Flame adjustment and gas pressure

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 60°–70°.

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 30°–40°.

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.

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 9.17Methods 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

260Repair 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


Angle of welding torch 40°–50°.

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 30°–40°.

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

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 9.18Edge preparation

262Repair of Vehicle Bodies

Figure 9.19Control 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


5 The use of chilling bars and chemical foam


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


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

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

6–10 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

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

264Repair 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 9.20Flame cutting process (BOC Ltd )

Figure 9.21Cutting 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.

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.


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 9.22Types of cutting torches (BOC Ltd )

Figure 9.23(a) Injector cutting torch and two-seat nozzle (b) three-seat nozzle (BOC Ltd )

266Repair 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


Table 9.2 gives material thickness, nozzle size

and pressures for use with Saffire equipment.

9.12 Gases: characteristics and colour


The following is a summary of gas characteristics

and cylinder colour codes.


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.


Cylinder colour: grey with black shoulder.

Characteristics: no smell. Does not burn. Inert, so

will cause asphyxiation in high concentrations.


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



Cylinder colour: bright red and bearing the words

‘Propane’ and ‘Highly flammable’.

Characteristics: distinctive fish-like offensive

Figure 9.25Cutting technique (BOC Ltd ) smell. Will ignite and burn instantly from a spark

Table 9.2Flame cutting data (BOC Ltd )

(a) ANM/ANM1E nozzle, 6.3 mm _ 10 m fitted hose, resettable flashback arresters

Mild steel Operating pressure Gas consumption


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


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

268Repair 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.


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.


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.


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