A comparison of the three most common cutting techniques.


There isn’t a lot of room for error in business today. Metal fabricators and steel service centers need to make smart choices to ensure the continued success of their business.

Those choices include selecting the right cutting method.

This article will explore three of the most common processes: laser, oxyfuel and plasma so that you can make a confident and informed decision.

First, let’s start with a very brief explanation of the three processes:

Oxyfuel uses a chemical or “exothermic” reaction between the oxygen and the iron found in mild (carbon) steel. This reaction is what causes a melting of the material. Oxyfuel is only used for cutting carbon steel (ferrous metal), and is typically used to cut plate greater than 2 inches thick.

Plasma combines electrical energy with gas to create a high-temperature, ionized gas that cuts through any electrically conductive material. Plasma is great for ferrous and nonferrous materials, no matter what condition it’s in. Rusted, painted and grated metal from up to 2 inches thick can all be cut with plasma.

Laser uses a high-power beam to heat, melt, and partly vaporize the material. Laser is good for all types of metal, though it does need to be in good condition (no rust). Laser is typically used for very thin plate, up to a quarter inch, though it can be used up to 1 inch thick.

The cutting method you choose depends on your individual needs and what areas are most critical to you: cut quality, productivity, operating costs, profitability or flexibility.

Differences

Each process produces different edge quality in terms of angularity. Angularity is measured by looking at edge deviation, or the amount of deviation the angle makes from a straight edge. Laser will typically give you the least amount of edge deviation or angle, oxyfuel will give you the most and plasma is somewhere in the middle.

Kerf is the width of the material that is removed during the process. For laser, the width of the kerf varies between 0.006 inches to 0.020 inches, depending on the thickness of the plate. Note that while the kerf is very small, it is wider at the top of the cut. If we were to compare half-inch plate, the kerf width for laser is 0.0138 to 0.0157 inches.




In comparison, the kerf width using plasma on half-inch can range anywhere from 0.053 inches to 0.340 inches, depending on the thickness. Oxyfuel will result in the greatest kerf width.

All three processes will produce a heat-affected zone on the edge of the cut. Laser gives the smallest depths - 0.004 to 0.008 inches - oxyfuel produces the largest, and again, plasma is in the middle.

For both laser and plasma, the hardness levels are somewhat dependent on the gases used.

All three processes can produce a certain amount of dross or slag. Oxyfuel produces the most, and since it is the slowest of the three processes, it is often the hardest to remove. Both laser and plasma offer virtually dross-free cutting up to certain thicknesses.

A comparison in the speed, parts production capabilities and speed of return on investment for cutting methods and machinery.

Tolerance is largely dependent on the accuracy of the cutting machine, so while we can provide numbers, it is really best to work with your table manufacturer. Thickness of material is also a factor to consider for tolerance levels.

In general, laser will produce tolerances anywhere from 0.006 inches to 0.015 inches. Plasma tolerances range from 0.015 to 0.030 inches, and oxyfuel ranges from 0.020 inches to 0.030 inches.

Speeds given are in inches per minute for half-inch-thick plasma and are provided by the manufacturer.

Productivity

Another area to consider is productivity, or put it a different way, the number of parts you can  make in a given time period. How critical is productivity to the success of your operation? It’s a seemingly simple question. However, some facilities are not equipped to handle an increase in output, so consider all factors when you think about this question.

Here’s how to calculate the productivity of the different processes.

One factor that is critical to the number of parts produced is speed. There are many other factors to consider as well: time spent waiting for preheat to occur, any delays associated with piercing, any necessary secondary operations, and any other productivity enhancers such as automated features. The table below provides speeds for a few selected thicknesses that are easily cut by all three processes. Notice that for thinner plate, Hypertherm’s HyDefinition plasma is the fastest cutting method, followed by laser, lower-amp plasma systems and finally, oxyfuel.

Speeds given are for optimum quality for all processes.

Comparing speed is a good first step; however, it doesn’t mean much if you aren’t able to cut more as a result. To figure out how much you can actually cut, you’ll want to multiply your cutting speed by 60 to come up with the number of linear inches produced in one hour.

Speeds given are in inches per minute for half-inch-thick plasma, and are provided by the manufacturer.

Calculations

The next step is to calculate the actual number of parts you can cut with each process. To determine the size of the part, calculate the linear inches first. For simplicity sake, we will use a 12-by-12 inch square. Feel free to do this with one of your own parts, though.

Take all sides and add them up to make one long, linear line. In this example, we get 48 inches.

Divide by 12 (inches) and you get 4 feet, which is the size of your part.

From the figures given above, divide the total number of feet cut in one hour by the size of your part, to get the total number of parts cut in one hour.

As you can see, Hypertherm’s HyDefinition plasma produces the greatest number of parts in one hour of cutting (212.5 parts). Laser is the next fastest process, producing 93.75 parts; the entry-level plasma system cuts 71.25 parts; and oxyfuel is the slowest process, producing 25 parts per hour.

The above calculations do not take into account the preheat or pierce time commonly associated with oxyfuel. Laser also has pierce delays, though shorter than oxyfuel. Of the three, plasma has the shortest amount of time associated with pierce delays.

All three processes use some type of method to control automatic gas flow. This removes the variability that is common with different operators trying to adjust the gas flows for each process.

One last area to consider is secondary operations. If cut quality is of great concern to you, you may need to allow time for secondary operations. This will cause a further reduction in part count using oxyfuel, though better plasma systems and laser will give you virtually dross-free cuts.

A look at the efficiency of each method.

Impacts on operating costs

A third factor to consider is operating costs, or how much will it cost you to operate this machine. Many factors - consumables, power, gas and spare parts - impact the overall operating cost of a thermal cutting machine.

Consumables make up the largest portion of operating costs when cutting with plasma. However, long-lasting consumables are now available to help keep operating costs low.

Power costs are negligible for oxyfuel, a small expense with plasma, and a bit higher for laser.

Gas is the largest cost associated with laser due to high flow rates.

Spare parts are mainly a consideration for laser. While items such as lenses and mirrors are not frequently changed, they do fail and can be costly to replace, in terms of both the cost to purchase and the downtime involved to replace them. You should include a portion of this cost when calculating your daily operational expense.

Besides these expenditures, the amount of time spent on secondary operations should also be considered when figuring out the cost to operate your system. The chart shows the estimated hourly cost for each cutting method based on manufacturer specifications.

While the operating cost for oxyfuel appears low, remember this is the cost per hour. The real number to consider is cost per part. This is the far better figure to use because if something costs you $20 per hour, but you only produce two parts, this is not nearly as efficient as something that costs $20 per hour and produces 100 parts.

To determine cost per part, divide the operating cost for one hour by the number of parts produced in that hour:

• Oxyfuel produced 25 parts, which equates to 25 cents per part.

• The entry-level plasma produced 71.3 parts, for a cost of 40 cents per part.

• The HyDefinition plasma produced 212.5 parts, or 13 cents per part.

•    The laser system produced 93.8 parts, or 50 cents per part.

Cost figures come from manufacturer specifications; actual costs may vary.

Investment worth

What if you discover that it would make more sense to use a different cutting method? Should you just go out and purchase a new cutting system? Not necessarily. You’ll first want to figure out if the particular system you are considering is worth the investment. To do this, multiply your profit per part, times your parts per hour. As an example, if you make $1 profit per part, you can easily see how the number of parts you produce in a given time period is critical. More parts equals more profit, which gives you a return on investment more quickly.

Here are some hypothetical examples based on data from the table.

• Oxyfuel produced 25 parts per hour or $25 profit per hour. Multiply that by an eight-hour workday and you’ll make $200 profit per day

• Plasma produced 212.5 parts, which equals $212.50 per hour or $1,700 profit per day

•    Laser produced 93.8 parts or $93.80 worth of profit per hour or $750 profit per day

The following figures present a good estimate of the components needed to cut half-inch-thick plate. Your actual numbers will be different, depending on individual preferences, but this should give a general idea of the investment needed for each metal cutting type.

• Oxyfuel using a 5-foot by 10-foot cutting machine with 1 oxyfuel torch, computer-based control, oxyfuel height control and nesting software. Cost: $50,000

• Plasma on a 5-by-10 precision cutting machine with PC-based control, arc voltage, nesting software, 130-amp plasma system. Cost: $100,000

• Laser-equipped with a 2.5-kilowatt system 5-by-10 precision cutting machine, comport-numeric control, nesting software. Cost: $300,000.

Recouping costs

The next thing you need to do is divide the cost of your investment by your expected daily profit to determine how long it will take to recoup your investment. To keep the math simple, assume that secondary operations are not required.

• Oxyfuel would take 250 days to recoup the cost of your investment; $200 profit per day thereafter.

• Laser would take 400 days to recoup the cost of your investment; $750 profit per day thereafter.

• Plasma equals 59 days to recoup the cost of your investment; $1,700 profit per day thereafter.

One last thing to keep in mind is the amount of flexibility offered with these cutting systems. Being able to cut thin and thick is a benefit if that fits with your cutting needs. But if the metal is rusty or painted, this will pose problems with laser, though both oxyfuel and plasma will cut it just fine. If you require marking, both plasma and laser can cut and mark, often with the same consumables, which saves time.

So which cutting method should you choose? It really does depend on your specific cutting operation. In general though:

• Plasma is considered the most versatile process because of its ability to cut a wide range of metal types and thicknesses.

• Laser is a good option for operations requiring tight tolerances on thin plate (less than a quarter inch), although you will need to balance the upfront capital costs and more expensive operating costs, before deciding if it makes sense.

• Oxyfuel should be considered if you only plan to cut greater than 2-inch-thick carbon steel, although even then you will still have to make trade-offs when it comes to cut speed and cut quality.

Kat McQuade is product manager for Hanover, N.H.-based Hypertherm. She travels extensively to better understand the challenges customers face when cutting metal. Her travels have taken her to all parts of the globe, including most recently to South America, Asia and India. She has been with Hypertherm for three years.

For reprints of this article, contact Jill DeVries at (248) 244-1726 or e-mail devriesj @bnpmedia.com.