202 THE APPLICATION OF ELECTRIC WELDING 



terial to the exclusion of marking off, punching, reaming, countersinking and rivet- 

 ing in the fabrication shops, and even on the ship itself, is obvious. It is also patent 

 that if the quantitative values of the above variables necessary to produce a perfect 

 weld are definitely determined for all thicknesses of material, the operation becomes 

 not only entirely mechanical but also uniformly efficient. 



Intensive research work is now being conducted along these lines by the General 

 Electric Company, which has built a heavy spot welding machine with a capacity of 

 100,000 amperes at 20 volts and a hydraulic pressure of 36 tons at the electrodes. 



Owing to the necessity of providing a gap large enough to deal with the width 

 of plates necessary in ship work, a number of practical difficulties were encountered 

 in the design of the machine. One important development in this connection has 

 been the elimination of the variable reactive drop caused by the varying inductance 

 created in the circuit due to the width of plate within the gap. This necessitated a 

 change in voltage for different widths, but by using two sets of electrodes and two 

 transformers, one on each side of the work with leads direct to the electrode on the 

 same side, the secondary windings of the transformers are in series and the effect 

 of varying widths of plates is eliminated. Moreover, the total reactive drop is 

 greatly reduced, which permits of a corresponding reduction in the size and weight 

 of transformer and generating apparatus. With the two sets of electrodes two spots 

 are welded at a time, or a dummy can be placed between the second pair of electrodes 

 if necessary. 



A careful series of experiments is being carried out with this machine, using 

 plates of from % inch to 3 inches total thickness, and that satisfactory spot welds 

 can be made within this range has already been demonstrated. 



It has been found that an appreciable range is permissible in the variables of 

 current, pressure and time without seriously impairing the efficiency of the weld for 

 a given thickness of material. 



Where considerable tensile pull is anticipated in an over-lap joint, it is consid- 

 ered desirable to have a double row of spots to prevent the tendency to bend and 

 tear out the spots in the same manner as the countersunk heads of rivets are some- 

 times torn through the holes in a single riveted over-lap. 



Plates 130 and 131 are illustrations of joints made during these experiments, 

 which, at the time of writing, are still proceeding with the probability of obtain- 

 ing definite and comprehensive data at a very early date. 



The 42-foot section of a 9,600-ton vessel to be built at the plant of the Federal 

 Shipbuilding Company, Kearny, N. J., and to which reference will be made later, 

 will be used to demonstrate the practicability or otherwise of this method of welding 

 as applied to the assembly of material in ship construction. 



A stationary spot welder will be used in the fabrication of the parts, and a large 

 60-inch gap portable spot welder is being completed which will be used both for 

 clamping the parts together in the field and spot welding them in place. This ma- 

 chine is illustrated in Fig. i, Plate 132. 



It will be seen, therefore, that although this method of welding has not yet 



