Resistance Spot Welding:
In spot welding the weld is effected by the heat produced due to resistance to the flow of current through two or more overlapping work pieces held pressed together between the electrodes. This is the simplest form of resistance welding and does not pose any problem for welding sheets ranging u to 12.5 mm in thickness. The majority of spot welding is however done with metal pieces less than 6 mm thick.
For best results the surfaces to be welded must be free from scales and foreign matter. Spot welds should not be made too close to the end of a workpiece or to each other. When a spot weld is attempted too close to the edge of the work pieces molten metal may flow out of the weld zone. Similarly if two spot welds are made too close to each other electric current may be shunted through a parallel path provided by an adjacent weld.
The resulting weld nugget is typically 5 to 10 mm in diameter, with a heat affected zone extending slightly beyond the nugget into the base metal.
Spot welding machines are available in three different varieties:
1. Stationary single spot welding machinesa. Rocker arm typeb. Direct pressure type
2. Portable single spot welding machines3. Multiple spot welding machinesRocker arm type:The rocker arm type is the simplest and cheapest but is limited to smaller sizes. It employs rocking motion of the upper arm for applying pressure and raising and lowering of the upper electrode.Direct pressure type:The direct pressure type employs a straight line motion of the upper electrode along the face of the machine column and can be used for larger sizes.Electrode Tip shape:The size and shape of the weld spot is determined by the electrode tip, the most common electrode shape being:1. Round (most common)2. Hexagonal3. Square and other shapesSpot Gun Machines:When the size of the work pieces to be welded increases, then spot welding machines are used on that occasion. A large number of welding guns are available for the purpose. These portable spot welders are connected to the transformers through a long cable and may be taken to the spot where welding is to be done. The electrodes used should have a contact area which gives the desired current density through the work pieces.Welding pressure in these guns may be applied manually, pneumatically or hydraulically depending up on the size and shape of the gun.Advantages of Resistance Spot Welding:1. Adaptability for automation in high rate production of sheet metal assemblies2. High speed3. Economical4. Dimensional accuracyLimitations of Resistance Spot Welding:1. Difficulty for maintenance or repair2. Adds weight and material cost to the product, compared with a butt joint3. Generally have higher cost than most arc welding equipment's4. Produces unfavourable line power demands5. Low tensile and fatigue strength6. The full strength of the sheet cannot prevail across a spot welded joint7. Eccentric loading condition
