Maintaining precision cutting is precisely the goal
July 8, 2011
Precise assist gas delivery is essential in CO2 laser cutting. The resonator is a key component, especially if gas delivery will be uninterrupted for unattended machine operation. Assist gas volume and pressure also must be appropriate for the application to ensure the best results.
Whether they’ve been through the challenge before or are addressing it for the first time, gas distributors and laser owners setting up new or second-hand cutting lasers all have one thing in common: They need precision.
Installing a cutting laser can be a complicated undertaking, especially if the system has material handling capability and will run unattended. With the more common CO2 cutting lasers (see Figure 1), it is important to keep in mind that half of the laser is a spec gas application, the other half is closer to an industrial application, and each has distinct requirements.
Let’s start the virtual checklist by breaking down the CO2 laser into its two main working parts.
1. Resonator. The resonator is the part of the laser that creates the cutting beam. It requires high-purity (grade 5 or better) CO2, nitrogen, and helium. Carbon monoxide sometimes is added to the resonator gas mix, depending on the resonator manufacturer. Some laser manufacturers have developed resonators that use such small amounts of these gases that they come supplied with the machine and do not require replacing for a year or more.
During new-cylinder hookup, inlet purging of resonator gases must take place to remove ambient air, moisture, and contamination before gas can enter the resonator (see Figure 2).
2. Cutting Head. The cutting head focuses the laser beam to the cutting surface of the material. It requires nitrogen or oxygen assist gas, from industrial to grade 4.5.
Pressure requirements can be from 100 to 400 PSI for oxygen and 100 to 500 PSI for nitrogen. Flow requirements at the back of the laser are 100 to 1,000 SCFH for oxygen and 500 to 3,000 SCFH for nitrogen. The installation manual typically indicates the gas pressure and flow requirements at the back of the laser for the maximum cut the machine can make.
It is worth noting that 80 percent of the people who purchase a laser cutting machine buy a second unit within the next 12 months. Typically, the learning curve dictates that laser owners start out with one job or parameter and evolve through the machine’s capabilities until they reach its cutting limit.
Though this is not always the case, “loaded for bear and shooting squirrel” is better than the other way around. Most laser owners start running as many jobs as possible through the machine, typically finding the upper limit of the laser rather quickly.
The type of resonator gas system required depends on whether the laser will have any material handling capability.
Those with a material handling system typically run the laser unattended, which requires an uninterrupted gas supply. Most commercial cutting lasers use either three separate resonator gas components or a single cylinder of premixed gases. A continuous supply of gases is needed or the laser will turn itself off when the gas runs out.
Whether that uninterrupted gas is supplied by a switchover system (see Figure 3) or by regulators on the cylinders, contamination is a critical issue. Again, inlet and line purging of resonator gas and systems is crucial to remove all ambient air before the resonator gas enters the back of the laser.
Purging is essential because of the resonator’s components, particularly the gold-plated mirrors in its chamber that bounce the beam back and forth within the chamber before the beam is directed to the cutting head. If moisture or particulate contaminants enter the resonator chamber with the gases, these impurities will get burned into the surface of the gold mirror as it reflects the laser beam back and forth, destroying the mirror.
Mirror destruction typically is indicated by poor cut quality and high power consumption, and it is not covered under warranty. Mirror replacement can cost $300 to $500, and a typical laser resonator has up to 10 mirrors. Labor costs can be $1,500 for the service technician, not to mention the lost production time.
Two other factors often ignored in the resonator gas system setup are relief valves to protect against a regulator failure and inline filters to ensure clean, uncontaminated gas. Most laser manufacturers recommend the use of 2-micron inline filters and appropriate relief valves installed in the line between the outlet of the resonator gas switchover or regulator and the back of the laser. Some manufacturers require these installations for warranty coverage, but regardless, both are good ideas for laser protection.
Failure to provide the correct assist gas pressure and flow to the back of the laser will cause it to shut down. Therefore, liquid cylinders or a bulk tank typically are used to maintain the high flow requirements for the gases.
A standard liquid cylinder of oxygen or nitrogen is rated at 200 to 300 SCFH for continuous gas use. Flow rates of 350 SCFH can be achieved for short periods. Laser-style liquid cylinders deliver up to 500 PSI and 1,000 SCFH of gas.
When gas flow requirements are higher, one option is to push the liquid out of the cylinder into a freestanding vaporizer to increase the gas volume. Another is to employ a bulk tank to source the gases. Inline filters and relief valves typically are recommended for these applications as well.
In considering gas devices for laser cutting, fabricators should remember that the whole is equal to the sum of its many diverse systems and equipment. Especially important throughout is the need for precision to ensure accuracy in jobs large and small.