How To Select and Operate a Hand-Held Plasma Cutter [Guide]

Plasma cutting benefits

One cut is often all it takes. Fabricators, contractors, maintenance personnel, artists and do-it-yourselfers who experience the benefits of a hand-held air plasma arc cutting machine rarely want to return to oxy-acetylene cutting or mechanical cutting processes such as saws, cut-off wheels, shears and snips.

Plasma cutting can improve productivity and lower the cost of cutting. Benefits of plasma cutting include:

• Faster cuts
• No pre-heat cycle required
• Cuts any metal that conducts electricity (unlike oxy-fuel, which cannot cut stainless steel or aluminum)
• Offers portability around jobsites
• Minimizes the heat-affected zone and yields a cut with a small kerf (cut width).
• Plasma units can also gouge, pierce, bevel, cut holes and trace shapes.

The actual process of operating a hand-held air plasma cutter is relatively simple. In fact, the hardest part comes before ever striking an arc — selecting a machine that best matches your application and choosing the right accessories.

What is plasma cutting?

Plasma looks and behaves like a high-temperature gas, but with an important difference: It conducts electricity and cuts any electrically conductive metal.

The plasma arc results from electrically heating a gas, typically air, to a very high temperature. This ionizes its atoms and enables them to conduct electricity. A plasma arc torch uses a swirl ring that spins the gas around an electrode. The gas is heated in the chamber between the electrode and torch tip, ionizing the gas and creating plasma. This causes the plasma gas to greatly expand in volume and pressure. The small, narrow opening of the torch tip constricts the plasma and accelerates it toward the workpiece at high speeds (20,000 feet per second) and temperatures (up to 30,000 degrees Fahrenheit).

The high-intensity plasma jet melts a very localized area. The force of the jet (or arc) pushes through the workpiece and removes the molten metal. This arc easily cuts through metals with poor heat conductivity (stainless steel) or excellent conductivity (aluminum).

Compared to plasma cutting, the flame created by an oxy-fuel torch lacks concentration and cuts stainless steel and aluminum poorly. Plasma arc cutting is considered the standard process for these metals.

Choosing a plasma cutter

There are several factors to consider when purchasing a plasma cutter. 

Material thickness

The thickness of metal you will routinely cut and maximum metal thickness are important in selecting the right plasma cutter. Like a welding power source, a plasma cutter's amperage and voltage capacities determine its size. The plasma process requires relatively high voltage and low amperage levels, the opposite of welding. Many people erroneously judge a plasma machine solely by amperage. While this is an important indicator, remember that total output power (in watts) equals amperage times voltage. Do the math to obtain a more accurate product comparison. The cutting capacity of a particular size plasma machine varies greatly by manufacturer.

Cutting speed

Knowing cutting speeds for the thickness of metal being cut allows you to calculate production rates, typically in parts per hour. This helps ensure that the cutting portion of the operation does not become a bottleneck. Many manufacturers provide cutting speed charts that allow you to compare cutting speed performance.

To determine the maximum rated cutting thickness of mild steel, follow the line from the 15 IPM (inches per minute) point on the cutting chart. The point at which this line intersects the cutting curve determines the maximum recommended production cutting thickness of the unit. Note: the rating is based on 15 IPM because that is the minimum speed at which the operator achieves a smooth, steady cut when using a hand-held torch.

While there is no universal standard, Miller facilitates comparison by qualifying capacity with two standards: rated cut and sever cut.

• Rated cut is the thickness of metal that an operator can manually cut mild steel at a rate of 15 IPM. This is considered the minimum speed at which an operator achieves a smooth, steady cut and the best possible cut quality.

Rated cut

• A sever cut rating means the operator is pushing the machine to its maximum thickness capabilities (1-1/4 inches for a 55-amp unit). Cutting speeds will be very slow and the cut will require significant clean-up. Fortunately, cutting speeds increase as the material gets thinner.

Severed cut

As with cutting speed, cutting thickness varies greatly between models.

Primary power

Plasma cutting requires two basic elements — air and electricity — so the next question to ask is what type of input power is available. Several 30-amp plasma cutters, such as the Spectrum® 375 X-TREME™, operate using 120- or 240-volt power. If your input circuit has a 30-amp breaker, you even get equal cutting capacity at both voltages (with a 20-amp breaker, cutting capacity drops by 20 percent). Miller offers a primary power management solution called Auto-Line™ technology, which allows a machine to accept input voltages from 190 to 630 volts, single or three-phase, 50 or 60 hertz. And even if the primary power spikes and dips but stays within the 190 to 630 volt range, units with Auto-Line technology provide a steady, consistent arc and full cutting power. If you’re working in the field and plan to use an engine drive’s auxiliary power, strongly consider a plasma cutter with Auto-Line technology. Similar units without Auto-Line experience erratic cutting arcs, frequent breaker trips, blown circuit boards and are prone to premature transformer failure. These problems typically occur because the plasma cutter, once triggered, places such a load on the line that voltage levels drop below the plasma cutter's operating range.

Environment and air supply

In environments with heavy dust and metal shavings (such as from grinding), Miller® machines with Wind Tunnel Technology™ and Fan-On-Demand™ provide better reliability. With Wind Tunnel Technology, the cooling air flows through the machine without blowing over the electronic components, so grinding dust can’t settle on critical components. Fan-On-Demand means the cooling fan runs only when needed, reducing the amount of debris entering the unit. Regarding air supply, most manufacturers of hand-held plasma cutters recommend using ordinary air as the cutting gas. In mobile applications, contractors often opt for bottled nitrogen because it costs less than bottled air. When cutting stainless steel, some people believe nitrogen produces slightly less oxidation, as it is drier than compressed air.

High frequency starts or contact starts

Plasma cutters use either high frequency (HF) start or contact start technology to initiate the pilot arc. If you plan to use a plasma cutter near telephones, computers, CNC machines or other electronic equipment, be aware that HF often interferes with electronic controls. To avoid potential HF problems, all Miller plasma cutters feature a contact start design that does not cause interference. The contact start method also creates a visible pilot arc that helps you better position the torch.

Consumables and accessories for plasma cutting

Different consumables and torch accessories enable configuring a plasma cutter for a variety of applications and operator skill levels.

• Regular and extended cutting tips: For cutting, select a regular or an extended cutting tip. With plasma units greater than 40 amps, a built-in drag shield protects the regular tip and automatically holds it 1/8 inch off the surface of the workpiece (with units less than 40 amps, the tip can be placed directly on the workpiece). For following straight edges or tracing stencils or patterns, many people select this arrangement because it allows them to simply brace the edge of the drag shield on a guide (a ruler, piece of wood, cardboard pattern, etc.) and start cutting.

• Extended tips: For beveling, cutting in corner and a better view of the cutting arc, some operators prefer an extended tip. This tip protrudes 1/2 to 1 inch beyond the retaining cup, enabling the operator to better direct the flow of plasma jet and reach into corners. Having a steady hand is important because touching the tip to the workpiece three or four times enlarges the hole in the tip to the point where it no longer constricts the arc sufficiently.

• Roller guides and circle cutting guides: The cut will reflect any hand movement, so if you don’t have a steady hand, consider using roller guides and circle cutting guides. These guides hold the torch in position, prevent the tip from touching the workpiece, and make it easier to travel in a straight line or cut a perfect circle.

• Gouging tips: For removing old or imperfect welds, use a gouging tip. The hole on a gouging tip is three to four times wider than a regular tip. Its cone shape pushes out the plasma arc, which removes much more material than the constricted orifice of the regular tip. Many people do not believe that plasma works well for gouging, but that perception is based on older machines with weaker arcs. More powerful Miller Spectrum plasma cutters feature a stronger design that creates an arc 1 to 1-1/2 inches long. Although carbon arc gouging removes metal faster, plasma arc gouging produces much less smoke and noise, as well as offers more control over the arc.
   

As with cutting capacity and speeds, consumable life varies greatly between manufacturers. In a test comparing the number of 12-inch long, 1/2-inch thick strips of mild steel cut with a single set of consumables, Miller outperforms many of its competitors.

Cutting technique

Now that you’ve selected your plasma cutter and completed your pre-cut checklist, here’s how to get started plasma cutting:

1. Place the drag shield on the edge of the base metal or hold the correct standoff distance (typically 1/8 inch) and direct the arc straight down.

2. Raise the trigger lock and press the trigger — and pilot arc starts immediately when the trigger is pressed.

3. Once the cutting arc starts, begin to slowly move the torch across the metal.

4. Adjust your speed so sparks go through the base metal and out the bottom of the cut.

5. At the end of the cut, angle the torch slightly toward the final edge or pause briefly before releasing the trigger to completely sever the metal.

6. To cool the torch, post-flow air continues for 20 to 30 seconds after you release the trigger. Pressing the trigger during post-flow instantly restarts the arc.

Making the switch to plasma cutting can deliver great results. If you select the appropriate plasma cutter and service it properly, you can experience years of trouble-free performance.