One of the advantages of fiber laser cutting is that the specific energy of light is high, generally > 10W / cm2. Because the specific energy is proportional to 4 / TD2, the focus spot diameter should be as small as possible to create a narrow slit; in addition, the focus spot diameter is also proportional to the relative aperture of the lens. The smaller the relative aperture of focusing lens is, the smaller the diameter of focusing spot is. However, there is splash in laser cutting, the lens is too close to the workpiece, which is very easy to destroy the lens. Therefore, it is very important to manipulate the relativity of focus on the part of cutting off the surface of raw materials. Considering the laser cutting quality, laser cutting rate other factors, under normal circumstances, the focus is on the surface layer of metal composite materials with the thickness of less than 6 mm; the focus of carbon steel with the thickness of more than 6 mm is above the surface layer; the focus of stainless steel plate with the thickness of more than 6 mm is below the surface. The actual specification is determined by test. In industrial production, there are three simple ways to clearly focus on the focus

(1) Copy method: make the laser cutting head exercise top to bottom, precisely cut the pipe, carry out particle beam copying on the plastic plate, focus on the point with the least copy diameter.

(2) Inclined column method: professional cutting pipe, with the vertical axis into a visual angle of the plastic plate, so that its level drive, to find the minimum point of particle beam focus.

(3) Dark blue flame method: remove the nozzle, blow gas, pulse light on the stainless steel plate, so that the laser cutting head top to bottom fitness movement, until the dark blue flame big point focus. For the automatic cutting machine of navigation laser path, because of the light emission angle, the laser cutting far end far side of the optical path length is the same