EP0188425B1

EP0188425B1 - Cutting torch

Info

Publication number: EP0188425B1
Application number: EP84903677A
Authority: EP
Inventors: Adolf Gunnar Gustafson
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-29
Filing date: 1984-10-01
Publication date: 1989-09-13
Also published as: SE444902B; EP0188425A1; SE8305331L; DE3479711D1; SE8305331D0; WO1985001462A1

Abstract

A cutting torch to generate from an electrode (2) placed in the cutting torch an arc which is transmitted to a workpiece (4), the said arc containing a plasma-generating gas and being guided by a gaseous medium (8) which is also arranged to cool the cutting torch. At least a portion (8b) of the said medium (8) is arranged to be forced into a cut (6) in the workpiece (4) generated by the cutting torch.

Description

The present invention is related to a plasma cutting torch to generate from an electrode placed centrically in the cutting torch an electric arc transmitted to a workpiece, which arc comprises a plasma-generating gas surrounding said electrode and which arc is controlled by a gaseous cooling medium which is also arranged to cool said cutting torch, which cutting torch includes an outer end sleeve and an inner guide sleeve of appropriate, electrically conductive material placed round the electrode, the inner surface of said inner guide sleeve being appropriately arranged to direct the plasma-generating gas.
Cutting torches utilizing gaseous cooling medium are already known. These, however, have not been capable of coping in a satisfactory manner with the actual cutting temperatures and thereby function less efficiently than could be anticipated on account of, among other things, insufficient cooling. These prior art cutting torches cannot for example cut thick workpieces at relatively low voltages and give a somewhat uneven cut in the workpiece.
The gaseous cooling medium used in known cutting torches, as shown for example in DE-A1-28 13 804, is used essentially to cool down the surroundings of the generated cut in the workpiece and thereby indirectly also for cooling the surface of the cut, a circumstance which impairs the stock removal ability. Further, a gaseous cooling medium is used to limit the extension of the arc but is linked off before the cut in the workpiece, whereby the arc is limited in extension, viewed across the direction of transfer from the torch to the workpiece, to such a high degree as is attainable in principle. The result of this is that the cut becomes unnecessarily wide, that a higher potential difference must be applied in order to cut a given cutting depth and/or than an uneven cut is generated.
The object of the present invention is to provide a cutting torch which is so well cooled by means of gaseous cooling medium that it is capable of withstanding the actual cutting temperature without being affected and is so elaborated that the used, gaseous cooling medium contributes not only to directing of the arc but also to removal of melted material from the cutting place, whereby a more efficient and concentrated arc can be generated, enabling both thicker materials to be cut and a more uniform cut to be obtained in the workpiece. A further object of the invention is to provide a cutting torch which does not need to be held at a distance from the workpiece in which a cut is to be cut without risking difficulties in passing the cutting torch over the surface of the workpiece which is to be cut.
In order to satisfy the aforesaid objects, the cutting torch according to the present invention exhibits the features as defined in claim 1. Other advantageous embodiments of the inventive concept are evident from the accompanying claims and from the description which now follows.
The invention will now be described in more detail and with reference to an embodiment of the invention selected as an example and which is illustrated by means of the accompanying drawings, wherein Fig. 1 shows a section through a design of a cutting torch according to the invention and Fig. 2 shows an enlarged subsection through the cutting torch according to Fig. 1, mainly its lower part.
The cutting torch shown in Fig. 1 is provided with a handle or holder 19 (not shown), through which runs a supply tube 18 for a plasma-generating gas 7 and a supply tube 17 for a gaseous cooling medium 8. Examples of plasma-generating gas 7 that can be used are argon, argon and hydrogen or nitrogen and examples of gaseous cooling medium are carbon dioxide, oxygen gas and compressed air, but obviously other suitable gases can be used. Moreover, the necessary voltages and potentials respectively can be supplied through the handle 19, for example by means of an electric conductor 20 and by means of the metal casing of the supply tube 18.
On account of the extremely concentrated arc obtained according to the present invention, in numerous application the need of using nitrogen, which gives relatively unpleasent residual products but higher heat than given by, for example, argon, can be avoided.
The gaseous cooling medium 8, for example oxygen gas under a certain pressure or compressed air, supplied by means of the supply tube is first supplied to a cooling chamber 13 provided in the cutting torch, which chamber is essentially limited by an upper casing 12 of suitable electrically conductive and heat-conductive material, such as copper or brass, by an isolating body 22, an isolating sleeve 16, for example of Teflon, capable of withstanding a temperature of approx. 250°C and by a guide device 23 consisting of electrically conductive material for a central electrode 2 placed in the latter, which electrode, for example via a screw sleeve 24 and a turnable button 21, is arranged to fix the electrode 2. The turnable button 21 is sealed and self-locking via a seal 28 against the isolated body 22.
The cooling chamber 13 opens out into a plurality of holes 15 in the isolating sleeve 16, which is fixed in an appropriate manner in the upper casing 12. Placed inside the said holes 15 is a device 26 made of an appropriate metal and provided with through-going ducts 27, relative to which a guide sleeve 9 made of a suitable electrically conducting material is fixed. Also attached to the isolating sleeve 16 - at a distance from the upper casing 12 - is an end sleeve 1 consisting of a suitable heat conducting material, for example copper or brass. The end sleeve 1 is provided in its lower part - as seen in the Figures - with an annular body 3, which is made to advantage of a ceramic material capable of withstanding high temperatures without changing shape or properties. If so deemed appropriate, the end sleeve 1 and the body 3 can obviously instead be made in one piece, for example of a ceramic material.
The electrode 2 is - as evident from Fig. 1 - guided at its upper end by the guide device 23 and at a relatively large distance from there by a holed electrode guide sleeve 25, which is fixed in the guide device 23, for instance by means of threads. The guide device 23 is provided with a cavity to which plasma-generating gas 7 is supplied via the supply tube 18. The supply tube 18, which is also fed with for example a minus potential and - if so required or desired - a high-frequency alternating current, is tightly fixed in the guide device 23 consisting of electrically conductive material, whereby the plasma-generating gas 7 is fed round the electrode 2 through the holes in the electrode guide sleeve 25 on to an outlet 30 in a guide sleeve 9. This guide sleeve 9 can - if so desired or required - be fed with a high-frequency alternating current from the conductor 20 via the metal device 26 in order to facilitate ignition of the arc and with a direct voltage potential which is slightly less negative than the potential of the electrode 2. This difference in direct voltage can be utilized to keep a small pilot flame burning between the electrode 2 and the guide sleeve 9. The inner surface 10 of the guide sleeve 9 - se especially Fig. 2 - and the outlet 30 throttle together the pressurized plasma gas 7, a workpiece 4, which is to be cut with a cut 6, is fed with a positive direct voltage potential, so that the pilot flame generated by the direct voltage potential between the electrode 2 and the guide sleeve 9 is transmitted via the plasma-generating gas 7 in the form of an arc between the electrode 2 - with a negative direct voltage potential - and the workpiece 4 - with a positive direct voltage potential. Other potentials can obviously be chosen instead.
The said generated arc has a relatively large diameter, viewed across the direction of transfer for the arc from the electrode 2 to the workpiece 4, and so a portion 8b of the said cooling medium 8 - supplied via the outlet 30 - is fed between on the one hand the inner surface 1a of the end sleeve 1 and the inner surface 3a of the annular body of ceramic material 3 and on the other hand the outer surface 11 of the guide sleeve 9 at an angle to the arc, for example directed between 15° och 75° to the latter. The said inner surfaces 1 a, 3a and the said outer surface 11 are so directed that they guide the said portion 8b of the cooling medium 8 to strike the arc just before the latter hits the workpiece 4, i.e. the said portion 8b of the cooling medium 8 throttles the cross section of the arc. The lower surface of the annular body 3- seen in Fig. 2 - is adapted to rest tightly against the workpiece 4, so that the said portion 8b of the cooling medium 8 is forced into the melt in the workpiece 4 generated by the arc and thus contributes to removal of the melt in the cut 6.
The remaining portion 8a of the cooling medium is discharged - if so required - during further cooling of the inner surface 1a of the end sleeve 1 through holes 29 in the upper part of the end sleeve 1 at such a distance from the workpiece 4 that the remaining portion 8a of the cooling medium 8 does not cool the workpiece 4. Possibly, one portion 8b of the cooling medium may be discharged through openings running in a turning direction between the surfaces 11 and 3a so that a whirl of the one portion 8b of the cooling medium 8 is created to further throttle the cross section of the arc.
The guiding inner surface 3a of the annular body 3 extends - as is especially evident from Fig. 2 - viewed in the direction of extension of the arc from the electrode 2 to the workpiece 4 far closer to the workpiece 4 than does the outer surface 11 of the guide sleeve 9, whereby the throttling effect of the cooling medium 8b on the cross section of the arc is large and the cooling medium 8b at the same time carries the arc through the ejector effect.
The upper casing 12 can be fed advantageously with a potential which agrees with the potential of the workpiece 4.
The lower part 1 of the end sleeve 1 is preferably made straight, enabling it to serve as a guiding surface at and for the movement of the cutting torch on the workpiece 4 which is to be cut.

Claims

1. A plasma cutting torch to generate from an electrode (2) placed centrically in the cutting torch an electric arc transmitted to a workpiece (4), which arc comprises a plasma-generating gas (7) surrounding said electrode (2) and which arc is controlled by a gaseous cooling medium (8) which is also arranged to cool said cutting torch, which cutting torch includes an outer end sleeve (1) and an inner guide sleeve (9) of appropriate, electrically conductive material placed round the electrode (2), the inner surface (10) of said inner guide sleeve (9) being appropriately arranged to direct the plasma-generating gas (7), wherein said outer end sleeve (1) is separated from the inner guide sleeve (9), in that at least a portion (8b) of said medium (8) is forced between the outer surface (11) of said inner guide sleeve (9) and the inner surface (1a) of said outer end sleeve (1) into a cut (6) in the workpiece (4) generated by the cutting torch, in that a closed space is achieved between said outer end sleeve (1) and said work- piec (4) by means of a heat resistant, electrically isolated body (3) fixed to the outer end sleeve (1) and placed in the outlet end of said cutting torch for the arc between said outer end sleeve (1) and said workpiece (4), which body (3) is annular- shaped and the open centre of which is placed opposite to the direction of transmission of the arc and in that said body (3) is arranged with the inner side (3a) thereof to guide said portion (8b) of the cooling medium (8) as well.

2. A plasma cutting torch according to Claim 1, which torch utilizes a surplus (8a) of the cooling medium in relation to the portion (8b) thereof which controls the arc, characterized in that said surplus (8a) of the cooling medium (8b) is also used to cool the cutting torch.

3. A plasma cutting torch according to Claim 2 with an annular flow of cooling medium (8b) around the arc, characterized in that the remaining portion (8a) of the cooling medium (8) is directed to flow along the outer end sleeve (1) and away from said outer end sleeve (1) at such a distance that the remaining portion (8a) of the cooling medium (8) partly cools the outer end sleeve (1), partly does not affect the workpiece (4), in any case not the generated cut (6) in the workpiece (4).

4. A plasma cutting torch according to any of the preceding Claims 1-3, characterized in that the inner surface (3a) of said body (3) extends to a position closer to the workpiece (4) viewed in the direction of transmission of the arc than does the outer surface (11) of the inner guide sleeve (9), viewed in the same direction.

5. A plasma cutting torch according to any of Claims 1-4, characterized in that said inner surface (3a) of the body (3) ia arranged to direct said portion (8b) of the cooling medium (8) to the edge situated closest to the cut generated by the the cutting torch.