Page 60 - MetalForming October 2010

Weld Appearance
Counts,
Even on Fabricated Prison Doors
At the Custom Products Division of Chief Industries, in Grand
Island, NE, an aging multi-gun resistance-spot-welding (RSW)
machine was causing just that scenario: huge power requirements
for welding, and hours upon hours of post-weld finishing. The divi-
sion manufactures hollow-metal prison doors, wall panels and
other products for the correctional industry and, yes—appearance
does count, even in prisons. Resistance-welding hat-section stiffeners
to sheetmetal doors and wall panels on its 20+-yr.-old machine
resulted in welds that “were so hot,” says Chief Industries maintenance supervisor Bill Peters, “that each weld caused a huge divot. We needed something better, to improve weld quality as well as increase speed and throughput.”
Custom Machine, New Welding Process
Something better came late in 2008 with the arrival of a new custom-designed and built RSW machine from T.J. Snow Co., Chattanooga, TN. Among its design enhancements is the use of push-pull circuitry rather than the series-style circuit previously employed, as well as automation features to improve throughput by as much as 20 percent.
Wall panels and doors comprise 12-gauge galvanneal panels 2 to 4 ft. wide. The machine joins a series of rolled hat-section stiffeners, of 16-gauge cold-rolled steel, to the panels in several rows spaced 6 to 12 in. apart. Resistance welds are placed every 3 in. along the stiffener flanges.
The new machine mounts 16 pneumatically operated weld cylinders and weld guns to a machined dovetail slide to the upper crown; guns can be independently
The series RSW circuit (top) previously used at Chief Industries employed one transformer to provide power to form four weld nuggets. Push-pull welding (bottom) now in use at Chief features opposing electrodes—four per transformer—to generate two spot welds simultaneously.
The move to push-pull circuitry, implemented on a custom resistance-welding machine used to fabricate prison doors, improves weld- process control and quality, slashes labor content while boosting throughput, while reducing power consumption by 20 percent.
BY BRAD F. KUVIN, EDITOR
Applying excessive heat in a resistance spot-welding (RSW) setup can cause excessive material shrinkage at the weld nugget. When weld appearance counts, as it often does, this means oper- ators have to go back and fill in the craters created due to shrink- age. After the Bondo comes grinding and sanding to smooth the area. End result: extra hours of wasted labor that fail to add value and instead just shrink the bottom line.
Series Welding
+–
Push-Pull Welding
+–
–+
58 METALFORMING / OCTOBER 2010
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