new design of resistance welding machine

8/10/2019 New Design of Resistance Welding Machine

1/10

N e w

D e s i g n

of

R e s i s t a n c e S p o t

W e l d i n g M a c h i n e

f o r

Q u a l i t y C o n t r o l

BY K.

I.

J O H N S O N

A N D J. C. N E E D H A M

S i n c e ,

at a

g iv e n c u r r e n t

a n d

w e ld d u r a t io n ,

t h e

e l e c t r o d e lo a d a f f e c t s

t h e

f i n a l n u g g e t

s i z e , t h e

l o a d v a l u e

c a n b e

u s e d

to

r e g u l a t e n u g g e t d e v e l o p m e n t

a n d

c o m p e n s a t e

for

v a r i a t i o n s in e f f e c t iv e h e a t in p u t . W e ld e x p a n s i o n e l e c t r o d e m o v e m e n t a p a r t ) is an

i n d e x of n u g g e t d e v e lo p m e n t

a n d c a n

t h e r e f o r e

b e

u t i l i z e d

a s

a s u i ta b l e s e n s o r to c o n t r o l

t h e e l e c t r o d e l o a d

A B S T R A C T . T he effect of electro de load

on the growth of the -weld nugget in

resistance spot welding has been investi

gated for 0.9 m m (0.03 6 in.) and 1.6 m m

(0 .064 in . )

mild

steel sheet using a

specially constructed low inertia, r igid

exper imenta l machine . I t has been shown

that spot welding can be made to lerant

to var ia t ions in welding current and t ime

by automat ica l ly cont ro l l ing the e lec t rode

load as a function of the weld ex pan sion

(which is an index of weld nugget de

v e l o p m e n t ) .

T he per fo rman ce range of th is self-

T he authors are associa ted wi th T he Weld

ing Ins t i tu te . A bington Hal l . C ambridge .

England .

load cont ro l has been examined for a

s imple sys tem in which the e lec t rode

head is bro ug ht into con tact with the

work and thereaf ter res t ra ined, so tha t

as weld expans ion occurs , the e lec t rode

load increases according to the stiffness

of the machine f rame ( locked head sys

t e m . T he result s show tha t und er these

condi t ions there i s an apprec iable in

crease in to lerance to var ia t ions in oper

a t ing current and weld dura t io n .

T he range of to lerance can be fur ther

increased by us ing a more complex com

pound res t ra in t se lf - loading sys tem. T his

system e mp loys a low init ial st iffness

which is cons iderab ly increased (with

expans ion) towards the end of the weld

per iod . T he res t ra ined head technique

gives adequately sized welds in spite of

a we l d i ng c u r r e n t va r ia t i on of + 1 5 % ,

2 0 , wh i l e no r m a l we l d i ng ma c h i ne

condi t ions g ive acceptable welds over a

range of only 1 0 % in 1.6 mm (0 .064

i n . ) ma t e r i a l . S i mi l a r l y , c ompound r e

s t ra in t g ives adequate welds when the

we l d t i me is va r ie d by + 1 0 0 % , 50% ,

whi le normal opera t ion only g ives ac

c e p ta b le w e ld s a t + 4 0 % , 2 5 % ( 8

cycle

w e l d ) .

A com pat ib le m oni tor sys tem for weld

quality assurance is also described which

util izes the increase in load for the re

s t ra ined head sys tem to conf i rm tha t a

nugget has been es tabl i shed .

T he scope for indus t r ia l appl ica t ion

of the locked head system is briefly dis

cussed and designs outlined for obtaining

locked head charac ter i s t ics on welding

machines of o therwise convent ional de

sign.

122-s | M A R C H 19 72


2/10

Introduction

In resistance spot welding, two

sheets are brought together, held u n

der pressure between two electrodes

and a high current passed via the

electrodes through the workpieces for

a short time to heat the interface

sufficiently to form a common weld

nugget. In normal practice the applied

load, welding current and weld cycle

durat ion are preset for a part icular

applicat ion, depending on the material

to be welded and on the diameter of

the electrode t ips . T hese external

parameters are, as far as possible,held

constant both from one weld to the

next and during the formation of any

one weld. In particular, care is taken

to ensure that the electrode head

moves freely in its guides so that the

electrode load is cons tant and no t

upset by stiction and the like.

However, even when the main vari

ables are held constant, there is often

considerable variation in weld size or

quality due to such factors as changes

in the surface condition of the

workpieces , electrode wear and the

shunting effect of alternative current

paths through adjacent spots . For th is

reason monitoring the quality of the

weld or control of the development of

the nugget is highly desirable. In the

past, several monitoring techniques

have been proposed

1

and in addition

in some cases a feedback control has

been developed in which the moni

tored parameter is used to sense the

development of the nugget and to

operate on one or more of the main

variables so as to maintain a desired

nugget size.

2

*

3

In the weld control

systems proposed previously the sens

ing parameter had been arranged ei

ther to operate on the welding cur

rent, or to determine the weld cycle

duration, or both with a substantially

constant electrode load .

Owing to the expansion associated

with the thermal heat ing of the mate

rial to be welded, including the forma

tion of the liquid nugget, the welding

electrodes are, under normal condi

tions, forced to move apart . T ypically

using electrode tips 5 mm

(

3

/ j

G

in .)

in diameter, this expansion (head

movement) amounts to about 0 .2 mm

(0.008 in.) for steel sheets 0.9 mm

(20 swg) th ick , and about 0 .3 mm

(0.012 in.) for steel sheets 1.6 mm

(16 swg) thick.

T his paper consider s the possibilities

of controlling the weld nugget by

means of the electrode load and in

particular of basing this control on the

head movement (patents appl ied for) .

In the s implest arrangement the ex

pansion directly governs the load ap

plied, according to the rigidity of the

welding machine. For th is the elec

trodes are brought together into con

tact with the work, and then locked or

restrained to prevent their free move

men t apar t . T he a t t empted expans ion

of the weld nugget then results in a

corresponding increase in the applied

load . T his is termed a ' locked head '

system to differentiate it from conven

tional resistance welding equipment in

which the moving electrode is rela

tively free.

Principle of Automatic Weld

Correction by Electrode Load

Effect of Load on Nugget Formation

T he new correct ion m ethod is based

on automatical ly varying the electrode

load during the weld cycle and in

02-

5

0-75

S010

< 3

005

6

ta

5S2

/

0

L 0

Splash

Normal

Electrode tip dia47 mm 0-18in.)

Transformer tap 4,

3-3Vo.c. approx.

6-OkA)

Weld duration 0-18 sec 9 cycles at 50 Hz)

^

i

I

I

0-5

/ 7-5

Electrode force, kN

IFace

I fracture

I

l

I

700 200 300

Electrode force, Ibf

400

500

Fig. 1Effect

of electrode load on nugget diameter for 0.9 mm (20 swg) mild steel

W E L D I N G R E S E A R C H S U P P L E M E N T I

123-s


3/10

particular with controlling the load as

a function of the development of the

nugget by means of the associated

expansion . T hat the electrode load or

pressure is significant in resistance

spot welding is well known and for

consistent welds in mild steel it has

been recommended that the pressure

be maintained constant with in

15 .

3

T hus for a g iven material and

thickness , part icular combinat ions of

weld current and durat ion are es tab

lished by trial and error for a prede

termined electrode load which itself is

based on the tip diameter which in

turn is related to sheet thickness.

T he effect of variation in electrod e

load on nugget development with oth

erwise constant welding conditions is

illustrated in Fig. 1 for 0.9 mm (20

swg) mild steel. [In these tests the

degree of phase shift (heat control) is

constant but the current can be varied

by altering the t ransform er tap .] T he

current at a given voltage tap setting

is nominally constant but does in

crease by 2 to 3% from a low to a

high electrode load owing to the de

crease in in terfacial res is tance.) A t a

given current and weld duration the

nugget size increases with decrease in

electrode load until weld splashing

occurs ; while on the o ther hand with

increase in load the nugget size de

creases until a face fracture sets in.

With further increase in load the nug

get size rapidly becomes insignificant

resulting in a stuck weld which has no

strength .

T he mechan isms by which electrode

load affects the nugget development

are believed to be threefold. First,

with increase in load the interfacial

resistance decreases and hence the

initial heating at the interface is re

duced at the early stages of the weld

dura t ion .

5

Secondly the electrode con

tact with the work is improved at

higher loads and the current distribu

tion from the electrode tip into the

workpiece is modified giving a lower

current densi ty and hence a reduced

effective heating. Finally, particularly

with thin sheet, the increased elec

trode load results in increased heat

conduct ion from the workpiece to the

water-cooled electrodes .

T hese three factors act in the same

direction and hence the nugget de

velopment is reasonably dependent on

the electrode load . T hus , as shown in

Fig. 1, the change in nugget diameter

with load is approximately linear be

tween the limits where weld splashing

sets in and where the nugget decreases

very rapidly in size to leave a low

strength or face fracture condition. In

this case a change of 25% in elec

trode load results in a 10% change in

nugget diam eter for a well develope d

nugget which is approximately the

same size as the electrode tip.

S imi la r nugge t d iameter / e lec t rode

load relations can be found for other

welding currents, as shown in Fig. 2

for 0.9 mm (20 swg) mild steel. In

this case the weld heat is altered in

relatively large steps by changes in the

transformer primary tapping, g iv ing

open circuit voltages ranging from

4.1v to 2 .8v or broadly + 2 5 % to

1 5 %

of the normal condition

( t rans fo rmer t ap 3 .3v ) . A s can be

seen from Fig. 2 there is a correction

zone for 0.9 mm (20 swg) mild steel

where, in spite of current changes

from about 5ka to 7ka, a substantially

0-2

0-75

0-70

3 *

Splash

Compound compliance

075kN 170 Ibf)

initial force

Free electrode

2 3 kN

500 Ibf)

force

Weld duration 0-16 sec

8 cycles at 50 Hz)

Electrode tip dia. 64mmfAin.)

8

Current, kA

10

11

12

03

r

8

02

c

0-7

6

ri

*~

cu

c

T

Compound compliance

075kN 170 Ibf)

initial force

x

-

T

*

I

o - ^ o Splash

Free electrode

23 kN 500 Ibf) force

Welding current 9-4 kA

b)

10 12 14

Time, cycles at 50 Hz

16 18 20

Fig.

fjNugget growth in 1.6 mm (16 swg) mi ld steel with free and co mpou nd com pliance restrained electrode systems: (a)

effect of weld current on nugget diameter for constant weld durat ion; (b) effect of weld t ime on nugget diameter for constant

current

distinctive with the dual or compound

spring rate system as evidenced by the

results given in Fig. 6a and 6b as well

as by the table on performance

toleran ce. T he principal feature in this

system is that the initial load is itself

low as well as having initially a low

spring rate. T he design of the elec

trode support to give initially a suffi

ciently free movement is simple. For

example a small degree of free move

ment can be provided in the locking

system itself or the electrode support

can include a thin pad of rubber or

other soft material or a flat spring

B elleville type washe r to give the de

sired low spring rate.

T he higher sp ring rate is then

provided by the complete locking of

the electrode or by ensuring that the

piston is suitably jammed. One method

of preventing free movement of the

piston is to use a hydraulic ram in

which th e fluid circuit is closed dur ing

the welding operat ion . A ny reverse

movement of the electrode tends then

to compress the hydraulic fluid (which

has a spring rate of the order of 15

N / m m

2

(2000 psi) for a 1% com

pression by volume) and hence in

creases the electrode loading.

T he degree of com pound compli

ance can also be readily provided in

an all hydraulic system by adding a

small freely compressible capsule in

the hydraul ic circuit . T he basic ar

rangements for an hydraulically actu

ated electrode are shown diagrammat-

ically in Fig. 7b in which a limited

W E L D I N G R E S E A R C H S U P P L E M E N T

129 s


9/10

Hydraulic brake

calipers

a

Electrode^^

assembly I J

Adjustable stop

Spring

Hydraulic fluid

Solenoid valve

Clamp

From

timer

y//////////////////^ Retract

Fig. 7Schematic

diagrams

of

systems

for

locked head operat ion:

a)

restrained

electrode by external c lam p; b) compound compl iance us ing hydraul ic ram

degree

of

free reverse movement

is

permit ted

by the

small expansion bel

lows

on the

high pressu re side

of the

cyl inder . Here the locking is provided

by

the

solenoid valve w hich

is

con

trolled

via the

weld t ime r .

Wi th

the

com pound res train t

the

initial load that is usedis low,typical

ly between 25 and 50 of normal

levels,

so as to

encourage nugget

de

velopment

at an

early stage

in the

weld cycle. Weld splash,

however,

is

prevented by the ensuing high spring

rate which prevents over-development

of

the

nugget

as

already described.

It

is this combination

of low

initial load

and virtually free head together witha

high final load, fixed head

(as a

func

tionof theexpansion) which resul tsin

the wide tolerance

to

variat ions

in

heat input (welding current)

and

weld

durat ion, shown

by the

nugget size

datain Fig. 6.

Automatic Weld Quality Monitor

A s already discussed,

the

control

of

nugget development

by

electro de load

is feasible and it involves a relatively

large change of electrode load to be

effective. A lso the load change is di

rectly caused

by the

weld expansion .

N ow

it has

been established previously

that there is a good correlation be

tween head movement (expansion)

and weld quality (since the head

movemen t

is a

direct indication

of the

nugget deve lopmen t ) . Hence

the ex

pansion measurement can beused as a

monitor

of

weld qual i ty

7

al though

there

is the

basic practical difficulty

of

registering such small movements in

industry, particularly on portablema

chines .

T his difficulty

is

circumvented

in

the restrained electrode system, since

in effect the load c hange serves as a

measure

of the

head movem en t

ob

t a ined . Thu s

the

electrode assembly

forms

a

cal iper

by

which

to

register

the weld expansion

and if the

elec

trode actuation point (or effective

cal iper fu lcrum)

is

locked

or re

strained from free movement then

the

stress developed

in the

locking system

corresponds directly with expansion.

It should be noted that the load

change is large and can be readily

detected especially with the hydraul ic

type

of

loading/ locking system. A lso ,

since with

the

locked head w ith du al

compliance the system is essentially

tolerant then only a relatively simple

weld quality monitorisneeded (viz,to

detect that

the

load increased signifi

cantly from

the low

in i tial level) . T his

corresponds to the formation of the

weld nugget which as already de

scribed develops early

in the

weld

cycle

and

then

is

held from splashing

by

the

increase

in

electrode load.

T h u s

for

critical weld applications

not only does the res trained electrode

system provide a greater degree of

reliabilitydue to itsinherent to lerance

to variations

in

effective w eld he atin g

bu t

it

provides also

a

ready means

for

weld quality assurance.

T ests have shown that this simple

monitor

of the

response

of the re

strained electrode correctly registers

which welds have well established

nuggets for a variety of external devi

ations

in the

welding condit ions ,

in

cluding change

of

current , weld dura

tion initial electrode load

and tip

diameter (for truncated cone elec

trodes) either singly or in any combi

nat ion .

It

also registers correctly

for

shunted welds even

in the

ex t reme

case of a spot placed between two

previous spot welds. The only limita

tion is that of a weld at the very edge

of

the

sheet which collapses w ithout

expansion

and

expels without increase

in load . Here the monitor indicates a

suspect weld (pressure failing to in

crease) even though a nugget was

formed before

it

splashed .

Applicationof Restrained

Electrode Systems

In

the

work reported

a

relatively

rigid spot welding machine

has

been

used

in

which

the

degree

of

restraint

with a locked head ishigh . T his condi

tion applies also to s tandard bench or

fixed station resistance welding

ma

chines

in

which

the

moving electrode

is carried directly

by the ram and is

130-s

I M A R C H 1972


10/10

not par t of an extension arm . T he

system could also be applied to por

table spot welding guns so long as the

electrode

support arms are relatively

short or of such a construction that a

reasonably rigid system is achieved. In

this connection it is noted that a load

change of at least 30% with weld

expansion is required .

T he rest ra ined e lec t rode system

should be applicable to roll spot seam

welding but with the addition of some

means for compensating for longer

term varia t ions such as wheel eccen

tricity and wear. For this a hydrauli-

cally

actuated head would be suitable

since it could provide both the long

term free electrode mechanism and

the short term restrained electrode

effect . For example , the hydraul ic ram

could be pressurized by an air cylinder

so as to main tain a long term mea n

load which would allow the system to

follow variations in sheet thickness or

in wheel radius . However , by provid

ing a constriction in the hydraulic

supply, the short term head movement

would be restrained and cause a cor

responding increase in pressure in the

hydraulic fluid which in turn increases

the electrode load.

T he system should a lso be appl ica

ble to stitch welding but in this case it

may not be necessary to incorporate a

dual compl iance rest ra int . Th is ar ises

since there is a stage of negligible

head movement or even slight collapse

during the initial part of a sti tch weld

before a positive expansion is regis

tered . This m ay be due to the overlap

of spots whereby the new spot falls on

the shoulder or edge of the indenta

tion produced by the previous spot .

T he deta i led mech anism is not known

but i t is believed that the shoulder

initially collapses and offsets the ex

pansion until a nugget is developing at

the new point . For the remainder of

the weld cycle a positive head move

ment is registered and with the re

strained electrode system this would

result in an increasing electrode load

to contain the nugget as already de

scr ibed.

For both sti tch welding and roll

spot seam welding, the weld could also

be mo nito red *in term s of the in creas e

in electrode load arising.

Finally it is believed that this new

concept in machine design for resist

ance welding

will

lead to a greatly

increased confidence in this, the

oldest, electric welding process for

crit ical applications such as in the

aerospace industry and for the struc

turally important welds in the auto

mobi le mass product ion indust ry . T he

increased tolerance in operation and

the faoility for weld quality assurance

has been clearly demonstrated for

mild steel sheet and these advantages

are expected to apply also to the wide

range of materials which are currently

resistance welded by the conventional

process .

Conclusions

1. Since at a given current and

weld durat ion the e lec t rode load

affects the final nugget size, the load

can in principle be used to

reguLate

the nugget development and to com

pensate for variations in effective heat

input due (for example) to changes in

current . T he e lec t rode movem ent

apart (weld expansion) which is an

index of the nugget development

provides a suitable sensor for control

of the load.

2. For weld correct ion during the

weld period, in the simplest arrange

ment the electrode movement is react

ed against a rigid support so that the

electrode load increases considerably

with expan sion accord ing to the

stiffness (spring rate) of the support .

On a r igid bench w elding m achine

(fit ted with stops to prevent move

ment of the electrode head with re

spect to the machine frame after the

workpiece is c lamp ed) the tolerance

to

changes

in weld current , though

limited, is twice that for the norm al

free system.

3. T he restra ined e lec t rode system

is applicable to many types of resist

ance welding machine but the max

imum spring rate obtainable is l imited

by the stiffness of the electrode sup

port system especially with portable

welding machines .

4 .

With a compound spring ra te

(giving initially a low degree of re

straint followed by a high stiffness) the

weld tolerance is greatly increased

over that from a convent ional ma

chine with a nominally free electrode

head. T he nugget diame ter for 1.6

mm (16 swg) mild steel is main tained

at about 6 1 mm (0 .25 0 .04

in . ) for current var ia tions of + 1 5 % ,

20%

and (a t the correct cu rrent)

for time varia t ions of 50% ,

+ 10 0% . A bnorm al ly low init ia l e lec

trode loads are used so that the nug

get is established early in the weld

cycle without splashing but thereafter

its further deve lopm ent is inhibited by

the dampening effect of the rapidly

increasing e lec t rode load.

5.

T he restra ined head provides a

means for registering the expansion of

the weld by the increased load and

hence serves as a monitor of weld

quality since, except for a weld at the

sheet edge, the nugget formation is

c losely corre la ted wi th expansion. T he

load moni tor reads correct ly for var i

ations in current, weld duration, elec

t rode size and for shunted welds .

Since the dual compliance system is

essentially tolerant, this results in sat

isfactory welds even with major chang

es in welding conditions.

6 . The combina t ion

of

the re

strained electrode approach with its

monitor facili ty is recommended for

weld

quality assurance in crit ical spot

welding applications for both the

aerospace and mass product ion indus

tries.

cknowledgements

T he authors express thei r thanks to

the Director General of the Welding

Institute for permission to publish this

paper and to thei r col leagues M.D.

Hannah and R . G . Newl ing who were

responsible for the experimental data

and machine design.

References

1. R o d e n . W . A . , E v a l u a t i o n o t R e s i s t

a n c e W e l d i n g I n - P r o c e s s M o n i t o r s .

Weld-

ing Journal,

V ol . 47 . N o .

11 .

pp. 515-s to

521-s, 1968.

2 . G r a n t . W . A . . E v a l u a t i o n o f E l e c

t r o n i c M o n i t o r e d R e s i s t a n c e W e l d i n g C o n -

t i -ol ,

Welding Journal,

V ol .

41 ,

N o . 6 .

p p .

563-567. 1962.

3. O r l o v .

B . D . . e t a l . A u t o m a t i c C o n

t r o l o f t h e S p o t W e l d i n g P r o c e s s ,

Avt.

Svarka,

V ol . 21 . N o . 6 . pp . 50 - 54 . 1968 .

4 . B e a t s o n . E . V . , T e c h n i q u e s f o r S p o t

W e l d i n g L o w C a r b o n S t e e l w i t h T r u n c a t e d

C o n e E l e c t r o d e s . I n t e r n a t i o n a l I n s t i t u t e

of Wel di ng D oc . 111-283-66. 1966.

5 . N i c h o l s o n . S . a n d A p p s . R . I . . . H e a t

B u i l d - u p a n d W e l d N u g g e t F o r m a t i o n i n

M i ld S t e e l S p o t W e l d s . A d v a n c e s in W e l d

i n g P r o c e s s e s C o n f e r e n ce .

H a r r o g a t e ,

W e l d

i n g I n s t i t u t e . 1 9 7 0 .

6 . E v r a r d . M . a n d H a s l e . E . . R e s i s t

a n c e W e l d i n g P a r a m e t e r s ,

British Weld-

ing Journal,

Vol . 5 , No . 1 , pp . 18-20, 1958.

7.

W a l l e r . D . N . a n d K n o w l s o n . P . M . ,

E l e c t r o d e S e p a r a t i o n A p p l i e d to Q u a l i t y

Conti'ol

in R e s i s t a n c e W e l d i n g ,

Welding

Journal,

Vol . 44 . No. 4 . pp . 168-s to 174-s.

1965.

APRIL IS

NATIONAL

WELDED

PRODUCTS < / I

MONTH

VISIT

THE A.W.S.

WELDING

SHOW

W E L D I N G R E S E A R C H S U P P L E M E N T

131 s

new design of resistance welding machine

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