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Resistance welding is a process that uses electrical energy and mechanical pressure together with the specific electrical conductivity of the workpieces to form a welded joint. Electric current is supplied to the workpieces by the electrodes and converted into heat which causes the workpieces to be instantly fused and joined. Resistance welders are typically used to join two plain metal pieces together. Electricity is supplied to the sheets or any workpieces joined by welding electrodes that exert a force on the sheets. This force is then converted into heat.
The heat is generated in such a way that it melts the metal where they join - the 'resistance' point between the surfaces. The electrode of the resistance welder then extracts heat from the molten area of the weld, which forms the weld lump where it solidifies. The force is applied before, during, and after the current is applied, which limits the contact area.
The power to the spot resistance welder is supplied to the primary winding of the welding transformer via a timer that controls the time the current flows. The primary part has a threading and a specific thread is selected with the S switch depending on the current required for welding. To obtain the maximum welding current, the selector switch must be set to position No. 1. The secondary conductor is connected to two copper or bronze electrodes and they are water-cooled. When the foot pedal is not depressed, the electrodes are held separately by a spring.
In reality, a projection resistance welder is a simplified type of spot welding system. One of the parts to be welded in this manner has pressure-generated predictions. There are flat electrodes. On the workpiece, the electrodes are mounted and the current flows through them. At the contacts, heat is produced and at these points, the workpiece gets welded. The welding of some sections that can not be welded by spot welding is simple. If two plates are of a different cross-section to be welded, then it is appropriate to provide the projections on the thicker plate to achieve the desired strength.
With a seam resistance welder, electrodes of the circular or roller type are used, and the pressure between them remains constant. Seam welding is performed to obtain a continuous joint that may be required for the construction of tanks, transformer refrigerators, gasoline tanks, aircraft, containers, and cylindrical components. A seam is a series of overlapping spot welds. Typically, the number of points obtained per meter of a weld is between 200 and 400. The two sheets on which the seams are required to be welded are overlapped. The current flow between the two moving electrodes, which are to be shaped like a continuously rotating wheel which also exerts pressure on the work. A seam resistance welder is used when a continuous connection between two overlapping sheets is required.
Important factors affecting the proper formation of the molten area between the metal pieces with a resistance welder are the magnitude of the current, the time it takes to pass it, and the force pressing the parts together. The optimal values of these parameters differ depending on the type of metal details and their thickness. Obtaining such a high electric current requires the use of a special resistance welding transformer, which converts the high factory voltage into a low and thus safer welding voltage. By transforming the voltage, it simultaneously converts low factory current into high welding currents.
In terms of process currents and welding times, resistance welder control is responsible for switching the current on and off and is a critical part of the system. A relay or manual switch may be considered but is inadequate given the relatively low speed of the operation. In the previous example, the current only needs to be on for 0.25 seconds, which is hard to achieve in such a short time, let alone consistent operation. Internally, in resistance welding control, the power device used to turn on the current is known as a silicon controlled rectifier. It works thanks to the fact that a small electrical signal applied to the device allows it to be turned on in a fraction of a second and conducts a large amount of current. By removing the electrical signal, the device can be switched off again.
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