Flexible automation application of the hottest tac

2022-08-16
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Flexible automation application of tack welding

the whole process of welding connection is becoming more and more automated, but it does not include tack welding. Because the flexible automation of welding process improves the productivity and quality, it can save time and cost. A positioning gripper is a workpiece gripper integrated with welding technology, which can solve the problem of using industrial robots to complete the positioning welder sequence

take the automobile industry as an example, the rationalization requirements for positioning and tack welding

the automobile body is not only completed by welding large deep drawn steel plates together, but also by fixing hundreds of formed small parts, such as classification plugs or ABS systems, seat fixing devices, doors, window hinges, distance finders, etc. Generally speaking, they should be reasonably positioned before resistance spot welding. This work is a headache for the production planning department. Due to the variety of parts and the often compact location, I can't imagine automating this positioning work. Therefore, the preparation before welding is mostly done manually. Design fixtures according to the actual application, or use non flexible special machinery. The disadvantages of this method are as follows:

■ the planning and design costs of production tools are disproportionately high

■ when rigid and non flexible production methods are used, if the shape and position of steel plate parts are changed in the production process, special equipment will be transformed

■ in the production process, the positional tolerance of tack welding is often too large, which brings difficulties to the full mechanization of metal electrode gas shielded welding

■ in the workplace with forced production beat, the adaptability of manual operation is poor

■ clamping device hinders the operation of welding work

■ in order to carry out tack welding, it is necessary to increase the working position of the assembly workshop

■ it is easy to get confused when the model is changed, and the wrong parts are positioned and welded

change the working mode

in order to realize the requirements of rationalization of the above work, the former staff of the franchoff Institute of production and automation technology (IPA) in Stuttgart developed a new production mode and required products. As a result, the invention of "plate arc tack welding with stroke arc strike" has been patented by the franhoff Institute in Munich. The positioning gripper developed with the industrial robot is mainly composed of standard parts. It is driven by 6BA compressed air, and the positioning welding process is adjusted by microprocessor. It is composed of the head of the positioning gripper, the welding power supply and the controller of the industrial robot. A tack welding program suitable for various weldments is set in the controller. Programmable welding parameters include:

- the size and time of the pre applied current

- size and time of main welding current

- size and time of welding stroke

- pressing force

- sinking depth

- characteristic curve of stroke movement

in addition, the mechanical tensile strength test can be carried out with the help of the positioning gripper to achieve the purpose of quality control. If necessary, repeat the spot welding with a new spot welding piece. In order to control the process, there is a function to compare the theoretical value of welding parameters with the measured value, so as to keep it within the preset allowable tolerance range

the whole process of one-time tack welding

of course, it is not to put a box full of small pieces in front of the industrial robot, and then command the robot to "work". In this way, the cost of identifying the position of the positioning weldment and performing the first level of high-pressure seal control on the clamping movement cannot be borne. The workpieces must be separated and arranged in the working area of the industrial robot one by one. According to the geometry of the workpiece and the required beat time, different working systems are required, which will not be further described here. A positioning gripper installed on the arm of an industrial robot is used to grasp various shapes of plates, and send them to the car shell according to the given coordinates

before pressing the tack weldment onto the car shell, move it for a distance X1 through linear transmission. Then the robot can put the plate on the designated position of the car shell. The programming of the robot target point should enable the positioning weldment to be pressed 25mm through the linear transmission of the positioning gripper after it contacts the car shell. This distance is half of the available transmission distance. Thus, the position and shape errors that always exist on the car body surface can be compensated. On the machining unit, the value is ± 5mm. The important point is that through this pressing movement, the connected part will not produce plastic deformation. Therefore, the air pressure of the usual servo pneumatic system is reduced

after the target point of the programming robot is reached, the formal positioning welding process begins. The first step is to measure the real position of the servo driver, so as to obtain the error of the car body, and temporarily store the coordinate displacement vector x2 related to the welding position. After starting the clamping pressure, the welding pre current begins to flow in the gripper caliper. Then, using the linear transmission, lift the positioning weldment from the body surface to a programmed welding stroke X3. This value is about 0.5 ~ 4mm depending on the tack welding task. Thus, a stationary arc is generated between the positioning weldment and the body steel plate. By increasing the pre current to the main welding current, the surface of the tack weldment and the body steel plate is melted. In line with the distance of the travel section, the welding voltage is adjusted by using the welding power supply to keep the welding current unchanged. When the welding energy reaches the required value, the positioning weldment of the servo drive mechanism accelerates to the body surface. When the two connecting surfaces contact, the arc goes out. Before the power supply is cut off, the short-circuit current flows. The target point of the stroke is a programmed penetration X4 into the molten pool, which is related to the generated penetration. After a short residence time, the positioning gripper can be released after the welding pool solidifies

the whole tack welding process consumes about 50 ~ 500ms. The working process is very similar to the stud welding process. The difference is that the shape of the positioning weldment can be varied. The typical feature of servo drive is that there is no impact when the weldment enters the molten pool. When two molten pools collide, the "soft" immersion of the positioned weldment is realized by deceleration. 3. Extensometer classification extensometer is a sensor that senses the deformation of the specimen. Compared with the traditional magnetic stroke mechanism, it greatly improves the welding quality

experience and development potential

at present, the effective elastic modulus and strength of the bit gripper have been significantly improved under the extremely low volume fraction of polycarbonate, which has been used in the automotive industry, and excellent results have been achieved in rationalization and flexibility. Since this positioning gripper actually does not need maintenance, it can achieve almost 100% utilization. Once a part breaks down, a standard part can be replaced quickly to ensure continued use. As for the reliability of the process, statistics show that within a few months, the probability of welding defects is only 0.03%

the tack welding head driven by servo adjustment also provides a new possibility for the welding of finished parts after tack welding. Due to the high positioning accuracy of the robot and the small shape error of the positioning weldment, the gas shielded welding torch can be installed on the robot arm to weld the finished parts. Because the position error of the vehicle body can be compensated at any welding position through the coordinate movement vector X2, the space point can be changed by a coordinate movement vector value in the continuous welding procedure, so the expensive seam tracking system can be abandoned. In the past, industrial robots were not suitable for gas shielded continuous welding, but now they can be realized reliably and economically

automatic tack welding is proposed in all fields of mechanical manufacturing. For example, it is used in agricultural machinery, steel structure and container manufacturing, heating and air conditioning, kitchen supplies and material handling technology. At the same time, the price of industrial robots has fallen sharply, making it more reasonable for small and medium-sized enterprises, which are increasingly struggling to reduce costs, to adopt this automatic positioning welding method. (end)

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