US3380271A - Structure for and method of tube expansion - Google Patents

Description

P 1968 E. P. ,HABDAS 3,380,271

STRUCTURE FOR AND METHOD OF TUBE EXPANSION Filed July 12, 1965 .5 Sheets-Sheet 1 IO 9 S z Q I n w I '9 I ,9;

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ATTORNEYS April 30, 1968 E. P. HABDAS 3,380,271

STRUCTURE FOR AND METHOD OF TUBE EXPANSION Filed July 12. 1965 5 Sheets-Sheet 2 76 3- f5 5; s if -:g :5 "5' I 5- a: 5-. 58 22 Wm w-v FIG. 5 INVENTOR. I w P. HABDAS BY m) ATTORNEYS April 30, 1968 E. P. HABDAS 3,330,271

STRUCTURE FOR AND METHOD OF TUBE EXPANSION Filed July 12, 1965 .3 Sheets-Sheet 3 VACCUM DRAWING APPARATUS INVENTOR. EDWARD P. HABDAS AT TOR NEYS United States Patent 3,380,271 STRUCTURE FOR AND METHOD OF TUBE EXPANSION Edward P. Habdas, Dearborn, Mich., assignor to Calumet & Hecla, Inc., Allen Park, Mich., a corporation of Michigan Filed July 12, 1965, Ser. No. 470,980 19 Claims. (CI. 72-56) ABSTRACT OF THE DISCLOSURE Structure for expanding plain sections of finned tubing or the like, including a die having an annular internal recess for positioning over the section of the tubing to be expanded, a source of electric energy, means connected to the source of electric energy and adapted to be extended into the tubing to the section thereof to be expanded for providing a mechanical tube expanding force on the section of the tube to be expanded in response to a signal from the signal source and such method of tube expansion. In one modification the means for providing an expanding force on the section of tube includes a radially expandable hollow coil haviiig a conduciing liquid therein connected to the source of electrical energy and means for concentrating the flux lines at the inner diameter of the coil on energizing the coil. In a second modification the means for providing a tube expanding force comprises a liquid within the tube and means for initiating an electric are at the section of tube to be expanded in response to a signal from the signal source.

The invention relates to structure for and a method of expanding tubes and the like and refers more specifically to a method of expanding finned tube plain sections by high energy rate techniques and structure for practicing the method.

In many applications of finned tubes it is required that plain sections be provided on a finned tube which have the approximate outer diameter of the tube fins. With fins of low height the fins can be produced by sinking the tube to reduce the internal diameter thereof whereby plain sections may be left with an outer diameter substantially equal to the outer diameter of the fins. With higher fin heights the sinking technique is difficult or impossible due to a tendency of tubes to collapse when deep sinking is attempted.

Therefore, where high fin heights are required along with plain sections of a diameter slightly greater than the fin diameter, alternate methods are generally used. One such method is to expand the plain ends of the tube with any of a variety of well-known mechanical expansion tools. However, the expansion of plain sections between finned sections along the length of the tube is difficult to achieve with known mechanical expansion tools. Another method for supplying the expanded ends is to weld or braze on sections that have been formed to the required shape in a separate operation. Again, this method is not readily adaptable to intermediate plain sections and is a relatively costly operation.

Electromagnetic forming of tubes has been previously known and is very successful with the coil producing the electromagnetic force positioned exteriorly to a tube that is to be reduced in diameter. However, when the coil is positioned within the tube so that it will produce radially outward forces on the tube as required for expansion of its diameter the design of the coil becomes more difiicult, especially in the range of 1" diameter and below Where existing internal coils are limited to relatively low energy input. This limitation is generally due to the design problems encountered in providing sufiicient structural strength to withstand coil destructive forces for a reasonable life span and in providing adequate cooling for high pulse repetition rates.

It is therefore one of the objects of the present invention to provide an internal coil of unique and improved design for expanding plain sections of finned tubes.

Another object is to provide improved structure for expanding plain sections of tubes.

Another object is to provide an improved method of expanding plain sections of tubes.

Another object is to provide structure for electromagnetic expansion of a plain section of a finned tube including a die having an annular recess therein positioned around the plain section to be expanded, a source of high energy rate electric signals and means within the tube at the plain section connected to the source of electric signals for producing a radially outward force on the plain section in response to an electric signal from the signal source.

Another object is to provide structure for electromag netic expansion of a plain section of a finned tube as set forth above wherein the means for producing a radial force on the tube includes a coil positioned within the plain section to be expanded which is expandable radially outwardly on receipt of a high energy rate electric signal.

Another object is to provide structure as set forth above wherein the expandable coil is wound with a hollow, resilient, electrically insulating material such as rubher which is filled with an electrically conducting material capable of a specific amount of nondestructive expansion, such as liquid mercury or specially braided copper.

Another object is to provide structure as set forth above wherein a core of high strength material having radially extending flanges at both ends thereof is provided on which the expandable coil is mounted.

Another object is to provide structure as set forth above wherein the outer surface of the central portion of the core and the axially inner surfaces of the flanges at the end of the core are coated with a material which is a good conductor of electricity.

Another object is to provide structure for the electrohydraulic expansion of a plain section of a finned tube as set forth above wherein the means for producing a radial force on the tube includes a shock wave transmitting fluid within the tube and means connected to said source of electric signals for producing an electric are within the fluid filled tube at the plain section thereof.

Another object is to provide a method of expanding plain sections of finned tubes including the steps of placing a die having an annular recess therein around the plain section of the tube, creating a high energy rate electric signal, and producing a mechanical force operable on the interior of the plain section of the tube in response to the high energy rate electric signal to expand the plain section of the tube into the annular recess.

Another object is to provide a method of expanding plain sections of finned tubes as set forth above wherein the electric signal is passed through a fiexi'b-le coil positioned within the tube at the plain section thereof and the coil is expanded into contact with the tube in response to the electric signal to provide the mechanical force.

Another object is to provide a method as set forth above and further including the step of filling the tube with a shock wave transmitting fluid medium and producing an arc Within the tube at the section to be expanded in response to the electric signal to produce a shock wave in the fluid which provides the mechanical force.

Another object is to provide structure for and a method of expanding plain sections of finned tubes and the like which is simple, economical and etficient.

Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings, illustrating a preferred embodiment of the invention, wherein:

FIGURE 1 is an elevation view of structure for practicing a method of expansion of a plain section of a finned tube constructed in accordance with the invention.

FIGURE 2 is an enlarged view of a portion of the structure illustrated in FIGURE 1 showing die members in position around the plain section of a finned tube to be expanded before expansion of the plain section.

FIGURE 3 is an enlarged view of a portion of the structure illustrated in FIGURE 1 similar to the view of FIGURE 2 but with the die members in an open position and the plain section of the finned tube expanded.

FIGURE 4 is an enlarged view of a portion of the die structure illustrated in FIGURE 1 showing the finned tube broken away and showing an expandable electromagnetic forming coil in longitudinal cross section.

FIGURE 5 is a reduced diagrammatic section view of the expandable electromagnetic forming coil of FIGURE 4, illustrating the concentration of the flux lines between the inner surface of the coil and highly electrically conductive core coating and the spreading of the flux lines between the coil and a poorly electrically conducting tube and die members produced in response to a pulse of electric energy received by the coil.

FIGURE 6 is a partly broken away view of an expandable electromagnetic fOrming coil similar to that illustrated in FIGURES 2 and 3 and showing the coil constructed of especially braided metal such as copper.

FIGURE 7 is an elevation view of modified structure for expanding a plain section of a finned tube.

FIGURE 8 is an enlarged broken away view of a portion of the tube expanding structure illustrated in FIG- URE 7.

With particular reference to the figures of the drawings, one embodiment of the present invention will now 'be considered in detail.

As shown in FIGURES 1-5, the structure 10 for expanding plain sections 12 of finned tube 14 includes the supporting structure 16 for the finned tube 14 and the die structure 18 for providing an annular die recess into which the plain sections 12 of the finned tube 14 may be expanded. A source of high energy rate electric signals 20 and an expandable electromagnetic coil 22 connected to the signal source by a coaxial cable 60 completes the tube expanding structure 10.

In operation, with the finned tube 14 supported on the supporting structure 16 and with the die structure 18 forming an annular die recess 24 around a plain section 12 of the finned tube, the coil 22 is energized by an electric signal from the signal source 20. The coil 22 expands radially into contact with the section 12 of the finned tube 14 with which it is aligned to expand the section of the tube into the annular die recess 24.

More specifically, the supporting structure 16 for supporting the finned tube 14 may include a longitudinally extending platform 26 having the supporting rollers 28 mounted thereon for rotation by pivot means 30. The finned tube 14 is placed on the rollers 28 which are provided in pairs and inclined to be in line with radii of the finned tube 14 for support of the tube during longitudinal movement thereof. Such supporting structures for finned tubes are well known and will not therefore be further considered herein.

The die structure 18 includes the upper and lower die members 32 and 34 each having a semi-circular internal recess 36 and 38 respectively therein. The semi-circular recesses '36 and 38 form the annular recess 28 with the die members 32 and 34 closed about a plain section 12 of the finned tube 14, as shown in FIGURE 2.

The die members 32 and 34 are supported on piston rods 40 and 42 for movement between a closed position,

.4 as shown in FIGURE 2, and an open position, as shown in FIGURE 3, on actuation of the piston and cylinder structures 44 and 46, respectively. Both hydraulic and pneumatic means for actuating the piston and cylinder structures 44 and 46 are well known and will not be considered in detail at this time.

The source of high energy rate electric signals 20, shown in FIGURE 1, includes a charging circuit 48 for charging a capacitor 50 when switch 52 is closed. On closing of the switch 54 with capacitor 50 charged, the charge on the capacitor 50 is placed across the electrodes 56 and 58 of the coaxial cable 60. The switches 52 and 54 may of course be actuated manually or automatically and other means for providing a pulse of electric energy across the conductors 56 and 58 of the coaxial cable 60 may be substituted for the specific source of high energy rate electric signals 20 indicated.

The electromagnetic coil 22, as best shown in FIGURE 4, includes a hollow helical tube 62 of expandable material, such as rubber, having wear characteristics to permit extended use thereof as a die forming member. As shown in FIGURE 4, the convolutions of the hollow tube seat on each other to form a helical coil. The hollow tube 62 is filled with a fluid medium 64 which is a conductor of electricity such as mercury. The mercury is in communication at the opposite ends of the tube 62 with the conductors 56 and 58 of the coaxial cable 60. In the modified coil 22 shown in FIGURE 6 the mercury is replaced with braided copper wire 65 which will expand and contract in length.

The tube 62 is wound on the core 66 of high strength material, such as steel, between the radially extending flanges 68 and 70 of the core 66 as shown best in FIG- URE 4. A thin layer or coating of highly conductive material, such as copper 72, is provided on the radially outer surface 67 of the central portion of the core 66 and on the radially extending axially inner surfaces 69 and 71 of the radially extending flanges 68 and 70 of the core 66 between the core 66 and the coil 22. The conductors 56 and 58 extend through insulating sleeves 74, 76 and 78 in core 66, as shown in FIGURE 4.

In operation, when it is desired to expand a plain section 12 of the tube 14, the tube 14 is positioned on the supporting structure 16 with the plain section to be expanded in alignment with the die members 32 and 34 which during the positioning of the tube 14 are open, as illustrated in FIGURE 3. The die members 32 and 34 are then closed around the plain section 12 of the tube 14 to be expanded, as shown in FIGURE 2, and a coaxial cable 60 connected to an expanding coil 22, as shown best in FIGURE 4, is inserted through the end 80 of the tube 14 into alignment with the section 12 of the finned tube 14 which it is desired to expand.

The coaxial cable 60 is connected to the source of high energy rate electric signals 20 with the switch 54 open. The switch 52 is closed to charge the capacitor 50 from the charging circuit 48. The switch 52 is then opened and the switch 54 closed to provide a pulse of electric energy through the expanding coil 22.

On a high energy rate electric signal passing through the mercury 64 of the expanding coil 22, flux lines 72, as shown in FIGURE 5, are produced about the coil 22 which are concentrated at the layer of high conductivity material 72 positioned between the core 66 and coil 22 due to a high resistance to transmission of flux lines produced by the coil 22 through the high conductivity material 72 offered by the material 72.

The high concentration of flux lines adjacent the radially inner surface of the expandable coil 22 will provide a force proportional to the density of the flux lines and their distance from the coil 22 to cause radial expansion of the convolutions of the coil 22 outwardly to expand the plain section 12 of the tube 14 into the annular recess formed between the die members 32 and 34 on contact of the coil tube 62 with the inner surface of the plain section of the tube.

The coil 22 is also subjected to an axial force tending to compress the coil during the radial expansion thereof due to the provision of the flanges 68 and 70 on the core and the coating of high conductivity material on the axially inner surface thereof and the consequent concentration of fiux lines at the ends of the coil, as shown best in FIGURE 5.

Since the flux lines which are exterior of the coil tend to resist radial outward expansion of the expandable coil, concentration of the exterior flux lines close to the radially outer surface of the coil is undesirable. Thus, expansion of highly electrically conductive materials is not as readily accomplished by the structure illustrated in FIGURES 14 as is the expansion of materials of relatively poor conduction which allow the magnetic field produced by the coil to travel outward with almost undiminished velocity and negligible flux concentration exterior to the coil.

To reduce the magnitude of the radially inward forces on the coil due to the concentration of flux lines external to the coil when an electrically conductive tube'to be expanded has been positioned adjacent to the radially outer surface of the expandable coil, an expandable coil having a relatively large outer thickness of the insulating expandable material of which the coil tube 62 is mademay be used. Thus since the same magnitude of flux appears inside coil 22 as outside coil 22, a difference in flux density is obtained since the insidefiux exists over the small cross-sectional area between the inside of the coil 22 and the electrically conductive coating 72 while the outside flux exists over a relatively larger cross-sectional area between the outer surfaces of the conductive core 64 and the inner surface of the tube 12 which is being expanded. Since the forces tending to radially expand the coil which are proportional to the inside flux density squaredare made considerably greater than the forces tending to radially reduce the coil which are proportional to the outside flux density squared, a net radially outward force is produced which accelerates the coil radially outward. The high energy rate electrical pulse is of such short duration that it has ended by the time the coil has expanded sufficiently to change the flux density unbalance significantly. The work of expanding the tube is thus performed by the inertial effects when the high velocity coil contacts the tube.

After expansion of the coil due to a high energy rate electric signal received thereby, the coil will resume its normal position, as illustrated best in FIGURE 4, wherein the convolutions thereof are positioned on the core 66.

The modified structure 90 illustrated in FIGURES 6 and 7 for expanding plain sections of finned tubes comprises a supporting structure 92 having rollers 94 thereon on which a finned tube 96 is supported in a position inclined to the horizontal, as shown in FIGURE 6. The structure 90 further includes the die members 98 and 100 again providing an annular recess 102 into which a plain section of the finned tube 96 may be expanded. The piston and cylinder structures 106 and 108 are provided as part of die structure 104 for reciprocating the piston .rods 110 and 112 to which the die members 98 and 100 are secured respectively.

The die members 98 and 100 further include a gasket seal 146 around the areas contacting the tube 96 and areas mating with one another which is capable of maintaining vacuum conditions around the tube section enclosed by the die members 98 and 100. Die member 100 further includes a passage 14! and fitting 143 for joining the enclosed space within the closed dies and tube to a vacuum pumping system 144.

The vacuum conditions within the die space around the tube are desirable to allow the tube to expand against the die contour at the extreme velocities encountered in the electro-hydraulic high energy rate process. Without vacuum conditions the air trapped within the die cavity would be compressed so rapidly as to cause detrimental heating of the air and a cushioning effect upon the tube.

Air vent strucure 114 is provided at end 116 of the finned tube 96. The structure 114 includes the sealing plug 118, air vent valve 120 and a connecting conduit 122.

At the other end 124 of the finned tube 96 fluid filling apparatus 126 is provided to fill the tube 96 with shock wave transmitting fluid, such as water. The fluid filling apparatus 126 includes a valve 128 for alternatively connecting the water line 130 and drain line 132 to the tube 96 through sealing plug 134 and conduit 136.

A coaxial cable 138 extends through the sealing plug 134 and into the finned tube 96 from a source of high energy rate electric signals 140 which may be the same as the source of electric signals 20 illustrated in FIGURE 1. As shown in FIGURE 7, the coaxial cable 138 terminates adjacent the die members 98 and 100 and an arc initiating wire 142 is connected across the conductors of the coaxial cable over substantially the entire length of the plain section of the tube to be expanded.

In operation the finned tube 96 is positioned so that the plain section to be expanded is aligned with the annular recess 102 formed by the open die members 98 and 100 at which time the cylinders 106 and 108 are actuated to clamp onto the tube and the vacuum pumping cycle automatically or manually initiated to evacuate the recess 102. The coaxial cable is inserted in tube 96 and the end aligned with the annular recess 102 formed by the die members 98 and 100 with the arc-initiating wire within the section of tube to be expanded. Seal 134 is then tightened into tube and around the coaxial cable and valve 128 actuated to fill the tube 96 with water with the air therein being purged through the air vent structure 114.

A high energy rate electric signal is then fed through the coaxial cable from the signal source 140 whereby an electric arc is produced within the water filled tube at the section to be expanded. The are in the Water will cause a shock wave which will expand the tube 96 into the annular recess formed in the die. While a subsequent pressure wave follows the initial shock wave created by the electric are formed in the water, most of the forming it is believed is produced by the shock wave.

The water may then be drained from the tube 96. The die members are retracted and the tube 96 removed or repositioned 0n the supporting structure 92.

While one embodiment of the present invention and a modification thereof have been disclosed in detail, it will be understood that other embodiments and modifications are contemplated by the inventor. It is the intention to include all embodiments and modifications as are defined by the appended claims within the scope of the invention.

What I claim as my invention is:

1. Structure for expanding plain sections of finned tubing or the like comprising die structure forming an annular internal recess into which it is desired to expand a section of a tube extending into the die structure, a source of electric signals including a pair of conductors extending into the tube within the section thereof to be expanded and means operable between the inner surface of the section of the tube and the signal source for providing a mechanical tube expanding force on the section of the tube in response to a signal from a signal source to expand the tube section into the recess in the die structure, comprising a radially expandable electric coil including a hollow helix of expandable material and an electric conductor within the hollow helix which is capable of alternate extension and retraction in length connected to the pair of conductors at the opposite ends thereof.

.2. An expandable electric coil comprising a hollow helix of expandable material wound on a core of high strength material having radially extending flanges at the opposite ends thereof extending over the ends of the coil, a fluid within the hollow helix which is a conductor of electricity and a layer of high electric conductivity material provided between the coil and core, whereby the velocity of propagation of electromagnetic waves toward the core would be considerably reduced to create an area of high magnetic flux density between the coil and the layer of material.

3. Structure for expanding plain sections of finned tubing or the like comprising die structure forming an internal recess into which it is desired to expand a section of a tube extending into the die structure, a source of electric signals including conductor means extending into the tube within the section thereof to be expanded and a radially expandable electric coil, the opposite ends of which are connected to the conductor means of the signal source positioned within the tube section to be expanded for providing a mechanical tube expanding force on the section of the tube to be expanded in response to a signal from the signal source.

4. Structure for high energy rate electromagnetic expansion of a plain section of a finned tube or the like comprising a supporting structure for the tube, opposing die members together forming an annular recess into which it is desired to expand the section of the tube, means for clamping the die members around the section of the tube into which it is desired to expand the tube section, a source of high energy rate electric signals, electrical conductor means connected to the signal source and extending into the tube to the section thereof to be expanded and a radially expandable electric coil connected across the conductor means, including a core, means connecting the conductor means to the opposite ends of the coil and a layer of material positioned between the core and the coil which is a good conductor of electricity for concentrating the flux lines produced on energization of the coil closely adjacent the inner diameter of the coil, whereby on energization the coil will be caused to expand radially into contact with the inner surface of the tube section to be expanded, to expand the tube section into the recess provided by the die members.

5. Structure for the high energy rate expanding of a plain section of a finned tube or the like electro-hydraulically comprising structure for supporting the finned tube in an inclined position with respect to the horizontal, a pair of die members having semi-circular recesses therein positioned on opposite sides of the section of the finned tube to be expanded, means for advancing the die members into contact with the section of the finned tube to be expanded to form an annular recess into which the section of the finned tube is to be expanded, a fitting for closing one end of the finned tube and allowing the escape of air therefrom, a fitting for closing the other end of the finned tube permitting entry into and drainage from the finned tube of shock wave transmitting fluid and entry into the tube of conductor means, a source of high energy rate electric signals, conductor means connected to the source of electric signals extending through the fitting at the other end of the finned tube and into the plain section of the tube, an arc initiating means extending for substantially the length of the plain section of the finned tube to be expanded connected to the conductor means within the tube section whereby in response to a signal from the signal source an arc is produced axially of the plain section of the finned tube and a shock wave results which proceeds radially outwardly into contact with the inner surface of the plain section of the finned tube to expand the tube section into the annular recess provided by the die members. i

6. Structure as set forth in claim 1 wherein the electric conductor is a braided wire of high electrical conductivity.

7. Structure as set forth in claim 1 wherein the electric conductor is a fluid.

8. Structure as set forth in claim 7 wherein the fluid is mercury.

9. Structure as set forth in claim 1 wherein the hollow helix is rubber tubing, rectangular in cross section.

10. Structure as set forth in claim 1 wherein the expandable coil is wound on a core of high strength material having radially extending flanges at the opposite ends thereof extending over the ends of the coil.

11. Structure as set forth in claim 10 wherein a layer of high electrical conductivity material is provided between the coil and the core whereby the velocity of propagation of electromagnetic waves toward the core would be considerably reduced to create an area of high magnetic flux density between the coil and the layer of material.

12. A radially expandable electric coil comprising a hollow helix of expandable material relatively fixed axially and a fluid within the hollow helix which is a conductor of electricity where-by on passage of electric energy through the fluid substantially entirely radial expansion of the coil will occur.

13. Structure as set forth in claim 12 wherein the hollow helix is rubber tubin rectangular in cross section.

14. Structure as set forth in claim 12 wherein the fluid is mercury.

15. Structure as set forth in claim 12 wherein the expandable coil is wound on an axially relatively fixed core of high strength material having radially extending flanges at the opposite ends thereof extending over the ends of the coil.

16. Structure for high energy rate electromagnetic expansion of a plain section of a finned tube or the like comprising a supporting structure for the tube, opposing die members together forming an annular recess into which it is desired to expand the section of the tube, means for clamping the die members around the section of the tube to form the annular recess into which it is desired to expand the tube section, a source of high energy rate electric signals, a coaxial cable connected to the signal source and extending into the tube to the section thereof to be expanded and a radially expandable coil connected across the conductors of the coaxial cable, including a core of high strength material having radially extending flanges at the opposite ends thereof, a hollow, rectangular, helical, expandable tube supported on the core between the end flanges thereof, fluid within the hollow tube which is a conductor of electric current, in-

' sulated means connecting the conductors of the coaxial cable to the opposite ends of the coil in electric contact with the fluid therein and a layer of material positioned between the core and the coil which is a good conductor of electricity for concentrating the flux lines produced on energization of the coil closely adjacent the inner diameter of the coil whereby on energization the coil will be caused to expand radially into contact with the inner surface of the tube section to be expanded, to expand the tube section into the recess provided by the die members.

17. Structure for the high energy rate expanding of a plain section of a finned tube or the like electro-hydraulically comprising structure for supporting the finned tube in an inclined position with respect to the horizontal, a pair of die members having semi-circular recesses therein positioned on opposite sides of the section of the finned tube to be expanded, means for advancing the die members into contact with the section of the finned tube to be expanded to form an annular recess into which the section of the finned tube is to be expanded, means for drawing in vacuum between the tube and recess, a fitting for closing the upper end of the finned tube and allowing the escape of air therefrom, a fitting for closing the lower end of the finned tube permitting entry into and drainage from the finned tube of shock wave transmitting fluid and entry into the tube of a coaxial cable therethrough, a source of high energy rate electric signals, a coaxial cable connected to the source of electric signals extending through the fitting at the lower end of the finned tube and into the plain section of the tube, an arc initiatin wire extending for substantially the length of the plain section of the finned tube to be expanded between the conductors of the coaxial cable whereby in response to a signal from the signal source an arc is produced axially of the plain section of the finned tube and a shock wave results which proceeds radially outwardly into contact with the inner surface of the plain section of the finned tube to expand the tube section into the annular recess provided by the die members.

18. A method of expanding a section of a tube comprising positioning an annular recess in a die into wh'ch it is desired to expand the section of the tube around the section of the tube, inserting a flexible substantially only radially expandable coil into the tube to the section to be expanded and providing a high energy rate electric signal through the coil to cause substantially only radial expansion of the coil outwardly into contact with the inner surface of the tube section and consequent mechanical con tact expanding of the tube section into the die,

10 19. A method as set forth in claim 18 and further including placing a core within the expandable coil and concentrating the radially inner flux produced on passing a high energy rate electric signal through the coil at the exterior of the core, thereby to increase the force of outward radial expansion of the coil.

References Cited UNITED STATES PATENTS 730,847 6/1903 Gilder et a1. 174-95 1,493,713 5/1924 Tykocinski 336-20 1,748,927 2/1930 Kiemer 174-95 2,252,919 8/1941 Eckard 336-20 3,203,212 8/1965 Simichich 72-56 3,206,845 9/1965 Crunip 72-56 3,256,846 6/ 1966 Keinanen 72-56 3,267,780 8/1966 Roth 72-56 3,286,497 11/1966 Cary 72-56 RICHARD J. HERBST, Primary Examiner.

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