Welder Training Program - BC Trades Modules · Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery

Welder Training Programlevel C

P5: arc Cutting and gouging (PaC) (aaC) (SmaC)Theory Competencies

Acknowledgements & Copyright Permission

The Industry Training Authority of British Columbia would like to acknowledge the Welding Articulation Committee and Open School BC, a division of the Queen’s Printer as well as the following individuals and organizations for their contributions in updating the Welder Training modules:

Welding Articulation Committee (WAC) Members and Consultants—“The Working Group”Jim Carson (Welding Articulation Committee Chair), University of the Fraser Valley (writer and senior reviewer)

Peter Haigh (Welding Curriculum Review Committee Chair), Northwest Community College (writer and senior reviewer)

Sheldon Frank, University of the Fraser Valley (writer and reviewer)

Greg Burkett, Okanagan College (writer and reviewer)

Randy Zimmerman (writer and reviewer)

John H.P. Little (reviewer)

Resource Training Organization (RTO)

BC Council on Admissions and Transfer (BCCAT)

The Queen’s PrinterThe Queen’s Printer, through its Open School BC unit, provided project management and design expertise in updating the Welder Training Level C print materials.

Open School BCSolvig Norman, Senior Project ManagerEleanor Liddy, Director/AdvisorDennis Evans, Production Technician (print layout, graphics & photographs)Christine Ramkeesoon, Graphics Media CoordinatorKeith Learmonth, EditorMargaret Kernaghan, Graphic Artist

Publishing ServicesSherry Brown, Director of Publishing Services

Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens’ Services, Province of British Columbia

Copyright Permission

The following suppliers have kindly provided copyright permission for selected product images:

Acklands-Grainger Inc.The Crosby GroupJ. Walter Company Ltd.Lincoln Electric CompanyNDT Systems, Inc.Praxair, Inc.Thermadyne Canada (Victor Equipment)The Miller Electric Mfg. Co.ESAB Welding & Cutting Products

Photo of welder walks the high steel at a construction site, Kenneth V. Pilon, copyright 2010. Used under license from Shutterstock.com

A special thank you to Lou Bonin and Jim Stratford at Camosun College (Welding department) for assisting us with additional photographs. An additional thank you to Richard Smith from England, for allowing us to use photographs of hydrogen bubbles.

ForewordThe Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Welder Program. It was made possible by the dedicated efforts of the Welding Articulation Committee of BC (WAC).

The WAC is a working group of welding instructors from institutions across the province and is one of the key stakeholder groups that support and strengthen industry training in BC. It was the driving force behind the update of the welding learning modules supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The WAC plays an important role in the province’s post-secondary public institutions as discipline specialists that share information and engage in discussions of curriculum matters, particularly those affecting student mobility.

ITA would also like to acknowledge the Resource Training Organization (RTO) which provides direction for improving industry training in the resource sector and which led consultation on changes related to the BC welder training program.

We are grateful to WAC and RTO for their contributions to the ongoing development of BC Welder Training Program Learning Resources (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA).

Industry Training AuthorityAugust 2010

DisclaimerThe materials in these modules are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Welding Articulation Committee, the British Columbia Industry Training Authority or the Queen’s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for welding trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

WelDer TrAInInG PrOGrAM — level C 5

P5: Arc Cutting and Gouging (PAC) (AAC) (SMAC)Theory Competencies

Table of Contents

Theory Competency P5-1: electric arc cutting and gouging processes and their applications 7

P5-1 Learning Task 1: Air carbon arc cutting . . . . . . . . . . . . . . . . . . . . . . . . 11

P5-1 Learning Task 2: Applications of CAC-A . . . . . . . . . . . . . . . . . . . . . . . 19

P5-1 Learning Task 3: Plasma arc cutting . . . . . . . . . . . . . . . . . . . . . . . . . 25

P5-1 Learning Task 4: Applications of PAC . . . . . . . . . . . . . . . . . . . . . . . . . 35

P5-1 Learning Task 5: Shielded metal arc cutting . . . . . . . . . . . . . . . . . . . . . 43

P5-1 Learning Task 6: Safety requirements. . . . . . . . . . . . . . . . . . . . . . . . . 47

Theory Competency P5-2: CAC-A equipment and its use 49

P5-2 Learning Task 1: CAC-A equipment . . . . . . . . . . . . . . . . . . . . . . . . . 53

P5-2 Learning Task 2: CAC-A electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . 63

P5-2 Learning Task 3: Electrode angles, cutting speeds and joint positions for CAC-A . . . 69

Theory Competency P5-3: PAC process and equipment 81

P5-3 Learning Task 1: PAC equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

P5-3 Learning Task 2: Main factors of PAC . . . . . . . . . . . . . . . . . . . . . . . . . 97

Answer Key 111

Theory CompeTenCy p5-1:electric arc cutting and gouging processes and their applications

p5

-1


Module P5 Theory CoMPeTenCy P5-1

OutcomesThis Theory Competency introduces electric arc cutting and gouging processes and their applications. These processes include air carbon arc cutting (CAC-A), plasma arc cutting (PAC) and shielded metal arc cutting (SMAC). There are two commonly used abbreviations for air carbon arc cutting: “CAC-A” (carbon arc cutting – air) and “AAC” (air arc cutting). Both abbreviations are used widely in the industry, but the shift is toward CAC-A, which is the AWS title. The three processes and the required equipment are relatively simple and easy to use. They are used in fabrication and repair work for cutting all types of metal.

Safety is a concern with any welding or cutting process, but CAC-A, PAC and SMAC have some special safety considerations. Understanding these safety concerns and the processes will help make your work safe and efficient, so you can avoid personal injury to yourself and others.

When you have completed the Learning Tasks in this Theory Competency, you should be able to describe:

• the CAC-A process• the principles of CAC-A• the components of a basic CAC-A system• the applications of CAC-A• safe work practices for CAC-A• the PAC process• the principles of PAC• PAC systems• PAC quality of cut• PAC metallurgical effects• safe work practices for PAC• the applications of PAC• the advantages and disadvantages of PAC• PAC machine cutting• PAC manual cutting• PAC gouging• the SMAC process

evaluationWhen you have completed all the Theory Competencies in module P5, you will take a written test. You must score at least 70% on this test. The test will include questions that are based on the following material about CAC-A, PAC and SMAC from Theory Competency P5-1:

• the processes• equipment required for cutting and gouging• type and capacity of welding power sources• compressed air and gas requirements• components of basic systems

10 WelDer TrAInInG PrOGrAM — level C


• torch and kerf angles• safe work practices• applications• advantages and disadvantages

resources

required: All required resources are contained within this Theory Competency.

Optional: The following resources are optional and are NOT required to complete this Theory Competency. The optional resources provide further information on a specific topic. If you want more information on these resources, please see your instructor.

Arcair® Air Carbon-Arc Guidehttp://www.thermadyne.com/arcair/

Arcair® SLICE Fire and Rescue Cutting Systemhttp://www.thermadyne.com/arcair/

TWI Air Carbon Arc Gouginghttp://www.twi.co.uk/content/jk12.html

Hypertherm®http://www.hypertherm.com/

Thermadyne®http://www.thermadyne.com/thermaldynamics/

Lincoln Electrichttp://www.lincolnelectric.com/knowledge/articles/content/plasma.asp

Miller® Plasma Cutting Resourceshttp://www.millerwelds.com/resources/improving-your-skills/plasma/

Notes



P5-1 learning Task 1:Air carbon arc cuttingThe air carbon arc process is one of several arc cutting processes. It is abbreviated as “CAC-A.” In the past it was abbreviated as “AAC.” The CAC-A process is widely used because it can cut and gouge all types of ferrous and non-ferrous metals more quickly than flame cutting.

Myron Stephan, a welding engineer, introduced the air carbon arc torch in 1948. The company Arcair® was founded in 1949. In the original experiments, one man would strike an arc with a carbon electrode and a second man would blast away the molten metal with an air gun. The modern air carbon arc gouging torch combines both the electrode holder and the air jet orifices.

CAC-A is considered to be a cutting process, but it does much more. CAC-A can do a partial cut, gouge or wash off a surface. This can prepare joints for welding, remove unwanted metal, remove weld faults or remove bolts, pins and other fasteners.

Principles of CAC-AAir carbon arc cutting works by melting the base metal with an electric arc (Figure 1). This arc is produced by a welding power source passing current through a carbon electrode held in a special holder. Jets of compressed air from holes in the lower jaw of the electrode holder blow the molten metal away to form a kerf or groove.

Air jets

Carbon electrode

Electrode holder

Figure 1—CAC-A gouging

Notes



CAC-A can be used on any kind of metal because it uses the intense heat of the arc to melt the metal. CAC-A does not cut the base metal by oxidizing it, as the oxy-fuel gas processes do. Unlike the oxy-fuel gas cutting processes, CAC-A can be used on any material that conducts electricity.

Other ways to gouge and cutFire and rescue teams use cutting systems that use a similar process. These have a hollow exothermic (self-burning) electrode that will continue to burn once ignited. They also have an oxygen supply and a small battery pack to ignite the electrode. This can be considered a type of oxygen lance. Variations of the design can be used underwater.

When a high-pressure air supply is not available, shielded metal arc cutting (SMAC) electrodes can be used to cut or gouge. When SMAC electrodes are used with a pushing motion, the expanding gases at the end of the electrode will provide a blowing force. This will push the slag aside rather than blow it clear of the gouge or cut.

Components of a basic CAC-A systemThe basic CAC-A system (Figure 2) consists of a welding power source, an electrode holder, a carbon electrode and a compressed air supply system. The power source can be any high-capacity welding power source that provides enough current.

Electrode holder

Air compressor Welding power source

Workpiece lead

Workpiece

Air line

Carbon electrode

Electrode leadGround clamp

– +

Figure 2—Typical CAC-A system

To strike the arc, you simply touch the carbon electrode to the workpiece the same as you would for SMAW. The circuit is now closed (or complete) and the welding current flows through the circuit. As the current crosses the arc, it creates tremendous heat. A small area of base metal just below the electrode melts.

The CAC-A electrode holder has small holes or orifices that direct compressed air at the cut to blow the molten metal out of the gouge or cut area (Figure 3). The electrode holder is commonly called a “gouging torch.”

Notes



Carbon electrode

Air jet holes (orifices)Air control valve

Figure 3—CAC-A gouging torch

Any standard welding power source, either AC or DC, can be used for CAC-A. It must be able to produce enough current for the size of the electrode. Module P5, Competency 2 (P5-2) covers power sources used for CAC-A in detail.

Safe work practices for CAC-ACAC-A has some extra safety hazards in addition to those for any arc welding process.

ventilationCAC-A is used to cut metals that are not usually cut with oxy-fuel gas. Many of these metals contain elements that produce toxic fumes when heated. Stainless steel, for example, contains chromium, which produces dangerous fumes when heated. Other metals, such as high-alloy steel and copper alloys, are also dangerous when heated.

You must wear a respirator when cutting any metals that contain chromium, zinc, copper or nickel. Never use CAC-A to cut beryllium, cadmium or lead. These metals produce extremely toxic fumes.

Heat from CAC-A can cause paint finishes and industrial coatings to produce toxic fumes. Grind or scrape the surrounding gouge area before gouging. Use active ventilation and a respirator.

noiseNoise levels from the compressed air used with CAC-A are very high. Wear ear protection (earplugs and/or earmuffs) when you are cutting with CAC-A. Ear protection will also prevent a stray spark from entering your ear canal and possibly perforating your ear drum. In extreme cases, infection from this kind of injury can cause complete hearing loss.

Notes



radiationAs with other arc processes, the high current and open arc used in CAC-A produce large amounts of radiation. When using this process, you will need a standard welding helmet that has a darker lens than you would use for SMAW with a comparable diameter electrode. A #12 filter lens shade is recommended for most jobs. Use a #14 filter lens shade for larger cutting jobs that require higher currents. You should also make sure that you properly cover all your skin surfaces. The best choice is leather or clothing treated with fire retardant.

FireThe blast of compressed air can throw molten metal and sparks up to 6 m (20 ft.) from a workpiece. The potential for fire is high. All combustible material must be removed from the area of the metal spray.

You can control the spray of molten metal by placing a sheet-metal shield around the workpiece. Make sure the spray points away from other workers and flammable materials. Always make sure you cut away from yourself.

Follow standard safe practices when near flammable material:

• Have a fire extinguisher or charged water hose nearby.• Maintain a fire watch during the operation and after it is completed.

Safety summary

As you prepare to use CAC-A equipment, remember these basic safety points:

• Turn off your welding power source when it is not in use.

• Wear leather welding gloves, flame-resistant clothing and ear protection when operating CAC-A equipment.

• Keep all equipment dry and do not work in damp or wet conditions.

• Make sure that the workpiece or workbench is properly connected to the workpiece lead.

• Never use current that is more than the rated capacity of the welding cables.

• Remove the electrode from the CAC-A electrode holder before putting the holder down.

Refer to Module P1, Competency 2 (P1-2), “Use Safe Work Practices,” for a more complete description of safe work practices.

Now complete Self-Test 1 and check your answers.

Notes



Answers



Self-Test 1Choose the correct response for each question and put it in the Answers column. Cover your answers when reviewing the test for study purposes.

1. Which metal should never be cut with CAC-A because it produces highly toxic fumes?

a. aluminum

b. lead

c. stainless steel

d. titanium

2. Which type of welding power source is used with CAC-A?

a. AC only

b. DC only

c. AC rectifiers only

d. either AC or DC

3. CAC-A electrodes are made of

a. carbon

b. silicon

c. low-carbon steel

d. tungsten

4. The effect of CAC-A on the base metal can be

a. extreme distortion

b. slight surface hardening

c. cracking

d. iron buildup

5. When cutting stainless steel with CAC-A, you need to be especially careful of which fumes?

a. zinc

b. chromium

c. iron

d. lead

Answers



6. CAC-A can be used to cut almost all ferrous and non-ferrous metals.

a. true

b. false

7. What is the purpose of the compressed air in CAC-A?

a. to remove the molten metal

b. to make the cut visible

c. to cool the workpiece

d. to heat the workpiece

8. CAC-A is like oxy-fuel gas cutting because both processes oxidize the metal.

a. true

b. false

9. Why does CAC-A cause little distortion in the workpiece?

a. Moves fast and does not allow heat buildup.

b. Arc temperature is low.

c. Thin material disperses heat rapidly.

d. Compressed air cools the workpiece.

10. CAC-A should never be used on metals containing cadmium, lead or

a. beryllium

b. nickel

c. zinc

d. copper

11. Which filter lens shade is recommended for most CAC-A applications?

a. #6

b. #8

c. #10

d. #12

now go to the Answer Key and check your answers

Notes



Notes



P5-1 learning Task 2:Applications of CAC-AAir carbon arc cutting (CAC-A) is widely used for the following reasons:

• CAC-A can remove metal much more quickly than oxy-fuel gas cutting or mechanical methods such as grinding or chipping.

• CAC-A equipment is relatively inexpensive because a standard welding power source can be used, and most shops have a compressed air supply.

• Proper cutting and gouging techniques can be learned in a short time. If they are done correctly, further edge preparation is not required.

• CAC-A is a very versatile process that can be used to cut metals that cannot be cut with the oxy-fuel gas cutting process.

• CAC-A can be used in situations where gas cylinders are not available or they are considered a safety hazard.

• Low heat input means fewer problems with distortion or surface hardening.

• CAC-A can be done manually or with an automated cutting machine.

Manual cuttingFor most CAC-A jobs you will use a manual torch. The equipment is versatile and portable, and the process can be done in the shop or on the job site.

Machine cuttingMachine cutting is more accurate than manual cutting. The cuts are straighter and cleaner than those produced manually. This is important when preparing a weld groove.

CAC-A cutting machines have a motorized carriage mounted on a track. They are similar to the machines used for OFC. These machines are portable and can be mounted in a vertical, horizontal or overhead position.

On a semi-automatic machine, the electrode must be manually fed into the holder to produce the desired cut. The carriage moves along the track at a preset speed. The amperage and air volume are also preset. These semi-automatic machines are simple to operate and produce clean cuts.

Fully automatic machines feed the carbon electrode automatically. They are used when you want even greater accuracy. One type of automatic machine uses a spring-loaded device that maintains a constant distance between the electrode and the workpiece. This provides a uniform groove depth.

Notes



More sophisticated machines use a voltage-controlled electrode feed. This maintains a constant arc length and produces grooves with a depth tolerance of ± 0.6 mm (± 0.025 in.).

Uses of CAC-ACAC-A can be used to cut metals that cannot be cut well with oxy-fuel gas. These metals include stainless steel, aluminum, cast iron, nickel and copper alloys. For example, CAC-A is used in refineries and pulp mills where stainless steel pipe or other high-alloy steel pipe must be cut.

Except for certain metals, CAC-A is seldom used for cutting through metals because there are other processes that can make these cuts more smoothly and accurately. The CAC-A process is used much more widely for gouging and for metal removal.

When properly done, CAC-A gouged grooves are very clean, smooth and suitable for welding with no further preparation (Figure 4). The groove itself is free of slag. Any minimal slag on the edge can be easily removed with a chipping hammer.

Figure 4—U-groove prepared with CAC-A

Making high-quality cuts and gouges with a manual CAC-A gouging torch is difficult because it is hard to hold the torch perfectly steady. Automatic and semi-automatic motor-driven equipment produces better quality cuts and gouges. The CAC-A melting process does not produce sharp edges like oxy-fuel gas cutting does.

When used correctly, CAC-A has very little adverse effect on the base metal. Since the CAC-A torch moves so rapidly over the workpiece, little heat is built up and there is little distortion.

CAC-A does produce changes in some base metals in much the same manner as the arc welding processes. In high-carbon steels and cast irons, CAC-A can produce a thin hardened zone. This hardened layer is only about

Notes



0.15 mm (0.006 in.) deep. Welding will remelt this layer and reduce the hardness. Also, if you preheat high-carbon steels and cast iron, you can avoid much of the hardening effect.

If the metal is to be machined after cutting or gouging, you need to remove any hardened areas. This removal is usually done by grinding.

Weld joint preparationOne of the most common uses of CAC-A is to prepare the edges of plates for welding. CAC-A can be used to bevel the edges of plates to make bevel and vee groove joints, but it is more commonly used to gouge U-groove joints. The plate is set up for a square butt joint, and a U-groove is cut into the joint in preparation for welding. The joint can be welded without further preparation.

CAC-A is also used for back-gouging. This is often required on vee and U-groove joints in thick material. In this procedure, one side is prepared and welded. The base metal is turned over and a groove is gouged to the root of the first weld. This back-gouging is done to make sure that the weld penetrates completely. Notes on drawings will often say “Back-gouge to sound metal.”

Weld defectsCAC-A is useful for removing defects in welds. A faulty weld can be gouged to sound metal and the joint can then be rewelded.

Disassembly and repairCAC-A is also commonly used in scrapyards to cut apart steel structures for demolition. Damaged or worn parts can be removed as the first step in a repair job. Railway maintenance workers use CAC-A to gouge cracks in railway tracks or rail cars before they are welded. Other applications include removing worn hardfacing material on equipment and removing the backing strips on completed welds.

Because CAC-A is effective in cutting cast iron, it is also used in foundries to remove cracks and defects, or excess metal from castings.


Answers




1. CAC-A cuts which of the following metals more effectively than oxy-fuel gas cutting?

a. cold rolled steel

b. stainless steel

c. low-carbon steel

d. carbon

2. CAC-A is commonly used to prepare what type of weld joint?

a. U-groove

b. flare

c. scarf

d. flange

3. When a weld joint requires a groove weld on both sides, CAC-A is used to __________ the other side of the joint after one side has been welded.

a. back strip

b. post-heat

c. cut

d. back-gouge

4. In foundries, CAC-A is commonly used to

a. fabricate new parts

b. remove castings from moulds

c. remove excess metal from castings

d. shape iron castings

5. Why is CAC-A not normally used to cut metal to length?

a. It is too slow.

b. It is too expensive.

c. It produces a ragged cut line.

d. It causes too much distortion.

Answers



6. How do automatic and semi-automatic CAC-A machines differ?

a. electrode feed mechanism

b. depth of cut

c. type of carriage used

d. type of motor used

7. Fully automatic CAC-A machines offer

a. greater speed

b. greater accuracy

c. less maintenance

d. lower initial cost


Notes



Notes



P5-1 learning Task 3:Plasma arc cuttingThe plasma arc cutting (PAC) process was developed mainly to cut metals that are difficult or impossible to cut using the oxy-fuel process. These metals include copper alloys, aluminum alloys, stainless steels and nickel alloys.

The first PAC equipment was so expensive that the process was only used for these special applications, even though the plasma arc can cut any metal, including low-carbon steel plate and sheet. With advances in technology, the plasma arc equipment is now more affordable and portable. It can be used for both cutting and gouging. Computer-controlled robotic arms are now being used with PAC. These are commonly called “robotics.” These automated machines are used to cut patterns, structural sections and copes and blocks on wide flange beams.

James A. Browning, an engineering professor, and his graduate student Merle Thorpe developed the plasma torch in the early 1950s. They formed the Thermal Dynamic Corporation in 1957.

The PAC process

The PAC process operates by transforming gas into plasma. Plasma is a fourth state of matter (after liquid, solid and gas). It is created by superheating a gas so that its molecules are torn apart into ionized atoms.

Inside the plasma arc torch (Figure 5), an electric arc heats the gas to such high temperatures that it changes into plasma. This plasma has several unique qualities that enable it to generate the high temperatures required in PAC:

• Plasma is capable of conducting an electric current. The ionized particles behave in much the same way as a metal conductor of electricity. Plasma, therefore, can carry and maintain the arc to the workpiece.

• More heat is generated when the stream of plasma hits the workpiece. As the stream strikes the workpiece, the plasma instantly changes back into a gas, releasing tremendous amounts of heat energy. The heat of the plasma arc therefore comes from two sources: the electric arc conducted by the plasma, and the heat released by the plasma as it changes back into a gas.

Notes



Workpiece

Electrode Cutting gasCutting nozzle Secondary gas

Shield cup

Plasma stream(+)

(–)

Figure 5—PAC process

Plasma arc torches with high-output plasma arc power sources are capable of generating temperatures up to 33 000 °C (60 000 °F), which is 10 times hotter than an oxyacetylene flame. These tremendous temperatures melt and even vaporize the base metal or anything that gets in the way of the plasma stream.

The cutting nozzle is very important. It focuses the plasma arc and makes it swirl. As the plasma is heated in the arc chamber, it expands. Because the orifice is very small, the amount of plasma that can exit is restricted, and the plasma accelerates in a swirling motion out of the torch. When the plasma leaves the orifice, it does so with tremendous energy.

The swirling motion and small orifice narrow (constrict) the plasma arc to a small spot on the workpiece. The tremendous heat melts and vaporizes the metal, and the high velocity of the blast blows the melted metal from the kerf. Because the temperatures created are so high, PAC torches must be cooled. A secondary cooling gas passes between the cutting nozzle and the heat shield (Figure 5). In some torch designs, water is circulated to cool the torch.

PAC systemsPlasma arc cutting systems vary a great deal, so there is no “typical” PAC system. The smaller portable systems are quite simple. They have a combination plasma arc power source and control unit. One gas (usually compressed air) is used both as the plasma gas and as the shielding and cooling gas.

The larger stationary PAC systems are much more complicated. They can have multiple mechanized torches using mixtures of gases. One gas is the plasma gas. The other gas is called the “secondary gas.” It shields as well as

Notes



cools. The plasma arc power source is joined with a robotic control unit that regulates plasma arc current, gas flow, direction of travel and travel speed.

Some of these larger systems use water to cool the torch or water injection in the cutting tip to increase the accuracy of the cut. The schematic representation in Figure 6 illustrates one type of gas-cooled PAC system.

Input power

PAC power source and control unit

Manual torch

Workpiece

Secondary gas

Plasma gas

Figure 6—Schematic of gas-cooled PAC system

PAC power sources are specially designed with a very high open circuit voltage, usually from 100 to 400 V.

Quality of PAC cutsThe quality of any cut surface is determined by:

• surface smoothness• kerf width• slag formation• parallel cut faces• sharp, square bottom and top edges

Used properly, PAC produces high-quality, economical, ready-to-weld cuts on a wide variety of metals and metal thicknesses (Figure 7). PAC makes slag-free cuts on such difficult-to-cut metals and alloys as nickel, aluminum, copper, stainless steel and cast iron.

Figure 7—High-quality PAC cut

Automatic PAC produces very clean and smooth cuts on plate up to 125 mm (5 in.) thick. The cuts are comparable in smoothness to those done with oxy-fuel gas. The top and bottom edges are fairly square.

Notes



PAC produces a kerf that can be up to 11⁄2 to 2 times as wide as a kerf cut with OFC. This wider kerf must be taken into account when you make precision cuts.

The most serious drawback with the cut quality is that the kerf tends to be slightly bevelled on one side. The plasma jet tends to remove more metal from the top part of the kerf than from the bottom. The result is a slight bevel on one of the kerf cut faces (Figure 8). One side of the kerf will be almost square and the other side will be slightly bevelled (2° to 4°).

2–4º bevel

Figure 8—Slight bevel on one kerf cut face

This slight bevel is not a serious problem in most cutting applications. Automated systems cutting thicker materials will travel so that the bevelled side of the cut is on the waste side at all times.

PAC metallurgical effectsEven though plasma arc cutting produces tremendous temperatures, there are very few adverse effects on the base metal. There is little or no distortion, even on light-gauge sheet metal. Because PAC is so fast, with cutting speeds up to 7620 mm (300 in.) per minute, there is very little heat buildup in the metal.

Surface hardening is also minimal. There is a microscopically thin heat-affected zone of 0.08 to 0.13 mm (0.003 to 0.005 in.). The small size of this zone is similar to that produced by CAC-A. As with CAC-A, some surface hardening does occur in this heat-affected zone in high-carbon steels, but this can be reduced by preheating the work.

There is a risk of nitrogen being introduced into the ferrite structure of the steel. When this occurs it is called “nitrided edges.” This is a concern when using nitrogen as the plasma gas and the secondary gas. Since nitrogen makes up approximately four-fifths of the atmosphere, PAC that uses compressed air can produce nitrided edges. The depth of nitride is less than 0.254 mm (0.010 in.). Nitriding of the cut surfaces can cause possible cracking when doing more critical welds. The nitride can easily be removed by a light grinding of the cut edge if it is a concern.

noTes



Safe work practices for PAC Safe work practices are important when working with any arc

process. You must take the usual precautions to protect yourself from electric shock, fumes, noise, arc radiation and metal spatter. As with CAC-A, the nature of the PAC process requires some special safety considerations. Review Theory Competency P1-2, Learning Task 2, “General safety precautions for welding.”

electric shockPlasma arc cutting power sources use high voltage. A 100 to 400 V open circuit voltage is common in PAC. By comparison, a SMAW power source has an OCV normally between 50 and 70 V. Some PAC power sources have even higher OCVs. Small inverter-type power sources that are designed to cut light-gauge sheet metal operate with OCVs of 200 to 300 V. You must be careful when working with high-voltage plasma arc equipment. An electric shock from these sources could be fatal.

WARNING!

Keep fingers, even when gloved, away from the plasma stream. It can vaporize anything in its path.

ventilationGood ventilation is important when you are plasma arc cutting. The intense heat of the plasma arc vaporizes some of the metal. This means that many more particles are suspended in the air than with other cutting processes.

Like CAC-A, the PAC process is used to cut metal not normally cut with OFC. The fumes produced by some of these metals, such as chromium from stainless steel, are very toxic. You should always be aware of the composition of the metal you are cutting and make sure that you are adequately protected from harmful fumes.

Arc radiationThe high voltages and currents in PAC produce large amounts of arc radiation. Wear a welding hemet with the recommended lens shade while plasma arc cutting. Be sure you are fully protected from the ultraviolet radiation. The following table gives an example of the lens shades recommended in the manufacturer’s operating manual for one plasma cutter (Figure 9). You should check the manufacturer’s manual for your PAC power source to find out what lens shade number they recommend you should use for each amperage.

noTes



PAC setting in amps recommended minimum lens shade

Less than 20 A #4

20 to 40 A #5

40 to 60 A #6

60 to 80 A #8

Figure 9—Shade numbers for PAC amperages

noiseWhen you use PAC, you must wear hearing protection. The blast of plasma hitting the workpiece produces hazardous noise levels, especially with the larger PAC machines. Even though the small, manual PAC machines produce much less noise, you still need some hearing protection, especially when you use the equipment for long periods of time.

Water-table cuttingWater can be used as a method of reducing the safety hazards of ventilation, arc radiation and noise. This is done by either performing the cut over water with the water touching the workpiece, or by performing the cut under water.

In cutting over water, the workpiece is laid on a specially designed water-filled work table so that the water just touches the bottom of the plate. As the plasma arc cuts through the metal, the water acts as a muffler to greatly reduce the noise level and as a filter to trap the fumes and slag.

An even safer alternative is to perform the cut underwater. The torch nozzle is positioned 25 to 50 mm (1 to 2 in.) underwater as the cut is performed. This process is clean and quiet, and little radiation escapes. The operator generally needs no eye, ear or respiratory protection. The cutting speed is slower, but underwater cutting is popular because of the improved safety.

Do not cut aluminum or similar alloys under water. The plasma operation could separate the oxygen from the water, leaving hydrogen bubbles under the plate. Hydrogen is very unstable and can explode.


noTes



Answers




1. What part of the plasma torch constricts the plasma arc?

a. heat shield

b. secondary gas

c. electrode

d. cutting nozzle

2. Plasma is a gas that has been superheated to the point where it becomes

a. ionized

b. energized

c. syncopated

d. molecular

3. What is the primary purpose of the secondary gas?

a. blows slag from the kerf

b. cools the torch

c. cools the workpiece

d. increases torch temperature

4. PAC reduces distortion of the workpiece because

a. The nitrided surface reduces expansion.

b. All electrical cutting processes reduce distortion.

c. Distortion is reduced by the extreme temperatures used.

d. High-speed cuts reduce heating of the base metal.

5. What are the main purposes of a water-filled table when cutting with PAC?

a. greatly increases the quality of cuts

b. speeds the cutting process

c. reduces noise, traps slag and fumes

d. cools the workpiece

Answers



6. PAC was originally developed to cut

a. metals that cannot be cut with OFC

b. titanium

c. uranium

d. thick steel plate

7. PAC can be used to cut

a. any metal

b. ferrous metals only

c. non-ferrous metals only

d. reactive metals only

8. How is a gas changed to a plasma?

a. allowed to expand

b. forced through a small opening

c. subjected to extreme pressure

d. heated to a high temperature by an electric arc

9. How do kerf widths in OFC compare to those in PAC?

a. The same.

b. PAC is 2 times wider.

c. OFC is 2 times wider.

d. PAC is 2 times narrower.

10. Slag is removed from the kerf in PAC by

a. a stream of water

b. the plasma jet

c. gravity as it falls to the floor

d. the secondary gases

11. Plasma, like a metal, has the ability to

a. oxidize at room temperature

b. cool quickly

c. conduct electric current

d. insulate the workpiece


noTes



noTes



P5-1 learning Task 4:Applications of PACPlasma arc cutting (PAC) has many different applications. It has become more widely used because the equipment is now more portable, easier to use and less expensive.

Advantages of PACPAC has some definite advantages over oxy-fuel gas cutting (OFC) and air carbon arc cutting (CAC-C) that help explain its increasing popularity:

• The plasma arc cuts very quickly. For example, on low-carbon steel less than 25 mm (1 in.) thick, PAC cuts up to five times faster than OFC and produces cuts of comparable quality. Even faster cutting speeds are possible on thin material. It can be hard to keep up the required cutting speed when hand cutting unless you use a non-conductive guide bar.

• There is no preheating time before the cut starts. With OFC the metal must first be heated to kindling temperature.

• The plasma arc can cut any metal. It is commonly used to cut metals that cannot be cut with OFC. These include stainless steel, cast iron, aluminum, magnesium, copper, brass, bronze, and nickel and its alloys.

• Plasma makes clean, high-quality cuts. Unlike OFC, which requires considerable heat input into the base metal, PAC puts very little heat into the metal. This means that there is little distortion, even in very thin material. The main advantage of using PAC on carbon steel is the fast and relatively distortion-free cuts PAC produces. When flame-cutting panels under 6 mm (¼ in.), buckling is a constant problem. With PAC, cuts can be made quickly while leaving a flat product. Previously these items had to be mechanically cut or drilled to prevent heat distortion.

• PAC can be used to stack cut. The cuts are often better than those done with OFC because there are no fused edges, which can occur with an oxy-fuel flame.

• There is very little risk of carbon contamination, unlike with CAC-A.

• Automatic cutting with PAC is popular when greater accuracy is required or distortion must be minimized. With numerically controlled cutting tables, PAC is the perfect match for producing multiple parts with accuracy. Parts can be produced to an accuracy of ± 0.254 mm (± 0.010 in.).

• PAC is fast and, if done right, it is slag-free. The time savings make up for the initial cost of the equipment.

• PAC can be done manually or with various automatic cutting machines.

noTes



Disadvantages of PACThe main barrier to using PAC has been the high initial cost. PAC was popular for cutting alloys that cannot be cut with OFC. It is now breaking into industries that only cut carbon steel.

Another disadvantage is that you must have a supply of high-quality filtered compressed air when using a hand-held PAC unit. Often when poor-quality compressed air and unskilled users damage the electrodes and nozzles, the cost rises and managers abandon PAC, changing back to OFC.

Electrical power is required as well as a pressurized gas (usually compressed air). OFC requires gases but no electrical power.

PAC machine cuttingPAC machines come in a wide variety of sizes and types. There are straight-line cutters mounted on self-propelled carriages, much like those used with OFC and CAC-A (Figure 10).

Figure 10—Straight-line cutting using PAC

Shape-cutting numerically controlled (NC) machines are commonly used for custom cutting plate to order. The accuracy and speed with which the cuts are made reduces the costs compared to manual cutting.

These NC units control the arrangement of the pieces cut from the plate, the sequence of operations, gas flow rates, carriage travel and coolant flow. The operator is not exposed to the high noise levels, radiation and hazardous fumes generated by the cutting process.

noTes



Figure 11—Automatic plasma arc cutting table

PAC portabilityOriginally, plasma arc cutting was difficult. The torch and equipment were large and hard to move around.

Today, modern, lightweight equipment that uses compressed air as both the plasma and secondary gas is very popular.

Applications of PACPAC can be used to make cuts, pierce holes, bevel edges and gouge grooves on any metal. This versatility gives it a wide range of applications. PAC is fast and precise. The only method that is more accurate is laser beam cutting (LBC).

PAC is finding wide application in the aluminum manufacturing industries for the fabrication and repair of products such as trailers, boats and rail cars.

Plasma arc gougingThe plasma arc gouging system is the same as the plasma arc cutting system except that a special gouging tip and heat shield are used. This allows a higher standoff distance. The choice of tips can also control the depth and width of gouge.

The advantage of plasma gouging is that it does not use carbon electrodes, which CAC-A does. But the plasma torch is not as versatile when gouging in narrow, cramped locations.

noTes



PAC using a gouging tip is ideal for aluminum because gouging aluminum with CAC-A leaves carbon contamination. The edges of aluminum that has been cut or gouged with plasma systems must still be cleaned to remove aluminum oxide before welding can start.

Weld preparation, salvage and repair can be done with plasma gouging.


noTes



Answers




1. Which of the following is an example of a metal that can be cut easily with PAC but not with OFC?

a. stainless steel

b. carbon steel

c. magnesium

d. both a and c

2. What is one advantage of PAC over CAC-A?

a. is more portable

b. can make clean cuts

c. uses lower temperatures

d. uses less voltage

3. There is little heat buildup in the base metal with PAC. This means that

a. Less heat is required.

b. Little distortion occurs.

c. Compressed air is not needed.

d. The cuts take place slowly.

4. On carbon steel less than 25 mm (1 in.) thick, the speed of PAC is __________ than OFC.

a. no different

b. slightly slower

c. up to 5 times faster

d. up to 10 times faster

5. One major drawback with a PAC system is the

a. lack of portability

b. high cost of personal protective equipment

c. high initial cost

d. slow cutting speed

Answers



6. Why do some automated PAC systems have a separate control unit away from the cutting area?

a. The unit is cheaper to operate.

b. To isolate the operator from health hazards.

c. To reduce manpower.

d. To increase the quality of cuts.

7. On carbon steel, PAC is used mainly for

a. bevelling plate in joint preparation

b. custom production cutting

c. short cuts on sheet steel

d. gouging weld defects

8. On which metal, in particular, is PAC better than CAC-A?

a. magnesium

b. copper

c. carbon steel

d. aluminum


noTes



noTes



P5-1 learning Task 5:Shielded metal arc cuttingShielded metal arc cutting (SMAC) is a metal arc process that uses coated electrodes for cutting, gouging or chamfering. It uses specially designed electrodes and some standard shielded metal arc welding (SMAW) electrodes.

SMAC is not commonly used, because in most cases it is not cost effective. SMAC is only used for small jobs or in emergencies.

Power sourcesThe equipment required for SMAC is the same as that used for SMAW. It requires a constant current power source with AC or DC output and uses a regular twist head or jaw-type electrode holder.

SMAC uses more current than SMAW when using the same size electrodes. To work properly, electrodes designed for gouging and scarfing can require up to three times the amperage. The power source and leads must be capable of handling these higher current requirements.

electrodesSMAC electrodes are designed for high-speed cutting and gouging applications. These are not the same as the standard electrodes used for SMAW. The coating in combination with high AC or DCEN amperages provides a high heat input into the base metal. This creates a molten metal pool and produces a forceful arc blow that removes the molten metal.

The special feature of these cutting electrodes is the high-velocity gas and particle stream they develop that cuts through the metal. The slow-burning ingredients in the electrode coating and the deep cavity in the electrode end (Figure 12) help develop this cutting action. This action is similar to that of CAC-A, but it does not require compressed air or oxygen. Electrodes designed for cutting are normally used for small gouging and chamfering applications where CAC-A or other means are not available.

SMAC electrodes are available in diameters of 2.5 mm (3⁄32 in.), 3.2 mm (1⁄8 in.), 4.0 mm (5⁄32 in.), 5.0 mm (3⁄16 in.) and 6.0 mm (1⁄4 in.). They are available in the standard lengths. The most common length is 350 mm (14 in.).

The size and depth of the groove are easily controlled, and the dross, or scum, is easily removed. Cuts are smooth and uniform and can be done in all positions. For clean high-speed cuts, DCEN is preferred. Hold the electrode at a low angle to the workpiece (10° to 15°). Point the electrode in the direction of the desired groove, strike an arc and push the electrode as fast as the metal is removed. Keep the electrode coating in contact with the workpiece when gouging. The electrode can be in contact with the base metal without any danger of it shorting out because of the heavy coating and the recessed

noTes



electrode wire. The maximum depth of the groove in a single pass should not be greater than the diameter of the core wire. For deeper grooves, use multiple passes.

Manufacturers have developed special electrodes for cutting, piercing and bevelling stainless steel, copper, aluminum, bronze, nickel, cast iron, manganese, carbon steels and alloy steels.

Kerf

PlateArc stream and gas jet from electrode covering and wire

Deeply recessed electrode

Steel core

Coating

+

–

Figure 12—Shielded metal arc cutting (SMAC) electrode

Cutting with SMAW electrodesStandard SMAW electrodes can also be used for cutting. If you must use a SMAW electrode, choose one with deep penetrating characteristics, such as E4310 (E6010) or E4311 (E6011). SMAW electrodes should be used with DCEN, and the current should be set much higher than normally used for welding. This provides a slight arc-blow force and creates a maximum amount of heat in the molten metal pool.

SMAW electrodes used for cutting should be dunked into water before use. The absorbed moisture will slow down the vaporizing of the coating and help produce a deeper cup at the end of the electrode. This will slow down the heating of the electrode coating and create a better cutting action.

Standard SMAW electrodes cause a high heat input into the base metal, which creates a molten metal pool that becomes large and unmanageable. The unmanageable molten metal falls away and creates a cut or hole.

The use of standard SMAW electrodes for cutting is normally limited to producing very rough, uneven cuts in thin metal and to cutting or blowing holes. For thicker material, a sawing action is required to allow the molten metal to fall away and make the cut.


noTes



Answers




1. SMAC electrodes produce a high-velocity gas and particle stream by using a special coating that

a. burns quickly

b. burns slowly

c. eliminates porosity

d. creates gas pockets

2. Which type of welding power source is required for SMAC?

a. constant current

b. constant amperage

c. constant voltage

d. constant potential

3. SMAC works best with which type of welding current?

a. AC

b. ACHF

c. DCEN

d. DCEP

4. Using a standard SMAW electrode for cutting requires

a. less current

b. more current

c. a long arc

d. a constant potential machine

5. When using E4310 (E6010) or E4311 (E6011) electrodes for cutting, which type of welding current is required?

a. AC

b. ACHF

c. DCEN

d. DCEP


noTes



P5-1 learning Task 6:Safety requirementsElectric arc cutting and gouging processes normally require intense heat and are quite often performed during salvaging or restoration operations. The intense heat combined with the removal of metal means that there will be larger than normal volumes of fumes and airborne particles around the work area.

Also, the very nature of salvaging and restoration implies that the weldments being worked on will be painted and most likely contaminated with foreign materials that will produce toxic fumes. This means that WorkSafeBC regulations should be followed. These include regulations on personal protective equipment (PPE), fire and explosions, confined spaces, ventilation, toxic fumes, Firewatchers, etc.

Review Module P1, Theory Competency 2 (P1-2), “Use Safe Work Practices.”

noTes



Theory CompeTenCy p5-2:CAC-A equipment and its use

p5

-2



OutcomesThis Theory Competency introduces the air carbon arc cutting (CAC-A) equipment you will use to produce high-quality cuts safely.


• the welding power source required for CAC-A• CAC-A electrodes and their uses• the air supply requirements for CAC-A• the CAC-A electrode holder• the main factors of CAC-A• the correct operating procedure for CAC-A• the correct technique for cutting, gouging and bevelling with CAC-A

evaluationWhen you have completed all the Theory Competencies in module P5, you will take a written test. You must score at least 70% on this test. The test will include questions that are based on the following material about CAC-A from Theory Competency P5-2:

• equipment required• type and capacity of welding power sources• welding cable sizes required• electrode holder design and use• compressed air supply requirements• carbon electrode travel angles • travel speeds• out-of-position cutting• troubleshooting

resources

All required resources are contained within this Theory Competency.


noTes



P5-2 learning Task 1:CAC-A equipmentCAC-A power sourcesAny standard welding power source, whether AC or DC, has the potential to be used as a power source for CAC-A. But because CAC-A uses such high currents compared to SMAW, not all welding power sources are appropriate.

Power ratingThe welding power source must be able to satisfy the high current demands of CAC-A. The minimum recommended open circuit voltage (OCV) is 60 V. The arc voltage required is 28 V or higher. The actual welding current (amperage) and voltages depend on the electrode size and the type of cutting job.

For most CAC-A applications, the welding power source should have a high current capacity at a 100% duty cycle rating. If your welding power source does not have a 100% duty cycle rating, check the manufacturer’s duty cycle specification chart to find out the amperage at which the power source can be operated at a 100% duty cycle.

Because carbon arc cutting and gouging use such high current, the welding power source should have overload protection in the output circuit. If you are not sure whether a particular welding power source is suitable for CAC-A, check the manufacturer’s specifications to see the rated duty cycles and if the machine is recommended for CAC-A.

Very few single-phase welding power sources produce enough current at a 100% duty cycle rating to be used for CAC-A. You are likely to permanently damage these units if you use them for gouging.

For the best results when doing CAC-A, you should use an industrial welding power source with the highest possible current capacity rated at a 100% duty cycle. The preferred choice is a large, three-phase transformer rectifier unit that has massive transformer windings and can take the extreme and violent surges in current. Similarly, if you use an engine-driven welding power source, it should have plenty of current capacity rated at a 100% duty cycle.

DC versus AC welding power sourcesBoth AC and DC welding power sources can be used for CAC-A. However, a DC welding power source is preferred for most CAC-A work because it produces a more stable arc and is more versatile. As for welding current efficiency, DC produces more current than AC for the same arc voltage (AC is only about 70% as efficient as DC).

Constant current (CC) welding power sources are the usual choice for manual (hand) gouging with CAC-A. You can use a CC power source with

noTes



automatic equipment that uses an electrode feeder controlled by an arc voltage sensor. Other automatic equipment can only be used on constant voltage (CV) with a continuous controlled electrode feeder. It is important to check the manufacturer’s specifications and recommendations.

Constant voltage (CV) welding power sources can be used if the carbon electrode is over 6 mm (¼ in.) in diameter. The CV power source produces a volt-ampere curve with a flat slope, which means that there is little variation in arc voltage regardless of the current being used. When you are hand gouging using CV, there is a risk that if the carbon electrode touches the base metal the current will take a sudden rise and the carbon electrode will explode off at the tip, contaminating the surface of the gouged area.

The use of CV for manual gouging with CAC-A is considered to be optional. CC is the common CAC-A mode. If you must use CV for CAC-A, it is important to read the welding power source manufacturer’s operating manual. Adjustments sometimes must be made to allow output current-limiting devices to stop excessive current surges and prevent disasters.

For most CAC-A applications on carbon, low-alloy and stainless steels, the polarity of the welding power source should be set to direct current electrode positive (DCEP).

For CAC-A on all cast irons it is recommended that AC or DCEN be used in combination with AC carbon electrodes. AC carbon electrodes are preferred as they have ingredients in them that let them work with AC. The end result of using AC carbon electrodes is a cleaner cut and less transfer of the carbon electrode to the base metal. Carbon transfer can be a nuisance during CAC-A on cast iron. However, in practice, most CAC-A on cast iron is done with DCEP using DC carbon electrodes and the carbon deposits are cleaned out as they build up.

On copper and nickel alloys, AC or DCEN should be used in combination with AC carbon electrodes. As an alternative, DCEP using DC carbon electrodes can be used on copper alloys.

Remember: For CAC-A on cast iron, copper alloys or nickel alloys, AC produces cleaner cuts than DC. Also, when you are using AC current, you must use AC-rated carbon electrodes.

Connecting two DC welding power sources in parallelLarge CAC-A electrodes have high current ratings. For example, a DC carbon copper-coated electrode with a 13 mm (½ in.) diameter requires a current of 600 to 1000 amps. When you use large electrodes, you might need to connect two DC power sources together to get the required current. Two identical DC rectifiers (DCEP) can be connected in parallel to double the supply of current (Figure 13).

noTes



– + – +Electrode holder

Air compressor

Welding power sources

Workpiece lead

Air line

Electrode leadGround clamp

Figure 13—DC welding power sources connected in parallel

You MUST follow the manufacturer’s recommendations when connecting two welding power sources in parallel. Connecting welding power sources incorrectly can permanently damage them.

When connecting DC welding power sources in parallel, you MUST connect the terminals positive-to-positive and negative-to-negative. The welding power sources should be identical in all respects and have the same OCV, polarity, manufacturer and model number. Even the length of the connecting cables must be considered.

Do not connect welding power sources in parallel unless you have checked the manufacturer’s operating manual and confirmed the procedure with your welding equipment representative.

Power cables for CAC-AAs with SMAW, the electrode and work lead cables for CAC-A must be able to carry the current needed for the job. Because CAC-A has such high current demands, a large size #4/0 copper lead is commonly used. If the cable is not large enough, it will overheat, and this will cause a voltage drop. High-nickel alloys such as Inconel® have a greater electrical resistance than carbon steels. For that reason, it is very important to locate the workpiece lead connection as close as practical to the location of the gouging.

The workpiece lead and connectors are the same as for SMAW, but the electrode lead is quite different. The electrode lead is usually combined with the compressed air hose. (Figure 14 shows a typical combination air/power line for CAC-A.)

noTes



Electrical lead

Insulating bootCombined air and electrical lead

Air hose

Figure 14—CAC-A combination electrode and compressed air connector

The electrode holderThe hand-held CAC-A electrode holder, commonly called a “torch” (Figure 15), is very similar to a heavy-duty SMAW electrode holder. The major difference is the air jet orifices in the lower jaw of the holder that direct the compressed air jets at the molten base metal.

Carbon electrode

Air jet holes (orifices)

Griphead

Air control valve

Clamp lever handle

Figure 15—Manual CAC-A torch

The air control valve on-off button usually has a lock-open feature so you can keep the air flowing and still keep your hand in a comfortable position. The jaws are spring-loaded and hold the carbon electrode in place. A clamp lever handle opens the jaws.

The electrode holder has a small, circular griphead with a vee groove that firmly grasps and locates the carbon electrode in relation to the air jet orifices. The griphead has two or three air jet orifices to direct the compressed air jet.

noTes



This griphead rotates 360 degrees so that you can adjust it to different positions and directions, such as left to right or right to left (Figure 16).

Figure 16—Swivel griphead

CAC-A air supplyIt is very important that the jets of compressed air have enough pressure to blow away the base metal as it melts. If the air pressure is too low, you will get a poor-quality cut. Although the cut edge or groove might look acceptable, it will contain too much carbon and slag. The carbon will appear as a black deposit in the bottom of the groove or cut.

This carbon buildup is a particular problem on joints that are being prepared for welding. The carbon will combine with the weld deposit and create a brittle and crack-prone weld.

The normal range for compressed air pressure in the CAC-A process is 550 to 690 kPa (80 to 100 psi). Light-duty torches designed for smaller diameter electrodes have smaller air jet orifices that require less air pressure, usually around 275 kPa (40 psi).

Compressed air for CAC-A is normally supplied by an air compressor and is delivered through air hoses. Standard air hoses are used, but these must be large enough to deliver the required volume. A hose with a 13 mm (1⁄2 in.) inside diameter is suitable for most applications. If you use an extra-long run of hose, it will need to have a larger inside diameter, 16 mm to 19 mm (5⁄8 in. to 3⁄4 in.), in order to deliver enough air.

noTes



Sometimes compressed air cylinders are used, especially for smaller jobs or when portability is necessary. If compressed air is not available, an inert gas such as nitrogen can be substituted, but good ventilation must be available.

never use oxygen instead of compressed air Oxygen will react violently in the CAC-A process and cause an explosion

Even when there is enough volume and pressure from the air source, the air flow can be restricted by clogged air passages or fittings. Check your equipment often and clean out any bits of metal slag or debris. Sometimes a flow problem can be as simple as a pinched hose.

Figure 17 shows the recommended pressures and volume flows.

Service application kPa psi l/min cfm

Light 280 40 227 8

Medium industrial 550 80 708 25

Heavy industrial 550 80 934 33

Automated process 414 60 1303 46

Figure 17—CAC-A pressure and volume


noTes



Answers




1. Why is it sometimes necessary to connect DC welding power sources in parallel?

a. to decrease cutting noise

b. to increase current capacity

c. to increase resistance

d. to decrease current capacity

2. What size welding lead cable is recommended for CAC-A applications?

a. 4/0

b. 3/0

c. 2/0

d. 1/0

3. The compressed gas or air for CAC-A is most often supplied by

a. a cylinder of compressed nitrogen

b. a cylinder of compressed oxygen

c. an air compressor

d. a cylinder of compressed air

4. What is the air pressure required for medium- to heavy-duty CAC-A?

a. 140 to 280 kPa (20 to 40 psi)

b. 280 to 400 kPa (40 to 60 psi)

c. 400 to 500 kPa (60 to 80 psi)

d. 550 to 700 kPa (80 to 100 psi)

5. CAC-A on carbon, low-alloy and stainless steels commonly uses

a. DCEP with a DC electrode

b. DCEN with a DC electrode

c. DCEP with an AC electrode

d. DCEN with an AC electrode

Answers



6. A welding power source used for CAC-A should have a minimum open circuit voltage of

a. 20 V

b. 40 V

c. 60 V

d. 80 V

7. You are connecting DC welding power sources in parallel. Which of the following statements about how to connect the terminals is correct?

a. positive to positive and negative to negative to increase voltage

b. positive to negative and negative to positive to increase voltage

c. positive to positive and negative to negative to increase amperage

d. positive to negative and negative to positive to increase amperage

8. Before connecting two DC welding power sources in parallel, you should make sure that the

a. power sources have the same length work leads

b. power sources have identical OCVs

c. power sources have the same brand names

d. manufacturer’s representative has been consulted

9. What is the main difference between a CAC-A electrode holder and a SMAW electrode holder?

a. the clamping device

b. the size

c. the weight

d. the air jet orifices

10. It is important that there is enough air pressure and volume to

a. maintain the correct arc length

b. prevent arc blow

c. blow away the molten base metal

d. maintain the cut temperature

Answers



11. The current requirements for CAC-A are generally for SMAW.

a. the same as

b. lower than

c. higher than

d. smoother than

12. Carbon buildup from the CAC-A process left in the bottom of a groove that will be welded can cause

a. poor penetration

b. a brittle weld

c. lack of reinforcement

d. a ductile weld

13. Which of the following must never be used as a compressed gas in CAC-A?

a. nitrogen

b. oxygen

c. air

d. inert gases


noTes



P5-2 learning Task 2:CAC-A electrodesTypes of CAC-A electrodesThe carbon electrodes used in CAC-A are made of carbon mixed with graphite, which is a crystalline form of carbon. There are three basic types of carbon-graphite cutting rods:

• DC copper-coated• DC uncoated• AC copper-coated

DC copper-coated electrodesThese are the most commonly used carbon electrode for cutting low-carbon steel. The copper coating helps to:

• keep the carbon electrode from overheating by increasing the electrode’s ability to carry higher currents, speeding up the cutting or gouging process

• improve heat dissipation, which helps concentrate the heat at the cutting end of the electrode

• maintain the shape of the electrode tip at the cutting end during gouging, which helps to keep the groove size and shape constant throughout

• reduce the oxidation rate of the electrode, making it last longer

DC copper-coated electrodes are very versatile and can be used to cut carbon, low-alloy and stainless steels and cast iron. They are available in a variety of diameters, lengths and shapes for different cutting and gouging applications. Diameters range from 4 mm to 25 mm (5⁄32 to 1 in.). Shapes include round, flat and semi-round. Normally, lengths vary from 300 mm to 450 mm (12 in. to 18 in.).

DC uncoated electrodesThese electrodes are similar to the coated electrodes, but they do not have a copper coating. They are not widely used for cutting or gouging because they tend to burn off very quickly. They are less accurate than coated electrodes because the end changes shape as the electrode is burned. This makes it very difficult to keep the groove size constant.

Bare electrodes are less expensive than coated electrodes, but they are not recommended for jobs where you need to be very accurate. They are mainly used in salvage work, where you don’t need to be so precise. The diameters of uncoated electrodes are usually 10 mm (3⁄8 in.) and smaller.

noTes



AC copper-coated electrodesSome copper-coated graphite electrodes include a rare-earth element to help stabilize the arc. These electrodes are normally used with an AC welding power source. They produce superior cuts on copper and nickel alloys and cast irons. They are available in diameters from 5 mm to 13 mm (3⁄16 to 1⁄2 in.). They can be used with DC when an AC welding power source is not available. If you must use direct current, DCEN is preferred for use on copper and nickel alloys and cast irons.

AC electrodes can be used with a DC welding power source, but DC electrodes cannot be used with an AC welding power source. Because the DC electrodes have no stabilizers, it is not possible to maintain an arc with AC.

electrode shapesCarbon electrodes are available as round, semi-round and flat. Round electrodes are the most common, and they are used for most grooves. Flat electrodes are mainly used to make wide, shallow grooves. Other shapes are also available for making special groove contours.

Some electrodes are designed to be joined together so the stub can be consumed to reduce waste. Round electrodes have tapered joints at their ends so they can be fitted together (Figure 18). They can be used on both manual and automatic equipment. This joining of electrodes is required for most carbon arc cutting operations with semi-automatic and automatic machines to permit continuous cutting and allow for longer cuts.

External taperInternal taper

Figure 18—Tapered electrode connections

Carbon electrode sizeWhen you are using CAC-A to prepare a weld joint, the correct choice of carbon electrode size depends on the size of the groove you want. When you are using CAC-A to cut through metal, the choice of electrode size also depends on the metal thickness.

The diameter of the electrode determines the width of the groove. The groove produced is usually 3 mm (1⁄8 in.) wider than the electrode diameter. For example, if you want to make a groove 16 mm (5⁄8 in.) wide, you would use a 13 mm (1⁄2 in.) electrode. Although the width of the groove depends mainly on the size of the carbon electrode, it is possible to make wider grooves with small carbon electrodes by weaving the electrode from side to side.

noTes



When choosing an electrode, you must make sure that the welding power source can deliver the current needed. Keep in mind that if you want to use a 13 mm (½ in.) carbon electrode, you need over 600 amps.

Once you have chosen an electrode size suitable for the thickness of the metal you will be cutting (or for the width and depth of the groove), you must set the welding current. You will need higher current settings if you use large electrodes and if you are removing larger amounts of metal.

The label on the manufacturer’s container for the electrodes usually states the amperage range for the various carbon electrode sizes. Figure 19 shows the suggested amperage settings for the most common AC and DC carbon electrodes. Note that AC electrodes cannot use as much DC current as they can AC current.

electrode diameter DC electrode with DCeP

AC electrode with AC

AC electrode with DCen

mm inches min/A max/A min/A max/A min/A max/A

4.0 5⁄32 90 150 --- --- --- ---

4.8 3⁄16 150 200 150 200 150 180

6.4 ¼ 200 400 200 300 200 250

7.9 5⁄16 250 450 --- --- --- ---

9.5 3⁄8 350 600 300 500 300 400

12.7 1⁄2 600 1000 400 600 400 500

15.9 5⁄8 800 1200 --- --- --- ---

19.1 3⁄4 1200 1600 --- --- --- ---

25.4 1 1800 2200 --- --- --- ---

Figure 19—Suggested current ranges for copper-coated carbon electrodes

After you have selected a carbon electrode, begin by setting the amperage in the middle of the range for that size of electrode. If the cut is progressing too slowly, increase the amperage. Do not increase the amperage above the setting recommended for the electrode. Instead, use a larger electrode and set the current to the appropriate higher setting. A larger electrode will require higher current settings, increase the rate of cutting and produce a bigger groove.

Experience will teach you which size of carbon electrode to use for different cutting jobs. You will also learn how to adjust the current setting to get the best results.


Answers




1. Which of the following is NOT a type of carbon electrode used in CAC-A?

a. AC bare electrode

b. DC bare electrode

c. AC copper-coated electrode

d. DC copper-coated electrode

2. The carbon electrodes used in CAC-A are actually made with

a. graphite

b. low-carbon steel

c. cast iron

d. aluminum

3. What is the purpose of the copper coating on some CAC-A carbon electrodes?

a. produces hotter temperature

b. keeps them from rusting

c. reduces their rate of burning

d. allows them to be used to cut nickel

4. For which CAC-A application are uncoated DC carbon electrodes still being used?

a. precision cutting

b. salvage work

c. cutting aluminum

d. preparing stainless steel for welding

5. A CAC-A carbon electrode that has graphite and arc stabilizers in it can be used with

a. DC only

b. AC only

c. DCEP and DCEN only

d. DCEP, DCEN and AC

Answers



6. Your carbon electrode burns away very quickly and starts to glow red after gouging for only a few centimetres. The correct action is to

a. order more electrodes for the job

b. increase the amount of electrode sticking out past the holder

c. decrease the amount of electrode sticking out past the holder

d. reduce the amperage setting

7. An AC electrode with an AC welding power source produces a superior cut on which metal?

a. titanium

b. aluminum

c. low-carbon steel

d. copper

8. Which type of groove preparation are flat electrodes designed for?

a. narrow, deep grooves

b. narrow, shallow grooves

c. wide, deep grooves

d. wide, shallow grooves

9. What is the recommended current setting for a 6.4 mm (1⁄4 in.) DC carbon electrode?

a. 150 to 200 A

b. 200 to 400 A

c. 350 to 600 A

d. 800 to 1200 A

10. If you need a groove 10 mm (3⁄8 in.) wide, which electrode size should you use?

a. 3 mm (1⁄8 in.)

b. 6.4 mm (1⁄4 in.)

c. 9.5 mm (3⁄8 in.)

d. 12.7 mm (1⁄2 in.)


noTes



noTes



P5-2 learning Task 3:electrode angles, cutting speeds and joint positions for CAC-ACarbon electrode angleAs with SMAW, in CAC-A the electrode angle includes two angles: the work angle and the travel angle. Both angles are taken from a line perpendicular to the weld axis at the weld pool. The work angle is taken in a transverse plane from the weld axis. The travel angle is taken in a longitudinal plane from the weld axis. The travel angle is also called the “drag angle” or the “push angle.” Drag angle is when the electrode is pointing away from the direction of travel. Push angle is when the electrode is pointing in the direction of travel. In air carbon arc cutting, you use a push angle.

By adjusting the push angle, you can control the shape and width of the groove (Figure 20). A small travel angle (electrode almost perpendicular to the base metal) will produce a more vee-shaped groove. On automatic gouging machines a change in the travel angle will produce a slight change in the depth of the cut.

The push angle will affect the direction of the molten metal blown out of the groove by the compressed air. Zero push angle (electrode perpendicular) will send molten metal to the sides as well as forward.

30º0ºTravel angle Travel angle

End view of groove End view of groove

Figure 20—effect of carbon electrode angle

Travel speedTravel speed is the speed at which you move the electrode along the weld joint. With all cutting and welding processes, keeping up the correct travel speed is important to getting good results. You might need to go faster or slower, depending on the groove position and the size of the groove required. With CAC-A, if the travel speed is too slow, the groove will be

noTes



wider, deeper, rough and uneven. If the travel speed is too fast, it becomes very difficult to maintain the correct depth and contour of the groove.

Travel speed depends on four factors:

• carbon electrode size• current setting• compressed air supply• base metal

A large-diameter carbon electrode removes metal faster than a smaller one, so with a smaller carbon electrode a slower travel speed is necessary to make a groove of the same size.

The current setting affects the travel speed in much the same way. A higher current setting will quickly remove more metal. The amperage setting for any carbon electrode is given as a range of possible settings. If you use a high current setting within this range, the metal will be removed faster than at a lower current setting, so you must increase the travel speed if you want to make the same size of cut.

If the compressed air supply is below the recommended pressure and volume, the molten metal will not be blown out as quickly, and you will need to reduce your travel speed to allow more time for this to occur.

The composition of the base metal and the depth of the groove also affect travel speed. Different metals have different melting temperatures and require different speeds. For example, the travel speed for aluminum would have to be faster than the travel speed for low-carbon steel of the same thickness. The thickness of the base metal and the depth of the groove to be cut also determine the appropriate travel speed.

Through experience, you will learn how to vary the travel speed for different work. If you are using the correct travel speed, the electrode will make a steady, hissing sound.

Correct positioning of the carbon electrodeThe carbon electrode must be positioned correctly in the electrode holder in order to get the best cuts. Insert the carbon electrode into the swivel disk on the electrode holder so that the air jets are parallel to the electrode. If the carbon electrode is not seated correctly, the air jets will not point precisely at the cut and the molten metal will not be blown out properly.

When inserting the carbon electrode, make sure to position it so that the air will blow from under the electrode (Figure 21).

noTes



Molten metal

Air jet

Carbon electrode

Figure 21—Air jet removing molten metal

Carbon electrode stickoutCarbon electrode stickout is the distance between the electrode holder and the hot tip of the electrode. This distance should be between 100 mm and 150 mm (4 to 6 in.). If the electrode sticks out more than 150 mm (6 in.), it overheats and burns back very quickly. The air jet will also have to travel too far to adequately blow away the slag.

The electrode stickout must be manually readjusted as the electrode is consumed. The electrode can be burned to within 38 mm to 50 mm (11⁄2 to 2 in.) of the torch before it needs to be repositioned.

Welder comfortTo produce a smooth cut, you must hold the electrode holder steady. Shaky movement results in a ragged, uneven cut with small bits of carbon left behind. If possible, hold the torch in both hands and support your arms by resting them on the work table.

When gouging, never point the carbon electrode toward yourself. You will be showered with hot molten metal and/or a blast of compressed air.

At some time you might have to pierce a large, heavy pin on a piece of equipment in order to remove it. In this case, make sure you wear all your personal protection equipment and stand to one side as you pierce. The slag will spray out of the hole at you, and if you are not prepared you will get burned.

noTes



Operating sequence for CAC-AOnce you have chosen the carbon electrode and welding power source for the particular job, you must follow the correct operating sequence. Follow all safety precautions. Be especially careful to make sure that the cutting area contains no combustible material. Make sure that other workers are well out of range of the molten metal.

1. Adjust the welding power source to the correct polarity. Set the current for the electrode selected and attach the workpiece connection.

2. Adjust the air pressure.

3. Insert the carbon electrode in the electrode holder. Check the following:

• the electrode is set in the jaws correctly• the amount of stickout is correct• the air jets are below the electrode and pointing directly along the

electrode

4. Completely open the air control valve on the holder before you strike the arc.

5. Strike an arc by touching the electrode to the base metal. It is not necessary to draw the electrode back once the arc is struck. The metal underneath the electrode immediately melts and blows away, establishing your arc length.

6. Immediately begin to move the electrode in the direction of the cut. Keep the arc length short in order to maintain a high current. Adjust the electrode angle and rate of travel to produce the size and shape of groove you need.

Be careful to avoid touching your work with the electrode. Touching the electrode to the workpiece will create a deposit of carbon that will contaminate any future weld.

7. Break the arc by pulling the electrode from the base metal.

8. Close the air control valve on the holder and remove the carbon electrode.

9. When you finish cutting, shut off the air supply, drain the air line and turn off the power source. Hang up the empty electrode holder in a safe position.

CAUTION:

Carbon electrodes remain hot for a long time after use

Notes



Out-of-position gougingThere are four basic welding positions in all welding and cutting processes: flat, horizontal, vertical and overhead. Welding and cutting techniques for the four positions vary according to the ease of depositing the metal. Welding and cutting in the flat position is generally faster and less tiring than in the other three positions. Whenever possible, you should try to place your workpiece in the flat position.

The term “welding out of position” means welding in any position other than flat.

Horizontal positionIn the horizontal position, the groove area is positioned horizontally and gouging can be done from right to left or left to right. Choose the direction that is most comfortable and convenient for you.

vertical positionIn the vertical position, the groove area is positioned vertically. The direction of travel should be downhill so that gravity helps to remove the molten metal from the groove (Figure 22).

Figure 22—Gouging in the vertical position

Notes



Overhead positionIn the overhead position, the groove area is positioned overhead. Maintaining a short, steady arc is most difficult in the overhead position. With the carbon electrode in the swivel disk holder, position the electrode so that it is pointed as parallel to the holder as possible (Figure 23). This positioning of the electrode gives you a better view of the cut in progress and keeps molten metal from dripping on you.

Figure 23—Gouging in the overhead position

Other techniques for using CAC-ACuttingThe technique for carbon arc cutting is the same as for gouging, except that the travel angle is almost perpendicular to the base metal. Keep the arc directed at one spot until it pierces through the base metal. To cut through thick material, you need to move the arc in an up-and-down sawing motion.

BevellingCAC-A is also used to bevel joint members in preparation for welding. The technique used is much the same as for gouging. Draw the electrode across the edge of the base metal, being sure to hold the electrode at the desired angle (Figure 24). A steady hand gives the best results. Be sure the air flow is between the carbon electrode and the workpiece, pointing in the direction of travel.

Notes



Figure 24—Making a 45º bevel cut

Washing“Washing” is the removal of large amounts of excess metal, such as worn hardfacing material or risers on castings. Torches with air jets on both sides of the electrode are best for this application. With other torch models, make sure that you keep the air jets behind the electrode.

Washing is often used to remove the heads of rivets or frozen bolts. Skillfully done, the head can be removed by washing. The shank of the bolt is then driven out with a hammer and punch.

In washing, the technique is to use a push angle, weaving the torch from side to side with a washing motion to remove the excess metal. As with all CAC-A applications, your steadiness as a Welder determines the smoothness of the surface produced.

noTes



Troubleshooting CAC-AFigure 25 summarizes some common problems that arise during CAC-A cutting.

Trouble Cause Solution

Hard, irregular start Air not on before striking arc

Be sure valve is open and air is on before striking arc.

Sputtering arc, with electrode slow to heat

Low current Increase current and check circuit for poor or loose electrical connections. Cables and ground clamp must match the amperage.

Sputtering arc, with electrode heating rapidly

Wrong polarity Change polarity (sometimes polarity switch on welding power source is incorrect).

Intermittent gouging action

Travel speed too slow Increase travel speed.

Carbon deposit Touching electrode to work

Hold and maintain short arc. Remove trace carbon by grinding.

Irregular groove Unsteadiness of Welder

Relax, loosen grip on torch. Use steady rest if possible.

Slag adhering to edges

Wrong stickout

Air jets not aligned with carbon electrode

Low air pressure

Shorten stickout.

Check alignment of air jets in relation to the carbon electrode.

Increase air pressure, check lines. If pressure cannot be raised, reduce travel speed, make lighter cuts. Remove by chipping or grinding.

Figure 25—Troubleshooting CAC-A


noTes



Answers




1. A large travel angle of the electrode produces what kind of groove?

a. wide

b. narrow, deep

c. wide, deep

d. high, wide

2. What is one method to increase the depth of the groove being cut?

a. reduce amperage

b. increase torch angle

c. increase air pressure

d. decrease travel speed

3. Which factor is the most important in determining the width of the groove?

a. electrode stickout

b. torch angle

c. rate of travel

d. electrode diameter

4. If the air pressure is too low to blow the molten metal from the cut, you should

a. increase travel speed

b. decrease travel speed

c. increase amperage

d. change to a larger electrode

5. What is the recommended electrode stickout when you use CAC-A?

a. 25 mm to 50 mm (1 to 2 in.)

b. 50 mm to 100 mm (2 to 4 in.)

c. 100 mm to 150 mm (4 to 6 in.)

d. 150 mm to 200 mm (6 to 8 in.)

Answers



6. How should the air jets on a CAC-A electrode holder be positioned?

a. 90° to the electrode

b. toward the electrode and the operator

c. between the electrode and the workpiece

d. away from the direction of travel

7. When using CAC-A, you should

a. turn on the welding power source, then strike an arc

b. strike the arc, then start the air flow

c. start the air before turning on the welding power source

d. start the air flow, then strike the arc

8. Why should the carbon electrode not touch the workpiece when you use CAC-A?

a. Arc blow could occur.

b. The welding cable will burn.

c. Carbon is deposited in the groove.

d. An erratic cut results.

9. In which welding position should the carbon electrode be pointing almost parallel to the electrode holder?

a. flat

b. vertical

c. horizontal

d. overhead

10. When too much molten metal sticks to the edges of a cut, what is the usual cause?

a. The polarity is wrong.

b. The electrode angle is too small.

c. The air pressure is too low.

d. The travel speed is too fast.


noTes



Theory CompeTenCy p5-3:PAC process and equipment

p5

-3



OutcomesTo produce high-quality cuts safely and efficiently with plasma arc cutting equipment, you need to know your equipment. It is essential that before you begin work you learn everything about the components and operating procedures of the particular plasma arc power source you will be operating.


• the power sources for PAC• the PAC control unit• PAC torch types• the basic components of PAC torches• plasma gases and their functions• secondary gases and their functions• the main factors of PAC• the correct operating procedures for PAC

evaluationWhen you have completed all the Theory Competencies in Module P5, you will take a written test. You must score at least 70% on this test. The test will include questions that are based on the following material about PAC from Theory Competency P5-3:

• equipment required• torches• compressed air requirements• gases used and their functions• main factors for cutting, setting up and operating PAC

resources

All required resources are contained within this Theory Competency.


noTes



P5-3 learning Task 1:PAC equipmentPAC power sourcesThe high voltages required for PAC are supplied by specially designed power sources. The open circuit voltage (OCV) of PAC power sources ranges from 100 to 400 V, depending on the capacity of the machine. These voltages are considerably higher than those used in CAC-A or SMAW.

PAC current is usually supplied by an inverter-type power source. DCEN is used instead of AC because the plasma arc is easier to maintain and control with DC power. Before the introduction of inverters, PAC power sources were massive DC transformer rectifier–type units that weighed more than 300 kg (660 lb.).

Inverter-type PAC units have the advantage of smaller size and weight (Figure 26). They are used with lightweight, manual cutting torches that use compressed air.

Figure 26—PAC inverter-type power source

The output ratings of PAC power sources vary depending on the thickness and type of metal being cut as well as on the desired cutting speed. If you are cutting thick plate, you need higher amperage and higher open circuit voltage.

PAC power sources have a current control for adjusting the amperage to suit a particular cutting job. The current ranges vary by manufacturer and model.

On a small manual PAC power source, the amperage can be set as low as 12 A. The line power can be 110 V single phase. Sometimes the unit will have a small air compressor built in.

noTes



With larger, heavy-duty PAC power sources, the amperage can be as high as 1000 A. The line power would be three-phase current.

Power sources for PAC contain a special circuit for the pilot arc. The main arc between the electrode and the workpiece has to be started by this pilot arc. This small pilot arc is struck between the electrode and the cutting nozzle. Ionized gas is then blown through the tip orifice, creating a low resistance for the main arc. The main arc between the electrode and workpiece ignites, and the pilot arc shuts off. To increase the life of internal components, the pilot arc often will shut off after 2 seconds, depending on the manufacturer of the system.

PAC control unitPAC systems require a control unit. In many cases, the control unit will sense whether there is an incorrectly assembled torch, lack of gas pressures or incorrect line voltage. If the settings are incorrect, the pilot arc will not start.

In large, mechanized applications, the PAC control unit is a separate console located between the power source and the torch. The control unit is linked to the operator’s display screen and to numerically controlled drives that move the torch.

On lightweight, manual equipment, the controls are combined with the power source into one unit. In most cases there are lights to indicate the operating status. The control prevents start up if settings are incorrect.

PAC torchesPAC torch designs vary. They can be controlled mechanically or manually and cooled by air, a special gas or water (Figure 27).

Figure 27—Manual PAC torch

Most manual torches are cooled by compressed air. Water-cooled torches are mainly used as automatic torches for production cutting of thick material.

Notes



Cutting nozzle

Heat shield

Swirl ring

Electrode

Outer retaining cap

Torch body

Figure 28—Manual PAC torch assembly

Modern torches can be dismantled without hand tools. The nozzle, electrode and swirl vanes are different for each design. Some require special lubricants for the O-ring seals. It is important to check the manufacturer’s operator manual for correct assembly and service.

Certain parts on a torch, called the “consumables,” must be replaced from time to time. The nozzle orifice may become enlarged or misshapen and should be replaced when worn. A damaged cutting nozzle will produce a poor-quality cut.

The heat shield also has to be replaced occasionally due to spatter buildup and heat damage. A ceramic shield is very fragile, so handle it carefully. Placing a hot ceramic shield down to rest on cold metal will cause it to crack.

The consumable that most often needs to be replaced is the electrode. Normally the electrode needs to be replaced if the end develops pitting deeper than 0.8 mm (1⁄32 in.). Check the manufacturer’s specification for the PAC torch you are using.

These consumables will need replacement often if they are handled carelessly. They are relatively inexpensive, but they can become an expensive part of the operation.

Air-cooled PAC torchesThe air-cooled PAC torch was a major design breakthrough. No longer were separate cylinders of compressed gas required. Well-filtered and dry compressed air could be used. The key to this was the change from a tungsten electrode in the centre of the torch to a small piece of hafnium embedded in the tip of a copper electrode. Hafnium allows the PAC process to operate with air or oxygen as the plasma and secondary gas.

Hafnium is a rare metal found in zirconium ore. Since it is so expensive, only a small piece is used in the copper electrode.

Notes



The compressed air should be filtered so that there are no particles larger than 0.85 micron. (One micron is one-millionth of a metre.) Good filtration will increase the life of consumables, which are the electrode and the nozzle.

The torch consists of an electrode mounted in a holder, with the electrode tip centred above the orifice of the cutting nozzle or tip. Surrounding the tip is a ceramic or copper heat shield. A cooling compressed air or gas blows between the tip and the shield (Figure 29).

The electrodes and cutting nozzle come in different sizes for different amperage ranges. Tips with a larger orifice are used for cutting thicker material at higher currents.

Workpiece

Electrode Cutting gasCutting nozzle Secondary gas

Shield cup

Plasma stream(+)

(–)

Figure 29—Air-cooled PAC torch

There are several tip designs, depending on the manufacturer. Some tips can be operated while in contact with the base metal (Figure 30). With these contact tips (or “drag tips”), the operator does not have to worry about maintaining a consistent tip-to-work (standoff) distance.

Drag tip - direct contact with plate

Plasma torch

Relief groove

Ori�ce

Figure 30—Drag-style plasma cutting tip

Notes



Non-contact tips must not touch the base metal. The operator must maintain a constant standoff distance.

Some torches are available with a ceramic or insulated standoff (Figure 31), which works both as a heat shield and as a mechanism for maintaining the standoff distance. Others use a nozzle with small fingers sticking higher than the nozzle orifice.

Some makes of torches using a standard nozzle tip can be used in a drag mode at lower amperage settings and must be used with a standoff at higher amperages.

Plasma torch

Ceramic cup

Cutting ori�ce

Stando�

Figure 31—Insulated standoff

Water-cooled PAC torchesWater-cooled PAC torches come in a wide variety of designs. Some are cooled by recirculating water, which must operate at the recommended pressure and flow rate in order to provide enough cooling. Only the manufacturer’s recommended coolant should be used in recirculating systems to prevent mineral deposits from forming in the passages.

Water-injected PAC torchesA water-injected PAC torch is similar to an air-cooled torch except that water is used instead of a shielding gas (Figure 32). Using water instead of a secondary and cooling gas produces a narrow, sharply defined cut at higher speed than the regular air-cooled torches. The bevel is also considerably smaller. The quality of cut is very close to that of a laser beam cut.

Notes



Electrode

Water injection

Copper

Ceramic

Workpiece

Nozzle

+

–

Cutting gas N2

Figure 32—Water-injected plasma torch

The water cools the workpiece, increases the nozzle life and constricts the plasma gas. Water-cooled PAC torches are used with automatic cutting systems.

Cable connectionsThe cable connections from the PAC power source to the torch are similar to the combination cables used in gas tungsten arc welding (GTAW). The cables are self-contained and carry both gas supply and current.

PAC electrodesThe electrode in the torch head is the most important part of the PAC system. You will have to replace it regularly.

Each manufacturer uses a slightly different arrangement in the torch head to hold the parts. The electrode and nozzle combinations will change according to the gases used and the power settings of the unit.

Most electrode and nozzle assemblies are designed to give the gas a spin as it discharges from the nozzle.

Modern electrode designs have a self-centring copper tip with a very small piece of hafnium embedded in the tip. When the tip is eroded, you must change the electrode.

The life of the electrode depends on the number and type of starts the operator makes. You can shorten the life of an electrode from hours to

minutes if you are not careful.

Notes



Plasma gasesEvery PAC system, whether air-cooled or water-cooled, uses compressed air or gas to create the plasma. The plasma gas (also called “cutting gas” or “orifice gas”) varies, depending on the type of metal to be cut. Plasma gas can be supplied directly from compressed gas cylinders or through a manifold system or a compressed air line system.

If your PAC system uses gases other than compressed air, you will need a gas selection chart to help you choose the best plasma gas and secondary gas for the type of metal you are cutting. The chart will also tell you the correct pressure for the metal thickness and speed of travel.

You might have noticed how many times you have been told to consult the manufacturer’s charts and instructions. This is very important with PAC because there are so many different system designs.

The following are used as plasma gases:

• nitrogen• argon-hydrogen• oxygen• compressed air• nitrogen-oxygen• argon-nitrogen-hydrogen

Compressed air is by far the most commonly used plasma gas. Argon-hydrogen mixes have also been popular for plasma arc gouging a large variety of metals.

Carbon steel is commonly cut with compressed air as the plasma gas. Air in the atmosphere is 80% nitrogen and 20% oxygen and produces high-quality, fast cuts less expensively than compressed gases supplied in cylinders.

Secondary gases (shielding gases)Air-cooled torches require a secondary gas, often called the “cooling gas” or “shielding gas.” This gas flows between the cutting tip and the heat shield so that the heat of the plasma arc does not melt the cutting tip. Compressed air is the preferred choice in modern PAC systems.

In addition to cooling the torch, the secondary gas can produce a shielding “blanket” around the arc plasma, protecting the work from oxidization.

Notes



The following are used as secondary gases:

• carbon dioxide• nitrogen• compressed air• nitrogen-oxygen• oxygen• air-methane

The choice of the secondary gas, like that of the plasma gas, is determined by the type of metal to be cut and the design of the cutting head.

Single-gas systemsModern PAC systems often use a single gas, which simplifies gas selection.

• Oxygen is best for cutting carbon steels.• Nitrogen is best for aluminum and stainless alloys.• Air is the universal second choice because of its convenience and

economy.

Automated PAC systems use a variety of gases, depending on:

• the type of metal being cut• the thickness of the metal• the accuracy required• the travel speed required• the design of the cutting head• the type and design of torch consumables


noTes



Answers




1. What is the advantage of inverter-type power sources for PAC?

a. They can be used underwater.

b. They require very little input current.

c. They are small and lightweight.

d. They produce superior cuts.

2. The usual PAC power source is either an inverter or

a. a high-voltage AC transformer

b. a high-voltage transformer rectifier

c. an AC rectifier with minimum 100 OCV

d. a CAC-A power source

3. Which polarity is used for PAC?

a. DCEN

b. DCRP

c. DCEP

d. AC/DC

4. The purpose of the pilot arc is to

a. preheat the base metal

b. initiate the main arc

c. heat the secondary gas

d. burn the cutting nozzle

5. The ceramic heat shield on a PAC torch must not be subjected to

a. high temperatures

b. rough handling

c. the flow of secondary gas

d. toxic fumes

Answers



6. The cooling gas in an air-cooled torch blows

a. from the torch handle

b. between the electrode and the cutting nozzle

c. between the heat shield and the cutting tip

d. on the outside of the heat shield

7. Cutting nozzles are normally made of which material?

a. copper

b. aluminum

c. stainless steel

d. ceramic

8. An insulated standoff is used to

a. increase cutting speed

b. maintain the correct torch angle

c. maintain consistent tip-to-work distance

d. hold the base metal firmly in place

9. Regardless of design, every PAC torch uses a

a. cooling gas

b. plasma gas

c. secondary gas

d. water-cooling system

10. The choice of a plasma gas is decided by the

a. type of metal being cut

b. thickness of the metal

c. type of cooling gas

d. size of the cutting tip

11. Which plasma gas produces the best results on stainless steel?

a. argon

b. nitrogen

c. compressed air

d. oxygen

Answers



12. Which type of plasma gas is commonly used to cut carbon steel?

a. argon

b. nitrogen

c. compressed air

d. nitrogen/hydrogen mixture

13. PAC torches that use compressed air as the plasma and cooling gas are commonly used to cut

a. titanium

b. cast iron

c. aluminum

d. low-carbon steel

14. Consumable PAC torch components can be interchanged between different equipment manufacturers’ torches.

a. true

b. false

15. Which gas is often used as a secondary gas for cutting stainless steel?

a. carbon dioxide

b. methane

c. oxygen

d. argon


noTes



P5-3 learning Task 2:Main factors of PACPlasma arc cutting is capable of producing high-quality, slag-free cuts at high speeds. In order to use PAC equipment to its full capabilities and produce good results, you must be aware of the factors that control cut quality. These factors are:

• material being cut• equipment setup• operating variables

Material being cutThe first things you must consider are the type and the thickness of the metal to be cut. These workpiece factors must be considered first because they determine the equipment setup.

PAC equipment often comes with data charts for the three most commonly cut metals—low-carbon steel, stainless steel and aluminum. The charts are used for setup. They also provide information for some of the operating variables.

equipment setupA typical torch cutting data table or graph for an air plasma arc cutting torch will provide you with the suggested amperage setting and travel speed for each thickness of metal (Figure 33). The information from each equipment manufacturer will be different.

3.2 mm 0.125"

0

0.520

1.040

1.560

2.080

2.5100

6.4 mm 0.250"

9.5 mm 0.375"

12.7 mm 0.500"

15.9 mm 0.625"

19.1 mm 0.750"

25.4 mm 1.00"

25 A35 A

55 A80 A

Reco

mm

ende

d to

rch

trav

el s

peed

(m/m

in) (

IPM

)80

% o

f max

imum

spe

ed

Base metal thickness

Figure 33—low-carbon steel cutting capacity

noTes



Follow the manufacturer’s setup suggestions in order to get the best cut. If you do not properly set up the equipment for the type of metal and thickness, you might have difficulty making cuts and you could damage the equipment.

Operating variablesAfter you have set up the equipment correctly, you, as the operator, still control important variables that determine cut quality. Since the life of the consumable parts is critical, there are also things you can do to reduce costs. These are listed later under “Efficient cutting.”

Travel speedTravel speed is perhaps the most important factor in producing good cuts. PAC cutting data tables show the suggested travel speeds for various metals and thicknesses. When using a manual torch, you will have to learn by experience how to vary the travel speed for different metals. As a rule, carbon steel can be cut faster than stainless steel, and stainless steel slightly faster than aluminum.

It is usually easy to tell if your cutting speed is too fast. You will usually “lose” the cut and have to restart it. Too high a speed will also increase the bevelling effect of the plasma swirl.

The effects of too slow a cutting speed are also very obvious. Too much slag will accumulate, and the cuts will be very rough, with highly rounded edges. If the speed is very slow, the torch could extinguish. When the cut is made at the correct speed, a fine spray of molten metal is produced directly beneath the workpiece.

The drag lines from the cut should angle at about 10° to 15°, much like OFC on carbon steel.

On aluminum and stainless steel, your speed is correct when the drag lines angle at about 30°.

Double arcingDouble arcing occurs when the cutting current does not pass through the nozzle opening. Instead, the arc travels from the electrode to the nozzle, then from the outside of the nozzle to the workpiece. Double arcing damages the nozzle and the electrode (Figure 34).

Notes



Workpiece (+)

Electrode (–) Cutting gasNozzle Secondary gas

Shield cup

Double arcing

(+)

Figure 34—Double arcing

Do not touch the workpiece with the cutting tip when using a non-contact tip. If the nozzle orifice is blocked by resting the cutting tip on the work or by slag, double arcing results. Low gas pressure or too high a current setting can also cause double arcing.

The pilot arc at startup is like double arcing. It will also shorten the life of the nozzle and the electrode.

Standoff (tip-to-work distance) Maintaining a consistent tip-to-work distance is important. The thicker the metal, the larger the standoff you need. Generally, a low standoff produces the best cuts.

If the standoff is too great, the top edge of the cut will be rounded with too much slag. If the initial standoff is too great, the arc might not strike at all. When the standoff is too short, the cuts are rough and too much slag sticks to the bottom edge of the cut.

Follow the PAC equipment manufacturer’s recommendation for tip-to-work distance. On automatic equipment and manual equipment with a drag tip, this distance will be maintained for you. When operating manual equipment without a drag tip, you will need some practice to maintain a standoff distance that produces a good-quality cut.

Travel directionThe swirling action of the plasma gas produces one edge that is reasonably straight and another edge that has a slight bevel. It is important to plan the direction of your cut so that the straight edge is on the desired piece and the kerf bevel edge is on the scrap piece.

Notes



As you face the direction of the cut, the kerf bevel edge will be on the left side of the cut and the square edge on the right side (Figure 35). When cutting shapes or holes, you would normally cut clockwise for outside cuts (squares, rectangles or disks) and counter-clockwise for inside cuts (holes).

Torch

Straight edge

Beveled edge (2-4º)

Direction of travel

Figure 35—Torch and travel direction

This direction seems common among PAC equipment manufacturers. But some manufacturers supply sets of alternative nozzles that spin the plasma in the opposite direction, changing the side that has the kerf bevel edge.

In some cases torch direction is not a factor. The kerf bevel is not large (2° to 4°), and it can often be accommodated during welding.

Cutting torch maintenanceIn order to get quality cuts, you must properly maintain the consumable parts of the torch, such as the heat shield, electrode and cutting nozzle.

Check the heat shield regularly. A damaged shield will not adequately protect the cutting tip. A damaged cutting tip produces very poor cuts with a lot of slag and poor penetration. Inspect the cutting tip often. The electrode should also be inspected for wear. It can become badly burned and pitted. Remove any spatter or replace the electrode if necessary. Anti-spatter spray will reduce spatter buildup on the nozzle and shield.

Operating procedure for PACAlthough PAC systems vary a great deal, there are certain basic operating procedures that should be followed with any system. Follow the manufacturer’s manual carefully to assemble and adjust the torch front-end components.

Anytime you are adjusting or performing maintenance on a PAC torch, make sure the power source is turned off

Notes



Using guides and patterns when hand cuttingWhen making manual cuts with OAC, it is common to use guide bars and patterns. Guides and patterns can also be used with PAC. However, there are some differences.

The guide or pattern should be made of non-conductive material such as wood or plywood. This is to prevent arcing between the nozzle of the gun and the guide.

PAC is so fast and the heat is so concentrated that the wood guides and patterns will survive repeated use. For cutting curves, pieces of plywood cut to the needed shape work well.

Thermoplastic will work as a guide or pattern, but it can melt and leave plastic sticking to the surface. This depends on your speed of cut and your skill.

One method is to use a drafting tool called a “flexible curve” to draw curves. These are made of plastic but are suitable for quick cuts on light-gauge material. They can be bent to the desired curve before starting the cut.

When freehand cutting, one method is to place a full-size drawing of the item directly onto the metal. The paper can be held in place with some small magnets or strategically placed masking tape. Pick a small hole through the paper for your start position. The torch can be guided around the lines on the drawing, burning through the paper and the metal underneath.

efficient cuttingThe quality of the compressed air supply is very important. An extra air filter and water separator on the air line will increase the life of the electrode and tip assembly. Buying extra filters and air/water separators is cost efficient. Contaminated compressed air will shorten the life of your consumables.

To test the quality of the air, set the PAC power source to the “set” mode. If the unit you are using does not have a set mode, then you will have to turn off the “plasma arc power” switch. Pull the trigger on the plasma torch and start the compressed air flow while pointing the torch tip at a welding lens or a piece of glass. If you see any moisture deposited on the glass, the air is not suitable for PAC use and you must replace or install air filters and water separators.

Make sure your compressed air or gas pressure regulator is set correctly.

Avoid unnecessary pilot arc starts. Unnecessary starts happen when you press the plasma torch trigger when the torch is in the air away from the base metal being cut. Doing so will start a secondary arc between the electrode and the nozzle. This is only intended for startup. The sooner the arc reaches the base metal, the longer the consumables will last.

Notes



The main factor deciding how long your electrode and nozzle will last is how many starts you do. The time that the unit is actually cutting is not as critical. Fewer starts equals longer life.

Always start the pilot arc at the edge or on the surface of the base metal. Do not run your arc completely off the edge of the base metal. One manufacturer suggests that each time the pilot arc is started, you shorten the life of your consumables by 10 to 15 cuts.

Some nozzles can be used with direct drag on the surface of lighter materials without a standoff or drag tip. Other nozzles are not designed for dragging on the surface. Check the manufacturer’s operator manual to confirm what is allowed.

The most efficient procedure for piercing material is to tilt the torch over at least 45°. Lower the nozzle to the standoff distance as you pull the trigger to start the arc. Rotate the torch until the nozzle is vertical and you have successfully pierced the plate. Generally, piercing shortens the life of your consumables. It also increases the risk of getting spatter on the nozzle or blocking the nozzle.

Keep a record of each time you change the nozzle and the electrode. This will tell you if your consumable consumption increases and that something is wrong. This could be as simple as putting 10 sets of tips and nozzles out in the storage tray on Monday morning, then checking to see how many are left on Friday afternoon. If the worn-out parts are kept for the week, they can be inspected and problems can be identified.

The exact assembly and adjustment procedures vary with the brand and type of PAC torch. To successfully operate any torch, you must carefully follow the specific manufacturer’s operating procedure for that torch.

Make sure that your workpiece connection is secure. A ground clamp without enough clamping pressure or with burned jaws will reduce your cutting amperage.

TroubleshootingThe troubleshooting list below (Figure 36) is typical for most PAC units. You must familiarize yourself with the manufacturer’s operating manual for the unit you are using so you know where the troubleshooting chart is located.

Notes



Insufficient penetration Main arc extinguishes

• cutting speed too fast• torch tilted too much• metal too thick• worn torch parts• cutting current too low• wrong brand of parts used• incorrect gas pressure

• cutting speed too slow• torch standoff too high from

workpiece• cutting current too high• work cable disconnected• worn torch parts• wrong brand of parts used

Too much slag formation Short torch parts life

• cutting speed too slow• torch standoff too high from

workpiece• worn torch parts• improper cutting current• wrong brand of parts used• incorrect gas pressure

• oil or moisture in air source• exceeding system capability

(material too thick)• pilot arc on too long • gas pressure too low• improperly assembled torch• wrong brand of parts used

Difficult starting

• worn torch parts• wrong brand of parts used• incorrect gas pressure

Figure 36—PAC troubleshooting

Hand operationsThe following instructions give the sequence of hand operations you should follow when using PAC. The hand skills you developed when using OFC will make this easier than for someone starting out with no experience at cutting metals.

Notes



90°

1/8 in(3.2 mm)

Wt

Always connect work clamp to a clean, paint-free location on workpiece, as close to cutting area as possible.

Maintain approximately a 90º angle to the workpiece surface for proper cutting results.

When doing extended (non-shielded)cutting, maintain approximately 1/8 in stando� between electrode and surface.

DO NOT put pressure on shield when drag cutting; instead, slide shield along the surface for proper cutting results.

Pulling rather than pushing the torch makes cutting easier. Use a proper guide or template for accurate cutting operations.

Sparks should pass through the workpiece and out the bottom when cutting.

If sparks �are back from surface, this usually is an indication that either travel speed is too fast or amperage is set too low.

Set correct air pressure for process:75 PSI (517 kPa) for cutting,55 PSI (379 kPa) for gouging.

DO NOT start pilot arc without cutting or gouging as this shortens the service life of the nozzle and electrode.

The pilot arc starts immediately when trigger is pressed.

The Miller Electric Mfg. Co. information included in this manual are reproduced with the permission of the Miller Electric Mfg. Co., © Miller Electric Mfg. Co., All Rights Reserved.

Figure 37—Plasma cutting system practices

Notes



90°

Connect work clamp to a clean, paint-free location on a workpiece, as clost to cutting area as possible.

For standard (shielded) cutting, place drag shield on edge of metal. For extended (non-shielded) cutting, use 1⁄8 in. (3.2 mm) stando� distance (dragging tip will reduce tip life).

Raise trigger lock and press trigger. Pilot arc starts.

Adjust torch speed so sparks go through metal and out bottom of cut.

Pause brie�y at end of cut before releasing trigger.

Post-�ow continues for approx. 20 seconds after releasing trigger; cutting arc can be instantly restarted during post-�ow by raising trigger lock and pressing trigger.

After cutting arc starts, slowly start moving torch across metal.

Set air pressure to 75 PSI (517 kPa) for cutting.



Figure 38—Sequence of cutting operation

Notes



45

Connect work clamp to a clean, paint-free location on workpiece, as close to cutting area as possible.

Hold torch at approximately 45º angle to workpiece.

Raise trigger lock and press trigger. Pilot arc starts. Move tip to within approximately 3⁄16 in. (4.8 mm). Start gouging across workpiece surface. Maintain approximately a 45º angle to surface.

Release trigger. Post-�ow continues for approx. 20 seconds after releasing trigger; arc can be instantly restarted during post-�ow by raising trigger lock and pressing trigger.

Set air pressure to 55 PSI (379 kPa) for gouging.



Figure 39—Sequence of gouging operation

Notes



90°90°

Connect work clamp to a clean, paint-free location on workpiece, as close to cutting area as possible.

Rotate torch to upright position approximately 90º to surface. When arc has pierced through workpiece, start cutting.

Maintain approximately 90º torch position to surface, and continue cutting.

Release trigger. Post-�ow continues for approximately 20 seconds after releasing trigger; arc can be instantly restarted during post-�ow by raising trigger lock and pressing trigger.

Set air pressure to 75 PSI (517 kPa) for cutting.

Hold torch at an angle to the workpiece. Raise trigger lock and press trigger. Pilot arc starts.



Figure 40—Sequence of piercing operation

The diagrams are universal for all makes of plasma cutters. Notice how all starts and stops are made at the end of the plate rather than in the air. This reinforces the points made earlier under the heading “Efficient cutting.”


Answers




1. Which factor or factors must you consider when preparing to make a plasma arc cut?

a. type of torch to use

b. travel speed

c. thickness and type of metal

d. desired slag buildup

2. When you cut thicker metal, you might have to

a. increase the current setting

b. lower the voltage setting

c. increase the travel speed

d. reduce the plasma gas pressure

3. The best cuts on thin material are produced with

a. high currents

b. a low standoff distance

c. a carbon electrode

d. a low cutting speed

4. One cause of double arcing is

a. current too low

b. gas pressure too great

c. a blocked tip orifice

d. a power surge

5. A damaged heat shield can cause

a. the tip to overheat

b. double arcing

c. a cut that’s bevelled too much

d. the base metal to overheat

Answers



6. The parts of a PAC torch that must be periodically inspected and replaced are called

a. operational

b. adjustables

c. consumables

d. maintenance points

7. “Double arcing” describes what happens when a small arc appears between the electrode and the

a. workpiece

b. cutting nozzle

c. heat shield

d. secondary gas

8. Double arcing is harmful because it

a. produces a rough cut

b. causes too much gas consumption

c. cracks the heat shield

d. damages the cutting nozzle

9. Which operating variable has the most effect on cut quality?

a. tip-to-work distance

b. travel speed

c. operator position

d. travel direction

10. Insufficient gas pressure can cause what problem?

a. rounded cut edges

b. too much slag

c. double arcing

d. bevelled cut edge


noTes



Answer Key

An

swer K

ey

noTes

Module P5 Answer Key


Answer SheetTheory Competency 1

Self-Test 11. b. lead

2. d. either AC or DC

3. a. carbon

4. b. slight surface hardening

5. b. chromium

6. a. true

7. a. to remove the molten metal

8. b. false

9. a. Moves fast and does not allow heat buildup.

10. a. beryllium

11. d. #12

Self-Test 21. b. stainless steel

2. a. U-groove

3. d. back-gouge

4. c. remove excess metal from castings

5. c. It produces a ragged cut line.

6. a. electrode feed mechanism

7. b. greater accuracy

Self-Test 31. d. cutting nozzle

2. a. ionized

3. b. cools the torch

4. d. High-speed cuts reduce heating of the base metal.

5. c. reduces noise, traps slag and fumes

noTes



6. a. metals that cannot be cut with OFC

7. a. any metal

8. d. heated to a high temperature by an electric arc

9. b. PAC is 2 times wider.

10. b. the plasma jet

11. c. conduct electric current

Self-Test 41. d. both a and c

2. b. can make clean cuts

3. b. Little distortion occurs.

4. c. up to five times faster

5. c. high initial cost

6. b. To isolate the operator from health hazards.

7. b. custom production cutting

8. d. aluminum

Self-Test 51. b. burns slowly

2. a. constant current

3. c. DCEN

4. b. more current

5. c. DCEN

Notes



Theory Competency 2Self-Test 11. b. to increase current capacity

2. a. 4/0

3. c. an air compressor

4. d. 550 to 700 kPa (80 to 100 psi)

5. a. DCEP with a DC electrode

6. c. 60 V

7. c. positive to positive and negative to negative to increase amperage

8. d. manufacturer’s representative has been consulted

9. d. the air jet orifices

10. c. blow away the molten base metal

11. c. higher than

12. b. a brittle weld

13. b. oxygen

Self-Test 21. a. AC bare electrode

2. a. graphite

3. c. reduces their rate of burning

4. b. salvage work

5. d. DCEP, DCEN and AC

6. d. reduce the amperage setting

7. d. copper

8. d. wide, shallow grooves

9. b. 200 to 400 A

10. b. 6.4 mm (1⁄4 in.)

Notes



Self-Test 31. a. wide

2. d. decrease travel speed

3. d. electrode diameter

4. b. decrease travel speed

5. c. 100 mm to 150 mm (4 to 6 in.)

6. c. between the electrode and the workpiece

7. d. start the air flow, then strike the arc

8. c. Carbon is deposited in the groove.

9. d. overhead

10. c. The air pressure is too low.

Theory Competency 3Self-Test 11. c. They are small and lightweight.

2. b. a high-voltage transformer rectifier

3. a. DCEN

4. b. initiate the main arc

5. b. rough handling

6. c. between the heat shield and the cutting tip

7. a. copper

8. c. maintain consistent tip-to-work distance

9. b. plasma gas

10. a. type of metal being cut

11. b. nitrogen

12. c. compressed air

13. d. low-carbon steel

14. b. false

15. a. carbon dioxide

Notes



Self-Test 21. c. thickness and type of metal

2. a. increase the current setting

3. b. a low standoff distance

4. c. a blocked tip orifice

5. a. the tip to overheat

6. c. consumables

7. b. cutting nozzle

8. d. damages the cutting nozzle

9. b. travel speed

10. c. double arcing

noTes



Documents

Welder Training Program - BC Trades Modules · Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery