Press brake crowning may not be a familiar term to most people, but it is a crucial process that ensures the precision and straightness of the bent part in metal manufacturing.
When a metal sheet is bent using a press brake, the deformation force is concentrated in the middle, causing the ram and work table to deform along with the upper and lower die.
This can result in irregularities along the length of the die edge, affecting the quality of the bent part. 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 adapted to the actual work, thus compensating for 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 is essential to understand which method is best suited for different types of press brakes.
If you are 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 press brake deformation
during flexion. In fact, the elastic structure of the machine may cause a
variation in the Y axis in the middle of the ram on a 3m press brake.
To compensate for this deformation, crowning creates an opposing force so that, during bending, the press brake applies the same force throughout the entire sheet metal. 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 bent part is concentrated in the middle. Therefore, the ram and 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 bent part. Therefore, it is necessary to take corresponding measures to reduce or eliminate the deflection caused by deformation.
The deflection compensation device is designed to counteract this deformation. It is preset to deform in the opposite direction to the force deformation on the ram and upper die, or the worktable and lower die worktable. The amount of deformation must correspond to the actual work, thus compensating for the deformation.
Therefore, to realize the compensation of the relative deformation of the ram with respect to the work table, the pressure distribution between the dies is more uniform and the bending quality of the sheet material is improved.
Modern press brakes are sometimes equipped with automatic crowning systems, such as tables with gearmotor-driven wedges or tables with hydraulic cylinders with sensors connected to the CNC (this is called active crowning). In this case, sensors monitor pressure changes and immediately compensate them to maintain deformation.
2. Importance of crowning in press brake operations
The primary purpose of crowning in press brake operations is to provide flexibility to accommodate material variations and ensure accurate and consistent bending. Helps maintain an even distribution of force on the base and ram throughout the bending process. This minimizes errors that can arise due to deflection and promotes better overall shape.
In summary, crowning plays a crucial role in press brake operations by:
- Compensating for deformation and deflection of the ram and base during bending
- Ensure uniform distribution of force between workpieces
- Improving bending accuracy and consistency across different materials and lengths
- Minimizing errors and improving the overall form.
Crowning methods can range from manual adjustments using an Allen wrench 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, press brake crowning mainly has three types:
- Geometric coronation
- Hydraulic crowning
- Mechanical coronation
(1) Geometric coronation
Generally, press brake manufacturers do not adopt this type of crowning method. The worktable is fixed and convex, which means that during manufacturing, the worktable is machined into an arc shape with a slight convexity in the middle to compensate for the deflection caused by bending.
To make proper correction for the upper mold, the central part of the mold is slightly curved. Therefore, when the slider undergoes upward deflection deformation, the upper edge of the die tends to be basically straight, thus keeping each bending point along the bending line to generate the same bending force for the plate.
The advantages of the geometric compensation method are low cost and ease of manufacturing, but there are some disadvantages. It can only perform fixed deformation compensation and has little compensation flexibility. Furthermore, the arc correction amount of the compensation block needs to be calculated accurately.
The calculation method based on mechanical theory and finite element calculation has a certain error. Therefore, although this crowning method can achieve the effect of deflection compensation, it is very difficult to realize.
(2) Hydraulic Crowning
Mainly used in electro-hydraulic synchronous CNC press brakes, hydraulic crowning is preferred because the compensation value needs to be controlled by the controller, such as DA52S, DA66T and others.
Hydraulic crowning is achieved by installing two hydraulic cylinders on each side of the press brake structure 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 moves down to generate downward deflection for compensation.
An automatic crowning system is formed by installing the auxiliary hydraulic cylinder at the bottom of the work table, generating an upward force on the work table during the bending process.
The pressure compensation device is composed of several small oil cylinders, a motherboard, an auxiliary board, a pin shaft and a compensator cylinder on the work table, with a proportional relief valve forming the pressure compensation system . During operation, the auxiliary plate supports the oil cylinder, and the oil cylinder supports the motherboard just enough to overcome the deformation of the ram and 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 opening of the die and the tensile strength of the material when bending different sheet materials.
Hydraulic crowning has the advantage of realizing deflection compensation for continuously variable deformation with great compensation flexibility. However, it also has some disadvantages of complex structure and relatively high cost.
(3) Mechanical Coronation
The most widely used crowning method for common press brakes 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 an oblique triangular wedge structure. The principle is that two triangular wedge blocks with angles α are used, and the movement of the upper wedge is fixed in the X direction and can only move in the Y direction. When the wedge moves the distance △x along the X direction, the upper wedge rises the distance H under the force of the lower wedge.
In relation to the existing mechanical compensation structure, two reinforcement plates are placed along the entire length of the work table. The upper and lower plates are connected using a disc spring and screws. The upper and lower plates consist of several oblique wedges with different inclinations. The motor causes them to move relatively, forming an ideal curve for a set of convex positions.
4. Coronation Process
Configuring the crowning system
The crowning process involves compensating for the deformation of the press brake during bending. Crowning systems are essential for maintaining precision when 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 control system. By analyzing these parameters, the control system automatically determines the real deflection of the table and ram, thus obtaining the necessary preload for each curve.
There are three common ways to perform the coronation:
- Manually operated with Allen key
- Manually operated using digital readouts
- Programmable crowning systems
For manual crowning methods, it is necessary to shim the die into the base or adjust the wedges to correct the alignment and maintain ideal bending accuracy. Programmable crowning systems, on the other hand, automate this process and eliminate potential errors.
Working with the workpiece
The part to be formed must be loaded into the press brake and carefully aligned with the die. Before bending occurs, it is crucial to ensure that the workpiece is positioned correctly and that all necessary adjustments have been made to the crowning system.
When the press brake is activated, the ram exerts force on 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 part is formed, the crowning system ensures that the bending force is properly distributed along the entire length of the part, resulting in precise and consistent bends.
In short, the crowning process in press brakes involves setting up the crowning system, aligning the part, and shaping it with the help of the press brake ram. Achieving precise and consistent bends is highly dependent on a properly adjusted and functional crowning system that compensates for any deformation during the bending process.
5. Design principle and implementation of convex work table
When the press brake 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 deformations in the ram and the work table, resulting in inconsistencies between the two ends of the piece and its central angle.
To analyze the bending machine, the finite element method is widely used due to its speed and accuracy.
Convex curve of a 100-ton, 3-meter press brake obtained by the finite element method:
There are several methods to compensate for deflection deformation:
- Saddle-shaped work table with fixed deflection compensation;
- Superior punch wedge compensation;
- Work table cylinder compensation; pressure control mode
- Mechanical compensation of the work table; position control mode
Work table cylinder compensation
The worktable features a three-layer splint design, with compensating oil cylinders located throughout the structure.
When the system applies pressure to the compensation cylinders, it pushes up the middle splint of the three-layer splint, resulting in compensation of the deformation.
Mechanical compensation of the work table
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 set of wedges with inclined planes, which can provide reverse compensation.
Before bending loading, pre-convex state
After the flexure is loaded, the actual compensation state is changed
Loading simulation animation of convex worktable
Driving mode
Why does the press brake need a crowning system?
Bending Accuracy
When it comes to bending parts, there are two main factors that determine its accuracy:
- Angle Accuracy: This is mainly related to the deviation in the height direction of the die system, as represented by Ty in Fig.
- Dimensional accuracy: This is mainly related to the deviation in the front and back direction of the die system, as shown by Tx in Fig.
Fig. 1 Tx and Ty scheme
The greater the depth of the upper die of the press brake into the lower die, the smaller the bending angle will be.
Based on Figure 2, it can be calculated that when bending a 2mm carbon steel plate to 135° using the V12 bottom die, a height direction deviation of 0.045mm can result in an angle deviation of 1.5 °.
Fig. 2 Influence of height direction deviation on angular deviation
Further reading:
- What is press brake crowning and 3 types of it
Deflection deformation of the press brake
When a workpiece is bent using a press brake, the upper and lower beams may deflect and deform due to their structural characteristics and the applied bending force, as illustrated in Figure 3.
Fig.3 Deflection and deformation diagram of the upper and lower beams
There is currently an inconsistency in the depth of the upper die entering the lower die opening along the entire length of the part. This inconsistency can cause excessive deviation of the part's bending angle along its full length direction.
This inconsistency typically results in a part with a large average angle and smaller angles at both ends, as illustrated in Figure 4.
Fig. 4 Schematic diagram of the bending angle
Therefore, to ensure the consistency of the bend angle along the entire length of the part, a crowning system needs to be introduced into the press brake.
Why does the press brake need a mechanical crowning system?
As mentioned above, when the press brake bends the part, the upper and lower beams, due to their structural characteristics, undergo deflection deformation under the bending force. This can lead to excessive deviation of the bending angle of the workpiece in the direction along the entire length.
However, the crowning system can effectively compensate for the deflection deformation of the press brake. By using the crowning system on the top or bottom beam, the consistency of the bend angle can be guaranteed over the entire length of the workpiece.
The crowning system is divided into two categories:
- Hydraulic crowning system
- Mechanical crowning system
1. Hydraulic crowning system
The hydraulic crowning system operates based on the principle of incorporating several hydraulic cylinders into the lower beam of the press brake. Each hydraulic cylinder can be controlled separately, causing the lower beam to form a certain bulge, as illustrated in Figure 1.
Theoretically, using more hydraulic cylinders increases the number of compensation points, resulting in greater compensation accuracy.
Hydraulic crowning is an integrated discrete compensation method.
To achieve high-resolution compensation effect and high bending accuracy, the number of hydraulic cylinders and its hydraulic control system must meet higher requirements, resulting in a more complex overall structure and higher cost of the press brake.
It is not possible to modernize the hydraulic crowning system on a customer's existing press brake.
Fig. 1 Schematic diagram of the hydraulic crowning system
2. Mechanical crowning system
The mechanical crowning system uses the filling method to compensate the lower beam/lower matrix. Its main principle involves generating several compensation curves through the mutual movement of a pair of deflection compensation wedges, as shown in Figure 2.
Fig. 2 Schematic diagram of the mechanical crowning system
There are many types of mechanical crowning systems available on the market.
Let's look at the example of Wila's mechanical compensation bench. It falls into the category of relatively continuous external compensation. This system can be installed directly on the bottom beam of the press brake and is suitable for both new and old press brakes.
The compensation curves of this system can be continuously adjusted for different applications, as shown in Fig.
Further reading:
- Press Brake Hydraulic Crowning vs Mechanical Crowning (Detailed Comparison Analysis)
Great bending machine crowning
The length of a sheet metal bend greatly affects its bending accuracy. The longer the sheet metal, the greater the bending force required, leading to greater equipment tilts and ram deformations, making it more difficult to ensure accuracy. This bending accuracy, including the total length of the bend, is called “straight-line accuracy”.
Without effective measures, inconsistent amounts of concave die entering the full length direction of the folded upper die may cause the folded portion to have a “boat beauty” effect. To solve this problem, a finite element simulation method was used to analyze the ram force and deformation displacement. The deflection compensation curve was extracted and modified, and combined with empirical data to design and manufacture a new mechanical deflection compensation device.
The linear accuracy of large bending machines can be improved by using a drive motor or manual adjustment to compensate for deflection along all or part of the length.
C characteristics the analysis of the ram carry
Modeling
The ram of the press brake is made of steel plates of various shapes. During the modeling process, only the main structure of the ram is considered, while details that have little impact on the results are ignored. The main body dimensions are 8000mm x 2500mm x 120mm.
The modulus of elasticity is defined as 2 x 10 5 MPa, Poisson's ratio to 0.27 and density to 7.8 x 10 3 kg/m 3 . Given the structural characteristics of the ram, a solid95 element defined by 20 nodes was selected for analysis.
This element has the ability to adapt to curved contour models and accurately analyze the elastic deformation of the ram, as it has arbitrary 3D orientation.
Load application and restraint of the ram block
(1) C restrictions
In real-world conditions, the battering ram is always moving. However, to perform a static ram analysis, it is necessary to simplify and approximate the ram constraints. To do this, symmetric constraints are imposed on nodes located in the central symmetry plane of the ram.
The ram is fixed by connecting the guide rail of the structure to its back, where a full restraint is applied. This ensures that the ram remains in a fixed position during analysis.
(2) I load condition
The surface load is applied to the contact area between the bottom of the hydraulic cylinder and the ram block. As the vertical deformation of the ram block is small compared to its total length, it is considered a small elastic deformation. As a result, a uniform load is applied to the tension surface at the bottom of the ram block in the model.
To ensure that the force is transmitted evenly from the ram block to the upper die, the lower part of the ram block is connected to the upper die by a connecting block. This ensures that the load is distributed evenly and does not cause imbalances in the system.
Extraction and analysis of simulation results
The displacement diagram of the ram block under load is shown in Figure 1. The path is defined in ANSYS for processing results, and the deformation-deflection curve of the tension surface at the bottom of the ram is extracted and shown in Figure 2 .
As can be seen in the figure, the maximum displacement appears at the center of the ram and gradually decreases on both sides in a parabolic shape. At the same time, the deformation displacement at any position along the bending length direction can be obtained, providing data support for designing wedges with different arrangement angles to form the deflection curve.
Mechanical deflection compensation device
Analysis shows that when a press brake is loaded, its stress surface on the ram produces parabolic deflection deformation due to its own structure, resulting in inconsistent bending angles of the part along its entire length. Furthermore, local wear in the bending die also affects the straightness of the bent part.
Currently, there are two common methods to solve this problem. The first method is to install a hydraulic upper cylinder in an appropriate position on the upper ram or lower working table of the press brake, and control the ejection height of each upper cylinder to compensate for the deformation. The second method is to use a mechanical deflection compensation device on the lower worktable, which compensates for deformation by adjusting the wedge blocks with different angles.
The hydraulic top cylinder method is easy to operate and meets the general precision requirements of bending production. However, for large-size and high-precision bent parts, the mechanical deflection compensation method is mainly used.
Traditional deflection compensation device
The traditional method of mechanical crowning involves manually adjusting the compensation block or adding a gasket to worn areas, which is time-consuming, labor-intensive and inefficient, making it difficult to guarantee accuracy.
More advanced bending machines, on the other hand, have automatic or semi-automatic deflection compensation mechanisms, such as the commonly used wedge and pull rod type compensation devices. The wedge-type device can ensure consistent angles and improve bending accuracy, but it requires a lot of manual labor and is not very efficient. The tie-rod device, on the other hand, easily compensates for deflection along the entire length, but does not solve the problem of local wear.
Figures 3 (a) and (b) represent two types of deflection compensation devices.
Fig. 3 Common mechanical deflection compensation device
New deflection compensation device
To solve the problem of local wear, our mold company designed a four-piece wedge deflection compensation device. This device not only automatically compensates for all part deflection, but also allows manual adjustment to compensate for local die wear.
Figure 4 is a two-dimensional sectional view of the device, and its operating principle is described below:
Fig. 4 Four-piece wedge-type deflection compensation device
- 1 top cover plate
- 2 IV wedges
- 3 wedges III
- 4 wedges II
- 5 wedges I
- 6-tie rod
- 7 spacers
- 8 right wedge
- 9-pin screw
- 10 – bearing pedestal
- 11 – bearing
- 12 bases
- 13 nut
(1) A rectangular groove is placed along the length direction (i.e., longitudinally) on the base. In this groove, odd groups of wedge mechanisms are evenly distributed longitudinally. Each group consists of two pairs of four wedges, namely Cunha I, Cunha II, Cunha III and Cunha IV, stacked from bottom to top.
(2) In each group of wedges, the lower pair, Wedge I and Wedge II, form a local adjustment mechanism. The inclined planes of each pair are corresponding and arranged in a transverse direction.
Screw holes are placed in the middle of the front and back walls of the base seat, corresponding to the large end of the Wedge I. Adjustment screws are installed on the outside of the base wall and each extend into the base to fit connect to Cunha I.
To achieve local compensation, the screw can be manually adjusted to move Wedge I back and forth (transversely), thereby adjusting the upper cover plate and causing the worktable to move up and down.
(3) The upper pair, Wedge III and Wedge IV, form an integral adjustment mechanism. They are placed longitudinally in each group and form an integrally adjusting inclined wedge device.
Each pair of Wedges III is matched to the inclined plane of Wedges IV, with the greatest inclination located in the middle of the rectangular groove at the base. The slope gradually decreases toward the left and right sides of the groove. When Wedges III move equidistantly along the length direction, the intermediate lift is substantial, forming a curve that adjusts deflection based on the movement of the Wedges. This performs overall deflection compensation.
The short axis of each Wedge IV is arranged symmetrically on the front and rear side walls. A vertical groove is arranged in the upper part of the front and rear side walls of the rectangular groove of the base, corresponding to the short axis. The short shaft of each Wedge IV slides in each groove, allowing only up and down movement and ensuring the lifting effect of the Wedge IV.
(4) The longitudinal screw holes are placed in the Wedge III on the right end, while the longitudinal through holes with the same centerline as the screw holes are placed in the other Wedge III. A hollow spacer sleeve is installed between each adjacent wedge III pair. A pull rod is installed in each wedge III and hollow spacer sleeve. The right end of the pull rod is threaded into wedge III on the right end. An adjusting screw is installed on the right part of the screw hole of the Wedge III on the right end, and a motor is installed on the end of the adjusting screw to start the motor, which can achieve automatic overall deflection compensation.
Figure 5 shows an 8 meter long four-piece wedge deflection compensation device with double pull rod.
Fig. 5 8m double support wedge type deflection compensation device
Wrap it up
In this post, the small elastic deformation of the ram in a press brake is simulated and analyzed, and the deflection deformation data of the tension surface at the bottom of the ram is extracted.
Based on the experiment data, a four-piece wedge deflection compensation device was designed. It not only automatically adjusts the overall deflection compensation of processed parts, but also allows manual adjustment of local die wear compensation.
The device has a well-designed structure, is convenient and reliable to use, improves the quality and production efficiency of sheet metal bent parts, and provides a new solution for high-precision bending compensation.
