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The Great Debate: Plasma Cutter or Oxyfuel Torch?

Whether you should put your trust in a flame or an arc depends on what you do

(As seen in the Welding Journal, March 2005)




For increased oxyfuel cutting safety, Smith Equipment's Dual Guard torches (pictured at left) feature a flash arrestor module built into the head of the torch and in-tip mixing technology that provides double protection against flashback fires getting beyond the tip and into the torch itself. This helps reduce torch damage, downtime, and repairs.

Plasma or oxyfuel? The answer to which tool is better for your cutting needs depends on a number of factors - alloy, thickness, location, variety of work, power resources, cost, etc. This article first introduces each process, and then sheds some light on what tool will work better for you. It is written from the basis of handheld units and for professionals who use such equipment.

Plasma Arc Cutting

Plasma - 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 arc cutting machines 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.


The Basics. With a power source, plasma torch, compressed air, electrical power, and a supply of tips and electrodes, plasma arc cutting machines 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 in. thick, along with expanded and stacked metals.

TABLE 1

PLASMA ARC CUTTING

OXY/FUEL CUTTING

Metals

Plasma cuts any electrically-conductive metal (steel, aluminum, copper, stainless steel, etc.) from very thin (<3/8 in.) up to 2in.

Oxy/fuel cuts ferrous (iron-containing) steels up to 24 in. thick.  Metals such as aluminum and stainless steel cannot be cut with Oxy/fuel due to the formation of an oxide that prevents oxidation from fully occurring.

Industries

Common in metal fabrication, construction, agricultural, maintenance, automotive repair, metal art, sculpting, home, hobby and DIY applications.

Agricultural, fabrication, construction, maintenance repair, mining, automotive, hobbyists, DIY applications

Preheating

Not required

Required

Productivity

Plasma cutting works exceptionally well on thinner materials (<1/2 in.), depending on the output of the power source. It can also cut stacked material and works well on expanded metals. Prep time is minimal with no preheating requirements. Produces a small and precise kerf (cut) width. Features a small heat-affected zone that prevents warping and damage. Cleanup is rarely needed as dross is cleanly blown away. 

Oxy/fuel is capable of efficiently cutting metals up to 24 in. thick. It is not dependent on a primary power source. Fuel gas and nozzle design options enhance performance.  

Portability

Portability varies based on technology, size of power source and size of air tanks (if not using a built-in air compressor). Many units can work efficiently in the field off of engine-driven generators and a variety of primary power sources thanks to primary power management technologies offered in some plasma cutters.

Highly portable and is not dependent on a primary power source – capable of cutting anywhere with gas tanks and torches.

Versatility

Stack cutting, beveling, shape cutting, gouging and piercing of metals is possible with plasma cutters. 

With a combination torch, oxy/fuel systems can be used for heating, brazing, soldering, welding,gouging, riser cutting and bending metals.

Safety Concerns

(Operated properly, each tool is safe and effective. Improper use can lead to the following problems.

Electrocution, fires started by sparks, arc flash to the eyes, cutting in and around combustible materials, damage to protective clothing, skin and tissue if used improperly. Always wear appropriate clothing.

Flashbacks, fires started by sparks, cutting in and around combustible materials, damage to protective clothing, skin and tissue if used improperly. Always wear appropriate clothing.

Table 1 compares the features and capabilities of the two cutting systems.

Oxyfuel Cutting

With oxyfuel 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. Oxyfuel cutting is used for ferrous (iron-containing), mild and low-alloy steels in thicknesses up to 24 in.

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

Other Cutting Methods

This article, while focusing on hand-held oxyfuel and plasma arc cutting, would be remiss if it did not make brief mention of other cutting methods. Band saws are extremely effective in accurately and efficiently cutting tube and pipe. Lasers, along with mechanized plasma arc and oxyfuel, are frequently used in large production applications such as the manufacturing of construction equipment and shipbuilding to cut massive, intricate sheets of steel - these systems eliminate 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 hack saws (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.

This welding operator uses a Spectrum 375 plasma arc cutting machine from Miller Electric Mfg. Co. to trim tubing. The machine's weight of 55 lb makes it portable.

Making the Selection: Plasma Arc or Oxyfuel?

The following two questions need to be answered before choosing plasma or oxyfuel cutting tools:

The Argument for Plasma

Plasma arc cutting has been praised for its ability to cut thin metals (<3/8 in.) quickly, neatly, and with a minimal heat-affected zone - minimizing distortion and warpage. It is also typically a clean cut, with minimal cleanup as any dross is blown clear of the workpiece. 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 arc systems can cut all electrically conductive metals, including aluminum and stainless steel. These two metals cannot be cut by oxyfuel 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 arc cutting system is the right tool for you. Equally as important as the type of alloy, however, is the thickness.

Each plasma arc cutting machine features a rated output and a rated cut; the higher the amperage (output), the thicker the cut. For instance, a machine with a rated output of 40 A at 140 V DC at 50% duty cycle features a rated cut of 1/2 in., a maximum quality cut of 5/8 in., and the ability to sever steel as thick as 7/8 in. Compare that to a plasma arc machine with a rated output of 100 A at 120 V DC at 80% duty cycle, and the capabilities increase: rated cut of 11/4 in., maximum quality cut of 11/2 in., and a sever rating of 13/4 in.

The three ratings of cutting abilities tell you how quickly you can cut with a given plasma machine: A "rated" cut represents a machine's ability to cut steel at a rate of 10 in./min. A "maximum quality" cut represents 7 in./min at the given thickness, while the "sever" rating simply points out the thickest metal that machine can cut - it does not have the amperage to adequately blast the arc through steel any thicker. See Table 2 for a sample chart of outputs and cutting abilities - you will notice there is a wide variety of output powers and cutting abilities. Using these examples, if 90% of your work is 1/2-in. steel and, on occasion, you have the need to sever steel 7/8 in. thick, the machine in row 3 of Table 2 would be your ideal choice.

TABLE 2 – Sample Chart of Plasma Cutting Outputs/Cutting Capacities

RATED OUTPUT

INPUT POWER

RATED

CUTTING CAPACITY

MAXIMUM QUALITY

SEVER

PIERCING

12 amps at 110 volts DC

120 V, 1-PH, 60 HZ

1/8 in. (3.2 mm)

3/16 in. (4.2 mm)

1/4 in. (6.4 mm)

12 ga.

(2.6 mm)

27 amps at 90 volts DC, 35 % duty cycle

115 or 230 VAC, 1-PH, 60HZ

5/16 in. (8 mm)

3/8 in. (9.5 mm)

1/2 in. (13 mm)

3/16 in. 

(5 mm)

40 amps at 140 volts DC, 50 % duty cycle

208 or 230 V, 1-PH, 60 HZ

1/2 in. (13 mm)

5/8 in. (16 mm)

7/8 in. (22 mm)

1/4 in.  (6mm)

55 amps at 140 volts DC, 60 % duty cycle for 3-phase, 50 % duty cycle for 1-phase

208 to 575 VAC, 1- or 3-PH, 50/60 HZ

7/8 in. (22 mm)

1 in. (25.4 mm)

1-1/4 in. (32mm)

3/8 in. 

(9.5 mm)

80 amps at 140 volts DC, 60% duty cycle for 3-phase, 40% duty cycle for 1-phase

208 to 575 VAC, 1- or 3-PH, 50/60 HZ

7/8 in. (22 mm)

1-1/4 in. (32 mm)

1-1/2 in. (38 mm)

1/2 in.

(13 mm)

100 amps at 120 volts DC, 80 % duty cycle

200/230/460 VAC, 3-PH, 60 HZ

1-1/4 in. (32 mm)

1-1/2 in. (38 mm)

1-3/4 in. (44 mm)

1 in.

(25 mm)

The Argument for Oxyfuel

Oxyfuel 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 oxyfuel cannot cut stainless steel and aluminum. Oxyfuel is particularly effective on ferrous (iron-containing) metals at thicknesses up to 24 in. with a handheld torch. Oxyfuel can efficiently cut even the thickest metals. For example, given a 2-in.-thick chunk of metal requiring a 4-in.-long cut, a 120-A plasma arc machine would take roughly 1 minute to complete the cut, while an oxyfuel torch could make that same cut in 15 to 20 seconds.

Whereas plasma arc cutting machines feature "ratings" to help choose the right machine, oxyfuel cutting varies by the type and style tip and fuel gas used. Oxygen is the constant - 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 comsumption 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 the fuel gases at approximately 4700°F and the lowest BTU output of 1050 BTU/ft3. By comparison, propane burns at a considerably lower temperature (5000°F) 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 2600 BTU/ft3 vs. 1450 BTUs 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 maintains a relatively high BTU output of 2400 BTU/ft3 with a lower consumption of oxygen than propane, propylene, or natural gas.

For more information on these five gases and how they affect your cutting operations, consult Table 3 then contact your local welding supply distributor for further insight into your specific application.

TABLE 3 – Oxy/fuel Fuel Gas Basics

Acetylene: Produces the highest flame temperature of all common fuel gases. The hotter the flame – the faster piercing occurs. The higher calorific value of the inner flame vs. that of other gases makes acetylene the fastest cutting gas with the least amount of distortion and the narrowest heat affected zone – which is why it is arguably the most popular gas. Also features the lowest oxygen-to-gas ratio of (1.2:1).

Propane: Lower flame temperature than Acetylene but has a greater total heat of combustion, making it equally fast at cutting. But with most of the heat being generated in the secondary flame, the flame is less focused, causing slower piercing times. Suffers from a relatively high oxygen-to-fuel gas ratio (4.3:1) required to meet maximum flame temperature. 

Methylacetylene-propadiene (MAPP): Slower piercing and cutting times than Acetylene due to lower flame temperature, larger distributed heat source and gas flows – but can be used at higher pressures than Acetylene (i.e., less combustible).  Oxygen-to-gas fuel ratio is (2:5:1).

Propylene: Similar in temperature to MAPP, hotter than propane, cooler than acetylene but requires a high oxygen-to-fuel gas ratio (3.7:1).

Natural Gas: Slowest gas for piercing, with lowest flame temperature and lowest total heat value.


Versatility of Plasma Arc and Oxyfuel Cutting

Rarely would you buy equipment for plasma arc or oxyfuel cutting just for its cutting abilities. That may be the primary objective, but both methods have benefits outside of cutting steel.

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

Oxyfuel cutting equipment offers metalworkers even more versatility. Straight handheld torches for cutting are widely used, with some capable of cutting steel up to 24 in. 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 multiorifice heating tips, oxyfuel systems can be used to 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 sheet metal cutting. While the cutting capacity of combination torches is reduced considerably (8-12 in.), 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 arc and oxyfuel cutting have positives - and drawbacks - when it comes to cutting outside of the shop.

This operator uses a plasma arc cutting system to cut a piece of 1⁄2-in. steel that will be used to repair a large snow thrower.

With plasma arc 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, concerns about using plasma arc cutting machines with high-frequency starting that might interfere with electrical equipment such as computers is no longer a concern as units are now built that start without high frequency. 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 arc cutting machine in the back of his truck and drive out to a fence line without an engine-driven generator to power it. Running plasma arc cutting machines off of engine-driven generators 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 arc machines that harness incoming power and ensure the output is consistent, which guarantees a smooth and easy cut.

Inverter-based technology and self-contained air compressors have made plasma arc cutting machines increasingly smaller. The most common plasma arc systems fit neatly in most shops and vary in weight, some as low as 43 lb and other, more stationary units, as heavy as 410 lb. Let's take a look at a 60-lb plasma arc cutting machine that can be 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 in. thick, running off of either 208/230 VAC (one-phase) or 8500 W of generator power. This is a relatively recent development in the plasma arc cutting industry, making the technology considerably more portable and trustworthy "in the field."

Oxyfuel 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 adequate hose, torch, and consumables can perform all of the functions possible with oxyfuel systems. Oxygen and fuel tanks can weigh in excess of 200 lb, and portable cutting and welding outfits are available at less than 35 lb. In some cases, oxyfuel's strongest benefit can also be a negative. You can run out of fuel or oxygen, whereas a plasma arc machine plugged into primary power can cut all day barring a blackout or other loss of power.

Start-up Costs

Making a decision on plasma arc or oxyfuel needs to include more than just upfront costs. "Time is money" - and the costs of using the wrong tool can drastically cost more than the initial start-up costs, depending on your application.

An example of the cut quality on 1⁄2-in. steel produced by the Spectrum 625. The other piece of steel was used as a template to guide the plasma arc torch during cutting.

At face value, plasma arc cutting is more expensive to start from scratch. The initial investment into a handheld plasma arc cutting power source can run from $1100 to $3000 depending on the size of the machine. Now, 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 application, but figure roughly $15 to replace the pair. Taking stock of your day-to-day operations is just as important in the cost of operation for plasma arc cutting machines as the overhead investment in equipment is. You must also consider your application and the thickness of metal you will be cutting. While oxyfuel is less expensive it may be unsatisfactory for cutting very thin sheet metal due to warping.

The same is true for oxyfuel - buying a plasma arc machine when your application and metal thickness calls for oxyfuel can be expensive. Luckily, oxyfuel systems are more cost effective up front, with a full torch setup costing roughly $250 and oxygen and fuel gas cylinders costing approximately $250. Add in cutting tip replacement ($10 each) and the refilling of empty gas cylinders ($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 oxyfuel system vs. trying to use a plasma system is the real difference. Conversely, if your main job is cutting stainless steel or aluminum, an oxyfuel 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 arc system.

Safety Considerations

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 coworkers familiar with the process), there are some inherent dangers with each of these processes you should consider.

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

Conclusion: Take Your Pick

This article is intended to give 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 you may still have some questions. Experience is the best resource in picking between a plasma arc and an oxyfuel system - we encourage you to contact manufacturers, local distributors, or others in your field to find out what works best in your niche of the metalworking world. We simply can't cover every situation or challenge you might come across, but we hope we have pointed you in the right direction.

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