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 focal spot diameter is as small as possible to produce a narrow slit. Because the smaller the focal depth of the focusing 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 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 great influence on the cutting speed, the choice of 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. The spot size is very small after the beam is focused by a short focal length lens, and the power density at the focus is very high, which is very beneficial for material cutting; but its disadvantage is that the focal depth is very short and the adjustment margin is small. It is generally more suitable for high-speed cutting of thin materials. Since the long focal length lens has a wider focal depth, as long as it has sufficient power density, it is more suitable for cutting thick workpieces.
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. Since the power density is highest at the focus, in most cases, the focus position is just on the workpiece surface or slightly below the surface during cutting. During the entire cutting process, ensuring that the relative position of the focus and the workpiece is constant is an important condition for obtaining stable cutting quality. Sometimes, the lens is heated due to poor cooling during operation, causing the focal length to change, which requires timely adjustment of the focus position.
When the focus is in the best position, the slit is the smallest, the efficiency is the highest, and the best cutting result can be obtained at the best cutting speed. In most applications, the beam focus is adjusted to be just below the nozzle. The distance between the nozzle and the workpiece surface is generally about 1.5 mm.
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: Move the cutting head from top to bottom, print the laser beam on the plastic plate, and the point with the smallest print diameter is the focus.
(2) Inclined plate method: Use a plastic plate placed at an angle to the vertical axis to pull it horizontally, and find the smallest point of the laser beam as the focus.
(3) Blue spark method: Remove the nozzle, blow air, and hit the pulse laser on the stainless steel plate. Move the cutting head from top to bottom until the point with the largest blue spark is the focus.