The clamps vary in design in different types of machines. Figures 16 and 19 show clamps for various work. They are generally of heavy copper, to allow for the passage of the large volume of current. The clamps are not rigid in a welding machine, but are pivoted or mounted on a straight sliding groove, and are made to move toward each other by a lever, wheel-and-screw, or by hydraulic pressure. Recently the hydraulic pressure has been made automatic, so that the machine will throw on its own current as soon as the pieces to be welded are clamped, will squeeze the pieces together at the temperature of plasticity, and will throw off the current at the same time.

Wheels. Approximate weight 7,500 pounds.

Fig. 20. - Thomson special machine for welding hubs and spokes in agricultural squeeze the pieces together at the temperature of plasticity, and will throw off the current at the same time.

Where heavy bars are to be welded, and a good electrical contact is needed at the clamps, the clamps are operated hydraulically.

As will be guessed, the clamps are liable to get very hot,

Representative Thomson Welders

Representative Thomson Welders - Continued

Copper wire B. & S. gauge. Iron and steel wire W. & M. gauge.

Right angles § of sizes of rounds and flats can be welded on Nos. 16 and 17 welders, also disks up to 2½ x ⅜ on No. 16 and 4 x \ on No. 17 welders, not less than 10 inches in diameter.

Squares and ovals can be welded in all welders equal in sections to rounds and flats, except No. 10 welder in which the limit is ¾ x 7/16 oval.

Reactive coils are not required with the following welders numbered 1, 4, 5, 9, 10, 12, 13, 16, 19, 20, 21. When only the maximum sizes of stock are to be welded, no reactive coil is required with any welder; but when the minimum sizes, or all sizes within the capacity of the machine are to be welded, a reactive coil is required: Type 2 EE with welders numbered 2, 3, 6, 7, 8; Type 4 EE with welder numbered 11; Type 10 EE with welders numbered 14, 15, 17, 18; and Type 20 EE with welder numbered 22. When less than 220 volts is used, or when the welder is to be operated to its utmost capacity in output on maximum size of work, the next size reactive coil is recommended.

Weight Of Reactive Coils

An alternating current is absolutely required; a direct current cannot be used. The welders are built for 50 cycles, but up to and including 5A they are sometimes used on a circuit as high as 125 cycles, but never lower than 50 cycles. Welders larger than 5A can be used to the best advantage on circuits between 40 and 60 cycles.

Any voltage from 100 to 350 can be used for the smaller welders, and from 200 to 350 for the larger; but it must be constant and not drop during the time of heating the stock.

Especially on continuous runs of heavy work. As heat affects the conductivity of the clamps, they are often water-cooled.

Some of the automatic machines are equipped with swages working between the electrodes. These swages are brought together on the weld immediately after the hot metal is upset. They compress the upset or bur and give the weld any desired shape. Besides which, their pressure amounts to working of the metal and makes the weld much stronger.

Fig. 21. - Thomson type 5 AA electric welder for copper wire from No. 6 to 1-4 inch. Time in heating from 2 to 6 seconds. 7500 Watt alternating current, not lower than 50 cycles, from 100 to 350 volts.

Practice

The operation of the Thomson electric welder is very simple, calling for much less skill than the hot-flame processes. For stock welds the current is first calculated for the size of the piece to be welded and the kind of metal in the piece. Tables of current value, cross-section, and time have been worked out for iron, steel, brass, and copper (see page 52). The machine being adjusted for stock welds will handle them rapidly without readjustment, much the same as a printing press will run off many impressions of the same form.

The operator places his pieces in the clamps, closes the clamps, and advances the clamps toward each other until the two pieces touch closely. He turns on the current and as soon as the pieces, become plastic or semi-molten at the point of contact he turns off the current and squeezes the clamps toward each other until the pieces become welded and upset at the contact.

Figure 21 shows a semi-automatic machine. The copper contacts, carrying the clamping device, move to and from each other: the left is moved by a screw to get the required opening between clamps; the right is held apart by the lever. The wire is inserted and tightly held in the clamps, the side lever raised, the circuit closed through the hand-automatic break-switch in the base of the welder, and the pieces, instantly heating at the joint, are forced together by the weights; the circuit being automatically opened by the adjustable cut-out device.

In making the joint, the metal is upset, the extent of which depends largely upon the weights and the adjustment of the cutout device.