by Jeff Henderson
Air carbon-arc gouging (CAC-A) is the process of cutting/removing metal through intense heat generated from a carbon arc. Using a carbon electrode, compressed air and a welding power source (either a constant current, constant voltage, DC or AC), the arc created between the carbon electrode and the work piece melts the material, while the compressed air blows away the molten metal leaving a clean groove. The rate of metal removal is based on the efficiency of air stream in removing molten metal and the melting rate.
The carbon arc gouging process is easy to learn, has a high metal removal rate, and can be closely controlled. Furthermore, it can be applied to a wide range of metals—basically any metal that can conduct electricity and be melted with an arc, including magnesium, iron, copper, aluminum and stainless and carbon steels. Typical applications include the back-gouging of weld seams to reach the deposited weld metal from the other side of the work piece, removal of gates and risers from castings and the removal of old or excess weld metal so that equipment can be dismantled.
1.) When using copperclad carbon electrodes, position the carbon in the torch so the bare end is pointing towards the work piece. Set the air pressure to 80 to 100 P.S.I. to prevent trapping carbon in the gouge. In normal conditions, the carbon should be positioned so that no more than 18 cm (7 in.) of carbon sticks out past the torch head. For aluminum, this extension should be 76.5 mm (3 in.). The air blast is always positioned between the carbon and work piece, not above the carbon. If there is sufficient airflow, there will not be a problem cleaning up the joint.
2.) Strike an arc by lightly touching the carbon electrode to the work, let the arc start, and slowly move it forward or side to side as needed to accomplish your goal. Striking the arc is a little bit different and slightly easier than with a welding electrode. Prior to striking, take the necessary time to get into a comfortable position, and do not draw the carbon back once the arc is struck.
3.) Hold the torch so the carbon electrode slopes back from the direction of travel with the air blast behind the tip of the carbon. The proper torch angle to work is 35° to 45°. The depth and contour of the groove produced are controlled by the electrode diameter and travel speed. Groove depths greater than 1-1/2 times the diameter must be made in multiple passes. The width of the groove is determined by the electrode diameter used and is usually 3.2 mm (1/8 in. ) wider than the diameter. A wider groove may be made with a small electrode by oscillating the electrode in a weaving motion.
4.) Remember, travel speed determines the depth of the gouge. The faster the travel speed, the shallower the gouge. A slow travel speed will produce a deeper gouge. A short arc must be maintained by progressing in the direction of the cut fast enough to keep up with the metal removal and carbon electrode consumption. The steadiness of the progression controls the smoothness of the resulting surface.
5.) Always use a push technique. Travel speed is continuously forward with the air blowing behind the arc. Never back up. This will avoid carbon deposits in the base material that cannot be welded without first re-gouging or grinding to completely clean the base material.
6.) When back-gouging a weld joint, always watch the parting line right at the end of the carbon electrode while moving forward. The joint line will be visible just in front of the carbon, allowing the operator to follow the weld seam.
With this advice in mind and a little bit of practice, air carbon-arc gouging is a simple, inexpensive and highly effective way to remove almost all metals in variety of metal fabrication and repair applications. SMT
Jeff Henderson is the Victor Arcair brand manager, Victor Technologies, St. Louis, MO