Miller - Plasma

Plasma Cutting and

Gouging

Plasma Cutting and

Gouging

ITW Welding ProductsWelding Process Training Series

TOPIC

#11

Table of Contents

Introduction 3

Process Description 4

Plasma Cutting vs Oxy-Fuel Cutting 5

Plasma Arc Equipment Requirements 6

The PAC Power Source 6

Rated Cutting Capacity 6, 7

Product Guide 7

Cutting Speed 7

Torch Consumables 7, 8

Extended Cutting vs. Drag Cutting 8

Cutting Tips for Different Amperages 9

Replacing Consumables 10

Cutting Gases 11

Plasma Cutting and Gouging Operating Sequence 11

High Frequency Starts 12, 13

Contact Starts 14

Pilot Arc Control Methods 14

Starting The Cut 14, 15

Plasma Arc Gouging 16

Elements of a Quality Cut 17

Six Steps To Good Cut Quality 18

Improving Consumable Life 18

Safety 19

Summary 19

ITW Welding Products | Welding Process Training Series

2

3

Introduction

Welding Process Training Series:Plasma Cutting and Gouging

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Process DescriptionExperiments with a plasma arc date back to early in thetwentieth century, but it was in the 1950’s when PACtorches were patented. The equipment was large andbulky using a variety of cutting and cooling gases. Theintroduction of cutting with clean, compressed air ornitrogen is replacing many other types of cutting equipment.

There are four states in which physical matter may befound: solid, liquid, gas or plasma. Changes from onephysical state to another occur, by either supplying orsubtracting energy, in the form of heat. Water can be usedas an example of these four states of matter. In the solidstate it is ice at temperatures of 32° F (0° C) or colder.With the addition of heat the ice melts and changes towater, the liquid state. The addition of more heat — totemperatures of 212° F (100° C) or hotter — convertsthis liquid to its gaseous state, steam. The fourth state ofmatter, plasma, looks and behaves like a high temperaturegas, but with an important difference; it conducts electricity.The plasma arc is the result of the electrical arc heatingany gas to a very high temperature so that its atoms areionized (an electrically charged gas due to an unequalnumber of electrons to protons) and enabling it to conductelectricity. The major difference between a neutral gasand plasma is that the particles in a plasma can exertelectromagnetic forces on one another.

If you happen to be readingthis by the light emittedby a fluorescent lamp you see plasma in action.Within the glowing tubeof the lamp is a plasmaconsisting of low pressuremercury or sodium vapor.It is ionized by a high volt-age across electrodes atthe ends of the tube and

conducts an electric current which causes the plasma toradiate which in turn causes the phosphor coating on theinner surface of the tube to glow.

The plasma arc torch has a space or area surroundingthe circumference of the electrode, between the insidecircumference of the torch tip or nozzle. It is in thisplenum or chamber that the plasma gas is heated andionized. This heat causes the plasma gas to greatly expandin volume and pressure. The plasma gas exits from the

constricting orifice of the torch nozzle or tip at very highspeeds and temperatures; up to 30,000° F (16,000° C) at20,000 ft/s (6000 m/s). The intensity and velocity of theplasma is determined by several variables including thetype of gas, its pressure and volume, the flow pattern, theamount of electric current, the size and shape of the con-stricting tip or nozzle orifice, and the tip to work distance.

The PAC process uses this high temperature, constricted,high velocity jet of ionized gas exiting from the constrictingorifice of the torch tip to melt a very localized area andremove the molten material from the metal being cut by the force of the plasma jet. The force of the arc pushes the molten metal through the workpiece and severs thematerial. Extremely clean and accurate cuts are possiblewith PAC. Because of the tightly focused heat energy,there’s very little warping, even when cutting thin gaugesheet metal thickness. PAC also offers quality gouging and piercing capabilities.

6,100° C

Gas Tungsten ArcNon-Constricted

3,300° C

Plasma ArcConstricted

10,000° C –14,000° C10,000° C

Electrode

Tip

Shield

PAC Torch Cut-Away

TIG and Plasma Arcs

Plasma Jet

PAC Arc

4


5

Plasma Arc Cutting and Gouging

Before the PAC process became commonplace, if you wantedto cut carbon steel, stainless steel or aluminum, chances wereyou would be using several means or methods of cutting.Perhaps you would use oxy-fuel gas flame cutting for steel,but that process is not recommended for cutting stainlesssteel and aluminum. Bandsaws could be used, or shears,abrasive cut-off wheels, or power hacksaws. And you’d needspecial blades to cut the stainless steels and alloy steels.

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

The PAC process is primarily compared to the oxy-fuel gas (OFC)cutting process. The OFC cutting process severs or removesmetal by the chemical reaction of oxidation. It is known as“burning” or oxidation. This occurs when pure oxygen is appliedto the hot, preheated metal. This temperature of the metal ismaintained at elevated temperatures with a flame from theburning oxy-fuel gas mixture. It requires a high purity oxygenand fuel gas, which comprises an explosive fuel gas mixture. Thisis usually supplied from high-pressure compressed gas cylinders.A properly installed Air Plasma Arc Cutting setup can be saferthan OFC system. Safety precautions on the PAC torches canbe safer than oxy-fuel gas torches where there is a chance offlashback and the danger of flammable gases in exposed hoses.

The advantage of OFC is the capability of cutting up to seven-(7) foot (2 m) thick carbon steel with relatively inexpensiveequipment requiring no electricity. The major disadvantage ofOFC is its limitation to carbon steel cutting only.

PAC requires minimum training to operate the equipment safelyand efficiently. One of the major advantages of PAC is speed.Because PAC operates at a much higher heat energy level, itcan cut faster than OFC. This is especially true on metal that is less than 2 inches in thickness. Cutting speed makes a significant difference in production time and operator comfort.Besides the faster cutting speeds, a major advantage of PAC is that it requires no preheating of the metal like the OFC process.Because of this, PAC results in less distortion of the metal beingcut. This is also due to a very narrow heat-affected zone (areachanged in characteristics near the cut). The clean, dross (theoxidized material that melts during cutting and adheres) freecut produced with the PAC process can eliminate the secondaryoperations of other cutting methods, such as cleaning uprough edges and dross on the bottom or backside of the cut.

Payback-per-cut with PAC can be better with compressed air ornitrogen than oxy-fuel gas and other cutting methods. Besidesthe faster cutting speeds, air and consumables are relativelyinexpensive versus, for example, the cost of gases for OFC.

Many of the limitations or disadvantages of PAC can be overcomeby using the proper equipment or procedure. The initial costof PAC equipment may be greater than other cutting methods,but the ease of operation and the cutting speed will often makethe equipment pay for itself. When compared to OFC, PAC in someareas will not be as portable, due to its dependence on primaryelectrical power from a utility line or engine driven generator.

Plasma Cutting vs Oxy-Fuel Cutting

PressureRegulators

Fuel GasCylinder Oxygen

Cylinder

Cutting Torch

Gas ControlValves

Cutting Tip

Hoses

Oxy-Fuel System

6


Plasma Arc Equipment Requirements

A typical air-cooled PAC system consists of the followingcomponents:

■ A power source■ Either a hand held or a machine style torch■ A supply of compressed air or nitrogen

The PAC Power Source

The power source used for PAC is a direct current powersource. The polarity setting for the power source is direct current electrode negative (DCEN). In most systems there isalso a positive connection to the torch tip in which the currentis limited by a resistor. This circuit is used to establish a “pilotarc” which is used to establish the cutting arc.

The power source is a constant current power source with ahigh open circuit voltage (250– 400 volts). The amperage isusually adjustable within the range of the power source. Thehigher the amperage is set, the faster the travel speed can beor the thicker the metal that can be cut.

Rated Cutting Capacity

There is no industry standard for determining the rated cuttingcapacity of a PAC power source. Miller Electric Mfg. Co. qualifiesits PAC power sources cutting capacity through the use ofthree standards.

Rated Cut Capacity is achieved using a travel speed of 10 inches per minute. This is considered the minimum speedat which an operator achieves a smooth; steady cut using ahand-held torch at the machine’s rated output.

Maximum Quality Cut Capacity maintains the same requirements for cut quality but uses a travel speed less than 10 inches per minute.

Sever Cut Capacity is the maximum metal thickness cutachieved in ideal conditions. There are no allowances for overall cut quality or travel speed.

PowerSourceWork

Clamp

WorkLead

Work

AirCompressor

Air Input Air Filter

Torch Lead Torch

Typical Plasma Cutting System

Typical Plasma CuttingPower Source

1-1/4 in (32 mm)

1 in (25 mm)

Rated Cutting Capacity10 inches per minute

(minimum speed at which an operator achieves a

smooth, steady cut using a hand-held torch).

PAC Power Source Cutting Capability Ratings(Mild Steel, Thickness to Scale)

Maximum Quality Cutting Capacity

Good-quality cut achieved at lower speeds.

Sever Cut CapacityMaximum cut achieved

in ideal conditions.

7/8 in (22 mm)

7


The rated cutting capacity shown on the previous page is forsteel and stainless steel. Rated cutting capacities for aluminummay be different due to aluminum’s high thermal conductivity.

Since aluminum will draw heat away from the cut area morerapidly than steel or stainless steel, the rated cutting capacityfor aluminum may be about 25% less than steel or stainless steel.

Cutting Speed

As indicated previously, the cutting speed will impact thethickness of the material that can be cut. The slower thespeed, the thicker the material that can be cut. The faster thetravel speed, the thinner the material that can be cut.

The following chart comes from the literature sheet of a PACpower source. It can be used to show how thick a materialcan be cut at a given cutting speed. Each power source has itsown cutting speed chart.

Torch Consumables

The plasma torch is designed to generate and focus the plasma cutting arc. In either hand held or machine torches,the same parts are used: an electrode to carry the currentfrom the power source, a swirl ring to spin the compressedair, a tip that constricts and focuses the cutting arc, and ashield and retaining ring to protect the torch.

1/8 1/4 3/8 1/2 5/8 3/4 7/8 1

MILD STEEL THICKNESS (inches)

CUTT

ING

SPEE

D (in

./min

.)

0

2010

40

60

80100120

140160

180200

CUTTING SPEED VS. MATERIAL THICKNESS

Cutting Speed Chart

Torch Breakdown

Rated Output

12 A at 35% Duty Cycle




55 A at 60% Duty Cycle, 3-Ph55 A at 50% Duty Cycle, 1-Ph

80 A at 60% Duty Cycle, 3-Ph80 A at 40% Duty Cycle, 1-Ph


Model

Spectrum® 125C

Spectrum® 375

Spectrum® 375X-TREME™

Spectrum® 625

Spectrum® 2050

Spectrum® 1000

Spectrum® 1251

Mild Steel

1/8 in (3.2 mm)

3/8 in (9.5 mm)

3/8 in (9.5 mm)

1/2 in (13 mm)

7/8 in (22 mm)

1 in (25.4 mm)

1-1/4 in (32 mm)

Shield RetainingCup

Tip

Electrode

SwirlRing

Plasma Torch

Stainless Steel

1/8 in (3.2 mm)

3/8 in (9.5 mm)

3/8 in. (9.5 mm)

1/2 in (13 mm)

7/8 in (22 mm)

1 in (25.4 mm)

1-1/4 in (32 mm)

Aluminum

1/16 in (1.6 mm)

1/4 in (6.4 mm)

1/4 in. (6.4 mm)

3/8 in (9.5 mm)

5/8 in (15.9 mm)

5/8 in (15.9 mm)

1 in (25.4 mm)

Rated Cutting Thickness

Comm

ercia

lIn

dust

rial

Product Guide This chart illustrates the different rating cutting thickness for the different metals.


8

Swirl RingThe swirl ring is designed tospin the cutting gas in a vortex.The swirl ring is made of ahigh temperature plastic withangled holes that cause thegas to spin. Spinning the gascenters the arc on the electrodeand helps to control and constrict the arc as it passesthrough the tip. The swirl ring on Miller plasma cuttingequipment causes the gas toswirl in a clockwise direction.

ElectrodeThe purpose of the electrodeis to provide a path for theelectricity from the power sourceand generate the cutting arc.The electrode is typically madeof copper with an insert madeof hafnium. The hafnium alloyedelectrodes have good wear lifewhen clean, dry compressedair or nitrogen is used (although,electrode consumption maybe greater with air plasmathan with nitrogen).

TipThe purpose of the torch tipis to constrict and focus theplasma arc. Constricting the arcincreases the energy densityand velocity. The tips are madeof copper, with a specificallysized hole or orifice in thecenter of the tip. Tips are sizedaccording to the amperagerating of the torch that theyare to be used in.

Retaining CupThe retaining cup serves twofunctions. First, it holds theother consumable parts firmlyin place. Second, it insulatesand keeps the other consumableparts from making contactwith the work piece.

ShieldsThere are two types of shields used on Miller plasma torches: a drag shield and a deflector. The drag shield insulates the frontend of the torch from the work piece and protects the torch tipfrom spatter. Additionally, the drag shield allows the operator toset the torch directly on the work piece.

The deflector insulates the electrode and protects it from spatter.It is used when extended cutting consumables are needed.

The use of extended cutting consumables require the operatorto maintain a torch stand off of about 1/8". “Torch stand-off” isthe distance from the outer face of the torch tip or constrictingorifice nozzle to the base metal surface. Extended cutting is used insituations where the operator needs extra control of the cuttingarc such as when cutting in a corner or when a machine torchis used.

The drag shield is constructed so that the required standoff ismaintained inside the torch. Using drag cutting consumablesallows the operator to drag the torch on the work piece whilecutting at full output, which increases operator comfort andmakes template cutting easier.

Extended Cutting vs. Drag Cutting

1/8 in.Standoff

Extended Cutting Drag Cutting

Drag Shield

Torch Shields

Extended vs Drag Cutting

Deflector

Torch Consumables

9


Consumable Parts Diagram

Cutting Tips for Different AmperagesSome models of plasma torches will have tips available that arerated for different amperages. For example, in the illustrationbelow the torch has both 80 amp tips which are standard forthe torch along with 40 amp tips. The 40 amp tips can beused any time that the amperage setting on the power sourceis less than 40 amps. Using these tips at 40 amps or less will

produce a smaller kerf than using an 80 amp tip at the samesetting resulting in less material loss. Caution using 40 amptips at an amperage setting greater than 40 amps will overpower the tip causing the tip to be unusable and possiblydamaging the torch.

Gouging80 Amp

Apply silicone grease (#169 231)

before installing.

40 Amp Cutting

Machine Cutting

Standard Cutting 80 Amp

Extended Cutting 80 Amp

Extended Cutting 40 Amp

Shield#212 732

O-Ring#212 735

Electrode#212 724

Retaining Cup#212 733

Swirl Ring #212 734

Drag Shield #212 730

Tip #212 726

TipDrag Shield Retaining Cup #212 725#212 730 #212 733

Deflector Tip Retaining Cup #212 736 #212 728#212 733

Deflector#212 736

Tip #212 729

Retaining Cup #212 733

Shield#212 731

Tip #212 727

Retaining Cup #212 733

40 AmpCutting Tip

80 AmpCutting Tip

Good preventive maintenance (PM) requires keeping a supplyof electrodes, tips, and shield cups on hand and to replacethose in the torch as wear appears. Inspection of the shieldcup, tip and electrode should be done before each use, hourlyduring operation, or whenever the cutting speed has been significantly reduced. Do not operate the PAC torch without atip or an electrode in place. A tip and electrode that are wornbeyond the manufacturers recommended values or operationwithout the tip or electrode in place can damage torch.

New Tip and Electrode Worn Tip and Electrode


10

Consumable Diagram

Replacing Consumables

11


Cutting Gases

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

The gas pressure and flow rates must be properly set to theequipment manufacturer’s recommendation. The gas supplypiping and hoses to the cutting unit must be of sufficient sizeto carry the pressure and gas volume required. It is recommended that a minimum 3/8" ID (inside diameter) piping or nonconductive hoses be used to provide the necessary pressure and volume of gas to the plasma arc cutting power source. If the piping or hose is more than 40 feet in length the minimum ID should be 1/2".

Plasma Cutting and Gouging Operating Sequence

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

The status lights advise the operator if the system is ready tocut or if there is a problem that will keep the unit from operatingproperly. There may be two to four status lights depending onthe model of the power source.

Typically, the top light is labeled power. When this light is on itindicates the the power source is on. If the power light is theonly light on that indicates that the system is ready to cut. Ifan additional light is on, that is the indication of a problem inthe system.

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

Should an additional status light come on, check to make surethat the the torch is properly assembled, the air pressure is setto the recommended setting, and that if the power source hasbeen cutting continuously for more than the rated duty cycletime period it has a chance to cool. Once these problems havebeen fixed the status light will turn off and the system will beready to cut.

After verifying that the status lights indicate that the system isready to cut purge the gas lines for a minute to get rid of anymoisture that may have formed inside the lines.

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

PowerSource

WorkClamp

WorkLead

Work

AirCompressor

Air Input

Torch Lead

Torch

Air Filter

PowerSource

WorkClamp

Work Lead

Work

AirInput

Torch Lead

Torch

Air PressureRegulator

CompressedAir or Nitrogen

Air Filter

Cutting Gasses

Status Lights

High Frequency Starts

The most common initial arc starting technique or method has been to strike a high-frequency spark between the electrode and thetorch tip. The arc is created by a high frequency, high voltage, spark gap oscillator, arc starter (high frequency generator).

A major concern with high frequency (HF) is that it can interfere with telephones, computers or machine (CNC etc.) controls if the systemis not properly installed.


12

Work

Resistor

HighFreq.

Generator

High Frequency Arc

Non-Transferred Arcs

Electrode

Torch

Tip

PowerSource

Pilot ArcRelay

Contact

+

-

Work

Resistor

HighFreq.

Generator

Cutting Arc

TransferredArc

Electrode

Torch

Tip

PowerSource

Pilot ArcRelay

Contact

+

-

Pilot Arc

50 ft(15 m)

Cutting Zone

3

2

2

1

1

3

1

3

3

HF Start Sequence

Sources Of High-Frequency Radiation From Incorrect Installation

1. Sources Of Direct High-Frequency Radiation

High-frequency source (PlasmaArc Cutting power source),cables, torch, work clamp,workpiece, and work table.

2. Sources Of Conduction of High Frequency

Input power cable, line disconnect switch, and input supply wiring.

3. Sources Of Reradiation of High Frequency

Ungrounded metal objects,lighting, wiring, water pipe and fixtures, external phoneand power lines.

13


In order to eliminate problems associated with high frequency interference steps need to be taken at the time of the installation of thesystem. The illustration below is taken from an owner’s manual and shows the necessary steps involved in setting up a groundingsystem for high frequency.

1 Plasma Arc Cutting Source

Ground metal machine case, line discon-nect switch, input supply, and workpiece (ifrequired).

2 Center Point Of Cutting Zone

Midpoint between high-frequency sourceand cutting torch.

3 Cutting Zone

A circle 50 ft (15 m) from center point in alldirections.

4 Torch And Work Cables

Keep cables close together.

5 Conduit Joint Bonding And Grounding

Electrically join (bond) all conduit sectionsusing copper straps or braided wire. Groundconduit every 50 ft (15 m).

6 Water Pipe And Fixtures

Ground water pipe every 50 ft (15 m).

7 External Power Or Telephone Lines

Locate high-frequency source at least 50 ft(15 m) away from power and phone lines.

8 Metal Building Panel BondingMethods

Bolt or weld building panels together, installcopper straps or braided wire acrossseams, and ground frame.

9 Grounding Rod

Consult the National Electrical Code forspecifications.

10 Windows And Doorways

Cover all windows and doorways withgrounded copper screen of not more than1/4 in (6.4 mm) mesh.

11 Overhead Door Track

Ground the track.

8

9

11

10

1

2

3 50 ft(15 m)

Cutting Zone

4

7

50 ft(15 m)

Metal Building

Nonmetal Building

9

9

5

9

6

Ground AllMetal ObjectsAnd All Wiring

In Cutting ZoneUsing #12 AWG Wire

GroundWorkpieceIf RequiredBy Codes

Correct High Frequency Ground Installation

1. Plasma Arc Cutting SourceGround metal machine case, line disconnect switch, input supply, and workpiece (if required).

2 Center Point Of Cutting Zone Midpoint between high-frequencysource and cutting torch.

3. Cutting Zone A circle 50 ft (15 m) from center point in all directions.

4. Torch And Work CablesKeep cables close together.

5. Conduit Joint Bonding And GroundingElectrically join (bond) all conduit sections using copper straps or braidedwire. Ground conduit every 50 ft (15 m).

6. Water Pipe And FixturesGround water pipe every 50 ft (15 m).

7. External Power Or Telephone LinesLocate high-frequency source at least 50 ft (15 m) away from power andphone lines.

8. Metal Building Panel BondingMethods Bolt or weld building panelstogether, install copper straps or braidedwire across seams, and ground frame.

9. Grounding RodConsult the National Electrical Code for specifications.

10. Windows And DoorwaysCover all windows and doorways withgrounded copper screen of not morethan 1/4 in (6.4 mm) mesh.

11. Overhead Door TrackGround the track.

Miller Electric’s contact start “pilot” arc (without using highfrequency) is formed between the nozzle and the inner electrode. This is created when the electrode is in contact with the torchtip and retracts when the trigger is pulled. The airflow pulls or forces the electrode to retract while electricity is flowingthrough the electrode.

After the initial arc is created, the gas flow pushes the arcthrough the orifice in the tip and reestablishes it on the outsideof the tip. This forms a J-shaped arc called the pilot arc. The pilot arc forms a path to the metal surface that is to becut. When the torch is close enough to the metal the arc willtransfer from the pilot arc between the electrode and the tip to a cutting arc between the electrode and the work piece.

Contact Starts

Pilot Arc Control Methods

On some power sources the pilot arc remains on even afterthe cutting arc is established. An advantage to this is that ifthe operator is cutting over a piece of expanded metal, forexample, the cutting arc is maintained as the arc moves fromone piece of metal to the other. One disadvantage of leaving thepilot arc on at all times during the cutting process is that it canlead to faster consumable wear. To help address these issues,some power sources have ways of controlling the pilot arc sothat it is on when needed and can be shut off when not needed.

In some cases the power sourcehas a switch that gives the operatora choice of settings for the pilotarc. As shown in the picture theoperator can select the expandedmetal position for a continuouspilot arc or the tip saver positionwhere the pilot arc shuts off afterthe cutting arc is established.

Other power sources are equipped with a feature called Auto-Refire™. This circuit automatically controls the pilot arc.The pilot arc will switch in and out as fast as needed whencutting expanded metal or multiple pieces of metal. When cutting on a solid piece of metal the pilot arc will drop outafter the cutting arc has been established.

Starting The Cut

With a hand held torch there are two methods for starting thecut: edge starts and pierce starts. To use an edge start, placethe torch directly over the edge of the work piece. With the tipcentered on the edge of the metal, start the arc and beginmoving the torch along the cut line.

Work

Resistor

No Arc (Electrode touching the tip)

Torch

PowerSource

Pilot ArcRelay

Contact

+

-

Work

Resistor

Pilot Arc (Non-Transferred Arc –electrode has pulled away from tip)

Torch

TipPowerSource

Pilot ArcRelay

Contact

+

-

Electrode

Pilot ArcCutting

ArcTip

Electrode

Work

Resistor

Torch

PowerSource

Pilot ArcRelay

Contact

+

-

Cutting Arc (Transferred Arc)

TipElectrode

Contact Start Sequence

Edge Start

14


15


Pierce starts are a little more difficult. The torch will need to be angled slightlyover the starting point. This will prevent the molten metal from the beginning ofthe cut from being blown back into the tip and electrode. Once the cutting arc haspierced through the metal, move the torch to a vertical position and continue alongthe cut line. The thicker the metal, the longer it will take the cutting arc to piercethrough the metal. The process of piercing a hole in the metal will cause a blowhole that is wider than the normal kerf, so the initial pierce should be done in thescrap portion of the part not on the cut line.

°

°

°°

°

Plasma Cutting Sequence-Edge Start

Plasma Cutting Sequence-Pierce Start

Pierce Start

Plasma Arc Gouging is a variation or an adaptation of the PACprocess. Gouging utilizes a different torch tip that produces areduction in the arc constriction, which results in a lower arcstream velocity. Note the larger diameter orifice of the gougingtip. This larger diameter orifice provides the reduction in arcconstriction, which results in a lower arc stream velocity. It givesa softer, wider arc and proper stream velocity. Gouging may beused for edge preparation (J or U-grooves), removal of welds, ordiscontinuities in welds. It may be used in all positions.

A major difference comparing PAC with Air Carbon ArcGouging (CAC-A) is that the gouge surface is bright and clean.This is particularly true with the aluminum alloys and stainlesssteels. There is virtually no clean up required because thegouges are clean and absent of carbon contamination, as is thecase when using the CAC-A process. Because of this fact CAC-Ais not recommended for weld preparation for stainless steeland aluminum without subsequent and sufficient cleaning.

The technique for plasma arc gouging requires the torch beangled 30° to 45° from the base metal surface. This torchangle and the speed of travel will determine the gougingdepth. It is important that not too much material be removedin a single pass. It is better to remove by gouging to therequired depth and width by using multiple passes.

Plasma Arc Gouging

Hold torch at approximately 45° 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 45° angle surface.

Release trigger. Postflow continues for approximately 20 seconds after releasing trigger; arc can be instantly

restarted during postflow by raising trigger lockand pressing trigger.

Plasma Tip Comparison

Plasma Gouging Sequence

Cutting Tip Gouging Tip


16


17

Elements of a Quality Cut

Good quality cuts result in less time and effort being spent on cleaning up the part before it goes to the next step in themanufacturing process. If the part is to be welded, a clean cut is important in order to produce a good weld. There areseveral terms that are important in defining a quality cut.These terms will be referred to as elements of a quality cutand are shown in the illustration below.

The kerf is the width of the cut, or the amount of metalremoved by the cutting process. All cutting processes producea kerf. It is important to account for the kerf when cutting apart to specific dimensions or when determining the numberof parts that can be cut from a piece of metal.

Factors that affect the size of the kerf include cutting speed,amperage setting, amount of stand off, and the size of the orifice in the tip.

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

The direction of travel and the swirl of the gas determinewhich side will be straight and which side will be beveled. Ona torch with a clockwise swirl (this includes all Miller torches)the straight side of the cut will be the right side of the cut inthe direction of travel. Being aware of this characteristic ofplasma cutting will insure that the part being produced hasstraight edges while the scrap piece has the beveled edge.

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

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

Dross is resolidified metal that attaches to the bottom of thepart being cut. The formation of dross is dependent on thetravel speed, amperage, and the type and thickness of metal.Either too high or too low a travel speed can cause dross toform. There is usually a range of travel speed that will producedross free or minimal dross cuts. Adjustments in travel speedwill enable the operator to find this range.

4°– 6° Kerf Angle

On a torch with a clockwise swirl, the straightside of the kerf will be the right-hand side,

in the direction of travel.

Scrap

Scrap

Production Piece

Top Edge Rounding

Kerf Cut Surface Drag Lines

Dross

Bevel Angle

Elements of Cut Quality

Direction of Travel


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Six Steps To Good Cut Quality

1. Use quality consumable partsYou will not get quality cuts without quality parts. Use theparts recommended by the manufacturer. These parts aremade to the exact tolerances required to ensure qualitycuts. To further insure quality cuts, always start with a newset of consumables.

2. Assemble the torch properlyAssemble the torch carefully, making sure the parts areproperly aligned and fit together snugly. This will insuregood electrical contact and cutting performance. Keep partsin their storage containers until needed in order to preventcontamination from dirt and dust.

3. Set the amperage no higher than necessaryThe amperage should be set based on the metal thicknessbeing cut. Using more amperage than needed will shortenconsumable life.

4. Square the torch with the part to be cutIf the torch is not aligned perpendicular with the part thecut may form a beveled edge instead of a straight edge.Check the squareness of the torch visually before startingto cut.

5. Verify the direction of travelThe square side of the cut will be the right hand side in thedirection of travel. Perform a test cut to verify the locationof the square edge.

6. Adjust the travel speedDuring the test cut, determine the travel speed that willprovide either a dross free or minimal dross cut.

Improving Consumable Life

1. Maintain proper gas pressure settingSetting the correct amount of gas pressure is very importantto consumable life. If the pressure setting is too high, electrodelife will be shortened. If the pressure setting is too low, thetip life will be shortened.

2. Maintain the correct stand offIf extended cutting consumables are being used, make sureto maintain the recommended amount of stand off. Too littlestand off may damage the torch and consumables. Toomuch stand off will result in inconsistent arc starts.

3. Pierce within the limits of the plasma systemDo not try to pierce metals that are too thick for the plasmacutter. The typical rating is to pierce up to half the ratedcutting thickness of the plasma cutter. For example, if theplasma cutter is rated to cut 1" steel it could pierce 1/2" steel.

4. Make sure the gasused is clean and dryPlasma cutting systemsrequire clean, dry gasto operate properly.Moisture in the gas lineis the cause of manysystem problems. Itcan cause shortenedconsumable life andpremature torch failure.To check for moisture inthe gas line, set the systemto the gas/air set position and hold a mirror under the tip. Ifany moisture appears on the mirror, inspect the system forthe source of the moisture or install an air dryer in the system.

5. Use edge startsUse edge starts whenever possible instead of pierce starts.Edge starts improve consumable life since there is lesschance for molten metal to be blown back into the tip.

6. Use the tip saver settingWhenever possible use the tip saver position for the pilotarc. More pilot arc time than is necessary will lead to shorterconsumable life. Use the expanded metal setting only whenabsolutely necessary.

7. Remove build up from shieldsInspect the shields on the end of the torch frequently andremove any slag from the shield. Slag can cause doublearcing, which shortens tip life.

8. Purge gas after changing consumablesPurge gas lines for 2–3 minutes after changing consumablesor extended periods of little or no use. This will ensure thatany moisture built up in the lines is removed.

9. Keep torch and consumables cleanAny type of contamination in the torch or consumables canaffect the performance of the cutting system. When changingconsumables, keep the new parts on a clean rag.

Setting Gas Flow Rate

Checking for Moisture

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Safety

As it is with any cutting or welding process, safety is theprime consideration. The equipment Owner’s Manuals will provide safety recommendations that must be followed.

Eye protection is necessary for PAC. Safety glasses should be worn, and a welding helmet, face shield or goggles are necessary. The shade filter lens required would be dependenton such factors as a person’s sensitivity to light, the amperagebeing used, and whether or not the cut itself is hidden by thework piece.

Proper clothing should be worn, and provide enough coverageto prevent burns by molten metal, sparks, or ultraviolet andinfrared light radiation.

When cutting thicker materials, it may be necessary to wearear protection.

Also, water tables are sometimes used beneath cutting tables.If a water table is used, strict guidelines must be followed toavoid such problems as hydrogen gas buildup beneath the platebeing cut. This is especially the case when cutting aluminumand also when argon/hydrogen mixtures are used as the cutting gas.

When used in the open air, PAC can generate a lot of smokeand vaporized metal particulate matter. Good ventilation isessential, and point of source fume capture is recommended.In some cases, respirators may be recommended for the cutting operator.

Operators and maintenance people should keep in mind thatPAC equipment operates with a higher output voltage thantypical welding equipment. Always follow recommended safetyprocedures as outlined by the equipment manufacturer.Further information on safety can be found in the AmericanWelding Society publications “Safety in Welding and Cutting,ANSI ASC Z49.1.”

Summary

If hand-held or machine torch, straight-line, shape or stackcutting, beveling, piercing or gouging is a requirement, PlasmaArc Cutting (PAC) and Gouging can be used to provide speed,economy and, quality cuts. And, this is done with minimumdistortion on any electrically conductive metal.


Miller Electric Mfg. Co.An Illinois Tool Works Company

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Miller - Plasma

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