U.S. patent application number 12/187747 was filed with the patent office on 2008-11-27 for electrode and electrode holder with threaded connection.
This patent application is currently assigned to The ESAB Group, Inc.. Invention is credited to Wayne Stanley Severance, JR..
Application Number | 20080293320 12/187747 |
Document ID | / |
Family ID | 35431646 |
Filed Date | 2008-11-27 |
United States Patent
Application |
20080293320 |
Kind Code |
A1 |
Severance, JR.; Wayne
Stanley |
November 27, 2008 |
ELECTRODE AND ELECTRODE HOLDER WITH THREADED CONNECTION
Abstract
A threaded connection for an electrode holder and an electrode
in a plasma arc torch is provided. The threaded connection has
relatively low height, and the engaged portion of a male threaded
portion of the electrode and a female threaded portion of the
electrode holder are positioned at least partially within a nozzle
chamber. In one inventive aspect, the nominal pitch diameter of the
electrode is less than the minor diameter of the electrode. In
another, the width of the root area of the electrode thread is
wider than the width of the root area of the electrode holder
thread by at least about 35%. The width of the root area of the
electrode is at least about 15% wider than the width of the crest
portion of the electrode. As such, the less consumable of the two
parts, the electrode holder, is provided with a thread that is less
likely to be worn and damaged. In one particular embodiment, the
crest profile of the electrode is that of a Stub Acme thread
separated by a larger root profile.
Inventors: |
Severance, JR.; Wayne Stanley;
(Darlington, SC) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA, 101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
The ESAB Group, Inc.
|
Family ID: |
35431646 |
Appl. No.: |
12/187747 |
Filed: |
August 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11419405 |
May 19, 2006 |
7423235 |
|
|
12187747 |
|
|
|
|
Current U.S.
Class: |
445/35 |
Current CPC
Class: |
H05H 1/34 20130101; Y10T
403/556 20150115; Y10T 29/49117 20150115; H05H 2001/3478
20130101 |
Class at
Publication: |
445/35 |
International
Class: |
H01J 9/02 20060101
H01J009/02 |
Claims
1. A method of manufacturing the body of an electrode for a plasma
arc torch, the method comprising the steps of: forming an electrode
blank from a base material and defining at least one external
cylindrical surface; removing material from the cylindrical surface
so as to define at least one helical thread form in the electrode
blank, the removing step comprising the steps of; removing material
so as to form flanks defining the thread form, the flanks defining
at least one line when viewed in cross section that intersects at a
crest apex with a line defined by another of the flanks and also
intersects at a root apex with a line defined by yet another of the
flanks, and discontinuing the removal of material at a depth from
the cylindrical surface that is above a depth halfway between the
root apex and the crest apex.
2. A method of manufacturing as defined in claim 1 wherein the
material removing steps define a crest profile for the thread form
that is consistent with the crest profile of a Stub Acme
thread.
3. A method of manufacturing as defined in claim 1 wherein the
material removing steps define a root profile for the thread form
that is not consistent with a root profile of a Stub Acme thread.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 11/419,405, filed May 19, 2006, which is currently pending and
is hereby incorporated herein in its entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1) Field of the Invention
[0003] The present invention relates to plasma arc torches and, in
particular, to plasma arc torches wherein an electrode and an
electrode holder are held to each other or to the torch by way of a
threaded connection.
[0004] 2) Description of Related Art
[0005] Plasma arc torches are commonly used for the working of
metal including cutting, welding, surface treatment, melting and
annealing. Such torches include an electrode that supports an arc
that extends from the electrode to a workpiece in a 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 envelop the gas and arc in a swirling jet of
water.
[0006] 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 copper
alloy. The forward or discharge end of the tubular electrode
includes a bottom end wall having an emissive element embedded
therein that supports the arc. The opposite end of the electrode
holds the electrode in the torch by way of a threaded connection to
an electrode holder. The electrode holder is typically an elongate
structure held to the torch body by a threaded connection at an end
opposite the end at which the electrode is held. The electrode
holder and the electrode define a threaded connection for holding
the electrode to the electrode holder.
[0007] The emissive element of the electrode is composed of a
material that has a relatively low work function, which is defined
in the art as the potential step, measured in electron volts (eV),
which promotes thermionic emission from the surface of a metal at a
given temperature. In view of this 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 alloys thereof.
[0008] A nozzle surrounds the discharge end of the electrode and
provides a pathway for directing the arc towards the workpiece. To
ensure that the arc is emitted through the nozzle and not from the
nozzle surface during regular, transferred-arc operation, the
electrode and the nozzle are maintained at different electrical
potential relative to each other. Thus, it is important that the
nozzle and the electrode are electrically separated, and this is
typically achieved by maintaining a predetermined physical gap
between the components. The volume defining the gap is most
typically filled with flowing air or some other gas used in the
torch operation.
[0009] The heat generated by the plasma arc is great. The torch
component that is subjected to the most intense heating is the
electrode. To improve the service life of a plasma arc torch, it is
generally desirable to maintain the various components of the torch
at the lowest possible temperature notwithstanding this heat
generation. A passageway or bore is formed through the electrode
holder and the electrode, and a coolant such as water is circulated
through the passageway to cool the electrode.
[0010] Even with the water-cooling, the electrode has a limited
life span and is considered a consumable part. Thus, in the normal
course of operation, a torch operator must periodically replace a
consumed electrode by first removing the nozzle and then
unthreading the electrode from the electrode holder. A new
electrode is then screwed onto the electrode holder and the nozzle
is reinstalled so that the plasma arc torch can resume
operation.
[0011] The design of the threaded connection between the electrode
holder and the electrode must take into account various
constraints. First, the threaded connection must be structurally
strong enough to securely hold the electrode to the electrode
holder. Second, in the case of water-cooled torches, the threaded
connection should allow for sealing between the electrode holder
and the electrode so that the cooling water cannot escape. The
sealing is typically achieved by way of an o-ring, and so the
threaded connection should allow sufficient room for such an
o-ring. Third, a considerable current is passed through the
electrode holder to the electrode, in some cases up to 1,000
amperes of cutting current. Thus, the threaded connection should
provide sufficient contact surface area between the electrode and
the electrode holder to allow this current to pass through.
Finally, the cost of manufacturing the electrode should be as small
as possible, especially because the electrode is a consumable part.
Similar considerations exist with respect to the threaded
connection holding the electrode holder to the torch body.
[0012] One way that this cost can be reduced is to make the
electrode shorter, thus reducing material cost and manufacturing
cost. This can be achieved by making the electrode holder longer to
compensate for the shorter length of the electrode so that the
total length of the electrode holder and electrode remains the
same. However, the length of the electrode holder is limited by the
nozzle geometry because the threaded connection between the
electrode holder and the electrode in many conventional torches is
too large to extend into the nozzle chamber and still meet the
design constraints noted above.
[0013] In particular, the threaded connection in present designs
sometimes comprises an enlarged female-threaded portion at the end
of the electrode holder that is radially larger than the adjacent
male-threaded end of the electrode. Thus, if such a conventional
threaded connection were designed to extend into the nozzle, then
the gap between the electrode holder and the nozzle would decrease.
As noted above, the electrode and electrode holder are at one
electrical potential and the nozzle is at a different electrical
potential. Thus, the decrease in the gap might cause undesired
arcing within the torch from the nozzle to the electrode
holder.
[0014] This particular problem has been resolved in part in some
prior torches by forming a threaded connection using a male thread
for the electrode holder and a female thread for the electrode. One
advantage of this approach is that the electrode holder is
protected from damage because any arcing that does occur inside the
torch extends from the outside of the electrode to the nozzle, and
not from the electrode holder to the nozzle, because the outer
surface of the female-threaded portion of the electrode is radially
closest to the remainder of the torch. Because the electrode must
be periodically replaced when the emissive end is spent in any
event, damage to the threaded end of the electrode is less of a
concern than it is to the electrode holder.
[0015] One disadvantage of this approach, however, is that female
threads are generally more difficult to machine and thus are more
expensive than male threads. Even though the electrode holder can
sometimes be a consumable part, the rate of consumption is
typically less than that of the electrode, and thus this
configuration can have an undesirable cost structure. The more
frequently replaced part must be subjected to the more expensive of
the two machining operations necessary for making a threaded
connection.
[0016] Another way to resolve at least some of these design
constraints is to use a fine thread. A fine thread allows a shorter
thread height (i.e. the dimension of the thread in the radial
direction) than a corresponding coarser thread as used in
conventional torches. This reduced thread height allows more of a
gap between the threaded connection and the nozzle. However, fine
threads are more difficult to machine and thus can be more
expensive. In addition, fine threads are more delicate, are quicker
to become unusably worn on the electrode holder when electrodes are
repeatedly replaced, and are more likely to be improperly
cross-threaded when an operator is installing a new electrode.
[0017] Thus, there is a need in the industry for an electrode and
an electrode holder where the threaded connection therebetween is
capable of meeting all of the electrical, structural and sealing
constraints required in a plasma arc torch, but yet which is
capable of being positioned at least partially within a nozzle of
the plasma arc torch without detrimental arcing occurring between
the threaded connection and the nozzle. Such a threaded connection
would preferably be relatively easy to manufacture and would
involve limited risks of cross-threading when the electrode is
attached to the electrode holder.
[0018] In addition, it would be desirable to provide an electrode
that can be secured to the electrode holder by way of a threaded
connection where the machining and material costs, and the
possibilities of premature wear and damage, are reduced for the
electrode. Because the costs and possibility for damage in such an
arrangement would be distributed more to the more-consumable
electrode than to the less-consumable electrode holder, the
long-term costs of operating the plasma arc torch would be reduced.
Similar advantages would also be beneficial for the threaded
connection between the electrode holder and the torch body.
BRIEF SUMMARY OF THE INVENTION
[0019] These and other objects and advantages are provided by the
present invention, which includes an electrode holder and an
electrode that is removably held to the electrode holder by a novel
threaded connection. The novel threaded connection has relatively
low height and, in another aspect of the invention, the engaged
portion of a male thread of the electrode and a female thread of
the electrode holder can be positioned at least partially within a
nozzle chamber of the plasma arc torch. In one embodiment of the
novel threaded connection, the width of the root portion of the
electrode thread is wider than the width of the root portion of the
electrode holder thread by at least 35%. As such, the
less-consumable of the two parts, the electrode holder, is provided
with a more robust crest for its thread that is less likely to be
worn and damaged relative to the crest of the thread of the
more-consumable electrode. In a particular embodiment, the crest
profile of the electrode thread and the root profile of the
electrode holder thread are consistent with those of a Stub Acme
thread.
[0020] More specifically, the electrode has a male threaded portion
for removably holding the electrode in the plasma arc torch and
defines at least one thread form extending helically and at least
partially around a thread axis. This threaded portion defines a
major diameter comprising a larger diameter of the threaded portion
and a minor diameter comprising a smaller diameter of the threaded
portion. At least two flanks define at least one crest profile of
the thread form, and each flank extends between the major diameter
and the minor diameter. Each of the flanks of the crest profile
defines at least one line when viewed in cross section that
intersects at a crest apex with the line defined by the other of
the flanks of the crest profile. In addition, the lines of adjacent
flanks of adjacent crest profiles intersect at a root apex. Thus, a
nominal pitch diameter can be defined as lying halfway between the
diameter of the crest apex and the diameter of the root apex.
[0021] According to one inventive aspect of the threaded connection
of the present invention, the crests of the male thread are
narrower than the roots of the male thread. This can be
geometrically defined by saying that the nominal pitch diameter of
the electrode is not greater than the minor diameter of the
electrode. In another, the nominal pitch diameter of the electrode
is smaller than the minor diameter of the female thread of the
electrode holder. In a conventional thread, the nominal pitch
diameter as defined herein would be closer to or at the midpoint
between the minor and major diameters of the respective components.
Another advantage of the present invention is that the electrode
holder can be held to the plasma arc torch body by a male thread at
the opposite end from the electrode, which male thread corresponds
at least in shape to the male thread of the electrode and provides
similar advantages inasmuch as the electrode holder can also be
consumable, at least relative to the plasma arc torch body.
[0022] Another way of defining the novel threaded connection of the
electrode and the electrode holder that embodies the benefits of
the invention is to recognize that each defines a mean diameter
between the major diameter and the minor diameter. As such, a crest
portion extends in one direction from the mean diameter, and a root
area extends in an opposite direction from the mean diameter and
defines a width along the mean diameter. Advantageously, the width
of the root area of the thread of the electrode is wider than the
width of the root area of the thread of the electrode holder, and
in particular is at least about 35% wider. The root area of the
electrode may be at least about 45% wider than the root area of the
electrode holder, and further can be at least about 55% wider than
the root area of the electrode holder. In addition, with regard to
the threaded portion of the electrode, the width of the root area
is greater than the width of the crest portion by at least 15%, and
can be at least about 55% greater than the width of the crest
portion, and may be 95% wider or more.
[0023] In another aspect of the present invention, a method of
manufacturing the body of an electrode for a plasma arc torch
comprises the steps of: [0024] forming an electrode blank from a
base material and defining at least one external cylindrical
surface; [0025] removing material from the cylindrical surface so
as to define at least one helical thread form in the electrode
blank, the removing step comprising the steps of; [0026] removing
material so as to form flanks defining the thread form, the
flanks
[0027] defining at least one line when viewed in cross section that
intersects at a crest apex with a line defined by another of the
flanks and also intersects at a root apex with a line defined by
yet another of the flanks, and discontinuing the removal of
material at a depth from the cylindrical surface that is above a
depth halfway between the root apex and the crest apex.
[0028] Thus, the present invention solves the problems recognized
above in that the novel threaded connection provides for the
more-consumable electrode to be formed with less material relative
to the electrode holder. Some electrodes can be made much shorter
as compared to conventional electrodes for corresponding torches.
In addition, any threading damage or wear as between the electrode
and electrode holder is less likely to be suffered by the less
consumable of the two parts, the electrode holder. Advantageously,
the present invention also provides for an electrode and electrode
holder threaded engagement to be positioned at least partially
within the nozzle chamber of the torch with the male thread on the
electrode.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0029] Having thus described the invention in general terms,
reference will now be made to the accompanying drawings, which are
not necessarily drawn to scale, and wherein:
[0030] FIG. 1 is a sectioned side view of a conventional shielding
gas plasma arc torch illustrating an electrode assembly as used in
the prior art;
[0031] FIG. 2 is a sectioned side view of the torch taken along a
different section from FIG. 1 to illustrate coolant flow
therethrough;
[0032] FIG. 3 is an enlarged view of the lower portion of the torch
as seen in FIG. 1 and illustrating the conventional electrode
assembly;
[0033] FIG. 4 is an enlarged view of the lower portion of torch as
seen in FIG. 1 but showing the advantageous electrode and electrode
holder according to the present invention;
[0034] FIG. 5 is a sectional view of the electrode and electrode
holder according the invention;
[0035] FIG. 6 is a greatly enlarged view of the threaded connection
between the electrode holder and the electrode according to the
invention;
[0036] FIG. 7 is a sectional view of the electrode;
[0037] FIG. 8A is a greatly enlarged view of the male thread of the
electrode;
[0038] FIG. 8B is the same view as FIG. 8A but provides some other
dimensional references;
[0039] FIG. 9 is a sectional view of the electrode holder;
[0040] FIG. 10A is a greatly enlarged view of the female thread of
the electrode holder; and
[0041] FIG. 10B is the same view as FIG. 10A but provides other
dimensional references corresponding to those in FIG. 8B.
DETAILED DESCRIPTION OF THE DRAWINGS
[0042] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings in which
some but not all embodiments of the invention are shown. Indeed,
these inventions may 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
satisfy applicable legal requirements. Like numbers refer to like
elements throughout.
[0043] With reference to FIGS. 1-3, a prior plasma arc torch that
benefits from the invention is broadly indicated by reference
numeral 10. A plasma arc torch 10 using an electrode and electrode
holder according to the present invention is illustrated in FIG. 4.
The torch 10 is a shielding gas torch, which provides a swirling
curtain or jet of shielding gas surrounding the electric arc during
a working mode of operation of the torch. The torch 10 includes a
generally cylindrical upper or rear insulator body 12 which may be
formed of a potting compound or the like, a generally cylindrical
main torch body 14 connected to the rear insulator body 12 and
generally made of a conductive material such as metal, a generally
cylindrical lower or front insulator body 16 connected to the main
torch body 14, an electrode holder assembly 18 extending through
the main torch body 14 and front insulator body 16 and supporting
an electrode 20 at a free end of the electrode holder assembly, and
a nozzle assembly 22 connected to the insulator body 16 adjacent
the electrode 20.
[0044] A plasma gas connector tube 24 extends through the rear
insulator body 12 and is connected by screw threads (not shown)
into a plasma gas passage 26 of the main torch body 14. The plasma
gas passage 26 extends through the main torch body 14 to a lower
end face 28 thereof for supplying a plasma gas (sometimes referred
to as a cutting gas), such as oxygen, air, nitrogen, or argon, to a
corresponding passage in the insulator body 16.
[0045] A shielding gas connector tube 30 extends through the rear
insulator body 12 and is connected by screw threads into a
shielding gas passage 32 of the main torch body 14. The shielding
gas passage 32 extends through the main torch body 14 to the lower
end face 28 for supplying a shielding gas, such as argon or air, to
a corresponding passage in the insulator body 16.
[0046] The insulator body 16 has an upper end face 34 that abuts
the lower end face 28 of the main torch body. A plasma gas passage
36 extends through the insulator body 16 from the upper end face 34
into a cylindrical counterbore 38 in the lower end of the insulator
body 16. As further described below, the counterbore 38, together
with the upper end of the nozzle assembly 22, forms a plasma gas
chamber 40 from which plasma gas is supplied to a primary or plasma
gas nozzle of the torch. As such, plasma gas from a suitable source
enters the plasma gas chamber 40 by flowing through the plasma gas
connector tube 24, through the plasma gas passage 26 in the main
torch body 14, into the plasma gas passage 36 of the insulator body
16, which is aligned with the passage 26, and into the chamber
40.
[0047] The nozzle, which is illustrated as a two-part nozzle
assembly 22, includes an upper nozzle member 42, which has a
generally cylindrical upper portion slidingly received within a
metal insert sleeve 44 that is inserted into the counterbore 38 of
the insulator body 16. An O-ring 46 seals the sliding
interconnection between the upper nozzle member 42 and the metal
insert sleeve 44. A lower nozzle tip 48 of generally frustoconical
form also forms a part of the nozzle assembly 22, and is threaded
into the upper nozzle member 42. The lower nozzle tip 48 includes a
nozzle exit orifice 50 at the tip end thereof. The lower nozzle tip
48 and upper nozzle member 42 could alternatively be formed as one
unitary nozzle. In either configuration, the nozzle channels the
plasma gas from a larger distal opening 49 to the exit orifice 50.
A plasma gas flow path thus exists from the plasma gas chamber 40
through the nozzle chamber 41 for directing a jet of plasma gas out
the nozzle exit orifice 50 to aid in performing a work operation on
a workpiece.
[0048] The plasma gas jet preferably has a swirl component created,
in a known manner; by a hollow cylindrical ceramic gas baffle 52
partially disposed in a counterbore recess 54 of the insulator body
16. A lower end of the baffle 52 abuts an annular flange face of
the upper nozzle member 42. The baffle 52 has non-radial holes (not
shown) for directing plasma gas from the plasma gas chamber 40 into
a lower portion of the nozzle chamber 41 with a swirl component of
velocity.
[0049] The electrode holder assembly 18 includes a tubular
electrode holder 56 which has its upper end connected by threads 11
within a blind axial bore 58 in the main torch body 14. The
electrode holder 56 is somewhat consumable, although usually less
so than the electrode itself, and thus the electrode holder and the
axial bore 58 can also be provided with a threaded connection
according to the present invention as discussed below. The upper
end of electrode holder 56 extends through an axial bore 60 formed
through the insulator body 16, and the lower end of the electrode
holder 56 includes an enlarged internally screw-threaded coupler 62
which has an outer diameter slightly smaller than the inner
diameter of the ceramic gas baffle 52 which is sleeved over the
outside of the coupler 62. The electrode holder 56 also includes
internal screw threads spaced above the coupler 62 for threadingly
receiving a coolant tube 64 which supplies coolant to the electrode
20, as further described below, and which extends outward from the
axial bore of the insulator body 16 into the central passage of the
electrode 20. To prevent improper disassembly or reassembly of the
coolant tube 64 and the electrode holder 56, the screw thread
connection between those items may be cemented or otherwise secured
together during manufacture to form an inseparable electrode holder
assembly 18. The electrode 20 may be of the type described in U.S.
Pat. No. 5,097,111, assigned to the assignee of the present
application, and incorporated herein by reference.
[0050] The prior art electrode 20 comprises a cup-shaped body whose
open upper end is threaded by screw threads 63 into the coupler 62
at the lower end of the electrode holder 56, and whose capped lower
end is closely adjacent the lower end of the coolant tube 64. A
coolant circulating space exists between the inner surface of the
wall of the electrode 20 and the outer surface of the wall of the
coolant tube 64, and between the outer surface of the wall of the
coolant tube 64 and the inner surface of the wall of the electrode
holder 56. The electrode holder 56 includes a plurality of holes 66
for supplying coolant from the space within the electrode holder to
a space 68 between the electrode holder and the inner wall of the
axial bore 60 in the insulator body 16. A seal 69 located between
the holes 66 and the coupler 62 seals against the inner wall of the
bore 60 to prevent coolant in the space 68 from flowing past the
seal 69 toward the coupler 62. A raised annular rib or dam 71 on
the outer surface of the electrode holder 56 is located on the
other side of the holes 66 from the seal 69, for reasons which will
be made apparent below. A coolant supply passage 70 (FIG. 2)
extends through the insulator body from the space 68 through the
outer cylindrical surface of the insulator body 16 for supplying
coolant to the nozzle assembly 22, as further described below.
[0051] During starting of the torch 10, a difference in electrical
voltage potential is established between the electrode 20 and the
nozzle tip 48 so that an electric arc forms across the gap
therebetween. Plasma gas is then flowed through the nozzle assembly
22 and the electric arc is blown outward from the nozzle exit
orifice 50 until it attaches to a workpiece, at which point the
nozzle assembly 22 is disconnected from the electric source so that
the arc exists between the electrode 20 and the workpiece. The
torch is then in a working mode of operation.
[0052] For controlling the work operation being performed, it is
known to use a control fluid such as a shielding gas to surround
the arc with a swirling curtain of gas. To this end, the insulator
body 16 includes a shielding gas passage 72 that extends from the
upper end face 34 axially into the insulator body, and then angles
outwardly and extends through the cylindrical outer surface of the
insulator body. A nozzle retaining cup assembly 74 surrounds the
insulator body 16 to create a generally annular shielding gas
chamber 76 between the insulator body 16 and the nozzle retaining
cup assembly 74. Shielding gas is supplied through the shielding
gas passage 72 of the insulator body 16 into the shielding gas
chamber 76.
[0053] The nozzle retaining cup assembly 74 includes a nozzle
retaining cup holder 78 and a nozzle retaining cup 80 which is
secured within the holder 78 by a snap ring 81 or the like. The
nozzle retaining cup holder 78 is a generally cylindrical sleeve,
preferably formed of metal, which is threaded over the lower end of
a torch outer housing 82 which surrounds the main torch body 14.
Insulation 84 is interposed between the outer housing 82 and the
main torch body 14. The nozzle retaining cup 80 preferably is
formed of plastic and has a generally cylindrical upper portion
that is secured within the cup holder 78 by the snap ring 81 and a
generally frustoconical lower portion which extends toward the end
of the torch and includes an inwardly directed flange 86. The
flange 86 confronts an outwardly directed flange 88 on the upper
nozzle member 42 and contacts an O-ring 90 disposed therebetween.
Thus, in threading the nozzle retaining cup assembly 74 onto the
outer housing 82, the nozzle retaining cup 80 draws the nozzle
assembly 22 upward into the metal insert sleeve 44 in the insulator
body 16. The nozzle assembly 22 is thereby made to contact an
electrical contact ring secured within the counterbore 38 of the
insulator body 16. More details of the electrical connections
within the torch can be found in commonly-owned U.S. Pat. No.
6,215,090, which is incorporated by reference herein in its
entirety.
[0054] The nozzle retaining cup 80 fits loosely within the cup
holder 78, and includes longitudinal grooves 92 in its outer
surface for the passage of shielding gas from the chamber 76 toward
the end of the torch. Alternatively or additionally, grooves (not
shown) may be formed in the inner surface of the cup holder 78. A
shielding gas nozzle 94 of generally frustoconical form
concentrically surrounds and is spaced outwardly of the lower
nozzle tip 48 and is held by a shield retainer 96 that is threaded
over the lower end of the cup holder 78. A shielding gas flow path
98 thus extends from the longitudinal grooves 92 in retaining cup
80, between the shield retainer 96 and the retaining cup 80 and
upper nozzle member 42, and between the shielding gas nozzle 94 and
the lower nozzle tip 48.
[0055] The shielding gas nozzle 94 includes a diffuser 100 that in
known manner imparts a swirl to the shielding gas flowing into the
flow path between the shielding gas nozzle 94 and the lower nozzle
tip 48. Thus, a swirling curtain of shielding gas is created
surrounding the jet of plasma gas and the arc emanating from the
nozzle exit orifice 50.
[0056] With primary reference to FIG. 2, the coolant circuits for
cooling the electrode 20 and nozzle assembly 22 are now described.
The torch 10 includes a coolant inlet connector tube 112 that
extends through the rear insulator body 12 and is secured within a
coolant inlet passage 114 in the main torch body 14. The coolant
inlet passage 114 connects to the center axial bore 58 in the main
torch body. Coolant is thus supplied into the bore 58 and thence
into the internal passage through the electrode holder 56, through
the internal passage of the coolant tube 64, and into the space
between the tube 64 and the electrode 20. Heat is transferred to
the liquid coolant (typically water or antifreeze) from the lower
end of the electrode (from which the arc emanates) and the liquid
then flows through a passage between the lower end of the coolant
tube 64 and the electrode 20 and upwardly through the annular space
between the coolant tube 64 and the electrode 20, and then into the
annular space between the coolant tube 64 and the electrode holder
18.
[0057] The coolant then flows out through the holes 66 into the
space 68 and into the passage 70 through the insulator body 16. The
seal 69 prevents the coolant in the space 68 from flowing toward
the coupler 62 at the lower end of the holder 56, and the dam 71
substantially prevents coolant from flowing past the dam 71 in the
other direction, although there is not a positive seal between the
dam 71 and the inner wall of the bore 60. Thus, the coolant in
space 68 is largely constrained to flow into the passage 70. The
insulator body 16 includes a groove or flattened portion 116 that
permits coolant to flow from the passage 70 between the insulator
body 16 and the nozzle retaining cup 80 and into a coolant chamber
118 which surrounds the upper nozzle member 42. The coolant flows
around the upper nozzle member 42 to cool the nozzle assembly.
[0058] Coolant is returned from the nozzle assembly via a second
groove or flattened portion 120 angularly displaced from the
portion 116, and into a coolant return passage 122 in the insulator
body 16. The coolant return passage 122 extends into a portion of
the axial bore 60 that is separated from the coolant supply passage
70 by the dam 71. The coolant then flows between the electrode
holder 56 and the inner wall of the bore 60 and the bore 58 in the
main torch body 14 into an annular space 126 which is connected
with a coolant return passage 128 formed in the main torch body 14,
and out the coolant return passage 128 via a coolant return
connector tube 130 secured therein. Typically, returned coolant is
recirculated in a closed loop back to the torch after being
cooled.
[0059] In use, and with reference to FIG. 1, one side of an
electrical potential source 210, typically the cathode side, is
connected to the main torch body 12 and thus is connected
electrically with the electrode 20, and the other side, typically
the anode side, of the source 210 is connected to the nozzle
assembly 22 through a switch 212 and a resistor 214. The anode side
is also connected in parallel to the workpiece 216 with no resistor
interposed therebetween. A high voltage and high frequency are
imposed across the electrode and nozzle assembly, causing an
electric arc to be established across a gap therebetween adjacent
the plasma gas nozzle discharge. Plasma gas is flowed through the
nozzle assembly to blow the pilot arc outward through the nozzle
discharge until the arc attaches to the workpiece. The switch 212
connecting the potential source to the nozzle assembly is then
opened, and the torch is in the transferred arc mode for performing
a work operation on the workpiece. The power supplied to the torch
is increased in the transferred arc mode to create a cutting arc,
which is of a higher current than the pilot arc. Although
illustrated herein with a torch that uses a high-frequency pilot
signal to start an arc, the electrode and electrode holder
according to the invention can also be used with blowback-type
torches.
[0060] The electrode holder assembly 18 and novel threaded
connection according to the present invention are illustrated in
FIGS. 4-10. The electrode holder assembly 18 includes the tubular
electrode holder 56, which has its upper end connected by threads
11 within the blind axial bore in the main torch body, as discussed
above. The coolant tube 64 supplies coolant to the cup-shaped
electrode 20, which has an open distal end secured to the electrode
holder 56 by the advantageous threads 15 according to the present
invention.
[0061] The threads 15 securing the electrode 20 to the electrode
holder 56 can be seen in FIG. 5. The electrode holder 56 has a
female threaded portion 17 formed therein and the electrode 20 has
a male threaded portion 19 formed thereon. An O-ring 31 is provided
to ensure adequate sealing and to prevent coolant from escaping
from the electrode and electrode holder. The electrode 20 and the
electrode holder 56 can be formed from a variety of different
electrically conductive materials, but in one embodiment the
electrode holder 56 is made of brass or a brass alloy and the
electrode 20 comprises a body made of copper or a copper alloy. The
coolant tube 64 can also be seen in FIG. 5, and it is illustrated
with a distal end have a constant diameter in the axial direction.
However, a coolant tube 64 having a distal end with an external
diameter larger than a more medial portion of the coolant tube,
such as the coolant tube 64 illustrated in FIGS. 1-3, could also be
used. Advantageously, the external diameter of the distal end of
the coolant tube 64 is less than internal diameter of the passage
in the electrode holder through which coolant tube extends, and the
threaded portion of the electrode holder is at least partially
within the nozzle chamber 41 as seen in FIG. 4.
[0062] FIG. 6 is an enlarged view of the female threaded portion 17
of the electrode holder and the male threaded portion 19 of the
electrode threadingly engaged together. The manufacturing
clearances between the threads are illustrated. Although the
electrode 20 is illustrated herein as being removably held in the
plasma arc torch by way of an electrode holder 56, it is within the
realm of the invention that the electrode 20 could be held within
the torch by being threaded directly to the torch body 14 or some
other component.
[0063] The electrode 20 as shown in the enlarged view of FIG. 7,
comprises a generally cup-shape having the male threaded portion 19
at a proximal end thereof. An emissive element 23 and a relatively
non-emissive separator 25 are held at the opposite end of a body 21
from the male threaded portion 19. The emissive element 23 is the
component of the electrode from which the arc extends to the
workpiece and is formed from an emissive material, such as hafnium.
The relatively non-emissive separator 25 is formed from a
relatively non-emissive material such as silver, and serves to
prevent the arc from emanating from the body 21 of the electrode 20
instead of the emissive element 23.
[0064] A greatly enlarged view of the male threaded portion 19 can
be seen in FIGS. 8A and 8B. The male threaded portion 19 defines at
least one thread form extending helically and at least partially
around the axis of the electrode 20. Although one thread form is
illustrated, double-thread forms can also be used in some
situations consistent within the scope of the invention. The thread
form has a crest portion 27 and a root area 29 and which together
define a crest profile for each helix of the thread form.
[0065] As shown in FIG. 8A, the male threaded portion 19 defines a
minor diameter K and a major diameter D. A crest portion 27 defines
a crest flat 33 and the root area 29 defines a root flat 35.
Although illustrated as having flats 33, 35, it should be
understood that threads can be formed in accordance with the
principles of the present invention that have rounded or
partially-rounded roots and crests.
[0066] The male threaded portion also defines flanks 37 that extend
between the crest flats 33 and the root flats 35. The flanks 37 are
shown as being straight in the drawing, and each defines a line
that can be extended as shown by a broken line in the drawings.
These extension lines extend towards each other and, at their
points of intersection, define a crest apex c.sub.a and a root apex
r.sub.a. It is to be understood that at least one of the apices
could comprise an actual apex of a thread profile for some
configurations, but in the illustrated embodiments these apices are
theoretical. A nominal pitch diameter D.sub.p is illustrated and is
defined as the diameter that lies halfway between the crest apex
c.sub.a and the root apex r.sub.a. Reference here is made to
Machinery's Handbook; Oberg, Jones and Horton; Industrial Press,
Inc.; 1979.
[0067] For many conventional thread configurations, the nominal
pitch diameter D.sub.p lies roughly halfway between the minor
diameter K and the major diameter D. However, with the special
thread configuration of embodiments of the present invention, where
the thread root is much wider than the thread crest (in the male
form), the nominal pitch diameter D.sub.p lies much closer to the
thread axis. Indeed, while the nominal pitch diameter D.sub.p of a
conventional thread may pass through the radial middle of the
flanks of the thread, in the present invention the nominal pitch
diameter D.sub.p is much smaller and may be no greater than the
minor diameter K of the female threaded portion of the electrode
holder (shown in FIGS. 10A & 10B), and in some embodiments may
be no greater than the minor diameter K of the electrode. In
others, the nominal pitch diameter D.sub.p maybe no more than about
105% of the minor diameter K of the electrode.
[0068] Another way of defining the benefits and advantages of the
threaded connection according to the present invention is to
consider the mean diameter of the threaded portions. The mean
diameter allows definition of the invention without relying upon
nominal pitch diameters, theoretical apices and extension lines and
is helpful in a case, for example, where one or more of the thread
forms has a curving profile but still embodies the advantages
discussed herein. Although the flanks are illustrated herein as
having a flat profile, the flanks could also be curved or
segmented, or have some other shape, and still achieve the
advantages of the invention. The mean diameter for the electrode is
shown in FIG. 8B, where a mean diameter d.sub.m is halfway between
the minor diameter K and the major diameter D. The mean diameter
d.sub.m passes through the flanks of the thread and defines both a
root area width r.sub.w and a crest portion width c.sub.w extending
along the mean diameter d.sub.m. As can be seen, the root area
width r.sub.w of the male threaded portion is larger than the crest
portion width c.sub.w.
[0069] In one particular embodiment of the invention designed for
use in the PT-19XLS torch available from Esab Cutting & Welding
Products of Florence, S.C., the electrode 20 can have the following
dimensions. The flanks of the threaded portion relative to the axis
of the electrode 20 are manufactured so as to provide an included
angle 2.alpha. that is 29.degree.. The pitch p of the thread is
0.0833'', which provides a thread count of 12 threads per inch
(tpi). The length of the threaded portion can be 0.193'' in the
axial direction so that only a small amount of turning is necessary
to seat the electrode 20, which can assist in rapid assembly. The
minor diameter K is 0.389'' and the major diameter D is 0.441''.
The crest apex c.sub.a thus lies at a diameter of 0.526'' and the
root apex r.sub.a lies at 0.203'', and the nominal pitch diameter
D.sub.p halfway between these two diameters is 0.364''. Thus, the
nominal pitch diameter D.sub.p is less than the minor diameter K of
the electrode threaded portion.
[0070] The width of the root area r.sub.w is 0.055'' and the width
of the crest portion c.sub.w is 0.028''. Thus, the width of the
root area r.sub.w is greater than the width of the crest portion
c.sub.w by at least 15%, and may be 55% wider, or 95% wider or
more.
[0071] The profile of the thread crest may be consistent with a
standard Stub Acme thread (as defined in ASME/ANSI standard for
Stub Acme threads, No. B 1.8, which is incorporated herein by
reference) even though the root profile is wider than a standard
Stub Acme thread. In particular, while the crest flat 33 has a
width of 0.022'', the root flat 35 has a width of 0.048'', which is
greater than 0.4224 times the pitch of threaded portion, and does
not meet the ASME/ANSI standard. The thread form can be machined
using tooling designed for a Stub Acme thread of 8 tpi even though
the thread count for the final thread is 12 tpi due to the enlarged
root profile relative to the crest profile of the thread form.
Thus, the advantageous threaded connection according to the present
invention can be made using conventional tooling.
[0072] Such a method can comprise an initial step of forming an
electrode blank from a base material, such as copper, and defining
at least one cylindrical surface on the exterior of the blank.
Thereafter, material is removed from the cylindrical surface so as
to define at least one helical thread form in the electrode blank.
In particular, material is removed so as to form flanks defining
the thread form; the flanks defining at least one line when viewed
in cross section that intersects at a crest apex with a line
defined by another of the flanks and also intersects at a root apex
with a line defined by yet another of the flanks. The removal of
material is discontinued at a depth that is above a depth halfway
between the root apex and the crest apex. While machining is a
practical way of forming the electrode from the blank, especially
when using the conventional tooling as noted above, the electrode
can also formed using other manufacturing methods, such as casting,
etc.
[0073] A corresponding electrode holder 56 is illustrated in FIGS.
9, 10A and 10B. In particular, using the same terminology for FIGS.
8A and 8B, the major diameter D has a value of 0.449'' and the
minor diameter K has a value of 0.395''. It should be noted here
that the nominal pitch diameter of the electrode (0.364'') is not
greater the minor diameter of the electrode holder. The crest apex
c.sub.a of the electrode holder thus lies at a diameter of 0.235''
and the root apex r.sub.a lies at 0.557'', and thus the nominal
pitch diameter D.sub.p of the electrode holder halfway between
these two diameters is 0.396'', which is larger than the minor
diameter of the electrode holder. The profile of the thread root is
consistent with a standard Stub Acme thread even though the crest
profile is wider than a standard Stub Acme thread. The crest flat
33 has a width of 0.041'', which is greater than 0.4224 times the
pitch of threaded portion, and does not meet the ASME/ANSI standard
for Stub Acme threads, No. B1.8. The root flat 35 has a width of
0.028''. The crest portion width c.sub.w is 0.048'', and is larger
than the root area width r.sub.w of 0.035''. However, the thread
form can be machined using tooling designed for a Stub Acme thread
of 14 tpi even though the thread count for the final thread is 12
tpi due to the enlarged crest portion relative to the root area of
the thread. The electrode holder can be formed using a similar
method to that described above for the electrode.
[0074] As between the electrode and the electrode holder, the width
of the root area r.sub.w of the electrode is 0.055'' and the width
of the root area r.sub.w of the electrode holder is 0.035'' as
noted above. The width of the root area of the electrode is greater
than the width of the root area of the electrode holder by at least
35%, and may be 45% wider, or 55% wider or more.
[0075] The electrode holder 56 also has an opposite male threaded
portion 11 as shown in FIG. 5. The dimensions are similar to those
of the male threaded portion of the electrode. The width of the
root area r.sub.w is 0.055'' and the width of the crest portion
c.sub.w is 0.028''. Thus, the width of the root area r.sub.w is
greater than the width of the crest portion c.sub.w by at least
15%, and may be 55% wider, or 95% wider or more.
[0076] Certain dimensions for the new threaded connections
according to the invention are set forth in the table below, and
can be compared to conventional 3/8''-24 tpi UN (Unified) and
1/2''-20 tpi UN threaded connections using dimensions and
calculations from the applicable ANSI standard.
TABLE-US-00001 Conventional Conventional New - (1/2'') - (3/8'') -
Male New - Male Male Male Electrode/ Electrode Electrode/ Electrode
Female Holder/ Female Holder/ Electrode Female Electrode Female
Holder Torch Body Holder Torch Body Threads per Inch 12 12 20 24
Male D.sub.p 0.364 0.294 0.464 0.345 Male K 0.389 0.317 0.437 0.322
Male D 0.441 0.369 0.495 0.370 Female D.sub.p 0.396 0.324 0.470
0.350 Female K 0.395 0.323 0.452 0.335 Female D 0.449 0.377 0.506
0.381 P 0.083 0.083 0.050 0.042 2.alpha. (deg.) 29 29 60 60 Male
d.sub.m 0.415 0.343 0.466 0.346 Female d.sub.m 0.422 0.350 0.479
0.358 Female r.sub.w 0.035 0.035 0.020 0.017 Female c.sub.w 0.048
0.048 0.030 0.025 Male r.sub.w 0.055 0.055 0.026 0.022 Male c.sub.w
0.028 0.028 0.024 0.020 Female Crest Flat 0.041 0.041 0.014 0.012
Female Root Flat 0.028 0.028 0.004 0.003 Male Crest Flat 0.022
0.022 0.007 0.006 Male Root Flat 0.048 0.048 0.009 0.008
All Dimensions are Inches Except as Noted
[0077] Given the space constraints available, the present invention
advantageously provides a threaded connection that can be made
between the electrode holder 56 and the electrode 20 with
relatively low crest/root height compared to conventional designs.
Although illustrated with the narrower crest profile being provided
on the male thread portion of the electrode and the male thread
portion of the electrode holder, the same relative compactness can
be achieved by forming the narrower crest profile on a
corresponding female threaded portion of the electrode holder
and/or a female threaded portion of the torch body. Similarly, the
positions of the male and female threads as between the electrode
and the electrode holder and/or as between the electrode holder and
the torch body can be reversed from those illustrated and still
provide advantages of the type discussed above. The compact
threaded connection provides an advantageous dimensional
relationship within the torch.
[0078] The present invention also includes a more distal position
for the electrode holder in the torch, and the threaded portion of
the electrode holder engaged with the threaded portion of the
electrode is advantageously partially or wholly within the nozzle
chamber 41, as can be seen in FIG. 4. As a result, the electrode 20
is much shorter than prior art electrodes of this type, which
reduces manufacturing costs. This is especially important because
the electrode is a consumable part and is the most frequently
replaced part of a plasma arc torch. The electrode holder 56 may
also need to be periodically replaced. However, the replacement
rate is much less often than that of the electrode 20.
[0079] Also, the "unequal" thread profiles of the electrode 20 and
the electrode holder 56 allow for detrimental wear of the threads
to be allocated more to the consumable electrode 20 than to the
electrode holder 56. In other words, it is more important for the
electrode holder to have wider crests for its threaded portion than
for the electrode because the electrode holder is expected to
securely hold many electrodes as the electrodes are consumed and
replaced. This can cause wear and other damage to the threaded
portions by repeated replacements, and the wider crests of the
electrode holder (which are provided by the threaded portions of
the electrode according to the invention) provide this additional
durability.
[0080] Many modifications and other embodiments of the inventions
set forth herein will come to mind to one skilled in the art to
which these inventions pertain having the benefit of the teachings
presented in the foregoing descriptions and the associated
drawings. Therefore, it is to be understood that the inventions are
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. It should also be
understood that reference to dimensions and angles of the various
parts mentioned herein, including relative dimensions, are intended
to relate to nominal dimensions representing a target value in a
manufacturing processes. Thus, absolute values deviating from the
nominal values by manufacturing tolerances are intended to be
included within the scope of the dimensional and angular
references.
* * * * *