Document 58E10JqkGjekgErLRo3zVbX5

Types A SPOT weld is made by applying heat and pressure at one point, usually on overlapped parts A SEAM weld is a senes of overlapping spot welds, spaced close enough to make a single, continuous joint A projection weld is a spot weld made between two parts, one or both of which have an embossment or projection, which localize the welding heat and pressure Flash and upset welds are welds made by butting two parts together and forming a weld for the full cross-section of the con tacting faces In flash welding, pressure is applied after heating is substantially com pleted In upset welding, pressure is main tained throughout the welding cycle Percussion welding is similar to flash and upset welding except that the welding heat is obtained from a rapid discharge of stored electrical energy Resistance welding machines are operated manually, semiautomatically, and auto matically (Fig 31-5) Fixed installation is most common, but portable machines are also available They use manual, electrical, me chanical, pneumatic, or hydraulic power to apply pressure They are often equipped with intricate control and timing devices Modem mechanical handling devices for the work add to their efficiency Expert main tenance is required for these welding matchines, even though they may be operated by unskilled labor Power supply Resistance welding usually employs 60Hertz alternating current which is fed to the primary of the water-cooled welding trans former The input can vary from 150 to 10,000 amp, at 240, 440, or 550 volts The output, at the secondary of the transformer is a low voltage (max 30 volts) and high current (up to 200,000 amp) used for welding The welding current is sometimes fur nished by the "stored energy" type of equip ment, m which energy is built up and stored either in capacitors or in a combination trans former-reactor during the nonwelding period, and then is discharged to form the weld This process involves low currents and high volt ages, which must be guarded against To facilitate servicing the equipment, a safety type disconnecting switch or a circuit breaker of the correct rating for opening supply circuits should be installed near the welding machine Permanent injuries, and several fatalities have been caused by neg lecting to use the line-disconnecting switch before making adjustments The use of single-pole primary circuit breakers and elec tronic contactors which leave one line to the welder "hot" makes this precaution im perative Current and timing controls are of two types, mechanical and electronic If worn parts throw the mechanical control out of synchronism with the current and voltage waves, transients may cause severe damage to the equipment Synchronism should be carefully maintained, particularly on highpower equipment The electronic controls are placed m the primary circuit and involve high voltages which must be guarded by proper insulation, lockouts, and safety switches All electrical work should conform to ap plicable codes Cables The high current of the primary circuits (up to 10,000 amp) in resistance welding does not permit the use of plugs except for control circuits, usually 120 volts or less Power lead terminals should be securely and perma nently attached with screws or bolts for the duration of service The magnitude of the secondary welding current is such that the cables for a portable welding machine must be heavy Even with sizes from 200,000 to 1,000,000 circular mils, copper losses are high and in many instances water cooling is required, especially for high production portable, as well as multiple-spot station, welding machines The length of these cables vanes from 1 to 14 ft, the latter being the maximum considered reasonably economical The covenng for the cable used with a portable welding machine is thick, high tensile rubber Abuse of the cables for resistance welding is severe The production requirements de mand the utmost of the cable matenals used, and even the best cables need frequent re- 931