Setting up equipment for use
The most important points to be considered in setting
up the equipment are:
1 Thickness of the sheet metal to be welded
2 Accessibility of the parts to be welded
3 State of the surface of the sheet metal to be
welded (this must be as clean as possible, and
any rust, scale or paintwork should be removed).
The setting-up procedure is as follows:
1 With the equipment switched off, check that the
electrode tips are aligned and correctly shaped.
If domed tips are used (these generally give
the best results), the tip radii should be 51 mm
and 77 mm. When truncated cone tips are used,
check that the diameter is correct for the gauge
of material to be used: the diameter should be
5_t, where t is the thickness of single sheet in
millimetres. Check that the tip force is enough
without bending the electrode arms or sliding
the tip one upon another.
2 Switch on mains supply and check that the
mains light (red) is on.
3 Switch function selection switch to resistance
spot welding.
4 Switch on Spotrite control and check that the
ready light is on.
290Repair of Vehicle Bodies
If the reject alarm is heard (a high pitched pulse
note: red light illuminates) for more than three
welds, reduce the spot size setting until the
reject alarm is only activated two or three times.
10 Increase the heat step by step to reduce the
actual weld time until splashing occurs. Then
set back heat one step to stop splashing. The
equipment is now fully set.
Welding procedure
1 Prepare the panel surface by removing any paint,
primer and in general any insulating material
covering the surface to be welded. This preparation
is a key factor to good-quality spot welding.
Paint must be removed by either paint remover or
sanding. If rust is present on the panel surface,
sanding is a better method of preparation as it
will leave the surface with a bright metallic finish
which facilitates the flow of electric current.
2 Obtain the correct adjustment of welding gun
electrodes and arms.
3 Determine a suitable weld pitch for the panel
assembly to be welded to obtain maximum
strength.
4 Make sure that the correct distance is set from
the edge of the sheet metal panel to the nearest
spot weld.
10.10 Single-sided spot welding
As a result of the operational limitations of conventional
double-sided spot welding, single-sided spot
welding equipment has become more widely used in
the vehicle body repair industry. This system offers
the benefits of conventional spot welding without
the inherent problems of accessibility with doubleskinned
panel sections. Single-sided spot welding is
therefore an alternative to conventional double-sided
spot welding, or can be used in conjunction with it.
In the single-sided spot welding process the operator
manually forces the single electrode against the
panel, with the electrical circuit being completed
by an earth clamp and the cable back to the transformer.
This allows welds to be made in positions
where access is possible only from one side.
The manufacturers are now using different types
of steel to construct their vehicle bodies, the main
three being low-carbon steel, galvanized steel and
high-strength steel. This has led to confusion and
difficulty in identification and welding. The main
problem with high-strength steel is that it is
heat sensitive and difficult to identify on vehicles
without data (MIRRC Thatcham Methods Manual).
In double-sided welding this problem is solved
by using pulse welding, which keeps the heat in
the weld as low as possible. Some single-sided
equipment, especially that which operates at high
current, does not require pulse control welding.
Instead these machines use a massive burst of
power at 8000 A DC in a very short interval, which
keeps the temperature of the spot weld below the
recommended temperature. The other advantage
of this system is that it does not need to identify
whether the steel is high strength or low carbon.
The problem with galvanized steel is that it has a
very high contact resistance, making it difficult to
weld; therefore the high DC machines use a special
preheat function for this type of steel. In the first
stage of the weld the machine lowers the resistance
of the steel by melting the coating, causing it
to flow from the weld. Then, once the resistance
is low enough, it will automatically carry out the
second stage weld at the correct setting.
Single-sided spot welding
Equipment
This equipment (Figure 10.21) ranges from 2500
to 9200 amperes as follows:
1 2500 A using two phases at 415 V
2 8000 A using three phases at 415 V
3 9200 A using three phases at 415 V.
The equipment can be used to carry out the
following operations:
Single-sided spot welding Ideal where access is
difficult from both sides of the material. It is suitable
for welding on wings, front panels and rear
quarter panels (Figure 10.22).
Two-electrode single-sided spot welding Suitable
when unable to attach the earth clamp satisfactorily.
It also allows two spot welds to be made simultaneously
and is suitable for welding sill sections
in place.
Pulse control roller spot welding This gives
a continuous spot weld along an overlapped
edge of metal. It is ideal for roof gutters and for
welding patches to vehicle panels when dealing
with corrosion repair without creating distortion
(Figure 10.23).
Electric resistance welding 291
Welding copper ring washers for pulling This
allows rows of washers to be welded to the panel
surface. Ideal for pulling out large dints by using
a slide hammer with a special hook attachment
which fits through the rings, which are later broken
off by twisting.
Rapid puller This is designed for pulling out small
dints quickly and effectively by welding the puller
to the panel, pulling out the dint, then twisting the
tool to release it from the panel (Figure 10.24).
Figure 10.21Single-sided and double-sided spot
welding equipment (Stanners Ltd )
Figure 10.22Single-sided spot welding used in
panel repair (Stanners Ltd )
Figure 10.23Pulse controlled roller spot welding
(Stanners Ltd )
Figure 10.24Rapid spot puller (Stanners Ltd )
Copper shrinking of high spots This uses a copper
tool for shrinking stretched panels which have
been overworked by hammering (Figure 10.25).
Carbon shrinking for over stretched panels This
uses a carbon pencil rod and is used for retensioning
a panel surface that is only slightly
stretched (Figure 10.26).
292Repair of Vehicle Bodies
Welding captive nuts to vehicle This piece of
equipment can be used to weld captive nuts, trim
studs and threaded bolts to panel surfaces when
they need to be replaced (Figure 10.27).
Single-sided spot welding equipment has a low
current intensity, which ensures safety for the operator.
The equipment uses direct current at a maximum
of 12 volts. All electrodes can be quenched
in water, without danger to the operator, when they
become overheated, and this extends their life.
Single-sided welding with one electrode and one
earth clamp results in current loss in the metal
panel, and also the metal panel is a bad conductor
of electricity. To overcome this problem, Stanners
Ltd use a system which gives good results by using
two earth clamps positioned correctly on the panel
being welded. With this system 20 per cent more
current is passed through the weld, resulting in
better and stronger welds.
Setting up equipment for
Single-sided spot welding
1 Set the weld timer dial.
2 Set the welding mode switch.
3 Set the welding power dial.
4 Connect the earth clamp plate to the negative
side of the welding output cable holder, and the
single-sided electrode to the positive side of the
cable holder.
5 Connect two earth clamp plates as close as
possible to the area to be welded.
6 Ensure that both pieces of metal are clean and
making good contact. Also make sure that the
lower piece of metal is well supported to allow
pressure to be applied to the electrode by the
operator.
7 Press the electrode against the workpiece to be
welded. Apply pressure to both pieces of metal
and press the switch to carry out the weld.
8 It is important that the electrode is quenched in
water after prolonged welding to prevent it
becoming hot or overheating.
9 The electrode tip should maintain its original
profile if good spot welds are to be achieved.
Therefore, when necessary, the electrode should
be redressed back to its correct shape.
Figure 10.25Copper shrinking attachment
(Stanners Ltd )
Figure 10.26Carbon pencil shrinking attachment
(Stanners Ltd )
Figure 10.27Captive nuts welded on to panel
(Stanners Ltd )
Electric resistance welding 293
Questions
1 What is meant by the term ‘resistance welding’?
2 Name and describe three methods of resistance
welding.
3 Give an example where resistance welding is
used in vehicle manufacture.
4 Resistance welding uses various electrodes:
select and sketch one type.
5 How is a workshop test carried out on a spot
weld, to test its strength?
6 Suggest one application of projection welding in
the assembly of a vehicle body.
7 Illustrate types of joints that would be suitable for
spot-welding applications.
8 Describe the three stages in the production of a
spot weld.
9 Which important points should be considered
when reshaping a resistance welding electrode?
10 Explain the importance of resistance welding in
the repair of vehicle bodies.
11 Which metals are used for making electrodes in
resistance welding?
12 What is the difference between butt welding and
flash welding?
13 Name the principal parts of resistance welding
equipment that would be used in the body repair
shop.
14 State the qualities required of a good spot weld.
15 Explain the term ‘robotic welding’.
16 What are the important factors to be considered
when spot welding high-strength steel.
17 Explain the importance of the adaptive self-setting
timing control unit when welding high-strength
steels.
18 Explain the working procedure of a pincer-type
welding gun.
19 Describe, with the aid of a sketch, the formation
of a weld nugget during the spot welding cycle.
20 Describe a method of resistance welding that
would be used on the assembly line to weld
a roof panel into position.
21 What is the purpose of ‘hold time’ when carrying
out the process of spot welding?
22 Describe typical repair situations that would use
a pincer welding gun and a single-sided welding
gun.
23 State where resistance seam welding could be
found on a vehicle body.
24 Show, with the aid of a sketch, the principle of
twin-spot welding.
25 Describe, with the aid of a sketch, the resistance
projection welding process.
26 List the advantages of the resistance spot
welding technique used in the repair of vehicle
bodies.
27 Sketch one type of electrode arm sets used in
vehicle repair.
28 With the aid of a sketch, explain how heat is
generated to form the spot weld.
29 State the treatment that should be carried out
before replacing a spot welded panel.
30 State why water-cooled electrode tips are used
in the manufacture of vehicle body shells.
Manual metal arc
welding
In the sphere of welding the electric arc has
become an efficient and reliable means of welding
sheet and metal plate. It is useful for welding the
heavier-gauge plates used for commercial vehicle
body building and also for the type of metal plate
processes in which the metal ranges in thickness
from 3 mm to 75 mm.
The use of arc welding depended naturally upon
the development of electricity, and dynamos or
generators were not developed until 1880. The first
actual arc welding, meaning the melting of metal
by means of electrodes and thus fusing them
together, was developed by Bernardoz in 1885; he
created a mechanism using a carbon electrode
which produced an arc between the carbon and
metal, melting the edges and thus performing a
weld. The arc form of welding, using the metallic
electrodes, was discovered by Slavinoff in 1892,
but had very little success because of the use of
bare metal electrodes. However when Kjellberg, a
Swedish inventor, developed the flux electrode in
1907, the success of the metallic electrode was
assured. Progress accelerated as a result of the
First World War, when productivity and speed of
welding was of prime importance. However, it was
not until the 1930s that good-quality joints could
be reliably and consistently produced by the arc
welding process. This was achieved by the development
of coatings which gave adequate protection
and improved arc stability whilst transferring
metal between the electrode and the parent metal.
From that time arc welding gradually displaced
gas welding techniques, especially when joining
heavy-gauge metal, although the major development
in arc welding was due to the production of
portable and automatic welding machines.
Although metal arc welding is only used a small
amount in private car construction for heavy-gauge
assemblies in cars which have chassis, it is still used
in the commercial vehicle body building industry for
the assembly of the fully welded, trailer-type bodies
in mild steel, stainless steel and aluminium.
11.1 Principles of manual metal arc
welding
Manual metal arc welding (MMAW) is used
extensively in modern practice. It is based on the
principle whereby intense heat is obtained from an
electric current which creates an arc between a
metal electrode and the plates which are to be
welded (Figure 11.1a). The heat produced is sufficient
to fuse the edges of the plates at the joint,
forming a small pool of molten metal. Additional
molten metal from the tip of the electrode is
deposited into the molten pool, and when solidified
it results in a strong welded joint. With this process
the electrode from which the arc is struck is made of
the same metal as the parent metal which is being
welded; metal from the electrode is transferred to
the weld, partly as drops under the influence of
gravity and partly as high-velocity particles. The
maintenance of a stable arc between a bare metal
electrode and the workpiece is an unreliable procedure,
and for this reason (among others) various
coatings are applied to the electrode wire. Not only
do these coatings help stabilize the arc, but they also
perform the important functions of stopping oxidation
of the heated electrode tip and molten workpiece.
By their slagging properties they dissolve and
segregate oxides and other impurities whose presence
might otherwise adversely affect the quality of
the welded joint. When the weld has cooled the slag
forms a brittle coating which can be fairly easily
removed by chipping and brushing (Figure 11.1b).
The electric supply for this type of welding may be
either direct current (DC) or alternating current
(AC); both systems possess certain advantages,
depending on the purpose for which they are
employed. Many different types and sizes of electric
welding machines are now available, and two of the
most generally used are the DC generator and the
AC transformer.
11.2 Electrical terms used in arc welding
Circuit A circuit is the path along which electricity
flows. It starts from the negative (_) terminal of the
generator where the current is produced, moves
along the wire or cable to the load or working
source and then returns to the positive terminal (_).
Amperes Amperes or amperage refers to the
amount or rate of current that flows through a circuit.
Voltage The force (electromotive force or EMF)
that causes electrons to flow in a circuit is known as
voltage. This force is similar to the pressure used
to make water flow in pipes. In the water system the
pump provides the pressure, whereas in an electrical
circuit the generator or transformer produces the
force that pushes the current through the wires.
Direct and alternating current There are two
kinds of current used in arc welding: DC and AC.
In DC the current flows constantly in one direction.
In AC the current reverses its direction in the circuit
a certain number of times per second. The rate of
change is referred to as frequency and is indicated
as 25, 40, 50, 60, cycles per second (hertz).
Voltage drop Just as the pressure in a water system
drops as the distance increases from the water
pump, so does the voltage lessen as the distance
increases from the generator. This fact is important
to remember in using a welding machine, because
if the cable is too long, there will be too great a
voltage drop. When there is too much drop, the
welding machine cannot supply enough current for
welding.
Open-circuit voltage and arc voltage Open-circuit
voltage is the voltage produced by the welder when
the machine is running and no welding is being
done. After the arc is struck, the voltage drops to
what is known as the arc or working voltage. An
adjustment is provided to vary the open-circuit
voltage so that welding can be done in different
positions.
11.3 Metal arc welding equipment
DC generator
With a DC welding machine the electric current is
produced by means of a generator, which is driven
by a petrol or diesel engine (Figure 11.2) or alternatively
by an AC or DC electric motor. The motor
driven type of generator set is chiefly used for the
type of welding work performed inside a workshop
and is therefore often permanently mounted on the
floor, but types are available for site work. The
electric motor provides a good constant-speed
drive for the generator and is not affected by the
load imposed upon it. The generator is specially
designed for welding purposes so that it generates
about 60 volts on open circuit; this drops to about
20 volts immediately the arc is struck.
Generators are built in various current ratings
ranging from about 100 to 600 amperes, and most
Manual metal arc welding 295
Figure 11.1(a) Principles of metallic arc welding
(b) cross-section of a coated electrode in the process
of welding
296Repair of Vehicle Bodies
modern types incorporate means for automatically
adjusting the voltage to meet variations in the
demand of the arc. Although the arrangement of
the control unit may vary for machines made by
different manufacturers, most consist of a wheel or
lever control which selects the correct current for
the right size of electrode and thickness of plate
being welded. DC generators usually have a polarity
switch which enables the welder to reverse the
polarity, as is occasionally required when welding
with special electrodes.
AC transformers
The AC welding machine employs a transformer
instead of a generator to provide the required welding
current. The AC transformer, as its name implies, is
an instrument which transforms or steps down the
voltage of the normal mains electrical supply to a
voltage suitable for welding between 60 and 100
volts (Figure 11.3). In its simplest form an AC transformer
consists of a primary and a secondary coil
with an adjustment to regulate the current output. The
primary coil receives the alternating current from the
source of supply and creates a magnetic field which
constantly changes in direction and strength. The secondary
coil has no electrical connection to the power
source but is affected by the changing lines of force
in the magnetic field and, through induction, delivers
a transformed current at a higher value to the welding
arc. The output current is controlled either by an
Figure 11.2Diesel engined DC generator and circuit
Figure 11.3AC transformers and circuit (Murex
Welding Processes Ltd)
Manual metal arc welding 297
adjustable reactor in the output circuit of the transformer
or by the adjustment of the internal reactance
of the transformer. The current adjuster is operated
by turning a handwheel or crank. As the control handle
is moved, a calibrated dial shows the current setting
in amperes. Unlike the DC generator, the AC
transformer has no moving parts and for this reason
is usually referred to as a static plant. AC welding
equipment has many advantages, namely:
1 Low initial cost
2 No moving parts and therefore negligible
maintenance
3 Higher electrical efficiency
4 Ease of transport
5 AC can be converted to DC by means of a motor
generator or rectifier, thus making both types
available.
The disadvantages of the AC system are:
1 Coated electrodes must always be used
2 Voltage higher than in DC system and therefore
risk of shock greater
3 Welding of non-ferrous metals more difficult
than in DC system.
Choice of system
As far as the quality of the weld is concerned there
is little to choose between AC and DC systems. An
AC system gives a smoother arc when using very
high current, although a DC system is essential
when welding certain non-ferrous metals. The availability
of the mains supply is obviously a criterion.
Welding accessories
In addition to the actual welding generator or
transformer, the following accessories should form
a part of every welder’s equipment (Figure 11.4):
Electrode holder fitted with a length of flexible
cable for connection to the plant. It should be of sufficient
capacity to hold the largest electrode to be
used, light in weight to reduce fatigue, well balanced,
and able to locate and eject the electrode easily. It
should also be able to carry the maximum welding
current without overheating. It can be a partially or
fully insulated type, the latter being by far the safer.
Welding earth A length of cable which is flexible
and connects the work to the plant. The diameters
of these cables are governed by the voltage and
distance to be carried from the machine. From the
safety point of view it is essential that the welding
circuit is efficiently earthed. The earth clamp
(Figure 11.5) which is fixed on the end of the earth
Figure 11.4Welding accessories (Murex Welding
Products Ltd )
Figure 11.5Welding accessories showing electrode
holders and earthing clamps (Murex Welding
Products Ltd)
298Repair of Vehicle Bodies
cable should be as near to the work as possible. Its
function is to keep the work at earth potential to
safeguard personnel against any breakdown of the
welding set. If the work is earthed and the return
current lead is broken, no welding current can flow.
Head shield or face screen fitted with special
coloured lenses is an absolute necessity, because
an electrode arc produces a brilliant light and gives
off invisible ultraviolet and infrared rays which are
very dangerous to the eyes and skin. Never attempt
to look at the arc with the naked eye. The helmet
type of head shield fits over the head and leaves
both hands free (Figure 11.6). The face screen
provides adequate protection but needs holding by
hand. The coloured lenses are classified according
to the current used.
flexible enough to permit proper hand movement,
yet not so thin as to allow any heat penetration.
Leather aprons are often used by beginners in
order to protect their clothes (11.6), although most
experienced welders seldom wear an apron on the
job except in situations where there may be an
excessive amount of metal spatter resulting from
awkward welding positions.
Goggles should be worn when chipping slag
from a weld; this is a thin crust which forms on the
deposited bead during the welding process. Whilst
removing slag, tiny particles are often deflected
upwards, and without proper eye protection these
particles may cause a serious eye injury.
Cleaning tools (Figure 11.4) To produce a strong
welded joint, the surface of the metal must be free
of all foreign matter such as rust, oil and paint. A
wire brush is used for cleaning purposes. After a
bead is deposited on the metal, the slag which
covers the weld is removed with a chipping hammer.
The chipping operation is followed by additional
wire brushing. Complete removal of slag is
especially important when several passes must be
made over a joint. Otherwise, gas holes will form
in the bead, resulting in porosity which weakens
the weld.
Welding booth for use in production welding is
designed to protect all other personnel from the arc
glare. It comprises a steel bench, insulated from
the booth, and a wooden duckboard to safeguard
the welder from damp floors.
11.4 Electrodes used in welding: BS
coding
Most modern electrodes are coated or covered and
consist of a metal core wire surrounded by a thick
coating applied by extrusion or other processes.
The coating is usually a mixture of metallic oxide
with silica which, under the heat of the electric arc,
unites to form a slag which floats on the top of the
molten weld metal. To a large degree the success
of the welding operation depends on the composition
of the coating, which is varied to suit different
conditions and metals. The principal functions of
the coating are to:
1 Stabilize the arc and enable the use of arc welding
2 Flux away any impurities present on the surface
being welded
Figure 11.6Welder wearing headshield and
protective clothing (Murex Welding Products Ltd )
Gloves (Figure 11.6) are another important item
for arc welding. These must be worn to protect the
hands from the ultraviolet rays and from hot metal.
Gloves, irrespective of the type used, should be
Manual metal arc welding 299
3 Speed up the welding operation by increasing
the rate of melting
4 From a slag over the weld in order to protect the
molten metal from oxidation by the atmosphere,
slow the rate of cooling of the weld and so
reduce the chances of brittleness, and provide a
smoother surface.
In the past each manufacturer employed different
means of identifying the properties of his electrodes.
To overcome the confusion arising from this the
British Standards Institution drew up a scheme for
classifying all electrodes from a code number which
enables the user to know the main features of an
electrode irrespective of the source of supply.
The BS classification of an electrode is indicated
by the following coding, in the order stated.
Strength, toughness and covering (STC) code
1 The letter E indicates a covered electrode for
manual metal arc welding.
2 Two digits indicate the strength (tensile, yield
and elongation properties) of the weld metal
(see Table 11.1).
3 A digit indicates the temperature for a minimum
average impact value of 28 J (see Table 11.2).
4 A digit indicates the temperature for a minimum
average impact value of 47 J (see Table 11.3).
5 A letter or letters indicate the type of covering:
E — — — — B basic
E — — — — BB basic, high efficiency
E — — — — C cellulosic
E — — — — R rutile
E — — — — RR rutile, heavy coated
E — — — — S other types.
The covering should be free from defects, such as
cracks and abnormalities which would affect the
operation of the electrode. The gripped end of
each electrode should be free from covering for a
minimum distance of 15 mm and a maximum of
40 mm.
Additional coding
The following additional coding must follow the
STC coding:
1 When appropriate, three digits indicate the nominal
electrode efficiency, which is the ratio of the
mass of weld metal to the mass of nominal diameter
core wire consumed for a given electrode.
2 A digit indicates the recommended welding positions
for electrodes:
1 all positions
2 all positions except vertical/down
3 flat and, for fillet welds, horizontal/vertical
4 flat
5 flat, vertical/down and, for fillet welds,
horizontal/vertical
6 any position or combination of positions not
classified above
3 A digit indicates the power supply requirement
(see Table 11.4).
4 Where appropriate, a letter H indicates a
hydrogen-controlled electrode.
Coding example
A typical classification for an electrode is therefore:
E51 54 BB [160 3 0 H]
the STC code is identified as follows:
E51 strength (510–650 N/mm2)
5 temperature for minimum average impact strength
of 28 J (_40 °C)
Table 11.1Designation for tensile properties (British Standards Institution)
Minimum elongation (%)
Electrode Tensile Minimum When digit of When digit of When digit of
designation strength yield stress Table 11.2 Table 11.2 is 2 Table 11.2
digit (N/mm2) (N/mm2) is 0 or 1 is 3, 4 or 5
E43— — — 430–550 330 20 22 24
E51— — — 510–650 360 18 18 20
300Repair of Vehicle Bodies
4 temperature for minimum average impact strength
of 47 J (_30 °C)
BB covering (basic, high efficiency)
The additional code, is as follows:
160 efficiency
3 welding positions
0 welding current and voltage conditions
H hydrogen controlled
Electrode dimensions and tolerances
The size of an electrode should be the specified
nominal diameter of the core wire. The length of
an electrode should be within _2 mm of the nominal
value given. Table 11.5 gives the nominal
lengths of electrodes.
Table 11.2First digit for an impact value (British
Standards Institution)
Digit Temperature (°C) for minimum
average impact value of 28 J
using 4 mm diameter electrodes only
E— — 0 — — Not specified
E— — 1 — — _20
E— — 2 — — 0
E— — 3 — — _20
E— — 4 — — _30
E— — 5 — — _40
Table 11.3Second digit for an impact value (British
Standards Institution)
Digit Temperature (°C) for minimum
average impact value of 47 J
using 4 mm diameter and largest
diameter electrodes submitted for
classification
E— — — 0 — Not specified
E— — — 1 — _20
E— — — 2 — 0
E— — — 3 — _20
E— — — 4 — _30
E— — — 5 — _40
E— — — 6 — _50
E— — — 7 — _60
E— — — 8 — _70
Table 11.4Welding current and voltage conditions
(British Standards Institution)
Digit Direct current: Alternating current:
recommended minimum open-circuit
electrode polarity voltage (V)
0 See manufacturer Not suitable for use on AC
1 _ or _ 50
2 _ 50
3 _ 50
4 _ or _ 70
5 _ 70
6 _ 70
7 _ or _ 80
8 _ 80
9 _ 80
Table 11.5Nominal lengths of electrodes (British
Standards Institution)
Diameter (mm) Nominal length (mm)
1.6 200
2 200
2.5 250
3.2, 4, 5 350
6 to 8 350
Electrode and bundle identification
The STC code should be marked on the covering
of each electrode as near as possible to the gripped
end, except on electrodes of 1.6 mm and 2 mm
diameter where it is not practicable.
Each bundle or package of electrodes should be
clearly marked with the following information:
1 The number and date of the British Standard
Classification (STC code)
2 Name of manufacturer
Manual metal arc welding 301
3 Trade designation of electrodes
4 Size and quantity of electrodes
5 Batch number
6 Recommended current range, polarity and power
supply
7 Recommendations for special storage conditions
8 Any other significant information on electrode
characteristics or limitations on use
9 Health warnings.
11.5 Arc welding positions
Arc welding operations can be carried out with
work in almost any position, but the degree of skill
required of the welding operator will vary considerably
depending on the position of the joint,
which may be flat or downhand, horizontal, vertical
or overhead (Figure 11.7).
Flat position (downhand)
Although welding can be done in any position the
operation is simplified if the joint is flat. The speed
of welding is then increased because the molten
metal has less tendency to run, better penetration
can be secured, and the work is less fatiguing for
the welder in this position. Some jobs may at first
glance appear to require horizontal, vertical or
overhead welding, but upon examination it may be
possible to change them to the easier and more
efficient flat position, and for this reason positioning
jigs are used in mass production. When welding
in this position the electrode should be at an
angle of 60–70° to the plate surface or horizontal.
Horizontal position
Occasionally the welding operation must be done
while the work is in a horizontal position, which
means that the welder must use a slightly shorter
arc than for flat position welding. The shorter arc
will minimize the tendency of the molten pool to
sag or run down and cause overlap. An overlap
occurs when the pool runs down to the lower side
of the weld and solidifies on the surface without
actually penetrating the metal. For horizontal welding,
hold the electrode so that it points 5–10° and
slants approximately 20° away from the depositing
weld. When welding, use a narrow weaving motion
which will still further reduce the tendency of the
molten weld pool to sag. Figure 11.7Arc welding positions
302Repair of Vehicle Bodies
Vertical position
Vertical welding is done by depositing a weld in an
upward or downward direction. Downward welding
is very good for welding light-gauge metals because
penetration is shallow, therefore forming an adequate
weld without burning through the metal. Downward
welding can also be performed more rapidly, which
is important in production work. On heavier plates of
6 mm or more, vertical upward welding is more
practical since deep penetration can be obtained.
Welding upwards also makes it possible to create a
shelf upon which successive layers of weld can be
placed. For vertical downward welding the electrode
should be held at 10–15° tilted from the horizontal,
and starting at the top of the seam, moving downwards
with little or no weaving motion. For vertical
upward welding start with the electrode at right
angles to the plate, then lower the rear of the electrode
10–15° with the horizontal.
Overhead position
Welding in an overhead position is probably the
most difficult of operations. It is difficult because
the welder must assume an awkward stance, and at
the same time work against gravity, which means
that the molten pool has a tendency to drop, making
it more difficult to secure a uniform weld and
correct penetration. Heavily coated rods must not
be used because of the continual dropping of the
slag, and for this reason medium coated rods are
generally used. The most important points about
overhead welding are to obtain the correct control
of the current and keep a very short arc. Correct
current control gives a pool that is sufficiently
molten to ensure good penetration. Hold the electrode
at right angles to the seam, then tilt the rear of
the electrode until it forms an angle of 10–15° to
the horizontal.
11.6 Essential factors of arc welding