Basic Principles of Arc Welding

Shielded metal-arc welding with the transformer welding machine depends upon this fundamental fact: that when one side of the welding circuit is attached to a piece of steel, a welding electrode connected to the other side and the two brought into contact, an arc will be established. 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.

Shielded metal-arc welding with the transformer welding machine depends upon this fundamental fact: that when one side of the welding circuit is attached to a piece of steel, a welding electrode connected to the other side and the two brought into contact, an arc will be established. 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 (0.10-0.14) 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.

In order to utilize these principles in metal-arc welding, some means of controlling the power is essential. The power in a welding circuit is measured by the voltage and current. However, the voltage is governed by the arc length and in turn depends on the electrode diameter. Therefore, the practical measure of the power, or, heat, is in terms of the current, generally measured in amperes. Obviously 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.

The exact current selected for a job depends upon the size of the pieces to be welded and the position of welding. Generally 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 heavier sections.

Striking the Arc - Running Beads

In learning to weld there are certain fundamental steps which must be mastered before one can attempt to weld on actual work. Preparatory to the actual striking of an arc, it is necessary to insert the electrode in the holder. In this method the striking end of the electrode is dragged across the work in a manner much the same as striking a match. When the electrode touches the work, the welding current starts. If held in this position, the electrode would "freeze" or weld itself to the work and to overcome this, the electrode is withdrawn from the work immediately after contact has been made.

The amount that the electrode is withdrawn is small and depends on the diameter; this distance is known as the arc length. If in striking an arc, the electrode freezes, it may be freed by a quick twist of the wrist.

Another method of establishing the arc is available. In this the electrode in the holder is brought straight down on the work and immediately after contact, is withdrawn to the proper arc length. Practice striking the arc using both methods. Generally the scratching method is preferred for a-c welding.

Determination of the correct arc length is difficult since there is no ready means of measuring it. As a preliminary guide, use about 1/16" arc length on 1/16" and 3/32" electrode; for 1/8" and 5/32" electrodes use about 1/8" arc length. When skill is acquired, the sound of the arc will be a good guide. A short arc with correct current will give a sharp, crackling sound. Examination of the deposited bead will give a further check.

Once the knack of starting and holding an arc has been learned, turn next to depositing weld metal. In the beginning it is best to run beads of weld metal on flat plates using a full electrode. Practice moving from left to right and from right to left. The electrode should be held more or less perpendicular to the work, except that tilting it ahead, in the direction of travel will prove helpful.


When it is necessary to cover a wider area in one pass of the electrode, a method known as weaving is employed. In this the electrode is moved or oscillated from side to side in a set pattern. In order to be sure of uniform deposits, it is necessary to use a definite pattern. While weaving is helpful, particularly when building up metal, it should be limited to weaves not exceeding 2-1/2 times the diameter of the electrode.

Butt Joints

Up to this point the discussion has covered only the deposit of beads on the flat plates. While such operations are helpful in building up worn parts or applying hard-facing materials, they do not help in learning to weld pieces together. In making bead welds, previously described, it was probably noted that the depositing of weld metal on one side of the plate caused it to "curl" up towards the weld; this is called distortion and will almost always be found when heat is applied locally to a metal plate. Similarly in making a butt weld distortion will cause the edges of the plate to draw together ahead of welding. This is caused by the contraction of the deposited weld metal on cooling. It may be overcome by spreading the edges apart on a long taper of about 1/8" per foot.

In making welds in a butt joint, preparation of the edges may be necessary to insure good results. In metal arc welding it is common practice to weld thin materials up to 3/16" thick without any special preparation using the square groove butt joint. For thickness of 3/16" and over the "V" groove either single or double is employed. Generally the single "V" groove will be satisfactory on thicknesses up to3/4" and in those cases, regardless of thickness, where one can work on the weld from one side only.


The best means for beveling steel for welding is by means of the oxyacetylene cutting torch. The work may be done with a hand guided torch or special oxyacetylene cutting machine. However, in performing this cutting, a scale will adhere to the plates. This must be removed by grinding or chipping before welding as it is likely to become entrapped and thus produce an unsound weld. Where oxyacetylene cutting equipment is not available, grinding will probably be the best means of preparing bevels. The angles of these bevels should be about 30 degrees and the bottom edge may be left square for a distance of about 1/16".

Practice making butt welds starting on thin material about 1/8" thick. Avoid very thin material (around 1/16" thick) in the beginning as this requires a fair degree of skill. Separate the squared edges of the 1/8" material about 1/16" and make a butt weld all the way through with a 1/8" electrode.

Probably the first attempts will fail to penetrate the sheet or may burn through. Keep trying by adjusting the current within the recommended range; also vary the travel speed to give the desired weld. Having mastered the 1/8", proceed to a similar exercise on 1/4". This time however deposit a bead on each side of the joint and try to fuse one to the other. Since the weld from one side is in effect on 1/8" thickness, no bevel is needed.

When making practice butt welds it is wise to check the results occasionally. When elaborate testing equipment is not available, this may be done with a hammer and vise. Grip a short, welding piece with the weld just above the jaws. A good weld will not break under this test but will bend over.

Tee and Lap Joints

The other basic type of weld, the fillet weld, is used for making tee and lap joints. For this type of welding no special preparation other than squared edges is necessary.

Considering the tee joint first, it will be seen immediately that the different locations of the pieces creates a problem. The method of holding the electrode for butt welds will not be satisfactory. This will provide fusion into the corner and a fillet, the sides of which will be approximately equal.

For maximum strength a fillet weld should be deposited on each side of the upright the lap joint, while involving the same fundamental weld type, the fillet has metal distributed differently and therefore requires still another technique.

Welding Vertically, Horizontally and Overhead

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. However, occasions will arise when it is necessary to work on parts fixed in position under which condition welds must be deposited horizontally, vertically and overhead. It must be realized at the very beginning that welding in these positions is difficult and will require constant practice to develop skill.

As in the case of welding in the flat position, it is best to start practicing by first running bead welds in the various positions. Then as facility is gained on these operations practice may be continued on butt and fillet welds (tee and lap joints) in these positions.

One of the first facts noted when welding in these positions is that the force of gravity tends to cause the molten metal to drip (fall) down. The technique used, therefore must be designed to overcome this and since it is difficult it is best to approach this by steps. To accomplish this, start by making horizontal bead welds on plates inclined at 45 degrees. When this has been mastered so that uniform beads can be made consistently, practice on welding vertically may be started. Again begin with an easy operation such as running beads vertically on plates set at 45 degrees.

To progress with this practice it is necessary now to move the plates into vertical position. Welding vertically may be performed either by carrying the weld upward or starting from the top and welding down. It is generally conceded that working upward is easier and therefore, bead welds in this manner should be practiced. Since bead welds are of limited practical value, this experience must be extended by practicing on butt welds in the vertical and horizontal patterns.

In use, the beveled plate edges should be spaced on the backing strip and the strip tack welded to the plates on the reverse side.


It may be appreciated that no printed instruction can impart to the beginner the skill necessary for successful welding. Personal instruction by an experienced welding operator is the best means devised to date for accomplishing this end. Therefore, an effort should be made to secure some facility for instruction and practice under competent supervision. In any event the beginner should at least secure the benefit of criticism of finished welds by a qualified welder.

January, 2007
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