Spot welding machine in India and things need to know

Fundamentals of resistance spot welding

Principle

Resistance welding is accomplished when current is caused to flow through electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electrical current flow causes localized heating in the joint, and the weld is made.

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The resistance spot weld is unique because the actual weld nugget is formed internally in relation to the surface of the base metal.

The gas tungsten-arc spot is made from one side only. The resistance spot weld is normally made with electrodes on each side of the workpiece. Resistance spot welds may be made with the workpiece in any position.

The resistance spot weld nugget is formed when the interface of the weld joint is heated due to the resistance of the joint surfaces to electrical current flow. In all cases, of course, the current must flow or the weld cannot be made. The pressure of the electrode tips on the workpiece holds the part in close and intimate contact during the making of the weld. Remember, however, that resistance spot welding machines are NOT designed as force clamps to pull the workpieces together for welding.

Heat Generation

A modification of Ohm’s Law may be made when watts and heat are considered synonymous. When current is passed through a conductor the electrical resistance of the conductor to current flow will cause heat to be generated. The basic formula for heat generation may be stated:

H = I2R

H = Heat

I2 = Welding Current Squared

R = Resistance

The secondary portion of a resistance spot welding circuit, including the parts to be welded, is actually a series of resistances. The total additive value of this electrical resistance affects the current output of the resistance spot welding machine and the heat generation of the circuit.

The key fact is, although current value is the same in all parts of the electrical circuit, the resistance values may vary considerably at different points in the circuit. The heat generated is directly proportional to the resistance at any point in the circuit.

Squeeze Time

Time between pressure application and weld.

Heat Or Weld Time

Weld time in cycles.

Hold Time

Time that pressure is maintained after weld is made.

Off Time

Electrodes separated to permit moving of material for next spot.

Resistance spot welding machines are constructed so minimum resistance will be apparent in the transformer, flexible cables, tongs, and electrode tips. The resistance spot welding machines are designed to bring the welding current to the weldment in the most efficient manner. It is at the weldment that the greatest relative resistance is required. The term “relative” means with relation to the rest of the actual welding circuit.

There are six major points of resistance in the work area. They are as follows:

1. The contact point between the electrode and top workpiece.

2. The top workpiece.

3. The interface of the top and bottom workpieces.

4. The bottom workpiece.

5. The contact point between the bottom workpiece and the electrode.

6. Resistance of electrode tips.

The resistances are in series, and each point of resistance will retard current flow. The amount of resistance at point 3, the interface of the workpieces, depends on the heat−transfer capabilities of the material, the material’s electrical resistance, and the combined thickness of the materials at the weld joint. It is at this part of the circuit that the nugget of the weld is formed.

(Source: internet)

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