FIELD OF THE INVENTION
[0001] The present invention relates to plasma arc torches and, more particularly, to an
electrode for supporting an electric arc in a plasma arc torch.
BACKGROUND OF THE INVENTION
[0002] Plasma arc torches are commonly used for the working of metals, including cutting,
welding, surface treatment, melting, and annealing. Such torches include an electrode
which supports an arc which extends from the electrode to the workpiece in the transferred
arc mode of operation. It is also conventional to surround the arc with a swirling
vortex flow of gas, and in some torch designs it is conventional to also envelope
the gas and arc with a swirling jet of water.
[0003] The electrode used in conventional torches of the described type typically comprises
an elongate tubular member composed of a material of high thermal conductivity, such
as copper or a copper alloy. The forward or discharge end of the tubular electrode
includes a bottom end wall having an emissive element embedded therein which supports
the arc. The element is composed of a material which has a relatively low work function,
which is defined in the art as the potential step, measured in electron volts (ev),
which permits thermionic emission from the surface of a metal at a given temperature.
In view of its low work function, the element is thus capable of readily emitting
electrons when an electrical potential is applied thereto. Commonly used emissive
materials include hafnium, zirconium, tungsten, and their alloys. The emissive element
is typically surrounded by a relatively non-emissive separator, which acts to prevent
the arc from migrating from the emissive element to the copper holder.
[0004] A problem associated with torches of the type described above is the short service
life of the electrode, particularly when the torch is used with an oxidizing gas,
such as oxygen or air. More particularly, the gas tends to rapidly oxidize the copper
of the electrode that surrounds the emissive element, and as the copper oxidizes,
its work function decreases. As a result, a point is reached at which the oxidized
copper surrounding the emissive element begins to support the arc, rather than the
element. When this happens, the copper oxide and the supporting copper melt, resulting
in early destruction and failure of the electrode.
[0005] The assignee of the present application has previously developed an electrode with
significantly improved service life, as described in U.S. Patent No. 5,023,425, the
entire disclosure of which is hereby incorporated by reference, and a method for making
such an electrode, as described in U.S. Patent No. 5,097,111, the entire disclosure
of which is hereby incorporated by reference. The '425 patent discloses an electrode
comprising a metallic tubular holder supporting an emissive element at a front end
thereof, and having a relatively non-emissive separator or sleeve surrounding the
emissive element and interposed between the emissive element and the metallic holder.
The sleeve thereby separates the emissive element from the holder. The '425 patent
describes the sleeve as preferably being composed of silver, which has a high resistance
to formation of an oxide. The silver and any oxide thereof which does form are poor
emitters, and thus the arc will continue to emit from the emissive element rather
than from the sleeve or the metallic holder. Service life is thereby significantly
extended.
[0006] The '111 patent discloses a method for making an electrode which includes the step
of forming a single cavity in the front face of a cylindrical blank of copper or copper
alloy, the cavity including an annular outer end portion for receiving a non-emissive
member. In particular, a metal blank of relatively non-emissive material, preferably
silver, is formed to substantially fit within the cavity. The non-emissive blank is
then metallurgically bonded into the cavity by first inserting a disk of silver brazing
material into the cavity, then inserting the non-emissive blank. The assembly is then
heated to a temperature only sufficient to melt the brazing material, and during the
heating process the non-emissive blank is pressed into the cavity, which causes the
brazing material to flow upwardly and cover the entirety of the interface between
the non-emissive blank and the cavity. The assembly is then cooled, resulting in the
brazing material metallurgically bonding the element into the non-emissive blank.
Next, the non-emissive blank is axially drilled and a cylindrical emissive element
is force-fitted into the resulting opening. To complete fabrication of the electrode,
the front face of the assembly is machined to provide a smooth outer surface, which
includes a circular outer end face of the emissive element, a surrounding annular
ring of the non-emissive blank, and an outer ring of the metal of the holder.
[0007] In addition, the torches described by the '425 and '111 patents define a rear cavity
that extends forwardly towards the front end of the holder such that the emissive
element, non-emissive separator, and a portion of the metallic holder form a cylindrical
post extending into the rear cavity. A cooling medium, such as water, is circulated
in the rear cavity and about the cylindrical post so that heat is transferred from
the arc to the cooling water and out of the torch. More specifically, heat is transferred
from the arc through the emissive element, non-emissive separator, the copper holder,
and any layers of brazing material therebetween to the cooling water. Although this
design allows greater heat transfer compared to having no rear cavity, several materials
and material interfaces must be crossed, which decreases efficiency.
[0008] One particular design defines a rear cavity wherein the cylindrical post includes
no portion of the copper holder so that the silver separator is exposed directly to
the rear cavity and cooling water circulated therein. For example, Figure 10 shown
in both the '425 and '111 patents discloses a plasma arc torch wherein the holder
16b has a through bore in the lower wall, and the non-emissive insert
32b extends through the bore and is exposed so as to directly contact the cooling water
in the rear cavity of the holder. This design is advantageous for two reasons: first,
silver has a greater thermal conductivity than copper, which increases the heat transfer
between the arc and the cooling water; second, the interface between the silver separator
and the copper holder is eliminated, which further improves heat transfer. However,
the torch shown in Figure 10 of the '425 and '111 patents is not easily formed in
that, in addition to the rear cavity being formed in the holder, the lower wall of
the holder is bored out and the non-emissive separator is press fit therein.
[0009] Thus, while both the electrode described in the '425 patent and the method of making
an electrode described by the '111 patent provide substantial advances in the art,
further improvements are desired. In particular, one method described by the '425
and '111 patents provides boring or drilling out a portion of the non-emissive blank,
which is typically silver, along a central axis so that the emissive element or insert
can be press-fitted therein. While providing a close-fitting relationship between
the emissive element and the non-emissive separator, this method disadvantageously
results in a loss of silver drilled from the separator to accommodate the emissive
element.
[0010] Another method used in forming conventional torches provides securing the emissive
element in the non-emissive blank or separator by way of brazing. According to this
method, the temperature of the silver alloy brazing material must be above its melting
point, and thus the temperature of the silver or silver alloy separator is raised
almost to its melting point, which can soften the separator material. If this approach
were tried in connection with the embodiment of Figure 10 of the '425 patent or 111
patent, however, the softened silver separator may be unable to adequately radially
restrain the emissive element when inserted into the silver separator, which could
result in the emissive element being "off-center" relative to the central longitudinal
axis of the electrode.
SUMMARY OF THE INVENTION
[0011] The present invention was developed to improve upon conventional electrodes and methods
of making electrodes, and more particularly electrodes and methods of making electrodes
disclosed in the above-referenced '425 and '111 patents. It has been discovered that
the difficulties of the electrodes described above, namely the loss of silver from
the relatively non-emissive separator and the positioning of the emissive element
along the central longitudinal axis of the electrode, can be overcome by positioning
the emissive element in the metallic holder before the separator is installed. In
one advantageous embodiment, the present invention provides an electrode and method
of making an electrode having an emissive element and a generally non-emissive separator
disposed in a front cavity defined by the metallic holder, whereby a brazing material
is disposed therebetween such that the emissive element's position along the central
longitudinal axis is not affected by the brazing process. In another embodiment, the
present invention provides an electrode and method for making an electrode wherein
the metallic holder also defines a rear cavity that is sized so that a portion of
the separator is exposed to the rear cavity, which thereby improves heat transfer
between an arc and a cooling fluid circulated in the rear cavity.
[0012] More particularly, in accordance with one preferred embodiment of the invention,
an electrode for supporting an arc in a plasma arc torch comprises a metallic holder
having a front end and rear end, the front end defining a front cavity. A generally
non-emissive separator is positioned in the front cavity and includes an inner peripheral
wall. An emissive element is also positioned in the front cavity and includes an outer
peripheral wall that is only partially surrounded by the inner peripheral wall of
the separator. According to one embodiment, part of the brazing material is disposed
between the emissive element and the separator, and also between the separator and
the metallic holder. The brazing layer has a melting temperature no greater than the
melting temperature of the separator. Thus, the separator and emissive element are
metallurgically bonded together such that separator totally separates the emissive
element from contact with the outer surface of the metallic holder.
[0013] The separator which surrounds the emissive element is preferably composed of a metallic
material, such as silver, which has a high resistance to the formation of an oxide.
This serves to increase the service life of the electrode, since the silver and any
oxide which does form are very poor emitters. As a result, the arc will continue to
emit from the emissive element, rather than from the metallic holder or the separator,
which increases the service life of the electrode.
[0014] In one embodiment, the rear end of the metallic holder defines a rear cavity that
extends towards the front end of the holder to expose the separator. The rear cavity
can be formed by trepanning or other types of machining, and the exposed separator
provides an improved medium for heat transfer from the arc to the cavity, particularly
if a cooling medium, such as water, is circulated in the cavity while the torch is
in operation.
[0015] The present invention also includes a method fabricating the above-described electrode
which comprises the steps of forming a front cavity in a generally planar front face
of a metallic blank and fixedly securing an emissive element in the front cavity.
A relatively non-emissive separator is then positioned in the front cavity of the
metallic holder such that the separator is interposed between and separates the metallic
holder from the emissive element at the front face of the holder. In one embodiment,
the separator has a tubular shape and sized such that the separator and the emissive
element have a close-fitting relationship. In addition, the emissive element and separator
can be brazed together using a brazing material, such as silver.
[0016] Preferably, the front face of the metallic holder is then finished to form a substantially
planar surface which includes the metallic holder, the emissive element, and the separator.
In one embodiment, a rear cavity is formed in the rear face of the metallic holder
such that the separator is exposed to the cavity. In this regard, the metallic holder
is trepanned or machined to remove a portion of the holder to thereby expose the separator,
which improves the heat transfer from the arc to the cavity. Water or other cooling
medium can be circulated within the cavity to further conduct and remove heat from
the electrode.
[0017] Thus, the electrode of the present invention provides an electrode and method of
making an electrode having improved heat transfer properties over conventional plasma
arc torches. By positioning the emissive element in the metallic holder before the
separator is installed, the position of the emissive element is not affected by a
subsequent brazing process. In addition, by exposing the silver separator by trepanning
or machining the rear cavity, the front end of the holder is not required to be bored
out and the silver separator press fitted therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Having thus described the invention in general terms, reference will now be made
to the accompanying drawings, which are not necessarily drawn to scale, wherein:
Figure 1 is a sectioned side elevational view of a plasma arc torch which embodies
the features of the present invention;
Figure 2 is an enlarged perspective view of an electrode in accordance with the present
invention;
Figure 3 is an enlarged sectional side view of an electrode in accordance with the
present invention;
Figures 4-8 are schematic views illustrating the steps of a preferred method of fabricating
the electrode in accordance with the invention;
Figure 9 is an end elevational view of the finished electrode; and
Figure 10 is an enlarged sectional side view of an electrode according to an alternative
embodiment of an electrode in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present invention now will be described more fully hereinafter with reference
to the accompanying drawings, in which preferred embodiments of the invention are
shown. This invention may, however, be embodied in many different forms and should
not be construed as limited to the embodiments set forth herein; rather, these embodiments
are provided so that this disclosure will be thorough and complete, and will fully
convey the scope of the invention to those skilled in the art. Like numbers refer
to like elements throughout.
[0020] With reference to Figure 1, a plasma arc torch
10 embodying the features of the present invention is depicted. The torch
10 includes a nozzle assembly
12 and a tubular electrode
14. The electrode
14 preferably is made of copper or a copper alloy, and is composed of an upper tubular
member
15 and a lower cup-shaped member or holder
16. The upper tubular member
15 is of elongate open tubular construction and defines the longitudinal axis of the
torch
10. The upper tubular member
15 includes an internally threaded lower end portion
17. The holder
16 is also of tubular construction, and includes a lower front end and an upper rear
end. A transverse end wall
18 closes the front end of the holder
16, and the transverse end wall
18 defines an outer front face
20 (Figure 2). The rear end of the holder
16 is externally threaded and is threadedly joined to the lower end portion
17 of the upper tubular member
15.
[0021] With primary reference to Figures 2 and 3, the holder
16 is open at the rear end
19 thereof such that the holder is of cup-shaped configuration and defines an internal
cavity
22. The internal cavity
22 has a surface
31 that includes a cylindrical post
23 extending into the internal cavity along the longitudinal axis. Advantageously, the
cylindrical post
23 is formed to have improved heat transfer properties compared to conventional designs,
as discussed below. Two coaxial cavities
24, 25 are formed in the front face
20 of the end wall
18 and extend rearwardly along the longitudinal axis and into a portion of the holder
16. The cavities
24, 25 are generally cylindrical, wherein the first cavity
24 has a diameter less than the second cavity
25. The cavities
24, 25 include inner side surfaces
27a, 27b, respectively.
[0022] An emissive element or insert
28 is mounted in the small cavity
24 and is disposed coaxially along the longitudinal axis. The emissive element
28 has a circular outer end face
29 lying in the plane of the front face
20 of the holder
16. The emissive element
28 also includes a generally circular inner end face
30 which is disposed in the small cavity
24 and is opposite the outer end face
29. The inner end face
30, however, can have other shapes, such as pointed, polygonal, or spherical, in order
to assist in securing the emissive element to the small cavity
24, as discussed below. The emissive element
28 is composed of a metallic material which has a relatively low work function, in a
range of about 2.7 to 4.2 ev, so that it is adapted to readily emit electrons upon
an electrical potential being applied thereto. Suitable examples of such materials
are hafnium, zirconium, tungsten, and alloys thereof. According to one embodiment,
the emissive element
28 is secured to the small cavity
24 by an interference fit, although other securing methods can also be used, such as
pressing or crimping.
[0023] A relatively non-emissive separator
32 is positioned in the large cavity
25 coaxially about the emissive element
28. The separator
32 has a peripheral wall
33 (Figures 4-5) extending the length of the emissive element
28. The peripheral wall
33 is illustrated as having a substantially constant outer diameter over the length
of the separator, although it will be appreciated that other geometric configurations
would be consistent with the scope of the invention, such as frustoconical.
[0024] The separator
32 also includes an outer end face
36 which is generally flush with the circular outer end face
29 of the emissive element
28, and is also generally flush with the front face
20 of the holder
16. The separator
32 preferably has a radial thickness of at least about 0.25 mm (0.01 inch) at the outer
end face
36 and along its entire length, and preferably the diameter of the emissive insert
28 is about 30-80 percent of the outer diameter of the end face 36 of the separator
32. As a specific example, the emissive element
28 typically has a diameter of about 0.08" and a length of about 0.25", and the outer
diameter of the separator
32 is about 0.25".
[0025] The separator
32 is composed of a metallic material having a work function that is greater than that
of the material of the holder
16, and also greater than that of the material of the emissive element
28. More specifically, it is preferred that the separator be composed of a metallic material
having a work function of at least about 4.3 ev. In a preferred embodiment, the separator
32 comprises silver as the primary material, although other metallic materials, such
as gold, platinum, rhodium, iridium, palladium, nickel, and alloys thereof, may also
be used.
[0026] For example, in one particular embodiment of the present invention, the separator
32 is composed of a silver alloy material comprising silver alloyed with about 0.25
to 10 percent of an additional material selected from the group consisting of copper,
aluminum, iron, lead, zinc, and alloys thereof. The additional material may be in
elemental or oxide form, and thus the term "copper" as used herein is intended to
refer to both the elemental form as well as the oxide form, and similarly for the
terms "aluminum" and the like.
[0027] With reference again to Figure 1, the electrode
14 is mounted in a plasma torch body
38, which includes gas and liquid passageways
40 and
42, respectively. The torch body
38 is surrounded by an outer insulated housing member
44. A tube
46 is suspended within the central bore
48 of the electrode
14 for circulating a liquid cooling medium, such as water, through the electrode
14. The tube
46 has an outer diameter smaller than the diameter of the bore
48 such that a space
49 exists between the tube
46 and the bore
48 to allow water to flow therein upon being discharged from the open lower end of the
tube
46. The water flows from a source (not shown) through the tube
46, inside the internal cavity
22 and the holder
16, and back through the space
49 to an opening
52 in the torch body
38 and to a drain hose (not shown). The passageway
42 directs injection water into the nozzle assembly
12 where it is converted into a swirling vortex for surrounding the plasma arc, as further
explained below. The gas passageway
40 directs gas from a suitable source (not shown), through a gas baffle
54 of suitable high temperature material into a gas plenum chamber
56 via inlet holes
58. The inlet holes
58 are arranged so as to cause the gas to enter in the plenum chamber
56 in a swirling fashion. The gas flows out of the plenum chamber
56 through coaxial bores
60 and
62 of the nozzle assembly
12. The electrode
14 retains the gas baffle
54. A high-temperature plastic insulator body
55 electrically insulates the nozzle assembly
12 from the electrode
14.
[0028] The nozzle assembly
12 comprises an upper nozzle member
63 which defines the first bore
60, and a lower nozzle member
64 which defines the second bore
62. The upper nozzle member
63 is preferably a metallic material, and the lower nozzle member
64 is preferably a metallic or ceramic material. The bore
60 of the upper nozzle member
63 is in axial alignment with the longitudinal axis of the torch electrode
14.
[0029] The lower nozzle member
64 is separated from the upper nozzle member
63 by a plastic spacer element
65 and a water swirl ring
66. The space provided between the upper nozzle member
63 and the lower nozzle member
64 forms a water chamber
67.
[0030] The lower nozzle member
64 comprises a cylindrical body portion
70 which defines a forward or lower end portion and a rearward or upper end portion,
with the bore
62 extending coaxially through the body portion
70. An annular mounting flange
71 is positioned on the rearward end portion, and a frustoconical surface
72 is formed on the exterior of the forward end portion coaxial with the second bore
62. The annular flange
71 is supported from below by an inwardly directed flange
73 at the lower end of the cup
74, with the cup
74 being detachably mounted by interconnecting threads to the outer housing member
44. A gasket
75 is disposed between the two flanges
71 and
73.
[0031] The bore
62 in the lower nozzle member
64 is cylindrical, and is maintained in axial alignment with the bore
60 in the upper nozzle member
63 by a centering sleeve
78 of any suitable plastic material. Water flows from the passageway
42 through openings
85 in the sleeve
78 to the injection ports
87 of the swirl ring
66, which injects the water into the water chamber
67. The injection ports
87 are tangentially disposed around the swirl ring
66, to impart a swirl component of velocity to the water flow in the water chamber
67. The water exits the water chamber
67 through the bore
62.
[0032] A power supply (not shown) is connected to the torch electrode
14 in a series circuit relationship with a metal workpiece, which is usually grounded.
In operation, a plasma arc is established between the emissive element
28 of the electrode, which acts as the cathode terminal for the arc, and the workpiece,
which is connected to the anode of the power supply and is positioned below the lower
nozzle member
64. The plasma arc is started in a conventional manner by momentarily establishing a
pilot arc between the electrode
14 and the nozzle assembly
12, and the arc is then transferred to the workpiece through the bores
60 and
62.
METHOD OF FABRICATION
[0033] The invention also provides a simplified method for fabricating an electrode of the
type described above. Figures 4-8 illustrate a preferred method of fabricating the
electrode in accordance with the present invention. As shown in Figure 4, a cylindrical
blank
94 of copper or copper alloy is provided having a front face
95 and an opposite rear face
96. A pair of generally cylindrical coaxial bores are then formed, such as by drilling,
in the front face
95 so as to form the small cavity
24 and large cavity
25, as described above. The emissive element
28 is then fixedly secured to the small cavity
24 by press-fitting the emissive element therein. Other methods of securing the emissive
element into the small cavity
24 can also be used, such as crimping, radially compressing, or utilizing electromagnetic
energy. The emissive element
28 extends outwardly from the small cavity
24 towards the front face
95 of the cylindrical blank
94 and defines an open space
97 between the emissive element and inner wall
27b of the large cavity
25.
[0034] As previously described, a separator
32 is composed of a silver alloy material. In one embodiment, for example, the silver
alloy material comprises silver alloyed with about 0.25 to 10 percent of copper. The
separator
32 is configured and sized to substantially occupy the open space
97 defined by the inner wall
27b of the large cavity
25 and the emissive element
28. In this regard, the separator
32 may be shaped by machining or forming.
[0035] Next, as shown in Figure 5, the separator
32 is inserted into the large cavity
25 such that the peripheral wall
33 of the separator slideably engages the inner wall
27b of the large cavity, and the cylindrical cavity
35 defined by the separator is disposed about the emissive element
28 to define an interface therebetween. In one embodiment, the separator
32 is disposed about the emissive element
28 in a close fitting or interference fit, although other methods of securing the separator
to the emissive element can be used, as described below.
[0036] According to one embodiment shown in Figure 6, a tool
98 having a generally planar circular working surface
100 is placed with the working surface in contact with the end faces
29 and
36 of the emissive element
28 and separator
32, respectively. The outer diameter of the working surface
100 is slightly smaller than the diameter of the large cavity
25 in the cylindrical blank
94. The tool
98 is held with the working surface
100 generally coaxial with the longitudinal axis of the torch
10, and force is applied to the tool so as to impart axial compressive forces to the
emissive element
28 and the separator
32 along the longitudinal axis. For example, the tool
98 may be positioned in contact with the emissive element
28 and separator
32 and then struck by a suitable device, such as the ram of a machine. Regardless of
the specific technique used, sufficient force is imparted so as to cause the emissive
element
28 and the separator
32 to be deformed radially outwardly such that the emissive element is tightly gripped
and retained by the separator, and the separator is tightly gripped and retained by
the large cavity
25, as shown in Figure 7.
[0037] In a preferred embodiment, the separator
32 is metallurgically bonded to the emissive element
28. Advantageously, the emissive element
28 is already secured to the small cavity
24 when the brazing step is performed (as discussed above) so that the emissive element
remains centered along the longitudinal axis even if the separator is softened by
the high temperatures associated with brazing. The brazing process is preferably conducted
by first inserting a ring
99 (Figures 5 and 7) of silver brazing material about the emissive element
28 after the emissive element has been secured to the small cavity
24 such that the ring occupies a portion of the open space
97 between the emissive element and inner wall
27b of the large cavity
25. In one example, the brazing material comprises an alloy composed mostly of silver
with one or more other elements, such as nickel, lithium, and/or copper. Also, a small
amount of flux may be included, so as to remove oxides from the surface of the copper.
[0038] The separator
32 is introduced after the ring
99 is inserted into the open space
97, and the resulting assembly is then heated to a temperature only sufficient to melt
the brazing material, which has a melting temperature no greater than the separator
32. However, with the present invention, the temperature does not have to be significantly
lower than the melting temperature of the separator because the emissive element
28 is secured to the small cavity
24 as described above. During the heating process, the separator
32 is pressed into the large cavity
25, which causes the melted brazing material to flow upwardly and cover the entirety
of the interface between the separator and the emissive element
28 and between the peripheral wall
33 of the separator
32 and the inner wall
27b of the large cavity
25. Upon cooling, the brazing material provides a relatively thin coating which serves
to bond the separator
32 to the emissive element
28, with the coating having a thickness on the order of between about 0.001 to 0.005
inches. Alternatively, the brazing step can be performed by melting a disk of brazing
material that is placed on the separator
32 and the emissive element
28 after the two have been pressed into the cavities. In this manner, capillary action
pulls the brazing material between the separator
32 and emissive element
28 so that the a relatively thin coating is disposed therebetween as discussed above.
[0039] To complete the fabrication of the holder
16, the rear face
96 of the cylindrical blank
94 is machined to form an open cup-shaped configuration shown in Figure 8 defining the
cavity
22 therein. Advantageously, the cavity
22 includes an internal annular recess
82 which defines the cylindrical post
23 and coaxially surrounds portions of the separator
32 and emissive element
28. In particular, the internal annular recess
82 includes an internal surface
83 comprising a portion of the peripheral wall
33 of the separator
32. In other words, the internal annular recess
82 is formed, such as by trepanning or other machining operation, so that a portion
of the peripheral wall
33 of the separator
32 is directly exposed to the cavity
22. As such, the exposed separator
32 improves the heat transfer between the cooling medium circulated in the cavity
22 and the arc. Further, the brazing material surrounding the peripheral wall
33 of the separator
32 at the internal surface
83 of the annular recess
82 is preferably eliminated, thus further improving heat transfer.
[0040] The improved heat transfer properties mentioned above result from two primary circumstances.
First, silver has a greater thermal conductivity than copper, namely 4.29 W/(cm·K)
versus 4.01 W/(cm·K), respectively. Second, there are fewer boundaries over which
the heat must pass. More specifically, by eliminating the boundary or interface between
the separator
32 and the brazing material (as well as the boundary between the brazing material and
the blank
94), the rate of heat transfer of the electrode according to the present invention is
significantly greater than conventional electrodes. In addition, the path the heat
must travel is shorter than conventional electrodes since the separator
32 is directly exposed to the cavity
22.
[0041] As discussed above, the surface
31 of the internal cavity
22 includes the cylindrical post
23. In one embodiment shown in Figures 3 and 8, the surface
31 includes a cap-shaped portion
92 of the blank
94 disposed about the emissive element
28. The portion
92 is tightly secured to the emissive element
28, although not directly attached to the remainder of the blank
94. Thus, the portion
92 is formed by the trepanning operation for ease of manufacturing, since by leaving
the portion
92 the post
23 has a uniform cylindrical shape. However, the portion
92 can also be partially or completely machined away to expose the emissive element
28 to the cavity
22 (see Figure 10).
[0042] The external periphery of the cylindrical blank
94 is also shaped as desired, including formation of external threads
102 at the rear end
19 of the holder
16. Finally, the front face
95 of the blank
94 and the end faces
29 and
36 of the emissive element
28 and separator
32, respectively, are machined so that they are substantially flat and flush with one
another. Any brazing material present on the front face
95 and end faces
29 and
36 is also removed during this machining process.
[0043] Figure 9 depicts an end elevational view of the holder
16. It can be seen that the end face
36 of the separator
32 separates the end face
29 of the emissive element
28 from the front face
20 of the holder
16. The end face
36 is annular having an inner perimeter
104 and an outer perimeter
106. Because the separator
32 is composed of the silver alloy material having a higher work function than that
of the emissive element
28, the separator
32 serves to discourage the arc from detaching from the emissive element and becoming
attached to the holder
16.
[0044] Thus, the present invention provides an electrode
14 for use in a plasma arc torch and a method of making an electrode wherein the emissive
element
28 is secured along the longitudinal axis and thus prevented from moving while brazing
the emissive element to the separator
32. In addition, the separator
32 has a tubular shape, thus eliminating the need for drilling an opening in the separator,
which results in a loss of silver.
[0045] Many modifications and other embodiments of the invention will come to mind to one
skilled in the art to which this invention pertains having the benefit of the teachings
presented in the foregoing descriptions and the associated drawings. For example,
the separator and or emissive element can have other shapes and configurations, such
as conical or rivet-shaped, without departing from the spirit and scope of the invention.
Therefore, it is to be understood that the invention is not to be limited to the specific
embodiments disclosed and that modifications and other embodiments are intended to
be included within the scope of the appended claims. Although specific terms are employed
herein, they are used in a generic and descriptive sense only and not for purposes
of limitation.
1. A method of fabricating an electrode adapted for supporting an arc in a plasma torch,
comprising the steps of:
forming a front cavity in a generally planar front face of a metallic holder, the
front cavity extending along an axis generally normal to the front face;
fixedly securing an emissive element to the front cavity of the metallic holder;
positioning a relatively non-emissive separator in the front cavity of the metallic
holder such that the separator is interposed between and separates a portion of the
metallic holder from the emissive element at the front face of the holder; and
forming a rear cavity in the metallic holder such that a portion of the separator
is exposed to the cavity.
2. A method according to Claim 1, wherein the separator is positioned in the front cavity
of the holder such that only a portion of the emissive element is in contact with
the separator.
3. A method according to Claim 1, wherein the emissive element is fixedly secured to
the front cavity of the holder by press-fitting.
4. A method according to Claim 1, wherein positioning the separator comprises positioning
a separator having a tubular shape over the emissive element in a close-fitting relationship.
5. A method according to Claim 1, wherein forming the front and rear cavities comprises
machining the metallic holder.
6. A method according to Claim 1, wherein forming the rear cavity includes exposing the
emissive element to the cavity.
7. A method of fabricating an electrode adapted for supporting an arc in a plasma torch,
comprising the steps of:
forming a front cavity in a generally planar front face of a metallic holder, the
front cavity extending along an axis generally normal to the front face;
fixedly securing an emissive element to the front cavity of the metallic holder;
inserting a relatively non-emissive separator into the front cavity of the metallic
holder such that only a portion of the emissive element is in contact with the separator;
and
introducing a brazing material in the front cavity such that some of the brazing material
is between the emissive element and the separator and also between the separator and
the metallic holder.
8. A method according to Claim 7, wherein the brazing material is introduced by inserting
a disk of brazing material about the emissive element in the front cavity, heating
the brazing material until the brazing material becomes at least partially flowable,
and pressing the separator into the front cavity.
9. A method according to Claim 7, wherein the emissive element is fixedly secured in
the front cavity before the separator is inserted in the front cavity.
10. An electrode adapted for supporting an electric arc in a plasma arc torch, comprising:
a metallic holder having a front end and a rear end, the front end defining a front
cavity;
a generally non-emissive separator positioned in the front cavity, the separator having
an inner peripheral wall;
an emissive element also positioned in the front cavity, the emissive element having
an outer peripheral wall that is only partially surrounded by the inner peripheral
wall of the separator; and
a brazing material disposed between the emissive element and the separator, and between
the separator and the metallic holder.
11. An electrode according to Claim 10, wherein a portion of the outer peripheral wall
of the emissive element is free from any contact with the brazing material.
12. An electrode according to Claim 10, wherein the front cavity comprises a proximal
portion and a distal portion, the proximal portion having a diameter smaller than
the diameter of the distal portion, the brazing material being disposed only in the
distal portion of the cavity.
13. An electrode according to Claim 10, wherein the separator is constructed of silver
alloyed with an additional material selected from the group consisting of copper,
aluminum, iron, lead, zinc, and alloys thereof.
14. An electrode according to Claim 10, wherein the emissive element has a cylindrical
shape and the separator has a tubular shape.
15. An electrode adapted for supporting an electric arc in a plasma arc torch, comprising:
a metallic holder having a front end and a rear end, the front end defining a front
cavity and the rear end defining a rear cavity;
a generally non-emissive separator positioned in the front cavity, the separator having
an outer peripheral wall; and
an emissive element also positioned in the front cavity coaxially with the separator,
the emissive element having an outer peripheral wall that is only partially in contact
with the separator, wherein a portion of the outer peripheral wall of the separator
is exposed to the rear cavity.
16. An electrode according to Claim 15, wherein the emissive element has a cylindrical
shape and the separator has a tubular shape.
17. An electrode according to Claim 15, wherein the front cavity comprises a proximal
portion and a distal portion, the proximal portion having a diameter smaller than
the diameter of the distal portion, wherein the emissive element and the proximal
portion of the front cavity have an interference fit therebetween.
18. An electrode according to Claim 15, wherein the separator is constructed of silver
alloyed with an additional material selected from the group consisting of copper,
aluminum, iron, lead, zinc, and alloys thereof.
19. An electrode according to Claim 15, wherein a portion of the emissive element is exposed
to the rear cavity.
20. A plasma arc torch, comprising:
an electrode which includes:
a metallic holder having a front end and a rear end, the front end defining a front
cavity and the rear end defining a rear cavity;
a generally non-emissive separator positioned in the front cavity, the separator having
an outer peripheral wall; and
an emissive element also positioned in the front cavity coaxially with the separator,
the emissive element having an outer peripheral wall that is only partially in contact
with the separator;
a nozzle mounted adjacent the front end of the holder and having a flow path therethrough
that is aligned with the longitudinal axis;
an electrical supply for creating an arc extending from the emissive element of the
electrode through the nozzle flow path and to a workpiece located adjacent the nozzle;
and
a gas supply for creating a flow of a gas between the electrode and the nozzle and
so as to create a plasma flow outwardly through the nozzle flow path and to the workpiece.
21. A plasma arc torch according to Claim 20, further comprising a brazing layer disposed
between the emissive element and the separator, and between the separator and the
second cavity.
22. A plasma arc torch according to Claim 20, wherein a portion of the emissive element,
the portion being in contact with the first cavity, is substantially free of the brazing
material.
23. A plasma arc torch according to Claim 20, wherein the separator is constructed of
silver alloyed with an additional material selected from the group consisting of copper,
aluminum, iron, lead, zinc, and alloys thereof.
24. A plasma arc torch according to Claim 20, wherein the emissive element and the separator
are flush with the front end of the metallic holder.
25. A plasma arc torch according to Claim 20, wherein the metallic holder includes a rear
end defining a rear cavity that is at least partially shaped such that the separator
is at least partially exposed to the cavity.
26. A plasma arc torch according to Claim 25, wherein the metallic holder is at least
partially shaped such that the emissive element is at least partially exposed to the
cavity.
27. A method of fabricating an electrode adapted for supporting an arc in a plasma torch,
comprising the steps of:
forming an opening in a front end of a holder;
securing an emissive element into the opening of the holder such that a portion of
the emissive element extends frontwardly from the holder;
securing a relatively non-emissive member about the emissive element in a position
such that the non-emissive member and the emissive element will together define at
least part of a front face of the electrode for supporting an arc; and
forming a cavity in a rear end of the holder such that at least a portion of the non-emissive
member is exposed to the cavity and cooling liquid can be circulated therethrough
to cool the non-emissive member.
28. A method according to Claim 27, wherein the method causes the emissive element to
be also at least partially exposed to the cavity.
29. A method of fabricating an electrode adapted for supporting an arc in a liquid cooled
plasma torch, comprising the steps of:
providing an emissive element and a relatively non-emissive member defining an opening
therein sized to fit around the emissive element;
bonding the non-emissive member to a holder having a front opening therein such that
the emissive element extends from the non-emissive member and into the opening of
the holder; and
forming a rear cavity in the holder such that at least a portion of the non-emissive
member is exposed to the cavity and cooling liquid can be circulated therethrough
to cool the non-emissive member.
30. A method according to Claim 29, wherein the method causes the emissive element to
be also at least partially exposed to the cavity.
31. A method according to Claim 29, further comprising removing at least a portion of
the non-emissive member to define a front face where the emissive element and the
non-emissive member are substantially flat and flush with one another at the front
face of the non-emissive member.
32. A method according to Claim 29, wherein said bonding step comprises thermally bonding
the non-emissive member to the holder.
33. A method according to Claim 32, wherein said bonding step comprises brazing the non-emissive
member to the holder.
34. A method according to Claim 29, wherein said providing step includes providing a metallic
holder.
35. An electrode adapted for supporting an arc in a liquid cooled plasma arc torch, comprising:
a holder having a front end and a rear end, the front end defining a front opening
and the rear end defining a rear cavity;
an emissive element positioned such that a portion of the emissive element is within
the front opening of the holder; and
a relatively non-emissive member being secured to said holder and surrounding a portion
of the emissive element in a position such that the non-emissive member and the emissive
element together define at least part of a front face of the electrode for supporting
an arc,
wherein a portion of the non-emissive member is exposed to the rear cavity and
cooling liquid can be circulated therethrough to cool the non-emissive member.
36. An electrode according to Claim 35, wherein at least a portion of the emissive element
is also exposed to the rear cavity.
37. An electrode according to Claim 35, further comprising a brazing layer disposed between
the emissive element and the non-emissive member.
38. An electrode adapted for supporting an arc in a plasma arc torch, comprising:
a holder defining a rear cavity therein, the rear cavity extending along a longitudinal
axis defined by the holder;
a relatively non-emissive member secured to the holder and at least partially exposed
to the rear cavity defined by the holder, the non-emissive member defining an opening
at least partially therethrough;
an emissive element positioned in the opening defined by the non-emissive member such
that the emissive element and the non-emissive member together define at least part
of a front face of the electrode for supporting an arc; and
a brazing material disposed between at least a portion of the emissive element and
the relatively non-emissive member.
39. An electrode according to Claim 38, wherein the relatively non-emissive member is
brazed to the holder.
40. An electrode according to Claim 38, wherein the non-emissive member is formed of silver
or alloys thereof and the brazing material is formed of silver or alloys thereof,
and the non-emissive member and brazing material have melting points that are approximately
the same.