Give It Some Gas
Tips for laser cutting with assist gas
March 14, 2011
Cutting with oxygen, nitrogen, and shop air.
When buying and using a laser cutting machine, some manufacturers forget a very important variable: assist gas.
Not only must manufacturers select the proper type of gas for their operations, but they must also consider the cost of the gas and the ancillary equipment necessary to deliver and store it. Ultimately, assist gas practices can either help or hurt profits, so optimizing them is crucial.
Choosing the correct type of assist gas, maintaining the proper pressure and flow, and identifying the most cost-efficient delivery and storage methods require knowledge of several factors. While the type and thickness of the metal are the most obvious variables, less apparent factors such as production volume, number of lasers in use, shop floor environment, and the required surface quality of the finished workpiece should influence manufacturers’ assist gas choice.
Assist Gas Types
It’s important to have an understanding of the various assist gases available and the ideal usage scenario for each.
The three primary types of assist gas used in laser cutting are oxygen, nitrogen, and air.
1. Oxygen. Oxygen is the most commonly used assist gas for laser cutting mild steel. Oxygen causes an exothermic reaction that helps provide the heat to make laser cutting possible. It is best for mild steel that is 1 inch thick or less. Typically, cutting mild steel with an oxygen assist gas requires the lowest amount of pressure, usually about 28 PSI or lower, and a flow rate of less than 60 standard cubic feet per hour (SCFH), depending on the thickness of the metal. The thicker the metal, the less oxygen is needed to cut it.
When cutting with oxygen, manufacturers must also decrease the feed rate as the thickness of the steel increases.
There are benefits and drawbacks to using oxygen instead of nitrogen or air. First, oxygen requires a much lower flow and pressure than nitrogen, meaning the consumption — and ultimately the costs — will be lower. Also, in thicknesses of more than 0.08 in., oxygen cuts faster.
Oxidation is one of the drawbacks of using oxygen as the assist gas. The reaction causes oxidation that can negatively affect surface quality, remove surface coatings, and prevent the ability to paint finished workpieces. Oxide that is not cleaned off will chip away and cause surface corrosion. In general, the overall edge quality of workpieces cut with oxygen is inferior to that of workpieces cut with nitrogen.
2. Nitrogen. Nitrogen provides a clean, precise cut without oxidation, because it doesn’t cause a reaction to increase heat and aid cutting, but rather serves only to remove the molten metal.
Nitrogen is well-suited for cutting stainless steel and aluminum. It also can cut mild steel much faster than oxygen when the metal thicknesses are less than 0.08 in. While nitrogen requires higher pressure and flow than oxygen on thick metals, the amount of nitrogen required, and the pressure and flow, drops as metal thickness decreases.
In most situations, using nitrogen instead of oxygen comes with a much higher price tag. Also, except in very thin metals, nitrogen is the slowest assist gas. For this reason, it is typically used only for mild steel cutting applications in which surface quality is extremely important.
3. Air. Shop air can be a viable alternative in some applications, with the obvious bonus being that there are no direct consumption costs.
The reaction air cutting causes creates plasma, an extremely effective heat conduit that allows the fastest speeds in mild steel and aluminum laser cutting. While compressed air does leave behind oxide, it is less than straight oxygen. Also, the edge quality of the resulting workpiece is not as good as with nitrogen.
While shop air is cheap and effective for thin metals — it works best with mild and stainless steel less than 0.06 in. thick and aluminum less than 0.08 in. thick — establishing a system that maintains the required air pressure and air filtration can be an obstacle for many manufacturers.
The process to collect, store, and deliver shop air can involve a large upfront investment in specialized equipment, and even then contamination can be an issue. If the shop air is not clean and dry it must be made so by a filtration system. If not, it can degrade the focal lens, which can lead to cutting problems or optics failure.
Efficient Assist Gas Setup
Choosing the ideal type of assist gas is just the beginning. For optimal production, manufacturers must mold assist gas delivery and storage strategies to fit individual needs and applications.
Before manufacturers buy a laser machine, they should consider the gas delivery and storage setup. They also should assess this situation when they purchase new machines, or if their production situations change.
For example, if source tanks are too small, a manufacturer may face frequent production interruptions as gas runs out. On the other hand, buying a too-large, bulk gas system can result in unnecessary equipment rental fees and storage space problems.
Another common issue is redundant gas equipment, which often happens when each laser has its own assist gas supply. Manufacturers can streamline assist gas delivery through a single setup shared among several machines, saving space and money.
Manufacturers can choose from gas cylinder banks, liquid gas cans, microbulk, and bulk gas systems.
Cryogenic bulk gas systems are the most economical, but require more space and supplementary equipment, so they may not be suitable for small or midsized manufacturing operations. Microbulk systems are the next best thing, as they still offer the economy of bulk tanks but on a smaller scale.
Another advantage of cryogenic systems is that they eliminate contamination and labor that come with refilling gas tanks.
For shop air setups, manufacturers typically use compressor/filtration systems to serve their lasers. Lasers that use shop air should be housed in a clean area to avoid contamination and moisture.
Ultimately, manufacturers need to assess their consumption and application needs before choosing a gas storage and delivery method. It’s important to ensure that the system can deliver the proper pressure and flow rates. Finding a reliable gas supplier is also crucial.
Technologies now exist to help manufacturers manage their assist gas inventories and avoid interruptions in supply. Transmitter systems can measure gas tank levels on a regular basis and send the information to the gas vendor to ensure perfectly timed deliveries. Other control technologies can automatically monitor and optimize pressure and flow levels to ensure the best cutting performance.
Manufacturers should speak with their gas suppliers to develop the best possible assist gas system.
Assist gas is an important consumable in the laser cutting process — one that manufacturers often neglect to optimize.
If not considered carefully, assist gas setups can cost businesses thousands of extra dollars in equipment and gas supply, not to mention the cost of inferior cuts caused by faulty assist gas systems. On the other hand, with the ideal combination of the appropriate assist gas, delivery, and storage method and pressure and flow management, laser cutting operations can become more productive and profitable than manufacturers may have thought possible.
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