Next, push the nozzle of the bellows through the hole S1, and screw the bellows down tightly to the shelf B. Join the lead pipe to the nozzle of the bottom bellows, and make an air-tight joint with glue and tape bound round. Between the top of the upper bellows D and the top of the packing case at V, a stout sofa spring E is fixed to keep the top bellows shut down tight till air is pumped in from the bellows below.
From a screw near the top of the case stretch a house-bell coil spring M, and attach its lower end to the foot-plate or pedal N. This spring tends to keep the lower bellows open. When pressure is put on the foot-plate N, air is pumped from the lower to the upper bellows, and thence along the flexible pipe R to the bunsen blast nozzle described in detail later. If it were not for the upper bellows, the air would come to the nozzle in puffs, but the spring E keeps the pressure constant, and a steady blast is secured. The rubber pipe R should be of sufficient length to reach the hand conveniently, and allow room for movement.
The hearth K can be made from sheet-metal, with the edges bent upwards to form a tray; or an old frying-pan will answer very well. Whichever is employed, four iron stays or legs H must be used to raise it above the top of the packing case as shown. These legs should be screwed at one end to the sides of the case, and at the other to the pan, and if a frying-pan is used, the handle may be cut and bent to form a hook as at L; it then acts as a support for the blast nozzle.
Fig. 63.—Blowpipe or Blast Gas Nozzle
for Brazing Hearth
For use with the brazing hearth, a blast gas nozzle on the bunsen principle is required, and this is easily made from two pieces of gas-piping, a right-angle joint, and two mouthpieces to take flexible pipes. First, get 1 ft. of 3⁄4-in. brass (or iron) pipe and an elbow, internally threaded at both ends. Cut 2 in. off the brass pipe, and cut a thread at one end of the short piece and one end of the long piece, to screw into the elbow as shown in [Fig. 63]. At the other end of the long tube solder in a cock or mouthpiece to take a large diameter flexible pipe from the gas bracket. Next get 1 ft. of 1⁄4-in. brass pipe, and bend it to the shape shown at B, soldering a mouthpiece at F to take the smaller diameter air-pipe R ([Fig. 62]) from the bellows. Bore a hole through the elbow C, and push the pipe in, making a tight fit, and so that it passes centrally through the larger brass tube until it nearly reaches the end as shown at D. Run a shoulder of solder to hold the small tube firmly in the larger one as at E, and unite with a drop of solder the large tube A and the small tube B where they cross at G.
The gas passes up the large pipe, and out at D, and a blast of air is forced through the centre of the flame through the small pipe B. The shape of the small pipe allows of it being readily hung up on the hook L ([Fig. 62]) when not in use. Instead of the mouthpiece shown at H ([Fig. 63]), a cock will be found more convenient; but it should not be too small, or it will restrict the flow of gas, which should be as great as possible. No gas-cock should have less than 1⁄4-in hole.
The “fuel” (heat conservers) consists of chunks or cubes of asbestos, and when these are blown upon with the gas flame, the heat is quite sufficient for moderately heavy brazing. Be careful not to get any kinks in the flexible tubes, or the air and gas will be reduced in quantity, if not stopped altogether.
A square of thick asbestos (sheet) is useful for laying on small articles whilst brazing, and a piece or two of charcoal will be handy for silver-soldering.