Because the laser power density has a great influence on the cutting speed, the selection of lens focal length is an important issue. After the laser beam is focused, the spot size is proportional to the focal length of the lens. After the beam is focused by the short focal length lens, the spot size is very small, and the power density at the focal point is very high, which is favorable for material cutting; But its disadvantage is that the depth of focus is very short and the adjustment margin is small, which is generally suitable for high-speed cutting thin materials. As long focal length lens has wide focal depth, it is more suitable for cutting thick workpieces as long as it has sufficient power density.
After determining which focal length lens to use, the relative position of the focus and the workpiece surface is particularly important to ensure the cutting quality. Because of the high power density at the focus, in most cases, the focus position is just on the workpiece surface or slightly below the surface when cutting. In the whole cutting process, it is an important condition to ensure that the relative position between the focus and the workpiece is constant to obtain stable cutting quality. Sometimes, the lens is heated due to poor cooling during operation, resulting in a change in focal length, which requires timely adjustment of the focus position.
When the focus is in a better position, the kerf is smaller, the efficiency is higher, and better cutting speed can obtain better cutting results. In most applications, the beam focus is adjusted just below the nozzle. The distance between the nozzle and the workpiece surface is generally about 1.5mm.
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