Spot Welding for the Electrical Rebuilder Part Ill By Richard R. Vensel Electrode Tip Size. When you consider that it is through the elec- trode that welding current is permitted to flow into the work piece it is logical that the size of the electrode tip point controls the size of the resistance spot weld. Actually, the weld nugget diameter should be slightly less than the diameter of the electrode tip point. If the electrode tip diameter is too small for the application, the weld nug- get will be small and weak. If, how- ever, the electrode tip diameter is too large, there is danger of overheating the base metal and developing voids and gas pockets. In either instance, the appearance and quality of the finish weld would not be acceptable. The resistance factors involved for different materials will certainly have some bearing on electrode tip diam- eter determination. A general formula has been developed for low carbon steel. It will provide electrode tip di- ameter values that are usable for most applications. CAUTION! The tip diameter dis- cussed in this test refers to the elec- trode tip diameter at the point of con- tact with the work piece. It does not refer to the major diameter of the total electrode tip. The formula generally used for low carbon steel is (not copper to copper or copper to steel): Electrode tip diameter = 0.100″ + 2t This applies where “‘t’ is the thick- ness in inches of one thickness of the metal to be welded. This formula is applicable to the welding of metals of dissimilar thicknesses. The formula is applied to each thickness individually and the proper electrode tip diameter selected for each size of the joint. For example, if two pieces of 0.062″ sheet steel are to be joined, the electrode tip diameter would be the same for both sides of the joint. The calculation would be: Electrode tip dia. = 0.100 + 2t = 0.100 + 2 x 0.62″ = 0.100 + 0.124″ Electrode tip dia. = 0.224″ If the two pieces were unequal in thickness, such as one piece 0.062″ and the other 0.094″, the calculations would have to be made. Each thick- ness would be treated as the basis for one electrode tip diameter determina- tion, as: Electrode tip dia. = 0.100 + 2t = 0.100 + 2 x 0.062″ = 0.100 + 0.124″ Electrode tip dia. =0.224″ (one side only) For the other side, the calculation is. Electrode tip dia. = 0.100 + 2t = 0.100 + 2 x 0.094″ = 0.100 + 0.188″ Electrode tip dia. =0.228″ (one side only) Remember that the formula is ap- plicable to low carbon steels and may not be correct for other materials. Pressure or Welding Force. The pressure exerted by the tongs and the electrode tips on the work piece have a great effect on the amount of weld current that flows through the joint. The greater the pressure, the higher the welding current value will be within the capacity of the resistance spot weld- ing machine. Setting pressure on the resistance APRIL, 2001 spot welding machines is relatively easy. Normally, samples of the mate- rials to be welded are placed between the electrode tips and checked for ad- equate pressure to make the weld. If more or less pressure is required the maintenance and operating manual for the resistance spot welding machine will give explicit directions for making the correct setting. As part of the set- ting up operation, the tong and elec- trode tip travel should be adjusted to the minimum required amount to pre- vent “hammering” the electrode tips and tips holders. MISCELLANEOUS DATA: This section of the text is designed to pro- vide information regarding several of the variables that occur in some resis- tance spot welding applications. Heat Balance. There is no particu- lar problem of heat balance when the materials to be welded are of equal type or thickness. The heat balance in such cases, is automatically correct if the electrode tips are of equal diam- eter, type, etc. Heat balance may be defined as the conditions of welding in which the fusion zones of the pieces to be joined are subjected to equal heat and pressure. When the weldment has parts of unequal thermal characteristics, such as copper and steel, a poor weld may result for several reasons. The metals may not alloy properly at the interface of the joint. There may be a greater amount of localized heating in the steel than in the copper. (The reason being copper has low electrical resistance and high thermal transfer characteris- tics, while steel has high electrical re- sistance and low thermal transfer char- acteristics.) Correct heat balance may be ob- tained in a weldment of this type by one of several methods. Figure 1 illus- Page 1 of 3