Plasma welding torch

Details Plasma welding torch Plasma welding torch

1999-05-28

2001-04-10

Walberg, Teresa (Department: 3742)

Electric heating

Metal heating

By arc

C219S075000, C315S111010

Reexamination Certificate

active

06215089

ABSTRACT:

The invention relates to a plasma welding torch.
In known such welding torches the outlet port which extends in the zone of the tapering section of the electrode is provided with a cylindrical zone which is immediately adjacent to the chamber and which is followed downstream by a tapering section extending up to the orifice. The interior wall of the tapering section of the outlet port, however, is provided with a cone angle which is considerably larger than that of the conical end of the electrode.
Despite the convergence of the plasma jet by the geometry of the outlet port and the electrode, a very considerable divergence of the plasma jet and thus a relatively large arc spot is observed on the workpiece to be machined in such known welding torches. This leads to a mostly undesirable increase of the zone of the workpiece which is subjected to the heat, and frequently leads to warping of the same, particularly when relatively thin sheet-metal parts are to be processed.
The large arc spot also causes a high thermal stress on the plasma torch, which as a result of the high temperatures must be cooled respectively well. As this can be brought about already at relatively small outputs of the plasma torch only with water flowing through cooling ports, respectively unwieldy designs are obtained for the plasma welding torch. This leads to the disadvantage, however, that it is not possible to work on workpieces that have a more complex shape, and it is particularly not possible to work at locations where T-seams are to be produced, and that it is already necessary to take into account the welding with such plasma torches in the design of the workpiece.
As a result of the formed larged arc spot, it is also necessary to work with conventional plasma welding torches from a very small distance from the workpieces, thus increasing the stress on the torch and subjecting it to high wear and tear.
A further disadvantage of known plasma torches is also that as a result of the very high thermal stress on the plasma torch it is not possible to have high outputs and the torches can therefore only be operated with not more than approx. 100 A. At higher strengths of current the plasma arc begins to burn between the electrode and the wall of the chamber of the outlet port of the plasma torch and from its free face side in the zone of the outlet port to the workpiece, which leads to a practically immediate destruction of the plasma torch.
It is only possible to work with relatively low welding speeds of not more than 25 mm per second or 30 mm per second with the known plasma welding torches, as otherwise the plasma arc would bend too strongly and begin to jump or even disintegrate.
It is the object of the present invention to avoid this disadvantage and to provide a plasma welding torch of the kind mentioned above which is also suitable for higher outputs and is characterized by a high service life.
This is achieved in accordance with the invention in a plasma welding torch comprising a non-consumable electrode arranged in a chamber provided in the torch and having a free flat end face extending substantially perpendicularly to a longitudinal axis of the electrode and having a diameter of 15% to 35% of the maximum diameter of the electrode. An inlet port leads to the chamber for delivering a plasma gas thereto, and an outlet port leads from the chamber to an orifice at an outer end of the outlet port. The outlet port has an interior wall, the electrode having a free tapering end section reaching at least to the outer end of the outlet port, and the interior wall of the outlet port being spaced from, and surrounding, the free tapering end section of the electrode and extending to the orifice substantially parallel to the tapering end section of the electrode.
The proposed measures ensure that the plasma gas emerging from the outlet port keeps the plasma jet of the torch together, or constricts it, outside of the outlet port. This is caused by the fact that the plasma gas emerges substantially along the envelope of a cone having a specific wall thickness and that thus turbulences are substantially avoided. This leads to a respectively small arc spot on the workpiece and to a concentration of the energy on a very small zone. This also allows guiding the plasma torch at a higher distance from the workpiece and, as a result of the higher energy density on the workpiece, also processing it with a higher rate of feed. Moreover, substantially lower wear and tear of the torch is obtained and it is also possible to work with a higher current load, e.g. with 1000 A.
Furthermore, it is also possible to omit water cooling of the welding torch which would be required in many cases, e.g. in pulsed operation as is required in overhead welding or in some types of steel, and is mandatory in conventional welding torches in such cases. The system can make do with cooling produced by the plasma gas flowing into the chamber.
The thermal stress as compared with conventional welding torches is also substantially lower when operating the welding torch in accordance with the invention with flow plasma, i.e. with a continuously flowing plasma, so that the efforts for water cooling can be reduced dramatically and the welding torches per se can thus be built with a substantially smaller size. This allows working with the welding torches in accordance with the invention at locations which formerly were inaccessible for conventional plasma welding torches as a result of their size which was caused by cooling.
The plasma gas emerging from the orifice of the outlet port ensures as a consequence of the conical shape of the gas envelope a very favourable constriction of the plasma by the ambient air despite the deceleration at its outer side, and thus ensures a very small size of the arc spot produced on the workpiece. This considerably reduces the danger of warping.
By avoiding the divergence of the plasma jet as occurs in conventional welding torches and as a result of the fact that the plasma jet does not touch the welding torch, it is possible to work with substantially higher welding speeds of 300 mm per second and more without any jumping of the plasma occurring on the workpiece. A working position of the welding torch in accordance with the invention can also be provided in which said plasma torch is inclined in the feeding direction, so that the orifice of the outlet port is leading in the feeding direction.
Moreover, the distance between the orifice of the outlet port and the workpiece, which formerly had to be set very precisely in known torches to mostly 2.5 mm to 3 mm, is uncritical in the welding torches in accordance with the invention and can also fluctuate between 2 mm and 6 mm, for example. As a result, the time consumed during the dressing and the setup of the workpieces to be welded can be reduced considerably.
To obtain a very precise guidance of the plasma gas jet, with the same accelerating towards the orifice of the outlet port as a result of the reduction of the free cross section between the electrode and the wall of the outlet port, the tapering end section may be cone-shaped, the cone angle being between 15° and 35°.
Particularly favourable conditions are obtained by the features of claim
3
.
These measures moreover lead to a very stable root of the arc at the electrode in applications where the workpiece to be machined is connected to a pole of a current source, and heat can be favourably discharged from the same. This leads to only very low wear and tear of the electrode.
In a known plasma torch its electrode is held in a chamber for plasma gas, e.g. argon, helium, hydrogen, etc., and penetrates the same, with said chamber being provided with an inlet port and an outlet port which is provided on one face side of the plasma torch with an orifice and the transition from the chamber to the outlet port is disposed in the zone of the minimum distance between the electrode and a wall encompassing the same in the zone adjacent the orifice of the plasma torch. The outlet port is enclosed in sections by further p

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