Electrode for a plasma arc torch having an improved insert configuration

An electrode for use in a plasma arc torch has an insert designed to improve the service life of the electrode, particularly for high current processes. The electrode comprises an elongated electrode body formed of a high thermal conductivity material and having a bore disposed in a bottom end of the electrode body. The bore can be cylindrical or ring-shaped. An insert comprising a high thermionic emissivity material, and in some embodiments, a high thermal conductivity material, is disposed in the bore. The insert can be ringed-shaped or cylindrical.

FIELD OF THE INVENTION 
The invention relates generally to the field of plasma arc torches and 
systems. In particular, the invention relates to an electrode for use in a 
plasma arc torch having an improved insert configuration. 
BACKGROUND OF THE INVENTION 
Plasma arc torches are widely used in the processing (e.g., cutting and 
marking) of metallic materials. A plasma arch torch generally includes a 
torch body, an electrode mounted within the body, a nozzle with a central 
exit orifice, electrical connections, passages for cooling and arc control 
fluids, a swirl ring to control the fluid flow patterns, and a power 
supply. The torch produces a plasma arc, which is a constricted ionized 
jet of a plasma gas with high temperature and high momentum. The gas can 
be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air. 
In process of plasma arc cutting or marking a metallic workpiece, a pilot 
arc is first generated between the electrode (cathode) and the nozzle 
(anode). The pilot arc ionizes gas passing through the nozzle exit 
orifice. After the ionized gas reduces the electrical resistance between 
the electrode and the workpiece, the arc then transfers from the nozzle to 
the workpiece. The torch is operated in this transferred plasma arc mode, 
characterized by the conductive flow of ionized gas from the electrode to 
the workpiece, for the cutting or marking the workpiece. 
In a plasma arc torch using a reactive plasma gas, it is common to use a 
copper electrode with an insert of high thermionic emissivity material. 
The insert is press fit into the bottom end of the electrode so that an 
end face of the insert, which defines an emission surface, is exposed. The 
insert is typically made of either hafnium or zirconium and is 
cylindrically shaped. 
While electrodes with traditional cylindrical inserts operate as intended, 
manufacturers continuously strive to improve the service life of such 
electrodes, particularly for high current processes. It is therefore a 
principal object of the present invention to provide an electrode having 
an insert configuration that improves the service life of the electrode. 
SUMMARY OF THE INVENTION 
A principal discovery of the present invention is the recognition that 
certain inherent limitations exist in the traditional cylindrical insert 
design. These limitations serve to limit the service life of the 
electrode, particularly for high current processes. For a traditional 
cylindrical insert, the size of the emitting surface is increased for 
higher current capacity operations. The high thermionic emissivity insert, 
however, has a poor thermal conductivity relative to the electrode body 
(e.g., hafnium has a thermal conductivity which is about 5% of the thermal 
conductivity of copper). This makes the removal of heat from the center of 
the insert to the surrounding electrode body, which serves as heat sink, 
difficult. 
It is known to limit the diameter of the insert to a specified dimension, 
and this approach is successful up to a particular current level. When the 
torch operates at a current that exceeds that level, the centerline 
temperature of the insert exceeds the boiling point of the insert 
material, causing rapid loss of the insert material. 
The present invention features an electrode having an insert designed to 
facilitates the removal of heat from the insert resulting in an improved 
service life of the electrode. In one aspect, the invention features an 
electrode for a plasma arc torch. The electrode comprises an elongated 
electrode body formed of a high thermal conductivity material. The 
material can be copper, silver, gold, platinum, or any other high thermal 
conductivity material with a high melting and boiling point and which is 
chemically inert in a reactive environment. A bore is disposed in a bottom 
end of the electrode body. The bore can be cylindrical or ringed-shaped. A 
ring-shaped insert, comprising a high thermionic emissivity material 
(e.g., hafnium or zirconium), is disposed in the bore. In one embodiment, 
the insert also comprises the high thermal conductivity material. 
In one embodiment, the insert comprises a closed end which defines an 
exposed emission surface. In another embodiment, the insert comprises a 
first ring-shaped member formed of the high thermionic emissivity material 
and a second cylindrical member formed of high thermal conductivity 
material disposed in the first ring-shaped member. In yet another 
embodiment, the insert comprises a first ring-shaped member comprising the 
high thermionic emissivity material disposed in a second ring-shaped 
member formed of high thermal conductivity material. In another 
embodiment, the insert comprises a rolled pair of adjacent layers, the 
first layer comprising the high thermal conductivity material and the 
second layer comprising the high thermionic emissivity material. 
In another aspect, the invention features an electrode for a plasma arc 
torch comprising an elongated body and an insert. The elongated body has a 
bore formed in an end face. The insert is disposed in the bore and 
comprises a high thermal conductivity material and a high thermionic 
emissivity material. 
In one embodiment, the insert comprises a rolled pair of adjacent layers, 
the first layer comprising the high thermal conductivity material and a 
second layer comprising the high thermionic emissivity material. The first 
layer can be in the form of hafnium plating and the second layer can be a 
copper foil. In another embodiment, the electrode body has a ring-shaped 
bore, and the insert is ring-shaped. The insert can further comprise a 
closed end which defines an exposed emission surface. 
In another embodiment, the insert comprises a cylindrically-shaped, high 
thermal conductivity material. The material has a plurality of parallel 
bores disposed in a spaced arrangement An element, comprising high 
thermionic emissivity material, is being disposed in each of the plurality 
of bores. 
In still another aspect, the invention features a method of manufacturing 
an electrode for a plasma arc torch. A bore is formed at a bottom end of 
the elongated electrode body, which is formed of a high thermal 
conductivity material, relative to a central axis through the electrode 
body. The bore can be cylindrical or ring-shaped. An insert comprising a 
high thermionic emissivity material is inserted into the bore. The insert 
can be cylindrical or ring-shaped and can also comprise high thermal 
conductivity material. 
In one embodiment, the insert is ringed-shaped and can have one closed end 
which defines an exposed emission surface. In another embodiment, the 
insert is formed from a first ring-shaped member comprising high 
thermionic emissivity material and a second cylindrical member comprising 
high thermal conductivity material disposed in the ring-shaped first 
insert. 
The insert can be disposed a cylindrical bore formed in the electrode body 
having an inner bore and a deeper outer bore, such that the first member 
fits in the outer bore and the second member fits in the inner bore. 
Alternatively, the insert can be disposed in a cylindrical bore formed in 
the electrode body having an outer bore and a deeper inner bore, such that 
the first member fits in the outer bore and the second member fits in the 
inner bore. 
In another embodiment, the insert is formed by sintering a composite powder 
mixture of a high thermal conductivity material and a high thermionic 
emissivity material. For example, the composite powder mixture comprises 
grains of the thermal conductivity material coated with the high 
thermionic emissivity material. In another embodiment, the insert is 
formed of a cylindrically-shaped, high thermal conductivity material. The 
material has a plurality of parallel bores disposed in a spaced 
arrangement An element, comprising high thermionic emissivity material, is 
being disposed in each of the plurality of bores. 
In another embodiment, the insert is formed by placing a first layer 
comprising the high thermal conductivity material adjacent a second layer 
comprising the high thermionic emissivity material and rolling the 
adjacent layers. 
An electrode incorporating the principles of the present invention offers 
significant advantages of existing electrodes. One advantage of the 
invention is that double arcing due to the deposition of high thermionic 
emissivity material on the nozzle is minimized by the improved insert. As 
such, nozzle life and cut quality are improved. Another advantage is that 
the service life is improved especially for higher current operations 
(e.g., &gt;200A).

DETAILED DESCRIPTION 
FIG. 1 illustrates in simplified schematic form a typical plasma arc 
cutting torch 10 representative of any of a variety of models of torches 
sold by Hypertherm, Inc. in Hanover, N.H. The torch has a body 12 which is 
typically cylindrical with an exit orifice 14 at a lower end 16. A plasma 
arc 18, i.e. an ionized gas jet, passes through the exit orifice and 
attaches to a workpiece 19 being cut. The torch is designed to pierce and 
cut metal, particularly mild steel, the torch operates with a reactive 
gas, such as oxygen or air, as the plasma gas to form the transferred 
plasma arc 18. 
The torch body 12 supports a copper electrode 20 having a generally 
cylindrical body 21. A hafnium insert 22 is press fit into the lower end 
21a of the electrode so that a planar emission surface 22a is exposed. The 
torch body also supports a nozzle 24 which spaced from the electrode. The 
nozzle has a central orifice that defines the exit orifice 14. A swirl 
ring 26 mounted to the torch body has a set of radially offset (or canted) 
gas distribution holes 26a that impart a tangential velocity component to 
the plasma gas flow causing it to swirl. This swirl creates a vortex that 
constricts the arc and stabilizes the position of the arc on the insert. 
In operation, the plasma gas 28 flows through the gas inlet tube 29 and the 
gas distribution holes in the swirl ring. From there, it flows into the 
plasma chamber 30 and out of the torch through the nozzle orifice. A pilot 
arc is first generated between the electrode and the nozzle. The pilot arc 
ionizes the gas passing through the nozzle orifice. The arc then transfers 
from the nozzle to the workpiece for the cutting the workpiece. It is 
noted that the particular construction details of the torch body, 
including the arrangement of components, directing of gas and cooling 
fluid flows, and providing electrical connections can take a wide variety 
of forms. 
For conventional electrode designs, the diameter of the insert is specified 
for a particular operating current level of the torch. However, when the 
torch operates at a current that exceeds that level, the centerline 
temperature of the insert exceeds the boiling point of the insert 
material, causing rapid loss of the insert material. 
Referring to FIG. 2, a partial cross-sectional view of an electrode having 
an insert designed to facilitate the removal of heat from the insert 
resulting in an improved electrode service life is shown. The electrode 40 
comprises a cylindrical electrode body 42 formed of a high thermal 
conductivity material. The material can be copper, silver, gold, platinum, 
or any other high thermal conductivity material with a high melting and 
boiling point and which is chemically inert in a reactive environment. A 
bore 44 is drilled in a tapered bottom end 46 of the electrode body along 
a central axis (X1) extending longitudinally through the body. As shown, 
the bore 44 is U-shaped (i.e., characterized by a central portion 44a 
having a shallower depth than a ringed-shaped portion 44b). An insert 48 
comprising high thermionic emissivity material (e.g., hafnium or 
zirconium) is press fit in the bore. The insert 48 is ring-shaped and 
includes a closed end which defines an emission surface 49. The emission 
surface 49 is exposable to plasma gas in the torch body. 
FIG. 3 is a partial cross-sectional view of an electrode having another 
insert configuration. The electrode 50 comprises a cylindrical electrode 
body 52 formed of high thermal conductivity material. A ring-shaped bore 
54 is drilled in the bottom end 56 of the electrode body relative to the 
central axis (X2) extending longitudinally through the body. The bore 54 
can be formed using a hollow mill or end mill drilling process. A 
ring-shaped insert 58 comprising high thermionic emissivity material is 
press fit in the bore. The insert 58 includes an end face which defines 
the emission surface 59. 
Referring to FIG. 4, a partial cross-sectional view of an electrode having 
another insert configuration is shown. The electrode 60 comprises a 
cylindrical electrode body 62 formed of high thermal conductivity 
material. A bore 64 is drilled in a tapered bottom end 66 of the electrode 
body along a central axis (X3) extending longitudinally through the body. 
As shown, the bore 64 is two-tiered (i.e., characterized by a central 
portion 64a having a deeper depth than a ringed-shaped portion 64b). A 
ring-shaped insert 68 comprising high thermionic emissivity material is 
press fit in the bore. The insert 68 includes an end face which defines 
the emission surface 69. A cylindrical insert 67, comprising high thermal 
conductivity material, is press fit into the central portion 64a of the 
bore 64 adjacent the insert 68. 
FIG. 5 is a partial cross-sectional view of an electrode having another 
insert configuration. The electrode 70 comprises a cylindrical electrode 
body 72 formed of high thermal conductivity material. A cylindrical bore 
74 is drilled in a tapered bottom end 76 of the electrode body along a 
central axis (X4) extending longitudinally through the body. A cylindrical 
insert 77, comprising high thermal conductivity material portion 78a and a 
ring-shaped high thermionic emissivity material portion 78b, is press fit 
into the bore 74. The ring-shaped portion 78b includes an end face which 
defines the emission surface 79. 
Referring to FIG. 6, a cross-sectional view of another insert configuration 
incorporating the principles of the present invention is shown. The insert 
80 is a composite structure comprising adjacent layers of high thermal 
conductivity material and high thermionic emissivity material. More 
specifically, a layer 82 of high thermal conductivity material is placed 
on a layer 84 of high thermionic emissivity material. The two layers are 
rolled up to form a "jelly roll" structure. In one embodiment, the layer 
of high thermal conductivity material is a copper foil. The foil is plated 
with a layer of high thermionic emissivity material such as hafnium. The 
composite structure is rolled to form a cylindrical insert. 
FIG. 7 is a cross-sectional view of another insert configuration. The 
insert 86 is a composite structure comprising both high thermal 
conductivity material and high thermionic emissivity material. The insert 
includes a cylindrical member 86 formed of high thermal conductivity 
material. A plurality of parallel bores 88 disposed in a spaced 
arrangement are formed in the member 86. An element 90, comprising high 
thermionic emissivity material, is disposed in each of the plurality of 
bores 88. 
Referring to FIG. 8, a cross-sectional view of another insert configuration 
is shown. The insert 92 is formed by sintering a composite powder mixture 
of a high thermal conductivity material and a high thermionic emissivity 
material. The result is a composite material including grains of high 
thermal conductivity material 94 and grains of high thermionic emissivity 
material 96. 
FIG. 9 a cross-sectional view of another insert configuration for an 
electrode. The insert 98 is formed of composite powder mixture comprising 
grains 100 of the thermal conductivity material coated with the high 
thermionic emissivity material 102. 
The dimensions of the inserts 48, 58, 68, 78, 80, 86, 92 and 98 are 
determined as a function of the operating current level of the torch, the 
diameter (A) of the cylindrical insert and the plasma gas flow pattern in 
the torch. 
EQUIVALENTS 
While the invention has been particularly shown and described with 
reference to specific preferred embodiments, it should be understood by 
those skilled in the art that various changes in form and detail may be 
made therein without departing from the spirit and scope of the invention 
as defined by the appended claims. For example, although the steps for 
manufacturing the electrode are described in a particular sequence, it is 
noted that their order can be changed. In addition, while the various 
inserts described herein are characterized as ringed-shaped, cylindrical 
and the like, such inserts can be substantially ringed-shaped, cylindrical 
and the like.