New Design of Resistance Welding Machine
Transcript of New Design of Resistance Welding Machine
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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.
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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
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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
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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
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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
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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