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Melting Point of Metal.—Continued.
..2400 Wrought iron
From the foregoing it will be evident that the metals of which the average solders are composed have melting points but little lower than that of aluminum, and careful manipulation will be necessary to insure heating the metal sufficiently to melt the solder, and at the same time not weaken the aluminum or cause it to flow. To successfully solder aluminum demands considerable preparation and careful manipulation, and authorities differ as to the best methods and solders to use. All agree that the metal must be dry and absolutely clean, and while this is easily said and advised it is difficult of accomplishment. If one cleans aluminum in ordinary temperatures, no matter how carefully, and obtains a bright surface, it is impossible to retain this as new oxide forms on its surface as soon as brightened. For this reason, some skilled in the arts recommend immersing the parts to be soldered in a strong solution of hydrosulphate of soda for several hours before joining them. The best solder to be used depends to a certain extent upon the alloy of aluminum, the same as with other metals. One that has been used with good success is made as follows: Ten parts each by weight of tin, cadmium and zinc and one part of lead all of which are melted together.
Dampness or salt air are the bane of the process of aluminum soldering and the rapid oxidization makes "tinning” which is of such benefit in uniting other metals, practically impossible. As moisture will hasten oxidization, the safest method of proceeding will include the drying of the surfaces, assuming that these have been properly prepared, as well as the solder, in an oven or other suitable container at a high enough temperature to thoroughly heat all parts, but lower than that required to melt the solder. When the parts are thoroughly dry, the next thing to do is to scratch the surfaces vigorously with a metal brush, bringing out the untarnished metal, and removing all traces of oxide. The next step is the heating of the aluminum parts nearly to the melting point of the solder, and after applying the alloy to the surfaces, brush it into intimate relation with the surfaces, using the metal brush. If the solder does not adhere there is still some oxide on the surface, and the operation of cleaning should be repeated. When the surfaces to be joined are properly coated with solder, the rules which govern ordinary soldering work will apply, it being merely necessary to heat the surfaces, melt the solder and join the surfaces. Either a bunsen burner or blow torch can be used to heat the metal, and the important points to remember are that the work must be done quickly and that the surfaces to be joined be as clean and dry as possible, and as free from aluminum oxide as conditions will permit.
If the surface is of such a shape that it cannot be readily cleaned by scraping, it can be cleaned by dipping it into a solution of nitric acid in three times its bulk of hot water containing about 5 per cent. of commercial hydrofluoric acid. This causes a slight action on the surface of the metal as shown by bubbles. Rinse the metal after removing from the acid bath and dry in hot sawdust. There are various compounds on the market for soldering aluminum, but this operation depends more on the workman than on the solder and unless considerable experience has been had it is probably better to purchase solder than attempt making it. Zinc can be used but does not form a very strong joint. Tin can also be used; is more nearly the color of aluminum, is stronger than zine, but is very difficult to work. A small proportion of phosphor tin added to pure tin makes it work more readily and is the basis of most aluminum solders. The chief difficulty in soldering aluminum is that the heat is dissipated so rapidly that it cools the soldering iron and furthermore aluminum oxidizes instantly upon exposure to the air. This extremely thin film effectually prevents a perfect union being made. If the parts are well heated and melted solder kept hot while the iron is allowed to stand on it, the surface can be scraped beneath the melted solder by the point of the soldering iron, thus preventing to a certain extent the oxidization. In this way the metal can be tinned. When both parts are brought to
gether and are well tinned, they can be united with some chance of success, nitrate of silver, resin, or zinc chloride being used as a flux. A soldering tool of nickel gives more satisfactory results than a copper one as the latter alloys with the tin and soon becomes rough.
Another authority advises as follows: Use zinc and Venetian turpentine for soldering small surfaces. Place the solder on the metal and heat very gently with a blowpipe until entirely melted. Another is to clean the surfaces by scraping and covering with a layer of paraffine wax as a flux then coating the surfaces by fusion, using a layer of alloy of zinc, tin and lead, preferably in the following proportions : Zinc, five parts; tin, two parts; and lead one part. Metallic surfaces thus prepared can be soldered by means of zinc or cadmium, or alloys of aluminum with these metals. Twenty-eight ounces of block tin, three and one-half ounces of lead, seven ounces of spelter, and fourteen ounces of phosphor tin, containing 10 per cent. of phosphorus, will make a good aluminum solder. Clean off all dirt and grease with benzine, apply the solder with a copper bit, and when the molten solder covers the metal, scratch through the solder with a wire scratch brush. A good solder for low grade work is composed of tin, 95 parts, and bismuth, five parts. A good flux in all cases is either stearin, vaseline, paraffine, copaiva balsam or benzine. Small tools made of aluminum or nickel should be used in the operation of soldering. These facilitate at the same time the fusion of the solder and its adhesion to the previously prepared surfaces. Copper or brass tools should be avoided as they would form colored alloys with the aluminum and the solder. To sweat aluminum to other metals, first coat the aluminum surface with a layer of zinc, on top of which is melted a layer of alloy of one part aluminum to two and one-half parts of zinc. The surfaces are then placed together and heated until the alloy between them is liquefied.
How to Braze Steel and Iron.-As with soldering, it is important in brazing to clean the work thoroughly. Sand blasting is an ideal method of cleaning for brazing, although the work may be done with a file and emery cloth. The sand blast not only cleans the metal of all scale, but penetrates the pores, leaving it in condition to receive and hold the brass. It also costs less than the hand method. There are several compounds in the market that will make a better flux than borax, burnt and ground fine, but if wanted for a quick job, mix borax with wood alcohol, or, better still, “Columbian Spirits.” Clean water is nearly as good. Mix to a thin paste and apply with a thin brush, so as to wet thoroughly every part of the joint. The flux is held in place by painting the joint with a mixture of machine oil and black lead. The joint should be pinned to hold it in place while being brazed. Run a No. 29 drill through the job and hold in place with an eightpenny wire nail. The fire should be clean, whether of gas, coal or oil. The gas fire is best, though it costs more. Put the heat on the heavy part of the work first, so as to bring it up nearly to the brazing point. When the heat is put onto the joint the heavy part will absorb it and cool off the part to be brazed. Bring it up slowly to a bright yellow heat, and as the spelter and flux begin to melt, dip the brazing wire in the brazing compound and apply to the joint. Before dipping the wire, however, it should be held in the flame so as to heat it as near as possible to the melting point and yet not melt it. As the flux and spelter melt turn the work so it will run to all parts of the joint, and while still turning remove it from the fire and keep it in motion until it sets. If it is a large job, turn off the heat and let the blast strike the work and cool it.
Nothing equals the sand blast for cleaning work after it is finished. The next best method is pickling in a weak solution of sulphuric acid and water, about one quart of acid to a barrel of water. The old-fashioned method of dipping the work in a pail of soapsuds is not recommended. Almost any broken joint in cast iron can be brazed, and if properly done it will be stronger than be. fore breaking. To make a good job first heat the work to a dull red, taking the dirt and grease out of the pores of the metal. Nest clean the work with a sand blast or with a wire brush, after which apply the flux. Fasten the broken parts firmly together, place in the fire and bring up to a bright yellow heat, in fact almost to the melting point, and apply the brazing compound. Shut off the gas and allow it to cool without moving. Brazing is possible even if the pieces are of irregular form though large work should be pre
heated before brazing as described in speaking of the autogenous welding process.
Simple Methods of Testing Lubricating Oils.—To find if an oil contains certain solid impurities, add kerosene to half a cup of the oil until the mixture becomes quite thin. This thin fluid is now passed through filter paper or ordinary colorless blotting paper. As soon as all of the thinned oil has passed through, the blotting or filter paper is washed with kerosene. The residue that remains, if there is any, will show whether the oil contains any solid impurities. Impurities of this kind may also be determined in a coarse way by smearing a piece of common correspondence or pad paper with the suspected oil and holding it against the light. If the oil is free from solid impurities the blot of oil will be equally transparent everywhere. If not, the solid particles of sediment will be plainly visible.
To test whether an oil becomes resinous or not, it must be poured in a shallow dish, and it is then to be left for about a week in some warm place. If at the end of this period there is not the slightest evidence of a crust you may consider the lubricant to be all right. These oils may also be tested by mixing them with nitric acid. If the oil is pure, a thick mass will form in a few hours. Oils that resinify do not thus clot, but remain very thin.
Among other impurities in oils are to be found injurious acids. When acids occur in lubricating oils they destroy the parts of machines and other apparatus that they lubricate much more quickly than should be the case. A test for such impurities is found in mixing the lubricating oils with copper oxide or copper ash. These are added to the oil in a glass container. When, if the oil is free from acid, it retains its original color. If acids are present their action on the copper makes the color greenish or bluish. This test
also be made by dropping the oil on a sheet of copper or brass. Here it should be left for a week, when at the end of that time if acid is present, a greenish discoloration will be seen on the metal. Almost any of the chemical test for acid as with colored solutions and litmus paper will indicate the occurrence of acid. Litmus paper turns pink in the presence of acid. In its absence a blue color will be apparent.