14
7.1
Fundamentals of Manual Metal Arc
(MMA) Welding
Welding Technique
Successful MMA welding depends on the following
factors:
•
Selection of the correct electrode.
•
Selection of the correct size of the electrode for
the job.
•
Correct welding current.
•
Correct arc length.
•
Correct angle of electrode to work.
•
Correct travel speed.
Correct preparation of work to be welded.
Electrode selection
As a general rule, the selection of an electrode
is straight forward, in that it is only a matter of
selecting an electrode of similar composition to
the parent metal. However, for some metals there
is a choice of several electrodes, each of which
has particular properties to suit specific classes
of work. Often, one electrode in the group will be
more suitable for general applications due to its all
round qualities.
Electrode size
The size of the electrode generally depends on
the thickness of the section being welded, and
the thicker the section the larger the electrode
required. In the case of light sheet, the electrode
size used is generally slightly larger than the work
being welded. This means that, if 2.0 mm sheet is
being welded, 2.5 mm diameter electrode is the
recommended size.
Welding current
Correct current selection for a particular job is an
important factor in arc welding. With the current
set too low, difficulty is experienced in striking and
maintaining a stable arc. The electrode tends to
stick to the work, penetration is poor and beads
with a distinct rounded profile will be deposited.
Excessive current is accompanied by overheating
of the electrode. It will cause undercut and burning
through of the material, and will give excessive
spatter. Normal current for a particular job may be
considered as the maximum, which can be used
without burning through the work, over-heating
the electrode or producing a rough spattered
surface (i.e. the current in the middle of the range
specified on the electrode package is considered to
be the optimum).
Arc length
To strike the arc, the electrode should be gently
scraped on the work until the arc is established.
There is a simple rule for the proper arc length;
it should be the shortest arc that gives a good
surface to the weld. An arc too long reduces
penetration, produces spatter and gives a rough
surface finish to the weld. An excessively short
arc will cause sticking of the electrode and rough
deposits that are associated with slag inclusions.
For down hand welding an arc length not greater
than the diameter of the core wire will be most
satisfactory. Overhead welding requires a very
short arc so that a minimum of metal will be lost.
Electrode angle
The angle that the electrode makes with the work
is important to ensure a smooth, even transfer of
metal.
Correct travel speed
The electrode should be moved along in the
direction of the joint being welded at a speed that
will give the size of run required. At the same
time, the electrode is fed downwards to keep the
correct arc length at all times.
Correct travel speed for normal welding
applications varies between approximately 100
and 300 mm per minute, depending on electrode
7.0 Basic Welding Techniques
