US3852985A

US3852985A - Device for processing elongated tubular workpieces by pressure waves

Info

Publication number: US3852985A
Application number: US00876773A
Authority: US
Inventors: J Haeusler; G Marz; H Seiffert
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1968-11-15
Filing date: 1969-11-14
Publication date: 1974-12-10
Anticipated expiration: 1991-12-10
Also published as: DE1809098A1; DE1809098B2; CH515757A; GB1292290A; AT298206B

Abstract

Device for processing elongated tubular workpieces by subjecting them to pressure waves produced in a liquid medium by spark discharge includes a discharge circuit having condenser means and a spark gap formed of a pair of spaced electrodes electrically connected to the condenser means, an ignition device disposed in the spark gap between the electrodes thereof, structure means coaxially surrounding the spark gap and forming part of the discharge circuit, the structure means being encirclable by a tubular workpiece so that the workpiece surrounds the spark gap, the structure means being pervious to pressure transmitted through a liquid medium receivable therein and generated by an electrical discharge at the spark gap.

Description

United States Patent [191 Haeusler et al.

[ DEVICE FOR PROCESSING ELONGATED TUBULAR WORKPIECES BY PRESSURE WAVES [75] Inventors: Jochen Haeusler; Giinter Miirz;

Helmut Seiffert, all of Nurnberg, Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin & Munich, Germany [22] Filed: Nov. 14, 1969 [21] Appl. No.1 876,773

[30] Foreign Application Priority Data Nov. 15, 1968 Germany 1809098 [52] U.S. Cl. 72/56 [51] 'Int. Cl B2ld 26/12 [58] Field of Search, 72/56; 29/421 [56] References Cited UNITED STATES PATENTS 3,203,212 8/l965 Simicich 72/56 3 ,418,835 12/1968 Erlandson..... 72/56 3,593,551 7/l97l Roth 72/56 Dec. 10, 1974 OTHER PUBLICATIONS High-energy Rate Forming;" by W. W. Wood; pp. 83-89; October 1963; Tool & Manufacturing Engineer.

Primary ExaminerRichard J. Herb st Attorney, Agent, or Firm-Herbert L. Lerner [57] ABSTRACT Device for processing elongated tubular workpieces by subjecting them to pressure waves produced in a liquid medium by spark discharge includes a discharge circuit having condenser means and a spark gap formed of a pair of spaced electrodes electrically connected to the condenser means, an ignition device disposed in the spark gap between the electrodes thereof, structure means coaxially surrounding the spark gap and forming part of the discharge circuit, the structure means being encirclable by a tubular workpiece so that the workpiece surrounds the spark gap, the structure means being pervious to pressure transmitted through a liquid medium receivable therein and generated by an electrical discharge at the spark gap.

2 Claims, 9 Drawing Figures wEr-mnm 10 1911 3852-985 SHEET 3 BF 5 m n mum 10 1974 sum 5 OF 5 SL852 985 5m Fig.5

DEVICE FOR PROCESSING ELONGATED TUBULAR WORKPIECES BY PRESSURE WAVES Our invention relates to device for processing elongated tubular workpieces by pressure waves. Such workpieces can have either a straight or a bent or curved axis. More specifically our invention relates to device for processing such workpieces by pressure waves produced in a liquid medium i.e., underwater for example, by spark discharge.

In conventional high-speed deforming processes by means of pressure waves which are produced for example, by underwater spark discharge of a condenser battery electrically connected with a spark gap in a discharge circuit, such pressure waves are employed for processing workpieces. The spark-over occurs between the electrodes of the spark gap that are immersed in a liquid medium, such as water, after switching on the condenser battery which has been charged with a high voltage, when the electric field strength, which is a function of the spacing of the spark gap electrodes and the condenser voltage, is adequate to effect ionization of the water or other liquid transmitting medium by impact or collision. If a processing problem requires an electrode spacing which can not be jumped by a free spark-over or which requires the spacing between the electrodes, on the one hand, and the workpiece, on the other hand, to be so small that an undesired free sparkover is possible between the electrodes and the workpiece, the ignition of the underwater spark gap must be effected by other means than by free spark-over.

The best known measure to be taken in such cases is to employ thin wires that are stretched between the electrodes and vaporized explosively by discharging the condenser. The use of ignition wires has a series of disadvantages associated therewith that makes it difficult to automatize this high-speed deforming process which is rather vital for carrying out production-line processing of workpieces by underwater pressure pulses. Thus, for example, a component of the device employed for carrying out this known high-speed deforming process, namely the wire which is exploded, must be replaced after each discharge, so that the performance of a processing cycle takes a relatively long time.

The length of the processing cycle can, however, still be kept within bearable limits in the case of devices wherein workpieces that are being processed do not surround the wire serving as ignition device, because the introduction of the workpiece into the processing device and the insertion of the ignition device between the electrodes of the spark gap can then be carried out independently of one another and, hence, simultaneously. If, in accordance with a heretofore known proposal, a filament formed of a nonconductive core and a conductive coating thereon is employed as the ignition device instead of the explosive wire, the fact that the nonconductive core is intact after the discharge reduces the problem of inserting the ignition device to a simple process of advancing the filament so that a succeeding length thereof having conductive coating on the nonconductive core thereof newly bridges the electrodes of the spark gap.

In contrast thereto, closed or tubular workpieces, such as profile tubes for example, are always so interlinked with the discharge circuit of the condenser battery in the known devices wherein the electrodes of the spark gap are bridged by an ignition device, that the insertion of the ignition device and of the workpiece can not be effected independently of one another but rather only by maintaining them in sequence i.e., by carrying out the insertions of the ignition device and the workpiece one after the other. In this regard reference can be had to H. Muller, Hydroelektrische Umformung, das Umformverfahren mit Hilfe einer Stross- Stromanlage, Mitteilung Forschungsgesellschaft Blechverarbeitung (1965) pp. 102 to 106; M. C. Noland et al. High-Velocity Metalworking A Survey NASA Sp-5062, Washington, D. C. (1967), PP. to 43; and E. M. Callendar Electric Hydroforming by Wire vaporization, ASTME-Creative Manufacturing Seminars (1961/62) technical paper SP- 62-81. In such devices especially, the advantage of the fact that the nonconductive core of the aforementioned filamentary ignition device always remains after a discharge cannot be fully utilized because, after each discharge, the discharge circuit must ordinarily be broken between the electrodes to remove the processed workpiece. With these known devices, furthermore, insulating and sealing members as well as the electrodes proper must frequently be placed in the operative position thereof after the workpiece has been introduced into the processing form therefor. In this regard, also, besides the expense resulting from employing pneumatic or hydraulic drive means or burdensome additional masses, the expenditure of time associated therewith must also be taken into consideration.

It is accordingly an object of our invention to provide device for processing elongated tubular workpieces by pressure waves which avoids the foregoing disadvantages of the heretofore known devices of this general type and which more specifically permits the insertion of the ignition device and the introduction of the workpiece into the device independently of one another and, consequently, simultaneously.

It is another object of our invention to provide device for processing elongated tubular workpieces by pressure waves with an ignition device formed of a nonconductive filamentary core and a conductive coating thereon which simplifies the insertion of the ignition device between the spark gap electrodes after a spark discharge by advancing the filamentary core which remains intact.

It is also an object of our invention to provide such device wherein the advancement of the filamentary core after a spark discharge is the only mechanical I movement that must be carried in the discharge circuit to make it ready anew for further operation i.e., the insulating and sealing components as well as the spark gap electrodes of the device together with all of the other current conducting parts of the discharge circuit remain in their operating positions.

With the foregoing and other objects in view we provide in accordance with our invention device for processing elongated tubular workpieces by subjecting them to pressure waves produced in a liquid medium by spark discharge comprising a discharge circuit including condenser means and a spark gap formed of a pair of spaced electrodes electrically connected to the condenser means, an ignition device disposed in the spark gap between the electrodes thereof, structure means. coaxially surrounding the spark gap and forming part of the discharge circuit, the structure means being encirclable by a tubular workpiece so that the workpiece surrounds the spark gap, the structure means being pervious to pressure transmitted through a liquid medium receivable therein and generated by an electrical discharge at the spark gap.

In accordance with another feature of our invention, the structure means forming part of the discharge circuit has a bird cage construction, and, more specifically, is made up ofa metal cover plate, a base member of insulating material and a plurality of metallic rods extending between the cover plate and the base member.

In accordance with a further feature of our invention, the rods of the structure means are displaceable and adjustable so that the current path therethrough is capable of being set for varying processing lengths.

In accordance with an added feature of the invention, one of the rods of the structure means is formed with an elongated bore for supplying the ignition device i.e., the conductively coated filament, therethrough to the spark gap. 7

In order to construct the birdcage-like electrically conductive structure means from the very start also for relatively short spacings between the electrodes of the spark gap, we provide, in accordance with the invention, the metallic cover plate and the insulating base plate with respective central projections directed toward the interior of the structure means, the central projections being moreover conical in shape.

In accordance with a concomitant feature of our invention, in the case where a bent or curved workpiece is to be processed, we provide structure means forming part of the discharge circuit which matches the bend or curve in the workpiece. Moreover, we provide insulating perforated discs in the mounting means, spaced at predetermined distances from one another in planes that are substantially parallel to the direction of propagation of the pressure wave.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in device for processing elongated tubular workpieces by pressure waves, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view, partly schematic, of the device for processing elongated tubular workpieces by means of pressure waves according to the invention;

FIGS. la and lb are cross-sectional views of two different embodiments respectively of the cage-like structure means for the workpiece according to FIG. 1;

FIG. 2 is a longitudinal sectional view similar to that of FIG. 1 of another embodiment of the invention having a longitudinal bore in a bar of the structure means, taken along the line IIII in FIG. 2a and including a workpiece mounted therein;

FIG. 2a is a cross-sectional view of FIG. 2 with the workpiece omitted;

FIG. 3 is a longitudinal sectional view similar to those of FIGS. 1 and 2 of yet another embodiment of the invention wherein the electrically conductive structure means is constructed for relatively short spacings of the spark gap electrodes;

FIG. 4 is a longitudinal sectional view similar to those of FIGS. 1, 2 and 3 of a further embodiment of the invention wherein the electrically conductive structure means is adjustable to the respective processing length;

FIG. 5 is a longitudinal sectional view similar to those of FIGS. 1, 2, 3 and 4 of an additional embodiment of the invention for processing bent or curved workpieces; and

FIG. 5a is a cross-sectional view of FIG. 5 with the workpiece omitted.

Our invention is based on the surprising fact that the widening of sleeve-like or tubular workpieces, such as cylindrical tubes, for example, through radially disposed narrow bars which, as part of the electrical discharge circuit, are surrounded by a tubular workpiece and extend parallel to the axis of the tubular workpiece, is not nonuniform in spite of the fact that parts of the wall of the tubular workpiece are located behind the bars away from the ignition device and in so-called pressure shadows. Thus, no measurable waviness could be detected in the wall of brass, aluminum and steel tubes (60mm inner diameter, 2mm wall thickness, 180 or mm length), which were widened or expanded in free deformation to 20mm diameter with a cage-like structure which forms part of the discharge circuit with energies up to 6 kW and condenser charge voltages up to 25 kV. Also, when tubes of the afore-mentioned dimensions were forced into a thick-walled steel cylinder, only a waviness of less than 0.0lmm was produced for deformation paths of several tenths of a millimeter.

Referring now to the drawings, and first particularly to FIGS. 1, la and 1b thereof, there is shown an embodiment of the device for processing elongated tubular workpieces by means of pressure waves constructed according to our invention with a cage-like electrically conductive structure means 11 formed of a metallic cover plate 12, a plurality of metal rods or bars 13 and a base 14 of insulating material through the center of which a high-voltage electrode 15 extends. The cover plate 12 and the bars 13 which join an electrically conductive base plate 16 provide a current conductive path forming part of a hereinafter-described discharge circuit for the device of ourcinvention. The bars 13 of the current-conductive structural member 11 are of relatively narrow construction, with the lateral surfaces thereof extending substantially in radial direction of the current-conductive structural member 11, so as to be pervious to and permit passage through the spaces therebetween of pressure waves, generated by an ignition device 17 in a suitable liquid medium provided in the structural member 11. The ignition device 17, in the illustrated embodiment of FIG. 1, is in theform of a filament having a nonconductive core covered by an electrically conductive coating. As shown in the crosssectional views of FIGS. la and 1b, the cross section of the rods 13 may be conveniently wedge-shaped or lensshaped, respectively with the side surfaces thereof extending in a substantially radially direction from the center of the current-conducting structural member 11. The total cross-sectional area of all the rods 13 must be sufficiently large, for reasons of stability, that the generated pressure also exerted on the cover plate Furthermore, the device of our invention can also be constructed for processing or deforming profile tubes such as tubes having rectangular or other polygonal cross sections for example. Furthermore, to accommodate respective deformation problems, the cross sections of such profile tubes can also be variable in the direction of the respective axes thereof, the only limitation thereon being that such tubes be capable of being slid over the cage-like, current-conductive structure means 11 of the device.

When the workpiece 18 has been slid over the cagelike current-conductive structure 11 so that the latter is encircled by the workpiece 18, the inner space of the structure 11 can be filled with pressure-transmitting medium, such as water for example. The inner space of the structure 11 is sealed by annular sealing rings 19 fitted in suitable recesses formed at opposite ends of the structure 11. The pressure-transmitting liquid can be supplied to the interior of the structure 11, for example, through a bore 110 formed in the electrode 15, through which the ignition device 17 also extends. The base plate 16 and the cage-like current-conductive structure 11 and the workpiece l8 therewith are electrically connected at ground potential. The base plate 16 is furthermore connected to the ground terminal of a high voltage condenser 111, while the high-voltage terminal of the latter is, in turn, connected through a switch 112, symbolically shown in FIG. 1 as a threeelectrode spark gap, with a connector clamp or clip 114 which is insulated from the base plate 16 by a suitable sheathing 113 while being in electrical contact engagement with the high voltage electrode 15.

If the discharge of the high voltage condenser 111 is effected by means of an ignition device 17 in the form of a filament having a nonconductive core and a conductive coating thereon then, after a condenser discharge and the conductive coating within the interior space of the structure 11 has been burned away leaving the nonconductive core of the filament 17 within the interior space of the structure 11, the thus-bared core can then be withdrawn through a bore ll5formed in the cover plate 12 and can be severed from that length of the conductively coated filament 17 now disposed within the interior space of the structure 11 so as not to interfere with the introduction to the device of our invention of the next workpiece to be processed. It is of course possible to draw the filament 17 in the opposite direction, namely through the bore 110 of the highvoltage electrode 15, as long as a roll of the filament 17 can be suitably mounted on, in or adjacent the base plate 16 so as not to hinder the feeding of the workpieces to the device of our invention.

In the emodiment of FIGS. 2 and 2a, the cage-like currentconductive structure is formed of a bipartite metallic cover plate 22, a plurality of metallic rods or bars 23 and a base member 24 of insulating materialformed with a central bore through which a highvoltage electrode 25 extends. The bars 23 terminate in a metallic base plate 26 and are encircled by a workpiece 28 that is to be processed.

The filamentary ignition device 27 is supplied from a roll 21 mounted on the base plate 26 and passes through a bore 29 formed in the base plate 26 and continuing through one of the bars 23 and through the lower part, as viewed in FIG. 2, of the bipartite cover plate 22. This lower part of the cover plate 22 is also formed with a groove connecting the bore 22 with a central bore formed in the center of the lower part of the cover plate 22. The filamentary ignition device 27 thus does not extend above the cover plate 22 but rather only to the upper surface of the lower part of the cover plate 22 and then returns through the groove and central bore formed therein through the interior space of the cage-like structure and through a bore formed in the high-voltage electrode 25. Pressure-transmitting liquid, such as water for example, is supplied through a channel 230 formed in the base plate 26 and having an outlet opening 231 located between a pair of the bars 23. Through the channel 23, the supply of water is at ground potential. The high-voltage electrode 25 and the bore 29 are sealed against leakage of the water under normal pressure conditions. A connector clamp or clip 232 with an insulating jacket connects the highvoltage electrode 25 to the base plate 26 through a switch and condenser in a manner similar to that in which the switch 112 and condenser 111 of FIG. I are connected. The embodiment of FIG. 2 is provided with annular sealing rings 234 serving a similar purpose as the sealing rings 19 of the embodiment shown in FIG.

In FIG. 3 there is shown an embodiment of the device of our invention wherein the cage-like currentconducting structure 31 is constructed for short spacings of the

electrodes

35 and 35 and thereby for a free spark-over between the electrodes. The structure 31 is formed of a metallic cover plate 32, metallic bars 33 and a base 34 consisting of insulating material. The respective parts of the cover plate 32 and the base 34 that extend into the interior water-supplied space of the structure 31 are provided respectively with a central projection 37. The projections 37 in the embodiment of FIG. 3 are of conical shape. The electrodes 35 and 35' are adjustably inserted in suitable aligned bores formed in the cover plate 32 and in the base 34 in order to equalize the burn-off of the electrodes 35 and 35' in the contact regions of the spark-over. The bars 33 are inserted in suitable recesses formed in an electrically conductive base plate 36 which is at ground potential and is connected with the ground terminal of a highvoltage condenser not shown in FIG. 3. The highvoltage side of the nonillustrated high-voltage condenser is connected through a switch, in the manner shown schematically in FIG. 1, with a connector clamp or clip 310, covered with insulation 311 and connected, in turn, to the high voltage electrode 35 Annular sealing rings 39 are provided in a manner similar to the aforedescribed embodiments, and a workpiece 38 is shown introduced into the embodiment of FIG. 3.

FIG. 4 shows an embodiment of the device of our invention wherein the cage-like, electrically-conductive structure 41 is adjustable to match the respective processing length. The structure 41 is formed of a metallic cover plate 42, metallic bars 43 and of a base 44 consisting of insulating material. As in the aforedescribed embodiments, there are further provided a high-voltage electrode 45 extending through the base 44, a metallic base plate 46, a filamentary ignition device 47 extending through a bore formed in the electrode 45, annular sealing rings 49 suitably disposed in recesses formed in the cover plate 42 and in the base plate 46, respectively, an insulated connector clamp or clip 410 connected to the high-voltage electrode 45, and a tensioning or clamping ring 411 mounted around the bars 43 and located adjacent the base plate 46. In FIG. 4, a workpiece, for example a cylindrical tube, is shown mounted in the device of our invention.

As can be seen in FIG. 4, the cage-like structure member 41 can be adjusted to different processing lengths, provided that the bars 43 are so formed that they have good sliding, electrically-conductive contact with the base plate 46 and can be firmly adjusted. This can be effected for example by the use of the tensioning or clamping ring 411. Consequently, in an extreme case, such short spacings between the

electrodes

42 and 45 are obtainable that even a free spark-over between the deck plate electrode 42 and the high-voltage electrode 45 is possible.

In FIGS. and So there is shown an embodiment of the device of our invention so constructed as to be used for processing bent or curved tubular members such as tube turns or els.

The curved cage-like structure member 51 of the embodiment of our device, shown in FIG. 5, is formed of a metallic cover plate 52, bars 53 and a base 54 ofinsulating material. The bars 53 end in an electrically conductive base plate 56. A high voltage electrode 55 extends through a bore in the base 54, and a filamentary ignition device 57, in turn, extends through a bore formed in the electrode 55 and into the interior of the structure member 51. A curved workpiece 58 is shown installed in the embodiment of FIG. 5, and sealing rings 59 are also provided in this embodiment and serve the same purpose as the corresponding sealing rings employed in the aforedescribed embodiments of the device of the invention. Also as in the other embodiments, a connector clamp or clip 510 covered by insulation 511 is connected to the high-voltage electrode 55. In addition to all of the foregoing components of the embodiment of FIG. 5 which have their counterparts in the afore-described embodiments, there are also provided along the axis of the structure member 51 of FIG. 5, insulating discs 512 in which respective metal rings 513 are force-fitted. To balance or compensate for axial pressure components that may arise from the wire-vaporization discharge in the curved embodiment of FIG. 5, perforations 514 are formed in the discs 512, as shown more clearly in FIG. 5a.

From FIG. 5, it can be seen that the current conducting structure member 51 matches the curvature of the workpiece 58.. Furthermore, the perforated discs 512, which have little effect on the pressure formation resulting from the spark-over are disposed at spaced intervals along the curved axis of the structure member 51 so as to provide a curved guidance for the filamentary ignition device 57. The metal rings 513 that are force-fitted into the center holes of the respective discs 512 provide a wear-resistant guiding surface for the ignition device 57 which consists, for example, of a nonconductive filamentary core and a conductive coating applied thereto. The remaining components of the electric discharge circuit for the embodiment of FIG. 5 correspond to those shown schematically in FIG. 1.

In those cases wherein highly sensitive or delicate workpieces are to be processed, it is advisable to seal the inner space for the water or other transmitting liquid with a jacket for example of strong elastic material such as plastic material or rubber over which the current-conductive, cage-like structure member is guided. In this manner, the workpiece vwill not come into contact with the pressure-transmitting medium. Furthermore, by the use of such an elastic jacket, one filling with water or other pressure-transmitting fluid will suffice for effecting repeated discharges, so that a number of workpieces can be processed successively without having to refill the device of our invention with pressure-transmitting fluid.

It must also be noted that the pressure-pervious cagelike structure member can also be used when the processing pressure is produced by an underwater explosion for example, instead of a spark-over or a wirevaporization. In such a case, a cage-like structure member having the same construction as those of the aforedescribed embodiments may be employed, but instead of having no ignition device between the electrodes, as for spark-over, or an ignition device in the form ofa vaporizable wire, there can be provided a cord or strip of explosive material that is stretched between the cover plate and a base which is made of metal instead of insulating material, and the explosive material can accordingly be exploded under water or other pressuretransmitting liquid in a conventional manner by an ignition or firing cable.

Note should also be taken that the workpiece has been characterized hereinbefore as being tubular which is to say that it is a laterally closed sleeve-like body. It should also be understood that by the term tubular workpiece there is also meant a workpiece having an axial length that is less than the cross sectional diameter or the smallest cross-sectional dimension of the workpiece. Furthermore, by tubular workpiece there is also meant any suitable closed-wall member irrespective of its cross-sectional shape.

We claim:

1. Device for processing elongated tubular workpieces by subjecting them to pressure waves produced in a liquid medium by spark discharge comprising a discharge circuit including condenser means, and a spark gap formed of a pair of spaced electrodes electrically connected to said condenser means, an ignition device disposed in said spark gap between said electrodes thereof, structure means comprising a plurality of bars connected electrically with one another and being disposed parallel to one another in the form of a cage-like structure coaxially surrounding the spark gap and forming part of said discharge circuit, said structure means being encircleable by a tubular workpiece so that the workpiece surrounds the spark gap, said structure means being pervious to pressure transmitted through a liguid medium receivable therein and generated in the medium by an electrical discharge at the spark gap, at least one of said bars being formed with an elongated bore for feeding said ignition device therethrough.

2; Device for shaping elongated curved tubular workpieces by subjecting them to pressure waves produced in a liquid medium by spark discharge comprising a discharge circuit including condenser means, and a spark gap formed of a pair of spaced electrodes electrically connected to said condenser means, an ignition device disposed in said spark gap between said electrodes thereof, structure means comprising a plurality of members connected electrically with one another and being disposed parallel to one another in the form of a cage-like structure coaxially surrounding the spark gap and forming part of said discharge circuit, said structure means being encircleable by a tubular workpiece so that the workpiece surrounds the spark gap, said structure means being pervious to pressure transmitted through a liquid medium receivable therein and generated in the medium by an electrical discharge at the through.

Claims

2. Device for shaping elongated curved tubular workpieces by subjecting them to pressure waves produced in a liquid medium by spark discharge comprising a discharge circuit including condenser means, and a spark gap formed of a pair of spaced electrodes electrically connected to said condenser means, an ignition device disposed in said spark gap between said electrodes thereof, structure means comprising a plurality of members connected electrically with one another and being disposed parallel to one another in the form of a cage-like structure coaxially surrounding the spark gap and forming part of said discharge circuit, said structure means being encircleable by a tubular workpiece so that the workpiece surrounds the spark gap, said structure means being pervious to pressure transmitted through a liquid medium receivable therein and generated in the medium by an electrical discharge at the spark gap, said structure means having a curvature corresponding to the curvature of the workpiece, and including a plurality of perforated discs formed of insulating material and disposed in said structure means at spaced intervals from one another in respective planes that are substantially parallel to the direction of propagation of a pressure wave, and metal rings respectively press-fitted in a perforation of said discs so as to form a wear-resistant guide for said ignition device, said discs being formed with a plurality of further perforations so as to equalize axial pressure components therethrough.