The adjustment of a good welding flame is affected
by the pressure of the gas entering the welding
torch. If the pressure from either the acetylene or
the oxygen regulator is excessive, the result will be
a fierce or harsh flame which is very difficult to
manipulate, especially when welding panel steel
with the neutral flame. Excessive pressure also
increases the tendency to blow the edges of the
steel away as they are melted. On the other hand, if
the pressure of either gas is too low a backfiring of
the welding torch will result.
Gas welding, gas cutting and plasma arc cutting 259
9.7 Methods of welding
Leftward or forward welding is the technique or
method of welding in which the welding torch
flame is directed towards the uncompleted joint.
When the flame is directed towards the completed
weld, this is termed rightward, backward or backhand
welding. The positions in which welding is
performed are termed respectively flat, downhand
or underhand when the weld is made on the upper
side of a horizontal or flat surface, overhead when
on the underside, and vertical when on an upright
or vertical surface.
Leftward and rightward welding are shown in
Figure 9.17.
and welding proceeds towards the left. The welding
torch is given a forward motion, with a slight sideways
movement to maintain both edges melting at
the desired rate, and the welding wire is moved progressively
along the weld seam. The sideways motion
of the welding torch should be restricted to a
minimum. The advantages of the leftward welding
technique are that it is faster because the flame, in
preceding the weld, has a preheating effect; the
molten metal is easily controlled; and penetration
(complete fusion of the edges) is easily obtained over
the range of application of the technique. One of the
important features of a good weld is complete penetration.
A disadvantage of the leftward method of
welding is that if incorrect manipulation of the flame
occurs, molten metal will flow ahead of the molten
pool at the bottom of the weld and adhere to the comparatively
cold plate, thus causing poor penetration.
The characteristics of the leftward welding technique
are as follows:
Position Flat.
Direction of welding Flame points away from the
finished weld.
Angle of welding torch 6070.
Movement of welding torch Straight along seam,
with side to side movement reduced to a minimum.
Gas consumption of the welding torch 86 l/h
(litres per hour) and 3 ft3/h (feet cubed per hour)
for 1.6 mm thickness.
Type of flame Neutral.
Position of filler rod Precedes the welding torch.
Angle of filler rod 3040.
Movement of filler rod Slight movement along
seam in and out of the molten pool.
Size of filler rod Approximately equal to the plate
thickness up to 2.4 mm, and then 3.2 mm for thickness
above 2.4 mm.
Plate preparation Up to 1.00 mm: edges flanged.
From 1.00 mm to 3.2 mm: square edge preparation.
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From 3.2 mm to 4.8 mm: 80 V preparation.
Rightward and all-position
Rightward welding
Rightward welding is recommended for steel
plates over 4.8 mm thick. Plate edges from 4.8 to
7.9 mm need not be bevelled. Plate over 7.9 mm
should be bevelled to 30 to give an included angle
of 60 for the welding V. The weld is started at the
left-hand end of the joint and the welding torch is
Figure 9.17 Methods of welding (a) the leftward
method and (b) the rightward method
Leftward welding
This is used on steel for flanged edge welds, for bevelled
steel plates up to 3.2 mm, and for bevelled
plates up to 4.8 mm. It is also the method usually
adopted for cast iron and non-ferrous metals.
The weld is started on the right-hand end of the joint
260 Repair of Vehicle Bodies
moved towards the right, the welding torch preceding
the filler rod in the direction of travel. The wire
is given a circular forward action and the welding
torch is moved steadily along the weld seam. It is
quicker than leftward welding and consumes less
gas. The V-angle is smaller, less welding rod is
required and there is less distortion. The advantages
of the rightward technique are as follows: the
direction of the flame holds back the molten pool,
preventing any tendency to adhesion; the flame,
which points directly towards the root of the weld,
causes a hole to form as the edges melt, thus ensuring
complete penetration; no preparation is necessary
on thicknesses up to 7.9 mm.
The characteristics of the rightward technique
are as follows:
Position Flat.
Direction of welding Flame points towards finished
weld.
Angle of welding torch 4050.
Movement of welding torch Straight along seam,
with sufficient side swing to ensure complete fusion
of the edges.
Gas consumption of the welding torch 140 l/h
which is 5 ft3/h for 2.6 mm thickness.
Type of flame Neutral.
Position of filler rod Follows the welding torch.
Angle of filler rod 3040.
Movement of filler rod Circular forward action.
Size of filler rod Half the plate thickness up to a
maximum of 6.4 mm.
Plate preparation From 4.8 mm to 7.9 mm: square
edge, with gap of half thickness. From 7.9 mm to
15.9 mm: 60 bevel, gap of 3.2 mm, and weld made
in one pass. From 15.9 mm upwards: as above but
step weld, using multipass technique.
In the rightward and leftward techniques it is seen
that edge preparation becomes necessary at 7.9 mm
and 3.2 mm respectively, in order to obtain complete
fusion without fear of adhesion. This involves
extra cost and additional filler material. If therefore
the bevel edge preparation can be obviated, the cost
of the welding is reduced.
9.8 Edge preparation and types of joint
In order to produce a satisfactory butt weld it is
essential for the plate edges to be joined throughout
their entire thickness; this necessitates complete
melting of the edges. With thin materials this result
can be achieved with square edged plates. However,
the same procedure applied to thicker metal is
unlikely to result in complete fusion, and the fusion
faces must be bevelled to enable the torch flame to
be directed into the root of the weld in order to
obtain complete fusion.
Square edge preparation is used for material up to
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3.2 mm thickness. The edges are left square and
separated by a distance equal to one-half the thickness
of the sheet used. Such joins are termed open
butt joint.
Single V preparation consists of bevelling the
edges of each plate so that a V is formed when they
are brought together. For thicknesses between
3.2 mm and 4.8 mm an 80 bevel is used, leaving a
small gap at the bottom edges. For material over
7.9 mm thickness the angle of V should be 60. It
is not necessary to bevel materials up to 7.9 mm
thick if the rightward method of welding is used.
Double V preparation is used for material thicker
than 15.9 mm, which must be welded from both
sides of the plate; for this reason a V must be provided
on each side. The top V should be 60 and
the bottom V 80, and the edges separated by a gap
of 3.2 to 4.0 mm.
Figure 9.18 gives details of edge preparation.
Control of distortion
A proper understanding of the problems of distortion
or buckling associated with the welding of
sheet metal is of the utmost importance to the
panel beater working on thin sheet metal. This
distortion results from the shrinkage which occurs
when molten iron passes from the liquid to the
solid state. In consequence the welded seam tends
to shorten in length, causing the parent metal to
buckle. If the welded seam is so placed that it
may be planished then the hammering along the
weld will correct the distortion. As iron shrinks
approximately 10 mm per metre when solidifying
from the molten condition, the tendency to produce
distortion is considerable. Where the work
to be welded is of thick steel plate, the plate itself
may be sufficiently strong to counteract or minimize
some of the distortion, but where the work is
of thin mild steel the distorting stresses take full
effect in buckling the sheet. In the process of
Gas welding, gas cutting and plasma arc cutting 261
welding it is necessary to consider and make
allowances for this distortion which, unless controlled,
may buckle the work to such an extent
that it is useless. When the joint edges are heated
they expand, and as welding proceeds, contraction
of the deposited weld metal takes place
owing to the loss of heat by radiation and condition.
The rate at which cooling takes place
depends on various factors such as the size of the
work; the amount of weld metal and the speed at
which it is deposited; the thermal conductivity of
the parent metal; and the melting point and specific
heat of the weld metal.
Some methods of controlling distortion are as
follows (Figure 9.19):
1 Efficient tacking or clamping, which maintains
the edge positions of the plates
2 Tapered spacing of plates so that they pull
together the process of welding
3 Use of intermittent weld technique and backstep
weld technique
Figure 9.18 Edge preparation
262 Repair of Vehicle Bodies
Figure 9.19 Control of distortion
Gas welding, gas cutting and plasma arc cutting 263
4 The offsetting and presetting of plates so that
they are pulled correct by the contraction of the
welds
5 The use of chilling bars and chemical foam
barriers
6 The use of planishing when the weld is in the
cold state
7 The use of jigs and fixtures.
9.9 Welding technique: butt joint in mild
steel
To master the skill of welding with an oxy-acetylene
torch, you will have to practise a series of operations
in a definite order. The torch may be held in either
one of two ways, depending on which is the more
comfortable for you. When welding light-gauge
metal, most operators prefer to grasp the handle of
the welding torch with the hose over the outside of
the wrist, which is the way in which a pencil is usually
held. In the other grip, the torch is held like a
hammer, with the fingers lightly curled underneath.
In either case the torch should balance easily in the
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hand to avoid fatigue. Hold the torch in the direction
in which you are going to weld and at an angle of
about 65 with the completed part of the weld. If
you are right-handed, start the weld at the right edge
of the metal and bring the inner cone of the neutral
flame to within 3 mm of the surface of the plate. If
you are left-handed, reverse this direction.
Hold the torch still until a pool of molten metal
forms, then move the puddle across the plate. As
the puddle travels forward, rotate the torch to form
a series of overlapping ovals. Do not move the
torch ahead of the puddle, but slowly work forward,
giving the metal a chance to melt. If the
flame is moved forward too rapidly, the heat fails
to penetrate far enough and the metal does not melt
properly, but if the torch is kept in one position too
long the flame will burn a hole through the metal.
On some types of joints it is possible to weld the
two pieces of metal without adding a filler rod, but
in most instances the use of a filler rod is advisable
because it builds up the weld, adding strength to
the joint. The use of a filler rod requires coordination
of the two hands. One hand must manipulate
the torch to carry a puddle across the plate, while
the other hand must add the correct amount of
filler rod. Hold the rod at approximately half the
angle of the torch, but slant it away from the torch.
It is advisable to bend the end of the rod at right
angles, since this permits holding the rod so that it
is not in a direct line with the heat of the flame.
Melt a small pool of the base metal and then
insert the tip of the rod in this pool. Remember that
the correct diameter rod is an important factor in
securing perfect fusion. If the rod is too large the
heat of the pool will be insufficient to melt the rod,
and if the rod is too small the heat cannot be
absorbed by the rod, with the result that a hole is
burned in the plate. As the rod melts in the pool,
advance the torch forward. Concentrate the flame
on the base metal and not on the rod. Do not hold
the rod above the pool, as if you do the molten
metal will fall into the puddle, combining with
oxygen in the air as it falls so that part of it burns
up and will cause a weak, porous weld. Always dip
the rod in the centre of the pool.
Rotate the torch to form overlapping ovals and
keep raising and lowering the rod as the molten
puddle is moved forward. An alternative torch
movement is the semicircular motion. When the
rod is not in the puddle, keep the tip just inside
the outer envelope of the flame. Work the torch
slowly to give the heat a chance to penetrate the
joint, and add sufficient filler rod to build up the
weld about 2 mm above the surface. Be sure that
the puddle is large enough and that the metal is
flowing freely before you dip in the rod. Watch
the course of the flame closely to make sure that
its travel along both edges of the plate is the
same, maintaining a molten puddle approximately
610 mm in width. Advance this puddle about
2 mm with each complete motion of the torch.
Unless the molten puddle is kept active and flowing
forward, correct fusion will not be achieved.
Keep the motion of the torch as uniform as possible,
as this will produce smooth, even ripples and
so complete the weld.
9.10 Welding various metals
Mild steel Select the appropriate method and
always use a neutral flame. Fluxes are not necessary.
Carbon steel A flux must be used, and the flame
kept in a neutral condition. After welding is completed
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the metal should be cooled slowly to avoid
the weld metal becoming brittle.
Alloy steels It is very important that the correct
welding rod is used for the appropriate alloy. The
264 Repair of Vehicle Bodies
metal should be first preheated and then welded. It
is essential to cool the metal slowly after welding.
Stainless steel A welding rod having the same
composition as stainless steel should be used.
A flux should be used, and the welding flame kept
in the neutral position. Do not interrupt the welding
sequence, and carry out the weld as quickly as possible.
On completion of the weld allow it to cool
slowly. Remove all oxide and scale from the weld
when it is finished.
Cast iron A silicon cast iron welding rod should
be used, together with a cast iron welding flux. The
metal must be preheated to dull red before the
welding commences. It is very important that the
cast iron cools very slowly, or the metal will crack.
Aluminium For pure aluminium sheet, either
paint both sides of the metal with flux or dip the
hot rod into the flux and allow the flux to coat the
rod like varnish. Before welding, remove all traces
of oil or grease and brush the edges to be welded
with a wire brush. Tack the weld at frequent intervals
using a neutral or slightly carburizing flame.
The welding technique is leftward and should be
carried out more quickly than when welding mild
steel. On completion of the weld, wash it thoroughly
to remove the remaining flux as this could
be harmful.
Aluminium castings Use cast aluminium alloy
rods with silicon and flux. Preheat before welding.
Melt the rod well into the weld, using the welding
rod to puddle the molten metal. On completion
cool slowly.
9.11 Gas cutting
The oxy-acetylene process is widely used to cut
metal, especially in the body building industry
where heavy sections have to be cut to special
shapes for construction, and also in the repair side of
the industry where special nozzles have been developed
for cutting away damaged parts of sheet metal
body sections. Cutting may be done by means of a
simple hand cutting torch, or by a more complicated,
automatically controlled cutting machine.
Flame cutting
Flame cutting, known also as oxygen cutting, is
made possible by the fact that oxygen has a
marked affinity for ferrous metals which have been
previously heated to their ignition temperature.
Thus the cutting of iron and steel merely involves
the direction of a closely regulated jet or stream of
pure oxygen on to an area that has been previously
heated to ignition temperature (1600 C, or when
the metal has reached a bright cherry-red colour).
As the iron is oxidized, the oxygen jet is moved at
a uniform speed so that a narrow cut is formed.
Since only the metal within the direct path of the
oxygen jet is acted upon, very accurate results can
be obtained if close control is exercised; when
hand cutting 150 mm thick steel, the working tolerance
is about 1.5 mm (Figure 9.20).
Figure 9.20 Flame cutting process (BOC Ltd)
Figure 9.21 Cutting torches (Murex Welding
Products Limited)
Cutting torch
This differs from the regular welding torch in that it
has an additional lever for the control of the oxygen
used to burn the metal (Figures 9.21 and 9.22). It is
possible to convert a welding torch into a cutting
torch by replacing the mixing head with a cutting
attachment (Figure 9.7). The torch has conventional
Gas welding, gas cutting and plasma arc cutting 265
oxygen and acetylene valves, and these are used to
control the passage of oxygen and acetylene when
heating the metal. The cutting tip has an orifice in
the centre for the oxygen flow, surrounded by
several smaller holes for a preheating flame which
generally uses acetylene, propane or hydrogen. The
preheating flame has two purposes.
1 To provide sufficient heat to raise a small area
of the steel surface to the ignition temperature
2 To transmit sufficient heat to the top surface of
the steel to offset the thermal conductivity of
the metal.
A wide variety of cutting torches are available, but
in essentials they are all similar. Oxygen and a fuel
gas for the preheat flame, for example acetylene,
enter the torch separately and are mixed in the body
of the torch or in the nozzle. Their respective flow
rates are adjusted by two hand valves on the torch.
The cutting oxygen stream is bled off inside the
torch through a lever operated valve (Figure 9.22).
Two basic methods of gas mixing are employed.
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In the first type, the two gases enter the torch at
approximately the same pressure and are mixed in
a separately designed chamber either in the body
of the torch or in the nozzle. This is termed the
equal-pressure or high-pressure cutting torch.
Alternatively, oxygen enters the torch at a very
much higher pressure than the fuel gas and sucks
the fuel gas in through an injector which also
mixes the two gases. These are termed injector
cutting torches (Figure 9.23). Both designs are
equally good for flame cutting.
Nozzles
A number of different designs of nozzles are available
to suit the various combinations of fuel gases
and torch design. In principle, however, nozzles are
the same: they have a central orifice for the cutting
oxygen stream, surrounded by a ring of orifices for
the preheat flame.
Cutting torches in which oxygen and the fuel
gas are mixed in the body of the torch (injector
type) have two tubes leading to the nozzle, one
for cutting oxygen and one for mixing oxygen
Figure 9.22 Types of cutting torches (BOC Ltd)
Figure 9.23 (a) Injector cutting torch and two-seat nozzle (b) three-seat nozzle (BOC Ltd)
266 Repair of Vehicle Bodies
and fuel gas. A so-called two-seat nozzle fits this
design (Figure 9.23a). Some torches are designed
for mixing oxygen and fuel gas in the nozzle
itself. These torches have three tubes leading to
the nozzle, and require special three-seat nozzles
(Figure 9.23b).
A typical oxy-acetylene nozzle has circular
pre-heat holes surrounding a central circular cutting
oxygen orifice, and is frequently of a onepiece
construction, made of copper and often
chrome plated (Figure 9.24a). On the other hand
a liquid propane gas (LPG) or natural gas nozzle
is mainly of two-piece construction: it has fluted
ports for the preheat flame, and the central part
of the nozzle is recessed: (Figure 9.24b). Nozzles
should not be interchanged between different
fuel gases.
The nozzle is one of the main keys to good quality
efficient cutting. There are different sizes of
nozzle for different metal thicknesses, and the nozzle
manufacturers will state the correct size on
their data sheets (Table 9.2).
Figure 9.24 (a) One-piece acetylene nozzle
(b) two-piece LPG nozzle (BOC Ltd)
oxygen cutting supply, and move the cutter steadily
and at a speed which produces a smooth cut. Keep
the white cone just clear of the work surface, and
ensure that the cut is penetrating the surface.
Whenever possible the operator should draw the
cutter towards him.
Machine cutting
Much oxygen cutting is done with machines, particularly
if the cuts are long. Machine cutting has
many advantages over hand cutting and results in
greater accuracy and better edge finish, particularly
when used for making single and double V edge
preparation. The principles of machine cutting are
similar to those for hand work. A wide variety of
types of machines are available, including stationary,
general-purpose or universal models, multiburner
machines, straight-line and circle cutting
and joint cutting machines. Some incorporate pantograph
or electronic devices, enabling profiles to
be accurately copied from templates or direct from
drawings.
Table 9.2 gives material thickness, nozzle size
and pressures for use with Saffire equipment.
9.12 Gases: characteristics and colour
coding
The following is a summary of gas characteristics
and cylinder colour codes.
Oxygen
Cylinder colour: black.
Characteristics: no smell. Generally considered
non-toxic at atmospheric pressure. Will not burn
but supports and accelerates combustion. Materials
not normally considered combustible may be
ignited by sparks in oxygen-rich atmospheres.
Nitrogen
Cylinder colour: grey with black shoulder.
Characteristics: no smell. Does not burn. Inert, so
will cause asphyxiation in high concentrations.
Argon
Cylinder colour: blue.
Characteristics: no smell. Heavier than air. Does
not burn. Inert. Will cause asphyxiation in absence
of sufficient oxygen to support life. Will readily
Hand cutting procedure
First remove any oxide or scale from the line of the
cut. Set the preheating flame to neutral and hold
the torch with two hands, one to act as a steady and
the other to control the oxygen flow, and position
the cutting torch so that the white cone is 6 mm
from the work surface (Figure 9.25). When the
metal reaches a bright red colour, switch on the
Gas welding, gas cutting and plasma arc cutting 267
collect in the bottom of a confined area. At high
concentrations, almost instant unconsciousness
may occur followed by death. The prime danger is
that there will be no warning signs before unconsciousness
occurs.
Propane
Cylinder colour: bright red and bearing the words
Propane and Highly flammable.
Characteristics: distinctive fish-like offensive
Figure 9.25 Cutting technique (BOC Ltd) smell. Will ignite and burn instantly from a spark
Table 9.2 Flame cutting data (BOC Ltd)
(a) ANM/ANM1E nozzle, 6.3 mm _ 10 m fitted hose, resettable flashback arresters
Mild steel Operating pressure Gas consumption
plate
Thickness Oxygen Fuel gas Cutting oxygen Heat oxygen Fuel
mm in Nozzle size bar lbf/in2 bar lbf/in2 l/min ft3/h l/min ft3/h l/min ft3/h
6 1.4 20 0.3 4 14.15 30 8.5 18 8 17
13 2.1 30 0.35 5 30.7 65 10.4 22 9.4 20
25 1 2.8 40 0.4 6 67.5 143 13.2 28 11.8 25
50 2 3.1 45 0.4 6 78.3 166 13.2 28 11.8 25
75 3 3.5 50 0.4 6 88.7 188 13.2 28 11.8 25
100 4 3.1 45 0.31 4.5 121 256 14.6 31 13.2 28
150 6 3.1 45 0.4 6 175 370 20 43 18.4 39
200 8 4.1 60 0.45 6.5 283 600 26 55 23.5 50
250 10 4.8 70 0.45 6.5 377 800 26 55 23.5 50
300 12 6.2 90 0.45 6.5 434 920 26 55 23.5 50
Sheet A-SNM 1.4 20 0.14 2 14.15 30 2.4 5 2.4 5
(b) AFN nozzle, Saffire Lite cutting attachment (valved version), 6.3 mm _ 10 m fitted hose, resettable flashback arresters
Mild steel Operating pressure Gas consumption
plate
Thickness Oxygen Fuel gas Cutting oxygen Heat oxygen Fuel
mm in Nozzle size bar lbf/in2 bar lbf/in2 l/min ft3/h l/min ft3/h l/min ft3/h
6 2 30 0.14 2 11.8 25 4.2 9 3.8 8
13 2 30 0.2 3 23.5 50 4.2 9 3.8 8
25 1 3 45 0.28 4 56.6 120 4.2 9 3.8 8
50 2 3.8 55 0.35 5 75.5 160 5.2 11 4.7 10
Sheet A-SFNM 1.7 25 0.4 6 14.2 30 2.1 4.5 1.9 4
268 Repair of Vehicle Bodies
or piece of hot metal. It is heavier than air and will
collect in ducts, drains or confined areas. Fire and
explosion hazard.
Acetylene
Cylinder colour: maroon.
Characteristics: distinctive garlic smell. Fire and
explosion hazard. Will ignite and burn instantly
from a spark or piece of hot metal. It is lighter than
air and less likely than propane to collect in confined
areas. Requires minimum energy to ignite in
air or oxygen. Never use copper or alloys containing
more than 70 per cent copper or 43 per cent silver
with acetylene.
Hydrogen
Cylinder colour: bright red.
Characteristics: no smell. Non-toxic. Much lighter
than air. Will collect at the highest point in any
enclosed space unless ventilated there. Fire and
explosion hazard. Very low ignition energy.
Carbon dioxide
Cylinder colour: black, or black with two vertical
white lines for liquid withdrawal.
Characteristics: no smell but can cause the nose to
sting. Harmful. Will cause asphyxiation. Much
heavier than air. Will collect in confined areas.
Argoshield
Cylinder colour: blue with green central band and
green shoulder.
Characteristics: no smell. Heavier than air. Does
not burn. Will cause asphyxiation in absence of
sufficient oxygen to support life. Will readily
collect at the bottom of confined areas.
9.13 Safety measures
