Abstract : In order to help everyone better understand bending machines, this article describes the function and motion process of bending machines, analyzes two types of hydraulic control systems, and introduces the high-performance hydraulic components used.
Keywords : CNC bending machine; proportional valve, hydraulic control valve block; CNC controller.
1. Introduction
Tianjin Luosheng, a leading brand service provider in the field of transmission control, serves industries including machinery manufacturing, factory automation , aerospace, power, metallurgy, petrochemicals, and medical. Its product series primarily covers industrial hydraulics, electronic transmission and control, linear transmission and control, and pneumatic transmission and control. Luosheng's Dipma hydraulic products are widely used in sheet metal bending machine control. A bending machine is a specialized machine tool used to bend sheet metal. Dipma hydraulic products are widely used in modern bending machines. These hydraulic control systems mostly utilize proportional control technology, and the machine tools possess flexible CNC systems specifically designed for bending machines. This allows for highly automated operation and high-precision sheet metal bending. Using CNC bending machines, high production efficiency and high-quality parts can be achieved when processing complex-shaped sheet metal, resulting in significant economic benefits.
1.1 Functions and Operation of the Bending Machine
CNC bending machines are primarily upper piston presses. Their main operating characteristic is that two parallel-moving hydraulic cylinders generate downward pressure to drive the die on the bending beam for bending operations. The CNC-controlled Diplomat hydraulic control system mainly controls the synchronous operation of the bending process and the positioning of the hydraulic cylinders at the bottom dead center when the machine is under full load. The typical bending process cycle stages of the upper bending beam include:
(1) The stationary position of the bending machine in a static state is when the upper bending beam is at the top dead center. In order to prevent uncontrolled downward movement, the system uses a leak-free back pressure valve to seal the rod chamber of the hydraulic cylinder.
(2) Downward Movement: The downward rapid closing movement of ordinary bending machines mostly adopts a free-fall form, that is, the movement is generated by the self-weight of the bending machine beam and its accessories. During this process, the rodless chamber of the hydraulic cylinder is replenished with oil through a filling valve, at which time the rod chamber will generate back pressure. The rapid advance speed can reach 250mm/s, which is controlled by the open-loop or closed-loop proportional directional valve. The rapid advance movement starts from the top dead center, and after a brief braking phase, it ends at the maximum distance of 6mm from the bending plate. Different bending machines require different rapid advance end positions.
(3) Stamping/Bending: The stamping/bending stage begins with pressure build-up in the rodless chamber. The stamping speed is limited by the oil pump supply and can be adjusted by the proportional and directional valves. Simultaneously, the directional valves also control the synchronous operation of the bending beam and the positioning of the bottom dead center. The stamping pressure is limited by the proportional relief valve, which restricts the pump pressure. The corresponding speed, synchronization, positioning, and pressure values are all provided by the CNC controller.
(4) Pressure reduction in the rodless chamber begins either at the bottom dead center or after a short holding time, allowing the deformable material to flow for a certain period, thus further improving the dimensional accuracy of the workpiece. Both holding and pressure reduction are performed by the proportional directional valve according to the CNC controller's instructions. The pressure reduction time must be as short as possible according to the required operating cycle time; however, it must be long enough to avoid unloading impacts throughout the system. In short, the pressure reduction curve must be smooth and not too steep. Optimization of the entire process is achieved through the proportional directional valve.
(5) The flow rate of the return pump and the pressure-bearing area of the rod chamber of the hydraulic cylinder determine the maximum return speed, which is close to the high speed in most cases. The return stroke begins from the decompression of the rod chamber and ends at the top dead center. The return stroke also requires synchronous operation.
1. Several types of hydraulic control systems for CNC bending machines
In CNC bending machines, the hydraulic control system requires a high degree of automation and standardization in manufacturing. Therefore, the bending machine should have a high integration rate, integrating the hydraulic system within the machine itself. Thus, the frame becomes the foundation for mounting the hydraulic components, and it integrates the oil tank into the stamping frame. This integration rate varies depending on the user's specific requirements. Generally, there are two models as follows:
(1) Three-control valve block: This type has three control valve blocks. Two main control valve blocks with their respective inter-plate filling valves are directly mounted on the hydraulic cylinder, realizing a pipeless connection between the main control block and the hydraulic cylinder. The main control valve block mainly consists of a proportional directional valve, a reversing valve with position monitoring, and a back pressure cartridge assembly. The proportional reversing valve plays a key role in the bending machine, and together with the CNC, it determines the movement direction, speed, synchronization accuracy, and positioning accuracy of the upper bending beam. The current control block series are all equipped with proportional reversing valves with integrated electronic amplifiers, which can be directly controlled by the CNC. The back pressure assembly (generally composed of a reversing valve and two relief valves) is a leak-free type according to customer requirements. The suction valve is responsible for filling the rodless chamber with oil during rapid advance and draining the large flow of oil from the rodless chamber during return. The above two control blocks plus the third control block—the pump control block—form a complete three-control valve block structure. The most important mounting components here are the proportional relief valve and a maximum pressure shut-off valve, as well as the position detection reversing valve, which serves as the pilot valve for the filling valve. The proportional relief valve sets the maximum stamping pressure according to the set value of the CNC, and the manually adjustable relief valve connected in parallel is responsible for the pressure protection of the whole system, especially for excessively high set values from the CNC on the pump.
The diagram of a typical three-control-block hydraulic system is as follows:
(2) Central Control Block: The central control block combines three control valve blocks into one. It is mainly used in bending machines with special structures (e.g., where machine height is limited). Due to control requirements, the connecting pipes between the control block and the two hydraulic cylinders must be symmetrically arranged, and the maximum allowable distance between the two cylinders (approximately 3m) must be ensured. Therefore, the central control block should be placed in the center of the machine as much as possible. The control block is connected to the hydraulic cylinders via pipes. In this type, a flange-type filling valve is used, directly mounted on the hydraulic cylinder, and connected to the oil pipe via a suction pipe. The compact structure of the filling valve is suitable for bending machines with very low overall height.
A typical hydraulic system diagram with a central control valve is shown below:
2. Introduction to Dipma Hydraulic Bending Machine Product Series
2.1 Dipma IGP Series Internal Gear Pump
This pump employs both radial and axial pressure compensation, resulting in low operating noise and high volumetric efficiency. It also features an optimized load distribution design and special sliding bearings, enabling continuous operation under high pressure and extending its service life. The operating pressure is 33 MPa, with a peak pressure reaching 34.5 MPa. Displacement ranges from 3.6 to 251.7 cm³/rpm. All sizes and displacements can be freely combined.
2.2 Position Detection Type Solenoid Directional Valve DS*M The DS*M series spool-detection type solenoid directional valve is equipped with a proximity sensor to indicate the spool position. In particular, the normally closed PNP type sensor indicates the spool's home position (when the solenoid valve is de-energized). When connected to a CNC circuit, the state of the directional control valve can be identified, allowing for integrated function control. The return port (T-port) has high pressure resistance, up to 100 bar under static conditions and up to 50 bar under dynamic conditions.
2.3 Integrated Digital Proportional Directional and Pressure Valves The DS*J series valve is a direct-acting, electrically controlled, servo-controlled proportional directional valve with spool displacement detection. This valve can be used for position and speed control of high-dynamic hydraulic actuators. The valve opening and flow rate are continuously adjustable and proportional to the input reference signal. The PRE*G series proportional pressure valve is an integrated electrically controlled, pilot-operated proportional relief valve, typically used for precise hydraulic system pressure control. The pressure is continuously adjustable and proportional to the reference signal. The maximum control pressure can reach 350 bar.
2.4 DXJ* series direct-acting servo valves and DS*J series servo proportional valves
The DXJ* series valve is a four-way spool servo valve. Driven by a torque motor, it boasts excellent dynamic performance and is unaffected by system pressure. The valve spool position is fed back via a displacement sensor (LVTD), forming a closed-loop system that ensures high accuracy and repeatability. This valve features very low power consumption, making it ideal for closed-loop control of position, speed, and pressure. When power is cut off or the reference signal is zero, the valve spool automatically returns to the neutral position, with minimal leakage at the neutral position, a value dependent on system pressure.
4. Conclusion
Hydraulic CNC bending machines are highly automated sheet metal processing machine tools, and therefore require high-performance hydraulic components. Luosheng Company can supply all the high-performance hydraulic components needed for bending machines. The company is committed to good cooperation with suppliers and customers, and will continue to provide hydraulic components and systems with excellent cost performance based on customer needs.
