Brazing is essentially a form of soldering. In brazing a comparatively infusible alloy such as brass is substituted for the conventional alloy used in soldering. The brazing alloy must have a melting point lower than that of the metal being bonded but higher than the melting point of solder.

Welds are made by fusing the metal at the site of the bond. Generally the process requires the addition of metal, which must be the same as the metal being bonded. Both brazing and welding are done at temperatures much higher than those developed by soldering irons and blowtorches. Special torches that attain the necessary temperatures cost more than the average layman is willing to spend in view of the limited use he is likely to make of the equipment.

The problem of cost appears to have been solved by Robert Cushman of Princeton Junction, N.J. He is a specialist in metal bonding. Cushman recently improvised a torch in the form of an electric arc. The device can weld steel plates up to a quarter of an inch thick. It can be made easily at home. Cushman writes: "Although the twin-electrode arc torch has been around for many years, it has never received the attention it merits. When properly used, it is astonishingly effective.

"Essentially the device consists of a pair of carbon rods that meet at an angle of about 45 degrees. When the carbons are supplied with an appropriate electric current, touched together momentarily and then separated, an electric arc appears in the form of a V-shaped flame that is much hotter than the melting point of any metal. The carbons are supported by a pair of copper rods clamped in a wooden handle [see illustration at left]. The lower rod is fixed in the handle; the upper one can be moved in or out by a knob. Movement of the rod facilitates striking and adjusting the arc. By means of the handle the arc can be manipulated much like the flame of an oxyacetylene torch.

"The arc operates on 120 volts at currents ranging from three to 60 amperes, depending on the amount of heat wanted. I use carbon electrodes as small as 1/16 inch in diameter and as large as 1/2 inch. The size depends on the amount of heat needed and on the thickness of the material to be bonded.

"Welding carbons differ from conventional carbon electrodes in two respects. First, they are coated with a film of copper to minimize the change in resistance that would otherwise result as the carbon burns away. Second, the center of each electrode is filled with a core of rare-earth material. The core tends to sustain the arc during the portion of each cycle when the voltage falls to zero

"The torch can be operated on either alternating current or direct current. When it is operated on direct current, one electrode burns away more rapidly than the other. Special welding carbons are available from dealers in welding supplies and from the larger mail-order houses. They cost between 25 cents and $1, depending on the size. The carbons last for many hours of welding and so are relatively inexpensive. Flux must be applied in brazing or welding just as in soldering. A can of general-purpose flux should be bought along with the electrodes.

"Power is supplied to the arc through a network of eight resistors. Parts of the network can be switched in and out of the circuit for regulating the current. The network consists of 1,000-watt Nichrome heating elements, which are available in most hardware stores. The resistors are interconnected by toggle switches rated at 10 amperes. One of the switches is of the double-pole, double-throw type. The others are of the single-pole, single-throw type. Suitable switches can be bought for about 60 cents each at radio supply houses.

"The switching scheme enables the experimenter to connect two resistors in series, use a single resistor or, in six steps, progressively connect all eight resistors in parallel. Current is thereby controlled in steps ranging from three to 60 amperes. If the house wiring is fused for a maximum of 15 amperes, all except the first two resistors of the network (and their companion switches) can be eliminated. In such a case the torch will be less useful, but it will still find many applications that involve metals up to about an eighth of an inch in thickness.

"The resistance elements of the network are supported by ceramic insulators fixed to a base of Transite, which is a stiff board of asbestos composition. It is available from dealers in building supplies. The ceramic insulators, known as nail-and-knob insulators, can be bought from dealers in electrical supplies. The nail and the leather washer that come with the insulator are replaced by a machine screw and a spring washer. The toggle switches are also mounted on the Transite base [see Figure 4].

"Electrical connections between the heating elements and the switches are best made by straightening a few turns of the coiled wire at the ends and twisting the ends together. Additional connecting wire can be obtained by straightening an extra heating element. A stud bolt equipped with seven pairs of nuts can be used as a bus bar for interconnecting the common end of all heating elements. Clamp between the nuts the leads to be connected. Do not use copper wire for interconnecting the unit Copper will oxidize rapidly at the temperature reached by the heating elements.

"The Transite base rests at the sides on brackets of angle iron attached every six inches by machine screws. A cover of perforated steel or expanded metal that shields the heating elements can be attached to the angle brackets. The angle brackets should be joined on the bottom by two or more lengths of strap iron. The installation of the strap iron gives you an excellent opportunity to make your first braze. The completed metalwork should be connected electrically to the grounded lead of the torch.

"An electrical outlet of the 'mogul' type, rated at 60 amperes, should be installed near the location at which the welder will be used. Identify the grounded lead of the power line and make sure the polarized plug of the welder is connected so that the upper electrode of the torch is at ground potential. It is possible, of course, to connect the apparatus directly to the main circuit of the fuse box by means of storage-battery clips, thus avoiding the expense of installing the special outlet. The reader is advised against adopting this expedient, which is not only hazardous but also illegal in most communities.

"Because the arc emits intense radiation in both the ultraviolet and the infrared portions of the spectrum, a protective face mask fitted with dark glasses must be worn. Moreover, the experimenter must protect his skin with opaque covering. Neglect of these precautions invites radiation burns. Face masks designed for arc welding cost about $10. An adequate mask can be made of fiberboard and equipped with welding goggles of dark glass. The goggles cost $3.50.

"The selection of carbon electrodes and the optimum current for a given job of welding or brazing must be determined by experiment. In general electrodes 1/8 inch in diameter operated at a current of three to five amperes are appropriate for joining wires up to 1/16 inch in diameter and for bonding even smaller wires to metallic foils. Steel plates 1/4 inch in diameter require 1/2 inch electrodes and a current of 60 amperes.

"The operating procedure is simple. Set the switches for a current of 15 amperes. Check to make sure your clothing is buttoned, then don the face mask and a pair of gloves that cover your wrists. Move the electrodes together slowly, by means of the slide button, until the tips just touch. After one or two seconds a glow will appear at the point of contact. When the glow is well developed, slowly move the electrodes apart about a sixteenth of an inch and let the arc operate for five seconds. The electrode separation should then be increased until the arc emits a soft hum. Normally the sound will develop at a separation of about a quarter of an inch.

"The heat generated by the arc increases with its length. This is a desirable characteristic not shared by most arc-welding equipment. The arc starts easily and is stable in operation. Both characteristics derive from the substitution of resistors for the special transformer commonly used in welding equipment of other types. Because resistors are used for limiting the current, full-line voltage exists across the electrodes at the moment the arc is struck. Once the arc has become established, the current can be increased or decreased by operating the switches. Usually the arc cannot be struck unless the switches are set for a current of about 15 amperes.

"Welding is an art and its mastery comes only with practice. Beginners will doubtless discover that the most successful initial bonds are made by brazing I find brazing at least 100 times easier than welding. Moreover, a good braze is usually far stronger than a poor weld. In general when two parts are welded, at least one of the metals must be heated to its melting point, whereas in brazing neither part need be heated to more than a few hundred degrees below its melting point. The brazing alloys become fluid at these temperatures and form smooth joints with a pleasing appearance, much like a conventional solder connection. The parts to be brazed, as well as the brazing alloy, must be coated with an appropriate flux. The brazing temperature is not particularly critical.

"In contrast, any perceptible variation from optimum temperature in the case of a weld results in either a poor bond or damage to the metals that are being joined. The artisan must create a pool of molten material bounded by the solid metal. The material must reach the fluid state or a poor bond results. On the other hand, if all the metal becomes molten, the material flows out of control and the work is damaged. For this reason thick pieces are much simpler to weld than thin ones Before attempting to weld thin wires and foils the beginner should consult one of the references listed in the bibliography.

"The resistance network can also be used for making bonds by means of a single metal electrode. In this method of welding the arc is struck between the electrode and the workpiece, which has been connected to the grounded lead. I do not recommend the technique, particularly for beginners.

"A tool of the suggested design can be assembled for about $30, a modest figure compared with the cost of commercial welders of equal performance. On the other hand, the apparatus is grossly inefficient in its consumption of power. At least 50 percent of the energy is lost in heating the resistors; compared with about 5 percent in commercial equipment that uses transformers and inductors for limiting the current. Power is cheap, however. At the rate at which the average amateur makes welds a lifetime of use would be required for the low efficiency to offset the saving in first cost.

Experimenters often have occasion to bond wires and sheet metal less than .040 inch thick, particularly when making electrode assemblies for use in gas-discharge tubes and similar apparatus. With practice such welds can be made in a matter of minutes using the apparatus described by Cushman. A spot-welding machine can join thin materials in seconds and its operation makes little demand on craftsmanship. The parts to be joined are first clamped between a pair of copper jaws. An electric current is passed through the small area where the pieces make contact. The zone of contact acts as an electrical resistance that converts the current into heat. The heat melts the surfaces. They fuse, forming the bond. A successful bond results only when the proper amount of current is applied for a proper interval to surfaces of proper resistance. The spot-welding machine is designed to take these three variables into account.

The editor of this department improvised a satisfactory spot-welder about a year ago, largely from the contents of his scrap box. My improvisation made use of such items as an old rattrap, a knife switch and a hinge from a barn door. The essential component consists of a set of four electrolytic capacitors connected in parallel by copper bars. They are also connected to a pair of copper jaws by flexible storage-battery cable of the kind used in automobiles. A quick-acting knife switch closes the circuit automatically when the metal parts to be welded have been clamped between the jaws. The charged capacitors send a pulsed current through the jaws; the current makes the weld. Accessory components include circuits for charging the capacitors with an appropriate amount of energy and for the operation of the jaws and switch.

Spot-welders of this type were made possible by the development of electrolytic capacitors. The energy stored by a capacitor is equal (in watt-seconds) to half the product of the capacity in farads multiplied by the square of the voltage to which the capacitor is charged. When connected in parallel, the four electrolytic capacitors of my unit total 6,800 microfarads, or .0068 farad. They can be charged to a maximum potential of 180 volts. At this potential the stored energy amounts to 180 x 180 x .0068/2, or 100 watt-seconds. I bought the capacitors from Barry Electronics Mail Order Corporation, 512 Broadway, New York, N.Y. 10012. They cost $2.25 each. Capacitors with different voltage and capacitance ratings can be substituted if they are capable of storing about 100 watt-seconds of energy.

The welding fixture consists of a pair of threaded copper rods supported in vertical alignment by the hinge and a wooden base [see Figure 5]. When the hinge is lowered, the tips of the copper rods meet and clamp the metal to be joined. The hinge is operated by a treadle that applies force on the workpieces through a spring. The amount of force can be adjusted by altering the position of a nut that controls the compression of the spring and also by substituting weaker or stronger springs. The electrical resistance of the contact between the workpieces to be joined varies with the applied pressure. Hence the adjustment of the spring partly determines the extent to which the current will heat the metal. At the lower limit of its travel the treadle activates a small switch. This switch in turn trips a knife switch that discharges the capacitors. In this way the operator can use both hands to manipulate the pieces to be welded.

The knife switch must be closed quickly to prevent the blade from welding to its seat at the point of initial contact. Any fast-acting device can be used to close the blade. I used a rattrap that is tripped by a solenoid [see Figure 6]. Although resetting the trap is inconvenient, the alternative is to pay about $65 for a quick-acting, automatically resetting contactor of the kind used in commercial spot-welders. The rattrap gadget can be built for less than $5.

The temperature developed in the weld is determined in part by the amount of stored energy; this quantity depends in turn on the voltage to which the capacitors are charged. The voltage can be varied as desired by inserting either a potentiometer or a variable-voltage transformer in the circuit ahead of the diode that provides direct current for charging the capacitors. All diodes can be lN1763 or the equivalent. The solenoid is a Guardian Type 11 that operates on 24 volts (direct current) and develops a pull of 21 ounces. This unit is available from Allied Radio, 100 North Western Avenue, Chicago, Ill. 60680, as are equivalent solenoids that operate directly on 120 volts (alternating current). To minimize the hazard of accidental shock an isolation transformer was inserted between the power line and the power supply. Equivalent isolation can be provided inexpensively by interconnecting the secondary windings of two filament transformers and using the primary winding of one as the input and the primary winding of the other as the output.

The optimum force with which to clamp the parts to be welded and the optimum voltage to which the capacitors should be charged must be determined experimentally for each kind and size of weld. In general both the clamping force and the voltage increase with the size of the weld. A record should be made of the proper settings established for each kind of weld. The record will serve as a rough guide to the best trial settings for similar welds.

Carl Fromer, a high school student in Staten Island, N.Y., has hit on an effective method of cleaning the soft-coated dielectric mirrors used in continuous lasers of the helium-neon type. The coatings accumulate a film of grime after some weeks of exposure to the air. The film seriously reduces their reflectivity. Any chemical treatment that dissolves the grime destroys the coatings.

Fromer removes the grime by electronic bombardment. He inserts the soiled mirrors face up in a simple gas-discharge tube, exhausts the tube to a pressure of about one torr, backfills with argon or with a 7:1 helium-neon mixture to a pressure of 10 torr and operates the tube for one minute at a current of 60 milliamperes [see Figure 7]. The mirrors emerge looking like new.

In addition to grime, the coatings are vulnerable to moisture. Hence the mirrors should be stored in a dry place. A good place is an airtight four-ounce jar. Put an ounce of anhydrous calcium sulfate on the bottom and then add a layer of absorbent cotton.

Sam Epstein of Los Angeles, who designed the spectrograph described in this department in September, calls attention to the fact that the instrument was designed for a diffraction grating with a focal length of 53 centimeters. If a grating of differing focal length is substituted, appropriate changes must be made in the dimensions of the instrument.

Bibliography

WELDING FOR ENGINEERS. Harry Udin, Edward R. Funk and John Wulff. John Wiley & Sons, Inc., 1954.

THE WELDING OF NON-FERROUS METALS. E. G. West. John Wiley & Sons, Inc., 1952.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.