Air Carbon Arc Cutting Process (CAC Process)

Last Updated on January 2021

Introduction

Carbon Arc Cutting (CAC), also known as metal arc gouging or air arc cutting, is a metal cutting method that uses high temperature to heat a metal to its melting state. Here, the heat is generated by a carbon electrode arc.

The molten metal is blown off by a high-velocity jet of compressed air. This air jet is external to the flammable carbon electrode. It blasts the molten metal immediately behind the carbon arc.

Comparison to Other Processes

Carbon arc cutting differs from machine plasma cutting as it employs a pen arc that’s independent of the air blast. The process differs from oxy-fuel cutting processes as the air jet physically removes the molten metal instead of chemically. It’s also flexible as no oxidation is required before the cutting is done.

Equipment

Conventional welding machines make use of a constant current mechanism. However, the CAC process can utilize a constant voltage technique. In such a case, you must be keen as to operate the machine within its rated amperage output and duty cycle.

Alternatively, you can use an AC type electrode in special applications thanks to its conventional drooping characteristics. There are heavy-duty high-current machines specially designed for the CAC process. That’s because of the high currents used for large carbon electrodes.

Essentially, the key equipment includes:

Electrode Holder

The electrode holder designed for the carbon arc cutting process has a smaller circular-grip-head, with many significances. The head contains the air blasts to direct high-velocity compressed air along the carbon electrode, as well as, a grip groove for holding the electrode. This head is rotatable to allow for different angles of the carbon electrode with respect to the holder.

A terminal block connects an air supply hose and a heavy electrical lead to the holder. There’s also a valve that serves to turn the compressed air on and off.

Electrode holders come in varying sizes depending on:

  • The welding current
  • The duty cycle of the welding to be performed
  • The size of the electrode used.

If the welding in question is extra heavy-duty, use a water-cooled holder.

Electrode

Carbon electrodes and other welding consumables are available in a variety of ways. The commonly used type is the carbon graphite electrodes – a blend of carbon and graphite bound together by a baked binder to produce a homogenous structure.

The carbon graphite electrodes can be uncoated or copper-coated. The uncoated electrode carries less current but it’s cheaper and easier to start. Copper coated electrode, on its end, provides better electrical conductivity between the electrode and the holder. Besides lasting longer, the copper-coated electrode carries a higher current making it ideal for maintaining the original diameter when welding.

Copper-coated electrodes can be a DC or an AC. While the DC type is more common, the AC electrode is specially designed to stabilize the arc.

The alternating current electrode can serve as a DC negative when cutting or gouging cast irons. It comprises graphite, carbon, and a special binder (made of rare earth materials).

Carbon electrodes vary in diameter from 4mm to 25.4mm. Their diameter determines the:

  • Amount of amperage output: the larger the diameter the more the output.
  • Depth and radius of the cut/gouge: the larger the diameter the deeper the cut; however, smaller electrodes can yield deeper gouges after multiple passes.

When it comes to length, carbon electrodes are normally 300mm long; however, some electrodes are available in lengths of 150mm.

Air Pressure

For best results, the air pressure should be 552 – 690 kPa, and the volume of compressed air in the range of 2.5 liters/min to 24liter/min.

A one-horsepower compressor is adequate for small-sized electrodes; however, large-sized electrodes require up to a 10-horsepower compressor to supply a sufficient amount of air.

Torch

The torch holds the carbon electrode in a swivel head with one or multiple breaths of air holds. As such, the air jets remain aligned with the carbon electrode regardless of the angle of the carbon electrode to the torch.

Note, two-headed torches or those with a fixed angle between the torch and the electrode are available. They are particularly useful in pad washing or when removing pads.

Normally, the torch is sufficiently cooled by the surrounding air. However, as is the case with high-current applications, water-cooling can be used.

CAC works by sending electrical current via the carbon electrode. The electric current heats the carbon arc, which in turn; heat and liquefies the metal. Next, a high-velocity air blast of compressed air that’s parallel to the electrode strikes the molten metal just behind the arc and blows it out of the melting zone.
CAC process is used to:

  • Cut metal.
  • Gouge out defective metal.
  • Prepare grooves for welding.
  • Remove inferior or old welds.
  • Carry out root gouging or full penetration welding

The process is particularly beneficial when slightly ragged edges are unobjectionable.
The cutting area is small, hence the need to remove the molten metal immediately. In so doing, the surrounding area remains relatively cool, which reduces metal cracking and distortion tendencies. The process is more common for gouging than it is for cutting.

When used to prepare edges on plate stock, it leaves a unique radius groove.
You can use the Carbon Arc Cutting to cut or gouge the following metals:

  • Aluminum
  • Copper-based alloys
  • Magnesium
  • Carbon
  • Cast Iron
  • Steel
  • Stainless steel
  • Nickel alloys

Advantageously, the process works in all positions; nonetheless, the overhead position calls for a high degree of skill. For more information on welding angles and positioners [Read our full Guide]

CAC is extremely loud, which calls for hearing protection. It also results in higher volumes of hot dross and sparks (when compared to plasma arc and gas cutting), which calls for a welding helmet with a number 10 shade lens. Note, darker lenses are a must in cases of higher amperages.

The process is not recommended for titanium, stainless steel, zirconium, and other metals without sanding. That’s because small amounts of carbon may be transferred to these metals. If that happens, the metals must be cleaned to remove the carbon residue.

  • Select the planning current, the diameter of the electrode, and the polarity of the power supply as per the type and thickness of the metal in question.
  • Adjust the extension length of the carbon electrode accordingly.
  • Adjust the air jets to align with the groove
  • If cutting in a vertical position, open the gas valve before igniting the arc – to avoid carbon trapping defects when planning. While at it, cut from the top to bottom, to easily discharge the slag or spatter.
  • Set the inclination of the electrode relative to the depth of the groove to 45°. Ensure the center of the electrode coincides with that of the groove. Failure to will result in an asymmetrical groove.
  • When planning, ensure the arc is stable and the plaining speed uniform. Also, draw the arc on the pit when connecting each segment. Note, the compressed air can be blown slightly, which, in turn, turns the slag to the outside tank – opposite the operator.
  • To cool the electrode, cut off the arc at the end of each plaining, then cut off the gas a few seconds after.

Do not allow any remnants of the molten metal (or any metals) in the plaining tank.

To recap, air pressure of 552 – 690 kPa is best for electrodes sized 13 – 16mm. However, you can use multiple passes and a combination of settings to cut grooves deeper than 19mm.

For a cutting or gouging operation, the cutter strikes the arc and simultaneously commences the airflow.

The carbon electrode is aligned in the direction of travel and an approximately 45° with the groove’s axis.

The electrode diameter dictates the width of the groove. On the other end, the depth of the groove is dictated by the size of the current, the size of the electrode, the electrode angle, and the speed of travel.

The usual safety precautions linked to other types of welding also apply to CAC processes. In addition, you shade take serious note of the following:

  • Placing a metal deflection plate in front of the cutting/gouging operations. That helps to stop the flowing molten metal.
  • Placing all flammable objects and materials away from the welding site. That is because the size of molten metal can be too large (especially at high-current levels), which makes it a possible fire hazard if not contained carefully.
  • The cutter should wear earmuffs, earplugs, or other ear protection when cutting. The reason being, the CAC process generates high noise levels (especially at high-currents and air pressure).

Carbon arc cutting is a metal gouging or cutting process with numerous applications. It has its benefits and downside, as discussed above.

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