SPOT AND ARC WELDING

BY H. A HORNOR

J. B. LIPPINCOTT COMPANY , 1920
 

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PREFACE



CONTENTS

I. Materials
II. Electric Welding Systems
III. Spot Welding
IV. Demonstration of Heavy Spot Welding
V. General Applications of Arc Welding
VI. Discussions on Arc Welding
VII. The Arc Welder
VIII. The All-Welded Ship
IX. Theories of Electric Welding 178


SPOT WELDING

There are four important points to be considered in either light or heavy spot welding: (1) The electric current requisite for producing the welding temperature; (2) the time in which this current is utilized for making the weld; (3) the mechanical pressure necessary for the electrical contact as well as for squeezing the materials during the application of heat; (4) the condition of both surfaces of the two pieces of material that are to be joined. In the spot welding of thin (or light) sheet steel apparently the wearing away of the electrode points is not of prime importance, but in heavy spot welding this feature becomes a serious matter when viewed from a shop production standpoint. This point of difference as well as others will be set forth when the results of the demonstration of heavy spot welding are discussed. In general a heavier current must be employed for the welding of greater thicknesses of steel plates, more time must be consumed, the pressure must be increased, although to what degree is questionable, and the contact surfaces, i.e., those next to the electrode as well as those impinging upon each other, must receive the attention of the engineer responsible for the results.

Apparatus for Light Spot Welding - For this class of work the design of machine may take various forms in conformity with the special nature of the product. For the process alternating electric current is employed because a high current at a low voltage is needed to produce the welding heat. The usual voltage supplied to shops in this country for power purposes is 220, and this requires a transformer usually integral with the machine. In smaller-sized machines the pressure may be secured by levers operated by hand or foot; in larger machines the practice is to employ either water or air pressure. In like manner small apparatus may require no water-cooling arrangement for the copper electrodes, but in larger machines this is essential. The lighter machines may also permit of the offsetting of the electrode for performing in close quarters or accomplishing some special object, but in the heavier designs the pressure must come directly in line with the electrodes. As will be seen in Fig. 1, the secondary of the transformer which in this case is composed of thin copper strips, conducts the induced current from the transformer to the electrodes. This machine is used for plate work and is designed with the necessary gap. The electrical connections are arranged so that adjustments for current may be made for varying conditions of work. Automatic features may be included in the design, so that repetition work can be done with great rapidity and with uniform results. Fig. 2 illustrates another type of spot-welding machine having a capacity of 25 to 30kw and capable of doing fairly heavy spot welding. This machine has a small gap and could not be used for extending over wide plates. It is to be noted that the pressure operates in a direct line through the electrodes.

Applications of Light Spot Welding - As the pressure required for light sheet steel (say 1/16 inch thick) is approximately very low, about 200 to 300 pounds, the application of this method to the fabrication of small articles is very large. As noted, the electrodes may be placed in various positions and offset in the electrode holder in any manner required by the special job. Thus kitchen utensils, like coffee pots, saucepans, etc., are made by spot welding the spout on to the body, thus facilitating the finishing operation by leaving a smooth surface. House fittings, such as doorknobs, sash pulleys, etc., are made by the thousand in a very short time. Small chains for various purposes are made on special and very interesting machines, and the use of this process in the bicycle and automobile industries has been responsible for decreased cost. Many special applications are of interest, such as the welding of the two magnet bars in a telephone receiver. The difficulty of doing this by other methods of welding is that the temper of the magnet steel would be drawn and so destroy the purpose for which it was intended.

Possibilities of Light Spot Welding - Many operations not strictly spot welding may be performed on a light spot-welding machine. By preparing the materials in a special way, by the use of a button placed on the materials, by means of special electrode, many small articles can be easily and quickly welded. Sheets may be welded to studs, bolt heads to body in a spot-welding lyiachine, although the operation bears a strong likeness to butt welding. In the same manner screws may be welded to sheet tubing. The spot-Welding machine may be utilized to heat rivets in place and squeeze them "home," thus performing as an automatic riveting machine. A wrongly punched hole may be corrected by introducing a proper-sized stud and then spot welding it in place. This also may be done with heavy spot-welding machines. By arranging the edge of thin sheets with projections which act to localize the heat, and with special electrodes or multiple electrodes, a number of spot welds may be made in one operation. It is stated that there are machines made with a solid electrode of copper against which a single electrode is made to move at designed intervals. This apparatus is capable of making thousands of spots a minute and connecting thin sheets. It only requires a fraction of time to make a spot in such materials. It is not believed that all the possibilities for light spot welding are by any means exhausted and undoubtedly the introduction of heavy spot welding will result in the further extension of spot welding in general.

Possibilities of Spot Welding in Ship Construction - Although spot welding was applied in the industries only for steel plates not exceeding  inch in thickness, the results had led many engineers to a belief in its extension to greater thicknesses. So in 1911 the American Car & Foundry Company built a portable spot-welding machine with a gap of 66 inches, so as to extend across wide plates and with which they constructed a gondola freight car.

In view of this wide variety of work that could be done by spot welding and the large saving to be secured by its use led those who were making this investigation to extend their trials to the building of a water-tight bulkhead door. Fig. 4 shows the spot welding of the 3-in. X 3-in. X 5/16-in. angle frame to the door plate. In the endeavor to secure water-tightness with a great number of spots and the use of flux about 141 spots were made. It was noted that a smaller number of spots and the edges of the angle arc welded would give more satisfactory results. After the door was completed it was placed with the edges of the angle upon a steel plate, securely clamped and made water-tight by the customary methods used on shipboard. A water connection was then made and pressure gradually applied.

As has been intimated there are two distinct conditions under which riveting is done in shipbuilding: that which secures the principal members of the hull structure called " field riveting," and accomplished in this country by a group of men in which one man uses an air-driven rivet hammer, and that which is done in the shops by semi-portable machines. These latter machines are of large size, usually suspended from jib-cranes, and are portable in the sense that they may be brought to the work and moved from rivet to rivet by the operator. These portable riveting machines are equipped with air cylinders and connected by flexible hose to a common source of supply. They are as in field riveting dependent upon the forge fire for the heating of the rivet, and on the rivet boy for supplying the rivets at a correct temperature and at the proper time. This practice in ship-building shops differs from that in bridge shops where the machines are stationary, usually mounted with their jaws in a vertical position and to which the overhead crane brings the material and moves it for the continuous performance of riveting. The rapidity with which such work is accomplished in this latter case is astonishing. A close study of this problem is like the run of spot-welded work, merely requiring as much time and attention devoted by those who understand the process as has been given by those who understand riveting. In short, it is a question of shop production which in the case of riveting has been developed to a high state of efficiency.

All these points and many more suggested that spot welding machines similar to a pneumatic riveting machine could be employed with advantage in shipbuilding and would hasten the construction of steel ships. The idea was proposed to convert the pneumatic riveting machine as now used into spot-welding machines by adding a transformer and the proper flexible connections. This was the inception of the design of two machines for spot welding which were built for the Emergency Fleet Corporation of the U. S. Shipping Board and with which a practical demonstration was made the early part of last year. The builders of this apparatus had previously experimented with a stationary apparatus of large size to determine if there were any obstructions to the welding of thick steel plates, and as their researches showed no hindrance to the process up to the spot welding of three thicknesses of 1-inch boiler plate, they accepted an order for two portable spot welders and one stationary spot welder for the purpose of fabricating the steel parts for the hull structure of ships/ This apparatus will be fully described in connection with the discussion in the next chapter.


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