Neil's News

Spot Welder

6 September 2014

Let's do some spot welding. First, find a microwave oven you don't love anymore and remove its transformer. These heavy duty transformers are wired to produce in excess of 2000 volts. We only want 2 or 3 volts, so the first step is to gently remove the secondary coil using a hacksaw and a hammer.

[Microwave oven transformer]

Since safety ought to make a token appearance somewhere on our list of priorities, it might be a good idea to build a wooden box to house the 120 volt components.

[Wooden box]

Tests showed that wrapping a cable around the secondary core two and a half times produces 2.7 volts. Doubling up the cable doubles the available amperage. A light switch turns on and off main power to the primary coil.
Update: the black 12 AWG cables have since been replaced by thicker 8 AWG cables which don't heat up as much.

[Wiring complete]

Finally a pair of copper electrodes are added. The upper one is hinged to create a jaw.

[Completed spot welder]

Somewhat to my surprise, the machine functioned perfectly the first time it was used. Two nice spot welds on galvanized steel.

Notice that the first weld is bigger and brighter (and stronger) than the second weld. This is due to the current for the second weld being shunted through the first weld. One of the limitations of spot welding.

After making over a hundred welds, I've found that the best technique is to grip the work pieces very lightly when applying current. This causes much greater resistance and a dramatic increase in heat. Once the area is glowing red hot, tighten the grip on the work pieces, cut the power, and wait a second before releasing. This presses the molten metal together to form a tight bond.

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