Correct choice of electrode

In selection, consideration must be given to position

of weld, type of metal, size of electrode, type of joint,

and current setting. The main group of electrodes

include mild steel, high-carbon steel, special alloy

steel, cast iron and non-ferrous aluminium, classified

as bare and flux-covered. The bare electrodes have

a very light coating which affords some protection

against oxidation of the surface. The flux-covered

or shielded electrode has a very heavy coating of

several chemical substances which protect the

molten metal from the oxidation and help to keep

a steady arc. When this type of electrode melts, the

coating produces a shield of gas around the molten

metal, safeguarding it against the atmosphere. Some

of the coating also forms a slag over the molten part,

which also serves as a protection against oxidation.

Correct arc length

This will depend upon the type of electrode used

and the nature of the welding operation. If the arc

is too long, a wide irregular bead is produced with

insufficient fusion between the plate and weld

metal. When the arc is too short, insufficient heat is

generated to melt the plate properly and the electrode

will stick frequently, resulting in a very

uneven bead (Figure 11.8).

Correct speed of travel

This must be slow and even to ensure sufficient

penetration without excessive build-up of bead.

When the speed is too fast, the molten pool solidifies

rapidly and impurities are retained in the

weld. If the speed is too slow, excessive metal

produces a high and wide bead (Figure 11.9).

Correct current

The correct current setting in accordance with the

size of electrode used is most important (Table 11.6).

When the current is too great the electrode melts fast,

causing a large pool of metal and excessive spatter. If

the current is too low the heat generated will be

insufficient to melt the plate and the molten pool will

be small, resulting in lack of fusion (Figure 11.9).

Figure 11.8Correct arc length

Manual metal arc welding 303

Striking the arc

The successful step towards electric arc welding is

learning to strike and maintain the arc and run a

straight bead of weld metal. First set the control

unit to the correct setting specified for the size of

electrode being used. Then bring the electrode into

contact with the plate by one of the following two

methods.

Methods of welding

Tapping method

The tapping motion method is as shown in Figure

11.10. The electrode is brought straight down on

the plate and instantly withdrawn a distance of

3–5 mm, this distance being equal to the core

diameter of the electrode.

Scratching method

The scratching method (Figure 11.11) is where the

electrode is tilted at an angle and is then given a

slight circular movement similar to that of striking

a match. As in the previous method the electrode is

promptly raised a distance equal to its diameter,

otherwise it will stick to the plate.

Figure 11.9Correct speed and current setting

Table 11.6Current setting for electrode

Sheet thickness Size of electrode Current

(mm) (mm) (A)

1.0 1.6 30

1.2 1.6 35

1.6 2.0 50

2.0 2.5 80

2.5 3.2 110

3.2 4.0 120–160

3.2–9.5 5.0–6.3 250–400

9.5 and over 8 mm 400–600

Figure 11.10Tapping method of striking the arc

Figure 11.11Scratching method of striking the arc

Welding currents

Welding currents may vary from 20 to 600 A. For

striking a DC arc an open circuit of 55–60 volts is

required, whilst an AC set requires 80–100 volts.

Once the arc is struck the arc voltage will drop to

20–25 volts. Before striking the arc the operator

should have his head shield or face screen in position

and observe the arc through the glass filter.

304Repair of Vehicle Bodies

Weld defects

Undercutting

This is a condition that results when the welding

current is too high. The excessive current leaves a

groove in the base metal along both sides of the

bead, which greatly reduces the strength of a weld

(Figure 11.12).

and double-U. T-joints also have several variations

which can be applied in appropriate places. The

other joints commonly used in manual metal arc

welding are the lap joint and corner joints.

Welding joint design

Joint configuration

The three basic joint types are usually classified as

fillet, butt or groove, and lap (Figure 11.15). These

may be further subdivided depending on the joint

detail and the joining process being used.

Figure 11.12Undercutting

Figure 11.13Overlapping

Overlapping

This occurs when the current is set too low. In this

instance the molten metal falls from the electrode

without actually fusing with the base metal, resulting

in a defective weld (Figure 11.13).