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Plasma Arc Cutting


1.0.0 Plasma Arc Cutting Process

2.0.0 Equipment and Consumables

3.0.0 Cutting and Gouging Operating Sequence

4.0.0 Plasma Arc Gouging

5.0.0 Qualities of a Plasma Cut

6.0.0 Safety Procedures

Review Questions

As a steelworker/welder, you will be expected to become familiar with the Plasma Arc Cutting (PAC) process. To achieve optimum performance of your plasma cutting system, first you must know what plasma is and understand the basic plasma cutting process.

Plasma is a physical state of matter. In fact, plasma is the most abundant form of matter in the universe. Physical matter may be found in four states: solid, liquid, gas, or plasma. Changes from one physical state to another occur by either adding or removing energy. Plasma looks and behaves like a high temperature gas, but with an important difference: it conducts electricity. Lightning is a naturally occurring example of plasma.

A plasma arc is created by electrically heating a gas to a very high temperature; this ionizes the atoms, which enables the gas to conduct electricity. This is the major difference between a neutral gas and plasma; the particles in plasma can exert electromagnetic forces on one another.

This course presents an introductory explanation of plasma arc cutting. Since your company purchases equipment from different manufacturers, always refer to the manufacturer’s manuals for specific operating and maintenance instructions.

When you have completed this course, you will be able to:

  1. Describe the Plasma Arc Cutting process.
  2. Describe plasma arc equipment and consumables.
  3. Identify the plasma cutting and gouging sequence.
  4. Describe the steps in arc gouging.
  5. Identify the steps of a quality plasma cut.
  6. Describe the safety procedures for plasma arc processes.


Plasma arc cutting is such a simple process you could almost take it out of the box and start using it. However, as with any piece of equipment you need to know how and why it does what it does and the necessary precautions to do the job safely.

1.1.0 Description

Materials in nature exist in one of four different states: solid, liquid, gas, or plasma. Plasma is very rare on Earth because of its very high temperature; however, most of the matter in the universe is plasma. The Sun, stars, and galaxies are made of plasma. On Earth, you will find naturally occurring plasma in lightning and a few other places (Figure 1). Neon tubes and florescent lights generate low-temperature plasma. It is the energy from ionization that you are actually seeing.

Figure 1 — Naturally occurring plasma.

Experiments with plasma arcs date back to early in the twentieth century but it was in the 1950s when PAC torches were patented. The equipment was large and bulky and used a variety of cutting and cooling gases. Today, the introduction of cutting with clean, compressed air or nitrogen is replacing many other types of cutting equipment.

Temperature makes the difference between water ice, liquid water, and water vapor. In each of these states, temperature energy pushes the molecules of water away from each other to change the water’s state. At very high temperature and pressure, the water molecules themselves break apart and the atoms begin to ionize.

Normal atoms consist of protons and neutrons in a nucleus surrounded by a cloud of electrons. In plasma, the negatively charged electrons separate from the nucleus leaving behind their positively charged nuclei known as ions. When the fast-moving electrons collide with other electrons and ions, they release vast amounts of energy. This energy is what gives plasma its cutting

power. Plasma cutters work by electrically charging a gas within a plenum (chamber) that surrounds the electrode (Figure 2). This charge superheats and ionizes the gas, which is now a greatly expanded (in volume and pressure) plasma gas. The electrically charged plasma then exits the torch nozzle through a constricting orifice and arcs to the surface of the grounded workpiece, creating a stream of directed plasma, approximately 30,000° F (16,649° C) moving at approximately 20,000 feet per second (6,096 m/sec), reducing metal to molten slag. The plasma itself conducts electrical current. The cycle of creating the arc is continuous as long as power is supplied to the electrode and the plasma stays in contact with the grounded metal being cut.

Figure 2 — Plasma arc.

The PAC process uses this high temperature, high velocity jet of gas (exiting from the constricting orifice of the torch tip) to melt a localized area, and removes the molten material by the force of the plasma jet. The force of the arc pushes the molten metal through the workpiece and severs the material (Figure 3). You can make extremely clean and accurate cuts with PAC, and because of the tightly focused heat energy, there is very little warping, even when cutting thin sheet metal. PAC also offers quality gouging and piercing capabilities.

Figure 3 — PAC torch cutaway. ionized

1.2.0 Plasma vs. Oxy-Fuel Cutting

Before the PAC process became commonplace, if you wanted to cut carbon steel, stainless steel, or aluminum, chances were you would be using several means or methods of cutting. Perhaps you would use oxy-fuel gas flame cutting for steel, but that process is not recommended for cutting stainless steel and aluminum due to the formation of an oxide that prevents oxidation from fully occurring. You could use bandsaws, shears, abrasive cut-off wheels, or power hacksaws, but you would need special blades to cut the stainless steels and alloy steels.

With engineering advances in PAC equipment, all metals that conduct electricity, whether they are common or exotic metals, can be cut economically with one process. Since the plasma arc cutting process is capable of hand-held or machine torch cutting, metals ranging from thin gauge aluminum to 1 1/2-inch carbon or stainless steel can be plasma cut. It can be used in many applications, including stack and shape cutting, beveling, gouging, and piercing in all positions. The PAC process is used in industries such as metal fabrication, construction, maintenance, metal salvage (scrap and recycling), automotive repair, metal art, and sculpting.

The PAC process is compared primarily to the oxy-fuel gas cutting (OFC) process. The OFC process severs or removes metal by the chemical reaction of oxidation. It is known as “burning” or rapid oxidation. This occurs when you apply pure oxygen to hot, preheated metal and maintain the elevated temperature with a flame from a burning oxy-fuel gas mixture. It requires a high purity oxygen and fuel gas, which comprises an explosive fuel gas mixture usually supplied from high-pressure compressed gas cylinders.

A properly installed Air Plasma Arc Cutting setup can be safer than an OFC system. Safety precautions on the PAC torches can be safer than oxy-fuel gas torches where there is a chance of flashback and the danger of flammable gases in exposed hoses (Figure 4).

Figure 4 — Oxy-fuel cutting setup.

OFC’s advantage is its capability of cutting very thick carbon steel with relatively inexpensive equipment that does not require electricity. OFC’s disadvantage is its recommended limitation to cut carbon steels only.

PAC requires minimum training to operate the equipment safely and efficiently. One of PAC’s major advantages is speed. PAC operates at a much higher heat energy level, so it cuts faster than OFC, especially on metal less than 2 inches thick, and cutting speed makes a significant difference in production time and operator comfort. Also, unlike the OFC process, PAC does not require preheating, another major advantage besides the faster cutting speed. Because of this, PAC results in less distortion of the metal being cut. This is due also to a very narrow heat-affected zone (area changed in characteristics near the cut). The clean, dross-free cut produced with the PAC process can eliminate the secondary operations of other cutting methods such as cleaning up rough edges and dross on the bottom or backside of the cut (Figure 5).

Figure 5 — Clean cut.

When compared to OFC, PAC in some areas will not be as portable, due to its dependence on primary electrical power from a utility line or engine-driven generator.

Test Your Knowledge

1. What happens to an atom when it is exposed to very high temperatures?

A. It adds a valence shell
B. It becomes an ion
C. It disintegrates
D. It remains stable

What characteristic makes plasma different than a gas?

A. It has a greater atomic weight
B. It is incandescent
C. It can conduct electricity
D. It has a distinct odor


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A pilot arc between the electrode and the constricting tip initiates the plasma arc process. The tip is connected to ground through a current-limiting resistor and a pilot arc relay contact in the torch assembly. One of two methods, either a high frequency generator connected to the electrode and tip or an internal contact start, initiates the pilot arc. The welding power supply then maintains a low current arc inside the torch. Ionized orifice gas from the pilot arc is blown through the constricting tip orifice by a compressed gas. This forms a low resistance path to ignite the main arc between the electrode and the workpiece (Figure 6). When the main arc ignites, the pilot arc relay may be opened automatically to avoid unnecessary heating of the constricting tip, which helps extend the life of the tip and electrode.

Figure 6 — Basic PAC setup.

Equipment Requirements

A typical air-cooled PAC system consists of the following components:

2.1.1 Power Source

Plasma arc cutting uses a direct current power source. The polarity setting for the power source is direct current electrode negative (DCEN). In most systems there is also a positive connection to the torch tip in which the current is limited by a resistor. This circuit establishes a "pilot arc," which then establishes the cutting arc.

The power source is a constant current power source with a high open circuit voltage (250-400 volts). The amperage is usually adjustable within the range of the power source, and amperage is directly proportional to the thickness and speed in which the metal can be cut.

Most manual PAC systems now use switch-mode or inverter technology. These sophisticated, electronically-controlled or microprocessor-controlled devices are better able to tolerate variations in line voltage, take more abuse in the field, and deliver better cutting performance while consuming less power with a longer duty cycle.

The term “duty cycle” identifies the number of minutes out of a 10-minute period that you can operate a plasma cutter at its rated capacity. For example, a 300-amp welder with a 60% duty could operate at 300 amps for 6 minutes and then needs to cool with its fan running for 4 minutes. Manufacturers rate their products based on ambient air temperature, so if a cutter is rated at 104º F and the ambient temperature where you are working is 84º F, the duty cycle of the machine increases. Conversely, if the ambient temperature is hotter than the manufacturer’s initial rating, the duty cycle decreases. You need to know what ambient temperature the manufacturer used to rate its PAC in order to operate the equipment at the appropriate duty cycle and prevent damage.

2.1.2 Rated Cutting Capacity

Selection of the PAC is based on the type and thickness of the metal to be cut and the speed at which the metal needs to be cut. The higher the PAC ampere and duty cycle rating, the thicker and faster it will cut (Figure 7). While there is no standard for PAC cutting speeds in the welding/cutting industry, some manufacturing companies qualify their PAC rated cutting capacity by three (3) standards (Figure 8):

Figure 7 — Cutting capacity rating

Figure 8 — Rated cutting capacity.

2.1.3 Cutting Speed

As indicated previously, the cutting speed will affect the thickness of the material that can be cut. The slower you move the torch, the thicker the material that can be cut, but if you move the torch too slow the plasma arc will remove all of the material directly underneath it and the arc will bend to the side of the kerf, causing a jagged cut. The faster the travel speeds, the thinner the material that can be cut, but if you increase the torch speed too much, the torch will be unable to cut completely through the workpiece. Cutting speed is measured in inches per minute. Maximum cutting speed is determined by the arc current, nozzle diameter, and metal thickness.

2.2.0 Consumables

The plasma torch is designed to generate and focus the plasma cutting arc (Figure 9). In either hand held or machine torches, the same parts are used: an electrode to carry the current from the power source, a swirl ring to spin the compressed air, a tip that constricts and focuses the cutting arc, and a shield and retaining ring to protect the torch.

Figure 9 — PAC torch assembly.

2.2.1 Swirl Ring

The swirl ring, made of a high temperature plastic, is designed with angled holes to spin the cutting gas in a vortex (Figure 10). Spinning the gas centers the arc on the electrode and helps control and constrict the arc as it passes through the tip. Some plasma cutting equipment swirls the gas in a clockwise direction, others in a counter-clockwise direction. Check the manufacturer’s manual; the direction of flow will indicate which side of the cut will be beveled.

Figure 10 — Swirl ring.

2.2.2 Electrode

The purpose of the electrode is to provide a path for the electricity from the power source and generate the cutting arc (Figure7-11). The electrode is typically made of copper with an insert made of hafnium. The hafnium-alloyed electrodes have good wear life when you use clean, dry compressed air or nitrogen, although electrode consumption may be greater with air plasma than with nitrogen.

Figure 11 — Electrode.

2.2.3 Tip

The purpose of the torch tip is to constrict and focus the plasma arc (Figure 12). Constricting the arc increases the energy density and velocity. The tips are made of copper, with a specifically sized hole or orifice in the center of the tip. Tips are sized according to the amperage rating of their respective torch.

Figure 12 — Tip.

2.2.4 Retaining Cup

The retaining cup serves two functions (Figure 13). First, it holds the other consumable parts firmly in place. Second, it insulates and keeps the other consumable parts from making contact with the work piece.

Figure 13 — Retaining cup.

2.2.5 Shields

There are two types of shields used on plasma torches: a drag shield (Figure 14) and a deflector (Figure 15). The drag shield insulates the front end of the torch from the work piece and protects the torch tip from spatter. The deflector insulates the electrode and protects it from spatter. It is used when extended cutting consumables are needed.

7-14 — Drag shield.

7-15 — Deflector shield.

2.2.6 Consumables Used During Extended Cutting vs. Drag Cutting

The use of extended cutting consumables requires the operator to maintain a torch standoff of about 1/8". “Torch stand-off” is the distance from the outer face of the torch tip or constricting orifice nozzle to the base metal surface (Figure 16). Extended cutting is used in situations where the operator needs extra control of the cutting arc, such as when cutting in a corner or when a machine torch is used.

Figure 16 — Extended vs. drag.

The drag shield is constructed so that the required standoff is maintained inside the torch. Using drag cutting consumables allows the operator to drag the torch on the work piece while cutting at full output, which increases operator comfort and makes template cutting easier.

2.2.7 Consumable Tips for Different Amperages

Tip size is directly proportionate to amperage; the higher the amperage, the larger the tip you would use. As you can see in Figure 17, the 40-amp tip opening is smaller than the 80-amp tip. Exercise caution and be sure to use the correct tip for the amperage. If you use an 80-amp tip for a 40-amp machine, the plasma arc will not constrict enough and will cut an uneven wide kerf. If you use a 40-amp tip on an 80-amp machine, internal arcing will damage the tip and electrode, decreasing their service life.

Figure 17 — Consumables chart

2.2.8 Replacing Consumables

Good preventive maintenance (PM) requires keeping a supply of electrodes, tips, and shield cups on hand and replacing them as wear appears. You should inspect the shield cup, tip and electrode before each use, hourly during operation, or whenever the cutting speed has reduced significantly. Do not operate the PAC torch without a tip or an electrode in place. A tip and electrode that are worn beyond the manufacturer’s recommended values, or operating a torch without the tip or electrode in place can damage the torch. Refer to Figure  18 for a comparison of new and worn consumables. Figure 19 shows what to look for in the inspection process.

Figure 18 — New and worn consumables.

Figure 19 — Consumable inspection process.

2.2.9 Cutting Gases

Plasma arc cutting gases must have high ionization potential (energy), high thermal conductivity to deliver high heat energy to the work piece, and high atomic weight to produce the energy to blow or push out metal from the cut. Compressed air (approximately 80% nitrogen) with its high ionization potential and density is commonly used to minimize gas costs. Compressed air may require installation of filters or line dryers to remove oil vapors and moisture. Clean, dry, compressed air may be purchased in cylinders. As a plasma gas, nitrogen is considered to be the gas that creates the least slag or dross.

The gas pressure and flow rates must be properly set to the equipment manufacturer’s recommendation. The gas supply piping and hoses to the cutting unit must be of sufficient size to carry the pressure and gas volume required. Use a minimum 3/8" ID (inside diameter) piping or nonconductive hoses to provide the necessary pressure and volume of gas to the PAC power source. If the piping or hose is more than 40 feet in length, use a minimum 1/2" ID (Figure 20).

Figure 20 — Cutting gasses.

2.3.0 Improving Consumable Life

  1. Maintain proper gas pressure setting.
    Setting the correct amount of gas pressure is very important to consumable life. If the pressure setting is too high, electrode life will be shortened. If the pressure setting is too low, the tip life will be shortened.
  2. Maintain the correct stand off.
    If extended cutting consumables are being used, make sure to maintain the recommended amount of standoff. Too little standoff may damage the torch and consumables. Too much standoff will result in inconsistent arc starts.
  3. Pierce within the limits of the plasma system.
    Do not try to pierce metals that are too thick for the plasma cutter. The typical rating is to pierce up to half the rated cutting thickness of the plasma cutter. For example, if the plasma cutter is rated to cut 1" steel, it could pierce 1/2" steel.
  4. Make sure the gas used is clean and dry.
    Plasma cutting systems require clean, dry gas to operate properly. Moisture in the gas line is the cause of many system problems. It can cause shortened consumable life and premature torch failure. To check for moisture in the gas line, set the system to the gas/air set position and hold a mirror under the tip (Figure 21). If any moisture appears on the mirror, inspect the system for the source of the moisture or install an air dryer in the system.

Figure 21 — Checking for moisture.

  1. Use edge starts.
    Use edge starts whenever possible instead of pierce starts. Edge starts improve consumable life since there is less chance for molten metal to be blown back into the tip.
  2. Use the tip saver setting.
    Whenever possible, use the tip saver position for the pilot arc. More pilot arc time than is necessary will lead to shorter consumable life. Use the expanded metal setting only when absolutely necessary.
  3. Remove buildup from shields.
    Inspect the shields on the end of the torch frequently and remove any slag from the shield. Slag can cause double arcing, which shortens tip life.
  4. Purge gas after changing consumables.
    Purge gas lines for 2–3 minutes after changing consumables or extended periods of little or no use. This will ensure that any moisture built up in the lines is removed.
  5. Keep torch and consumables clean.
    Any type of contamination in the torch or consumables can affect the performance of the cutting system. When you change consumables, always try to keep the new parts on a clean rag.
Test Your Knowledge

3. How many minutes make up a duty cycle?

A. 5
B. 10
C. 15
D. 20

4. What is hafnium used for?

A. Shield material
B. Grounding clamp
C. Electrode tip
D. Insulator


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The first step in operating a PAC system is to perform a system check. Make sure that the torch is assembled properly. Turn the power source and air supply on. Next, check the status lights on the power source.

The status lights advise the operator if the system is ready to cut or if there is a problem that will keep the unit from operating properly. There may be two to four status lights depending on the model of the power source (Figure 22).

Figure 22 — Control panel.

Typically, the top light is labeled “power.” When this light is on, it indicates the power source is on. If the power light is the only light on, that indicates that the system is ready to cut. If an additional light is on, that is the indication of a problem in the system.

Three parts of the system are monitored and when not functioning properly cause the additional status light to turn on and keep the system from cutting. These parts are the torch assembly, the air pressure setting, and the internal temperature of the power supply.

Should an additional status light come on, check to make sure the torch is properly assembled, the air pressure is set to the recommended setting, and it has had a chance to cool if the power source has been cutting continuously for more than the rated duty cycle time period. Once these problems have been fixed, the status light will turn off and the system will be ready to cut.

After verifying that the status lights indicate that the system is ready to cut, purge the gas lines for a minute to get rid of any moisture that may have formed inside the lines.

When the trigger has energized the circuit, a preflow of gas will flow through the torch for a few seconds. This is done to ensure that the right amount of gas flow is available before an arc is created. The cutting arc is created by one of two different starting methods: high frequency starts or contact starts.

 3.1.0 High Frequency Starts

The tried-and-true method is a high-frequency (HF) starting circuit built into the power supply. This system uses a high-voltage transformer (similar to a bug zapper), capacitors, and a spark-gap assembly to generate a high-voltage spark at the torch (Figure 23).

Figure 23 — HF starting circuit.

The spark ionizes the plasma gas, enabling current to flow across the air gap  between the nozzle and electrode. The resulting arc is called the pilot arc. High-frequency starting systems are simple, relatively dependable, and require no moving parts in the torch. However, they do need periodic maintenance to prevent hard-starting problems. Another potential problem is the high frequency that radiates from the system, creating electrical noise that may interfere with sensitive electronic equipment.

3.2.0 Contact Starts

A contact start torch uses a moving electrode or nozzle to create the initial spark that enables the pilot arc (Figure 24). When the torch is fired, the electrode and nozzle are in contact in a dead short, or short circuit. When the operator depresses the trigger, gas enters the plasma chamber; it blows the electrode back (or the nozzle forward) creating a spark. This process is similar to the spark created when an electrical plug is pulled quickly from a receptacle.

After the initial arc is created, the gas flow pushes the arc through the orifice and reestablishes it on the outside of the tip. This forms a J-shaped arc called the pilot arc. The pilot arc forms a path to the metal surface to be cut. When the torch is close enough to the metal, the arc will transfer from a pilot arc between the electrode and the tip to a cutting arc between the electrode and the workpiece.

Contact start torches produce much less electrical noise than HF systems, and they are instant-on torches, which reduces cycle time because of the lack of preflow.

3.3.0 Pilot Arc Control Methods

On some power sources the pilot arc remains on even after the cutting arc is established. An advantage to this is that if the operator is cutting over a piece of expanded metal, for example, the cutting arc is maintained as the arc moves from one piece of metal to the other. One disadvantage of leaving the pilot arc on at all times during the cutting process is that it can lead to faster consumable wear. To help address these issues, some power sources have ways of controlling the pilot arc so that it is on when needed and can be shut off when not needed. In some cases the power source has a switch that gives the operator a choice of settings for the pilot arc. The operator can select the expanded metal position for a continuous pilot arc or the tip saver position where the pilot arc shuts off after the cutting arc is established.

Figure 24 — Contact start.

Other power sources are equipped with a circuit that automatically controls the pilot arc. The pilot arc will switch in and out as fast as needed when cutting expanded metal or multiple pieces of metal. When cutting on a solid piece of metal, the pilot arc will drop out after the cutting arc has been established.

3.4.0 Starting the Cut

With a hand-held torch there are two methods for starting the cut: edge starts and pierce starts. To use an edge start, place the torch directly over the edge of the work piece (Figure 25). With the tip centered on the edge of the metal, start the arc and begin moving the torch along the cut line (Figure 26).

Figure 25 — Starting an edge cut.

Figure 26 — Edge cut process.

Pierce cuts are a little more difficult. The torch will need to be angled slightly over the starting point (Figure 27).This will prevent the molten metal from the beginning of the cut from being blown back into the tip and electrode. Once the cutting arc has pierced through the metal, move the torch to a vertical position and continue along the cut line. The thicker the metal, the longer it will take the cutting arc to pierce through the metal. The process of piercing a hole in the metal will cause a blow hole that is wider than the normal kerf, so the initial pierce should be done in the scrap portion of the part not on the cut line (Figure 28).

Figure 27 — Pierce cut.

Figure 28 — Pierce cut process.


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Plasma Arc Gouging is a variation or an adaptation of the PAC process. Gouging utilizes a different torch tip that produces a reduction in the arc constriction, which results in a lower arc stream velocity. Note the larger diameter orifice of the gouging tip (Figure 29). This larger diameter orifice provides the reduction in arc constriction, which results in a lower arc stream velocity. It gives a softer, wider arc and proper stream velocity. Gouging may be used for edge preparation (J or U-grooves), removal of welds, or discontinuities in welds, and it may be used in all positions.

 Figure 29 — Cutting vs. gouging tips.

When comparing PAC with Air Carbon Arc Gouging (CAC-A), one major difference is that with PAC, the gouge surface is bright and clean. This is particularly true with the aluminum alloys and stainless steels. There is virtually no cleanup required because the gouges are clean and absent of carbon contamination, as is the case when using the CAC-A process. Because of this, CAC-A is not recommended as a weld preparation for stainless steel and aluminum without subsequent and sufficient cleaning. The technique for plasma arc gouging requires the torch be angled 30° to 45° from the base metal surface. This torch angle and the speed of travel will determine the gouging depth. It is important that not too much material be removed in a single pass. It is better to remove by gouging to the required depth and width by using multiple passes (Figure 30).

Figure 30 — Gouging process.


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Good quality cuts result in less time and effort being spent on cleaning up the part before it goes to the next step in the manufacturing process. If the part is to be welded, a clean cut is important in order to produce a good weld. It takes several terms to define a quality cut (Figures 7-31and 7-32).

Figure 31 — Elements of a quality cut.

Figure 32 — Direction of cut.

5.1.0 Kerf

The kerf is the width of the cut, or the amount of metal removed by the cutting process. All cutting processes produce a kerf. You must account for the kerf when cutting to specific dimensions or determining the number of parts that can be cut from a piece. Factors that affect the size of the kerf include cutting speed, amperage setting, amount of standoff, and the size of the orifice in the tip.

5.2.0 Bevel Angle

As the plasma gas cuts through the metal, it has a swirling motion. As a result of this motion the arc has more energy on one side of the cut than the other. This leads to a cut that is straight on one side and has a bevel angle (typically four to six degrees) on the other side.

The direction of travel and the swirl of the gas determine which side will be straight and which side will be beveled. On a torch with a clockwise swirl (this includes all Miller torches) the straight side of the cut will be the right side of the cut in the direction of travel. Being aware of this characteristic of plasma cutting will ensure that the part being produced has straight edges while the scrap piece has the beveled edge.

5.3.0 Drag Line

Drag lines are ripples along the surface of the cut. The travel speed and amperage setting will have the most effect on the appearance of the drag lines.

5.4.0 Top Rounding

Top edge rounding is a slight rounding over of the metal at the top of the cut. It is caused by the fact that the arc is hotter at the top of the cut than at the bottom. There is usually some top edge rounding in any plasma cut part. It is most affected by material thickness and is more apparent on thicker metals.

5.5.0 Dross

Dross is re-solidified oxidized molten metal that is not fully ejected from the kerf during cutting. It is the most common cut quality problem of plasma cutting. Dross may form as a thick bubbly accumulation along the bottom edge of the plate, a small, hard bead of uncut material (high-speed dross), or a light coating along the top surface of the plate (top spatter).

Dross is affected by the material’s variables, such as thickness and type, grade, chemical composition, surface condition, flatness, and even temperature changes as the material is cut. However, the three most critical variables to consider in dross formation are cutting speed, amperage, and standoff distance.

If the cutting speed is too slow, the plasma jet begins to look for more material to cut. The arc column grows in diameter, widening the kerf to a point where the high velocity portion of the plasma jet no longer ejects the molten material from the cut. As a result, this molten material begins to accumulate along the bottom edge of the plate in a thick globular form. This is called low-speed dross. At extremely low speeds the arc extinguishes because there is not enough metal to sustain a transferred arc. Increasing the amperage or decreasing the standoff (while keeping material thickness and speed constant) have a similar effect on the cut as slowing down the cut speed. Both of these changes cause more energy from the plasma jet to contact a given area of the material in a given period of time. Excessive amperage or low standoff can also cause low-speed dross. Some low speed dross in the corners of a plasma cut is normal since velocity does not remain constant through a sharp turn.

To prevent low-speed dross form forming, increase the cut speed in 5 ipm increments, increase the standoff in 1/16-inch increments or 5 volt increments, or decrease the amperage in 10 amp increments. If none of these measures improves the cut, consider a smaller nozzle size.

If the cutting speed is too fast, the arc begins to lag back in the kerf, leaving a small, hard bead of uncut material or rollover dross along the bottom of the plate. This high-speed dross is more tenacious and usually requires extensive machining to remove. At extremely high speeds, the arc becomes unstable and begins oscillating up and down in the kerf, causing a rooster tail of sparks and molten material. At these speeds, the arc may fail to penetrate the metal or may extinguish. High standoff or low amperage (for a given material thickness and cutting speed) can also cause high-speed dross since both of these changes cause a reduction in the energy of the plasma jet.

To prevent high speed dross, first check the nozzle for signs of wear (gouging, oversize or elliptical orifice), decrease the cutting speed in 5 ipm increments, decrease the standoff in 1/16-inch increments or 5 volts increments, or increase the amperage (do not exceed 95% of the nozzle orifice rating). Top spatter is an accumulation of re-solidified metal that sprays along the top of the cut piece. It is usually very easy to remove. The usual cause is a worn nozzle, excessive cutting speed, a high standoff, or the swirling flow of the plasma jet, which at a certain angle of attack flings molten material out in front of the kerf rather than down through it.

To eliminate top spatter, check the nozzle for signs of wear, decrease the cutting speed in 5 ipm increments, or decrease the standoff in 1/16-inch increments or 5 volt increments.

Test Your Knowledge

5. What is dross?

A. Excess plasma formation
B. Resolidified molten metal
C. Angle on the top of a cut
D. Preferred shielding material

6. What is the most likely reason the plasma torch would not cut through the work piece?

A. Incorrect angle
B. Wrong shielding gas
C. Rapid torch speed
D. Inexperienced technician

5.6.0 Six Steps to Good Cut Quality

  1. Use quality consumable parts.
    You will not get quality cuts without quality parts. Use the parts recommended by the manufacturer. These parts are made to the exact tolerances required to ensure quality cuts. To further ensure quality cuts, always start with a new set of consumables.
  2. Assemble the torch properly.
    Assemble the torch carefully, making sure the parts are properly aligned and fit together snugly. This will ensure good electrical contact and cutting performance. Keep parts in their storage containers until needed in order to prevent contamination from dirt and dust.
  3. Set the amperage no higher than necessary.
    The amperage should be set based on the metal thickness being cut. Using more amperage than needed will shorten consumable life.
  4. Square the torch with the part to be cut.
    If the torch is not aligned perpendicular with the part, the cut may form a beveled edge instead of a straight edge. Check the squareness of the torch visually before starting to cut.
  5. Verify the direction of travel.
    Depending on the manufacturer and the direction of the air flow, the square side of the cut may be the right-hand side in the direction of travel. Perform a test cut to verify the location of the square edge.
  6. Adjust the travel speed.
    During the test cut, determine the travel speed that will provide either a dross-free or minimal-dross cut. Increasing travel speed will also insure that you do not have a negative bevel angle.


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As with any cutting or welding process, safety is the prime consideration. The equipment owner’s manuals will provide safety recommendations that must be followed.

The plasma arc emits intense visible and invisible radiation (ultraviolet and infrared). Operators need to be fully clothed with dark leather or woolen clothing. Ultraviolet radiation can cause rapid disintegration of cotton-based clothing.

Dark clothing reduces reflection, particularly underneath the welding helmet where reflected ultraviolet burns can occur to the face and neck.

To provide adequate protection for the eyes, use filter lenses conforming to ANSI Z49.1 (Table1).

Table1 Suggested filter glass shades for plasma.

When cutting thicker materials, it may be necessary to wear ear protection. Also, water tables are sometimes used beneath cutting tables. If a water table is used, strict guidelines must be followed to avoid such problems as hydrogen gas buildup beneath the plate being cut. This is especially the case when cutting aluminum and also when argon/hydrogen mixtures are used as the cutting gas.

The PAC process produces fumes and gases that can harm your health. The composition and rate of generation of fumes and gases depend on many factors including arc current, cutting speed, material being cut, and gases used. The fume and gas by-products will usually consist of the oxides of the metal being cut, ozone, oxides of nitrogen, and phosgene gas.

Adequate ventilation is required during the plasma arc cutting process due to the brightness of the plasma arc, which causes air to break down into ozone. These fumes must be removed from the work area or eliminated at the source by an appropriate exhaust system.

Take the proper precautions to avoid being burned by hot molten material; sparks can travel in excess of 35 feet during the cutting process. Do not wear any clothing with cuffs or uncovered pockets, and always wear the proper insulated gloves.

Handle compressed gas cylinders carefully. Secure them when stored or in use; knocks, falls, or rough handling can damage cylinders and valves, causing leakage and potential accidents.

Use the following guidance when setting up and using cylinders of gas:

  1. Properly secure the cylinder.
  2. Before connecting a regulator, purge the valve of dust and debris.
  3. When you attach a regulator to a cylinder, be sure it is in a fully closed condition. Once you have opened the cylinder valve slowly, adjust the screw on the regulator slowly until you obtain the correct pressure.
  4. When the cylinder is not in use, close the valve and the regulator.

Operators and maintenance people should keep in mind that PAC equipment operates with a higher output voltage than typical welding equipment. Always follow recommended safety procedures as outlined by the equipment manufacturer. Read Material Safety Data Sheets (MSDSs) for metals, consumables, and coatings.

Further information on safety can be found in the American Welding Society publications “Safety in Welding and Cutting, ANSI ASC Z49.1.”


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This course introduced you to the basics of plasma arc cutting, a very easy method of cutting all conductive metals, which requires very little training to use. It discussed the formation of plasma and its properties, explained the equipment used for plasma arc cutting, and gave some proper cutting techniques.

It also presented some advantages and disadvantages of plasma arc cutting over other cutting methods. The main theme of the course was to select the right size PAC for the job at hand based on the type and thickness of the metal to be cut, while keeping a constant eye on the torch consumables to ensure proper production efficiency is maintained. Finally, it cannot be overemphasized to follow all of the manufacturer’s recommended operating and safety procedures.

Review Questions

1. What is the most common form of matter in the universe?

A. Solid
B. Liquid
C. Gas
D. Plasma

2. What action is visible during an electrical arc?

A. Ionization
B. Fusion
C. Fission
D. Transpiration

3. What is responsible for the difference between the different states of the same matter?

A. Atomic weight
B. Chemical composition
C. Number of protons
D. Temperature

4. What causes atoms to break apart?

A. Intense light
B. Chemical reaction
C. Weak valence shells
D. Extremely high pressure and temperature

5. What causes the release of vast amounts of energy between electrons and ions?

A. Their collisions
B. Their velocity
C. Their atomic structure
D. The reaction of their protons

6. How is plasma produced in a plasma cutting torch?

A. High pressure gas
B. Extreme heat
C. Shielding gas
D. Chemical reaction

7. What must be created between the torch and workpiece to maintain cutting?

A. Contact
B. Air pressure
C. Electrical pathway
D. Heat transfer

8. What attribute makes plasma different from steam?

A. Conductivity
B. Directionality
C. Pressure
D. Atomic structure

9. What controls the radius of the plasma arc?

A. Voltage
B. Speed
C. Tip constriction
D. Type of electrode

10. What removes molten metal from the cut area?

A. Shielding gas
B. Plasma jet
C. Gravity
D. Skilled technician

11. What is the main reason PAC is used on aluminum?

A. It uses less energy than oxy-fuel
B. It causes little to no oxidation
C. It is easier to use
D. It results in minimal warping

12. For a plasma cutter to function on metal, what physical condition must?

A. It must be grounded
B. It must conduct electricity
C. It must be fully submerged in water
D. It must be preheated

13. What is a disadvantage of plasma cutting?

A. Its speed
B. Overall weight of the system
C. Necessity of having a source of electricity
D. Its highly flammable nature

14. Why does plasma cutting cause less workpiece distortion than oxy-fuel?

A. No preheating is required
B. A smaller torch is used
C. A smaller standoff is used
D. Plasma is not as hot as oxy-fuel

15. In the transferred arc mode where is the arc struck?

A. Between the tip and the nozzle
B. Between the electrode and the shield
C. Between the electrode and the workpiece
D. Between the tip and the workpiece

16. How do you avoid unnecessary heating of the constricting tip during cutting operations?

A. Open the pilot arc relay
B. Increase the output gas pressure
C. Decrease the output current
D. All of the above

17. What type of kerf is produced by a plasma torch?

A. Heavy dross
B. Double bevel
C. Bevel and straight
D. Double straight

18. What component does an inverter power supply use to adjust the frequency of incoming AC?

A. Transformer
B. Capacitor
C. Diode
D. Microprocessor

19. How many minutes can an 80-amp plasma arc cutter operate continuously with a duty cycle of 70%?

A. 3
B. 7
C. 30
D. 70

20. What does a rating of 104º F refer to in regard to a PAC?

A. Ambient temperature
B. Maximum operating temperature
C. Preheat temperature
D. Plasma temperature

21. How is PAC cutting speed measured?

A. Feet per second
B. Inches per second
C. Feet per minute
D. Inches per minute

22. What is the purpose of a swirl ring in a PAC torch?

A. Conduct electricity
B. Focus the plasma arc
C. Spin the compressed air
D. Control the temperature

23. What are the two most common torch systems to initiate the plasma pilot arc?

A. FM and contact
B. HF and contact
C. HF and automatic
D. CW and automatic

24. What enables current to flow across the air gap between the tip and electrode?

A. Spark
B. Gas pressure
C. Heat transfer
D. Contact with the workpiece

25. What type of torch is also known as an instant-on torch?

A. Solid state
B. Contact start
C. High frequency
D. Hafnium

26. What torch component is made of high temperature plastic?

A. Tip
B. Electrode
C. Swirl ring
D. Retaining cup

27. What type of shield is used for extended cutting applications?

A. Drag
B. Deflector
C. Directional
D. Dimensional

28. What is the recommended torch standoff of an extended tip, in inches?

A. 1
B. ½
C. ¼
D. 1/8

29. The tip size of the torch is directly proportional to what PAC characteristic?

A. Voltage
B. Amperage
C. Speed rating
D. Material composition

30. What is the recommended pierce starting position of the PAC torch in relation to the workpiece?

A. Perpendicular to the workpiece
B. Parallel to the workpiece
C. Slightly angled away from you
D. Slightly angled toward you

31. What is the PAC current selection based on?

A. Ambient temperature
B. Type of electrode
C. Thickness of the workpiece
D. PAC manufacturer

32. The PAC should be inspected at the beginning of what?

A. Project
B. Workday
C. Workweek
D. Month

33. The condition of torch consumables is directly related to what torch characteristic?

A. Cutting speed
B. Production efficiency
C. Output amperage
D. Duty cycle

34. What does a kerf refer to on the workpiece?

A. Molten material left on the bottom of the workpiece
B. Amount of material removed by the cutting process
C. Direction of travel of the gas swirl
D. Bevel angle on the edge of the cut

35. What are drag lines on the surface of the cut?

A. Ripples along the surface of the cut
B. Scratches left by the torch tip
C. Gouges left by the grounding strap
D. Measurement marks

36. What causes top edge rounding on a cut edge?

A. Torch is too close to the workpiece
B. Torch is too far away from the workpiece
C. The arc is hotter at the top of the cut
D. The arc is cooler at the top of the cut

37. How often should an electrode be replaced?

A. Once a day
B. Once every ten hours of operation
C. After significant wearing appears
D. After every job

38. Which gas is considered to produce the least dross?

A. Compressed air
B. Hydrogen
C. Argon
D. Nitrogen

39. What is the effect of oxidation on a workpiece?

A. Increases melting temperature
B. Causes the metal to warp
C. Increases welding quality
D. Causes a crystalline deposit

40. How do you determine the maximum cutting speed of a PAC torch?

A. Arc current, nozzle diameter, and metal thickness
B. Arc current, electrode material, and type of metal
C. Gas pressure, nozzle diameter, and metal thickness
D. Gas pressure, electrode material, and type of metal

41. Why is very little workpiece preparation necessary after plasma cutting?

A. Preciseness of the arc
B. The lack of chemical reactions
C. The low temperature used
D. Potential harm to material

42. What causes a bevel angle on one side of a workpiece?

A. Amperage and angle of the torch
B. Direction of swirl of the plasma gas
C. Size and standoff of the torch
D. Standoff and amperage of the torch

43. How do you correct a negative bevel angle?

A. Decrease the torch speed
B. Increase the torch speed
C. Decrease the torch angle
D. Increase the torch angle

44. What occurs to the workpiece when the cutting speed is too slow?

A. Kerf gets sharper edge
B. Less dross is formed
C. Kerf gets wider
D. Oxidation increases

45. What has the greatest effect on the appearance of drag lines?

A. Amperage and angle of the torch
B. Height and speed of the torch
C. Speed and amperage of the torch
D. Standoff and amperage of the torch

46. What ANSI standards should be followed when selecting the proper filter glass shade numbers?

A. Z49.1
B. Z59.1
C. Z69.1
D. Z79.1


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