Characteristics And Development Of The Process

The use of the electric current to heat metals to the welding point, by passing the current through the joint until the metal becomes plastic and then applying sufficient pressure to cause the pieces to adhere, was first proposed by Elihu Thomson in 1877; and the present day process of "resistance welding", in its various forms, is the result of his work. The process is based upon the phenomenon that a poor conductor of electric current will heat if current is forced through it, or that a good conductor will also heat if enough current is passed through it, and that the heating effect will be greater if alternating current is used than if direct current is used. Since an imperfect joint between two pieces of metal is a poor conductor and offers resistance to the passage of current, it will naturally heat and finally cause the metal to soften sufficiently to weld. In practice the operaing" 89, and this is today one of the principal uses of the system. Later a modification of the system was made in order that pieces of sheet tion is very rapid because comparatively large amounts of current are used and heavy pressures are applied.

Fig. 88. Welded Ban Showing Upeet.

Courtesy of Toledo Electric Welder Company.

Originally this process was used to weld bars and strips together, end to end, Fig. 88, performing the operation known as "butt weldsteel could be welded; this resulted in the development of the operation known as "spot welding", by means of which lapped joints can be made, Fig. 89, particularly for thin material. Heavy plates cannot be spot welded so readily as thin ones but they are frequently welded edge to edge, forming a butt weld, or a combination butt and lap weld, as in Fig. 90. Special machines have been developed by the various manufacturers for numerous operations and for a wide range of articles, a few of which are shown in subsequent illustrations, and the work done with them is of the highest class. Practically every kind of metal can be welded by this process and many different kinds may be joined to each other, as shown in Table X. The great advantage of this process lies in the amount of work which can be done in a short time, but it is limited almost exclusively to the production of new articles instead of being also good for repair work like the arc system.

Fig. 89.

Fig. 90. Single Strap Butt Joint, Spot Welded Courtesy of ToIedo Electric Wilder Company.

Table X. Metals, Alloys, And Combinations Of Different Metals Actually Welded By The Thomson Process

From a study of Table X it will be seen that practically all of the commercial combinations of metals can be made with butt or spot welding apparatus and, it may be added, there is no other system in use today that will do welding on as wide a range of metals, alloys, or combinations. This system operates on a very low voltage - about 3 volts - and the important factor, as with arc welding, is the amount of current.

Equipment Required

Butt or spot welding requires practically a separate machine for each class of work to be done, these machines consisting of a main frame, Figs. 91 and 92, containing a transformer and some means for clamping the article to be welded, together with a device for applying the pressure required to force the parts together when heated. Unless the machine is designed for one special sort of articles, it is necessary to have a reactive coil to adjust the current to suit the work and a switch to control this coil. As with any other electrical device, a main switch for connecting the welder to the power circuit is necessary, of course, and the larger sizes of machines are water cooled.

The illustrations scattered through the text of this section show the more important types of machines, and the pipes for carrying the cooling water to the copper contacts are clearly shown on most of them. These machines are built for welding pieces with cross sections as small as fine wires, Fig. 93, and as large as 7 or 8 square inches in section and may require as much as 200 horsepower for large work. Butt- or spot-welding machines can be operated from any single-phase power circuit supplying current .at a constant voltage by providing the proper transformer, but direct current cannot be used.

Butt welders are comparatively low machines, Fig. 94, and have the clamps for the work on top, generally in the form of jaws with a lever for operating each pair, Fig. 95, and another lever or a hydraulic cylinder to bring the pieces together and to apply the required pressure when properly heated. The current is carried into the pieces through the jaws and is usually turned on autorustically after the parts are clamped into position. Foot levers are also provided on some forms of machines, Fig. 96, for clamping in order to leave the workman's hands free to handle the work. Spot welders, Figs. 97 and 98, are usually higher but smaller and have a pair of arms, Fig. 99, extending to one side for carrying the two welding tips or contacts; the pieces are laid together and placed on the lower contact and the upper one is forced down against it. The current is automatically switched on, when the contacts close, and the pressure is applied by a hand or a foot lever. Special machines are also made for rail welding, etc. (See Figs. 104 and 107).

Fig. 93. Simples of Butt Welding Courtesy of Toledo Electric Welder Company.