Q.Show the procedure of spot welding?

The steps involved in making a spot weld are listed below and shown but before spot welding one must make sure that

(i) The job is clean, i.e. free form grease, dirt, paint, scale, oxide etc.

(ii) Electrode tip surface is clean, since it has to conduct the current into the work with as little loss as possible. Very fine emery cloth may be used for routine cleaning.

(iii) Water is running through the electrodes in order to

(a) Avoid them form getting overheated and  thus damaged, (b) Cool the weld.

(iv) Proper welding current has been set on the current selector switch.

(v) Proper time has been set on the weld-timer.

Step 1. Electrodes are brought together against the overlapping work pieces and pressure applied so that the surfaces of the two work pieces under the electrodes come in physical contact after breaking any unwanted film existing on the work pieces.

Step 2. Welding current is switched on for a definite period of time. The current may be of the order of 3000 to 100,000 A for a fraction of second to a few seconds depending upon the nature of material and its thickness.

As the current passes through one electrode and the work pieces to the other electrode, a small area where the work pieces are in contact is heated. The temperature of this weld zone is approximately 815°C to 930°C.To achieve a satisfctory spot weld, the nugget of coalesced metal should from with no melting of the material between the faying surfaces.

Step 3. At this stage, the welding current is cut off. Extra electrode force is then applied or the original force is prolonged. This electrode force or pressure forges the weld and holds it together while the metal cools down and gains strength.

Step 4. The electrode pressure is released to remove the spot welded work pieces