In the laser cutting machine, several key technologies must be mastered and solved for parts with high cutting accuracy or large thickness. Focus position control is one of them. One of the advantages of laser cutting is that the energy density of the beam is high, so the diameter of the focus spot should be as small as possible to produce a narrow slit. Because the smaller the focal depth of the focus lens, the smaller the focal spot diameter. For high-quality cutting, the effective focal depth is also related to the lens diameter and the material to be cut. Therefore, it is important to control the position of the focus relative to the surface of the material to be cut.
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 laser beam is focused by a short focal length lens, the spot size is very small, and the power density at the focal point is very high, which is very beneficial to material cutting; However, its disadvantages are that the depth of focus is very short and the adjustment margin is small. It is generally suitable for high-speed cutting of thin materials. Because the long focal length lens has a wide focal depth, as long as it has sufficient power density, it is more suitable for cutting thick workpiece.
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. Due to the high power density at the focus, in most cases, the focus position is just on the surface of the workpiece or slightly below the surface when cutting. During the whole cutting process, it is an important condition to ensure that the relative position of 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 changes in focal length, which requires timely adjustment of the focus position.
When the focus is in the proper position, the cutting seam is small, the efficiency is high, and the cutting speed can obtain good cutting results. In most applications, the beam focus is adjusted to just below the nozzle. The distance between nozzle and workpiece surface is generally about 1.5mm.
In the process of laser application, focus and other problems are often encountered. There are three simple methods to determine the focus position:
(1) Printing method: make the cutting head move from top to bottom, and carry out laser beam printing on the plastic plate, with the smaller printing diameter as the focus.
(2) Oblique plate method: use the plastic plate placed at an angle with the vertical axis to pull it horizontally, and find the smaller part of the laser beam as the focus.
(3) Blue spark method: remove the nozzle, blow air, hit the pulse laser on the stainless steel plate, make the cutting head move from top to bottom, until the larger blue spark is the focus.
