2.1 Gouging parameters

Air carbon arc gouging removes metal by melting it with an electric arc struck between a carbon-graphite electrode and the workpiece. A jet of compressed air then blows the molten metal out of the groove. The quality of the groove depends on a small group of settings: current, arc force, polarity, air supply, electrode angle, stick-out and travel speed. If they are wrong, the groove is too shallow, too wide, carbon-rich or full of slag. A welder who understands these parameters can remove a defective weld root or cut out a worn section cleanly, without scrapping the plate. This topic explains how to set them.

2.1.1 Setting parameters

Setting parameters means choosing the machine and torch settings before the first gouge and then refining them on a trial piece. The settings are not independent. A larger electrode needs a higher current. A higher current needs a stronger air flow to clear the extra molten metal. A stiffer arc needs a steadier hand and a more even travel speed. Start from the electrode maker's chart, make a short trial groove on scrap of the same material and thickness, then adjust one setting at a time.

For work up to 16 mm thickness, the aim is usually a smooth, U-shaped groove of even depth, with a bright metal surface and little slag on the edges.

Procedure for setting up before gouging:

1. Identify the job. Note the material, the plate thickness, the depth of groove required and the position of the work. For example, a welder back-gouging a 12 mm mild steel butt weld needs a groove about half the plate thickness, so about 6 mm deep.
2. Select the electrode diameter. Choose a diameter that suits the groove width and the plate thickness. Selection of the electrode itself is covered in the Electrodes topic.
3. Set the polarity. Connect the machine as described under polarity and power source below, and check the electrode packaging if you are using a special type.
4. Set the current. Pick a value within the maker's range for the chosen diameter, starting in the middle of the range, and adjust it on the trial piece.
5. Set the arc force. Set the arc force or dig control to a medium value and refine it after the first trial.
6. Set the air supply. Check that the air pressure at the torch matches the torch maker's recommendation. Open the air valve and listen for a steady, strong jet.
7. Fit the electrode. Clamp it with the correct stick-out and angle, with the air jets running along the underside of the electrode, as described under the torch settings below.
8. Make a trial groove. Gouge about 150 mm on scrap, then measure the depth and width and inspect the surface.
9. Adjust and record. Change one setting at a time, repeat the trial, and write down the final values for the next job.

2.1.1.1 Current

Current is the main setting in gouging because it decides how much heat enters the metal and so how fast metal is removed. It is chosen mainly by the electrode diameter. A large electrode carries more current without overheating, and a high current gives a wider, deeper groove. If the current is too low, the arc is unstable, the electrode sticks and the groove is shallow. If it is too high, the electrode overheats, the copper coating burns off, the carbon rod breaks and the groove becomes wide and rough.

The following ranges are indicative for copper-coated DC electrodes. Always check the electrode maker's chart before setting the machine.

Factors that affect the current chosen:

• Electrode diameter: Each diameter has a working range. Staying inside it protects the electrode and gives a steady arc.
• Groove depth and width: A deeper or wider groove needs more current, but it is better to change to a larger electrode than to push a small one above its range.
• Material: Steel gouges easily because the oxides and molten iron are easily blown away. Metals that conduct heat strongly, such as copper and aluminium, draw heat away from the groove and usually need a higher current. The maker's chart for that metal should be followed, and the surface should be cleaned afterwards because of carbon pick-up.
• Plate thickness: Thin plate overheats and distorts at high current, so use the lower part of the range. Thick plate and cold plate can take the upper part.
• Position: In vertical and overhead positions, molten metal is harder to control, so a slightly lower current helps keep the groove tidy.
• Machine capacity: The setting must stay within the duty cycle of the power source. Running above it overheats the machine.

Example: A farm workshop needs to remove a cracked weld from a 10 mm mild steel trailer drawbar. The welder chooses a 4.8 mm electrode and sets 180 A, which is inside its 150 to 200 A range. A trial on scrap gives a clean groove about 7 mm wide and 4 mm deep per pass, so the welder keeps 180 A and makes a second pass for the extra depth.

2.1.1.2 Arc force

Arc force is the stiffness or "digging" action of the arc. On many welding machines it is a control (also called dig or inductance) that briefly raises the current when the arc voltage drops, for example when the electrode comes close to the plate. In gouging, the same word also describes the physical push of the arc and air jet on the molten pool. Both meanings matter: the arc melts the metal and pushes into the groove, and the air jet carries away what has melted.

Effects of the arc force setting:

• Low arc force: The arc is soft and the electrode tends to stick or "freeze" to the plate. Metal removal is slow and the groove is shallow.
• Medium arc force: The arc is stable and responds to small changes in arc length. The groove is smooth and even. This is the best starting point.
• High arc force: The arc is stiff and penetrates deeply, which helps in deep grooves and on thick plate. Too much gives a harsh arc, extra spatter, a rough groove and faster electrode wear.
• Interaction with current: A high current with a high arc force can overheat the electrode. If you raise the current, check whether the arc force needs lowering.
• Interaction with air: A strong arc produces more molten metal. If the air flow is weak, the metal stays in the groove and the surface becomes slag-covered.

How to adjust arc force on a trial groove:

1. Set the current from the chart and set arc force to the middle of its range.
2. Strike the arc and gouge about 150 mm at a steady speed.
3. If the electrode sticks or the arc stutters, increase the arc force slightly.
4. If the groove is rough, with heavy spatter and a harsh crackling arc, reduce the arc force slightly.
5. Repeat the trial after each change until the arc runs smoothly and the groove surface is even.

Example: A fabrication workshop is cutting out a worn weld on a 14 mm steel gate frame. At the first setting the 6.4 mm electrode keeps sticking, so the welder raises the arc force by one step. The arc then holds steadily and the groove comes out clean and U-shaped.

Polarity and power source

Polarity decides in which direction current flows between the electrode and the plate. Standard copper-coated graphite electrodes are used on DC with the electrode connected to the positive terminal. This gives a stable arc and efficient metal removal, while the wrong polarity gives an unstable arc and a poor groove.

• DC electrode positive: This is the usual setting for steel with standard DC electrodes.
• Constant-current machine: The power source should hold the current steady while the arc length changes, which is why a DC constant-current welding machine is normally used.
• AC electrodes: Special electrodes are made for AC machines. Use them only when the machine and the electrode packaging say so.
• Cable size: Heavy gouging currents need cables of sufficient size and sound connections, because loose connections overheat and make the arc unsteady.
• Work clamp: Fix the work clamp close to the area being gouged on clean bare metal for a good return path.

Air supply, electrode angle, stick-out and travel speed

These settings are made at the torch rather than at the machine. They decide how well molten metal is cleared and how accurately the groove is shaped.

• Air pressure: Compressed air at the torch is commonly around 550 to 700 kPa, but follow the torch maker's figure. Too little pressure leaves molten metal in the groove, and the surface becomes slag-covered.
• Air flow: The air supply must deliver enough volume as well as pressure. A small compressor with a long, narrow hose may not keep up, and the pressure then drops as soon as the torch is opened.
• Electrode angle: For a normal groove, hold the electrode at about 35° to 45° to the plate. A steeper angle gives a deeper, narrower groove, and a shallower angle gives a wider, shallower one.
• Stick-out: Keep about 100 mm of electrode extending from the torch. Too long a stick-out makes the electrode overheat and break. Too short brings the torch head too close to the heat. Re-adjust the electrode when it burns down.
• Travel speed: A slow speed gives a deeper and wider groove, and a fast speed gives a shallower one. The speed should keep the arc just ahead of the molten pool so that the air jet clears it. The groove is usually a little wider than the electrode, by about 1.5 to 3 mm.
• Direction: Push the electrode forward along the line of the groove, with the air jet behind the arc so that it blows the molten metal forward, ahead of the arc and away from the operator. Make sure sparks and molten metal are directed away from people and from flammable materials.

Safety and checking the setting

Gouging produces intense light, loud noise, sparks, fumes and large amounts of molten metal, so the setup must include protection as well as machine settings.

• Eye and face protection: Wear a welding helmet with the correct filter shade for the current in use.
• Hearing protection: Gouging is very loud, so ear defenders or plugs are needed.
• Clothing: Wear flame-resistant overalls, leather gloves, boots and a leather apron or sleeves. Avoid open pockets or turn-ups that catch sparks.
• Ventilation and fire control: Provide extraction or good airflow for the fumes, remove flammables from the area, and keep a fire extinguisher nearby.
• Checking the groove: After each trial, inspect for uniform depth, a smooth U-shape and a bright surface. Remove any carbon deposit by grinding before welding.

Chapter Summary

This chapter focused on the essential aspects of carrying out gouging in arc welding processes. It began by examining gouging parameters, emphasizing the importance of correctly setting current and arc force to achieve effective material removal and control the gouge profile. The discussion then shifted to electrodes, particularly the selection of graphite electrodes, which play a critical role in maintaining arc stability and ensuring clean gouging results. Finally, the chapter detailed the gouging procedure for materials up to 16 millimeters in thickness, outlining the practical steps required to perform gouging safely and efficiently while maintaining the quality of the workpiece. Understanding these components ensures that welders can prepare surfaces properly for subsequent welding operations.