ESAB Miniarc 150, Instruction Manual, Page: 20 / 48

20
Principles of Shielded Metal Arc Welding
GENERAL
Shielded Metal-Arc welding depends upon the fundamental fact that when one side of the secondary welding circuit is at-
tached to a piece of steel and the other side of the circuit is connected to an electrode, an arc will be established when the
electrode touches the steel. If the arc is properly controlled, the metal from the electrode will pass through the arc and be
deposited on the steel. When the electrode is moved along the steel at the correct speed, the metal will deposit in a uniform
layer called a bead. The electrodes used in welding are carefully manufactured to produce strong, sound welds. They consist
of a core of steel wire, usually called mild since it contains a low percentage of carbon. Around this core is applied a special
coating which assists in creating the arc and at the same time protects the molten steel as it transfers across the arc.
To utilize these principles in shielded metal-arc welding, some means of controlling the power is essential. The power in a
welding circuit is determined by the voltage and current. The arc voltage is governed by the arc length and the electrode
diameter. Therefore, the practical measure of the power or heat, is in terms of the current, measured in amperes. A small
electrode requires less current than a large one. To simplify operations, the scale on the front of the welding machine is
marked off for the various current values and electrode diameters.
The exact current required for a job depends upon the size of the pieces to be welded and the position of welding. Gener-
ally a lower current will be sufficient for welding on a small part than would be necessary to weld on a large piece of the
same thickness. Similarly, with a given size of electrode, a lower current should be used on thin metals than on the thicker
sections.
TABLE 3.1 - CURRENT REQUIREMENTS FOR MILD STEEL ELECTRODES
Figure 3.7. Flat Position Welds
The importance of welding in the flat position, whenever possible, cannot be stressed too strongly. The quality of the weld
is better, the operation easier and faster (Figure 3.7). However, occasions will arise when it is necessary to work on parts
SECTION 3 OPERATION
Electrode Diameter Amperage
Inches/Meteric Minimum Maximum
5/64" (2.0mm) 20 50
3/32" (2.4mm) 40 80
1/8" (3.2mm) 65 125