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Induction Brazing Technical

The induction brazing system

Because high-frequency current causes metals to reach a relatively low temperature quickly, low-temperature solders (melting at temperatures of 250 to 500 ˚F) for the induction joining of metal parts can be bonded at exceptional speeds. With little difference in time, however, harder solders (having a melting range of 500 to 700˚F) are also quickly melted into joints more effectively than by any other means. Induction soldering operations that ordinarily might require a half minute by irons or gas flames can be performed by induction heating equipment in a second or two.

High frequency induction brazing with silver alloys provides an excellent means of joining ferrous, nonferrous, and dissimilar metals. An outstanding advantage of using silver alloys lies in the fact that the fluidity of silver alloys provides penetration into restricted openings, which other alloys might not reach. Capillary attraction results in their spreading evenly over the surfaces to be induction brazed, thus forming a solid bond.

To some extent, silver induction brazing is a form of hard soldering. However, the joint usually becomes stronger than the alloying material, thus making exceedingly strong bonds possible. In many cases, silver induction brazing, using high-frequency induction heat for the melting of the alloy and the heating of the parts to be joined, will produce joints with strengths equal to those obtained by copper brazing, and when considered as a substitute for copper brazing, the need of a controlled-atmosphere furnace can be eliminated.

Silver alloys melt at temperatures ranging from 1100 to 1500˚F, according to their composition. They have tensile strengths of 40,000 to 70,000 lb. per square inch, but with some designs can provide joints with strengths in excess of 100,000 lb. per square inch. To obtain the maximum strength, the spacing between the parts to be jointed must be closely held. The better the fit, the stronger the joint.

In the long run, induction brazing with silver alloys is more economical than other methods. It’s quick, produces a uniform alloy flow, uses a relatively small amount of alloy, and produces a joint that requires practically no finishing. Briefly, a sliver induction brazing joint combines strength, smoothness, and ductility; it assures leak proof assemblies, and withstands temperatures that ordinarily would melt solder. High frequency heat for silver induction brazing of joints is suitable for more uses than other means of heating. The acetylene torch, for example, gives excessive heat which might have a tendency to distort parts adjacent to the brazed surfaces. The virtue of high frequency induction brazing heat lies in the closer control of heat to restricted surfaces that it provides.

Miscellaneous induction brazing joints

Fig. 93 presents various types of silver-brazed joints that are adaptable to the induction brazing process. In example A, a preformed silver-alloy ring is brazed at the joint as indicated. When heat is applied around the bottom surface of the shaft, the alloy flows in such a way that it fills the surface between the two parts.

B shows a joint in which the induction brazing material is placed at the edge of the flange, so when it melts it runs down through the jointed area. In example C, the silver-alloy washer, as indicated at D, is placed between the shoulder of the shaft and the edge of the flange. With this type of induction brazing application, it is necessary to apply pressure on the shafts so when the silver rung melts, the excess metal will go down into the joint. E shows a similar application, although in this case, the insert is made in the form of a bushing. Like example D, E requires pressure applied to it, so when the silver ring melts, the flow of the material will go into the joint to provide metal-to-metal contact.

When applying bushings of this type, it’s also possible to use a preformed ring of silver material placed under the shoulder, as shown at F. In example G, a ring is placed under the head of the bushing which, with an induction braze application, will flow into the joint. If an insert of this type has sufficient weight, it’s likely the part will settle into position as soon as the alloy is heated to its flowing point, and thus form a metal-to-metal contact. However, with light pieces it’s preferable to apply suitable pressure to insure a tight metal-to-metal joint.

Another group of induction braze joints is illustrated in Fig. 94. The example at A represents a shouldered flange induction brazed to an upset a sheet-metal opening. For this application, a washer of silver alloy is inserted prior to the assembly of the two parts. At B is illustrated a method for mounting a tube to a flange, in which the silver-alloy disk is placed under the edge of the tube and then squeezed down into position after it is melted. The alloy then runs up through the joint and forms a solid induction brazing bond.

Another way to assemble a tube to a flange is shown at C. Here, the performed silver-alloy ring is placed at the upper portion of the joint. When induction braze heat is applied, the alloy will flow down the sides of the tubing to the bottom edge, thus covering the entire surface. Another way to perform this operation is to provide a counterbore at the upper portion of the flange and set the silver ring into it, as shown at D. One of the most satisfactory methods, however, is to machine a groove either into the flange or the member to be inserted into it, and to place a preformed ring of silver alloy into the groove prior to the assembly of the two parts. The example shown at E has a groove cut into the flange and, when the alloy reaches melting temperature from induction brazing, it will tend to flow up and down, so that the entire joining surface are covered. Still another way to braze such a joint is indicated at F, in which a preformed ring of silver alloy is placed within the tube. When the alloy melts it will run out along the edge of the tube and up the sides of the joint.

In joining together sheet-metal assemblies, such as tubing and covers or drawn shells and caps, often required for containers and the like, there are many varieties of induction braze joints well adapted to the application of induction heat. Fig. 95 shows a variety of these joints. In example A, both edges of the assembly are formed out, so a preformed silver-alloy ring can be inserted between them, at assembly, as shown. For the joint at B, however, the silver-alloy ring is placed underneath the cover and, when induction brazing heat is applied, the alloy will flow up through the joint. One of the better types of joint, however, is illustrated at C. Here the inner body is preformed with a groove, into which a silver-alloy ring is placed before assembling the cap. After induction brazing heat has been applied to melt the alloy, it will flow upward toward the top of the joint and also downward, so both contacting surfaces become completely covered, thus forming an effective bond. For the joint at D, the preformed wire alloy is placed on the inside surface and, when the induction braze method melts it, flows downward through the joint.

The reverse of this induction brazing is shown at E, where the alloy ring is placed on the outside edge and, when melted, flows through the inside joint. In example F, the cover is inserted on the inside of the sleeve and, when melted, flows through the inside joint. In example F, the cover is inserted on the inside of the sleeve and a preformed ring of alloy is placed on the inside which, likewise, flows through the joint. Example G is also arranged so the cover has a groove rolled into it. Into this the alloy ring is placed prior to assembly, so that upon being heated the metal will flow up and down. Another form of induction braze, in a similar assembly, can be seen at H, where the ring is place on the inside and, when induction brazing is melted, it will flow upward. Example J is similar, though here the cover is placed over the outside of the body.

Another type of assembly, comprising a cap to a sleeve, is shown at K, in which the preformed alloy ring is placed inside, as shown. As soon as the surrounding metal is heated sufficiently to flow the alloy, it will run down through the joint. In example L, before induction braze, however, the ring is placed on the inside and must flow up into the joint. With this type of induction brazing it’s important that the ring remain in position, and usually it would be better to turn the part upside down, to ensure a perfect bond, although because of the characteristics of silver alloys, they will flow upward practically as well as downward.

The five examples of induction brazing shown at the lower position of the illustration are representative of the method used for applying shallow covers to tubular bodies. The preformed rings are mounted in various ways somewhat along the lines already discussed. With the induction braze joints at M and N, however, it’s necessary to apply pressure to the outer sleeve in order to form a metal-to-metal contact, as soon as the preformed silver alloy ring begins to flow.

Production induction brazing setups

Fig. 96 illustrates a variety of induction brazing operations for miscellaneous types of metal parts, which are representative of some of the joints that can be made by high frequency induction brazing heating equipment generators. The assembly at A includes a tube to which a solid type insert is brazed. The silver induction brazing ring is placed within a groove, and cut into the plug prior to assembly, as shown. The tube is then placed over the plug, and induction braze heat is generated to the surface by the single turn inductor as shown. For example, two stations are used, with one at the right of the fixture during the induction brazing operations. The fixture at the left however, is open and ready for loading. There are eight pieces induction brazed simultaneously, and each assembly is held in place firmly by means of individual spring plungers, located on the upper cross plate of the fixture. After the proper heat has been applied to the joint, the silver induction brazing alloy flows both upward and downward, so that a uniform induction brazing action results. Pressure on the tube against the shoulder of the insert assures correct alignment and eliminates the possibility of the tube moving out of place while being induction braze heated.

An example of induction brazing setup for joining a nose cup and a spacer is also shown. Here, six pieces are handled simultaneously. For this process, an internal type coil is used, details of which may be seen in Fig. 96 at B. The silver alloy ring is placed underneath the cup section, and when heat is applied to the inside surface of the spacer, the molten alloy is drawn up through the joint, completely surfacing the contacting areas of both parts.

The operation illustrated at C in Fig. 96 represents the induction brazing of a tube to a body, in which the preformed silver induction brazing alloy ring is placed at the joint on top. This table is arranged with two fixtures and, four pieces are induction brazed at one time. The brazing coil is arranged so that heat is generated from the outside surface. When both parts attain a temperature slightly above the melting point of the alloy, the silver runs down through the joint, thus completing the induction braze.

Induction brazing an insert to a drawn cup, in which an internal type brazing coil is used, is also possible. Details of this assembly are illustrated at D in Fig. 96. Each insert is held firmly in place during the induction brazing operation, so that after the ring of alloy has melted, the excess material will be squeezed out and distributed uniformly, thus assuring a perfect metal-to-metal joint.

For induction soldering eight bourdon tubes (used in the manufacture of pressure gauges) to the body component, two parallel type inductors are used and arranged so that the current comes in on one bar and goes out on the other. Details of the operations, shown at E in Fig. 96, illustrate the relation of the inductor to the parts to be induction brazed. The inductors, when energized, generate heat to both sides of the body from where it conducts to the tube joints.

When induction brazing tubing to a flange, internal type brazing coils are used, as may be seen at F in Fig. 96. For this operation, the fixture is arranged to handle four pieces at one time. The silver alloy ring is located in the chambered section of the flange and, upon reaching melting temperature, flows down throughout the entire joint. The outer sleeve is used as a guide to centralize the work over the brazing coil.

Preparation of surfaces

In any form of induction joining, whether an induction solder or a silver alloy is used, clean surfaces are essential. Even though a flux will have a tendency to dissolve oxide films on the surface of metal parts, it is better to clean the surfaces thoroughly before applying heat. This will assure a stronger bond. Also, surfaces to be joined should be as smooth as can be commercially produced. This procedure assures a better flow of the alloy and a more equal distribution than would result with irregular surface contact.

In connection with silver alloy induction brazing, it’s important that a suitable amount of flux be applied around the areas to be jointed, as a means of protection against oxidation. This flux, when heated, also provides a free flowing surface for the alloy, so that uniform bonding is made possible. Usually fluxes having a borax base are used and are applied in liquid form, which results in a bubbling action when subjected to heat. The fluidity of the flux must be sufficient for it to flow at a temperature below that which is required to melt the silver alloy. With a fluidity temperature differential of 50 to 100˚F, the flux will serve as a good temperature indicator during the induction brazing operation.

Induction heating equipment can be used effectively when a small temperature differential exists between the metal parts to be joined and the solder used for bonding. This is due to the fact that the heating coil can be placed far enough away to bring the heat up gradually to the melting temperature. Then, through the automatic timer, the heat can be cut off at the exact moment when the solder flows.

Multiple internal induction brazing

Multiple internal induction brazing is used when joining a steel ring to the open end of a sheet metal windshield. The ring is provided with a groove into which the induction brazing alloy is inserted prior to assembly. The parts are then assembled and placed in the fixture and they are arranged to join six pieces at one time. The fixture is provided with a hinged bar at the top, arranged with a plunger and individual spring plungers, to hold the windshields in position.

Jumper connections are used between the brazing coils to provide for a series connection, whereas copper tubing is jointed over the ends of the brazing coil connections to provide for the continuous passage of cooling water. The brazing coils themselves are made from a section of large copper tubing, formed into an oval shape.

When the operation consists of a soldering three inserts onto a sheet metal body, as well as the two different brazing coils which are used for induction brazing, the upper brazing coil is arranged so that two inserts are soldered simultaneously. The lower brazing coil, which is also of the single turn type is arranged to solder the side insert.

Continuous induction soldering

For continuous induction soldering, a turntable fixture for soft induction soldering small parts is placed on an asbestos disk, which is mounted on a slowly revolving spindle. As the parts pass to the rear over a pancake-type induction coil located under the disk, they become heated to the temperature needed to make the preplaced solder flow uniformly around the joint. After the work has passed the heating zone, it is ejected by means of a plate and falls through the chute into a tote box. A fixture of this design will find many uses in plants where much soldering is encountered. The drive to the spindle should be provided with a variable control to make it possible to adjust the heating for parts varying in size and shape. Such a design is also feasible for coils of different styles, so the length of the heating zone can be made shorter or smaller as desired. The asbestos disk offers no resistance to the passing of high frequency current. If desired, or if the part that is being soldered so requires the induction coil, it can be placed on top, permitting the work to pass under it.

Another type of continuous induction soldering and induction brazing fixture for small condenser can cover is the soft soldering process. A preformed ring of solder is place on the cover and the assembly is laid on the asbestos belt, which is power driven by a small motor with a variable feed drive. A section of asbestos board is placed under the belt to support it between pulleys. The pulleys are made of hard wood, so there are no metallic parts close enough to the induction coil to absorb any of the high frequency magnetic flux.

This type of fixture is suitable for a variety of soldering operations through the application of different coils and changing their location as needed.

Multi-induction brazing operations

For this operation, a simplified form of fixture and a series-type coil is used for silver induction brazing a sheet-steel dome to a steel base plate in which six pieces are jointed together simultaneously. The parts are prepared for induction brazing by assembling them with a ring of silver alloy at the joint, to which is applied a suitable quantity of induction brazing flux. The work assembly and its location in relation to the brazing inductor, which is of the solid type made of a copper plate, is bored to provide a coupling about 1/8 inch from the work piece. The plate is provided with saw cuts so the high frequency current follows the path of the arrows, coming in at one lead and going out at the other. A copper tube cooling coil is induction brazed around the outside edge of the inductor plate, the ends of which are attached to a hose connection providing the water supply. The six assemblies will receive the induction braze in 30 seconds.

A tandem type brazing coil used for induction brazing two assemblies at one time is made of a flat copper plate, bored out with two openings large enough to provide correct coupling from the work. This coupling is made to 1/8 inch. The plate is then slotted by a 1/32 inch cutter, so the high frequency current will pass continuously around the openings. A single copper tube will then get an induction braze on the outside to provide cooling.

When operating the changeover switch used with two station induction brazing or hardening tables, it’s possible to actuate it by means of a solenoid, which in turn is controlled by the master timer. In operation, the solenoid is actuated immediately after the completion of the heating operation on one side of the table, so the setup on the opposite side is automatically engaged, thus eliminating any manual work.

Vertically operated fixture

When using this two-station induction brazing setup, equipped with fixtures having air operated elevating platforms, the two parts will get induction brazed simultaneously by a series type brazing coil of solid inductor design. In our example, a flange and a sleeve will be induction brazed together, as well as the silver alloy rings which are used as the bonding agent. Two assemblies are placed on the fixture platform while it’s at its lowest position. Then, through operation of the air valve, the work pieces are elevated into the brazing coil to the correct induction brazing position. While one pair of parts gets the induction braze, the other fixture, is loaded. With this type of setup, a generator is in almost constant service, its only down time is the time required to throw the changeover switch and engage the start push button.

Strength of joints

There is definite relation between the strength of the joint and the thickness of the alloying agent. Usually, the closer the fit between the surfaces to be joined, the higher the tensile strength of the joint. This relationship is illustrated graphically in the chart shown in Fig. 97. The lower curve represents alloy soldered joints having clearances of from 0.001 to 0.0016 inches, for which the theoretical tensile strengths in pounds per square inch are listed in the left-hand column. Tensile up to about 8,000 lb. can be obtained when the clearance or fit is held to 0.002 inches. When the thickness of the joint increases, the tensile strength falls off until it reaches the strength of the solder.

The upper curve represents a silver induction brazing alloy, for which the tensile-strength calculations are shown in the right hand column. Silver alloy makes it possible to obtain strengths above 100,000 lb. per square inch, where the fit or clearance between surfaces is held to 0.001 to 0.003 inches. As in the case of solder, the strength drops off as the thickness of joint increases to a point where it equals the tensile of the alloy. Thin films of silver alloy offer much more ductility than heavier sections and, of course, are stronger and more economical. Clearances below 0.001 inch often results in bare spots, inasmuch as the silver alloy is unable to flow and, as shown on the curve, the strength of the joint falls off sharply at this point.

The figures represented in this chart are for tubular sections having uniform circumferential spacing, as well as for flat parts, where parallel surfaces are maintained. Usually with flat surface soldering and brazing it is best to apply a slight amount of pressure to the joint as it heats, in order to force out excess alloy and thus assure a thin film, which offers the strongest joint.

Copper brazing

Small gun sights induction brazing operations are carried out within a quartz tube, which is provided with a hydrogen atmosphere to prevent oxidation of the surface being joined. The parts to be brazed are fed over the rod. Seven pieces are inserted into the chamber at one time. The brazing coil is located within the quartz tube. Each group of parts requires a separate time cycle for induction brazing. After one group has received the induction braze, the door at the right is opened, and the next charge is inserted into the correct induction brazing position. This causes the previously brazed group to advance into a cooling chamber, which is located at the left. The operation is practically continuous and produces a total of three thousand pieces per day.

Until brazing, the parts are held together by a pin. The brazing coil used is of the hairpin type and straddles the part at each side, so that heat is induced to the joints only. One of the advantages of induction brazing in a quartz tube of this type lies in the fact that the part can be observed during the induction brazing operation. The heating cycle, however, is automatic and is controlled by a timer at the upper portion of the panel.

When joining a small tube and screw machine flange, twelve pieces will receive induction braze simultaneously. The parts are located and centralized over a stud, mounted in the base of the fixture. Since the generator is used almost continuously, the only lost time is the time required to throw the changeover switch from one station to the other, a setup of this type allows maximum output.

Conveyor soldering

This is a continuous type setup for induction soldering covers onto condenser cans. The cans are completely assembled when soldered, and they’re comprised of alternate layers of paper and tinfoil. Preparation includes fluxing the bottom edge of the can body by dipping it in a solution. A preformed ring of solder is then placed within the cover, and finally the cover is assembled to the condenser body. The fit between both parts is important so the proper joint may be obtained.

With the assembly ready for induction soldering, it’s placed on the conveyor and fed between inductors at a speed of approximately 10 feet per minute. As the work passes the inductor, the high frequency current flows around the entire edge of the condenser assembly, causing the solder to melt and flow within the joint. The heat is inducted to the joint area only, and therefore the condensers cool rapidly after they pass the heating coils.

Because of the shape of the part, it’s sometimes preferable to join one piece at a time with the coil used for brazing a bourdon tube to a flanged casting. For this operation a focus type inductor with a connected coil surrounding the entire surface to be heated is used. The heating coil is made from a flat tubular shape of copper, so that water cooling can be maintained.

With a fixture used for induction soldering a series of plates to the shaft of a small variable radio trimmer condenser, by means of a single turn inductor coil, the assembly is first placed within a fixture, then a strip of solder material is placed on top. With the assembly in place, power is applied to the coil, so the high frequency current is induced into the shaft and plates, heating them sufficiently to melt the solder, and to solder all joints at one time.

When induction brazing four lugs to a cylindrical receiver body, the fixture is arranged to hold two pieces so eight joints are completed at one setting. The parts are assembled on to the fixture body when the locating plate is at its lowest position. By raising the cross handle, the work pieces are elevated so the lugs are surrounded by the brazing coil. The brazing coil in this case is of the solid series type, 3/8 inches thick and formed out on the underside to clear the shape of the work piece. A preformed ring of silver alloy is placed over each lug before the parts are placed on the fixture.

Continuous feed for induction brazing

The assemblies for induction braze are placed on a continuous conveyor that is arranged with a series of work holding stations. The parts enter the compartment in the center, where the induction heating brazing coil is located. The parts are heated as they pass the brazing coil and then make a complete circuit around the conveyor. This process provides sufficient time for cooling, so they can be removed at the same position in which parts requiring induction brazing are loaded.

Induction heating equipment is widely used in progressive feeding operations for induction soldering, induction brazing, and hardening, in which various types of conveyors can be used. As a rule such operations should be arranged so a variable speed is available, as well as a certain amount of adjustment to the heating coil. With this combination, it’s possible to make adjustments to compensate for incorrect heating temperatures. For example, if during their travel the parts should become a little too hot in relation to the heating coil, the conveyor system can be speeded up slightly. On the other hand, if loading difficulties should make this procedure undesirable, the coupling of the coil might be increased slightly to reduce the generation of heat proportionately.

For induction brazing alloy steel inserts to cast-iron valve bodies, the setup can be made in the center, when the bushing is of sufficient size to permit the use of an internal brazing coil. The two leads coming from the brazing coil should be kept very close together, however, so there will be minimum heat loss along the flanged surface, where the leads enter the work.

Sometimes the insert of these parts is of such proportion that an internal type brazing coil cannot be used. In this case, it’s possible to use a single loop cylindrical brazing coil. On the other hand, when the flange of the insert is wider, requiring a broader distribution of heat, it’s possible to use a multi-turn pancake coil.

The induction brazing of valve inserts is more economical than the former method mentioned, which required heating of the entire valve body. In operations of this kind, however, there are limitations and restrictions usually because of the shape and size of the part. Yet, with slight modifications in the design of valve, induction brazing is possible.

When induction brazing fins to a sleeve, the assembly is arranged so six pieces are brazed at one time. The brazing coil is of the internal hairpin type, over which the sleeve assembly is placed. Each work holding station is provided a locating sleeve for alignment of the assembly, to assure concentricity with the brazing coil. The fins are spot welded to the sleeve, as a preceding operation, to assure a firm metal-to-metal assembly. For induction brazing, a preformed silver-alloy ring is placed over the sleeve, so that when the assembly is properly heated, the alloy flows throughout the joining surfaces.

Spot welding and induction brazing provide an excellent means of joining a large variety of assemblies, especially those that do not hold together by virtue of their construction. Many flat assembles can be joined by this combined method. Usually a small piece of silver wire, placed on top or adjacent to the joint, will flow into the contacting surfaces by capillary action, thus producing an exceptionally strong bond.

Induction brazing of tools

The application of high frequency heat provides a fast and dependable method for the silver alloy induction brazing of tungsten carbide cutting tools. The process is usually carried out by forming a brazing coil according to the shape of the tool and is arranged so heat will be applied only to the area surrounding the joining surfaces. The coils used can be shaped in many ways and can be of the single turn or multi-turn type, depending upon the size of tool. With single shank tools, the carbide tip and recess should be provided with a good fit; then, a piece of silver induction brazing material may be placed either on top of or directly under the tip.

Another way to induction braze tools is by means of a multi-turn brazing coil which surrounds the entire end of the shank to be brazed. With this method, the tool is inserted into the brazing coil and withdrawn after heating has completed, so that as the alloy sets, the tip can be pressed firmly into place.

When using this form of induction brazing, the heating coil can be more loosely coupled around the straight shank tool located within the brazing coil because of the high intensity of the flux generated within the coil. As soon as the tool has become heated beyond the melting temperature of the silver alloy, it is withdrawn to one side, so that the tip can be pressed down into position to form a firm metal-to-metal contact while squeezing out any excess induction brazing material.

For the induction brazing of tungsten carbide inserts to milling cutters, a copper tube brazing coil of the double hairpin type can be used, so that heat is generated around the entire area of the tip and the adjacent area of the cutter body. With this type of setup, each tooth is brazed separately.

A variety of tungsten carbide tipped tools, some of the single shank type and other of the mutli-blade type are shown. One of these is the reamer within the brazing coil at the upper section, which can be fabricated by induction brazing. With multi-brazing, such as reamers would require, it’s necessary to hold the tips in place by wire or some other means, so the inserts will be held firmly against the recesses of the cutter body during the induction heating equipment operations.

For an example of using induction heating in tool induction brazing in which an inert atmosphere is used, a setup is arranged for the brazing of tungsten carbide tipped reamers. The induction brazing operation is carried out by means of a copper strip, which requires a melting temperature of about 2100˚F. The brazing coil is located around the outside of a cylindrical quartz chamber into which hydrogen is brought from the underside and burns off through the tube. The use of the quartz chamber permits observation of the reamer during the heating cycle.

In some instances, it’s possible to copper induction braze by means of induction heat without the use of a controlled atmosphere tub. For the most part, such operations are limited to very small parts that heat up at exceptionally fast rates. Usually, however, the steel part being brazed develops a scale at temperatures exceeding 1800˚F. The result is that the brazed surfaces become contaminated and will not produce a satisfactory bond.

When using a high frequency inductor, comprising two parallel bars, like those used for the soldering of condenser cans with an inserted type cover, a preformed ring of solder is placed around the inside joint. When the inductor is energized, heat is concentrated to the open portion of the assembly, causing the solder to flow into the joint and provide a smooth, tight joint. For the soldering of parts, such as condensers, the work can be fed on a conveyor under the coil, or a series of condensers can be placed in a fixture having a vertical travel then elevated to proper relation to a long heating inductor, comprising two parallel bars.

For the induction brazing of six tubes and flanges simultaneously, the brazing coil is located at the uppermost section of the panel. The parts to be induction brazed are inserted through the coil and are located by means of a plate just below the brazing coil. Alignment of the tubes at the bottom end is accomplished by cylindrical bushings, into which the tubes fit. The upper portion of the tube is flared out or beaded, so that it fits into the counterbore of the flange. A ring of silver alloy is placed at the joint; when the proper temperature has been reached, it flows freely through the area requiring joining.