Since the laser power density has a great influence on the cutting speed, the choice 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 small, and the power density at the focus is high, which is good for material cutting. However, its shortcoming is that the depth of focus is very short and the adjustment margin is adjusted. Small, generally suitable for high-speed cutting of thin materials. Since the telephoto long lens has a wide focal depth, it is 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 to the surface of the workpiece is particularly important to ensure the quality of the cut. Due to the high power density at the focus, in most cases, the focus position at the time of cutting is just at the surface of the workpiece, or slightly below the surface. Ensuring a constant relative position of the focus to the workpiece throughout the cutting process is an important condition for obtaining a stable cutting quality. Sometimes, the lens is heated due to poor cooling and causes a change in focal length, which requires timely adjustment of the focus position.
When the focus is in a better position, the slit is smaller and the efficiency is higher, and the 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 surface of the workpiece is generally about 1.5 mm.