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Laser Cutting Machines Courtesy: GSI Lasers Laser Cutting Basics Laser cutting is the process of using a laser to cut materials, usually in industrial manufacturing. Laser cutting works by directing the output of a high power laser, and focussing it to a small spot on the material to be cut. The material then either melts or vapourises. Removal of molten material is achieved by using a relatively high pressure assist gas. As the beam moves relative to the material a cut channel (the kerf) is formed, having an edge with a high quality surface finish. Laser Cutting Market In 2008, 29% by revenue of all lasers were sold for use in Materials Processing applications, amounting to over $2 billion in systems revenue. This is by far the largest application area for non- diode lasers (which are mainly used for communications and storage applications). Of all the units sold into industrial laser applications, 22% are used for laser cutting applications, and the vast majority of these are used for the cutting of metal sheets. This is a global market, with 45% of the flat sheet cutting systems being sold into Asia, 37% to Europe and 17% to North America. Cutting Machine Basics A laser cutting system has three major sections: The Laser: The type of laser is chosen dependant on the material, its thickness and the size of the parts to be cut. For fine cutting in thin metal sheets a Fiber Laser will offer the finesse needed. In thicker metal sheets a high power fast axial flow CO2 laser will provide the speed needed for economic processing. For non-planar metal components where a robot might be employed for 3D cutting an Nd:YAG solid state laser utilising Fibre Optic Beam Delivery will be the laser of choice. For plastic, textile and wooden components a sealed CO2 laser (either DC or RF excited) will provide a very economic cutting solution. The Beam Delivery: The laser beam is transmitted to the focusing head via a series of mirrors or through an optical fibre. The focusing, or cutting head uses a lens to focus the beam down to a spot down to 10µm in case of fine Fiber Laser cutting up to a few hundred microns in the case of thicker section high power CO2 laser cutting. The focused laser beam will vaporize and or melt a small area of the work piece. A pressurized gas, or "assist gas", will be introduced beneath the lens, coaxially with the laser beam, to push the molten material out of the cut zone. The Motion system: n Various configurations are used to provide the relative motio between the focussed laser spot and the workpiece to allow the desired cut path to be followed. Overview of Industrial Laser Cutting System Types There are generally three different configurations of industrial 2D laser cutting machines based on the way that the laser beam is moved over the material to be cut or processed: Moving material, Hybrid, and Flying Optics systems. For all of these, the axes of motion are typically designated as the X and Y axes, for the long and short direction of travel respectively. If the height of the cutting head over the workpiece may be controlled, it is designated as the Z-axis. Moving material lasers have a stationary cutting head and move the material under it. This method provides a constant distance from the laser generator to the work piece and a single point from which to remove cutting effluent. It requires fewer optics, but requires moving the work piece, which can be an issue for large or heavy items. Hybrid lasers provide a table which moves in one axis (usually the X-axis) and move the head along the shorter (Y) axis. This results in a more constant beam delivery path length than a flying optic machine and may permit a simpler beam delivery system. This can result in reduced power loss in the delivery system and more capacity per watt than flying optics machines. Flying optics systems feature a stationary table and a cutting head (with laser beam) that moves over the work piece in both of the horizontal dimensions. Flying-optics cutters keep the work piece stationary during processing, and often don't require material clamping. The moving mass is constant, so dynamics aren't affected by varying size and thickness of work piece. Flying optics machines are the fastest class of machines, with higher accelerations and peak velocities than hybrid or moving material systems. Flying optic machines must use some method to take into account the changing beam length from near field (close to lase output) cutting to far field (far away from laser) cutting. Common methods for controlling this include collimation, adaptive optics or the use of a constant beam length axis. The above is written about X-Y systems for cutting flat materials. The same discussion applies to five and six-axis machines, which permit cutting formed work pieces. In addition, there are various methods of orienting the laser beam to a shaped work piece, maintaining a proper focus distance and nozzle standoff, etc. For 3D cutting applications the Process Head from a solid state Nd:YAG laser can be mounted on an industrial robot. Full flexibility in the movement of the robot is provided by using a Fibre Optic Beam Delivery system. Specific Types of Cutting Systems Flat bed plastics/textiles/wood These systems are generally of the moving optic type. They range from table-top sized systems using low power CO2 lasers, to systems with cutting area of 2 x 3m using higher powered sealed off DC CO2 lasers. The photograph shows a typical system of this type. 2D Flat Bed Metal Cutting This is the biggest sector for cutting systems with around 3000 systems being sold per year , with most of these being of the moving optic type, though some of the systems for larger sheets are of the hybrid type. The CO2 laser flat bed cutters typically use a 5” (127mm) or 7.5” (190mm) focal length lens. These produce a 0.2mm to 0.4mm spot size on the work piece. For cutting of materials more than a mm or so in thickness, focussing to a very small spot size does not help as the kerf is too small for ejection of molten material and to allow separation of the parts. Metal cutting speeds for a given laser depend on the edge quality required, and on the thickness and type of material. Typically cuts will be at a few m/min for material around 6mm thick, increasing to around 10m/min for 2mm thick steel. Most of the sheet metal applications can be addressed with a system using a 2kW to 4kW CO2 laser. Due to their better beam quality a slab laser will have a faster processing performance to a similarly powered fast axial flow laser. Interestingly the Fiber Laser is beginning to penetrate the flat sheet m cutting sector for millimetre etal type thicknesses which has long been the total preserve of CO2 lasers. Only a few systems have been installed so far this application, but it does indicate how the Fiber Laser is penetrating mature laser metal processing applications. For thinner sheets (sub-mm thickness) Fiber Lasers are well established as the laser of choice. A particular application they have adressed is cutting solder paste screens used in manufacture of printed circuit boards. Typically these screens are made from 0.2mm thick Stainless Steel. The advantages of the Fiber Laser in cutting applications are generally better efficiency, lower consumable costs and mch lower process gas cost than using a CO2 laser. The Fiber Laser is maturing rapidly as a technology for use in all aspects of metal fabrication from high finesse to thick section, and easily covers the typical job shop application areas. This shows that its transition from its scientific and low power telecoms heritage into the metal fabrication arena is real and will only strengthen in the coming years. Tube cutting systems Specialised tube cutting systems are now available for use with round, oval and rectangular section tubes. Generally they are of the Hybrid type, where the tube is rotated under the laserbeam, which t will move along the tube in the cut ing area. A special type of tube cutting machines are directed at the manufacture of medical stents used in the treatment of blocked arteries. These are made from small diameter (~1mm) tubes, either of stainless steel or Nitinol (a ‘memory metal’). They demand very high precision cutting with fine features and low levels of dross from the cut. Dedicated systems for this application provide high accuracy cutting using a fixed beam delivery point and rotating and fedding the tube longitudinally through the laser focus. Robot Cutting In the applications above the parts have an essentially flat cutting area. This does not cover all parts to be cut, and there are growing number of 3D components to be cut. In these areas the use of a robot arm to direct the output from a solid state laser fed through a fibre optic beam delivery system offers unparalled flexibility. The ability to easily integrate with existing robots is particularly attractive to the automotive industry as it allows capital investment to remain relatively low while providing versatility and redeployment for future needs. Hydroformed Cutting n Hydroformed and rectangular cross-sectio tubes have become popular for new truck and sport utility vehicle (SUV) frames, engine cradles, roof pillars, and suspension members. The ability to achieve i high stiffness, exotic shapes, and minimal weight from these structures could drive their appl cations to even more platforms. For the manufacturer, the challenge is in cutting these closed tubes with only single-sided access. In the past, the members were produced from folded and spot-welded assemblies that could be punched easily with conventional tooling as flat sheet before forming. Laser robotic cutting cells with fiber- delivered laser beams have been helpful in this area, and end users also are looking to laser robot systems for option holes, short-run production, and prototype systems in sheet metal parts. Remote Cutting A new cutting format that has become available with the advent of excellent beam quality fiber lasers with reasonable power levels, is the use of a galvanometer scan head to enable the very high speed cutting of thin metal sheets and foils over a scan area of order 100mm square. This type of application is limited to thin sheets because there is no possibility to introduce the high pressure assist gas usuallly employed in cutting operations, which is vital for clearing the dross from thicker section kerfs. Typically a Scan Head Systems has been used with lasers for marking applications, and so the scan head driver software and interfaces have been optimised for these applications. Using these applications directly in remote cutting applications does not produce the optimum cycle times, but system driver software that has knowledge of the laser performace parameters, as well as the scan head characteristics will produce the fastest processing times. Laser Glass Separation Laser Glass separation (also sometimes called Zero Width Cutting) is a special cutting technique for thin sheet glass, often used in the manufacture of LCD, TFT and PDP screens. Generally these cutting systems are of the flying optic design of 2D cutting, but have the added complication of the need for a water mist spray to be following the cutting head. Thus the cutting head arrangement must be rotated round as the direction of cutting changes.
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