design of welding fixture for head end sub assembly of motor case

7/27/2019 Design of Welding Fixture for Head End Sub Assembly of Motor Case

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INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 3, ISSUE 6, JUNE 2014 ISSN 2277-8616

276IJSTR2014www.ijstr.org

Design of Welding Fixture forHead End Sub-Assembly of Motor Case

Naveen A M, V A Girish

Abstract This paper deals with the design of the welding fixture for the head end sub-assembly of a rocket motor case. The head end

sub-assembly consists of four parts namely Y-Ring, Dome, Igniter Boss and the Fore skirt ring that have to be welded to each other with a

specified tolerance and weld quality. The material used in the manufacture of different parts of head end sub-assembly is Maraging steel

which is one of the most commonly used material in the field of aerospace. All the welding processes are carried out in the presence of the

Copper Arsenic back-up support bar to ensure the quality of weld is as per the requirement. The optimum thickness of the back-up support

bar is determined and with in-process purging facility in the copper-arsenic bar ensure sound weld. Modeling of all the parts of head end

sub-assembly and the welding fixture is carried out using UNIGRAPHICS NX8.0.

Index Terms Welding fixture, Maraging steel, Motor Case, Copper-Arsenic, Weld back-up support, head end sub-assembly.

1 INTRODUCTION

fixture is a device for locating, holding and supporting awork piece during a manufacturing operation. Fixturesare essential elements of production processes as they are

required in most of the automated manufacturing, inspection,and assembly operations. Fixtures must correctly locate awork piece in a given orientation with respect to a cutting toolor measuring device, or with respect to another component, asfor instance in assembly or welding. Such location must beinvariant in the sense that the devices must clamp and securethe work piece in that location for the particular processingOperation. Fixtures are normally designed for a definiteoperation to process a specific work piece and are designedand manufactured individually.

he correct relationship and alignment between thecomponents to be assembled must be maintained in thewelding fixture. o do this, a fixture is designed and built tohold, support and locate work piece to ensure that eachcomponent is !oined within the specified limits. " fixtureshould be securely and rigidly clamp the component againstthe rest pads and locator upon which the work is done.

Fixtures vary in design from relatively simple tools toexpensive, complicated devices. Fixtures also help to simplifymetalworking operations performed on special equipments.Fixtures play an important role on reducing production cycletime and ensuring production quality, by proper locating and

balanced clamping methods #$%.herefore to reduceproduction cost, fixture design, fabrication and its testing iscritical.&enerally, all fixtures consist of the following elements' (ocators' " locator is usually a fixed component of a

fixture. )t is used to establish and maintain the position ofa part in the fixture by constraining the movement of thepart.

*lamps' " clamp is a force actuating mechanism of afixture. he forces exerted by the clamps hold a partsecurely in the fixture against all other external forces.

Fixture +ody' Fixture body, or tool body, is the ma!orstructural element of a fixture. )t maintains therelationship between the fixture elements namely (ocator,clamps, supports, and the machine tool on which the partis to be processed.

Supports' " support is a fixed or ad!ustable element of afixture. hen severe part displacement is expected underthe action of imposed clamping and processing.

elding fixtures for work piece comprise the usual locatingand clamping elements as used in other fixtures. -owever, theeffect of heat and prevalence of welding spatter must be takeninto account while designing hot !oining fixtures. he designconsiderations for welding fixtures are given below'

xpansion of the heated work piece and resulting

distortion should not affect proper location, clamping,

loading and unloading. here should be adequate

clearance between the work piece and locators to permit

expansion, contraction and distortion of the work piece

without !amming the fixture. -andles sub!ected to heating

should be made of insulating materials such as wood.

elding spatter should not be allowed to fall on the

threaded parts of the clamping elements. he parts near

the welding area should not to be threaded.

Spatter grooves must be provided below the line of

welding to prevent the work piece from getting welded to

the base plate.

*are should be taken to check that the !oined work piece

do not get locked in the fixture after welding.

For work piece requiring welding from a number of sides,

a provision for easy tilting or rotating the fixture should

be made to ease welding from the various sides.

o protect the weld from the atmosphere, the purging

facility can be designed.

"

Naveen A M is currently pursuing masters degree program in Toolengineering in RVCE,Bangalore /-09448588886.0mail' [email protected]

V A Girish iscurrently Assistant Professor, Department of MechanicalEngineer, RVCE, Bangalore


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` 277IJSTR2014www.ijstr.org

o have rapid cooling to achieve less defects in weld, the

backup bar facility must be designed.1*opper0"rsenic bar

has been used in this paper2

2 INTRODUCTION TO ROCKET SYSTEM

Solid rocket systems are used extensively in situations in

which the total impulse is known in advance and restart is not

required. Structurally, a solid rocket motor 1see Fig. $2 consists

of the solid propellant grain itself, the liner, whose primary

purpose is to provide an adhesive bond between the

propellant grain and the case insulation3 the case insulation

which provides thermal protection to the case from

combustion products and also structurally supports the

propellant grain with the motor case. "ny one of these

structural elements may become the weakest link in the

propellant grain structure. " rocket motor case is used onlyfor a short duration during flight#4%.

Fig 1 *ross0section of a typical rocket motor' 1"2 chamber3 1+2head end dome3 1*2 no55le3 162 igniter3 12 no55le convergentportion3 1F2 no55le divergent portion3 1&2 port3 1-2 inhibitor3

1)2 no55le throat insert3 172 lining3 182 insulation31(2 propellant3 192 no55le exit plane31:2 S);* system3

1O2 segment !oint.

-ead end sub0assembly is a part of motor case is as shown inthe Fig 4. )t consists of four main parts 1$2 2, 142 6ome1shown in fig.?2 ,1@2 )gniter boss1shown infig.A2 and the 1>2 fore0skirt ring 1shown in fig.B2 which have tobe welded together with a specified tolerance and quality.)t isthe main part of the motor case where the actual initialignition to produce the thrust in the rocket motor case takesplace. )t is made up of 9araging steel material. )t is one of themost popular material used in the aerospace field. he cadmodel of the -ead end sub0assembly is as shown in fig 4.

)n order to keep in track with the tolerance and the qualityof weld a proper welding fixture has to be designed. hecopper0arsenic back0up support bar with purging facilitywhich blows out the hot gas with the influence of highpressurised inert gas blown into the groove machined exactlybelow the weld line 1a tiny hole is provided for heat to escapeduring welding2 is used in order to maintain the quality of theweld by rapid cooling. he heat evolved during the welding is

taken by the copper0arsenic bar due to its property of highthermal conductivity and the provided purging holes willmake way for the heat to escape into the atmosphere#@%.

Fig 2 -ead nd Sub0"ssembly

3 LITERATURE REVIEW

Siva Sankara =a!u = et. al #>% have reported the materialproperties of the maraging steel and :i6) #B% has been used towork on the welding and its weld parameters of maragingsteel.

9 * 9ittal et al #@% have experimented successfully onthe &"1&as ungsten "rc2 welding of maraging steel usingthe copper back0up plate with the purging facility to analy5ethe fracture toughness.

9.;arul et al #?% have examined the effect of weldingfixture to prevent the weld distortion, which are very muchtaken into consideration while designing the welding fixture.

-ui ang et al #A% have reported the direct impact offixture on the product manufacturing quality, productivityand cost. he methodology of building a fixture has beenreported, which will be a guideline to design the weldingfixture.

)ain +oyle et al #C% have stated the basic fixture designprinciple, the flow chart of the design steps to design the weldfixture has been stated and @040$ principle of location has beenexplained in this paper that has been considered whiledesigning the fixture.

"ccording to -.. Sanche5 and 9. strems. F, the finalprecision with which work piece are formed strongly dependson good design and correct operation of fixtures #D%. he mainerrors introduced by fixtures are related to positioning,indentation and structural deformation that has been reportedin this paper.

/atricio. F. 9ande5 et al #$E% has given the different typesof welding techniques and their parameters that are used inthe "eronautical industry. he paper has been studied and therequired weld parameters have been extracted beforedesigning the fixture.

F. Sikstrom et al #$$% reported in his paper the influence ofthe clamping force on the structural integrity of the &"welded work piece. he paper has been studied andconsiderable clamping force calculations have to be carriedout in designing of the welding fixture.


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=. Scott Funderburk #$4% has reported the key concepts inwelding operation and in his paper the expression for heatinput has been stated.

&. =. Stoeckinger et al #$@% have conducted a successful

experiments on computeri5ed prediction of heat distributionin welding tool and they have used the expressions ofthermodynamics to arrive at the heat transfer efficiency andheat energy.

4 METHODOLOGY

he design of the welding fixture for head end assembly iscarried out as per the flow chart given in fig@.

Fig 3Flow *hart to design the fixture)n this flow chart, the initial step starts with the material

information ,machine specifications,geometric dimensions and

tolerances required to be achived on the component , anddifferent parts of the head end sub0assembly and their caddrawings which are modeled using the software:)&="/-)*S :G C.E.

+efore the design of the welding fixture the fixturerequirements have to be considered.Some of the requirements are stated below&eneric requirement' "bstract sub0requirement examples. /hysical' he fixture must be physically capable of

accommodating the work piece geometry and weight. hefixture must allow access to the work piece features to bemachined.

olerance' he fixture locating tolerance should be

sufficient to satisfy part design tolerances. *onstraining' he fixture shall ensure work piece stability

1i.e., ensure that work piece force and momentequilibrium are maintained2. he fixture shall ensure that

the fixtureHwork piece stiffness is sufficient to preventdeformation from occurring that could result in designtolerances not being achieved.

"ffordability' he fixture cost shall not exceed desiredlevels. he fixture assemblyHdisassembly times shall notexceed desired levels. he fixture operation time shall notexceed desired levels.

*ollision prevention' he fixture shall not cause tool pathfixture collisions to occur. he fixture shall cause workpiecefixture collisions to occur 1other than at thedesignated locating and clamping positions2.he fixtureshall not cause fixturefixture collisions to occur 1otherthan at the designated fixture component connection

points2. sability' he fixture weight shall not exceed desired

levels. he fixture shall not cause surface damage at thework pieceHfixture interface. he fixture shall providetool guidance to designated work piece features. I hefixture shall ensure error0proofing1i.e., the fixture shouldprevent incorrect insertion of the work piece into thefixture2. he fixture shall facilitate chip shedding1i.e., thefixture should provide a means for allowing machinedchips to flow away from the work piece and fixture2#C%.

4.1 MARAGING STEEL

9araging steels are leading members of the ultra highstrength steel family and derive their superior properties likehigh strength and toughness due to a combination of two solidstate reactions' 9"= J "&):& meaning martensitictransformation and subsequent ageing. hese steels areprimarily based on the Fe:i system. he composition is sobalanced that on cooling from the austenitic region ittransforms to soft martensite and on ageing, precipitation of Kinter0metallic compounds occurs on a fine scale to increasestrength. hese steels have attracted material scientists andaerospace structural designers because of their uniquestrength0toughness combination, ease of fabrication and heattreatment, good weld ability and minimum dimensionaldistortion. ighteen percent of :i maraging steel is the mostwidely used ultra0high tensile strength maraging steels. )t is

classified into 904EE,904?E, 90@EE and90@?E gradesaccording to their E.4L proof stress levels, namely 4EE, 4?E,@EE and @?E ksi. eld ability is one of the most importantproperties of ultra0high tensile strength steels andconsequently steels with poor weldability cannot be applied topractical use. he weldability of $CL :i maraging steel differsgreatly from that of the other martensitic ultra0high tensilestrength steels in that the former does not need preheating orpost0heating, has little hardening in the heat affected 5one andproduces few cracks during welding. )n addition, the ageingtemperature after welding is low, and therefore thecontraction on ageing is very little #4%.


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he compostion used in head end sub0assembly is 96:04?E which has a composition of $CL:i and the properties of96:04?E is as stated below#$>%'

6ensity' C.$ gHcm1E.4DlbHin2 Specific heat, mean for E$EE * 1@44$4 F2' C$@ 7Hkg8

9elting point' 4,?B? F, $,>$@ *

hermal conductivity' 4?.? Hm8

9ean coefficient of thermal expansion' $$.@$EA

-=* 1grade @EE23?C -=* 1grade @?E2

4.2 DESIGN OF WELDING FIXTURE

he given information about the head end sub0assembly are asgiven below'

Table-1 Input information

Fig 4 Igniter boss Fig 5 Dome

Fig 6 Y-Ring Fig 7 Fore Skirt Ring

The models of the above mentioned parts of the head endsubassembly are given below from fig > to fig B. he optimumprocedure of sequential of welding process using the designedwelding fixture has been established as shown in the abovetable2gives clear cut information:

able02Sequence o weld

he model of the welding fixture for the head end sub0assembly is shown in the fig C. his fixture is casted withaluminium considering the casting does not have a ma!oreffect due to the temperature rise because of the welding.Since, the heat evolved during welding is thrown out as soon

as it is produced because of the purging facility providedunder the weld bead. he critical part of the casting is theprofile. his casted round profile has to match the profile ofthe dome ,


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INTERNATIONAL JOURNAL OF SCIENTIFIC & T

the welding fixture is carefully casted anis finali5ed to use as a welding fixture.

4.3 ARSENIC COPPER WELD B

+ackup supports are used for welthickness for easy penetration controlcarried out from one side only. he role oalso essential to protect the under beaoxidation. his is achieved by gas purgithe under bead is smooth and of highsupport can be divided into three categtype of welding'

$. +ack0up support for4. +ack0up support for@. +ack0up support for

he width 1w2 of the back supportdistance between two clamps, width of pr

of the clamps and additional width to hol"dditional rigidity to the back suppoperipheral aluminum "lloy or mild steel rin weight. "ll the above categories of bacdivided on the basis of type of grooves pro

he selection of grooves' he groove type 1)2 is selected for largel0seam welding where the shell thicknesthickness2 is not less than >mm and undewith parent material. he groove widthwith respect to the sheet 1component2 thic he groove type 1ii2 is selected for small1circular2 welding where under bead nflushed to parent material. he groovevariable factors.

CHNOLOGY RESEARCH VOLUME 3, ISSUE 6, JUNE 201

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inspected before it

CUP

ding of significantwhere welding isf backup support is

from atmosphericg and ensures thatuality. he backup

ories depending on

0seam weldingir0seam weldinglanetary weldingis the sum of the

essure exerting area

d the !oint properly.rt is provided bying, which is lightersupport can be sub

vided.

diameter cream and1skinH component

bead to be Flushedis a variable factorness.diameter planetary

ed not have to beidth and depth are

he groove type 1iii2 iswhere the purging call froweld, and the gas used ikgHchum2. he groove wid

factors with respect to the s

ith reference to the paare extracted from thesedetermining the mass of tspecific heat of both 9aragi

*alculation For Finding Outhe weld parameters

eld trials on the test specicarried out before proceewelding the components. the results of each iteratio

weld parameter among theproceed with the actual cofixture.he results of all the iteratithe test specimen has beentable the optimum welding p

established and it is further use

`able @ elding parametehe feasible one out o

the theoretical calculation foelding parameters'

;oltage, N $4 ;*urrent, ) N $CE "mravel speed, vN4C

-eat )nput #-%')n an arc welding, the totalsource is given by

-N)H; in

here,N;oltage N $4;)N*urrent N $CE"

;Nravel SpeedN4CEmmHminN4CEHAEN >.AA mmHs

Therefore, the heat input:

-= 1$4 x$CE2H >.AAHN >A4.C? 7Hmm

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280

generally deployed to "* weldm both top and bottom faces of

"rgon 1specific gravity is $.ADth depth and radius are variable

eet thickness

ers #$4% and #$@%, the expressionspapers and are used in thee back0up support bar using theg steel and *opper "rsenic.

he 9ass Of eld +ack p ool'ave to be established by doingens. he number of iterations are

ing with the actual process ofese iterations are carried out andis documented and the feasible

different iteration is finali5ed tomponent welding using welding

ons that have been carried out onshown in the below table. In thisrameters of the TIG welding is also

.

rs iterationsf the above iteration is taken andr the same is carried out below'

psmmHmin N>.AA mmHs

eat energy -, generated by power

Hmm


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Let Hn be the actual heat transferred to the work piececonsidering small electrical losses in arc. Then,

Hn N 1f$ x x )2Hv

N f$ x -

here, f$ N -eat transfer efficiency

N E.B for )& elding(considered as 70% efficient)-nN E.B x >A4.C?-nN@4>

7Hmm-eat nergy sed o 9elt 9araging Steel1-m2:The expression for heat energy is given by:

-m N#-f 1m0o2%H(here,

-f N -eat of fusionN$$4Ax$E @7Hkg 1for 904?E2

* NSpecific heat of weld metal

N E.C$@ x $E@7Hkg 1for 904?E2mN9elting temperature of base metal

N $>$@ E*o N temperature of base metal prior to weld

N 4EE* N eight of the deposited weld metal

N "(here,

"NCross sectional "rea of weld beamN $>.?B mm4 1from bead cross0section2

(N (ength of weld metal depositedN 4 x x r N x d

here,

d N 6iameter of the component to be welded.( N x >$C.>N $@$>.>> mm

N 6ensity of weld metal 1904?E2N C.$ x $E@ 8gHm@

N "(N E.$?? 8g

herefore,

-m N#-fJ* 1m0o2% H( in 7Hmm

-m N #$$4A x [email protected]$@ x $E@1$>$@04E2P% x E.$?? H $@$>.>>

-m N 4AA.?> 7Hmm

-eat dissipated to component and tool is given by the

expression:

QN -n+-mN ?B.>?7Hmm

"lso,Q N-eat absorbed by component J -eat absorbed by tool.

Q N 1m * d2904?EJ 1m * d2"s *uN -1904?E2J -back

here,9 904?ENmass of maraging steel,

here effective heat transfer is taking place*904?EN Specific heat of 9araging Steel

N C$@ 7H8g8(derived using the J-MAT PRO)For Arsenic-Copper,

9back N 9ass of back up bar*back N Specific heat of back up bar N @CE 7H8g8

dt N *hange )nemperature Of +ack p +ar

9ass Of 9araging Steel 1m904?E2 is found out using a relation,9 1904?E2 N x ;

here,N 6ensity of 904?E; N ;olume N " x L

N H> 1>$C.>40>$@.>42 x E.$?,here,

(N $?Emm, effective heat transfer length9 1904?E2N E.E$?C 8g

-eat "bsorbed +y /art (-)i.e Maraging Steel

1- :et2 N 1m * d2904?E N @@??.B 7

For unit length, 1- part2 N 1- :et2 H1x >$C.> 2 N 4.?? 7Hmm

-eat "bsorbed +y "rsenic *opper +ack p +ar:ow,

-back N Q 1-2904?EHback N mbackx @CE x 1$$@0@C)mback = Hback/ (380 x 75 )

+ack p 9ass, mback N 4.?025 8g

*onsidering the weld parameter the mass of the backup toolis approximated as 4.58gs. =ise in temp in back up tool is D@Eand hence it is recommended not to touch the backup toolafter welding. "llow the tool to cool and then disassemble thetool set up used for welding.

From the above calculations, it can be concluded that for$4; R $CE", a minimum of 4.?04?8g of back0p mass issufficient to dissipate the heat satisfactorily. +ut, the thicknesswill be very less, arising the strength R rigidity concerns.-ence, a nominal 4Emm thick minimum is sufficient toaddress the thermal as well as strength requirements as for thecalculations.

4.4 MINIMUM GAP BETWEEN WELD EDGEAND CLAMP EDGE

he thickness of the weld between each of the parts is?.ABmm. using this thickness value the minimum gap betweenthe weld edge and the clamping edge has to be determinedusing below expressions.

"n examination of bead shape1weld pool2, welded withdirect polarity shows the bead width d to a depth ofpenetration prelationship of the form'

d N$.Bp


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d N $.B x ?.AB

d N D.A@ mm

his is the normal polarity for )& process

)t tis the thickness of the material tosafe gap 1a2 between one edge of weld b$.4?t.-ence,

=equired gap to =- clamp N $.4?=equired gap to (- clamp N $.4?

he distance between the clamps for 6* di

polarity weld N d J 4a

-ere, t N ?.ABmm

herefore the distance between the cla

polarity weld N D.A@ J 4B.EC

N [email protected] approximately 4>hereby, the minimum gap between theweld edge is half of the total distance i.e 4>

4.5 WELD SETUPS

$steld set0up

he first weld setup in the sequence sable04 is


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Step 3: Weld Dome Y-Ring to Igniter Boss.

@rdeld set0upntil this part the welding fi

flexibility to weld with the "rsenic *opbar. )n this setup, providing a back0up

special design to fit into the gap betweenthe Emm. -ence, desig

up is a challenge and the figure belo

problem.o tackle this problem ,design of a coll

up support "rsenic *opper ring that hasfore skirt ring before the weld setup anoperation the fasteners are removed to e!the "rsenic *opper back0up support .shown in the below figure and is the stepweld.Step $' Fastening the collapsible back0upSkirt =ing.he 406 drawings of these collapsible bhas been shown below'

CHNOLOGY RESEARCH VOLUME 3, ISSUE 6, JUNE 201

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ture had enougher back0up supportsupport needed a

he upper surface ofe fore skirt ring ising a support back0

shows the exact

psible type of back0een fastened to theafter the welding

ct out the pieces ofhis arrangement

$ for the @rdsetup of

support to the Fore

ck0up support ring

Fig. D +ottom half

Fig11 Ejectinhe 40d drawings are relatithe bottom half of the backto the top half which is shothe e!ection of this collapsremoving this is easier since

of each of the aligning singlwhen assembling and is rering has to be disassembled.Step 4' /lace the Fore Skirtfastened on the pre0welded

Step @' *lamp the Fore Skithe thrust pad.

Step >' he final welded heathe figure below.

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283

Fig. $E op hal

halvesof the ring

e to each other as the fig. D showsup support ring which is aligned

n in the fig. $E and in the fig. $$ible backup support is designedthere is a straight edge at one end

e part and this pair is aligned lastmoved first when the collapsible

ring with the collapsible back0up


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5 ACHIEVING THE TOLERANC

he specified tolerances on the head eshown in the figure12.

o achieve the above tolerances as menthe top surface of the )gniter +oss is pmaterial which is machined parallel to ththe % Siva Sankara =a!u =, 8aru

and "nalysis of 9otor *a

)SS:' 44>D0CD?C, ;olume04,)

#?% 9. ;arul , -. F. 9u5affero

Fixtures on elding 6istort

Feb 4EEB ,pp ?$$0?$>.

#A% -ui ang , E1

#B% $C percent :ickel 9ar

/ublication :o. >>$D by :ic#C% )ain +oyle , % www.wikipedia.comHmara

#$?% 9. strems, -. . Sanche5

6imensional "ccuracy

4EE@,pp@C>0@DE.

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284

ces that are specified on the headvery tight and hence this specifiedieved upto an extent using the

xture. Since, in this case theion or rework will be moreobability of acceptance a specifiedon > will provide the morethe acceptance than re!ection.

can be opted for achieving the

Force Optimi5ation For 9inimum

by 6ynamic "nalysis of orkpiece0

?4.)6OS) 4E$E. pp C>E0C>Akata =ao , +. :ageswara =ao ffect of

+urst /ressure of 9araging Steel =ocketailure "nalysis $4 14EE?2 pp @4?0@@A.

n "nalysis of Fracture oughness in the

araging Steel elding =esearch

D,pp >?B0>A$.

8umar $04?4, 7an

rends )n elding in the "eronautic

stitute of echnology, *ambridge , 9"

nsson , +. (ennartson =ole of Fixture

ungsten "rc elding an xperimental proc. )9ech ;ol.44? /art +'7

. $>E0$>C.,"ugust 4E$E.pts of welding Operation

brese and =. F. 9enaul *omputeri5ed

in elding ooling paper based on >Cth

"pril 4>04C.Science direct pp $>04A, 7an $DBE.

ingsteel

and F.Faura )nfluence of Fixtures on

n 9achining /rocesses springer0

design of welding fixture for head end sub assembly of motor case

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