Patent Publication Number: US-3875543-A

Title: Toroidal core-coil combination with in situ molded end rings

Description:
United States Patent 1191 Gostyn et al.  
 [ TOROIDAL CORE-COIL COMBINATION WITH IN SITU MOLDED END RINGS [75] lnventors: Ernest Gostyn, El Paso. Tex.;  
 Francis J. Fratar, Springfield, Mass.  
 [73] Assignee: General Instrument Corporation,  
 Newark, NJ.  
 [22] Filed: Aug. 29, I974 [2|] Appl. No.: 501,687  
  Related U.S. Application Data [62] Division of Scr. No. M4978. Nov. [2, i973.  
 Primary E.ramim&#39;rG. Harris [57] ABSTRACT A toroidal core-coil combination has a core compris- Apr. 1,1975  
 ing a toroidal body formed of magnetic material and having a central opening therethrough and ends. End rings formed of insulating plastic material are molded in situ on each of the ends substantially covering the axially exposed surfaces thereof, each end ring having an exposed surface provided with a series of grooves circumferentially exposed therearound. The coil comprises a wire conductor wound on the body in turns passing through the central opening, the turns being received in and located by the grooves. The dimensions of the grooves and the conductor are so related as to permit such winding and enable such location of the turns.  
 In the process for manufacturing the toroidal core-coil combination. the end rings are molded either simultaneously or successively. In the latter instance. one of the end rings is molded and the grooves formed therein. and then the resultant sub-assembly is placed in a mold which uses the grooves for rotatably fixing the position of the sub-assembly in the mold. the second of the end rings then being molded in the mold. A layer of insulating plastic material may also be molded on a radially exposed inner or outer surface of the body simultaneously with the molding of one or both of the end rings.  
 14 Claims, 18 Drawing Figures CIT 3 SF 3 FIG/4 FIG /5 TOROIDAL CORE-COIL COMBINATION WITH IN SITU MOLDED END RINGS This is a division of application Ser. No. M4978, filed Nov. 12, 1973, entitled TOROIDAL CORE-COIL COMBINATION WITH IN SITU MOLDED END RINGS AND METHODS OF MAKING SAME.  
 BACKGROUND OF THE INVENTION In order to obtain predictable and reproducible control of the magnetic field within the opening of a toroidally wound ferro-magnetic core, the turns of the winding must be disposed in predetermined locations and secured there throughout the life of the core. In the early state of the art this was accomplished by the formation ofgrooves or channels in the annular core itself (by abrasion or cutting of the core material) and subsequent laying of the winding turns in such grooves. However, as the ferro-magnetic core material is extremely brittle in nature, great care had to be exercised in the precision machining of such grooves, and the grooving process was accordingly both time consuming and laborious. Furthermore. as the ferro-magnetic core material may be an electrical conductor, electrical insula tion had to be provided between the core and the wind ings to insure proper operation of the wound core and to reduce the possibility of short circuits.  
  To remedy the aforementioned defects, in the current state of the art the core grooves are replaced by grooved plastic end rings which are affixed on the ends of the core and provide with grooves to receive and secure the turns of the winding. While providing a significant improvement over machined core grooves, the use of grooved plastic end rings has not proven to be entirely satisfactory in operation for a number of reasons. Great care must be taken to insure proper axial separation of the end rings and proper registration of the grooves of one end ring with the grooves of the other end ring To insure proper spacing of the end rings on the core, it is generally necessary to precision machine the outer surface of the core at both ends so that the end rings can be slipped onto the core and press-fit towards one another until the proper axial separation is obtained. If for mechanical or electrical reasons the end rings are to extend part way into the annular open ing of the core, machining of the portion of the inner surface of the core to be covered by the end ring is also required. Furthermore, as the two end rings differ considerably in diameter, alignment of the grooves of the two end rings requires a precise indexing or registration assembly technique.  
  A further disadvantage of this system is the continuing need to provide a layer of insulating material about the core to insulate it from the winding, the application of the insulating material typically being an additional step in the process. Furthermore, if the layer of insulating material extends all the way to the end rings, as required for complete protection, it tends to interfere with the proper placing of the end rings on the core. Typically in order to insure maintanence of the proper spacing and registration of the end rings during the subsequent winding step and throughout use of the wound core, it is considered desirable to utilize an adhesive cement to secure the end rings in place on the core. Each of the above steps for applying and securing the end rings on the core is further complicated by the inherent fragility of the end rings.  
  Accordingly, it is an object of the present invention to provide a toroidal core-coil combination in which proper axial spacing of the end rings and registration of the grooves thereof are invariably obtained and retained, all without precision machining of the core structure or the use of adhesives.  
  It is another object of the present invention to provide such a toroidal core-coil combination in which a layer of insulating material may be provided between the coil and core, integral with one or both of the end rings and without requiring an additional process step.  
  It is also an object of the present invention to provide such a toroidal core-coil combination having a simple and rugged construction which is durable in use and able to withstand repeated shocks and jostling without modification of the spacing and registration of the end rings.  
  A further object of the present invention is to provide a method of making a toroidal core-coil combination which insures proper axial spacing of the end rings and registration of the grooves of one end ring with the grooves of the other end ring.  
  Another object of the present invention is to provide such a method in which one grooved end ring may be utilized to ensure the proper spacing and registration of the other end ring.  
  Still another object is to provide such a method which does not require precision machining of the core to obtain and secure proper spacing and registration of the end rings.  
  Yet another object of the present invention is to provide such a method which permits an insulating layer of material to be applied to a radial surface of the core simultaneously with the formation ofone or both of the end rings.  
 SUMMARY OF THE INVENTION It has now been found that the above and related objects of the present invention are provided in a toroidal coil-core combination including a core comprising a toroidal body formed of magnetic material and having a central opening therethrough and ends. End rings formed of insulating plastic material molded in situ on each of the ends substantially cover the axially exposed surfaces thereof. each end ring having an exposed surface provided with a series of grooves circumferentially exposed therearound. The coil comprises a wire conductor wound on the body in turns passing through the central opening, the turns being received in and located by the grooves. The dimensions of the grooves are so related to the dimensions of the conductor as to permit such winding and enable such location of the turns.  
  In a preferred embodiment, a layer of the insulating plastic material is disposed on a radially exposed surface of the body and extends from at least one of the end rings. The layer is molded in situ integrally with the aforementioned at least one end ring, and preferably both of the end rings. The layer may be disposed on the inner or outer radial surface of the body and may cover only a minor fraction of the area of that surface or substantially all of such surface of the body.  
  The body is generally in the form of a frustrum, and the body ends and the end rings are preferably provided with interlocking parts. The grooves of the end rings may be disposed at least in part on the radially exposed surfaces of the end rings, or at least in part on axially exposed surfaces of the end rings; preferably they are disposed at least in part on both radially and axially exposed surfaces of the end rings.  
  The method of making the toroidal coil-core combination comprises the steps of providing a toroidal body formed of magnetic material having a central opening therethrough and ends. An end ring ofinsulating plastic material is molded in situ onto each of the ends to substantially cover at least the axially exposed surface of the corresponding body end. Registering grooves in an exposed surface of each of the end rings are formed, and a wire conductor is wound around the body in turns which are received in and circumferentially located by the grooves. The dimensions of the grooves are so related to the dimensions of the conductor as to permit such winding and enable such location of the turns.  
  The end rings may be molded simultaneously or sequentially, and the grooves may be formed in the end rings either during molding of the end rings or after the end rings have been molded. A layer of the insulating plastic material may also be molded on the radially exposed surface of the body simultaneously with the molding of one or both of the end rings, the layer being molded on either the inner or outer surface of the body and covering either a minor fraction or substantially all of such surface.  
  In a preferred embodiment of the method, first one of the end rings is molded and the grooves formed therein. Subsequently the resultant sub-assembly is placed in a mold which uses the grooves for rotative fixing the position of the sub-assembly in the mold, the second of the end rings then being molded in the mold and the grooves thereof being formed simultaneously.  
 BRIEF DESCRIPTION OF THE DRAWING FIG. I is a side elevation view, in cross section, showing a mold suitable for use in forming in situ on a toroidal core body the end rings of the present invention;  
  FIG. 2 is a side elevation view of the toroidal core with in situ molded end rings (as formed in the mold of FIG. 1);  
  FIG. 3 is a front elevation view of the core taken along the line 3-3 of FIG. 2;  
  FIG. 4 is a rear elevation view of the core taken along the line 4-4 of FIG. 2;  
  FIG. 5 is a fragmentary sectional view, to an enlarged scale, taken along the line 5-5 of FIG. 3, and shows the interlocking nature of the front or large end ring and the front of the core body;  
  FIG. 6 is a fragmentary sectional view, to an enlarged scale, taken along the line 6-6 of FIG. 4 and shows the interlocking nature of the rear or small end ring and the rear of the core body;  
  FIG. 7 is a side elevation view of a core according to the present invention having grooved end rings and three strips of insulating material extending between the end rings on the outer surface of the core body;  
  FIG. 8 is a front elevation view taken along the line 8-8 of FIG. 7, and schematically indicates representative windings of a coil on the core;  
  FIG. 9 is a rear elevation view taken along the line 9-9 of FIG. 7, and schematically indicates representative windings of a coil on the core;  
  FIG. 10 is a fragmentary section view, to an enlarged scale, taken along the line 10-10 of FIG. 8, and shows the interlocking relationship between the front or large end piece and the front of the core body (one turn of the coil being indicated in phantom line);  
  FIG. 11 is a fragmentary front elevation view, to a greatly enlarged scale, of the front end ring taken along the line 11-11 of FIG. 10;  
  FIG. 12 is a fragmentary top plan view, to a greatly enlarged scale, of the front end ring taken along the line 12-12 of FIG. 10;  
  FIG. 13 is a fragmentary sectional view, to an enlarged scale, taken along the line 13-13 of FIG. 9, and shows the interlocking relationship between the rear or smaller end ring and the rear end of the core body (one turn of the coil being indicated in phantom line);  
  FIG. 14 is a fragmentary rear elevation view, to a greatly enlarged scale, taken along the line 14-14 of FIG. 13;  
  FIG. 15 is a top plan view of the rear end ring, to a greatly enlarged scale, taken along the line 15-15 of FIG. 13;  
  FIG. 16 is a perspective view of an insulated core according to the present invention having three strips of insulating material on the inner surface thereof extending between the end rings thereof and integral therewith, the grooves of the core being omitted to facilitate overall viewing of the core construction;  
  FIG. 17 is a perspective view of an insulated core according to the present invention having a layer of insulating material completely covering the outer surface of the core body between the end rings, the grooves of the core being omitted to facilitate overall viewing of the core construction; and  
  FIG. 18 is perspective view of an insulated core according to the present invention having a layer ofinsulating material completely covering the inner surface of the core body between the end rings, the grooves of the core being omitted to facilitate overall viewing of the core construction.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, in particular to FIG. 2 thereof, therein illustrated is a core generally designated by the numeral 10 suitable for use in the present invention. More specifically, the core 10 comprises a toroidal core body 12 with a central opening 14 therethrough, a large or front end 16, and a smaller or rear end 18. The core body 12 has the general configuration of an annular frustrum and is formed of suitable magnetic material such as the ferro-magnetic compound conventionally used for electrical cores in transformers and the like. Molded in situ on the core body 12 are a pair of end rings; the large or front end ring 26 substantially covers the axially exposed surface of the large or front end 16 of the core body 12, and the smaller or rear end ring 28 substantially covers the axially exposed surface of the smaller or rear end 18 of the core body 12. The end rings 26, 28 are formed of electrically insulating plastic material of either a thermoplastic nature (such as polypropylene, polyethylene, and polystyrene) or a thermosetting nature (such as the phenolics and polyesters).  
  Referring now to FIG. 1, therein illustrated is a two part injection mold generally designated by the numeral 30 and having a right half 34, a left half 32 and openings 36, 38 which communicate respectively with cavities 40, 42 therein to permit the injection of plastic insulating material from without the mold 30 into the cavities 40, 42. In operation, the core body 12 is placed in the right half 34 of the open mold 30, the mold is securely closed by affixing left half 32 to right half 34, and the desired plastic insulating material is injected through injection openings 36, 38 into end ring cavities 40, 42, respectively, to mold in situ on the core body ends 16, 18 the end rings 26, 28 respectively. The mold halves 32, 34 are finally opened to permit removal of the core with in situ molded end rings 26, 28. As the details of injection molding are well known in the art, there is no need to go into the detailed operating parameters of the injection molding process. It will also be noted, however, that the configuration and dimensions of the surfaces of the core body ends 16, 18 eventually covered by the end rings 26, 28 are not critical (as in prior art processes using independently fabricated end rings) as deviations therein are automatically adapted to by the end rings 26, 28 as the latter are molded in situ on the former. The end rings 26, 28 formed in situ by the molding process are strongly bonded directly to the core body ends l6, l8 and require no adhesives to secure them thereto. As the end ring pair is formed in a single operation using a single mold 30, it will be obvious that the desired spacing between the end rings 26, 28 (and the appropriate registration of any grooves thereon) will be easily, infallibly and automatically obtained in a single step due to the inherent precision of the injection molding operation.  
  While the strength of the bonds occuring between the plastic end rings 26, 28 and the core body ends 16, 18 is generally sufficient to preclude rotative play of the end rings 26, 28 on the core body end l6, 18, in instances where it is expected that the toroidal core-coil combination will be subjected to unusually large or repeated vibrations or jarring, a mechanical interlock arrangement may be provided between the core body ends 16, 18 and the end rings 26, 28 to insure that proper spacing and registration of the end ring pair is maintained. FIG. 5 illustrates a suitable mechanical interlock between the large end ring 26 and the large core body end 16, while FIG. 6 illustrates a suitable mechanical interlock between the small core body end 18. In both instances, the core body ends 16, 18 are formed with a circumferential pattern of axially-extending inward recesses or depressions 50 as well as axiallyextending peripheral tabs or projections 52. The end ring 26, 28 will naturally and automatically be formed, as part of the molding process, with a series of correspending inward bulges or projections 54 to fill the recesses or depressions 50 of the core body ends 16, 18 and a series of axially-extending recesses or depressions 56 to receive the peripheral tabs or projections 52 of the core ends l6, 18. In this manner not only is the total frictional and bonding area between the mating surfaces of the end rings 26, 28 and the core body ends 16, 18 increased, but an actual physical interlock is provided to guarantee that the end rings 26, 28 are not rotatable relative to the core body ends 16, 18 in the absence of an actual breaking of each of the mating elements 50-54 and 52-56. Generally about eight sets of mating elements 50 and 54, 52 and 56 regularly spaced 45 apart about each end ring-core body end set are used, although a greater or lesser number may be used for particular applications.  
  As will be immediately apparent to those skilled in the art, the end ring pair may be molded in situ onto the core body 12 using a conventional plastic injection mold having an interior surface which is the negative of any desired shape for the core 10. Thus the desired complex surface of the core 10 with the required wireguiding grooves, electrical insulating layers and dimensional characteristics are simply obtained in a single process step, with the desired end ring spacing and groove registration being insured by the precision of the mold. Accordingly, while the illustrated mold 30 is adapted to produce the core 10 illustrated in FIGS. l-6 and characterized both by the absence of grooves on the end ring pair and by the absence of layers or strips of insulative plastic material on the core body 12 between the end ring pair, it is obviously a simple matter to modify the design of the interior of the mold 30 to produce the core 10 of FIG. 7, which core 10 is characterized both by the presence of registering circumferentially exposed grooves on exposed surfaces of the end ring pair and by the presence of a plurality of bridging strips of insulative plastic material on the outer radial surface of the core body 12 integral with and extending between the end ring pair.  
  Referring now to FIGS. 7-15, therein illustrated is a core 10 having grooved end rings 26, 28 and three bridging strips of plastic insulating material extending over the outer surface of the core body 12 between the end ring pair. Referring now in particular to FIGS. 7-8 and 10-12, a plurality of circumferentially exposed grooves 76 substantially cover the exposed axial and radial surfaces of the front end ring 26. Referring now in particular to FIGS. 7, 9, and 13-15, a plurality of circumferentially exposed grooves 78 substantially cover the exposed axial surface of the rear end ring 28. While FIG. 7 illustrates a preferred pattern of grooves 76, 78, other patterns will be useful for particular applications and for different configurations of the core body 12. Generally the grooves 76, 78 are disposed at least in part on radially exposed surfaces of the end rings, or at least in part on axially exposed surfaces of the end rings, the larger end ring 26 (and sometimes the smaller end ring 28) usually having grooves disposed at least in part on both the radially and the axially exposed surfaces thereof.  
  A wire conductor 80 is wound on the core 10 in turns passing through the central opening 14 thereof and being received in and located by the grooves 76, 78 of the front and rear end rings 26, 28, respectively. The dimensions of the grooves 76, 78 are so related to the dimensions of the conductor 80 as to permit such winding and enable such locations of the turns. The grooves 76, 78 not only facilitate the initial winding of the core 10 with the conductor 80, but also insure that the turns of the conductor 80 remain in place despite any vibrations or shock to which the toroidal core-coil combination may be subjected in use (so long as the conductor 80 remains intact and unstretched).  
  The pattern of the winding of conductor 80 on the core 10 is determined by the desired function of the toroidal core-coil combination, and the winding illustrated in the drawing is to be taken only as exemplary of one of the many patterns which may be utilized. By way of example, a plurality of separate wire conductors 80 may be utilized to provide the dual toroidal deflection yoke used in the cathode ray or picture tube of a television set. It is to be noted, however, that the grooves 76, 78 of the core 10 permit the standard core 10 to be wound in any of a multitude of patterns, with the turns of the winding passing through the central opening 14 of the core body 12 and being received in and located by the grooves 76, 78 in a secure and reli able fashion. While the representative winding illus trated in FIGS. 8-10 and 13 shows a single groove receiving only a single turn ofthe conductor 80, it will be obvious that one or more turns of the conductor 80 could also be received in and located by a single groove 76 or 78. Where a large number of turns ofthe conductor 80 are to be received by a single groove 76, 78, the dimensions of the grooves 76, 78 may be increased to accommodate the dimensions of the accumulated turns passing therethrough. Furthermore. while the illustrated small end ring 28 has grooves 78 only on its exposed axial surface obviously communicating grooves could also be provided on its exposed radial surface for particular applications.  
  lfdesired, the configuration of the mold 30 which receives the core body 12 may be modified (and the mold 30 possibly provided with one or more additional injection openings) to permit the molding of one or more layers or strips 70 of insulating plastic material on the inner and/or outer radial surfaces of the core body 12 simultaneously with the molding of one or both of the end rings 26, 28. Preferably, the modifications in the main cavity will communicate with both of the cavities 40, 42 (which participate in the formation of the end rings 26, 28) so that a layer 70 of the insulating plastic material formed therein bridges the end ring pair and is integral therewith. Such a layer 70 may extend over one complete inner or outer surface of the core body, and preferably both the inner and outer surface thereof, between the end ring pair and be integral with the end ring pair 24 to preclude any possibility of elec trical shorting involving the core body 12 and the conductor 80. Alternatively, the mold 30 may be modified to provide for the molding (in situ and simultaneously with the molding of at least one of the end rings 26 or 28, and preferably both) of one or more strips 70 of plastic insulating material extending on the inner and- /or outer radial surfaces of the core body 12 from such at least one end ring and integral therewith. Preferably the strips extend between and are integral with both end rings 26, 28, such bridging strips 70 permitting the attachment of extraneous electrical componenets such as circuit boards to the toroidal core-coil combination in a convenient manner. Thus. the insulating plastic material may be applied as an inner layer covering substantially all of the inner radial surface of the core body (as shown in FIG. 18). as an outer layer covering substantially all of the outer radial surface of the core body (as shown in FIG. [7) or merely as strips or other patterns integral with at least one of the end rings 26, 28 (preferably both) and covering only a minor fraction of a surface of the core body 12; for example, as strips ex tending between the end rings 26, 28 on the inner radial surface of the core body 12 (as shown in FIG. 16) or on the outer radial surface of the core body 12 (as shown in FIG. 7).  
  The grooves 76, 78 may be formed in the end rings 26, 28 in several different ways. Where possible, the simplest technique is obviously to form the grooves 76, 78 as part of a single molding operation forming both end rings 26, 28 simultaneously, as described hereinbefore. Another technique is to mold both blank or ungrooved end rings 26, 28 in a single molding operation (as shown in FIGS. 1-6) and then form the grooves 76, 78 therein in a single post-molding machining operation. By way of example, a stepping motor may be used to progressively rotate the core 10 by angular increments as a phased-in-time cutting tool machines the de sired grooves 76, 78 by conventional embossing, broaching or cutting techniques. A third technique is to first form one end ring and the grooves therein (the grooves being formed either as part of the end ring molding operation or in a separate machining operation) and then utilize the grooved end ring to index the location of the partial core in a second mold where the other end ring and its grooves are formed in a single separate molding operation. More specifically, one end ring (hereinafter arbitrarily referred to as end ring 26) is formed first in a molding process using a mold 30 similar to that shown in FIG. 1 except that there is only one opening 36 and one cavity 40. Grooves 76 may be formed on the single end ring 26 as part of the molding operation, or, alternatively, the partially formed core (that is, the core body 12 with a single end ring 26) may be removed from the mold 30 and machined to provide the desired grooves 76 on the end ring 26. In either case, the partially formed core (including a single grooved end ring 26) is placed in a second mold provided with a modified cavity 40 characterized by interior projections which engage the machined grooves 76 of the single end ring 26 to rotatively and axially fix its position within the second mold, and hence the posi tion of core body 12 within such mold. The second mold is further provided with an opening 38 and cavity 42 for simultaneously forming not only the other end ring, but also the grooves 78 thereof. Molding the sec ond grooved end ring 28 in this manner insures both that the appropriate spacing between the end rings 26, 28 is achieved and that the grooves 76, 78 of the various end rings 26, 28 are formed in proper registration.  
  Thus it can be seen that the present invention provides a toroidal core-coil combination in which proper axial spacing of the insulating plastic end rings and registration of the grooves thereof are invariably obtained and retained, all without precision machining of the core structure or the use of adhesives. Furthermore. the present invention permits a layer or strips of plastic insulating material to be placed on the core body integral with one or both of the end rings and without requiring an additional process step. Because the end rings (and any desired layers or strips of plastic insulating material) are molded in situ onto the toroidal core body, a simple and rugged construction is provided which is durable in use and able to withstand repeated vibrations and shocks without modification of the spacing and registration of the end rings.  
  The present invention is directed primarily and especially towards the peculiar problems of winding toroidal magnetic deflection yokes, where by virtue of opposing fluxes in the core there is obtained a magnetic field within a central opening of limited diameter passing through the core body; however, the principles of the present invention are also applicable in a broader sense to such other toroidal structures as indicators and transformers. where the magnetic field is intended to be contained within the ferro-rnagnetic material of the core body.  
  Now that the preferred embodiments of the present invention have been shown and described in detail, various modifications and improvements will be readily apparent to those skilled in the art. For example, instead of forming strips of layers of insulative plastic material which communicate with and are integral with one or both of the end rings. the mold may be provided with one or more cavities adjacent the main cavity and communicating therewith, but spaced from the end ring cavities and fed by a separate opening, to provide a distinctive pattern of insulative plastic material on the radial surface of the core body. Such a pattern of plastic insulative material is retained on the radial surface of the core body through bonding although it is not integral with either of the end rings. Also, the end rings may be formed in situ on the core body ends using casting rather than injection molding techniques. Accordingly. it is to be understood that the spirit and scope of the present invention is to be limited not by the foregoing disclosure, but only by the appended claims.  
 We claim 1. A toroidal core-coil combination,  
 A. said core comprising a toroidal body formed of magnetic material and having a central opening therethrough and ends, and end rings formed of insulating plastic material molded in situ on each of said ends substantially covering the axially exposed surfaces thereof. each said end ring having an exposed surface provided with a series of grooves circumferentially exposed therearound; and  
 B. said coil comprising a wire conductor wound on said body in turns passing through said central opening, said turns being received in and located by said grooves, the dimensions of said grooves being so related to the dimensions of said conductor as to permit such winding and enable such location of said turns.  
 2. The combination of claim 1, in which a layer of said insulating plastic material is disposed on a radially exposed surface of said body and extends from at least one of said end rings.  
  3. The combination of claim 2, in which said layer is molded in situ integrally with said at least one end ring.  
  4. The combination of claim 2, in which said layer and both of said end rings are integral and molded in situ.  
  5. The combination of claim 2, in which said layer is disposed on the inner surface of said body.  
  6. The combination of claim 2, in which said layer is disposed on the inner surface of said body and covers only a minor fraction of the area of said inner surface.  
  7. The combination of claim 2, in which said layer is disposed on the outer surface of said body.  
  8. The combination of claim 2, in which said layer is disposed on the outer surface of said body and covers only a minor fraction of the area of said outer surface.  
  9. The combination of claim 1, in which said body ends and said end rings are provided with interlocking parts.  
  10. The combination of claim 9, in which a layer of said insulating plastic material is disposed on a radially exposed surface of said body and extends from at least one of said end rings.  
  11. The combination of claim I, in which said grooves are disposed at least in part on radially exposed surfaces of said end rings.  
  12. The combination of claim 1, in which said grooves are disposed at least in part on axially exposed surfaces of said end rings.  
  13. The combination of claim 1, in which said grooves are disposed at least in part on both radially and axially exposed surfaces of said end rings.  
 14. The combination of claim 1, in which said body is in the form of a frustrum.