Document G68gr3xEE0b0emGp7kpezr2jV

WELDING AND CUTTING the welding machine Permanent injuries, and several fatalities have been caused by neglecting to use the hne-disconnecbng switch before making adjustments The use of single-pole primary circuit breakers and electronic contactors which leave one line to the welder "hot" makes this precaution imperative Current and tuning 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 in 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 weldmg does not permit the use of plugs except for control circuits, usually 120 volts or less Power lead terminals should be securely and permanently attached with screws or bolts for the durabon of service The magnitude of the secondary welding current is such that the cables for a porta ble weldmg machine must be heavy Even with sues from 200,000 to 1,000,000 circular mils, copper losses are high and m many in stances water cooling is required, especially for high production portable, as well as mulbple-spot station, weldmg machines The length of these cables vanes from 1 to 14 ft, the latter beuig the maximum considered reasonably economical The covering for the cable used with a portable weldmg machine is thick, high tensile rubber. Abuse of the cables for resistance weld ing is severe The producbon requirements demand the utmost of the cable materials used, and even the best cables need frequent replacement during a producbon year The cables are subjected to electrical pulsation, bending, and twisting Electncal pulses at the rate of 3,600 per minute cause a con tinuous whipping of the two cables which leads to fatigue and eventual breakdown. Strapping will check the movement of the cables and will prevent excessive wear, kicking of the operator, and unnecessary muscular strain The secondary voltage presents little shock hazard, since the maxi mum voltage is about 30 volts, but the op erator can be hurt by a cable blowout caused by steam pressure from faulty water cooling curculabon A periodic check for weak spots in the cable covering is good pracbce. The use of concentric weldmg cables is advised because they do not have the undesirable features of the pulasbng cables Portable weldmg machines, including the cables, should have proper weight balance to permit operabon without undue strain to the operator Electrodes and holders For resistance weldmg, copper or copper alloy electrodes are in general use Made in various shapes for particular apphcabons, they are, with few excepbons, water-cooled Resistance-welding electrodes, unlike those used in arc welding, are not deposited on the work. However, the high current densi ty and continuous apphcabcm of mechanical force finally cause wear and necessitate re dressing and replacement Electrodes are placed in holders connect ed to the secondary circuit The weldmg pressure is also applied through the holders Their designs cover a mulbtude of apphca bons and vary considerably with the weld mg methods used Electrode holders aod electrodes should be water-cooled to mini mize softening of the metal from heat and to reduce the chance of heat bums to the operator Machine installation Installabon of resistance welding equip ment should conform to the Nattonal Elec tncal Safety Code and the Nattonal Elec tncal Code. Some items worthy of special attenbon are listed below. 1 The frame cabinets for welding machines and for auxiliary equipment should be grounded The ground wire should be omitted, however, on AC power sources that are grounded or where the manu facturer does not recommend grounding Where water-cooling is provided, the weldmg machine is considered adequately grounded when electrically connected to water pipes. 2 Control circuits should operate on low 29-14