Comparison to Mechanical Cutting
The advantages of laser cutting over mechanical cutting vary according to the situation, however two important factors are
There is also a reduced chance of warping the material that is being cut, as laser systems have a smaller heat affected zone. Some materials are also very difficult or impossible to cut by more traditional means. Although a disadvantage of laser cutting includes the high energy required.
Main Types of Lasers
There are three main types of lasers used in laser cutting. The CO2 laser is suited for cutting, boring, and engraving. The neodymium (Nd) and neodymium yttrium-aluminum-garnet (Nd-YAG) lasers are identical in style and differ only in application. Nd is used for boring and where high energy but low repetitions are required. The Nd-YAG laser is used where very high power is needed and for boring and engraving. Both CO2 and Nd/ Nd-YAG lasers can be used for welding.
Common variants of CO2 lasers include fast axial flow, slow axial flow, transverse flow, and slab.
CO2 lasers are commonly “pumped” by passing a current through the gas mix (DC-excited) or using radio frequency energy (RF-excited). The RF method is newer and has become more popular. Since DC designs require electrodes inside the cavity, they can encounter electrode erosion and plating of electrode material on glassware and optics. Since RF resonators have external electrodes they are not prone to those problems.
In addition to the power source, the type of gas flow can affect performance as well. In a fast axial flow resonator, the mixture of carbon dioxide, helium and nitrogen is circulated at high velocity by a turbine or blower. Transverse flow lasers circulate the gas mix at a lower velocity, requiring a simpler blower. Slab or diffusion cooled resonators have a static gas field that requires no pressurization or glassware, leading to savings on replacement turbines and glassware.
|Nd||High energy pulses
Low repetition speed (1kHz)
|Nd-YAG||Very high energy pulses
Boring Engraving Trimming
Generation of the laser beam involves stimulating a lasing material by electrical discharges or lamps within a closed container. As the lasing material is stimulated, the beam is reflected internally by means of a partial mirror, until it achieves sufficient energy to escape as a stream of monochromatic coherent light. The coherent light then passes through a lens that focuses the light into a highly intensified beam generally less than 0.0125 in (0.3175 mm). in diameter. Depending upon material thickness, kerf widths as small as 0.004 in (0.1016 mm) are possible. In order to be able to start cutting from somewhere else than the edge, a pierce is done before every cut. Piercing usually involves a high power pulsed laser beam which slowly (taking around 5–15 seconds for half-inch thick stainless steel, for example) makes a hole in the material.
There are many different methods in cutting using lasers, with different types used to cut different material. Some of the methods are vaporization, melt and blow, melt blow and burn, thermal stress cracking, scribing, cold cutting and burning stabilized laser cutting.
The parallel rays of coherent light from the laser source may be 1/16 in. to 1/2 in. (1.5875mm to 12.7mm) in diameter. This beam is normally focused and intensified by a lens or a mirror to a very small spot of about 0.001 in. (0.0254mm) to create a very intense laser beam. Recent investigations reveal that the laser beam has a distinctive polarization. In order to achieve the smoothest possible finish during contour cutting, the direction of polarization must be rotated as it goes around the periphery of a contoured work piece. For sheet metal cutting, the focal length is usually between 1.5 in. and 3 in. (38.1mm and 76.2mm).]
Setup and Equipment
The laser machining system consists of a power supply for producing a laser beam (Power requirements below), a workpiece positioning table, laser material, a method of stimulation, mirrors, and a focusing lens. The workpiece is held stationary by clamps, straps, hold down tabs, pressure blocks, positioning tabs, magnets, or suction cups. The focusing unit moves around the workpiece to cut the desired shape.
There are generally three different configurations of industrial laser cutting machines: Moving material, Hybrid, and Flying Optics systems. These refer to way that the laser beam is moved over the material to be cut or processed. For all of these, the axes of motion are typically designated X and Y axis. If the cutting head 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 workpiece and a single point from which to remove cutting effluent. It requires fewer optics, but requires moving the workpiece.
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 lasers 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 workpiece 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 workpiece. Flying optics machines are the fastest class of machines, with higher accelerations and peak velocities than hybrid or moving material systems.
Pulsed lasers which provide a high power burst of energy for a short period are very effective in some laser cutting processes, particularly for piercing, or when very small holes or very low cutting speeds are required, since if a constant laser beam were used, the heat could reach the point of melting the whole piece being cut.
Most industrial lasers have the ability to pulse or cut CW (Continuous Wave) under NC program control.
Double pulse lasers use a series of pulse pairs to improve material removal rate and hole quality. Essentially, the first pulse removes material from the surface and the second prevents the ejecta from adhering to the side of the hole or cut.
Effects on Work Material Properties
The effects on the workpiece materials is rather minimal due to the small zone of metal affected by the laser beam. However, the effects are due to the high temperature of the laser that change the hardness and the creation of a narrow heat-affected zone.
|Work material properties||Effects of laser beam cutting|
|Mechanical||May affect hardness
Narrow heat-affected zone
|Physical||Grain size may change|
It is necessary to keep the medium that generates the laser, and the lens cool to a safe working temperature. In both cases the cooling is done by water. In each case, water is constantly pumped around the heated object. By so doing, the heated water is pushed out and recycled for another use.
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