plasma arc maching
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INTRODUCTION:Plasma arc cutting was developed 20 years ago primarily for cutting stainless steel and
aluminum. Although favorable economically, mild steel was seldom cut with this process
because of three fundamental limitations: relatively poor cut quality, equipment
reliability, and inability of the earlier cutting machines to handle plasma cutting speeds.
As a result of these limitations, plasma cutting did not encounter rapid growth until after
Water-injection Plasma Cutting was introduced in 1970. The plasma arc process has
always been seen as an alternative to the oxy-fuel process. In this part of the series the
process fundamentals are described with emphasis being placed on the operating features
and the advantages of the many process variants.
THE basic principle behind plasma arc cutting is that the arc formed between the
electrode and the workpiece is constricted by a fine bore, copper nozzle. This increases
the temperature and velocity of the plasma emanating from the nozzle. The temperature
of the plasma is in excess of 20,000C and the velocity can approach the speed of sound.
When used for cutting, the plasma gas flow is increased so that the deeply penetrating
plasma jet cuts through the material and molten material is removed in the efflux plasma.
The process differs from the oxy-fuel process in that the plasma process operates by
using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidises the
metal and the heat from the exothermic reaction melts the metal. Thus, unlike the oxy-
fuel process, the plasma process can be applied to cutting metals which form refractory
oxides such as stainless steel, aluminium, cast iron and non-ferrous alloys.
The Plasma arc cutting process
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A plasma cutter is a relatively easy-to-use tool to cut steel and other electrically-
conductive metals. These cutters work by using a high-voltage electrical arc and a
compressed gas, usually air. An electrical arc generated by an internal electrode ionizes
gas passing through a nozzle, creating a concentrated arc of plasma at the cutter's tip. The
arc's contact with the working surface makes a high heat circuit which melts a section
less than 1/16" (1.6mm) wide. The force of the plasma flow then literally blows out the
molten area on the work piece, creating a fairly clean cut with little or no slag. The
plasma arc travels through the nozzle at a speed of up to 20,000 feet per second, and at
temperatures as high as 30,000 degrees Fahrenheit (16,600 Celsius)!
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PLASMA:In physics and chemistry, plasma is a gas in which a certain portion of the particles are
ionized. The presence of a non-negligible number of charge carriers makes the plasma
electrically conductive so that it responds strongly to electromagnetic fields. Plasma,
therefore, has properties quite unlike those of solids, liquids, or gases and is considered to
be a distinct state of matter. Like gas, plasma does not have a definite shape or a definite
volume unless enclosed in a container; unlike gas, in the influence of a magnetic field, it
may form structures such as filaments, beams and double layers Some common plasmas
are flame, lightning, and the Sun.
Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in
1879 (he called it "radiant matter").[1] The nature of the Crookes tube "cathode ray"
matter was subsequently identified by British physicist Sir J.J. Thomson in 1897,[2] and
dubbed "plasma" by Irving Langmuir in 1928,[3] perhaps because it reminded him of a
blood plasma. Langmuir wrote:
Plasma lamp, illustrating some of the more complex
phenomena of a plasma, including filamentation.
The colors are a result of relaxation of electrons in
excited states to lower energy states after they have
recombined with ions. These processes emit light in
aspectrum characteristic of the gas being excited.
Degree of ionization
For plasma to exist, ionization is necessary. The term "plasma density" by itself usually
refers to the "electron density", that is, the number of free electrons per unit volume. The
degree of ionization of a plasma is the proportion of atoms which have lost (or gained)
electrons, and is controlled mostly by the temperature. Even a partially ionized gas in
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which as little as 1% of the particles are ionized can have the characteristics of a plasma
(i.e., response to magnetic fields and high electrical conductivity). The degree of
ionization, is defined as = ni/(ni + na) where ni is the number density of ions and na is
the number density of neutral atoms. The electron density is related to this by the average
charge state of the ions through ne = ni where ne is the number density of
electrons.
Plasm
a cutters work by sending a pressurized gas, such as nitrogen, argon, or oxygen, through
a small channel. In the center of this channel, you'll find a negatively charged
electrode. When you apply power to the negative electrode, and you touch the tip of the
nozzle to the metal, the connection creates a circuit. A powerful spark is generated
between the electrode and the metal. As the inert gas passes through the channel, the
spark heats the gas until it reaches the fourth state of matter. This reaction creates a
stream of directed plasma, approximately 30,000 F (16,649 C) and moving at 20,000 feet
per second (6,096 m/sec) that reduces metal to molten slag. For example
Whe
n energy, in the form of heat, is applied to ice, the ice melts becoming water. The H2O
transforms from the solid-state, ice, to the liquid state, water.When more heat is appliedto the water, the water vaporizes becoming steam. The H2O transforms from the liquid
state, water, to the gas state, steam (H2& O2).Finally, when additional heat is applied tothe individual gases, the gases ionize. The ionization of the gases is the final change in
states. The gases are now in an electrically conductive state called a plasma.
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PLASMA ARC TERMINOLOGY :
PLASM ARC TORCH TERMINOLOGYA plasma torch works by projecting an electric arc produced by polarized electrodes
through gas that is being forced through a small opening, the nozzle. As the gas becomes
electrically charged its temperature elevates until the gas enters the plasma state. The
electric charge is transferred to the metal causing it to melt, and the high velocity gas cuts
through the molten material. Nitrogen, oxygen and argon can all be used, but the most
popular gas is simply forced air. A plasma torch can cut cheaper, faster and more
accurately than the traditional oxy-acetylene torch.
A plasma torch can be effective for both cutting and welding. As with cutters, plasma
welding torches have high speeds and high quality performance for creating pore-free
welding seams. These tools come with a standard electrical cord, a chamber for the air or
gas, and a nozzle with an electrode placed behind it. A gas source, such as an air
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compressor or bottled air or gas, is attached to the torch. For most metals, compressed air
is sufficient, though nitrogen or argon may be used for cutting stainless steel or other
exotic metals.
The automotive and construction industries use a computer numerical control (CNC)
plasma torch to make custom auto shapes required for chassis or frames, and to cut large
steel beams. The versatility and accuracy of this device makes it a great tool for the
creating metal art, jewelry and ornamental iron work. Many CNC torches are large,
expensive equipment designed to operate on the assembly line of a large shop, such as an
automobile factory; however, smaller and more affordable models are now available for
small shops and individual craftsmen. For applications which do not require CNC
equipment, hand-operated units are available at a relatively inexpensive price. The air
pressure capacity required varies depending upon the size of the plasma torch, so the
purchaser should select the torch prior to determining what size air compressor to buy.
Replacement parts, such as air filters, nozzles and electrodes are available from a number
of sources.
A plasma torch, like any other power tool, can cause serious injury if appropriate safety
measures are not followed. The light from the plasma torch can seriously damage the
naked eye, so safety glasses with side shields should be worn at all times. Gloves and
fire-resistant clothing can protect against sparks. These cutters operate at extremely high
temperatures which can burn through gloves, so hands should be kept away from the
nozzle, arc and metal workpiece. To prevent fires or explosions, the torch should only be
used in well-ventilated areas away from any other flammable material.
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PLASMA ARC CUTTING SYSTEM:
A plasma jet can either be operated in the transferred mode, where the power supply is
connected between the electrode and the workpiece, or in the nontransferred mode
wherethe power supply is connected between the electrode and the nozzle. Both modes of
Operation are illustrated in Figure 2. Although a stream of hot plasma emerges from the
Nozzle in both modes of operation, the transferred mode is always used in plasma cutting
because the usable heat input is most efficiently applied when the arc is in electrical
contact with the workpiece.
Schematic diagram for non transferred and transferred arcs
The thermal efficiency is low for non transferred up to 65-75% while for transferred arcs
it is up to 85-90%.
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CONVENTIONAL PLASMA ARC CUTTING:The plasma jet that is generated by conventional "dry" arc constriction techniques can be
used to sever any metal at relatively high cutting speeds. The thickness of plate can range
from 1/8 inch to a maximum thickness depending on both the current capacity of the
torch and the physical properties of the metal. A heavy duty mechanized torch with a
current capacity of 1000 amps can cut through 5 inch thick stainless steel and 6 inch thick
aluminum. However, in most industrial applications the plate thickness seldom exceeds
1-1/2 inch. In this thickness range, conventional plasma cuts are usually beveled and have
a rounded top edge.
Fig.- Pojitive cut angle
Beveled cuts are a result of an imbalance in heat input into the cut face. As shown in
Figure , a positive cut angle will result if the heat input into the top of the cut exceeds
the heat input into the bottom. One obvious approach to reduce this heat imbalance is to
apply the arc constriction principle described in Figure 1: increased arc constriction will
cause the temperature profile of the plasma jet to become more uniform and,
correspondingly, the cut will become squarer. Unfortunately, the conventional nozzle is
limited by the tendency to establish two arcs in series---electrode to nozzle, and nozzle to
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work. This phenomenon is known as "double arcing" and can damage both the electrode
and nozzle. Conventional plasma cutting can be cumbersome to apply if the user is
cutting a widevariety of metals and plate thickness. For example, if the conventionalplasma process is used to cut stainless steel, mild steel and aluminum, it will benecessary to have three different cutting gases on hand if optimum cut quality is to be
obtained. This requirement not only complicates the process, but necessitates stocking
expensive cutting gases such as 65% argon - 35% hydrogen.
PROCESS VARIENT OR PROCESS REFINEMENT:
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The process variant have principally been designed to improve cut quality and arc
stability, reduce the noise and fume or to increase cutting speed.
DUAL GAS PLASMA ARC:
The same features as conventionalplasma cutting except that a secondary shield gas isadded around the nozzle. Usually the cutting gas is nitrogen and the secondary shielding
gas is selectedaccording to the metal to be cut. Secondary shield gases typically used are:
mild steel--either air or oxygen; stainless steel--CO2; aluminum--argon--hydrogen
mixture. Cutting speeds are slightly better than with conventional cutting on mild steel;
however, cut quality is inadequate for many applications. Cutting speed and quality on
stainless steel and aluminium are essentially the same as with the conventional process.
The major advantage of this approach is that the nozzle can be recessed within a ceramic
shield gas cup as shown in Figure 4, thereby protecting the nozzle from double arcing. If
no shield gas were present, the ceramic shield gas cup could deteriorate because of the
high radiative heat load produced by the plasma jet.
Schemetic diagram of dual gas plasma torch
Air cutting was introduced in the early 1960's for cutting mild steel. The oxygen in the air
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provides additional energy from the exothermic reaction with molten steel. This
additionalenergy increases cutting speeds by about 25%. Although the process can be
used tocut stainless steel and aluminum, the cut surface will be heavily oxidized and
unacceptablefor many applications.
Special electrodes, made of zirconium or hafnium, must be used since tungsten will erode
in seconds if the cutting gas contains oxygen. Even with these special electrodes, the
service life is must less than what can be achieved with the conventional plasma cutting
process.
The beneficial effects of the secondary gas are increased arc constriction and more
effective 'blowing away' of the dross. The plasma
forming gas is normally argon, argon-H2 or
nitrogen and the secondary gas is selected
according to the metal being cut.
Steel
air, oxygen, nitrogen
Stainless steel
nitrogen, argon-H 2, CO 2
Aluminium
DUALGAS TORCH
argon-H2, nitrogen / CO 2
The advantages compared with conventional
plasma are:
Reduced risk of 'double arcing'
Higher cutting speeds
Reduction in top edge rounding
WATER INJECTED PLASMA TORCH:
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Earlier it was stated that the key to achieving improved cut quality is through increasing
arc constriction. In the Water-injection Plasma Cutting process, water is radially injected
into the arc in a uniform manner as shown in Figure 6. The radial impingement of the
water around the arc provides a higher degree of arc constriction than can be achieved by
conventional means. Arc temperatures in this region are estimated to approach 50,000
degrees K, or roughly nine times the surface temperature of the sun. The net result is
improved cut squareness and increased cutting speeds. This radial water-injection
techniquewas developed and patented by Hypertherm, Incorporated.Another approach to
constricting the arc with water is to develop a swirling vortex ofwater around the arc.
This technique does not perform as well as radial injection becausethe degree of arc
constriction is limited by the high swirl velocities needed to produce astable water vortex:
the centrifugal force created by the high swirl velocity tends to flatten the annular film of
water against the inner bore of the nozzle.
Schemetic diagram of water injection plasma torch
Despite the extremely high temperatures generated at the point where the water impinges
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the arc, less than 10% of the water is vaporized. The remaining 90% of the water exits
from the nozzle in the form of a conical spray which cools the top surface of the
workpiece. This additional cooling prevents the formation of oxides on the cut surface.
Little water is evaporated at the arc because an insulating boundary layer of steam forms
between the plasma and the injected water. This steam boundary layer, usually referred to
as a "Lindenfrost Layer", is the same principle that allows a drop of water to dance
around on a hot skillet rather than immediately vaporizing.
Nozzle life is greatly increased with the Water-injection technique because the steam
boundary layer insulates the nozzle from the intense heat of the arc, and the water cools
the nozzle at the point of maximum arc constriction. The protection afforded by the
water-steam boundary layer also allows a unique design innovation: the entire lower
portion of the nozzle can be ceramic. Consequently, double arcing from the nozzle
touching the workpiece--the major cause of nozzle destruction--is virtually eliminated.
Nitrogen is normally used as the plasma gas. Water is injected radially into the plasma
arc,Fig. , to induce a greater degree of constriction. The
temperature is also considerably increased, to as high as
30,000C.
The advantages compared with conventional plasma are:
Improvement in cut quality and squareness of cut
Increased cutting speeds
Less risk of 'double arcing'
Reduction in nozzle erosion
WATER SHROUD PLASMA TORCH:
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The plasma can be operated either with a water shroud, Fig. 2c, or even with the
workpiece submerged some 50 to
75mm below the surface of the water.
Compared with conventional plasma,
the water acts as a barrier to provide
the following advantages:
Fume reduction
Reduction in noise levels
Improved nozzle life
In a typical example of noise levels at
high current levels of 115dB for
conventional plasma, a water shroud was effective in reducing the noise level to about
96dB and cutting under water down to 52 to 85dB.
As the water shroud does not increase the degree of constriction, squareness of the cut
edge and the cutting speed are not noticeably improved.
AIR PLASMA TORCH:
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The inert or unreactive plasma forming gas (argon or nitrogen) can be replaced with air
but this requires a special electrode of hafnium or zirconium mounted in a copper holder,
Fig. The air can also replace water for cooling the torch. The advantage of an air plasma
torch is that it uses air instead of expensive gases.
It should be noted that although the electrode and nozzle are the only consumables,
hafnium tipped electrodes can be expensive compared with tungsten electrodes.
Schematic diagram of air plasma torch
OXYGEN INJECTED PLASMA TORCH:This process refinement circumvented the electrode life problem associated with air
cutting by using nitrogen as the cutting gas and introducing oxygen downstream in the
nozzle bore as shown in Figure
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Schematic diagram of oxygen-injected torch construction
This process is used exclusively on mild steel and increases cutting speed by about 25%
if the optimum gas mixture is used (80% N2 - 20% O2). The major disadvantages are
lack of cut squareness, short nozzle life, and limited versatility (mild steel only).
Although this process is still being used at some locations, it has been almost entirely
displaced by Water-injection cutting.
HIGH TOLERANCE PLASMA:In an attempt to improve cut quality and to
compete with the superior cut quality of laser
systems, High Tolerance Plasma Arc cutting
(HTPAC) systems are available which operate
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with a highly constricted plasma. Focusing of the plasma is effected by forcing the
oxygen generated plasma to swirl as it enters the plasma orifice and a secondary flow of
gas is injected downstream of the plasma nozzle,Fig. 2e. Some systems have a separate
magnetic field surrounding the arc. This stabilises the plasma jet by maintaining the
rotation induced by the swirling gas. The advantages of HTPAC systems are:
Cut quality lies between a conventional plasma arc cut and laser beam cut
Narrow kerf width
Less distortion due to smaller heat affected zone
HTPAC is a mechanised technique requiring precision, high-speed equipment.
The main disadvantages are that the maximum thickness is limited to about 6mm and the
cutting speed is generally lower than conventional plasma processes and approximately
60 to 80% the speed of laser cutting.
NON SHIELDING PART ISSUE:
Double Arcing:
During the pierce, droplets of the molten metal can form a conductive path to the nozzle,
causing the nozzle to be at positive potential.This can cause a path of least
resistancefrom the electrode to the nozzle to the plate known as a double arc. This can
also occur if the nozzle contacts the plate during a cut.
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Nozzle Damage:
Contact to work piece damage and blow-back of spatter during cutting damages the
nozzle by pitting and ovaling the orifice.normalovaling
TORCH SHIELDING TECHNOLOGYShielded Front End Nozzle is protected by an electrically isolated shieldSignificantly reduces double
arcingProlongs parts lifeFacilitates drag cutting and template tracingDramatic
improvement in nozzle life
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ISOLATED SHIELD
Shielded Front End Benefits
During mechanized torch cutting:
Greater nozzle life
Lower cost of operation
Longer service life with consistent cut quality
Thicker Pierce Capacity
Protects the nozzle from occasional contact withplate.
CUTTING TERM:
Kerf:
Opening created by the metal removed
by the plasma arc. The width of the
kerfis determined by:
amperage
gases
nozzle orifice size
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consumable condition
torch-to-work distance Cutting
Dross:
The resolidifiedmetal on the bottom or top of the cut.
Dross formation and its condition is determined by
many factors
:travel speeds
amperage
gases used
type and thickness of metal
torch-to-work distancematerial surface coatings
LAG LINES:These are the ripples on the cut face or surface. The more consistent the power produced
by power supply is, the smoother the cut. Depending on the process, normal lag lines are
curved and slanted at about 15with properspeeds.
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TORCH STATING METHOD:
High Frequency
High voltage (5,000V -10,000V), high
frequency AC forms a spark between
electrode and nozzle
Gas is forced to flow through this spark,
raising it to its ionization temperature
This is an effective starting method, but
generates electrical noise that may affect
sensitive electronicequipment
Contact Starting
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MATERIAL TO BE CUT GAS OR GAS MEXTURE USED
Aluminum Nitrogen,nitrogen-hydrogen.argon-
hydrogen
magnesium Nitrogen,nitrogen-hydrogen.argon-
hydrogen
Stainless steel and some othernon ferrous metals
Nitrogen-hydrogen. argon hydrogen
Carbon and alloy steels, cast
iron
Nitrogen-hydrogen. Compressed air
COMPARISON B/W PAM AND LASER CUTTING:
Fundamental process difference:
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Safety consideration and operating environment :
PLASMA ARC CUTTING ADVANTAGE:
Automated plasma arc cutting systems provide several advantages over other cutting methods
such as oxyfuel and laser.
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Rapid cutting speed:
Plasma arc cutting is faster than oxyfuel for cutting steel up to 2 inches thick and is competitive
for greater thicknesses. Plasma cutting achieves speeds greater than those of laser cutting
systems for thicknesses over 1/8 inch. CNC controls allow speeds of up to 500 inches per minute
(ipm) to be achieved on gauge thicknesses. These fast cutting speeds result in increased
production, enabling systems to pay for themselves in as little as 6 months for smaller units.
Wide range of material thikness
Plasma cutting systems can yield quality cuts on both ferrous and nonferrous metals.
Thicknesses from gauge to 3 inches can be cut effectively.
Easy to use
Plasma cutting requires only minimal operator training. The torch is easy to operate, and new
operators can make excellent cuts almost immediately. Plasma cutting systems are rugged, are
well suitable for production environments, and do not require the potentially complicated
adjustments associated with laser cutting systems.
Economical
Plasma cutting is more economical than oxyfuel for thicknesses under 1 inch, and comparable up
to about 2 inches. For example, for inch steel, plasma cutting costs are about half those of
oxyfuel.
LIMITATION AND CONCERN:
A chief concern about plasma arc technology is ensuring that gaseous emissions are kept
to a minimum and cleaned before being released to the atmosphere.
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Concerns have been raised regarding the reliability of plasma torch technology.
The water-cooled copper torch must be replaced periodically to prevent burn-through at
the attachment point of the arc and a subsequent steam explosion due to rapid heating of
the released cooling water.
other limitation are
Large heat affected zone.
Rough Surfaces
Difficult to produce sharp corners.
Smoke and noise.
Burr often results.
APPLICATION:
Automated plasma cutting systems are being chosen over oxyfuel, hand tools, and laser
cutting in the following areas:
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Sheetmetal
Plasma cutting is commonly used to cut sheet metals from 24 gauge up to 1/8 inch thick
at high speeds on carbon steels, aluminum, and stainless steels.
Plasma cutting is widely used in the transportation industry to form the outer
skins of tractor trailers, buses, and agricultural equipment.
Plasma cutting systems are also used in the heating, ventilating, and air
conditioning industry to cut complex duct work.
PlateThicknesses
Industries involved in cutting plate thicknesses also find many applications for plasma
cutting. Plasma systems cut plate thicknesses from 1/8 to 3 inches, but the most common
applications are for carbon steel plate to inch thick.
Steel service centers cut large plates of steel down to size with plasma.
Makers of large construction machinery, mining equipment, and material
handling equipment utilize plasma cutting to produce cranes, bulldozers, and
other large equipment.
Plasma cutting also produces structural steel framework for railroad cars, trucks,
and other heavy equipment.
Other applications include cutting metal for ship building and the production of
pressure vessels.
OtherApplications
Plasma cutting is not limited to flat sheets of metal. Plasma torches placed on robots are
being used increasingly for contour cutting of pipes and vessels, removal of sprues and
risers from castings, and cutting of formed shapes, angles, and curves in various planes.
BIBLIOGTAPHY:
Advance Machining Process ------ VIJAY K. JAIN
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Modern Machining Process --------- P.C. PANDYE,H.S. SHAN
Plasma Arc Cutting----------------- AHMAD HASSAN SAYED
Plasma Processes of Cutting and Welding------ J.A. Hogan and J.B. Lewis
Plasma Arc Cutting----------------- Bill Lucas in collaboration with DerrickHilton,
Plasma Arc Cutting------------------ WIKIPADIA
Basic Plasma Theory--------------------- HYPERTHERM PRIVATE LIMITED