Plasma Cutting or Oxy/Fuel Cutting - The Great Debate - Tech

To be or not to be, it's one of those questions where thoughts collide, brain matter disintegrates, and before you know it you're left with a headache hoping you just made the right decision. But the fact of the matter is we have to decide between two objects on a daily basis. Here's just a few of them: Coke or Pepsi? McDonald's or Burger King? Coors Light or Bud Light? Cheerios or lasagna? Work or the Dodger's game? It never ends, so with that being said we're going to throw another debate into the mix, plasma or oxy/fuel? Each of them has their benefits and each has loyal followers, but when it comes time for you to choose, what side do you stand on? The good news is, if you don't know by the end of this article, you should at least have some idea where to mark your territory.

Plasma Cutting-An IntroductionPlasma-the fourth state of matter-is an ionized gas that conducts electricity. Plasma is created by adding energy to an electrically neutral gas. In this case, the gas is compressed air and the energy is electricity. By adding electricity through an electrode (hafnium), the gas becomes imbalanced and conducts electricity. The more electrical energy added, the hotter the plasma arc becomes. Plasma cutters control this powerful energy by constricting the arc and forcing it through a concentrated area (the nozzle). By increasing air pressure and intensifying the arc with higher voltages, the arc becomes hotter and more capable of blasting through thicker metals and blowing away the cuttings, with minimal cleanup required.

With a power source, a plasma torch, compressed air, electrical power, and a supply of tips and electrodes, plasma cutters can rapidly and precisely cut through, gouge, or pierce any electrically conductive metal without preheating and with a minimal heat-affected zone. Plasma can cut metals such as aluminum, stainless steel, brass, and copper in excess of 2 inches thick-along with expanded and stacked metals.

Oxy/Fuel Cutting-An Introduction
With oxy/fuel cutting, an oxygen/fuel gas flame preheats the steel to its ignition temperature. A high-power oxygen jet is then directed at the metal, creating a chemical reaction between the oxygen and the metal to form iron oxide-also known as slag. The high-power oxygen jet removes the slag from the kerf. Cut quality, preheating times, and thicknesses can be influenced by the type of fuel gas used. Oxy/fuel cutting is used for ferrous (iron-containing), mild, and low-alloy steels in thicknesses up to 24 inches.

With a fuel tank, oxygen tank, and a torch, you can cut steel anywhere-except for non-ferrous metals such as aluminum and stainless steel. Oxy/fuel cutting allows metalworkers to cut extremely thick metals with ease-and a variety of torch options allow users to braze, solder, fusion weld, gauge, preheat, and bend metals as needed.

Other Methods-An Overview
This article, while focusing on hand-held oxy/fuel and plasma cutting, would be remiss if it did not make brief mention of other methods of cutting metal and steel. Bandsaws are extremely effective in accurately and efficiently cutting tube and pipe. Lasers, along with mechanized plasma and oxy/fuel, are frequently used in large-production applications such as the manufacturing of construction equipment and shipbuilding to cut massive, intricate sheets of steel-eliminating operator error and operator fatigue-but are only necessary for certain high-flow work applications. Other more antiquated and labor-intensive tools, such as shears and hacksaws (with the appropriate blade), are used in some applications, but are hardly efficient given the lowering costs and overabundance of power tools to do the job.

Making The Selection: Plasma Or Oxy/Fuel?
The two key questions that need to be asked before choosing plasma or oxy/fuel cutting tools are:What do you cut on a day-to-day basis?What is the thickest metal, within reason, that you'll ever cut?The answer to these two questions will point you in the right direction.

The Argument For Plasma
Plasma cutting has been praised for its ability to cut thin metals (less than 3/8 inch) quickly, neatly, and with a minimal heat-affected zone-minimizing distortion and warpage. It is also typically a very clean cut, with minimal cleanup, as any dross is blown clear of the work piece. This method continues to gain market share in the metal cutting industry as equipment becomes less expensive, units become smaller due to inverter-based technology, and it continues to prove itself as a viable cutting option. Plasma cutting cuts all electrically conductive metals, including aluminum and stainless steel. These two metals cannot be cut by oxy/fuel cutting systems due to an oxide layer that prevents oxidation from occurring. If your day-to-day business involves cutting aluminum and stainless steel, a plasma cutting system is the right tool for you. Equally as important as the type of alloy, however, is the thickness

Each plasma cutter features a rated output and a rated cut-the higher the amperage (output), the thicker the cut. For instance, a plasma cutter with a rated output of 40 amps at 140 volts DC at 50 percent duty cycle features a rated cut of 1/2 inch, a maximum quality cut of 5/8 inch, and the ability to sever steel as thick a 7/8 inch. Compare that to a plasma cutter with a rated output of 100 amps at 120 volts DC at 80 percent duty cycle, and the capabilities increase: a rated cut of 1 1/4 inches, a maximum quality cut of 1 1/2 inches, and a sever rating of 1 3/4 inches.

The three cutting ability ratings tell you how quickly you can cut with a given plasma cutter. A "rated" cut represents a machine's ability to cut steel at a rate of 10 inches per minute (IPM). A "maximum quality" cut represents 7 IPM at the given thickness, while the "sever" rating simply points out the thickest metal the machine can cut-it does not have the amperage to adequately blast the arc through steel any thicker.

The Argument For Oxy/Fuel
Oxy/fuel cutting, arguably one of the most time-honored and trusted methods of cutting steel, is the tool for you if you'll be cutting thicker metals. One limitation to keep in mind, however, is that oxy/fuel cannot cut stainless steel and aluminum. Oxy/fuel is particularly effective on ferrous (iron-containing) metals at thicknesses up to 24 inches with a handheld torch, and cuts even the thickest metals in an efficient manner. For example, given a 2-inch-thick chunk of metal where a 4-inch-long cut is required, a 120-amp plasma cutter would take roughly one minute to complete the cut, while an oxy/fuel torch could make the cut in 15 to 20 seconds.

Whereas plasma cutters feature "ratings" to help choose the right machine, oxy/fuel cutting varies by the type and style of tip and fuel gas used. High-purity oxygen is required to maintain cutting speeds and provide superior cut edge quality. The five fuel gases used most frequently in combination with oxygen are acetylene, propane, methylacetylene-propadiene (MAPP), propylene, and natural gas.

Fuel gases are characterized and chosen by flame temperature, fuel gas to oxygen ratio, and the heat of combustion (BTU output). A gas such as acetylene, with a high inner (primary) flame temperature and a lower outer (secondary) flame temperature relative to other gases, preheats quickly and allows the operator to fusion weld steel. Natural gas, conversely, produces the coolest flame of all of the fuel gases at approximately 4,700 degrees Fahrenheit and the lowest BTU output of 1,050 BTU/cubic foot. By comparison, propane burns at a considerably lower temperature (5,000 degrees Fahrenheit) than acetylene but cuts at roughly the same speed due to a greater total heat of combustion (heat of combustion is greater in the secondary flame, where propane produces approximately 2,600 BTU/ cubic foot. vs. 1,450 BTU for acetylene.

The added heat that comes from the outer flame causes a less focused flame and makes piercing somewhat slower. Gases such as MAPP produce a higher flame temperature than propane and maintain a relatively high BTU output of 2,400 BTU/cubic foot, with a lower consumption of oxygen than propylene or natural gas.

Versatility Of Plasma And Oxy/Fuel Cutting
Rarely would you buy equipment for plasma or oxy/fuel cutting just for its cutting abilities. That may be the primary objective, but both disciplines have benefits outside of cutting steel.

Handheld plasma cutters are extremely effective in gouging out steel. Many welding repairs call for gouging out damaged metal, whether it's a crack in a trailer hitch, a damaged fence rail, or a rusted piece of metal. Or maybe you want to perform edge preparation or remove an imperfection in your weld? Each plasma cutter has the ability to gouge by simply replacing the cutting tip with a gouging tip. A larger-diameter orifice in the tip for plasma gouging reduces the arc constriction, which results in a lower stream velocity. This gives a softer, wider arc and proper stream velocity for gouging.

Oxy/fuel cutting equipment offers metalworkers even more versatility. Straight handheld torches for cutting are widely used, with some capable of cutting steel up to 24 inches thick. But combination torches are also available that allow operators to braze, solder, and fusion weld. By utilizing special tips, such as rosebud tips or large multi-orifice heating tips, oxy/fuel systems can straighten and bend parts as well as heat-treat metals. Special-purpose tips are also available for cutting torches that allow gouging, riser and bulkhead cutting, as well as thin sheetmetal cutting. While the cutting capacity of combination torches is reduced considerably (8-12 inch), the cutting capabilities matched with the added benefits are likely to satisfy most metalworkers' needs.

Portability-Where Can You Go?
Many people in the metalworking industry don't have the luxury of having all of their work located in one place. Farmers, equipment maintenance crews, and mobile welding shops often need to cut metal away from the shop, where power may be limited and relocating equipment might not be the easiest option. Both plasma and oxy/fuel cutting have positives-and drawbacks-when it comes to cutting outside of the shop.

With plasma cutting, a reduction in size, self-contained air compressors, and a drastically improved ability to run off of engine drives have made this tool increasingly more portable. Also, using plasma cutters with high-frequency starting that might interfere with electrical equipment such as computers is no longer a concern as units without high frequency are now built. The only drawback? Primary power, whether one or three phase or produced by an engine-driven generator, is needed. A farmer could never simply throw a plasma cutter in the back of his truck and drive out to a fence line without an engine drive to power it. Running plasma cutters off of engine drives on rural, often dirty primary power was not the easiest or most efficient way to cut metal in the past. But some manufacturers today have built technologies into plasma cutters that harness incoming power and ensure that the output is consistent, which guarantees a smooth and easy cut.

Inverter-based technology and self-contained air compressors have made plasma cutters increasingly smaller. The most common plasma systems fit neatly in most shops and vary in weight, some as low as 43 pounds and other, more stationary units as heavy as 410 pounds. For the sake of our article, let's take a look at a 60-pound plasma cutter, easily loaded into a truck or onto a cart by two people. This machine can perform quality cuts on some metals up to 1/2-inch thick, running off of either 208/230 VAC (1-phase) or 8,500 watts of generator power. This is a relatively recent development in the plasma cutting industry, making the technology considerably more portable and trustworthy in the field.

Oxy/fuel has one distinct advantage in terms of portability-it is not, in any way, dependent on electricity. In the middle of nowhere, a farmer with his oxygen and fuel tanks on a trailer or cart with an adequate hose, torch, and consumables can perform all the functions possible with oxy/fuel systems. Oxygen and fuel tanks can weigh in excess of 200 pounds, and portable cutting and welding outfits are available that weigh less than 35 pounds.

In some cases, oxy/fuel's strongest benefit can also be a negative. You can run out of fuel or oxygen, whereas a plasma cutter plugged into primary power can cut all day-barring a blackout or other loss of power.

Start-Up CostsMaking a decision on plasma or oxy/fuel needs to include more than just upfront costs. Time is money-and the having the wrong tool can cost drastically more than the initial start-up costs, depending on your application.

At face value, plasma cutting is more expensive to start from scratch. The initial investment into a handheld plasma cutting power source can run from $1,100 to $3,000 depending on the size of the machine. With built-in air compressors, the air is free-but you still have to replace tips and electrodes. The frequency at which these consumables need to be replaced will vary based on the application, but figure roughly $15 to replace the pair. Taking stock of your day-to-day operations is just as important as the overhead investment in equipment is.

ou must also consider your application and the thickness of metal you will be cutting. While oxy/fuel is less expensive up front, it may be disadvantageous to operators that cut very thin sheetmetal.

The same is true for oxy/fuel-buying a plasma cutter when your application and metal thickness calls for oxy/fuel can be expensive. Luckily, oxy/fuel systems are more cost-effective up front, with a full torch setup costing roughly $250, and oxygen and fuel gas bottles, which can cost approximately $250. Add in cutting tip replacement ($10 each) and refilling empty gas bottles ($30), and your initial start-up cost is less. The real savings come if your job consistently requires cutting thicker metals-the time (and money) you can save by quickly cutting through thick metal with an oxy/fuel system versus trying to use a plasma cutter is the real difference.

Conversely, if your main job is cutting stainless steel or aluminum, an oxy/fuel system isn't going to make you any money, as it is not reasonably possible to cut these metals with this tool-your money is better spent on a plasma cutter.

Safety-Take Your Surroundings Into Account When Making Selection
Safety is all too often taken for granted, but when dealing with steel, electricity, flames, and combustible gases, it is important to take full stock of your operations and weigh the effects of each tool (although by now, equipment capabilities have likely made the choice easy for you). Outside of the typical operator safety issues (wear your protective eyewear and clothing, follow the instructions and lead of co-workers familiar with the process), there are some inherent dangers with each of these processes you should consider.

Both plasma arc and oxy/fuel cutting are capable of causing fires started by sparks. Cutting in and around combustible materials is a danger with both processes. And high-temperature arcs and flames can burn unprotected skin. Each process also has unique dangers. There is a risk of electrocution and arc flash burns to the eyes with plasma cutting, and the open flame when oxy/fuel cutting can be potentially dangerous-but prevention and smart equipment usage will trump these dangers every time.

Conclusion: Take Your Pick
This article was designed to give you a comprehensive overview of each process and the inherent benefits of both as they relate to factors most important to professionals in the metalworking industry. The discussion of alloys and thicknesses may have made the choice easy for you, but many of you may still have questions. Experience is the best resource in picking between a plasma cutter and an oxy/fuel system-we encourage you to contact manufacturers, local distributors, or fellow members of your profession to find out what works best in your niche of the metalworking world. We simply can't fit into this article every single situation or challenge you might come across-but we hope we can point you in the right direction.