force between the rods, the resistance at the contact point will increase. The energy produced at this joint is covered by the following formula: Joules of energy = I2 r t where I = current, r = resistance, and t = time. If you reduce the force at the weld joint area, you increase the resistance and therefore increase the energy expended on that joint. That’s the good news. But you have to be careful when lowering electrode force. One of the purposes of electrode force is to forge the parts together. Lower force will reduce the amount of forging and reduce the reliability of the weld. Another purpose of the force is to keep metal within the nugget area during the weld process. Low force will start to produce metal expulsion. This removes some of the metal at the joint and the flying sparks of hot metal can be a danger to the operator and others in that area. Extremely low force will cause voids inside the weld nugget to lower the total joint area and reduce weld strength, and metal expulsion between the wires at the nugget area might need to be ground down as an expensive and unnecessary secondary process. Additionally, if you lower the electrode force, you also lower the force where the electrodes contact the outside of the parts being welded. This heats up the area under the electrodes more than needed and will rapidly degrade the electrode. The result will be greatly lowered electrode life. Consult a good cross-wire welding chart to get starting values to use. Take the weld force and weld time on the chart, and adjust the weld current only until you reach good stable results. You can find a good chart in the RWMA Resistance Welding Manual, 4th edition, available from the American Welding Society at pubs.aws.org/p/ 323/rwma-resistance-welding-manualrevised 4th-ed. The chart is in Table 6.1. This chart is for welding steel rods. For stainless steel rods, increase the electrode force about 10%. Using this chart, you can even adjust the percent setdown (final height of completed joint). Q: My company is a threeshift operation. We weld large coldrolled steel door panels on deep throat welding machines. This includes welding of stiffeners as well as hinge plates and latch brackets. There are two problems that we have observed. First, the welds produced during the third shift seem to be stronger than those from the other two shifts even though the welding programs are locked into all of our welding machines. The second is that if we set up a weld when the door panel is pushed into the throat of the welding machine (between the machine’s arms) and then weld a front reinforcement channel when the door panel is pulled almost out of the welding machine’s throat, the welds are different in appearance and strength. We try to make our setups in the middle position of the door, but the results are not reliable. This also makes production of strong welds that do not mar the outer “show” surface very difficult. A: Modern welding controls can easily solve these problems. 1. Line voltage variation. In answer to your first problem, the voltage going into your welding machine from the power company is not constant. It will rise and fall during the day and evening in response to the amount of power being used by others on the line. Typically, voltage at night is the highest when most industrial use is low and, in the summer, air conditioning use is minimized. The voltage available between the electrodes on the welding machine secondary is directly proportional to the JULY 2016 / WELDING JOURNAL 23 Fig. 2 — Stainless steel wires being cross welded. For info, go to aws.org/adindex
Welding Journal | July 2016
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