If the drawing ([Fig. 70]) be regarded as 4 in. to 1 ft., and the lamp made accordingly, it will be of ample size to braze small and model boilers. It must be understood, though, that the size of the lamp alone does not govern the size of the flame, this depending entirely on, firstly, the size of the nipple, and secondly, the pressure of air in the container forcing the vapour out. The larger the container, the longer the lamp will burn without refilling, and, incidentally, the larger it is the weaker the container will be, and vice versa.
Should this lamp be required for brazing and silver-soldering articles about 1⁄2 in. in diameter, it must be altered, as it is obviously much too large. The same container will do, of course, although perhaps rather unwieldy. If preferred, another container just half the size could be made; this will be handier in use, although it will not allow the lamp to burn for so long a period without recharging as the larger pattern. For the tubing, 3⁄16-in. copper tube, preferably solid drawn, will be suitable. This should be heated to a dull red and then quenched in water to anneal it, this making the task of bending it much simpler. If it kinks too much in the bent portions, pour some molten resin into the tube, one end being plugged up for this purpose, and when set, bend to the shape desired. The resin can be heated and run out afterwards. The flame tube may be half the diameter and length of the one illustrated, but no hard and fast rule can be given for the dimensions of this part of the lamp, as the size of the nipple hole has everything to do with this. Make the latter just big enough to admit a fine needle. A simple method is to drill the cap almost through, and then punch the rest through with the point of a needle. By experimenting, it will be easy to find the right proportions of flame tube and nipple hole. The screwed joints should be a metal-to-metal fit as it is termed, that is, they should fit so tightly when screwed home that no leakage is possible. This can be ensured by seeing that all male threads are a tight fit in their respective holes.
CHAPTER XV
Electric and Thermit Welding Briefly Considered
Contact Welding.—The electric welding of iron strips and sheets is usually done by the Thomson process of “contact-welding.” In this process the metal is brought to a welding heat by passing a very large current through the joint to be welded, which, by virtue of its high resistance in relation to other parts of the circuit, develops great local heat. When the correct temperature for welding is reached, the joint is pressed together by mechanical means, and the current interrupted. In [Fig. 71] the necessary arrangements for the welding of a steel rim are sketched. A is the iron core of an alternating current transformer and B the primary winding supplied with alternating current either from a works dynamo or a public supply, and controlled by a double-pole switch C. The transformer has a secondary winding consisting of a single copper strip of very heavy section D, in which secondary currents of low voltage but very large volume are induced. This winding D terminates in two heavy metal clamps E, one fixed and the other capable of movement by rack and pinion or screw, and the clamps must be shaped to the contours of the work F they are intended to hold, so as to fit well and present as little electrical resistance there as possible. The butt ends of the wheel rim are brought into contact, current switched on at the transformer primary, and immediately a very heavy secondary current passes round the “winding” D, generating intense heat at the junction of the metal rim G held in the clamps, where the electrical resistance is comparatively high. In a few moments the joint will arrive at welding heat and the screw feed is then operated, driving the joint together and completing the weld, except so far as a little hand dressing may be found necessary. Directly the weld is established the current is switched off at the mains and the job allowed to cool out. Nothing less than 5 kilowatts to 10 kilowatts is likely to be very satisfactory for hoops about 3⁄4 in. by No. 16 gauge, and the current must be alternating. The primary voltage and frequency is immaterial, as the transformer can be wound to suit the circuit conditions whatever they may be.
Fig. 71.—Electric Contact Welding
of Steel Rim
The method of welding by resistance, that is, by raising locally the welding point to the temperature required by bringing the two surfaces into contact until their high resistance produces a welding heat and then squeezing them together, is by far the most manageable and satisfactory commercial process of the two electrical processes. It is adapted for “spot-welding” or producing local adhesions between metal plates after the manner of riveting, for butt or end-on welds, for seams, chains, rings, etc., and automatic welding machines are now made that can deal with no less than 1,500 welds and upwards per hour with semi-skilled labour, with the least possible percentage of failures and a very low cost for electrical energy. Alternating current is essential with this type of weld, and is used to energise a step-down transformer of special construction.
Arc Welding.
—Notwithstanding the superiority of the resistance welding process to most commercial work, particularly that of a small kind necessitating rapid repeat work, the “arc” method, which has been in use for many years, and was probably the first experimented with, has now become largely used on work where it was thought impossible to adapt it a few years ago. The system is extensively employed in iron and steel works, shipyards, and boiler works, and the class of work it is employed on varies from the dismantling of iron and steel buildings, by fusing and cutting through the structural ironwork and girders, to the filling up of blowholes in castings. The metal to be welded is connected to one pole, and the electrode handled by the operator forms the other pole, an arc being struck between the two. Broken castings and forgings can be satisfactorily repaired by running new fused metal round them.