Plasma Arc Cutting

RecommendedPractices forPlasma Arc Cuttingand Gouging

AWS C5.2:2001An American National Standard

COPYRIGHT American Welding Society, Inc.Licensed by Information Handling ServicesCOPYRIGHT American Welding Society, Inc.Licensed by Information Handling Services

550 N.W. LeJeune Road, Miami, Florida 33126

AWS C5.2:2001An American National Standard

Approved byAmerican National Standards Institute

May 4, 2001

Recommended Practices

for Plasma Arc Cutting

and Gouging

Supersedes AWS C5.2-83

Prepared byAWS C5 Committee on Arc Welding and Cutting

Under the Direction ofAWS Technical Activities Committee

Approved byAWS Board of Directors

AbstractThis revised recommended practice provides information regarding current practices for plasma arc cutting and gouging.The document explains the basic principles of operation, methods of operation, system components and their installa-tion, optimization of cut quality, and cost considerations. Safety aspects associated with the process are also discussed.

Key Words—Plasma arc cutting, gouging, constricted arc, ionizing gas, nozzle, safety


Statement on Use of AWS American National Standards

All standards (codes, specifications, recommended practices, methods, classifications, and guides) of the AmericanWelding Society are voluntary consensus standards that have been developed in accordance with the rules of the AmericanNational Standards Institute. When AWS standards are either incorporated in, or made part of, documents that areincluded in federal or state laws and regulations, or the regulations of other governmental bodies, their provisions carrythe full legal authority of the statute. In such cases, any changes in those AWS standards must be approved by thegovernmental body having statutory jurisdiction before they can become a part of those laws and regulations. In allcases, these standards carry the full legal authority of the contract or other document that invokes the AWS standards.Where this contractual relationship exists, changes in or deviations from requirements of an AWS standard must be byagreement between the contracting parties.

International Standard Book Number: 0-87171-648-8

American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126

© 2001 by American Welding Society. All rights reservedPrinted in the United States of America

AWS American National Standards are developed through a consensus standards development process that bringstogether volunteers representing varied viewpoints and interests to achieve consensus. While AWS administers the processand establishes rules to promote fairness in the development of consensus, it does not independently test, evaluate, orverify the accuracy of any information or the soundness of any judgments contained in its standards.

AWS disclaims liability for any injury to persons or to property, or other damages of any nature whatsoever, whether spe-cial, indirect, consequential or compensatory, directly or indirectly resulting from the publication, use of, or reliance on thisstandard. AWS also makes no guaranty or warranty as to the accuracy or completeness of any information published herein.

In issuing and making this standard available, AWS is not undertaking to render professional or other services for or onbehalf of any person or entity. Nor is AWS undertaking to perform any duty owed by any person or entity to someoneelse. Anyone using these documents should rely on his or her own independent judgment or, as appropriate, seek the adviceof a competent professional in determining the exercise of reasonable care in any given circumstances.

This standard may be superseded by the issuance of new editions. Users should ensure that they have the latest edition.

Publication of this standard does not authorize infringement of any patent. AWS disclaims liability for the infringementof any patent resulting from the use or reliance on this standard.

Finally, AWS does not monitor, police, or enforce compliance with this standard, nor does it have the power to do so.

Official interpretations of any of the technical requirements of this standard may be obtained by sending a request, in writ-ing, to the Managing Director Technical Services, American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126(see Annex A). With regard to technical inquiries made concerning AWS standards, oral opinions on AWS standards maybe rendered. However, such opinions represent only the personal opinions of the particular individuals giving them. Theseindividuals do not speak on behalf of AWS, nor do these oral opinions constitute official or unofficial opinions or interpre-tations of AWS. In addition, oral opinions are informal and should not be used as a substitute for an official interpretation.

This standard is subject to revision at any time by the AWS C5 Committee on Arc Welding and Cutting. It must bereviewed every five years and if not revised, it must be either reapproved or withdrawn. Comments (recommendations,additions, or deletions) and any pertinent data that may be of use in improving this standard are required and should beaddressed to AWS Headquarters. Such comments will receive careful consideration by the AWS C5 Committee on ArcWelding and Cutting and the author of the comments will be informed of the Committee’s response to the comments.Guests are invited to attend all meetings of the AWS C5 Committee on Arc Welding and Cutting to express their com-ments verbally. Procedures for appeal of an adverse decision concerning all such comments are provided in the Rules ofOperation of the Technical Activities Committee. A copy of these Rules can be obtained from the American WeldingSociety, 550 N.W. LeJeune Road, Miami, FL 33126.

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Personnel

AWS C5 Committee on Arc Welding and Cutting

J. R. Hannahs, Chair ConsultantN. A. Sanders, 1st Vice Chair Hypertherm

D. B. Holliday, 2nd Vice Chair Northrop Grumman Corp.M. O. Kulp, Secretary American Welding Society

*E. R. Bohnart Welding Education & ConsultantH. A. Chambers TRW Nelson Stud Welding Division

C. Connelly Poly-WeldJ. DeVito ESAB Welding & Cutting Products

R. M. Dull Edison Welding InstituteD. A. Fink The Lincoln Electric Co.

I. D. Harris Edison Welding Institute*R. T. Hemzacek Consultant

G. K. Hicken Sandia National LaboratoryR. P. Munz The Lincoln Electric Co.S. R. Potter SSP Consulting Services

*B. L. Shultz The Taylor Winfield Corp.R. L. Strohl Tweco-ArcairE. G. Yevick Weld-Met International Group

A. P. Yost The Lincoln Electric Co.

AWS C5J Subcommittee on Plasma Arc Cutting

N. A. Sanders, Chair HyperthermA. Oseitutu, Secretary American Welding Society

J. D. Colt Mechanized Product Manager*J. R. Hannahs Process Equipment Co.

I. D. Harris Edison Welding InstituteS. D. Hidden Miller Electric Mfg. Co.

C. Landry Centricut LLCJ. Lu W. A. Whitney Co.

M. Manohar Bethlehem Steel Corp.A. F. Manz A. F. Manz Associates

V. Nemchinsky ESAB Welding & Cutting Products*Advisor


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Foreword

(This Foreword is not a part of AWS C5.2:2001, Recommended Practices for Plasma Arc Cutting and Gouging, but isincluded for information purposes only.)

These recommended practices were first prepared by the AWS C5J Subcommittee on Plasma Arc Cutting of the AWSCommittee on Arc Welding and Cutting in 1973. The document was revised and substantially expanded in 1983. Thiscurrent revision provides additional updated information on the process principles, methods, equipment, and safety andhealth aspects.

Comments and suggestions for the improvement of this standard are welcome. They should be sent to the Secretary,AWS C5 Committee on Arc Welding and Cutting, American Welding Society, 550 N.W. LeJeune Road, Miami, FL33126.

Official interpretations of any of the technical requirements of this standard may be obtained by sending a request, inwriting, to the Managing Director, Technical Services Division, American Welding Society (see Annex A). A formalreply will be issued after it has been reviewed by the appropriate personnel following established procedures.


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Table of Contents

Page No.

Personnel .................................................................................................................................................................... iiiForeword .......................................................................................................................................................................vList of Tables................................................................................................................................................................ixList of Figures ..............................................................................................................................................................ix

1. General ..................................................................................................................................................................11.1 Scope.............................................................................................................................................................11.2 Introduction...................................................................................................................................................11.3 General Process Description .........................................................................................................................11.4 Principles of Operation .................................................................................................................................31.5 Individual Process Descriptions....................................................................................................................31.6 Commonly Processed Base Metals...............................................................................................................61.7 Safety and Health..........................................................................................................................................7

2. Applicable Documents ..........................................................................................................................................7

3. Plasma Arc Cutting and Gouging Methods...........................................................................................................73.1 Manual Plasma Arc Cutting..........................................................................................................................73.2 Mechanized Plasma Arc Cutting ..................................................................................................................83.3 Deep Underwater Plasma Arc Cutting..........................................................................................................83.4 Plasma Arc Gouging.....................................................................................................................................9

4. Equipment, Consumables, and Gases ...................................................................................................................94.1 Torches ..........................................................................................................................................................94.2 Arc Initiation Methods................................................................................................................................114.3 Power Sources.............................................................................................................................................114.4 Controls.......................................................................................................................................................124.5 Electrodes and Nozzles (Consumable Hardware).......................................................................................134.6 Cutting Machines ........................................................................................................................................134.7 Pollution and Hazard Control .....................................................................................................................184.8 Plasma Gas Selection..................................................................................................................................20

5. Installation Considerations..................................................................................................................................225.1 Site Selection and Planning ........................................................................................................................225.2 Electrical Power and Safety Grounding......................................................................................................225.3 Gas Supply ..................................................................................................................................................225.4 Water Supply...............................................................................................................................................22

6. Optimizing Plasma Arc Cut Quality ...................................................................................................................236.1 Cut Angle ....................................................................................................................................................236.2 Dross ...........................................................................................................................................................246.3 Metallurgical Effects...................................................................................................................................256.4 Kerf Width ..................................................................................................................................................266.5 Quality Improvement Suggestions..............................................................................................................26

7. Economics ...........................................................................................................................................................277.1 Plasma Arc Cutting Equipment ..................................................................................................................277.2 Accessory Equipment Costs .......................................................................................................................27


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7.3 Operational Costs........................................................................................................................................277.4 Process Comparisons ..................................................................................................................................27

8. Safety and Health Hazards ..................................................................................................................................298.1 Electrical Shock ..........................................................................................................................................298.2 Explosion ....................................................................................................................................................298.3 Fumes and Gases ........................................................................................................................................308.4 Noise ...........................................................................................................................................................308.5 Radiated Light.............................................................................................................................................318.6 Radiated Heat/Spatter and Sparks...............................................................................................................318.7 Chlorinated Solvents ...................................................................................................................................31

Annexes—Nonmandatory InformationAnnex A—Guidelines for Preparation of Technical Inquiries for AWS Technical Committees..................................33Annex B—Terms and Definitions ................................................................................................................................35Annex C—Selected Reading List ................................................................................................................................39

AWS List of Documents on Arc Welding and Cutting .................................................................................................41


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List of Tables

Table Page No.

1 Recommended Plasma/Shield Gas Combinations .......................................................................................102 Conventional Plasma Arc Kerf Widths for Mild Steel Plate........................................................................263 Recommended Shade Densities for Plasma Arc Cutting Filter Lenses .......................................................31

List of Figures

Figure Page No.

1 Plasma: The Fourth State of Matter ...............................................................................................................22 Temperature Differences Typical GTAW vs. Typical PAC ............................................................................23 Plasma Arc Torch Circuitry ...........................................................................................................................34 Single-Gas Plasma Arc Cutting .....................................................................................................................45 Dual-Flow Plasma Arc Cutting ......................................................................................................................46 Water Injection Plasma Arc Cutting ..............................................................................................................57 Manual Plasma Arc Cutting...........................................................................................................................88 Different Methods to Achieve High-Current Density..................................................................................119 Gantry Crane Cutting Machine ....................................................................................................................14

10 Punch/Plasma Press......................................................................................................................................1511 Plasma Cutting Robot—Classic Six-Axis Design .......................................................................................1512 Cantilever Cutting Machine with Optical Tracing Head..............................................................................1613 Sample Part Program....................................................................................................................................1714 Water Muffler ...............................................................................................................................................1815 Underwater Plasma Cutting .........................................................................................................................1916 Collection Cup System—Wet Type .............................................................................................................2017 General Features of Plasma Arc Cut............................................................................................................2318 Direction of Cut with Clockwise Swirl........................................................................................................2419 Typical Dross-Free Window for 200 Amp Oxygen Plasma Cutting Process on Mild Steel........................2520 Typical High- and Low-Speed Dross and Drag Lines .................................................................................2621 Typical Cutting Cost Breakdown—200 Amp O2 Plasma Cutting Operation ..............................................2822 Comparison of Cost—200 Amp Plasma Arc Cutting vs. Oxyfuel Cutting .................................................28


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1. General

1.1 Scope. This document describes the recommendedpractices of plasma arc cutting and gouging. It provides abrief history of its application process development andprovides a general description of components used for theprocess, controls, power supplies, and process consum-ables. The types of cutting machines and the related con-trol systems are described, along with associated pollutionand hazard control methods. Methods of optimizing cutquality and economics associated with the process are dis-cussed. Safety and health aspects associated with theplasma arc cutting and gouging process are discussed.

These recommended practices, prepared by the AWSC5J Subcommittee on Plasma Arc Cutting of the AWSCommittee on Arc Welding and Cutting, are intended toguide plasma arc operators and supervisors in the use ofplasma arc cutting and gouging equipment. These rec-ommended practices are not intended to take the place ofsound engineering principles and should be supple-mented with the equipment manufacturers operation andsafety instructions.

This standard makes use of the U.S. Customary Units.Approximate mathematical equivalents in the InternationalSystem of Units (SI) are provided for comparison in paren-theses ( ) or in appropriate columns in tables and figures.

1.2 Introduction. A nontechnical description of plasmais “the fourth state of matter,” the three familiar statesbeing solid, liquid, and gas. For a common substancesuch as water, these three states are ice, water, and steam.When energy in the form of heat is added to ice, the icemelts and forms water, as shown in Figure 1. When moreenergy is added, the water vaporizes into steam. By add-ing more energy to steam the water vapor will disassoci-ate into hydrogen and oxygen. Additional energy willcause the hydrogen and oxygen to become ionized.These ionized gases are electronically conductive. Theresulting high-temperature conductive gas is known asplasma.

1.3 General Process Description. Plasma arc cutting(PAC) is a process that severs metal by using the heat of anozzle-constricted plasma arc to melt a localized area.The molten metal is removed by a high-velocity streamof high-temperature, ionized gas. Plasma arc gouginguses the same process to remove material from the sur-face of a plate without fully penetrating the workpiece.Welding research scientists discovered in the early 1950sthat the characteristics of the open arc used in gas tung-sten arc welding (GTAW) could be altered to performcutting operations instead of welding. The arc was con-stricted by directing it through an orifice in a water-cooled copper nozzle located between the electrode andthe workpiece, which greatly increased the arc’s powerdensity. They discovered that constricting the arc into awell-focused column raised the arc’s voltage signifi-cantly, making it more concentrated than the open arcused for welding.

Figure 2 shows arcs operating at the same current.The plasma jet on the right is only moderately con-stricted by the nozzle, but it operates at about twice thevoltage and it acts considerably hotter than the open arc(GTAW) on the left. Even higher energy intensity can beachieved by constricting the arc with a boundary layer ofnonionized gas inside the nozzle bore. The effectivenessof this boundary layer can be increased by swirling theplasma gas and raising the flow rate, which forces thecool, non-ionized gas outward radially into a thickerboundary layer. This protects the nozzle orifice allowingit to operate at higher energy levels and prolonging itslife.

The characteristics of the plasma jet vary according tothe gas flow rate, nozzle orifice size and arc current, aswell as the type of gas used (gases are discussed in 4.8).A low flow rate, for example, reduces the jet’s momen-tum and forms a highly concentrated heat source idealfor welding, but inadequate for cutting or gouging. Con-versely, if the gas flow is increased sufficiently, themomentum of the plasma jet will be high enough toexpel the metal melted by the heat of the plasma arc,resulting in gouging or cutting.

Recommended Practices forPlasma Arc Cutting and Gouging


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Figure 1—Plasma: The Fourth State of Matter

Figure 2—Temperature Differences Typical GTAW vs. Typical PAC


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1.4 Principles of Operation. The circuitry of the basicplasma arc system is shown in Figure 3. The operatingcycle begins by causing the plasma gas to flow aroundthe electrode and out through the orifice in the nozzle.A DC open circuit potential of 250 V–400 V (negativepolarity) is applied to the electrode, with the workpiecebeing at ground potential. After an ionized path is cre-ated between the electrode and the nozzle (see “Arc Initi-ation Methods” in 4.2), a main plasma arc (transferred arc)is then generated between the electrode and the work-piece. The cutting or gouging process can then proceed.

1.5 Individual Process Descriptions

1.5.1 Conventional Plasma Arc Cutting. The firstplasma arc cutting process, developed during the 1950s,could cut any metal at relatively high cutting speeds. Dis-advantages of early conventional plasma arc cuts in-cluded rough, angled cut surfaces and short electrode andnozzle life. Originally limited to mechanized cutting,conventional plasma arc cutting system designs havebeen significantly refined. Conventional plasma arc cut-ting systems are now available for both mechanized andhand-held cutting systems that employ single-gas, multiple-gas, and other process variations. There are now a number

of different types of plasma arc cutting systems. Descrip-tions of the different systems follow, listed in their orderof chronological development.

1.5.2 Single-Gas Plasma Arc Cutting. A schematicof the basic single-gas torch is shown in Figure 4. Afterthe plasma gas flows through the nozzle plenum whichsurrounds the electrode, and exits the nozzle’s orifice, theelectrode receives a DC voltage. An ionized path is createdbetween the electrode and the nozzle by a pilot arc orsome other means. A main plasma arc is then generatedbetween the electrode and the workpiece, after which thecutting or gouging process can proceed.

1.5.3 Dual-Flow (Shielded) Plasma Cutting. Thedual-flow technique, developed in the early 1960s, addeda second, outer nozzle (“shield cup”) to provide a pas-sageway for a concentric flow of a shielding gas aroundthe plasma gas. This is illustrated in Figure 5. Nitrogen,air, carbon dioxide, argon, oxygen and argon-hydrogenmixtures can be used as a shielding gas. The selection ofthe shielding medium is based on the type of metal to becut or gouged, the metallurgical properties desired for thefinished edge, and the physical properties of the edge, suchas dross, squareness (bevel), smoothness, and flatness.

Notes:R = ResistanceS = SwitchHFI = High-Frequency Ignition Source

Figure 3—Plasma Arc Torch Circuitry


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Figure 4—Single-Gas Plasma Arc Cutting

Figure 5—Dual-Flow Plasma Arc Cutting


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The main advantages were reduced double-arcing,improved consumables life, faster cutting speeds on mildsteel and improved cut surfaces by countering differentmetals’ chemical reactive tendencies by a variety ofgases available for shielding. Cut quality was not signifi-cantly improved, however, and cutting speeds on stain-less steel and aluminum were essentially the same as thatachieved by conventional, single-gas plasma arc cutting.Modern dual-flow systems are available for either mech-anized or manual cutting.

1.5.4 Air Plasma Arc Cutting. Researchers in theearly 1960s discovered that the oxygen content of aircould increase mild steel cutting speeds by 25%, whichwould reduce operating costs. But there were disadvan-tages with oxidizing plasma gases: electrode life wasvery short, and oxidized cut surfaces were a disadvantagein some cases. Air plasma arc cutting systems did notgain wide use until the 1980s, when electrode improve-ments and cutting at lower currents helped overcomethese problems.

1.5.5 Water Shield Plasma Arc Cutting (Mecha-nized only). This mid-1960s variation on dual-flowmechanized plasma arc cutting substituted water for the

shield gas. The cooling effect of the water improved noz-zle life and the appearance of the cut surfaces (mainly onstainless steels), but squareness, speed and the tendencyto form dross along the kerf were not measurablyimproved.

1.5.6 Water Injection Plasma Arc Cutting (Mecha-nized only). In the late 1960s, a process was developedthat used water to constrict the arc as well as to shield it.The water strikes the arc as a high-velocity spray, eitherradially or in a tangential (swirling) manner, as illus-trated in Figure 6. Greater arc constriction can beachieved through water injection than by the nozzlealone. This advance increased cut speeds and squareness,and eliminated dross on mild steel.

Nitrogen became the preferred gas for water-injectionplasma arc cutting of all metals, offering good parts life,eliminating double-arcing, and improving squarenessand cutting performance. Air or oxygen may also be usedas the plasma gas. The cooling provided by the injectionwater permits the use of a ceramic material in the lowerportion of the nozzle. This feature extends nozzle life,improves piercing capability of the torch, and acts as abarrier against double-arcing between the nozzle and theworkpiece.

Figure 6—Water Injection Plasma Arc Cutting


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1.5.7 Low-Current Air Plasma Arc Cutting. Themain disadvantage of short consumable parts life for airplasma arc cutting was overcome in the early 1980s withthe introduction of lower current air plasma systems.These plasma systems were typically 100 amps or lessand gas cooled. Many companies introduced smaller,easier-to-operate, less expensive systems—includinghand-held cutting systems—and helped plasma arc cut-ting become the process of choice for the first time inmany industries.

1.5.8 Oxygen Plasma Arc Cutting (Mechanizedonly). Oxygen plasma arc cutting’s major disadvantage—unacceptably short parts life—was overcome in the early1980s, with the introduction of the water-injected oxygenplasma arc cutting process for cutting mild steel. Cuttingspeeds were significantly increased and lower cuttingcurrent levels produced smooth, square cuts that could beeasily welded without rework. The use of hafnium andzirconium electrodes allowed the use of oxygen for cut-ting mild steel. The virtual elimination of dross, high-cutspeeds, and cut surface quality outweighed the shorterthan desired consumable parts life.

1.5.8.1 Extending Consumables Life for OxygenPlasma Arc Cutting Techniques. Plasma arc cuttingsystems manufacturers’ R&D efforts concentrated onmaking longer-lasting consumable parts for air and oxy-gen plasma arc cutting during the late 1980s. Becausemild steel cutting dominates the industry, and oxygen-based plasma arc cutting provides the fastest method forcutting mild steel, interest in air and oxygen plasma arccutting has always been high. Plasma system manufac-turers achieved extended electrode and nozzle life withtechniques that control the cutting current and gases farmore precisely than previous processes. Plasma arc cut-ting operators using systems that incorporate these newtechniques can greatly increase their productivity and re-duce the cost of cutting: they use fewer parts and reducedowntime by changing parts less frequently.

1.5.9 High-Current-Density Plasma Arc Cutting(Mechanized only). The plasma arc cutting industry’sresponse to laser cutting came in the early 1990s with theintroduction of a new generation of dual-flow plasma arccutting that offered cleaner, squarer, more precise cuttingthan conventional, shielded, or injected plasma arc cut-ting processes. These precision systems have some capa-bilities and characteristics in common. Each involves“super-constriction” to increase the arc’s current densityand reduce the kerf width, as well as precise control bythe system over the gas flow and current to extend con-sumable parts life.

These dual-gas systems can cut all metals well, usingoxygen, air or nitrogen as the plasma gas, and oxygen,air, nitrogen, methane, propane, carbon dioxide and com-

binations as shield gases. Key characteristics of torch as-semblies, particularly alignment of the consumable parts,are designed with tighter tolerances than other types ofplasma systems. The advantages of high-current-densityplasma arc cutting include narrow kerfs, square, dross-free cuts, long consumable parts life and a choice ofshield gases to produce uncontaminated cuts.

While not comparable to all laser cutting, high-current-density plasma arc cutting fills an important middleground between plasma arc and laser cutting. In metalsup to 1/2 in. (13 mm) thick, high-density plasma arc cut-ting significantly outperforms conventional plasma arccutting in terms of cut edge quality. The cut edge qualityis comparable to that of laser cutting. High-current densityplasma arc cutting systems represent lower purchase andmaintenance costs than laser cutting systems, and requireless operator training than laser cutting systems.

1.6 Commonly Processed Base Metals. Although mostconductive metals can be cut, mild steel, stainless steel,and aluminum alloys represent the majority of materialsprocessed in the worldwide welding and cutting indus-tries. The rapid increase in plasma arc cutting that startedin the 1980s has been due to its ability to cut most mildsteel—the single dominant metal—faster and more eco-nomically than oxyfuel gas cutting, and its ability to cutstainless steel or aluminum alloys, which oxyfuel gascutting cannot cut.

1.6.1 Mild Steel. Prior to the 1980s, most mild steelplate was cut using oxyfuel processes, for two main rea-sons: the first was that, even though conventional plasmaarc cutting offered higher cutting speeds, it also requireda sacrifice in cut quality. The second reason—perhapsmore significant—was that many of the shape tracingmachines could not operate reliably at the higher speedsat which the plasma arc cutting process was capable ofcutting.

Since then, the quality of plasma arc cuts has beengreatly improved, and the introduction of numericallycontrolled shape cutting machines has made high-speedplasma arc cutting routine.

As a result, the most rapid plasma arc cutting growthhas been in mechanized cutting of steel using air or oxy-gen as the plasma gas. The high-cutting speeds and cutquality achievable with air and oxygen plasma arc cut-ting, plus the capabilities of computer numeric control(CNC) cutting machines, contribute to increased produc-tivity at high quality levels.

The cost of cutting with plasma, oxyfuel, and lasersystems is discussed in Section 7, Economics.

1.6.2 Stainless Steel, Nickel and Cobalt Base andAluminum Alloys. Another reason for the rapid growthof plasma arc cutting is the increased use of stainlesssteel, nickel and cobalt base and aluminum alloys in


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manufacturing industries. The plasma arc process cutsstainless and aluminum alloys faster and more economi-cally than other methods—and oxyfuel gas cutting meth-ods cannot cut these important metals without specialtechniques. For example, oxyfuel gas cutting can cutstainless steel, by a process of adding metal powder tothe gas in the torch, a method that is seldom used.

1.7 Safety and Health. Plasma arc cutting and gougingsystems involve high-voltage electrical current and com-pressed gases that pose significant safety risks, if im-properly handled. The potential hazards specificallyassociated with plasma arc cutting and gouging includeelectrical shock, fumes and gases, noise, and ultravioletradiation, as well as such obvious hazards as fires andburns from the heat of the plasma arc.

Other, less obvious, potential hazards exist, such ashigh-pressure gas and water supplies that can cause per-sonal injury or damage to nearby objects if leaks or otherunexpected conditions arise. Follow established guidelinesfor the safe handling of process gas mixtures containinghydrogen and oxygen, which can present explosion andfire hazards.

Proper instruction and training for system operators isessential for safety in the workplace. Plasma arc cuttingand gouging operators should always read and under-stand the manufacturer’s safety and operating instruc-tions before operating the equipment. See Section 8 fordetailed safety information on plasma arc cutting andgouging.

2. Applicable DocumentsThe following standards have either been cited in this

standard or deemed to contain information that would beuseful in meeting the requirements of this standard.

The following AWS publications are available through:

Global Engineering Documents15 Inverness Way EastEnglewood, CO 80112-5776

(1) AWS A5.32/5.32M, Specification for WeldingShielding Gases

(2) AWS C5.10, Recommended Practices for Shield-ing Gases for Welding and Plasma Arc Cutting

(3) AWS F2.2, Lens Shade Selector Chart(4) AWS Safety and Health Fact Sheets(5) ANSI Z49.1, Safety in Welding, Cutting, and Al-

lied Processes

Other Standards:

(1) NFPA 70, National Electrical Code®

(2) NFPA Standard 51B, Standard for Fire PreventionDuring Welding, Cutting, and Other Hot Work

Available Through:National Fire Protection AssociationOne Batterymarch ParkP.O. Box 9101Quincy, MA 02269-9101

(3) OSHA Safety and Health Standards, 29CFR Part1910

Available Through:Occupational Safety and Health Administration200 Constitution Avenue NWWashington, DC 20210

3. Plasma Arc Cutting and Gouging Methods

3.1 Manual Plasma Arc Cutting. Manual plasma arccutting systems are designed to handle all position cut-ting jobs unsuited to mechanized cutting systems. Theportability of manual plasma arc cutting systems is obvi-ously an advantage for cutting jobs at construction sitesor for repair and maintenance assignments. Some plasmaarc cutting systems are designed specifically for manualoperation (see Figure 7). Maximum arc current is typi-cally 200 A or less, since higher arc currents usually ra-diate too much heat for the operator. Plasma arc cuttingsystems with hand-held torches have made significantgains over oxyfuel gas cutting as users have learnedabout the advantages of plasma arc cutting.

The widespread use of compressed air as the plasmagas has expanded manual plasma arc cutting by makingthe process more economical and convenient. For mildsteel application, hand-held plasma arc cutting providesan attractive alternative to manual oxyfuel gas cutting.Additionally, the plasma arc cutting process can cutstainless steel, aluminum alloys and other nonferrousmetals, which gives the process an important advantageover oxyfuel gas cutting.

Hand-held plasma arc cutting torches differ frommechanized plasma arc cutting torches only in size andshape; the basic technology is the same in manual andmechanized systems. Manual plasma arc cutting is notdifficult to learn, and in many ways is easier to use thanoxyfuel gas cutting. For instance, although standoff,(also known as torch-to-work distance) is required formost brands of plasma arc cutting and for all oxyfuel gascutting—some manufacturers’ designs allow the operatorto contact and actually drag the torch on the workpiecewithout destroying the torch front-end parts. In addition,plasma arc cutting’s higher cutting speeds help increaseoperator efficiency and performance.


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All manufacturers of plasma arc cutting equipmentprovide procedure guidelines for manual plasma cuttingwith their equipment, including the type and thickness ofmaterial that can be cut, appropriate power source outputsettings, torch travel speeds, and standoff distances ifrequired. In mild steel less than 2 in. (51 mm) thick,manual plasma arc cutting is faster than oxyfuel gas cut-ting, and oxyfuel gas cutting cannot cut aluminum at all,or stainless steel without using a special powder addition.The quality of the cut, however, is influenced as much bythe skill of the operator as by the system capabilities. Avariety of aids are available for operators, such as circle-cutting attachments, angle-cutting guides, torch guides tostabilize standoff, and templates. A hand-held torch canalso be used in conjunction with motorized cutting at-tachments for speed and standoff control, as discussed in4.3.

For the best combination of cut speed and quality, sizethe system or set the current according to the thickness ofthe metal to be cut. For example:

• up to 1/2 in. (13 mm) can be cut manually at 20 A to70 A

• greater than 1/2 in. (13 mm) can be cut above 70 A

These are the approximate thicknesses that manualplasma arc cutting systems can cut when set at maximumoutput current. See Figure 19 for typical speeds vs. mate-rial thickness for a 200 A cutting process. It should benoted that a manual system can pierce metal approxi-mately twice as thick as a mechanized system of similaroutput, since a manual torch can be easily tilted for pierc-ing. Edge starts, which eliminate piercing, allow eithermethod to cut thicker workpieces. Each manufacturerprovides safety instructions concerning power source op-eration and safe plasma arc cutting practice. The manu-facturer should be contacted if there is any questionabout equipment operation. Safety issues associated withplasma arc cutting are also discussed in Section 8 of thisdocument.

3.2 Mechanized Plasma Arc Cutting. MechanizedPlasma systems are designed for automated cutting ofmetals and can be used for a variety of cutting jobs fromlow to very high-volume production. Mechanized plasmasystems can be mounted on a variety of equipment, in-cluding X-Y cutting machines, industrial robots, pipecutting machines, and specialized production fixtures.

Section 4 describes components, equipment and re-quirements for mechanized plasma cutting.

3.3 Deep Underwater Plasma Arc Cutting. Speciallymodified plasma cutting systems have been used in deepunderwater cutting operations. The primary applicationto date has been the repair and/or dismantling of radio-active components of nuclear reactors. Water is used inthe nuclear industry as a shield which protects the envi-ronment from radioactive emissions. In order to work onradioactive materials it is sometimes necessary to sub-merge equipment into this protective water shield.

Manual plasma systems have been used by profes-sional divers in the disassembly and maintenance of radio-active components of nuclear reactors in Europe, Japanand the United States. The primary modification made tothe manual plasma systems has been to increase the elec-trical isolation of the hand torch and the leads from thepower supply to the torch. Divers typically wear “Dry”type diving suits and typically work in depths of less than50 ft.1 General safety concerns for divers using high-voltage

1. J. Lawton, “Underwater Diver Operated Plasma Cutting in theDecommissioning Nuclear Power Plants,” Underwater Construc-tion Corp., Essex, CT, 1998.

Figure 7—Manual Plasma Arc Cutting


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electrical devices in water have limited the number ofapplications outside of the nuclear area.

Mechanized plasma systems have been used in nu-clear dismantling operations to sever and cut into piecesof thick (3 in.–4 in.) radioactive stainless steel compo-nents of nuclear reactors at depths up to 30 ft. The pri-mary modification to the mechanized plasma systemshas again been to increase the electrical isolation of thetorch and leads. In addition, companies have developedspecially angled torches and torch remote manipulationdevices for specific cutting tasks.2, 3, 4, 5

For more information about the use of plasma cuttingsystems in deep underwater applications contact plasmacutting system manufacturers.

3.4 Plasma Arc Gouging. Plasma arc gouging is a varia-tion of plasma arc cutting that removes metal from thesurface of the plate without full penetration. Gouging isused in fabrication, manufacturing, repair, maintenance,and many other industrial applications, but compared tocutting, gouging is a relatively minor plasma arc applica-tion. The torch is tilted for gouging so the heat of theplasma arc melts a zone on top of the workpiece, aheadof the torch. The gas flowing through the nozzle blowsthe molten metal forward and creates a groove, or gouge,in the workpiece. Conversion from cutting to gouging isusually simple. A special gouging nozzle is required andother torch front end parts may have to be changed,which produces less orifice constriction and current den-sity, producing a “softer” arc, which lowers the quantityof fumes produced and improves the quality of thegouged surface.

The plasma and shielding gas flows require adjust-ment, and the torch must be tilted, typically about 35 de-grees from the surface of the workpiece. The same gasesused for plasma arc cutting may be used for gouging,with the same criteria for selection (type of metal, sur-face quality desired, etc.). For gouging carbon steel, thelow cost, effectiveness, and universal availability of com-pressed air make it the primary plasma gas choice. Al-though various gases can be used for plasma arc gouging,

2. C. L. Child, M. S. McGough, “Segmentation of the YankeeReactor Internals,” Nuclear Plant Journal, Sept–Oct 1995.3. M. S. McGough, W. E. Austin, G. J. Knetl, “Performance ofthe Automated Cutting Equipment System During the PlasmaCutting of the Three Mile Island Unit 2 Lower Core SupportAssembly,” Nuclear Technology, Vol. 87, Nov 1989.4. M. S. McGough, W. E. Austin, H. Kwech, T. J. Litka,“Robotic Delivery System for Plasma Cutting of the TMI-2Lower Core Support Assembly,” ANS Annual Meeting, LosAngeles, CA, Nov 1988.5. Editors K. Pflugrad et al., “Decommissioning of NuclearInstallations,” ISBN 1-85166-523-4, Elsevier Applied Sciences,1989.

the most common plasma gas for stainless steel and non-ferrous materials is H35 (a 65% argon, 35% hydrogenmixture, AWS A5.32/5.32M designation SG-AH-35),which works well for gouging most alloys. With dual-gassystems, the shield gas can stabilize the plasma gougingarc, shield the groove and molten metal from the atmo-sphere, and aid in blowing metal from the groove. Argon,nitrogen and air are commonly used for shielding. Moreinformation regarding shielding gases can be found inAWS A5.32/5.32M, Specification for Welding ShieldingGases, and AWS C5.10, Recommended Practices forShielding Gases for Welding and Plasma Arc Cutting.See Section 2 for information on where to obtain thesedocuments and 4.8 of this document for gas selection.

Plasma arc gouging has several advantages over aircarbon arc gouging, its nearest process competitor. Al-though plasma systems generally cost more than carbon-arc systems,

(1) the plasma arc process works equally well withcarbon steel, stainless steel, aluminum alloys, and mostother metals;

(2) inert process gases produce bright, clean surfacesthat have no carbon contamination and require little re-work;

(3) the quantity of fumes produced by the plasma pro-cess is less.

(4) noise levels generated by plasma arc gouging, al-though fairly high, are considerably lower than for aircarbon arc gouging.

4. Equipment, Consumables, and Gases

4.1 Torches. There are four basic types of plasma arccutting torches, each distinguished by variations in themethod of constricting the arc:

(1) Single-gas (conventional plasma arc cutting)torches

(2) Dual-flow (shielded plasma arc cutting) torches(3) Water-injected torches(4) High-current density plasma arc cutting torches

4.1.1 Single-Gas Torches. In a single-gas plasmatorch—also known as “conventional plasma,” the elec-trode’s arc ionizes the plasma gas that surrounds it toform a plasma jet (see Figure 4). When the jet is con-stricted by passing through the nozzle orifice, its temper-ature and arc current density increase and the arc iselongated in a straight stream.

The electrode generally consists of a hafnium or thori-ated tungsten insert (emitter) embedded in a copperbody. Hafnium electrodes are used with air or oxygencutting gases; thoriated tungsten electrodes are used withnon-oxidizing cutting gases (used with non-oxidizing


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cutting gases such as nitrogen and H35). Electrodes andnozzles must be kept cool, either by gas or water cooling.

Single-gas plasma arc cutting or gouging is suitablefor manual and mechanized systems.

4.1.2 Dual-Flow (Dual-Gas and Water Shield)Torches. This design incorporates the same features asconventional or single gas plasma arc cutting, except thata shielding gas (secondary gas) flows around the nozzlethrough a shield cup or cap, as shown in Figure 5. Theshield gas covers the cut zone to improve performanceand cools the nozzle and shield cap. The major advantageof dual-gas cutting is that a different gas from the plasmagas can shield the plasma and the cutting zone, whichcan reduce or eliminate cut-surface contaminations. SeeTable 1 for recommended plasma and shield gases com-binations and cut quality information for various materi-als. Another advantage of the dual-flow process is thatthe nozzle can be recessed within an outer cap, prevent-ing the nozzle from contacting the workpiece and reduc-ing double arcing. Double arcing usually occurs when anunprotected nozzle is destroyed by connecting the nozzleto ground potential via the molten spatter produced bypiercing, having the workpiece tip up, or other unex-pected events.

Water shield plasma arc cutting is a variation of themechanized dual-flow plasma arc cutting process inwhich water is substituted for the shield gas. Watershielding does not provide additional arc constriction,but does improve cut appearance of stainless steels andincreases nozzle life due to the cooling action of the water.

4.1.3 Water Injection. In the water injection plasmaarc cutting process, water is injected around the arc, ei-ther as a high-velocity radial spray, or tangentially, to im-part a swirling action (see Figure 6). The impingement ofthe water around the arc provides a higher arc constric-tion than the conventional process, improving cut square-ness, increasing cutting speeds, and minimizing drossformation. The cooling action of the injected water per-mits the use of a ceramic section, which nearly elimi-nates double arcing between the nozzle and workpieceand facilitates piercing operations. This process is usedonly with mechanized cutting applications, and generallyfrom 250 A to 750 A.

4.1.4 High-Current Density Plasma Arc CuttingTorches. These plasma arc cutting torches produce an ex-tremely high-current density. They cut with narrower kerfwidths, using a much tighter arc constriction than is possi-ble with other plasma arc processes. This class of plasmaarc torches employs a variety of techniques to achievea super-constricted arc and excellent performance. Estab-lished techniques are high-flow vortex nozzles, high-velocity mixing chambers, and the use of a magnet forstabilizing the arc (see Figure 8). High-current densityplasma arc cutting systems can actually produce laser-likecut quality in the proper application.

High-current density plasma arc cutting is generally adual-flow process, using oxygen or air plasma gases anda variety of shield gases for cutting almost any metal upto 1/2 in. (13 mm) with excellent cut-edge qualities. Cur-rent levels range from 15 A–100 A or more.

Table 1Recommended Plasma/Shield Gas Combinations

Material Air/Air O2/Air N2/CO2 N2/Air H35/N2

MildSteel

Most EconomicalGood Cut Quality

Good SpeedGood Gouging

Good Weldability

Best Cut QualityMaximum Cut Speed

Best Weldability

Some DrossLong Electrode Life

NotRecommended

Best GougingLong Electrode Life

Some Dross

Stainless Steel

Most EconomicalGood SpeedSome Dross

NotRecommended

Good Cut QualityGood GougingMinimal Dross

Long Electrode Life

Long Electrode LifeLowest Shield Gas

Cost

Best Cut QualityBest GougingMinimal Dross

Long Electrode LifeCuts Thicker Material

Aluminum Most EconomicalGood SpeedSome Dross

NotRecommended

Good Cut QualityGood GougingMinimal Dross

Long Electrode Life

NotRecommended

Best Cut QualityBest GougingMinimal Dross

Long Electrode LifeCuts Thicker Material


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High-current density plasma arc cutting systems canbe utilized to their full cut quality potential only on high-performance mechanized cutting machines. They are notintended for gouging applications.

4.2 Arc Initiation Methods. Various methods are usedto produce the ionized path that must be created beforethe main plasma arc can be generated. These methods in-clude contact starting, discharge starting without pilotarc, and discharge starting with pilot arc.

4.2.1 Contact Starting. Contact starting is a simplemethod used on some types of manual torches, in whicha voltage is applied between the electrode and the nozzle.The electrode and the nozzle are brought into momentaryelectrical contact by pushing the torch tip against thework. A low-current pilot arc is generated between theelectrode and nozzle after a spring in the torch forces theelectrode and nozzle apart.

In a different type of contact starting torch, the elec-trode and nozzle are forced together by spring pressure.The power supply is turned on and current flows throughthe electrode and nozzle in a short circuit condition.Then a solenoid valve opens to let the plasma gas pres-sure force the electrode and nozzle apart, which gener-ates a low-current pilot arc.

In both cases, the low-current pilot arc heats theplasma gas significantly, making the plasma gas moreconductive. The highly-ionized gas flows out through thenozzle orifice, and the main arc is generated (transferred)when the workpiece is close enough to the torch for cur-rent to flow from the electrode to the workpiece. Cuttingor gouging can begin when the pilot arc turns off auto-matically and the main arc current is increased.

4.2.2 Discharge Starting Directly to the Workpiece.Discharge starting, another method to start a manualplasma arc cutting system, requires that the nozzle befirst brought into contact with the workpiece, after whicha spark or high-frequency discharge between the elec-trode and the nozzle is used to ionize the plasma gas. Themain arc is generated directly between the workpiece andthe electrode and cutting or gouging can begin.

4.2.3 Discharge Starting with a Pilot Arc. Dischargestarting with a pilot arc is the most common method ofstarting all types of plasma arc cutting torches. Theplasma arc power source generates a high voltage with ahigh-frequency oscillation. This high frequency is dis-charged between the electrode and the nozzle, ionizingthe plasma gas. A pilot arc is then established between theelectrode and the nozzle and is maintained for short pe-riod of time necessary for the arc to transfer to the work-piece. Once this main arc is established, the pilot arc isturned off and the cutting or gouging operation can begin.

4.3 Power Sources. Most plasma arc power sources con-tain the power components and control circuitry neces-sary to provide power, gas flow, and coolant to the torchin the proper sequence. Some power sources can alsomonitor the consumables’ condition and automaticallychange current, gas or other operating parameter settingsbased on signals from the cutting machine’s CNC.

Power sources have a constant current DC output andusually exhibit a drooping volt-amp curve, with open circuitvoltage between 250 V and 400 V. Stable output currentrequires high arc, or cutting, voltage—in the 100 V–250 Vrange.

Figure 8—Different Methods to Achieve High-Current Density


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Most power supplies fall into four basic categories:transformer-rectifier, silicon controlled rectifier (SCR),chopper, and inverter. The SCR unit has been popularsince the 1980s, but the chopper and inverter types showsome strong advantages over SCR types.

Chopper and inverter models have high-speed semi-conductor switching techniques that provide a more con-stant current volt-amp curve, low levels of DC ripple, andhave very fast response times, which yield superior per-formance. Inverter units have the additional advantage ofsignificantly reduced size and weight.

4.3.1 Power Source Configurations. Plasma arcpower sources take many different forms, from suitcase-sized portable inverter units to large mechanized systems,which can include multiple power sources and severalconsoles, such as remote control, high-frequency control,and gas control units.

4.3.2 Cutting Capacity. Power source units are avail-able in various output current capacities, ranging from20 A–600 A. Some large power sources may be used inparallel to increase their cutting capacity, up to 1000 A.Plate thickness, plasma gas type, and desired cut qualityare used to determine the required current capacity. Cut-capacity claims vary, but a 50 A system will generallysever metals up to 3/4 in. (19 mm) thick, 200 A systemswill sever up to 2 in. or 3 in. (51 mm or 76 mm), and400 A to 500 A systems will sever up to 5 in. (127 mm).

It is important to note that “cut capacity” may refer tothe thickest metal the system can cut (a “severance” cut,when maximum thickness is more important than thequality of the cut), or to the maximum thickness that canbe cut with repeatable high quality (a “production” cut).A severance cut may be as much as twice the thicknessof a production cut.

4.3.4 Duty Cycle. Many power sources rated to sup-ply a certain current at a 100% duty cycle may supply agreater current at a lower duty cycle. For example, somepower supplies rated at 600 A with a 100% duty cycleand 750 A with a 60% duty cycle can be operated at600 A continuously or at 750 A for a total of 6 minutesduring a 10 minute period.

4.3.5 Power Source Control. Manual plasma arc cut-ting systems include controls for setting current level andgas pressure or flow. Mechanized systems require addi-tional controls, such as arc voltage control, gas type se-lection, and travel speed. The feed rate, or torch travelspeed, is specified as part of the shape-cutting program inthe CNC software.

Additionally, “motion start delay” and torch heightcontrol are necessary in piercing operations, as the arcmust penetrate the workpiece before the torch motion be-gins. In the most programmable systems, all operating

parameters can be set and monitored by the CNC systemleaving the operator free to concentrate on materialhandling and periodic consumable parts inspection andreplacement.

4.4 Controls

4.4.1 Torch Initial Height Sensing. Initial height(IH) refers to the distance between the torch and work-piece prior to starting the cutting arc. Standoff control re-fers to a means of maintaining the distance between torchand workpiece during the cutting process. While thesefunctions are often available within the same device theytypically work independently.

The optimum initial height (IH) is one that allows thegreatest distance between the torch and workpiece whilestill allowing consistent arc transfer (usually 1-1/2–2 times the height used during the cutting process). Suchan IH minimizes damage to the front end of the plasmatorch and its consumables from the molten metal blow-back that occurs during piercing. At the same time thisoptimum IH avoids misfires which lead to unnecessarypilot arc time which can shorten consumable life.

IH can be set manually with a simple rack and pinionmounting arrangement that holds the plasma torch. It canalso be set using an up/down switch connected to a torchlifter motor. Automatic initial height sensing (IHS) isavailable through a variety of technologies includinglifter motor stall and retract, inductive sensing probes,and electrical contact sense and retract.

A standoff control system maintains the desired gapbetween the torch and workpiece (see Figure 4). Thisprovides consistent cut quality (bevel angle, top edgerounding, dross, kerf width) regardless of variations inthe surface of the cutting table and waviness of the work-piece. Optimum torch standoff settings for different ap-plications are found in the cut charts of most plasmasystem instruction manuals. Since torch standoff is a cru-cial parameter in mechanized plasma arc cutting it mustbe maintained with some precision to achieve high-cutquality. Torch standoff in conventional plasma arc cut-ting is usually maintained within 0.020 in. (0.50 mm). Inhigh-current density plasma arc cutting the standoff mustbe held within 0.005 in. (0.13 mm).

The most basic standoff control is a mechanical fol-lower device that suspends the torch inside a housing thatis in contact with the workpiece through ball casters.These are usually found on sheet metal cutting systemsthat travel at speeds in excess of 300 inches per minute.A limitation of mechanical followers results from thefact that they ride along the surface of the workpiece dur-ing the cut. Standoff variations and vibrations are trans-mitted to the torch when moving over adjacent cutoutsand can result in poor cut quality. Surface marking of the


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workpiece due to the physical contact may also be unde-sirable in certain applications.

The most common type of standoff control is the arcvoltage control device. This system monitors the cuttingarc and signals a lifter motor to raise or lower the torch tomaintain a preset voltage. Since plasma arc cutting is anelectric arc process the arc voltage is directly related tothe distance between the torch and workpiece (a higherstandoff means a higher arc voltage). The desired stand-off, or arc voltage, is usually loaded into the controlthough an adjustable dial or an automatic sampling cir-cuit. Most systems allow standoff adjustment while thecut is in process giving the operator the chance to finetune his conditions. (Note: Other factors that impact arcvoltage in the plasma arc cutting process include cuttingspeed, plate thickness, gas flow, and electrode wear. Anychange to these conditions during the cut may require acorresponding change to the arc voltage setting to main-tain the desired standoff.)

4.4.2 Advanced Control Features. Some systemscan monitor and predict when consumable parts should bechanged. This capability, in conjunction with the develop-ment of quick-change torch heads, further reduces opera-tor involvement in mechanized applications. It is possibleto change torch heads robotically, thereby installing newconsumables, to achieve uninterrupted processing.

4.5 Electrodes and Nozzles (Consumable Hardware).Parts at the cutting end of a plasma arc torch, includingthe electrode, nozzle, swirl ring, and retaining or shieldcaps, are known as consumable parts. They are consid-ered “replaceable consumables” because they wear outfrom use, these parts are critical to success in plasma arccutting and gouging.

Each consumable part plays an important role: theelectrode “strikes the arc” and is the point from whichthe electric arc travels to the workpiece. Electrodes aregenerally made of two metals: the copper body and asmall center insert, from which the arc is emitted. De-pending on the type of metal to be cut and the gases used,the electrode may have a hafnium or thoriated tungsteninsert. The nozzle shapes the stream of plasma gas into acolumn, focusing it in the same way that a nozzle on ahose can “focus” a large stream of water into a smallerstream with greater velocity. The swirl ring organizes theflow of gas into a clockwise (or counterclockwise, in aspecial case) direction, which helps concentrate the arcinto a narrow stream as it exits the nozzle, and protect thenozzle from the heat of the plasma arc. The retaining capusually screws onto the front end of the torch and servesto protect the other consumable parts. In some torch de-signs, the retaining cap also serves as a safety device,preventing the torch from firing until all the parts are se-curely in place. A shield cap is available for some torch

systems, which helps prolong nozzle life by protecting itfrom physical hazards such as double arcing during cut-ting or gouging operations, and splashing of hot metalonto the nozzle.

Each consumable part must be designed for the particu-lar torch and power supply with which it will be used. Inaddition, each part must be compatible with the metalbeing cut, as well as the output current and the gas or gasesbeing used. Plasma arc cutting requires a stable arc withhigh-current density. All systems employ a copper nozzleto physically constrict the plasma arc after it is emittedfrom the electrode and as it exits the nozzle. Plasma’s hightemperatures, gas velocities and flying particles of meltedmetal all contribute to the consumption of the parts at thecutting end of the torch. Electrodes and nozzles are knowas consumables because they are particularly vulnerable towear and require timely replacement in order to providethe best possible cut quality. Other torch parts, such asswirl rings, shields, insulators, and O-rings, also need in-spection and periodic replacement.

Some cutting torches can also be used for gouging.Conversion from cutting to gouging involves installingspecial gouging electrodes, nozzles, swirl rings and capsdesigned for this process. Gouging nozzles, for instance,have a larger orifice designed to produce a wider, lessfocused plasma stream with less density.

4.6 Cutting Machines. The quality of mechanizedplasma arc cutting depends on the quality of the plasmaarc cutting system and the machine on which the cuttingsystem is mounted. End users must select the best ma-chine configuration for their application. Plasma arc sys-tems can be mounted on many different types of cuttingmachines. Brief discussions of some of the commontypes of cutting machines follow.

A fundamental element of machine configuration iswhether the torch moves, or whether the workpiece movesunder a stationary torch. Positioning and moving only thetorch results in notable cost and performance advantages,since it represents only a small mass. However, when thetorch is used in conjunction with another tool, which can-not be easily positioned, such as a punching head or turret,it is more practical to position the workpiece.

Efficient material utilization and well-designed mate-rial handling systems can make any cutting machinemore productive. Load and unload operations will in-crease overall cycle time unless multiple cutting tablesare used. Some machine configurations allow the use ofmultiple cutting torches on a single machine for evenmore productivity. The best method to increase the pro-ductivity of mechanical cutting systems is to performload/unload operations while the torches continue cuttingon another table. This avoids the torches being idle whilethe plate is loaded or parts/skeleton (scrap) is removedfrom a single table.


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4.6.1 Gantry (Bridge). This type of cutting machineis designed to process flat materials. The workpiece isplaced on a stationary table and the torch is mounted on atraveling bridge (see Figure 9).

Gantry machines accommodate either oxyfuel gas orplasma arc cutting systems. Because plasma arc torchesare heavier and cut at higher speeds, plasma arc cuttingsystems require heavier-duty machines with stronger mo-tion drive systems to operate accurately and effectively atplasma arc cutting speeds. A highly responsive torchlifter (Z-axis) is necessary to maintain the desired torchheight and to produce good cut quality. Heavy-constructiongantries, with accurate motion and process control capa-bilities, are used widely for all types of applications ofplate and sheet metal plasma arc cutting.

The thickness ranges from very thin gauge sheet(28 gauge; 0.4 mm) to very heavy plate (6 in.; 152 mm)stainless steel. Plasma arc cutting systems applied tothis type of cutting machine include air plasma (20 A–200 A), nitrogen plasma (40 A–750 A), oxygen plasma(15 A–400 A) and argon/hydrogen (200 A–1000 A). Theplasma arc cutting system is normally sized for a particu-lar application, and the current level and type of plasmagas will be selected accordingly.

Mechanisms capable of extremely accurate torch posi-tioning for bevel cutting and gouging are available in awide range of sizes, and are often produced to customer

specifications. Structure and drive configurations alsovary widely. For example, a machine may have one motordriving one side of the longitudinal axis (single-sidedrive), one motor connected to a shaft driving both sides,or one motor electronically synchronized with a secondmotor driving the other side (dual-side drive). The mostcommon design, though generally not optimal for high-speed contouring, is the single-side drive design whereone motor drives the long axis via rack and pinion and theother side, connected by a rigid beam, follows on a railassembly. For highest quality cuts, a very rigid machineconstruction is necessary or feed rates may be limited toavoid overshoot or whipping of the undriven side. In ad-dition, ballscrew or rack and pinion drives are preferred,unless ruled out due to cost or length considerations.

Gantry machines combine well with water tables ordown-draft tables for containment of the polluting by-products of plasma arc cutting, as described later in thissection.

4.6.2 X-Y Table. An X-Y table processes flat materialby positioning it under a stationary torch head. The X-Ytable is commonly used only on machines such aspunch/plasma arc units where the torch is installed adja-cent to another tool that is stationary such as a turret orsingle-station punching head.

A punch press with plasma arc cutting (see Figure 10)is a two-function machine: one operation punches out

Figure 9—Gantry Crane Cutting Machine


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precision holes while the plasma arc cutting system isused to cut out the parts’ shapes and large holes. Punchedpilot holes increase productivity, since the life of un-shielded nozzles is extended by eliminating molten metalsplashback when piercing at the start of the cut.

The workpiece is held by two or more work clamps,which slide in the X-axis direction over ball transfersembedded in the table. The entire worktable, with theclamp assembly, is generally driven in the Y-axis direc-tion. Z-axis torch height control is available, but is oftenreplaced by a “plate follower” arrangement where thetorch rides across the surface of the workpiece whilecutting. Due to the additional weight of the workpieceand table assembly, a rigid machine design and a high-performance drive system are required to produce goodquality cuts at high-plasma cutting speeds. The ability topunch or nibble small holes and intricate patterns, and toplasma cut larger contours, in one setup, makespunch/plasma arc cutting machines an attractive option.High-performance servo systems are also needed to over-come wide variations in frictional load and weight ofworkpieces. High overall performance requires rapidacceleration, high-torch travel speed capabilities, littlevibration as the material traverses the worktable, and no“overshoot” when changing velocity and direction.

These capabilities can add significantly to the cost ofthe machine tool, but make excellent plasma cuts possi-ble in terms of edge smoothness and contour definition.

4.6.3 Robotics. Six-axis articulated tool manipulators,almost universally known as robots, can use plasma arc tocut or gouge non-flat stock material in any location or posi-tion within the machine’s range of motion (see Figure 11).

Proper torch standoff is generally maintained throughsix-axis articulation based on arc voltage feedback. Moreadvanced dynamic height sensors, based on “look ahead”techniques, are also available, which allow better torchheight control while traversing shape changes.

4.6.4 Cantilever. Designed for cutting shapes in flatmaterials by tracing shapes or following CNC programs,

Figure 10—Punch/Plasma Press

Figure 11—Plasma Cutting Robot—Classic Six-Axis Design


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a cantilever machine generally features a torch mountedon an unsupported beam extending over a cutting table(see Figure 12).

Cantilevers are primarily intended for oxyfuel gas cut-ting and do not perform well in high-speed cutting appli-cations due to the vibration generated on the cantileveredbridge and speed limitations imposed by optical tracingsystems. Machines equipped with high-speed tracers andcoordinate drive systems can operate successfully atspeeds up to 150 in./min (64 mm/sec), which is adequatefor many plasma arc cutting applications. Though mostcantilevers are of lightweight construction, some are suf-ficiently heavy-duty to handle the size and weight ofplasma arc cutting torches and leads.

4.6.5 Other Cutting Machines

4.6.5.1 Magnetic Template Tracer (TractionDrive Tracer). This first generation of shape-cutting ma-chines was configured so that the torch was held by apantograph arm assembly and its motion was guided by aseries of arms to a tracing stylus that followed a precutpattern. Either manual or automatic controls could beused. These units were generally portable.

4.6.5.2 Cutting Tractor (Track Machine; PortableCarriage). Capable of moving a torch in a straight orcurved path on a track, these units are used primarily forcut off and trimming operations, and for manual cuttingof large layout work. The plasma system is usually oper-ated independently of the tractor and the machine aids inmaintaining constant speed and stand-off. A variation ofthis type of unit allows hand-guided cutting.

4.6.5.3 Pipe Cutting Units. These devices are por-table units that are used for cut off or beveling operations

on large-diameter pipes. Once tightly attached to thepipe, drive rollers propel the unit around the circumfer-ence of the pipe, while the plasma arc torch performs acutting or beveling operation.

4.6.6 Cutting Machine Control Systems. Develop-ments in high-speed plasma arc cutting have been paral-leled by improvements in cutting machines andcontrollers, which now make extensive use of computertechnology.

4.6.6.1 Optical Tracing Systems. Photo opticaltracing systems, evolved from the traction drive system,and are still used on some gantry and cantilever machines.The system uses an electronic eye to follow a part outline,generally on paper, and it is possible to trace silhouetteand line drawings. For gantries, tracing is generally doneoff-line, and production of very large parts requires ratiotracing. For high-speed plasma arc cutting, a CNC mayreplace or supplement a tracer. A CNC used with a trac-ing unit may include a trace-record feature to developparts programs from traces for subsequent production.

As with any complex equipment, errors and break-downs can occur. As a result, today’s CNCs offer moreand more diagnostic and fault recovery features. For ex-ample, a diagnostic message may direct the operator toreplace a certain sensor, or a lost-cut-recovery featuremay direct the plasma arc system to restart in the middleof a cut in order to avoid scrapping a part.

4.6.6.2 Numerical Control (NC). Numeric Con-trol systems, which preceded CNCs, were characterizedby lack of memory, punched paper tapes for programstorage, and the use of hardware rather than software toimplement control functions. In addition, NC systemswere bulky, generated excessive amounts of heat, and

Figure 12—Cantilever Cutting Machine with Optical Tracing Head


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had reliability problems. Commercial development ofmicroprocessors and semiconductor memory led to thedevelopment of the CNC.

4.6.6.3 Computer Numerical Control (CNC).Almost all cutting machine direction commands are nowgenerated by computers. CNCs offer reliability, ex-panded capacity, and the ease and efficiency of connect-ing several units to a central computer in a direct numericcontrol (DNC) system. The parallel evolution of plasmaarc cutting techniques and machine motion controlsystems has allowed plasma arc cutting to develop into ahigh-speed production process capable of cutting anymetal.

The CNC is the brain of today’s machine tools and themotion control of cutting machines. Nearly all directionand speed commands are software-derived from part pro-grams that are stored in computer memory. CNC tech-nology can now provide positioning information withsuch speed and precision that part program processingdelays are no longer a factor to limit cutting machine per-formance, even with complex, high-speed motion. In ad-dition to controlling motion and other machine functions,the CNC can provide many features to simplify operationand maintenance.

CNC programs can provide “motion and guidancecodes” for full automation of the cutting operation, orallow the operator to input commands several levelsbeyond start/stop.

Manual data input by the operator, calling up shapeprograms from data storage libraries, and such functions

as part transformation, rotation, scaling and mirror imag-ing are often accessible through pull-down menus, whichincrease the operator’s programming power.

Programming can also be done away from the ma-chine on an off-line part programming system. Thesesystems can post-process part programs to make a gen-eral program work with specific machines and can runnesting software to maximize efficient use of materials(see Figure 13). Part programs generated in the office canbe transferred to the cutting machine through a directnumerical control (DNC) remote communications link.Loading part programs via punched paper tape or floppydisk is also possible. Once in the CNC, graphics pack-ages allow the operator to view the part program as it willbe processed on the machine, and to monitor progress inreal time as the machine is cutting.

Compensation features are also increasingly common,such as kerf width compensation (kerf width offset). Thisfeature automatically cuts parts slightly larger to com-pensate for kerf width. Fixture offsets (work zero offsets)are used on machines with multiple tools to allow opera-tion of a secondary tool in a different location without al-tering dimensions in the part program.

For maximum part accuracy, many machine toolbuilders use positioning compensation. When differencesbetween actual and programmed positions are foundduring calibration, the errors are recorded in an error-correction table in CNC memory. This table is used tocompensate for hardware inaccuracy in real time duringpositioning operations.

Figure 13—Sample Part Program


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4.6.7 Drives. A highly capable CNC requires an ef-fective way to translate motion commands into move-ment. A typical drive system consists of several parts: aservo amplifier to convert CNC commands into the highpower levels necessary to run the drive motors; the drivemotors themselves, also known as servo motors; a feed-back device, such as a resolver or encoder, to supply po-sition information to the CNC; and a system ofmechanical components such as ballscrews, gear boxes,rack and pinions, etc., to provide linear motion.

A basic system uses a permanent magnet DC servomotor and a PWM (pulse width modulated) servo ampli-fier with analog control. Digital servo amplifiers achievethe highest performance that CNCs can provide, includ-ing such features as on-the-fly gain changing and back-lash compensation. The power and flexibility of digitalservo systems lets machine builders extract high perfor-mance from a mechanical system while maintaining sta-bility over a wide range of operating conditions.

4.7 Pollution and Hazard Control. Plasma arc cuttingand gouging systems produce fumes, gases, noise, andultraviolet light, which can cause harm to the operatorand others nearby if not suitably mitigated. Many pollu-tion control systems are available for mechanized sys-tems, several of which involve water table techniques.

Introduced in the 1970s, water tables can be used withtwo mechanized cutting methods: cutting with a watermuffler (water shroud) or underwater cutting.

Primary water table design requirements include ade-quate workpiece support, sufficient scrap capacity, andwater level controls. For underwater cutting, the operatormust be able to raise and lower the water level rapidly tomaintain good productivity.

4.7.1 Water Muffler. In this method, the cutting tablemaintains a bed of water that contacts the bottom surfaceof the workpiece. An annular-shaped water muffler noz-zle mounted on the torch body generates a thick shroudof water around the arc, as shown in Figure 14.

Water pumped from the cutting table through thewater muffler nozzle surrounds the arc, removing thefumes and airborne particulates by capturing them in thewater. The water muffler also functions as a sound bar-rier, reducing noise by approximately 20 dBA. The watershroud can also act as a light-absorbing shield when dyeis used in the water table, although it is still necessary toprovide operator eye protection in case the water flow isinterrupted. Water injection and water shielding must notbe confused with water muffling, as neither water injec-tion nor water shielding uses a sufficient flow of waterfor noise reduction.

Figure 14—Water Muffler


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4.7.2 Underwater Cutting. This system uses a waterbed in which the workpiece is submerged 2 in.–3 in.(51 mm–76 mm) below the surface of the water, asillustrated in Figure 15. Underwater plasma arc cuttingprovides greater noise reduction than the water mufflerbecause the arc and the entire working end of the torchoperate below the surface of the water. The water depthalso reduces ultraviolet radiation; therefore, the additionof dye is usually not required.

Underwater plasma cutting may require a watershroud nozzle even though it is operating underwater,creating an air bubble to eliminate all water in the cutzone. Such an underwater air-assisted water muffler iscalled for when oxygen is used as the plasma gas for cut-ting steel plate.

The efficiency of underwater cutting requires a watertable designed to raise and lower the water level rapidlyfor unloading cut parts and reloading new material forcutting.

Since the operator cannot gauge torch height visuallywhen cutting underwater, quick and accurate torch posi-tioning can be achieved with inductive probes or othermeans that locate the torch at the correct height for pierc-ing or cutting underwater. Automatic torch height controlis recommended for use with CNC systems.

4.7.3 Down Draft Table. This method is well suitedfor controlling fume and particulate matter when usingdry plasma arc systems. The fumes are exhausted fromthe bottom of the cut area to a bag house or other filter toclean the air. Very large volumes of air must be moved inorder to make this technique effective. Manufacturers’specifications for the correct type of exhaust system willinclude the volume of air removal required for theplasma arc type, current output, and number of torches inuse.

4.7.4 Collection Cup for X-Y Tables. For cuttingtable systems that move the workpiece instead of thetorch, a collection cup system as illustrated in Figure 16is required instead of a water table or down-draft ex-haust. Cup systems generally consist of a water-cooledcollector positioned to envelop the tail of the arc beneaththe workpiece during cutting. The collector may be a wetor dry type, and can be fixed or may move up verticallyto a position nearly touching the workpiece during cut-ting. Dry collectors allow cutting slag to drop into a slagtray and sparks and airborne particulates to settle into aspark trap as the air from the collector travels to the filter.

Wet collectors use a curtain of water along the insidesurface that prevents slag from adhering to the collectorand washes it into a slag tank. Fumes are carried by an air

Figure 15—Underwater Plasma Cutting


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flow from the collector and tank to a filter. Water in theslag tank may be circulated through a heat exchanger be-fore again flowing through the collector. Large slag parti-cles and slugs are often removed from the tank by aconveyer system. Particulates suspended in the water canbe removed by filtration. As with water tables, the watermust be replenished and conditioned to maintain waterlevel, proper pH, etc., and the tank water, slag, sludge,and filter dust must be disposed of in accordance withapplicable regulations.

4.8 Plasma Gas Selection. Four plasma gases are incommon use in plasma arc cutting and gouging: air,oxygen, nitrogen, and mixtures of argon and hydrogen.Selection of the plasma gas starts with considering thetype of material to be cut, but may also be influenced byequipment selection, desired cut quality, and operatingcost considerations.

4.8.1 Air Plasma Gas. Compressed air is widely usedas a plasma gas for a variety of reasons. The main advan-tages are convenience, lower operating costs, and elimina-tion of expensive and potentially hazardous compressedgases. It is used primarily for cutting carbon steel, but isalso acceptable for some aluminum alloys and stainlesssteel applications. Air plasma gas is used for manual andmechanized cutting in the 10 A–300 A current range. Theuse of air or oxygen as the plasma gas requires specialefforts to obtain satisfactory electrode and nozzle life.

4.8.2 Oxygen Plasma Gas. Oxygen as a plasma gas isvery useful for cutting of steels, which is the most widelyused metal in many industries. Oxygen’s primary advan-

tages are its high-speed cutting capabilities and its abilityto cut all grades of steel over a wide speed range. In addi-tion, less dross is produced when cutting with oxygen,cut quality can be excellent, and the level of nitrides inthe cut edges is very low. This last feature means that thecut edge can be welded with less porosity in the weldmetal.

As with air, electrode life is shortened when using oxy-gen as the plasma gas. Various methods are used to in-crease the life of the hafnium electrode with oxygenplasma gas, including arc starting and stopping tech-niques to preserve the oxide layer on the hafnium, andspecial electrodes designed to control erosion of thehafnium insert.

Pure oxygen combines chemically with the iron insteel in a reaction that liberates a great deal of heat. Inplasma arc cutting, this allows high-speed, high-qualitycutting that must be mechanized to take full advantage ofthe speeds possible with an oxygen plasma gas process.The cutting machine should be numerically or computercontrolled, and capable of at least 250 in./min (635 cm/min). Both wet (water injected) and dry oxygen plasmaarc cutting systems are available for cutting at currentlevels up to 400 A.

4.8.3 Nitrogen Plasma Gas. Nitrogen plasma gascuts a wide variety of metals, including good quality cutson carbon steel, aluminum alloys, and stainless steel.Cutting speeds, though not as high as with oxygenplasma, are still quite high. The inert nature of nitrogenallows tungsten to be used as the electrode material, pro-ducing relatively long electrode and nozzle life.

Figure 16—Collection Cup System—Wet Type


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Nitrogen plasma cutting has these advantages overoxygen plasma arc cutting:

(1) the cost per foot of cut is less (because of longerconsumable parts life)

(2) nitrogen produces better cut-edge quality on non-ferrous metals.

Nitrogen plasma gas is most commonly used withwater-injection cutting, but it can also be used in dry(non-water injection) processes. There are two disadvan-tages of using nitrogen. Theses are the much lower toler-ance range of cutting speeds to produce dross-free cutedges, and the tendency for nitride formation resulting inporosity when the edge is welded.

4.8.4 Argon-Hydrogen Plasma Gas. Argon-hydrogenas a plasma gas (most commonly 65% argon, 35% hydro-gen, which is also called H35), is best known for its abil-ity to produce excellent cut-edge quality in stainless steeland aluminum alloys.

The presence of hydrogen in the plasma gas raises arctemperature. The reducing nature of the argon-hydrogenmixture produces clean, bright cut edges and long elec-trode and nozzle life.

Tungsten electrodes can carry high currents with thistype of plasma gas. Applications include high-currentcutting (up to 1000 A) of 3 in.–6 in. (76 mm–152 mm)thick plate, and cutting and gouging various nonferrousmetals. The chief disadvantages of argon-hydrogen mix-tures are the high gas cost and a tendency to producedross when cutting steels.

4.8.5 Plasma Gas/Shield Gas Combinations. Theprocess capabilities are heavily influenced by the plasmagas and shield gas selected for cutting with dual-flowsystems. The selection process is more complicated thanfor single-gas systems, but the larger number of possiblecombinations allows a wider choice among the variablesof speed, consumables life, cut-edge quality, dross, weld-ability, and economy.

4.8.5.1 Dual-Plasma Gas Combinations for MildSteel

(1) Air Plasma/Air Shield. This gas combination givesgood cut speed, low dross levels, and is most economical.Current range: 20 A–200 A. Thickness range: 1/8 in.–2 in.(3 mm–51 mm).

(2) N2 Plasma/CO2 Shield. This gas combination isused when cut-edge quality and surface nitriding are lessimportant. Cutting or gouging with nitrogen extendselectrode life. Current range: 100 A–600 A. Thicknessrange: 3/16 in.–3 in. (5 mm–76 mm).

(3) N2 Plasma/Water Injection Shield. When cut edgeand surface roughness are less important, this plasma gascombination gives excellent electrode and nozzle life.Current range: 400 A–750 A. Thickness range: 3/16 in.–2 in. (5 mm–51 mm).

(4) O2 Plasma/Water Injection Shield. This combina-tion gives very good cut speed, no surface nitriding, min-imum amount of dross, and very good weldability, alongwith good electrode and nozzle life. The current rangeis from 240 A–340 A. Thickness range from 1/4 in.–1-1/2 in. (6 mm–38 mm).

(5) O2 Plasma/Air Shield. This combination gives su-perior cut speed, minimum dross, minimum amount ofsurface nitriding, and very good weldability. Currentrange is from 15 A–260 A. Thickness range from verythin gauge to 1 in. (25 mm).

(6) Argon Hydrogen (H35) Plasma/N2 Shield. Thisplasma/shield combination reduces dross, extends elec-trode life, and gives the best gouging results. CurrentRange: 100 A–1000 A. Thickness Range: 1/8 in.–4 in.(3 mm–102 mm).

4.8.5.2 Dual-Plasma Gas Combinations forStainless Steels

(1) Air Plasma/Air Shield. The air/air combinationgives good cut speed, low dross levels, and is most eco-nomical. Some surface nitriding and surface oxidation ofalloy elements can occur. Current range: 20 A–200 A.Thickness range: light gauge to 1-1/2 in. (38 mm).

(2) N2 Plasma/CO2 Shield. These gases are usedwhen surface nitriding and surface oxidation of alloyingelements are less important. Extended electrode life, re-duced dross. Current range: 100 A–600 A. Thicknessrange: 3/16 in.–4 in. (5 mm–102 mm).

(3) N2 Plasma/Water Injection Shield. This combina-tion is used when cut edge, angle, and surface roughnessare less important. There is minimal surface nitriding andoxidation of alloying elements. Nitrogen water-injectioncutting gives excellent electrode and nozzle life. Currentrange: 400 A–750A. Thickness range: 3/16 in.–5 in.(5 mm–127 mm).

(4) Argon-Hydrogen (H35) Plasma/Water Shield.This combination gives maximum thickness capability,reduced dross levels, minimal surface contamination, ex-cellent weldability, excellent cut-edge quality, and ex-tended electrode life. Current range: 750 A–1000 A.Thickness range: 2 in.–6 in. (51 mm–152 mm).

(5) Argon-Hydrogen (H35) Plasma/N2 Shield. Thisgas combination gives reduced dross, minimal surfacecontamination, excellent weldability, excellent cut-edgequality, and extended electrode life. Current Range: 100 A–1000 A. Thickness Range: 1/8 in.–4 in. (3 mm–102 mm).

4.8.5.3 Dual-Plasma Gas Combinations forAluminum Alloys

(1) Air Plasma/Air Shield. This gas combination givesgood cut speed, low dross levels, and is most economi-cal. Some surface nitriding and surface oxidation of alloy


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elements can occur. Current range: 20 A–200 A. Thick-ness range: light gauge to 1-1/2 in. (38 mm).

(2) N2 Plasma/CO2 Shield. These gases are usedwhen surface nitriding and surface oxidation of alloyingelements is less important. Electrode life is extendedwhen using this gas combination. Current range: 100 A–600 A. Thickness range: 3/16 in.–4 in. (5 mm–102 mm).

(3) Argon-Hydrogen (H35) Plasma/Water Shield.This combination gives maximum thickness capability,reduced dross levels, minimal surface contamination, ex-cellent weldability and cut edge quality, and extendedelectrode life. Current range: 750 A–1000 A. Thicknesscapability: 2 in.–6 in. (51 mm–152 mm).

(4) Argon Hydrogen (H35) Plasma/N2 Shield. Thisgas combination gives reduced dross, minimum amountof surface contamination, excellent weldability, excellentcut-edge quality, and extended electrode life. Currentrange: 100 A–750 A. Thickness capability: 1/8 in.–4 in.(3 mm–102 mm).

5. Installation ConsiderationsInstallation of a plasma arc cutting system is similar to

that of other major equipment installations. However, at-tention should be paid to the high voltages, compressedgases, sparks, noise, arc radiation, and fumes and gaseswhich are generated or used by the equipment. Consultequipment manufacturers’ instruction manuals for details.

5.1 Site Selection and Planning. Efficiency and safetyare very important factors in site selection and planningfor any plasma arc cutting installation. For best results,both should be considered early in the planning process. Itis important to note that all the potential hazards associ-ated with plasma arc cutting and gouging can be handledsafely. Each of these topics is covered in Section 8. Ingeneral, plasma arc equipment should be situated in aclean, dry location with adequate space provided aroundall system components to provide access for maintenanceand proper air flow for cooling. It is important to considerthe availability and location of three utilities vital toplasma arc cutting and gouging: electric power, gas sup-plies, and water. Efficient material handling should also becarefully considered, since the equipment involved is animportant element in the total cost of cutting operations.

5.2 Electric Power and Safety Grounding. The plasmaarc equipment manufacturers’ specifications should beconsulted for specific recommendations on primary wiresize and fuse requirements. Proper safety grounding ofthe plasma arc cutting operation is essential, to provideoperator safety and protection of the equipment. Mostplasma arc units use three-phase input power. Thissource of power should have either a fuse disconnect

switch or a circuit breaker at the cutting location. Ade-quate electric service and proper cable size for the unit’spower is required.

High-frequency circuitry is required for starting thearc in many plasma arc systems, and without properequipment grounding, high-frequency electromagnetic(EM) radiation may be emitted. High-frequency (EM)emissions can interfere with nearby electronic equip-ment, or produce radio and TV interference.

Consult the equipment manufacturer’s installation in-structions and all applicable local and national codes forinstalling suitable protective earth grounds. Compliancewith electromagnetic compatibility (EMC) limits shouldbe confirmed.

5.3 Gas Supply. One or more gases are required for theplasma arc cutting system, with delivery pressure andflow rates dependent on the type of equipment used. Al-ways consult the system manufacturer’s gas supply rec-ommendations and specifications. Consult your gassupplier to determine the best method of supply and dis-tribution. The gas may be supplied from a single high-pressure cylinder, a multiple-cylinder manifold system,or liquid containers (bulk or portable). Gases should beobtained from a reputable supplier to ensure gas purityand acceptable moisture content. When liquid containergas supplies are used, particular attention should be paidto the evaporation rate of the system: be sure that pres-sure and flow rate can be maintained at required levelsfor extended operating periods.

Compressed air used as the plasma gas or the second-ary gas must be clean, dry, and free of oil for optimumperformance. Compressed air containing moisture, oil, orother contaminants will contribute to poor torch partslife, reduced cut quality, and reduced ability to cutthicker metal.

The air filter and regulator supplied with most airplasma arc systems are not designed for conditioninglarge volumes of air, therefore when on-site compressorssupply the air, additional filtration and drying are oftennecessary to provide acceptable air quality and quantity.Loss of sufficient gas flow in a plasma arc cutting systemcan cause serious damage to the torch. Delivery pressuremust be maintained at the manufacturer’s specifications.Some plasma arc equipment will automatically shutdown the system if the pressure drops too low, but thesesafety circuits are not available with all systems, and arenever completely fail-safe.

Consult AWS C5.10, Shielding Gases for Weldingand Plasma Arc Cutting, for further information.

5.4 Water Supply. Most mechanized plasma arc cuttingsystems employ water for torch cooling, noise or pollu-tion control, or as a component of the cutting itself, as inthe case of water injection plasma arc cutting. Some


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water systems circulate and reuse the water from a watertable or reservoir, which reduces water use and allowseasier control of the water purity and supply. Water puritylevels vary with equipment and intended use of the water.Water injection and torch cooling systems have higherpurity requirements than a pollution control system. Thisis due to the increase in water conductivity and the po-tential for electrolysis as “impurity” or ion levels in-crease. Deionized water or special water filtration isoften required for water delivered to the torch, especiallyfor water injection systems. The equipment manufactur-ers’ recommendations and specifications should be fol-lowed in each case.

6. Optimizing Plasma Arc Cut QualityThere are four factors to consider in evaluating the qual-

ity of cuts obtained by any cutting process: cut angle,dross, metallurgical effects, and kerf width (see Figure 17).

6.1 Cut Angle. The heat of the plasma arc removesslightly more metal from the surface of the plate closestto the source of the arc, creating a beveled cut. The de-gree of bevel is known as the cut angle. Cut angle will beminimized by swirling the plasma gas before it exits thenozzle, and by achieving the best combination of the cut-ting parameters for the metal type and thickness beingcut. These include cutting current, torch travel speed and

height control, and selecting the appropriate gas or gasesfor each metal. Plasma arc torches with clockwise orificegas swirl (see Figure 18), viewed from the torch towardthe work, produce cuts with the right side of the kerf (inthe direction of travel) square, and the left side slightlybeveled. The swirling plasma gas is less disturbed at theright, where the swirl offsets the forward travel; at theleft side, drag is encountered. These factors contribute tomore uniform arc energy being delivered to the right sideof the kerf.

For axial-flow plasma arc torches, the cut angle (asshown in Figure 17) on both edges is typically 4° to 8°;in the case of tangential-swirl plasma torches, cut angleon one side (“good side”) is 1° to 3°, and 3° to 8° on theother (“scrap side”). With optimized parameters in con-ventional plasma arc cutting, cuts with one side showingan angle of 1° to 3° are considered “square” if this slightangle is within the tolerance of the processing steps thatfollow the cutting stage. In plasma arc torches that cutwithout swirling the gas, cut direction does not affect cutangle: both sides of the cut are beveled.

In shape-cutting applications, this means the directionof cut can be selected to produce a square cut on the pro-duction part. On the annular-shaped production pieceshown in Figure 18, the outside cut is made in a clock-wise direction to place the saved piece on the right sideof the kerf. Similarly, the inside cut must be made in acounter-clockwise direction to maintain a square inside

Figure 17—General Features of Plasma Arc Cut


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edge. In most applications, the beveled side of the cut isdisregarded when evaluating squareness as this edge ison the “scrap” side of the cut. Swirl rings that producecounterclockwise orifice gas swirl are available for appli-cations such as mirror-image cutting where the high-quality side must be on the left for one of the parts.

It is important to keep the torch perpendicular to pro-duce square cuts. The torch can be perpendicular to thecutting bed, but not be perpendicular to the workpiece,which degrades squareness. The table must be main-tained in good repair, and slag and scrap pieces must beremoved from the cutting table. Hoses and cables thatpull tight when the cutting machine carriages extends totheir travel limits may also cause off-square torches. Pro-cess variables which affect cut squareness are cuttingspeed and arc voltage/standoff. Cutting at too fast aspeed or too high a voltage will result in a positive cutangle (a beveled part), while too slow a speed or too lowa voltage will result in a negative cut angle (undercut). Ofthe two, speed control is the most important, and it iscritical for uniformity of cut angle around the part perim-eter. Developments in high-current-density plasma arcshave improved the control of the cut angle substantially.

Higher current densities emerging through the nozzleorifice are achievable, which maintain melt rate unifor-mity through the thickness of the material. High-current-density plasma arcs can produce cut angles that arenearly perpendicular, not only on one side, but both sidesof the cut, in metals up to 3/8 in. (10 mm) thick. With in-creased current densities, current levels are lower andspeeds are considerably slower, allowing much more in-tricate and highly defined cuts. The end result is cuts thatare similar to cuts produced by a CO2 laser operating inthe 1.5 kW to 2 kW range.

6.2 Dross. Dross is resolidified, oxidized metal that ad-heres to the edge of the cut—either at the top or bottom.

Bottom-edge dross is one of the most common prob-lems faced by fabricators and for a given type of gas, isinfluenced mainly by cutting speed, current and arc volt-age. Other factors influencing bottom-edge dross includethe condition of consumables, grounding, surface condi-tions of the plate, nozzle diameter and standoff. Thereare two kinds of bottom-edge dross: low-speed dross andhigh-speed dross. Low-speed dross, richer in oxides, iseasy to remove. High-speed dross, however, contains asignificant proportion of resolidified molten metal thatrequires grinding or other rework to remove, which re-duces productivity. Optimum cut quality results fromcutting in the “dross-free window” shown in Figure 19.

Although the dross free cutting window widens as thecurrent increases, the wear rate of the consumables gen-erally sets the upper limit for current. In cases where thewindow is very narrow or nonexistent, it is preferable tostay in the low-speed range, since low-speed dross is eas-ier to remove. Steels are the most difficult metals to cutwithout dross, especially when cutting with nitrogenplasma, because of the steel’s chemical composition—especially its silicon content. Recent developments incutting equipment and consumables have considerablywidened the dross-free window for steels. Stainless steelis tolerant in terms of dross formation and has a rela-tively wide dross-free window. Nickel and copper alloys,by contrast, tend to form dross more easily.

Top-edge dross generally indicates excessive standoffand can be easily eliminated by reducing standoff to themanufacturer’s recommended settings. Top dross issometimes evident when cutting steel with air or nitro-gen, depending on chemical composition and surface fin-

Figure 18—Direction of Cut with Clockwise Swirl


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ish. Although top dross is difficult to avoid or reduce incertain types of material, it is seldom a problem withoxygen-plasma arc cutting.

Drag lines on a plasma arc cut edge, illustrated inFigure 20, are useful in fine-tuning the cutting speed toensure cutting is well within the dross-free window, andto balance required quality with maximized productivity.Although generalizations are difficult because the char-acteristics of drag lines depend on what cutting process isbeing used, a trained eye can use drag lines and theirangle to diagnose cutting problems. Too high a dragangle, for example, indicates excessive travel speed forthe current used.

6.3 Metallurgical Effects. The heat-affected zone(HAZ) of plasma arc cuts is much narrower than in oxy-fuel gas cutting because plasma arc cuts at higher torchtravel speeds. This can be a significant factor in subse-quent welding or finishing operations. The thickness ofthe heat-affected zone is typically 0.010 in. (0.25 mm)for austenitic stainless steel and 0.020 in. (0.51 mm) forcold-rolled or low-carbon steel. Carbon steels generallyexhibit heat-affected zones from 0.015 in.–0.030 in.

(0.4 mm–0.8 mm) thick. Aluminum alloys cuts exhibitHAZ in the range of 0.040 in.–0.100 in. (1 mm–2.5 mm)thick.

When cutting austenitic stainless steel, the high cut-ting speed and the quenching effect of the base platecause the cut face to pass rapidly through the criticalmetallurgical temperature of about 1200°F–1500°F(600°C–800°C) minimizing metallurgical effects. Thisrepresents the visible HAZ but significant hardening islimited to a much narrower zone. The degree of harden-ing can be reduced by preheating the workpiece beforecutting, which reduces the quench effect of the metalmass immediately behind the cut face. Contact the steelmanufacturer for recommended preheating procedures.For oxygen plasma arc cut edges, the degree of harden-ing is much less because of the following factors: first,the arc current is generally lower, thus less energy is putinto the material. Secondly, the oxygen gas does not formdetrimental, hard compounds with the steel, like thoseformed when using air or nitrogen. Typical hardness lev-els are about 20 Rockwell C points lower for A 36 steelcut with oxygen than those cut with nitrogen or air.

Figure 19—Typical Dross-Free Window for 200 AmpOxygen Plasma Cutting Process on Mild Steel


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6.4 Kerf Width. As with all cutting methods, a kerfallowance must be applied to the templates or programsgenerating the desired shape. Some cutting machinescompensate for kerf width automatically with a preset kerfallowance of one-half the kerf width. The width of a con-ventional plasma arc cut is greater than cuts made withoxyfuel gas processes, by one-half to as much as fourtimes, depending on cutting power and plate thickness,but kerfs are significantly narrower with high-current den-sity plasma arc cutting. The allowance required is difficultto quantify in general terms, because every variable in aplasma arc cutting system has some effect on kerf width,and each installation is unique. One variable that is truefor all situations, however, is that kerf width increaseswith plate thickness, as shown in Table 2. Actual kerfwidths will vary.

6.5 Quality Improvement Suggestions. It is essentialthat the equipment manufacturer’s recommendations forinstallation, operation and maintenance be strictly fol-lowed, in order to produce and maintain the highest pos-sible cut quality that a system can provide. A list ofsuggestions for troubleshooting is offered below as ageneral guideline, but note that all the following sugges-tions may not be applicable for all plasma systems, or forall cutting situations.

6.5.1 Gas-Related Items(1) Monitor gas flow rate and pressure closely and

consistently. If using compressor air and the pressuredrops during the cut, check compressor capacity.

(2) Check for leaks or blockages in the system.(3) Check purity of gases used; consider higher purity

gases, if all else has been checked.

6.5.2 Torch-Related Items(1) Measure and maintain torch standoff accurately—

do not use visual estimation (“eyeballing”).(2) Maintain consistent travel speed, for a particular

material type and thickness.(3) Reducing standoff distance may reduce dross, but

consumables life may be reduced and lower current mayhave to be used.

(4) Dross is more commonly found in shape cuttinginvolving curved edges; straight cuts made on the sameplate using the same parameters often produce less dross.

6.5.3 Consumables-Related Items(1) Use appropriate consumable parts for the cutting

gas.(2) Check all consumables for wear or damage.(3) Check electrodes and nozzles for heavy or uneven

wear.(4) Follow manufacturer’s guidelines on replacement

of consumables.

6.5.4 Workpiece-Related Items(1) Use automatic voltage/standoff control; if unavail-

able, try to ensure the plate is flat.(2) Check for differences in grounding, total weight

of the plate, etc. Try to position all the plate at the samelocation on the table for each cut, rather than at random.Ensure that the plate is not supported by dross from pre-vious cutting.

(3) Invert a “bad” cutting plate and see if there is anydifference when cutting it with the bottom side up.

(4) Some plates may be magnetized, a condition thatcan affect arc stability.

(5) Try cutting with the plate surface slightly belowwater level to reduce heat warping.

(6) Check for distortion of plate as cutting proceeds;even a small amount of distortion affects torch standoff.This could be a factor during cutting of hot strip products

Figure 20—Typical High- and Low-Speed Dross and Drag Lines

Table 2Conventional Plasma Arc

Kerf Widths of Mild Steel Plate

Plate Thickness Kerf Width

in. mm in. mm

1/8 to <11 to <22 to 5<

3 to 2525 to 5151 to 127

3/323/165/16

2.44.88.0


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(“coil memory”) that is related to residual stresses in thestrip; there may be no practical solution. Consult equip-ment manufacturer.

(7) If feasible, try substituting similar material from adifferent vendor for the “problem plates.”

6.5.5 Power-Related Items(1) Check that the line input power for the power

source does not fluctuate beyond the limits recom-mended by the cutting system’s manufacturer.

(2) Check output current level.(3) With water-cooled torches, check the conductivity

of the deionized coolant water. Conductivity higher thannormal will decrease the power available for cutting.

6.5.6 Cutting Procedure-Related Items(1) Run cutting trials to find the best power/cutting

speed combination for each type and thickness combina-tion of material being cut—this needs to be consistentwith the required edge quality (while generally needingto maximize productivity).

(2) Don’t try to use the same cutting speed/power fortoo wide a range of plate thickness.

(3) Consider the cost of removing dross in the overallcutting costs. This can often justify the use of oxygenplasma arc cutting rather than nitrogen PAC despitehigher consumable costs.

7. EconomicsPlasma arc cutting has become a top choice in manu-

facturing and metal industries because it provides high-quality cutting at a low cost per foot. Cutting mild steelless than 2 in. (50 mm) thick is the most common indus-trial practice. Industrial users now prefer plasma arc cut-ting over oxyfuel gas cutting because of its faster cuttingand its ability to cut stainless steels and aluminum alloys.Cutting costs also favor plasma arc cutting: oxyfuel gascutting is typically 2–5 times more expensive thanplasma arc cutting for cutting mild steel up to 1 in.(25 mm) thick, and laser systems are significantly moreexpensive to install. Operating costs associated withmetal cutting fall into three categories:

(1) Buying the plasma arc cutting system,(2) The cost of work tables, ventilation and other

accessory equipment, and(3) The costs associated with operating the system.

7.1 Plasma Arc Cutting Equipment. Currently plasmaarc cutting systems typically cost from $1,500 for a smallhand-held air plasma arc cutting system to $40,000 for aprecision (high-current-density) plasma arc cutting system.The purchase price is higher for systems that can producehigher quality cuts, or cut faster by providing higherpower outputs. In general, conventional plasma arc and

oxyfuel gas cutting systems are the least expensive andprecision plasma arc cutting is medium priced. Lasersystems cost typically more than any plasma arc cuttingsystems.

7.2 Accessory Equipment Costs. Accessory equipmentcosts are dictated by the type of cutting setup required.Cutting tables with CNCs and ventilation equipmentoften account for the largest portion of the expense of acomplete cutting system when full automation is re-quired. Other costs such as material handling systems,water chilling equipment, and gas and water supply linesmay also be required.

7.3 Operational Costs. Operational costs consist oftorch parts, gas, electric power, operator labor, and over-head associated with operating and maintaining theplasma arc cutting system. Operational costs vary ac-cording to the type and size of system and the materialbeing cut. Some materials, for instance, can be cut wellusing air as the plasma and shield gases, while somematerials require significantly more expensive gas forhigh-quality cutting. Labor and overhead is the largestcomponent in the operational costs category—as muchas 85% of the total operating cost, as shown in Figure 21.Higher cutting speeds reduce the per-foot cutting cost,and the ability to produce square, clean cut edges reducethe overall cost of parts for manufacturing. The cost oflabor for any secondary operations such as dross removalalso needs to be considered for systems with narrow win-dows of dross-free operating conditions.

7.4 Process Comparisons. Conventional plasma arc, pre-cision plasma arc, oxyfuel gas and laser cutting systemseach excel at some aspect of cutting, such as type or thick-ness of metal, or cut-edge characteristics. In the areaswhere there is not a clear choice of which type of cuttingsystem is best, the two major considerations are cut qual-ity and the cost per foot of cutting with each system.

7.4.1 Plasma Arc Cutting vs. Oxyfuel Cutting.Plasma arc cutting systems provide cost-per-foot savingsfor all thicknesses up to 1.5 in. (38 mm). Oxygen plasmaarc cutting produces better quality cuts than air plasmaarc cutting, so it is often preferred for cutting mild steelbelow 2 in. (50 mm) even though oxygen is a more ex-pensive plasma gas. The comparative cost of air plasmaarc, oxygen plasma arc, and oxyfuel gas cutting for cut-ting mild steel up to 2 in. (50 mm) thick are shown inFigure 22. This cost comparison graph generalize thecosts of typical applications of plasma arc cutting. Everyactual application includes so many variable factors thatspecific cost information should be obtained from opera-tors’ actual costs or from equipment manufacturers esti-mates for more exact comparisons of cost for a particularinstallation.


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Figure 21—Typical Cutting Cost Breakdown—200 Amp O2 Plasma Cutting Operating

Figure 22—Comparison of Cost—200 Amp Plasma Arc Cutting vs. Oxyfuel Cutting


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7.4.2 “Precision Plasma” Arc Cutting vs. LaserCutting. The cost of precision plasma arc cutting(another name for high-current-density plasma arc cut-ting) can be compared with laser cutting since precisionplasma arc cutting now competes with laser in quality ofcut produced. Precision plasma arc cutting systems pro-vide advantages in both equipment and operational costs.A typical precision plasma arc cutting setup costsapproximately half the cost of a laser cutting system.Although the cost of the cutting table, control system andventilation equipment is similar, the higher capital costof the laser accounts for the difference.

7.4.3 Plasma Arc Gouging vs. Air Carbon ArcGouging. Plasma arc gouging and carbon arc gougingare comparable processes when removing welds, prepar-ing plates for welding, or other gouging tasks. Plasma arcgouging, however, is the less expensive gouging method.In addition, it is cleaner and more versatile, being able togouge aluminum alloys and stainless steel as well as mildsteel.

8. Safety and Health HazardsAs discussed in the Introduction and other sections of

this booklet, plasma arc cutting and gouging systems in-volve components and aspects that pose safety risks, in-cluding electrical shock, flammable gases, fumes andgases, noise, radiated heat, fire and burns. Proper instruc-tion and training for system operators is essential forsafety in the workplace. Plasma arc cutting and gougingoperators should always read and understand the manu-facturer’s safety and operating instructions before operat-ing the equipment. This technical document does notaddress all welding and health hazards. However pertinentinformation can be found in the following documents:

(1) NFPA 70, National Electrical Code®

(2) ANSI Z49.1, Safety in Welding, Cutting, and Al-lied Processes

(3) OSHA Safety and Health Standards, 29CFR Part1910

(4) American National Red Cross First Aid andSafety Handbook

(5) NFPA Standard 51B, Standard for Fire PreventionDuring Welding, Cutting, and Other Hot Work

(6) AWS Safety and Health Fact SheetsInformation regarding the publisher of these documentscan be found in Section 2 for the standards and AppendixC for other references.

The following paragraphs discuss in greater detailhazards associated with plasma arc cutting and gougingsystems.

8.1 Electrical Shock. Voltages present in plasma arc cut-ting and gouging equipment may be as high as 400 V DCoutput, which represents a potentially lethal shock haz-ard. The equipment must be properly grounded as rec-ommended by the manufacturer and applicable electricalcodes, and safe work practices must be followed. Emer-gency first aid should always be available, since promptand appropriate emergency response may reduce the ex-tent of injury caused by an accidental electrical shock.Untrained, unskilled personnel should not be permittedto operate or maintain the equipment.

Additional safety reminders for reducing the shockhazard and chance of equipment damage are listed below:

(1) Keep all electrical circuits dry, since moisture mayprovide an unexpected path of current flow. Equipmentcabinets that contain water and gas lines, as well as elec-trical circuits, should be checked periodically for leaks.

(2) All electrical connections should be clean, dry,and tight. Poor electrical connections can generate heat,and may represent a fire risk.

(3) Consult the manufacturer’s specifications forproper cable and wire sizes, to prevent cable damage andreduce the chance of equipment failure. The cable shouldbe rated at no less than 600 V DC. Keep cables and wiresin good repair to reduce the possibility of electrical shockor equipment damage.

(4) Do not touch live circuits, keep equipment accessdoors closed, and never bypass or disable safety interlocksystems.

(5) The risk of electrical shock is present when in-specting or replacing electrodes, nozzles, and other torchparts. Operators must always disconnect the primarypower and the power to the control circuitry before in-specting or replacing torch parts, to prevent electricalshock.

(6) Wear dry, hole-free clothing and gloves. Be insu-lated from the work and ground.

(7) Operators and maintenance personnel should beaware that plasma arc equipment presents a greater haz-ard than typical welding equipment, due to the highervoltages involved. Only properly trained personnelshould operate or service the equipment.

(8) Plasma arc power sources often contain largecapacitors that store hazardous levels of electrical energy.Capacitors should be allowed five minutes to dischargeenergy to safe levels and should never be discharged witha screwdriver or other implement.

8.2 Explosion. Plasma arc cutting systems use com-pressed gases, some of which may be flammable. Properprecautions must be observed when handling and usingcompressed gases and pressurized systems. Equipmentmanufacturers should furnish complete instructions foroperating plasma arc equipment safely.


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Operators must be properly trained in safe operatingpractices and follow ANSI, OSHA and other local andnational safety codes for safe handling of flammable orpotentially explosive gases such as oxygen, hydrogenand methane, and gas mixtures. Other important safetyreminders are these:

(1) Oxygen—now widely used in plasma arc cut-ting—accelerates combustion dramatically and must behandled with care. An oxygen hose should never be usedfor any other gas.

(2) Do not operate a plasma arc cutting system in anyatmosphere that may contain explosive vapors or dust.

(3) Do not cut pressurized or closed containers. Theymay explode.

(4) Make sure all regulators are operating properly.(5) Keep flames away from hoses and containers car-

rying flammable gases. Explosion may result.(6) Inspect all hoses regularly and replace immedi-

ately any hose that appears damaged. Leaks may causeexplosion or asphyxiation.

8.2.1 Explosion Hazards when Cutting AluminumAlloys. When cutting aluminum alloys underwater, orwith the water touching the underside of the aluminumalloys, free hydrogen gas may collect under the work-piece, due to hydrogen released by the interaction ofmolten aluminum alloys with the oxygen component ofthe water molecules. The hydrogen can accumulate inpockets under the workpiece and ignite when the plasmaarc passes near or over a pocket of hydrogen gas. Hydro-gen gas accumulation can be minimized by aerating oragitating the water in the water table. Contact the equip-ment manufacturer for recommended practices for alu-minum alloys cutting.

8.3 Fumes and Gases. Plasma arc cutting and gougingproduce by-product fumes and gases which can causeharm to operators or persons nearby. The compositionand rate of generation of the fumes and gases depend onmany factors, including arc current, cutting speed, platematerial, orifice gas and others. The fume and gas by-products will usually consist of the oxides of the ele-ments in the metal being cut, as well as ozone and oxidesof nitrogen. These fumes must be eliminated at thesource or removed from the work area through an ex-haust system. Local codes may require that the exhaustbe cleaned before being vented into the atmosphere. Dif-ferent types of fume removal systems are available formechanized cutting systems, including air vacuum/filtersystems, collecting the fumes in water, or both. See 4.6for more complete descriptions of these pollution controlsystems and Annex C for reference information.6

6. See Annex C, References 15 and 17.

(1) Water Table and Water Muffler. This system en-capsulates the plasma cutting arc completely and cap-tures fumes in the water, which requires periodiccleaning or conditioning.

(2) Underwater Cutting. This system submerges theworkpiece 2 in. to 3 in. (51 mm to 76 mm) below thewater surface and offers the best pollution containment.

(3) Down-Draft Exhaust Table. This dry method col-lects the fumes and particulates from the area of the cuttingarc using powerful exhaust fans. It is particularly effectiveat controlling fumes when using plasma arc cutting sys-tems that do not employ water injection or water shield.

(4) Collector Cup and Torch Housing. This systemcan be used for fume containment on cutting machineswhere a water table or down-draft table is not practical,such as an X-Y table on which the workpiece is inmotion.

8.4 Noise. The amount of noise generated by a plasmaarc cutting or gouging system varies according to thecurrent being used. For example, one torch operating at400 A generates approximately 100 dBA, measured at adistance of 6 ft. At 1000 A, the noise level is about135 dBA at the same distance.

Much of the noise is in the frequency range of5000 Hz to 20 000 Hz, which can damage hearing un-less hearing protection is worn where the noise levelexceeds specified limits. The maximum acceptable noiselevel, according to 1995 U.S. Occupational Safety andHazards Act (OSHA) regulations, is 90 dBA for 8 hourscontinuous exposure, 95 dBA for 4 hours, and 100 dBAfor 2 hours.

The water muffler system, described in 4.6.1, reducesnoise effectively in mechanized cutting applications. Theshroud of water acts as a sound-deadening enclosurearound the torch nozzle, and the water below the platekeeps noise from escaping through the kerf opening.Noise reduction is typically 20 dBA, sufficient for opera-tors in many situations without requiring additional hear-ing protection. The noise-reducing capabilities of watermufflers should not be confused with water injection andwater shielding. Neither of these processes uses a suffi-cient flow of water for noise reduction. Underwaterplasma arc cutting, however, provides greater noise re-duction than a water muffler because the arc and theentire working end of the torch are submerged. Noiselevels for 400 A to 600 A underwater plasma arc cuttingoperating are typically 85 dBA, which is the quietest pos-sible plasma arc cutting process. With greater than 3 in.(75 mm) thick materials, however, underwater or watertable/water muffler techniques cannot be used. Startingat this thickness level, water must be eliminated from thecut zone, which also removes effective control of fume,smoke, and radiation. Operators and other personnel inthe general area of the cutting operation must wear ap-


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propriate safety equipment. The collector cup/vacuumhood system provides some noise reduction, due to thephysical containment of the cut zone, but water mufflertechniques are usually more effective.

8.5 Radiated Light. Besides heat, a plasma arc radiatesintense visible and invisible light. In addition to the po-tential harm to the eyes and skin, the radiation may pro-duce ozone and oxides of nitrogen. Eyes and skin mustbe protected against arc radiation hazards. Operators andothers nearby must wear eye protection designed toguard against ultraviolet and infrared radiation, as shownin Table 3.

Mechanical barriers such as walls or welding curtainswill reduce radiation exposure in the vicinity of cutting or

gouging operations. A water muffler can function as alight-absorbing shield when dye is added to the water ta-ble, but operators should wear eye protection even whenusing water dyes in case of accidental interruption ofwater flow through the water muffler. In addition, waterdyes do not filter ultraviolet radiation effectively. Under-water plasma arc cutting reduces the amount of radiationbecause of the depth of the water in which the arc is oper-ating, but operators and observers should wear eye pro-tection as shown in Table 3. The torch housing also servesas a radiation shield, since it encloses the arc. However,eye protection is still required since sparks and light mayescape the housing under a variety of conditions.

8.6 Radiated Heat/Spatter and Sparks. Plasma arc cut-ting and gouging produce significant by-product heat, aswell as metal sparks and hot spatter, which create a firehazard in the surrounding area. Personnel should wearfire-retardant clothing and cover exposed skin to preventburns from heat or particles, and fire extinguishersshould be placed near the cutting station. In addition tothe fire hazard, there may be an explosion hazard whencertain gases are used, such as hydrogen mixtures. Referto the National Fire Protection Association Standard51B, Standard for Fire Prevention During Welding, Cut-ting, and Other Hot Work, concerning these hazards.

8.7 Chlorinated Solvents. Some chlorinated solvents,e.g., trichloroethylene, methylene chloride, and ethylchloride can decompose into phosgene gas—a poisongas—when exposed to ultraviolet radiation. Keep cuttingareas ventilated, and keep chlorinated solvents awayfrom the cutting area.

Operators should be sure to read and understand theMaterial Safety Data Sheet of any solvent in use.

Table 3Recommended Shade Densities

for Plasma Arc Cutting Filter Lenses(1)

Current Range (A) Minimum Shade Comfortable Shade(2)

Less than 300 8 9

300 to 400 9 12

>400 10 14

Notes:(1) Adapted from ANSI Z49.1, Safety in Welding, Cutting and Allied

Processes. See Section 2 for information in obtaining this document.Also see AWS F2.2, Lens Shade Selector Chart.

(2) As a rule of thumb start with the shade that is too dark to see thecutting zone. Then go to a lighter shade which gives a sufficientview of the cutting zone without going below the minimum.


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A1. IntroductionThe AWS Board of Directors has adopted a policy

whereby all official interpretations of AWS standardswill be handled in a formal manner. Under that policy, allinterpretations are made by the committee that is respon-sible for the standard. Official communication concern-ing an interpretation is through the AWS staff memberwho works with that committee. The policy requires thatall requests for an interpretation be submitted in writing.Such requests will be handled as expeditiously as possi-ble but due to the complexity of the work and the proce-dures that must be followed, some interpretations mayrequire considerable time.

A2. ProcedureAll inquiries must be directed to:

Managing Director, Technical ServicesAmerican Welding Society550 N.W. LeJeune RoadMiami, FL 33126

All inquiries must contain the name, address, and af-filiation of the inquirer, and they must provide enough in-formation for the committee to fully understand the pointof concern in the inquiry. Where that point is not clearlydefined, the inquiry will be returned for clarification. Forefficient handling, all inquiries should be typewritten andshould also be in the format used here.

A2.1 Scope. Each inquiry must address one single provi-sion of the standard, unless the point of the inquiryinvolves two or more interrelated provisions. That provisionmust be identified in the Scope of the inquiry, along with

the edition of the standard that contains the provisions orthat the Inquirer is addressing.

A2.2 Purpose of the Inquiry. The purpose of the inquirymust be stated in this portion of the inquiry. The purposecan be either to obtain an interpretation of a standard re-quirement, or to request the revision of a particular provi-sion in the standard.

A2.3 Content of the Inquiry. The inquiry should beconcise, yet complete, to enable the committee to quicklyand fully understand the point of the inquiry. Sketchesshould be used when appropriate and all paragraphs, fig-ures, and tables (or the Annex), which bear on the in-quiry must be cited. If the point of the inquiry is to obtaina revision of the standard, the inquiry must provide tech-nical justification for that revision.

A2.4 Proposed Reply. The inquirer should, as a pro-posed reply, state an interpretation of the provision that isthe point of the inquiry, or the wording for a proposed re-vision, if that is what inquirer seeks.

A3. Interpretation of Provisions of the Standard

Interpretations of provisions of the standard are madeby the relevant AWS Technical Committee. The secre-tary of the committee refers all inquiries to the chairmanof the particular subcommittee that has jurisdiction overthe portion of the standard addressed by the inquiry. Thesubcommittee reviews the inquiry and the proposed replyto determine what the response to the inquiry should be.Following the subcommittee’s development of the re-sponse, the inquiry and the response are presented to theentire committee for review and approval. Upon approval

Annex A

Guidelines for Preparation of Technical Inquiriesfor AWS Technical Committees

(This Annex is not a part of AWS C5.2-2001, Recommended Practices for Plasma Arc Cutting and Gouging, but isincluded for information purposes only.)


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by the committee, the interpretation will be an official in-terpretation of the Society, and the secretary will transmitthe response to the inquirer and to the Welding Journalfor publication.

A4. Publication of InterpretationsAll official interpretations will appear in the Welding

Journal.

A5. Telephone InquiriesTelephone inquiries to AWS Headquarters concerning

AWS standards should be limited to questions of a gen-eral nature or to matters directly related to the use of thestandard. The Board of Directors’ policy requires that allAWS staff members respond to a telephone request foran official interpretation of any AWS standard with theinformation that such an interpretation can be obtained

only through a written request. The Headquarters staffcannot provide consulting services. The staff can, how-ever, refer a caller to any of those consultants whosenames are on file at AWS Headquarters.

A6. The AWS Technical CommitteeThe activities of AWS Technical Committees in regard

to interpretations, are limited strictly to the interpretationof provisions of standards prepared by the committee orto consideration of revisions to existing provisions on thebasis of new data or technology. Neither the committeenor the staff is in a position to offer interpretive or con-sulting services on: (1) specific engineering problems; or(2) requirements of standards applied to fabrications out-side the scope of the document or points not specificallycovered by the standard. In such cases, the inquirershould seek assistance from a competent engineer experi-enced in the particular field of interest.


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Both the plasma arc cutting process and the gas tung-sten arc welding process, from which it derived, sharesome terminology. However, due to the greater complex-ity and recent developments in plasma arc cutting newterms have come into common usage. Familiarity withthese terms will provide a common language for opera-tors, service personnel, and management, and will alsohelp readers make useful comparisons between the sys-tems available. Refer to AWS A3.0, Standard WeldingTerms and Definitions, for a complete list of terms usedin this document. A term preceded by an asterisk is notcurrently defined in AWS A3.0 or is a variation of theA3.0 definition.

*arc length, plasma arc cutting. The distance from theelectrode to the attachment point on the workpiece.

*backlash. A mechanical situation where a reversal ofdriving motion does not result in the immediate rever-sal of the driven assembly. Excessive backlash isgenerally caused by damaged, worn or poorly manu-factured parts.

*ball transfer. A ball bearing assembly used to reducethe friction experienced by an object that must slideover or under another object. Ball transfers are com-monly used in work tables.

*ballscrew. A mechanical drive system componentresembling a threaded rod which is used to convertrotary motion to straight line motion.

*bevel cutting. A plasma arc cutting technique that usesa tilted torch to produce an angle on the edge of partsbeing cut.

*chopper. A high-performance plasma arc power sourcedesign using power semiconductor switching techniques.

*computer numeric control (CNC). Microprocessor-based electronic machine control used to coordinateand manage the cutting machine functions.

*consumables, plasma arc cutting. Torch parts that areeroded or otherwise worn during normal cutting orgouging operations, such as electrodes, nozzles,shields, caps and swirl rings.

*coordinate drive. Machine motion system wheremovements are electronically controlled. Axis motioncan be executed independently as with point to pointpositioning or can be synchronized as with contourcutting.

*current density, plasma arc cutting. A measure of thedegree of arc constriction achieved with a plasma arctorch. Units are Amperes/in.2 or mm2.

*cut angle/bevel angle, plasma arc cutting. The angleformed between the cut surface and a theoreticalplane perpendicular to the plate surface. Plasma arccutting tends to remove more metal from the top thanfrom the bottom, producing a cut angle.

*cycle time. The total amount of time needed to processa part, including the time needed for material handlingand other noncutting activities.

*diagnostics. Messages issued by a control which indi-cate faults and aid in troubleshooting.

*direct numeric control (DNC). The practice of creatingand maintaining part programs in one location and down-loading them to the cutting machine location whenneeded.

double arcing. A condition in which the welding or cut-ting arc of a plasma arc torch does not pass throughthe constricting orifice but transfers to the inside sur-face of the nozzle. A secondary arc is simultaneously

Annex B

Terms and Definitions(This Annex is not a part of AWS C5.2-2001, Recommended Practices for Plasma Arc Cutting and Gouging, but is

included for information purposes only.)


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established between the outside surface of the nozzleand the workpiece.

*drag, plasma arc cutting. The offset distance betweenthe entrance and exit points of the gas stream on theplate being cut, measured on the cut edge.

*drag angle, plasma arc cutting. The angle between theimpinging jet stream centerline and a direction perpen-dicular to the plate surface.

*drive. A term used to describe any system of compo-nents used to produce machine motion, including suchcomponents as motors, ballscrews, servo amplifiers,etc.

*dross, plasma arc cutting. Resolidified molten metaland oxides adhering to the top or bottom edge duringthermal cutting.

*duty cycle, plasma arc cutting. A power source specifi-cation describing the percentage of time a system canbe operated at a given current level. Based on a tenminute cycle.

*electrode, plasma arc cutting. The plasma arc torchpart from which arc current is emitted.

*feedback. A signal used by a controller to monitor theevents being controlled and modify output commands.

*fumes. Airborne solid particulate matter generated bythe welding process. Fume particles are usually sub-micron in size, and thus tend to remain airborne anddrift with the air currents.

*hafnium. The metal used most commonly for the elec-trode emitter for air or oxygen plasma gases.

*heat-affected zone, plasma arc cutting. The region onthe edge of the plasma cut where the heat of the archas changed the properties of the material. Generallya hardening of the material is the primary concern.

*high-frequency discharge. High-voltage power sup-plied to the plasma torch, which breaks down the airgap between the nozzle and electrode in order to ini-tiate the plasma stream.

*inverter power source. A high-performance plasmapower source design which takes advantage ofadvanced power semiconductor circuitry to reduce thesize and weight of the transformer and, hence, theoverall size of the power source.

*kerf, plasma arc cutting. The void left by the linearremoval of material by any kind of cutting process(saw, plasma arc, oxyfuel gas, etc.)

*manual data input (MDI). A CNC operating mode inwhich the operator may enter part program informa-tion or short commands.

*nesting. The creation of an array of parts on a plate,generally arranged to minimize the amount of scrapafter cutting.

*nibbling. A method of producing parts using a punchpress to rapidly punch the outline of the desired shape.

*nontransferred arc. An arc established between theelectrode and the constricting nozzle of the plasma arctorch. The workpiece is not in the electrical circuit.

*nozzle, plasma arc cutting. A “consumable” torch partcontaining a hole, or orifice, through which the arcpasses.

*nozzle diameter, orifice diameter. The diameter of thenozzle through which the plasma arc passes.

*off-line. Not under machine control. Operations such asguidance programming are developed before produc-tion time.

*open circuit voltage, plasma arc cutting. The voltagepresent at the electrode in the brief internal before thetorch ignites and whenever the arc is extinguishedwith the power source still active.

*overshoot. A condition where a controlled variablesuch as standoff or workpiece position will brieflyexceed or hunt around its commanded value.

oxyfuel gas cutting (OFC). A group of oxygen cuttingprocesses that uses heat from an oxyfuel gas flame.

*pantograph. A mechanism which follows a template toreproduce the shape of the template to scale.

*photo optical tracing. A shape-cutting technique inwhich the cutting machine takes its motion controlinformation from a photoelectric sensor that scans adrawing of the desired shape during the cutting process.

*piercing. A method of starting a plasma arc cut inwhich the arc plunges into and through the workpiecebefore cutting begins.

pilot arc. A low current arc between the electrode andthe constricting nozzle of the plasma arc torch to ionizethe gas and facilitate the start of the welding arc.

*pilot hole. On a punch/plasma press, a punched hole atthe edge of which the plasma arc cut is started.

*plasma, arc plasma in AWS A3.0. A gas that has beenheated by an arc to at least a partially ionized condi-tion, enabling it to conduct an electric current.


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*plasma arc cutting (PAC). An arc cutting process thatuses a constricted arc and removes the molten metal ina high-velocity jet of ionized gas issuing from theconstricting orifice. Plasma arc cutting is a direct cur-rent electrode negative (DCEN) process.

*plasma gas/cutting gas/orifice gas. A gas directed intothe torch to surround the electrode, which becomesionized by the arc to form a plasma, and issues fromthe torch nozzle as the plasma jet.

*plenum chamber. The space between the electrode andthe inside wall of the constricting nozzle of theplasma arc torch.

*proximity probe. Electronic inductive proximity sensorscapable of detecting the location of the workpiece,often used for initial height sensing.

*punched tape. A method of storing and transportingpart programs in which characters are recorded on thepaper or Mylar tape using standardized patterns ofpunched holes. Programs generated off-line areloaded into the machine controller using a tape reader.Punched tape for data storage is being phased out infavor of disks and DNC.

*rack & pinion. A mechanical drive system used to con-vert rotary motion to straight line motion. These unitsconsist of a relatively small round pinion gear whichengages a length of toothed rectangular bar.

*SCR power source. A type of power source that uses asemiconductor device known as the silicon-controlledrectifier in the main power circuit.

*secondary gas/shielding gas, plasma arc cutting.Unlike the plasma gas, the secondary gas does not

pass through the orifice of the nozzle. It passes aroundthe nozzle and forms a shield around the arc.

*servo system. Electronic machine tool positioning system.

*shape library. A CNC feature that provides a collectionof commonly used shapes for part programs, whichcan be scaled, rotated and translated as necessary tocreate the desired cut-part shapes.

*standoff/torch-to-work distance, plasma arc cutting.The distance between the outer most portion of thetorch and the work surface.

*thoriated tungsten. A metal used as the plasma cuttingelectrode emitter for a non-oxidizing plasma gas, suchas nitrogen.

transferred arc. A plasma arc established between theelectrode of the plasma arc torch and the workpiece.

*turret. An assembly which holds punching tools on apunch press. The turret rotates to select different tools.

*ultraviolet light. Short wavelength light emitted duringarc cutting and welding processes that is harmful tothe eyes and skin.

*vortex. An intense swirling gas similar to a tornado.Most plasma arc cutting systems develop a vortex ofsome extent in the nozzle during cutting.

*workclamp. An assembly used to hold the workpiece,generally using hydraulic pressure for clamping force.

*workpiece, plasma arc cutting. The piece of material tobe cut or gouged.

*zirconium. An element used in some electrodes as aplasma arc cutting electrode emitter. Although similarto hafnium, zirconium provides a shorter service life.


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(1) R. Finch, Welder’s Handbook: A Complete Guideto Mig, Tig, Arc & Oxyacetylene Welding, February,1997.

(2) American Machinist, “Technology Trends,” July1993, p. 22.

(3) ASM Handbook, Vol. 6, 1993, pp. 1166–1171.(4) AWS Welding Handbook, Vol. 2, 8th Edition,

1994, pp. 482–489.(5) Colt, J. D., “Longlife® Oxygen Plasma Arc Cut-

ting,” Hypertherm Inc., Hanover, NH, 1994.(6) DIN 2310 – Teil 4 (German) Thermal Cutting;

Plasma Arc Cutting, Principles of Process, Terms, Quality,Dimensional Tolerances, September 1987.

(7) Engblom, G. and Falck K., Welding in the World,Vol. 28, No. 11/12, pp. 204–215, 1990.

(8) Fernicola, R. C., Welding Journal, “New OxygenPlasma Arc Process Rivals Laser Cutting,” June 1994,pp. 65–69.

(9) Handal M. D., Kathleen, The American Red CrossFirst Aid and Safety Handbook, Little Brown & Co.ISBN: 0316736465

(10) Harris, Ian, “Plasma Arc Cutting of BridgeSteels,” Edison Welding Institute, 1997.

(11) Houlcroft, P., John, R., “Welding and Cutting, AGuide to Fusion Welding and Associated Cutting Pro-cesses,” Woodhead-Faulkner Ltd., 1988.

(12) Lindberg, B., Welding & Metal Fabrication, April1990, p. 166.

(13) Linde Publication: “Plasma Cutting Fundamen-tals,” reprinted in Australian Welding Journal, Winter1987, pp. 5–8.

(14) Manohar, M., Snyder, J. P. II, “Dross FormationDuring Plasma Arc Cutting of Steel,” Welding Journal,November 1994.

(15) G. Pilot et al, “Measurements of Secondary Emis-sions from Plasma Arc and Laser Cutting in StandardExperiments,” Decommissioning of Nuclear Installations,ISBN:1-85166-523-4, Elsevier Applied Sciences, 1989.

(16) Plasma-Arc Cutting and Welding, PAC and PAW,Welding Design and Fabrication, 65(6), pp. 23–56, June1992.

(17) Trainor, R. S., “Fume Emissions Testing forPlasma Arc Cutting,” Hypertherm Inc., Etna Road, Ha-nover NH, 1998.

(18) Wu, Y., Hackett, C., Eickhoff, S., “The Effects ofPlasma Arc Cutting on Mild Steel,” Hypertherm Inc.1997.

(19) Physik und Technologie des Plasmastrahls in derSchweiss-, Schneid und Spritztechnik: Vortrèage d. 3.Kolloquiums Physik u. Technologie d. Plasmastrahls ind. Schweiss-, Schneid u. Spritztechnik in Aachen am 20.u. 21. Mèarz 1975.

Annex C

Selected Reading List(This Annex is not a part of AWS C5.2:2001, Recommended Practices for Plasma Arc Cutting and Gouging, but is

included for information purposes only.)


AWS C5.2:2001

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AWS List of Documents on Arc Welding and Cutting

AWS Designation Title

C5.1 Recommended Practices for Plasma Arc Welding

C5.2 Recommended Practices for Plasma Arc Cutting and Gouging

C5.3 Recommended Practices for Air Carbon Arc Gouging and Cutting

C5.4 Recommended Practices for Stud Welding

C5.5 Recommended Practices for Gas Tungsten Arc Welding

C5.6 Recommended Practices for Gas Metal Arc Welding

C5.7 Recommended Practices for Electrogas Welding

C5.10 Recommended Practices for Shielding Gases for Welding and Plasma Arc Cutting

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