Spot Welding for the Electrical Rebuilder BY RICHARD R. VENSEL There are six major points of re- sistance in the work area. They are: 1. The contact point between the electrode and top work piece. 2. The top work piece. 3. The interface of the top and bot- tom work pieces. 4. The bottom work piece. 5. The contact point between the bottom work piece and the electrode. 6. Resistance of electrode tips. The resistances are in series and each point of resistance will retard cur- rent flow. The amount of resistance at point three, the interface of the work pieces, will depend on the heat trans- fer capabilities of the material, its elec- trical 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. The Time Factor. Resistance spot welding depends on the resistance of the base metal and the amount of cur- rent flowing to produce the heat nec- essary to make the spot weld. Another important factor is time. In most cases, several thousands of amperes are used in making the spot weld. Such amper- age values, flowing through a relatively high resistance, will create a lot of heat in a short time. To make good resis- tance spot welds it is necessary to have close control of the time the current is flowing. Actually, time is the only con- trollable variable in most single impulse resistance spot welding applications. Current is very often economically im- practical to control. It is also unpre- dictable in many cases. Most resistance spot welds are made in very short time periods. Since alternating current is normally used for the welding process, procedures may be based on a 60 cycle time (60 cycles = one second). Figure 2 shows the resistance spot welding time cycle. Previously, the formula for heat gen- eration was used. With the addition of the time element the formula is com- pleted as follows: H=I°’RTK where H = Heat I?= Current Squared R=Resistance T=Time K = Heat Losses Control of time is important. If the time element is too long, the base metal in the joint may exceed the melting (and possibly the boiling) point of the mate- rial. This could cause faulty welds due to gas porosity. There is also the pos- sibility of expulsion of molten metal from the weld joint which could de- crease the cross section of the joint, weakening the weld. Shorter weld times also decrease the possibility of excessive heat transfer in the base metal. Distortion of the welded parts is minimized and the heat affected zone around the weld nugget is substantially smaller. FLEXIGLE CONDUCTOR WELDING TRANSFORMER CONTACTOA SINGLE PHASE POWER MARCH, 2001 Figure 2 – Spot Welding Time Cycle. Pressure. The effect of pressure on the resistance spot weld should be carefully considered. The primary pur- pose of pressure is to hold the parts to be welded in intimate contact at the joint interface. This action assures con- sistent electrical resistance and con- ductivity at the point of weld. The tong and electrode tips should NOT be used to pull the work piece together. The resistance spot welding machine is not designed as an electrical “C” clamp! The parts to be welded should be in intimate contact BEFORE pres- sure is applied. Investigations have shown that high pressures exerted on the weld joint decrease the resistance at the point of contact between the electrode tip and the work piece surface. The greater the pressure, the lower the resistance factor. Proper pressures, where there is intimate contact of the electrode tip and the base metal, will tend to conduct ELECTAODE HOLDER ELECTAODE HORN ELECTROCE SPACING ELECTRODE HOLDEA THROAT GEPTH Page 1 of 3