The process of welding by the resistance method is the simplest of all methods of joining metals, and is also the quickest and cheapest method for work which may be easily handled. While it is true that it is limited almost exclusively to new work and to articles of moderate and small size, it is possible to do work in such quantity and of such variety that this system is used in numerous lines of production. The operation consists mainly in clamping the pieces in the machine, passing the current through the point of contact until it heats, and then squeezing the pieces together until they join. At the proper temperature, the metal will be in a plastic state and the molecules of the two pieces will amalgamate, or mix, so as to unite and form one piece. If the pieces are of different metals, the result will be an alloy of the two metals at the joint. When the nature of the metal is such that it is injured at high temperatures - as, for instance, brass or tool steel - it should be heated quickly and pushed together hard enough to squeeze the burnt metal out of the weld. Experience will soon show the correct heat for the best results on each metal, but the makers of the various welders will be glad to state the proper current, time, and temperature for any sort of work.
Butt welding, as its name indicates, is the operation of welding two pieces of metal end to end or side to side, and spot welding is the operation of welding two pieces by lapping them and welding them in spots only, Fig. 100, instead of full width as with butt welding. Numerous modifications of both these processes have been developed for special shapes and sizes of pieces, among which may be mentioned lap welding, tee welding, cross welding, seam welding, upsetting, jump welding, annealing, and brazing. The two latter applications of resistance-welding machines are not welding in the true sense but are practical applications of the apparatus, which should be understood. Hardening and tempering may also be done through the use of this apparatus and rivet heads, heated ready for shaping, can easily be formed by pressure. light or small work can be done on large machines, of course, but not very economically, and large work can be done on small machines but it is a dangerous practice on account of the excessive current required and should be discouraged.
Fig. 100. Examples of Spot-Welded Courtesy of Toledo Electric Welder.
Butt welding is applicable to welding metals of practically the same cross section by bringing them together end to end or edge to edge; all of the energy passing through the joint is effective because it is confined to a limited area of contact. A slight projection or fin will be raised at the joint, Fig. 88, due to the flowing of the soft metal, but this is easily removed.
Spot welding is the operation of joining sheets by heating and softening the metal in spots only, each about the size of a rivet, and applying pressure while the metal is plastic.
The operation causes a slight thinning of the metal at the weld, No. 3, Fig. 89, due to the pressure, but when it is properly done the joint should be as strong as the rest of the sheet.
Lap welding consists in making a joint by overly lapping the edges of the sheets to be welded, heating the joint, and applying pressure by means of rolls which pass along the seam. This makes a continuous seam, instead of merely a series of spots, and gives a tighter and stronger joint than spot welding. It is used to a lesser degree than either of the other methods, but is coming into more general use for special purposes such as welding-in barrel heads, making grease cans, oil tanks, etc. Seam welding is a similar operation. Butt Seam Welding. Butt seam welding is an operation similar to ordinary butt welding in one respect and like lap welding in another. It is used for making the seam in steel plate articles of moderate thicknesses, such as range boilers; it is done in a machine which brings the edges of the plates together and does the welding by pressure while hot, in the usual manner, Fig. 101. The special feature of the machine lies in the fact that the current is carried into the plates and across the seam by means of rolls each side of the joint. As the work progresses through the machine, the rolls keep the current always passing across the joint just ahead of where the pressure is applied.
Fig. 101. Electric Welding Machine (or Tubes with Tube in Position.
Tee welding is the process of making a weld in the shape of a "T" by welding one piece to the side of another. The peculiar feature of this process is that the head of the tee must be heated first in order that the shank may not be burned before the parts are both soft enough to weld.
Jump welding is similar to tee welding and is used on light stock which does not require preheating before welding. This process is also used for pipe work, when forming tees or welding branches into larger pipes, and the hole in the header or main pipe should be made before welding the branch.
Cross welding is done when forming wires or bars into screens or when making any other article requiring the crossing of strips. The pieces are merely laid together and a welded joint formed at the crossing in either a butt-or a spot-welding machine. The pieces will flatten at the joint, of course.
Upsetting is the operation of forming an enlarged section on a bar for the purpose of increasing its strength or for reforming it to another shape. The bar is placed between the jaws of a butt welder, the current passed through the space to be upset, and pressure applied when the bar heats. The pressure will squeeze the metal up so that the bar will expand, after which it may be hammered into the desired shape while hot. This process must be distinguished from ordinary butt welding in which the joint swells and shows an "upset". Deliberate "upsetting" is usually carried much further. A "flash weld" is a similar operation and is used on wide stock or on rectangular pieces and when ever the ends of the stock cannot be trimmed square. It consists in squeezing the metal so hard and rapidly while soft that it almost "squirts" apart and forms feathery fins around the joint which must be ground off. It is used when welding brass and copper and an amount equal to the diameter or thickness of the material is taken up in the weld.
Electric annealing can be done with either a butt welder or a spot wilder and consists in passing the current through the part to be annealed, heating it until soft, and allowing it to cool slowly. Hardened steel plates, springs, dies, tools, chilled rolls, etc., may be treated this way and may then be drilled or cut very readily. When much of this work is to be done, it is better to have a special machine made for the article because it will reduce the cost.
Hardening and tempering can also be done by heating the pieces in a butt welding machine and, when the desired color is reached, chilling in the usual manner. The advantage in this process lies in the fact that the work is always in sight.
Electric brazing can be done quickly by placing the parts in a butt- or seam-welding machine, heating the joint, and then applying the spelter and flux, allowing them to run into the joint. The temperature can be controlled more easily than by any other process and the work is always in sight. It would seem, however, that welding would be preferable to brazing.
Electric riveting is another recent variation of the spot-welding process and consists in making the holes in the pieces, inserting the rivets, heating them between the tips of a spot welder, and then pressing them to form heads while soft. It is very quickly done and eliminates the rivet heating furnace; for heavy work it is good, but is more expensive than straight spot welding for thin plates.
When doing either butt or spot welding or any of their variations, it is important to see that the surfaces are cleaned thoroughly before starting to weld them because the presence of grease, dirt, or other matter between the surfaces will prevent a perfect joint. The cleaner and better the stock the easier it is to weld, the less current it takes, and the less wear on the dies. Dirt, grease, and scale are insulators, in most cases, and it takes only a small amount, at the low voltage used, to prevent the flow of current; if there is any undue heating in any part of the machine where there, is a joint in the circuit, it should be carefully examined for dirt and grease and then cleaned. Bolts frequently work loose and, allowing oil to carry dirt under their heads, cause heating. There is no danger of a shock from the welding circuit because of the low voltage but the line, or primary side of the machine, should be avoided, if possible or handled with proper precaution.
Fig. 103. Spot Welder Showing Simple Construction Courtesy of Geuder. Poeschke and Prey Company.
If the machine has been properly installed, there should be no trouble so long as the cooling water flows through the welding dies, Fig. 102, and everything is kept clean and all connections tight. The only moving parts on most contact welders are the clamps and jaws, Fig. 103, and these are easily watched; the transformer is so simple it should never get out of order except in cases of accident. The switches should be enclosed to prevent accidental contact and ordinarily are automatic and out of reach.