STEEL CUTTING N2 VS. O2: TO USE OR NOT TO USE, THAT IS THE QUESTION
When cutting with nitrogen assist, your overall costs are determined by time and secondary operations. That’s not all: N2 cutting of gage material should be done on a solid state laser if or a high powered CO2 laser. Here’s why.
When deciding to cut with oxygen versus nitrogen, it is important to consider the many factors involved. Laser power, laser type (solid state or CO2), cutting speeds, gas consumption, gas costs, material thickness, edge quality, and secondary operations all contribute to this decision.
To begin, think in reverse. Ask what additional processes will be required after your parts are cut. Will they need to be welded and/or painted? If so, you have to acknowledge the costs associated with cleaning the oxide edges that result from an oxygen cut as well as the labor and machine costs to perform the task. Cleaning the outside geometry of a part might be easy, but perhaps the inside geometry is not so simple and requires an additional machine. Once these factors are considered, move to the laser cutting process.
First, consider your laser power and how each gas influences speed and cut quality when processing steel. Cutting thin steel with oxygen will result in the same cutting speed whether you are using a 1.5 kW or 6 kW laser because you are burning material in the process and the power supplied to the cut is limited. If you apply more power, your cut quality will suffer without an increase in speed. When cutting with nitrogen, power can increase cut speeds dramatically based on the amount of power applied, type of laser and material thickness being cut. Nitrogen acts as a coolant to the material allowing utilization of the maximum wattage available while maintaining cut quality. Here, nitrogen has a clear advantage.
The next consideration in determining oxygen versus nitrogen is material thickness. In thinner material with nitrogen and high laser power or solid state lasers, cutting speed can be up to seven times faster than with oxygen. Typically, the cutoff point where nitrogen is no longer a consideration is at approximately 1/8 in. For example, cutting .080 in steel with oxygen and a 3 kW laser occurs at approximately 200 ipm and requires roughly 65 cfh of gas. On the same laser, cutting with nitrogen, speed remains constant but gas consumption increases to 465 cfh. Comparing the cost of each gas, your requirements are approximately $1 per hour in oxygen as opposed to $2.50 per hour in nitrogen. The benefits of cutting with nitrogen are seen only in edge quality and reduced secondary operations, which we will discuss later.
The laser type is also a deciding factor. A solid state laser that is cutting with nitrogen –at the same 3 kW of power – increases cutting speed by a magnitude of three, but also increases assist gas usage to 1500 cfh or approximately $6 per hour. However, if you use a 6 kW CO2 laser with nitrogen as the assist gas, cutting speeds are two and a half times greater and processing occurs at approximately 550 ipm while requiring only 408 cfh of nitrogen. In this case, you are producing parts much faster with a high quality edge at about $2 per hour in nitrogen gas consumption. Also factor in the additional operating costs of a higher power laser, which can go from $5 per hour on a 3 kW to $9 per hour on a 6 kW.
Now, you can start to consider total costs in determining the right choice in laser cutting with assist gas. Identify costs while keeping in mind that total cost per part is also influenced by the speed at which you can produce the parts.
Let’s consider a sample run part that is .080 in thick, cut with oxygen on a 3 kW laser with 250 parts required. Cutting 100 parts per hour, it will take 2.5 hours of laser time to complete the job. If the laser operating cost is $5 per hour and gas consumption is $2.50, the 250 parts will cost $15.00 to produce. But we must also consider the cost of labor to run the parts. Let’s assume the operator, with benefits, makes $20 per hour over the 2.5 hours needed to produce the part. These wages, combined with the gas and operating costs, means laser cutting these parts will cost $65.00.
The price of the next step, removing the oxide edge, depends on the method used. Chemical removal produces hazardous waste and requires labor. Manual grinding, depending on the geometry of the part, could take excessive time. Special machines are effective in finishing the edge but can be expensive to own, maintain and have someone operate. No matter the method, there is a high cost involved in oxide removal.
Use the same example except cut with nitrogen instead of oxygen. At the same laser power (3 kW) with a CO2 laser, speed gains are minimal but the oxide edge is eliminated and costs are not much higher – approximately $68 for time, gas, and operating costs. However, if you stay with the CO2 laser and increase the laser power to 6 kW, cutting speed is 2.5 times faster, as mentioned before. Therefore, the same 250 parts are now completed in an hour. Estimating operating cost at $9.00 and gas consumption at $2.00, the cost is $11.00. Now the operator can complete the job in an hour, so total cost is reduced from to $65.00 to $31.00. Furthermore, the parts are finished and ready for the next operation – without any additional labor, machining, or handling.
For our last example, consider a 3 kW solid state laser which cuts three times faster in the .080 in material. The parts are produced in 45 minutes at an operating cost of roughly $1.50, N2 assist gas at $4.50 and an operator wage of $15. Total laser cost is $21.00, much less than the oxygen example and without the oxide edge. Use a 5 kW solid state laser and production occurs five times faster with less assist gas. In 30 minutes, 250 parts are produced for approximately $14.00. This method is the best choice of our examples.
In conclusion, when cutting with nitrogen assist, your overall costs are determined by time and secondary operations. Furthermore, it is safe to say that N2 cutting of gage material should be done on a solid state laser if possible, or a high powered CO2 laser as a close, second choice.
About the Author: Shane Simpson is a product manager at Trumpf Inc., 111 Hyde Road, Farmington Industrial Park, Farmington, CT 06032, 860-255-6039, Fax: 860-255-6421, email@example.com, www.us.trumpf.com.