Because laser power density has great influence on cutting speed, the choice of lens focal length is an important problem. The spot size of laser beam after focusing is proportional to the focal length of lens. The spot size of laser beam after focusing by short focal length lens is smaller and the power density at the focus is higher, which is beneficial to material cutting. But its shortcoming is the focal depth is short, the adjustment allowance is small, generally is suitable for the high speed cutting thin material. As long focal length lens has wide focal depth, as long as it has enough power density, it is suitable for cutting thick workpiece.
After determining which lens length to use, the relative position of the focus and the workpiece surface is particularly important to ensure the cutting quality. Due to the high power density at the focal point, in most cases the focal point is just below or just below the surface of the workpiece when cutting. In the whole cutting process, ensuring the relative position of the focus and the workpiece is constant is an important condition to obtain stable cutting quality. Sometimes, the lens is heated due to poor cooling and thus the focal length changes, which requires timely adjustment of the focus position.
When the focus is in the right position, small slot, high efficiency, cutting speed can get good cutting results. In most applications, the beam focus is adjusted to just below the nozzle. The distance between the nozzle and the workpiece surface is generally about 1.5mm.
In the process of laser application, focusing and other problems are often encountered. There are three common simple methods to determine the focus position:
(1) printing method: make the cutting head move from top to bottom and print the laser beam on the plastic plate, with the smaller diameter as the focus.
(2) inclined plate method: use the plastic plate placed at an Angle with the vertical axis to pull it horizontally and look for the smaller part of the laser beam as the focus.
(3) blue spark method: take out the nozzle, blow the air, and hit the pulse laser on the stainless steel plate, making the cutting head move from top to bottom until the larger blue spark is the focus.