Introduction of laser cutting machine (2)

by:Transon     2021-04-01

Several key technologies of laser cutting machine are integrated technology of light, machine and electricity.

In the laser cutting machine, the parameters of the laser beam, the performance and accuracy of the machine and the numerical control system all directly affect the efficiency and quality of the laser cutting. Especially for parts with high cutting precision or thicker thickness, the following key technologies must be mastered and solved: 1. Focus position control technology: One of the advantages of laser cutting is the high energy density of the beam, generally 10W/cm2. Since the energy density is proportional to 4/πd2, the focal spot diameter is as small as possible to produce a narrow slit; at the same time, the focal spot diameter is also proportional to the focal depth of the lens. The smaller the focal depth of the focusing lens, the smaller the focal spot diameter. However, there are spatters in cutting, and the lens is too close to the workpiece to easily damage the lens. Therefore, the focal length of 5'~7.5' (127~190mm) is widely used in general high-power CO2 laser cutting machine industrial applications. The actual focal spot diameter is 0.1~0.4 mm. For high-quality cutting, the effective focal depth is also related to the lens diameter and the material to be cut. For example, when cutting carbon steel with a 5' lens, the focal depth is within +2% of the focal length, which is about 5mm. Therefore, control The position of the focus relative to the surface of the material to be cut is very important. Taking into account factors such as cutting quality and cutting speed, in principle, 6mm metal materials, the focus is on the surface; 6mm carbon steel, the focus is on the surface; 6mm stainless steel, the focus Below the surface. The specific size is determined by experiment.

There are three easy ways to determine the focal position in industrial production:

(1) Printing method: The cutting head is moved from top to bottom, and the laser beam is printed on the plastic plate, and the spot with the smallest printing diameter is the focus.

(2) Inclined plate method: Use a plastic plate placed obliquely at an angle to the vertical axis to pull it horizontally to find the smallest point of the laser beam as the focus.

(3) Blue spark method: remove the nozzle, blow air, hit the pulse laser on the stainless steel plate, and make the cutting head move from top to bottom until the largest blue spark is the focus.

For the cutting machine of the flying light path, due to the beam divergence angle, the length of the optical path is different between the proximal and distal ends of the cutting, and the beam size before focusing is different. The larger the diameter of the incident beam, the smaller the diameter of the focal spot. In order to reduce the change of the focal spot size caused by the change of the beam size before focusing, the manufacturers of laser cutting systems at home and abroad provide some special devices for users to choose:

(1) Parallel light pipe. This is a commonly used method, that is, a collimator is added to the output end of the CO2 laser for beam expansion. After the beam expands, the beam diameter becomes larger and the divergence angle becomes smaller, so that the proximal and distal ends of the cutting work range The beam size before focusing is nearly the same.

(2) Add an independent lower axis of the moving lens to the cutting head, which is two independent parts from the Z axis that controls the distance from the nozzle to the surface of the material (stand off). When the machine tool table 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 beam spot diameter remains the same in the entire processing area after the beam is focused. As shown in Figure 2.

(3) Control the water pressure of the focusing lens (usually a metal reflective focusing system). If the beam size before focusing becomes smaller and the focal spot diameter becomes larger, the water pressure is automatically controlled to change the focus curvature to make the focal spot diameter smaller.

(4) Add x and y direction compensation optical path system on the flying optical path cutting machine. That is, when the optical path at the distal end of the cutting is increased, the compensation optical path is shortened; on the contrary, when the optical path at the proximal end of the cutting is reduced, the compensation optical path is increased to keep the optical path length consistent.

2. Cutting perforation technology: Any kind of thermal cutting technology, except for a few cases, which can start from the edge of the board, generally a small hole must be pierced in the board. Earlier on the laser stamping compound machine, a punch was used to punch out a hole, and then a laser was used to start cutting from the small hole. There are two basic methods of perforating for laser cutting machines without punching devices:

(1) Blast drilling: (Blast drilling), the material is irradiated by a continuous laser to form a pit in the center, and then the molten material is quickly removed by the oxygen stream coaxial with the laser beam to form a hole. Generally, the size of the hole is related to the plate thickness. The average diameter of the blasting hole is half of the plate thickness. Therefore, the blasting hole of the thicker plate is larger and not round, and it is not suitable for use on the parts with higher requirements (such as oil screen pipe ), can only be used on scrap. In addition, since the oxygen pressure used for perforation is the same as that used for cutting, the splash is larger.

(2) Pulse drilling: (Pulse drilling) uses a high peak power pulse laser to melt or vaporize a small amount of material. Air or nitrogen is often used as an auxiliary gas to reduce the expansion of the hole due to exothermic oxidation. The gas pressure is higher than the oxygen pressure during cutting. small. Each pulse laser only produces small particle jets, which gradually penetrate deeper, so it takes a few seconds for the thick plate to perforate. Once the perforation is completed, immediately change the auxiliary gas to oxygen for cutting. In this way, the perforation diameter is smaller and the perforation quality is better than blast perforation. For this reason, the laser used should not only have a higher output power; more importantly, the time and space characteristics of the time beam, so the general cross-flow CO2 laser cannot meet the requirements of laser cutting. In addition, pulse perforation also needs a more reliable gas path control system to realize the switch of gas type, gas pressure and the control of perforation time. In the case of pulse perforation, in order to obtain a high-quality cut, the transition technology from pulse perforation when the workpiece is stationary to continuous cutting of the workpiece at constant velocity should be paid attention to. Theoretically speaking, it is usually possible to change the cutting conditions of the acceleration section: such as focal length, nozzle position, gas pressure, etc., but in fact, it is unlikely that the above conditions will be changed due to too short time.

In industrial production, it is more realistic to use the method of changing the average power of the laser. There are three specific methods as follows:

(1) Change the pulse width;

(2) Change the pulse frequency;

(3) Change the pulse width and frequency at the same time. The actual results show that (3) has the best effect.

3. Nozzle design and airflow control technology: When laser cutting steel, oxygen and the focused laser beam are shot through the nozzle to the material to be cut, thus forming an airflow beam. The basic requirement of the air flow is that the air flow into the incision should be large and the speed should be high, so that there is enough oxidation to make the incision material fully carry out the exothermic reaction; at the same time, there is enough momentum to spray the molten material out. Therefore, in addition to the quality of the beam and its control directly affecting the cutting quality, the design of the nozzle and the control of the airflow (such as the nozzle pressure, the position of the workpiece in the airflow, etc.) are also very important factors. At present, the nozzle used for laser cutting adopts a simple structure, that is, a tapered hole with a small round hole at the end. Usually design with experiment and error method. Since the nozzle is generally made of red copper, the volume is small, and it is a vulnerable part that needs to be replaced frequently, so fluid mechanics calculation and analysis are not performed. When in use, a certain pressure Pn (gauge pressure is Pg) gas is introduced from the side of the nozzle, which is called the nozzle pressure, which is sprayed from the nozzle outlet and reaches the surface of the workpiece after a certain distance. Pa. Research work shows that with the increase of Pn, the air flow rate increases, and Pc also increases.

The following formula can be used to calculate: V=8.2d2(Pg+1)

V-gas flow rate L/min

d-Nozzle diameter mm

Pg- nozzle pressure (gauge pressure) bar

There are different pressure thresholds for different gases. When the nozzle pressure exceeds this value, the gas flow is a normal oblique shock wave, and the gas flow velocity transitions from subsonic to supersonic. This threshold is related to two factors, the ratio of Pn, Pa, and the degree of freedom of gas molecules (n): for example, n=5 for oxygen and air, so the threshold Pn=1bar×(1.2)3.5=1.89bar. When the nozzle pressure is higher Pn/Pa=(1+1/n)1+n/2 (Pn; 4bar), the normal airflow shock wave seal becomes a positive shock wave, the cutting pressure Pc drops, the airflow speed decreases, and The formation of vortex on the surface of the workpiece weakens the effect of the air flow to remove the molten material and affects the cutting speed. Therefore, the nozzle with a tapered hole and a small round hole at the end is used, and the nozzle pressure of the oxygen is often below 3bar.

In order to further increase the speed of laser cutting, according to the principle of aerodynamics, without generating a positive shock wave under the premise of increasing the nozzle pressure, a zoom-type nozzle, namely Laval nozzle, can be designed and manufactured. For the convenience of manufacturing, the structure shown in Figure 4 can be used. The Laser Center of the University of Hannover in Germany used a 500W CO2 laser with a lens focal length of 2.5〃, and a small hole nozzle and a Laval nozzle were used for experiments, as shown in Figure 4. The test results are shown in Fig. 5: It respectively represents the function relationship between the surface roughness Rz and the cutting speed Vc of the NO2, NO4, and NO5 nozzles under different oxygen pressures. It can be seen from the figure that the cutting speed of the NO2 small hole nozzle can only reach 2.75m/min (the thickness of the carbon steel plate is 2mm) when the Pn is 400Kpa (or 4bar). The cutting speed of NO4 and NO5 Laval nozzles can reach 3.5m/min and 5.5m/min when Pn is 500Kpa to 600Kpa. It should be pointed out that the cutting pressure Pc is also a function of the distance between the workpiece and the nozzle. Because the oblique shock wave is reflected multiple times at the boundary of the airflow, the cutting pressure changes periodically.

The first high cutting pressure zone is next to the nozzle outlet, and the distance from the workpiece surface to the nozzle outlet is about 0.5~1.5mm. The cutting pressure Pc is large and stable, which is a commonly used process parameter for cutting handles in industrial production. The second high cutting pressure zone is about 3~3.5mm of the nozzle outlet, and the cutting pressure Pc is also larger, which can also achieve good results, and is beneficial to protect the lens and increase its service life. Other high cutting pressure areas on the curve cannot be used because they are too far away from the nozzle outlet and difficult to match the focused beam.

In summary, CO2 laser cutting machine technology is being used more and more in my country's industrial production. Foreign countries are researching and developing cutting technologies and devices for higher cutting speeds and thicker steel plates. In order to meet the increasing requirements of industrial production for quality and production efficiency, we must pay attention to solving various key technologies and implementing quality standards, so that this new technology can be used more widely in our country. Laser cutting technology  Laser cutting technology is widely used in the processing of metal and non-metal materials, which can greatly reduce processing time, reduce processing costs, and improve workpiece quality. There are two types of laser cutting technology: One is pulsed laser which is suitable for metal materials. The second is that continuous laser is suitable for non-metallic materials, and the latter is an important application field of laser cutting technology. Modern lasers have become the 'sword' of 'cutting iron like mud' that people dream of pursuing.
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