DESIGN OF INDUCTION HEATING COILS

Coils for high frequency induction heating can be made in a wide variety of styles, shapes, and sizes.  Their design usually is governed by the nature of the component to be heated and by the type of generator used.  Briefly, coils can be of the single-turn or multiturn type, fabricated, multiple, series-connected, or cast to shape.  However, some high frequency generators are limited to certain specific types of coils, because of the impedance of their tank circuits which may limit their use to multiturn coils.  In view of this condition, the power source must be taken into account before a heating coil is designed.  Other generators, however, are provided with means to match practically any type of heating coil so that a much broader selection of coil designs is made possible.

In describing different types of heating coil for inductive operations, it would be difficult to cover all the different types of condition which arise, such as the shape and size of the work piece, the nature of the surface to be heated, and its relation to the heating coil.  However, from the variety of heating coils shown, representing the designs most commonly used, together with some of the basic principles involved, a good conception of the importance of coil construction will be gained.  The designs represent coils that will service the need for a wide variety of heating operations, and that with slight modifications will cove the majority of heating applications encountered.

Perhaps nothing is more important to induction heating than the coil itself.  Next is the source of power, which must be suited to particular operation contemplated.  It is here that frequency and volume of power show their possibilities as well as their limitations.  But with suitable high frequency power it is on the coil that we depend for the correct distribution of heat within a metal part.

If we consider a heating coil as a means of transferring heat by eddy currents, and then consider the shape of the coil in relation to the work's surface, we can see that the heat pattern will closely resemble the shape of the coil, more so with a close coupling than where the work and the coil are farther apart.  The problem therefore is to make the coil of correct shape to surround a surface to be treated and then space it according to the amount of heat needed.  As the coil is placed farther from the work, the eddy currents spread out in wider form and cover a larger surface.  Coupling, therefore, is an important consideration.

Not always is maximum heat transfer the most desirable factor.  Often a slow, even heat distribution is outstanding need.  For general hardening purposes a quickly and rapidly heated surface usually is desired.  For soldering a slower penetrating heat might be more desirable.  For brazing a

Fig. 37 - A variety of heating coils used in connection with induction heating equipment, including those of the single and multiturn types.

somewhat in between heat might be required, since rapid heat  might cause blistering and a slow heat would result in lost time or the heating of unwanted areas.

Types of Coils.  In Fig. 37 are illustrated some of the more commonly used coils as applied to induction heating equipment.  The general purpose type most used is one shown at A representing the multiturn design, which is would or formed from copper tubing, either symmetrical in contour or shaped to suite the outline of the part to be heated.  Solid-type inductor coils, like the one shown at B, are also widely used and are particularly suitable for heating of parts where a restricted heat zone is desired.  The coil shown at C is of series design, which makes possible the heating of several pieces at onetime.  The coil at D is made of flat strip, which, as may be seen, may be used in a variety of heating operations.

Regardless of the types of coil used, it is necessary to provide cooing, which in the case of the multiturn coil is accomplished by circulating a flow of water through the coil itself.  With the single-turn inductors, cooling is accomplished by adding a copper tube to the outside or by other suitable cavities for the passage of water.

Fig 38. - Multiturn coils can be formed to various shapes and usually have a wide latitude of applications in connection with induction heating work.

All heating coils, regardless of their design or shape, should be made of copper having a conductivity of 90 per cent or more.  Pure copper unquestionably serves as the best material for coil construction.

Multiturn Coils.  In making multiturn coils of copper tubing, a wide variety of shapes is possible, as shown in Fig. 38.  The most common is a cylindrical coil, as at A, which is suited to surface-heating of shafts and round parts.  The rectangular or square coil at B, as well as the cam-formed coil at C, also is used for heating the outer surfaces of bars or shafts and can be easily formed and would over a wooden block.  The pancake coil at D is used for heating flat surfaces, such as clutch jaws, or ends of shafts, while the spiral-helical coil E is used for heating conical surfaces, such as bevel gears.  The coil at F is of the internal type and is used for heating inner surfaces of holes.

Other suggested types of induction coils made of copper tubing, arranged for heating irregular surfaces, may be seen in Fig. 39.  The coil at A heats the surface around the slot, while the one at B heats the end section of a formed steel bar.  Coil C, somewhat similar in shape, heats on edge and the fillet of a plate, one view of which illustrates the coil in position during heating.  With irregular surfaces such as these, requiring coils of intricate shape, it is often necessary to experiment until

Fig. 39 - Copper tubing can be formed into various odd-shaped coils, as shown, for the localized heating of metal parts.

the correct heat pattern is obtained.  Sometimes only the spreading of the coil, one way or another, might offer the results desired, whereas in other cases a completely new shape may have to be wound.

In Fig. 40 suggestions are shown for the construction of copper-tube coils.  Such coils usually are made of tubing ranging from 1/8 to 1/4 in. in diameter.  The 1/8 in. size should be used very sparingly, however, because of its small area and since the flow of cooling water is likely to be too small to prevent overheating.  For short heating cycles, where a slight heating of the coil may have no effect, this size could be considered.  However, the two larger sizes are preferable.

Round copper tubing can be used for many types of coils, as shown in A, although it is usually preferable to flatten the tubing as illustrated at B.  Another practical form is the square or rectangular shape shown at C.  It also is possible to use a larger diameter tubing, such as 5/8 or 3/5 in., as shown at D, and to produce a flat coil similar to a solid inductor previously mentioned.

In making helical copper-tube coils where a restricted heat zone is desired, the coils should be made with an offset, as shown at E, so that uniform heating can be provided.  At F is shown

Fig. 40 - Constructional details of copper-tube heating coils of the multiturn types are shown here.

a means of providing a brace for large-diameter coils, which might have a tendency to sag or go out of shape.  These braces provide unusual stiffness and, as shown, are attached by brazing small rivets to the coil section and heading the rivet over on the outside of the braces.  These braces can be made of thin asbestos board, or a laminated plastic material.

A suggested form of tube flattener for use in the making of heating coils requiring square, oval, or other formed sections, is illustrated in Fig. 41.  This device consists of two formed rolls, through which the copper tubing is passed. Feeding of the tubing through the rolls is done by means of the hand crank.  The upper roll is mounted on a hinged bracket, which is adjusted. to correct relation with the lower roll by means of the hand knob B.  At C is shown the type of roll which can be used for forming a square-shaped section from a round tube, whereas at D is illustrated the roll used for providing an oval - or flat - shaped tubing.

Fig. 41 - A hand - operated tube flattener, provided with forming rolls, can be used in the construction of heating coils requiring flat tubing.

Heating Rates.  Because of the path of magnetic flux around a cylindrical coil like that at A, Fig. 42, the greatest field strength will be obtained within the coil itself, rather than on the outside.  For this

Fig. 42 - Three most popular types of multiturn coils are the external, pancake, and internal designs shown here.

reason outside surface-heating of parts, such as steel shafts, can be preformed exceedingly fast.  With a flat- or pancake - type coil, as at B, heating will be somewhat slower per square inch of area, or roughly 75 per cent that of the example at A.  With internal-type coils, like those shown at C, heat transfer is still slower, because less magnetic flux is concentrated on the work surface, and may be heated only 50 to 60 per cent as fast as outside heating.  These estimates are based on the use of a high frequency generator of a given output power.  for non-ferrous materials heating times will be somewhat  greater than for ferrous materials, usually in the order of 1 1/2 or 2 to 1.

Internal Coils.  since the density of the magnetic flux is less on the outside than on the inside of a coil, and because the greatest strength lies next to the coil, it is advisable to make internal coils in

 Fig. 43 - Constructional features of internal coils are shown here.  Coils of this type should be made as thin as possible and should be located close to the work's surface.

such a way that the over-all distance from the surface of the hole being heated to the inner surface of the coil is held to an absolute minimum.  In A, Fig. 43, is shown a multiturn coil of usual design, which, when applied within a hole, results in the dissipation of heat energy because of the excessive over-all depth between the work surface and the inner diameter of the coil.

At B the condition is greatly improved, since the coupling between the work and coil is reduced and since the coil is flattened.  This means that the over-all depth is held to a minimum, thus assuring maximum heat energy, the magnetic flux being more constricted to the heated area.  The coupling for internal heating coils should be made no more than 1/16 in., and less if possible.

Internal cols for hole heating usually are limited in size because of their mechanical construction, and usually a 5/8 in. hole is the smallest that can be heated with a multiturn coil, as shown at C.  Another form of internal coils is the double-turn coil, illustrated at D, in which the tubing is flattened to provide for maximum heat transfer.  In making coils of this type, care should be taken to avoid closing in of the tubing, so that the water passage will not be restricted.  Also, because the hat pattern within the hole is likely to resemble four longitudinal bands, the work piece should be rotated during heating.  The coil at E provides still another design using formed tubing, thus permitting the coil to be small in diameter.

The hairpin coil at F also is practical for small-hole heating, but the part must be rotated during heating to assure uniformity of heat transfer.  Usually internal coils of the types shown are limited to surfaces in which the height of the heated surface is not more than twice the diameter.  Where the length is in excess of the ratio progressive heating should be provided if possible.  In this case the heat is concentrated to a small area of the hole, while the work itself feeds at a uniform rate, depending upon available output capacity of the generator, the size of the hole, and the area being heated.

When heating a tapered surface, the coil is usually made to conform with the taper, although exceptions may be considered as at A, Fig. 44, which shows a fixed-diameter coil arranged so that the pitch of the winding is wider at the large end than at the small end.  With this design, a greater concentration of heat will be provided at the small end, because of the variation in pitch.

It is well to remember that with a fine-pitch coil the penetration of heat will be deeper than with one having a coarser pitch.  Usually the spacing between cols should not exceed half the diameter of the coil, while smaller spacing is preferable.

In heating conical parts, such as bevel gears, a coil with a consistent spacing or coupling will result in a more intense heat at the small end, as shown at B.  To compensate for this condition, the coil should be made with a wider angle at the small end, as shown at C, so that heating will be uniformly distributed.

The same procedure is followed when using a solid-inductor coil, shown at D.  In heating flat surfaces, like that illustrated at E, a coil made parallel to the face will have a tendency to create the greatest heat toward the center.  Coils for such surfaces, therefore, should be made slightly conical or angular, as shown at F.

Fig. 44 - Coils provided with variations in spacing between turns can be arranged to transfer heat uniformly to tapered surfaces.  Usually in heating tapered surfaces the coils should be made with a wider angle than that of the work piece.

Distribution of Heat.  An example of a multiturn copper-tube coil used for hardening a projectile on which differential-hardness readings are desired is illustrated in Fig. 45.  The coil is made so that the hardness readings from the point of the projectile to the base will taper gradually from 60 to 35 Rockwell C.  In operation the part is heated and then quickly dropped into a quench.  The variation in heat and hardness is attained by a variation in coupling, and in the pitch of the coil turns, which

Fig. 45 - A coil provided with variations in turn spacing, which is used for the hardening of a projectile on which a gradient of hardness ranging from 35 to 60 Rockwell C is obtained.

as can be seen are wider at the bottom than at the top.  This type of coil provides for maximum heat where the hardest area is desired, and gradual diminishing of heat for less hardness.

Fig. 46 -  A single-turn heating coil used for brazing a steel tube and an insert together.  It is necessary to arrange the coil so that the transfer of heat is concentrated on the heaviest section of the assembly.

In designing coils for heating purposes, it is often necessary to analyze the application to determine the best means of heat distribution.  For the application shown in Fig. 46, which requires the brazing of a steel insert to a drawn-steel shell, the coils as shown at A might cause overheating of the then material before the proper amount of heat could be conducted to the heavier insert.  This being the case, the outside surface also might blister and become badly warped.

To overcome this condition, a coil such as that shown at B would be desirable, since the generation of heat would be distributed more to the heavier section, particularly at the bottom, and then would travel by conduction to the remaining surfaces directly above.  For heating operations of this kind, a solid inductor of the single-turn, or even-series, type, is to be preferred.  The coil can be made somewhat higher than might be necessary.  Then after observation of the resulting heat the coil can be trimmed down one way or the other until the exact heat pattern has been produced.

Fig. 47 - It is not good practice to make long multiturn coils, because uneven heating is very likely to take place.  In heating long areas, it is better to use a short coil and feed the work progressively through it.

Length of Coils.  There is a limit to the length of surface which can be heated at one time, and while no fixed rule applies, a helical coil usually should not be more than three to four times its diameter, whereas a single-turn coil will be found more effective when the length is not more than half the diameter.  The example shown at A in Fig. 47 obviously is too long, since too great a surface is being heated at one time, causing possible deflection as well as an uneven distribution of heat.  For a surface of this type, assuming that hardening is required, it would be much better to use a shorter coil and progressively feed the part through it, such as is shown at B.  The available output power of a generator also is relative to the amount of surface that can be heated at one time.  IN this same example a generator with considerable power would be required to heat the entire shaft at one time, as compared with one of nominal power for the progressive-heating method.

Multiturn coils exceeding the proportions given can be sued for the heating of this sections, such as steel wire and rod, especially in annealing operations.  For hardening and brazing requirements,

Fig. 48 - In connecting heating coils to generators, it is well to keep the leads as close together as possible in order to avoid inductance losses, which would detract from maximum heating in the work.

 however, exceptionally long heating cols may offer difficulties.  Sometimes it is this consideration that decides whether a job can best be performed by inductive heating or some other method.

Inductance Losses.  When connecting the leads of a heating coil to a generator, especially  those of the quick-change type, it is desirable to keep them as close together as possible, in order to avoid inductance losses between the leads, as might be represented by the example in Fig. 48.  The spacing, as represented at A, would result in some dissipation of the high - frequency current, so that maximum heating of the work within the coil would not be attained.  By providing leads as shown, where the space B is held to a minimum, there is a better assurance of maximum heating of the work within the coil.

This same condition would be true in a series-connected coil, where substantial losses will occur if the proper technique is not provided.  The coil shown at A in Fig. 49, for example, represents a very poor design, where owing to its shape undesired inductance is set up between the coil sections.  On the other hand, the coil at B, which represents the same general type, is very well laid out and would offer the assurance of maximum heat concentration to the work pieces located in the openings.

Fig. 49 - In connecting series-type coils, it is essential to make the connections in such a way that minimum inductance losses will occur.

Pitch of Coil Windings.  In applying multiturn coils for the heating of steel shafts and other round parts, the pitch of the turn windings will have a direct relation to the depth of heat penetration.  With a find-pitch coil, such as that illustrated at A in Fig. 50, the magnetic flux penetration will be more highly concentrated on the surface of the work, with a resultant deeper layer of heat.  With an open-pitch coil, where the space between windings is increased as at B, the penetration of heat to the surface of the work will be somewhat less affected by the fact that the magnetic flux spreads out over a wider area and causes certain losses.  Usually in making multiturn coils it is best to arrange the windings so that the space between turns will not be more than one-half the thickness of the tubing itself.  Beyond this limit, or with loosely wound coils, non-uniform heating is likely to occur, and unless the piece being heated is rotated the heat pattern on the work's surface will follow the spiral of the coil.

When a steel part, such as a shaft, is heated at a comparatively fast rate by a multiturn coil, there is likely to be a variation in temperature on the work's surface because of the field set up by the spiral windings.  The more open the windings of the coil, the more pronounced will be the bands of heat, to a point where the heated surface will follow the spiral form of the coil.  For this reason, the coupling, or spacing from the work to the coil is an important consideration and, likewise, bears a direct relation to the pitch winding of the coil.

Fig. 50 - Coils provided with fine-pitch windings have a tendency to throw a deeper layer of heat than those would with loose windings.

With a coil like the one shown at A in Fig. 51, where the coupling is close and the pitch winding of the coil loose, the heated zone will take on a bright and dark pattern, following the exact shape of the coil windings.  By increasing the coil coupling, as at B, the heating becomes more uniform, though it increases somewhat because of less flux-density transfer to the work's surface.

This heating effect is an important consideration when using multiturn coils.  It is most obvious when the spacing between coil turns is about equal to the coupling, and diminishes as the tube spacing are reduced, in proportion to coupling.  The best procedure is to rotate the work within the coil so that uniform heating is assured.  This can be done in many cases by hand, if the part permits, or by a motor-driven spindle, using a speed as low as 25 r.p.m., or just enough to break up the uneven distribution of current density.

Fig. 51 - Multiturn coils should be correctly proportioned as to space between windings and their distance from the surface of the work, so that an even distribution of heat will be assured.

Naturally, the coil spacing around a part to be heated should be uniform, especially if a uniform distribution of heat is desired.  In some cases, however, a slight amount of eccentricity between

Fig. 52 - If an eccentric coil is used to heat a section of round steel, the variation in depth of heat penetration will be proportionately greater with narrow coils than with high.

the work and the coil is not objectionable and may have no effect on the heated area.  This is usually the case when heating a small part and where an ample amount of power is available.  Where the rate of heating may be slower, however, eccentricity of the coil may produce a variation in heat, the side nearest the coil receiving the greatest amount.

With a high coil, like that shown at A in Fig. 52, a slight amount of eccentricity will be hardly noticeable.  However, with a narrow coil, as shown at B, any appreciable amount of eccentricity or unevenness in coupling will be more apparent in the form of heat variations to the work's surface.  This is quite noticeable in heating such parts as gears, especially when the thickness is small in proportion to the diameter.  Rotation of the work, of course, offsets irregular heating due to slight

Fig. 53 - When heating coils overlap the edge of a piece of work, there is likely to be overheating on the end surface.  Placing the coil slightly below the edge will produce a more uniform heat pattern.

variations in coil couplings, as would be the case with an eccentrically located inductor.

Heating Effects. When a cylindrical-type heating coil is placed around the end of a shaft in such a way that it projects over the edge, as indicated at A in Fig. 53, there will be a pronounced heating over the face as indicated at B.  If, however, the coil is arranged as shown at C, in which case the edge is even with or slightly below the end of the work, the heat pattern on the surface will be more uniform, as shown at D.

This same condition exists when heating a disk, or the flanged area of a piece, as shown at E.  The heat pattern will be such that the depth of penetration will be less in the middle of the blank than at the ends.  With some parts it may be necessary to use a formed coil, as shown at F, with the inner face made convex.  This design provides for a uniform layer heat, as shown at G, and eliminates the overheating of the edges which might occur with a straight cylindrical-type coil.  These same conditions apply to use of multiturn heating coils.

In applying heat to a surface which may have a shoulder or flange at each end, as illustrated in Fig. 54, the spacing of the heating coil in relation to the various surfaces becomes an important consideration.  If the coil were proportioned as shown at A, where the edges of the coil are closer to the flanges than the central part is to the body, some heat would be applied to the edges.  This

Fig. 54 - In applying heat to flanged surfaces, the relationship of the coil to the surface of the work is an important consideration.

would result in the absorption of much of the flux density, so that the body of the part would receive only  a small amount of heat.

For such a surface, therefore, the coil should be made as shown at B, arranged so that the coil is closest to the body requiring heat.  In any case, a split-type inductor would be required in order that the heat-producing surface might be properly placed in relation to the surface requiring heat.  For such a part there is the possible use of a larger coil such as shown at C, through which the work is inserted, and then positioned eccentrically as shown.  Rotation of the work is thus provided.  With such a setup the flanges are more liable to absorb some of the eddy currents, although the body will become more predominantly heated.

Two-piece Coils.  A split-type coil like that used to heat a part having flanges on each side of the bearing surface is illustrated in Fig. 55.  In each section of the coil are separate cooling tubes, which in turn  are series connected for the continuous flow of water.  Both sections are held together by a clamp, although various other means of connecting may be used.  One of the pieces to be heated is shown at the right of the coil.

A hinged-type inductor of solid design used for the hardening of crankshaft bearings is illustrated in Fig. 56.  The coil is arranged so that after the work has been properly located and centralized within

Fig. 55 - A typical single-turn heating coil of the split type, which is used to heat the small diameter of a flanged part.

it, the upper half of the coil is brought into contact with the lower half, thus completing the electrical connection.  Both coil sections are held together with pressure during the heating operation.  Also, both sections of the coil are provided with internal passages for cooling purposes, with suitable quenching chambers to make the quenching water pass directly through the area of the coil's heating surface.  The cycle for the operation is controlled by a timer and functions as follows:

1. The coil is closed.

2. High-frequency heat is generated to the work's surface.

3. The quench is applied.

4. Finally, the coil is opened so that the work can be removed.

When applying high-frequency current to surfaces which include keyways, or holes like those illustrated in Fig. 57, the eddy currents concentrate at the corners, probably casing some overheating.  With the keyways  as shown the edges will heat rapidly and a slight burning action might take place.  To overcome this, the part can be fitted with a copper key to bridge this gap, thus making the heating  more uniform.

Fig. 56 - A well proportioned hinged-type induction heating coil used for hardening crankshafts bearings.

In case a part having a hole through the surface is to be heated, as shown at A, the magnetic flux from the coil will tend to enter the hole and develop maximum heat around the edges.  To reduce this condition the hole can be provided with a copper plug which shunts the opening and thus reduces the absorption of eddy currents at this point.

In heating a slotted part within a cylindrical coil, as shown at A in Fig. 58, the circulating current on the surface of the work piece naturally is interrupted by the slot, but the current will continue to flow

Fig. 57 - Overheating of sharp corners, such as keyways and holes, often takes place and the use of copper inserts may be advisable to shunt the path of high frequency current.

on the inside because of the nature of high frequency, which must form a closed circuit.  When treating pats of this design, therefore, a somewhat higher heat usually is produced at the edge of the slot, as indicated at S.  With the multi-slotted part as shown at B, the circulating current around

Fig. 58 - In heating shaped parts such as those shown in this example, the high frequency current tends to crowd into the slots resulting in overheating.

the work piece will have a tendency to crowd into the slots T, and again an overheating condition is most likely to occur on the corners.  The thinner the coil in relation to its diameter, the more pronounced the heating at the slots.  On the other handle, with the use of a heating coil having a height greater than its diameter, this condition will be less pronounced.

Single-turn Coils.  Solid-type induction coils are made of sheet copper, as illustrated in Fig. 59, and can be arranged for single or multiple operation.  The coil shown at A, which is typical for heating

Fig. 59 - Constructional details of single-turn coils made from flat copper plate and provided with means for cooling.

parts simultaneously, is made of a thick copper plate,  bored to provide coupling sufficient for the diameter.  Two connecting blocks are brazed to this plate, and then the plate is sawed out, as shown, so that the high frequency current will follow the path of the arrows, coming in at one block and going out at the other.

At B is a single-station coil of similar design, showing a suitable method for providing the necessary cooling.  A groove is milled around the coil, and a copper tube is brazed in place.  At C is shown another solid-type inductor, in which the cooling chamber is provided by milling a slot around the outside edge and brazing a sheet-metal cover over it.  The water outlet to the two connectors is made by drilling a hole through them and connecting a small tube, as shown.  Another type of solid inductor is illustrated at D.  This includes a band of copper to which a section of flattened tubing is brazed to the outer edge.

In making the so-called solid-type inductors, which comprise a single-turn coil, it is best to consider the proportions shown in Fig. 60.  It will be seen that the height of the coil is equal to one half the diameter.  These proportions can be exceeded in certain cases and various modifications are

Fig. 60 - In making single-turn coils it is well to keep the over-all height to approximately one-half the diameter as shown.

possible, but it is usual to limit the use of solid inductors to applications in which the height or length of the heat zone is less than the diameter of the coil.  When a longer area requires heating, multiturn coils may be used to  better advantage.  If the area to be heated is exceptionally long, progressive feeding may be the solution.

the length of the heated zone of a shaft placed within a cylindrical coil will usually exceed the over-all height of the coil at both ends by a distance equal to the coupling C, as shown in Fig. 61.  This overlap is indicated at E in the illustration.  When the thickness of the coil T is increased, as shown, there is a tendency for the heating zone to spread out still farther on each end of the coil, as indicated at F.  This heating action is the result of a wider distribution of magnetic flux, which develops from a thicker heating coil.  The same condition also exists in either case, when using multiturn copper-tube heating coils.

Series-type Coils.  The coil used for heating the inner surface of a rocker arm which required hardening the coil comprises two turns positioned so that they lie adjacent to the surface requiring heating.

A good many applications of induction heating require the use of coils which might have two or more windings spaced widely apart to suit surfaces of varying size.  In this case it is important that the paths of the current flow in the same direction.  Assuming that a double-turn coil is desired, like that shown at A in Fig. 63, the winding arrangement should be such that the current flows in the same direction, as indicated by the arrows.  If a coil were made such as that as shown at B, with the current flowing in one direction in one coil and opposite in the other, the magnetic flux of one would have a tendency to cancel that of the other, so that practically no heating would result.  This is an important consideration in the construction of coils of this type, particularly where multiturn and series type heating coils are needed.

 A multiturn coil of the series type arranged for hardening two surfaces is illustrated in Fig. 64.  While applications of this type can be provided for as shown, there are bound to be some losses due to the length of the leads between each coil section and, before the adoption of multiple heating as illustrated, the advisability of hardening the surfaces separately should be considered.  In any case, a coil of this type should be so arranged that the leads are close together and never as indicated by the dotted lines, for the inductance resulting from the jumper and the input leads which would cause heat losses.

Fig. 63 - When making series-type coils, particularly those of the flat type, it is essential that all turns be made to carry the current in the same direction, as shown at the left.

Fig. 64 - In constructing a series-type coil as shown here, it is important that the leads be kept close together in order to assure maximum heating to the areas on which heat is required.

Fig. 65 - A series-type coil assembly, comprising four multiturn coils, arranged for internal heating.

Multiturn copper tube coils of the series type can be made of a single piece of tubing, as has been shown, whereas another practical design is that shown in Fig. 65.  Here the coils are made separately and joined together by bus bar jumpers, or connectors.  The hose shown at the top is used for providing a continuous flow of water through all four coils.  The two supporting members, through which the copper tubing is assembled, are made of asbestos board.

In Fig. 66 is illustrated a series-type internal-heating coil showing another means of making a connection.  Each coil is made separately and then connected to a jumper plate A, through

Fig. 66 - Double-type internal-heating coils arranged with a bus-bar connector and a hose for the continuous passage of water.

which the high frequency current passes from one coil to another.  The ends of the coil tubing are then connected with a section of hose, to provide continuous passage of cooling water.  The other two ends of the coils are connected to the output leads of the high-frequency generator.  This design can be used for a greater number of coils, which may have to be joined together and, where operating conditions permit, operated in series.

A multiturn coil used for brazing together two steel sections in which heat is desired from the inside surface is illustrated in Fig. 67.  The part has been cut in half  to show the relative location of the coil.  When long leads are required, as is true in this case, it is necessary to keep the leads close together, particularly where they enter the bottom opening, like that shown. It must be remembered

Fig. 67 - A series-type internal-heating coil with two separate coil units, which is used for the brazing of two steel cases simultaneously.

that if the leads lie adjacent to a metallic surface, thy will generate heat where it may not be wanted and dissipate some of the energy needed for a particular portion.  In this case, the leads are mounted in insulating blocks attached together but separated by mica insulation.  for the operation shown, the two pieces are handled in one setting and the two coils are series connected.  All lead connections are made on the underneath side of the table

The coils shown in Fig. 68 is an excellent example of a solid-type inductor for heating two parts at one time.  The coil is made from a plate of copper 3/8 in. thick and bored out to provide suitable clearance around the two work pieces to be heated.  The copper tube, used for cooling, may be seen around the outside edge of the coil.  The coil is supported by two angle plates connected to the front panel of the worktable.  Since the panel is made of insulating material, the brackets have no means of shorting the high-frequency current.  When making supports for coils it is important to provide proper insulation.  The coil is used for brazing two steel tubular parts together, like those shown at the right.

A series-type coil made from several copper bushings attached together  around the upper and lower portions of the bushing are brazed the copper tubes used for cooling.

Fig. 68 - Tandem design heating coil of the single-turn type used for the brazing of steel tube assemblies.

Saw cuts are provided between the bushings in order to provide for a continuous path of high frequency current.  The operation in this case is the brazing of a steel tube to a cap.  The coil and fixture are arranged so that eight parts are completed simultaneously.  A coil of this type is relatively easy to build and may be employed in a variety of operations requiring multi-setups.

The same general principle is applied to series - type coils made from a flat copper plate, like the one illustrated in Fig. 70.  Here the cooling tube is brazed on the underneath side of the plate and shaped to conform to the coil openings.  The tubing connections are brought out at one side, to provide a suitable connection to the output leads of the generator.  The coil is provided with small end plates, as at A, to which supporting braces can be attached, to provide for rigid mounting.

Fig. 70 - A heating coil made from a flat copper plate and provided with a cooling tube, placed on the underneath side, is illustrated above.

General-type Coils.  The heating path, or area of surface contact, can be increased by means of a flat disk brazed to a copper-tube coil, as illustrated in Fig. 71.  When the heating area desired is greater than can ordinarily be obtained by means of a single-turn copper-tube coil, this method can be well applied.  It is possible, of course, to use a multiturn pancake-type coil in place of the single-loop coil illustrated, but usually the application of a wide heating disk on a single-turn coil, as shown, will result in a more even distribution of heat to the work's surface.  The same principle can be applied to series-type coils as well as step coils, in which a coil of smaller diameter may be located within another of larger diameter.

Another form of single-turn inductor for internal heating is illustrated in Fig. 72.   The coil comprises a copper ring A, which is slotted as shown.  A groove then is milled around the top face, as shown at B, after which the cover plate C is assembled.  The leads D are then connected to the under side of the ring.  A section through these connections is shown at X.

A coil of this type has many applications, and often the general principle of the design can be arranged for the heating of restricted areas.  The leads can be readily connected to the outside, if desired, so that the coil can be used for external heating.

Such a coil for the heating of a small stud is illustrated at A in Fig. 73.  A section is shown at B.  This type of coil can easily be connected in series so that several areas will be heated simultaneously.  At C is shown another modification of this design.  Here the coil is formed out to localize heat at the base of a stud.  A coil of this type can be arranged to heat hard-to-get-at corners and, since it is made of solid ring, re-modification is possible, by machining of some kind, should slight changes be required to produce a certain heat pattern.

An example of this type of solid-inductor coil, connected in series, is illustrated in Fig. 74.   It is applied for the brazing of two flanges to the outside of the tube requiring inside heating.   It will be seen that the high-frequency connections are arranged so that the current circulates in both coil section in the same direction, as indicated by the arrows at the right.  A coil of this type will provide

Fig. 73 - A representative design of solid inductor, which can be made with  various modifications to localize the transfer of heat to restricted areas.

Fig. 74 - A series-type heating coil for internal heating is illustrated above.  The coil sections are connected, so that the current flows in the same directions in both coils.

uniform heat to both area, while the copper tube connections are formed back toward the center in order to avoid unnecessary heating on the surface of the tubing between the joints, where heat along is required.

A modification of this coil is shown in Fig. 75.  Here the construction comprises a slotted copper sleeve, as at A, on the inside of which is brazed a single copper tube B.  This provides for the passage of the cooling water to the generator leads as well as for the connections between coil units.  This series-type coil is arranged for heating two surfaces of different diameters, and is for internal heating, such as the coil shown in the preceding example.

Parallel Inductors.  High-frequency current will circulate around the surface of the metal part even though this is not completely surrounded by a coil.  Naturally there are limits in the extent to which this principle can be applied, but for average small parts, usually requiring high-frequency heating, it is possible to use two parallel inductors and to pass the work underneath, or arrange it in approximate relation to the inductors, so that heat will be absorbed around its entire outer surface.  This principle is illustrated in Fig.76.  Here, at A may be seen the relation of two parallel inductors used for soldering a cover plate to the body of a round condenser can.  In this case the heat is

Fig. 75 - Another form of series type internal heating coil.  Copper tubing is used for connecting both sections, as well as to provide for the flow of cooling water.

Fig. 76 - An example of parallel-type inductor, comprising two bars, which is used for the soldering of condenser cans.

concentrated to the edges only.  For the example shown at B, the inductor bars are located directly above the joints to be soldered, which in some cases will be found preferable.  The principle of heating is shown below.  It will be seen that the high-frequency current circulates through the bars of the coil which, in turn, is induced into the work located underneath in the opposite direction.  In handling operations of this kind, the work can either be placed in a fixture that provides correct relation to the inductors, or conveyor-fed progressively under the inductors.

Another form of inductor, comprising two bars, is illustrated in Fig. 77.  Here the longer bar A is adjustable by means of the jumpers B.  As will be seen, the bars are provided with cooling

Fig. 77 - A two-bar parallel inductor of the adjustable type is shown above.

tubes and have hose connections at their ends for the continuous passage of cooling water.

The coil shown in fig. 78 also is of the two-bar type and is arranged so that work can be passed through the opening, as illustrated at A.  The part represents the end of a drawn-steel shell which requires annealing.  The operation is performed by feeding and rotating the work through the bars, starting at one end and leaving at the other.  In mounting coils of this type, it is necessary to provide suitable supports, such as stand-off insulators, in order that rigid mounting can be obtained.

In Fig. 79 is illustrated a two-bar inductor of the parallel type used for the heating of a long steel bar requiring hardening at one edge only.  In this example the inductors are cut out to conform to the shape of the part, as shown in the cross-sectional view at A.  Holes are drilled lengthwise through the inductors for the passage of cooling water.  At one end a jumper is provided, whereas at the other end of each bar the terminals connected with the generator are brought out.  Coils of this type can be insulated by means of mica, in which case it is possible to provide a means for clamping them firmly together.  The sectional view taken through the inductors shows the work in the heating position.  Only the edge of the bar is heated as it is progressively fed through the inductors.  A spray

Fig. 78 -  A two-bar-type inductor arranged so that the work can be rolled between the plates as may be required for annealing the ends of shells or tubes.

quench unit, not shown, is located at the left of the inductor, in order to complete the hardening cycle.

In making heating coils for continuous feed operations, it is often desirable to use copper tubing

Fig. 79 - A solid-type parallel inductor used for the transfer of heat to the edge of a long shear blade, requiring progressive hardening.

made into the form of a hairpin, under which the parts to be heated are fed.  With such a coil it is often necessary to  bend the end of the coil upward, as illustrated in Fig. 80.  If the coil remains on a true horizontal plane, as illustrated at A, there may be a likelihood of excess heating  on the edge of the work, as at B, especially is sharp corners are encountered.  Usually the thinner the work, the more necessary it is to provide this bend at the end of the coil.

Fig. 80 - When using hairpin-type coils for heating the edge of narrow strips, it is advisable to turn the end of the coil upward to avoid overheating on the corners of the work.

Miscellaneous Coils.  In Fig. 81 is illustrated a flat type coil, which has broad uses in the heating of parts requiring uniform heat.  This coil is made from a flat piece of copper.  First, four holes are drilled through the entire length, then the slots are cut as shown, so that the current will circulate back and forth, entering through the connection at A and leaving at the connection B.  At the end of the coil, where provision is made for circulation of the cooling water, the small plates P can be attached, as indicated.  Coils of this type can be made in various sizes and styles, and are particularly suitable in cases where a single hairpin coil is applicable.

Fig. 81 - A flat-type coil made from a block of copper, with drilled holes for passage of water and slots cut between sections to distribute the passage of high-frequency current.

The coil shown in Fig. 82 is used for the heating of the end of a drawn cup which requires forging.  The coil in this case is made from a strip of copper, to which a cooling tube is brazed, as may be seen.  The area of the part to be heated is approximately 2 in. in length and the temperature required is 1800˚F.  The coupling of the coil is approximately 3/16 in.

In using coils of this type, a very even distribution of heat can be produced, but solid coils of the same type are limited in their height.  The proportions shown in this particular coil are very  satisfactory, however, and the example is representative of the possibility of providing a high temperature heat to a localized surface.  In other methods it should be necessary to heat the entire part, which often is objectionable.  With high-frequency heating the temperature change is so localized and so quickly applied that the part can be removed by hand and inserted into the forging press without insulating precautions that normally would be required.

Fig. 82 -  A large single-turn coil used for heating the end of a drawn steel shell, for a forging operation.

A single-turn coil used for soldering a socket to a reflector is illustrated in Fig. 83.  The socket is assembled to the reflector body and a ring of solder is placed in the joint.  Because of the thickness of the joint of the parts to be heated, the heat is quickly applied and as a rule the coupling of the coil can be made somewhat greater then for the average heating operation, particularly where a greater

Fig. 83 - A small single-turn coil used for the soldering of a socket to a reflector.

mass of metal is to be considered.  The spacing of the joint where the coil meets should be held to a minimum, so that even distribution of flux density will prevail.  Soldering operations of this kind can be effectively handled in multiple, using a series-type coil.

In Fig. 84 is shown a double hairpin-type coil used for joining a steel ring.  This coil practically

Fig. 84 - A double hairpin-type coil used for the joining of a steel ring.

surrounds the surface to which heat is to be applied, as will be seen.  A soil of this type can be made to heat a wide variety of surfaces and because of its simplicity in design can be quickly formed.

Sometimes it is necessary to apply heat to the corners of an assembly such as shown at A in Fig. 85.   In this case an analysis has to be made of the magnetic-flux dissipation from the coil to insure uniform heating.  The use of a hairpin coil, as shown at B, would result in the absorption of some of the energy on the end plate C rather than in the corner.  This would indicate that the coil should be provided with a formed end as shown at D, in order to provide for an extra amount of flux to this corner area.

On other parts presenting a similar condition, it would be possible to use a coil formed on the end, as shown at E.  Still another way to insure maximum flux density in corners is to make a heating inductor as illustrated at F.  In this design, the ends of the connecting tubing are fastened to a small copper block.  This block is provided with a saw cut at G, so that the high-frequency current will pass down to the corners.  The end of the block is milled out for the circulation of cooling water, as shown at H.  A cover plate is then attached to the face of the block to complete its structure.

In Fig. 86 is represented a piece of work requiring the heating of three inserts for brazing purposes, for which a specially formed coil should be provided.  Assuming, however, that a symmetrical coil were used, as shown at the upper portion of the illustration, there would be a tendency for the

Fig. 85 - When heat has to be thrown into a corner, it is usually necessary to provide a coil which will transfer an extra amount of heat to this joint.

Fig. 86 - Principles involved in brazing three lugs to a cap by means of a single-turn coil, showing the correct and incorrect methods.

circulating current to crowd through the narrow portion of the work, as shown at A, and excessive heating would take place.  Also, there would be insufficient heat at the inside portion of the insert.

To handle such an operation a series-type heating coil, as shown at B would be required.  In that case the high-frequency current would enter at one connection, then completely surround the three inserts, and pass out through the other connection.  It would be necessary, however, to use a coil inset, as shown at C, which would act as a flux concentrator.  In making such a coil it would be necessary to provide cooling not only for the outer section, but also for the insert.

When hardening the outer surface of a part having a variation in depth, such as the sprocket shown in Fig. 87, there would be a tendency for the outer portion of the teeth to absorb more heat than at

Fig. 87 - When heating irregular surfaces such as sprocket teeth, it is essential to use a formed coil, usually of the cast type, in order to assure uniform heating.

the root.  Assuming that a single-turn flat-type induction coil were used, we would get a heat pattern such as that shown at A, on which the points might become overheated.  To overcome this condition, the coil should be made to conform to the approximate outline of the teeth, like that shown in the lower portion of the illustration.  A coil of this type can be cast to size and with a carefully made pattern should require very little cleaning to assure uniform heat.  With a formed  coil of this type, the heated zone will be produced as shown at B, with a more uniform density of heat around the entire profile of the tooth, especially at the root.

Sometimes it is necessary to provide a tubular-type coil for maximum heat concentration to the surface of a part, where a solid-type inductor might be preferred to a multiturn coil.  The design of a coil for such an operation is illustrated in Fig. 88.  A copper tube of the size required is cut off as shown at A, then a section of small tubing is wound around the outside, as illustrated at B.  Finally the coil is saw-cut with a spiral, as shown at C.

An induction-coil assembly for the heating of continuous strip this coil is longer in relation to its diameter than would normally be necessary for hardening operations, but in cases where continuous heating of material is desired, a coil of this type works satisfactorily.  In this application, the heating-coil unit is placed adjacent to a power press.  The material being heated is a magnet-steel strip which is heated to 1800˚F, then blanked, formed, and hardened in one operation.  The temperature attained in the steel as it enters the press is slightly higher than that required for hardening, but during the blanking and forming operation the temperature drops to approximately 1525˚F, then quenched in oil.  The steel is 1/8 by 5/8 in. in size and is heated by a 30 kva input high-frequency unit.  As a result of high-frequency heating, the steel receives a higher indexing magnetism and a better magnet is produced.  Another direct saving is a marked reduction, or practical elimination, or rejects.

Fig. 88 - Constructional details of a spiral-type coil made of a copper sleeve to which copper tubing is attached for cooling purposes.

Flexible leads can be used with induction heating setups, although their application should be limited to heating requirements which might make them necessary.  These leads can be made from sections of high-pressure metallic tubing of the flexible type, as illustrated at A in Fig. 90.  It is essential, however, that the braided cover, as well as the inner bellows, be made of a pure copper, having high electrical-conductivity qualities.  For safety it is well to provide a rubber hose on the outside, as indicated.  Leads of this kind provide for the passage of high-frequency current from the generator to the work coil, as well as for the passage of cooling water.

Fig. 90 - Two types of flexible leads which can be used in connection with induction-heating generators.

Another way to make a flexible lead is illustrated at B.  Here the braided-copper lead is brazed to the connector and then covered by a rubber hose as shown.  With this type of lead, it is necessary to have a similar connection at the other end for attaching the work coil.

In Fig. 91 is illustrated a multiturn coil provided with insulating in the form of woven glass sleeving.  About the only advantage of insulation of this type is the elimination of metal-to-metal contact of the coil windings, which in this case are very close together.  Where intense heat is to be applied, however, insulating material of this kind does not give long service and other means of insulation are more practical.   A copper-tube coil can be lacquered, or coated with insulating varnish, then baked, if its use is not in connection with flux as used for brazing.  Plating is not usually recommended, since high-frequency current travels on the surface of a coil, and plating materials, such as chrome, are not good conductors.  Ceramic cement provides good insulation around a coil and may be used where protection is desired.

Fig. 91 - A multiturn heating coil covered with woven-glass sleeving.  Coil insulation can also be obtained by applying a baked coat of insulating varnish.

Fig. 92 - Large heating coils can be supported by stand-off insulators, in order to assure rigid mounting.

Heating coils sometimes require outboard-supports, because of their sizes and shapes.  While such supports can be provided in various ways, it usually is best to consider an insulator that will prevent the loss of high-frequency current from the coil.  A satisfactory type of support is a stand-off insulator, as illustrated  in Fig. 92.  Small extensions can be brazed to the bottom, so that a rigid coil mounting is provided.  It also is possible to support induction heating coils from their sides, as shown in the lower portion of the illustration.  In this case, the end of the stand-off insulator is brazed to the side of the coil, then the base of the insulator is mounted to some convenient vertical support at the sides.

Many other forms of brackets and supports can be used for coils and, in cases where insulating material such as asbestos board is used for the top of a work table, there is no objection to providing copper feet or legs in the coils for alignment purposes.  Other means of supporting coils may be seen in various illustrations showing hardening and brazing installations.