In the laser cutting machine, some key technologies must be mastered and solved for the parts with high cutting accuracy or large thickness. Focus position control is one of them. One of the advantages of laser cutting is the high energy density of the beam, so the diameter of the focus spot should be as small as possible to produce a narrow slit. Because the smaller the focal depth is, the smaller the focal spot diameter is. For high-quality cutting, the effective focal depth is also related to the lens diameter and the cut material. Therefore, the position of the control focus relative to the surface of the material to be cut is very important.
Because the laser power density has a great influence on the cutting speed, the choice of lens focal length is an important problem. The spot size is directly proportional to the focal length of the lens after the laser beam is focused. The spot size is very small after the laser beam is focused by the short focal length lens, and the power density at the focal point is very high, which is very beneficial to the material cutting. However, its disadvantage is that the focal depth is very short, and the adjustment margin is small, which is generally applicable to the high-speed cutting of thin materials. Because the long focal length lens has a wide focal depth, as long as it has enough 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 workpiece surface or slightly below the surface when cutting. In the whole cutting process, it is an important condition to ensure the relative position of focus and workpiece is constant to obtain stable cutting quality. Sometimes, the lens is heated due to poor cooling, which results in the change of focal length, so it is necessary to adjust the focal position in time.
When the focus is in the right position, the slit 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 under the nozzle. The distance between nozzle and 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 to determine the focus position:
(1) Printing method: make the cutting head move from top to bottom, print with laser beam on the plastic plate, and focus on the place with smaller printing diameter.
(2) Inclined plate method: use the plastic plate placed at an angle with the vertical axis to pull it horizontally to find the smaller part 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, so that the cutting head moves from top to bottom, until the big blue spark is the focus.leapion