3.0 MMA Process
3.1 Process
Manual Metal Arc welding is the process of joining metals where an
electric arc is struck between the metal to be welded (parent metal) and
a flux-coated filler wire (the electrode). The heat of the arc melts the
parent metal and the electrode which mix together to form, on cooling,
a continuous solid mass.
Before arc welding can be carried out, a suitable power source is
required. Two types of power sources may be used for arc welding, direct
current (DC) or alternating current (AC).
The essential difference between these two power sources is that, in the
case of DC, the current remains constant in magnitude and flows in the
same direction. Similarly, the voltage in the circuit remains constant in
magnitude and polarity (i.e. positive or negative).
In the case of AC however, the current flows first in one direction and
then the other. Similarly, the voltage in the circuit changes from positive
to negative with changes in direction of current flow. This complete
reversal is called a ‘half cycle’ and repeats as long as the current
flows. The rate of change of direction of current flow is known as the
‘frequency’ of the supply and is measured by the number of cycles
completed per second. The standard frequency of the AC supply in
Australia is 50 Hz (Hertz).
3.2 Welding machine
The most important consideration when contemplating the use of arc
welding for the first time is the purchase of a suitable welding machine.
BOC supplies a popular range of arc welding machines. Machines
range from small portable welders that operate from standard
240 Volt household power to heavy-duty welders used by the largest
steel fabricators.
Basic Welding Machine and Cables
The choice of welding machine is based mostly on the following factors:
→
primary voltage, e.g. 240 Volt or 380 Volt
→
output amperage required, e.g. 140 amps
→
output required, e.g. AC or DC +/-
→
duty cycle required, e.g. 35% @ 140 amps
→
method of cooling, e.g. air-cooled or oil-cooled method of output
amperage control, e.g. tapped secondary lugs
→
or infinitely variable control.
For example, the Smootharc 170 connects to 240 Volt supply (15 amps
Input), has an output of 170 amps DC @ 50% duty cycle.
Having decided on a welding machine, appropriate accessories are
required. These are items such as welding cables, clamps, electrode
holder, chipping hammer, helmet, shaded and clear lenses, scull cap,
gloves and other personal protective equipment.
BOC stocks a huge range of personal protective equipment. This
combined with BOC’s extensive network ensures fast reliable service
throughout the South Pacific.
Schematic of MMA process in operation
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BOC Smootharc Advance II MIG 400R Operating manual
