Setting up the equipment for use

1 Select the electrode type according to the cutting

gas used. Check that the gas nozzle and electrode

of the torch are not damaged, and set the

electrode distance as required for the selected

electrode type using the nozzle tool. For the best

cutting results, the electrode distance should be

regularly checked and adjusted.

2 Before performing any work in the torch,

always disconnect the main voltage.

3 Check that the correct pressure (approximately

5 bar or 75 psi) is set on the regulator, and that

any valves ahead of the regulator are open.

4 Connect the earth lead, ensuring as good a contact

as possible to the workpiece (if necessary,

clean off any surface coating rust). The welding

clamp must not be moved during cutting; the

machine should be disconnected.

Figure 9.29Plasma cutting unit: single phase,

30 amperes (SIP (Industrial Products) Ltd)

Figure 9.30An air-cooled plasma cutting torch

(SIP (Industrial Products) Ltd )

1 Handle

2 Electrode

3 Insulator bush

4 Spring

5 Plasma nozzle

6 Protection nozzle

Figure 9.31Plasma arc cutting torch nozzle,

showing position of electrode (Motor Insurance

Repair Research Centre)

Medium-output units (approximately 30 amperes)

operating on input voltages of 220/240 V single

phase. These cutters are normally available with

air-cooled torches using clean workshop or bottled

air (see Figures 9.29 and 9.30).

274Repair of Vehicle Bodies

5 Set the main switch of the machine. The cooling

unit and the fan will start and the indicator

‘on’ lamp will light.

6 Check for correct gas flow by activating the

switch on the torch handle. The gas valve will

open and the gas will start to flow from the

nozzle (3.5–4.1 bar). After this gas test, reset

the switch to its initial position.

7 Press the trigger button once only. Air should

flow from the torch; if it does not, check the

warning lamps on the machine. The red lamps

should not be illuminated. The left-hand one

indicates low air pressure, and the right-hand

one that thermal cut-outs have operated. If

either is lit then appropriate action should be

taken. The green lamp should be illuminated,

as it indicates that the machine is on. The

green light is switched off when a red light

goes on.

8 Whilst the air is flowing, check that the pressure

is 3.5 bar (50 psi). If not, adjust the regulator

to give 3.5 bar.

9 Set the machine to either maximum or minimum

according to the thickness to be cut.

Caution: do not attempt to cut material

beyond the range specified, as this will damage

the torch.

10 Position the torch on the workpiece, ensuring

that the tip has a good contact.

11 Press the trigger button once and release it.

Allow the air to flow for a few seconds.

Press the button again and then, within one

second, press and hold the button. The arc

will strike and a hole will be cut in the

metal.

Caution: if the arc does not penetrate the

metal then stop, as the machine is either on

the wrong setting or not sufficiently powerful

for the thickness being cut.

12 Move the torch along so that the metal is cut

right through. If the metal is not cut, reduce

the speed of travel until it is. The torch head

should be held at right angles to the work

(Figure 9.32).

13 When the cut is complete, release the trigger

button.

Caution: do not switch off the machine until

the air has stopped flowing, otherwise the

torch will be damaged, as the air is required to

cool the torch.

14 The operator must be constantly aware of the

risk of fire and fumes, and therefore proper

health and safety precautions must be adhered

to at all times.

Cutting methods

The equipment can be used for contact or distance

cutting. When cutting is started, the techniques differ

between nitrogen and argon/hydrogen plasma

cutting and air plasma cutting.

Contact cutting

Contact cutting is used for materials up to 5 mm

in thickness. In this method the torch nozzle is in

contact with the workpiece. The torch is tilted at an

angle with the work surface to obtain plasma flow,

and gradually straightened up until it lies flush

with the workpiece when cutting.

Distance cutting

This is used for materials over 5 mm thick. With

this method the nozzle is not in contact with the

workpiece, and an even distance must be maintained

between them when cutting.

Process advantages

Plasma arc cutting is one of the most effective

processes for high-speed cutting of many kinds of

ferrous and non-ferrous metals. The quality of the cut

is very high, with little or no dross and reduced distortion.

Better economy is also a feature of plasmaarc

cutting as compared with oxy-fuel methods.

Figure 9.32Plasma arc cutting machine in use

(SIP (Industrial Products) Ltd )

Gas welding, gas cutting and plasma arc cutting 275

With plasma arc equipment a saving in time and

energy can be achieved in certain areas of repair,

particularly in the removal of damaged panels and

structural members. Plasma arc cutting will prove

of benefit in a modern body repair shop as an alternative

cutting technique to speed up damaged

panel removal and to complement traditional

mechanical methods.

Safety

General

1 Do not operate the machine with any of the

panels removed.

2 Ensure that the machine is connected to the

correct voltage supply, that the correct fuse is

used and that the equipment is earthed.

3 Under no circumstances must the plasma nozzle

be removed or any other work be carried

out on the torch with the machine switched on.

Ignoring this precaution could lead to contact

with high DC voltage.

4 The torch should be kept free of slag at all

times to ensure a free passage of air.

5 The tip and electrode will require changing

when they have become pitted and the arc will

not strike. The life of these components depends

upon the current used and any misuse of the

torch.

Fire

1 All inflammable materials must be removed

from the area.

2 Have a suitable fire extinguisher available close

by at all times.

3 Do not cut containers which have held inflammable

materials or gases.

Glare and burns

1 The electric plasma arc should not be observed

with the naked eye. Always wear goggles of the

type used for oxy-acetylene welding.

2 Gloves should be worn to protect the hands

from burns.

3 Non-synthetic overalls with buttons at neck and

wrist, or similar clothing, should be worn.

4 Greasy overalls should never be worn.

5 Do not wear inadequate footwear.

Compressed air

Compressed air is potentially dangerous. Always

refer to the relevant safety standards for compressed

gases (see Chapter 2).

Ventilation, fumes, vapours

Ventilation must be adequate to remove the smoke

and fumes during cutting. Toxic gases may be given

off when cutting, especially if zinc or cadmium

coated steels are involved. Cutting should be carried

out in a well ventilated area, and the operator

should always be alert to fume build-up. In small or

confined areas, fume extractors must be used.

Vapours of chlorinated solvents can form the

toxic gas phosgene when exposed to UV radiation

from an electric arc. All solvents and degreasers

are potential sources of these vapours and must be

removed from the area being cut.

Questions

1 Describe the general precautions which should

be taken in the storage and handling of oxygen

and acetylene cylinders.

2 State the safety precautions to be taken when

assembling oxy-acetylene equipment.

3 State how gas cylinders are visibly identifiable.

4 State the correct pressures for full cylinders of

oxygen and acetylene.

5 Compile a list of safety measures which should

be applied in the preparation and use of gas

welding equipment.

6 What method should be used to find the location

of a leak in an acetylene connection?

7 What safety precautions should be taken if

an acetylene cylinder was to take fire

internally?

8 State the reason why copper or high-coppercontent

alloy tubing should not be used for an

acetylene connection.

9 Why is it dangerous to allow grease or oil to

come into contact with the oxygen cylinder valve

or fittings?

10 What is meant by the following terms: (a) highpressure

welding (b) low-pressure welding

system?

11 What are the essential components of a highpressure

welding system?

276Repair of Vehicle Bodies

12 Describe the reasons for both leftward and

rightward welding methods, and state any

benefits derived from these methods.

13 What is meant by an oxidizing flame?

14 What is meant by a carburizing flame?

15 Suggest two reasons for a welding torch

backfiring.

16 Explain the meaning of the term ‘penetration’ in a

welded joint.

17 Sketch and describe the best condition for a

welding torch flame for welding low-carbon steel

sheet.

18 When making gas-welded butt joints in sheet

steel, which common faults should be avoided?

19 Sketch and name the type of flame suitable for

welding each of the following: (a) aluminium

(b) brass.

20 Describe the technique which would be most

suitable for welding low-carbon steel up to 5 mm.

21 With the aid of a sketch, give a detailed

explanation of the working and function of the

hose check valve on oxy-acetylene welding

hoses.

22 List the procedure sequence necessary to shut

down a gas welding plant high-pressure system.

23 Explain the difference between line pressure and

contents pressure as shown on the regulator.

24 Describe the process of gas welding pure

aluminium sheet.

25 Explain the precautions to be taken when welding

aluminium.

26 State two disadvantages which limit the use of

oxy-acetylene welding in vehicle body repair.

27 What principle makes possible the cutting of

metal by means of oxy-acetylene?

28 Explain the basic principle of plasma arc cutting.

29 Explain the type of gases used in plasma arc

cutting.

30 Describe how the plasma arc process can be

used in vehicle body repair.

31 Name the oxy-acetylene flame that would burn

equal quantities of the gases.

32 State the purpose of the two pressures that are

shown on the gauge of an acetylene gas cylinder

regulator.

33 Explain the safety device used on oxy-acetylene

welding equipment to stop the risk of the flame

travelling back down the torch supply hose.

34 With the aid of a sketch, outline two methods of

limiting distortion when welding thin sheet metal.

35 Explain the difference in welding low-carbon steel

and aluminium of equal thickness.

36 List the consumables used in plasma arc cutting.

37 State the welding process that is not

recommended when welding HSLA steels.

38 What is meant by ‘contact cutting’ when using

plasma arc?

39 What is the electrode made from in a plasma arc

cutting torch?

40 What are the two gases that can be used in the

flame cutting process?

Electric resistance

welding

10.1 Resistance welding in car body

manufacture

Resistance welding is a joining process belonging to

the pressure welding sector. With its locally applied

heat and pressure it has an obvious relationship with

the forge welding technique practised by blacksmiths

when joining metal. The resistance welding process

was invented in 1877 by Professor E. Thomson of

Philadelphia, USA, when an accidental short circuit

gave him the idea for what was originally termed

short circuit welding. From the beginning of the

twentieth century it was used on a small scale in

industry, but it was only after the Second World War

that resistance spot welding had its real beginning in

the automobile industry. It has since grown to be the

most important method of welding used in the construction

and mass production of vehicle bodies.

Resistance welding is extensively used for the

mass production assembly of the all-steel body and

its component sheet metal parts (see Figure 10.1).

Its wide adoption has been brought about by its

technical advantages and the reductions in cost.

Most mass produced car bodies are assembled

entirely by welding steel pressings together to produce

an integral rigid chassis and body structure.

Low-carbon steel thicknesses used in this unitary

construction range from 0.8–1 mm for skin or floor

panels to 3 mm for major structural pressings such

as suspension brackets. Intermediate gauges such as

1.2 mm are used for hinge reinforcements, 1.6 mm

for chassis structural members, and 1.8 mm to

2.5 mm for suspension and steering members. With

the introduction of high-strength steels (HSLA

steels), car manufacturers are producing body panels

as thin as 0.55 mm, and structural members with

gauges of between 1.2 and 2 mm. This reduction in

thickness can be made without loss of strength.

There are a number of resistance welding

processes. Resistance spot welding is the most widely

used welding process in car body construction; there

are approximately 4500–6000 spot welds per body,

which accounts for approximately 80 per cent of the

welding used. A further 10 per cent is composed of

other resistance welding processes: seam, projection,

flash and butt welding. The remaining 10 per cent is

divided between MIG/MAG welding and gas welding.

Of the many welding techniques used in the

mass production of car bodies, resistance welding

dominates the field.

The fundamental principle upon which all

resistance welding is based lies in the fact that the

weld is produced by the heat obtained from the

resistance to flow of electric current through two

or more pieces of metal held together under pressure

by electrodes which are made from copper

or copper alloys. The engineering definition of

heat (heat being the essence of all welding) is

(energy) _ time. This indicates a balance between

Figure 10.1Automatic spot welding using robots on

body framing assembly line (Vauxhall Motors Ltd )

278Repair of Vehicle Bodies

energy2 input and weld time; therefore the faster

the welding, the greater the clamping force. However,

the engineering definition of resistance is such

that the higher the clamping force, the greater the

current needed to produce a constant heat. Heat is

generated by the resistance of the parts to be joined

to the passage of a heavy electrical current. This

heat at the junction of the two parts changes the

metal to a plastic state. When the correct amount

of pressure is then applied fusion takes place.

There is a close similarity in the construction

of all resistance welding machines, irrespective of

design and cost. The main difference is in the type

of jaws or electrodes which hold the object to

be welded. A standard resistance welder has four

principal elements:

Frame The main body of the machine, which

differs in size and shape for robot, stationary and

portable types (Figures 10.2, 10.3, 10.4).

Electrical circuit Consists of a step-down transformer,

which reduces the voltage and proportionally

increases the amperage to provide the

necessary heat at the point of weld.

Electrodes Include the mechanism for making

and holding contact at the weld area.

Timing control Represents the switches which

regulate the volume of current, length of current time

and the contact period. Many now include adaptive

in-process control units (pulsing weld timer).

The principal forms of resistance welding are

classified as: resistance spot welding; resistance

projection welding; resistance seam welding; resistance

flash welding; and resistance butt welding.

Figure 10.3Stationary pedestal spot welding

machine (SIP (Industrial Products) Ltd )

Figure 10.4Portable spot welding machine in use

(SIP (Industrial Products) Ltd )

Figure 10.2Robot spot welder on an assembly line

(Vauxhall Motors Ltd)

Electric resistance welding 279

10.2 Resistance spot welding

Resistance spot welding (Figure 10.5) is basically

confined to making welds approximately 6 mm

diameter between two or more overlapping sheet

metal panels. This type of welding is probably the

most commonly used type of resistance welding.

The art of production planning for spot welding is

to simplify the presentation of panels in the region

of mutual panel overlap. The limitation of spot

welding is that the electrode assemblies have to

withstand applied forces ranging from 2200 N to

4450 N for the range of sheet steel thicknesses

used in vehicle construction and repair. Product

design of planning must account, therefore, for the

requirement that the electrodes, which are constructed

from relatively weak copper alloys, need

normal access to both sides of the overlapping

panels to withstand such electrode forces.

The material to be joined is placed between two

electrodes, pressure is applied, and the electric current

passes from one electrode through the material

to the other electrode. There are three stages in

producing a spot weld. First, the electrodes are

brought together against the metal and pressure is

applied before the current is turned on. Next the

current is turned on momentarily. This is followed

by the third hold time, in which the current is

turned off but the pressure continued. The hold

time forges the metal while it is cooling.

Regular spot welding usually leaves slight

depressions on the metal which are often undesirable

on the show side of the finished product.

These depressions are minimized by the use

of larger-sized electrode tips on the show side.

Resistance spot welding can weld dissimilar metal

thickness combinations by using a larger electrode

contact tip area against the thicker sheet. This

can be done for mild steel having a dissimilar

thickness ratio of 3:1.

There are three kinds of distortion caused by

resistance spot welding which are relevant to

car body manufacture and repair. The first is the

local electrode indentation due to the electrode sinking

into the steel surface. This is a mechanical distortion

– a byproduct of the spot welding process.

Second, there is a small amount of thermal distortion

which is troublesome when attempting to make spot

welds on show surfaces such as skin panels without

any discernible metal distortion. Last, there is the

gross distortion caused when badly fitting panels are

forced together at local spots. This is a mechanical

distortion totally unconnected with the spot welding

process; the same type of distortion would occur

with rivets or with any similar localized joining

method. A combination of all these distortions contributes

to the general spot-weld appearance, which

is virtually unacceptable on a consumer product. The

technique on car bodies is to arrange for spot-welded

flanges to be either covered with trims (door apertures),

or with a sealing weather strip (window and

screen surrounds). The coach joint is one of the features

that distinguish a cheap, mass produced body

from an expensive hand built one.

Figure 10.5(a) Resistance spot welding system

(b) relationship between weld formation, current and

pressure in welding

280Repair of Vehicle Bodies

Spot welders are made for both DC and AC. The

amount of current used is very important. Too little

current produces only a light tack which gives

insufficient penetration. Too much current causes

burned welds. Spot welds may be made one at a

time or several welds may be completed at one

time, depending on the number of electrodes used.

One of the most significant advantages of resistance

spot welding is its high welding production

rate with the minimum of operator participation.

Typical resistance spot-welding rates are 100 spots

per minute. To dissipate the heat at the weld as

quickly as possible, the electrodes are sometimes

water cooled. Although many spot welders are of

the stationary design, there is an increased demand

for the more manoeuverable, portable type. The

electrodes which conduct the current and apply the

pressure are made of low-resistance copper alloy

and are usually hollow to facilitate water cooling.

These electrodes must be kept clean and shaped

correctly to produce good results. Spot welders

are used extensively for welding steel, and when

equipped with an electronic timer they can be used

for metal such as aluminium, copper, stainless and

galvanized steels.

10.3 Resistance projection welding

Projection welding (Figure 10.6) involves the

joining of parts by a resistance welding process

which closely resembles spot welding. This type of

welding is widely used in attaching fasteners to

structural members. The point where the welding

is to be done has one or more projections which

have been formed by embossing, stamping or

machining. The distortion on the embossed metal

face is low and is negligible on heavy metal thicknesses,

although in the case of dissimilar thicknesses

it is preferable to emboss the projection on

the thicker of the two sheets. The projections serve

to concentrate the welding heat at these areas and

permit fusion without the necessity of employing

a large current. The welding process consists of

placing the projections in contact with the mating

fixtures and aligning them between the electrodes.

The electrodes on the machine are not in this case

bluntly pointed as in spot welding, but are usually

relatively large flat surfaces which are brought to

bear on the joint, pressing the projections together.

The machine can weld either a single projection

or a multitude of projections simultaneously. The

many variables involved in projection welding,

such as thickness, kind of material, and number of

projections, make it impossible to predetermine the

correct current. Not all metals can be projection

welded. Brass and copper do not lend themselves

to this method because the projections usually

collapse under pressure.

10.4 Resistance seam welding

Seam welding (Figure 10.7) is like spot welding

except that the spots overlap each other, making a

continuous weld seam. In this process the metal

pieces pass between roller-type electrodes. As the

electrodes revolve, the current supply to each one

is automatically turned on and off at intervals

corresponding to the speed at which the parts are

set to move. With proper control it is possible

Figure 10.6Projection welding system Figure 10.7Continuous resistance seam welding

Electric resistance welding 281

to obtain air-tight and water-tight seams suitable

for such parts as fuel tanks and roof panels. When

spots are not overlapped long enough to produce a

continuous weld, the process is sometimes referred

to as roller spot or stitch welding (Figure 10.8).

In this way the work travels the distance between

the electrodes required for each succeeding weld

cycle. The work stops during the time required to

make each individual weld and then automatically

moves the proper distance for the next weld cycle.

Intermittent current is usually necessary for most

seam welding operations. Each individual weld

contains a number of overlapping spot welds,

resulting in a joint which has good mechanical

strength but which is not pressure tight.

Mild steel, brass, nickel, stainless steel and

many other alloys may be welded by this method,

although its use is largely limited to operations on

mild and stainless steel. A maximum economy is

obtained if the combined thickness of sheets to be

joined does not exceed 3.2 mm. Before welding,

the surfaces must be clean and free from scale,

and this may be done by sand blasting, grinding

or pickling.

10.5 Resistance flash welding

In the flash welding process (Figure 10.9) the two

pieces of metal to be joined are clamped by copper

alloy dies which are shaped to fit each piece and

which conduct the electric current to the work. The

ends of the two metal pieces are moved together

until an arc is established. The flashing action

across the gap melts the metal, and as the two

molten ends are forced together fusion takes place.

The current is cut off immediately this action is

completed.

Flash welding is used to butt or mitre sheet, bar,

rod, tube and extruded sections. It has unlimited

application for both ferrous and non-ferrous metals.

For some operations the dies are water cooled

to dissipate the heat from the welded area. The

most important factor to be considered in flash

welding is the precision alignment of the parts. The

only problem encountered in flash welding is the

resultant bulge or increased size left at the point of

weld. If the finish area of the weld is important,

then it becomes necessary to grind or machine the

joint to the proper size.

10.6 Resistance butt welding

In butt welding (Figure 10.10) the metals to be

welded are brought into contact under pressure,

an electric current is passed through them, and the

edges are softened and fused together. This process

differs from flash welding in that constant pressure

Figure 10.8Seam welding (stitch welding)

Figure 10.9Flash welding

Figure 10.10Resistance butt welding

282Repair of Vehicle Bodies

is applied during the heating process, which eliminates

flashing. The heat generated at the point of

contact results entirely from resistance. Although

the operation and control of the butt welding process

is almost identical to flash welding, the basic difference

is that it uses less current, has a constant

pressure and allows more time for the weld to be

completed.

10.7 Resistance welding in body

repair work

The object of car body repair is to put damaged

vehicles back into a pre-accident condition. Today’s

chassisless bodies hold engine, suspension and

steering in the right places and are designed to

absorb the impact of crashes by crumpling, thus

shielding the passenger compartment (and its

passengers) from shock and deformation. From the

viewpoint of safety as well as mechanical efficiency,

proper welding is vital in this kind of repair.

Car body design demands careful choice of the

sheet metal, which was, until recent years, all mild

steel. Tensile strength and ductility, which are good

in mild steel, are vital to ‘crumplability’ (the ability

to absorb the impact energy), and this is why resistance

spot welds are used. The average body shell

is joined together by approximately 4500–6000

such spot welds. These remain ductile because the

welding process does not alter the original specifications

of the steel. Lighter body weight reduces

the load on the car engine and therefore has a

direct influence on petrol consumption.

For weight and fuel saving reasons, high strength

steels have been introduced for some panel assemblies

on body shells. As these steels have a different

character from low-carbon steel (mild steel), which

still accounts for 60 per cent of the body shell (see

Figure 10.11), they cause repair welding problems.

Higher-strength steels have been made specifically

for motor car manufacturers to produce body shells

from thinner but stronger steel sheet. These steels

are less ductile and are harder. Above all, they do

not tolerate excess heat from bad welding, which

makes them brittle or soft or can cause panel

distortion. Low-carbon steel tolerates excess heat

well. While older all-mild-steel bodies essentially

needed only to have the welding machine set for the

metal thickness, current new bodies can contain

steel of up to four different strengths, hardnesses

and ductilities, some coated on one side, some

coated on both sides, and some uncoated. Zinc

coated sheet materials require the use of heavier

welding equipment capable of producing a higher

current to penetrate the zinc coating, and the electrodes

must be constantly maintained by cleaning

to avoid zinc pick-up when welding.

The repair of bodies incorporating low-carbon

steels and HSLA steels therefore demands very different

welding routines from those for low-carbon

steel alone. Low-carbon steel bodies can be resistance

spot welded or gas (TIG) welded or arc

(MIG) welded; but higher-strength steels should

not undergo the last two processes, because they

involve nearly three times the heat of resistance

spot welding. The temperatures generated are over

3300 °C for gas or arc welding but only 1350 °C

for resistance spot welds, for joints of similar

strength. Higher-strength steels, however, with their

higher tensile strength, limited ductility and greater

hardness, are particularly vulnerable to heat, and

are apt to lose strength and change ductility when

overheated.

Developments in welding equipment combined

with the use of electronic controls have opened

the way to new body repair welding techniques that

help to simplify the practical problems posed by

bodies made from a mixture of low-carbon and highstrength

low-alloy steels. The traditional resistance

welding method has had to be improved to join

higher-strength steels. Because the weld current flow

is hindered by the steel’s coating, these may require

higher temperatures to break them down before a

weld can be formed. For producing consistently

good welds it is necessary to use two or three stages

for welding, the duration of each stage being adapted

to the nature of the steel and its coating.

10.8 Resistance spot welding

of high-strength steels

The rigidity of the body and its ability to withstand

high torsional and other stresses depend on the

assembly method used to bring the various body

panels together. Spot welding is used throughout

the industry (see Figure 10.12) for two reasons:

first, because it is the strongest and most reliable

method of joining two pieces of metal; and second,

because of the total absence of panel distortion

through the welding. In order to effect a satisfactory

Electric resistance welding 283

Figure 10.11Body panels made from high-strength/galvanized steels (Motor Insurance Repair Research Centre)

Material

1 Hood panel, outer SGAC35R

2 Hood panel, inner SGACC

3 Upper frame, outer SGACC

4 Upper frame, inner SGACC

5 Front side member SPRC35

6 Front end gusset SPRC35

7 Front fender SENCE

8 Front skirt panel SGACC

9 Grille filler panel SGACC

10 Front end cross member SPRC35

11 Side sill, inner front SGACC

12 Front upper frame extension SGACC

13 Cowl top outer panel SGACC

14 Front door outer panel SGACC

15 Front door inner panel SGACE

16 Rear door outer panel SGACC

17 Rear door inner panel SGACE

18 Roof panel SGACC

19 Front pillar, outer, lower SPRC35

Material

20 Front pillar, outer, upper SPRC35

21 Front Pillar, inner, upper SPRC35

22 Centre pillar, inner SPRC35

23 Centre pillar, outer SPRC35

24 Front floor, side sill, outer SGACC

25 Front floor, side sill, inner SGACC

26 Side sill, outer rear extension SGACC

27 Quarter panel, inner SENCE

28 Rear pillar, inner SPRC35

29 Rear shelf panel SPRC35

30 Trunk lid inner panel SGACE

31 Trunk lid outer panel SGAC35R

32 Quarter panel, outer SENCE

33 Rear skirt panel SGACC

34 Rear floor cross member SGACC

35 Rear floor side member SGACC

36 Rear floor side sill SGACC

37 Roof reinforcement SGAHC

SPRC: phosphorus added

SGACC, E

Galvanized steel plate

SGAHC

SGAC35R: phosphorus added (also galvanized)

SENCE: SPCE with electrogalvanized zinc-nickel coating

The numbers in the material codes indicate the tensile strength (kg/mm2)

_

284Repair of Vehicle Bodies

repair, vehicle welds having the same characteristics

as the original are essential.

A comparison between an electrical spot weld

and a forged weld shows that in both these

processes a union is formed by an amalgamation of

the metal molecules. These have been consolidated

in each of the two pieces, despite the difference in

the two processes employed. In the case of forge

welding the pieces are heated in the forge furnace

and then hammered until a homogeneous material

results. In the case of the spot-welding process the

gun must have a pressure device which can be

operated by the user and transmits the pressure to

the electrodes, and a transformer to enable current

at high intensity to be fed to the electrodes.

Spot welding is the primary body production

welding method. Most production welding is

carried out on metal gauges of less than 2.5 mm,

although greater thicknesses can be spot welded.

With the introduction of high-strength steels, car

manufacturers are producing body panels as thin

as 0.55 mm and using gauges of between 1.2 and

1.5 mm for structural members. By using the

correct equipment, two-sided spot welding can be

successfully accomplished on these steels.

Weld quality

Careful control of three factors is necessary to

make a good-quality spot weld:

Squeeze time

Weld time (duration of weld flow)

Hold time.

The main disadvantage associated with welding in

general is that any visual inspection of the weld gives

no indication of the weld quality. It is therefore

essential, that spot-welding equipment used in motor

body repair takes outside circumstances automatically

into account, such as rust, scale, voltage drop in

the electric main supply, and current fluctuations.

Figure 10.12Weldmaster 2022 in use (ARO Machinery Co. Ltd )

Electric resistance welding 285

The quality of a spot weld depends on:

1 The strength of the weld must be equal to the

parent metal. This is a problem of tensile strength

and homogeneity of the weld nugget itself.

2 The welding heat must not in any way alter the

inherent qualities of the parent metal.

3 The welding pressure must prevent parent metal

separation so as not to cause panel distortion

and stresses in the assembly.

Heat application

With resistance welding the welding time is controlled

by either electronic or electro (electrical)

devices. Hand-held pincer-type weld guns, as supplied

to the car body repair trade, are usually

controlled electronically with the minimum of

presetting. Weld time can range from a fraction

of a second for very thin gauges of sheet steel, to

one second for thicker sheet steel. Weld time is

an important factor, because the strength of a weld

nugget depends upon the correct depth of fusion.

Sufficient weld time is required to produce this

depth of fusion by allowing the current time to

develop enough heat to bring a small volume of

metal to the correct temperature to ensure proper

fusion of the two metals. If the temperature reached

is too high, metal will be forced from the weld

zone and may induce cavities and weld cracks.

In some grades of high-strength steels, cracking

within the vicinity of the heat affected zone has

appeared after the welding operation.

Heat balance

In the resistance welding of panels of the same

thickness and composition, the heat produced will

be evenly balanced in both panels, and a characteristic

oval cross-sectional weld nugget will result

(see Figure 10.13). However, in the welding of

panels produced from steels of a dissimilar composition,

i.e. conventional low-carbon steel to highstrength

low-alloy steels, an unbalanced heat rate

will occur. This problem will also arise when spot

welding two different thicknesses of steel; more

heat will usually be lost by the thicker gauges,

resulting in unsatisfactory welds. To compensate,

the welder must either select an electrode made

from materials that will alter the thermal resistance

factor, or vary the geometry of the electrode tip,

or use pulse equipment.

Electrodes

The functions of the electrode (chromium/copper)

are to:

1 Conduct the current to the weld zone

2 Produce the necessary clamping force for the

weldment

3 Aid heat dissipation from the weld zone.

The profile and diameter of the electrode face

is dependent upon the material to be welded

(Figure 10.14). The diameter of the face will directly

influence the size of the nugget. The proper maintenance

of the electrode tip face is therefore vital to

ensure that effective current flow is produced. For

example, if a tip diameter of 5 mm is allowed through

wear to increase to 8 mm, the contact area is virtually

doubled; this will result in low current density and

weak welds. Misalignment of the electrodes and

incorrect pressure will also produce defective welds.

Electrode tip dressing is best carried out with

a fine-grade emery cloth. Coated steels will often

cause particles of the coating to become embedded

in the surface of the electrode tip face (pick-up). This

necessitates frequent tip dressing; when the faces

Figure 10.14Types of electrode profile (Motor

Insurance Repair Research Centre)

Figure 10.13Welding together panels of the same

thickness (Motor Insurance Repair Research Centre)

286Repair of Vehicle Bodies

have become distorted, the profile or contour must be

reshaped. Cutters are available to suit most profiles

and tip diameters. Domed tips are recommended;

they adapt themselves best to panel shapes and last

longest. The adjustment of electrode clamping force

is critical for any type of steel, particularly when

long-arm arrangements are used and complex configurations

of arms are used for welding around

wheel arches. It should also be noted that the tip

force is difficult to maintain with long arms fitted to

hand-held pincer-type guns (see Table 10.1).