Spot Welding for the Electrical Rebuilder Part I By Richard R. Vensel Resistance weld- ing is one of the old- est of the electric welding processes in use by industry today. The weld is ss made by a combination of heat, pres- sure and time. As the name resistance welding implies, it is the resistance of the material to be welded, to current flow, that causes a localized heating in the part. The pressure exerted by the tongs and electrode tips, through which the current flows, holds the parts to be welded in intimate contact before, dur- ing and after the welding current time cycle. The required amount of time current flows in the joint is determined by material thickness and type, the amount of current flowing and the cross-sectional area of the welding tip contact surfaces. In Figure 1, page 11, a complete secondary resistance spot welding cir- cuit is illustrated. For clarity the vari- ous parts of the resistance spot weld- ing machine are identified. Fundamentals of Resistance Spot Welding. ELECTRODE TIPS w/Work Resistance spot welding is accom- plished when current is caused to flow through electrode tips and the sepa- rate pieces of metal to be joined. The resistance of the base metal to electri- cal current flow causes localized heat- ing in the joint and the weld is made. The resistance spot weld is unique because the actual weld nugget is formed internally with relation to the surface of the base metal. Figure 2 shows a resistance spot weld nugget compared to a gas tungsten-arc spot weld. 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 workplace. Resis- tance 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 cur- rent 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 though, that resistance spot welding machines are NOT designed as force clamps to pull the workpieces together for weld- ing. Heat Generation. A modification of Ohm’s Law may be made when watts and heat are con- sidered synonymous. When current is passed through a conductor the elec- trical resistance of the conductor to current flow will cause heat to be gen- erated. The basic formula for heat gen- eration may be stated: H = I2H where H = Heat 12 = Welding Current Squared R = Resistance The secondary portion of a resis- FEBRUARY, 2001 RESISTANCE SPOT TUNGSTEN ARC SPOT Figure 2 – Resistance and Tig Spot Weld tance 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 genera- tion 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 gener- ated is directly proportional to the re- sistance at any point in the circuit (see Figure 3). Squeeze Time: Time between pressure application and weld. Weld or Heat 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. The resistance spot welding ma- chines manufactured by Miller Elec- tric Co. are constructed so minimum resistance will be apparent in the trans- former, flexible cables, tongs and elec- HEAT OR WELD TIME Figure 3- Spot Welding Times Page 1 of 2°4]§{)!