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).