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BOC RAPTOR 200C & 250R MIG Operating manual
15.0 MMA Process
Schematic of MMA process in operation
15.1 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).
15.2 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.
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