Laser cutting explained...
Laser cutting is one of the largest applications of lasers in metal working industry. It is based on the precise sheets cutting by focused laser beam. The laser beam is a new universal cutting tool able to cut almost all known materials.
Laser cutting is the process of vaporizing material in a very small, well defined area. Laser cutting process utilizes coherent light developed within an optical resonator cavity. An electrical discharge, through premixed He, N2, CO2 causes photon emission between reflecting mirrors mounted perpendicular to axis of the resonator.
Beams exit a 50 percent reflecting, 50 percent transmitting mirror at one end of the resonator cavity. The photon energy derived from the excited CO2 molecule has a 10,6 |im wave length (infrared), phase aligned to permit focusing through processing optics. The output beam typically focused to a spot diameter of 0,1 mm using a 63,5 mm focal length lens. Cutting temperature is approximately 18000 F and energy concentration is 10 MW/cm2 (enough to vaporize most materials).
However, 90 percent of the light beam is reflected from the surface of steel, necessitating an oxygen assist. Cutting speed ranges from 2 to 10 m/min for laser power 1,5 kW.
The laser beam' effect upon a workpiece material can be divided into several characteristic phases:
Absorption of the laser radiation in the workpiece surface layer and transformation of the light energy into the heat one,
Heating of the workpiece surface layer at the place subjected to the laser beam,
Melting and evaporation of the workpiece material,
Removal of the break-up products, and,
Workpiece cooling after the completion of the laser beam' effect.
Part of the laser beam power is lost due to its passing through the workpiece, by the molten material and process gas. Still, its greatest part is absorbed and used for inducing melting and evaporation of the material at the cut point. The absorbed energy quantity is mostly depending on thermal and physical properties of the workpiece material; the choice of material also depends upon them.
Absorption is essential only at the first moment of the interaction between the laser beam and the workpiece material. Later on, heat diffusion is of crucial influence. In cutting, the aim is to vaporize the material as quickly as possible and to produce as narrow a heat-affected zone as possible with minimum distortion of the workpiece. Most industrial laser cutters employ a gas stream coaxial with the laser beam.
The gas stream helps to remove molten material from the region of the cut. In the laser cutting operation, in addition to the heat obtained by focusing the laser beam, the process gas is used for removing the molten material from the cutting zone, to protect the lenses from evaporation and to aid the burning process. The useful power can be increased in the case that the process gas is oxygen due to the exothermic reaction. The gas blowing increases the feed rate for as much as 40%.
By combining the laser as the light radiation source and the machine providing motion, in addition to the applied numerically controlled system, it is possible to provide for a continual sheet cutting along the predetermined contour. The laser is a cutting tool has been successfully applied to a large number of materials. Since the laser beam exerts no force on the part and is a very small spot, the technology is well suited to fabricating high occuracy parts, especially flexible materials. The materials which are currently being cut by lasers include: metals, wood, cardboard, fabrics, plastics, composites, ceramics, glasses and quartz. The part keeps its original shape from start to finish Laser cutting systems have enjoyed the reputation of producing parts that are below 0,1 mm tolerance with narrow heat affected zones and cutting speeds to 5 m/min and greater in gauge thickness materials.
The downside of laser technology was it's relatively high capital cost. While current developments in laser technology and motion systems have not reduced the capital cost, capabilities have improved in areas of system reliability, speed, material thickness capacity and even tighter tolerances in finished parts. In industrial application nowadays various types and constructions of laser cutting machines can be met. By contour cutting two dimensional formed thin sheet workpieces the use of machines with X-Y table coordinate is effective and real when CNC control unit is used for control. By contour cutting three dimensional formed thin sheet workpieces the laser robots are often used. Technological problems related to the application of laser machines to continual sheet cutting are in insufficient knowledge of the laser technique application as well as due to absence of sufficiently reliable practical data and knowledge about the parameters influencing the work process itself.