Handling the plasma jobs

Before choosing a plasma cutting system, determine how and where it will be used.

One of the benefits of an air plasma system is that it is easy to use. Basically, plug it in, hook up compressed air, and go. In fact, the most difficult thing about a plasma system can be choosing the right one.

You have a number of factors to consider and questions to ask. Cut capacity and cut quality are at the top of most lists, but other factors like system reliability, duty cycle, ease of use, and operating cost also should be considered.

What Type of Cutting Do I Plan to Do?

Are you planning to cut by hand, on a table, or do you foresee switching between the two methods? Many air plasma systems are capable of doing both, which gives you the most versatility.

Some systems include a CNC interface, giving you even more options. If you foresee doing both types of cutting, think about how often you'll switch from one method to the other. That's an important question to ask, because switching between your hand and mechanized torch is easier on some systems than others.

What Thickness of Metal Do I Want to Cut?

Once you know what type of cutting you are going to do, then you should consider the thickness of the material you plan to cut. Most experts recommend the 80/20 rule, which calls for choosing a system with a recommended cut capacity that matches the material thickness you plan to cut 80 per cent of the time. For example, if you mainly plan to cut ½-in.-thick material and only occasionally cut metal that’s a little thicker, say, ¾ in., you can safely choose a ½-in. system.

People often equate the thickness of metal they can cut with the amperage required. The thinking is that the system with the highest amperage will provide the most power and therefore the greatest cut capacity. That isn't necessarily true as amperage is only part of the equation.

The thing that really determines power output is watts (or kilowatts). Multiply amps by volts to get that number. It’s possible for a 30-amp system to have more power than a 40-amp system if the 30-amp system has more volts, or a 65-amp system can have more power than an 80-amp system.

Another tip: Look for cut speed ratings when comparing the capacities of plasma cutting systems. If a manufacturer says a system can cut ½ in., you’ll want to find out how fast the torch was actually moving to cut that thickness. This is because some manufacturers overstate cut thickness by moving the torch at a very slow speed.

Where Will I Use My System?

The next factor to consider is the system’s physical size and weight. If you don't plan to move it around a lot, you can get away with a large system; however, if you plan to move your system frequently or use it in the field, you'll want to take size and weight into account. Fortunately, engineering advances mean you can get a lightweight system without sacrificing power and performance.

What Kind of Electrical Power Do I Have?

Is 220-V available or only 110-V? It is important to know how you’ll power your system so you can plan accordingly. For example, if you plan to use the system with a 110-V outlet, you’ll need to buy a system the meets that specification.

Consider ease of use and operating cost when purchasing a plasma cutting system.

If it will run in a shop from fixed power, is the incoming voltage single-phase or 3-phase, and what is the breaker size limit? If it will be used with a generator, look at the generator’s peak output power (kW or kVA) to see if it can operate the plasma system at its maximum output. Some plasma models are limited to operating with just one power source, while others can operate on either power source and have auto-voltage technology with a 200- to 600-V range to handle either phase.

How Much Cutting Do I Plan to Do?

What duty cycle, defined as the percentage of time a unit can operate continuously, do I need?

The most reliable systems are engineered with fewer parts, use software instead of hardware when possible, meet ISO standards, and have been thoroughly tested.

Duty cycle is an approximation of how long a system can operate in a continuous cut before overheating within a 10-minute period. Most plasma systems are programmed to shut off once they have reached a certain temperature. A duty cycle of 50 per cent means a system will cut continuously for five minutes at full output before needing five minutes to cool down. A 50 per cent duty cycle is normally sufficient for hand cutting. Be aware, though, that not all manufacturers determine duty cycle in the same way. Some manufacturers use a lower ambient temperature rating and/or a lower load voltage rating when testing to make the duty cycle appear higher.

Ideally, you will want a centralized fan configuration to bring in cool air through the center of the system, where the most thermally sensitive components are located. This will result in more efficient and consistent cooling and enable a higher duty cycle.

Final Considerations

Finally, though not a question, you should consider ease of use and operating cost. Simple operation is a requirement in a plasma tool as it allows the job to get done faster with less headache and frustration. It also means people with little or no experience can get good results.

When it comes to cost, it’s important to understand how much a system will cost over time. For instance, a system that costs less money to buy initially, but cuts slowly, inconsistently, or even stops working a few years after you buy it, is no bargain. You also need to look at consumable cost and life since they can vary significantly from one brand of plasma to another. Long consumable life is important as it reduces downtime for changeouts and lowers the cost you pay for new consumables. The best brands utilize patented technology and precision machining to extend consumable life, while delivering high-quality cuts.

Another way to lower operating cost is to use your system for more than cutting. Some plasma systems come with dedicated gouging and marking modes, providing extra versatility, saving setup time, and eliminating the need to purchase separate fuel gases.

If you make your selection intelligently, based on your answers to these questions and considerations, your plasma cutter will give you years of reliable performance and lessen the chance of running into situation where it doesn’t meet your needs.

WHAT THE SPARKS AND EDGE HAVE TO SAY

Though the learning curve for operating a plasma cutting system is short, novices and experts alike sometimes need to fine-tune their technique and cut speed to get the best cut quality. The correct speed can be determined by paying close attention to the sparks the plasma arc produces.

Look for cut speed ratings when comparing the capacities of plasma cutting systems.

Ideally, the sparks should spray out from the underside of the material at a 15- to 30-degree angle. If the sparks are coming out at a 90-degree angle, that’s a sign that your torch travel speed is too slow. If most of the sparks are flying back up toward you, the plasma arc isn’t going all the way through the metal because your torch travel speed is too fast.

Once you determine the correct speed, you can use other techniques, such as keeping the torch perpendicular to the metal, maintaining a fluid motion, and dragging the torch on the metal using shielded consumables to achieve a quality cut.

Here are some tips for addressing other common issues:

Wide cut angle—This issue occurs when the angle of the torch is too great. Correct it by holding the hand torch completely perpendicular to the metal while cutting. If the torch is machine-mounted, adjust its mounting bracket.

Jagged cut edge—If the cut edge is jagged and the lag lines (fine lines on the side of the metal) are overly prominent, adjust the speed following the guidelines listed previously. If you are using mechanized cutting, double-check that your torch-to-work distance, travel speed, output current, and consumables are correct.

Dross on top—Dross on the top of the metal likely means the torch speed is too slow. Speed up. If the torch is paired with a CNC, the voltage may be too low for the torch height control. Adjust the torch or the voltage in small increments until the dross is reduced.

Dross on bottom—If a heavy, bubbly deposit forms on the bottom of the material, the torch motion could be too slow or the arc could be angled ahead of the cut. Increase the torch speed or adjust the angle. If you notice that dross has formed in a thin line, then your torch motion could be too fast or the arc angled behind the cut. Reduce the torch speed or decrease the torch-to-metal distance.

Harry Mellott is senior product marketing associate at Hypertherm, 877- 607-6892, www.hypertherm.com.