Electrodes for spot welding. Selecting electrodes for resistance welding
We decided to separate the story about electrode holders and electrodes for spot welding into 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 unit being welded. As a rule, the electrode holder is a copper or brass pipe with a conical hole for installing an 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 being welded. In some low-power machines, electrode holders may not be included in the package at all, since their functions are performed by welding trunks.
In standard machines, straight electrode holders are most often used (Fig. 1), as they are the simplest. 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 shaped electrode holders with simple straight-shaped electrodes. They are attached to electrode holders using a conical fit, pins or screws. The electrode is removed from the holder by lightly tapping it with a wooden hammer or a special extractor.
Electrodes for spot welding
Electrodes for spot welding 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 unit, and its durability determines the quality of welding and the duration of uninterrupted operation of the machine. There are straight (Fig. 4) and shaped electrodes (Fig. 5). Some examples of the use of straight electrodes are given in Table 1. Many straight electrodes are manufactured in accordance with GOST 14111-77 or OST 16.0.801.407-87.
For shaped electrodes, the axis passing through the center of the working surface is significantly shifted relative to the axis of the seating surface (cone). They are used for welding parts of complex shapes 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 electrode body there is an internal channel into which the cooling water supply tube 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 being 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 to dismantle the electrodes. Manufacturers use various techniques to calculate electrode sizes. In the USSR, according to OST 16.0.801.407-87, standard size ranges were established:
D el = 12, 16, 20, 35, 32, 40 mm
L = 35, 45, 55, 70, 90, 110 mm
Depending on the maximum compression force of the machine:
D el = (0.4 - 0.6)√F el (mm).
Where: F el - maximum compression force of the machine (daN).
The seating part (3) must be tapered to fit tightly into the electrode holder and prevent cooling water leaks. 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 at least 1.25D el. The landing part is treated with a cleanliness of at least class 7 (R z 1.25).
The diameter of the internal cooling channel is determined by the flow of cooling water and the sufficient compressive strength of the electrode and is:
d 0 = (0.4 - 0.6) D el (mm).
The distance from the working surface of the electrode to the bottom of the internal channel significantly affects the operational characteristics of the electrode: durability, service life. The shorter 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 made of 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 shell are also used (Fig. 4h) or copper electrodes with a steel spring-loaded sleeve.
Materials for spot welding electrodes
The durability of electrodes is their ability to maintain the dimensions and shape of the working surface (end), to resist the mutual transfer of metal from the electrodes and the parts being 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 cone angle, the properties and thickness of the material being welded, the welding mode, and the cooling conditions of the electrode. The wear of the electrodes depends on the design of the electrodes (material, diameter of the cylindrical part, cone angle of the working surface) and welding mode parameters. Overheating, melting, oxidation when working in a humid or corrosive environment, deformation of the electrodes under high compression forces, misalignment 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 during cyclic heating.
Compared to pure copper, alloys based on it have 3-5 times greater resistance to mechanical loads, so 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 a heated state, and 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 given in Table 2.
Selecting 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 brand of the material being welded, the combination of thicknesses of the sheets being welded, the shape of the welded unit, the requirements for the surface after welding and the design parameters of the welding mode.
There are the following types of shape 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, so they are recommended for use on radial-type machines and suspended machines (pincers) and for shaped electrodes operating with large deflections. 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 electrodes on a hinge help avoid distortion and therefore can replace spherical electrodes (Fig. 8). However, these electrodes are mainly recommended for welding sheets ≤1.2 mm thick.
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 point, it becomes clear that there is a direct relationship between the diameter of the electrode and the diameter of the core of the weld point. The electrode diameter determines the contact surface area, which corresponds to the fictitious diameter of the resistance conductor r between the sheets being welded. The contact resistance R will be inversely proportional to this diameter and inversely proportional to the pre-compression of the electrodes to smooth out surface micro-irregularities. Research by the ARO company (France) has shown that the diameter of the working surface of the electrode can be calculated using the empirical formula:
d el = 2t + 3 mm.
Where t is the nominal thickness of the sheets being welded.
It is most difficult to calculate the diameter of the electrode when the thickness of the sheets being welded is unequal, when welding a package of three or more parts, and when welding dissimilar materials. Obviously, when welding parts of different thicknesses, the diameter of the electrode must be selected relative to the thinner sheet. Using the formula for calculating the diameter of the electrode, 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 simultaneously welding a package of parts, the diameter of the working surface of the electrode is selected based on the thickness of the outer parts. When welding dissimilar materials with different thermophysical characteristics, less penetration is observed in the metal with lower electrical resistivity. In this case, on the side of the metal part with lower resistance, an electrode with a larger diameter of the working surface d el or made of a material with greater thermal conductivity (for example, BrKh chromium bronze) is used.
Valery Raisky
Magazine "Equipment: market, supply, prices", No. 05, May 2005.
Literature:
- Knorozov B.V., Usova L.F., Tretyakov A.V. Metal technology and materials science. - M., Metallurgy, 1987.
- Mechanical Engineer's Handbook. T. 5, book. 1. Ed. Satele E.A. - M., Mashgiz, 1963.
Electrodes intended for contact welding are made from metal rods, the diameter of which ranges from 12 to 40 mm. Their working surface is either flat or spherical. To connect the workpieces together into a rather complex structure, they use electrodes that have an offset surface - the so-called shoe products. Such products are secured using a special shank having a cone of 1:10 or 1:5.
You can also find electrodes on sale that have a cylindrical surface, thanks to which they will be fixed to work in special structures with a conical thread. In addition to them, products are produced with a replaceable working part - it is installed on the cone using a standard union nut or simply pressed.
Electrodes for resistance welding of relief type in their shape will directly depend on the method of connection and the final shape of the product. In most cases, the size of the working surface of a given electrode does not play a special role. This is due to the fact that the contact area and the selected welding current directly depend on what shape the workpieces will have at the points of contact.
There are also electrodes for connecting elements with very complex topography. Suture equipment uses products that are a disk with a flat working surface. Moreover, these products may even have asymmetrical bevels. Such discs are fixed to the equipment by veneering or pressing.
Inside the electrodes themselves there are certain cavities through which coolant will circulate during the welding process. Electrodes for resistance spot welding are solid, so in this case the so-called external cooling is used.
To ensure that the electrode material is consumed to a minimum, the roller is made replaceable. The electrode itself is made from a special alloy based on a metal such as copper. The result is a product that has virtually no resistance to electric current, is an excellent heat conductor, and is resistant to even fairly high temperatures. In addition, when hot, this electrode will retain its original hardness, and interaction with the workpiece metal will be minimal.
Types of resistance welding equipment
The main feature of this technology is the connection of workpieces over the entire area. Optimal heating is achieved through reflow using a welding machine. However, in some cases they resort to heating due to the resistance of the part to the passage of electric current.
Resistance spot welding can occur either with metal melting or without this technological feature of the process. Resistance welding can be used to connect metal elements whose cross-section is in the range from 1 to 19 mm, and in most cases resistance welding is used, since the consumption of electrode material will be significantly lower, and the final connection is much more durable. This welding is used when performing fairly precise work, for example, in the process of producing rails to create a railway track.
Features of resistance spot welding
This technology is perfect for connecting metal elements together, and the connection is carried out both at one and at several points on these workpieces. It is extremely popular not only in industry (in particular, it is often used in agriculture, in the construction of aircraft, automobile transport, and so on), but also in everyday life.
The principle of operation of this method is quite simple: electric current, when passing through parts that are in direct contact with each other, very much heats up their edges. The heating is so strong that the metal begins to quickly melt, and the workpieces are immediately compressed with considerable force. As a result of this, a welded joint is formed.
Equipment designed to use this technology is designed to connect sheets, rods and other metal products together. The key advantages of this method are the following:
- Absence of a welded joint in the traditional sense;
- There is no need to use filler material, gas or flux;
- The equipment is very easy to use;
- The speed of work is quite high.
The main and only drawback of this method is that the seam is completely unsealed.
What are electrodes for resistance welding made of?
The material from which the electrodes will be made is selected depending on the requirements for the operating conditions of the product. It is worth noting that the electrodes must be able to withstand compression, temperature changes, exposure to high temperatures, and stress that will be generated inside the electrode itself, which is under serious load.
In order for the products to be of the highest quality, it is necessary that the electrode retains the original shape of its working surface, which will be in direct contact with the parts to be connected. Melting of this consumable material accelerates its wear.
Usually copper is taken as the main element, and other elements are added to it - magnesium, cadmium, silver, boron, and so on. The result is a material that excellently resists even very severe physical stress. Electrodes with tungsten or molybdenum coating practically do not wear out during operation, which is why they have recently gained the greatest popularity. However, they cannot be used for welding products made of aluminum and other materials with a soft structure.
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 studying the tabular data, you will be able to draw your own conclusions.
TIP PROFILE
WELDING SPOT
REQUIRED CURRENT, A
RESULT
CORRECT MAINTENANCE OF ELECTRODES FOR RESISTANCE SPOT AND RELIEF WELDING
Electrodes for projection welding
To ensure the precise alignment required for good contact and quality welds, projection welding electrodes should be positioned directly on the center line of pressure application. In addition to producing poor-quality welds, poor alignment of the electrodes can lead to damage to their surfaces [Fig. 1].
Another serious cause of poor welding is non-parallelism of the electrode surfaces. It entails uneven pressure on the electrodes, which leads to molten metal splashing out of the weld area during the welding cycle. If welding goes through the supporting part of the electrode, the reliefs are damaged and the insulation may burn out. In addition, non-parallelism leads to the biting of the electrode tips by their supporting parts during welding, which results in a burn on the workpiece at the point of contact with displaced reliefs, and a possible shift relative to the mating parts of the welding equipment [Fig. 2].
SHOULD
... keep a supply of electrodes on the machine to minimize downtime due to electrode replacement,
... sharpen electrodes on a lathe,
...use special grade 3 copper for electrode tips.
DO NOT DO IT
... file down the electrodes (an uneven surface will lead to either partial welding or metal splashing out of the welding zone),
Electrodes for spot welding
In resistance 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 current passes. Small diameter spot welding electrode tips erode or wear away much more quickly than their projection welding counterparts and therefore must be sharpened regularly to maintain proper contact [Fig. 3].
SHOULD
... keep a supply of electrodes on the machine,
... periodically sharpen the electrodes on a specialized machine,
... change the diameter of the tips when working with different thicknesses of the metal being welded.
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 checked by inserting a piece of carbon and a piece 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 contact plane between the two surfaces.
2. Use a water jacket if necessary and place it as close to the welding surface as possible.
3. Keep the material being welded clean: free of oil, film, dirt and other foreign matter.
4. Follow the prescribed welding procedure.
WELDING ELECTRODES AND HOLDERS
RECOMMENDED |
PROHIBITED |
| 1. Use electrodes made of material suitable for your application. 2. Use standard electrodes wherever possible. 3. Use tips of the optimal diameter for the given thickness of the materials being welded. 4. Use transparent hoses to constantly monitor the flow of water through the electrodes. 5. Connect the water supply hose to the corresponding inlet on the holder so that water flows into the central cooling pipe first. 6. Cool the electrodes with water flowing at a rate of at least 7 liters per minute through each tip. 7. Make sure that the inner cooling tube of the holder is inserted into the water hole on the tip to a depth of 6mm. 8. Adjust the inner tube of the holder cooling system in height when changing to a different length of tip. 9. Make sure the top end of the holder coolant tube is cut at an angle that will not cause the tip to jam and cut off the water supply. 10. Apply a thin layer of special lubricant to the tip rod before inserting it into the holder to make it easier to pull it out. 11. Use ejector type holders for easy removal of tips and to avoid damage to tip rods. 12. Keep the tip and holder clean, smooth and free of foreign substances. 13. Grind spot welding electrodes often enough to maintain weld quality. 14. Grind the electrodes on a lathe to the original shape whenever possible. 15. Use a piece of leather or a rubber mallet when leveling the holder or tip. 16. Apply coolant to both sides of the disc when seam welding. 17. Use specially designed knurling discs to maintain proper shape of the seam welding disc electrode. |
1. Never use unknown electrodes or electrode materials. 2. Avoid specialty, offset or custom tips when the job can be done with a standard straight tip. 3. Do not use small tips for welding work with heavy large workpieces and vice versa. 4. Be sure to turn on the cooling water supply to full power before starting welding. 5. Never use a hose that does not fit tightly onto the water nipple on the holder. 6. Avoid leaking, clogging or damaging water equipment. 7. Avoid using holders with leaking or deformed tubing. 8. Never use electrode holders that do not have adjustable internal cooling tubes. 9. Do not allow the tube to become clogged due to the accumulation of impurities. A few drops of oil at reasonable intervals will help keep the tube working. 10. Do not allow electrodes to remain idle in holders for long periods of time. 11. Do not use adjustable wrenches or similar tools to remove electrodes. 12. Avoid using white lead or similar compounds to seal leaking adapters. 13. Never allow the tip of a spot welding electrode to become so flat that pointing becomes difficult. 14. Never use rough discs to sharpen electrodes. 15. Do not hit the holder or tip with a steel hammer when leveling the tool. 16. Avoid using seam welding discs that are too thin for the given thermal or physical load. 17. Do not allow the welding discs to extend beyond the workpieces being welded. |
The design of the electrodes must have a shape and dimensions that provide access to the working part of the electrode to the place where parts are welded, be adapted for convenient and reliable installation on the machine, and have high durability of the working surface.
The simplest to manufacture and operate are straight electrodes, made in accordance with GOST 14111-69 from various copper electrode alloys, depending on the grade of metal 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 connections, the electrodes must have a fairly low electrical thermal conductivity (30...40% of copper). If the entire electrode is made from such metal, it will heat up intensely 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 connections. Working part 3 can be replaceable (Fig. 1, a) and secured with a nut 2 on base 1. The use of electrodes of this design is convenient, as it allows you to install the desired working part when changing the thickness and grade of the metal of the parts being welded. The disadvantages of an electrode with a replaceable part are the possibility of using it only when welding parts with good approaches and insufficiently intensive cooling. Therefore, such electrodes should not be used in heavy welding conditions at high speeds.
Rice. 1 . 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-in tip (Fig. 1, c). The 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 base of the electrode. When welding parts made of stainless steel 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 durability of the electrode are observed. The working surface of the insert can be flat or spherical.
When designing electrodes, special attention should be paid to the shape and dimensions of the seating part. The most common is a conical landing part, the length of which should be at least. Electrodes with a shortened cone should only be used when welding using low forces and currents. In addition to the conical fit, electrodes are sometimes fastened to threads using a union nut. This connection of electrodes can be recommended in. multi-point machines, when it is important to have the same initial distance between the electrodes, or in clamps. When using shaped electrode holders, electrodes with a cylindrical seat are also used (see Fig. 8, d).
When spot welding parts with complex contours and poor approaches to the joint, a wide variety of shaped 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 shaped electrodes when welding is generally impossible without them. The dimensions and shape of the shaped electrodes depend on the size and configuration of the parts, as well as the design of the electrode holders and consoles of the welding machine (Fig. 2).
Rice. 2. Various types of shaped electrodes
During operation, shaped electrodes usually experience a significant bending moment from off-axis application of force, which must be taken into account when selecting or designing electrodes. The bending moment and the usually small cross-section of the cantilever part create significant elastic deformations. In this regard, mutual displacement of the working surfaces of the electrodes is inevitable, especially if one electrode is straight and the other is shaped. Therefore, for shaped electrodes, the spherical shape of the working surface is preferable. In the case of shaped electrodes that experience large bending moments, deformation of the conical seating part and the electrode holder socket is possible. The maximum permissible bending moments for shaped 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× cm. If the conical part of the shaped electrode experiences a moment greater than permissible, then the maximum diameter of the cone should be increased.
When designing complex spatial shaped electrodes, it is recommended to first make a model of them from plasticine, wood or easily machined metal. This allows you to establish the most rational dimensions and shape of the shaped electrode and avoid alterations when manufacturing it directly from metal.
In Fig. 3 shows some examples of welding assemblies in places with limited access. Welding of the profile with the shell is performed using a lower electrode with an offset working surface (Fig. 3, a).
Rice. 3. Examples of using shaped electrodes
An example of using an upper electrode with oblique sharpening and a lower, shaped one is shown in Fig. 3, b. 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 will be deformed. If it is impossible to install the upper electrode with a slope, then it can also be shaped. The shaped electrode is bent in two planes to reach a hard-to-reach welding spot (Fig. 3, c-e). If the machine does not have or has limited horizontal movement of the consoles for welding the parts shown in Fig. 3, e, two shaped electrodes with equal projections are used.
Sometimes shaped electrodes perceive very large bending moments. To avoid deformation of the conical seating part, the shaped electrode is additionally secured to the outer surface of the electrode holder using a clamp and a screw (Fig. 4, a). The strength of shaped electrodes with a long reach increases significantly if they are made of composite (reinforced) electrodes. For this purpose, 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 made using soldering, and with a steel console - using screws. A design option is possible when a shaped electrode made of a copper alloy is supported (reinforced) with steel elements (bars), 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 shaped electrodes that experience 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 supply
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 heating, 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 shaped electrodes, since it is not always possible to supply 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 a shaped electrode of a fairly large size. Considering 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 pliers for welding in hard-to-reach places, as well as the need to frequently replace electrodes, use the electrode mounting shown in Fig. 5. This fastening provides good electrical contact, convenient regulation of electrode extension, good stability against lateral displacement, and quick and easy removal of electrodes. However, due to the lack of internal cooling in such electrodes, they are used when welding at low currents (up to 5...6 kA) and at a low speed.
Rice. 5. Methods for attaching electrodes
For ease of operation, electrodes with several working parts are used. These electrodes can be adjustable or rotary (Fig. 6) and significantly simplify and speed up the installation of electrodes (aligning working surfaces).
Rice. 6. Multi-position adjustable (a) and surface (b) electrodes:
1 - electrode holder; 2 - electrode
The electrodes are installed in electrode holders, which are fixed to the cantilever parts of the welding machine, transmitting compression force and current. In table For reference, the dimensions of straight electrode holders of the main types of spot welding machines are given. Electrode holders must be made of sufficiently strong copper alloys with relatively high electrical conductivity. Most often, electrode holders are made of Br.Kh bronze, which must be heat-treated to obtain the required hardness (HB not less than 110). In the case of welding steels, when low currents (5...10 kA) are used, it is advisable to make electrode holders from Br.NBT bronze or silicon-nickel bronze. These metals ensure long-term preservation of the dimensions of the conical mounting hole of the electrode holder.
Table. Dimensions of electrode holders for point machines in mm
|
Electrode holder dimensions |
MTPT-600 |
MTPT-400, MTK-75 |
MTP-300, MTP-400 |
MTK 6301, MTP-200/1200 |
MTPU-300, MTP-150/1200 MTP-200, MTP-150, MT 2507 |
MT 1607, MTP-75 MTP-100, MTPR-75 (50, 25) MTPK-25, MT 1206 |
|
Outside diameter |
||||||
|
Cone diameter for electrode |
||||||
|
Taper |
1: 10 |
1:10 |
1:10 |
The most common are straight 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 changes 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 a spring 2. It is important that the end of the ejector hitting the end of the electrode does not have damage on its surface, otherwise the seating part of the electrode will quickly fail, jamming when it is removed from the electrode holder. It is convenient for operation to make the end of the electrode holder 1 in the form of a replaceable threaded bushing 2, in which the electrode 3 is installed (Fig. 7, c). This design makes it possible to make sleeve 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 when jammed by knocking it out with a steel drift from inside the sleeve.
Rice. 7. Straight electrode holders:
a – normal;
b – with ejector;
c – with replaceable sleeve
If shaped electrodes are more often used when welding parts that have small dimensions of the elements being connected, then for larger sizes it is advisable to use special shaped electrode holders and simple electrodes. Shaped electrode holders can be composite and provide installation of electrodes at different angles to the vertical axis (Fig. 8, A). The advantage of such an electrode holder is the easy adjustment of the electrode extension. In some cases, the shaped electrode can be replaced with electrode holders shown in Fig. 8, b. Also of interest is the electrode holder, the tilt of which can be easily adjusted (Fig. 8, c). The design of an electrode holder bent at an angle of 90° is shown in Fig. 30, g, it allows you to attach electrodes with a cylindrical seat. A special screw clamp ensures quick fastening and removal of the electrodes. In Fig. Figure 9 shows various examples of spot welding using shaped electrode holders.
Rice. 8. Special electrode holders
Rice. 9. Examples of the use of various electrode holders
When spot welding large-sized components such as panels, it is advisable to use a four-electrode rotating head (Fig. 10). The use of such heads allows you to quadruple the operating time of the electrodes before the next stripping, without removing the panel to be welded from the working space of the machine. To do this, after each pair of electrodes is contaminated, the electrode holder 1 is rotated 90° and secured with a stopper 4. The rotating head also makes it possible to install electrodes with different shapes of the working surface for welding an assembly with parts changing, for example, stepwise in thickness, as well as to provide mechanization of stripping the electrodes with special devices. The rotating head can be used when spot welding parts with large differences 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 quickly wears out and is replaced by turning the head with a new one. It is convenient to use a roller as an electrode on the side of a thick part.
Rice. 10. Rotating electrode head:
1 – rotary electrode holder; 2 – body; 3 – electrode; 4 – stopper
When spot welding, the axes of the electrodes must be perpendicular to the surfaces of the parts being welded. To do this, welding of parts that have slopes (smoothly varying thickness), or are manufactured using overhead machines, in the presence of large-sized components, is performed using a self-aligning rotary electrode with a spherical support (Fig. 11, a). To prevent water leakage, the electrode has a seal in the form of a rubber ring.
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 - plate electrode for welding mesh: 1, 7 - machine consoles; 2-fork; 3 - flexible tires; 4-swinging electrode; 5 - welded mesh; 6 - bottom electrode
On conventional spot machines, welding of steel parts of relatively small thickness can be performed at 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 axis 2 under the action of the compression force of the machine.
To weld a mesh of steel wire with a diameter of 3...5 mm, plate electrodes can be used (Fig. 11, c). The upper electrode 4 swings on an axis to evenly distribute forces between the connections. The current supply for the purpose of its uniformity is carried out by flexible busbars 3; fork 2 and the swing axis are isolated from the electrode. When electrodes are up to 150 mm long, they can be non-oscillating.
Rice. 12. Sliding wedge electrodes inserts
When welding panels consisting of two skins and stiffeners, there must be an electrically conductive insert inside that absorbs the force of the machine electrodes. The design of the insert must ensure its tight fit to the inner surface of the parts being welded without a gap, in order to avoid deep dents on the outer surfaces of the parts and possible burns. For this purpose, a sliding insert shown in Fig. 12. The movement of the wedge 2 relative to the stationary wedge 4, ensuring their compression to the welded parts 3, is synchronized with the operation of the machine. When electrodes 1 and 5 are compressed and welding occurs, air from the pneumatic drive system of the machine enters the right cavity of the cylinder 8 mounted 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 raising electrode 1, the air leaves the right one and begins to enter the left cavity of the cylinder 8, reducing the distance between the surfaces of the wedges, which allows the panel to be welded to be moved relative to the electrodes of the machine. The wedge insert is cooled by air that enters through tube 6. The use of such an insert allows you to weld parts with an internal distance between them of up to 10 mm.
Most of the metal products that surround us are made using resistance welding. There are different types of welding, but contact welding allows you to create fairly strong and aesthetically beautiful seams. Since the metal is not welded using the traditional method, this process requires resistance welding electrodes.
Resistance welding is only possible for welding two metal parts superimposed on one another; they cannot be joined end-to-end using this method. At the moment when both parts are clamped by the conductive elements of the welding machine, an electric current is briefly supplied, which melts the parts directly at the point of compression. This is mainly possible due to current resistance.
Electrode designs
Electrodes are also used to work with electric arc welding, but they are fundamentally different from the conductive elements for contact welding, and are not suitable for this type of work. Since at the time of welding the parts are compressed by the contact parts of the welding machine, electrodes for resistance welding are able to conduct electric current, withstand compression loads and remove heat.
The diameter of the electrodes determines how firmly and efficiently the parts will be welded. Their diameter should be 2 times thicker than the welded joint. According to state standards, they range in diameter from 10 to 40 mm.
The metal being welded determines the shape of the electrode used. These elements, which have a flat working surface, are used for welding conventional steels. The spherical shape is ideal for joining copper, aluminum, high carbon and alloy steels.
The spherical shape is most resistant to combustion. Due to their shape, they are able to make a larger number of welds before sharpening. In addition, the use of this form allows you to weld any metal. At the same time, if you weld aluminum or magnesium with a flat surface, dents will form.
The electrode seat is often cone-shaped or threaded. This design avoids current losses and effectively compresses parts. The landing cone can be short, but they are used with low forces and low currents. If a threaded fastener is used, it is often through a union nut. Threaded fastening is especially important in special multi-point machines, since the same gap between the claws is required.
To perform welding deep into the part, electrodes of a curved configuration are used. There is a variety of curved shapes, so if you are constantly working in such conditions, it is necessary to have a selection of different shapes. However, they are inconvenient to use, and they have lower durability compared to straight ones, so they are used last.
Since the pressure on the shaped electrode is not along its axis, it is subject to bending during heating, and this must be kept in mind when choosing its shape. In addition, at such moments, it is possible that the working surface of the curved electrode may shift in relation to the flat one. Therefore, in such situations, a spherical working surface is usually used. Non-axial load also affects the seat of the electrode holder. Therefore, if there is excessive load, you need to use electrodes with an increased cone diameter.
When welding deep into a part, you can use a straight electrode if you tilt it vertically. However, the angle of inclination should be no more than 30°, since with a greater degree of inclination, deformation of the electrode holder occurs. In such situations, two curved conductive elements are used.
Using a clamp at the point where the shaped electrode is attached allows you to reduce the load on the cone and extend the service life of the welding machine seat. When developing a shaped electrode, you must first make a drawing, then make a test model from plasticine or wood, and only then begin its manufacture.
In industrial welding, cooling of the contact part is used. Often this cooling occurs through an internal channel, but if the electrode is of small diameter or increased heating occurs, then the coolant is supplied externally. However, external cooling is allowed provided that the parts being welded are not susceptible to corrosion.
The most difficult thing to cool is the shaped electrode due to its design. To cool it, thin copper tubes are used, which are located on the side parts. However, even under these conditions, it does not cool well enough, so it cannot cook at the same pace as a straight electrode. Otherwise, it overheats and its service life is reduced.
Welding in the depths of a small part is carried out with shaped electrodes, and with large parts it is preferable to use shaped holders. The advantage of this method is the ability to adjust the length of the electrode.
During contact welding, the axis of the two electrodes should be 90° relative to the surface of the part. Therefore, when large-sized parts with a slope are welded, rotary, self-aligning holders are used, and welding is performed with a spherical working surface.
Steel mesh with a diameter of up to 5 mm is welded with a plate electrode. Uniform load distribution is achieved by free rotation of the upper conductive contact around its axis.
Although the spherical shape of the working surface is the most stable of the other shapes, it still loses its original shape due to thermal and power loads. If the working surface of the contact increases by 20% of the original size, then it is considered unusable and must be sharpened. Sharpening of resistance welding electrodes is carried out in accordance with GOST 14111.
Electrode materials for resistance welding
One of the decisive factors in the quality of a weld is tensile strength. This is determined by the temperature of the weld point and depends on the thermophysical properties of the conductor material.
Copper in its pure form is ineffective because it is a very ductile metal and does not have the necessary elasticity to recover to its geometric shape between welding cycles. In addition, the cost of the material is relatively high, and with such properties, the electrodes would require regular replacement, which would make the process more expensive.
The use of hardened copper was also not successful, since a decrease in the recrystallization temperature leads to the fact that with each subsequent weld point the wear of the working surface will increase. In turn, alloys of copper with a number of other metals turned out to be effective. For example, cadmium, beryllium, magnesium and zinc added hardness to the alloy during heating. At the same time, iron, nickel, chromium and silicon allow it to withstand frequent heat loads and maintain the pace of work.
The electrical conductivity of copper is 0.0172 Ohm*mm 2 /m. The lower this indicator, the more suitable it is as an electrode material for resistance welding.
If you need to weld elements from different metals or parts of different thicknesses, then the electrical thermal conductivity of the electrode should be up to 40% of this property of pure copper. However, if the entire conductor is made of such an alloy, it will heat up quite quickly, since it has high resistance.
By using composite construction technology, significant cost savings can be achieved. In such designs, the materials used in the base are selected with a high electrical conductivity, and the outer or replaceable part is made of heat and wear-resistant alloys. For example, metal-ceramic alloys consisting of 44% copper and 56% tungsten. The electrical conductivity of such an alloy is 60% of the electrical conductivity of copper, which allows heating the weld point with minimal effort.
Depending on the working conditions and assigned tasks, alloys are divided into:
- Difficult conditions. Electrodes operating at temperatures up to 500 o C are made of bronze, chromium and zirconium alloys. For welding stainless steel, bronze alloys alloyed with titanium and beryllium are used.
- Average load. Welding of carbon, copper and aluminum parts is usually carried out using electrodes made of alloys, in which the grade of copper for the electrodes is capable of operating at temperatures up to 300 o C.
- Lightly loaded. Alloys, which include cadmium, chromium and silicon-nickel bronze, are capable of operating at temperatures up to 200 o C
Electrodes for spot welding
The spot welding process explains itself from its name. Accordingly, a mini welding seam is one point, the size of which is determined by the diameter of the working surface of the electrode.
Electrodes for resistance spot welding are rods made of alloys based on copper. The diameter of the working surface is determined by GOST 14111-90, and is manufactured in the range from 10-40 mm. Electrodes for spot welding are carefully selected because they have different properties. They are made with both spherical and flat working surfaces.
Electrodes for spot welding with your own hands can theoretically be made, but you need to be sure that the alloy meets the stated requirements. In addition, you need to maintain all sizes, which is not so easy at home. Therefore, when purchasing factory-made conductive elements, you can count on high-quality welding work.
Spot welding has a lot of advantages, including an aesthetic weld spot, ease of operation of the welding machine and high productivity. There is also one drawback, namely the lack of a sealed weld seam.
Electrodes for seam welding
One of the varieties of resistance welding is seam welding. However, electrodes for seam welding are also an alloy of metals, only in the form of a roller.
Rollers for seam welding are of the following types:
- without bevel;
- with a bevel on one side;
- with bevel on both sides.
The configuration of the part being welded determines what shape of roller should be used. In hard-to-reach places, it is unacceptable to use a roller with a bevel on both sides. In this case, a roller without bevels or with a bevel on one side is suitable. In turn, a roller with a bevel on both sides presses the parts more efficiently and cools faster.
The use of roller welding helps to achieve hermetically sealed welds, which allows them to be used in the manufacture of containers and tanks.
So, resistance welding allows you to produce high-tech seams, but in order to achieve a high-quality result, you need to carefully follow the values indicated in the tables. Which welding option you choose, spot or seam welding, depends on your needs.
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