spot welding machine
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The ends to be welded touch each other before the current is switched on. A heavy current is then passed from one piece to another and the contacting faces are heated up due to the contact resistance. The two pieces are pressed together firmly after the desired welding temperature of 870 to 925 oC is reached. The pressing action which results in the increase in lateral dimension of the workpieces is called upsetting. Upsetting takes place both during and after the current flow. The upsetting action results in welding of end faces with squeezing of a part of the softened metal to form a fin, which can be removed later, if required, by machining.
Resistance butt welding is used for end joining of rods, tubes, bars and similar other sections. However, an important application of this process is the large scale production of tubes and pipes at a high rate of production and the process is then referred to as resistance butt-seam welding or simply as electric resistance welding (ERW). In ERW process, the strip for tube making is continuously edge shared and rolled into tube for forming a longitudinal seam.
Current of upto 40,000 amperes at 5 volt is introduced across the joint by split electrode rollers and the force is applied by the pressure rolls. In this process, both the work motion and current and supply are continuous. The flow of current through the shunt path is avoided or reduced by the use of ferrite or wrought iron ‘impeder’ placed inside the tube.
The maximum speed of production is controlled by the frequency of the current as that decides the number of zero-current periods per unit time. Frequencies upto 350 Hz are commonly used which results in the production rate of about 36 m/min. The fin formed due to extruded metal is continuously removed by cutters and the desired length of the tube or pipe is cut on the production table without any interruption in the process of welding.
Spot Welding Process
All resistance welding operations are automatic and therefore all process variables are pre-set and maintained constant. Once a welding operation has been initiated there is no way in which its progress can be controlled and, thus, the weld cycle is completed as per the pre-set times.
The welding cycle for spot welding machine, seam welding machine and projection welding machine consist basically of four elements viz., squeeze time, weld time, hold time, and off time. These timing are pre-set for a particular metal and a thickness range and the shop operator normally cannot change them on his own. Each one of these four time phases has its own role to play in achieving a sound weld of the required size.
The time interval between the application of electrode pressure to the work and switching on the welding currents called the squeeze time. This time interval is provided to assure the contact between the electrode and the work and to initiate the application of force on it.
It is the time for which the welding current actually flows to melt the metal at the interface.
It is the time for which the electrodes are kept in position, after the welding current is switched off, to assure the application of pressure so as to consolidate the molten metal into a nugget which is then cooled by the dissipation of heat to the surrounding work material. If the applied force is excessive it may result in expulsion of molten metal from in-between the sheets.
The time allowed to shift the work to the next location before the cycle is repeated is referred to as the off time. The electrodes are kept off the work during this time interval.
Welding current, time of current flow and the electrode pressure are recognized as the fundamental variables of resistance spot welding machine. For achieving quality welds in most metals, these variables are required to be kept within very close limit.
The size of the weld nugget and in fact whether it will form or not depends upon the heat being generated faster than it is dissipated by conduction. Welding current is, thus , the most critical variable.
Both a.c. and d.c. are used to produce spot, seam, and projection welding machine. Most applications use single phase a.c. of mains frequency i.e. 50 hertz. However, d.c. is used for applications that need heavy current and the load for which can be balanced on a 3-phase power line. Also, with direct current machines the rate of current rise and fall can be programmed as per requirements. The current rise period or upslope and current decay period or down slope can be programmed with electronic control systems.
Control of upslope helps to avoid overheating and expulsion of molten metal at the beginning of the weld time as the interface resistance at that time is high. Down slope helps to control weld nugget solidification to avoid cracks in weldments particularly in metals that are prone to quench-hardening and hot tearing.