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Weld Appearance Counts, Even on Fabricated Prison Doors

October 1, 2010

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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.

Applying excessive heat in a resistance spot-welding (RSW) setup

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.

can cause excessive material shrinkage at the weld nugget. When weld appearance counts, as it often does, this means operators have to go back and fill in the craters created due to shrinkage. 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.

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 division 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 positioned along the length of the crown to accommodate custom weld patterns. Lower tooling comprises 16 high-pressure platen-mounted electrode holders. A toggle table lowers the tooling to allow indexing of the panel assemblies, between weld sequences, to avoid dragging the material across the top of the electrodes.

“That’s just one advantage we gain from the new machine,” says Peters, noting that premature electrode wear from the previous machine led to excessive weld-tip dressing, and caused unplanned downtime. Now, once a set of 16 welds has been completed and the toggle table drops to move the electrodes a from the material, the assembly shifts by of two gauge bars, one on each side of the machine.

Since order size for each door can be as little as one or two, setups change frequently and automating the gauge-bar setup was a key machine specification, adds Peters, allowing speed and throughput to climb. To manufacture wall panels—which the firm hopes to move into in a big by virtue of the improved productivity and quality it’s enjoying with the new machine—typical order size is 12 panels/cell.