Page 61 - MetalForming October 2010

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 exces- sive 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 elec- trodes away from the material, the assembly shifts by way 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 fre- quently 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 way by virtue of the improved productivity and quality it’s enjoying with the new machine—typi- cal order size is 12 panels/cell.
How it Works
To run the machine, an operator loads a pre-tacked panel onto the machine and positions it against the
left-hand gauge-bar assembly and underneath a series of laser-dot indica- tors. These indicators help the operator align the material to ensure proper weld placement along the stiffener flanges.
The operator then initiates the weld sequence. The right-hand gauge bar extends and forces the panel against the opposing gauge bar. With the panel locked into weld position, the toggle table extends to bring the 16 lower elec-
The push-pull setup provides better weld control and nicer-looking nuggets, with much less heat input.
trodes up. The upper welding guns then extend downward and the first series of 16 welds are completed. The guns retract, the toggle table drops, the gauge bars shift the panel and the sequence repeats.
The setup uses 1.25-in.-dia. zirconi- um-copper electrodes (preferred for welding galvanneal and other coated materials, to avoid electrode sticking)— Tuffaloy TA-25CZ male caps on top and a flat-face lower electrode. Reduced weld heat and power, combined with the abil- ity of the toggle table to prevent electrode marring during indexing of the assembly, have improved electrode life between dressings by 20 percent, says Peters.
To fabricate a prison-door panel (shown), Chief Indus- tries’ new automated resist- ance-welding machines fea- tures 16 opposing sets of welding guns to spot-weld mild-steel top-hat-shaped stiffeners to a galvanneal sheet. Bottom electrodes mount on a toggle table that lifts the electrodes into weld- ing position as the top elec- trodes index downward. With a weld cycle completed, the toggle table retracts to allow the panel to automatically index to the next weld posi- tion without dragging over the bottom electrodes.
Turn Down the Power
Along with its automation capabili- ties, the machine’s push-pull circuitry is a big hit at Chief Industries. T. J. Snow engineers designed each of the machine’s eight push-pull circuits, which run along the width of the machine, to include four weld guns and two transformers. The machine’s upper crown and its lower knee each hold a bank of eight transformers to power the circuits. This differs considerably from the previous machine’s series- welding circuitry, which used only four huge transformers mounted to the upper crown, Each transformer had to generate enough power to complete a set of four welds per cycle.
“We really had to crank up the power to pull the nuggets and form the spot welds,” says Peters. “This created quite a divot at each weld location. Now, the push-pull setup provides better weld control and nicer-looking nuggets, with much less heat input, so the resulting divots are barely visible.
“Yes, we have twice as many trans- formers as we used to, but the payback is definitely there,” Peters adds. “Weld quality is improved, overall power con- sumption is down, and we have practi- cally eliminated the added, wasteful process of filling each weld divot with Bondo and then grinding and polishing each repair.” MF
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