1. Numerical control positioning and marking method of laser focus
Numerical control positioning and marking method: use a flat and smooth white cardboard, lay it flat on the workbench, set the laser cutting head above it, and set the height of the focus lens from the cardboard at a position about 10mm smaller than the focal length of the focus lens. For example, if the focal length of the focus lens is 127mm, set the focus lens at about 117mm from the cardboard. The CNC system sets the cutting head to move every 10mm along the x-axis or y-axis, and the z-axis rises by 1mm at the same time of each movement. The distance of 20 consecutive movements can be set. Each time when it is moved in place, 20 holes are punched with 20 times, and the height of Z axis is increased by 20mm. Observing these 20 holes, we can find that the diameter of the holes changes from large to small, and then from small to large bamboo slips. Find the position with the smallest aperture, which is the focal position, and record this. The distance between the paperboard and the lens when measured at this position is the actual focal position of the laser beam.  
2. Inclined focus cauterization method
Oblique focus cauterization method: the schematic diagram of this method is as follows: place a flat wooden board on the workbench obliquely, with a slope of about 10 degrees. Set the cutting head at point a, and the height of point a from the focus lens is 20mm smaller than the focal length of the focus lens. The CNC system sets the cutting head to move continuously horizontally along the X axis or Y axis by 230mm. At the beginning of the movement, the laser outputs 200W continuous laser light. When the movement of the cutting head stops, the laser also stops. At this time, it can be seen that there is a burning mark of the laser beam on the wood board, which has changed from wide to narrow and from narrow to narrow. Take the narrowest part of the trace as the focal position, record this point, and measure the distance between the wood plate at this position and the lens, which is the actual focal position of the laser beam.   
3. Laser cutting direct burning method
Direct cauterization method: this principle is to hold a flat wooden board, immediately make an angle of 85 degrees on the cutting workbench, and raise the cutting head to a position where the focus lens is about 1.5 times from the surface of the workbench. Open the laser shutter, output 200W laser beam continuously, and move the wood plate horizontally and rapidly to the lower part of the focusing mirror. You can see a burning trace on the surface of the wood plate before and after the focusing of the laser beam from wide to narrow, and from narrow to wide. This trace is very close to the change of the focusing process of the laser beam. Take the narrowest part of the trace as the focal position, record this point, and measure the distance between the wood plate at this position and the lens, which is the actual focal position of the laser beam. Because this method requires manual operation, special attention should be paid to safety to avoid human injury.