Plasma arc torch

A plasma arc torch is disclosed which comprises front and rear coaxially aligned electrodes, and vortex generator for generating a vortical flow of gas, such as air, between the two electrodes. A power supply system is operatively connected to the two electrodes for generating an arc which is adapted to extend axially from the rear electrode through the vortical flow of gas and to an attachment point located on the front electrode. The bore of the front electrode includes a cup-shaped outer end portion which defines a forwardly facing radial shoulder. By proper coordination of the power and gas delivery systems, the arc may be made to attach to the radial shoulder of the front electrode, and such that the erosion of the front electrode occurs along an axial direction rather than a radial direction. The vortex generator also includes structure for continuously varying the pressure of the gas delivered between the electrodes, which serves to distribute the arc attachment point and the resulting erosion on each of the electrodes.

The present invention relates to a plasma arc torch of the type wherein an 
electric arc is employed to heat a gas to a high temperature, and which is 
useful for example in the cutting or welding of metals, or the heating of 
various materials. 
Plasma arc torches are usually designed for operation in one of two modes, 
which are commonly referred to as the transfer arc mode and the 
non-transfer arc mode. For the transfer arc mode of operation, the torch 
typically comprises a tubular rear electrode having a closed inner end, a 
tubular front electrode which acts as a collimating nozzle, and a gas 
introducing chamber between the two electrodes. The electric arc extends 
from the rear electrode through the gas introducing chamber and front 
electrode, and the arc extends forwardly from the torch and attaches or 
"transfers" to an external grounded workpiece. The prior patents to Baird, 
U.S. Pat. No. 3,194,941 and Camacho, U.S. Pat. Nos. 3,673,375 and 
3,818,174 illustrate torches of the transfer arc type. 
In the case of a plasma arc torch adapted for operation in the non-transfer 
arc mode, the electric arc extends from the rear electrode through the gas 
introducing chamber, and it attaches to the front electrode. A torch of 
this general type is illustrated in the patent to Muehlberger, U.S. Pat. 
No. 3,740,522. 
In existing non-transfer plasma arc torches, the front electrode comprises 
a tubular metal member having a central bore to which the arc attaches. 
The arc will naturally tend to attach to the bore at a single point, and 
the attachment of the arc results in wear or erosion of the metallic 
material at that point. The erosion moves through the wall of the 
electrode in a radially outward direction, and since the wall of the front 
electrode is necessarily somewhat thin, the front electrode has a very 
short operating life by reason of the fact that the erosion moves 
completely through the wall relatively quickly. 
Rapid erosion and short operating life are also problems with respect to 
the rear electrode, in torches adapted for either the transfer or 
non-transfer modes of operation. Here again, the arc will naturally tend 
to attach to and wear at a single point within the bore of the rear 
electrode, and the arc will quickly erode through the wall at that point. 
In the above referenced patent to Baird, it is suggested that alternating 
current be employed to power the electrode, which is said to cause the arc 
attachment point to move along the length of the rear electrode and 
thereby disperse the wear. Also, the Baird patent suggests that a field 
coil be placed about the rear electrode to cause the arc to rotate, but 
these proposed improvements involve a relatively complex and expensive 
electrical system. 
It has also been previously known that rotation of the arc attachment point 
in the rear electrode can be achieved aerodynamically, which is more 
efficient in that no specially designed electrical power supply system is 
required for this purpose. The known aerodynamic system includes the 
tangential injection of the gas into the gas introducing chamber to 
produce a vortical flow of gas in the chamber. Some of this gas moves 
rearwardly into the rear electrode, creating a well defined point within 
the rear electrode at which the pressure of the entering gas equals the 
back pressure in the electrode. At that point, the entering gas turns 
around and goes back out, creating a low pressure zone where the arc 
attaches. It has also been proposed to manually vary the pressure and thus 
the gas flow rate at periodic intervals, so that the point at which the 
arc attaches will move axially within the electrode upon each pressure 
change. Thus some operators of plasma torches have installed a manual 
pressure valve in the gas delivery system, with the operator periodically 
manually regulating the valve in order to change the arc attachment 
location. However, this procedure does not produce uniform erosion, and it 
results in localized wear points. 
It is accordingly an object of the present invention to provide a plasma 
arc torch of the type adapted for operation in the non-transfer mode, and 
wherein the problem of rapid erosion and failure of the front electrode is 
substantially alleviated. 
It is also an object of the present invention to provide a plasma arc torch 
of the described type and which is operable in either the transfer or 
non-transfer modes of operation, and which has provision for the efficient 
and uniform distribution of the wear of the rear electrode, to thereby 
extend the life of the rear electrode. 
These and other objects and advantages of the present invention are 
achieved in the embodiment illustrated herein by the provision of a plasma 
arc torch which comprises a torch housing, a rear electrode mounted within 
the housing and which includes a closed inner end and an open outer end, 
and a front electrode comprising a tubular metal member mounted within the 
housing and in coaxial alignment with the rear electrode. Vortex 
generating means is provided for generating a vortical flow of a gas at a 
location intermediate the rear and outer electrodes, and power supply 
means is provided for generating an arc which extends axially from the 
rear electrode and through the vortical flow of gas. In accordance with 
one aspect of the present invention, the front electrode has a bore which 
includes an outer end portion which is cup-shaped in cross section to 
define an outwardly facing radial shoulder, and the power supply means is 
operatively connected to the front electrode so that the arc attaches at a 
point located on the radial shoulder of the bore of the front electrode. 
Thus the attachment of the arc to the radial shoulder results in erosion 
of the material of the front electrode along an axial path of travel, 
rather than radially through the electrode. Since the axial length of the 
front electrode is substantially greater than the radial wall thickness of 
the electrode, the life of the front electrode is thus significantly 
extended. 
In accordance with another aspect of the present invention, the vortex 
generating means includes programmed control means for varying the 
pressure of the gas back and forth between predetermined limits and in 
accordance with a predetermined program. This variation in pressure is 
preferably continuous, which results in the attachment point of the arc 
being continuously moved axially back and forth along the length of the 
bore of the rear electrode by the changing pressure, while the arc is 
being rotated by the vortical flow of gas, to thereby distribute the 
erosion of the rear electrode and extend the life thereof. In the case of 
a non-transfer torch, the continuously varying pressure and the vortical 
flow of the gas also serve to distribute the arc attachment point on the 
radial shoulder of the cup-shaped front electrode to distribute the 
erosion thereof, and to further extend its life.

Referring more particularly to the drawings, there is illustrated a plasma 
arc torch 10 which is adapted for operation in the non-transfer arc mode, 
and which embodies the features of the present invention. In the 
illustrated embodiment, the torch comprises an outer housing, which 
includes a metal cylindrical rear housing section 12 and a coaxial metal 
extension 13 at the forward end of the section 12. 
A rear electrode 14 is mounted within the outer housing and comprises a 
tubular metal member having a closed inner end 15 and an open outer end 
16. The inner end 15 of the electrode is threadedly mounted in one end of 
a metal electrode holder 18. The holder 18, in addition to serving as a 
means for supporting the rear electrode, also serves as a means for 
delivering electrical current from an external power source to the rear 
electrode as further described below. The holder 18 also serves as a fluid 
conduit for the fluid cooling system, and for this purpose the rear end of 
the holder includes a tubular bore 19 which is threadedly coupled to a 
copper tube 20. The tube 20 in turn is connected to an external fluid 
supply, such as a municipal water system. The bore 19 in the rear end of 
the holder 18 also includes radial apertures 21 for the passage of the 
water therethrough, and in the manner further described below. 
The holder 18 is supported within a coaxial rear sleeve 24 by means of the 
bolts 25, and the forward end portion of the rear sleeve 24 mounts a 
tubular body member 26. The sleeve 24 and body member 26 are both formed 
of an electrically insulating material, such as a suitable phenolic resin. 
The body member 26 includes a number of radial apertures 27 therethrough, 
and it mounts an annular gas vortex generator 28. The generator 28 
includes a plurality of tangentially directed apertures 29 through the 
wall thereof, and which is threadedly mounted to the outer end of the rear 
electrode 14. The tubular body member 26 also includes a plurality of 
axially directed gas passages 30 which communicate with the apertures 29 
of the vortex generator as further described below. A water guide 32 in 
the form of a thin walled metal tube, is interposed between the holder 18 
and rear sleeve 24, and the water guide 32 extends forwardly between the 
rear electrode 14 and the rear sleeve 24 while defining a narrow annular 
water passage 33 therebetween which is part of the fluid cooling system as 
further described below. 
The rear end portion of the rear sleeve 24 is threadedly mounted to an 
insulator sleeve 36, which in turn is supported within the rear end cap 37 
of the torch. The insulator sleeve 36 also mounts a coaxial metal inner 
gas shroud 38 which closely overlies the exterior surface of the insulator 
sleeve 36 and rear sleeve 24, and the end cap 37 mounts a coaxial outer 
gas shroud 40 which overlies the inner shroud in spaced relation so as to 
define an annular gas passage 41 therebetween. The gas passage 41 
communicates with the gas inlet duct 42 via the radial aperture 43 in the 
end cap 37. The forward end of the passage 41 communicates with the axial 
passages 30 in the tubular body member 26, and such that gas delivered 
from the inlet duct 42 is directed to the tangential apertures 29 in the 
wall of the vortex generator 28. 
The plasma arc torch 10 further comprises a front electrode 46 comprising a 
tubular metal member having a bore therethrough. The front electrode 46 is 
mounted within the housing and in coaxial alignment with the rear 
electrode 14, with the inner end of the front electrode disposed adjacent 
and slightly spaced from the open outer end 16 of the rear electrode 14. 
The bore of the front electrode 46 includes an inner cylindrical end 
portion 48 and an outer end portion 50 which is cup-shaped in cross 
section to define an outwardly facing radial shoulder 51 and a cylindrical 
portion 52. The diameter D' of the cylindrical portion 52 is preferably 
between about at least one and one half to four times the diameter D of 
the inner cylindrical end portion 48 of the bore of the electrode, such 
that the radial shoulder 51 has a width of substantial dimensions. In the 
illustrated embodiment, the radial shoulder 51 is in the form of a frustum 
of a cone with the wall thereof being inclined forwardly at an angle A of 
about 10.degree.-12.degree. from a plane disposed perpendicularly to the 
axis of the bore of the electrode 46. 
The axial length L of the inner end portion 48 will be seen to be 
substantially longer than the axial length L' of the cup-shaped outer end 
portion 50. Also, the radial thickness of the wall of the front electrode 
is greater than the radial dimension of the outwardly facing radial 
shoulder 51, over at least the majority of the axial length of the front 
electrode extending rearwardly from the radial shoulder. Thus a 
substantial mass of material is located rearwardly or axially behind the 
radial shoulder 51. 
The front electrode 46 is releasably mounted to a tubular front sleeve 55 
by means of the threaded interconnection 56, and the front sleeve 55 
coaxially overlies a substantial portion of the length of the front 
electrode 46, with the front sleeve being spaced from the front electrode 
along substantially its entire length to define an annular water passage 
57 therebetween. The rear end of the front sleeve 55 engages and supports 
the end of the tubular body member 26, and the rear end of the sleeve is 
threadedly mounted to the forward end of the outer gas shroud at 58. The 
front sleeve 55 also includes a plurality of radial passages 59, so that 
the passage 57 communicates with the space 60 between the tubular body 
member 26 and outer gas shroud 40. Also, the front end of the sleeve 55 
supportingly engages the forward end of the electrode 46, and a plurality 
of radial apertures 61 extend through the forward end of the front sleeve 
for the purposes set forth below. In addition, an annular insulating block 
62 is mounted in the gap between the rear end of the front sleeve 55 and 
the vortex generator 28. 
The forward extension 13 of the outer housing will be seen to overlie the 
front sleeve 55 to define an annular passage 64 therebetween, and the 
forward end of the extension 13 engages and supports the forward end of 
the front electrode 46. Also, the rear section 12 of the housing is spaced 
from the outer gas shroud 40 to form a continuation of the passage 64, 
which communicates with the cooling system fluid outlet duct 66 which is 
attached to the rear end cap 37. 
From the above description, it will be seen that the plasma torch of the 
present invention includes a coolant flow path which extends so as to be 
in serial heat exchange relation with the rear electrode 14 and then the 
front electrode 46. Thus a fluid coolant may be circulated through the 
coolant flow path to remove heat from the torch during operation thereof. 
More particularly, the coolant flow path includes the copper tube 20, 
which delivers the water or other coolant to the rear bore 19 of the 
holder 18. The water then passes through the radial apertures 21 and into 
the annular passage 33 along the outside of the rear electrode. The water 
then passes through the apertures 27 in the tubular body member 26 to the 
passage 60, and then through the passages 59 in the front sleeve 55 to the 
annular passage 57 along the outside of the front electrode. The water 
then moves through the apertures 61 at the forward end of the sleeve 55, 
and it then moves through the passage 64 rearwardly to the outlet duct 66. 
A gas such as air may be delivered to the vortex generator 28 from the gas 
inlet duct 42, and so that the gas will pass along the annular passage 41 
between the inner and outer shrouds. Upon reaching the tubular body member 
26, the gas will pass through the axial apertures 30, and to the vortex 
generator 28. The gas then passes through the tangential apertures 29 in 
the vortex generator, so as to form a vortical flow of gas in the space 
between the rear and front electrodes, and which is in coaxial alignment 
with the two electrodes. 
It will also be apparent from the above description that the front 
electrode 46 is releasably connected to the tubular front sleeve 55 so as 
to permit the separation and replacement of the front electrode without 
replacement of the sleeve. More particularly, the front electrode 46 may 
be removed by gripping the bore of the electrode with an internal wrench, 
and unthreading the electrode from the sleeve. A new front electrode may 
then be installed by reversing this procedure. 
As best seen in FIG. 5, the plasma arc torch 10 of the present invention 
further includes power supply means 70 operatively connected to the rear 
and front electrodes for generating an arc which is adapted to extend 
axially from the rear electrode 14 through the vortical flow of gas and to 
an attachment point located on the radial shoulder 51 of the front 
electrode 46. Thus any erosion of the material of the front electrode will 
occur along an axial path of travel rather than radially through the 
electrode, to thereby extend the life of the front electrode. As 
illustrated, the positive side of the direct current power supply is 
connected to the copper tube 20, such that the current may be delivered 
through the electrode holder 18 and to the rear electrode 14. The negative 
or grounded side of the power supply is connected to the end cap 37, which 
is electrically connected to the front electrode 46 via the outer gas 
shroud 40 and front sleeve 55. 
As also illustrated schematically in FIG. 5, the vortex generating means 
includes a pressurized source of gas 72, and programmed control means 73 
for continuously varying the pressure of the gas between predetermined 
limits. Thus upon delivery of the gas to the vortex generator 28, the 
vortical flow of gas will cause the attachment point P of the arc to the 
bore of the rear electrode 14 to be rotated, while being moved axially 
back and forth along a substantial portion of the length of the bore by 
the varying pressure of the gas. As illustrated, the arc attachment 
location moves between the point H, representing the high pressure 
location, and the point L, representing the low pressure location. As a 
result, the erosion will be uniformly distributed along a substantial 
portion of the bore, thereby extending the life of the rear electrode. 
With respect to the front electrode, it is believed that the arc will 
attach at the low pressure point within the cup-shaped portion of the 
bore, and the attachment point may be established on the shoulder 51 by 
proper coordination of the gas flow rate (i.e. pressure) and power level. 
The continuous variation in pressure will cause the attachment point p on 
the radial shoulder 51 to move radially between the points h (high 
pressure location) and 1 (low pressure location) as seen in FIG. 5, and 
the vortical flow pattern of the gas will cause the attachment point to be 
rotated around the bore. Thus the varying pressure and vortical flow 
pattern cooperate to move the attachment point p along a spirally directed 
path on the shoulder 51 and as seen in FIG. 6, with the attachment point p 
spiraling inwardly as the pressure increases and spiraling outwardly as 
the pressure decreases. By this arrangement, the erosion along both the 
bore of the rear electrode and the radial surface of the front electrode 
is continuously moved and distributed over a relatively large surface 
area, to effectively extend the life of each electrode. 
Referring again to the front electrode 46, it will be seen that the erosion 
caused by the attachment of the arc may extend axially for a substantial 
distance before failure of the electrode, by reason of the substantial 
mass of material rearwardly of the radial shoulder. The only effective 
limitation on the wear distance is the fact that in order to maintain the 
arc attached to the radial shoulder 51, it is believed that the ratio of 
the axial length L of the inner bore portion to the diameter thereof must 
be greater than about four. Thus the erosion may continue until the 
critical length/diameter ratio is approached, at which point the arc will 
transfer to the adjacent workpiece. 
As a specific non-limiting example, a torch was constructed in accordance 
with the present invention and which had a power capacity of 150 KW. The 
bore of the rear electrode 14 had a length of 7 inches and a diameter of 
0.90 inches. The bore 48 of the front electrode 46 had a diameter D of 
0.60 inches and a length L of 6.68 inches, and The cup-shaped portion 50 
had a diameter D' of 2.20 inches and a length L' of 1.32 inches. The air 
was introduced into the vortex generator 28 at a pressure which oscillated 
between about 20 to 50 psi, which resulted in an oscillating mass flow 
rate of between about 5 to 40 cubic feet per minute. The rate of change in 
the pressure was about 4 psi per second. 
In the drawings and specification there has been set forth a preferred 
embodiment of the invention, and although specific terms are employed, 
they are used in a generic and descriptive sense only, and not for 
purposes of limitation.