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Introduction
Do you need a cutting tool for occasional repair and maintenance
work? Have you recently embarked on a new project that requires
higher cutting volumes? Or, are you looking for a new alternative
to your current mechanical saw? All of these scenarios provide
great reasons to investigate plasma cutting. With the cost of
machines on the decline, smaller-sized, portable machines flooding
the market and technology offering increased benefits and easier
usage -- it may be time to take a serious look at plasma for
your cutting applications. The benefits of plasma cutting include
ease of use, higher quality cuts and faster travel speeds.
What is Plasma Cutting Technology?
In simplest terms, plasma cutting is a process that uses a high
velocity jet of ionized gas that is delivered from a constricting
orifice. The high velocity ionized gas, that is, the plasma,
conducts electricity from the torch of the plasma cutter to
the work piece. The plasma heats the workpiece, melting the
material. The high velocity stream of ionized gas mechanically
blows the molten metal away, severing the material.
How Does Plasma Cutting Compare to Oxyfuel
cutting?
Plasma cutting can be performed on any type of conductive metal
- mild steel, aluminum and stainless are some examples. With
mild steel, operators will experience faster, thicker cuts than
with alloys.
Oxyfuel cuts by burning, or oxidizing, the
metal it is severing. It is therefore limited to steel and other
ferrous metals which support the oxidizing process. Metals like
aluminum and stainless steel form an oxide that inhibits further
oxidization, making conventional oxyfuel cutting impossible.
Plasma cutting, however, does not rely on oxidation to work,
and thus it can cut aluminum, stainless and any other conductive
material.
While different gasses can be used for plasma
cutting, most people today use compressed air for the plasma
gas. In most shops, compressed air is readily available, and
thus plasma does not require fuel gas and compressed oxygen
for operation.
Plasma cutting is typically easier for the
novice to master, and on thinner materials, plasma cutting is
much faster than oxyfuel cutting. However, for heavy sections
of steel (1 inch and greater), oxyfuel is still preferred since
oxyfuel is typically faster and, for heavier plate applications,
very high capacity power supplies are required for plasma cutting
applications.
What Can I Use a Plasma Cutter for?
Plasma cutting is ideal for cutting steel, and non-ferrous
material less than 1 inch thick. Oxyfuel cutting requires that
the operator carefully control the cutting speed so as to maintain
the oxidizing process. Plasma is more forgiving in this regard.
Plasma cutting really shines in some niche applications, such
as cutting expanded metal, something that is nearly impossible
with oxyfuel. And, compared to mechanical mean of cutting, plasma
cutting is typically much faster, and can easily make non-linear
cuts.
What are the limitations to Plasma Cutting?
Where is Oxyfuel preferred?
The plasma cutting machines are typically more expensive than
oxyacetylene, and also, oxyacetylene does not require access
to electrical power or compressed air which may make it a more
convenient method for some users. Oxyfuel can cut thicker sections
(>1 inch) of steel more quickly than plasma.
What to Look for When Purchasing
a Plasma Cutting Machine
Once you have determined plasma cutting is the right process
for you, look at the following factors when making a buying
decision.
1. Determine The Thickness of the Metal that You
will Most Frequently Cut
One of the first factors you need to determine is the thickness
of metal most frequently cut. Most plasma cutting power sources
are rated on their cutting ability and amperage. Therefore,
if you most often cut ¼" thick material, you should consider
a lower amperage plasma cutter. If you most frequently cut
metal that is ½" in thickness look for a higher amperage machine.
Even though a smaller machine may be able to cut through a
given thickness of metal, it may not produce a quality cut.
Instead, you may get a sever cut which barely makes it through
the plate and leaves behind dross or slag. Every unit has
an optimal range of thickness -- make sure it matches up with
what you need. In general, a ¼" machine has approximately
25 amps of output, a 1/2” machine has a 50-60 amp output while
a ¾" - 1" machine has 80 amps output.
2. Select Your Optimal Cutting Speed
Do you perform most of your cutting in a production environment
or in an atmosphere where cutting speed isn't as critical?
When buying a plasma cutter, the manufacturer should provide
cutting speeds for all thickness of metal measured in IPM
(inches per minute). If the metal you cut most frequently
is ¼", a machine that offers higher amperages will be able
to cut through the metal much faster than one rated at a lower
amperage, although both will do the job. For production cutting,
a good rule of thumb is to choose a machine, which can handle
approximately twice your normal cutting thickness. For example,
to perform long, fast, quality production cuts on ¼" steel,
choose a 1/2” class (60 amp) machine.
If you are performing long, time-consuming
cuts or are cutting in an automated set-up, be sure to check
into the machine's duty cycle. Duty cycle is simply the time
you can continuously cut before the machine or torch will
overheat and require cooling. Duty cycle is rated as a percentage
of a ten-minute period. For example, a 60 percent duty cycle
at 50 amps means you can cut with 50 amps output power continuously
for six minutes out of a 10-minute period. The higher the
duty cycle, the longer you can cut without taking a break.
3. Can the Machine Offer an Alternative
to High Frequency Starting?
Most plasma cutters have a pilot arc that utilizes high
frequency to conduct electricity through the air. However,
high frequency can interfere with computers or office equipment
that may be in use in the area. Thus, starting methods that
eliminate the potential problems associated with high frequency
starting circuits may be advantageous.
The lift arc method features a DC+ nozzle
with a DC- electrode inside. Initially, the nozzle and the
electrode physically touch. When the trigger is pulled, current
flows between the electrode and the nozzle. Next, the electrode
pulls away from the nozzle and a pilot arc is established.
The transfer from pilot to cutting arc occurs when the pilot
arc is brought close to the work piece. This transfer is caused
by the electric potential from nozzle to work.
Lincoln Electric's Pro-Cut line offers patented
Dual Winding Technology with separate windings for the pilot
and cutting arc. With Dual Windings, the pilot arc is optimized
during current transfer for a fast, positive transfer without
the use of a resistor. Dual Windings work by creating the
electric potential for a transfer - they create a voltage
difference to snap the arc to the work piece. Because Lincoln
has eliminated the large resistor usually found in plasma
cutting machines, it can offer units that are smaller in size
with increased portability.
4. Compare Consumable Cost Versus Consumable
Life
Plasma cutting torches have a variety of wear items that
require replacement, commonly called consumables. Look for
a manufacturer that offers a machine with the fewest number
of consumable parts. A smaller number of consumables mean
less to replace and more cost savings. For example, Lincoln
Electric's Pro-Cut line has only three front-end parts in
the torch and only two of those are consumables: the electrode
and the nozzle. Lincoln also offers tool-less changeovers
when replacing these consumables.
Look in the manufacturer's specifications
for how long a consumable will last - but be sure when comparing
one machine against another that you are comparing the same
data. Some manufacturers will rate consumables by number of
cuts, while others will use the number of starts as the measurement
standard.
5. Test the Machine and Examine Cut Quality
Make test cuts on a number of machines, traveling at the
same rate of speed on the same thickness of material to see
which machine offers the best quality. As you compare cuts,
examine the plate for dross on the bottom side and see if
the kerf (the gap left by cut) angle is perpendicular or angular.
Look for a plasma cutter that offers a tight,
focused arc. Lincoln Electric offers its Pro-Cut line with
VORTECH Technology consumables which are specially designed
to concentrate the plasma swirl, offering a tighter arc and
concentrating more cutting power on the work piece.
Another test to perform is to lift the plasma
torch up from the plate while cutting. See how far you can
move the torch away from the work piece and still maintain
an arc. A longer arc means more volts and the ability to cut
through thicker plate.
6. Pilot to Cut and Cut to Pilot Transfers
The transfer from pilot arc to cutting arc occurs when
the pilot arc is brought close to the work piece. A voltage
potential from nozzle to work is mechanism for this transfer.
Traditionally, a large resistor in the pilot arc current path
created this voltage potential. This voltage potential directly
affects the height at which the arc can transfer. After the
pilot arc transfers to work a switch (relay or transistor)
is used to open the current path.
Look for a machine that provides a quick,
positive transfer from pilot to cutting at a large transfer
height. These machines will be more forgiving to the operator
and will better support gouging. A good way to test transfer
characteristics is by cutting expanded metal or gratings.
In these instances, the machine will be required to quickly
transfer from pilot to cut and back to pilot very quickly.
To get around this, they may recommend you cut expanded metal
using only the pilot current.
Lincoln's Pro-Cut products excel at this
process because they employ Dual Winding Technology™. This
technology utilizes two separate power systems (windings):
one tailored for pilot arcs, the other for cutting. This patented
configuration creates the nozzle to work voltage potential
without a large resistor. Additionally, the control system
can rapidly select which winding is required for the task.
The result is instantaneous positive transfers from up to
¼" away from the work. At the end of the cut, the control
system maintains the arc by instantly retracting back into
a pilot arc.
*Pro-Cut 55 and 80 only.
 7.
Check the Machine's Working Visibility
As you are working on an application, you want to be able
to see what you are cutting, especially when tracing a pattern.
Visibility is facilitated by the geometry of the torch - a
smaller, less bulky torch will enable you to better see where
you are cutting, as will an extended nozzle.
8. Look for the Portability Factor
Many consumers use their plasma cutter for a variety of cutting
applications and need to move the machine around a plant,
job site or even from site to site. Having a lightweight,
portable unit and a means of transportation for that unit
- such as a valet style undercarriage or shoulder strap -
make all the difference. Additionally, if floor space in a
work area is limited, having a machine with a small footprint
is valuable.
Also, you want a machine that offers storage
for the work cable, torch and consumables. Built-in storage
drastically improves portability since these items will not
drag on the ground or get lost during machine transport.
9. Determine the Ruggedness of the Machine
For today's hard job site environments, look for a machine
that offers durability and has protected controls. For example,
fittings and torch connections that are protected will wear
better than those that aren't. Some machines offer a protective
cage around the air filter and other integral parts of the
machine. These filters are an important feature since they
ensure oil is removed from the compressed air. Oil can cause
arcing and reducing cutting performance. Protection of these
filters is important as they ensure oil and water, which reduces
cutting performance, is removed from the compressed air.
10. Find Out if the Machine is Easy to
Operate and Feels Comfortable
Look for a plasma cutter that has a big, easy-to-read
control panel that is user-friendly. Such a panel allows someone
who does not normally use a plasma cutter to be able to pick
it up and use it. In addition, a machine with procedural information
clearly printed on the unit will help with set-up and troubleshooting.
How does the torch feel in your hand? You
want something that has good ergonomics and feels comfortable.
11. Look for Safety Features
Look for a machine that offers a true Nozzle-in-Place
safety sensor. With such a feature, the plasma cutter will
not start an arc unless the nozzle is in place. Some safety
systems can be fooled into thinking the nozzle is in place
(i.e. shield cup sensing), even when it is not. If the output
is turned on, the operator will be exposed to 300 VDC, a very
unsafe condition. This cannot happen with the Lincoln Nozzle-in-Place
safety sensor.
Look for a machine that provides a pre-flow
sequence. This feature provides an advanced warning to the
use before the arc initiates. In addition, look for a machine
which provides a three-second pre-flow safety which gives
users advanced warning to make sure all body parts are clear
of the nozzle before the arc initiates.
How Can I Make the Most of This Cutting Tool?
After you have selected the plasma cutting machine that is right
for you, here are some tricks-of-the-trade that will help beginners
make the best possible cut.
1. Set-Up Procedures
Before you start, check for the following items:
-
A clean compressed air supply, without water
or oil. Consumables that wear quickly, or black burn marks
on the plate, may indicate that the air is contaminated
-
Correct air pressure - this can be checked
by looking at the gauges on the unit
-
A nozzle and electrode are correctly in place
-
A good connection of the work lead to a clean
portion of the work
2. Safety Gear
Some basic safety practices should be observed. You should read
your instruction manual thoroughly to understand the machine.
Wear long sleeves and gloves while cutting since molten metal
is generated during the cutting process. Eye protection such
as dark goggles or a welding shield is required to protect your
eyes from the cutting arc. Typically a darkness shade of #7
to #9 is acceptable. Finally, follow all safety tips and guidelines
that are detailed in your instruction manual.
3. Piercing the Work
Many inexperienced users try to pierce the metal by coming straight
down, perpendicular (90 degrees) to the work. This results in
molten metal being blown back into the torch. A better method
is to approach the metal at an angle (60 degrees from horizontal,
30 degrees from vertical) and then rotate the torch to the vertical
position. This way, the molten metal is blown away from the
torch.
4. Don't Touch the Nozzle to the Work Piece
Do not touch the nozzle to the work when using current levels
of 45 amps or more. Doing so will drastically reduce the nozzle
life as the cutting will double arc through the nozzle. Double
arcing can also occur if the torch is guided by dragging it
against a metal template. The result is the same as dragging
the nozzle on the work -- prematurely worn nozzles.
5. Beginners Should Use a Drag Cup to Facilitate
the Cut
Many systems offer an insulated drag cup, which snaps over the
nozzle. This allows the torch to rest on the work piece and
dragged along to facilitate a consistent cut.
6. Travel at the Right Speed
When moving at the right cutting speed, the molten metal spray
will blow out the bottom of the plate at a 15 to 20 degree angle.
If you are moving too slowly, you will create slow speed dross,
which is an accumulation of molten metal on the bottom edge
of the cut. When moving too fast, high-speed dross on the top
surface is created since you are not allowing time for the arc
to completely go through the metal. Traveling too fast or too
slow will create a low-quality cut. Typically, low speed dross
can be distinguished from high-speed dross by ease of removal.
For example, low speed dross can be removed by hand whereas
high-speed dross typically requires grinding.
 7.
Set the Current to Maximum As You Begin
When setting the current, put it on the maximum output of the
machine, then turn it down as needed. More power is usually
better, except when doing precision cutting or when you need
to keep a small kerf.
8. Minimize Pilot Arc Time
Because of the wear it creates on the consumables, try to minimize
the amount of time spent in pilot arc mode. To do this, position
the plasma torch by the edge of the work before starting the
arc so you can get right to cutting.
9. Maintain A Constant Work Distance
Optimally, you should maintain a 3/16" to 1/8" distance from
the nozzle to the work. Moving the torch in an up and down fashion
will only hinder your efforts.
10. Travel in the Direction that will Give
You the Best Finished Work
If you are making a circular cut and plan to keep the round
piece as your finished work, move in a clockwise direction.
If you plan to keep the piece from which the circle was cut,
move in a counterclockwise direction.
As you push the torch away from you, the better
cut will appear on the metal that is on the right hand side,
since it will tend to have a better, squarer edge.
11. End with a Push Angle on Thick Material
One trick to use on thicker material is to rotate the torch
slightly, increasing the torch orientation to a push, rather
than drag angle as you cut through the last section of material.
This increase in the push angle at the finish will cut through
the bottom first and get rid of the bottom corner that is usually
left at the end of thick plate. Never finish a cut by using
the torch to hammer away the last corner of the work.
After finding the right machine for your application and learning
some of the tricks of the trade, you should be ready to cut.
Remember that plasma cutting offers a number of benefits and
should provide you with faster, higher quality cuts.
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