In all kinds of metalwork a knowledge of how metals may be joined together is essential. Joints are either permanent or temporary. Permanent joints can be made by burning, autogenous welding, ordinary welding, brazing, silver soldering, soft soldering, folding, riveting. Temporary joints are screwing, bolting, wedging, taper fitting, force and shrink fitting. In many cases, though we call brazing, silver soldering, soft soldering, and riveting permanent joints, they can with care be taken apart again.

Burning

The method of burning a joint together is as follows: The two pieces of metal to be joined are placed in position and fixed so that a stream of molten metal may be run between the joint until the ends to be joined are at the same heat; the molten metal that has been flowing to waste is then checked, the joint filled up and left to cool. The joint is usually embedded in sand. When cool it is removed and the lumps trimmed off, leaving the metal in one piece and without any sign of a joint. Nearly all metals can be joined by this method.

Autogenous Welding, or thermit welding, is done as follows: Thermit, which consists of finely divided aluminium and oxide of iron, is placed in a crucible which has a hole in the bottom fitted with a fusible plug. The crucible is placed in position over the ends to be joined, and the ends are enclosed in a mould which has an outlet that can be closed after sufficient metal has passed through to heat the joint. When everything is ready the fusing action is started by igniting a special powder, or magnesium powder, which can be lit with a match. The heat generated by the chemical action reduces the iron oxide to a fluid mass of iron; this melts the fusible plug, allowing the metal to enter and fill the mould. This is also called aluminothermy.

Blowpipe Welding is carried out by means of various gases, such as acetylene, oxygen, hydrogen, and is now fairly general in its application. The gases are under pressure and drawn from cylinders. The mixing of them takes place in the mixing chamber of the special blowpipes which are used. The edges of the metals to be joined are filed clean and bevelled, and while the fusion of the edges is taking place the actual joint is fed with a metal of the kind that is being welded, in the form of rod, to fill up the spaces in the joint. The advantages of this method are that the appliances are portable, so that welds can be made"in situ"and the heat is localized. The perfection of the weld depends on the skill of the operator.

Electric Welding is very suitable for repetition work, is very quick, and dissimilar metals may be welded. But it requires special plant and fittings for each particular job. The operation is somewhat as follows: The pieces to be welded are held tightly one by each arm or guide, and these are insulated one from the other; the electric current is turned on, and the ends are brought close together but not quite touching. This causes an electric arc to form at the ends to be welded, and they heat from the centre to the outside, and when it is at the right temperature, the ends are forced together either by manual or hydraulic power.

Ordinary Welding which is done by the smith consists of thickening up or up-setting, as it is called, the ends to be joined to allow for their subsequent reduction by hammering and then raising the pieces to a welding heat, laying one on top of the other, and hammering them together. The work is heated in a coal or coke fire assisted by a blast of air. Wrought iron and mild steels are usually brought to a bright white heat and tool steel to a dull white or yellow heat. There are various forms of welds such as scarf welds, tongue welds, split welds, and jump welds, but the scarf weld is the strongest and most common. A flux is generally used to protect the surfaces of the metal to be joined from the action of impurities in the fuel and from oxidation-silver sand for wrought iron and mild steel, Venetian red and borax for tool steels.

Brazing and Soldering consists of joining metals by means of alloys which are heated and fused together with the edges of the work, so that they alloy with the metals being joined. There are three essentials for this kind of work.

The joints must be clean.

A flux must be used.

The solder or alloy used must melt before the material that is to be joined.

A coal-gas blowpipe flame assisted by an air blast is generally employed, but a forge fire, or spirit blow lamp would very often answer the same purpose. The only difference between brazing or hard soldering, as it is sometimes called, and silver soldering, is that in the one case spelter is used and in the other silver solder is used. (For the composition of these see p. 124.) The following method of procedure should be adopted when brazing or silver soldering. The joint should be clean and the edges fit together. When fitted, the parts should be held together by ties of iron binding wire, when the joint is difficult to adjust one or more iron dowel pins should be used. These with the wire ties prevent any shifting of the joint during the progress of the operation. The work should now be carefully adjusted on the bed of the forge, care being taken to support it so that no strain is on the tied joint. When arranging the coke around joint to conserve the heat, place it so that all the parts are open to observation, otherwise while the solder is fusing, some adjoining part may be melted. The difference of the melting temperatures being very slight, spelter should be mixed with water and powdered borax in a small pot. When the work is in position the joint should be moistened with the borax water and the joint"charged,i.e. just sufficient of the moistened spelter laid on the joint to fill it when fused. The work should now be heated in the vicinity of the joint, and when there are unequal thicknesses the heaviest parts should be heated first, gradually bringing the heat up to the joint; watch that both sections receive the same amount of heat, otherwise it will not be a good joint. When a red heat is reached the borax will melt; dry powdered borax should now be dusted on from time to time to prevent the solder perishing or being converted into oxide; as the heat increases the flame should be localized over the solder and a fiercer blast given until the spelter fuses and flows into and fills up the joint. When this point is reached remove the flame and allow the work to cool. It should not be moved until all traces of red heat have disappeared; then place in pickle (for composition of this see p. 177) to dissolve the borax and oxide.