Basic Principles of Arc Welding

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Introduction to Arc Welding Fundamentals

Shielded metal-arc welding with transformer welding machines operates on a fundamental principle: when one side of the welding circuit attaches to a steel piece and a welding electrode connected to the other side makes contact, an arc establishes. With proper control, metal from the electrode passes through this arc and deposits on the steel. Moving the electrode along the steel at the correct speed creates a uniform layer called a bead.

The electrodes used in welding undergo careful manufacturing to produce strong, sound welds. They consist of a mild steel wire core containing a low percentage of carbon (0.10-0.14%). A special coating surrounds this core, which serves two purposes: it helps create the arc and protects the molten steel as it transfers across the arc.

Effective metal-arc welding requires precise power control. While welding circuit power is measured by voltage and current, voltage is determined by arc length, which depends on electrode diameter. Therefore, the practical measure of power (or heat) is current, measured in amperes. Smaller electrodes require less current than larger ones. For operational simplicity, welding machines feature scales marked with various current values.

The appropriate current selection depends on the size of the workpieces and the welding position. Generally, smaller parts require less current than larger pieces of the same thickness. Similarly, when using a specific electrode size, thinner metals need lower current than heavier sections.

Essential Welding Techniques

Striking the Arc and Running Beads

Mastering fundamental steps is necessary before attempting actual welding work. Before striking an arc, insert the electrode into the holder. One method resembles striking a match—drag the electrode across the work surface. When contact occurs, welding current begins. To prevent the electrode from "freezing" or welding to the work, immediately withdraw it after contact.

The withdrawal distance, known as arc length, is small and depends on electrode diameter. If the electrode freezes during this process, a quick wrist twist can free it.

An alternative method involves bringing the electrode straight down onto the work and immediately withdrawing it to proper arc length after contact. Practice both methods, though the scratching method is generally preferred for AC welding.

Determining correct arc length requires practice, as there's no ready measurement method. As a preliminary guide, use approximately 1/16" arc length for 1/16" and 3/32" electrodes; for 1/8" and 5/32" electrodes, use about 1/8" arc length. With experience, the arc's sound becomes a reliable indicator—a short arc with correct current produces a sharp, crackling sound. Examining the deposited bead provides further verification.

After learning to start and maintain an arc, practice depositing weld metal. Begin by running beads on flat plates using a full electrode. Practice moving both left-to-right and right-to-left. Hold the electrode roughly perpendicular to the work, though tilting it forward in the travel direction proves helpful.

Weaving Techniques

To cover wider areas in a single electrode pass, weaving is employed. This technique involves oscillating the electrode side-to-side in a set pattern. For uniform deposits, follow a definite pattern. While weaving helps when building up metal, limit weaves to no more than 2-1/2 times the electrode diameter.

Joint Types and Preparation

Butt Joints

Beyond depositing beads on flat plates, welding frequently involves joining pieces together. When making bead welds, you may notice the plate "curling" toward the weld—this distortion occurs when heat applies locally to metal plates. Similarly, when making butt welds, distortion causes the plate edges to draw together ahead of welding due to cooling contraction of the deposited weld metal. Overcome this by spreading edges apart on a long taper of approximately 1/8" per foot.

Butt joint welding may require edge preparation for optimal results. For materials up to 3/16" thick, metal arc welding typically uses square groove butt joints without special preparation. For thicknesses of 3/16" and greater, either single or double "V" grooves are employed. Single "V" grooves generally work well on thicknesses up to 3/4" and in cases where welding is only possible from one side.

Beveling Techniques

The oxyacetylene cutting torch provides the best means for beveling steel for welding. This work can be performed using either a hand-guided torch or a specialized oxyacetylene cutting machine. However, cutting creates scale that adheres to the plates, which must be removed by grinding or chipping before welding to prevent entrapment that would produce unsound welds. When oxyacetylene cutting equipment isn't available, grinding offers the best alternative for preparing bevels. These bevels should have approximately 30-degree angles, with the bottom edge left square for about 1/16".

Begin practicing butt welds on 1/8" thick material. Avoid very thin material (around 1/16") initially, as this requires considerable skill. Separate the squared edges of the 1/8" material about 1/16" and make a butt weld throughout with a 1/8" electrode.

Initial attempts may fail to penetrate the sheet or might burn through. Continue trying by adjusting the current within recommended ranges and varying travel speed to achieve the desired weld. After mastering 1/8" material, proceed to a similar exercise on 1/4" material. For this thickness, deposit a bead on each side of the joint, ensuring fusion between them. Since the weld from one side effectively works on 1/8" thickness, no bevel is needed.

When making practice butt welds, periodically check results. Without elaborate testing equipment, this can be done using a hammer and vise. Grip a short welded piece with the weld just above the jaws—a quality weld will bend rather than break under this test.

Tee and Lap Joints

The fillet weld, another basic weld type, is used for creating tee and lap joints. These joints require no special preparation beyond squared edges.

For tee joints, the different piece positions create a challenge. The electrode holding method used for butt welds won't suffice. Proper technique provides fusion into the corner and creates a fillet with approximately equal sides.

For maximum strength, deposit a fillet weld on each side of the upright. Lap joints involve the same fundamental weld type (fillet), but with differently distributed metal, requiring an adjusted technique.

Advanced Welding Positions

Welding Vertically, Horizontally and Overhead

While flat position welding is strongly preferred for better quality, easier operation, and faster completion, circumstances sometimes require welding in fixed positions—horizontally, vertically, and overhead. Recognize that welding in these positions presents difficulties and demands consistent practice to develop proficiency.

As with flat position welding, begin by practicing bead welds in various positions. As skills develop, progress to practicing butt and fillet welds (tee and lap joints) in these challenging positions.

When welding in non-flat positions, gravity tends to cause molten metal to drip downward. The technique must counteract this tendency. Approach this challenge gradually, starting with horizontal bead welds on plates inclined at 45 degrees. After mastering this to consistently produce uniform beads, progress to vertical welding practice. Again, begin with an easier operation like running beads vertically on plates set at 45 degrees.

To advance your practice, move the plates to a fully vertical position. Vertical welding can be performed either by carrying the weld upward or starting from the top and welding downward. Generally, the upward approach proves easier, so practice bead welds using this method. Since bead welds have limited practical applications, extend your experience by practicing butt welds in vertical and horizontal orientations.

When using beveled plate edges, space them on the backing strip and tack weld the strip to the plates on the reverse side.

Conclusion

Printed instructions alone cannot impart the necessary skills for successful welding. Personal instruction from an experienced welding operator remains the most effective method for skill development. Therefore, seek facilities for instruction and practice under competent supervision. At minimum, beginners should obtain qualified feedback on their finished welds from experienced welders.