Arc Welding.

Fig. 71.—Electric Contact Weldingof 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 failuresand 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.

—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.

Recently the arc system has been applied with success for making welds on tramway rails, the resistanceof the welded joint being found very much lower than when made with the usual fishplates and bonded joints. Continuous current gives better results than alternating for the arc system, and a generator designed for use with this process has a “drooping characteristic,” that is, the volts at the terminals fall rapidly with an increase in the current output. In this way the current is automatically limited to some extent, when the short-circuiting effect of the operation comes into play. Successfully to weld by this process a current of 300 to 500 amperes at 80 volts is necessary, and every precaution has to be taken to protect the workman from the intense glare of the arc.

Thermit.—Thermit is an aluminium alloy whose combustion generates so much heat that the substance can be used for the welding of iron and steel. It is the patented invention of Dr. Hans Goldschmidt. With thermit as a means of melting and welding, and with the use of special clamps and devices, a number of operations, otherwise difficult, can be performed, and thermit has come into general use for repairing broken or defective parts. By the use of a portable jacket and clamp, the joints of gas, water, and steam pipes may be welded with the pipes in position; and the advantages of such a material to an engineer far removed from supplies and repair shop, as at sea, can hardly be enumerated. New journals have been welded to heavy rolls, broken pump-rods have been joined, and a number of structural parts successfully united by its aid.

Thermit is made up as follows: Iron oxide is intimately mixed up with about one-third as much in weight of powdered aluminium, according to the equation Fe2O32 Al = Al2O32 Fe. The metals to be united are placed together and surrounded by a little clay or similar substance and the mixture is placed all around the joint. The mixture is fired by a little magnesium, and the chemical change that follows creates such an intense heat that iron is readily welded. When the joint is cold and cleaned up it is hardly perceptible.

According to instructions published by Thermit, Ltd., it is of the utmost importance that the moulds, crucibles, etc., should be kept thoroughly dry, and they may advantageously be warmed before use to ensure the absence of any dampness. The parts to be welded must previously be brought to red heat, which is best effected by means of a gas and air flame. The proper design of the mould is of the utmost importance. It should have a runner and riser, and the metal should be allowed to flow between as well as around the ends of the pieces to be welded. The expense and inconvenience of making wooden patterns may be obviated by making the model of wax, ramming the sand round this, and subsequently melting out the wax.

Of late years oxy-acetylene welding and cutting have made great strides, and have placed at the disposal of the metal-working trades a means of doing many things that hitherto were impossible. Purified acetylene and oxygen, both under pressure, are supplied to a special blowpipe or torch. The flame in its hottest part has a temperature of about 4,000° C. and is therefore sufficiently high to melt any metals with which it may be brought into contact. The torch is fitted with all necessary adjustments to vary the supply of either of the gases, and constitutes a handy tool with which the intelligent worker soon acquires great dexterity. In a special form of the torch there is a means of introducing a further supply of compressed oxygen, which makes it possible for the blowpipe flame to cut its way rapidly through thick metal, the particles of which are actually consumed in the path of the oxygen.

It is out of the question in a single chapter of a handbook covering such a large scope as the present work to do more than indicate some of the uses to which the oxy-acetylene torch or blowpipe may be put. This chapter is obviously no attempt whatever at providing a complete working guide to oxy-acetylene welding. All that will be here attempted is to present abrief description of a typical outfit and some notes on working the blowpipe, and then to give some practical instruction from the pen of an oxy-acetylene welder on the treatment of copper, aluminium and cast-iron. It may here be pointed out that there is now a large number of firms specialising in the manufacture of oxy-acetylene welders’ appliances, and most of them publish illustrated catalogues which anyone proposing to equip himself for oxy-acetylene welding would do well to obtain.

The source of the oxygen used in welding is now always cylinders, which are obtainable in various sizes, either by purchase or on hire from the gas-compressing companies, to whom they have to be returned for recharging. The acetylene also can be had in the compressed form, but in this case the gas is not simply compressed into steel cylinders because, if it were, any simple shock would be likely to cause explosion. The acetylene is therefore dissolved in liquid acetone, the cylinders containing some porous substance such as fossil meal, which is saturated with the acetone and the acetylene then pumped in. These also can be bought or hired from the gas-compressing companies. A tremendous amount of welding is done, however, with acetylene generated on the spot, and there are on the market quite a number of approved appliances that can be recommended, the best form of generator being that in which the calcium carbide is dropped into the water instead of the water dripping into the charge of calcium carbide. It is essential that the gas bepurified before use. As most people doubtless know, acetylene is one of the hydro-carbon series of gases and is evolved by the action of water on calcium carbide, a substance which is one of the products of the electric furnace.

Fig. 72.—Diagram ofOxy-acetylene Welding Apparatus

The particulars and instructions onpp. 136to139are due to the Acetylene Corporation, Ltd.Fig. 72presents a diagrammatic illustration of a complete oxy-acetylene blowpipe equipment with the exception of the acetylene generator and holder, which apparatus may be placed in any suitable position (preferably outside) at any reasonable distance from the blowpipe.Ais an ordinary gas tap connecting the hydraulic back pressure valveBwith the acetylene supply pipe from the acetylene holder. The blowpipe is connected at valveCby means of a flexible tube with the outlet tapDof the hydraulic back-pressure valve. This forms the acetylene supply pipe to the blowpipe. The blowpipe is connected at valveEby means of a special canvas-covered strong rubber pipe with the outlet tapFof the oxygen pressure regulator, which is fixed, as shown, on the oxygen cylinder.Gis a pressure gauge. This pipe conveys the oxygen supply to the blowpipe, and should be securely attached, as it is subject to pressures varying from 5 lb. to 40 lb. per sq. in. The hydraulic back-pressure valve should have been previously charged with water, and the gas regulator screwed into the oxygen cylinder. The blowpipe apparatus is now ready for use, with the tapsAandDclosed and the tapsC,EandFopen.

First, slowly open the oxygen cylinder valve (not shown) with the key supplied for that purpose. By means of the thumb-screwH, adjust the gas pressure to the correct working pressure for the blowpipe used. The approximate pressure of oxygen required for each blowpipe is as follows: No. 2, 8 lb. per sq. in.; No. 3, 10 lb.; No. 4, 11 lb.; No. 5, 12 lb.; No. 6, 14 lb.; No. 7, 16 lb.; No. 8, 19 lb.; No. 10, 20 lb.; No. 12, 25 lb.; No. 15, 30 lb. Then open the acetylene tapsAandD, and when acetylene is unmistakably smelt at the nozzle of the blowpipe, ignite the gases by means of a gas jet, candle, or taper. Then by means of the tapCslowly throttle down the acetylene until the small white cone of flame at the nozzle of the blowpipe shows a clearly defined outline. As some indication of the correct size of the cone, it may be mentioned that when working with the No. 10 blowpipe this should be about1⁄4in. diameter by5⁄8in. long. This cone in the other blowpipes is greater or less according to the relative size.

The tapAmust never be used to regulate the supply of acetylene; in fact, after the hydraulic back-pressure valve has been charged with water, it is best to leave this tap always on.

The working pressure for oxygen previously given should not be too rigidly adhered to. Even in the same sizes of blowpipes the conditions must vary slightly, and a little practical experience with each blowpipe will soon indicate the best working conditions. If the flame is not properly regulated it may fire back and go out. If so, the tapsCandEshould be shut off at once, and a few seconds allowed to elapse before relighting. When work is carried on for a long time at a stretch and the burner becomes warm, it will be found necessary to slightly open the acetylene tapCfrom time to time. If work is being done which involves the nozzle of the blowpipe being held in a confined space, it is advantageous to cool this end of the blowpipe by immersing it from time to time in a bucket of water. While this is done the gases must be turned off atCandE.

Welding should be done at the apex or outer extremity of the small white cone.

If the hole in the nozzle of the blowpipe gets obstructed at any time through beads of iron being splashed into it, or from any other cause, it may be cleared with a piece of copper wire and cleaned with a wire brush. No steel reamer or other sharp instrumentshould be used in the hole, which otherwise will be altered in size.

On stopping work the acetylene tapCshould be closed first and then the oxygen tapE. When work is completely stopped, the oxygen cylinder should be shut off. The oxygen cylinder valve should never be opened until tapsFandEare open, and it should then be opened slowly. In this way sudden impact of oxygen in the regulator is obviated.

The following instructions on the methods of welding copper, cast-iron and aluminium are contributed by a foreman welder.

—Copper to be welded should have its edges bevelled to enable the welding to penetrate the entire thickness of the metal. Bevelling is not generally practised below a thickness of3⁄32in. From3⁄32in. to3⁄16in., a slight open bevel is sufficient;3⁄16in. thick and over, the angle of the bevel should be about 90°. It is not necessary to go beyond this even with great thickness. The bevelling should be regular, especially at the bottom, so as not to produce holes or excess of thickness at the bottom of the bevel.

The edges to be welded and their immediate neighbourhood should be thoroughly cleaned. This can be done with a file, scraper, or sheets of emery. Chemical agents such as spirits of salt or nitric acid are sometimes employed; but it is preferable to precede their use by a mechanical cleaning.

Before beginning the welding the parts should be carefully arranged so that during the welding operationthey remain perfectly in position. Owing to the high conductivity of copper, a relatively larger blowpipe tip must be used than when welding either iron or mild steel of the same thickness. The power of a blowpipe of 225 litres with an approximate consumption of 7·75 cub. ft. of acetylene per hour would be suitable, with economical results, for iron or mild steel1⁄8in. thick, whereas for copper of the same thickness the power of the blowpipe should be of 300 litres, having an approximate consumption of 10·5 cub. ft. of acetylene per hour. Also, a blowpipe which is too strong tends to melt the metal too rapidly. This should be as carefully avoided as that of melting too slowly.

A pure copper welding rod may be employed for filling in, but it is not so effective as a welding rod made of phosphor copper. The phosphorus is incorporated in a very small quantity, so that none remains in the weld after its execution. A filler rod which contains too much phosphorus lacks fluidity, and melts at a temperature much lower than that of the copper to be welded, thus facilitating adhesion. Moreover, the welds in which the phosphorus remains lack elongation, and therefore do not possess the same mechanical properties as pure copper. The welding rod after1⁄16in. of its diameter should be about equal to the thickness of the weld, although in practice feeders about1⁄4in. in diameter are not generally employed. Welds made on copper without a deoxidising welding rod properly prepared have a tendency to oxidise, and therefore do not possess the required qualities. In addition, thesurface of the metal must be covered with a carefully prepared mixture of potassium phosphate and potassium carbonate to a depth of about1⁄16in. Upon the application of the flame, the mixture will melt and form a glaze over the surface of the copper, thus preventing oxidation and assuring good work.

A flux consisting of chloride of sodium, sodium borate, and boracic acid is also recommended. The flux should be sparingly applied by dipping the end of the welding rod into the vessel containing the flux. The end of the rod should be warmed in order that the flux adheres.

Before beginning the actual operation of welding, it is essential to raise the edges of the weld and the parts in the vicinity to a high temperature. The high conductivity of the metal necessitates this, as any supply of molten welding rod before the edges are in a molten state inevitably produces adhesion. The flame of the blowpipe should be perfectly regulated and maintained without excess of either acetylene or oxygen. In executing the weld, care must be taken to avoid contact of the white jet of the blowpipe flame with the metal just about to be melted. The distance of the white jet should vary according to the power of the blowpipe, say from3⁄16in. to3⁄8in. If this distance is increased, the gases resulting from the second phase of combustion, carbonic acid and water vapour, influence the weld. Care must be taken that the fusion of the metal should not be undertaken until the edges of the weld and the parts near have been raised to ahigh temperature. At this moment the welding rod and the parts to be joined should be melted simultaneously. It is essential that the welding rod should be regularly incorporated in the line of welding, and must not be allowed to fall in drops. The operation should be continuous, taking care to attack regularly the two edges of the metal. The welding is thus executed rapidly.

It is well known that internal strains are always set up in every process of welding, due to the expansion and contraction when a metal is heated and cooled. Copper lacks tenacity when heated; hence contraction of the metal, whose coefficient of expansion is also fairly high; fractures thereby are often produced, especially in the welded part. However, pre-heating the article to a high temperature, maintaining the heating after the operation of welding and slow cooling, enables one in many cases to avoid fractures due to contraction. It is also necessary to hammer the line of welding and its vicinity. After the hammering operation it is essential to reheat the copper, raising it to redness (500° C. to 600° C.). Then plunge into cold water, or cool as rapidly as possible. The structure of the weld is not quite as homogeneous as other parts of the piece welded. This is, however, controlled largely by the skill and workmanship of the operator, who can, at will, make the weld more or less homogeneous.

It is impossible to enumerate in anything like detail all the work in copper which may be executed byoxy-acetylene autogenous welding. However, copper-smiths are advantageously making great use of the system, thereby replacing their old methods of brazing and riveting.

—In preparing aluminium to be welded, the edges must first be thoroughly cleaned and the welding rod very pure, so as to avoid the incorporation of impurities, which is apt to bring about rapid disintegration in the line of welding. Bevelling the edges to be joined is not necessary below a thickness of1⁄8in. From1⁄8in. to3⁄16in. a slight open bevel is sufficient,3⁄16in. thick and above angle of bevel should be about 90°. For thin sheets up to a maximum of3⁄32in., welding is facilitated by flanging the edges at right angles. The depth of the flange should be slightly deeper than the thickness of the metal. By this method no welding rod is required, the edges being simply fused. The weld should afterwards be hammered level.

Aluminium should never be welded without a flux. If welding is attempted without a flux, globules consisting of aluminium within and a coating of alumina (oxide of aluminium) will appear. In order to eliminate these by the blowpipe flame it would be necessary to raise the temperature to the melting point of the oxide of aluminium, which is nearly 3,000° C., whilst the melting point of metallic aluminium is only 657° C. To produce a flux which will dissolve the oxide at the low melting point of the metal and at the same time protect the hot metal from contact with the air hasobviously not been a simple problem to the chemist and engineer. However, several good fluxes are now obtainable which enable any experienced welder to effect satisfactory welds in aluminium.

A flux consisting of the following ingredients can be recommended: sodium chloride 30 parts, potassium chloride 45 parts, lithium chloride 15 parts, potassium fluoride 7 parts, and bisulphate of potassium 3 parts.

When making fluxes for the welding of aluminium, great care is necessary in order to completely dry the ingredients, thus avoiding their combination with each other. On aluminium above3⁄32in. thick, the flux is best applied by dipping the end of the welding rod into the vessel containing the flux. The end of the rod should be first warmed in order that the flux adheres. The welding rod after1⁄16in., its diameter should be just about equal to the thickness of the weld, although in practice feeders above1⁄4in. diameter are not advisable.

In executing the weld, care must be taken to avoid contact of the white jet of the blowpipe flame with the metal just about to be melted, because the high temperature of this part tends to produce holes which are difficult to fill in. The distance of the white jet should vary according to the power of the blowpipe, say from1⁄4in. to3⁄4in. The flame should be so adjusted as to furnish an excess of acetylene. There need be but little fear of carbonising the metal, for the reason that the temperature of the work is comparatively low. For thin welds, up to1⁄8in. thick, it is preferable to holdthe welding rod in front of the blowpipe in the direction of the edges to be welded. As soon as the latter begins to melt it is heated rapidly, and should be lowered to form one molten bath with the metal of the piece. The welding is thus done very rapidly. For great thicknesses it is preferable to obtain fusion of the welding rod, previously heated in the molten bath of the bevel. Directly after welding, the weld should be thoroughly washed in clean warm water in order to remove all remaining traces of the flux, which would otherwise continue to have a chemical action on the metal, thereby setting up corrosion.

—The edges of the weld should be bevelled when the thickness exceeds1⁄8in.; this enables the welding to penetrate the entire thickness of the metal. Both edges must be bevelled to an angle of 45°, so as to form a right angle at the weld. The bevelling should be regular, especially at the bottom, so as not to produce holes or excess thickness at the bottom of the bevel. Workers who attempt to effect welds on cast-iron above, say,1⁄4in. in thickness, without bevelling, invariably obtain poor results, as it is impossible to get regular and thorough penetration. The bevelling of the edges may be done by chipping or grinding, etc. Grinding wheels made from a carbide of silicon abrasive are very effective for cast-iron. The edges to be welded and their immediate neighbourhood must be free from sand, dirt, and rust.

It is known that internal strains are always set up in every process of welding, due to the expansion andcontraction when a metal body is heated and cooled. These strains are not unavoidable, but their effect may be minimised or nullified. In the case of cast-iron, the tendency to crack will be greatly increased if the cooling of the metal after fusion is rapid or irregular. Consequently, the article to be welded should be pre-heated slowly to about 700° F. to 1,000° F. Generally speaking, the higher the temperature of pre-heating, the less the danger of cracking. Preferably, pre-heating and subsequent slow cooling should be carried out in a muffle, particularly where light and intricate castings have to be dealt with.

In all cases care should be taken in the selection of the proper size of blowpipe tip to be used on any particular job. Therefore, the size of tip recommended by the manufacturers should be employed. The total heat of fusion of cast-iron being high, it is necessary to use a blowpipe with a greater power than for the same thickness of welds on mild-steel or wrought-iron. In the actual operation of welding, the blowpipe flame should be played on the edges to be welded until the melting of the iron just takes place. It is essential to avoid contact of the white cone of the blowpipe flame with the metal just about to be melted; the point should be kept at a distance varying from3⁄16in. to3⁄4in., according to the thickness of the work. The two edges to be joined should melt simultaneously. As soon as the first fusion is obtained, a little flux or scaling powder must be added; this is usually applied by dipping the extremity of the welding rod into thevessel containing the flux, the rod having been previously heated. Avoid throwing the powder into the molten metal whilst executing the weld, as the supply from the welding rod is always sufficient.

Many kinds of fluxes for cast-iron are furnished by the manufacturers of welding apparatus, which vary considerably in composition. The principle of all of them is to provide some chemical which, at the high temperature involved, will break up the oxide into its component parts. The following combinations will perform these functions, and can be recommended: (1) Boracic acid 80 parts, powdered chlorate of potash 20 parts, ferric carbide 15 parts. (2) Equal parts of carbonate and bicarbonate of soda, to which is added from 10 to 15 per cent. of borax and 5 per cent. of precipitated silica. (3) Carbonate of soda 50 per cent. and bicarbonate of soda 50 per cent. The necessity for using a flux may not be thoroughly appreciated; but if it is attempted to weld cast-iron without it difficulty will at once be experienced.

Do not add any metal from the welding rod until the bottom of theVis filled from the sides. It is found that by employing silicon in the welding rod, in the form of ferro-silicon, the iron combines with the silicon in preference to the carbon, allowing the carbon to take the form of graphite, and thus facilitate the formation of grey iron. The welding rod should contain about 4 per cent. of silicon and as low as possible in manganese. The purchase of such a welding rod is not at all difficult, and may be obtained from the samemanufacturers as the flux, from1⁄8in. to1⁄2in. in diameter.

One criticism of cast-iron welding has been directed against the hardness of the weld. This hardness may be due to a number of causes, such as inefficiency of the operator, unsatisfactory fluxes and welding apparatus, rapid cooling, etc. Therefore, as stated previously, in order to get good workable welds, there must be slow cooling after the welding is complete; and there is no reason why the worker who carefully follows the instructions given, and applies himself diligently to the task, should not be able to weld cast-iron of any thickness in an efficient and workmanlike manner.

This method of welding cast-iron successfully solves an unlimited variety of manufacturing and repair problems in the engineering industry, and can be relied on to make homogeneous welds on cast-iron. It is impossible to enumerate in anything like detail all the work in cast-iron which may be executed by oxy-acetylene welding; but the following are some of the applications for which it has already been advantageously employed: For repairing broken machine parts, gear boxes, motor cylinders, crank cases, tanks, manifolds, flywheels, etc., filling blowholes and defects in castings. Castings impossible or difficult to mould can be made in parts and united. Teeth broken from gear wheels can be renewed, and adding metal in any desired quantity to worn parts of cast-iron articles. As a concrete example of its economical and positive aid to the engineering industry, the following may beof interest. A cast-iron belt-wheel would have gone on the scrap heap, a total loss, with four of the six spokes broken, three entirely out. It was 5 ft. in diameter, and weighed about 500 lb., but was not worth much as scrap metal. Scrapping it meant the purchase of a new wheel, and perhaps a long delay in getting one cast. But with the oxy-acetylene process the three spokes that were fractured were welded into place; the fourth spoke broken near the hub was also welded. There were seven welds, each about 11⁄2in. by 4 in.; the job was done profitably at a cost of £5, ready for delivery in two days, and was considerably better than buying a new wheel, and waiting two weeks or two months for delivery. The process is particularly suitable for this class of work, and cannot fail to give satisfaction if performed by an experienced welder. The cost of welding a given job depends not only on its thickness, but on the skill of the workman. For example, the same class of job may vary as much as 50 per cent. if executed by different operators.

Lead-burning or flaming is the autogenous welding of lead by means of either an aero-hydrogen or oxy-coal-gas blowpipe flame. In the past the apparatus required included a hydrogen-gas generating chamber (called the “lead-burning machine”) and a blower or air chamber. The hydrogen was made by the action of dilute sulphuric acid on zinc. That system is now, or should be, obsolete, having been superseded by the cleanly and altogether more convenient process of employing two cylinders, one of compressed coal-gas and the other of compressed oxygen, in conjunction with an injector-pattern blowpipe. Gauges and regulators are required as in the oxy-acetylene process.

The oxy-acetylene process may be successfully applied to lead-burning in spite of the great heat of such a flame. The consumption of acetylene, according to Mr. D. Richardson’s translation of Granjon and Rosenberg’s French work, is only 1 to 2 cubic feet per hour for lead1⁄16in. to3⁄16in. thick, and the process is stated to have “considerable advantages over all other methods of autogenous soldering.”

In lead-burning it is customary to employ a triangular stick of refined lead for filling up the seams.By being burnt or joined together in this way, the lead becomes homogeneous, and the various parts of it equally withstand the same chemical action and heat. For this reason it is used for joining the seams of chemical and acid tanks, and for the joints of pipes used for the conveyance of such chemicals. Solder being an alloy, the acid would have a solvent action on it, eating it away and rendering it useless, and it would also give rise to electrical action, practically impossible when only one metal is exclusively employed. Lead-burning is also often used on external or roof work.

The seams burnt on sheet-lead are of two kinds: one forming a butted joint, the other a lapped joint.

In burning a butted seam, the two edges of the lead to be joined are butted together, and shaved about1⁄4in. to3⁄8in., or slightly less, on each side. The gas and oxygen are turned on and adjusted so as to produce a flame from about 5 in. to 6 in. long, and tapering to a fine point. The hottest part of the flame is the centre of the thickest portion, about 1 in. or 11⁄2in. from the jet. Hold the jet in the right hand, and a strip of lead in the left, and allow the flame to play on the end of the strip, which is held just above the seam. As the strip melts, the jet is diverted on to the seam so as to fuse the edges together, the additional lead forming a thickened portion. The strip is again melted, and joined to the edges, and also to the thickest part; and so on along the length. Care should be taken to burn the lead through, but not for the metal to flow beneath the seam. After a little practice, the operatorwill know exactly when to apply and when to remove the jet.

Fig. 73shows a flat butted joint partly burnt. The stick of lead is just nipped with the flame, and a bead of lead dropped on the seam. The flame is then directed on to this bead until it is fused with the seam. When bead and seam are melted together, the flame is immediately raised. The next bead of lead is then dropped on the seam so as to half cover the previous bead, as shown atM(Fig. 73). The flame is then directed on the second bead, the flame being immediately raised after these are fused together, and this operation is repeated until the whole of the seam is burnt.

A flat lapped joint, partly burnt, is shown byFig. 74. In burning this joint, the stick of lead is only required to fill up any irregularities in the burning, and is not required to form the seam in the same way as it is in a butted joint, because in lapped burning the overcloak is burnt down on to the undercloak, as shown inFig. 75. In horizontal and vertical burning, lapped joints only should be used.

Fig. 75shows a specimen of horizontal or side burning, andFig. 76one of vertical or upright burning. In burning both of these, the stick of lead is not required at all, the overcloak being in each case burnt down on to the undercloak. Care must be taken that both the overcloak and undercloak of a lapped joint are well shaved.

Fig. 73.--Butted Seam Partly BurntFig. 75.--Horizontal or Side BurningFig. 74.--Lapped Seam Partly BurntFig. 76.--Vertical or Upright BurningFig. 77.--Burning Upright JointFig. 78.--Branch Joint Ready for Burning

Fig. 73.--Butted Seam Partly BurntFig. 75.--Horizontal or Side Burning

Fig. 73.--Butted Seam Partly Burnt

Fig. 75.--Horizontal or Side Burning

Fig. 74.--Lapped Seam Partly BurntFig. 76.--Vertical or Upright Burning

Fig. 74.--Lapped Seam Partly Burnt

Fig. 76.--Vertical or Upright Burning

Fig. 77.--Burning Upright JointFig. 78.--Branch Joint Ready for Burning

Fig. 77.--Burning Upright Joint

Fig. 78.--Branch Joint Ready for Burning

The seams should not be soiled or greased, and care must be taken not to tarnish them in any way. If the lead is not shaved quite clean, or it becomes tarnished after it is shaved, it will be found difficult to burn it together successfully. No tallow or smudge is necessary. The operator will soon detect the presence of any foreign substance or dirt on the lead, and the shavehook should be kept handy to remove it.

In burning a vertical lapped seam, starting at the bottom, the lapping lead is melted, and as it runs is turned on to the back portion and fused into it. A slight projection is formed, which holds the next melting, and so on, each layer forming a base for the next, and adding to the height until the top is reached.

In practising either horizontal or vertical burning, the student should first place his work at an easy angle—say, at about 25° or 30°—gradually raising it as he becomes proficient until the seam is in a horizontal or vertical position as desired. Two surfaces can be burned together in any position—horizontal, vertical, or even overhead, where soldering would be impossible.

Pipe joints can also be made by burning. First one pipe is opened to form a socket like a slip joint. The male part, which must enter at least3⁄4in., must be well shaved and made to fit tight.Fig. 77shows an upright joint prepared and partly burnt.Fig. 78shows a section of a branch joint as prepared for burning. Care must be taken to work up a good thick shoulder for the socketN.

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