One must bear in mind that when a forge fire is used for brazing, the fire should be free from green coal on account of the presence of sulphur. A reducing fire is best for that purpose. It is well to mention here that a forge fire that has been used for brazing must be thoroughly cleaned before it can be used again for any welding purposes. The brass will run down and clog the tuyere. If the least bit of smelter is left on the forge it is apt to mix with the coals. In that case it would find its way into the fire, melt, and collect on any iron that may be in the fire heating. The success of any of these operations depends absolutely upon a clean fire and a clean piece of work to be done.
To prepare the pieces to be brazed: All parts to be brazed should be free from any foreign matter. They should be filed or polished with emery cloth or sand-paper. This refers not only to the parts that come into contact with each other, but both sides of the contact edge for the depth of at least 1⁄2 in. When the parts are clean, put them together. See that the joints fit. Paint the joints with a flux made of boracic acid. Mix one part boiling water with one part borax and two parts boracic acid. Keep it pasty so that it can be handled with a brush. Ordinary borax can be used if the other is not at hand. The smelter (brass) can be bought at supply stores. However, brass filings, common sheet brass, or brass wire is just as good. If brass filings are used these should be mixed with the flux. Any old tin can will do for the mixing. A spoon for dipping the flux out when wanted can be made on the end of a rod 1⁄4 in. in diameter, about 20 ins. long, by flattening out an inch of the end, drawn out quite thin and fashioned spoon shaped. Place the parts to be brazed in the fire and heat till they are red hot. Dip the spoon into the brazing material and put a little of the smelter on the joints. Increase the heat. When the smelter begins to run add more. When it shows a bright streak all around the parts to be joined shut off the blast and let it cool before removing it from the fire. When it is black hot remove it from the fire and file into shape. Be very careful to see that the pieces to be brazed are so fastened together that they will not move apart when once put into place. This can be done by packing brick or clay around them or by clamping them in place. Do not put any weight that would cause any strain upon them, for when heated this would change the position and might cause a bend in the brazed parts.
Brazing of cast-iron: Cast-iron is prepared for brazing in the same way as steel or iron is prepared. The flux, however, is not the same. It is made up of boric acid, 8 oz.; pulverized chlorate of potash, 4 oz.; carbonate of iron, 3 oz. Mix these together thoroughly, breaking up all lumps that may be there. Keep this mixture in a glass jar. Place the pieces to be brazed in a fire as you did the iron and steel. Heat it all red hot. Mix the flux with the smelter and put it on. Do not put this on at any time before the metal is red hot. Increase the heat and add more flux and smelter. When the brass has flowed into the joints, shut off the heat and let the metal cool off in the fire. Cast-iron requires a higher temperature for brazing than iron or steel. If a brass rod is used in place of the smelter, push the end of the rod in the joint to be brazed. It will melt off as fast as it is pushed in. When brazing some cast-iron, the break should be reinforced on the unexposed side by a plate of wrought iron or steel. If this is cleaned off thoroughly and placed against the break the smelter will not only fill up the break but it will hold this piece of metal fast.
Brazing steel band saws: Band saws break very often while in use. They can be repaired only by brazing the broken ends together. Do so by filing each end of the broken pieces, tapering to a very sharp edge 3⁄4 in. in length for lapping over. The length of this lapping depends upon the width of the teeth of the saw. When the ends are filed, place them in a vise that can be bought for this purpose. If no vise is at hand two wood clamps, or iron clamps, or an ordinary shop vise will answer the purpose. Put the two ends overlapping in place, and clamp close enough to these ends to keep them in a straight line and, also, for shifting while brazing. Paint the joint with flux, using the first flux mentioned in the article. Put a piece of silver solder between the laps. Now heat the jaws of a pair of close tongs white hot, and grasp the joints between the hot jaws, and pinch them together. The heat in the tongs will melt the silver and braze the two parts. Hold the tongs steady; if not, they will bend, and the saw will be rather hard to straighten when the tongs are removed. Do not remove the tongs until they are black hot. Now place the saw in a vise and file the teeth to correspond to those on either side of the joint. Smooth the roughness from the joint.
The value of brazing: A knowledge of how to braze is most useful to any one working with the metals. Many things break that can be repaired only by this process. For instance, tubing, automobile parts, very complex parts of machinery, etc., when broken cannot be welded; they must be mended by brazing. The success of the process depends upon the care taken to make the parts perfectly clean and well fitted together. If this is well done the brazing material will find its way to the closest joints and fix them.
PROCESSES OF STRETCHING IRON OR STEEL
(1) Drawing: Stretching a piece of metal in one or more directions either by hammering, or pulling, or rolling, is called drawing. In the blacksmiths' shop the term drawing always means a decrease in the size of a cross section of the piece and an increase in the length.
(2) Bending: Bending means making the metal take a shape from a straight line. It may be made into a circle or curved or bent at right angles.
(3) Twisting: In the process of twisting no change is made in the axis of the piece. The faces of the piece are changed; as for instance, when you twist a piece of paper holding one end fast and turning the other in the direction of its axis.
Upsetting: Upsetting means increasing the thickness of a piece of iron by shortening its length.