In CO2 laser cutting machines, for parts with high cutting accuracy or large thickness, several key technologies must be mastered and solved. Focus position control technology is one of them. One of the advantages of laser cutting is the high energy density of the beam, so the diameter of the focal spot should be as small as possible to create a narrow slit. Because the smaller the depth of field of the focusing lens is, the smaller the diameter of the focal spot will be. For high-quality cutting, the effective depth of field is also related to the diameter of the lens and the material being cut. Therefore, it is very important to control the position of the focus relative to the surface of the material being cut.
Since the laser power density has a significant influence on the cutting speed, the selection of the lens focal length is an important issue. The spot size of the laser beam after focusing is proportional to the focal length of the lens. After the beam is focused by a short-focal-long lens, the spot size is very small and the power density at the focus is very high, which is very beneficial for material cutting. However, its drawback is that the depth of focus is very short and the adjustment margin is small. Generally, it is more suitable for high-speed cutting of thin materials. Due to the wide depth of field of long focal length lenses, as long as there is sufficient power density, they are more suitable for cutting thick workpieces.
After determining the focal length of the lens to be used, the relative position of the focus to the workpiece surface is particularly important for ensuring the cutting quality. Due to the highest power density at the focal point, in most cases, when cutting, the focal point position is just on the surface of the workpiece or slightly below the surface. Throughout the entire cutting process, ensuring that the relative position of the focus and the workpiece remains constant is an important condition for achieving stable cutting quality. Sometimes, during operation, the lens gets heated due to poor cooling, causing changes in focal length. In such cases, it is necessary to adjust the focal position in a timely manner.
When the focus is at the optimal position, the slit is the smallest and the efficiency is the highest. The optimal cutting speed can achieve the best cutting result. In most application cases, the beam focus is adjusted to just under the nozzle. The distance between the nozzle and the workpiece surface is generally about 1.5mm.
In the process of laser application, problems such as focusing are often encountered. There are three simple methods commonly used to determine the position of the focus:
(1) Printing method: Move the cutting head from top to bottom to print the laser beam on the plastic plate. The point with the smallest printing diameter is the focus.
(2) Inclined plate method: Place a plastic plate at an Angle to the vertical axis and pull it horizontally to find the smallest point of the laser beam as the focus.
(3) Blue Spark method: Remove the nozzle, blow air, and direct the pulsed laser onto the stainless steel plate to make the cutting head move from top to bottom until the point where the blue spark is the greatest becomes the focus.