US 3885083 A
This improved coaxial cable is made by molding dielectric discs to a center conductor at spaced locations along the conductor, and then folding a metal strip longitudinally around the discs to form an outer, butt seam, tubular conductor having a diameter greater than the diameters of the discs. The seam is welded and the tube is reduced in diameter at a sinking station to compress the discs. A flash heating of the outer tube fuses the circumferences of the discs to the inside of the tube without softening the discs beyond their circumferential edge regions. This maintains the pressure between the discs and tube so that the discs are held in place by a combination of friction under pressure and fusion. Induction heating concentrates the heat in the metal of the tube; and a quench is used to stop further heat flow following fusion. Refinements include continuous movement of the conductors and movement of the mold with the inner conductor during molding and cooling of the discs; and also include the use of discs of different kind of material in a repeated sequence to improve the attenuation qualities of the cable.
Claims available in
Description (OCR text may contain errors)
United States Patent 1 Jachimowicz et al.
[111 3,885,083 51 May 20, 1975 Primary Examiner-Arthur T. Grimley  ABSTRACT This improved coaxial cable is made by molding dielectric discs to a center conductor at spaced locations along the conductor, and then folding a metal strip longitudinally around the discs to form an outer, butt seam, tubular conductor having a diameter greater than the diameters of the discs. The seam is welded and the tube is reduced in diameter at a sinking station to compress the discs. A flash heating of the outer tube fuses the circumferences of the discs to the inside of the tube without softening the discs beyond their circumferential edge regions. This maintains the pressure between the discs and tube so that the discs are held in place by a combination of friction under pressure and fusion. Induction heating concentrates the heat in the metal of the tube; and a quench is used to stop further heat flow following fusion. Refinements include continuous movement of the conductors and movement of the mold with the inner conductor during molding and cooling of the discs; and also include the use of discs of different kind of material in a repeated sequence to improve the attenuation qualities of the cable.
9 Claims, 10 Drawing Figures PRESSURE FUSED WATERTIGHT DISC COAXIAL CABLE  Inventors: Ludwik Jachimowicz, Elizabeth;
Jerzy A. Olszewski, Edison, both of NJ.  Assignee: General Cable Corporation,
Greenwich, Conn.  Filed: Dec. 14, 1973 [2|] Appl. No.: 424,904
Related US. Application Data  Division of Ser. No, 321,641, Jan. 8, I973, Pat. No.
 [1.8. CI 174/28; l74/102 R; 174/110 PM [5l Int. Cl H0lb 9/04; HOlb ll/l8  Field of Search 174/28, 29, H1, H0 F, l74/l 10 PM, 102 R; 333/96  References Cited UNITED STATES PATENTS 2,439,916 4/l948 Werner et al l74/28 X 3,480,724 [1969 Garner l74/l [0 F 3,515,909 6/1970 Trump [74/28 X I f 2 3a :2
I $2 I I I I PRESSURE FUSED WATERTIGI-IT DISC COAXIAL CABLE ,RELATED PATENT BACKGROUND AND SUMMARY OF THE INVENTION An air space between the inner and outer conductors of a coaxial cable provides an extremely effective and inexpensive insulation. There have been many constructions in the prior art using different means at spaced locations along the length of a coaxial cable for holding the inner conductor centered in a tubular outer conductor with air space for insulating the conductors from one another between the centering means.
One common means used for centering the inner conductor has been discs made of dielectric material which can be molded around or otherwise secured at spaced locations along the length of the inner conductor. The outer conductor has been commonly formed by longitudinally folding a metal strip around the discs.
This invention provides a spaced disc type of coaxial cable which is of greater strength, protected from moisture penetration lengthwise of the cable, more reliable in service, and economical to manufacture.
The method of this invention folds the outer conductor into a tube having a butt seam and which is of substantially larger diameter than the diameters of the discs. The butt seam is welded without producing an internal flash. The weld is made with the seam spaced from the discs so that the heat of the weld does not adversely affect the discs.
The welded tube is reduced in diameter by passing it through sinking die means to reduce the diameter of the tube so that it compresses the discs radially. The circumferential edge portions of the discs are then fused to the tube and it is a feature of the invention that the fusing heat is applied by flash heating the metal so that heat is transferred to the circumferential regions of the tubes quickly to produce fusion without softening the discs beyond the circumferential edge regions. By thus avoiding the softening of the discs, the pressure of the outer metal conductor against the discs can be maintained, though slightly reduced by the circumferential fusing.
The preferred method of flash heating is by means of an induction coil which generates heat in the metal of the tube without generating any substantial heat in the material of the discs. The flash heated tube heats the circumferential edge regions of the discs to promote the fusion and the tube is preferably quenched immediately after fusion so that residual heat cannot flow into the discs to soften them and reduce their pressure against the inside surface of the tubular outer conductor. This produces an extremely strong cable because the discs are held in place not only by the fusion but also by friction resulting from the pressure of the outside tube against the discs.
The discs forrn moisture proof partitions across the inside of the coaxial cable so that in the event that the cable is damaged at any location, and water can penetrate into the compartment between two successive discs, this water or moisture cannot progress lengthwise LII of the cable in either direction and the operation of the cable is impaired very little, if at all, by the damage to a compartment between two successive discs.
Other features of the invention relate to the way in which the discs are molded on a continuously moving inner conductor, and on parallel conductors with the discs on the different conductors formed from dielectric plastic material from a common header in the mold. The tailings from the mold are broken loose from the discs by transverse pressure against the tailings while the discs travel with their tensioned conductors along parallel courses.
Another feature of the invention relates to the use of discs of different dielectric properties interposed between other discs in repeating sequences so that strongly bonded discs of good dielectric properties have other discs with better dielectric properties spaced between the first discs to improve the attenuation characteristics of the coaxial cable.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.
BRIEF DESCRIPTION OF DRAWINGS In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views;
FIG. 1 is a diagrammatic view showing apparatus for molding spaced discs to inner conductors in accordance with this invention;
FIG. 2A is a diagrammatic view showing the way in which the inner conductor with the spaced discs thereon is advanced through a tube forming station in which the outer conductor is wrapped around the discs and the seam of the outer conductor is welded followed by a reduction in diameter of the outer conductor and a fusion of the outer conductor to the discs;
FIG. 2B shows the fusion station of FIG. 2A followed by a quench beyond which the coaxial cable is pulled to a spooling station;
FIG. 3 is a diagrammatic view showing the lower half of a mold in which discs are formed around parallel inner conductors by dielectric material supplied from a common header;
FIG. 4 is a diagrammatic view of a modified mold for molding discs of different kind of material to the inner conductor at different locations;
FIG. 5 is an enlarged sectional view taken on the line 5-5 of FIG. 1;
FIG. 6 is a view taken on the line 66 of FIG. 5;
FIG. 7 is a greatly enlarged sectional view at the welding station of FIG. 2A;
FIG. 8 is a greatly enlarged sectional view taken through part of the tube sinking station and through the fusion station of FIG. 2A ,and through the quenching head of FIG. 2B; and
FIG. 9 is a sectional view on the line 9-9 of FIG. 8.
DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows a wire spool 10 from which a conductor 12 is withdrawn by a puller 14. A subsequent puller l6 maintains the conductor 12 under tension while it passes through a cleaner l8 and preheater 20, both of which are of conventional construction.
The pullers 14 and 16 are preferably constructed so that they can pull a plurality of parallel wires, usually from different spools, and these wires pass through the 3 same cleaner l8 and preheater 20, the second wire being shown in FIGS. 3 and and designated by the reference character 12'.
Referring again to FIG. 1, the wire 12 and the parallelwire 12' pass from the preheater into a mold designated generally by the reference character 24. This mold 24 includes an upper section 26 and a lower section 28 which fit together to form a plurality of mold cavities for molding discs to the conductors l2 and 12'. In the diagrammatic showing of the mold 24, there are dowels 30 held by frame sections 32 and used to maintain the upper mold section 26 in register with the lower mold section 28. A hydraulic mechanism 34 is shown diagrammatically with an arrow indicating that the hydraulic mechanism moves the upper section 26 of the mold into contact with the lower section for molding, and away from the lower section 28 as necessary for moving successive groups of discs and the conductors l2 and 12'. The lower mold section 28 is also movable up and down by mechanism not illustrated; and when the mold sections 26 and 28 are together, as shown in FIG. 1, the parting line of the mold 24 is a plane coincident with the plane through the longitudinal axis of the conductors l2 and 12. When the mold opens, and both of the mold sections 26 and 28 move away from one another, they separate by distance sufficient to permit the conductors 12 and 12' and discs 38 which are molded on the conductors, to clear the mold cavities while moving in the direction in which the wires 12 and 12' extend.
In the preferred embodiment of the invention, the wires 12 and 12' move with continuous uniform motion and the mold 24 is also movable back and forth in the direction of movement of the conductors l2 and 12. FIG. 1 shows the mold 24 with wheels 40 on which it moves along a track 42. Motor means, preferably a hydraulic motor 44 moves the mold 24 in the same direction as the movement of the cables 12 and 12' while the mold is closing, during the actual molding operation, during cooling of the molded discs, and while the mold is opening to release the molded discs. The mold 24 is then moved rearwardly to its original position and its direction of movement is again changed to that of the direction of movement of the wires 12 and 12'. The mold is accelerated so that it is moving at the exact same speed as the conductors l2 and 12' while the mold closes.
The reverse movement of the mold 24 preparatory to each successive molding operation, is timed with respect to the speed of movement of the conductors 12 and 12' and their attached discs 38 so that each time the mold closes it molds a new group of discs to the conductors l2 and 12' with the first disc of the new group spaced from the last disc of the preceding group by the same distance as the discs are spaced from one another along the conductors.
FIG. 3 is a diagrammatic view of the lower mold section 28. There are cavities 48 at spaced locations along a groove 50 for molding the lower half of successive discs 38. Complementary mold cavities in the upper section of the mold register with the mold cavities 48 to mold the upper half of each disc 38. The groove 50 accommodates the conductor 12 and is located in such position that the discs 38 are molded concentric with the conductor 12.
There are similar mold cavities 48' along a groove 50' so that other discs 38' can be molded concentrically about the conductor 12'.
Each of the grooves 50 and 50' flares outwardly at its 5 juncture with both the front and rear of the cavity 48 so as to leave a fillet 52, best shown in FIGS. 7 and 8 where the disc is somewhat thicker in contact with the conductor 12. The grooves 50' and corresponding grooves of the upper mold section have similar flares so that the fillet 52 extends all the way around the wires 12 and 12' as clearly shown in FIGS. 7 and 8.
Molten dielectric material is supplied to the cavities 48 and 48' from a header 56 formed with half of its cross-section in the lower mold section 28 and the other half in the upper mold section in accordance with conventional molding practice. This header 56 has branch passages 58 leading to the different mold cavities with the branch passages reduced to very small cross-section where they enter the mold cavities, all in accordance with conventional molding practice.
The discs 38 and 38' which are made in the mold illustrated in FIG. 3 are all made of the same material since they are supplied from a common header 56. FIG. 4 shows a different mold construction in which some discs are made of different kind of material from that of other discs.
FIG. 4 shows a mold section 280 which has mold cavities 48a and 62. The mold cavities 48a and 62 are the same except that they are supplied with molten dielectric material from different headers. For example the mold cavities 480 are supplied from a header 64 having branches 66 leading to the mold cavities 48a; whereas the mold cavities 62 are supplied with molten dielectric material from a header 68 through branch passages 70.
In the construction shown in FIG. 4, two-thirds of the cavities are supplied from the header 64 and one-third from the header 68. The arrangement is such that the discs molded along an inner conductor 12a are equally spaced from one another with every third disc formed of material from the header 68. Discs formed of material from the header 64 are designated by the reference character 38a in FIG. 4; and those formed of material from the header 68 by the reference character 38b.
The purpose in forming some discs from different material from other discs will be explained in connection with the bonding of the discs to the outer conductor of the coaxial cable. It will be understood that discs made in a mold with different headers 64 and 68, as shown in FIG. 4, can also be made so as to mold discs on parallel conductors but each conductor has to be supplied with mold cavities fed from more than one header so that the construction shown in FIG. 3 cannot be used. The construction shown in FIG. 4 is used for parallel conductors by duplicating the apparatus shown in FIG. 4 for as many conductors as are to be used.
When using the mold shown in FIG. 3, the discs 38 and 38' come from the mold connected together by the tailings 72 shown in FIG. 5. These tailings are formed of the solidified material from the header 56 and the branch passages 58.
The tailings 72 are broken loose from the discs 38 and 38 by means of belts 76 and 78 which are endless and reverse their direction of run around pulleys 80 supported by axles 82. These belts 76 and 78 bear against the tailings 72 between the discs 38 and 38' as shown in FIG. 5 and 6. Instead of extending in the same direction as the movement of the wires 12 and 12',
however, the belts 76 and 78 extend off in a transverse direction, as shown in FIG. 6 so that as the tailings 72 advance with the wires 12 and 12', the belts 76 and 78 push the tailings to one side and break them loose from the discs 38 and 38' in the manner shown in FIG. 6. It has been found in practice that the tailings break cleanly from the discs 38 and 38 so that no smoothing of the circumferences of the discs is necessary to obtain a cylindrical surface on the circumference of the disc.
The conductors 12 and 12' pass through the puller 16 which is shaped to accommodate the discs and continue to advance to a tube forming station 86 shown in FIG. 2A.
A strip of electrically conductive material 88 is withdrawn from a supply reel 90 and is passed through successive roll passes 92 at a strip roller station. These roll passes 92 smooth the strip 88 and bring it to a more uniform thickness than is found in the usual commercial strip. The strip 88, which constitutes the outer conductor of the coaxial cable, produces a cable of better electrical characteristics if the thickness of the cable outer conductor is kept within close tolerances. In the preferred embodiment of this invention, the strip 88 is passed through a strip smoothing station 92, consisting of stationary flat dies, which is designed to remove thickness irregularities particularly those that appear periodically. The thickness is maintained to a tolerance of not more than +/-0.000l inches against periodic change.
The strip 88 passes from the smoothing roll passes 92 to an edge trimmer 94 which trims the edges of the strip so as to obtain a uniform width of the strip 88. This is important because it avoids the variations in the seam weld which would occur if the strip were not of uniform width. Such variations in the weld could result in the formation of flash on the inside of the weld and this would not only affect the operational qualities of the coaxial cable but it would interfere with the bonding of the discs to the outer conductor.
Beyond the edge trimmer 94, the strip 88 passes through a plurality of roll passes 96 at the tube forming station 86; and these roll passes 96 form the strip 88 into a butt seam tube designated by the reference character 88a. It will be understood that each of the wires 12 and 12' with the discs molded thereon, is supplied with its own strip 88 longitudinally folded around the discs to make the outer conductor of a coaxial cable. Since there is no correlation between the individual wires after they have passed beyond the apparatus for removing the tailings, shown in FIGS. 5 and 6, the invention from here on will be described as applied to a single wire conductor 12. The tube 88a which is formed at the tube forming station 86 is of substantially larger diameter than the discs 38 as is clearly shown in FIG. 7. The diameter of the disc 38 is indicated in FIG. 7 by the dimensional arrow D. The diameter of the inner conductor wire 12 is designated by the dimension d. The ratio of D to d is preferably between 3.3 to l and 4.5 to l.
The thickness of the disc at its circumference is indicated by the dimension T and the ratio of T to D is preferably between 0.25 to l and 0.08 to 1; discs of larger diameter having the smaller ratio. The portion of the disc which is adjacent to the conductor 12 is preferably about 2T, each fillet 52 having a base width of about T/2 the fillet 52. In the construction illustrated, the ratio of the radial extent of the substantially flat annular sides beyond the fillets 52 to the radial extent of the fillets is from about 3.3 to l and 4.5 to l; the same value range as the D/d ratio. The examples herein are for discs between about A; and inches diameter, but can be used on other sizes.
In the preferred embodiment, the diameter of the tube 88, as it comes from the tube forming station, requires a reduction in diameter of between 15 and 30 percent to bring the tube into contact with the discs around the full circumference of the discs 38.
The bottom of the tube 88a, as it comes from the tube forming station is preferably in contact with the bottom of each of the discs 38. This leaves a maximum distance above the discs for protecting the discs from the heat of the seam welding.
As the formed tube 88a travels from the tube forming station 86 to a seam welding station 100, the tube 88a passes through a ring 102 which holds the tube to an accurate circular shape just prior to the welding of the seam. The seam is preferably welded by a nondepositing electrode 103 with a shield 104 around the electrode for use with a shielding gas such as are commonly used for shielded arc welding. The heating of the are from the electrode 103 is correlated with the speed of travel and the thickness of the tube 88a so as to weld the abutting edges together without forming any flash.
Beyond the welding station 100, the tube 88a passes over guide rolls 106 and through a tube sinking station 110.
FIG. 8 shows the conductor 12 with the spaced discs 38 molded thereon as the outer conductor 88a passes through the tube sinking station 110. The sinking of the tube 88a to a smaller diameter can be accomplished in a stationery die but is preferably performed by sinking die means comprising successive rollstands 112, 113, and 114, all of which are shown in FIG. 2A. FIG. 8 shows only the last two roll passes 113 and 114.
At the tube sinking station 110, the diameter of the tube 88a is reduced to something less than the diameter D" of the discs 38. The last roll pass 114, therefore, causes the discs 38 to be somewhat compressed radially; but the discs are hard and this compression sets up a substantial pressure between the discs 38 and the inner surface of the reduced diameter outer conductor tube 880. There being no flash at the weld of the tube seam and the outer circumference of each disc 32 being a cylindrical surface, the tube 880 contacts with the circumference of each disc 38 around the entire extent of the disc circumference. This makes each disc a vapor proof partition across the inside of the outer conductor tube 880. To provide greater strength for the coaxial cable when subject to bending stresses, the discs 38 are not only held in position with respect to the outer conductor tube 88 by the friction generated by the pressure between the tube and disc circumference, but also by fusion at a fusion station 116.
In practice, the fusion step presented great difficulty. It was found that the fusing heat softened the disc sufficiently so that the substantial pressure of the tube against the disc was greatly reduced or completely lost. This invention overcomes the difficulty by a unique fusing step.
In the preferred construction, the tube 88a passes through an induction coil which surrounds the tube and which generates heat in the metal tube without generating heat in the discs 38. The heat of the tube 88a is transmitted by conduction to the circumference of the disc 38. The tube is heated by what is termed herein as flash heating. That is, the heating of the tube is so rapid that the circumference of the disc 38 reaches a fusing temperature before the material of the disc adjacent to the circumference has time to soften. The fusion is performed so quickly that the pressure between the disc 38 and the tubular outer conductor 88a undergo no substantial reduction in pressure against one another.
There is another influence that improves the fusion bond between the discs 38 and the outer conductor tube 880 without substantial reduction in the pressure between the circumference of the disc and the surrounding tube. When the conductor 12 is pulled through the smallest diameter portion of the sinking die means that is, through the roll pass 114 of FIG. 8, each disc 38 resists being pulled into the roll pass and in addition to the radial compression of the disc by the roll pass, there is a slight dishing of the disc 38. The circumferential portion of the disc is held back slightly by the resistance encountered in the roll pass 114 so that the disc has what amounts to a slight dish toward the rear. When the circumferential edge of the disc 38 is brought to fusion temperature by heating of the surrounding tube 88a, the disc gets an opportunity to recover from this dished condition because the fused circumferential surface of the disc can move forward and relieve the stress which caused the dishing. This action is evident from the fact that there is a small fillet 126 around the circumference of each disc after fusion but this fillet is only on the rearward side of the disc, the circumference of the disc having moved forward slightly at the time of fusion.
Even though the outer conductor tube 880 is heated by a flash heating, there would be a soaking of heat into the disc 38 if the heat in the metal of the tube were not instantly eliminated after fusion by a quenching ring 130. This ring 130 surrounds the coaxial cable and sprays cooling jets 132 against the metal tube 880 in sufficient volume and at sufficient velocity to instantly chill the metal of the tube below the softening temperature of the discs 38.
The discs 38 are made of dielectric material. In the preferred embodiment of the invention, the dielectric material is polyethylene. By using polyethylene containing carboxyl groups, an extremely tenaceous bond can be obtained between the disc 38 and the metal of the outer conductor tube 88a. The carboxyl groups make the polyethylene polar so that a chemical bond between the disc and the metal is obtained which gives the coaxial cable great strength and which insures a moisture tight partition even though the coaxial cable in the region of a particular disc is subject to substantial stresses when the cable is bent.
Lower density polyethylene, without carboxyl groups or other modification can be used to make the discs 38. Polyethylene without the carboxyl groups has better electrical characteristics and if the discs are made of polyethylene without carboxyl groups, the attenuation characteristics of the cable are somewhat improved. However, the polyethylene without the carboxyl groups does not fuse strongly to the metal of the outer conductor tube and this reduces the strength of the coaxial cable and has the more serious disadvantage of reducing the ability of the cable to withstand penetration of moisture lengthwise of the cable in the event that the outer conductor is damaged at some point.
The construction shown in FIG. 4 has the advantages, to a large extent, of both the discs with the carboxyl groups and those without. By making every third disc 38b of polyethylene with carboxyl groups, the coaxial cable can have the high insurance against penetration of moisture lengthwise of the cable in the event that the outer tube becomes damaged at some location so as to admit moisture into the interior of the cable. The distance between these moisture proof partitions provided by the discs 38b is not sufficient to cause the operation of the cable to be substantially impaired if the space between two successive discs 38b should have moisture enter into it. The distance between the discs 38b is sufficient, however, to require other spacer discs for maintaining the concentricity of the center conductor 12a and the outer conductor tube which surrounds the discs in the finished coaxial cable. These intervening discs 38a which are necessary to maintain concentricity can be made of polyethylene which gives the best attenuation characteristics to the cable.
The spacer discs of this invention can be made of materials other than polyethylene. For example discs can be made of dielectric material consisting of polypropylene, polystyrene and other dielectrics of low loss.
The preferred embodiment of the invention has been illustrated and described, but changes and modifications can be made and some features can be used in different combinations without departing from the invention as defined in the claims.
What is claimed is:
l. A coaxial cable comprising a center conductor and an outer conductor of substantially larger diameter than the center conductor and spaced therefrom, discs on the center conductor holding the outer conductor in spaced relation to the inner conductor including imperforate discs of dielectric material bonded to both the inner and outer conductors at spaced regions along the cable and forming separate watertight compartments therealong, and other discs of dielectric material having better attenuation characteristics than the first discs and located between the first discs in positions for providing additional mechanical support for the outer conductor at regions between the discs that form the watertight compartments.
2. The coaxial cable described in claim 1 characterized by the discs that form the waterproof compartments being of polyethylene with reactive carboxyl groups, and the other discs being made of polyethylene without reactive carboxyl groups and having better dielectric properties than the first discs.
3. The coaxial cable described in claim 1 characterized by the first group of discs being made of dielectric material having polar characteristics, and the second group of discs being made of dielectric material having non-polar characteristics.
4. The coaxial cable described in claim 1 characterized by the discs being molded around the inner axial conductor to provide a fusion bond thereto, and the outer circumferences of the discs being fusion bonded to the inside surface of the outer tubular conductor.
5. A coaxial cable including in combination an inner axial conductor, an outer tubular conductor spaced from the inner axial conductor by circular insulating discs, said discs being spaced from one another lengthwise of the cable and bonded to the inner conductor, and said discs having imperforate annular portions extending from the inner conductor to the outer conductor and secured in place on the outer conductor by hoop stresses in the outer conductor, the outer conductor exerting a substantial pressure on the circumferences of the discs to form a waterproof juncture with the discs.
6. The coaxial cable described in claim characterized by the outer tubular conductor having a longitudinally extending, welded, butt scam, the weld being free of inside flash and the discs contacting with the inside of the welded tube with substantially uniform pressure around the entire extent of the circumferences of the discs.
7. The coaxial cable described in claim 5 characterized by the outer conductor being a metal strip longitudinally formed to a circular tube with a butt seam, the metal of the strip being smoothed to uniform thickness with a tolerance of about plus or minus 0.0001 inches to eliminate repetitive changes in thickness of the outer conductor and thereby avoid periodicity in the coaxial cable.
8. The coaxial cable described in claim 5 characterized by the outer conductor being a tube that is sunk clown into contact with the circumferences of the discs so that there are longitudinal stresses between the outer conductor and the circumferences of the discs, and fillets of material displaced axially from most of axial width of the circumferential portions of the discs and on the rear sides and only the rear sides of the discs opposite to the direction in which the discs were moving when the outer conductor was brought into compressing contact with the circumferences of the discs by the sinking operation.
9. The coaxial cable described in claim 8 characterized by each disc having a front side that is free of any fillet corresponding to the fillet on the rear sides of the disc around the circumferences of the disc at the region where the disc contacts with the outer conductor.
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