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Technology

Laser Cutting Systems for Dieboards

by Mike Adams, Adams Technologies, Inc.

November-December, 2013

When looking at dieboard laser components, the four main components are the laser source (also sometimes called laser resonator), the beam delivery (the path the laser beam follows from the laser source to the work), movement (the method of moving the laser, the beam or the work piece) and the controller (the computer that controls the laser and the movement).

Each of those components is critical to being able to maintain the kerf width.

Beam delivery and movements

The beam delivery and movement are tied together, and the four different types are moving material/fixed beam, flying optics, moving laser and hybrid. Each beam delivery movement combination has advantages and disadvantages.

Moving Material/Fixed Beam. In the case of moving material/fixed beam, the laser beam is a constant length and is stationary, while the material moves underneath the beam. Typically, this combination utilizes a mirror at the top of the L-shaped configuration and then a lens toward the bottom. This is the gold standard in dieboard lasers as it produces the highest quality kerf. There are fewer optical components Ė as few as one mirror and one lens. A constant beam length allows for a sealed and pressurized optical path. This pressurized optical path means that dust doesnít get on the optical components, which can degrade the laser quality and the laser cutting resolution. The movement also typically lasts 10-15 years with routine maintenance. However, it also has a larger footprint than some other configurations. The maximum speed and acceleration can be slower than other configurations and it is more expensive to manufacture.

Flying Optics. With flying optics, a series of mirrors bounce the laser light to the desired location and the beam length is varied. This type of beam delivery movement system is cheaper to manufacture than other configurations, while offering higher accelerations, higher peak velocities and a smaller footprint.

However, if the beam length changes, then itís difficult to control the width of the kerf, causing the width of the kerf to vary across the workpiece. The optical path canít be sealed and pressurized, which results in dirty mirrors and lenses. As smoke and dust foul the optics, divergence increases, which leads to the degradation of kerf quality over time. In addition, as the optical components move, alignment of the beam becomes more difficult over time and in order to get a perfect kerf, the laser beam must be kept perfectly centered in the optics.

Moving laser. In this configuration, the laser source is mounted on a gantry system. The laser moves in the X and Y axis, and the material stays stationary. With this type of system, a constant beam length is achieved, it has a smaller footprint and it allows for a sealed and pressurized optical path (keeping the lens and mirror cleaner). The disadvantages are that the laser resonator is a precision instrument. The acceleration/deceleration stresses imposed on the resonator may result in more maintenance. Obviously, positional accuracy can suffer, as a lot of weight is moving around on the gantry. Finally, system integrators are limited in the type of laser resonator that can be used because the resonator must be light enough to be carried by the gantry or the gantry has to be huge.

Hybrid movements. Hybrid movements are a little bit of both. In one direction, the material may move, and in the other direction, either the laser moves or the beam is moved with flying optics. Advantages include the following: if the laser is moved in the Y axis, a constant beam length is achieved. These systems have a smaller footprint and allow for a sealed and pressurized optical path. Disadvantages include the following: if flying optics are used, the beam length changes as the system cuts in the Y axis, which causes the width of the kerf to vary. Also, it is difficult, if not impossible, to seal and pressurize the optical path. This causes fouled optics, which results in increased divergence.

A note on dieboard lasers: The reason the kerf width varies when using a laser is because the laser beam diverges; that means that when using the laser, a diameter of beam is emitted from the laser. For example, that diameter in most fast axial flow lasers is somewhere in the 20mm to 25mm range, or about one inch in diameter. With a constant beam length, youíve got a kerf that is constant. But, as you get further away from the laser resonator, that diameter increases (see Diagram 1). Thatís the reason flying optics makes it difficult to keep kerf width consistent.

Movement systems

For moving lasers, the key factors are the type and weight of the laser resonator (and how well it stands up to the acceleration/deceleration); the controller (and how well it controls the laser while the laser is experiencing this acceleration/deceleration); and the movement (and how well it stands up to the forces imposed by moving the laser).

The different components of a movement system are ball screw, lead screw, rack and pinion and either a moving belt or moving cables. But very, very few lasers of any brand use cables or belts.

The movement in most fixed beam/moving material systems is servo motors and ball screws. This type of movement has been around the industry for a long time, and itís been known to last 10 to 15 years before needing significant maintenance. The moving part of the mechanism is the ball nut. It contains ball bearings that ensure minimum friction in the movement, which results in minimum wear.

A cheaper alternative to the ball screw is the lead screw. The mechanism is similar to the ball screw, but the nut that rides in the threads of the screw has no ball bearings. Instead, the nut typically is made of a low-friction device or a low-friction plastic or polymer, like nylon, or it may be made of a low-friction composite material. Lead screw assemblies are much cheaper than ball screws; however, they require more maintenance. Some lead screws are Teflon-coated. This coating wears as well, and then the screw itself must be replaced, not just the following nut. Lead screw nuts and screws mate with rubbing surfaces, and consequently they have a relatively high friction and stiction compared to mechanical parts, which mate with rolling surfaces and bearings. Efficiency typically is only between 25 percent and 70 percent, with higher pitch screws tending to be more efficient.

Even cheaper alternatives are the rack and pinion drives typically used in light duty applications like X, Y plotting tables or samplemakers (see Diagram 2). The rack and pinion system is found in the steering mechanism of a lot of automobiles. Unlike ball screws, even with a new rack and pinion system, there is backlash unless an anti-backlash modification is used. Some of those include using plastic pinion gears that will compress as they go into the rack. Another option is to split the gear and spring and load it in two different directions. As an example of backlash, letís say you move 10" (254mm) in one direction and then go back the other direction. When you go back, there is a little bit of slop in the movement.

When it comes to accuracy, Iíve seen one system in which the accuracy specification on the rack and pinion system was .002" (.0508mm) per 12 inches (304.8mm) of travel. That sounds sufficient, but if you consider this over the entire bed of the laser, if itís a 60" x 96" (1,524mm x 2,438.4mm) laser, youíve got diagonally 113.2" (2,875.28mm). That means the error would be around .018" (.4572mm). Thatís almost twice the acceptable plus or minus .010" (.254mm) thatís demanded within the diemaking industry. So as far as movements go, while there are alternatives to the fixed beam ball screw-driven moving beds system, for dieboard lasers, this approach has been and continues to be the gold standard by which all others are judged.

Laser sources

There are several types of laser sources as well. L.A.S.E.R. actually stands for Light Amplification by Stimulated Emission of Radiation. A CO2 laser is a device that amplifies light (electromagnetic radiation) through a process stimulating carbon dioxide molecules. Laser light is notable for its narrow beam and single wavelength or color that canít be achieved using other technologies.

Types of CO2 resonators

In dieboard lasers, the three types most commonly used are slow axial flow, fast axial flow and slab sealed-off/semi-sealed.

Most slow axial flow designs fold the beam into a shorter working length using mirrors, but the design essentially is a long tube with mirrors at each end. Laser gas goes in one end, and there's a vacuum pump on the other end. The reason the gas has to be vacuumed away is because when working with the laser gases, they are excited (brought to a higher energy level). When that happens, CO2 disassociates into CO and 02. That's carbon monoxide and oxygen. Wherever it disassociates, you lose laser power, and you have to either get rid of that gas or somehow recombine the gases.

Slow axial flow lasers are limited in their wattage. Essentially, it is a function of the length of the tube. The folding mirrors on the slow axial flow are needed because in order to get higher wattage, you have to fold the beam or its going to be the length of the room. And the maximum that you are going to get from a slow axial flow laser is about 1,500 watts. If you want to translate the formula for the speed of cutting, youíve got about 1mm per watt per minute. These lasers are cooled by convection.

Fast axial flow lasers cool the gas by recirculating the gas through a heat exchanger. These resonators recirculate the gas at a high rate of speed in order to keep the gas cooler. The cooler it is, the less the disassociation into CO and O occurs.

For fast axial flow, the maximum speed of cutting is in the tens of thousands of watts. You can get into 20,000 to 30,000 watts of power, but in most cases for dieboard the maximum is about 3,000, because it becomes very difficult to control the kerf at higher wattages.

For both the fast flow and the slow flow, a laser gas must be provided that consists of helium, nitrogen and CO2. As a matter of fact, even though it is called a CO2 laser, the CO2 is the smallest percentage of the gas, and that is what actually does the generation of the light (electromagnetic radiation).

The slab is a relatively new laser that uses resonating frequency excitation. The gases have to be excited somehow. With the slow and the fast axial flow types, the power supply usually is a high-voltage power supply, with electrical current running through the gas to excite it. In the case of the slab, resonating frequency, basically like a radio beam, is used to excite the gas. There are two different types of slab: sealed or semi-sealed.

One advantage of the slab laser is lower maintenance. It does not use as much gas. If it is completely sealed, then it uses no gas. If itís semi-sealed, it uses a small amount of gas every year.

A disadvantage is that the slab laser produces a square beam. It must go through a beam shaper to produce a round beam. That beam shaping is not perfect, but can be compensated by the controller. As a matter of fact, even when you are working with the axial flow lasers, the beam itself is not perfectly round but is elliptical, and that has to be compensated by the controller as well. So the beam shaping is a little more difficult with the slab laser.

A big disadvantage of the slab lasers is that it can be very difficult, if not impossible, to get field service. That means that when a slab laser gets contaminated, in many cases the entire laser has to be shipped back to the factory for recharging. Axial flow lasers, whether they are slow or fast, can be field serviced.

As far as laser sources go, the right one is really up to you. One may have a lower cost of operation, but a higher cost of maintenance. It may be important to you to have a source that is field serviceable; on the other hand whatever your reason, the important thing is that everyone make informed decisions about these laser resonators.

Dieboard laser controllers

The fourth component is the dieboard laser controller. Laser controllers in general have matured along with the CNC industry. Many of these general purpose controllers are used to control CNC machine tools like routers or milling machines, waterjets and anything needing an XY and possibly Z movement.

Some diemaking operations have purchased lasers with general purpose controllers only to find that they cannot control the cutting width Ė the kerf Ė adequately. This is one essential element of a true board laser Ė a controller that can properly control kerf width. Dieboard laser controllers must control acceleration, cutting speed and deceleration, while at the same time controlling laser power to keep the kerf width constant. Itís not a trivial task. At the same time, the controller must adjust focal height for different pointages or even different directions of cut.

When cutting dieboard in particular, an operator typically will cut a wider kerf in one direction than the other if the operator does not compensate for the fact that the grain of the dieboard will affect the kerf width. In general, the kerf width at the bottom of the board is controlled by speed and power, and the kerf width at the top of the board is controlled by focus. Even with a good controller, dieboard lasers require skilled operators. In Diagram 3, the laser operator would test the board and determine that kerf #1 is correct at the top but wide at the bottom, so the operator would increase the speed. On kerf #2, it is correct at the bottom, but wide at the top. The operator would adjust the focus. For Kerf #3, itís correct at the bottom, but too narrow at the top. The operator would adjust the focus to make the entry wider. And so on and so forth. An operator must control speed and focus to get the correct kerf width.

When choosing a dieboard laser, evaluate each of those components to decide which system is right for you.

Mike Adams is president of Adams Technologies. Inc., a provider of automation technologies to the diemaking and diecutting industry and whose philosophy is to develop and seek out products that create new solutions to existing problems; to be innovators and not imitators. Contact Adams at 303.798.7710 or via email at mike@adamstech.com or visit the companyís website at www.adamstech.com.

This portion of a more extensive article was reprinted with permission from The Cutting Edge and was adapted from a presentation at the 2011 IADDēFSEA Odyssey.