In spot-welding, the heat is generated by electric resistance: Copper electrodes pass a current between sheets of steel and the steel becomes so hot that a spot melts between the sheets to fasten them together. The challenge of the 9090 is precision. “Sapper invented a new closure,” Alessi remembers, “which can be done with one hand—–the handle on the top half swings down to clip over the lower boiler, holding it all together.”
Two bent-steel fittings spot-welded onto the boiler body form the seal. A jig helps to line them up, and the worker presses a pedal, which clamps the electrodes. “This closure has tight tolerances,” explains Alliata. “Otherwise the seal will be inadequate and the consequences are drastic.”
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