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.
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.
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.
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.