In households, a spot welding device is often required, but it is difficult to buy because of the high price. Meanwhile, there is nothing complicated in it, and you can make the device yourself. The basis of the welding machine is a transformer. For personal needs, do-it- yourself spot welding from a microwave can be made. To better understand the device, you first need to understand how it works.
The principle of operation of the spot welding device
Metal parts are placed between the electrodes of copper or brass, which they are pressed against each other. After that, an electric current is passed through them, heating the welding spot red-hot. Parts become plastic, and at the junction a liquid bath is formed with a diameter of about 12 mm. Under pressure, the joint is welded.
The current supply and heating occur in the form of a pulse, after which the parts continue to remain locked in one position, until slightly cooled.
Imagining the principle of work, it is easier to figure out how to do spot welding yourself.
Advantages and disadvantages of spot welding
The main advantages of spot welding are:
- profitability;
- high bond strength;
- simplicity of the device;
- the ability to do it yourself;
- the ability to automate the process in a production environment.
Resistance welding does not ensure the tightness of the seam, which is the main disadvantage.
Welding Machine Requirements
- The ability to change the duration of the process.
- Creation of significant pressure at the welding site with a maximum at the end of heating.
- The presence of electrodes with high electrical and thermal conductivity. For this, electrolytic copper, its alloys with chromium and tungsten, bronze with cadmium and cobalt additives are suitable. For the home craftsman, copper and its alloy of the EV brand are most accessible. The contact area of the working end of the electrode should be 2-3 times smaller than the size of the weld .
DIY welding machine
From the description of the principle of operation, it becomes clear that manual spot welding should first of all ensure heating of the parts at the contact point to the melting temperature. The heating power of the devices is different, and you need to imagine for what purposes a home-made device will be used.
Parts for manufacturing:
- transformer;
- insulated wire with a diameter of 10 mm;
- electrodes
- switch;
- tips;
- bolts
- improvised material for the manufacture of the body and welding tongs (plywood, wooden bars).
The devices are made mainly desktop. Portable devices are widely used and often not inferior to stationary ones.
Spot Welding Pliers
The electrodes are inserted into the tips, and the latter are mounted on the ends of the welding tongs, isolated from each other. The easiest way to make them from wooden blocks with a body made of plywood.
Only the upper lever moves, and the lower is attached to the base. The compression force needs to be provided as much as possible, especially when thick sheets of metal need to be welded. To do this, you need a powerful lever. It must be spring-loaded so that the electrodes in the initial state are open. At home, it is advisable to provide a pressing of no more than 30 kg. The handle can be up to 60 cm long, and the electrodes are attached closer to the axis of rotation so that the shoulder is 1:10. In industry, pneumatic and hydraulic devices are used for the necessary pressing of parts with adjustable force.
It is convenient to bring the switch to the handle. It will be connected to the primary winding, through which a small current flows. The device is also controlled by a magnetic starter through the foot pedal. The secondary winding and transformer housing are grounded.
Transformer assembly
When do-it-yourself spot welding from a microwave is done, the main part of the device is a transformer with a power of 700-1000 watts. The higher it is, the better. The microwave transformer has a welded construction. He needs to remove only the secondary winding, without damaging the primary. At the output, it is necessary to obtain a welding current of at least 500 A. For this, you need to wind a new winding on top of the primary winding, from a wire with a diameter of at least 1 cm. For it, a gap remains in the transformer slots, through which 2-3 turns of a thick insulated wire that fits tightly into opening between the core and the primary winding. A 1 kW device is suitable for welding plates up to 3 mm thick.
Spot welding from a microwave with your own hands creates a current in the secondary winding of up to 2 thousand A. With its larger value, surges in the network will be noticeable and in the apartment this can have a negative impact on the operation of electronic devices. In a private house, you can use a more powerful device.
How to disassemble the microwave is not a problem. It is important that its transformer provides the necessary power. Often, two identical voltage converters are used to increase it , connected in parallel to each other. For this, the spot welding circuit is created from two identical secondary windings with the connection of the same terminals at the input and output. In this case, a 2-fold increase in power is achieved without changing the voltage. Welding current will also double. It is important not to confuse the findings so that a short circuit does not occur. As a result, it will be possible to weld plates up to 5 mm thick.
When the secondary windings are connected in series, the output voltage of each of them is added up. In this case, erroneous connection in antiphase should also be avoided. To do this, the load is connected to the output and an alternating voltage is measured with a voltmeter.
Transformers are attached to the base of the chassis and grounded.
Electrode Making
For the manufacture of electrodes, it is easiest to find copper rods. For a small device, they can be made from the tip of a powerful soldering iron. Electrodes quickly lose their shape and need to be periodically grinded. A hole is drilled in them, through which bolts are connected to the wires of the secondary winding.
Electrode Requirements:
- strength at operating temperature;
- ease of machining;
- high heat and electrical conductivity.
Most of all, copper alloys with tungsten and chromium or bronze additives containing cobalt and cadmium satisfy these requirements. EV is considered the best alloy.
The lower electrode is fixedly mounted, and the upper is attached to the upper arm. It is important to ensure their reliable isolation.
The device is connected to the network by a 20 A circuit breaker.
Wire connection
Wires are connected to the electrodes, the length of which should be as short as possible. They are soldered to copper tips. Separate cores of the wire are also soldered together, because with a large current the contact points can oxidize and power is lost. It is not recommended to compress the tips, as additional resistance occurs at the contact points.
Resistance welding technology
Welding is performed only after pressing the electrodes, otherwise they may burn. The main welding parameters are as follows:
- current strength;
- pulse duration;
- electrode pressing force;
- shape and size of electrodes (sphere, plane).
Maximum compression is created by passing current and a short time after it. In this case, the metal has time to crystallize, and the connection is more durable.
The device is preferably cooled by a fan. It is necessary to monitor the temperature of the electrodes, wires and windings of the transformer. In the event of their heating, a break is made in the work.
The spot welding time depends on the current value and is selected empirically. This is usually a few seconds. Mostly sheet material is connected, but there may also be rods.
The welding mode can be hard and soft. In the first case, a large current is supplied and a large compression force is created at a short pulse duration (not more than 0.5 sec). Hard mode is suitable for welding alloys of copper and aluminum, as well as alloy steels. Soft mode is characterized by a longer pulse duration. It is more suitable for home environments where it is not always possible to achieve the required power. Workpieces made of ordinary carbon steels are welded.
The pressure, current density, and spot size of the weld depend on the diameter of the contact surface of the electrode.
In home designs of spot welding machines, the current value is usually not regulated. Basically, the orientation is on the duration of heating, and control is carried out by changing the color of the parts. If voltage regulation is required, a laboratory autotransformer connected at the input can be used. In order for its winding to not burn out at high current when it is closed by a wheel, devices with stepwise voltage regulation are used.
Spot welding application
The home master needs contact welding for minor repairs, when it is necessary to connect small sheets of metal. It is used to replace batteries for power tools, laptops, and similar equipment to reduce battery replacement costs. The high speed of the process does not allow parts to overheat.
Conclusion
The choice of spot welding machines on the market is large, but their cost remains high. In addition, it is difficult to choose the right parameters. The device can be made independently, and you can find everything you need at home or in your workshop. Self-assembled spot welding from a microwave, works reliably and provides the necessary minor repair of parts, if all parameters are correctly selected. It is important to understand for what purposes it is made.