How to determine the focus position of laser cutting machine?

When the laser cutting machine is engaged in the cutting operation, there will be some differences in the cutting quality with different focus positions. 

Therefore, in order to further improve the cutting quality of the laser cutting machine, it is very important to adjust the focus position of the laser cutting machine. 

Sometimes, there are often some problems around you. 

Why is there always sticky slag in cutting? 

The cutting surface of the plate is always rough. 

How to determine the focus position?

Before we solve these problems, let’s first understand the importance of focusing spot. 

For the parts with high cutting quality or large thickness in the laser cutting machine, several key technologies must be mastered and solved. 

Focus position control technology is one of them. 

The main advantage of laser cutting is the high energy density of the laser beam, so the diameter of the focus spot is as small as possible in order to produce a narrow slit. 

Because the smaller the focal depth of the focus lens 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 material to be cut. 

Therefore, it is very important to control the position of the focus relative to the surface of the cut material. 

When the focus of the laser cutting machine is in the best position, the slit is small, the quality cutting efficiency is the highest, and the best cutting effect can be obtained. 

Different cutting focus positions correspond to different cutting surface roughness, and the cutting quality of laser cutting machines is largely determined by the relative position of cutting focus and cutting plate.

Focusing Position 

The focus position of the laser cutting machine is theoretically the relative position between the focus and the upper surface of the workpiece, based on the machined material surface, above the workpiece surface is positive and below is negative.

NameFocusing positionFeaturesApplicable Material
Positive FocusingFocusing position on the surfaceThe focus is far away from the cutting surface, and compared with the cutting point, the cutting point is larger on the workpiece surface. In this mode, the cutting airflow is small, the temperature is sufficient, and the cutting piercing time is relatively long.This mode is often used in cutting carbon steel plates.
Negative FocusingFocusing position under the surfaceThe focus is located below the surface of the cutting material. This method is mainly used in cutting materials with high hardness. In this way, the focus is located at the bottom of the cutting material surface, mainly because the cutting width of the plate is large, otherwise the auxiliary gas transported by the nozzle is prone to insufficient flow and the cutting slag can not be blown off. The disadvantage is that the cutting surface is rough, so it is not suitable for high precision cutting.This mode is often used in cutting stainless steel plates.
0 FocusingFocusing position in the surfaceThe smoothness of the upper and lower surfaces of the workpiece is different. Generally speaking, the cutting surface close to the focus is relatively smooth, while the lower surface far away from the cutting focus appears rough.It is commonly used when cutting materials such as SPC,SPH,SS41.

The influence of focus position on cutting quality

The focus position is the distance between the laser focus and the workpiece surface, which directly affects the roughness of the section, the slope and width of the slit and the adhesion of the molten residue. 

If the focus position is too advanced, it will increase the heat absorbed by the lower end of the cut workpiece, and when the cutting speed and auxiliary pressure are constant, it will cause the cut material and the melted material near the cutting seam to flow in liquid state on the lower surface, and the melted material will adhere to the lower surface of the workpiece as a ball after cooling. 

If the position lags behind, the heat absorbed by the lower end face of the cut material is reduced, so that the material in the slit cannot be completely melted, and some sharp and short residues will be adhered to the lower surface of the plate. 

In general, the focus position should be on the surface of the workpiece or a little lower, but the requirements of different materials are different, when cutting carbon steel, the focus is on the surface of the plate, the cutting quality is better, while when stainless steel is cut, the focus should be about 1max 2 of the thickness of the plate.

How to determine the Focus position?

NC positioning and dotting method. 

Make the focus of the cutting head move from top to bottom, print the laser beam on the board, and the minimum print diameter is zero focus (focus reference point). 

Oblique focus burning method. 

Pull it horizontally with a plastic sheet tilted at an angle vertically to find the smallest point of the laser beam as the focus. 

Blue flame method. 

Remove the nozzle, blow the air, hit the pulsed laser on the stainless steel plate, and operate the cutting head from top to bottom until the blue spark is the focus. 

Parallel light tube method. 

This is a commonly used method, that is, a collimator is added to the output end of the laser to expand the beam, so that the diameter of the beam becomes larger and the divergence angle becomes smaller, so that the size of the beam before focusing at the near end and the far end is close to the same within the working range of cutting. 

Add an independent moving lens. 

An independent moving lens lower axis is added to the cutting head, which is independent of the Z axis which controls the distance between the nozzle and the material surface. 

When the worktable of the machine tool moves or the optical axis moves, the beam moves from the proximal end to the distal F axis at the same time, so that the spot diameter of the focused beam is consistent in the whole processing zone.

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