Analysis of Common Problems in Laser Cutting of Pipes

With the rapid development of modern science and technology as well as the industrial field, pipe parts are playing an increasingly important role in various industries such as engineering machinery, transportation, agricultural and animal husbandry machinery, kitchenware, fitness, etc. At the same time, with the emergence and rapid development of CNC laser pipe cutting machines, Laser cutting of pipes has many advantages, such as narrow cut width, small thermal impact, fast cutting speed, clean cut, good flexibility, and no obvious restrictions on the shape, type, thickness, etc. of the cut pipes, making Laser cutting pipe technology more prominent compared to traditional pipe processing methods. Laser cutting pipe technology can improve the quality of cutting and processing, ensure processing efficiency, simplify processing processes, and greatly improve the economic benefits of laser pipe cutting users. However, laser cutting of pipes is a complex process, and there are many factors that affect the cutting effect. We need to analyze the problems generated during the cutting process and find certain methods to solve them.

Imperfect cutting or unsmooth incision

There are four main reasons for incomplete or unsmooth cuts during laser cutting.

Cutting speed too fast

Especially when cutting special pipes or high thickness pipes, such as profiles, irregular pipes, etc., when the cutting head cuts to the corner, due to the increase in thickness at the corner and the fast cutting speed, it is easy to encounter the phenomenon of incomplete cutting or unsmooth cutting, as shown in Figure 1. In this situation, the cutting speed is generally reduced at the cutting corner while increasing the pressure of the auxiliary gas.

Figure 1 Unsmooth incision

Improper adjustment of the focal length of the cutting head or incorrect laser path of the cutting head

The focal length of the cutting head also affects the cutting quality of pipes. The normal cutting head focuses on three states: negative focus, zero focus, and positive focus. Positive focus cutting is suitable for oxygen cutting of carbon steel materials, zero focus is suitable for cutting thin plates, and negative focus is suitable for nitrogen/air cutting of stainless steel, carbon steel, galvanized steel, aluminum, copper, and other materials. This requires us to choose the correct cutting head focal position when cutting different pipes.

Improper light path of the cutting head can also have a significant impact on pipe cutting. Therefore, during trial cutting, it is necessary to continuously adjust the position of the light spot of the cutting head to make a very round light spot on the laser test paper.

Cutting head protection mirror, focusing mirror contamination, and unclean water source for cooling system

When the cutting head is cutting normally, the auxiliary gas continuously cools and removes slag. However, due to the high temperature during cutting, the protective lens and focusing lens of the cutting head can still be easily contaminated, as shown in Figure 2. Once the protective lens or focusing lens of the cutting head is contaminated, it will cause the pipe to cut poorly or cannot be cut off.

Figure 2 Protective lenses that need to be replaced due to contamination

If the laser cutting system uses unclean water, it can affect the cooling effect of the entire cutting system, causing blockage of the cutting head and generator water path, affecting the quality of pipe cutting. In severe cases, it can cause damage to the generator and cutting head, leading to greater economic losses. Therefore, the laser cutting system must use pure water, preferably distilled water, to avoid blockage of the cutting head and generator water path and affecting the cooling effect.

Excessive laser output power or improper selection of cutting head nozzle

When laser cutting circular pipes, square pipes, and other pipes with symmetrical surfaces, due to excessive laser output power, molten edges usually appear at the edge of the cutting seam after cutting on the front side of the cutting surface. The slag will fall along the cutting seam on the inner side of the symmetrical surface, equivalent to adding a layer of thickness on the inner side of the pipe. When the cutting head cuts its symmetrical surface, the slag will fall on the previous cutting seam, causing the entire pipe to cut continuously; Even if it has been cut, the surface of the cut will still be quite rough, affecting the overall cutting quality of the pipe. Therefore, before cutting, it is necessary to conduct perforation and trial cutting tests to test the optimal laser output power and auxiliary gas pressure.

The choice of nozzle for the cutting head also affects the cutting quality. Usually, double spray nozzles are suitable for fine slotted pipes, mainly used for cutting pipes with small thickness, high accuracy, and fast speed, while single spray nozzles are suitable for cutting pipes with large thickness, complex materials, and low accuracy requirements. Therefore, during the trial cutting, selecting a cutting head nozzle that is suitable for the current pipe can ensure the overall cutting quality of the pipe and improve cutting efficiency. The nozzles for single and double spraying of the cutting head are shown in Figure 3.

Figure 3 Single and Double Spray Nozzles

The starting and ending lines of the cutting do not coincide or the cutting pattern does not meet the requirements

Inaccurate parameter settings for each axis of the machine tool

In general, a laser pipe cutting machine consists of several basic control axes, including X-axis guide rail rack transmission, Y-axis guide rail screw transmission, Z-axis cutting head cutting, and W-axis clamping and feeding chuck rotation. When setting the parameters of each axis of the machine tool, errors in any of the parameters such as the size of the gear tooth top circle, the lead screw of the guide rail, the lead screw of the cutting head, and the gear transmission ratio of the W-axis can have a significant impact on the final cutting effect, and some even affect the good operation status of the entire laser pipe cutting machine. So after starting the machine tool, first check if the setting parameters of each axis are correct, and then put the machine tool into operation.

The X-axis of the machine tool is not perpendicular to the Y-axis or the Z-axis is not perpendicular to the X-axis

When the X-axis and Y-axis of the laser pipe cutting machine are not perpendicular, the circular pattern cut on the surface of the pipe will appear out of circle, as shown in Figure 4, and the square pattern cut will become a parallelogram; When the Z-axis is not perpendicular to the X-axis, the cutting path of the cutting head will change, causing it to be inconsistent with the simulated cutting stroke in the cutting system, ultimately resulting in a mismatch between the cutting starting line and the ending line, as shown in Figure 5, which is also one of the reasons for continuous pipe cutting. Therefore, when cutting pipes, if the above phenomena are found, a dial gauge should be used for calibration as soon as possible. At the same time, the installation screws of the guide rail, gear rack, and lead screw can be checked for looseness and gear wear.

Figure 4 Cutting Not Round enough

Figure 5: The starting and ending lines of the cutting do not coincide

Conclusion

With the increasing status of laser cutting pipes in the entire laser cutting industry, especially the laser pipe cutting machine, which has gone through a series of development processes of manual cutting, semi-automatic cutting, semi-automatic cutting, and fully automatic cutting, more and more people have discovered the enormous potential of laser pipe cutting machines in the production and processing field. This requires us to be able to identify various problems that arise in the laser cutting of pipes while using laser cutting machines, so as to propose improvement measures for these problems, continuously improve the laser cutting system and laser cutting process, and further promote the development of laser cutting machines towards high speed, high precision, high automation, flexibility, and diversified functions, This is of great significance for the rapid development of the entire laser cutting industry.