What is Spot Welding | SPOT welding machine | Spot Welding Advantages and Disadvantages | Spot Welding parameter
What is Spot Welding:
It is used for joining two or more sheets of metal. The sheets to be welded are held overlapping between the two electrodes, one moveable and the other fixed. Heavy current is passed through the electrodes. Pressure is applied from the top electrode by moving it downward.
The fusion of material takes place at the spot. The sheet is then moved to have another spot weld at the desired place. The time period for each weld may be 1/100th of a second or more. The current may be of the order of 5000 amperes or more. The voltage between electrodes about 2 volts. The open circuit voltage is less than 12 volts.
Fig. 4.4 shows how spot welding is done. Rods, wires and small pieces can also be welded by this method. For the sheets the time required is about 1/50sec. for each 0.25 mm of total thickness of the two sheets to be welded.
Spot Welding Machine:
A spot welding machine is shown in Fig. 4.5 AC supply is given to the primary of the welding transformer through the timer which controls the time for which the current flows. The primary has got tappings and a particular tapping is selected by the selector switch S depending upon the current required for welding.
For maximum welding current the selector switch should be kept in position No. 1. The secondary is connected to the two electrodes made of copper or bronze and they are cooled by water. When pressure is not applied on the foot pedal, the electrodes are kept apart by the spring.
The sheets to be welded are placed over-lapping each other between the two electrodes and pressure is applied by pressing the foot pedal. The spot weld is made between the sheets. Thickness and composition of the plates determine the duration and magnitude of the current. It varies from 1000 to 10,000 amperes and may flow for a fraction of a second to some seconds. Spot welding is mostly employed in the manufacture of automobiles, refrigerators and other metal stamping assemblies.
Advantages of Spot Welding:
(i) It has low initial cost.
(ii) Higher productivity.
(iii) Less skilled worker can operate this welding.
(iv) No edge preparation is required.
(v) Operation may be made automatic or semi-automatic.
Disadvantages of Spot Welding:
1. Job of more thickness cannot be welded.
2. High initial cost of equipment.
3. Skilled labour is required for maintenance purpose.
Applications of Spot Welding:
(i) It can be frequently used in automobile and aircraft industries.
(ii) It is used for making containers.
(iii) Fabrication of steel household furnitures.
Main Parameters for control Spot Welding are
1 . SQUEEZE TIME TIME REQUIRED FOR THE ELECTRODE TO CLOSE ON METAL AND APPLY PROPER PRESSURE.
2. UPSLOPE TIME TIME TAKEN FOR THE CURRENT TO REACH FROM ZERO THE PEAK VALUE.
3. WELD TIME TIME REQUIRED FOR PEAK CURRENT TO FLOW AND HEAT THE METAL
4. DOWN SLOPE TIME TIME TAKEN FOR THE CURRENT TO CUT OFF i.e, FROM PEAK TO ZERO
5. HOLD TIME TIME TAKEN BY THE ELECTRODES TO HOLD THE SHEETS TOGETHER AFTER THE CURRENT FLOW STOPS. PRESSURE STILL APPLIED TO ALLOW THE MOLTEN METAL TO SOLIDIFY.
6. OFF TIME IT IS THE TIME BETWEEN THE END OF ONE SPOT SEQUENCE TO START OF NEXT SPOT SEQUENCE
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