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Electrodes and Tooling

. Electrodes and Tooling. Lesson ObjectivesWhen you finish this lesson you will understand: The various electrode materials and design Electrode and part holders and fixtures Typical Applications. Learning ActivitiesView Slides; Read Notes, Listen to lectureDo on-line workbookDo Homework. KeywordsResistance Welding Electrodes, RWMA Classification, Electrode Geometry, Multiple Welding Tools, Fixtures.

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Electrodes and Tooling

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    3. Electrode Functions Conduct the welding current to the work Transmit the proper electrode pressure or force to the work in order to produce a satisfactory weld Help dissipate heat from the weld zone Resistance welding electrodes perform three important functions, listed in the above slide, in all resistance welding processes. The first of these functions is electrical. If the application of force did not have to be considered, suitable electrode selection could be made almost entirely on the basis of electrical and thermal conductivity, taking into account the resistance of the electrode itself and the resistance at the area of contact between the electrode and the work surface. The second function is mechanical. During welding operations the electrodes are subjected to stresses which are often of considerable magnitude. They must withstand these stresses at elevated temperatures without excessive deformation. This is because the current must not only be conducted to the work, but must also be localized within a fixed area. The transmitted electrode force not only forges the heated workpieces together, but also concentrates the welding current to a localized area. In the third function the electrodes must process higher thermal conductivity than that of the metals being welded. Generally, thermal and electrical conductivities of various metals are proportional. Because of their higher thermal properties the electrodes conduct heat away from the exterior surfaces of the material being welded. This last function sometimes assumes considerable importance when dissimilar metals are being welded, and it becomes necessary or desirable to obtain a thermal balance. In such cases an electrode with a lower heat conductivity may be chosen to prevent too rapid heat dissipation from one part of a dissimilar combination. This procedure is sometimes resorted to in the welding of heat-treatable components.Resistance welding electrodes perform three important functions, listed in the above slide, in all resistance welding processes. The first of these functions is electrical. If the application of force did not have to be considered, suitable electrode selection could be made almost entirely on the basis of electrical and thermal conductivity, taking into account the resistance of the electrode itself and the resistance at the area of contact between the electrode and the work surface. The second function is mechanical. During welding operations the electrodes are subjected to stresses which are often of considerable magnitude. They must withstand these stresses at elevated temperatures without excessive deformation. This is because the current must not only be conducted to the work, but must also be localized within a fixed area. The transmitted electrode force not only forges the heated workpieces together, but also concentrates the welding current to a localized area. In the third function the electrodes must process higher thermal conductivity than that of the metals being welded. Generally, thermal and electrical conductivities of various metals are proportional. Because of their higher thermal properties the electrodes conduct heat away from the exterior surfaces of the material being welded. This last function sometimes assumes considerable importance when dissimilar metals are being welded, and it becomes necessary or desirable to obtain a thermal balance. In such cases an electrode with a lower heat conductivity may be chosen to prevent too rapid heat dissipation from one part of a dissimilar combination. This procedure is sometimes resorted to in the welding of heat-treatable components.

    4. RWMA Electrode Material Standards Group A - Copper Base Alloys RWMA Class 1 Zirconium Copper Cadmium Copper Chromium Copper RWMA Class 2 Chromium-Zirconium Copper Chromium Copper RWMA Class 3 Cobalt-Beryllium Copper Nickel-Beryllium Copper Beryllium-Free Copper RWMA Class 4 Beryllium Copper RWMA Class 5 Aluminum Copper RWMA Class 1. Because of its high electrical and thermal conductivity, class 1 is specially recommended for spot welding aluminum alloys, magnesium alloys, some coated materials, brass and bronze. RWMA Class 2. Class 2 material has slightly lower conductivity and higher mechanical properties than Class 1. It is a general purpose resistance welding electrode material recommended for production spot and seam welding of most materials. It is suitable for spot, projection, seam, or flash welding electrodes, shafts and bearings, current-carrying structural members, and it is also available as high strength and high electrical conductivity castings for use as welding platens and secondary circuit structural members. RWMA Class 3. Class 3 material has lower conductivity and higher mechanical properties than Class 2. It is recommended for projection welding electrodes, flash and upset welding electrodes, current-carrying shafts and bushings. RWMA Class 4. Class 4 material has lower conductivity and significantly higher mechanical properties than Class 3. It is generally recommended as electrode material for special flash, upset and projection welding applications where forces are extremely high and wear is severe but where heating is not excessive. RWMA Class 5. Class 5 is a copper base material available as castings having lower electrical conductivity than Class 4 and mechanical properties similar to Class 1. It may be used where high strength, wear resistance and nonmagnetic properties are required.RWMA Class 1. Because of its high electrical and thermal conductivity, class 1 is specially recommended for spot welding aluminum alloys, magnesium alloys, some coated materials, brass and bronze. RWMA Class 2. Class 2 material has slightly lower conductivity and higher mechanical properties than Class 1. It is a general purpose resistance welding electrode material recommended for production spot and seam welding of most materials. It is suitable for spot, projection, seam, or flash welding electrodes, shafts and bearings, current-carrying structural members, and it is also available as high strength and high electrical conductivity castings for use as welding platens and secondary circuit structural members. RWMA Class 3. Class 3 material has lower conductivity and higher mechanical properties than Class 2. It is recommended for projection welding electrodes, flash and upset welding electrodes, current-carrying shafts and bushings. RWMA Class 4. Class 4 material has lower conductivity and significantly higher mechanical properties than Class 3. It is generally recommended as electrode material for special flash, upset and projection welding applications where forces are extremely high and wear is severe but where heating is not excessive. RWMA Class 5. Class 5 is a copper base material available as castings having lower electrical conductivity than Class 4 and mechanical properties similar to Class 1. It may be used where high strength, wear resistance and nonmagnetic properties are required.

    5. RWMA Electrode Material Standards (CONT.) Group B - Refractory Metals and Refractory Metal Composites RWMA Class 10 Copper Tungsten RWMA Class 11 Copper Tungsten RWMA Class 12 Copper Tungsten RWMA Class 13 Tungsten RWMA Class 14 Molybdenum Group C - Specialty Material RWMA Class 20 Dispersion-Strengthened Copper RWMA Class 10. Class 10 material is recommended as facing material or insert material for projection welding electrodes and flash and upset welding electrodes when relative high electrical and thermal conductivity is desirable and some malleability is desired. RWMA Class 11. Class 11 material is a harder, lower conductivity material than Class 10. It is recommended as a facing material or insert material for flash and upset welding electrodes, and general purpose projection welding electrodes where welding forces are moderate. It may also be used as seam welding bearing inserts and facings for upsetting. RWMA Class 12. Class 12 material is harder and has lower conductivity than Class 11. It is specially recommended for heavy-duty projection welding electrodes and for electrode facings in upsetting applications. RWMA Class 13 and 14. Class 13 and 14 materials have lower conductivity than Class 12 material. Class 13 is extremely hard with relatively low ductility. It cannot be machined but contours may be ground. Class 14 is not as hard as Class 13. It can be drilled or machined. Both are used for welding or resistance brazing nonferrous metals having relatively high electrical conductivity. RWMA Class 20. This material has similar conductivity and higher mechanical properties than Class 2. It is often recommended for use on coated materials, and it is suitable for spot welding, projection welding, and seam welding electrodes. Class 20 material must be cold worked by upsetting.RWMA Class 10. Class 10 material is recommended as facing material or insert material for projection welding electrodes and flash and upset welding electrodes when relative high electrical and thermal conductivity is desirable and some malleability is desired. RWMA Class 11. Class 11 material is a harder, lower conductivity material than Class 10. It is recommended as a facing material or insert material for flash and upset welding electrodes, and general purpose projection welding electrodes where welding forces are moderate. It may also be used as seam welding bearing inserts and facings for upsetting. RWMA Class 12. Class 12 material is harder and has lower conductivity than Class 11. It is specially recommended for heavy-duty projection welding electrodes and for electrode facings in upsetting applications. RWMA Class 13 and 14. Class 13 and 14 materials have lower conductivity than Class 12 material. Class 13 is extremely hard with relatively low ductility. It cannot be machined but contours may be ground. Class 14 is not as hard as Class 13. It can be drilled or machined. Both are used for welding or resistance brazing nonferrous metals having relatively high electrical conductivity. RWMA Class 20. This material has similar conductivity and higher mechanical properties than Class 2. It is often recommended for use on coated materials, and it is suitable for spot welding, projection welding, and seam welding electrodes. Class 20 material must be cold worked by upsetting.

    6. Electrode Materials Electrodes must be basically capable of three functions: conducting current to the workpiece, mechanically constraining the workpiece and conducting heat away from the workpiece. These materials must sustain high loads at elevated temperatures while maintaining adequate thermal and electrical conductivity. Depending on the application, a range of copper, copper-alloy, copper-tungsten or tungsten electrode materials are used. A list of common electrode materials is given in the above slide. These materials commonly trade off thermal and electrical conductivity for high temperature strength.Electrodes must be basically capable of three functions: conducting current to the workpiece, mechanically constraining the workpiece and conducting heat away from the workpiece. These materials must sustain high loads at elevated temperatures while maintaining adequate thermal and electrical conductivity. Depending on the application, a range of copper, copper-alloy, copper-tungsten or tungsten electrode materials are used. A list of common electrode materials is given in the above slide. These materials commonly trade off thermal and electrical conductivity for high temperature strength.

    7. Typical Hardness-Temperature Curves During the early development of resistance welding, pure copper was the only electrode material available, but as the art progressed, welding current and forces increased. It soon became evident that superior electrode materials were necessary to keep pace with the rapidly developing art. The ideal electrode material for most applications would possess the compressive strength of tool steel and the electrical conductivity of silver, but no such material was available. This demand brought about the development of a series of electrode materials designed to overcome the shortcoming of pure copper. In fact, this is a continuing development as the state of the art advances with regard to processes and equipment. All of the RWMA recommended electrode materials have higher annealing or softening temperatures than pure copper, together with improved compressive strength and wear resistance which has necessarily been accompanied by some sacrifice in conductivity. The above slide shows typical room temperature hardness after exposure to elevated temperatures for pure copper are selected RWMA material. This Chart also indicates the approximate softening temperature of these materials. The best choice of electrode materials for a given application is the material which has sufficient conductivity to prevent overheating and/or alloying of the electrode face with the work. It must also possess adequate strength to resist deformation or change under operating conditions. Electrodes in service are subjected to the heat of the welding process which has a marked effect on the hardness of the electrode material. It must be remembered that the surface temperature of the electrode, where it contacts the work, may exceed the annealing temperature for the material being used.During the early development of resistance welding, pure copper was the only electrode material available, but as the art progressed, welding current and forces increased. It soon became evident that superior electrode materials were necessary to keep pace with the rapidly developing art. The ideal electrode material for most applications would possess the compressive strength of tool steel and the electrical conductivity of silver, but no such material was available. This demand brought about the development of a series of electrode materials designed to overcome the shortcoming of pure copper. In fact, this is a continuing development as the state of the art advances with regard to processes and equipment. All of the RWMA recommended electrode materials have higher annealing or softening temperatures than pure copper, together with improved compressive strength and wear resistance which has necessarily been accompanied by some sacrifice in conductivity. The above slide shows typical room temperature hardness after exposure to elevated temperatures for pure copper are selected RWMA material. This Chart also indicates the approximate softening temperature of these materials. The best choice of electrode materials for a given application is the material which has sufficient conductivity to prevent overheating and/or alloying of the electrode face with the work. It must also possess adequate strength to resist deformation or change under operating conditions. Electrodes in service are subjected to the heat of the welding process which has a marked effect on the hardness of the electrode material. It must be remembered that the surface temperature of the electrode, where it contacts the work, may exceed the annealing temperature for the material being used.

    8. Electrode Geometry The basic electrode geometry is usually selected to improve the electrical-thermal-mechanical performance of the electrode. This is usually some geometry in which the cross-sectional area of the electrode increases rapidly with increasing distance from the workpiece. Truncated cone, bullet or A nose, and radius-faced electrodes are common, as shown in the above slide. The large increase in area, with increasing distance from the workpiece implicit in these electrode design, enhances cooling and current flow effects and provides a high degree of mechanical support for the electrode face. Also implicit in the electrode design is the diameter of the electrode contact area. This diameter must be considered carefully: too small an area will lead to sub-sized welds and insufficient weld strength; too large an area will lead to unstable and inconsistent weld growth characteristics.The basic electrode geometry is usually selected to improve the electrical-thermal-mechanical performance of the electrode. This is usually some geometry in which the cross-sectional area of the electrode increases rapidly with increasing distance from the workpiece. Truncated cone, bullet or A nose, and radius-faced electrodes are common, as shown in the above slide. The large increase in area, with increasing distance from the workpiece implicit in these electrode design, enhances cooling and current flow effects and provides a high degree of mechanical support for the electrode face. Also implicit in the electrode design is the diameter of the electrode contact area. This diameter must be considered carefully: too small an area will lead to sub-sized welds and insufficient weld strength; too large an area will lead to unstable and inconsistent weld growth characteristics.

    9. Electrode Size

    10. Spot Welding Electrode Nose Geometries

    13. Interchangeable Die Set Figure (a) shows a simple single spot welding die set mounted in a standard press welding machine. Figure (b) shows a standard projection welding machine provided with three interchangeable fixtures for welding three types of automotive truck running boards. These particular fixtures have their own pressure springs and current-carrying flexible bands in the upper part for each individual upper electrode. In the lower member, the electrodes which are stationary need no bands nor springs. Figure (c) shows a typical projection welding die set mounted on a standard press welding machine.Figure (a) shows a simple single spot welding die set mounted in a standard press welding machine. Figure (b) shows a standard projection welding machine provided with three interchangeable fixtures for welding three types of automotive truck running boards. These particular fixtures have their own pressure springs and current-carrying flexible bands in the upper part for each individual upper electrode. In the lower member, the electrodes which are stationary need no bands nor springs. Figure (c) shows a typical projection welding die set mounted on a standard press welding machine.

    14. Tooling for Multiple Welding Since it is necessary for multiple or parallel welding electrodes to have separate or individual pressure application, a single fixture for multiple welding is more complicated. While the preceding statement is true for spot welds, it is seldom true for projection welds. This is because the collapse of the various projections act as their own equalizer, both as to current and welding pressure or force. Several methods of providing this equalization are available, as follows: 1. Bar equalizer as shown in Figure (a). 2. A variation of this in which the bar equalizer is replaced by springs as shown in Figure (b). 3. Another variation is a grease or oil filled equalizer as shown in the following slide.Since it is necessary for multiple or parallel welding electrodes to have separate or individual pressure application, a single fixture for multiple welding is more complicated. While the preceding statement is true for spot welds, it is seldom true for projection welds. This is because the collapse of the various projections act as their own equalizer, both as to current and welding pressure or force. Several methods of providing this equalization are available, as follows: 1. Bar equalizer as shown in Figure (a). 2. A variation of this in which the bar equalizer is replaced by springs as shown in Figure (b). 3. Another variation is a grease or oil filled equalizer as shown in the following slide.

    16. Tooling for Multiple Welding (CONT.) Figure (a) shows a commonly used method of equalizing. Individual spring-loaded electrodes are mounted in a workholding fixture which is, in turn, mounted in a standard spot or projection machine. In the case of a grease or oil filled equalizer as shown in Figure (b), each electrode is attached to a piston operating in the bore of a common block or manifold. All the cavities are connected together, allowing free flow to all, thus providing equal pressure to each one. The principal disadvantages to this type include cost, and lack of individual adjustable electrode force. The spring-loaded type has the advantage of adjustable the force exerted by each electrode.Figure (a) shows a commonly used method of equalizing. Individual spring-loaded electrodes are mounted in a workholding fixture which is, in turn, mounted in a standard spot or projection machine. In the case of a grease or oil filled equalizer as shown in Figure (b), each electrode is attached to a piston operating in the bore of a common block or manifold. All the cavities are connected together, allowing free flow to all, thus providing equal pressure to each one. The principal disadvantages to this type include cost, and lack of individual adjustable electrode force. The spring-loaded type has the advantage of adjustable the force exerted by each electrode.

    17. Tooling for Multiple Welding (CONT.) In large multiple electrode machines, the same principle is carried out by the use of individual air or hydraulic guns where each gun is supplied from the same air or hydraulic source. The variety of workholding fixtures is almost endless, and very little generalization can be made. Some fixtures are merely workholding devices, as in Figure (a), in which the fixture merely holds the workpiece permitting manual indexing. Figure (b) shows a special fixture mounted on a 600 kVA press welding machine. This fixture clamps the web of a truck brake shoe with the table or shoe properly located over it. The machine is automatically cycled for about nine projection welds. After the last weld, the fixture opens and returns to the starting position permitting unloading and repeating the cycle.In large multiple electrode machines, the same principle is carried out by the use of individual air or hydraulic guns where each gun is supplied from the same air or hydraulic source. The variety of workholding fixtures is almost endless, and very little generalization can be made. Some fixtures are merely workholding devices, as in Figure (a), in which the fixture merely holds the workpiece permitting manual indexing. Figure (b) shows a special fixture mounted on a 600 kVA press welding machine. This fixture clamps the web of a truck brake shoe with the table or shoe properly located over it. The machine is automatically cycled for about nine projection welds. After the last weld, the fixture opens and returns to the starting position permitting unloading and repeating the cycle.

    18. Press Welding Machine Fixtures A example of a common application for projection welding would be welding a plate, washer or stamping to the end of a rod. Such jobs are best done in a standard press type welding machine with platens for mounting the dies and fixture. The rod is clamped in a hand or air-operated fixture mounted on the lower platen (preferably). The upper die or fixture is mounted on the upper platen. The stamping or plate to be welded is held in the upper fixture by detents or other means. The upper holding device must be self-releasing to permit the upper platen to return without the plate. Alternately, the plate may be placed on the lower platen and held by locators. In this case, it may be necessary to support the plate on springs to permit it to settle as the projection collapses. The clamping fixture on the lower platen holds the rod. It has replaceable dies (electrodes) and usually, where possible, a backup to take welding thrust. The above slide shows a typical fixture of this type mounted on a RWMA Size 3 press welding machine. A example of a common application for projection welding would be welding a plate, washer or stamping to the end of a rod. Such jobs are best done in a standard press type welding machine with platens for mounting the dies and fixture. The rod is clamped in a hand or air-operated fixture mounted on the lower platen (preferably). The upper die or fixture is mounted on the upper platen. The stamping or plate to be welded is held in the upper fixture by detents or other means. The upper holding device must be self-releasing to permit the upper platen to return without the plate. Alternately, the plate may be placed on the lower platen and held by locators. In this case, it may be necessary to support the plate on springs to permit it to settle as the projection collapses. The clamping fixture on the lower platen holds the rod. It has replaceable dies (electrodes) and usually, where possible, a backup to take welding thrust. The above slide shows a typical fixture of this type mounted on a RWMA Size 3 press welding machine.

    19. Workholding Devices: Dial Mechanisms Various types of workholding devices are readily adapted to standard press welding machines. Generally, the only change necessary is the substitution of the device for the standard lower knee. These workholding devices are of three general types (not including feeding devices) as follows: 1) dial mechanisms, 2) shuttle devices, and 3) chain or conveyor movements. Dial Mechanisms. This is the most commonly used workholding device, and consists of an indexing table containing from four to twelve sets of fixtures or tooling. Movement is imparted to the table by means of a Geneva or ratcheting drive. The number of stations on the table will depend on the size of the parts and the ease and method of loading. Series weld on dial tables are desirable wherever practical, as these simplify the secondary circuit by eliminating collector rings and brushes, contact shoes or current-carrying bearings. The dial table is synchronized with the welding. The most common control is a foot switch. The above slide shows a 75 kVA---RWMA Size 1 projection welding machine with an eight station Geneva dial table. Each station is provided with dual lower dies for series welding. The machine is also provided with a vibrating type of hopper feed with dual delivery tracks. Accurate positioning of the table is assured by an air-operated shot pin. Most dial table machines are provided with automatic unloaders. These may be operated by mechanical (or magnetic) fingers or by air blast.Various types of workholding devices are readily adapted to standard press welding machines. Generally, the only change necessary is the substitution of the device for the standard lower knee. These workholding devices are of three general types (not including feeding devices) as follows: 1) dial mechanisms, 2) shuttle devices, and 3) chain or conveyor movements. Dial Mechanisms. This is the most commonly used workholding device, and consists of an indexing table containing from four to twelve sets of fixtures or tooling. Movement is imparted to the table by means of a Geneva or ratcheting drive. The number of stations on the table will depend on the size of the parts and the ease and method of loading. Series weld on dial tables are desirable wherever practical, as these simplify the secondary circuit by eliminating collector rings and brushes, contact shoes or current-carrying bearings. The dial table is synchronized with the welding. The most common control is a foot switch. The above slide shows a 75 kVA---RWMA Size 1 projection welding machine with an eight station Geneva dial table. Each station is provided with dual lower dies for series welding. The machine is also provided with a vibrating type of hopper feed with dual delivery tracks. Accurate positioning of the table is assured by an air-operated shot pin. Most dial table machines are provided with automatic unloaders. These may be operated by mechanical (or magnetic) fingers or by air blast.

    20. Workholding Devices: Shuttle Devices Shuttle Devices. Large or bulky components are often loaded into a fixture outside of the welding station, shuttled into position for welding, then back for unloading and reloading, A double shuttle which may be loaded and unloaded on either side may also be used. The shuttling may be accomplished manually or by an air or hydraulic cylinder. Figure (a) shows a typical shuttle fixture for spot welding file cabinet drawers. The machine is a multiple spot machine with all welds being made simultaneously at one positioning of the fixture. Figure (b) is another interesting example of a shuttle fixture. The machine is a hydraulic multiple spot welding machine with one terminal of all transformers connected to a common anvil. The fixture reacts for loading the components, then shuttles into welding position and back again for unloading and reloading. Shuttle Devices. Large or bulky components are often loaded into a fixture outside of the welding station, shuttled into position for welding, then back for unloading and reloading, A double shuttle which may be loaded and unloaded on either side may also be used. The shuttling may be accomplished manually or by an air or hydraulic cylinder. Figure (a) shows a typical shuttle fixture for spot welding file cabinet drawers. The machine is a multiple spot machine with all welds being made simultaneously at one positioning of the fixture. Figure (b) is another interesting example of a shuttle fixture. The machine is a hydraulic multiple spot welding machine with one terminal of all transformers connected to a common anvil. The fixture reacts for loading the components, then shuttles into welding position and back again for unloading and reloading.

    21. Workholding Devices: Conveyor Type Fixtures Conveyor Type Fixtures. Another type of loading or positioning device is some type of conveyor, usually a chain. This usually takes the form of a continuous device containing a loading position, welding position and unloading position. In addition, there may be more than one welding station, or one or several mechanical operations, such as riveting, staking, milling, etc., which may be incorporated in the machine. The above slide shows a machine for welding terminals to electric coil leads, then in a second station, crimping the terminal. The coil is loaded in a chain conveyor, and the terminal is fed from a hopper. The conveyor moves the coil from the welding position to the staking position, then ejects.Conveyor Type Fixtures. Another type of loading or positioning device is some type of conveyor, usually a chain. This usually takes the form of a continuous device containing a loading position, welding position and unloading position. In addition, there may be more than one welding station, or one or several mechanical operations, such as riveting, staking, milling, etc., which may be incorporated in the machine. The above slide shows a machine for welding terminals to electric coil leads, then in a second station, crimping the terminal. The coil is loaded in a chain conveyor, and the terminal is fed from a hopper. The conveyor moves the coil from the welding position to the staking position, then ejects.

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