—Before attempting to braze either iron or steel the surfaces should be thoroughly cleaned by filing or grinding, etc. Brass or copper may be cleaned by dipping in a solution of 1 part nitric acid and 2 parts of sulphuric acid. This same solution can be used to remove the scale after brazing. The parts should be fastened together in the position they are to occupy when joined. The fastening may be effected by the use of wires, screws, bolts, clamps, etc. If practicable, the parts should be held in such a way that they can be turned over during the brazing process without disturbing the relation of the parts, thus affording a better chance to apply the flux and brazing material.
In brazing sheet metal, if the seams are not required to stand much working after soldering, they may be joined edge to edge. When seams are formed in this way, little nicks, about 1⁄2 in. apart, should be filed out along the edges, so that the solder flowing through the nicks will render the joint sound. If the seam is to be worked after soldering, a small lap is necessary to ensure adequate strength. To form seams of this type, first thin the edge of the metal along the ends that are to form the seams, about 1⁄8 in. in from the edge, so that when the two edges are lapped over each other the combined thickness at the seams will be the same as the single thickness of the metal at other parts. Cut a small cramp at the top and bottom of the seam, and fit the opposite edge in these cramps. After preparing the seams by either of the above methods, fasten binding wire round the articles so as to hold the seams securely in position. Now powder some borax flux, mix equal parts of the borax paste and grains of spelter, and along the seams place sufficient of the mixture to solder them when melted. Some dry borax should also be kept ready at hand, so that a little may be taken and thrown on the solder at any point where the material does not appear to be flowing freely. Gently heat the article by some suitable means, such as foot bellows and blowpipe, so that it will expand equally, and not disarrange the seam; increase the temperature until the metal is a dull red, and the spelter runs. If necessary, with a piece of wire flattened at one end gently rub the solder along the seam until every part is joined.
Brazing Copper Rod.
—For uniting two pieces of copper rod, 1⁄4 in. or 3⁄8 in. in diameter, first prepare the joint as at A in [Fig. 64], and file the surface of the copper clean in the immediate vicinity of the joint. A mixture of borax and water and spelter should now be applied to the joint, which should rest on a small heap of broken coke, the coke being also built round it. The flame of the blow-lamp should be directed at first on the coke surrounding the joint, and then gradually brought to bear on the joint itself. If necessary, add a little more spelter before any of it fuses, and when the copper begins to get red hot, throw just a pinch of dry borax on the joint to facilitate matters.
Fig. 64.—Dovetailed Joint in Copper Rod
Fig. 65.—Dovetailed Joint in Key Stem
Brazing Key Stems.
—In brazing together the broken parts of a key stem, first it is necessary to file the fractured ends quite true; this may entail the shortening of the key by 1⁄4 in. or 1⁄2 in., and as another 1⁄4 in. will be lost in making the joint, it may be advisable to use another key bow having a longer piece of stem than the one that was broken off. With a warding file cut a dovetail on each of the ends to be joined, as shown by [Fig. 65]. A small, half-round file will assist in making the edges true and square. The pieces must interlock perfectly, and when this is the case, very lightly hammer the joint, round which then bind seven or eight turns of brass wire to act as spelter. Wet the joint, sprinkle powdered borax on it (this is to serve as the flux), and, holding the key in a pair of tongs, place it in a clear part of a forge fire made with charcoal, small coke, or coal cinders, and commence to blow steadily the forge bellows or blower. Failing a forge fire, use a blowpipe, the key being placed on a piece of charcoal or pumice-stone whilst the heat is being applied. If the forge fire is used it is as well to support the key on a guard of thick iron plate having a hole in its centre over which is the joint to be brazed. By this means the necessary local heating is obtained, and much labour in cleaning the key afterwards is avoided. On being heated, the borax swells and boils up, and should be pressed down with a spatula, previously dipped in cold water to prevent the hot borax adhering to it; a suitable spatula is made by flattening one end of a 1-ft. length of a 1⁄4-in. round rod, having at its other end an eye by which it may be hung when not in use. With this spatula, also, powdered spelter may be added to the joint if required. When the brass wire begins to run, assist the flow by adding powdered borax, and when all the brass has run into the joint, rub off superfluous molten metal from underneath and allow the joint to cool gradually. When cold, file up and clean the stem of the key until only a thin bright line of brass can be seen.