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

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