Arc Cutting Principles and Arc Cutting Practice: Jobs 17-J1-J7

Arc Cutting Principles and Arc Cutting Practice: Jobs 17-J1-J7
Chapter 17

Objectives Describe arc cutting principles.
Identify plasma arc cutting (PAC) techniques. Identify air carbon arc cutting (CAC-A) techniques. Perform visual inspection of cuts and gouges. Describe equipment setup and use.

Objectives Perform troubleshooting of cut and gouge quality.
Perform straight line, curve square cuts, bevel cuts, and gouges on a variety of shapes and metals with PAC. Perform gouges with CAC-A.

Manual Arc Cutting Universal tool Increases speed of fabrication
Widely used in foundries for removal of risers from castings, and scrapping obsolete metal structures Used in fabrication of structures requiring heavy thicknesses of metal from rolled metal Increases speed of fabrication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Miller Electric Mfg. Co.

Arc Cutting Process melts metal along desired line of cut with heat generated by electric arc Number also use oxygen, compressed air, or an inert gas in addition to arc Compare favorably with oxy-fuel gas cuts Advantage – can be used on all types of metal Also used for hole piercing, rivet cutting, gouging, and other special uses Done with plasma arc, carbon arc and metal arc

Plasma Arc Cutting (PAC)
Similar to gas tungsten arc Introduced by Linde division of Union Carbide in 1955 Produce economical, high quality, ready-to-weld cuts Can cut aluminum, magnesium, and stainless steels Arc stream much hotter than melting temperatures of both metals and their oxides Praxair, Inc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

PAC High speed process Requirement that metal being cut must be able to conduct electricity Kerf formed by blowing out molten metal Plasma Fourth state of matter Ionized so can conduct electric current Purpose of gas is to maintain electric arc

This video clip shows real life and animation of how PAC works.
Miller Electric Mfg. Co.

Plasma Arc Torch Use transferred arc for cutting
Material being cut and torch act as electrodes in electric circuit Tip of electrode located within nozzle Nozzle has small opening that constricts arc High pressure gas flows through electric arc Heated to plasma temperature (25,000ºF) Expanding gas force through small opening in nozzle and emerges at high velocity Hot jet of gas can melt any known metal

Schematic Diagram of Plasma Arc Cutting Equipment
Gives high quality cut Use good ventilation when doing plasma arc cutting to remove all fumes from area. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Mechanized Plasma Arc Cutting
Equipment needed for system Torch Cutting control box Gas regulators Power supply Carriage unit Supply of cutting gases Water

Torch Used for cutting all metals Water cooled
Variety of nozzles to permit use of different gases Amount of current affects nozzle selection Single-port nozzles used with argon- hydrogen and nitrogen-hydrogen mixtures Multiport and dual flow nozzles produce better results using compressed air or oxygen as cutting gas Praxair, Inc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Torch Multiport plasma arc cutting torch
Dual flow plasma arc cutting torch Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Torch Can be supplied by two gases
Plasma gas Shielding gas (gas to help cool torch head) Operate at extremely high temperatures Various parts considered to be consumable Electrode and tip vulnerable to wear during cutting Cutting performance deteriorates as they wear Current torches are self-aligning and self-adjusting Periodic inspection of torch parts should be performed per manufacturer’s maintenance recommendations

Control Unit Provides sequence of operations and control of all functions Such as arc starting, varying gas flow, varying power level, carriage travel and flow of water Interlocks used with PAC systems Shut systems down if not adequate or working properly Gas and coolant systems ESAB Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Environmental Controls
Generates tremendous amount of noise and fumes that needs to be overcome Cut over water table and surround arc with water shroud Circulating pump to circulate water from table through shroud Work under 3 inches of water Working end of torch and cut completely submerged Flow of compressed air prevents water from entering torch when not cutting Coloring agents added to water to reduce glare

Electrodes Special tungsten electrodes used in cutting torch
Held in place by collet Type, shape, location critical for proper operation

Power Supply Direct current, electrode negative constant current (drooper) type used for power Open circuit voltage: range of 150 to 400 volts Heavy cutting: 400 open-circuit volts and 200 kilowatts or more Transformer-rectifier and inverter type power units available May be connected in series for higher voltage requirements ESAB Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Regulators Required for controlling flow of plasma and shielding gas
Gas flows through main port of cutting torch at 60 to 350 cubic feet per hour

Cutting Gases Nonoxidizing gas such as mixture of argon-hydrogen or nitrogen Cuts aluminum, stainless steels, and other nonferrous metals Oxidizing gas Provides additional heat from iron-oxygen reaction Cuts carbon steel, cast iron, and certain alloy steels Nitrogen, oxygen, or compressed air Life of electrodes, operating in oxygen, short

Water Supply High pressure and high rate of flow necessary to dissipate heat generated in torch Necessary to install circulatory system for adequate flow

Manual Plasma Arc Cutting
For workpieces that cannot be adapted to mechanized setup For remote locations For specialized work on odd-shaped pieces Equipment needed Manual torch, electrodes power source, control unit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Praxair, Inc.

Torch Air-cooled with 100-ampere capacity Right-angle head
Pilot arch established by pressing switch on torch handle Cutting arc established when brought to within 1/2 inch of workpiece Arc immediately extinguished when welder releases torch switch

Plasma Arc Starting with High Frequency
Power turned on and torch switch turned on, 2–3 seconds of preflow gas before pilot arc starts Pilot arc occurs between electrode and torch tip Nontransferred, noncutting arc Cutting gas superheated to over 30,000ºF when reaches pilot arc Pilot arc brought close to workpiece and completes electric circuit Referred to as transferred or cutting arc Pilot arc relay will open and shut pilot power off

Plasma Arc Starting with High Frequency
Most common starting method Strike high frequency spark between electrode and torch tip Pilot arc created by high frequency High voltage produced by transformer and spark gap oscillator Device referred to high frequency generator May interfere with telephones, computers, or machine controls if system not properly installed

Pilot and Cutting Arc Miller Electric Mfg. Co.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Plasma Arc Starting with Contact Starts
Contact start pilot arc formed between nozzle and inner electrode Created when electrode in contact with torch tip and retracts when trigger pulled Airflow pulls or forces electrode to retract Small narrow opening of torch tip accelerates expanding plasma toward workpiece Pilot arc crosses gap allowing main electric plasma current to follow becoming the transferred cutting arc

Control Unit Built into power source for manual cutting
Regulates electrical, gas and water supplies Separate flowmeters provided for argon and hydrogen Most manual cutting done with compressed air Solenoid valves provide flow of pure argon to torch to establish pilot arc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Praxair, Inc.

Cutting Gases Mixture of 80% argon and 20% hydrogen used for cut quality, brightness, fume generation, and cost Compressed air as plasma gas and secondary cooling gas Compressor external to power source or compressor built in Secondary or cooling gas can be argon or nitrogen Other gases can be supplied premixed in cylinders or supplied to gas mixer device

Advantages of Plasma Cutting
Dross-free cuts on carbon and stainless steels, nickel, Monel®, Inconel®, cast iron, clad steels, aluminum, copper, and magnesium Clean cuts on most metals up to 5" thick Precision cuts with a narrow kerf Minimum heat-affected zone Cutting speeds up to 300 inches per minute (many times faster than oxyacetylene cutting)

Advantages of Plasma Cutting
Cuts of such quality that machining or finishing is not needed in many cases Almost no distortion of metals No bowing or cambering. Magnetic permeability and hardness little affected Stack cutting of several sheets of 1/16 to 1/4 inch thickness possible Can be used for hole piercing, gouging, and scarfing

Cutting Carbon Steel Dross-free cuts with smooth surfaces and sharp edges No preheating required Stack cutting of sheets produces cuts comparable to those obtained when cutting one Superior results obtained when nitrogen and oxygen used Cutting speed higher than that for oxyacetylene See Table 17-1A

Cutting Stainless Steel
Completely dross-free Up to 2 inches thick without further finishing Radiographic quality welds produced without further cleaning of cut surfaces Cut with mixtures of argon and hydrogen or with nitrogen mixtures See Table 17-1B

Cutting Aluminum Equal or better quality at much faster speeds than other cutting methods Dross-free for thicknesses up to 5 inches Excellent cuts on magnesium using higher cutting speeds See Table 17-1C

Air Carbon Arc Method of cutting and gouging by melting work with electric arc and blowing away molten metal with strong jet of compressed air Can be used on all metals No oxidation Metals not harmed by process Arcos Corp. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Air Carbon Arc Arc struck between carbon electrode and metal
Metal melts instantly and high velocity jets of air blast molten metal away Air blast continuous and directed behind point of arcing Electrode travel speed Depends on size of electrode, type of material, amperage, and air pressure Depth and contour of groove controlled by electrode angle, travel speed, and current Width determined by size of electrode

Table 17-2 lists types of current and polarities for different metals.
Air Carbon Arc Cutting Commonly used in metal fabrication, can remove weld defects, gouge out cracks before damaged railroad cars and tracks are welded Air carbon arc electrode made of carbon and graphite; coated with copper Sizes range from 5/32 to 1 inch Flat electrode allows greater flexibility Table 17-2 lists types of current and polarities for different metals.

Air Carbon Arc Cutting Compressed air passes through holes in electrode holder which directs it parallel to electrode Holder may be air or water cooled Direct current, electrode positive polarity used for most applications Ordinary compressed air supplied by compressor (40 to 125 p.s.i.)

Air Carbon Arc Cutting Thermadyne Industries Inc./Arcair Co. Air carbon arc torch withflat electrode that makes it possible to cut with more precision. Thermadyne Inductries Inc./Arcair Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Mechanized Air Carbon Arc Torch and Carriage
Thermadyne Industries Inc./Arcair Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Gas Metal Arc Cutting (GMAC)
Developed from gas metal arc (MIG) welding Heat obtained from electric arc formed between electrode (continuous), metal wire, and plate Wire becomes white hot, vaporizes, establishing arch between itself and workpiece Inert gas shields operation and work from contamination Gas is chemically inactive

Forces result in metal running from kerf Pressure of shielding gas Jumping action of arc Metal vapor from electrode Arc extends between leading edge of wire and edge of plate along entire depth of cut Nature of cut influenced by voltage and wire-feed speed

Width of kerf increases with voltage Cutting speed determines amount of current Speed increases, current must be increased Wire: steel, stainless-steel and aluminum Argon most common gas used Process not used extensively

Gas Tungsten Arc Cutting (TIG Cutting) (GTAC)
Developed from gas tungsten arc (TIG) welding process Electric arc struck between tungsten electrode and metal workpiece to provide intense heat High velocity jet of gas comes through nozzle of torch and ejects melted metal, thus producing cut Arc started by high frequency spark or by making contact with plate Molten metal blown away by stream of gas to form kerf Higher currents used for arc cutting than for gas tungsten arc welding

Quality of cut good Brings all advantages of oxygen cutting to cutting of nonferrous metals and stainless steel Can cut aluminum, magnesium, copper, silicon, bronze, nickel, copper-nickel and stainless steels Can be used for manual or mechanized cutting Materials as thick as 1/2" aluminum Praxair, Inc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Same general equipment used for both TIG welding and cutting d.c. motor-generator or transformer-rectifier welding machine with straight polarity recommended Gas tungsten arc welding torches can be used for cutting Cap size of 3/8" recommended Nitrogen can be used as shielding gas Table 17-4 gives current requirements and gas mixtures for cutting metal plate

Carbon Arc Cutting (CAC)
Used for cutting of cast iron Also metals that can be welded without postheat Not used if smooth, accurate cut desired Electric arc drawn between carbon or graphite electrode and material to be cut Heat of arc melts away material Material removed by force of arc and gravity Done with d.c. welding machine on straight polarity

Carbon Arc Cutting (CAC)
Graphite or hard carbon electrodes best choice Retain point longer than soft carbon electrodes Special electrode holder used Have hand shield for protecting hand against intense heat May also be water cooled

Carbon Arc Cutting Technique
Arc advanced along section to be cut at rate equal to that at which metal melted away Best cuts made in flat position, but may cut in vertical position Overhead position hazardous due to molten dross Speed of cut Size of electrode, thickness of material, amount of current used, and skill of cutter Width of cut increases with increased current, larger electrode diameters

Shielded Metal Arc Cutting
Equipment more versatile Process consists of drawing arc between covered electrode and material being cut Covering forms gaseous shield protects metal being cut from atmosphere Material removed from cut by gravity and force of arc Electrode covering serves as electrical insulation Stabilizes arc so heat concentrated at front of cut

Shielded Metal Arc Cutting
Select electrodes range from 5/32 to 1/4 inch in diameter Direct or alternating current may be used Direct current, electrode negative preferred Speed of cut and width of cut Size of electrode being used Current values employed Thickness of material Kind of material Skill of cutter

Oxygen Arc Cutting Method of cutting, piercing, and gouging metals with electric arc and stream of oxygen Stream of oxygen directed into pool of molten metal Pool formed and kept molten by arc established between base metal and tubular coated electrode Consumed during cutting operation Coating on electrode provides insulation, acts as arc stabilizer, aids flow of molten metal from cut

Oxygen Arc Cutting Cutting action fast, preheating not required
Finished cuts require very little grinding Used to cut metals always considered nearly impossible by standard methods Electrodes first developed primarily for use in underwater cutting Possible to cut ferrous and nonferrous metals in any thickness and position

Schematic of Oxygen Arc Electrode in Operation
Arcos Corp. The oxygen arc electrode and holder Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Oxygen Arc Cutting Equipment
Oxygen arc electrode holder Oxygen arc coated tubular electrodes Ferrous metal tube with nonconductive coating Tube conducts current for establishment and maintenance of arc 3/16 and 5/16 inch diameters; bores 1/16 and 1/10 inch respectively a.c. or d.c. welding machine Tank of oxygen with oxygen-regulating gauges

Oxygen Arc Cutting Flow of oxygen controlled by valve built into holder and triggered by operator Cutting and piercing begin by tapping tip of electrode on work to establish arc and release oxygen Piercing: Electrode pushed into and through plate Cutting: Electrode dragged along plate surface Electrode coating burns off more slowly than electrode thus maintaining arc

Oxygen Arc Process at Point of Cutting

Oxygen Arc Cutting Speed of cut
Thickness and composition of plate Oxygen pressure Current Size of electrode Gouging performed by inclining electrode until almost parallel to plate surface and pointed away from operator along line of gouge Table 17-3A: Ampere settings and oxygen pressures for various types of metal cut by oxygen arc process

Protection Goggles Gloves
Must wear to prevent harm to eyes from sparks, hot particles of metal and glare Gloves Worn to protect hands Heat very intense Shower of sparks Hot material Keep free from grease and oil

Magnetic Burning Square
Makes it possible to cut straight lines with high degree of accuracy Base Perfect square with two strong magnets for holding tool in all positions Inscribed with 90º protractor in 1º increments, 180º swing Large knob for holding in position Contour Sales Corp. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Arc Cutting Machines Have device to hold cutting torch and guide it along work at uniform rate of speed Produce work of higher quality and greater speed than hand cutting torch Machines may be used for cutting straight lines, bevels, circles, and other cuts of varied shape. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hypertherm, Inc.

Arc Cutting Machines Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ESAB Gantry-type mechanized cutting machine with PAC speeds up to 400 inches per minute. They can be guided by hand or template. Cutting machines may be guided by various types of tracing devices.

Stack Cutting Cuts made through several thicknesses at same time
Plates in stack must be clean and flat and have edges in alignment where cut started Plates must be in tight contact Minimum of air space between them Usually clamp them together

Beam Cutter Portable structural fabricating tool
Can trim, bevel, and cope beams, channels, angles Beam rail positioned across flanges Two permanent magnets lock and square rail Variable speed power units on both horizontal and vertical drives Squaring gauge enables bevel to straight trim cuts Provides clean, accurate cuts in short time Weighs only 60 pounds

Practice Jobs: Plasma Arc Cutting
Type and thickness of metal to be cut will determine amperage setting and cutting speed Power source and equipment rated in amperage and thickness and type of material it is designed for Torch positioning and travel speed key variables to control for successful cutting Miller Electric Mfg. Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Torch Standoff Distance
Miller electric Mfg. Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Acceptable Plasma Arc Cut

Set Up for PAC Connect power source to primary power.
Check torch, compressed air, settings, and work clamp connection. Put on personal safety equipment. Check torch tip, electrode, and shield cup. Check compressed air pressure and flow requirements. Check power source amperage and control settings. Check torch operation. Turn on power source and begin cutting.

Plasma Cutting Sequence
Place drag shield on metal edge Press trigger. After 2 sec. of preflow, pilot arc starts Start moving torch slowly Miller Electric Mfg. Co. Adjust torch speed of sparks go through metal and out bottom Pause briefly at end of cut before releasing trigger Postflow continues for 20–30 sec. after releasing trigger Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Square Cutting with PAC: Job 17-J1
Objective To make square cut of acceptable quality in flat position with manual air plasma cutting system Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Job 17-J1 Cutting technique
Plasma torch positioned so work and travel angles are 90º angle to piece being cut Use template to keep cut straight Note one side of cut square and other side has slight bevel of 4º to 6º Due to swirling, cutting gas as it exits torch tip or nozzle

Job 17-J1 Plasma arc cutting quality of square cut
Miller Electric Mfg. Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

PAC Direction of Travel
Miller Electric Mfg. Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Bevel Cutting with PAC: Job 17-J2
Objective To make bevel cut of acceptable quality in flat position with manual air plasma cutting system Thickness to be cut determined not by metal thickness but by angle of bevel Cutting technique Plasma torch positioned so work angle 30º from vertical and with travel angle of 90º Use template to keep cut straight

Gouging with PAC: Job 17-J3
Objective To make gouge of acceptable quality in flat position with manual air plasma cutting system Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Job 17-J3 Gouging technique Variation of PAC process
Utilizes larger diameter orifice torch tip that produces reduction in arc constriction, which results lower arc stream velocity Gouging used for edge prep, removal tack welds, braces Travel angle of 90º and work angle of 30º to 45º from vertical Gouging Tip Cutting Tip Miller Electric Mfg. Co. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Plasma Gouging Sequence
Miller Electric Mfg. Co. This torch angle and the speed of travel will determine the gouging depth. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Hole Piercing: Job 17-J4 Objective
To be able to perform hole piercing to an acceptable quality level Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Job 17-J4 Hole piercing technique
Replace gouging tip from last job with proper cutting tip Set torch in same position used for gouging 30º to 45º from vertical Rotate torch into approximately 90º position as material blown out of hole area Move torch to cut proper size hole

Shape Cutting with PAC: Job 17-J5
Objective To make various shape cuts of acceptable quality in flat position with manual air plasma cutting system Shape cutting technique Use techniques already learned for square, bevel, and hole piercing to accomplish this job Templates can be used instead of laying out parts and generally preferable to free-hand cutting with PAC process

Examples of Shape Cutting

Air Carbon Arc Cutting Torch should be held to give work angle of approximately 90º and with travel angle of 45º Use push angle on electrode Maximum extension of air jets should not be > 7 inches and minimum never < 1.5 inches Locate air jet between work and electrode Keep arc length as short as possible Travel speed quite fast to keep up with molten pool begin blown away

Working Standard CAC-A Torch
American Welding Society (AWS) Welding Handbook Committee, 1991 Welding Processes, Volume 2 of Welding Handbook, 8th ed., Miami: American Welding Society, p. 489, Fig 15.4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Setting Up and Using CAC-A Equipment
Connect power source to primary power. Make sure electrical power turned off. Bolt electrode cable from power source to connector on carbon arc torch (use DCEP). Connect air hose to carbon arc torch. Make sure both electrical connection and air hose connection to torch are properly protected with appropriate “boot cover.”

Attach work clamp and cable to power source and workpiece (use DCEP). Turn on air supply to torch and check for proper pressure. Select proper size and shape of carbon electrode. Before starting make sure all safety precautions taken.

Put on personal safety equipment. Check power source amperage and control settings. Turn on power source, turn on air with torch on/off valve, and begin cutting.

Gouging with CAC-A: Job 17-J6
Objective To make gouge in butt joint to form U-groove, with acceptable quality to make complete joint penetration (CJP) weld that is capable of passing radiograph test Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Job 17-J6 General job information Use Table 17-9 for proper settings.
Make square groove butt joint with E7018 electrode (simulate making of CJP weld) Now gouge out weld to dimensions on Job Drawing CAC-A process produces little dross on finished cut Remaining dross (if any) rich in carbon and must be removed by chipping hammer or grinding Use Table 17-9 for proper settings.

CAC-A Torch Positions American Welding Society, C5.3-91, Air Carbon Arc Gouging and Cutting, p. 6 and 7, figs. 6 through 9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Weld Removal with CAC-A: Job 17-J7
Objective Remove weld metal with minimal gouging into base metal See Table for Job Outline Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Job 17-J7 General job information
Select appropriate fillet weld from scrap area Reduce electrode to approximately 30º Direct electrode toward center of fillet weld Use sufficient travel speed to maintain arc while removing weld with minimal gouging into base metal Will see very distinct line (joint root) if gouged deep enough

Shutting Down the CAC-A Equipment
Turn off air supply using torch valve. Allow airflow time to help cool carbon electrode Do not let electrode come into contact with workpiece until power source turned off Turn off power source. Turn off main air supply. Remove carbon electrode from holder. Place in proper storage area after cooled

Disconnect air supply and power cable to torch. Disconnect workpiece connection Wrap up work and electrode cables, air hose and store. Place carbon air arc torch in proper storage Mark on hot metal with “HOT.” Return all unused carbon electrodes to proper storage.

Dispose of all dross used electrodes, and scrap metal. Turn off main power to both power source and air compressor. Conduct fire watch for smoldering combustibles minimum of one hour after work stoppage.

Arc Cutting Principles and Arc Cutting Practice: Jobs 17-J1-J7

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