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Welding Machines
Many types and sizes of welding machines are used for shielded metal-arc welding. The current of a Constant-Current type does not change significantly in variations in arc length. This is preferred for manual welding operations.
The Constant-Potential type of welding machine adjusts
current according to
arc length. If used in a manual operation,
unavoidable variations in arc length will produce large fluctuations in
current, resulting in an unstable, non-uniform arc. For automatic operations,
constant electrode feed rate establishes a stable arc and uniform arc length.
The current rating of
machine is its maximum current output. A current rating of 400 amp means
machine can deliver up to 400 amps of welding current.
The duty cycle rating of
machine is
safe operating capacity for non-stop welding. This is expressed as a percentage over a 10-minute period over which a machine can deliver its rated maximum welding current output without damage or overheating. An 80 percent 400-amp machine is one, which can deliver 400 amps of welding current for a total of 8 minutes out of every 10, and must idle at least, 2 minutes out of every 10 for cooling.
Tools and Accessories
The tools for SMAW are: cables, holders, clamps, chipping hammer, wire brush. The protective equipment are face shield, goggles, apron, gloves, shoes, long sleeved shirt. The protective equipment is to avoid eye injuries, and burns. Also there is a need to prevent electric shocks, especially in wet areas.
Selecting an Electrode
Electrodes are classified by their core material: mild steel, high-carbon steel, special alloy steel, cast iron, and non-ferrous. Mild steel electrodes are
most commonly used. In general,
electrode core material is matched as closely as possible with
composition of
base metal. Electrode size varies with
thickness of
base metal.
The size and characteristics of
selected electrode determine
arc current settings on
welding machine. Normally,
range of recommended amperage for
electrode is given by
manufacturer.
Set up and Operation
Observe fire precautions before starting to weld. No combustible should be near
work area.
With
electrical power still off; clamp
earth wire to
work piece and
electrode on
holder. Turn on
power. To strike an arc, position
end of
electrode about 1 inch above
weld start point. Lower your face shield and with a rapid tapping of scratching motion, touch
electrode to
base of
metal. Immediately after
contact, raise
end of
electrode slightly to establish an arc whose length is approximately equal to
electrode diameter.
If you do not raise
electrode fast enough, it will stick to
work. Twist or bend
electrode to break it free. If
electrode does not break free, quickly release it from
holder. With a little practice, you can learn to strike an arc without
electrode sticking.
When
current settings and a proper arc length is maintained, a continuous cracking sound is heard while welding. A humming sound indicates that
arc length is too long or
current is too high. Arcs too short make a popping sound and may flash on and off, indicating
electrode is sticking and short circuiting to
base metal.
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