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Spot micro welding

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Spot micro welding

A portable small-sized electric welding machine with an external welding gun is designed for welding stainless sheet and ordinary steel with a thickness of 0.08 ... 0.15 mm to massive steel parts, as well as for welding steel wire up to 0.3 mm in diameter. It can find application in many sectors of the national economy, for example, in the manufacture of thermocouples, for welding to metal structures of strain gauges, pre-glued on steel foil, and in many other cases.

The appearance of the welding machine is shown on the 3rd p. tabs (above). The mass of the power unit of the device is about 8 kg, dimensions are 225x135x120 mm.
As can be seen from the circuit diagram (Fig. 1), the apparatus consists of two main components: an electronic relay on a V9 trinistor and a powerful welding transformer T2.


A welding electrode is connected to one of the conclusions of its low-voltage secondary winding, the second output is reliably connected to the more massive of the two parts to be welded.
The mains winding of the welding transformer is connected to the mains via a V5-V8 diode bridge, the diagonal of which is connected to an electronic relay trinistor V9. The low-power auxiliary transformer T1 feeds the trinistor control circuit (winding ///) and the welding spot lamp HI (winding //).

The device works as follows.
When closing the contacts of the switch S1 "On" the supply voltage of 220 V is supplied to the primary winding of the transformer T1 of the trinistor control unit. The capacitor C1, connected through the closed contacts of the switch S3 "Impulse" to the rectifier bridge V1-V4, is being charged. The primary winding of the welding transformer T2 is de-energized, since the V9 trinistor is closed. When the switch button S3 is pressed, the charged capacitor C1 is connected to the control electrode of the V9 trinistor via a variable resistor R1.
The discharge current of the capacitor opens the trinistor, and the mains voltage is supplied to the primary winding of the welding transformer T2. If the secondary winding of the welding transformer is connected to the parts to be welded, then a powerful current pulse arises in it, which causes a strong heating of the metal at the touch point of the welding electrode.
The duration of the current pulse depends on the parameters of the timing chain R1C1. When the nominal elements of this goal are shown in the diagram, the maximum pulse duration t and (without taking into account the internal resistance of the trinistor) is approximately 0.1 s. During this time, the current in the secondary winding can reach 300 ... 350 A. This is quite enough for durable welding to massive structures of parts made of foil with a thickness of up to 0.15 mm, for example, from alloyed 1X18H10T steel.
The device returns to its original state automatically at the end of the discharge of the capacitor C1. The optimal welding mode set trimmer R1 "Mode".
Structurally, the welding machine consists of two parts: the power unit and the welding gun, which are interconnected by a flexible cable using a multi-pin connector. On the chassis of the power unit placed almost all the elements of the device. The design of the chassis and its main dimensions are shown on the clutch.

Fig.2. Power unit casing
Fig.3. Power block design
On the basis of the chassis 3, a welding transformer 4 and strips with diodes V1-V8 are placed. A bracket 8 is attached to the front panel of the chassis with an auxiliary transformer 5, a capacitor 6 and a trinistor 7 mounted on it. On the front panel one of the connector parts (in a rectangular hole) of the connecting cable, a variable mode setting resistor, a network toggle switch, a pin part of the power cord connector are mounted and a clip for connection - more massive of the welded parts.

Casing 1 is made of 2.5 mm duralumin and equipped with a handle 2 for carrying. The device of the welding gun is shown in the figure.

Fig.4. Welding gun device
The body 7 of the gun is made in the form of two parts of the same shape milled from sheet PCB 12 mm thick. The case contains the holder 3 of the welding electrode 2. The illumination lamp 8 with the pushbutton switch 4 "Illumination", the microswitch 6 "Impulse". The connecting cable 5 is a flexible twenty-four-wire cable in rubber insulation with an outer diameter of 11 mm and a cross-section of each wire of 0.75 mm square.
Five wires of the cable are used to connect the microswitch and the backlight, and the remaining nineteen are soldered directly to the holder 3 electrodes. The holder is made of copper bar of rectangular or square section. The electrode 2 is a copper bar with a diameter of 8 mm. The electrode must be securely fixed in the holder. Along with this, it should be possible to change the electrode. To weld the foil, the electrode sting is sharpened with a cone moving into a sphere with a diameter of 1 ... 1.5 mm. For welding the wire used electrode with a flat highlander working.

Installation of the gun start with cable cutting. Nineteen cable conductors are carefully cleaned, twisted together, serviced and sealed into the hole of the holder 3 of the electrode. The remaining five wires are cut to the required length and soldered to the microswitch 6 and the backlight lamp 8. The second end of the cable is inserted into the insert of the type A connector for 20 contacts (cable construction, see photo on the tab). In the gun, the MPZ-1T microswitch, the SM-34 illumination lamp at 6 V, 0.25 A with fittings equipped with a small lens, the illumination lamp-on button - from a desk lamp are used.
On the front panel of the chassis of the power unit install the counterpart of the connector cable. Five corresponding pins of the connector are connected to one or another circuit of the device, and the rest are connected in parallel and connected to one of the terminals of the secondary winding of the welding transformer.

Fig.5. Appearance of the device

The magnetic core of this transformer is assembled from Sh40 plates, the set thickness is 70 mm. The primary winding contains 300 turns of the wire sew-2 0.8. The secondary winding of this transformer consists of 10 turns of insulated wire or bus with a cross section of at least 20 sq. Mm (in the described construction, this winding is made of two multicore conductors with a diameter of 4 mm, wound at the same time). The same cross-section is made of the "grounding" connecting conductor of the secondary winding. Its length should not be chosen greater than 2 ... 2.5 m. The transformer T1 can be any, providing on the secondary windings a voltage of 8 ... 10 V (for charging capacitor C1) and 3 ... 6 V (for powering the lamp) .
In this design, the magnetic circuit from the transformer of the children's railway was used (section 10x10, L-shaped plates). On it is placed the network winding / containing 8000 turns of wire sew-2 0.08, winding // - 330 turns of wire sew-2 0.3 and winding /// - 350 turns of wire sew-2 0.2. The clamp connected to the lower (according to the scheme) output of the secondary winding of the transformer T2 is mounted on the chassis without insulating gaskets.
In the manufacture of transformers, it must be borne in mind that the safety of working with the device depends on the quality of the insulation of their windings. Therefore, at least 4-6 layers of varnished cloth or paraffin-impregnated paper should be applied on the primary (mains) windings of transformers.

The welding resistor PPZ-11, the capacitor K50-3, the network toggle switch TP1-2 are used in the welding machine. It should be noted that the use of the PTL-50 trinistor is due solely to the desire to ensure high reliability of the device and reliable operation in harsh climatic conditions and with large fluctuations in the mains voltage. With some deterioration in the quality of welding in the apparatus, KU202 series trinistors with indices K, L, M, or H can be used. It is necessary to reduce the resistance of the resistor R1 to 50 Ohms, and the capacitance capacitor C1 to double. Properly assembled unit begins to work immediately, without any adjustment.
The quality of the weld (point) is checked as follows. A strip of steel foil with a width of 10 ... 12 mm is welded to the surface of the steel bar, cleared of scale, with three to five points, and then torn off with the help of pliers.
Holes with a diameter of 0.5 ... 0.8 mm should remain at the points of welding on the foil, which indicates that the gap does not occur at the place of welding, but around it. If the foil comes off at the place of welding, select the welding current by the "Mode" trimming resistor. When selecting the current, it is necessary to take into account that the quality of the weld deteriorates with increasing pressure on the electrode. It should also be noted that, according to reference data, the constant voltage that must be supplied to the control electrode of the PTL-50 trinistor to open it is 8 V. However, the quality of the seam is significantly improved if this voltage is increased to 12 ... 15 V (charged capacitor voltage C1).

The procedure for working with the device.

First of all, the welding machine casing and the structure to which the part is to be welded are “grounded”. Working with a welding machine must wear protective rubber gloves and stand on a rubber mat. The apparatus is turned on, the part being welded is applied to the structure and tightly pressed with the tip of the welding electrode of the gun in the place where it is necessary to obtain a weld point. Click on the "trigger" of the pistol (on the microswitch button), after 1 ... 1.5 s they remove the pistol from the part and set the sting to the next point. In those cases, when necessary, turn on the backlight lamp.

When operating the device in production, it must be adopted by the local safety committee. In conclusion, it should be noted that the capabilities of the device can be significantly expanded. If you use, for example, copper-clad graphite electrode with a diameter of 6 ... 8 mm, you can weld copper tinned conductors with a diameter of up to 0.3 mm.
Very well, these conductors are welded to any tinned and silver-plated parts, as well as to copper tin-free foil. It is possible, for example, to weld thin conductors to a foil of a printed circuit board without the use of flux. Good results are obtained when welding sheets of very thin copper foil. In this case, you need to experimentally choose the length and shape of the tip of the graphite electrode.

If it is necessary to weld parts from thicker sheet metals, the welding transformer will have to be replaced with more powerful ones. For example, for connecting steel sheets with a thickness of 0.5 ... 0.7 mm, a transformer with a section of a magnetic core of at least 65 ... 70 square cm is required.
The primary winding of such a transformer should contain 160-165 turns of a PETV wire with a diameter of 1.62 ... 1.7 mm, and the secondary winding - 4.5 turns of a copper bus with a cross section of at least 90 sq. Mm (based on a welding current of 1400 ... 1800 A). The diameter of the electrode should be increased to 18 ... 20 mm. At the same time in the primary winding of the transformer at the time of the welding pulse current flows around 45 A. Therefore, the V5-V8 diodes need to be replaced with more powerful ones, for example VL-50.
Trinistor V9 should also be designed for a direct current of at least 50 A. Experience, however, shows that for welding steel sheets with a thickness of 0.5 ... 0.7 mm, the use of a PTL-50 trinistor without an additional radiator is acceptable, since the welding the momentum is very short.
In order to ensure the nominal mode during welding of metals of different thickness (from 0.08 to 0.7 mm), it is necessary to provide for a wider control of the welding current in the apparatus. It is most expedient to use instead of capacitor C1 a set of three capacitors with a capacity of 1000 microfarads each, switched by switch either sequentially (for thin-sheet metals) or in parallel.