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Publication numberUS3654787 A
Publication typeGrant
Publication dateApr 11, 1972
Filing dateOct 15, 1968
Priority dateOct 15, 1968
Publication numberUS 3654787 A, US 3654787A, US-A-3654787, US3654787 A, US3654787A
InventorsBrower David F
Original AssigneeGulf Oil Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electromagnetic forming apparatus
US 3654787 A
An electromagnetic forming apparatus utilizes a forming coil having separable die portions for receiving a workpiece which is configured in such a manner that it could not otherwise be worked by a high intensity magnetic field. An improved design of the forming coil increases its operating life.
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Description  (OCR text may contain errors)

United States Patent Brower [151 3,654,787 {451 Apr. 11, 1972 [54] ELECTROMAGNETIC FORMING APPARATUS [72] Inventor: David F. Brower, San Diego, Calif.

[73] Assignee: Gulf Oil Corporation, San Diego, Calif.

[22] Filed: Oct. 15, 1968 [21] Appl. No.: 767,777

[52] US. Cl..... ..72/56 [51] Int. Cl ..B2ld 26/14 [58] Field of Search ..72/56; 330/225, 229, 232

[56] References Cited UNITED STATES PATENTS 3,347,074 10/1967 Eilers et al. ..72/56 Primary Examiner-Richard J. l-lerbst Attorney-Fitch, Even, Tabin & Luedeka [5 7] ABSTRACT An electromagnetic forming apparatus utilizes a forming coil having separable die portions for receiving a workpiece which is configured in such a manner that it could not otherwise be worked by a high intensity magnetic field. An improved design of the forming coil increases its operating life.

4 Claims, 5 Drawing Figures TO ENERGY SOURCE PAIENTEBAPRH 1912 3,654,787

SHEET 1 OF 2 Mil 1 w r llllllI INVENTOR DAVID F. BROWER PATENTEDAPR 11 I972 3, 654,787

sum 2 or 2 F I36 I50 TO ENERGY SOURCE INVENTOR DAVID F. BROWER ELECTROMAGNETIC FORMING APPARATUS The present invention relates generally to forming apparatus, and more particularly to apparatus by means of which conductive materials may be formed or swaged by energy acquired from a varying magnetic field.

Various methods and apparatus have heretofore been developed for forming or swaging conductive materials by employing a varying magnetic field of high intensity. In such methods and apparatus generally, an electrical current pulse of high amperage and short duration is passed through a conductor, typically formed into a work coil, to thereby produce a pulsed magnetic field of high intensity in the proximity of the conductor. A conductive workpiece is positioned in the pulsed magnetic field and a current pulse is thereby induced in the workpiece, which current pulse interacts with the magnetic field to produce a force acting on the workpiece. This force or magnetic pressure is made sufficiently great to cause the desired deformation of the workpiece, the swaging of one piece to another, etc. The manner of deformation is generally dependent upon the shape or configuration of the magnetic field and the position of the workpiece relative to the field. Repeated pulses of current may be applied to the conductor or coil, thus causing a series of deformingimpulses to be applied to the workpiece.

As indicated above, such apparatus and methods for electromagnetic forming of a workpiece typically employ a conductor in the form of a coil, i.e., a work or forming coil, which surrounds that part of the conductive or metallic workpiece to be formed or swaged. For example, in the typical case of magnetic swaging, it is generally possible to insert an elongated uniform workpiece into the forming coil aperture or opening which defines the magnetic field and controls the pressure pattern on the workpiece. However, when using such a forming coil with certain types or shapes of workpieces, a problem may arise in that the workpiece cannot be inserted within the coil aperture or the formed assembly cannot be withdrawn therefrom. This problem may arise in connection with workpieces having a generally dumbbell shape, wherein the end portions may be flanged or otherwise dimensioned so as to be larger than the coil aperture, making it impossible to insert or pass the workpiece therethrough, while the aperture is sufficiently small to provide an efficient forming operation. Further, other applications and workpiece configurations prevent the workpiece from being inserted axially into the forming coil, and examples of such configurations are those which are closed in themselves, such as a ring, or which, for some other reason, cannot be threaded into the coil. Examples of this latter type are swaging applications requiring work on pipelines or electrical conductors which, by reason of their indefinite or long lengths, or their installations, cannot be threaded through a coil.

Accordingly, it is an object of the present invention to provide apparatus for electromagnetically swaging or otherwise forming such workpieces which are not practicably formable by means of the type of forming coils generally described above.

A further object is to provide an improved magnetic form ing apparatus which is efficient in operation, relatively inexpensive to manufacture, durable in use, and yet may be used to form such cumbersome workpieces as described.

Other objects and advantages of the present invention will become apparent by reference to the following description and the accompanying drawings, in which:

FIG. 1 is a view partially in elevation and partially schematic of a forming apparatus in accordance with one embodiment of the present invention, showing the apparatus in its operating condition;

FIG. 2 is a partial elevational view of the apparatus of FIG. 1, showing the apparatus in its loading and unloading condition;

FIG. 3 is a sectional view taken along lines 3-3 in FIG. 2;

FIG. 4 is an elevational view, partially broken away, of a portion of a forming apparatus in accordance with a further embodiment of the invention, showing the apparatus in its operating condition; and

FIG. 5 is a partial elevational view of the apparatus of F I67 4, showing the apparatus in its loading and unloading condition.

Referring now to FIGS. 1 through 3, apparatus is shown for electromagnetically forming a metallic workpiece 10, illustrated as a pipe or tube 12 having a collar 14 generally concentrically arranged and disposed thereover so that it can be swaged to the tube. The tube 12 may, for example, be fixed in place with no access to its ends, or may have flanges or fittings on its ends which extend radially beyond the periphery of the collar 14. The fonning apparatus comprises a forming or work coil assembly 15 having a pair of relatively movable and engageable dies or coil-halves l6 and 18 formed of a conductive material, which are so arranged, when engaged as shown in FIG. 1, that they provide current paths around the workpiece 10. Operating and clamping means 20 is provided for maneuvering the dies 16 and 18 into and out of engagement, and an energy source 22, connected directly to the dies 16 and 18, is operative to supply high amperage current pulses to the dies in their engaged position so that the current flow through the dies creates high intensity magnetic fields for forming the workpiece 10 by, for example, swaging the collar 14 onto the tube 12.

The pair of relatively movable and engageable dies 16 and 18 have interfacing surfaces 26 and 28, respectively, which cooperate to define a common recess or workspace 24 to receive the workpiece l0, and the dies are arranged to form a single turn conductor for electric current around the recess 24 when the dies are engaged. The interfacing engageable and disengageable surfaces 26 and 28 are symmetrically formed to define the common recess 24 for the workpiece and may be suitable insulated from each other, as will be hereinafter described.

The energy source 22 shown schematically in FIG. 1, is connected directly to dies 16 and 18 by means of a transmission line assembly 29 which supplies the high amperage current pulses to the dies with suitable polarity or direction of current flow to produce additive high intensity magnetic fields within the recess or workspace 24 for forming the workpiece. The current pulses to each die follow a path around the recess 24 from one conductor to the other conductor of the transmission line assembly 29, through suitable interface contacts which will be later described.

The workpiece 10, comprising the conduit or tube 12 and the collar 14, is preferably suspended within the workspace 24 so as to provide a gapbetween the outer or peripheral surface of the collar and the inner surface or wall of the coil aperture. The collar 14 may be disposed on the conduit with some small clearance therebetween, although the particular fit used will generally depend on the material type and thickness, and on the operating parameters employed. The gap between the collar and the coil may be filled in some cases with a solid insulation or insulating film.

As shown in FIG. 2, the dies 16 and 18 are illustrated in their loading and unloading condition wherein they are disengaged or open for introduction or positioning of the workpiece 10 into the workspace 24 or for the removal of the completed or swaged assembly. The die 18, which is fixed, and the transmission line assembly 29 form an integral unit, as shown. The transmission line assembly 29 has a pair of conductor leads or plates 30 and 32 which are separated by an insulating layer or sheet 34, and serve as input and output current terminals for the work coil 15. The input or upper lead or bus 30, which is desirably in the form of a flat plate, strip, or bar has a generally cylindrical or semi-cylindrical contact surface 36 disposed near the end thereof adjacent the die 18 and fixed thereto. The contact surface 36, as illustrated, is formed by a generally circular cylindrical conductive metal bar or rod which is welded or brazed into a complementary hollow or groove 38 in the bus 30. The output or lower lead or bus 32, which may be in the form of a flat plate, strip or bar, is joined to the die 18 at surface 40 with a high conductivity joint, made in any known manner. The insulation 34, which may be of any suitable type, is disposed between the buses 30 and 32, and may extend to the workspace 24, although it need only extend to the contact end of the upper bus 30 to provide support therefor. The insulation between this end of the bus 30 and the workspace 24 may be formed by an air gap between the die surfaces 26 and 28 in this region.

The portion of the surface 28 on the opposite side of the workspace 24 has a second generally cylindrical or semi-cylindrical contact surface 42 formed in a manner similar to the first contact surface 36, and has a generally circular cylindrical rod welded or brazed in a mating hollow 44 in the die 18.

The corresponding surface 26 of the movable die 16 has generally complementary contact portions 46 and 48 which define contact surfaces with the fixed contacts 36 and 42, respectively. Thus, a complete current path is provided from the input terminal bus 30, through contact surfaces 36 and 46, die 16, contact surfaces 48 and 42, and die 18, to the output terminal bus 32.

The output of the energy source 22 is connected directly to the buses 30 and 32 of the transmission line assembly 29. The energy source 22 may include any suitable form of circuitry commonly utilized with electromagnetic forming apparatus, and, in the illustrated embodiment, the source includes a dc. power supply 50 which is serially connected with a switch 52, a current limiting resistor 54, and a storage capacitor 56 to form a charging circuit for the capacitor. The transmission line assembly 29, and consequently the coil 15, is connected across the capacitor 56 through an impedance matching transformer 58 and a firing switch 60. Thus, in operation, the charging switch 52 is closed until a sufficient charge is attained on the capacitor 56, after which the charging switch 52 is opened and the firing switch 60 is closed. This discharges the stored charge to the work coil 15. More particularly, the switch 60 may comprise a suitable thyratron or ignitron circuit which is selectively fired to discharge the capacitor 56 through the transformer 58 which supplies a high magnitude, short duration current pulse through the dies 16 and 18, and around the workspace 24, by applying a positive potential to input terminal bus 30 and a negative potential to the output terminal bus 32. An insulating material or air gap is provided, as previously mentioned, to prevent the work coil from constituting a closed loop or shorted turn. The contact surface portions 36, 46 and 42, 48 of the surfaces 26 and 28 on opposite sides of the recess 24 are in electrical contacting engagement, but the remaining portions thereof are electrically insulated from each other by an air gap 62 (FIG. 1) or by an insulating sheet, such as the extended portion 64 of insulation 34.

The large current pulse flowing in the single turn formed by the dies 16 and 18 generates an intense varying magnetic field in the workspace 24 which induces a current in the collar 14. This induced current interacts with the generated magnetic field to produce a very high opposing radial force or pressure between the collar and the coil assembly, causing the collar 14 to be swaged onto the tube 12. A suitable mandrel or rod may, if desired, be inserted into the tube 12 for additional support during the swaging operation and to maintain the tube dimensrons.

The discharge current from the capacitor 56 may, of course, be discharged through the coil 15 via transmission line assembly 29 without the matching transformer 58. However, the use of the transformer 58 to match impedances, may increase the efficiency of the energy transfer if the impedance of the work coil is other than optimum for the given capacitor bank.

The dies 16 and 18 may each comprise a field shaper portion 66 and 68, respectively, concentrically disposed therewithin, which together define the workspace orifice 24. The field shaper portions 66 and 68 are composed of conductive metallic material of high strength, and may be mounted within the outer die portions by any suitable means so that they are either in direct conductive relation thereto or inductively coupled thereto. That is, they may be insulated from the outer conductive die portions so as to receive current by induction or transformer action rather than by conduction. Alternatively, of course, the dies 16 and 18 may be each formed as a unitary mass extending to and defining the workspace 24.

Although the thickness of the dies 16 and 18 (i.e., in the direction parallel to the axis of the aperture or orifice 24) may be uniform, it is sometimes desirable for certain applications to reduce the effective thickness in the region about the orifice 24, as compared to the thickness of the outer portions. As shown in FIG. 3, the dies have a tapered portion between the peripheral region and the workspace so that each die is quite narrow in the workspace to produce the necessary high pressures required for forming. The cross-section then diverges towards the general region of the contacts, with the contacts being preferably about four times as wide as the working region. This serves to increase the magnetic force on the workpiece in the workspace for a given current density at the contact area.

The forming coil 15 is supported on a sturdy cabinet or frame structure 70 which resists the reaction forces produced when the coil is pulsed, and supports the workpiece to the extent that it is not otherwise supported. The frame 70 has a lower arm 72 which extends from the lower portion thereof and receives the lower or stationary die 18 which is suitably insulated from the arm 72 by insulation 74. An upper arm 76 extends from the upper part of the frame 70, and a C shaped bracket 78 is rigidly fixed to the cantilevered end thereof. The bracket 78 has a lower sleeve portion 80 and an upper ear portion 82 formed integrally therewith. A rigid shaft or rod 84 slidably engages the sleeve portion 80 for vertical motion therewithin. The lower end of the shaft 84 is coupled to the movable die 16 by any suitable means such as threaded assembly 86, which also desirably may insulate the movable die 16 from the shaft 84 by the use of an intermediate section of insulating material, or in any other convenient manner. A handle 88 is pivotably fastened to the ear portion 82 of the bracket 78 at pivot 90, which is preferably located in alignment with the slidable shaft 84. A link 92 has one end pivotably connected to a point 94 on the handle which is spaced radially from the handle pivot 90, and the other end of the link 92 is pivotably fastened at 96 to the upper end of the slidable shaft 84. Consequently, when the handle 88 is lifted or rotated in a counterclockwise direction, the linkage pulls the movable die 16 to its open or disengaged position for loading or unloading a work piece. When the handle 88 is lowered in the clockwise direction, the linkage lowers the movable die 16 into engagement with the stationary die 18. When the movable die 16 has reached its lowest or engaged position, continued movement clockwise of the handle 88 clamps the dies together, since the upper link pivot 94 moves into alignment with, or to the left of, the slidable shaft 84, and the handle 88 cannot move from this position by the application of the upward vector force produced by the reaction to the magnetic pressure at the workpiece. Thus, the dies 16 and 18 can only be unclamped by lifting the handle so that the upper link pivot 94 swings to the right of the slidable shaft axis. As previously mentioned, the clamping action on the dies 16 and 18 is necessitated by the production of opposing reaction forces on pulsation of the work coil 15, which tends to drive the dies apart. The handle and linkage assembly described, provides a simple means maneuvering the dies into and out of engagement, although other mechanisms may be employed for this purpose.

Referring now to FIGS. 4 and 5, there is shown a further embodiment of the electromagnetic forming apparatus of the invention, and parts corresponding to those in FIGS. l-3 are indicated with like reference characters, but having the suffix a."The forming coil 15a is shown in FIG. 4 in its closed or engaged position, and may be supported from the apparatus frame (not shown) by conductive member 100. The pair of relatively movable dies 16a and have complementary engaging faces 26a and28a (FIG. 5) which define the recess or workspace 24a. A transmission line assembly 29a is connected to a pulse energy source (not shown) of the same type as illustrated in FIG. 1. The transmission line assembly 29a comprises an input terminal conductor or bus 30a, an output terminal conductor or bus 32a, and an insulating sheet 34a disposed therebetween. Rigid conductive members 100, 102 and 104 are provided for structural strength, the former being at one potential and the latter two being at a different potential when the coil a is pulsed to produce a pulse current through the coil dies. The conductor 30a is of a flexible type and the conductive members 102 and 104 are spaced apart in the region of conductor portion 106 to permit the movable die 16a to be pivoted for disengagement of the surfaces 26a and 28a so that a workpiece may be introduced into or removed from the workspace 24a. In this manner, the dies are disengaged by a tilting motion of the supper die 16a, and complete separation of the dies is avoided.

The complementary surfaces 26a and 28a define electrical contact surfaces 108 and 110, respectively, which are in contact when the dies 16a and 18a are in engagement. The contact surface 108 of die 16a is formed by a fingerlike projection defined on one side by the workspace recess and on the other side by a hollow 112. This finger-like projection is dimensioned so as to be slightly flexible for a purpose to be explained later. The other contact surface 110 of die 18a is formed by a projection 114 from the complementary surface 280. Projection 114 is dimensioned so as to be rigid relative to the projection mentioned above. The projection 114 has an inclined contact face and nests in the hollow 112 with the surface 108 pressed against the inclined contact face when the dies are in engagement, as shown in FIG. 4. An alignment pin 116 protrudes from the surface 28a of the stationary die 18a and fits into a mating recess or bore 118 in the surface 26a of the movable die 16a. A clamping means 120 is provided which comprises an angular shaped arm 122 which has a relatively short portion 124 and a relatively longer portion 126. The end of the shorter arm portion 124 is pivotably fastened to the lower or stationary die 18a at a pivot point 127 directly beneath the workspace 24a. The longer arm portion 126 has a block 128 pivotably fastened to the upper end thereof at pivot 130. The block 128 has a threaded bore, and a threaded screw 132 is disposed in the bore and extends through the pivotable block 128 so that the screw may be turned downwardly until the tip 134 enters or engages a recess 136 in the outer top surface of the movable die 16a. The recess 136 is diametrically in line with the workspace 24a and the lower pivot point 127, and the screw 132 is aligned, due to the relative lengths of the arms and their angular relationship, to preferably be directly over the workspace 24a and in line with the lower pivotable connection.

In order to permit the pair of dies to be opened or disengaged from each other for the insertion or removal of a workpiece, the screw is threaded upwardly until the lower tip thereof is disengaged from the recess, and then the entire clamping assembly 120 is pivoted counterclockwise or to the left (as illustrated in FIGS. 4 and 5). This unclamps the upper die 16a from the lower die 18a and permits the upper die to be raised in the manner illustrated in FIG. 5. Since the upper conductor or supply lead 30a is flexible, it permits the pivoting action by bending. The movable die 16a may be maneuvered out of and into engagement with the stationary die 18a by merely lifting the die 16a by hand or by any suitable mechanism, either manual or motor driven.

Thus, the coil 150 may be separated to permit the ready removal or insertion of a general workpiece, but yet only one electrical contact is required to complete the current path between the separable dies, rather than the two contacts required in the embodiment of FIGS. 1 through 3. This is an additional advantage in that it is believed that contact failure is a principal cause of coil failure, and the elimination of one of the two contacts will generally increase the lift of the coil, i.e., increase the number of coil pulses before coil failure. The reaction produced to each pulse tends to separate the coil dies, which may cause arcing and burning of the interface contacts. The contact surface configuration 108 and results in minimal surface separation in response to pulsing, thus resulting in less arcing and longer contact life. The expanding magnetic forces within the workspace 24a act to exert a radially outward bending moment to the flexible finger-like projection containing the bearing surface 108 so as to greatly increase the contact pressure during the magnetic pulse, resulting in lower contact resistance and thus further lessening of the burning and pitting so as to increase the contact life. These contact portions may, of course, be reversed, with the flexible contact associated with or formed in the stationary die and the rigid contact portion associated with or formed in the movable die, rather than as shown. The coil life will therefore generally be increased somewhat from both the longer contact life and the use of a contact on only one side rather than on both sides of the workspace.

Thus, there has been described a method of electromagnetically forming a workpiece comprising the steps of installing the workpiece within a recess provided within at least one of two relatively movable dies formed of a highly conductive material, moving the dies into electrically insulated engagement so as to provide individual proximately parallel current paths, each extending around a portion of the workpiece, and connecting the dies individually to a source of high amperage opposite polarity current pulses so that the current flowing along the current path in one die is in the opposite direction of the current flowing in the other die so that additive high intensity magnetic fields are produced for forming the workpiece. The inductive dies closely surround the workpiece for effective magnetic coupling thereto and serve as conductors for the current flowing around the workpiece, the dies defining a recess for the workpiece when they are in engagement with each other. In this position the dies form a single turn conductor for the current since they are electrically isolated in one portion thereof to prevent their forming a continuous conductive loop. The coil dies may be satisfactorily made from copper, molybdenum or other highly conductive materials or combinations thereof. Also, suitable cooling tubes may be provided in the coil to remove the heat generated therein.

Although particular types of manual clamping and die maneuvering or operating devices have been herein described for clamping and unclamping the pair of conductive dies and for engaging and disengaging one from the other, it is understood that other forms of devices for these purposes may be used. Such devices may be motor driven and supplied by a source of energy and/or automatically operated, and thus, for example, may conveniently include an electrical solenoid, a hydraulic system, etc., as desired. Such a mechanism may, for example, have the connecting rod of a hydraulic piston assembly coupled to the handle 88 of the embodiment illustrated in FIGS. 1 through 3 through any suitable linkage arrangement. Further, the principles of the invention may be applied, of course, to other forming or metal working techniques than those specifically described herein, such as to various shaping operations.

Various modifications of the present invention will be apparent to those skilled in the art; and accordingly, the invention should be defined by the appended claims, and equivalents thereof.

Various features of the invention are set forth in the following claims.

What is claimed is:

l. A work coil for electromagnetically forming a metallic workpiece, and adapted for connection to a high amperage electric current supply, comprising a pair of relatively movable and engageable dies formed of a conductive material and having opposing interface surfaces with respective recesses cooperating to define a workspace for the workpiece and arranged to form a single turn for electric current around the workspace when the dies are engaged, a first electric contact portion associated with the interface surface of one of said dies on one side of said workspace formed by a projecting surface generally transverse to said interface surfaces, and a from said contact portions, and means associated with one of 10 said terminals to permit opening of said dies for the introduction or removal of a workpiece.

2. The work coil of claim 1 wherein one of said contact portions is rigid and the other of said contact portions is relatively flexible to provide an increased contact pressure between their respective contacting surfaces on pulsing of the coil.

3. The work coil of claim 2 wherein said rigid contact portion has an inclined projecting surface, said relatively flexible contact portion has a finger-like projecting surface, and said flexible contact portion is radially closer to the workspace than said rigid contact portion.

4. Apparatus for electromagnetically forming a metallic workpiece, comprising a pair of relatively movable and engageable dies formed of a conductive material and having the interface surfaces thereof cooperating to define a common recess for the workpiece and arranged to form a single turn for electric current around the recess when the dies are engaged,

a first electrical contact formed by a slightly flexible fingerlike projection from the interface surface of one of said dies on one side of said recess, and a second electrical contact formed by a relatively rigid projection from the interface surface of the other of said dies also on said one side of said recess and so arranged as to be in engagement with the fingerlike projection when the dies are in engagement with one of said projections being nested within a hollow located adjacent the other of said projections, a first terminal connected to one die in the region of the interface on the opposite side of the recess from said contacts, a second terminal connected to the other die on said opposite side of the recess, insulating means disposed between said first and second tenninals to prevent electrical contact therebetween, means associated with one of said terminals to permit pivoting of the die to which it is connected so as to enable the workpiece to be introduced in or removed from said recess, and an energy source connected to the dies through said first and second leads to supply a high amperage electric current pulse thereto so that the current around the single turn creates a high intensity magnetic field within said recess for forming the workpiece and said magnetic field within said recess acts to exert a radially outward bending moment on the flexible finger-like projection to increase the contact pressure against the rigid projection during the magnetic pulse.

Patent Citations
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Referenced by
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US4531393 *Oct 11, 1983Jul 30, 1985Maxwell Laboratories, Inc.Electromagnetic forming apparatus
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US5710536 *Feb 14, 1996Jan 20, 1998Electronic De-Scaling 2000, Inc.Adaptive coil wrap apparatus
US5824998 *Dec 20, 1995Oct 20, 1998Pulsar Welding Ltd.Joining or welding of metal objects by a pulsed magnetic force
US5953805 *Jul 31, 1997Sep 21, 1999Magnet-Physik Dr. Steingroever GmbhMagnet field concentrator for shaping metal parts
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US6591649Dec 29, 1998Jul 15, 2003Pulsar Welding Ltd.Method and apparatus for pulsed discharge forming of a dish from a planar plate
US6875964Mar 20, 2003Apr 5, 2005Ford Motor CompanyApparatus for electromagnetic forming, joining and welding
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US7467532Oct 18, 2005Dec 23, 2008Ford Global Technologies, LlcApparatus for electromagnetically forming a workpiece
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US7954221May 21, 2008Jun 7, 2011The Boeing CompanyElectromagnetic mechanical pulse forming of fluid joints for high-pressure applications
US20100275439 *Apr 22, 2008Nov 4, 2010Pablo PasqualeMultiple tube processing coil
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U.S. Classification72/56
International ClassificationH01F7/20, B21D26/14, B21D26/00
Cooperative ClassificationB21D26/14, H01F7/202
European ClassificationH01F7/20B, B21D26/14