In two words, the focal length is the distance from the lens to the focal plane. In our case, we take a metal sheet as the focal plane.
The position of the focused spot (focal point) above the focal plane is called positive, and the position below the workpiece is called negative.
VALUE OF FOCAL POSITION
The value of the focal length affects the size of the spot on the surface or inside the metal workpiece: as the focal length increases, the diameter of the spot increases and the width of the cut increases.
POSITIVE FOCUS
This is the focal length at which the laser beam is focused above the surface of the workpiece.
As a rule, such focusing is used for oxyfuel cutting of carbon steel workpieces.
This cutting method realizes slag removal and helps oxygen to reach the lower surface of the workpiece to participate in the complete oxidation reaction.
The greater the value of positive focus, the larger the diameter of the laser spot on the surface of the workpiece, the greater the heating and heat addition around the slot, and the smoother and brighter the carbon steel cutting surface.
NEGATIVE FOCUS
That is, the cutting focus is in the workpiece. In this mode, since the focus is far away from the cutting surface, the cutting width is relatively larger than the cutting point on the workpiece surface. At the same time, the cutting air flow is large and the temperature is sufficient.
When cutting stainless steel, it is reasonable to use negative focus cutting, the cutting surface has a uniform texture and good cross-section.
Plate perforation before cutting. Because the perforation has a certain height, the perforation uses negative focus, which can ensure that the spot size at the perforation position is the smallest and the energy density is the highest.
ZERO FOCUS
That is, the cutting focus is on the surface of the workpiece. Generally, the cutting surface close to the focus is relatively smooth, while the lower surface away from the cutting focus is rough. This case is mainly used for continuous laser cutting of thin wafers and pulsed laser vaporization with high peak power for cutting metal foil layers.
The choice of focus for laser cutting is not determined by the material of the cutting plate (stainless steel, carbon steel) but by the cutting method (oxidation cutting, molten cutting).
