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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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