CATEGORY: LASER
PUBLICATION: CIM

Give it some gas: Tips for laser cutting with assist gas

When buying and employing 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 costs of that 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 right 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, shop-floor environment and the necessary level of surface quality of the finished workpiece should influence manufacturers’ assist-gas choices.

Types of assist gasses

First, it’s important to have an understanding of the types of assist gasses and the ideal usage scenario for each type. The three primary types of assist gas used in laser cutting are oxygen, nitrogen and air.

Oxygen is the most commonly used assist gas for mild-steel cutting applications. Oxygen causes an exothermic reaction that helps provide the heat to make laser cutting possible, and is best for mild steel that is one-inch thick or less. Typically, cutting mild steel with an oxygen assist gas requires the lowest amount of pressure, usually around 28 PSI or lower, and a low flow of less than 60 SCFH, depending on the thickness of the metal. With oxygen, the thicker the metal, the less oxygen 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 and air. First, oxygen requires a much lower flow and pressure than nitrogen, meaning the consumption—and ultimately the costs—are lower. Also, at thicknesses of more than .08 inches, oxygen cuts faster.

Using oxygen as an assist gas also has its downsides—namely oxidation. The reaction from oxygen assist gas 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.

Nitrogen provides a clean, precise cut without oxidation, since it doesn’t cause a reaction to increase heat and aid cutting, but rather serves only to remove the molten metal. Nitrogen is ideal for stainless steel and aluminum, and can cut mild steel much faster than oxygen when metal thicknesses are less than .08 inches (see figure 1). While nitrogen requires higher pressure and flow than oxygen when cutting 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 with very thin metals, nitrogen is the slowest assist gas. For this reason, it is typically only used for mild-steel cutting applications in which surface quality is extremely important.

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 high as with nitrogen.

While shop air can be cheap and effective for thinner metals (it works best with mild and stainless steel less than .06 inches thick and aluminum less than .08 inches thick) establishing a system that maintains proper air pressure and air filtration can be an obstacle for many manufacturers.

The process to collect, store and deliver the 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 (or made so by a sophisticated filtration system), it can degrade the focal lens, which can lead to cutting problems or optic failure.

Efficient assist-gas setups

Choosing the ideal type of assist gas is just the beginning. For the most optimal production, manufacturers must mold assist-gas delivery and storage strategies to fit individual needs and applications.

Before a manufacturer buys his first laser machine, he should consider the gas delivery and storage setup. Manufacturers should also 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 contrary, buying a too-large bulk-gas system can result in unnecessary equipment rental fees and shop-floor space issues.

Another common issue is redundant gas equipment, which often happens when each laser has its own assist-gas supply unnecessarily. 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, micro-bulk or bulk gas systems. Cryogenic bulk gas systems are the most economical, but require more space and supplementary equipment, so they many not be right for small or mid-sized manufacturing operations. Micro-bulk systems are the next best thing, as they still offer the economy of bulk tanks on a smaller scale. Another advantage to cryogenic systems is that they eliminate contamination and labor that comes with refilling gas tanks.

For shop-air setups, manufacturers typically use compressor/filtration systems to serve their lasers. Lasers that use shop air should also 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 at 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 monitor and optimize pressure and flow levels automatically to ensure the best cutting performance. Manufacturers should speak with their gas suppliers to develop the best 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 dollars in equipment and gas-supply costs—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.