Current Development Status and Research Dynamics at Home and Abroad

Sheet metal bending machines are widely used bending machines. They have long been hydraulic and quickly achieved numerical control in the 1980s. According to incomplete statistics, 18 bending machines were exhibited at CIMT95, of which 7 were foreign and 11 were Chinese (including 1 from Taiwan). Except for the one from Taiwan, all 17 are CNC bending machines. In China's previous international machine tool exhibitions, this session exhibited many bending machines, many of which are domestically produced and of high quality. Sheet metal bending machines can bend metal sheets at various angles with simple molds to obtain complex shapes of metal sheet parts. They are simple to operate, have strong mold versatility, and low operating costs, so they are widely used. Sheet metal bending machines can be divided into mechanical bending machines and hydraulic bending machines according to their transmission forms. At present, mechanical bending machines have been replaced by hydraulic bending machines. The advantages of hydraulic bending machines are that they have a large working stroke and can produce the maximum nominal force at any point in the stroke; the bending stroke, pressure, and speed are adjustable, making it easy to achieve numerical control; fast approach and slow bending can be achieved, meeting the process requirements of workpiece bending; a multi-cylinder synchronous system is adopted, greatly improving the bending accuracy and realizing the multi-machine linkage of the bending machine, which broadens the process range of the bending machine. CNC hydraulic sheet metal bending machines are metal sheet processing machine tools that have been on the market early, widely used, and have many domestic manufacturers. They have developed from 3-axis control to 8-axis control, and can be used with loading and unloading robots and bending robots to form a CNC bending unit.

The emerging high-end and mid-range building materials and decoration industries at home and abroad have put forward higher requirements for the metal decorative materials they need, namely, the arc radius of the edge of the bent metal sheet workpiece should be as small as possible, and the surface of the metal sheet workpiece should be free of glare; the bending cross-section shape of the metal sheet workpiece is more complex and better reflects the trendy design style of the designer.

Technical Requirements of This Design

It is desired to design a vertical sheet metal bending machine. The up and down movement of its slider and bending mechanism is to be driven hydraulically. The working cycle to be achieved through electro-hydraulic control is: rapid descent → slow pressure increase (bending) → rapid return. Maximum bending force Fmax = 0.5 × 106N, slider weight G = 12000N, rapid descent speed v1 = 20 mm/s, slow pressure increase (bending) speed v2 = 10 mm/s, rapid ascent speed v3 = 45 mm/s; rapid descent stroke l1 = 150 mm, slow pressure increase (bending) stroke l2 = 30 mm, rapid ascent stroke l3 = 180 mm; start-up and braking time Δt = 0.15 s. It is required to balance the weight of the slider and bending mechanism hydraulically to prevent self-weight sliding; slider guide friction can be ignored. As an actuator, it drives the slider and bending mechanism to perform bending operations on the sheet metal.

Task Analysis

Since the bending machine is vertically arranged, its slider moves reciprocally up and down in a straight line, and the stroke is small (nearly 180 mm), so a cylinder-fixed vertical single-rod piston type hydraulic cylinder can be selected (take the mechanical efficiency of the hydraulic cylinder In the transmission system of the vertical sheet metal bending machine, the up and down movement of its slider and bending mechanism is to be driven hydraulically. The working cycle to be achieved through electro-hydraulic control is: rapid descent → slow pressure increase (bending) → rapid return. Maximum bending force Fmax = 0.5 × 106N, slider weight G = 12000N, rapid descent speed v1 = 20 mm/s, slow pressure increase (bending) speed v2 = 10 mm/s, rapid ascent speed v3 = 45 mm/s; rapid descent stroke l1 = 150 mm, slow pressure increase (bending) stroke l2 = 30 mm, rapid ascent stroke l3 = 180 mm; start-up and braking time Δt = 0.15 s. It is required to balance the weight of the slider and bending mechanism hydraulically to prevent self-weight sliding; slider guide friction can be ignored. As an actuator, it drives the slider and bending mechanism to perform bending operations on the sheet metal.

Because the working cycle of the sheet metal bending machine consists of three stages: rapid descent, slow pressure increase (bending), and rapid return. The conversion between the stages is controlled by a three-position four-way electro-hydraulic directional valve. When the electro-hydraulic directional valve operates in the left position, rapid return is achieved. When it operates in the middle position, the hydraulic pump is unloaded, and when it operates in the right position, the hydraulic pump is fast and advanced. The speed change is controlled by the flow of the pressure-limiting variable vane pump, controlled by the limit switch. When the bending machine descends rapidly, its speed is required to be fast to reduce the empty stroke time. The hydraulic pump uses full-pressure oil supply. The piston stroke is controlled by a stroke valve. When the piston moves to a certain position at a constant speed, the stroke valve receives a signal and generates an action to achieve the conversion from fast feed to working feed. When the piston moves to the termination stage, the pressure relay receives a signal to change the electro-hydraulic directional valve. Because the bending machine has a high pressure, the pressure in the oil inlet chamber is high at this time. Therefore, a pre-relief circuit is required for the transition from working feed to rapid return to prevent high-pressure impact on the hydraulic components and ensure smooth oil unloading. Therefore, a pre-relief circuit can be designed in the rapid return oil circuit, and the speed of the circuit relief is adjusted by a throttle valve. At this time, the directional valve is in the middle position. When the pressure is relieved to a certain pressure, the directional valve is switched to the left position to achieve smooth unloading. In order to monitor the oil circuit pressure, a pressure gauge and a relief valve are installed at the hydraulic pump outlet, which also protects the system from overload. Because the slider is affected by its own gravity, the slider will slide down, so the oil circuit should design a unidirectional sequence valve as a balance valve to form a balance circuit, generating a certain amount of back pressure, and making the working process smooth. A check valve is designed at the outlet of the hydraulic pump, which can be used as a preset pressure valve, provide starting pressure for the electro-hydraulic directional valve switching, and prevent oil pressure from impacting the hydraulic pump, protecting the pump.