Comprehensive Guide To Plasma Cutting

Posted by Josh Legoza on Dec 20, 2017 10:44:17 AM

In Plasma Cutting

What is plasma cutting?

Image Credit: Dennisveninga licensed under CC BY-SA 4.0

At its most basic, plasma cutting is a process that cuts through electrically conductive materials with an accelerated jet of hot plasma.  So what is plasma then?  You can think of plasma as an electrically conductive superheated ionized gas.  Lightning is a common example of plasma.  In most plasma cutting systems, just as with lightning, the gas being ionized is air.  (Systems may use nitrogen or other gases, but air is the most common gas used so we will limit this discussion to air plasma systems.) In an air plasma cutting system, compressed air is directed through a narrow nozzle, and the power supply adds enough energy to the air stream to create a plasma arc with temperatures near 40,000°F.  This arc melts a conductive workpiece and the pressurized air flow blows the molten metal away.

Main components of a plasma torch cutting tip

Main components of a plasma torch cutting tip

What do I need to start flinging molten metal and making lawn ninjas?!?

First, you’ll need protective equipment for your safety.  But we’ll go into details on what you need to protect yourself a little later.  For now, let’s stick to the fun stuff!  To cut metal, you’ll need to have the two components mentioned above: 1) A compressed air supply with sufficient pressure and flow for the application; and 2) An electrical energy source to ionize the air and create plasma. 

Air

Clean, dry air is critical to good plasma cutting.  Moisture or oil in the air supply can affect cut quality and rapidly wear the cutting consumables (specifically the electrode and nozzle, which we will discuss in detail later).  Cleanliness isn’t the only consideration for your air, it’s also important to supply enough to ensure a clean cut.

All Forney plasma cutters include a moisture separator to reduce moisture in the air supply.  But if you work in a humid environment, it may be necessary to have additional moisture/particulate filtration upstream.  If you see a mist of moisture coming out of the cutting torch during the pre-arc flow, you will need additional filtration.  Just be sure that any filtration system you install is rated for the pressure and flow requirements of the machine.

Electrical energy power source

Any plasma cutting power source (plasma machine) on the market provides the needed energy to create a plasma arc and cut.  Most will connect to your air source and run the power and air through an included cable and torch.  The biggest consideration when purchasing a power source is how thick of material you will be cutting.  The machine you choose will need to have enough output amperage to cut that thickness.

  

What products does Forney Industries offer to get me started in plasma cutting?

All the plasma cutting power sources offered by Forney Industries provide the necessary electrical output to create a plasma arc.  The most significant difference between them is how much output power the machine has.  This directly correlates to the maximum material thickness you can cut.  If you know the thickest material you will be cutting, use the table below to pick the best machine for your usage.

Forney Plasma Cutting System

Maximum Clean Cut

Maximum Sever Cut

Output Amps

Input Power

250 P+

1/8”

1/4”

12 A

120 VAC

325 P (available 2017 & earlier)

1/4"

3/8”

20 A

120 VAC

20 P (available 2018)

1/4"

3/8”

20 A

120 VAC

700 P

1/2"

3/4"

30 / 40 A

120 / 240 VAC

When evaluating your cutting needs and this table, it is important to understand the difference between clean and sever cut quality:

  • A clean cut has excellent cut quality (smooth finish, square/straight cut surface):
    Clean plasma cut
  • A sever cut is performed at or near the limit of the system’s capabilities and has lower cut quality (wavy or uneven cut surface and finish):
    Sever plasma cut

After considering your cutting requirements, make sure that you have or are buying a compressor that can supply the necessary air.  Check this chart for each machine’s requirements.

Forney Plasma Cutting System

Minimum air supply pressure

Recommended air supply pressure

Required air supply flow rate

250 P+

No need to supply air.  Has integrated air compressor.

325 P (available 2017 & earlier)

65 PSI

90 PSI

3.5 CFM

20 P (available 2018)

60 PSI

90 PSI

1.5 CFM

700 P

65 PSI

90 PSI

4 CFM

Each Forney plasma cutting system comes with the power supply, a cutting torch (with assembled consumables), an extra set of consumables, and a ground cable/clamp.  Some Forney machines have detachable torches while some are hard-wired to the machine.  For those that can be removed, it is very important that only approved torches with the proper ratings are used as replacements.  Though they look very similar, there are subtle differences in the design of the torch and the consumables used that affect air flow, current carrying capacity, cut quality, etc.  The torch used with each power source is designed and selected for use with that power source.  Each machine’s user manual contains details on torch specifications and approved torches.  Using torches with improper ratings can lead to severe injury or death.  It is also important that the correct consumables are used when replacing worn consumables.  These too can affect performance, reliability, and safety.  Again, the user manual will guide you in the selection of proper consumables.  While we’re talking about consumables and their use, let’s dive into setting up and using a plasma cutter.

 

I’ve picked a plasma cutting system that suits my needs.  Now how do I get set up?

It is very important to start by selecting or creating a work area that will keep you safe from all hazards.  Choose a well-ventilated work area to avoid the inhalation of fumes.

  • Sparks from plasma cutting can easily travel far from the workpiece and ignite fires. Have at least 15 feet of clearance around the work area, ensuring no flammable materials (cardboard, wood, fabrics, etc.), liquids (gasoline or other hydrocarbons) or gases/fumes (propane, acetylene, oxygen, etc.) are in that area.
  • Place the machine on a flat stable surface with a foot of clearance around any openings on the machine.

Using a Forney plasma cutter

Of course, it’s important to protect not only the work area but your body as well. 

  • Wear flame-resistant clothing, along with leather gloves to leave no areas of your body exposed.
  • Protect your eyes and face with a face shield or helmet. Wear safety glasses underneath, and use a proper filter shade on one or the other to prevent damaging your eyes with arc rays. 
  • Ensure that your shoes don’t have flammable materials (like polyester). Leather shoes are the best to protect your feet from falling sparks.
  • Be sure to read the safety summary of your manual to protect yourself from all related hazards.

With your safe work environment and PPE in place, let’s turn our attention to setting up our workpiece and machine.  Connect the ground clamp to the workpiece or near it on a conductive work table.  The clamp needs to be attached on an area that is clean and free from any coatings such as paint, rust, oil/grease, or scale.  This ensures a good connection for a closed electrical circuit, which is essential to good cut quality.  On some machines, you will also need to connect the ground cable to the machine.

On the machine, we need to connect our air supply (except on the 250 P+) and set the output parameters. The 700 P and 325 P have an adjustable pressure regulator that needs to be set at 65 PSI.  The 250 P+ with its integrated compressor, and the 20 P with a factory-set regulator need no adjustment.  Some machines require adjustment of air pressure and output amperage depending on the thickness of the metal being cut.  However, Forney plasma cutting machines are designed to operate at a fixed input pressure, and we recommend setting the amperage adjustment at the maximum setting for most thicknesses.  The inverter based designs allow for the machine to adjust the arc many times a second according to the plasma arc characteristics.  For this reason, setting the machine at maximum amperage will still give you good cuts throughout the range of thicknesses that the machine can cut.  A common exception is when cutting very thin materials.  In this case, maximum amperage can produce an uneven cut edge or remove too much material.

Plasma Torch assembly

The last
 part of your setup comes pre-assembled, and it’s the part of your system that will require the most inspection and maintenance: the consumables.  These include: (1) the electrode, (2) the air diffuser, sometimes called a swirl ring, (3) the cutting tip, and (4) the shield cup.  (Note that the optional 700 P Max Performance consumables add a brass shield cap to the top of the shield cup.)

Any time you take apart this assembly, make sure all components are there and re-assembled correctly.  It’s easy to lose or forget one (especially the air diffuser, which can stick to a cutting tip you’re replacing and get thrown away).  If improperly assembled, you can have cut quality issues, arc instability, or even no arc at all.  So, what are you looking for when you inspect your consumables?  The hole in the cutting tip will wear with use.  If the hole becomes oblong (no longer round) or enlarged, it’s time to replace it.  An enlarged hole will lead to a larger kerf (the width of material removed during the cutting process) and lower cut quality.  The center of the tip of the electrode will slowly wear away with use.  When the tip of the electrode becomes heavily pitted rather than a flat surface the time has come to replace it.  (A 1/16” deep pit or deeper means you are overdue on replacing your electrode).  It is typical to replace cutting tips twice as often as you need to replace your electrode.

New Plasma Consumables Used Plasma Consumables Replace Plasma Consumables
New Used but still ok Replace

The electrode should have swirl marks on it after even light use.  This indicates that the air flow is swirling as it should.  Proper flow is crucial to the arc characteristics and therefore good cut quality.  If it doesn’t look like the air is swirling, but instead travelling straight along the electrode, there may be a blockage in one of the small holes in the shield cup or air diffuser.  That is the main reason to replace a shield cup or air diffuser.  They don’t wear with use, but sometimes problems while cutting can cause them to melt, deform, or burn.  If they are damaged, replace them.  See the table below for the Forney part numbers for replacement consumables.

Replacement Consumables Table

Everything is all set-up.  What do I need to know to start cutting?

Before we turn you loose let’s go over some tips on proper plasma cutting with a Forney machine. 

Cutting in contact

These machines are designed to be used by dragging the cutting tip across the workpiece, maintaining contact through the whole cut.  You might even hear the cutting tip referred to as a “drag tip” from time to time.  If you try to cut without the tip in contact with the workpiece, the circuit from the machine through the torch, the workpiece, and returning to the machine through the ground clamp will be open.  If the machine detects this open circuit for more than a few seconds, it will cut power to the torch and you will lose the arc.

Torch angle

When cutting, you want to hold the torch handle at a slight angle to the workpiece within the plane of the travel direction.  This will put the nozzle straight on the workpiece in the direction of the line of travel (See Figure A below).  As you cut material with a thickness close to the capacity of the machine, lifting the torch handle more so that the nozzle points slightly in the direction of travel can improve cut quality and get the most out of the machine.  On the other plane, you can hold the torch at an angle to the workpiece.  As the plasma travels through the workpiece, it does so with an oblong shape (imagine the shape of a candle flame).  This can leave the cut edge at an angle rather than square.  But by adjusting the angle of the torch in the direction shown in Figure B below, you can “move” all of that angle to the “drop” side of the cut and leave the good material with a clean, square edge.  Note that the angle in Figure B is exaggerated for clarity.  Your adjustment of this angle should be in the range of 8° or less for your good edge to be square.

Plasma cutting torch angle

Travel speed

Travel speed is critical in plasma cutting, especially as you cut near the limits of the capability of the machine.  Cutting too fast or too slow leads to excessive dross (re-solidified molten metal that is not fully ejected from the cut) on the bottom of the workpiece.  Dross from travel speeds that are too low result in the bottom edge of the cut “welding” itself back together due to dross.  So how do you tell if you are going the right speed?  You watch the spark trail angle, the angle that the molten metal is being blown away from your workpiece.  The angle between vertical and the spark trail should be about 30° (See Figure C).

Travel speed

Understanding duty cycle

The duty cycle of a machine is how long a machine can cut continuously out of a 10-minute period at a certain output amperage setting.  For example, on 230 VAC input power, the 700 P has a duty cycle of 30% at its maximum output setting of 40A.  This means that when the output is set at 40A, the machine can run for 3 minutes out of 10.  After 3 minutes of cutting, the machine will shut off the output power.  The machine will have to cool for the next 7 minutes before you can cut again.  At 20A, the 700 P has a duty cycle of 100%, meaning it can cut continuously for the full 10-minute period.

Having trouble with your machine or cut quality?

The first thing to note is that the most common issues with plasma machines exist outside of the power source itself.  These can be the inputs to the machine (air, power) or on the outputs of the machine (torch, cable, consumables).  The most common culprits are:

  • The consumables. Make sure they are all present and assembled correctly.  Make sure the shield cup is fully seated on the torch and tightened completely.  Problems with consumables can create quality issues or even prevent the machine from arcing.  Check their condition as well.  Badly worn consumables quickly create cut quality problems.
  • The pressure, flow rate, and dryness/cleanliness of the air input to the machine.
    • Check that the moisture filter does not have water in it, drain if necessary.
    • Make sure the filter drain is closed so none of your air flow is being lost there.
    • Add auxiliary filtration if you continuously have issues with moisture or dirty air.
    • Check your input pressure and flow rate specs of your compressor and ensure they meet the requirements of the machine.
  • The power input to the machine. See the manual for the input power requirements.  Reduced input power means reduced output power.  Make sure the plasma cutter is the only thing plugged into the circuit and if using a generator or extension cord, make sure it meets the requirements laid out in the manual.
  • The torch or cable.
    • Check the tip of the torch for loose parts which can lead to air leaks and poor quality.
    • Examine the cable for excessive wear or damage. Electrical wiring or the tubing for your air can be damaged.  The results can range from small quality issues to large quality issues or the inability to cut at all.
    • Check the torch connection to the machine. A lose connection here can have the same results as the last point.
  • There are a lot of variables in plasma cutting, and some of the most important ones are controlled by you! 
    • Be sure you are dragging the tip of the torch across the workpiece while cutting.
    • Torch angle (in both planes) and travel speed play significant roles in the quality of your cut.

As you can see, there are a lot of things that can make your plasma cutter not work like it should.  Most of these can look like the machine is not putting out enough power or refusing to work all together.  Even if that seems to be the case, check everything mentioned above.  The problem is almost always outside of the power source. 

Now your lawn ninjas should turn out exactly how you expected. You’ve learned all you need to know to get started with a Forney plasma cutter.  Now get out there and get it done with green!