CA1324189C

CA1324189C - Apparatus and process for cooling a plasma arc electrode

Info

Publication number: CA1324189C
Application number: CA000605319A
Authority: CA
Inventors: Nicholas A. Sanders; Richard W. Couch, Jr.
Original assignee: Hypertherm Inc
Current assignee: Hypertherm Inc
Priority date: 1988-09-26
Filing date: 1989-07-11
Publication date: 1993-11-09
Anticipated expiration: 2010-11-09
Also published as: EP0391984A4; WO1990003243A1; US4902871A; EP0391984A1; JPH03501467A; DE68907891T2; EP0391984B1; JP2568126B2; DE68907891D1

Abstract

Abstract of the Disclosure In a plasma arc torch, an electrode is movable axially into and out of electrical connection with an anode, typically a nozzle secured to one end of a torch body. A flow of pressurized plasma gas is directed to a plasma chamber between the electrode and the nozzle, preferably through a replaceable swirl ring that closely surrounds and guides the electrode at a larger diameter shoulder portion of the electrode. A gas flow passage, preferably a spiral passage machined on the outer side surface of the shoulder portion, diverts a portion of the gas flow from the plasma chamber to a region above the electrode where it is vented to atmosphere. The passage is sufficiently constricted that a substantial pressure drop appears along the path, while at the same time allowing a sufficient flow to produce the desired cooling. The revolutions of the spiral are preferably closely spaced to enhance the surface area of the electrode in a heat transfer relationship with the cooling gas flow.

Description

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APPARATUS A~ PR()CESS FOR COOLING
A PLASMA ARC l~LECTRODE

Scope of ~he Invention Thi~ invention relates to plasma arc cutting ~y~t~m~;, More specîfically, it relates to a novel ~lec~rode, a novel ele~trode ~ooling system, an~ a novel gas cosling proces~ or use wi~h the electrode.
Bxief De~cri~éion o ~h~ Drawi~q~
Fig. ~ a view i~ vertical ~ction of a plasma arc ~utting torch with a fluid orce contact . ætarting capability, a~d 0 Pig. 2 i~ a vi~w i~ vertical sec~ion corresponding l:o Fig. 1 showiIlg a plasma arc torch l with an el2ctrod~ cooling sy'stem according to the present invention.
~ound of the Invention Applicants have d~vised an apparatus and m~hod for corltac~ ~tarting a plasma ar~ ~orch which does no~ re~Iuire the ~orch to touch the snetal : workpiece b~ing ~ut or welded by the torch. The ~orch ha~ an electrode that move~ axially within th~ torch body under the influence of a spring and opposed gas , ...

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1 forces which act on the lower surfaces of ~he electrode adjacent the anode, typically ~he torch nozzle. On start up, it was found that gas pres~ure in the region between the ~lectrode and could build to a suficien~ level that ~hey could lift the electrode against the spriny. ~hen th~ gas is cut off to stop cutting~ the spring returns the cathode to a position where it contacts the anode and closes off the plasma exit port in the nozzle. ~his structure and mode of lo operation are described and claimed in the parent United States patent 4,791,268.

This development has been a signi f icant improvement in arc contact starting and has found :i. commercial accep~ance. Howev~r, several ~reas for improYement have com~ to ligh~. One relates ~o the fact that the ~lec~rode must be cooled. In earlier designs, applicants hav~ split: ~he plasma gas stream, directing a portion through a swirl ring to a reg;on I over the electrode, as shown i.n Flg. 1. After i 20 circulating around the upper ~;urfa~es of the ;, el~ctrode, the gas exits to the a~mosphere through .~ ports and passag~s formed in the torch body. This arrangement has several disad~an~ages. One is ~hat the presencP of pressuri~ed air above the electrodes, as well as below it, produces a delicate balancing of th~ ~luid forces ac~ing on the electrode. This adversely affect~ the reliability of the separation of , the electrode rom the nozzle on start up. ~nother is :1.
"1 ~1 l that ~his cooling is relatively inefficient sincQ the gas con~acts only a fraction of ~he surface are~ of the eleGtrode, and at a point most distant from the plasma arc. Finally, this cooling arrangement : 5 consumes more plasma gas since the portion used for cooling does not contri~ute to formlng the plasma arc.
~nother problem is that the mating surfaces of th~ housing twhich is intended to mean the o~her housing as well as componen~s which are mountqd on ~he housing and can guide and support ~he ~lec~rode), mus~
be machined to ~xtremely tight tolerances in order to allow a ~liding movement of the electrode, whil~ at the same time blocking the flow of pressurlzed gas , 15 past th~ electrode. Th~ machining requirements to j produce parts with the requislte tight ~ol~ranc~
j engender higher manufacturing costs, bo~h for the torch as well as for repl~cement electrodes.
therefore a pr:;ncipal object of this invention to provide a cooling ~yst~m and process for the electrode of a pla~ma arc torch, par~icularly on~
i~ whQre 1uid forces mov~ the electrode for contact i; starting, which cools more e~fectively than known .,; ~ystems and does not interfere with the balance o forces acting to create th~ arc~ ;
~nother objec~ is to provide a cooling system and process with the foregoing advantages that requires looser machining tolerance~, and therefore a lower cost o4 manufacture, than compara~le known ::
system~ 2nd processes.
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1 Y~t another object :is to provida a cooling system and process with the foregoing advantages that also consumes less plasma gas tban comparable known system~ and processes.
~ S ~ still further object is ~o provide a - cooling system and process with th~ foregoing ; advantages which Call be r~adily retrofit onto existing plasma arc torches.
Summary of the Invention lo A plasma arc cutting torch has a torch body ~hat mollnts a nozzle (an anode) and an electrode (a cathode) movable axially within ~h~ torch body between a star~-up po~ition, where i~ is in electrical conne~tion with the nozzle and closes an outlet '' 15 orifice of the nozzle, and an operating position where ' the electrode is separated rom the nozzle. A spring ', urges the electrode toward th~ nszzle. Plasma gas under pressure i8 guided through the torch body, and I preferably through canted por~s of a swirl ring, to a j 20 plasma chamber defined by ~he electrode, the nozzle and the torch body. Xn the preferred form the I electrode has an integral shoulder portion whose sid~
surface is guided by the swir:l ring.
' A ga~. flow passage i~ formed in the electrode .. ~ 25 with on~ end communicating with the pre~surized gas in the plasma chamber and a second end communicating wi~h a region opposite from the plasma chamber. This region is sealed from a direct 1uid communication ~:
with the plasma chamber by ~he close-fitting relation6hip of the shoulder portion side ~ur~ace and the surrounding swirl ring, but also by ~he cooling ~ ga~ 10w through thi~ ~lectrods passage. This region '~

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l is vented through suitable passages in the torch body to atmosphere. This gas flow passage in th~ electrod~
allows a portion of the plasma gas entering the plasma chamber to flow through the electrode to cool it, but s at the same time offers 7iuffici~nt resistance to ~he ' fluid flow to produce a substantial pressur~ along the '~ path and ~hereby allow the ga~ entering ~he plasma ~I chamber on s~art-up to reach a pressure levlal sufficient ~o move the elec~rode ou~ of electrical ,, 10 connectio~ with th~ anode agains~ thQ ~orce of the spring. The passage is preferably iA the form of a spiral groove with at least 6 revolutions psr inch, measured ~xially, i~ the direc~ion of mo~ement of the ~lectrodQ, machined in the side surface o~ the should~r portion of ~he electrode. Th~ electrode is :~ preferably ormed as a~ integral, one-pielce part of a .~ material with excellen~ heat conductivity charactQristics, such as copper.
Th~se and other features and objects of the 3 20 present invention will be mor!a ~ully understood from ~he following detailed de~crip~ion which shsuld b~
read in li~h~ o the accompanying drawings. :
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: .-De~ail~d Description o ~he Pre~erred Embodimen~
`~ Fig. l shows a plasma arc torch having a ~:~
3 25 torch body 1~ with an i~ner component 12a and an outQr .~

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component 12b, a plu~g~r 14 and a ~pring 16 that drive~ the plunger downwardly, as shown. Consumable parts of the ~orch 10 include a swirl rin~ 18 secured ~o ~he lower ~nd o the body component 12a, a nozzle 29 with a central plasma arc exit orifice 20a, an ~leotrod~ 22, and a caI? 24 threaded onto the body componeII~ 12b at its lower end. Th~ cap 24 capture~
the nozzle and holds it iIl place. The ~lectrode 22 is slidable axially (ver~ically a~ shown~ within the ::
0 swirl r ing 18. In a starting positiorl illustrated in ~i Figs . 1 and 2, the lower end f ace 22a ' closes o~f the exi~ orifice 20a. In a raised, opera~incJ posi~iorl~ ar u~?per surface 22a" of the bo~ portion either abuts or ~ .
, is ne~r the low~r end of the body component 12a and the OrifiCe 2OB is open. This rai~ing movQment i~
accomplished using ~luid orces.
A pressurized 3?lasma gas flow 26 enters the torch via passag~ 28, port or port 30, an annular ?assage 3~ and canted ports 34 in the swirl rillg 18~ ~:
finally entering a plasma chamber 36 d2fined by the eli3~trodQ, the ~wirl rin~ and ~he nozzle. In ~his ~igO 1 embsdlmen~, a portion 26a of the plasma ~a~
.1 1OW passes through B ~et o can~ed cooli~g ports 38 ormed in th~ swirl ring to impinge on the upper ; ~
~ur~acef~ o th~ electrode. Thif~ cooling gaæ flow 26a ~;
., exit~ the t~ch through hole~ 40 in the body portio~
`I 12~ ~d hole~ ~2 in the body portioll 12b.
I~ thi~ Fig. 1 embodiment, th~ main plasma 3~ ;~

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1 gas 10w ~6c entering the plasma chamber 36 pressuriz0s the chamber to create a fluid li~ing force acting on the lower surfaces of ~he electrode.
i, This orce overcom~s the spring orce, and the ba~k pressure o the cool;ng ga~ 10w 26a ac~ing in ~h~
upper surfaces of the electrode forcing the electrod~
to msve upwardly to its operating position. The pilot ~:! arc produced as the elec~rode breaks ele~trical ~, connection with the anode initiates a plasma arc which exits ~he torch through the ori~ice 20a where it attache~ to a metal wor~piece to be cut or w~lded.
When the el~ctrode is raised, the main gas flow 2Sc in ¦ ~h~ plasma chamber 26 swirls about th~ lower electrod~
. body portion 22a, as shown. A ~ortion 26b of the main . 15 yas flow 2S i~ al~o directed through holes 44 in the : ¢ap to cool tor~h parts other than the electr~de. A~
noted above, in this form there is a problem in ~:; balancing the fluid forces acting on the elec~rod@ and : there is cooling onl~ ~t fhe top suxfaces of the : 20 electrode.
Turning now to the Fig. 2 embodimen~, like ~ parts in Fig~. 1 and 2 are identified wi~h the sæm~
: ~ : re~erence nu~ers~ Most o the torch parts are -i identical to those described above wi~h r~ference to ~ig. 1, how~ver, the electrode 22 i~ of a di~rent ~1 design~ and the ~wirl ring 18 i~ constructed ``,J dlffe~ently, 80 that in comblnation th~y pro~duce very diferent cooling gas ~low pathæ within the torch 10 and eliminate the presence of a high pressure gas aboYe th~ el2ctrode. ThQ plasm~ gas stream 26, excep~

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1 for a portion 26b that exits the cap through the holes 44, passes entirely through the canted ports 34 to ~, enter the plasma chamber 36. There ~re no other opening~ in the swirl ring or other p~rts allowi~g a direct flow of plasma gas to a region 46 over the electrod~ (defined principally by ~h~ upper surfaces , of the electrode, the swirl ring and the lower surfaces of th~ body compon2nt 12a~. The flow 26b through the cap holes 44 serves the same cooling function described above wi~h respect to Fig. ~
A principal featur~ of this invention is a gas flow passag~ 48 formed in the elec~rod~ and extendi~g from a first end ~8a in fluid ~ommunication wi~h the plasma chamber 26 and a second end 48b in fluid communication with the region 46. In ~he ~:-preferred form illustrated/ th~ passage 48 is a ~piral ~roove ~ormed in the ou~er side wall of ~he shoulder portion 22b of the electrode. The cross-sectional dimensions, the length, and the conflguration of th~
~: 2~ passage are such tha~ a cooling gas flow ~d traYels up th@ passag~ to the region 46, but the passag~ is æuf1clently restrictive to th~ flow that there is a sub~tantial pressure drop along the passage.
.3 With the spiral groove configuration for the pa~sage ~8, the electrode "fins" 22c between ~he grooves act as heat transfer surface~. They grea~ly 3 ~ncrease the surface area of khe el~ctrode e~posed to the cooling gas and thus more efec~ively cool th~
electrode than with the arrangemen~ shown in ~ig. l.
Tha flow o cooling gas between the fins 22c also ~-.. ~ - .
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~ 3 7t ~ ~L '~ 9 1 produces a gas s~al ~hat blocks a flow of pressurized gas directly from th0 plasma cha~ber 26 to ~he region 46 along the interface bet~een the swirl ring and the electrode. This allows the elQctrode, par~icularly the side surfaces of ~he~ ~houlder portion, to bQ
machined to looser tolerances than in the Fig. 1 ~mbodiment. By way o~ illus~ra~ion, but not of limitation, or an electrode having a maximum diameter of 0.5 inch, and with a pressurized gas flow 26 at 40 psi, th~ aforementioned operational characteristics I can be observed with revolutions machined at a i frequency of tWQ~ty per inch (measured axially), and a ! groove ,naving a gen~rally con~tant depth of I approximat~ly 0.080 inch and a width (alæo m~asured axially a~ shown) of 0.0~5 inch. More generally, ! using a spiral groove on aa e;Lectrode of generally a ,1 ~alf inch maximum diameterO ~he gro~v~ should hav~ at ¦ least six re~olutions per i~ch, with the groove having a depth (measu~ed laterally a~ shown~ of at lea~t 0.040 inch and a "width" ~asured vertically as shown) of a~ least 0.0~0 i~ch.
Because ~here is a su~stanti~l pressure drop ~long the pas~age 48~ the gas flow 26c pr~ssuriz,Ps ~he :~:
chamb~r rapidly with only a small pressure a¢ting on thQ opposite surface~ of the electrod~ in the r~gion ~-~
46. This pressuri3atio~ "blows back" the electrode againæt the for~ of th~ spring 16 allowing the 10w 26c ln thQ pla_ma chamber to assume an unrestricted swirling pattern, a~ shown in Fig. 2, which is eonducive to th@ formation o a good ~uality plasma ,-~ 3~

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- ~ lo arc. The cooling arrangements of the presen~
inven~ion therefore provide both an effec~iva cooling of th0 ~lectrode as well a~ reliable contS~ct starting ~hrough a ~as blow-back of the electrode . ~ :-:, 5 Sta~ed in process terms, the presen~
inven~ion inYolyes supplying a pressurized f low 26 of plasma gas, formiIlg a gas flow passage t~rough the :. electrode, div~rting a portion 26d of the flow ~hrough the passage to cool th~ electrode~ whil~ a~ the same l lo time restricting the coolinSg gas flow through ~hes passag sufficiently to maint~in a substantisal pressure drop along the passagP.
There has been de~crib~d a cooling system and ~ ~:
proc~ss which provide~ an enhanc~d cooling o ~h~ :
el~c'crode and a reliable "blow back" o the electrode or gas-pressure actuated contact Sstarting. ~hi~
invention call ~e practiced on any ~xisting torch where ~he plasma gas is the cooling ~s by replacirlg .1 conslunable parts with ones having the features :~
.j 20 described and claimed herein.
While ~he invention has been described with re~p~ct to it~ preferred embodiment~ will be ~:
ulderstood that variou~ modiications and altera~ions will ocaur to tho$e ~l~illed in the art from ~he ..
~5 foregoing detailed de~cription and the accomparlying .~ drawings. For example, while the ;nv~n~ion has been ::
desaribed with respect to a spiral groov~ cut in t}le ou~er surface o the ~houlder portiont it could be formed by a non-spirat passage, multiple iIldependent passage~, or paæsages machined through th~ body of the ~:;
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1 electrode as well as the shoulder. Further, while the invention has heen described with respect to an .~ electrode that moves axially for contact ~tartiny, ~he ~- features of the presen~ invention could be applied to a s~ationary electrode. Further, while the electrod~
.. has been described as moving within a swirl ring as a guide ~d support elemen~, it will be understood that it could be mounted to move within the torch body sr .- some other replaceable torch component. ~herefore, as . lo used herein, "torch body" should be in~erpre~ed to ~...... include ~he swirl ring or other component acting a~ a -~` guide and suppor~ for the electrode. These and other modi~ic~tions and varia~ions are int nded to fall ~, within th~ scope of the pending claims.
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Claims

1 . In a plasma arc torch having (i) a torch body, (ii) an anode supported on the body, (iii) a cathode mounted for an axial sliding movement within the body, said cathode, anode and torch body defining a plasma chamber, (iv) resilient means mounted within the body that urges said anode and said cathode into electrical contact, (v) means for directing a pressurized flow of a plasma gas to said plasma chamber to produce a fluid force within said plasma chamber that separates said anode from said cathode, and (vi) a volume opposite the chamber that is vented to atmosphere which is at a substantially reduced pressure as compared to the gas pressure in said plasma chamber, the improvement comprising a cooling gas flow passage formed in said cathode that is in fluid communication between said plasma chamber and said volume, said passage being dimensioned and configured to restrict a gas flow therethrough to a sufficient degree to maintain a pressure differential between the plasma gas in said plasma chamber and the plasma gas in said volume, while at the same time passing a sufficient gas flow to said volume to cool the cathode.

2. The improved torch of claim 1 wherein said body includes a replaceable swirl ring secured to the body and closely surrounding said cathode at a side surface to allow its axial sliding movement while blocking any significant flow of gas from the plasma chamber to said volume except said flow through said gas flow passage.

3. The improved torch of claim 2 wherein said cooling gas flow passage comprises a spiral groove formed in said side surface of said cathode.

4. The improved torch of claim 3 wherein said spiral groove has at least 6 grooves per inch, measured axially, to place a large surface area of said cathode in contact with said cooling gas flow passage.

5. The improved torch of claim 4 wherein said cathode is formed of a material having a very good heat conductivity characteristics and said spiral groove has at least 10 grooves per inch, measured axially.

6. The improved torch of claim 4 wherein said groove has a depth of at least . 040 inch and a width of at least 0.020 inch.

7. An electrode for a plasma arc cutting torch having a torch body, an anode secured on the torch body with a central outlet orifice for said plasma arc, resilient means mounted within the body that urges said electrode into electrical connection with said anode, and means for directing a flow of pressurized plasma gas to a plasma chamber defined by said electrode, said anode, and said torch body to produce a liquid force within said plasma chamber that separates said electrode from said anode against the force of said resilient means, said electrode comprising, a body portion that seals said outlet orifice when said electrode and said anode are in electrical connection, a shoulder portion having an outer side surface in a closely spaced, sliding engagement with said torch body that also provides a seal against a flow of the pressurized gas from the plasma chamber along said outer side surface, and a cooling gas flow passage formed in said electrode and extending between a first end of said electrode open to said plasma chamber and a second end open to a region sealed from said plasma chamber by said shoulder portion, said cooling gas flow passage being dimensioned and configured to restrict he flow of the pressurized gas therethrough to maintain a substantial pressure differential between the gas pressures at the ends of said passage while at the same time allowing a sufficient cooling gas flow through said passage to cool said electrode.

8. The electrode of claim 7 wherein said body portion and said shoulder portion are formed integrally.

9. The electrode of claims 6 or 7 wherein said electrode is formed of a material having a large coefficient of heat conductivity.

10. The electrode of claim 9 wherein said material is copper.

11. The electrode of claim 9 wherein said cooling gas passage is a spiral groove formed in said side surface of said shoulder portion.

12. The electrode of claim 11 wherein said groove has at least 6 grooves per inch, measured axially.

13. The electrode of claim 12 wherein said groove has a depth of at least 0.04 inch and a width of at least 0.020 inch.

14. A process for cooling an electrode mounted in a torch body of a plasma arc torch at plasma chamber also defined by an anode mounted on the torch body, comprising directing a flow of pressurized plasma gas to said plasma chamber, forming a gas flow passage through said electrode extending from said plasma chamber to a region sealed from said chamber and at a lower gas pressure than said plasma chamber, and diverting a portion of said pressurized gas flow through said passage to cool said electrode, and restricting said diverted flow through said passage to produce a substantial pressure drop along the length of said passage.

15. The electrode cooling process of claim 14 wherein said forming produces a spiral groove.

16. The electrode cooling process of claim 14 wherein said electrode is movable against a spring force and said restricting produces a sufficient pressure differential to move said electrode against said spring force.

17. In a plasma arc torch having (i) a torch body, (ii) an anode supported on the body, and (iii) a cathode mounted for an axial sliding movement within the body, said cathode, anode and torch body defining a plasma chamber, the improvement comprising resilient means mounted within the body that urges said anode and said cathode into electrical contact, means for directing a pressurized of a plasma gas so said plasma chamber to produce a fluid force within said plasma chamber that separates said anode from said cathode, a volume opposite the chamber that is vented to atmosphere which is at a substantially reduced pressure as compared to the gas pressure in said plasma chamber, and a cooling gas flow passage formed in said cathode that is in fluid communication between said plasma chamber and said volume, said passage being dimensioned and configured to restrict a gas flow therethrough to a sufficient degree to maintain a pressure differential between the plasma gas in said plasma chamber and the plasma gas in said volume, while at the same time passing a sufficient gas flow to said volume to cool the cathode.