Tech tips for better plasma cutting

by Steve Hidden

Over the years, the air plasma arc process has been greatly refined so plasma provides good quality cutting, gouging and piercing at very high speeds for a much lower cost.  This cutting process is truly perfect for people in many industries.

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 workpiece.  Although the technology behind the plasma arc may seem complicated, the process itself is very easy to learn and perform.

Plasma, a cut above oxyfuel
While oxyfuel was the most common method of cutting carbon steel in the past, plasma cutting is gaining favor because it provides numerous advantages.  Plasma cuts faster than oxyfuel; a pre-heat cycle is not required; the kerf width (the width of the cut) produced is small; and, it has a smaller heat-affected zone that prevents the surrounding area from warping or damaging paint.

In addition, plasma cutting can be used on any type of electricity-conducting metal (the oxyfuel process cannot cut stainless steel or aluminum).  Moreover, plasma cutting is a cleaner, less expensive and more convenient method of metal cutting because clean, dry air is used for most plasma cutting applications.

A properly installed air plasma arc cutting setup is safer than oxyfuel gas cutting.  This is because there is a chance of flashbacks and the danger of flammable gases in exposed hoses with oxyfuel torches.

Plasma cutting is easier for hole piercing due to the plasma jet.  The plasma jet is the swirling force of air around the arc, in combination with the arc attachment, which focuses the heat into the metal.  In addition, plasma arc piercing does not require the metal to be preheated, which saves production time.  The plasma jet also makes for a better choice when cutting stacked materials.

Plasma cutting applications
The plasma process can be used to cut any material that is a good electrical conductor.  Unlike chemical cutting, it can be used on any metal for applications such as stack cutting, beveling, shape cutting, gouging and piercing.

Plasma cutting can be successfully performed on a variety of material sizes as well.  Plasma can be used to cut anything from thin gauge aluminum to stainless and carbon steel up to several inches, depending upon the power of the cutting machine.

Setting up a plasma arc cutter
To set up a plasma arc cutter, simply hook up the compressed air to the plasma cutting unit.  Three choices of air are available: bottled, an in-house air supply or a small air compressor.  Most plasma units have a built-in regulator to maintain the proper flow of air for the system.

To set the amperage (or heat) of the cutting unit to the proper level, make a few practice cuts with the amperage set high.  You can then adjust the amperage down according to your travel speed.  If the amperage is too high or your travel speed is too slow, the material you are cutting may become hot and accumulate dross.  Traveling at the right speed and using the right amount of heat will produce a very clean cut with less dress on the bottom of the cut, as well as little or no distortion to the metal.

Operating a plasma arc for cutting
Begin cutting by placing the torch as close as possible to the edge of the base metal.  Press the trigger to initiate the pre-flow air; the pilot arc will then light, followed by the cutting arc.  Once the cutting arc starts, move the torch slowly across the metal.  Adjust your speed so that the cutting sparks emerge from the bottom of the metal.  You should be able to see the bottom of the workpiece and the arc should be directed straight down.  If the sparks are not visible at the bottom of the plate, you have not penetrated the metal.  This is because your travel speed is too fast, you have insufficient amperage or the plasma stream is directed at an angle.

At the end of a cut, angle the torch slightly toward the end of the cut or pause briefly to completely finish the cut.  The post-flow air will continue for a short period of time after the trigger is released to cool the torch and consumable parts.  However, cutting can be resumed immediately.

Operating a plasma arc for gouging
Plasma arc gouging can be accomplished by placing the torch at approximately a 40-degree angle to the base metal.  Press the trigger for the pre-flow air and pilot arc.  When the cutting arc ignites, form the arc length a short distance from the workpiece.  Further adjust the arc length and travel speed as needed.  Do not gouge too deep, as several passes may be needed to accomplish the necessary gouge.  Again, after releasing the trigger, the post-flow air will continue for a short period of time, but gouging can be resumed immediately.

Operating a plasma arc for piercing
Perform piercing (creating a hole) by placing the torch at a 40-degree angle to the workpiece.  Press the trigger.  After initiating the cutting arc, bring the torch tip to a 90-degree angle and the arc will pierce the base metal.  A good rule to follow is that you can pierce up to half of the maximum cutting thickness provided by the machine.

What to look for in a plasma cutter
Output power: The output power needed in a plasma cutting machine depends primarily on the thickness and type of material to be cut.  The metals thickness will also determine the size of the nozzle opening, as well as the type and amount of gas or air required.

Determine the type of metal you are planning to cut and then check the cutting capacity of the machine you wish to purchase.  For example, Miller Electric's Spectrum 300 CutMate provides 25 amps of cutting output power and cuts metal up to three-eighth inches thick.  This type of machine works well for most fabrication, maintenance and automotive repair, as well as home workshop or hobbyist applications.  A more powerful machine may be necessary for operators planning to cut thicker materials.  For example, Miller Electric's Spectrum 1250 is a heavy-duty, industrial cutting machine that provides 100 amps of cutting output and can cut metal up to 1 1/4 inches thick.

Cutting speed: Also check the cutting speed of the machine.  This is usually noted as Inches Per Minute (IPM).  A machine that cuts half-inch material may take five minutes to do so, whereas another machine may take one minute.  Cutting speed makes a significant difference in production time.

Input power: Next, check the primary voltage and amperage that the power source requires.  Also, determine if you need a machine that can operate on multiple voltages and amperages.  Some machines can use either 115V, 230V or 460V, single-phase or three-phase primary power.

Duty cycle: Duty cycle is an important factor to consider when purchasing a cutting machine.  Duty cycle is the amount of time a machine can cut within a 10-minute cycle without overheating.  For example, if a machine's duty cycle is 60 percent, the machine can run continuously for six out of every 10 minutes and then needs to cool for the remaining four minutes.  A larger duty cycle is important when making long cuts, in high productivity applications or when using the machine in a hot environment.  The duty cycle is usually given for the maximum output of the unit.  If the machine is used at a lower output, the duty cycle will increase correspondingly.  The temperature of the surrounding air may also have an effect on duty cycle.  For example, Miller Electric establishes its duty cycle at 104 degrees F, however this may not be true for all manufacturers.  In fact, power sources with an established duty cycle at 75 degrees F will not be able to provide full duty cycle at 85 degrees F.

Torch: The type of torch to purchase depends upon the application for which the plasma cutter will be used.  The torch should always be heavy duty and be able to withstand the necessary work environment, as well as being dropped or thrown.  Several options are available for torches.  An ICE torch with an epoxy cup is more rugged than ceramic and is virtually unbreakable.  This torch also provides a standoff guide, or drag shield, that attaches to the cup and holds the tip 1/16 to 1/8 inches off of the workpiece.  This permits the operator to drag the torch on the workpiece while cutting at full output, which increases operator comfort and makes template cutting easier.  Torch standoff is the distance the outer face of the nozzle is to the base metal surface.  The torch standoff is determined by the thickness of the material and the amperage required.  Low amperage cutting may allow dragging the tip or nozzle on the metal.  High-amperage cutting, above 40 amps, requires a standoff distance of 1/16 to 1/8 inches.

You must also choose between a single-flow torch and a dual-flow torch.  A single-flow torch has only a flow of air for cutting.  This is because its use is limited to low-amperage, thin-gauge sheet metal applications where a shielding gas flow used to cool the torch is not necessary.  A dual-flow torch has a flow of air for cutting, as well as a shielding gas flow for cooling the torch.  Dual-flow torches are used for cutting thicker materials that require higher amperages.

Consumables: Besides compressed air or nitrogen, there can be as few as two consumables needed for plasma arc cutting.  These are the tip and the cutting electrode.  If either the tip or the electrode become worn or damaged, the quality of the cut will be affected.  The consumables will wear with each cut, but factors like moisture in the air supply, cutting excessively thick materials or poor operator technique will increase the deterioration of the consumables.  You will want to have consumables available when you need them, so the ability to order and receive them in a timely fashion is important.  Best practice is to replace the tip and the electrode together for optimal quality cuts.  It is especially convenient if the cutting machine has a storage compartment for these consumables to save on downtime.

Weight and size: Weight and size may be extremely important factors to consider when purchasing a plasma cutter if there is a need for portability.  There are many hand-portable units available that weigh less than 45 pounds.  There are also compatible running gears available for some larger, heavier machines.  In addition, there are inverter-based plasma cutters available that provide high cutting output power, yet weigh much less than regular cutting machines that offer the same cutting capacity.

Proper safety procedures
Safety procedures must be closely followed in any application of the plasma cutting machine.  Be aware of potential hazards involved with the process, including high voltages and temperatures, fumes, ultraviolet radiation and molten metal.  Proper welding clothing should be worn, as well as welding helmets with dark lenses, as specified by the manufacturer.

Before cutting, inspect the shield cup, tip and electrode and do not operate the unit without the tip or electrode in place.  Hitting the torch on a hard surface to remove spatter can damage the torch and stop proper operation.  In addition, avoid constant starting and restarting of the plasma arc to lengthen consumable life.  As with all industrial products, read the owner's manual for proper safety procedures.

When used properly, your plasma arc cutting unit will provide clean, quality cuts at very high speeds.

Steve Hidden is a product manager for Miller Electric Manufacturing Company, a manufacturer of welding equipment and related supplies.  Miller Electric is a wholly owned subsidiary of Illinois Tool Works.

MRO Today.  Copyright, 2000.

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