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Plasma
arc cutting is a process where an open arc, much like in TIG welding,
can be constricted by passing through a small nozzle, or orifice, from
the electrode to the work-piece. The gas used, typically air, combines
with an electrical current to create a high temperature plasma arc.
When placed in contact with an electrically conductive material, the
arc passes through the metal, melting a thin area. The force of the arc
pushes the molten metal through the work-piece and severs the metal.
Plasma
cutters
are used for cutting a wide variety of metals and are
extremely
popular for cutting operations and automotive work. The following guide
will review the applications for plasma cutting as well as suggest some
tips.
Why Use a Plasma Cutter
Faster Cutting
Plasma
cutting provides numerous advantages over oxy-fuel. It cuts faster;
does
not require a pre-heat cycle; produces a small and more precise
kerf (the width of the cut), and affects a smaller section of
the
metal being cut,
which prevents the surrounding area from warping or damaging the paint.
The arc for plasma cutting is around 20,000 degrees F, while the flame
from oxyacetylene is a typically around 6000 degrees F.
Greater Versatility
The
Plasma process also cuts any type of electrically conductive metal (the
oxy-fuel process cannot cut stainless steel or aluminum). Plasma
cutting
is a cleaner, more convenient method of metal
cutting because dry air is used for most plasma cutting
applications. Some high end plasma cutters use nitrogen gas instead of
oxygen.
Projects
such as hole piercing and cutting stacked materials are also ideal for
plasma cutters because the heat is concentrated in such a compact area.
Safety: Greater Heat Means Greater Hazards
With the
amount heat produced by a plasma cutter, the risk of electric shock is
much greater. While cutting, wear gloves and handle the torch according
to the manufacturer's guidelines. Sparks from plasma cutting can do a
lot of damage very quickly, causing severe burns. Therefore, make sure
you wear appropriate clothing to cover your skin, as well as a helmet
that protects your eyes from harmful rays and sparks.
Before
starting, make sure you have a good, solid grounding according to the
National Electrical Code (NFPA 70) and to OSHA standards. Make sure you
are not wearing anything synthetic and create a 35-foot safety zone
around your cutting area that is free of flammable materials. If
possible, stand on a rubber mat or piece of plywood to keep yourself
grounded--keeping in mind that either could catch a spark and begin to
burn. In addition, any damaged wires or other parts must be repaired
immediately on a plasma cutter.
Some users prefer plasma
cutters over oxy-fuel cutting setups because oxy-fuel is highly
flammable, and there is also a chance of a flashback while cutting.
Though plasma cutters carry the risk of a fatal electric shock, the
chance of an explosion is far less likely. Each user will have to weigh
the pros and cons of plasma vs. oxy-fuel cutting since both present
their own hazards and challenges.
Practical Concerns with a Plasma Cutter
Electrical Concerns:
There's no use in buying a tool that you can't use at home or in your
shop. Plasma cutters with a 27 amp output can typically be plugged into
a 110V outlet and will have a limited cut rate in the 1/8" to 1/2"
range. Machines for cutting a thicker range of metals will run at
higher amperages through a 220V outlet which will have to be installed
if your home or shop does not have one.
Running a Plasma Cutter
off a Generator:
It is possible to run some plasma cutters off a generator if you don't
have the right power set up. However, make sure that your cutter is
able to handle the dirty power that generators produce, which means
that the voltage fluctuates. If your plasma cutter is not equipped to
handle these fluctuations, there's a chance your machine could shut
down in the middle of a job or even break. Your plasma cutter will need
Line Voltage Compensation (LVC) in order to operate properly.
Plasma Cutting Tips
Torch Position: The
torch should remain at a 90-degree angle and depending on the machine
you can keep a standoff or drag either the tip or the drag shield on
the metal.
Settings:
There are only two settings on a plasma cutter: the
on/off switch and the amperage control. The amperage is adjusted
depending on the thickness of the metal being cut, though some
operators recommend simply leaving the amperage switch at the highest
level.
Compressed Air for Plasma
Cutting:
Each plasma cutter will specify a particular air pressure in order to
cut at certain levels. The manual will specify what each machine needs.
The only other gas commonly used is nitrogen for particularly clean
cuts on stainless steel.
Before Cutting:
The shield cup, cutting tip, and electrode should be inspected prior to
cutting. Also make sure all of the lines to the plasma cutter are in
excellent shape--always replace a damaged plasma cutter cord. Avoid
hitting the torch on a hard surface in order to remove spatter. In
addition, the consumable will last longer if the plasma cutter is not
restarted frequently while in use.
Operating A Plasma Arc
For Cutting:
Start cutting metal by placing the plasma cutter torch as close to the
edge of the metal workpiece as possible. by pressing the trigger, the
preflow air will begin, which will be followed by the lighting of the
pilot arc, and then the cutting arc. While cutting, move the torch at a
slow, steady rate and make sure that the sparks fall from the bottom of
the metal.
If you don't see sparks falling through the metal,
this means that the metal is not being cut all of the way through. Make
sure there is enough amperage, that your torch is angled properly, and
that the torch is not traveling too fast.
In
order to finish a cut, angle the torch slightly toward its end or pause
briefly at the end of the metal to create a clean cut. Don't forget
that some air will continue to pass through the torch for a short
period of time after releasing the trigger to the torch.
Operating A Plasma Arc
For Gouging:
A plasma cutter can be used to gouge metal by holding the torch at a 40
degree angle relative to the base metal. Begin by pressing the trigger
for the pre-flow air and pilot arc. The arc should be formed slightly
away from the workpiece in order to avoid gouging too deep into the
metal, though the arc length and travel speed will have to be adjusted
as needed.
Operating A Plasma Arc
For Piercing:
In order to pierce a hole with a plasma cutter, hold the torch once
again at a 40 degree angle to the workpiece. Press the trigger on the
torch and then bring to the torch to a 90 degree angle when the arc
forms in order to pierce the work piece.
The Disadvantages of Plasma Cutting
Besides
the risk of a fatal shock, there is a significant upfront investment
for a plasma cutter. Plasma cutters range anywhere from $1100 to $2000,
with plenty of more expensive models as well. However, once you've made
that purchase, you need to only replace your tips and consumable
electrodes for $11 per pair.
For those who do a lot of
cutting, plasma cutting's steep up front costs may pay off when
compared to an oxy-fuel setup that costs around $500 for getting
started and then regular replacement of tips ($5) and gas tank refills
($30). In addition, the versatility and speed of plasma cutting may
make a plasma cutter a worthwhile investment.
Plasma Cutting Resources
Written Exclusively for Baker's Gas and Welding by Ed Cyzewski
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