PLASMA, OXYFUEL AND LASER: WHICH CUTTING METHOD IS RIGHT FOR YOU’

The current economic reality means 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. So that you can make a confident and informed decision, this article explores three of the most common processes: laser, oxyfuel, and plasma.

First, let?s start with a very brief explanation of the three processes. Oxyfuel uses a chemical (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 very thick plate (greater than 2 in) (see Chart 1).

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 non-ferrous material, no matter what condition it?s in. Rusted, painted, and grated metal from gauge to 2 in can all be cut with plasma (see Chart 1).

Laser uses a high power laser 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 (gauge to ¼ in), though it can be used up to 1 in (see Chart 1).

CUT QUALITY COMES FIRST

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

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 in to 0.020 in, 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 ½ in plate, the kerf width for laser is 0.0138 in to 0.0157 in. In comparison, the kerf width using plasma on ½ in can range anywhere from 0.053 in to 0.340 in, depending on the thickness. Oxyfuel results in the greatest kerf width.

All three processes produce a heat affected zone (HAZ) on the edge of the cut. Laser gives the smallest depths (0.004 to 0.008 in); oxyfuel produces the largest. 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.

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 in to 0.015 in. Plasma tolerances range from 0.015 in to 0.030 in; and oxyfuel ranges from 0.020 in to 0.030 in.

PRODUCTIVITY MORE THAN SPEED

Another area to consider is productivity, or put a different way, the number of parts you can produce 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. If the answer is still yes, this section shows you 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. Table 1 provides speeds for a few selected thicknesses that are easily cut by all three 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 (see Chart 2).

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 in by 12 in 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 in. Divide by 12 (inches) and you get 4 ft, which is the size of your part (Illustration 1).

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 by Chart 3, 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 calculations in Table 2 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. This goes back to the first question we posed, 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.

OPERATING COST IMPACTED BY MANY FACTORS

A third factor to consider is operating cost, 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. Therefore, 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. Chart 4 shows the estimated hourly cost for each cutting method based on manufacturer specifications.

While the operating cost for oxyfuel appears quite low, remember this is the cost per hour. The real cost 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¢ per part

? The entry level plasma produced 71.3 parts, for a cost of 40¢ per part

? The HyDefinition plasma produced 212.5 parts, or 13¢ per part

? The laser system produced 93.8 parts, or 50¢ per part

As you can see, the HyDefinition plasma system is the most economical system to run when you compare it to the actual number of parts you can produce with it. Laser is the most expensive.

CALCULATING 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. Take our three examples based on data from the table above:

? Oxyfuel produced 25 parts per hour or $25 profit per hour. Multiply that by an 8-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 ½ in 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 x 10 cutting machine with 1 oxyfuel torch, PC-based control, oxyfuel height control and nesting software: $50,000

? Plasma on a 5 x 10 precision cutting machine with PC-based control, arc voltage THC, nesting software, 130 Amp HyPerformance plasma system: $100,000

? Laser equipped with a 2.5 KW System 5 x 10 precision cutting machine, CNC control, nesting software: $300,000

RECOUP INVESTMENT 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. We will assume that secondary operations are not required to keep the math simple.

Table 3 shows that 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. Also, 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? Hopefully the answer ?it depends? now makes sense as 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 ¼ in), although you will need to balance the upfront capital costs and more expensive operating costs, before deciding if it makes sense financially.

? Oxyfuel should be considered if you only plan to cut thick (greater than 2 in) carbon steel, although even then you will still have to make tradeoffs when it comes to cut speed and cut quality.

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Kat McQuade is the product marketing manager of mechanized systems at Hypertherm, Inc., Etna Road, P.O. Box 5010, Hanover, NH 03755, 603-643-3441 ext 1463, Fax: 603-643-5352, www.hypertherm.com.