—For satisfactory brazing, thoroughly clean the surfaces to be joined, first with a file and then with emery-cloth, and, if necessary, bind them together with thin iron wire. A flux of borax and water mixed up into a thick paste is smeared round the joint, which should then be warmed to get rid of the moisture. Heat the metal to a white heat, dip the spelter into the borax paste, and apply to the part to be joined, rubbing round the joint until the brass is seen to run, when the heat can be cut off. The work should be almost covered in the asbestos cubes, and the spelter applied all round and not only in one place; failures to unite the parts mostly result from insufficient heat or cleaning of the parts.
Brazing-lamps.
—These lamps are constructed to burn benzoline or paraffin, and the more powerful types are fitted with pumps so that air pressure may be exerted on the oil. The paraffin or benzoline is thus forced into the burner, and by passing through the previously heated tubular coil is converted into gas, which issues forth out of the gas nipple, mixes with air, and then burns with a blue atmospheric flame of high temperature. After filling the lamp, a torch should be held under the burner to vaporise the oil, and thus ignite the lamp, after which pressure is applied by means of the pump. Do not start pumping too soon—that is, before the burner is sufficiently hot to vaporise the oil—or the oil itself will be forced through the gas nipple. The flame may be regulated at will, to suit the work in hand, after the burner has become thoroughly hot to set the lamp fairly going. When brazing, lay the article to be brazed on some broken coke, charcoal, or firebrick, and if the article is comparatively heavy, cover it over with some small pieces (to conserve the heat) except where the brazing is required. After applying flux and spelter to the previously cleaned metal, direct the flame of the lamp on it, gently at first, until the spelter fuses and makes the joint.
Gas Blowpipe for Brazing.
—Brazing blowpipes should be fitted with a combination air and gas regulator, by means of which both the size and nature of the flame can be adjusted to suit the work in hand. When choosing a blowpipe it is always better to make one rather larger than the work in hand necessitates—the flame can be reduced when required; by this means a margin of reserve is provided which is useful in emergencies. A blowpipe whose dimensions are 10 in. long, having a 5⁄8-in. gas supply reduced to 7⁄16 in. at the nozzle, and a 3⁄8-in. air supply reduced to 3⁄16 in. at the nozzle, will braze ordinary latch keys and other similar work if used with a No. 3 size blower. As already stated, a paraffin, petrol or benzoline blowlamp is a cheap and effective apparatus, especially where the quantity of work to be done does not warrant the outlay of a foot blower, gas blowpipe, etc., but gas has advantages over the liquid fuel, inasmuch as the blowpipe is more under control. The quantities of gas and air can be readily and more delicately adjusted during the actual brazing process, then as soon as the spelter fuses and the gas is turned off, the stream of cold air soon counteracts any excessive heat.
Making a Gas Brazing Hearth.
—The brazing hearth can be bought ready-made, or can be cheaply put together by the worker himself, the necessary materials consisting of two bellows, some lead piping, and an old packing case to support a large size frying-pan—about 1 ft. 4 in. by 1 ft. The bellows are fixed one above the other, interconnected by a fixed lead pipe, one pair of bellows worked by the foot pumping air into the pair above it, from which the air is taken to a bunsen blast burner by flexible tube. This bunsen burner can also be easily and cheaply made, and as this works very well and will be found useful for both soldering and brazing, the following hints on making it are given. First, get two ordinary large house bellows about 1 ft. square, or larger if possible; the larger the better. Next a good strong packing case about 2 ft. long by 1 ft. 3 in. high and 10 in. wide is required. One end of the box will form the base, and to make it steadier two 1-in. boards should be screwed across to protrude about 6 in. on each side, the front one being considerably wider, as can be seen at T ([Fig. 62]), which shows the apparatus with one of the case sides removed. The packing case proper is denoted by the letter A; the part A S A was formerly the bottom of the case, but is now the back; and the top A V A was the end of the packing case. Half-way between the top and bottom a shelf B should be fixed, having a large hole cut out of the middle at G to accommodate the union piece (seen in section), which holds the lead air-pipe communicating from the lower bellows to the upper.
Fig. 62.—Home-made Brazing Hearth
In the back, near the bottom, at S, cut a hole for the nozzle of one of the bellows, and above the shelf cut another hole at S1 to take the nozzle of the other bellows. The bottom bellows C should have the top handle extended by a piece of stout bent iron N to act as a pedal, and should be screwed to the base. A central hole about 3 in. in diameter should be cut at P to allow the air to reach the valve. Now get a piece of lead pipe 1⁄2 in. inside diameter, and, having cut down the nozzle to just under that diameter, force the mouth of the pipe over it as at S, and bend the pipe F as shown, to reach the centre of the shelf B and enter the hole G until flush with the top. Remove the lead pipe and get a block of wood about 3 in. square. Cut off the corners and bore a hole in the middle, so that the lead pipe will just pass through it, and countersink the hole. Broach out the mouth of the lead pipe until it becomes bell-shaped and fits the block of wood, so that its edge is flush with the wood when pressed hard against it. This is clearly shown at G, where the dark shading represents the wood block. Cut a leather washer the size of the block, with a 1⁄2-in. central hole, and lay this over the hole in the bottom of the top bellows where the leather flap valve is. Bore two holes in the wood block, and screw this down tightly to the bottom of the bellows, so that the bell-mouth of the lead pipe faces the hole and has the leather between it and the block. This should make an air-tight joint for the fixing of the pipe to the bellows.