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Plasma Arc Cutting
PAC
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Objectives
Define plasma arc cutting (PAC).
Explain how a PAC cutter operates.
Identify the parts of a PAC cutter.
Explain advantages and disadvantages of thePAC system.
Identify materials that can be cut with thePAC.
Explain safety associated with using theplasma arc cutter.
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History
The plasma-arc process had its origin almost 50 years ago,during the height of World War II.
Plasma cutting was accidentally discovered by an inventorwho was trying to develop a better welding process.
In an effort to improve the joining of aircraft materials, amethod of welding was developed that used a protectivebarrier of inert gas around an electric arc to protect theweld from oxidation.
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History
It was discovered that by restricting the opening through which theinert gas passed, the heat produced by the process was greatlyincreased.
At the same time, the smaller opening caused the flow of gas to speedup dramatically, ultimately blowing out a channel in the work.
The plasma-arc cutting process started seeing commercial use in thefirst few years of the sixties.
It was an extremely expensive process to undertake, and most cuttingwas performed by large burning services.
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4thState of Matter
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Plasma
Plasma has two meanings.
The fluid portion of blood.
A state of matter that is found in the region of an electrical
discharge (arc).
Plasma created by an arc is an ionized gas that has both
electrons and positive ions whose charges are nearly equal
to each other.
Plasma is present in any electrical discharge.
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Plasma
Plasma consists of charged particles that conduct the electrons acrossthe gap.
Both the glow of a neon tube and the bright fluorescent light bulb areexamples of low-temperature plasmas.
Plasma results when a gas is heated to a high enough temperature to
convert into positive and negative ions, neutral atoms, and negativeelectrons.
The temperature of an unrestricted arc is about 11,000F
The temperature created when the arc is concentrated to from a plasma isabout 23,000F.
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Machines
Most, if not all, of the light portable plasma cutters are 110
volt machines.
Suited primarily for cutting sheetmetal and other light work.
The next level up are the 220 volt machines with 50 to 80amp output current.
These are portable from the standpoint that one person can put it
on a truck and take it to the job.
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How PAC works
Plasma cutters work by sending an electric arc through a gas that is passingthrough a constricted opening. The gas can be shop air, nitrogen, argon, oxygen. etc.
This elevates the temperature of the gas to the point that it enters a 4th stateof matter. Scientists call this additional state plasma. As the metal being cut is part of the
circuit, the electrical conductivity of the plasma causes the arc to transfer to thework.
The restricted opening (nozzle) the gas passes through causes it to squeeze byat a high speed. This high speed gas cuts through the molten metal.
The gas is also directed around the perimeter of the cutting area to shield thecut.
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How a Plasma Cutter works
A complete plasma cutter consists of a
power supply,
a ground clamp,
and a hand torch.
The main function of the power supply is to convert the AC line voltageinto a user-adjustable regulated (continuous) DC current.
The hand torch contains a trigger for controlling the cutting, and anozzle through which the compressed air blows. An electrode is alsomounted inside the hand torch, behind the nozzle.
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PAC System
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Operation
Initially, the electrode is in contact with(touches) the nozzle.
When the trigger is squeezed, DC current
flows through this contact. Next, compressed air starts trying to force its
way through the joint and out the nozzle.
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Operation
Air moves the electrode back and establishes a fixed gap between itand the tip. The power supply automatically increases the voltage in order to maintain
a constant current through the joint - a current that is now going throughthe air gap and turning the air into plasma.
Finally, the regulated DC current is switched so that it no longer flowsthrough the nozzle but instead flows between the electrode and thework piece. This current and airflow continues until cutting is halted.
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Starting the Arc
In many of today's better plasma cutters, apilot arc between the electrode and nozzle isused to ionize the gas and initially generate
the plasma prior to the arc transfer. Other methods that have been used are
touching the torch tip to the work to create aspark, and the use of a high-frequency startingcircuit.
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PAC versus Oxy-Fuel
In general, fabricators consider oxy-fuel to be superior to
plasma for cutting steel when thicknesses exceed about 1/2
inch.
This is because of the slight bevel (4 to 6 degrees) in the cut
face that plasma produces. It is not noticeable in thinner
materials, but becomes more so as thicknesses increase.
Also, at thicknesses above 1/2 inch, plasma has no cutting
speed advantage over oxy-fuel.
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PAC versus Oxy-Fuel
If you are planning to cut non-ferrous metals such as
stainless or aluminum, which cannot be cut by oxy-fuel,
consider a 50 to 80 amp, 220 volt plasma cutter.
Plasma cutting is by far the simplest and most economical
way to cut a variety of metal shapes accurately.
Plasma cutters can cut much finer, faster, and more
automatically than oxy-acetylene torches.
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PAC Cutting Examples
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Plasma Torch
A device depending on its design, which
allows the creation and control of the plasma
for welding and cutting processes.
Plasma torch supplies electrical energy to a
gas to change it into the high energy state of a
plasma
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Torch Body
Made of a special plastic that is resistant to
high temperatures, ultraviolet light, and
impact.
Provides a good grip area and protects the
cable and hose connections to the head.
Torch body is available in a variety of lengths
and sizes.
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Torch head
Torch head is attached to the torch body where the cables and hosesattach to the electrode tip, nozzle tip, and nozzle.
Torch and head may be connected at any angle such as 90, 75, 180(straight), or it can be flexible.
Because of the heat in the head produced by the arc, some provisions
for cooling the head and its internal parts must be made. Cooling for low power torches may be either by air or water.
High power torches must be liquid cooled.
It is possible to replace just the torch head on most torches if itbecomes worn or damaged.
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Power switch
Manual power switch used to start and stop
the power source, gas, and cooling water.
Thumb switch on the torch body most often
used.
Foot control can be used.
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Common Torch Parts
Parts of the torch
Electrode
Tip nozzle insulator
Nozzle tip
Nozzle guide
Nozzles and the metal partsare usually made out ofcopper, and they may beplated.
The plating of copper partswill help stay spatter-freelonger.
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PAC
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Electrode tip
Electrode tip is often made of copper electrode with atungsten tip attached.
Use of copper/tungsten tip has improved the quality ofwork they can produce.
By using copper, the heat generated at the tip can beconducted away faster.
Keeping the tip as cool as possible lengthens the life of thetip and allows for better quality cuts for a longer time.
Old torches you must grind the tungsten electrode
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Nozzle Insulator
Located between the electrode tip and thenozzle tip
Provides the critical gap spacing and electrical
separation of the parts. The spacing between the electrode tip and
nozzle tip called the electrode setback is
critical to the proper operation.
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Nozzle tip
Has a small, cone-shaped, constricting orifice in the center.
The electrode setback space, between the electrode tip and nozzle tip iswhere the electric current forms the plasma.
The preset close-fitting parts provide the restriction of the gas in the presenceof the electric current so the plasma can be generated.
The diameter of the constricting orifice and electrode setback are major
factors in the operation of the torch. As the diameter of the orifice changes, the plasma jet action will be affected.
When the setback distance is changed, the arc voltage and current flow willchange.
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Nozzle
Sometimes call the cup.
Made of ceramic or any other high-
temperature resistant substance.
Helps prevent the internal electrical parts
from accidental shorting and provides control
of the shielding gas or water injection if they
are used.
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Water shroud
Water shroud nozzle may be attached to some
torches.
Water surrounding nozzle tip is used to
control the potential hazards of light, fumes,
noise, or other pollutants produced.
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Power Requirements
Requires a drooping arc voltage or constant current, direct
current, high-voltage, power supply.
Drooping arc voltage allows for a rapid start of the plasma
arc at the high open circuit voltage and more controlled
plasma arc aas the voltage rapidly droops.
Ranges from 50-200 volts closed circuit
Ranges from 150-400 volts open circuit.
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Amperages
High voltage
Amperage range from 10-200 amps.
Some automated machines may have 1,000ampere capacities.
Higher the amperage capacity the faster and
thicker they will cut.
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Cutting speeds
High cutting speeds are possible
Up to 300 inches per minute
25 feet a minute
mile an hour
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Metals to be cut
Any material that is conductive can be cut using the PAC process.
In a few applications nonconductive materials can be coated with conductivematerial so that they can be cut.
Most popular materials cut Carbon steel up to 1
Stainless steel up to 4
Aluminum up to 6
Other metals commonly cut Copper
Nickel-alloys
High-strength, low alloy steels
Clad materials
Expanded metal
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Starting Methods
Two methods are used to establish a current
path through the gas
High frequency alternating current
Momentary shorting
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High frequency alternating current
Most common Uses a high frequency alternating current carried through the
conductor, the electrode and back from the nozzle tip.
High frequency current will ionize the gas and allow it to carry theinitial current to establish a pilot arc.
After the pilot arc has been started, the high frequency starting circuitcan be stopped.
When the torch is brought close enough to the work, the primary arcwill follow the pilot arc across the gap, and the main plasma is started.
Once the main plasma is started, the pilot arc power can be shut off.
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Momentary shorting
Requires the electrode tip and nozzle tip to be
momentarily shorted together.
This is accomplished by automatically moving
them together and immediately separating
them again.
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Safety
Electrical shock Because the open circuit voltage is much higher for this process than for any other,
extra caution must be taken.
The chance that a fatal shock could be received from this equipment is muchhigher than from any other welding equipment.
Moisture
Often water is used with PAC torches to cool the torch, improve the cuttingcharacteristics, or as a part of a water table.
Any time water is used its very important that there be no leaks or splashes.
The chance of electrical shock is greatly increased if there is no moisture on thefloors, cables, or equipment.
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Safety
Noise Because the plasma stream is passing through the nozzle orifice at a high speed, a loud sound
is produced.
The sound level increases as the power level increases.
High levels of sound can have a cumulative effect on ones hearing.
Light PAC produces light radiation in all three spectrums.
Large quantity of visible light, if the eyes are unprotected, will cause night blindness. Most dangerous of the lights is ultraviolet. This light can cause burns to the skin and eyes.
Infrared can be felt as heat, and it is not as much a hazard.
Fumes PAC produces a large quantity of fumes that are potentially hazardous.
A specific means for removing them from the work space should be in place.
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Safety
Gases
Some of the plasma gas mixtures includehydrogen.
Hydrogen is a flammable gas. Make sure that the system is leak-proof.
Sparks
Danger of accidental fire is present.
Use a fire watch person if excessive sparks arepresent.
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