SPOT WELDING
In this type of welding, after proper cleaning of the surfaces, the metals to be welded are kept one above the other and two copper electrodes are kept on both sides of the plates. The weld cycle consists of four periods.
A) Squeeze Time: A pressure is applied to the electrodes and maintained for a particular interval before further operations start and this time is called squeeze time.
B) Weld Time: This is the period during which the weld current is applied. The duration of application of current is always measured in terms of the number of cycles; each cycle corresponds to 20 ms. (1 / line frequency). The pressure is also maintained during this time.
C) Hold Time: After the passage of current is cut off, the pressure has to be maintained for a brief interval of time so that the heated metal solidifies and forms a weld nugget. This period is called hold time.
D) Off Time: This is the time for which we have to wait at the end of the spot welding before starting any other welding operation on the materials. Usually, resistance welding is carried out at line frequency (50 Hz) but for some applications high frequency (450 KHz) resistance welding is also employed.