It is sometimes difficult to weld small chips, or foreign materials with arc welding, it can also be difficult to weld with silver or tin, so occasionally spot welding may be convenient.
Spot welding is based on pressure and temperature. Two pieces are welded together when a part of them is heated to temperatures close to the melt and pressure is made between them. In the case of this welding the heating of the piece is made by electric current between two electrodes and the pressure is precisely performed by these electrodes in the form of a clamp.
As generally the resistance of the parts to be welded is very low the current that must pass through the zone to be welded must be very high of the order of the 500 amps, but nevertheless the voltages are very low, of 1 to 3 volts. The total power is therefore one or two kilowatts.
Obviously with a suitable transformer it is not difficult to achieve this with a little bit of skill.
Microwave ovens carry a transformer of about 2 kilowatts of power. The primary accepts the 220 V and has two secondary ones. One of them to feed the magnetron supplies about 2200 volts effective, another winding consists of one or two turns and serves to feed the filament of the magnetron.
In any scrapping you can buy an old microwave oven for less than 3 euros. Normally, the magnetron has been damaged and the transformer is intact. In others it may be broken the transformer, in that case the most normal is that the high voltage secondary has been shorted. If the primary is burned, it will not work.
Starting from a transformer that has the primary well.
When observing the transformer it can be seen that the primary and secondary high voltage windings are perfectly separated. The primary one is thicker thread. The secondary does not need it and is also dangerous because the high stresses it produces can be deadly. In addition we need the space that occupies for the secondary of our welding.
To eliminate it, cut the winding with a saw and remove the turns until it is completely removed. If you have an insulated reel let it, we can come well although it is not a must.
The secondary we need, it should produce about 2 volts and about 500 amps even temporarily. For this secondary we need a pair of wire turns of about 10 0 or 15 mm in diameter. In a supplied electrical material buy 1.5 m of cable of this coarse (copper). It will not cost us more than a couple of euros. Wind it directly into the core (a couple of turns) connect the primary to the grid (through a 10 amp fuse) and measure the voltage. There should be a couple of volts.
The electrodes have the mission of passing the current through the metals to be soldered and also imprisoned. You must be isolated from each other and must also approach so that their tips are aligned. The electrodes must be very well connected to the transformer secondary.
The electrodes are made with a copper bar of about 10 mm in diameter, we will need two electrodes of about 50 mm, or in total 10 cm. In A metal warehouse we can get it for less than one Euro. By means of a file, one of the two ends of each electrode is tapered. Connect the two terminals of the secondary to the electrodes by soldering them or by means of any very firm support.
In order to complete the welding in a semi-provisional manner, both electrodes can be held by a drill holder, so that their lever action is used to approximate them. One of them can be held in the base or in the jaw and the other in the chuck. The drill itself can be isolated. Note that the three volts that feed the tips are not dangerous.
A switch is needed to power the transformer. The simplest is to activate it with the foot. The hands are usually held holding the piece.
Before starting to flow, the parts must be trapped. Otherwise, the tips sizzle. After doing some tests you can decide to raise or lower the voltage, winding more or less turns in the transformer. When you have a final assembly shorten the secondary cables as much as possible.
This is a spot welding lab. With a similar I salary stainless steel sheets up to 1 mm. I have soldered strands of rods and tantalum, titanium, platinum, zirconium, but I have not been able to weld tungsten.
I take advantage of a transformer with the damaged secondary. On top of the transformer itself, using aluminum plates and fiberglass plates, built the mechanisms to hold the clamps, using a foot switch.
My spot welding equipment during construction.
Learn With a Video.
Improvements in welding. (04-14-2004)
Initially I did this welding to join threads and thin sheets, so the clamps are not very thick, but some fans wanted to weld thicker things so I thought how to extend their power. This expansion is mainly achieved by eliminating part of the transformer current limiting magnetic circuit.
To prevent the microwave generator from being burned by an excess of current during its operation the power transformer is designed to deliver a maximum power of the order of 1.5 kW, rather for a primary current of about 7 amps. This is achieved by partially closing the magnetic circuit of the primary with some sweet iron plates which can be seen precisely in the separation of the primary and secondary windings. The photo transformer has two, one in each of the holes. I have removed one of them and the primary current has gone from 7 amps to 18 amps, which in theory absorbed power is more than 3.5 kW. This effect has been very noticeable and I have managed to weld stainless steel sheets 1 mm thick.
If you still do not have enough power you can try to remove the second plates, I have not done it but I estimate that you can get another sensible increase of power, if at the expense of very high consumption that may not support an installation Electric household as we will walk for 25 or 30 amps.
In my case before removing the current limiting by the clamps passed more than 400 amps, then I could not make the measurement because my ammeter clamps did not reach out of scale, but I must assume that they would walk through the 1000 amps.
Although the transform provides so much energy, the results will be poor if the high current secondary circuit is not carefully realized. To do this, replace the aluminum clamps by copper, as thick as possible, increase the thickness of the secondary winding and ensure that all connections are forceful. Analyze with a polymer that there are no voltage drops at any point. In short, the whole power is dissipated between the two clamps and not in the rest of the circuit.
One thing still more, the objective of the electric current is to bring the temperature of the materials above 1000 degrees, but if these metals are not pressed together they will not be soldered, for that reason the pressure that is exerted on the pieces to be welded Must be as high as possible without the plates being pierced.