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Carbon-Arc Cutting

Metals can be cut cleanly with a carbon electrode arc because no foreign metals are introduced at the arc. The cutting current should be 25 to 50 amps above the welding current for the same thickness of metal.

The carbon electrode point should be ground so that it is very sharp. During the actual cutting, move the carbon electrode in a vertical elliptical movement to undercut the metal; this aids in the removal of the molten metal. As in oxygen cutting, a crescent motion is preferred. Figure 7-52 shows the relative positions of the electrode and the work in the cutting of cast iron.

The carbon-arc method of cutting is successful on cast iron because the arc temperature is high enough to melt the oxides formed. It is especially important to undercut the cast-iron kerf to produce an even cut. Position the electrode so the molten metal flows away from the gouge or cutting areas. Table 7-4 is a list of cutting speeds, plate thicknesses, and current settings for carbon-arc cutting.

Because of the high currents required, the graphite form of carbon electrode is better. To reduce the heating effect on the electrode, you should not let it extend more than 6 inches beyond the holder when cutting. If the carbon burns away too fast, shorten the length that it extends out of the electrode holder to as little as 3 inches. Operating a carbon electrode at extremely high tempera-tures causes its surface to oxidize and burn away, result-ing in a rapid reduction in the electrode diameter.

Carbon-arc cutting does not require special generators. Standard arc-welding generators and other items of arc-welding station equipment are suitable for use. Straight polarity direct current (DCSP) is always used.

Because of the high temperature and the intensity of the arc, choose a shade of helmet lens that is darker than the normal shade you would use for welding on the same thickness of metal. A number 12 or 14 lens shade is recommended for carbon-arc welding or cutting.

AIR CARBON-ARC CUTTING
Air carbon-arc cutting (ACC) is a process of cutting, piercing, or gouging metal by heating it to a molten state and then using compressed air to blow away the molten metal. Figure 7-53 shows the process. The equipment consists of a special holder, as shown in figure 7-54, that uses carbon or graphite electrodes and compressed air fed through jets built into the electrode holder. A push button or a hand valve on the electrode holder controls the air jet.

The air jet blows the molten metal away and usually leaves a surface that needs no further preparation for welding. The electrode holder operates at air pressures varying between 60 and 100 psig.

During use, bare carbon or graphite electrodes be-come smaller due to oxidation caused by heat buildup. Copper coating these electrodes reduces the heat buildup and prolong their use.

The operating procedures for air carbon-arc cutting and gouging are basically the same. The procedures are as follows:

1. Adjust the machine to the correct current for electrode diameter.

2. Start the air compressor and adjust the regulator to the correct air pressure. Use the lowest air pressure possible-just enough pressure to blow away the molten metal.

3. Insert the electrode in the holder. Extend the carbon electrode 6 inches beyond the holder. Ensure that the electrode point is properly shaped.

4. Strike the arc; then open the air-jet valve. The air-jet disc can swivel, and the V-groove in the disc automatically aligns the air jets along the electrode. The electrode is adjusted relative to the holder.

5. Control the arc and the speed of travel according to the shape and the condition of the cut desired.

6. Always cut away from the operator as molten metal sprays some distance from the cutting action. You may use this process to cut or gouge metal in the flat, horizontal, vertical, or overhead positions.

AIR CARBON-ARC GOUGING
Air carbon-arc gouging is useful in many various metalworking applications, such as metal shaping and other welding preparations. For gouging, hold the elec-trode holder so the electrode slopes back from the direc-tion of travel. The air blast is directed along the electrode toward the arc. The depth and contour of the groove are controlled by the electrode angle and travel speed. The width of the groove is governed by the diameter of the electrode.

When cutting or gouging a shallow groove on the surface of a piece of metal, you should position the electrode holder at a very flat angle in relation to the work. The speed of travel and the current setting also affect the depth of the groove. The slower the movement and the higher the current, the deeper the groove. An example of a V-groove cut made in a 2-inch-thick mild steel plate by a machine guided carbon-arc air-jet is shown in figure 7-55.

METAL ELECTRODE ARC CUTTING
Metal can be removed with the standard electric arc, but for good gouging or cutting results, you should use special metal electrodes that have been designed for this type of work, Manufacturers have developed electrodes with special coatings that intensify the arc stream for rapid cutting. The covering disintegrates at a slower rate than the metallic center. This creates a deep recess that produces a jet action that blows the molten metal away (fig. 7-56). The main disadvantage of these electrodes is that the additional metal produced must be removed.

These electrodes are designed for cutting stainless steel, copper, aluminum, bronze, nickel, cast iron, man-ganese, steel, or alloy steels.

A typical gouge-cutting operation is shown in figure 7-57. Notice that the angle between the electrode and plate is small (5 degrees or less). This makes it easy to remove the extra metal produced by the electrode.

The recommended current setting is as high as the electrode will take without becoming overheated to the point of cracking the covering.

• For 1/8-inch electrodes, the setting ranges between 125 and 300 amperes
• For 5/32-inch electrodes, the setting ranges between 250 and 375 amperes
• For 3/16-inch electrodes, the setting ranges between 300 and 450 amperes

Use a very short arc, and when cutting takes place underwater, the coat-ing must be waterproof.

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