What is press brake crowning?
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.
Types of press brake crowning
At present, the press brake crowning mainly has three types:
- Geometric crowning
- Hydraulic crowning
- Mechanical 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.
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.
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.