—A simple form of gas blowpipe is shown in [Fig. 68], the rubber tube connecting it with the gas supply being fixed on the pipe at the point of connection with the cock. To construct the appliance, one end of a piece of brass gas pipe of the required length with, say, a 3⁄8-in. bore, is bent as shown, whilst at the back of the curve thus made a hole is drilled to admit a tube A 5⁄16 in. in diameter. This should have one end (see dotted lines) bent to correspond with the angle previously formed in the larger tube, whilst its other extremity should be bent upwards. Make these pipes red hot where they are to be bent, and, if they are afterwards plunged in cold water, the material will to some extent be softened, and its tendency to split will be obviated. The smaller tube is passed through the hole in the bend of the larger one, the ends being almost flush and quite concentric. Solder the parallel portions of the tubes together, and then fix a gas-regulating cock to the larger one, as in [Fig. 68]. The end is then connected to an ordinary bracket or burner by means of an indiarubber tube G, and a short piece of tubing is fitted with a bone or other mouthpiece, and attached to the projecting end of the air tube. This instrument will do any soldering, and will be suitable for melting gold, silver, and brass, or brazing odd jobs in iron or steel. Of course, when used for the last-named purpose it would be in conjunction with asbestos tubes or other supports.
Fig. 68.—Simple Form of Gas Blowpipe
Fig. 69.—Larger and more efficient Blowpipe
Proportion of Air to Gas.
—As regards the relative volumes of gas and air for blowpipes, the late Mr. Thomas Fletcher said that, speaking roughly, but still sufficiently near to make a correct rule by which to work, a blowpipe requires one volume of gas to eight of air. If the gas is supplied at a pressure equal to 1 in. of water, and the air at eight times that pressure, then, to get the best effect, the area of the gas and air pipes should be equal. If the air supply is equal to 16 in. of water pressure, the gas pipe must be double the area of the air, and so on in proportion. Some makers assert that a better working flame is produced by using ten volumes of air to one volume of gas; but, of course, if the blowpipe is fitted with taps, the supplies can be adjusted easily. It will be found, however, that any practical departure from Fletcher’s rule will result in a loss of power.
Capacities of Blowpipes.
—It has been said, a blowpipe with a 1⁄8-in. air jet, if worked with an air pressure of 10 oz. per square inch—that is, 15 in. of water—will braze up to about 1⁄2 lb. total weight; or in other words, will securely unite two pieces of brass each weighing 1⁄4 lb. With the same pressure a 1⁄4-in. bore air-jet will braze a total weight of about 2 lb., and so on in proportion. It will be understood that the air jet is measured at the point at which the blast leaves the air tube, whilst the area of the gas supply is that of the annular space between the two tubes. When the air tube is thus carried inside the gas tube (see [Fig. 69]), the tool appears to be much larger than it really is, and this accounts for the fact that a 1⁄2-in. size blowpipe with the air tube fixed outside the gas supply is just as effective as one of the 3⁄4-in. size which carry the air tube inside the stem. All indiarubber tubing must be perfectly smooth inside, for if it is wired or in any way rough, the resultant friction will cause a loss of pressure. It should also be of as large a bore as is convenient.