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Taking the automotive industry as an example, the rationalization requirements for positioning and positioning welding
The car body is not only finished by welding large deep-drawn steel parts to each other, but also has hundreds of forming small parts fixed, such as: sorting plug or ABS-system, seat fixing device, door and window hinge, distance And so on. In general, rational positioning is required before resistance spot welding. This work is a very headache for the production planning department. Due to the variety of parts and the often compact position, it is hard to imagine automating this positioning work. Therefore, the preparation work before welding is mostly done by hand. Design fixtures according to the actual application, or use non-flexible special machinery, the disadvantages of this approach are as follows:
â– The cost of planning and design of production equipment is disproportionately high;
â– When using rigid and non-flexible production methods, if the shape and position of the steel plate are changed during the production process, special equipment transformation will be carried out;
â– The positional tolerance of the positioning welding in the production process is often too large, which makes it difficult to fully mechanize the metal gas shielded welding;
â– Poor adaptability by manual operation at the workplace where forced production is beaten;
â– The clamping device hinders the operation of the welding work;
â– In order to perform tack welding, it is necessary to increase the working position of the assembly shop;
â– It is easy to mess up when the model changes, and the wrong parts are positioned and welded;
Change the way you work
In order to meet the requirements of the rationalization of the above work, the former staff of the Fraunhofer Institute for Production and Automation Technology (IPA) in Stuart, developed a new production method and the required products. The result was the invention of "arc welding of sheet metal with stroke arc" and the patent of the Fraunhofer Institute in Munich. The positioning grippers developed in conjunction with the industrial robots are mainly composed of standard components. It uses 6ba compressed air drive to adjust the positioning welding process through the microprocessor. It consists of the head of the positioning gripper, the welding power supply and the controller of the industrial robot. A positioning welding program suitable for various welding parts is set in the controller. Programmable welding parameters include:
- the magnitude and time of the pre-charge current;
- the magnitude and time of the main current of the welding;
- the size and time of the welding stroke;
- pressing force;
- sinking depth;
- Characteristic curve of the stroke motion.
In addition, the mechanical tensile strength test can be carried out by means of a positioning gripper for quality control purposes. If necessary, repeat the spot welding with a new spot weldment. For process control, there is a function to compare the theoretical and measured values ​​of the welding parameters to a predetermined tolerance.
The whole process of a tack welding
Of course, instead of putting a small box full of industrial robots in front of the industrial robot, the robot is ordered to "work". In this way, it is impossible to determine the position of the position weldment and the cost of controlling the movement of the card. The workpieces must be separated and arranged one by one in the working order of the industrial robot. Depending on the geometry of the workpiece and the required tact time, different working systems are required and will not be described further here. A variety of differently shaped panels are grabbed with a positioning gripper mounted on the arm of the industrial robot and delivered to the car's casing at a given coordinate.
Before the positioning weldment is pressed onto the car casing, it is displaced by a linear drive by a distance X1. The robot then places the panel in the designated position on the car's outer casing. The robot target point should be programmed so that the positioning weldment can be pressed into the 25 mm after being in contact with the car casing. This distance is half the distance available for transmission. Thereby, it is possible to compensate for the positional and shape errors that always exist on the surface of the automobile body. On the processing unit, the value is ±5 mm. It is important that the press-fit motion does not cause plastic deformation of the coupled member. This reduces the air pressure of the usual servo-pneumatic system.
After the programmed robot target point is reached, the formal positioning welding process begins. The first step is to measure the actual position of the servo drive to derive the error of the car body and temporarily store the coordinate displacement vector X2 associated with the weld. After the clamping pressure is activated, the welding pre-current begins to flow in the gripper caliper. Immediately following the linear drive, the positioning weldment is lifted from the body surface by a programmed weld stroke amount X3. This value is about 0.5 to 4 mm depending on the positioning welding task. Thereby, a stationary arc is generated between the positioning weldment and the body steel plate. The positioning weldment and the surface of the body steel sheet are melted by increasing the pre-current to the main welding current. In accordance with the distance of the stroke section, the welding power source is used to adjust the welding voltage so that the welding current remains unchanged. When the welding energy reaches the required value, the servo drive mechanism positions the weldment to accelerate toward the vehicle body surface. When the two coupling faces are in contact, the arc is extinguished. The short circuit current flows before the power is cut off. The target point of the stroke is to enter a programmed penetration depth X4 in the molten pool, which is related to the resulting penetration. After a short dwell time, the positional gripper can be released after the weld pool has solidified.
The entire process of tack welding is consumed between 50 and 500 ms. The working process is much like the stud welding process. The difference is that the shape of the positioning weldment can be varied. A typical feature of a servo drive is that the weldment has no impact when it enters the weld pool. When the two molten pools collide, the "soft" immersion of the positioning weldment is achieved by deceleration. It greatly improves the quality of the weld compared to conventional magnetic stroke mechanisms.
Experience and development potential
The positioning grippers that have been used in the automotive industry have achieved excellent results in terms of rationalization and flexibility. Since this positioning gripper does not actually require maintenance, it can achieve almost 100% utilization. If a part is broken, you can quickly replace a standard part to ensure continued use. Regarding the reliability of the process, statistics show that the probability of welding defects is only 0.03% in a few months.
The positioning of the welding head driven by the servo adjustment also opens up new possibilities for the finished welding of the post-weld parts. Due to the high positioning accuracy of the robot and the small shape error of the positioning weldment, a gas shielded welding torch can be mounted on the robot arm to weld the finished part. Since the position error of the vehicle body can be compensated at any weld position by moving the vector X2 by coordinates, it is possible to change the space point by a coordinate movement vector value in the continuous welding procedure, thereby negating the use of an expensive weld seam tracking system. Originally, industrial robots were not suitable for gas-shielded continuous welding, but now they can be realized reliably and economically.
All areas of mechanical engineering have been proposed to use automatic tack welding. For example, in agricultural machinery, steel structure and container manufacturing, pick and air conditioning, kitchen supplies and material handling technology. Just the value of industrial robots has been greatly reduced, making it easier for SMEs, which are increasingly struggling to reduce costs, to adopt this automatic positioning welding method.
Flexible automation application for tack welding
The entire process of solder joints is becoming more automated, but does not include tack welding. Since the flexible automation of the welding process increases productivity and quality, time and expense can be saved. A positioning gripper is a workpiece gripper integrated with welding technology, which can solve the problem of using the industrial robot to complete the positioning welding process.