Oxygen generators are much more elaborate devices than acetylene generators and the methods of action of the large types have already been given on pp. 98 and 99. Most of the small plants, whose details may be easily understood, are for making oxygen from chlorate of potassium and consist of a generator, washer, gasometer, and compressor. The generator for the oxygen consists of a metal retort and a gas burner for heating it and the chlorate is mixed with a little manganese dioxide and heated in the retort. The vapor from this is usually carried through three scrubbers, consisting of barrels filled with a solution of sodium hydroxide, thence into a gasometer or tank to be stored until required. From the gasometer the oxygen is carried to a compressor, usually two-stage and there compressed for filling the cylinders at 300 pounds pressure per square inch. Oxygen of high purity can also be generated by wetting sodium peroxide; small outfits of this type have been put on the market under the name of "Oxone".
Fig. 114. Medium Welding Torch Courtesy of Davis-Bournonville Company.
The torch is the next item of importance in any good acetylene welding outfit and upon the development of this device alone depended the success of the oxy-acetylene process to a large extent. It has taken years to bring torches to their present state for the flame is very hot and the gases are highly explosive, yet they must be mixed and controlled accurately. The oxy-acetylene torch was probably invented by Fouche and was a high pressure device; so it was comparatively easy to get a good mixture but, later, it became necessary to develop a torch for low pressure work and it proved to be a difficult matter. It was done, however, and there are now three styles in use: the original high pressure torch, the medium or positive pressure torch, Fig. 114, and the low pressure torch. There is also a special torch for cutting with an extra oxygen feed in addition to the regular flame feed. The gases are regulated by cocks at the handle, and wire gauze is placed in the passages to prevent flashing back in case of too low pressure, somewhat on the principle of the miner's lamp. Torches are made in several sizes and have a series of removable tips to provide for various sized flames. (Joggles for the eyes and gloves for the hands of the operator are necessary for his protection.
A recent development in connection with oxy-acetylene apparatus is the use of special cutting machines and welding machines which work automatically and displace hand welding and cutting. An attendant is necessary, of course, to see that the material is properly placed but the machine does such a high quality of work at such a uniform rate that it can turn out work of a simple nature more quickly and more cheaply than when done by hand. With this machine any irregular pattern may be cut quickly and accurately and it will cut steel 3 inches thick at the rate of 6 inches per minute. The welding machines are especially valuable for work on pipes, barrels, cylinders, cans, and other articles which are all alike, and some very efficient special machines have been built for such service. These automatic machines are not of any value for repair work, to be sure, and do not warrant the cost for any other than repetition or straight work. The torch is carried on an arm which is moved by the mechanism of the machine.
The process of welding with the acetylene flame is similar to that of using the graphite electrode in electric-arc welding, as the flame is the source of heat and the filling material must be added as melted, but it has the disadvantage of being an open flame, which presents a certain element of danger. The first thing the operatorhas to do is to learn how to adjust his flame, and this is not easy because there is no rule for the exact proportion of oxygen and acetylene. It is approximately 1 part acetylene to 1.5 partsoxygen for most purposes. If the oxygen is as great as 2.5 against 1 of acetylene, an oxidizing flame will be produced which will probably cut the metal; if there is too much acetylene to be all consumed in the flame, it will split up and allow carbon to enter the weld and carbonize it. The flame should be so adjusted that the two cones formed in the flame unite into a single small one. In operation, the tip of the white cone in the flame should just touch the metal and the hand should be held steady because, if the tip of the torch should touch the work, it will cause a flash back and necessitate relighting, if nothing worse. The torch should be given a sort of rotary motion around over the surface of the weld, with a slight forward and upward movement, in order to blend the metal and reduce the liability to overheat it.
All welding operations, whether with gas or electricity, should be undertaken only after a careful consideration of the effects of expansion and contraction on both the joint and the piece welded. This is especially important when welding castings, and even more so with the oxy-acetylene system than with the electric-arc system, because of the necessity for heating a comparatively large surface around the weld.
Cast-iron pieces and articles of circular or closed shapes, such as wheels, should be preheated before welding and reheated afterwards to relieve any stresses which may be set up in them. Gas furnaces or oil burners make good preheaters as they are much cheaper than using the oxy-acetylene flame for heating preparatory to welding. Heating is necessary for practically all kinds of materials in order to prevent chilling of the flame and consequent loss of efficiency. It is also necessary to choose the proper sized tip to suit the work.
It is important to have the surfaces clean before starting to weld and, if the parts have been cut with the flame, they must be chipped off in order to remove the oxidized material before being welded. Plates, to be welded should be beveled on the edges, unless they are less than 1/8 inch thick and filling material of similar composition used for the joint. If the plates are over J inch thick, it is advisable to bevel from both sides towards the center in order to balance the shrinkage strains and reduce the amount of filling required. The edges of the bevel should be from 30 to 45 degrees angle and the plates should be spaced slightly apart to Insure filling clear through the joint. The opening should flare a little from the end where work begins to the opposite end in order to allow for the parts drawing together as the work progresses. Fluxes are an advantage in acetylene welding in order to absorb or reduce the oxide formed by the flame and to prevent burning out the carbon from high carbon steels, etc. They also protect copper and aluminum from oxidizing and reduce the liability of the zinc burning out of brass.