The development of modern manufacturing technology requires that the press not only can operate at high speed, high precision and large load, but also has greater flexibility, and can design the motion curve of the slider according to different materials and process requirements.
On the basis of abandoning the flywheel, clutch and other energy consuming components of the traditional mechanical press, the servo press adopts the AC servo motor as the power source of the press directly, and converts the rotating motion of the motor into the linear motion of the slider through the actuator such as screw, crank and connecting rod, so as to realize the controllable motion of the slider, so as to meet the needs of flexible and intelligent stamping processing.
Servo press can improve the technical level and manufacturing capacity of complex shape stamping parts, high-strength plates and aluminum alloy plates, which fully reflects the future development trend of forging press machine tools.
Production status of traditional press
Mechanical press is the most widely used stamping equipment in the field of metal plastic processing.
It uses DC motor, AC motor or variable frequency speed regulation motor as the power source and crank and multi connecting rod as the actuator to complete the stamping process by running the flywheel at high speed under no-load and releasing energy in the load stage.
Disadvantages of traditional mechanical press
Traditional mechanical presses have the following disadvantages.
Low production efficiency and large potential safety hazards
Fig. 1 shows the common assembly line production mode.
Traditional mechanical press assembly line operation
The operator is responsible for loading and unloading.
During the operation, the personnel have high labor intensity, easy fatigue and great potential safety hazards;
At the same time, due to manual operation in the production process, there is a certain amount of deformation in the production and transportation process, which has a great impact on the compliance rate and assembly performance of the parts.
Due to the assembly line operation, when one of the processes has a problem, the whole line is facing shutdown, and the production efficiency can not be guaranteed.
High energy consumption
The conventional crank press drives the motor to rotate all the time, drives the flywheel to rotate, and controls the slider movement through the clutch by the inertia of the flywheel.
At the same time, the utilization rate of the energy released by the high-power motor is only 65%, and a lot of energy is wasted in the movement process, increasing the production cost.
Large noise, large vibration and reduced die life
There are several kinds of noise generated by crank connecting rod machine tools:
The first is that the noise generated by mechanical press during plate processing and forming will be higher than 90dB, up to 93dB;
The second is the noise caused by the idling of the motor;
The third is the noise caused by the change of impact motion when the clutch and brake of the press work.
Therefore, operators must wear anti-noise earplugs to avoid hearing loss.
At the same time, the sliding block will produce a large vibration in the movement process, which has a great impact on the working parts of the die.
If the die works for a long time in an environment with frequent vibration, it will seriously affect its service life.
Product quality and performance cannot be guaranteed
The cold stamping process at room temperature will have many quality defects, such as cracking, wrinkling, necking, slip line, etc.
The influencing factors include: forming process, sheet properties, die parameters, process parameters, equipment accuracy and working conditions, etc.
Under the same forming process conditions, the rationality of drawing speed is also a factor that can not be ignored.
The traditional press can not effectively control the drawing speed, and the problems of unstable production and inconsistent quality often occur.
Advantages of servo press in stamping production
Improve production efficiency
Because the servo press retains the advantages of the crank press, especially its production efficiency is much higher than that of hydraulic press, which reflects “the processing quality of hydraulic press and the production efficiency of the mechanical press”.
Moreover, the crank press driven by the servo motor can also adjust the stroke of the slider according to different workpieces.
In a cycle, it does not need to complete 360 °rotation, but only swing at a certain angle to complete the stamping production, which further shortens the cycle time, minimizes the invalid stroke and greatly improves the production efficiency.
Due to the high precision and stability of the servo press, the quality of the parts is greatly guaranteed and the die maintenance time is reduced;
At the same time, the emergence of servo press also reduces the operational risk of staff.
Energy-saving and environmental protection
The conventional crank press drives the motor to rotate to drive the flywheel to rotate. Depending on the inertia of the flywheel, the slider movement is controlled by the clutch.
The servo motor drives the press (Fig. 2), which works completely by the torque of the motor. Through the mixing (screw rod, multi connecting rod, etc.) force increasing mechanism, the small motor can play a large force without clutch, and the motor driving unit controls the start and stop of the motor.
Fig. 2 Servo press
Because the flywheel is omitted, the motor rotates only when stamping, and the idle energy consumption of the motor and flywheel of the ordinary crank press can be saved.
Because there is no clutch, the energy consumption of the clutch is also reduced.
Compared with the traditional press of the same tonnage and the servo press, the servo press saves more than 35% energy.
Low noise and long service life of die
Due to the use of imported servo motor and CNC control system, special working characteristic curve can be designed to control the speed of punch during blanking, so as to reduce the vibration and noise of blanking and improve the service life of die.
The research results of Komatsu company in Japan show that the blanking noise of servo motor driven NC press is more than 20dB lower than that of the conventional crank press.
At the same time, because there is no idling of the motor and flywheel, there is no noise when blanking.
Production and application of servo press
Relationship between press and blank
The working speed of the press is the drawing speed of the sheet metal in the macro and the strain rate of the sheet metal in the micro.
According to the plastic forming theory, the increase of strain rate will cause material hardening, but when the deformation speed is further increased, the heat generated in the plastic deformation process will reduce the hardening effect (Fig. 3).
Fig. 3 Relationship between sheet plasticity and strain rate
According to the general trend of sheet plasticity changing with strain rate, when the strain rate is not very large (ab section), the plastic decrease caused by the increase of strain rate is greater than the plastic increase caused by temperature effect, that is, the sheet plasticity decreases with the increase of strain rate;
When the strain rate is large (cd section), due to the significant temperature effect, the plastic increase caused by temperature effect is equivalent to the plastic decrease caused by strain rate.
That is, the decrease of sheet plasticity is not significant;
When the strain rate increases to a certain extent (de section), the plasticity of the sheet decreases sharply and the sheet approaches the crack edge.
From the above analysis, it can be concluded that with the increase of the working speed of the press, the plasticity decreases due to the increase of deformation and tensile resistance in the sheet deformation area, which increases the stress in the force transfer area of the drawing part, and will increase the possibility of cracking there;
Therefore, for the maximum allowable drawing speed of different plates, the press speed in the drawing process must be checked during drawing to ensure that the working speed of the press is within the maximum allowable drawing speed of sheet metal.
Practical application of servo press
Under the premise of high production efficiency, how to ensure the formability and stability of parts puzzles many enterprises.
Automobile B-pillar reinforcing plate is a very important load-bearing structural part in the vehicle body.
The structural characteristics of B-pillar parts are: large forming depth, complex change of part section, high yield strength raw materials and large height difference of parts.
Such parts are easy to cause problems such as cracking, wrinkling and eccentric load in mold manufacturing and production, because these problems can not be completely avoided in process and mold design.
After putting into production, the requirements for the use of the press have been greatly improved.
There are five processing procedures from blank to finished product of automobile B-pillar reinforcing plate (Fig. 4).
Fig. 4 Production sequence of automobile B-pillar reinforcing plate in servo press
The emergence of servo press solves the problem that B-pillar reinforcing plate cannot be produced with high efficiency, high precision, high stability and high quality;
At the same time, during the production process, the intelligent monitoring system of the servo press can collect the stress changes of each process in real time.
In case of unqualified products or abnormal mold faults, it will give an alarm (Fig. 5), and the on-site personnel will deal with it in time to avoid the batch output of defective products and safety accidents.
Fig. 5 Intelligent monitoring stress change of servo press during production of B-pillar reinforcing plate
AC servo motor driven press can greatly improve the flexibility and intelligence of equipment and improve the monopoly characteristics of servo press.
It is the development direction of a new generation of forming equipment.
In view of its broad application prospects, the press manufacturing enterprises should speed up the research of this new technology and develop large servo presses with completely independent intellectual property rights.