Large and Efficient Blowpipe.
—A large and efficient blowpipe that can be made in a few minutes is shown by [Fig. 69], the only materials required being a T-coupling and diminishing socket, an elbow, and one or two pieces of pipe. The air tube A (represented for the most part by dotted lines) passes through the diminishing socket until it almost reaches the nozzle of the blowpipe, with which it is concentric. By using the elbow D, the two supply pipes are brought parallel to each other, so that the indiarubber connecting tubes can be more easily held in the hand like reins, as by simply squeezing them the flame can be readily regulated. Sometimes, in cases of emergency, a plug drilled to meet the air tube is used in place of the socket. The plug is thrust into the end of the T-socket; but in all cases it must be airtight. This blowpipe can be used efficiently only in conjunction with a foot blower.
CHAPTER XIII
Managing Blow-lamps
The stoves and lamps burning paraffin in the form of vapour have become very popular on account of their good heating properties, portability, and little attention required. They consist of a container holding paraffin, a burner with a heating tube attached is screwed to the container, and a tube leading almost to the bottom. A small air-tube, similar to a cycle-pump, is fixed in container, the handle and cap only being in sight. When the burner tubes are heated, a thumbscrew on the filler caps is closed, and a few strokes of the pump puts a slight pressure on the oil in the container. The oil is forced up the central tube to the burner; but before reaching this it has to flow round the heating coils, and in so doing is turned to vapour. The outlet at the nipple being very small, causes the vapour to issue with some force, and it mixes with the air, forming a mixture which burns with a non-luminous flame similar to that of a bunsen burner. This flame plays on the heating coils, and once started, the lamp is practically automatic; a stroke or two from the pump will keep it going until the oil is consumed.
There are patterns that use petrol or benzol, their action being slightly different. Petrol and benzol are light spirits, which give off inflammable vapour at a much lower heat than paraffin does. Advantage is taken of this fact by causing burner and nipple to be in one solid brass casting, so that when the nozzle is heated, the brass conducts the heat back to the nipple and so vaporises the petrol, which is fed to the nipple by a thick wick contained in a tube which reaches almost to the bottom of the container, the wick touching the bottom.
To start either paraffin or petrol lamps, the exit tube or nozzle is heated. The petrol lamp has no coils round the nozzle; but comes straight from the holder to the exit nipple. The size of flame is regulated by a needle valve fitted with a wood or fibre handle. The petrol lamp has no pump, except on high-power brazing lamps whose use requires expert handling.
The chief trouble with lamps using paraffin is that the burner becomes choked; this is shown by the lamp jumping out or the flame not attaining sufficient heat. The makers supply a proper cleaning needle, a sheet stamped to form a handle and a piece of fine steel wire fixed at one end. Nothing else, such as pins, etc., should be used, or the hole in the nipple becomes enlarged, and emitting too much gas, causes a smoky flame. The cleaner should be used each time before lighting. A good way to avoid this trouble is to use a small funnel with fine brass gauze soldered in the body when filling. White Rose is a quite satisfactory oil for these lamps.
After considerable use the heating coil becomes choked with carbon deposit. A new heater tube can be obtained, or the old one cleaned by drilling two or three holes in the ends and passing a piece of flexible wire (such as Bowden brake wire) through the heater and removing the obstruction. After getting it clean, tap out the holes and fix suitable screws, flat under the head, with a piece of asbestos to make a tight joint.
If a larger flame is not obtained by pumping, take the cap off the pump and draw out the plunger; the leather is probably worn. Fit a new one; or it may have become hard, in which case apply a little oil and open out carefully. The retaining valve is in the centre of the pump bottom, and is removed by using a long key down the pump barrel. The valve is in four pieces. See that the spring is free and that the cork is in good condition. When replacing, take care not to get it cross thread, and screw firmly home. The washer under the filler cap is of rubber and cuts through in time. Do not use pliers to screw down; it will go gas-tight with the fingers if the washer is good.
To remove the nipple from which the gas issues is almost impossible without a proper key. This has a universal joint, which allows it to be rotated, although the handle is almost at right angles with the burner. Keys and all other parts mentioned can be obtained from any dealer in these lamps.
With the lamps using petrol, the only parts requiring attention are the plate from which gas issues and the wick. Unlike the paraffin nipple, this is a circular stamping of brass approximately 3⁄4 in. in diameter with a fine hole in the centre. To remove this disc, pass a long flat screwdriver blade through two openings in the nozzle and turn to the left (anti-clockwise), holding the body of the lamp firmly on the bench. Before fitting a new disc, thoroughly clean recess and remove any deposit from the inside of the valve box. Unscrew the needle and gland if there is any leak there, and clean and repack with asbestos yarn. A little glycerine on the packing appears to be an advantage. Place the disc in position, dip an asbestos washer in water, and screw the nozzle firmly down.
The wick inside the lamp filters the spirit before reaching the nipple and occasionally needs replacing. Remove the cap from the bottom of the lamp, and with a piece of thick wire flattened at one end and filed to a hook, push it up the tube and withdraw the old wick. The new one is simply put in its place and the cap screwed tight. The washer under the filled cap is of cork and rarely gives any trouble.
CHAPTER XIV
Making Blow-lamps
A Paraffin Brazing Blow-lamp.—The brazing blowlamp shown in [Fig. 70] was made at a total cost of less than 4s. The illustration is printed to a scale of about one-quarter full size. The lamp illustrated is not a mere experiment, as the writer of this description had a similar one in use for over two years, and during that time brazed hundreds of jobs with its aid.
The container is of tinplate, and adapted from a workman’s tea can. When purchased this will have a wire handle and two hinges, and these should be removed and soldered up. The handle shown at H is made from a strip of iron, 1⁄8-in. by 5⁄8-in. section, bent round to the shape shown and riveted to the side of the container. These rivets should be well soldered over inside to prevent leakage. The joint of the longer strip is shown at X. The lid should next be taken in hand, a 5⁄16-in. hole being drilled at one side close to the handle, to take an ordinary Lucas cycle valve. A leather washer is fitted inside, and also one outside under the lock nut, the latter being then tightly clamped up.
At E is shown the filling cap, the body part of which was taken from an old paraffin oil-lamp container, and the screw cap made at the local brass works; but this fitting can be obtained in a finished state from many model-fittings manufacturers. This is soldered to the lid in the position shown, a hole being afterwards drilled in the tin to suit it.
Fig. 70.—Paraffin Brazing Blow-lamp, the container being shown in section
The cock shown at D is an ordinary gas-cock, with a length of 3⁄8-in. outside-diameter copper piping screwed and sweated in beneath. This is passed through a hole drilled in the lid for its reception, and the base of the cock is then sweated in position. Note that the length of this pipe is such that when the lid is in place it clears the container bottom by 1⁄8 in.
The coil of piping at P is 3⁄8 in. diameter copper tube coiled round to the shape shown, the lower end being tightly screwed into the top of the cock. The opposite end is screwed for a short length of 3⁄8-in. gas thread, and very tightly fitted with a screw plug such as that used by plumbers for shutting off a portion of gas piping. Before screwing this on, a small hole about 1⁄32 in. in diameter should be drilled in the middle of same; this is the nipple for the exit of gas to the burner.
The 3⁄8-in. copper tubing should not require filling with anything before bending; this operation is best done round a mandrel of wood. No heating is necessary. A certain amount of flattening of the tube will no doubt occur, but this is immaterial. The end of the flame tube is, of course, open, otherwise the flame could not emerge. The nipple end of the flame tube is also quite open, with the nipple end of the coil just projecting inside. The portion of tubing shown dotted in [Fig. 70] takes the vapour from the coils to the nipple; it does not pass through the coils, but at the back of them, and bends round as shown.
The lid of the container should now be carefully soldered down all round, and then the 3⁄16-in. brass stay rod shown at S must be fitted and both ends sweated over. It is essential that this stay is not omitted, as otherwise the pressure to which the container is subjected would bulge out the ends.
The flame tube A should now be made of 1⁄32-in. sheet-iron, being held in place by two or three clips riveted on and bent over the coil ends as shown at C, only one of which is shown for clearness. The flame tube does not taper, nor is the back end closed up. Its diameter should be 11⁄4 in. and length 31⁄2 in., but this depends on the size of the hole in the nipple. No holes are required in this tube, as all the air is drawn in at the end. The nipple has a gas thread cut inside it, and it is screwed tightly on to the end of the copper coil. The length of the pipe from tap to coil is not important; about 9 in. will be satisfactory.
The best means of joining the nipple to the tube is to screw the end of the tube before bending it round at the end, and then to screw the nipple on tightly. The pipe can afterwards be bent as shown.
The jet hole in the nipple should be about 1⁄32 in. bare, and should be so drilled that the issuing gas passes through the centre of the flame tubes. The tube is open at both ends, to allow air to be drawn down and complete the combustion of the paraffin vapour, the mixture igniting and burning properly when the vapour passes out at the other end and comes into contact with the atmosphere.
To use the lamp, it should be filled about two-thirds or one-half full of paraffin oil, and the container cap then screwed hard down with a leather cap as a washer. A cycle pump should then be connected to the valve V, and a few strokes given, the cock D being meanwhile closed. The copper coil should next be put into a gas flame or the fire for a few minutes until nearly red hot, and then the cock D should be opened a shade, which will allow the paraffin to rise up the pipe I and enter the coils P, where it will become vaporised, and the gas will then issue from the nozzle N, and burn at the mouth of the coils.
The lamp may then be applied to the job, and five or six more strokes given to the pump, when the flame should burn with an intense heat and give out a roaring noise.
The theory of action of lamps of this character is as follows: The pressure of air in the container forces the paraffin up the copper tube into the coil, where it is vaporised by the red-hot piping. The vapour then issues at some pressure from the nipple hole and, passing down the flame tube, an ejector action is caused which draws air in with it. This air mingles with the paraffin vapour, and when the mixture issues at the other end, and comes into contact with more air, combustion takes place.
The heat produced by the lamp should be quite sufficient to braze the bottom bracket of a motor-cycle frame and other similar jobs. The size of nipple with relation to the size of the flame tube is most important. If the flame tube be too large in proportion, the result is an excess of air, which cools the gas unduly and the flame dies out. A candle is put out in a draught from the same cause. On the other hand, if the flame tube is too small, or the nipple hole too large, the result is an excess of gas, causing incomplete combustion and a partly luminous and smoky flame, with less heating power. To ascertain if the nipple hole is too small, take a strip of tin and, whilst the lamp is burning, hold the tin partly over the end of the flame tube, so as to restrict the amount of air entering, and note the result.
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.