. Types of Press Brake Crowning
At present, the press brake crowning mainly has three types:
- Geometric crowning
- Hydraulic crowning
- Mechanical crowning
(1) Geometric crowning
Generally, press brake manufacturers do not adopt this type of crowning method. The working table is fixed convex, meaning that during manufacture, the worktable is machined into an arc shape with a slight convex in the middle to compensate for the deflection caused by bending.
To make the appropriate correction for the upper mold, the middle part of the mold is slightly curved. So, when the slider undergoes upward deflection deformation, the upper die edge basically tends to be straight, therefore keeping each bending point along the bend line to generate the same bending force for the plate.
The advantages of the geometrical compensation method are low cost and ease of manufacture, but there are some disadvantages. It can only realize the compensation of fixed deformation and has small compensation flexibility. Additionally, the compensation block arc correction quantity needs to be calculated precisely.
The calculation method based on mechanics theory and finite element calculation has a certain error. Therefore, while this crowning method can achieve the deflection compensation effect, it is very difficult to realize.
(2) Hydraulic Crowning
Mainly used on electro-hydraulic synchronous CNC press brakes, hydraulic crowning is preferred since the compensation amount needs to be controlled by the controller such as DA52S, DA66T, and others.
Hydraulic crowning is achieved by installing two hydraulic cylinders on either side of the press brake frame, and another two auxiliary hydraulic cylinders in the middle of the machine. During the bending process, the auxiliary cylinder is filled with hydraulic oil and goes downward to generate downward deflection for compensation.
An automatic crowning system is formed by installing the auxiliary hydraulic cylinder in the lower part of the worktable, generating an upward force on the worktable during the bending process.
The pressure compensation device is composed of several small oil cylinders, a motherboard, an auxiliary plate, a pin shaft, and a compensating cylinder on the worktable, with a proportional relief valve forming the pressure compensation system. During operation, the auxiliary plate supports the oil cylinder, and the oil cylinder holds up the motherboard just enough to overcome the deformation of the ram and the worktable.
The convex device is controlled by a numerical control system, and the preload can be determined based on the thickness of the plate, the die opening, and the tensile strength of the material when bending different sheet materials.
Hydraulic crowning has the advantage of realizing deflection compensation for continuous variable deformation with large compensation flexibility. However, it also has some disadvantages of complex structure and relatively high cost.
(3) Mechanical Crowning
The most widely used crowning method for the ordinary press brake is a good compensation method with low cost. In real operations, it is very convenient and easy for operators.
Mechanical crowning is a new deflection compensation method that generally uses a triangular oblique wedge structure. The principle is that two triangle wedge blocks with α angles are used, and the upper wedge moving is fixed at the X-direction and can only move in the Y-direction. When the wedge moves the △x distance along the X-direction, the upper wedge moves up the H distance under the lower wedge force.
Regarding the existing mechanical compensation structure, two bolster plates are placed in the full length on the worktable. The upper and lower plates are connected through the disc spring and bolts. The upper and lower plates consist of a number of oblique wedges with different slopes. The motor drives them to move relatively, forming an ideal curve for a set of convex positions.
4. Process of Crowning
Setting Up the Crowning System
The process of crowning involves compensating for the deformation of the press brake during bending. Crowning systems are essential in maintaining accuracy while working with a press brake. To set up the crowning system, operators need to input parameters such as sheet thickness, length, die opening, and material tensile strength into the machine’s control system. By analyzing these parameters, the control system automatically determines the real deflection of the table and ram, thereby obtaining the required preloading for each bend.
There are three common ways to perform crowning:
- Hand operated with an Allen key
- Hand operated using digital readouts
- Programmable crowning systems
For manual crowning methods, shimming the die on the bed or adjusting the wedges is necessary to correct the alignment and maintain optimal bend accuracy. Programmable crowning systems, on the other hand, automate this process and eliminate potential errors.
Working with the Workpiece
The workpiece to be formed must be loaded onto the press brake and carefully aligned with the die. Before bending occurs, it is crucial to ensure that the workpiece is properly positioned, and any necessary adjustments have been made to the crowning system.
When the press brake is activated, the ram exerts force onto the workpiece, causing it to bend. The crowning system plays a critical role in compensating for any deformations that may occur during this process. As the workpiece is being formed, the crowning system ensures that the bending force is properly distributed along the entire length of the workpiece, resulting in precise and consistent bends.
To summarize, the process of crowning in press brakes involves setting up the crowning system, aligning the workpiece, and forming it with the help of the press brake’s ram. Achieving accurate and consistent bends relies heavily on a properly adjusted and functioning crowning system that compensates for any deformation during the bending process.
5. Design principle and implementation of convex worktable
When the press brake machine is in operation, it will cause deformation, which is mainly due to the application of force at both ends of the machine. This force, generated during the bending process, causes deformation in the ram and the worktable, resulting in inconsistencies between the two ends of the workpiece and its central angle.
To analyze the press brake machine, the finite element method is widely used due to its speed and accuracy.
Convex curve of 100 ton 3-meter press brake machine obtained by finite element method:
There are several methods to compensate for deflection deformation:
- Saddle shaped worktable with fixed deflection compensation;
- Top punch wedge compensation;
- Worktable cylinder compensation; pressure control mode
- Mechanical compensation of worktable; position control mode
Worktable cylinder compensation
The worktable has a three-layer splint design, with compensating oil cylinders located throughout the structure.
When the system applies pressure to the compensating cylinders, it pushes up the middle splint of the three-layer splint, resulting in compensation for the deformation.
Mechanical compensation of worktable
To control the position, compensation is provided at the corresponding point during bending to counteract the elastic deflection deformation of the machine.
Mechanical compensation is achieved through a group of wedges with inclined planes, which can provide reverse compensation.
Before bending loading, pre convex state
After the bending is loaded, the actual compensation state is changed
Loading simulation animation of the convex worktable
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