ing control that provides some form of constant current function. This function can maintain the welding current at both extremes of part position by electronically changing the welding firing and using feedback to target a desired welding current. Some controls are more successful than others at reaching rock-solid current values between the electrodes. The best will hold the welding current variation to a range of about ± 1% with parts fully into and out of the throat. This becomes even more challenging when you are seam welding with rolling electrodes. One process I have worked on over the years is the joining of steel coil ends during the processing of sheet steel into coils. When one billet has been rolled out, the end of the steel strip has to be joined to the start of the next rolled sheet. This is often done by having a seam weld wheel assembly move over this splice while the metal stays still. The sheets can be 8 ft or wider and this changes the secondary impedance dramatically as the wheels travel deeper into the steel sheet. Without some form of constant current ability from the welding control, the welds produced at the end of a joint would be considerably weaker than the starting welds. This can cause these joints to fail as the steel sheet is processed through rollers under high stress on the way to the coiler. Ironically, many companies own modern resistance welding machines that have the ability to operate in the constant current mode but do not know it exists — Fig. 5. They paid for a quality control and are not taking full advantage of the control’s ability. Check your control to see if it includes a constant current mode. If not, it is probably time to upgrade your welding machine if you want consistent weld quality on large parts. WJ ROGER HIRSCH is a past chair of the Resistance Welding Manufacturing Alliance (RWMA), a standing committee of the American Welding Society. He is also president of Unitrol Electronics, Inc., Northbrook, Ill. Send your comments and questions to Roger Hirsch c/o Welding Journal, 8669 NW 36 St., # 130, Miami, FL 33166, or via email at roger@unitrolelectronics. com. JULY 2016 / WELDING JOURNAL 25 Fig. 5 — Setting a weld program in constant current mode using direct amps. For info, go to aws.org/adindex For info, go to aws.org/adindex
Welding Journal | July 2016
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