U.S. patent application number 11/247613 was filed with the patent office on 2006-10-26 for plasma arc torch and method for improved life of plasma arc torch consumable parts.
Invention is credited to Kevin D. Horner-Richardson, Jesse A. Roberts, David A. Small.
Application Number | 20060237399 11/247613 |
Document ID | / |
Family ID | 26889164 |
Filed Date | 2006-10-26 |
United States Patent
Application |
20060237399 |
Kind Code |
A1 |
Horner-Richardson; Kevin D. ;
et al. |
October 26, 2006 |
Plasma arc torch and method for improved life of plasma arc torch
consumable parts
Abstract
A plasma arc torch and method for improving the life of the
consumable parts of the plasma are torch, including the electrode,
the tip and the shield cap. The method includes turbulating gas as
it flows over the exposed surfaces of the electrode, tip and shield
cap to increase turbulence in the hydrodynamic boundary layer of
the gas flow, thereby enhancing convective heat transfer. The
result of enhanced cooling is improved consumable parts life. For
example, to increase the turbulence of the gas flow over the outer
surface of the electrode, the plasma arc torch electrode has a
roughened, or textured outer surface formed with dimples, axially
extending grooves or spiraling grooves formed in the outer surface
of the electrode. The inner and outer surfaces of the tip and the
inner surface of the shield cap are similarly textured.
Inventors: |
Horner-Richardson; Kevin D.;
(Cornish, NH) ; Small; David A.; (Strafford,
VT) ; Roberts; Jesse A.; (Milton, VT) |
Correspondence
Address: |
HARNESS, DICKEY, & PIERCE, P.L.C
7700 BONHOMME, STE 400
ST. LOUIS
MO
63105
US
|
Family ID: |
26889164 |
Appl. No.: |
11/247613 |
Filed: |
October 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09821868 |
Mar 30, 2001 |
6987238 |
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11247613 |
Oct 11, 2005 |
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60193820 |
Mar 31, 2000 |
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60193602 |
Mar 31, 2000 |
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Current U.S.
Class: |
219/121.51 |
Current CPC
Class: |
H05H 1/3478 20210501;
H05H 1/34 20130101 |
Class at
Publication: |
219/121.51 |
International
Class: |
B23K 9/00 20060101
B23K009/00; B23K 9/02 20060101 B23K009/02 |
Claims
1. An improvement to a textured electrode of a plasma arc torch,
the improvement comprising a textured surface defining a smaller
number of deeper grooves over a length of the textured
electrode.
2. The textured electrode according to claim 1, wherein the
texturing is disposed along a lower end of the electrode.
3. The textured electrode according to claim 1, wherein the
texturing is disposed along a front face of the electrode.
4. An electrode for use in a plasma arc torch of the type having a
gas passage defined at least in part by an outer surface of the
electrode and a tip surrounding the electrode in spaced
relationship therewith and working gas flowing through the gas
passage, the gas passage defining a flow having a laminar boundary
layer attached to the outer surface of the electrode, wherein: the
outer surface of the electrode is textured to promote turbulence of
the working gas flowing in the laminar boundary layer while not
changing a flow pattern in a bulk region above the laminar boundary
layer, wherein the working gas within the laminar boundary layer is
turbulated.
5. The textured electrode according to claim 4, wherein the
textured outer surface is disposed along a front face of the
electrode.
6. In a textured electrode for use in a plasma arc torch, an
improvement comprising modifying grooves of the textured electrode
such that the textured electrode defines fewer grooves and deeper
grooves.
7. The improved textured electrode according to claim 6, wherein
the grooves are disposed along a front face of the electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation of U.S.
application Ser. No. 09/821,868 titled "Plasma Arc Torch and Method
for Improved Life of Plasma Arc Torch Consumable Parts," filed Mar.
30, 2001, the contents of which are incorporated herein by
reference in their entirety and continued preservation of which is
requested.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to plasma arc
torches and, in particular, to consumable parts utilized in plasma
arc torches and methods for improving the useful life of such
consumable parts.
[0003] Plasma arc torches, also known as electric arc torches, are
commonly used for cutting and welding metal workpieces by directing
a plasma consisting of ionized gas particles toward the workpiece.
In a typical plasma torch, a gas to be ionized is supplied to a
lower end of the torch and flows past an electrode before exiting
through an orifice in the torch tip. The electrode, which is a
consumable part, has a relatively negative potential and operates
as a cathode. The torch tip (nozzle) surrounds the electrode at the
lower end of the torch in spaced relationship with the electrode
and constitutes a relatively positive potential anode. The gas to
be ionized typically flows through the chamber formed by the gap
between the electrode and the tip in a generally swirling or
spiraling flow pattern. When a sufficiently high voltage is applied
to the electrode, an arc is caused to jump the gap between the
electrode and the torch tip, thereby heating the gas and causing it
to ionize. The ionized gas in the gap is blown out of the torch and
appears as an arc that extends externally off the tip. As the head
or lower end of the torch is moved to a position close to the
workpiece, the arc jumps or transfers from the torch tip to the
workpiece because the impedance of the workpiece to ground is made
lower than the impedance of the torch tip to ground. During this
"transferred arc" operation, the workpiece itself serves as the
anode. A shield cap is typically secured on the torch body over the
torch tip and electrode to complete assembly of the torch.
[0004] In addition to the electrode, other parts of the plasma are
torch are typically consumed during repeated operation of the
torch, including the torch tip and the shield cap surrounding the
tip. These consumable parts are consumed as a result of the
destructive effects of the high heat environment, and effective
management of the heat generated in and on these parts is critical
to improving the useful life of the consumable parts. For example,
heat is generated in the body of the electrode primarily by
interaction with the heated plasma at its front face. Additional
heat is generated in the electrode body by ohmic heating resulting
from current flow. All of this heat in the electrode must be
dissipated by conduction through the electrode body to a cooling
mechanism.
[0005] To this end, it is known to provide a fluid cooled plasma
are torch in which the electrode is cooled primarily by high
velocity plasma gas swirling through a plasma chamber formed by a
gap between the electrode and surrounding tip. Plasma gas is
directed over the outer surface of the electrode before it is
ionized and exits through the tip orifice. A similar condition
exists for the torch tip and the shield cap of a plasma arc torch.
Heat developed in the tip and the shield cap is dissipated by
convection to plasma gas flowing on the inside of the tip and by
convection to secondary gas flowing on the outside of the tip. It
is well established that cooling of the tip and the electrode
during operation of the torch improves the useful life of these
components.
[0006] Convective heat transfer (i.e., cooling) as discussed herein
is the mechanism of heat removal in which heat in a body is
deposited into fluid flowing over the surface of the body. The
effectiveness of the cooling fluid flowing over the surface is
referred to as the convective heat transfer coefficient h, which is
impacted by velocity of the fluid flow, turbulence of the fluid
flow, physical properties of the fluid, and interactions with
surface geometry. In any convective cooling approach, a consequence
of the fluid-surface interaction is the development of a region in
the fluid adjacent to the surface, through which the fluid flow
velocity varies from zero at the surface to a finite value
associated with the bulk fluid flow near the center of the flow
passage. This region is known as the hydrodynamic boundary layer.
As illustrated in FIG. 13, in fully developed turbulent flow this
boundary layer consists of three sublayers: a laminar sublayer
adjacent the surface, an intermediate buffer layer and a turbulent
outer layer. Heat transport across the laminar sublayer is
dominated by conduction, while heat transport in the intermediate
and turbulent layers is substantially augmented by the convective
motion of the eddies present in these layers. The overall effect is
that heat transfer from the surface to be cooled is substantially
increased by the presence of turbulence in the boundary layer.
Effective means for increasing convective heat transfer thus rely
on increasing turbulence and mixing in the boundary layer, either
by increasing the flow velocity or by promoting mixing or
turbulence in the boundary layer as illustrated in FIG. 14.
SUMMARY OF THE INVENTION
[0007] Among the several objects and features of the present
invention is the provision of a plasma are torch which enhances
convective cooling of the consumable parts of the torch; the
provision of such a torch in which the useful life of the
consumable parts is increased; and the provision of such a torch in
which the electrode is capable of a threadless quick
connect/disconnect connection with the cathode of the torch.
[0008] Among additional objects and features of the present
invention is the provision of a method which increases the useful
life of the consumable parts of a plasma arc torch; and the
provision of such a method which enhances convective cooling of the
consumable parts of the torch.
[0009] Other objects and features will be in part apparent and in
part pointed out hereinafter.
[0010] In general, a plasma arc torch of the present invention
comprises a cathode and an electrode electrically connected to the
cathode. A tip surrounds at least a portion of the electrode in
spaced relationship therewith to define a gas passage. The gas
passage is in fluid communication with a source of working gas for
receiving working gas into the gas passage such that working gas
within the gas passage swirls about the outer surface of the
electrode. The tip has a central exit orifice in fluid
communication with the gas passage. The outer surface of the
electrode is textured to promote turbulence of working gas flowing
over the outer surface of the electrode as working gas swirls
within the gas passage for enhancing convective cooling of the
electrode.
[0011] In another embodiment, a plasma arc torch of the present
invention comprises a cathode and an electrode electrically
connected to the cathode. A tip surrounds a portion of the
electrode in spaced relationship therewith to define a primary gas
passage. The primary gas passage is in fluid communication with a
source of primary working gas for receiving primary working gas
into the gas passage such that the primary working gas flows over
an inner surface of the tip in the gas passage. The tip has a
central exit orifice in fluid communication with the gas passage.
The inner surface of the tip is textured to promote turbulence of
the working gas flowing through the gas passage over the inner
surface of the tip for enhancing convective cooling of the tip.
[0012] In yet another embodiment, a plasma are torch of the present
invention comprises a cathode and an electrode electrically
connected to the cathode. A tip surrounds a portion of the
electrode in spaced relationship therewith to define a primary gas
passage. The primary gas passage is in fluid communication with a
source of primary working gas for receiving primary working gas
into the gas passage. The tip has a central exit orifice in fluid
communication with the gas passage. A shield cap surrounds the tip
in spaced relationship with an outer surface of the tip to define a
secondary gas passage for directing gas through the torch over the
outer surface of the tip. The shield cap has at least one opening
therein for exhausting gas in the secondary gas passage from the
torch. The outer surface of the tip is textured to promote
turbulence of the gas flowing through the secondary gas passage
over the outer surface of the tip for enhancing convective cooling
of the tip.
[0013] Another plasma arc torch of the present invention generally
comprises a cathode and an electrode electrically connected to the
cathode. A tip surrounds a portion of the electrode in spaced
relationship therewith to define a primary gas passage. The primary
gas passage is in fluid communication with a source of primary
working gas for receiving primary working gas into the gas passage.
The tip has a central exit orifice in fluid communication with the
gas passage. A shield cap surrounds the tip in spaced relationship
therewith to define a secondary gas passage for directing gas
through the torch over an inner surface of the shield cap. The
shield cap has at least one opening therein for exhausting gas in
the secondary gas passage from the torch. The inner surface of the
shield cap is textured to promote turbulence of the gas flowing
through the secondary gas passage over the inner surface of the
shield cap for enhancing convective cooling of the shield cap.
[0014] In general, an electrode of the present invention for use in
a plasma are torch of the type having a cathode, a gas passage
defined at least in part by the electrode and a tip surrounding the
electrode in spaced relationship therewith and working gas flowing
through the gas passage in a generally swirling direction about an
outer surface of the electrode generally comprises an upper end
adapted for electrical connection to the cathode. A lower end face
of the electrode has a recess therein. An insert constructed of an
emissive material is disposed in the recess of the lower end face.
A longitudinal portion of the electrode intermediate the upper end
and the lower end face of the electrode defines at least in part
the gas passage through which working gas flows in a generally
swirling direction about the electrode. The outer surface of the
longitudinal portion of the electrode is textured to promote
turbulence of the working gas swirling within the gas passage over
the outer surface of the longitudinal portion of the electrode.
[0015] A torch tip of the present invention for use in a plasma arc
torch of the type having a cathode, a primary gas passage defined
at least in part by an electrode electrically connected to the
cathode and the tip surrounding the electrode in spaced
relationship therewith and working gas flowing through the primary
gas passage generally comprises a lower end having a central exit
orifice in fluid communication with the primary gas passage for
exhausting working gas from the primary gas passage. An inner
surface of the torch tip is exposed for fluid contact by working
gas in the primary gas passage. The inner surface of the tip is
textured to promote turbulence of the gas flowing through the
primary gas passage over the inner surface of the tip for enhancing
convective cooling of the tip.
[0016] In another embodiment, a torch tip of the present invention
for use in a plasma torch similar to that above and further having
a shield cap surrounding at least a portion of the tip in spaced
relationship therewith to define a secondary gas passage through
which working gas flows generally comprises a lower end having a
central exit orifice in fluid communication with the primary gas
passage for exhausting working gas from the primary gas passage. An
outer surface of the torch tip is exposed for fluid contact by
working gas in the secondary gas passage. The outer surface of the
tip is textured to promote turbulence of the gas flowing through
the secondary gas passage over the outer surface of the tip for
enhancing convective cooling of the tip.
[0017] A shield cap of the present invention for use in a plasma
arc torch of the type having a cathode, a primary gas passage
defined at least in part by an electrode electrically connected to
the cathode and a tip surrounding the electrode in spaced
relationship therewith and working gas flowing through the primary
gas passage, with the shield cap surrounding at least a portion of
the tip in spaced relationship therewith to define a secondary gas
passage through which working gas flows, generally comprises a
lower end having at least one exhaust orifice in fluid
communication with the secondary gas passage for exhausting working
gas from the secondary gas passage. An inner surface of the shield
cap is exposed for fluid contact by working gas in the secondary
gas passage. The inner surface of the shield cap is textured to
promote turbulence of the gas flowing through the secondary gas
passage over the inner surface of the shield cap for enhancing
convective cooling of the shield cap.
[0018] A series of electrodes of the present invention generally
comprises at least two interchangeable electrodes, with each
electrode corresponding to a different current level at which the
torch is operable. The outer surface of each electrode is textured
to promote turbulence of the working gas flowing over the outer
surface of the electrode as working gas swirls about the electrode
in the gas passage. The cross-sectional area of the textured outer
surface of each electrode increases as the current level at which
the torch can be operated decreases to thereby decrease the
cross-sectional area of the gas passage as the current level
decreases.
[0019] A series of torch tips of the present invention generally
comprisesat least two interchangeable tips, with each tip
corresponding to a different current level at which the torch is
operable. The central exit orifice of the tips substantially
decreases as the current level at which the torch can be operated
decreases. Each tip has an inner surface defining an inner
cross-sectional area of the tip. The inner cross-sectional area of
the tips substantially increases as the current level at which the
torch can be operated decreases.
[0020] In general, a series of electrode and tip sets of the
present invention comprises a plurality of electrode and tip sets,
with each set corresponding to a different current level at which
the torch is operable. Each set comprises an electrode having a
textured outer surface to promote turbulence of the working gas
flowing over the outer surface of the electrode as the working gas
swirls about the electrode, and a tip. The size of the central exit
orifice of the tip decreases for each set as the current level at
which the torch is operable decreases. The electrode and tip of
each set are sized relative to each other such that the
cross-sectional area of the gas passage defined therebetween
decreases for each set as the current level at which the torch is
operable decreases.
[0021] A method of the present invention for improving the useful
life of an electrode used in a plasma are torch generally comprises
directing working gas through a gas passage defined by an electrode
and a tip surrounding the electrode for exhaust from the torch
through a central exit orifice of the tip. The working gas swirls
within the gas passage about the electrode to flow over an outer
surface of the electrode as it is directed through the gas passage
to define a hydrodynamic boundary layer generally adjacent the
outer surface of the electrode. The boundary layer includes a
turbulent outer layer. Gas is turbulated in the hydrodynamic
boundary layer generally adjacent the outer surface of the
electrode as gas is directed through the gas passage to increase
turbulent flow in the boundary layer for enhancing convective
cooling of the electrode thereby to improve the useful life of the
electrode.
[0022] A method of the present invention for improving the useful
life of a torch tip generally comprises directing working gas
through a secondary gas passage of the torch for exhaust from the
torch through at least one opening of the shield cap. The working
gas flows over an outer surface of the torch tip as it is directed
through the secondary gas passage to define a hydrodynamic boundary
layer adjacent the outer surface of the torch tip. The boundary
layer includes a turbulent outer layer. Gas is turbulated in the
hydrodynamic boundary layer adjacent the outer surface of the torch
tip as gas is directed through the secondary gas passage to
increase turbulent flow in the boundary layer for enhancing
convective cooling of the torch tip thereby to improve the useful
life of the torch tip.
[0023] A method of the present invention for improving the useful
life of a shield cap generally comprises directing working gas
through a secondary gas passage of the torch for exhaust from the
torch through the least one opening of the shield cap. The working
gas flows over an inner surface of the shield cap as it is directed
through the secondary gas passage to define a hydrodynamic boundary
layer adjacent the inner surface of the shield cap. The boundary
layer includes a turbulent outer layer. Gas is turbulated in the
hydrodynamic boundary layer adjacent the inner surface of the
shield cap as gas is directed through the secondary gas passage to
increase turbulent flow in the boundary layer for enhancing
convective cooling of the shield cap thereby to improve the useful
life of the shield cap.
[0024] A method of the present invention for improving the useful
life of an electrode or tip of a plasma arc torch generally
comprises texturing the surface of at least one of the electrode
and tip to promote turbulence of working gas flowing within the gas
passage over the textured surface of said at least one of the
electrode and tip. The method also includes changing the level of
electrical current supplied to the electrode. One or more of the
following parameters is modified in response to the change in
current: (1) the standard volumetric gas flow rate through said
annular gas passage, and (2) the dimensions of the annular gas
passage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a vertical section of a torch head of a plasma
torch with an electrode of the torch head shown in full;
[0026] FIG. 2 is an exploded vertical section of the plasma torch
head of FIG. 1;
[0027] FIG. 3 is an exploded perspective of the plasma torch head
of FIG. 1;
[0028] FIG. 4 is a section taken in the plane of line 4--4 of FIG.
1;
[0029] FIG. 5 is an expanded vertical section of a portion of the
torch head of FIG. 1 showing respective connecting ends of the
electrode and a cathode;
[0030] FIG. 6 is a vertical section of a torch head of plasma torch
of a second embodiment of the present invention;
[0031] FIG. 7 is an exploded vertical section of the plasma torch
head of FIG. 6;
[0032] FIG. 8 is an exploded perspective of the plasma torch head
of FIG. 6;
[0033] FIG. 9 is an expanded vertical section of a portion of the
torch head of FIG. 6 showing respective connecting ends of the
electrode and a cathode;
[0034] FIGS. 10a-c are elevations of various embodiments of the
electrode of the plasma arc torch of FIG. 1, with the outer surface
of the electrode textured in accordance with the present
invention;
[0035] FIG. 11 is vertical section similar to FIG. 1, with an outer
surface of the tip textured in accordance with the present
invention;
[0036] FIG. 11a is a vertical section similar to FIG. 11, with an
inner surface of the tip textured in accordance with the present
invention instead of the outer surface of the tip;
[0037] FIG. 12 is a partial section of another embodiment of a
torch head of a plasma arc torch of the present invention with an
inner surface of a shield cap textured in accordance with the
present invention;
[0038] FIG. 13 is a schematic illustration of a conventional
hydrodynamic boundary layer comprising a laminar sublayer,
intermediate buffer Payer and outer turbulent layer;
[0039] FIG. 14 is a schematic illustration of a hydrodynamic
boundary layer for flow over a textured surface such as the
electrode of FIGS. 10a-c; and
[0040] FIG. 15 is a table of data from an experiment illustrating
the increase in useful lifetime of an electrode consumable of the
present invention; and
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] With reference to the various drawings, and in particular to
FIG. 1, a torch head of a plasma torch of the present invention is
generally indicated at 31. The torch head 31 includes a cathode,
generally indicated at 33, secured in a torch body 35 of the torch
at an upper end of the torch head, and an electrode, generally
indicated at 37, electrically connected to the cathode. A central
insulator 39 constructed of a suitable electrically insulating
material, such as a polyamide or polyamide material, surrounds a
substantial portion of both the cathode 33 and the electrode 37 to
electrically isolate the cathode and electrode from a generally
tubular anode 41 that surrounds a portion of the insulator.
[0042] The cathode 33 and electrode 37 are configured for a coaxial
telescoping connection (broadly, a threadless quick
connect/disconnect connection) with one another on a central
longitudinal axis X of the torch. To establish this connection, the
cathode 33 and electrode 37 are formed with opposing detents
generally designated 43 and 45, respectively. As will be described
hereinafter, these detents 43, 45 are interengageable with one
another when the electrode 37 is connected to the cathode 33 to
inhibit axial movement of the electrode away from the cathode.
[0043] The cathode 33 is generally tubular and comprises a head 51,
a body 53 and a lower connecting end 55 adapted for coaxial
interconnection with the electrode 37 about the longitudinal axis X
of the torch. A central bore 57 extends longitudinally
substantially the length of the cathode 33 to direct a working gas
through the cathode. An opening 59 in the cathode head 51 is in
fluid communication with a source of primary working gas (not
shown) to receive working gas into the torch head 31. The bottom of
the cathode 33 is open to exhaust gas from the cathode. The cathode
33 of the illustrated embodiment is constructed of brass, with the
head 51, body 53 and lower connecting end 55 of the cathode
preferably being of unitary construction. However, it is understood
that the head 51 may be formed separate from the body 53 and
subsequently attached to or otherwise fitted on the cathode body
without departing from the scope of this invention.
[0044] Referring to FIGS. 1 and 3, the connecting end 55 of the
cathode 33 comprises a set of resilient longitudinally extending
prongs 61 defined by vertical slots 63 in the cathode extending up
from the bottom of the cathode. The prongs 61 have upper ends 65
integrally connected to the body 53 of the cathode 33 and free
lower ends 67 which are offset radially outwardly so that each
prong has an upper radial shoulder 69 and a lower radial shoulder
71. The prongs 61 are sufficiently resilient to permit generally
radial movement of the prongs between a normal, undeflected state
(FIGS. 2 and 5) and a deflected state (FIG. 1) in which the prongs
are deflected outward away from each other and the central
longitudinal axis X of the torch to increase the inner diameter of
the cathode connecting end 55 to enable the electrode 37 to be
inserted up into the cathode, as will be described. The radial
outward movement of the prongs 61 is permitted by an annular gap 73
formed between the connecting end 61 of the cathode 33 and the
central insulator 39.
[0045] In the preferred embodiment, the detent 43 on the cathode 33
comprises a cap 75 of electrically insulating material fitted on
the lower end 67 of each prong 61. Thus, it will be seen that the
detent 43 is on the connecting end 61 of the cathode 33 for
conjoint radial movement with the prongs between an undeflected and
deflected state. As best illustrated in FIG. 5, the cap 75 is
generally J-shaped in vertical section, comprising an outer wall
77, an inner wall 79 and a bottom wall 81 which define a recess 83
for receiving the offset lower end 67 of the prong 61. The outer
wall 77 of the cap 75 and the lower end 67 the prong 61 have a
tongue and groove connection for securely holding the cap on the
prong. Significantly, the thickness of the inner wall 79 below the
lower radial shoulder 71 of the prong 61 is greater than the width
of the lower radial shoulder of the prong so that a portion of the
inner wall projects radially inwardly beyond the lower shoulder to
define a generally radial detent surface 85 of the cathode detent
43. A sleeve 87 of electrically insulating material is disposed on
the inside of the cathode 33 at a location spaced above the radial
detent surfaces 85, leaving a portion of the inside wall of the
metal cathode exposed to function as an electrical contact surface
89 for the electrode 37. An inner edge 91 of the bottom of the
cathode 33 (e.g., of the insulating end caps 75) is tapered outward
to provide a cam surface engageable by the electrode 37 upon
insertion of the electrode into the cathode to initiate outward
displacement of the prongs 61 to their deflected state. The amount
of insertion force required to deflect the prongs 61 may vary, but
approximately 5 lbs. of axially directed force has been found to be
suitable.
[0046] The inner diameter D1 (FIG. 5) of the cathode 37 at the
contact surface 89 is preferably about 0.208 inches; the inner
diameter D2 of the cathode at the insulating end caps 75 is
preferably about 0.188 inches; and each radial detent surface 85
preferably projects radially inward from the contact surface
approximately 0.01 inches. However, it will be understood that
these dimensions may vary. Also, in the preferred embodiment the
connecting end 55 of the cathode 33 comprises four resilient prongs
61, but this number may vary from one prong to many prongs without
departing from the scope of this invention. Moreover, the radial
detent surfaces 85 may be formed in ways other than by the caps 75.
For example, the caps 75 may be eliminated entirely, and the detent
surfaces 85 may be formed by machined radial grooves or recesses
(not shown) in the prongs 61, or by otherwise forming radially
inwardly projecting surfaces (not shown) on the prongs.
[0047] Referring again to FIGS. 1 and 3, the electrode 37 is
generally cylindric and has a solid lower end 101, an upper
connecting end 105 adapted for coaxial telescoping connection with
the lower connecting end 55 of the cathode 33 about the
longitudinal axis X, and a gas distributing collar 103 intermediate
the upper and lower ends of the electrode. The electrode 37 of the
illustrated embodiment is constructed of copper, with an insert 107
of emissive material (e.g., hafnium) secured in a recess 109 in the
bottom of the electrode in a conventional manner. The gas
distributing collar 103 extends radially outward relative to the
upper and lower ends 105, 101 of the electrode 37, defining a
shoulder 111 between the gas distributing collar and the upper
connecting end of the electrode. A central bore 113 of the
electrode 37 extends longitudinally within the upper connecting end
105 generally from the top of the electrode down into radial
alignment with the gas distributing collar 103. It is understood
that the collar 103 may be other than gas distributing, such as by
being solid, whereby the gas is distributed in another manner,
without departing from the scope of this invention.
[0048] The central insulator 39 includes an annular seat 115
extending radially inward to define an inner diameter of the
central insulator that is substantially less than the outer
diameter of the gas distributing collar 103 such that the shoulder
111 formed by the gas distributing collar engages the annular seat
115 to limit insertion of the electrode 37 in the cathode 33 and
axially position the electrode in the torch head 31. The top of the
electrode 37 is open to provide fluid communication between the
cathode central bore 57 and the electrode central bore 113 upon
coaxial interconnection of the electrode and cathode 33. Opening
117 extend radially within the gas distributing collar 103 and
communicate with the central bore 113 in the electrode connecting
end 105 to exhaust working gas from the electrode 37.
[0049] With reference to FIG. 5, the outer diameter of the
electrode connecting end 105 is predominately of a diameter less
than the inner diameter D2 of the connecting end 55 of the cathode
33 at the insulating end caps 75 (e.g., at the cathode detent 43).
However, the detent 45 on the electrode 37 comprises an annular
protrusion 119 projecting generally radially outward from the
connecting end 145 of the electrode such that the outer diameter of
the electrode connecting end at the detent is substantially greater
than the diameter of the inner surface of the cathode, including
the cathode inner diameters D2 at the cathode detent 43 and D1 at
the contact surface 89 above the cathode detent. For example, the
electrode connecting end 105 of the illustrated embodiment
preferably has an outer diameter of about 0.182 inches; and the
outer diameter of the electrode connecting end at the electrode
detent 45 is preferably about 0.228 inches.
[0050] The annular protrusion 119 constituting the electrode detent
45 is preferably rounded to provide an upper cam surface 121
engageable with the tapered inner edge 91 of the bottom of the
cathode 33 to facilitate insertion of the electrode connecting end
105 into the cathode connecting end 55. The rounded protrusion 119
also includes a lower radial decent surface 123 engageable with the
radial detent surfaces 85 of the cathode detent 43 to inhibit axial
movement of the electrode connecting end 105 out of the cathode
connecting end 55. It is contemplated that the electrode detent 45
may be other than annular, such as by being segmented, and may be
other than rounded, such as by being squared or flanged, and remain
within the scope of this invention as long as the detent has a
radial detent surface engageable with the radial detent surfaces 85
of the cathode detent 43. It is also contemplated that the detent
may be formed separate from the electrode and attached or otherwise
connected to the electrode, and may further be resilient, and
remain within the scope of this invention. The axial position of
the detent 45 on the connecting end 105 of the electrode 37 may
also vary and remain within the scope of this invention, as long as
the length of the electrode connecting end 105 is sufficient such
that when the shoulder 111 of the gas distributing collar 103
engages the annular seat 115 of the central insulator 39, the
electrode detent is disposed in the cathode 33 above the cathode
detent 43 in electrical engagement with the contact surface 89 of
the cathode.
[0051] As shown in FIGS. 1-3, a metal tip 131, also commonly
referred to as a nozzle, is disposed in the torch head 31
surrounding a lower portion of the electrode 37 in spaced
relationship therewith to define a gap forming a gas passage 133
between the tip and the electrode. The gas passage 133 is further
defined by a tubular gas distributor 135 extending longitudinally
between the tip 131 and the gas distributing collar 143 of the
electrode 37 around the lower end of the electrode in radially
spaced relationship therewith. The gas distributor 135 regulates
the flow of working gas through the gas passage 133. The tip 131,
electrode 37 and gas distributor 135 are secured in axially fixed
position during operation of the torch by a shield cup 131
comprising an exterior housing 139 of heat insulating material,
such as fiberglass, and a metal shield insert 141 secured to the
interior surface of the housing. The exterior housing 139 has
internal threads (not shown) for threadable engagement with
corresponding external threads (not shown) on the torch body
35.
[0052] The lower end of the central insulator 39 is radially spaced
from the gas distributor 135 and the electrode gas distributing
collar 103 to direct gas flowing from the openings 117 in the
collar into a chamber 143 defined by the central insulator, gas
distributor, tip 131 and shield cup insert 141. The gas distributor
135 has at least one opening (not shown) in fluid communication
with both the gas passage 133 and the chamber 143 to allow some of
the gas in the chamber to flow into the gas passage and out of the
torch through an exit orifice 145 in the tip for use in forming the
plasma arc. In the illustrated embodiment, working gas is directed
by the gas distributor 135 to flow through the gas passage 133 in a
generally swirling or spiral direction about the electrode 37
(e.g., in a generally clockwise direction from the upper end to the
lower end of the gas passage) as indicated by the flow arrow in
FIG. 1. The remaining gas in the chamber 143 flows through an
opening 147 in the shield cap insert 141 into a secondary gas
passage 149 formed between the shield cap exterior housing 139 and
metal insert for exit from the torch through an exhaust opening 151
in the shield cap. The shield cap 137, tip 131, gas distributor 135
and electrode 37 are commonly referred to as consumable parts of
the torch because the useful life of these parts is typically
substantially less than that of the torch itself and, as such,
require periodic replacement. Operation of the plasma arc torch of
the present invention to perform cutting and welding operations is
well known and will not be further described in detail herein.
[0053] To assemble the plasma torch of the present invention, such
as when the consumable electrode 37 requires replacement, the
electrode of the present invention is inserted, upper connecting
end 105 first, into the torch head 31 up through the central
insulator 39. As the electrode connecting end 105 is pushed upward
past the annular seat 115 of the central insulator, the cam surface
121 of the detent 45 on the electrode engages the tapered inner
edges 91 of the insulating end caps 75 on the lower ends 67 of the
prongs 61. The cam surface 121 of the electrode detent 45 urges the
cathode prongs 61 outward to move the cathode detent 43 radially
outward to its deflected state against the inward bias of the
prongs, thereby increasing the inner diameter D2 of the cathode
connecting end 55 at the cathode detent to permit further
telescoping movement of the electrode connecting end 105 into the
cathode to a position in which the radial detent surface 123 of the
electrode detent 45 is above the radial detent surfaces 85 of the
cathode detent 43.
[0054] Once the electrode detent 45 is pushed upward past the
cathode detent 43, the electrode detent comes into radial alignment
with the contact surface 89 of the cathode connecting end 55 above
the detent surfaces 85 where the inner diameter D1 of the cathode
connecting end is greater than the inner diameter D2 at the cathode
detent. The cathode prongs 61, being in their deflected state,
create inward biasing forces that urge the prongs to spring or snap
inward to move the cathode detent 43 toward its undeflected state.
The metal contact surface 89 of the cathode connecting end 55 is
urged against the electrode detent 45 to electrically connect the
cathode 33 and electrode 37. Inward movement of the cathode detent
43 generally axially aligns (e.g., in generally overlapping or
overhanging relationship) the detent surface 123 of the electrode
connecting end 105 with the detent surfaces 85 of the cathode
connecting end 55. In other words, the electrode radial detent
surface 123 is aligned with the cathode radial detent surfaces 85
so that in the event the electrode 37 begins to slide axially
outward from the cathode 33 during assembly or disassembly, the
electrode radial detent surface 123 engages the radial detent
surfaces 85 to inhibit the electrode from failing out of the torch
head 31. Since the outer diameter D2 of the electrode connecting
end lay at the electrode detent 43 is greater than the inner
diameter of the cathode connecting end 55 at the contact surface
89, the cathode prongs 61 remain in a deflected state after
interconnection of the electrode 37 and cathode 33 to maintain the
biasing forces urging the prongs inward against the electrode
detent 45 for promoting good electrical contact between the cathode
and electrode.
[0055] To complete the assembly, the gas distributor 135 is placed
on the electrode 37, the tip 131 is placed over the electrode to
seat on the gas distributor, and the shield cap 137 is placed over
the tip and gas distributor and threadably secured to the torch
body 35 to axially .English Pound.x the consumable components in
the torch head 31. Upon securing the shield cap 137 to the torch
body 35, the shoulder 111 of the gas distributing collar 103 of the
electrode 37 engages the annular seat 115 of the central insulator
39 to properly axially position the electrode in the torch
head.
[0056] To disassemble the torch, the shield cap 137 is removed from
the torch body 35 and the tip 131 and gas distributor 135 are slid
out of the torch. The electrode 37 is disconnected from the cathode
37 6y pulling axially outward on the lower end 101 of the
electrode. The electrode detent surface 123 engages the detent
surfaces 85 of the cathode detent 43 and, with sufficient axial
pulling force, the electrode detent surface urges the cathode
prongs 61 outward to move the cathode detent 43 further toward its
deflected state to allow withdrawal of the electrode connecting end
105 from the connecting end 55 of the cathode 33. The rounded
detent surface 123 of the annular protrusion 119 facilitates the
outward movement of the prongs 61 upon engagement with the detent
surfaces 85 of the cathode detent 43.
[0057] As illustrated in FIGS. 1-5 and described above, the plasma
torch of this first embodiment incorporates an interconnecting
cathode 33 and electrode 37 in which the electrode is inserted into
the cathode. Alternatively, the electrode 37 may instead be sized
and configured for surrounding the cathode 33, with the electrode
detent 45 extending radially inward from the electrode connecting
end 105 and the cathode detent 43 projecting radially outward from
the cathode connecting end 55 such that the cathode prongs 61 are
deflected inward upon relative telescoping movement of the cathode
and electrode.
[0058] FIGS. 6-9 illustrate a second embodiment of a plasma torch
of the present invention in which an electrode 237 (as opposed to
the cathode 33 of the first embodiment) has a connecting end 305
comprising resilient longitudinally extending prongs 361. As with
the first embodiment described above, the torch of this second
embodiment includes a cathode, generally indicated at 233, the
electrode 237, a central insulator 239, a gas distributor 335, a
tip 331 and a shield cap 337. The electrode 237 is configured for
coaxial telescoping insertion into the cathode 233 on a
longitudinal axis X of the torch for electrical connection with
cathode (again referred to broadly as a threadless quick
connect/disconnect connection).
[0059] In this second embodiment, the central insulator 239 and
electrode 237 are formed with radially opposed detents, generally
designated 243 and 245, respectively. These detents 243, 245 are
interengageable with one another when the electrode 237 is inserted
in the torch head 231 to inhibit axial movement of the electrode
relative to the central insulator outward from the torch.
[0060] As shown in FIG. 6, the cathode 233 is substantially similar
to the cathode 33 of the first embodiment, comprising a head 251, a
body 253 and a lower connecting end 255. A central bore 257 extends
longitudinally substantially the entire length of the cathode 233
to direct a working gas through the cathode. The connecting end 255
of the cathode 233 is generally of rigid construction and is formed
of brass, free of the electrically insulating sleeve 87 and end
caps 75 described above in connection with the first embodiment.
The diameter of the inner surface of the cathode connecting end 255
is jogged outward to define a shoulder 256 (FIG. 9) for seating a
plug 351 in the connecting end. The plug 351 is generally cylindric
and has a head 353 sized for seating in the connecting end 255 of
the cathode 233 up against the shoulder 256 in frictional
engagement with the inner surface of the cathode connecting end to
secure the plug in the cathode. A body 355 of the plug 351 extends
down from the head and has a substantially smaller diameter than
the head so that the outer surface of the body is spaced radially
inward from the cathode connecting end 255. The inner surface of
the connecting end 255 jogs further outward below the shoulder 256
and head 353 of the plug 351 and defines a contact surface 289 of
the cathode connecting end for electrical contact with the
electrode. The radial spacing between the contact surface 289 and
the plug body 351 defines an annular gap or recess 357 sized for
receiving the electrode connecting end 305 therein in electrical
contact with the contact surface 289 of the cathode connecting end
255. A lower end 359 of the plug body 351 tapers inward to define a
cam surface for urging the electrode connecting end 255 to seat in
the recess 357 in electrical contact with the contact surface
289.
[0061] The electrode 237 of this second embodiment is generally
cylindric and has a solid lower end 301, an upper connecting end
305 adapted for coaxial telescoping insertion in the cathode
connecting end 255 and interconnection with the central insulator
239 about the longitudinal axis X, and a collar 303 intermediate
the upper and lower ends of the electrode. The electrode 237 of the
illustrated embodiment is constructed of copper, with an insert
(not shown but similar to insert 107 of the first embodiment) of
emissive material (e.g., hafnium) secured in a recess (not shown
but similar to recess 109 of the first embodiment) in the bottom of
the electrode in a conventional manner. The collar 303 extends
radially outward relative to the upper and lower ends 305, 301 of
the electrode 237, thus defining a shoulder 311 between the collar
and the upper connecting end of the electrode. A central bore 313
extends longitudinally within the upper connecting end 305 of the
electrode 237 generally from the top of the electrode down into
radial aliment with the collar 303 of the electrode. The top of the
electrode 237 is open to provide fluid communication between the
cathode central bore 257 and the electrode central bore 313 upon
insertion of the electrode 237 in the cathode 233.
[0062] Referring to FIGS. 6 and 7, the upper connecting end 305 of
the electrode 237 comprises a set of resilient longitudinally
extending prongs 361 defined by vertical slots 363 in the electrode
connecting end extending generally the length of the central bore
313 of the electrode. These vertical slots 363 also exhaust working
gas from the electrode connecting end 305 in a manner substantially
similar to the openings 117 of the gas distributing collar 103 of
the first embodiment described above. The prongs 361 have lower
ends 365, integrally connected to the collar 303 of the electrode
237, and free upper ends 367. The prongs 361 are sufficiently
resilient to permit generally radial movement of the prongs between
a normal, undeflected state and a deflected state in which the
prongs are deflected inward toward each other and the central
longitudinal axis X of the torch to decrease the diameter of the
electrode connecting end 305 to enable insertion of the electrode
connecting end up into the cathode connecting end 255, as will be
described.
[0063] In the preferred embodiment, the electrode detent 245
comprises a radial projection 369 integrally formed with each prong
361 and extending radially outward from the free upper end 367 of
each prong. Thus, it will be seen that the detent 245 is on the
connecting end 305 of the electrode 237 for conjoint radial
movement with the prongs 361 between an undeflected and deflected
state. Each projection 369 is substantially square or rectangular
in cross-section (FIG. 9) to define an upper surface 371, a lower
radial detent surface 373 and an outer contact surface 375 for
electrical contact with the contact surface 289 of the cathode
connecting end 255. It is understood, however, that the shape of
the detent 245 may vary without departing from the scope of this
invention, as long as the detent has a lower radial detent surface
373 extending generally radially outward from the connecting end
305 of the electrode 237 and the electrode is capable of electrical
connection with the cathode 239. Also, in the preferred embodiment
the connecting end 305 of the electrode 237 comprises four
resilient prongs 361, but this number may vary from one prong to
many prongs without departing from the scope of this invention.
[0064] The central insulator 239 of this second embodiment includes
an annular seat 315 extending radially inward to a diameter
substantially less than the outer diameter of the electrode collar
303 such that the shoulder 311 formed by the collar engages the
annular seat to limit insertion of the electrode 237 in the cathode
233 and axially position the electrode in the torch head 231. The
detent 243 on the central insulator 239 is formed by an annular,
radially inward extending protrusion 381 located between the bottom
of the cathode 239 and the annular seat 315 of the central
insulator. As shown in the illustrated embodiment, the detent 243
is preferably positioned adjacent the bottom of the cathode 233. At
the lower end of the protrusion 381, the inner diameter of the
central insulator tapers inward to define a cam surface 383 for
initiating inward deflection of the electrode prongs 361 to their
deflected state upon insertion of the electrode through the central
insulator 239. The inner diameter of the central insulator 239
tapers back outward at the upper end of the detent 243 to define a
radial detent surface 385 of the central insulator in generally
radially and axially opposed relationship with the electrode detent
surface 373. The tapered detent surface 385 of the central
insulator detent 243 also provides a earn surface for deflecting
the electrode prongs 361 inward to facilitate withdrawal of the
electrode 237 from the cathode 233 upon disassembly of the torch.
The detent surface 385 of the central insulator 239 preferably
tapers outward to a diameter equal to or slightly less than the
inner diameter of the contact surface 289 of the cathode connecting
end 255 to guide insertion of the electrode connecting end 305 into
the cathode connecting end when installing the electrode 237 in the
torch.
[0065] As seen best in FIG. 9, the electrode detent 245 is sized
diametrically larger than the inner diameter of the contact surface
289 of the cathode connecting end 255 so that after insertion of
the electrode 237 through the central insulator 239 and into the
cathode connecting end, the prongs 261 and detent of the electrode
will remain in an inward deflected state. The inward deflected
prongs 361 create a biasing force that urges the prongs outward,
thereby urging the electrode detent 245 to move radially outward
into electrical engagement with the contact surface 289 of the
cathode connecting end 255 to electrically connect the electrode
237 and cathode 233.
[0066] To assemble the plasma torch of the second embodiment, the
electrode 237 is inserted, upper connecting end 305 first, into the
torch head up through the central insulator 239. As the electrode
connecting end 305 is pushed past the annular seat 315 of the
central insulator 239, the upper surfaces 371 of the radial
projections 369 on the prongs 361 of the electrode 237 engage the
tapered lower cam surface 383 of the central insulator detent 243.
The cam surface 383 urges the electrode prongs 361 inward against
the outward bias of the prongs to radially move the electrode
detent 245 inward to its deflected position, thereby decreasing the
outer diameter of the electrode connecting end 305 at the electrode
detent to permit further insertion of the electrode connecting end
through the central insulator 239 and into the cathode connecting
end 255 to a position in which the radial detent surfaces 373 of
the electrode detent 245 are above the radial detent surface 385 of
the central insulator detent 243.
[0067] Once the electrode detent 245 is pushed upward past the
central insulator detent 243 and into the cathode connecting end
255, the electrode detent 243 comes into radial alignment with the
contact surface 289 of the cathode connecting end 55 where the
inner diameter of the cathode connecting end is greater than the
inner diameter at the central insulator detent. The electrode
prongs 361, being in their deflected state, create outward biasing
forces that urge the prongs outward to move the electrode detent
243 toward its undeflected state. The outer contact surfaces 375 of
the radial prong projections 369 are urged outward against the
contact surface 289 of the cathode connecting end 289 to
electrically connect the cathode 233 and electrode 237. Outward
movement of the electrode detent 243 generally axially aligns
(e.g., in overlapping or overhanging relationship) the detent
surfaces 373 of the electrode connecting end 305 with the detent
surface 385 of the central insulator 289. In other words, the
electrode radial detent surfaces 373 are aligned with the central
insulator detent surface 385 so that in the event the electrode 237
begins to slide axially outward from the torch head 231 during
assembly or disassembly, the electrode radial detent surfaces 373
engage the radial detent surface 385 of the central insulator 239
to inhibit the electrode from falling out of the torch head 31.
[0068] Since the outer diameter of the electrode connecting end 305
at the detent 243 is greater than the inner diameter of the cathode
connecting end 255 at the contact surface 289, the electrode prongs
361 remain in an inward deflected state after insertion of the
electrode 237 in the cathode 233 to maintain the biasing forces
urging the electrode detent 245 outward against the cathode contact
surface for promoting good electrical contact between the cathode
233 and electrode. Where slight permanent inward deformation of an
electrode prong 361 is present, the outward bias of the prong may
not be sufficient to urge the electrode detent 245 into electrical
contact with the cathode contact surface 289. In that case, the
upper surface 371 of the radial projection 369 on the deformed
prong 361 will engage the tapered lower end 359 of the plug body
355 upon insertion of the electrode connecting end 305 into the
cathode connecting end 255. The tapered lower end 359 provides a
cam surface that urges the electrode prong 361 outward, thereby
moving the electrode detent radially outward to seat in the recess
357 between the plug body 355 and the contact surface 289 with the
prong projections 369 in electrical engagement with the contact
surface.
[0069] To complete the assembly, the gas distributor 235 is placed
on the electrode 237, the tip 231 is placed over the electrode to
seat on the gas distributor, and the shield cap 237 is placed over
the tip and gas distributor and threadably secured to the torch
body 235 to axially fix the consumable components in the torch head
231. Upon securing the shield cap 237 to the torch body 235, the
shoulder 311 of the collar 303 of the electrode 237 engages the
annular seat 315 of the central insulator 239 to properly axially
position the electrode in the torch head.
[0070] To disassemble the torch, the shield cap 237 is removed from
the torch body 235 and the tip 231 and gas distributor 235 are slid
out of the torch. The electrode 237 is removed from the torch by
pulling axially outward on the lower end 301 of the electrode. The
electrode detent surfaces 373 engage the tapered detent surface 385
of the central insulator detent 243 and, with sufficient axial
pulling force, the tapered detent surface urges the electrode
prongs 361 further inward to move the electrode detent 245 further
toward its deflected state to allow withdrawal of the electrode
connecting end 305 from the central insulator 239.
[0071] As illustrated in this second embodiment, the plasma torch
of the present invention incorporates an electrode 237 and central
insulator 239 having interengageable detents 245, 243 for
inhibiting axial movement of the electrode outward from the torch
during assembly of the torch. However, it is understood that
instead of the detent 243 extending radially from the central
insulator 239, the detent may instead extend radially from the
inner surface of the cathode connecting end 255 in a manner similar
to that described above with respect to the first embodiment,
without departing from the scope of this invention. Also, the
electrode 237 may instead be sized and configured for surrounding
the cathode 233, with the electrode detent 245 extending radially
inward from the electrode connecting end 305 and a corresponding
detent extending radially outward from the cathode connecting end
255 such that the electrode prongs 361 are deflected outward upon
relative telescoping movement of the cathode and electrode.
[0072] Now referring to FIGS. 10a-c, in accordance with the present
invention the electrode 37 of the plasma are torch of the first
embodiment (FIGS. 1-5) has a roughened, or textured outer surface
76 along substantially the entire length of the portion of the
electrode that partially defines (along with the torch tip) the gas
passage 133. The textured outer surface 76 of the electrode 37 may
be formed by circular depressions or dimples (indicated as 80 in
FIG. 10a), similar to those formed in the outer cover of a golf
ball, or by axially extending grooves (indicated as 82 in FIG. 10b)
or by one or more spiral, thread-like grooves (indicated as 84 in
FIG. 10c) in the outer surface of the electrode. The axially
extending grooves 82 of the electrode 37 of FIG. 10b and the spiral
grooves 84 of the electrode 37 of FIG. 10c are sized and oriented
for turbulating working gas swirling about the outer surface of the
electrode in the gas passage 133. As an example, the electrode 37
of FIG. 10b has a textured outer surface 76 formed by about 12-14
axially extending grooves 82 spaced equally about the outer surface
of the electrode, with each groove having a depth of approximately
0.015 inches. It has been found that forming the textured surface
by providing a smaller number of deeper grooves 82 is generally
preferred over a textured surface formed by providing a greater
number of shallower grooves since the deeper grooves are more
capable of turbulating working gas flowing over the outer surface
of the electrode.
[0073] The spiral grooves 84 of the textured surface 76 of the
electrode 37 of FIG. 10c also have a depth of about 0.015 inches.
The spiral grooves 84 extend downward within the outer surface of
the electrode 37 in a direction crosswise, or counter, to the
direction that working gas swirls about the electrode within the
gas passage 133. The pitch of each spiral groove 84 is preferably
equal to or less than the pitch of the swirling gas within the gas
passage 133 so that the longitudinal component of each groove is at
least as great as, or preferably greater than, the longitudinal
component of the swirling gas in the gas passage.
[0074] The grooves 82, 84 of the electrode 37 of FIGS. 10b, 10c may
be formed by various methods, such as by knurling, molding or
machining the grooves in the outer surface of the electrode. For
example, the axially extending grooves 82 of the textured surface
76 of the electrode 37 of the embodiment of FIG. 10b are preferably
formed by knurling the outer surface of the electrode. It is
understood that the textured outer surface 76 may be formed other
than as illustrated in FIGS. 10a-c without departing from the scope
of this invention. Also, while the textured electrode 37 of the
present invention is shown and described herein as being used in
connection with the plasma arc torch of the first embodiment (FIGS.
1-5), it is understood that the textured electrode may be used in
other plasma arc torches in which gas is directed through a gas
passage 133 in a generally swirling direction, without departing
from the scope of this invention.
[0075] In accordance with a method of the present invention for
improving the useful life of consumable parts of a plasma arc
torch, primary working gas is directed to flow downward through the
gas passage 133 in a swirling motion about the electrode 37,
flowing over the textured outer surface 76 of the electrode. As
with any fluid flow in an annular passageway, a hydrodynamic
boundary layer (FIG. 13) is established on the outer surface 76 of
the electrode 37. As the gas flows over the textured outer surface
76 of the electrode 37, the gas is tumbled or turbulated in the
boundary layer (FIG. 14) to increase turbulence in the boundary
layer near the outer surface of the electrode, thereby improving
the cooling effectiveness of the gas. Providing the textured outer
surface 76 of the electrode 37 to promote turbulence of the gas
swirling within the gas passage has been found to substantially
increase the useful life of an electrode. In particular, it has
been found that for a torch in which the working gas flows through
the gas passage 133 in a swirling direction (e.g., clockwise from
the upper end to the lower end of the gas passage as illustrated in
FIG. 1), the textured outer surface 76 of the electrode 37 is
preferably formed to extend within the outer surface of the
electrode in a direction other than the direction that working gas
swirls about the electrode within the gas passage 133. For example,
the axially extending grooves 82 of the electrode 37 of FIG. 10b
are oriented generally crosswise to the direction of swirling gas
in the gas passage 133. As another example, the spiral grooves 84
of the electrode 37 of FIG. I Oc spiral within the outer surface of
the electrode in the direction crosswise, or counter (e.g., in a
counter-clockwise direction) to the direction of swirling gas
within the gas passage 133.
[0076] It has also been found that under the conditions that exist
inside the gas passage 133, convective cooling of the textured
electrode 37 and the tip 131 generally increases with the flow
velocity through the annular gas passage between the outer diameter
of the electrode and the inner diameter of the tip. The gas flow
velocity is generally directly proportional to the volumetric flow
rate of the gas through the torch and generally inversely
proportional to the dimensions that define the annular space
forming the gas passage 133 between the tip 131 and the electrode
37. Thus, to further enhance consumable life (i.e. the useful or
working lives of the electrode 37 and tip 131), the beneficial
affect derived from the textured surface 76 may be augmented by
increasing volumetric flow rates and/or by decreasing the
cross-sectional area of the gas passage 133 defined by the
electrode and tip. Increasing the volumetric flow rate and/or
decreasing the cross-sectional area of the annular gas passage 133
will tend to increase the flow velocity of the gas flowing through
the gas passage. The cross-sectional area of the gas passage 133
may be decreased by increasing the outside diameter of the
electrode (e.g., by increasing the cross-sectional area of the
outer surface of the electrode) and/or by decreasing the inside
diameter of the tip (e.g, by decreasing the cross-sectional area of
the inner surface of the tip) to narrow the gap between the two
parts.
[0077] By way of example, the volumetric flow rate for the torch of
the present invention is preferably reduced, along with the
diameter of the exit orifice 145 of the tip 131, as the current
level at which the torch is operated is reduced. Absent a
corresponding decrease in the cross-sectional area of the gas
passage 133, the gas flow velocity in the gas passage would be
substantially reduced at lower volumetric flow rates, resulting in
decreased cooling of the consumable parts. This decrease in cooling
can be avoided by using the textured electrode 37 in combination
with a higher volumetric flow rate or, more preferably, a reduced
size of the cross-sectional area of the gas passage 133 defined by
the electrode and tip 131 to provide higher flow velocity in the
gas passage for greater cooling, or a combination of both. However,
it has been found that where a non-textured electrode is used,
increasing the flow velocity of the gas swirling within the gas
passage 133 by decreasing the cross-sectional area of the gas
passage provides little or no improvement in the useful life of the
non-textured electrode, and may even decrease its useful life.
EXPERIMENT
[0078] An experiment was conducted in which a series of tests were
performed using the plasma are torch shown in FIGS. 1-5 and
described above. For each test, the torch was fitted with an
electrode 37 and a tip 131 and operated at a predetermined current
level, such as 80 amps or 40 amps, and a predetermined standard
volumetric flow rate corresponding to the current level at which
the torch was operated, such as 90 standard cubic ft./hr. and 50
standard cubic ft./hr., respectively. As used herein, the standard
volumetric flow rate is measured using a conventional gas turbine
meter positioned at the, exit of the tip 131 at atmospheric
pressure and room temperature. In accordance with conventional
plasma arc torch design, the central exit orifice 145 of the tip
131 used for operating the torch at 80 amps (e.g., about 0.455
inches) was greater than the central exit orifice of the tip used
for operating the torch at 40 amps (e.g., about 0.031 inches).
[0079] For each test, the outer diameter (e.g., outer surface) of
the electrode 37 and the inner diameter (e.g., inner surface) of
the tip 131 were sized relative to each other to obtain a different
cross-sectional area of the gas passage 133 formed between the
electrode and the tip. In effect, varying the cross-sectional area
of the gas passage 133 resulted in variance of a standard flow
velocity of working gas swirling within the gas passage 133 about
the outer surface of the electrode 37. As used herein, the standard
flow velocity is a calculated velocity obtained by dividing the
standard volumetric flow rate by the cross-sectional area of the
gas passage. The cross-sectional area of the gas passage 133 as
used herein is calculated based on the outermost diameter of the
electrode 37 and does not reflect any additional spacing between
the electrode and the tip 131 resulting from the grooves 82 formed
in the outer surface of the electrode.
[0080] One set of tests was run at a current level of 80 amps using
electrodes 37 having axially extending grooves 82 in their outer
surface, with each groove having a depth of about 0.415 inches. A
similar set of tests was run at a current level of 40 amps. For
further comparison purposes, a third set of tests was run at a
current level of 80 amps using nontextured electrodes and a fourth
test was run at a current level of 80 amps using an electrode (not
shown) having grooves (not shown) extending substantially
circumferentially within its outer surface (e.g., by forming a
threaded outer surface having a high pitch, such as about 20
threads/inch to approximate circumferentially oriented
grooves).
[0081] Each test comprised repeated operation of the torch through
a working cycle including starting the torch, piercing a metal
workpiece, cutting the workpiece and shutting off the gas flow
through the torch. The duration of each working cycle was 11
seconds. Operation of the torch was repeated until a catastrophic
failure of the electrode resulted in the torch becoming inoperable
without replacement of the electrode. The number of working cycles
completed before failure of the electrode was recorded as the
useful lifetime of the electrode. The useful lifetime data reported
in the table of FIG. 15 is based on conducting each test three
times and averaging the resultant useful lifetime data.
[0082] According to the results of the experiment, the useful
lifetime of the textured electrode 37 incorporated in the torch
operated at a current level of 80 amps generally increased with the
increased standard flow velocity resulting from decreasing the
cross-sectional area of the gas passage 133 between the electrode
and the tip 131 while holding constant the current level and the
standard volumetric flow rate. While not as pronounced, the useful
lifetime of the textured electrode 37 incorporated in the torch
operated at 40 amps also generally increased with the increased
standard flow velocity resulting from decreasing the
cross-sectional area of the gas passage 133 while holding constant
the current level and the standard volumetric flow rate.
[0083] However, the test results also suggest that when a
non-textured electrode is used in the torch, increasing the
standard flow velocity of working gas swirling within the gas
passage 133 has little or no effect on, or more particularly may
actually decrease, the useful lifetime of the electrode where the
current level and the standard volumetric flow rate are held
constant. Consequently, the resultant advantages obtained by
increasing the standard flow velocity of working gas swirling
within the gas passage (e.g., by decreasing the cross-sectional
area of the gas passage) are achieved in combination with using a
textured electrode 37 capable of turbulating the gas flowing over
the outer surface of the electrode.
[0084] Also, where the electrode having substantially
circumferential grooves was incorporated in the torch the useful
lifetime of the electrode was substantially less than that of
textured electrodes 37 tested at similar standard flow velocities
and the same current level and standard volumetric flow rate. Thus,
for a plasma arc torch in which the working gas swirls within the
gas passage 133 about the electrode 37, the longitudinally
extending grooves yield a noticeably greater useful lifetime of the
electrode than substantially circumferentially oriented
grooves.
[0085] Comparing the data obtained for tests in which the torch was
operated at a current level of 80 amps with the tests in which the
torch was operated at a current level of 40 amps, it can be seen
that the standard flow velocity, and accordingly the useful
lifetime of the textured electrode 37, increased for the torch
operated at 40 amps by decreasing the cross-sectional area of the
gas passage 133 along with the current level and standard
volumetric flow rate. Thus, the decrease in standard volumetric
flow rate conventionally associated with the decrease in current
level is overcome by decreasing the cross-sectional area of the gas
passage 133 to maintain a desired standard flow velocity in the gas
passage. For example, the cross-sectional area of the gas passage
133 is preferably sized for a given current level at which the
torch is operated such that the standard gas flow velocity in the
gas passage is at least about 140 ft/sec, more preferably at least
about 164 ft/sec, and most preferably at least about 194
ft/sec.
[0086] Therefore, in accordance with a further aspect of this
invention, a series of electrodes 37 may be provided wherein each
electrode corresponds to a different current level and is has a
textured surface 76, such as by having grooves 82 (FIG. 10b)
extending axially therein, to promote turbulence of working gas
flowing over the outer surface of the electrode as the working gas
swirls within the gas passage. More particularly, the outer
diameter (e.g., outer surface) of the electrode 133 is increased,
or stated more broadly, the cross-sectional area of the electrode
is increased, as the current level at which the torch is operated
decreases. By increasing the cross-sectional area of the electrode
37, the cross-sectional area of the gas passage 133 is
correspondingly decreased as the current level decreases to
maintain the desired standard flow velocity in the gas passage.
[0087] In an alternative embodiment, a series of tips 131 may be
provided for a torch having a textured electrode 37 capable of
turbulating gas swirling within the gas passage 133 about the outer
surface of the electrode. Each of the tips 131 corresponds to a
current level at which the torch may be operated. More
particularly, the central exit orifice 145 of the tip 131 is
decreased as the current level at which the torch operates
decreases. The inner diameter (e.g., inner surface) of the tip 131
is decreased, so that the cross-sectional area of the gas passage
133 is correspondingly decreased, as the current level at which the
torch is operated decreases to maintain the desired standard flow
velocity in the gas passage.
[0088] In another embodiment, a series of electrode 37 and tip 131
sets can be provided, with each set including an electrode having a
textured outer surface 76 and one tip. Each set corresponds to a
particular current level at which the torch may be operated. The
central exit orifice 145 of the tip 131 is decreased as the current
level at which the torch operates decreases. The electrode 37 outer
diameter and tip 131 inner diameter are sized relative to each
other such that the cross-sectional area of the gas passage 133 is
correspondingly decreased as the current level at which the torch
is operated decreases to generally maintain the desired standard
flow velocity in the gas passage.
[0089] Thus, these sets are designed so that the dimensions of the
gas passage 133 for each set decreases as the current level
(amperage) decreases. Thus, if the standard volumetric flow rate is
decreased at lower current levels, the decreased dimensions of the
gas flaw passage 133 will result in a higher standard flow velocity
within the gas passage for good cooling even at the lower standard
volumetric flow rates. The cross-sectional area of the annular gas
passage 133 of each set can be varied by changing the dimensions of
either or both the electrode 37 and tip 131 to correspond to the
desired standard flow velocity through the gas passage for
increasing the useful lifetime of the electrode.
[0090] FIG. 11 illustrates the torch head 31 of the plasma arc
torch of FIG. 1 with an outer surface 90 of the torch tip 131 being
roughened or otherwise textured in accordance with the present
invention. In this embodiment, convective cooling of the torch tip
131 is accomplished by directing a flow of non-swirling gas through
the secondary gas passage 149 over the textured outer surface 90 of
the tip. It is understood, however, that the gas in the secondary
gas passage may instead have a swirling motion without departing
from the scope of this invention. The textured outer surface 90 of
the tip 131 may be formed by generally concentric grooves 92 in the
outer surface of the tip and spaced at intervals along the surface
or by one or more spiral grooves (not shown), oriented either
clockwise or counterclockwise, in the tip outer surface so that the
grooves are in a generally crosswise orientation relative to the
gas flowing through the secondary gas passage 149.
[0091] FIG. 11a illustrates the torch head 31 of FIG. 11 with an
inner surface 94 of the torch tip 131 being roughened or otherwise
textured in accordance with the present invention. In this
embodiment, convective cooling of the torch tip 131 is accomplished
by directing gas to flow down through the gas passage 133 in a
generally swirling direction over the textured inner surface 94 of
the tip. The textured inner surface 94 of the tip 131 may be formed
by axially extending grooves 96 in the inner surface o.English
Pound. the tip, or by dimples (not shown but similar to the dimples
80 of the electrode 37 of FIG. 10a) or one or more spiral grooves
(not shown but similar to the grooves 84 in the electrode 37 of
FIG. I Dc. In this manner the axially extending grooves 96 or
spiral grooves are oriented generally crosswise relative to the
direction that gas swirls about the electrode within the gas
passage 133 over the inner surface of the tip.
[0092] FIG. 12 illustrates another embodiment of a torch head 431
of a plasma are torch of the present invention. This torch is of a
dual-gas type in which a secondary working gas, separate from the
primary working gas, is utilized during operation of the torch. In
this torch, primary working gas enters the torch at an inlet 494
and is directed into and through the gas passage 433 formed by the
electrode 437 and tip 531 before being exhausted from the torch
through the central exit orifice 566 of the tip. The torch head 431
includes a shield cap assembly 596 comprising a shield cap 539
generally surrounding the torch tip 531 in spaced relationship
therewith to partially define a secondary gas passage 549. The
assembly 596 also includes a retainer 598 for use in securing the
shield cap assembly to the torch body 600. Secondary working gas is
received in the torch head 431 via a second inlet 602 and is
directed through the torch to the secondary gas passage 549 for
exhaust from the torch via a central exhaust opening 551 of the
shield cap 539.
[0093] As shown in FIG. 12, an inner surface 604 of the shield cap
539 is roughened or otherwise textured in accordance with the
present invention. Convective cooling of the shield cap 539 of the
illustrated embodiment is accomplished by directing non-swirling
secondary working gas through the secondary gas passage 549 in a
generally axial direction over the inner surface 604 of the shield
cap 539. However, it is understood that secondary gas may flow
through the secondary gas passage in a generally swirling motion
without departing from the scope of the invention. The textured
inner surface 644 of the shield cap 539 may he formed by concentric
grooves 606 in the inner surface of the cap and spaced at intervals
along the inner surface or by one or more spiral grooves (not
shown), oriented either clockwise or counterclockwise, such that
the grooves have a generally crosswise orientation relative to the
flow of secondary working gas through the secondary gas passage
549.
[0094] While the textured surfaces of the consumable parts of the
torch are generally shown and described above as being formed by
cutting into the surface of the consumable part, it is understood
that the textured surface may be formed by raising the surface of
the part, such as by forming bumps, fins or other suitable
formations on the surface of the part, without departing from the
scope of this invention.
[0095] The embodiments illustrated and described above can be used
in combination with each other to enhance the useful life of all of
the consumable parts of the plasma arc torch. For example, it is
contemplated that texturing the opposing surfaces that form act
annular gas passage 133 (e.g., the outer surface of the electrode
37 and the inner surface of the tip 131, or the outer surface of
the tip and the inner surface of the shield cap 549) will create
additional turbulence in the hydrodynamic boundary layer of the
cooling gas to further improve convective cooling of each
consumable part.
[0096] In view of the above, it will be seen that the several
objects of the invention are achieved and other advantageous
results attained.
[0097] As various changes could be made in the above constructions
without departing from the scope of the invention, it is intended
that all matter contained in the above description or shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense.
* * * * *