Spot welding
From Wikipedia, the free encyclopedia
Spot welding is a type of
resistance welding used to weld various sheet
metals. Typically the sheets are in the 0.5-3.0 mm thickness range. The process uses two shaped
copper alloy electrodes to concentrate welding current and force between the materials to be welded. The result is a small "spot" that is quickly heated to the
melting point, this forms a nugget of welded metal after the current is removed. The amount of heat released in the spot is determined by the amplitude and duration of the current. The current and duration are chosen to match the material, the sheet thickness and type of electrodes. Applying the current for too long can result in molten metal being expelled as weld splash, or can even burn a hole right through the materials being welded.
A miller sport welder
Applications
Spot welding is typically used when welding particular types of metal
steel sheet metal. Thicker
stock is difficult to heat up from a single spot, as the heat can flow into the surrounding metal too easily. Spot welding can be easily identified on many sheet metal goods, such as metal buckets. Aluminum
alloys can also be spot welded. However, their much higher
thermal conductivity and
electrical conductivity mean that up to three times higher welding currents are needed. This requires larger, more powerful, and more expensive welding
transformers.
Perhaps the most common application of spot welding is in the
automobile manufacturing industry, where it is used almost universally to weld the sheet metal to form a car. Spot welders can also be
completely automated, and many of the industrial
robots found on
assembly lines are spot welders (the other major use for robots being painting).
A further place where spot welding is used is in the orthodontist's clinic, where small scale spot welding equipment is used when resizing metal "molar bands" used in
orthodontics.
Spot welding: KUKA industrial robots welding a car body in the white section of a production line.[1]
Electrical notes
The voltage needed for the welding depends on the resistance of the material to be welded, the sheet thickness and desired size of the nugget. When welding a common combination like 1.0 + 1.0 mm sheet steel, the voltage between the electrodes is only about 1.5 V at the start of the weld but can fall as low as 1 V at the end of the weld. This drop in voltage stems from the resistance reduction caused by the steel melting. The open circuit voltage from the transformer is much higher than this, typically in the 5-10 V range, but there is a very large voltage drop in the electrodes and secondary side of the transformer when the circuit is closed.
Due to changes in the resistance of the metal as it starts to liquefy, the welding process can be monitored in real-time to ensure a perfect weld every time, using the most recent advances in monitoring/feedback control equipment. The resistance is measured indirectly, by measuring the voltage at and current through the electrodes.
External links
hobbyspotwelders.
http://www.philpem.me.uk/elec/welder/
