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Welder
Plasma Arc Cutting and Gouging
Workplace Safety and Tools
First Period
Table of Contents
Objective One ............................................................................................................................................... 2 Plasma Arc Cutting ................................................................................................................................... 2 Objective Two............................................................................................................................................. 14 Plasma Arc Cutting and Gouging ........................................................................................................... 14 Objective Three ........................................................................................................................................... 15 Arc Cutting.............................................................................................................................................. 15 Air Carbon Arc Cutting........................................................................................................................... 15 Objective Four ............................................................................................................................................ 24 CAC-A Set-Up and Operation ................................................................................................................ 24 Quality Cuts ............................................................................................................................................ 26 Troubleshooting ...................................................................................................................................... 27 Air Carbon Arc Cutting Exercise ............................................................................................................ 27 Other Arc Cutting Processes ................................................................................................................... 29 Self-Test ...................................................................................................................................................... 31 Self-Test Answers ....................................................................................................................................... 35 Plasma Arc Cutting and Gouging
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Rationale
Why is it important for you to learn this skill?
Plasma arc cutting and gouging are common processes used in metal fabrication shops.
As you gain experience in welding, you will use arc cutting processes for the preparation
and repair of a wide variety of metals on various fabrications. You must become
proficient in these processes to work in a fabrication shop.
Outcome
When you have completed this module, you will be able to:
Cut and gouge using the plasma arc and carbon arc cutting processes.
Objectives
1.
2.
3.
4.
Describe the plasma arc cutting process and equipment.
Observe plasma arc cutting.
Describe the carbon arc cutting process.
Gouge using the carbon arc cutting process.
Introduction
This module describes the plasma arc cutting (PAC) process and the carbon arc cutting
with air (CAC-A) process. Practical exercises and demonstrations are provided so that
you can work with or observe the PAC and CAC-A processes. Other arc cutting
processes are also included.
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Objective One
When you have completed this objective, you will be able to:
Describe the plasma arc cutting process and equipment.
Plasma Arc Cutting
To cut and prepare most metals used in welded fabrication, you can use various thermal
cutting processes. Oxyfuel gas cutting relies upon an oxidation process, which restricts its
range of application. You can use the plasma arc cutting and carbon arc gouging
processes for weld metal removal and to cut most ferrous and non-ferrous metals.
Plasma arc cutting (PAC) is an arc cutting process that uses a constricted arc to remove
molten metal with a high-velocity jet of ionized gas flowing through a constricting
nozzle. The process has been available as a commercial process since the mid-1950s.
Plasma arc cutting cuts all metals that conduct electricity. This includes ferrous metals
such as cast iron, steel, stainless steel, and non-ferrous metals such as nickel, copper,
aluminum and their alloys.
Terminology
You should be familiar with the following terms for plasma arc cutting.
Term
Definition
high frequency
current
An AC current with an operating frequency of thousands of cycles
per second. High frequency current uses several thousand volts and
only a fraction of an ampere superimposed onto the secondary
circuit and is used to initiate the pilot arc.
ion
An atom, molecule (or group of atoms or molecules) that has either
gained or lost one or more electrons.
non-transferred
arc
An arc established between the electrode and the torch tip. It does
not transfer to the workpiece.
pilot arc
Low current high voltage arc (non-transferred) established between
the electrode and the torch tip. The pilot arc is used to establish a
pathway to initiate the main cutting arc.
plasma
A gas which has been heated to an extremely high temperature by
an electric arc. This causes the gas to ionize, making it electrically
conductive. Plasma is considered to be the fourth state of matter;
the others are gas, liquid and solid.
stand-off
distance
The distance between the torch nozzle and the work.
transferred arc
Current flow between the electrode in the torch and the workpiece.
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Principles of Operation
The plasma cutting process (Figure 1) does not depend on oxidation to produce a cut.
Instead, it relies on a high temperature arc to melt the metal and a high velocity jet of
plasma gas to blow the metal away. Temperatures produced in the plasma arc stream
range from 10 000°C to 14 000°C (18 000°F to 25 000°F). Most metals and their surface
oxides melt well below these temperatures. This results in high-speed quality cuts when
you follow the proper operating variables. The cuts are clean and have little slag. There is
a minimum heat-affected zone with very little distortion. The PAC is an erosion process
that operates on direct current straight polarity (DCSP).
Figure 1 - Plasma arc cutting.
When the system activates, a pilot arc is established using high frequency current. The
gas becomes ionized or converts to plasma because of this high voltage. Plasma has high
electrical conductivity and the pilot arc passes to the metal. A series of relays contained
within the power source switch the current automatically from the pilot arc to a highcurrent transferred arc shortly after the pilot arc contacts the workpiece.
As the plasma develops, it heats rapidly, expands and is forced through a constricting
nozzle (tip) to the metal at supersonic speeds. The high temperature arc melts the metal
and the high-velocity jet of plasma blows the metal away.
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Figure 2 shows basic PAC circuitry.
Figure 2 - PAC circuitry.
The use of a secondary shielding gas protects the kerf walls from oxidation and isolates
the plasma stream from the atmosphere. Figure 3 illustrates a typical plasma cutting torch
head that shows the passageways for both plasma and shielding gas. If the PAC torch
system is set up with separate passageways for plasma and shielding gases, the torch is
referred to as a dual flow torch.
Figure 3 - Plasma dual flow torch.
(Courtesy ESAB Welding & Cutting Products)
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Safety
Plasma arc cutting presents the same safety hazards as other arc welding and cutting
processes. Proper personal protective equipment is required to shield you from:
 electrical shock,
 fumes,
 noise,
 radiation and
 gases.
Electrical Shock
The power sources for PAC use a voltage range from 120 to 400 volts. All workers using
the equipment must be aware of the greater risk for electrical shock that this higher
voltage creates.
Follow these safety precautions to prevent electrical shock.
 Keep all electrical circuits dry.
 Use high voltage cables.
 Ensure equipment is properly grounded.
 Keep electrical connections tight and in good repair.
 Turn off the power source before replacing torch parts.
 Keep access doors closed and do not touch live circuits.
 Do not operate PAC with wet gloves or clothing or while standing on a wet
surface.
DANGER
The operating range for PAC power sources is 120 to 400 open circuit
volts. Electrical shock from these voltage levels can be fatal. Only
qualified technicians should install equipment to ensure it meets
applicable electrical codes and standards.
Fumes
The fume particles generated from PAC are much smaller than those generated from
OAC and create a greater health risk. This is because the smaller particulate is more
easily absorbed into the body through the lungs. Stainless steels, aluminum and other
metals that the conventional OAC process cannot cut easily require the PAC process and
many of the fumes from these metals are carcinogenic or can lead to other other health
risks.
DANGER
Always use the appropriate safety precautions to avoid inhaling
vaporized metals and other harmful fumes.
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Metals alloyed with chromium, nickel, aluminum and manganese, or metals that have
lead, cadmium or zinc coatings are particularly hazardous. Ozone and oxides of nitrogen
produced by the intense ultraviolet radiation are a health hazard. Chlorinated solvents
break down to form toxic phosgene gas when they contact the ultraviolet light emitted
from a PAC arc.
DANGER
You must use proper ventilation and the recommended respiratory
protection when working on or near a plasma cutting operation. Fume
extraction or environmental control systems should be in use at all
times.
Noise
Noise level measurements as high as 100 dBA to 110 dBA can occur during the PAC
process. The sound wave frequency generated by PAC is 5000 HZ to 20 000 HZ. These
levels increase your risk for permanent hearing loss.
DANGER
Always wear appropriate hearing protection during PAC.
Radiation
The PAC process emits intense ultraviolet, infrared and visible light rays. Filter plates
should be a minimum shade #8 for less than 300 ampere current ranges and as dark as a
#14 for current settings up to 800 amperes. If the arc is hidden or under water, lighter
shades may be used.
Gases
Compressed gas cylinders used during PAC should be secured to prevent them from
accidentally falling over. Treat cylinders as pressure vessels.
Hydrogen, when used as a plasma or secondary gas, can cause explosions. Hydrogen can
also form when cutting into or under water. The water breaks down to form measurable
amounts of hydrogen. If inadequate ventilation allows hydrogen to accumulate, the
plasma arc can ignite the hydrogen, causing an explosion.
Plasma Cutting Equipment
Plasma arc cutting requires:
 a power supply,
 a torch,
 a process control system and
 an environmental control system.
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Power Supply
Plasma arc cutting machines range from 35 ampere output, which is suitable for auto
body and light sheet metal work, to 750 ampere output that is capable of cutting metals
up to 150 mm (6") thick. Power supplies designed for PAC are DCSP rectifier or
inverter-type constant current machines with open circuit voltages ranging from 120 to
400 volts. They contain special circuits to produce a pilot arc that shuts off when the
main arc initiates. Figure 4 shows a typical plasma arc cutting power supply.
Figure 4 - Plasma arc cutting package.
(Courtesy Miller Electric Mfg. Co.)
Torch
The torch transfers current to a fixed, non-consumable electrode and directs the flow of
plasma and shielding gases (Figure 5). Variations include hand-held torches for manual
PAC, semi-automatic machine-guided torches and fully automatic torches controlled by
computers or robotics. Internal torch variations include constricting orifice diameters,
water or gas-cooled torches and water or gas-shielded torches. Non-consumable
electrodes of tungsten, zirconium or hafnium are available. Hafnium has a high oxidation
resistance making them suitable for use with compressed air and oxygen plasma cutting
systems.
Figure 5 - Plasma cutting torch.
(Courtesy ESAB Welding & Cutting Products)
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Some torches are adaptable for plasma arc gouging applications. Adapting a PAC torch
for gouging operations involves changing to a nozzle that reduces arc constriction,
resulting in lower arc stream velocity. The design of the tip allows it to produce a wider
and less harsh arc. This softer arc melts the metal and the plasma gas stream expels the
metal. Orifice diameter requirements change when cutting different thickness of
materials. Figure 6 gives a detailed view of the internal parts of a plasma arc cutting
torch.
Figure 6 - Plasma torch parts.
(Courtesy ESAB Welding & Cutting Products)
Process Control Systems
Manual operation of the torch usually requires two hands for steady uniform cutting. Two
methods used to start the arc when cutting are the edge and the pierce start. The operator
controls the travel speed and the standoff distance of the torch. Semi-automatic
applications incorporate machine carriages and guidance equipment for straight line and
shape cutting applications. Fully automatic machines such as computer numerical
controlled (CNC) cutting machines and robots are becoming common with this process.
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Water injection plasma arc cutting is another mechanized system where water is injected
through the constricting nozzle that surrounds the plasma jet stream (Figure 7). Water
increases the constriction and narrows the cutting jet, improving squareness of the cut and
increasing travel speeds.
NOTES
Figure 7 - Water injection torch.
(Courtesy ESAB Welding & Cutting Products)
Environmental Control Systems
The large volume of light radiation, vapours and fumes produced by PAC must be
controlled to prevent injury to personnel within the work area. Local ventilation systems
with filtration devices and water tables have reduced many of these environmentally
hazardous materials. With water table PAC, the actual cutting operation takes place over
the surface of or under water, which eliminates fumes and gases and reduces noise levels
(Figure 8).
Figure 8 - Water table PAC.
(Courtesy of Metal Fabricators and Welding Ltd.)
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Operating Variables
Table 1 lists the typical operating variables for cutting stainless steel, such as amperage,
metal thickness and travel speed. These conditions vary depending on the type and brand
of PAC system you are using. Always refer to the equipment operating manual for
directions on setting machine variables for a given material type and thickness.
Thickness
mm (in)
Amperage
Speed
mmmin (in/min)
Orifice Diameter
mm (in)
6.4 (14)
300
5080 (200)
3.2 (18)
12.7 (12)
25.4 (1)
300
2540 (100)
3.2 (18)
400
1270 (50)
4 (532)
50.8 (2)
500
508 (20)
5 (316)
76.2 (3)
500
406.4 (16)
5 (316)
101.6 (4)
500
203.2 (8)
5 (316)
Table 1 - Conditions for cutting stainless steel.
Travel Speed
The type and thickness of metal, orifice diameter and output of the power source
determine the travel speed. The condition of the constricting nozzle and electrode and the
skill of the operator contribute to cut quality and production levels achieved. Quality cuts
are produced at speeds that leave minimum dross (slag) and a narrow kerf with smooth
edges. Consult manufacturers' recommendations for optimum cutting speeds for various
metals and metal thicknesses.
Standoff Distance
Standoff is the distance from the end of the PAC torch nozzle to the work. In most
applications, the standoff distance for cutting should be 6.4 mm to 10 mm
(1/4" to 3/8") (Figure 9). Some torches have a built-in standoff distance and are designed to
be used by dragging the nozzle on the workpiece. Refer to the torch's operating manual
for the suggested standoff distance. Improper standoff distances may result in excessive
nozzle wear, poor quality cuts and slow cutting speeds.
Figure 9 - Standoff distance.
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Standoff distances for plasma gouging may be increased beyond those recommended for
cutting. They generally use a forehand travel inclination of 30. The travel speed and
standoff distance determine the depth of gouge. Increasing standoff distance decreases
the depth of the groove (Figure 10).
NOTES
Figure 10 - Plasma gouging.
(Courtesy ESAB Welding & Cutting Products)
Gases and Flow Rates
Gases used for PAC are dependent on the type of torch used. The two types of torches are
compressed air torches and dual flow torches.
Compressed air torches are common in many fabrication shops because of the
availability and economy of compressed air. These torches provide quality cuts on carbon
steels. The electrodes generally have a shorter life span due to a higher rate of
deterioration through oxidation. Hafnium electrodes are for plasma arc cutting with
compressed air because they have a high oxidation resistance.
Dual flow torches use nitrogen, argon or helium mixed with up to 35% hydrogen or
oxygen for dual-flow gas plasma torches. Carbon dioxide or nitrogen is generally the
shielding gas in dual flow torches, although water may be used for some torches. Proper
shielding prevents oxidation along the kerf walls, as well as protecting the plasma stream
from atmospheric contamination. Consult an operating manual to select appropriate
plasma and shielding gas combinations for specific material types and thicknesses.
Table 2 lists a few typical gas combinations used in dual flow gas torches.
Material Type
Carbon steel
Aluminum
Stainless steel
Plasma Gas
Nitrogen
Argon 65%
Hydrogen 35%
Argon 65%
Hydrogen 35%
Shielding Gas
Plasma Gas
Pressures
kPa (psi)
Shielding Gas
Pressures
kPa (psi)
Carbon dioxide
140 - 280 (20 - 40)
280 (40)
Carbon dioxide
140 - 280 (20 - 40)
280 (40)
Nitrogen
140 - 280 (20 - 40)
280 (40)
Table 2 - Plasma and shielding gases and settings for a dual flow gas torch.
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Metallurgical Effects
As a result of high temperatures and fast cutting speeds, PAC produces a very narrow
heat-affected zone (HAZ) along the kerf. For example, the approximate depth of the HAZ
of a typical one inch thick stainless steel measures only 0.08 mm to 0.127 mm (0.003" to
0.005"). The small amount of heat input minimizes distortion or upset. Mechanical
removal of the HAZ prior to welding is normally not necessary unless specified.
Plasma Arc Cutting Procedures
Before you perform a plasma arc cutting operation, make sure you are wearing the
necessary personal protective equipment for the job and then follow these steps.
1. Refer to the manufacturer's specifications for the air pressure settings for the
machine you are using and then make sure the air pressure supply is sufficient.
2. Attach the ground clamp to the workpiece.
3. Turn on the plasma arc machine and adjust the amperage for the thickness of
material to be cut.
4. Position yourself so you are comfortable.
5. Position the heat shield at the correct standoff distance.
6. Raise the trigger lock (Figure 11) and then press the trigger causing the pilot arc
to start.
Figure 11 - Trigger lock and trigger.
7.
8.
9.
10.
Begin moving the torch across the material once the arc is established.
Adjust your travel speed to the desired quality of cut.
Release the trigger at the end of the cut to stop the machine.
Clean off any dross from the cut area by chipping or grinding.
The post flow setting on the machine cools the torch, making it ready for use again.
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Dross
Dross (Figure 12) is re-solidified oxidized molten metal that does not blow away during
the cutting process. The main causes of dross formation are:
 incorrect cutting speeds,
 incorrect amperage settings for the material being cut,
 incorrect standoff distance and
 a worn out nozzle.
Figure 12 - Dross.
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Objective Two
When you have completed this objective, you will be able to:
Observe plasma arc cutting.
Plasma Arc Cutting and Gouging
The following observation exercise is to familiarize you with typical job site applications
for the PAC process. Before you begin, set up in a location that provides a safe working
environment. Follow all safety precautions.
Observe your instructor set up a PAC unit and operate a hand-held or machine torch by
cutting and gouging on various metal types and thicknesses. Suggested metals are:
 low carbon and stainless steel,
 aluminum,
 cast iron,
 copper and
 brass.
For this project, you require a PAC power source, torch and gases for cutting and
shielding (if appropriate). You can use any of the metal types listed above with a
thickness that you can cut and gouge without exceeding the capacity of the equipment.
Figure 13 shows the quality of cuts and gouged areas on different metal types.
Figure 13 - Cut edges and gouged areas on aluminum, mild steel and stainless steel.
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Objective Three
When you have completed this objective, you will be able to:
Describe the carbon arc cutting process.
Arc Cutting
Arc cutting is a thermal cutting process where melting the base material is accomplished
using an electric arc established between the base metal and an electrode. The electrode
may be consumable or non-consumable, and compressed air or other gases may or may
not be required.
When selecting an arc cutting process for a particular job, you must consider the
following:
 the effectiveness of each cutting process with regard to costs, productivity and
required quality,
 the limitations of the process with respect to the type of metal being prepared,
 the type of power source and auxiliary equipment required for the job and
 the necessary precautions to avoid personal injury and damage to property.
The two most used arc cutting processes are air carbon arc cutting (CAC-A) and plasma
arc cutting (PAC).
Air Carbon Arc Cutting
Air carbon arc cutting (CAC-A), is a cutting process in which metals are melted by the
heat of an arc established between a carbon electrode and the base metal. Molten metal is
forced away from the cut by a jet stream of compressed air (Figure 14).
Figure 14 - Air carbon arc cutting.
(Courtesy Tweco/Arcair A Thermadyne Company)
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Process Applications
The CAC-A is used for gouging, cutting, bevelling and washing operations. The
following examples illustrate the versatility of the process. You can use the process to:
 cut or trim parts to size on ferrous and non-ferrous metals,
 flushing off (washing) rivets and bolts,
 gouge out defective weld metal for repair,
 prepare U groove joints,
 back gouge the root of a weld to clean metal prior to back welding,
 remove old surfacing material before a part is resurfaced or
 remove metal in and around cracks of a broken component to ensure full fusion
welds.
Air carbon arc is effective on most ferrous metals and non-ferrous metals, as listed in
Table 3.
Material
Plain carbon steels, alloy steels and stainless steels
Gray cast iron and malleable cast iron
Copper alloys
Nickel alloys
Aluminum and magnesium alloys
Electrode
Current
DC
DCRP
AC
AC
DC
DCRP (High Amperage)
AC
AC or DCSP
DC
DCRP
AC
AC or DCSP
AC
AC or DCSP
DC
DCRP
Wire brush or grind before welding
Table 3 - Metals compatible with CAC-A.
The CAC-A process provides low heat input into the base metal, minimizing warpage
and distortion. Under ideal conditions, air carbon arc gouging on ferrous-based metals
leaves a bright, clean surface ready for welding. With higher carbon steels, stainless
steels and non-ferrous metals, it may be necessary to first remove the carbon
contamination from the cut surface by grinding or machining.
Safety Considerations
The CAC-A process presents the same safety hazards as other arc welding and cutting
processes. The operator must wear proper personal protective equipment at all times and
must be aware of:
 fire hazards,
 radiation,
 noise and
 air contaminants.
Fire Hazards
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The air carbon arc process produces a shower of sparks and molten metal that can easily
reach distances of up to 6 m (20 ft) or more. This creates a potential fire hazard if
combustible materials are present in the work area. You must follow the proper fire
prevention procedures during the gouging operation. Workers in the area should be
protected from harmful rays, fumes and gases and flying slag by properly placed screens
and adequate ventilation.
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Radiation
The high current levels and the open arc of CAC-A both produce intense light radiation.
Suitable personal protective equipment is necessary. A No. 12 shade filter plate is
recommended when using CAC-A in current ranges up to 500 amperes and a No. 14 for
higher current ranges using automatic equipment.
Noise
The noise level of the air carbon arc process is between 95 to 115 decibels (dB).
In areas where high noise levels exist, the daily exposure without hearing protection must
not exceed the maximum permitted time duration set out by the Alberta Occupational
Health and Safety Act (Noise Regulation). Table 4 lists permissible noise exposure limits.
Sound Level
Maximum Permitted Duration
(dBA)
(hours per day)
82
16 hrs
83
12 hrs and 41 mins
84
10 hrs and 4 mins
85
8 hrs
88
4 hrs
91
2 hrs
94
1 hr
97
30 mins
100
15 mins
103
8 mins
106
4 mins
109
2 mins
112
56 seconds
115 and greater
0
Note: Exposure levels and exposure durations to be
prorated if not specified.
Table 4 - Permissible noise exposures.
NOTE
In the unit dBA, dB means decibels, a unit for measuring the loudness
of sound, and A means weighted sound level scale.
When any person is required to work in areas where the noise level exceeds the
permissible level, appropriate personal hearing protection equipment must be worn to
reduce the noise to acceptable levels. At normal working distances during CAC-A, noise
level readings exceed 115 dBA, which makes hearing protection mandatory while
working with this process.
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Air Contaminants
Fumes and gases produced from air carbon arc gouging are a potential health hazard.
Airborne contaminants from this process include ozone, nitrogen dioxide, carbon
monoxide and toxins produced from metal coatings or degreasers. When you use CAC-A
in an enclosed or semi-enclosed area, you should use exhaust ventilation. You should
also wear suitable dust filters or air respirators.
DANGER
CAC-A is not recommended for cutting beryllium, cadmium or lead
because these materials produce highly toxic fumes.
Equipment Requirements
Air carbon arc cutting requires:
 a welding power source,
 an electrode holder,
 carbon electrodes and
 a compressed air supply.
Power Source
Constant current power sources normally used for SMAW with an open circuit voltage of
at least 60 volts is adequate for manual operation of the air carbon arc cutting process.
You can use this type of machine for all electrode sizes, provided that it is capable of
handling the current requirements of the electrode size.
When the process is set up for automatic or semi-automatic operating modes, a 100%
duty cycle rated constant voltage power source may be required. A DC constant voltage
power source is suitable only for electrodes 7.9 mm (5/16") or larger. Using smaller
electrodes with a DC constant voltage power supply causes excess carbon deposits in the
metal.
It is also recommended that the machine's output circuit have overload protection to
prevent damage to the power source during high current surges, which tend to occur
while cutting or gouging. Where rectifier units do not have sufficient current output
individually, two rectifiers may be connected in parallel, effectively doubling the current
output (Figure 15).
Figure 15 - DC rectifiers in parallel.
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CAUTION
Follow the manufacturer's recommendations when connecting two or
more power sources for CAC-A. Power sources that are not
recommended or are connected incorrectly are at risk for severe
damage. Ensure that the cable size is capable of carrying the required
current.
Table 5 outlines the type of current and power sources recommended for CAC-A.
Current
DC
DC
Power Source
Constant current motor generator, rectifier or
multiple-operator equipment
Constant voltage motor-generator or rectifier
AC
Transformer
AC-DC
Rectifier
Remarks
Recommended for all electrode sizes.
Recommended only for electrodes 7.9 mm
(516") or larger.
Should be used only with AC electrodes.
DC supplied by three phase transformer
rectifier is satisfactory.
DC from single phase source not
recommended.
AC/DC power source is satisfactory if AC
electrodes are used.
Table 5 - Power sources for air carbon arc cutting and gouging.
Electrode Holder
Manual electrode holders for air carbon arc gouging (also known as torches) contain air
passages and orifices to direct the air stream to the end of the carbon electrode. A valve
located on the holder controls the flow of compressed air. To hold the electrode in place,
alligator type spring-loaded jaws are used. The swivel grip head has two or more orifices
to direct the air jet and a groove to grip the electrode. When working in the flat position,
the electrode is placed so that the air is directed along the underside of the electrode. This
allows the air to pass through the arc between the electrode and the workpiece, removing
the molten metal effectively.
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Manual torches (Figure 16) are usually air-cooled. Heavy duty torches designed to carry
high currents can be water-cooled.
Figure 16 - Electrode holder.
Semi-automatic electrode holders are designed for mounting on a machine carriage. The
operator feeds the electrode manually by an adjustment on the carriage as the electrode is
consumed. Automatic electrode holders are mounted on a machine carriage similar to the
semi-automatic electrode holders, but the electrode is fed automatically. Automatic
electrode holders maintain a constant arc length by sensing arc voltage. Consistent
groove depths can be obtained with a depth tolerance of 0.64 mm (0.025") (Figure 17).
Figure 17 - Automatic CAC-A system.
(Courtesy Tweco/Arcair A Thermadyne Company)
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Carbon Electrodes
The carbon electrode is described as a non-filler metal electrode used in arc welding and
cutting, consisting of a carbon and graphite rod. Carbon electrodes are available in
various shapes and sizes, the most common being round, flat and half-round (Figure 18).
The size of groove preparation required for the job and the type of equipment available
determines the electrode size and type.
Figure 18 - Carbon arc cutting electrodes.
These electrodes may be coated with copper or other materials to prevent rapid
vaporization of the carbon. The coatings may also help conduct the high currents for
higher cutting and gouging efficiency. Three types are used:
1. DC copper-coated,
2. DC plain and
3. AC copper-coated.
DC Copper-Coated Electrodes
These electrodes last longer and can carry higher currents than plain electrodes. The
copper coating helps maintain their original diameter while in use, resulting in a more
uniform groove width. Jointed electrodes shown in Figure 19 are available for continuous
operation without stub loss. This is an economical advantage when using larger diameter
electrodes on semi-automatic and fully automatic equipment.
Figure 19 - Jointed carbon electrodes.
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DC Plain Electrodes
These electrodes do not have a coating and are cheaper than copper-coated. One
disadvantage to the DC plain type is they do not perform well in diameters over
10 mm (38"). Another disadvantage is that the blast of air rushing past the heated bare
carbon electrode tapers the end of the electrode to a sharp point due to carbon
vaporization. This results in a groove that resembles a V rather than the desired U groove.
In addition, the width of cut decreases as the electrode tapers down.
AC Copper-Coated Electrodes
These electrodes are designed for use on an AC power source, although they can also be
used on DC straight polarity. Electrodes designed for DC do not work satisfactorily on
AC. The AC electrodes have rare earth materials added to stabilize the arc during use
with alternating current. These electrodes work well on copper, nickel and cast iron.
Compressed Air Supply
Compressed air of sufficient volume, with a pressure ranging from 550 kPa to 700 kPa
(80 psi to 100 psi) is normally required for air carbon arc gouging. Some light-duty
electrode holders operate successfully with pressures as low as 280 kPa (40 psi). Table 6
lists the recommended minimum air pressures. Since exact pressure is not critical, an air
line regulator is not normally required. Compressed nitrogen or inert gas can be used if
compressed air is not available, although the costs are considerably higher.
Electrode Diameter
mm
6.4 (light duty)
6.4
10.0
12.7
15.9
19.0
Minimum Air Pressure
Requirements
inches
kPa
psi
4 (and under)
1
4
3
8
1
2
5
8
3
4
280
40
550
80
550
80
550
80
550
80
550
80
1
Table 6 - Compressed air pressures for various electrode sizes.
Air hoses and fittings with an inside diameter of 6.4 mm (14") are sufficient for light-duty
electrode holders. A minimum of 10 mm (38") inside diameter is required when using
electrode holders having a capacity of 10 mm (38") diameter or larger carbon electrodes.
Semi or fully automatic holders require hoses and fittings with a minimum inside
diameter of 12.7 mm (12"). The hoses must be large enough to deliver air of sufficient
volume and pressure. If the air supply is inadequate, excessive slag adhesion occurs and
unnecessary joint cleaning is required.
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Metallurgical Effects
To avoid changing the physical properties of the base metal, air carbon arc gouging must
be done with care, especially when working on non-ferrous and alloy steels.
The two most common problems encountered during air arc cutting or gouging are:
1. carbonization of the base metal and
2. surface hardening.
Carbonization
Carbonization (sometimes called carburization) of the base metal occurs when the
process is used incorrectly. The DCRP current carries ionized carbon atoms from the
electrode to the base metal. These free carbon particles are absorbed rapidly by the
melted base metal. With proper air velocity and electrode movement, this carbonized slag
and molten metal is blown away, leaving the surface with a minimum of carbon
contamination. Grinding is one effective method for removing any remaining carbon
deposits, but is not normally required if the operation is done properly.
When the gouged surface is rough or irregular, the carbon and slag deposits become
difficult to remove and some carbon and copper residues may still remain even though
the ground surface appears shiny. The copper from the electrode coatings does not
transfer into the base metal if used correctly.
Surface Hardening
Surface hardening is a problem when arc air gouging cast irons and higher carbon steels.
The high temperature of the arc and the sudden cooling by the blast of air to remove the
slag leaves a non-machinable heat-affected zone to a depth of approximately 0.15 mm
(0.006"). Preheating helps reduce this hardness and subsequent welding normalizes the
affected area. Surface hardening is not normally a problem on mild steels because of their
low carbon content.
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Objective Four
When you have completed this objective, you will be able to:
Gouge using the carbon arc cutting process.
CAC-A Set-Up and Operation
To complete a project to an acceptable level of quality, you must consider a number of
variables. They are:
 amperage range,
 electrode stickout,
 starting the arc,
 electrode inclination and
 work angle.
Amperage Range
Table 7 lists the recommended amperage range, dependent on electrode size and type.
Electrode
and Current
Metric
(Imperial)
DC
Electrode
(DCRP)
AC
Electrode
(AC)
AC
Electrode
(DCSP)
Electrode Size
4 mm
(532")
5 mm
(316")
6.4 mm
(14")
7.9 mm
(516")
10.0 mm
(38")
12.7 mm
(12")
90 - 150
150 - 200
200 - 400
250 - 450
350 - 600
600 - 1000
-
150 - 200
200 - 300
-
300 - 500
400 - 600
-
150 - 180
200 - 250
-
300 - 400
400 - 500
Table 7 - Recommended current ranges for air carbon arc gouging.
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Electrode Stickout
Electrode stick-out is the distance between the electrode holder and the end of the carbon
electrode (Figure 20). This distance should not exceed 180 mm (7") for all metals except
when gouging aluminum, which has a maximum stick-out of 100 mm (4"). The electrode
may be burned to within 50 mm (2") of the electrode holder before repositioning or
discarding. Damage to torch parts can occur if the electrode is burned any shorter.
Figure 20 - Electrode stick-out.
Starting the Arc
You must turn on the air stream before touching the electrode to the work. Do not draw
the electrode back once the arc is established because the metal directly under the
electrode will immediately melt and blow away. The arc length must provide enough
clearance for this melting and removal action to be continuous. The progression and
quality of the cut depends upon the type of base metal being cut, electrode size, current
settings, available air supply and the experience of the operator.
Electrode Inclination
The forehand inclination with which you hold the electrode in relation to the direction of
travel determines the rate of travel and depth of cut (Figure 21A). A 30 angle results in a
wide shallow groove requiring a faster travel speed. The 30 angle is used for most
gouging operations. A 90 angle produces a deep, narrow groove requiring a slow travel
speed (Figure 21B). The 90 angle is used for most cutting operations.
Figure 21 - CAC-A torch inclination.
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When cutting, you may need to use a technique similar to a sawing motion. Washing
excess material from a surface may require a side to side (weave) pattern with possibly
more of a forehand inclination. If bevelling, hold the electrode at an angle equal to the
angle of the required bevel and direct the slag away from the surface you wish to have
prepared.
Work Angle
Figure 22 shows the positioning of the electrode in relation to the included angle of the
joint. This angle should be one half of the joint angle. Slight adjustments to the
inclination and angle of the electrode direct the slag away from the cut or gouged area.
Figure 22 - Work angle.
Quality Cuts
In manual operations of CAC-A, the smoothness and uniformity of the surface depends
on the skill of the operator. You need to maintain a close arc length with a travel speed
that is as uniform as possible. You can achieve higher quality cuts by using
semi-automatic or fully automatic motor-driven equipment.
To produce a smooth uniform cut, the electrode holder must advance at a steady, even
rate. Welders often use steady rests and both hands to steady the electrode movement
whenever possible. A loud, continuous tearing or hissing sound is an indication that a
smooth gouge or cut is being made.
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Troubleshooting
Table 8 is a troubleshooting guide to help you problem solve once you begin working
with CAC-A.
Trouble
Cause(s)
Carbon deposits
Touching electrode to the
work without air supply.
Difficult arc starting
Air not on prior to striking the
arc.
Travel speed too slow.
Inadequate air pressure.
Intermittent gouging action
Slag adhering to the edges
Incorrect electrode
inclination or work angle.
Air jets in the wrong position.
Irregular groove
Poor operator control.
Sputtering arc with slow
heating of the electrode
Low current setting or loose
connections.
Sputtering arc with rapid
heating of the electrode
Groove too shallow
Incorrect polarity.
Groove too deep
Electrode inclination too
steep.
Travel speed too slow.
Electrode inclination too flat.
Travel speed too fast.
Solution
Hold and maintain a short arc.
Turn on air supply before touching
electrode to work.
Ensure that the air valve is open
before attempting to strike the arc.
Increase travel speed.
Check lines for leaks or blockage.
Increase air pressure and volume.
Ensure the use of the appropriate
electrode angle and forehand
inclination.
Ensure air jets are positioned
under the electrode when working
in the flat position.
Readjust for comfort and use a
guide to steady yourself.
Check work lead and cable
connections or increase the current
setting.
Check and change polarity.
Increase electrode inclination
toward perpendicular.
Decrease travel speed.
Decrease electrode inclination
toward flat.
Increase travel speed.
Table 8 - Troubleshooting the CAC-A process.
Air Carbon Arc Cutting Exercise
Before you begin the CAC-A process, set up in a location that provides a safe working
environment. Follow all recommended safety precautions.
The purpose of the following exercise is to simulate possible job site applications for the
CAC-A process. It involves the removal of a surfacing weld and a fillet weld so that you
can reuse the material. Then you gouge the back of a groove weld joint to sound metal.
This ensures a joint has full penetration and fusion, which produces a sound weld.
Gouging out the back of a groove weld to sound metal is also known as back gouging.
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NOTE
When you work with welding symbols on shop drawings, you may
find the abbreviation GTSM in the tail of the welding symbol or on a
note elsewhere on the drawing. GTSM stands for gouge to sound
metal.
For this project, you require 3 coupons of 10 mm (38") mild steel flat bar.
1. Make the required welds as indicated in Figure 23.
a) Arrange and tack the coupons.
b) Complete the fillet welds and surfacing welds using available electrodes.
c) Complete Vee-groove butt joint (flat position).
Figure 23 - Prepared weldment.
2. Remove weld metal by gouging (Figure 24).
a) Remove fillet weld metal without over-gouging either plate. Watch for a
dark line appearing between the two workpieces indicating that you have
removed all the weld metal and you are at the bottom of the joint.
b) GTSM from the back side of the groove weld. Gouge just deep enough to
reach clean, sound weld metal. Root bead defects can be seen clearly as
you gouge out the root bead.
c) Remove weld metal from the centre of the surface welds, leaving
25.4 mm (1") on each end. Do not remove any parent material.
d) Hand the completed project in to your instructor for evaluation.
Figure 24 - Gouged weldment.
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Other Arc Cutting Processes
In addition to the PAC and CAC-A processes, you may use the following arc cutting
processes:
 shielded metal arc cutting,
 carbon arc cutting and
 oxygen arc cutting.
Shielded Metal Arc Cutting
Shielded metal arc cutting (SMAC) uses the high temperatures produced by the arc,
which range from 3600°C to 5500°C (6500°F to 10 000°F). This melting process leaves a
very rough edge surface. The arc force and gravity removes the molten metal from the cut
area.
You can use mild steel electrodes such as E4310 or E4311 (E6010 or E6011) (dampened
in water) for piercing holes or severing purposes that use DC straight polarity. Specially
designed electrodes suitable for cutting, chamfering and gouging are also available.
These electrodes operate on AC or DCSP, using a short arc length. The application of this
process is normally limited to demolition projects.
Carbon Arc Cutting
Carbon arc cutting (CAC) is similar to shielded metal arc cutting except a pointed carbon
electrode is used instead of a coated metal electrode. No compressed air is used. The arc
melts the metal and arc force and gravity remove the molten metal, which produces the
cut. DCSP is used because it has a tendency to vaporize the carbon electrode, causing
faster electrode consumption. The cut made with this process is rough and irregular,
limiting its range of application.
Oxygen Arc Cutting
Oxygen arc cutting (AOC) uses hollow or tubular electrodes. These electrodes may be
made of:
 steel with an extruded coating,
 a ceramic core material covered with a steel sheath or
 a glass cloth sleeve saturated with a thermoplastic insulator.
The glass cloth sleeve saturated with a thermoplastic insulator electrodes are used mainly
for underwater cutting to a maximum material thickness of 25.4 mm (1").
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The torch is designed so that current is carried to the electrode. The tubular passage in the
electrode carries oxygen controlled by a valve on the torch. The best choice is to use
DCSP, although you can use AC (Figure 25).
Figure 25 - Oxygen arc cutting.
You can use oxygen arc cutting at relatively high speeds on steels up to 76 mm (3") thick.
However, on stainless steel, the cut can only be as deep as the arc penetrates. Some
non-ferrous materials can be handled with success. The edges prepared by oxygen arc
cutting can be rough and uneven and may require further surface preparation before
welding.
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Self-Test
1. What is plasma?
2. In plasma arc cutting, plasma is the:
a) tungsten electrode.
b) electrons travelling across the arc.
c) ionized gas.
d) outer shielding gas.
3. The plasma arc cutting process:
a) can be used only on materials 6.4 mm (14") thick or less.
b) can be used only on mild steel and stainless steel.
c) is used on metals that are electrically conductive
d) is used mainly to clean up foundry spills.
4. List five (5) safety hazards associated with plasma arc cutting.
a) ____________________________________
b) ____________________________________
c) ____________________________________
d) ____________________________________
e)
5. Any available welding power source may be used for PAC.
a) true
b) false
6. What are the functions of the plasma cutting torch?
7. Compressed air is generally used for PAC on which materials?
a) carbon steels
b) copper alloys
c) magnesium
d) titanium
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8. What effect does improper PAC standoff distances have?
9. In the plasma arc process, what is dross?
a) a slag-like non-metallic by product
b) re-solidified oxidized molten metal
c) re-solidified non-metallic material
d) none of the above
10. What happens once the trigger lock is raised and the trigger is pressed?
a) Gas starts flowing through the torch.
b) The high frequency unit starts ionic bombardment.
c) Short circuit metal transfer begins.
d) The pilot arc starts immediately.
11. What is the minimum welding shade recommended when using the PAC process at
250 amperes?
a) 5
b) 8
c) 10
d) 14
12. PAC cutting produces no significant respiratory hazards.
a) true
b) false
13. The heat-affected zone of a plasma arc cut is extremely wide.
a) true
b) false
14. CAC-A may be used to cut most ferrous and non-ferrous metals.
a) true
b) false
15. List three (3) safety considerations when using CAC-A.
a) ____________________________________
b) ____________________________________
c) ____________________________________
16. What type of power source is required for CAC-A?
a) AC only
b) DC only
c) AC rectifiers only
d) either AC or DC
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17. What is the purpose of compressed air with CAC-A?
a) to remove molten metal
b) to improve cut visibility
c) to cool the workpiece
d) to heat the workpiece
18. List three (3) metals that give off toxic fumes when cut with the CAC-A process.
a) ____________________________________
b) ____________________________________
c) ____________________________________
19. Name the major components of the manual CAC-A electrode holder.
20. Electrodes for CAC-A are usually made of:
a) cast iron.
b) aluminum.
c) carbon steel.
d) graphite and carbon.
21. What is the maximum length that a CAC-A electrode should extend from the holder
to the workpiece when gouging carbon steels?
a) 180 mm (7")
b) 200 mm (8")
c) 250 mm (10")
d) 300 mm (12")
22. What is the range of air pressure normally used for CAC-A?
23. How are the air jets positioned when air carbon arc gouging in the flat position?
a) toward the operator
b) toward the work, above the electrode
c) toward the work, below the electrode
d) in any position because it has no effect on the metal removal
24. How can you reduce carbon deposits on the joint surface when using CAC-A?
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25. When is surface hardening a problem when using CAC-A?
a) when working on high carbon steels and cast irons
b) when air pressures are inadequate
c) when using AC current
d) when gouging nickel alloys
26. With CAC-A, when excessive slag adheres to the edges of the cut, what is the likely
cause?
a) incorrect polarity
b) electrode angle is too shallow
c) air pressure is too low
d) travel speed is too fast
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Self-Test Answers
1. Plasma is a gas that has been heated to an extremely high temperature by an electric
arc. This causes the gas to ionize, making it electrically conductive.
2. c) ionized gas.
3. c) is used on metals that are electrically conductive.
4. a)
b)
c)
d)
e)
electric shock (120V - 400V)
fumes (carcinogenic, ozone)
noise (100 dBA - 100 dBA)
radiation (visible, ultra-violet and infra-red light rays)
gases (compressed cylinders and hydrogen)
5. b) false;
Power supplies designed for PAC are DCSP rectifier or inverter-type
constant current machines with open circuit voltages ranging from 120 to
400 volts. They contain special circuits to produce a pilot arc that shuts
off when the main arc initiates.
6. The plasma cutting torch functions are to transfer current to an electrode, supply a
flow of orifice gas to the constricting nozzle and supply a flow of secondary
shielding gas. The torch transfers current to a fixed, non-consumable electrode and
directs the flow of plasma and shielding gases.
7. a) carbon steels
8. Improper PAC standoff distances results in poor quality cuts, excessive nozzle wear
and slow cutting speeds.
9. b) re-solidified oxidized molten metal
10. d) The pilot arc starts immediately.
11. b) 8
12. b) false;
Plasma arc cutting presents the same safety hazards as other arc welding
and cutting processes. The fume particles generated from PAC are much
smaller than those generated from OAC and create a greater health risk.
Stainless steels and aluminum require the PAC process and many of the
fumes from these metals are carcinogenic or can lead to other other
health risks.
13. b) false;
As a result of high temperatures and fast cutting speeds, PAC produces a
very narrow heat-affected zone (HAZ) along the kerf.
14. a) true
15. Any three (3) of the following are correct.
a) proper protective clothing
b) proper eye protection
c) adequate hearing protection
d) adequate ventilation
16. d) either AC or DC
17. a) to remove molten metal
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18. a) beryllium
b) cadmium
c) lead
19. The electrode holder components are the power cable, the means to grip the electrode
and the air passages and orifices to direct the air stream.
20. d) graphite and carbon.
21. a) 180 mm (7")
22. The range of air pressure for CAC-A is 550 kPa to 700 kPa (80 psi to 100 psi).
23. c) toward the work, below the electrode
24. Use the proper air velocity and electrode movement. Maintain a short arc without
touching the electrode to the work without an air supply.
25. a) when working on high carbon steels and cast irons
26. c) air pressure is too low
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Module Number 120101j
Version 4.0