Spot welding, thanks to the emergence of compact hand-held devices such as BlueWeldPlus, is becoming popular not only in industrial scale applications, but also in everyday life. The weak point of this technology is electrodes for contact welding: their low resistance in many cases scares off the consumer.

Reasons for the fragility of resistance welding electrodes

The contact welding process consists of the following stages:

1. Preliminary preparation of the surface of the parts to be joined - it should not be easily cleaned of dirt and oxides, but also very smooth in order to eliminate the unevenness of the resulting electric field voltage.
2. Manual or mechanical clamping of the workpieces to be welded - with an increase in the clamping force, the intensity of diffusion and mechanical strength increase weld.
3. Local melting of metals in the pressing zone by heat electric current, resulting in the formation of a welded joint. The clamping of the electrodes at this stage prevents the formation of welding spatter.
4. Switching off the current and gradual cooling of the weld.

Thus, the material of electrodes for contact welding undergoes not only significant thermal stresses, but also mechanical loads. Therefore, a number of requirements are imposed on it - high electrical conductivity, high thermal resistance (including from constant temperature fluctuations), increased values ​​of compressive strength, low heat capacity coefficient. A limited number of metals have such a complex of properties. First of all, it is copper, and alloys based on it, however, they do not always satisfy production requirements.

In connection with constant increase energy characteristics produced by many trade marks orient the consumer to the use of only "their", branded electrodes, which is not always observed. As a result, the quality of welds obtained using this technology decreases, and confidence in the process of contact electric welding itself is undermined.

These problems are overcome in two ways: by improving the types and designs of welding electrodes for spot welding, and the development of new materials used for the manufacture of such electrodes. For private users, the price of the issue also matters.

Electrode materials

According to GOST 2601, the criterion for the quality of the finished seam is its tensile or shear strength. It depends on the intensity of the thermal power in the electric discharge zone, and therefore is associated primarily with the thermophysical characteristics of the electrode material.

The use of copper electrodes is ineffective for two reasons. Firstly, copper, being a highly ductile metal, does not have sufficient elasticity to completely restore the geometric shape of the electrodes between work cycles. Secondly, copper is very scarce, and frequent replacement of electrodes causes high financial costs.

Attempts to use harder, hardened copper are not successful: for work-hardened material, in parallel with an increase in hardness, the recrystallization temperature decreases, therefore, with each work cycle, the wear of the working end of the resistance welding electrode will increase. That's why practical use received copper alloys with the addition of a number of other metals. In particular, the introduction of cadmium, beryllium, magnesium, zinc and aluminum into a copper alloy does not significantly change the thermal conductivity, but improves the hardness when heated. The stability of the electrode against dynamic thermal loads is increased by iron, nickel, chromium and silicon.

When selecting the optimal material for welding electrodes for contact welding, they are guided by the indicator of the specific electrical conductivity of the alloy. The less it differs (downward) from the electrical conductivity of pure copper - 0.0172 Ohm mm 2 / m, the better.

The most effective resistance to wear and deformation is shown by alloys, which include cadmium (0.9 ... 1.2%), magnesium (0.1 ... 0.9%) and boron (0.02 ... 0.03%).

The choice of material for spot welding electrodes also depends on the specific tasks of the process. Three groups can be distinguished:

1. Electrodes designed for resistance welding in harsh conditions (continuous alternation of cycles, surface temperatures up to 450 ... 500ºС). They are made from bronzes containing chromium and zirconium (Br.Kh, Br.KhTsr 0.6-0.05. Nickel-silicon bronzes (Br.KN1-4), as well as bronzes additionally alloyed with titanium and beryllium (Br.NTB), used for spot welding of stainless and heat-resistant steels and alloys.
2. Electrodes used at contact temperatures on the surface up to 250 ... 300ºС (welding of ordinary carbon and low alloy steels, copper and aluminum products). They are made from copper alloys of grades MS and MK.
3. Electrodes for relatively light operating conditions (surface temperatures up to 120 ... 200ºС). Cadmium bronze Br.Kd1, chromium bronze Br.Kh08, silicon nickel bronze Br.NK, etc. are used as materials. Such electrodes can also be used for roller contact electric welding.

It should be noted that, in descending order of electrical conductivity (with respect to pure copper), these materials are arranged in the following sequence: .NK →Br.Kd1→Br.KN1-4. In particular, heating up to the required temperature of an electrode made of bronze Br.KhTsr 0.6-0.05 will occur approximately twice as fast as that obtained from bronze Br.KN1-4.

Electrode designs

The least resistant place of the electrode is its spherical working part. The electrode is rejected if the increase in the dimensions of the butt exceeds 20% of the primary dimensions. The design of the electrodes is determined by the configuration of the surface to be welded. There are the following versions of the tool

1. With a cylindrical working part and a conical landing part.
2. With a conical landing and working part, and a transitional cylindrical section.
3. With a spherical working end.
4. With bevelled working end.

In addition, the electrodes can be solid and composite.

When self-manufacturing (or regrinding), it is recommended to maintain the following ratios of sizes, in which the tool will have maximum durability:

• To calculate the electrode diameter d, the dependence Р = (3…4)d 2 is used, where Р is the actual required compression of the electrodes during the resistance electric welding process. In turn, the recommended values ​​of the upsetting pressure, at which the highest quality joints are obtained, is 2.5 ... 4.0 kg / mm 2 of the area of ​​the resulting weld;
• For electrodes with a conical working part, the optimal taper angle varies from 1:10 (for a tool with a working part diameter up to 30 ... 32 mm) to 1:5 - otherwise;
• The choice of the cone angle is also determined by the greatest compression force: at maximum effort, it is recommended to take a taper of 1:10, as providing an increased longitudinal resistance of the electrode.

The main forms of electrodes for resistance welding are established by GOST 14111, therefore, using certain size ratios, one should take into account the dimensions of the seating space for the tool for a specific model of the resistance welding machine.

Significant savings in material can be achieved through the use of composite structures. At the same time, materials with high electrical conductivity values ​​are used for the manufacture of the housing, and a removable working part are made of alloys with high hardness and wear resistance (including thermal). In particular, ceramic-metal alloys from the Swiss company AMRCO of grades A1W or A1WC, containing 56% tungsten and 44% copper, have a similar combination of properties. Their electrical conductivity reaches 60% of the electrical conductivity of pure copper, which determines the low heating losses during welding. Bronze alloys with additions of chromium and zirconium, as well as tungsten, can also be recommended materials.

Electrodes for contact welding of light alloys, where significant clamping force is not required, are performed with a spherical working part, and it is advisable to use silicon bronzes for contact jaws of electric spot welding machines.

The mechanical characteristics of the electrodes must be within the following limits:

• Brinell hardness, HB - 1400 ... 2600;
• Young's modulus, GPa – 80…140;
• Ultimate bending moment, kgcm - not less than 750 ... 800.

Electrode designs should always be hollow to ensure efficient cooling.

We decided to separate the story about electrode holders and electrodes for spot welding in a separate article due to the large amount of material on this topic.

Electrode holders for spot welding machines

Electrode holders are used to install electrodes, regulate the distance between them, supply welding current to the electrodes and remove heat generated during welding. The shape and design of the electrode holders is determined by the shape of the welded assembly. As a rule, the electrode holder is a copper or brass tube with a conical hole for installing the electrode. This hole can be made along the axis of the electrode holder, perpendicular to the axis or at an angle. Often the same machine can be equipped with several options for electrode holders for each type of electrode, depending on the shape of the parts to be welded. In some low-power machines, electrode holders may not be included at all, since their functions are performed by welding trunks.
In standard machines, straight electrode holders are most often used (Fig. 1), as the simplest ones. Electrodes of various shapes can be installed in them. In the case of welding large parts with limited access to the welding site, it is advisable to use figured electrode holders with simple straight electrodes. They are fastened in electrode holders due to a conical fit, pins or screws. Removal of the electrode from the holder is carried out by light tapping with a wooden hammer or a special extractor.

Spot welding electrodes

Spot welding electrodes are used to compress parts, supply welding current to parts and remove heat generated during welding. This is one of the most critical elements of the welding circuit of a spot welding machine, because the shape of the electrode determines the possibility of welding a particular node, and its durability determines the quality of welding and the duration of the machine's trouble-free operation. There are straight (Fig. 4) and curly electrodes (Fig. 5). Some examples of the use of straight electrodes are shown in Table 1. Many straight electrodes are manufactured in accordance with GOST 14111-77 or OST 16.0.801.407-87.

Curly electrodes have an axis passing through the center working surface, is significantly displaced relative to the axis of the landing surface (cone). They are used for welding parts of complex shape and assemblies in hard-to-reach places.

Design of electrodes for spot welding

The electrode for spot welding (Fig. 6) structurally consists of a working part (1), a middle (cylindrical) part (2) and a landing part (3). Inside the body of the electrode, there is an internal channel, into which the tube for supplying the cooling water of the electrode holder is inserted.
The working part (1) of the electrode has a flat or spherical surface; the diameter of the working surface d el or the radius of the sphere R el is chosen depending on the material and thickness of the parts to be welded. The cone angle of the working part is usually 30°.
The middle part (2) ensures the strength of the electrode and the possibility of using extractors or other tools for dismantling the electrodes. Manufacturers use different methods to calculate electrode sizes. In the USSR, according to OST 16.0.801.407-87, standard series were established:

D el = 12, 16, 20, 35, 32, 40 mm

L = 35, 45, 55, 70, 90, 110mm

Depending on the maximum compression force of the machine:

D el \u003d (0.4 - 0.6) √ F el (mm).

Where: F el - the maximum compression force of the machine (daN).

The landing part (3) must have a taper for a tight fit in the electrode holder and to prevent leakage of cooling water. For electrodes with a diameter of 12-25 mm, the taper is 1:10, for electrodes with a diameter of 32-40 mm, the taper is 1:5. The length of the conical part is not less than 1.25D el. The landing part is processed with a purity of at least the 7th class (R z 1.25).

The diameter of the internal cooling channel is determined by the flow rate of cooling water and sufficient compressive strength of the electrode and is:

d 0 \u003d (0.4 - 0.6) D el (mm).

The distance from the working surface of the electrode to the bottom of the inner channel largely affects performance characteristics electrode: durability, service life. The smaller this distance, the better the cooling of the electrode, but the less regrinding the electrode can withstand. According to experimental data:

h = (0.75 - 0.80) D el (mm).

Refractory inserts made of tungsten W or molybdenum Mo (Fig. 4g) are pressed into copper electrodes or soldered with silver-containing solders; such electrodes are used when welding galvanized or anodized steels. Electrodes with a replaceable working part (Fig. 4i) and with a ball joint (Fig. 4k) are used when welding parts from different materials or parts of different thicknesses. The replaceable working part is made of tungsten, molybdenum or their alloys with copper and is attached to the electrode with a union nut. Steel or brass electrodes with a pressed copper sheath (Fig. 4h) or copper electrodes with a steel spring-loaded sleeve are also used.

Materials for spot welding electrodes

The resistance of electrodes is their ability to maintain the dimensions and shape of the working surface (end), to resist the mutual transfer of the metal of the electrodes and the parts to be welded (contamination of the working surface of the electrode). It depends on the design and material of the electrode, the diameter of its cylindrical part, the angle of the cone, the properties and thickness of the material being welded, the welding mode, and the electrode cooling conditions. The wear of the electrodes depends on the design of the electrodes (material, diameter of the cylindrical part, the angle of the cone of the working surface) and the parameters of the welding mode. Overheating, melting, oxidation during operation in a humid or corrosive environment, deformation of the electrodes with high compression forces, distortion or displacement of the electrodes increase their wear.

The electrode material is selected taking into account the following requirements:

• electrical conductivity comparable to that of pure copper;
• good thermal conductivity;
• mechanical strength;
• machinability by pressure and cutting;
• resistance to softening under cyclic heating.

Compared with pure copper, alloys based on it have 3-5 times greater resistance to mechanical stress, therefore, copper alloys are used for spot welding electrodes with their seemingly mutually exclusive requirements. Alloying with cadmium Cd, chromium Cr, beryllium Be, aluminum Al, zinc Zn, zirconium Zr, magnesium Mg does not reduce electrical conductivity, but increases strength in the heated state, while iron Fe, nickel Ni and silicon Si increase hardness and mechanical strength. Examples of the use of some copper alloys for spot welding electrodes are shown in table 2.

Selection of electrodes for spot welding

When choosing electrodes, the main parameters are the shape and dimensions of the working surface of the electrode. In this case, it is necessary to take into account the grade of the material to be welded, the combination of thicknesses of the sheets to be welded, the shape of the welded assembly, the requirements for the surface after welding, and the design parameters of the welding mode.

Distinguish the following types forms of the working surface of the electrode:

• with flat ones (characterized by the diameter of the working surface d el);
• with spherical (characterized by radius R el) surfaces.

Electrodes with a spherical surface are less sensitive to distortions, therefore they are recommended for use on machines of the radial type and hanging machines (pliers) and for shaped electrodes operating with a large deflection. Russian manufacturers recommend using only electrodes with a spherical surface for welding light alloys, which avoids dents and undercuts along the edges of the weld point (see Fig. 7). But you can avoid dents and undercuts by using flat electrodes with an enlarged end. The same hinged electrodes avoid distortion and can therefore replace spherical electrodes (Fig. 8). However, these electrodes are mainly recommended for welding sheets with a thickness of ≤1.2 mm.

According to GOST 15878-79, the dimensions of the working surface of the electrode are selected depending on the thickness and grade of the materials being welded (see Table 3). After examining the cross section of the weld spot, it becomes clear that there is a direct relationship between the diameter of the electrode and the diameter of the core of the weld spot. The electrode diameter determines the area of ​​the contact surface, which corresponds to the fictitious diameter of the resistance conductor r between the sheets to be welded. The contact resistance R will be inversely proportional to this diameter and inversely proportional to the preliminary compression of the electrodes to smooth out surface microroughness. Research by ARO (France) showed that the calculation of the diameter of the working surface of the electrode can be carried out according to the empirical formula:

d el = 2t + 3 mm.

Where t is the nominal thickness of the sheets to be welded.

It is most difficult to calculate the electrode diameter when the thickness of the sheets being welded is unequal, when a package of three or more parts is welded, and when dissimilar materials are welded. Obviously, when welding parts of different thicknesses, the electrode diameter must be selected relative to the thinner sheet. Using the formula for calculating the electrode diameter, which is proportional to the thickness of the sheet being welded, we form a fictitious conductor with a tapering diameter, which, in turn, moves the heating spot to the point of contact of these two sheets (Fig. 10).

When welding a package of parts at the same time, the choice of the diameter of the working surface of the electrode is made according to the thickness of the outer parts. When welding dissimilar materials with different thermophysical characteristics, less penetration is observed in a metal with a lower electrical resistivity. In this case, an electrode with a large diameter of the working surface d el or made of a material with higher thermal conductivity (for example, BrX chromium bronze) is used on the side of the metal part with lower resistance.

Valery Raisky
Magazine "Equipment: market, offer, prices", No. 05, May 2005

Literature:

1. Knorozov B.V., Usova L.F., Tretyakov A.V. Technology of metals and materials science. - M., Metallurgy, 1987.
2. Handbook of the machine builder. T. 5, book. 1. Ed. Satel E.A. - M., Mashgiz, 1963.

This table clearly shows the importance of electrode maintenance. This is important not only to maintain the quality of the welded joint, which is of paramount importance, but also to reduce unnecessary stress on the welding equipment. After examining the tabular data, you can draw your own conclusions.

TIP PROFILE

WELD SPOT

REQUIRED CURRENT, A

RESULT

CORRECT MAINTENANCE OF ELECTRODES FOR RESISTANCE SPOT AND RELIEF WELDING

Projection welding electrodes

To ensure the precise alignment required for good contact and quality of welded joints, the electrodes for projection welding should be located directly on the center line of application of pressure. In addition to the appearance of poor-quality welded joints, insufficient alignment of the electrodes can lead to damage to their surfaces [Fig. one].

Another serious cause of poor welding is the non-parallelism of the electrode surfaces. It entails uneven pressure on the electrodes, which causes molten metal to slosh out of the weld area during the welding cycle. In the event that welding has gone through the bearing part of the electrode, the reliefs are damaged, and the insulation may burn out. In addition, non-parallelism leads to biting of the electrode tips by their bearing parts during welding, resulting in a burn on the workpiece at the point of contact with displaced reliefs, and a shift relative to the mating parts of the welding equipment is possible [Fig. 2].

SHOULD
... keep a supply of electrodes on the machine to minimize downtime due to electrode replacement,
... grind the electrodes on lathe,
...use special Grade 3 copper for the electrode tips.
DO NOT DO IT
... file the electrodes (an uneven surface will result in either partial welding or metal splashing out of the welding zone),

Spot welding electrodes

In contact spot welding, the thermal concentration depends on the size and shape of the electrode tips. Welding is carried out over the entire area under the tip of the electrode through which the current passes. Tips of small diameter spot welding electrodes break down or wear down much faster than their relief welding counterparts and therefore need to be sharpened regularly to maintain proper contact [fig. 3].

SHOULD
... keep a supply of electrodes on the machine,
... periodically grind the electrodes on specialized machine,
... to change the diameter of the tips when working with different thicknesses of the welded metal.
DO NOT DO IT
... file the electrodes (an uneven surface will lead to lack of penetration),
... store electrodes in places where damage to their surfaces is possible,
... use an adjustable wrench to remove the electrodes.

1. To ensure perfect alignment, the surfaces and axes of the electrodes must be parallel. This can be tested by inserting a piece of charcoal and a sheet of clean white paper between the electrodes and running the electrodes in test mode. The resulting print on paper will show the size and uniformity of the plane of contact between the two surfaces.

2. Use a water jacket if necessary and keep it as close to the welding surface as possible.

3. Keep the material to be welded clean: free of oil, film, dirt and other foreign matter.

4. Follow the prescribed welding procedure.

WELDING ELECTRODES AND HOLDERS

The design of the electrodes must have a shape and dimensions that provide access to the working part of the electrode to the place of welding of parts, be adapted for convenient and reliable installation on the machine and have a high resistance of the working surface.

The simplest for manufacturing and operation are straight electrodes, made in accordance with GOST 14111-69 from various copper electrode alloys, depending on the metal grade of the parts being welded.

Sometimes, for example, when welding dissimilar metals or parts with a large difference in thickness, in order to obtain high-quality joints, the electrodes must have a sufficiently low electrical and thermal conductivity (30 ... 40% of copper). If the entire electrode is made of such a metal, then it will intensively heat up from the welding current due to its high electrical resistance. In such cases, the base of the electrode is made of a copper alloy, and the working part is made of metal with the properties necessary for the normal formation of joints. The working part 3 can be replaceable (Fig. 1, a) and fixed with a nut 2 on the base 1. The use of electrodes of this design is convenient, as it allows you to set the desired working part when changing the thickness and grade of the metal of the parts to be welded. The disadvantages of an electrode with a replaceable part are the possibility of using it only when welding parts with good approaches and insufficient cooling. Therefore, such electrodes should not be used in heavy welding at a high pace.

Rice. one . Electrodes with a working part made of another metal

The working part of the electrodes is also made in the form of a soldered (Fig. 1, b) or pressed tip (Fig. 1, c). Tips are made of tungsten, molybdenum or their compositions with copper. When pressing a tungsten tip, it is necessary to grind its cylindrical surface in order to ensure reliable contact with the electrode base. When welding stainless steel parts with a thickness of 0.8…1.5 mm, the diameter of the tungsten insert 3 (Fig. 1, c) is 4…7 mm, the depth of the pressed part is 10…12 mm, and the protruding part is 1.5…2 mm. With a longer protruding part, overheating and a decrease in the resistance of the electrode are observed. The working surface of the insert can be flat or spherical.

Particular attention in the design of electrodes should be given to the shape and dimensions of the landing part. The most common conical landing part, the length of which should be at least. Short cone electrodes should only be used when welding using low forces and currents. In addition to the conical fit, sometimes the electrodes are fastened to the thread with a union nut. Such connection of electrodes can be recommended c. many point machines ah, when it is important to have the same initial distance between the electrodes, or in clamps. When using figured electrode holders, electrodes with a cylindrical landing part are also used (see Fig. 8, d).

When spot welding parts of a complex contour and poor approaches to the junction, a wide variety of figured electrodes are used, which have a more complex design than straight ones, are less convenient to use and, as a rule, have reduced durability. Therefore, it is advisable to use figured electrodes when welding is generally not feasible without them. The dimensions and shape of the curly electrodes depend on the dimensions and configuration of the parts, as well as the design of the electrode holders and consoles of the welding machine (Fig. 2).

Rice. 2. different types curly electrodes

Curly electrodes during operation usually experience a significant bending moment from the off-axis application of force, which must be taken into account when choosing or designing electrodes. The bending moment and the usually small section of the cantilever create significant elastic deformations. In this regard, the mutual displacement of the working surfaces of the electrodes is inevitable, especially if one electrode is straight and the other is shaped. Therefore, for figured electrodes, the spherical shape of the working surface is preferable. In the case of shaped electrodes experiencing large bending moments, deformation of the conical landing part and the electrode holder socket is possible. The maximum permissible bending moments for figured electrodes made of Br.NBT bronze and electrode holders made of heat-treated bronze Br.Kh are, according to experimental data, for electrode cones with a diameter of 16, 20, 25 mm, respectively, 750, 1500 and 3200 kg× see. If the conical part of the figured electrode experiences a moment greater than the allowable one, then the maximum diameter of the cone should be increased.

When designing complex spatial figured electrodes, it is recommended to pre-make their model from plasticine, wood, or easily processed metal. This makes it possible to establish the most rational dimensions and shape of the shaped electrode and avoid alterations in its manufacture immediately from metal.

On fig. 3 shows some examples of welding knots in places with limited access. Welding of a profile with a shell is performed with a lower electrode with a displaced working surface (Fig. 3, a).

Rice. 3. Examples of the use of curly electrodes

An example of using the upper electrode with oblique sharpening and the lower one, curly, is shown in fig. 3b. The angle of deviation of the electrode holder from the vertical axis should not be more than 30°, otherwise the conical hole of the electrode holder is deformed. If it is impossible to install the upper electrode with an inclination, then it can also be curly. The shaped electrode is bent in two planes to reach a hard-to-reach welding spot (Fig. 3, cd). If there is no or limited horizontal movement of the consoles for welding parts shown in fig. 3e, two shaped electrodes with the same projections are used.

Sometimes shaped electrodes perceive very large bending moments. To avoid deformation of the conical seating part, the shaped electrode is additionally fixed to the outer surface of the electrode holder with a clamp and a screw (Fig. 4, a). The strength of figured electrodes with a long overhang increases significantly if they are made composite (reinforced). For this, the main part of the electrode is made of steel, and the current-carrying part is made of a copper alloy (Fig. 4, b). The connection of current-carrying parts to each other can be done by soldering, and with a steel console - by screws. A variant of the design is possible, when the curly copper alloy electrode is reinforced (reinforced) with steel elements (slats), which should not form a closed ring around the electrode, since currents will be induced in it, increasing the heating of the electrode. It is advisable to fasten figured electrodes experiencing large moments in the form of an elongated cylindrical part, for installation in a machine instead of an electrode holder (see Fig. 4, b).

Rice. 4. Electrodes that perceive a large bending moment:

a - with reinforcement for the outer surface of the electrode holder;

b - reinforced electrode: 1 - steel console; 2 - electrode; 3 - current lead

In most cases, spot welding uses internal cooling of the electrodes. However, if welding is performed with electrodes of small cross section or with high heat, and the material being welded is not subject to corrosion, external cooling is used in the tongs. The supply of cooling water is carried out either by special tubes or through holes in the working part of the electrode itself. Great difficulties arise when cooling curly electrodes, since it is not always possible to bring water directly to the working part due to the small cross section of the cantilever part of the electrode. Sometimes cooling is performed using thin copper tubes soldered to the side surfaces of the cantilever part of the figured electrode of a sufficiently large size. Given that shaped electrodes are always cooled worse than straight electrodes, it is often necessary to significantly reduce the welding rate, preventing overheating of the working part of the shaped electrode and reducing durability.

When using clamps for welding in hard-to-reach places, as well as the need for frequent replacement of electrodes, the electrode fastening shown in fig. 5. Such mounting provides good electrical contact, convenient adjustment of the electrode extension, good stability against lateral displacements, quick and easy removal of the electrodes. However, due to the lack of internal cooling in such electrodes, they are used for welding at low currents (up to 5 ... 6 kA) and at a low rate.

Rice. 5. Ways of fixing electrodes

For ease of use, electrodes with several working parts are used. These electrodes can be adjustable or swivel (Fig. 6) and greatly simplify and speed up the installation of electrodes (combination of working surfaces).

Rice. 6. Multiposition adjustable (a) and surface (b) electrodes:

1 - electrode holder; 2 - electrode

The electrodes are installed in electrode holders, which are fixed on the cantilever parts of the welding machine, which transmit the compression force and current. In table. for reference, the dimensions of the direct electrode holders of the main types of spot welding machines are given. The electrode holders should be made of sufficiently strong copper alloys with relatively high electrical conductivity. Most often, electrode holders are made of bronze Br.X, which must be heat-treated to obtain the required hardness (HB not less than 110). In the case of welding steels, when small currents (5 ... 10 kA) are used, it is advisable to make electrode holders from Br.NBT bronze or silicon-nickel bronze. These metals provide long-term preservation of the dimensions of the conical mounting hole of the electrode holder.

Table. Dimensions of electrode holders for spot machines in mm

The most common are direct electrode holders (Fig. 7). Inside the cavity of the electrode holder there is a tube for supplying water, the cross section of which should be sufficient for intensive cooling of the electrode. With a tube wall thickness of 0.5 ... 0.8 mm, its outer diameter should be 0.7 ... 0.75 of the diameter of the electrode hole. In the case of frequent change of electrodes, it is advisable to use electrode holders with ejectors (Fig. 7, b). The electrode is pushed out of the seat by hitting the striker 5 with a wooden hammer, which is connected to a stainless steel tube - ejector 1. The ejector and striker are returned to their original lower position by spring 2. It is important that the end of the ejector that strikes the end of the electrode does not have damage on its surface, otherwise the landing part of the electrode will quickly fail, jamming when it is removed from the electrode holder. Convenient for operation is the execution of the end of the electrode holder 1 in the form of a replaceable threaded sleeve 2, in which the electrode 3 is installed (Fig. 7, c). This design makes it possible to manufacture bushing 2 from a more resistant metal and replace it when worn and install an electrode of a different diameter, and also to easily remove the electrode in case of jamming by knocking it out with a steel punch from inside the bushing.

Rice. 7. Straight electrode holders:

a - normal;

b - with ejector;

c - with a replaceable sleeve

If shaped electrodes are more often used when welding parts that have small dimensions of the elements to be connected, then for large sizes it is advisable to use special shaped electrode holders and simple electrodes. Figured electrode holders can be composite and provide installation of electrodes at different angles to the vertical axis (Fig. 8, but). The advantage of such an electrode holder is the easy adjustment of the electrode extension. In some cases, the figured electrode can be replaced by the electrode holder shown in Fig. 8, b. Of interest is also the electrode holder, the slope of which can be easily adjusted (Fig. 8, c). The design of the electrode holder bent at an angle of 90° is shown in fig. 30, d, it allows you to fix the electrodes with a cylindrical landing part. A special screw clamp ensures quick attachment and removal of the electrodes. On fig. 9 shows various examples of spot welding using figured electrode holders.

Rice. 8. Special electrode holders

Rice. 9. Application examples of various electrode holders

When spot welding large assemblies such as panels, it is advisable to use a four-electrode rotary head (Fig. 10). The use of such heads makes it possible to quadruple the operating time of the electrodes until the next cleaning, without removing the welded panel from the working space of the machine. To do this, after contamination of each pair of electrodes, the electrode holder 1 is rotated by 90° and fixed with a stopper 4. The swivel head also makes it possible to install electrodes with a different shape of the working surface for welding an assembly with a variable, for example, stepwise thickness of parts, and also to provide mechanization of electrode cleaning with special devices. The swivel head can be used for spot welding of parts with a large difference in thickness and is installed on the side of the thin part. It is known that in this case the working surface of the electrode in contact with a thin part wears out quickly and is replaced by a new one when the head is turned. It is convenient to use a roller as an electrode on the side of a thick part.

Rice. 10. Rotary electrode head:

1 - rotary electrode holder; 2 - body; 3 - electrode; 4 - stopper

In spot welding, the axes of the electrodes must be perpendicular to the surfaces of the parts to be welded. To do this, welding of parts with slopes (smoothly changing thickness), or manufactured using hanging machines, in the presence of large-sized units, is performed using a self-aligning rotary electrode with a spherical support (Fig. 11, a). The electrode is sealed with a rubber ring to prevent water leakage.

Rice. 11. Self-aligning electrodes and heads:

a - rotary electrode with a flat working surface;

b - head for two-point welding: 1 - body; 2 - axis;

c - lamellar electrode for mesh welding: 1, 7 - machine consoles; 2-fork; 3 - flexible tires; 4-rocking electrode; 5 - welded mesh; 6 - bottom electrode

On conventional spot machines, welding of steel parts of relatively small thickness can be carried out with two points at once using a two-electrode head (Fig. 11, b). Uniform distribution of forces on both electrodes is achieved by rotating the housing 1 relative to the axis 2 under the action of the compression force of the machine.

Plate electrodes can be used for welding a mesh made of steel wire with a diameter of 3 ... 5 mm (Fig. 11, c). The top electrode 4 swings on the axis for even distribution of forces between the joints. The current supply for the purpose of its uniformity is carried out by flexible tires 3; fork 2 and the swing axis are isolated from the electrode. With electrode lengths up to 150 mm, they can be made non-swinging.

Rice. 12. Sliding wedge electrode inserts

When welding panels consisting of two skins and stiffeners, there must be an electrically conductive insert inside, which perceives the force of the machine's electrodes. The design of the insert should ensure its tight fit to the inner surface of the parts to be welded without a gap, in order to avoid deep dents on the outer surfaces of the parts and possible burns. For this purpose, the sliding insert shown in fig. 12. The movement of the wedge 2 relative to the fixed wedge 4, which ensures their compression to the parts to be welded 3, is synchronized with the operation of the machine. When the electrodes 1 and 5 are compressed and welding takes place, air from the pneumatic drive system of the machine enters the right cavity of the cylinder 8 fixed on the front wall of the machine and moves the wedge 2 through the rod 7, increasing the distance between the working surfaces of the wedges. When electrode 1 is raised, air leaves the right cavity and begins to flow into the left cavity of the cylinder 8, reducing the distance between the surfaces of the wedges, which allows you to move the welded panel relative to the machine electrodes. The wedge insert is cooled by air that enters through tube 6. The use of such an insert makes it possible to weld parts with an internal distance between them of up to 10 mm.

High electrode durability and good spot weld quality are not possible without proper care of the electrodes. From 3 to 10% of the welder's working time is spent on electrode maintenance. Proper care of the electrodes allows one pair of electrodes to make 30 ... 100 thousand weld points, while the consumption of the electrode alloy is only 5 ... 20 g per thousand welded points.

Caring for the electrodes of point machines consists of two operations - cleaning the electrodes directly on the machine and filling the removed electrode on a lathe or a special machine.

The frequency of stripping depends mainly on the material to be welded. When welding steel with a well-prepared surface, in some cases it is possible to do without stripping, in others the required stripping is carried out after welding several hundred points. When welding aluminum alloys, electrode cleaning is required after 30 ... The same phenomenon is observed when welding other materials with a lower melting point, such as, for example, magnesium.

Stripping should be carried out in such a way that, without removing a large amount of metal, a clean electrode surface is obtained. To simplify this operation and facilitate working conditions when stripping the electrodes, special devices are used.

The simplest device is shown in Fig. 1. It is a spatula with double-sided recesses into which sandpaper. The spatula is inserted between the compressed electrodes, and when rotated around the axis of the electrodes, it cleans their contact surfaces.

Rice. 1. Device for manual stripping of electrodes:

1 - skin; 2 - spherical recess.

Instead of such a spatula, you can use a steel plate for stripping electrodes with a flat contact surface or a piece of rubber for stripping electrodes with a spherical working surface. Electrodes with a flat contact surface are cleaned simultaneously or alternately, with a spherical one - simultaneously, with a small compressive force. After stripping, traces of abrasive dust are removed with a dry rag.

The desire to mechanize the process of cleaning the contact surface of the electrodes led to the creation of devices with an electric or pneumatic drive. On fig. 2 shows a pneumatic machine for stripping electrodes.

Rice. 2. Angle pneumatic electrode stripping machine

The need for cleaning the contact surface is determined visually, according to the state of the surface of the product to be welded, but attempts are known to determine the moment of cleaning with the help of special devices.

With the help of software control, not only the installation of the welded unit, welding current and welding time is carried out, but also a signal is given about the need to clean the electrodes.

It is proposed to determine the moment of stripping the electrodes by comparing the brightness light beam reflected from the contact surface of the electrode, with the brightness of the beam reflected from the surface of the standard. This method also makes it possible to stop the welding process under the action of a signal, the magnitude of which increases when the working surface of the electrode is contaminated.

Filling the working part of a worn electrode in order to restore its original shape can be done in several ways. The least quality is refilling with a small file. It is recommended to use special filling stations for these purposes. An example of a manual primer is shown in fig. 3.

Rice. 3. Manual electrode filler:

1 - body; 2 - screws. 3 - incisors; 4 - handle.

It is also recommended to use special pneumatic fillers equipped with a face mill, the profile of the cutting part of which corresponds to the profile of the working part of the electrode. A special cutter is inserted into the chuck of a conventional hand drill and allows you to simultaneously process the conical and flat surfaces of the working part of the electrode.

A good way to thread electrodes is threading on lathes with a gauge check.

At in large numbers refillable electrodes, it is advisable to use special type machines.

To quickly change the electrodes without damage, it is recommended to use turnkey flattened electrodes or use special pullers.

The simplest puller (Fig. 4) is a screw clamp of a special design.

Rice. 4. Puller of the simplest design:

1 - body; 2 - dies; 3 - clamping screw.

Recovery of worn electrodes for spot welding has not previously been practiced. Behind Lately the technology of restoration of electrodes of spot welding machines by arc surfacing has been developed. The hardness, electrical conductivity and resistance of the regenerated electrodes correspond to the properties of electrodes made from rods. Application of the method of recovery of the electrode by surfacing for only one multi-point machine allows saving up to 500 kg of bronze per year.