What is press brake crowning?
The press brake is pressurized by two working cylinders at both ends of the ram. The deformation force of the bending workpiece is in the middle, so the ram and the worktable are deformed together with the upper and lower die. It 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 so-called deflection compensation device has preset a deformation in the direction of the opposite force-deformation in the ram and the upper die or the worktable and the lower die working table, and the deformation has the same amount of deformation generated in the actual work.
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, the press brake manufacturer will not adopt this type of crowning method.
Working table fixed convex, that is, at the time of manufacture, the worktable was machined into arc shape with a slightly 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 occurs upward deflection deformation, the upper die edge basically tends to be straight, therefore it keeps each bending point along the bend line generate the same bending force for the plate.
The advantages of the geometrical compensation method are low cost and easy to manufacture, but there are some disadvantages that can only realize the compensation of fixed deformation and small compensation flexibility.
In addition, the compensation block arc correction quantity needs to pass through the precise calculation. The calculation method which is based on the mechanics’ theory and the finite element calculation all has a certain error, therefore, even this kind of crowning method can achieve the deflection compensation effect, but it is very difficult to realize.
Mainly used on the electro-hydraulic synchronous CNC press brake, because the amount of compensation needs to be controlled by the controller, such as DA52S, DA66T controller etc.
In addition, install two hydraulic cylinders at two sides of the press brake frame and install another two-auxiliary hydraulic cylinder in the middle of the machine. When the ram is down, the auxiliary cylinder is filled with liquid oil and goes downward. During the bending process, hydraulic oil inlet into the auxiliary cylinder, so that the ram generate downward deflection for compensation.
Install the auxiliary hydraulic cylinder in the lower part of the worktable. During the bending process, it generates an upward force on the worktable, which forms the automatic crowning system.
The pressure compensation device is composed of several small oil cylinders, comprising an oil cylinder, a motherboard, an auxiliary plate and a pin shaft, and a compensating cylinder is placed on the worktable. The pressure compensation system is formed with a proportional relief valve.
During working, the auxiliary plate supports the oil cylinder, the oil cylinder holds up the motherboard up, just overcomes the deformation of the ram and the worktable. The convex device is controlled by a numerical control system, so that the preload can be determined according to the thickness of the plate, the opening of the die and the tensile strength of the material when bending different sheet materials.
The advantage of hydraulic crowning is that it can realize the deflection compensation for continuous variable deformation with large compensation flexibility, but there are also some disadvantages of complex structure and in relatively high cost.
The most widely used crowning method for the ordinary press brake, with good compensation effect and low cost. In real operation, it is very convenient and easy for operators.
Mechanical crowning is a kind of new deflection compensation method, which generally uses a triangular oblique wedge structure.
The principle is that the two-triangle wedge block with α angles, the upper wedge moving is fixed at X-direction and can only move in 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 full length on the worktable, the upper and lower plates are connected through the disc spring and bolts. The upper and lower plates are consist of a number of oblique wedges with different slopes, through the motor drive to make them relatively moving and forming an ideal curve for a set of convex positions.