GB2099669A

GB2099669A - Plasma torch

Info

Publication number: GB2099669A
Application number: GB8117037A
Authority: GB
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1981-06-03
Filing date: 1981-06-03
Publication date: 1982-12-08
Also published as: GB2099669B

Abstract

A plasma torch for use in either the transferred or the non-transferred mode has a screen 21, 22 of material of high magnetic permeability around the nozzle or jet to form a magnetic shield around the arc. This reduces interaction between the arc and residual magnetic fields resulting from the workpiece 20 or its support or the current in the leads of the torch. The screen may be part of a reservoir for a fluid, the fluid being fed to slots located around the nozzle or jet to constitute a fluid jet supporting the weight or part of the weight of the torch. <IMAGE>

Description

SPECIFICATION Plasma torches for operating on a workpiece and methods of operating such devices This invention relates to plasma jet torches for operating on a workpiece and to methods of operation of such devices.

When a plasma jet is used in either the transferred or the non-transferred mode to cut or to mark or to weld a magnetic material or a workpiece supported on magnetic material, there can be interaction between the arc and residual magnetic fields resulting from the workpiece or its support. In addition the magnetic field generated by the current in the leads to the torch, which current will generally be a direct current with an alternating current ripple superimposed, will also interact with the arc. Variation in the arc voltage may have a magnitude up to typically 30% of the quiescent value, depending upon the operating conditions.

We have found that the variation in the power density in the plasma jet due to arc voltage/arc current variations results in a very substantial reduction in the possible cutting speed which may be employed if the jet is used to cut the material.

This reduction may be greater than 30%.

It is one of the objects of the present invention to reduce this effect and to prevent a reduction in cut quality which can result from stray magnetic fields disturbing the position and symmetry of the plasma.

According to one aspect of the present invention a plasma torch is provided with a screen of material of high magnetic permeability around the nozzle or jet to screen the arc.

The screen, which may for example be made of mu-metal, may comprise an annular disc or other structure around the nozzle. A disc with a lip fitting around the nozzle may be employed or a cylinder with a central hole in the base to permit exit of the plasma.

It is found that the smoother arc voltage waveform produced by screening the arc in this way gives an increased minimum power density which increases the speed of cutting or marking or welding and leads to a more consistent quality of cut or mark or welding.

The screen may be arranged to constitute a shroud to screen the jet of plasma from the ambient atmosphere. This reduces entrainment of air and reduces oxidation and thereby reduces any resultant marking of the workpiece in the cut region. This is particularly important for thin materials. The screen also reduces splash-back of material from the surface of the workpiece.

For shrouding the jet, a single or multiple screen may be used. The screen may be arranged to form or to be part of a reservoir for a liquid or gas, which liquid or gas may be fed through the reservoir to slots or the like to form a fluid shroud around the plasma jet. The use of such a fluid shroud with a continuous flow of fluid permits of flotation of the torch, the fluid jet supporting the weight or part of the weight of the torch. Such an arrangement is particularly desirable in ensuring that the jet is positioned at a fixed distance from the workpiece. Whilst with a rigid workpiece having a plain surface, it is relatively easy to ensure that the jet is positioned at a fixed distance from the workpiece, this is much more difficult in the case of flexible materials or materials when the thickness of the workpiece is not constant.

The use of a flotation device as described above enables the separation of the torch from the surface of the workpiece to be controlled by the rate of flow of the gas or liquid. Such flotation devices are inherently self-regulating tending, for a given flow rate, to maintain a fixed distance separation of the torch from the workpiece.

The following is a description of a number of embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 illustrates diagrammatically the use of a magnetic screening disc for a non-transferred plasma torch; Figure 2 illustrates diagrammatically the use of a screening cylinder on a transferred plasma torch; and Figures 3, 4, 5 and 6 illustrate preferred embodiments of the invention in which the magnetic screen constitutes also a shroud and flotation device.

Referring to Figure 1 there is shown diagrammatically a plasma torch having a cathode 10, an anode 11 with a main body 12 into which argon is fed, this body having a nozzle 1 3 which is cooled by water as indicated at 14.

Such a torch, as thus far described, may be of known construction. In accordance with the present invention, a screening disc 1 6 of a material having a high magnetic permeability, for example mu-metal is provided around the plasma jet at the nozzle outlet, this disc extending outwardly and having an upturned lip 19 around its outer periphery. The disc is arranged to form a magnetic screen for shielding the arc from stray magnetic fields arising from the workpiece or its support. As described above, by screening the arc in this way, a smoother arc voltage waveform is obtained enabling an increased average power density to be employed thereby enabling an increase to be obtained in the speed of cutting or marking or welding. It is found moreover that a more consistent quality of cut or mark or weld may be obtained.

Figure 2 illustrates the screening of a transferred plasma torch. In this figure the same reference numerals are used as in Figure 1 to illustrate corresponding components. In Figure 2 the workpiece constitutes the anode and is illustrated diagrammatically at 20. The screening cylinder illustrated in Figure 2 is a cylinder 21 attached to the body of the torch and having an end plate 22 in the form of a disc with a central aperture 23 through which the plasma passes as shown at 24. As in Figure 1, the magnetic screen screens the arc from stray magnetic fields from the workpiece or its support and also from fields due to the power supply leads.

In a preferred construction, as shown in Figure 3, a conventional torch is shown in chain-dotted lines and an added shroud shown in full lines. This shroud or at least part thereof is made of high permeability material to form the required magnetic screen. The shroud consists essentially of a cylindrical member 30 defining an annular gas reservoir 31 into which gas is fed through one or more inlet pipes 32. The gas from the reservoir escapes through an annular slot 33 or alternatively through a series of holes 34, the two sides of the drawing illustrating the two alternative possibilities. The slots or the holes define an angled opening so that the gas is directed downwardly and inwardly around the plasma jet. The use of separate holes enables the jet not only to converge inwardly but also, by the use of suitably shaped holes, to have a rotational component improving the stability.The gas is fed into the reservoir at such a rate that the output fluid flow provides flotation for the plasma torch.

The torch may be mounted for example so-that part of its weight is supported by this flotation jet.

Using such a jet, the torch will be kept at a fixed distance from the workpiece by the flotation effect.

Figure 4 illustrates a modification of the arrangement of Figure 3 in which there is a collar 40 around the plasma torch internally shaped to have an annular reservoir 41 lying between the collar and a screen 42, in the form of a disc of Mu-metal or other high permability material. A gap 43 between the screen and collar at the lower end of the collar forms an annular slot through which gas, fed into the reservoir at 45, 46, carl escape to give the required flotation effect four supporting or partially supporting the weight of the plasma torch.

Figure 5 illustrates another embodiment having a gas reservoir 50 into which gas is fed from a gas inlet 51, the reservoir being of annular form between an upper disc 52 around the nozzle of the torch and a lower disc 53 of mu-metal or other high permability material around the plasma jet. In Figure 5 the torch is operating in the transferred mode over a flat anode structure 56, the gas escaping through a narrow gap 54 between the lower surface 53 of the shroud and the anode 56 to give the required flotation.

Figure 6 illustrates a modification of the construction of Figure 5 with similar reference numerals being used to indicate corresponding components. In Figure 6 support rods 60 mounted on the torch carry a reservoir structure 61 which provides the magnetic shielding. The fluid escaping from the reservoir through the gap 53 provides flotation for the torch.

Claims

1. A plasma torch having a screen of material of high magnetic permeability around the nozzle or jet arranged to screen the arc.

2. A plasma torch as claimed in claim 1 wherein said screen is made of mu-metal.

3. A plasma torch as claimed in either claim 1 or claim 2 wherein said screen comprises an annular disc or other structure around the nozzle.

4. A plasma torch as claimed in claim 3 wherein said disc has a lip fitting around the nozzle.

5. A plasma torch as claimed in either claim 1 or claim 2 wherein said screen is a cylinder with a central hole in the base to permit exit of the plasma.

6. A plasma torch as claimed in any of the preceding claims wherein the screen is arranged to constitute a shroud to screen the jet of plasma from the ambient atmosphere.

7. A plasma torch as claimed in any of the preceding claims wherein said screen forms or is part of a reservoir for a liquid or gas, which liquid or gas is fed through the reservoir to slots or the like around the jet or nozzle to form a fluid shroud around the plasma jet.

8. A plasma torch as claimed in any of claims 1 to 6 wherein said screen forms or is part of a reservoir for a liquid or gas which liquid or gas is fed through the reservoir to slots or the like around the nozzle or jet to constitute a fluid jet supporting the weight or part of the weight of the torch.

9. A method of operating a plasma jet torch wherein magnetic screening is provided around the nozzle or plasma jet to screeen the arc from magnetic fields resulting from a workpiece or its support.

1 0. A plasma jet torch substantially as hereinbefore described with reference to Figure 1 or Figure 2 or Figure 3 or Figure 4 or Figure 5 or Figure 6 of the accompanying drawings.