laser cutting machine

laser cutter

The laser cutting machine is to focus the laser light emitted from the laser into a high-power density laser beam through the optical path system. The laser beam irradiates the surface of the workpiece to make the workpiece reach the melting point or boiling point, and the high-pressure gas coaxial with the beam blows away the molten or vaporized metal.

As the relative position of the beam and the workpiece moves, the material will finally form a slit, so as to achieve the purpose of cutting.

Laser cutting processing is to replace the traditional mechanical knife with an invisible beam. It has the characteristics of high precision, fast cutting, not limited to cutting patterns, automatic typesetting, saving materials, smooth cuts, and low processing costs. It will be gradually improved or replaced. Traditional metal cutting process equipment. The mechanical part of the laser cutter head has no contact with the workpiece, and will not scratch the surface of the workpiece during work; the laser cutting speed is fast, the incision is smooth and flat, and generally does not require subsequent processing; the cutting heat affected zone is small, the plate deformation is small, and the slit is narrow ( 0.1mm~0.3mm); the incision has no mechanical stress, no shearing burr; high processing accuracy, good repeatability, no damage to the surface of the material; CNC programming, can process any plan, can cut the entire board with a large format, without Opening the mold saves time and economy.

Introduction

Compared with traditional oxyacetylene, plasma and other cutting processes, laser cutting speed is fast, the slit is narrow, the heat-affected zone is small, the edge of the slit is perpendicular, and the cutting edge is smooth. At the same time, there are many types of materials that can be laser cut, including carbon steel. , Stainless steel, alloy steel, wood, plastic, rubber, cloth, quartz, ceramics, glass, composite materials, etc. With the rapid development of the market economy and the rapid development of science and technology, laser cutting technology has been widely used in the fields of automobiles, machinery, electricity, hardware, and electrical appliances. In recent years, laser cutting technology is developing at an unprecedented speed, with an annual growth rate of 15% to 20%. Since 1985, my country has grown at a rate of nearly 25% per year. At present, the overall level of laser cutting technology in my country still has a big gap compared with advanced countries. Therefore, laser cutting technology in the domestic market has broad development prospects and huge application space.

During the cutting process of the laser cutting machine, the beam is focused by the lens of the cutting head into a small focal point, so that the focal point can reach a high power density, and the cutting head is fixed on the z-axis. At this time, the heat input by the beam far exceeds the part of the heat reflected, conducted or diffused by the material, and the material is quickly heated to the melting and vaporization temperature. At the same time, a high-speed airflow will melt from the coaxial or non-coaxial side. And the vaporized material is blown out to form holes for cutting the material. With the relative movement of the focus and the material, the hole forms a continuous slit with a very narrow width to complete the cutting of the material.

At present, the outer optical path part of the laser cutting machine mainly adopts the flying optical path system. The light beam emitted from the laser generator reaches the focusing lens on the cutting head through the reflecting mirrors 1, 2, and 3, and forms a light spot on the surface of the material to be processed after focusing. The reflecting lens 1 is fixed on the fuselage without moving; the reflecting mirror 2 on the beam moves in the x direction with the movement of the beam; the reflecting lens 3 on the z axis moves in the y direction with the movement of the z axis. It is not difficult to see from the figure that in the cutting process, as the beam moves in the x direction and the z-axis part moves in the y direction, the length of the light path changes all the time.

At present, due to manufacturing cost and other reasons, the laser beams emitted by civilian laser generators have a certain divergence angle and are "conical". When the height of the "cone" changes (equivalent to a change in the optical path length of the laser cutting machine), the cross-sectional area of the beam on the surface of the focusing lens also changes. In addition, light also has the properties of waves. Therefore, diffraction phenomenon will inevitably occur. Diffraction will cause the beam to expand laterally during propagation. This phenomenon exists in all optical systems and can determine the performance of these systems. Limit value. Due to the "cone" of the Gaussian beam and the diffraction effect of light waves, when the length of the optical path changes, the diameter of the beam acting on the lens surface changes momentarily, which will cause changes in the focus size and depth, but affect the focus position Very small. If the focus size and focus depth change during continuous processing, it will inevitably have a great impact on the processing, for example, it will cause inconsistent cutting slit widths, incomplete cutting or ablation of the board under the same cutting power.

principle

Laser is a kind of light, like other natural light, it is produced by the transition of atoms (molecules or ions, etc.). However, it is different from ordinary light in that the laser only relies on spontaneous emission for a very short period of time, and the subsequent process is completely determined by the excitation radiation. Therefore, the laser has a very pure color, almost no divergent directionality, and extremely high luminous intensity. And high coherence.

Laser cutting is achieved by applying high power density energy generated by laser focusing. Under the control of the computer, the laser is discharged through pulses, thereby outputting a controlled repetitive high-frequency pulsed laser to form a beam with a certain frequency and a certain pulse width. The pulsed laser beam is guided and reflected by the optical path and focused by the focusing lens group. On the surface of the processed object, a small, high-energy density light spot is formed. The focal spot is located near the surface to be processed, and the processed material is melted or vaporized at an instant high temperature. Each high-energy laser pulse instantly sputters a small hole on the surface of the object. Under computer control, the laser processing head and the processed material perform continuous relative movement according to the pre-drawn graphics, so that the object will be processed into The shape you want.

The process parameters (cutting speed, laser power, gas pressure, etc.) and motion trajectory during slitting are controlled by the numerical control system, and the slag at the slit is blown away by a certain pressure of auxiliary gas.

Main process

1. Vaporized cutting.

In the laser gasification cutting process, the speed of the material surface temperature rising to the boiling point temperature is so fast that it is enough to avoid melting caused by heat conduction, so part of the material vaporizes into steam and disappears, and part of the material is sprayed from the bottom of the slit by auxiliary gas The flow blows away. In this case, very high laser power is required.

In order to prevent material vapor from condensing on the slit wall, the thickness of the material must not greatly exceed the diameter of the laser beam. This process is therefore only suitable for applications where the removal of molten material must be avoided. This processing is actually only used in areas where iron-based alloys are very small.

This process cannot be used for materials such as wood and certain ceramics that are not in a molten state and therefore are unlikely to allow the material vapor to recondense. In addition, these materials usually require thicker cuts. In laser gasification cutting, the optimal beam focus depends on the material thickness and beam quality. The laser power and the heat of vaporization have only a certain influence on the optimal focus position. In the case of a certain thickness of the sheet, the maximum cutting speed is inversely proportional to the vaporization temperature of the material. The required laser power density is greater than 108W/cm2 and depends on the material, cutting depth and beam focus position. In the case of a certain sheet thickness, assuming sufficient laser power, the maximum cutting speed is limited by the gas jet speed.

2. Melting and cutting.

In laser melting and cutting, the workpiece is partially melted and the molten material is sprayed out with the help of airflow. Because the transfer of the material only occurs in its liquid state, the process is called laser melting and cutting.

The laser beam is matched with a high-purity inert cutting gas to drive the melted material away from the kerf, and the gas itself does not participate in the cutting. Laser melting cutting can get a higher cutting speed than gasification cutting. The energy required for gasification is usually higher than the energy required to melt the material. In laser melting and cutting, the laser beam is only partially absorbed. The maximum cutting speed increases with the increase of the laser power, and decreases almost inversely with the increase of the thickness of the sheet and the increase of the melting temperature of the material. In the case of a certain laser power, the limiting factor is the air pressure at the slit and the thermal conductivity of the material. Laser melting and cutting can obtain oxidation-free incisions for iron materials and titanium metals. The laser power density that produces melting but not gasification is between 104W/cm2～105W/cm2 for steel materials.

3. Oxidation melting cutting (laser flame cutting).

Melting and cutting generally use inert gas. If it is replaced by oxygen or other active gases, the material is ignited under the irradiation of a laser beam, and a fierce chemical reaction with oxygen generates another heat source to further heat the material, which is called oxidative melting and cutting .

Due to this effect, for structural steel of the same thickness, the cutting rate that can be obtained by this method is higher than that of melting cutting. On the other hand, this method may have worse cut quality compared to fusion cutting. In fact, it will produce wider kerf, obvious roughness, increased heat-affected zone and worse edge quality. Laser flame cutting is not good when processing precision models and sharp corners (there is a danger of burning off the sharp corners). A pulsed laser can be used to limit the thermal influence, and the power of the laser determines the cutting speed. In the case of a certain laser power, the limiting factor is the supply of oxygen and the thermal conductivity of the material.

4. Control fracture cutting.

For brittle materials that are easily damaged by heat, high-speed and controllable cutting is performed by laser beam heating, which is called controlled fracture cutting. The main content of this cutting process is: the laser beam heats a small area of the brittle material, causing a large thermal gradient and severe mechanical deformation in this area, leading to the formation of cracks in the material. As long as a uniform heating gradient is maintained, the laser beam can guide cracks in any desired direction.

Key technology

There are two types of laser cutting technology: the first is pulsed laser for metal materials, and the second is continuous laser for non-metal materials. The latter is an important application field of laser cutting technology.

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 accuracy or large thickness, the following key technologies must be mastered and solved:

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 inversely proportional to the area, 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 splashes in cutting, and the lens is too close to the workpiece to 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 between 0.1~0.4mm. For high-quality cutting, the effective focal depth is also related to the lens diameter and the material being cut. For example, cutting carbon steel with a 5" lens, the focal depth is within +2% of the focal length, which is about 5mm. Therefore, it is very important to control the position of the focal point relative to the surface of the material to be cut. Taking into account factors such as cutting quality and cutting speed, the principle is The top 6mm metal material, the focus is on the surface; the 6mm carbon steel, the focus is above the surface; the 6mm stainless steel, the focus is below the surface. The specific dimensions are determined by experiments.

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 the air, hit the pulse laser on the stainless steel plate, 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 optical path length of the near end and the far end of the cutting are different, 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 to expand the beam. After the beam expands, the diameter of the beam 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 between the nozzle and 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 decreased, the compensation optical path is increased to keep the optical path length consistent.

The main parameters

X, Y working range: 1300mm*2500mm

Cutting focus lens: F=80mm

Maximum laser output power: 500W

Adjust the follow-up frequency: $300Hz

Power pulse width: 0.5ms-2ms

Laser: dual-lamp gold-plated condenser cavity

Cutting interface card: CNC 3000 control card

Cutting software: adapt to PLT, DXF and other formats

Cooling power: 4W

Repeat positioning accuracy: ±0.03/300mm

Idle speed: 0-20000mm/min

Cutting speed: 0-15000mm/min

Cutting quality

Cutting accuracy is the first element to judge the quality of a CNC laser cutting machine. Four factors affecting the cutting accuracy of CNC laser cutting machines:

1. The size of the laser condensate of the laser generator. If the light spot is very small after gathering, the cutting accuracy is very high, and the gap after cutting is also very small. It shows that the precision of the laser cutting machine is very high, and the quality is very high. But the light beam emitted by the laser is cone-shaped, so the slit cut out is also cone-shaped. Under this condition, the greater the thickness of the workpiece, the lower the accuracy, so the larger the slit.

2. The accuracy of the workbench. If the accuracy of the table is very high, the accuracy of the cutting will also be improved. Therefore, the accuracy of the worktable is also a very important factor to measure the accuracy of the laser generator.

3. The laser beam is condensed into a cone. When cutting, the laser beam is tapered downwards. At this time, if the thickness of the cut workpiece is very large, the cutting accuracy will be reduced, and the cut gap will be very large.

4. Different cutting materials will also affect the accuracy of the laser cutting machine. Under the same circumstances, the precision of cutting stainless steel and aluminum will be very different, the precision of stainless steel cutting will be higher, and the cut surface will be smoother.

Generally speaking, the quality of laser cutting can be measured by the following 6 standards.

1. Cutting surface roughness Rz

2. Cutting slag size

3. Cutting edge perpendicularity and slope u

4. Cutting edge fillet size r

5. Drag after stripes n

6. Flatness F

Cutting perforation

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 punching 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 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 diameter of the thicker plate is larger and not round. 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; the more important time and space characteristics of the beam, so the general cross-flow CO2 laser cannot meet the requirements of laser cutting.

In addition, pulse perforation also requires a more reliable gas path control system to realize the switching of gas types, gas pressure and perforation time control. In the case of pulse perforation, in order to obtain high-quality cuts, 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. In theory, 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 to change the above conditions due to too short time. In industrial production, it is more realistic to mainly adopt the method of changing the average power of the laser. There are three specific methods: (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.

Nozzle design

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 to form an airflow beam. The basic requirement for air flow is that the air flow into the incision should be large and the speed should be high, so that enough oxidation can 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 nozzle pressure, the position of the workpiece in the airflow, etc.) are also very important factors.

The nozzle used for laser cutting adopts a simple structure, that is, a tapered hole with a small round hole at the end (Figure 4). 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, and it is sprayed from the nozzle outlet and reaches the surface of the workpiece after a certain distance. The pressure is called the cutting pressure Pc, and finally the gas expands to atmospheric pressure. Pa. Research work has shown 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: Pn, Pa ratio and the degree of freedom of gas molecules (n): For example, oxygen and air have n=5, 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 oblique shock wave seal of the airflow becomes a positive shock wave, the cutting pressure Pc drops, and the airflow speed decreases, and A vortex is formed on the surface of the workpiece, which weakens the effect of the airflow to remove the molten material and affects the cutting speed. Therefore, a 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 laser cutting speed, according to the principle of aerodynamics, and 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". The test was carried out using a small hole nozzle and a Laval nozzle respectively, as shown in Figure 4. The test results are shown in Figure 5. The relationship between the surface roughness Rz of the notch and the cutting speed Vc under the oxygen pressure. It can be seen from the figure that the cutting speed of the NO2 small hole nozzle can only reach 2.75m/min (carbon steel plate thickness) when the Pn is 400Kpa (or 4bar) 2mm). 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 of this. The oblique shock wave is reflected multiple times at the boundary of the airflow, so that 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 from the nozzle outlet, and the cutting pressure Pc is also large, 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.

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