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 11.8 Correct 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the plate and instantly withdrawn a distance of
35 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
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promptly raised a distance equal to its diameter,
otherwise it will stick to the plate.
Figure 11.9 Correct speed and current setting
Table 11.6 Current 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 120160
3.29.5 5.06.3 250400
9.5 and over 8 mm 400600
Figure 11.10 Tapping method of striking the arc
Figure 11.11 Scratching 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 5560 volts is
required, whilst an AC set requires 80100 volts.
Once the arc is struck the arc voltage will drop to
2025 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.
304 Repair 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 11.12 Undercutting
Figure 11.13 Overlapping
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).
