Metal cutting basics
The number and speed of technological developments in laser metal cutting has increased dramatically in recent years as this application has gained popularity globally.
Laser cutting offers a cost-effective way to produce parts from sheet metal and plate
Editor’s note: ILS recognizes that some readers are new to laser materials processing. Therefore, over the coming months we will periodically publish basic technology features. This, the first of the series, is chosen because laser metal cutting represents the largest portion of annual laser systems revenues, making it one of the most important processing sectors for industrial laser technology.-DAB
by Erin Chasse and James Rogowski
The number and speed of technological developments in laser metal cutting has increased dramatically in recent years as this application has gained popularity globally. Manufacturers are continually looking for different ways to produce parts in the most cost-effective and time-efficient manner. In response, the suppliers of laser cutting machines have been making this happen with advancements resulting in system and process flexibility and improved production capabilities.
Manufacturers typically use a CO2 (gas) laser resonator ranging in output power from 1.5 to 6 kilowatts as a laser source for the cutting workstations. A CO2 resonator enables cost-effective manufacturing of parts and is the reason why so many manufacturers turn to laser machines to process material.
The first step in processing material is creating a nest of parts and generating a cutting code so that the machine knows what and where to cut. Most laser manufacturers, and many third party programming software companies, offer software programs for laser machines that can create programs off-line away from the machine to prevent production disruptions. Once completed, plans for production (over a shift or an entire day), consisting of many code programs, can be sent directly to the machine via a network drive. This enables designers to optimize the cutting path and material utilization while the machine operator is cutting other jobs. Both the programmer and machine operator can focus on their respective details. Smaller shops sometimes rely on a single person to perform both duties. This can be done reliably and efficiently as long as the person has enough time for each function.
Manually loading and unloading the machine and then sorting a large number of finished parts can consume a considerable amount of time in a typical job cycle. Automating such procedures can create additional flexibility and productivity in the manufacturing process and reduces downtime.
Most laser cutting machines have a basic pallet changer to introduce raw material to the machine while a second pallet is processing a prior job. A basic pallet changer can be important in reducing the time in-between cutting, unless the laser cutting machine is used for R&D purposes or in other situations in which production is not required.
Laser cutting is a cost-effective way to produce parts out of sheet metal.
Another level of automation may be a lifting device to automatically place raw material and remove finished parts and skeleton from the pallet changing system. This gives the user the ability to run unmanned and even “lights out” in some instances. Here, typically, the operator is then responsible for supplying a raw material stack to the machine and then sorting finished parts from scrap skeletons in the processed material stack.
Beyond rudimentary automation, complete systems can be built to individual specifications. Such systems may include integrated material storage towers (limited only by ceiling height), multi-cart systems to deliver raw material to the laser cutting machine and other types of machinery, and automatic loading devices that have the ability to sort and stack finished parts.
To maintain critical inventory levels of raw material, some manufacturers also use automatic weighing scales. In other systems, carts literally cross the factory floor to deliver parts to secondary operations (for example, press brakes). Some carts even have built-in sensors to detect objects in their path, transforming them into safe, reliable, self-guided fork trucks without the cost or manpower needed for operation.
Piercing is a very important, but often overlooked, part of laser cutting. The speed and quality of the part and nest produced can be directly affected by the piercing process.
Processing time can be significantly decreased when a machine has the ability to control two factors in piercing: power control and perspective. First, lasers are now able to drill through the material much quicker than older technology by automatically adjusting the power and frequency hundreds of times each second during a piercing cycle. Piercing is a complex cycle that requires further explanation and consequently it will be the subject of a future article in ILS. This factor may be one of the most important advancements in laser processing in the last five years.
Second, laser systems can now optically detect when the material has been successfully pierced, which allows the machine to begin cutting immediately after the pierce. Because the amount of energy introduced into the material is controlled, the quality is better, providing a smaller pierce, and the cutting is faster-cooling the material after the pierce is eliminated because the energy is controlled.
Another advanced piercing technique is piercing on the fly. These pierces are done while the cutting head is in motion when cutting thin gauge materials to increase processing speeds.
Many laser machines have built-in cutting programs to efficiently and easily process material, even by beginner laser operators. They offer proven processing techniques and data to eliminate heat distortion and increase cutting speeds.
Many modern laser cutting machines have “flying optics” where the material is stationary and the cutting head moves rapidly over the material. This enables the machine to accelerate to high cutting speeds with very consistent accuracy allowing high-speed cutting of thinner materials and reliable cutting of thicker materials. Power can now be controlled dynamically with the drive system in order to cut small contours and sharp corners for optimum quality. While cutting, a height sensing module automatically regulates the distance from the workpiece to the cutting head to maintain the distance so that operators can look away or work on other tasks without the risk of losing the laser cut due to a warped piece of material.
New cutting techniques are generated as the machines grow increasingly more capable and more powerful. One very promising technique is cutting with shop (compressed) air. In particular, the cutting of aluminum and stainless steel with compressed air as an assist gas has increased both the capacity and speed on each material. Perhaps the greatest benefit of compressed air cutting is the cost savings over bulk and bottled nitrogen. Compressed air cutting produces finishes with an edge quality similar to that of nitrogen cutting, but the only costs are the electricity to compress it and filter to keep the air clean.
For those not ready to delve into the advanced techniques of laser cutting, basic laser cutting technology still exists. Oxygen and nitrogen assist gas technical data appears on nearly every laser cutting machine to provide established parameters for cutting.
Laser manufacturers use different technology for the CO2 resonators on their cutting machines. Additionally, some manufacturers produce their own laser resonators and others purchase them as a component. Single-source suppliers, who manufacture both the laser resonator and machine, offer the advantages of greater quality and control over the direction of the laser and its communications to and from the machine. Vendor-supplied resonators can sometimes reduce the overall cost of the machine tool.
Most laser cutting machines employ a fast-axial-flow resonator where a turbine moves gasses quickly through an area where light (photons) is created. This light then passes through a series of optics to produce a beam. Designs may be similar, but maintenance requirements can vary significantly. Most resonators require an optical cleaning anywhere between 1000 hours and 10,000 hours. Because these requirements can vary, it is a good topic to discuss when investigating options. Different variables, such as gas purity and excitation methods, can determine optic maintenance. For example, any foreign particles that are introduced internally to the resonator will decrease the efficiency of the mirrors and thus the power of the laser beam. Turbine quality will also affect optic life. Almost all turbines run on a lubricated bearing system, which will decrease optic life if lubrication material makes its way to the internal resonator system.
Another type of resonator is a diffusion-cooled resonator, which operates without the turbine used in fast axial flow resonators. Typically diffusion-cooled resonators have lower maintenance requirements because they have fewer internal moving parts and do not have a turbine. While these resonators may have a maximum of 3.5 kilowatts, they offer a very high beam quality that lends itself to processing thinner materials very quickly. They can reduce the hourly cost (cost per part) because of the simple and efficient design.
Laser cutting has evolved over the past decade and has proven to be a cost-effective way to produce parts out of sheet metal and plate. As technology in the laser machine increases, it will add to its capacity, ease of use, increase in speed, and as an end result increase in your profits.
Erin Chasse and James Rogowski are with the laser cutting and automation group at TRUMPF Inc., Farmington, CT, www.us.trumpf.com.