what is press brake crowning?
During the bending process of the workpiece, due to the maximum force at both ends of the ram, the reaction force of the plate bending causes concave deformation on the lower surface of the ram, and the deformation of the middle part of the ram is the largest.
In order to eliminate the adverse effect of the ram deformation, it is necessary to compensate for the deflection deformation of the ram.
Hydraulic crowning and mechanical crowning make the middle part of the worktable produce upward elastic deformation, which can offset the deformation of the ram of the machine tool, ensure the accuracy of the machining joint surface and improve the accuracy of the workpiece.
At present, the main press brake manufacturers around the world are using mechanical compensation devices, such as Bystronic, etc.
Some of China’s press brake manufacturers use mechanical compensation, such as Shanghai ACL and YSD, and some use hydraulic compensation, such as Yangli, JFY, Yawei, etc.
2. Working principle
Because there are many kinds of CNC press brake control system, this article only takes the Netherlands DELEM DA6x series CNC system as an example to explain the working principle of hydraulic compensation and mechanical compensation.
2.1 Working principle of hydraulic compensation
Figure 1 shows the control schematic diagram of the compensation with DELEMDA6x CNC system and HO-ERBIGER hydraulic system.
Fig. 1 Block diagram of the control principle
In Fig. 2(a), it can be seen that the hydraulic proportional valve table is composed of three iron plates with one thick and two relatively thin side plates.
In Fig. 2(b) where the compensation cylinder is placed, the position of the middle main board is obviously more protruding.
When the workpiece is bent, the hydraulic valve opens the opening of the proportional valve according to the compensation value automatically calculated by the numerical control system, and the hydraulic oil is filled into the compensation cylinder.
The piston in the compensation cylinder will eject a small part to make the main board in the middle produce upward elastic deformation, thus improving the accuracy of the workpiece.
Of course, the bending accuracy of the workpiece is also related to the setting of the internal parameters of the CNC system.
Fig. 2 Installation of compensation cylinder
2.2 Working principle of mechanical compensation (Fig. 3)
Fig. 3 Block diagram of the mechanical compensation principle
The mechanical compensation mechanism is composed of upper and lower base plates and working table, which are connected by disc spring and bolt.
The upper and lower base plates are composed of a number of wedges with different slopes (Fig. 4).
Fig. 4 Base plate
As can be seen in Fig. 4, there is a significant difference in the slope between the two ends and the middle of the upper and lower backing plates, because the middle part of the press brake needs the largest amount of deformation compensation.
As shown in Figure 5, the slopes of the upper and lower base plates at A and B are the same.
Fig. 5 Location diagram of upper and lower base plates before compensation
Before compensation, they are fully fitted together.
When compensation is needed, the lower base plate moves along the direction shown in Fig. 5 under the drive of the motor.
The upper and lower base plates still stick together at A point and are separated at B point (Fig 6).
Fig. 6 Location diagram of upper and lower base plates after compensation
Due to the different slopes of the upper and lower base plates, the upper base plates will undergo upward convex elastic deformation under the action of the lower base plates.
This kind of mechanical compensation mechanism adopts integral upper and lower base plates, of course, split wedge with slope can also be used.
Then a screw rod with thread is used to drive the wedge to move left and right to realize the compensation for the deformation of the ram of the press brake and improve the bending accuracy of the workpiece.
Its electrical control principle is the same as the mechanical compensation mechanism control principle of the whole upper and lower base plates.
At present, this kind of split wedge mechanical compensation mechanism is quite common in large tonnage press brake machines.
3. Performance comparison
Hydraulic compensation mechanism
The CNC system automatically calculates the compensation amount according to the thickness and length of the bending material and the selected die, and outputs the corresponding analog voltage to the compensation amplifier card.
After amplifying the signal, the amplifier card controls the opening size of the hydraulic compensation proportional valve.
During the whole process from the sliding block entering the slow down state until the ram starts to return to the top dead center, the compensation proportional valve is always powered on.
If the pressure holding time of the ram at the bottom dead center of the press brake machine is changed at this time, when the actual deformation of the worktable under the press brake is in the case of shorter holding time and longer holding time, the machining accuracy of the bent workpiece may produce some errors.
At this time, if the press holding time of the machine tool changes a little, under the condition of the press holding time of the machine tool is a little longer, if the press holding time of the machine tool changes.
If there are impurities in the hydraulic oil, the valve core of the hydraulic proportional valve will be stuck, and the machining accuracy error will be greater.
Because the compensation pressure is separated from the pressure of the whole system, when the compensation valve works, the pressure of the whole system will be partially lost.
Due to the use of hydraulic oil control, oil leakage will cause pollution if the oil pipe is broken, the sealing ring of the compensation cylinder is aging or the joint is loose.
Mechanical compensation mechanism
When the ram stops the top dead center position, the CNC system will automatically calculate the compensation amount according to the thickness and length of the bending material and the selected die.
The system module controls the positive and negative steering of the compensation motor to adjust the compensation deformation, then feedback the position of the upper and lower base plates through a potentiometer.
After the compensation motor is adjusted in place, the compensation deformation will not change with the change of the holding time when the ram is at the bottom dead center position, and it is not necessary to power on each time when the ram enters the slow down position, so that the pressure of the whole system will not have energy loss.
As there is no hydraulic pipeline, there will be no oil leakage and pollution.
The press brake is using an integral worktable, which saves the manufacturing cost.
Through the above analysis and comparison of the working principle of hydraulic compensation and mechanical compensation, it can be confirmed that the latter is more superior.
With the continuous improvement of users’ requirements for press brake performance, new requirements are put forward for press brake manufacturers.
It is believed that this new type of mechanical compensation mechanism will be adopted by more machine tool manufacturers.