Basics Of CO2 Laser Cutting Machine And CNC Laser Cutting Machine

1. CO2 laser cutting machine

In the cutting range, the proportion of carbon dioxide laser has reached about 80%.

In addition to the supporting components, moving parts and corresponding motion control devices required by general machine tools, laser cutting equipment shall also have laser processing system.

Carbon dioxide laser cutting machine is composed of laser, focusing system, mechanical system and electrical system, as shown in figure 6-57.

carbon dioxide laser cutting device

Fig. 1 CO2 laser cutting device

(1) Laser

The rated continuous output power of high-power laser equipment suitable for production is up to 20kW.

It uses the rapid flow of laser gas to take away the waste heat in the laser cavity.

The typical electro-optical conversion efficiency of this laser is 10% ~ 15%.

Effectively removing waste heat is very important to ensure continuous operation.

In order to minimize the working cost, a gas-liquid heat exchanger is used and the gas medium is recirculated through the system.

Due to the continuous heat dissipation and the need to supplement a small amount of laser gas to prevent the accumulation of pollution caused by the decomposition of CO2 and N2 during discharge, a small amount of gas is consumed.

(2) Focusing system

Its function is to accurately focus the laser beam on the workpiece through the optical system.

At the same time, it also has the function of adjusting the focus position and observing the display.

The laser output from CO2 laser can only pass through the optical lens made of germanium single crystal and gallium arsenide.

In order to reduce surface reflection, speed increasing film shall be plated.

From the laser to the workpiece, the laser is divided into internal light path and external light path.

The laser oscillates in the laser to produce laser, which is called internal light path.

The optical path after the laser is emitted is called the external optical path.

The external optical path system is composed of gold-plated mirror and gallium arsenide focusing lens.

The angle of the mirror can be adjusted arbitrarily through the attached adjusting device.

The focusing lens device is in the cutting head, and the focus position can be adjusted through the adjusting device.

On the laser thermal paper, the laser spot needs the same area to meet the processing requirements.

After adjustment, install the laser nozzle and adjust the lens to make the laser not offset when cutting the workpiece.

(3) Electrical system

The electrical system includes laser power supply and control system.

Its function is to supply laser energy (high voltage DC power supply of CO2 laser) and control the output mode (such as continuous or pulse, repetition frequency, etc.).

In addition, the movement of the workpiece or laser beam is mostly controlled by CNC.

The system is composed of microprocessor, interface circuit, display, driving system and power supply.

The microprocessor is an industrial control computer suitable for the industrial environment.

The system has the characteristics of good reliability, strong anti-interference ability and complete functions.

In order to realize the automatic adjustment of the focus position, especially when the surface of the workpiece cut by laser is uneven, the control system of automatic focus tracking needs to be adopted.

It usually uses inductive or capacitive sensors to detect in real time and controls the position of the focus point through feedback. The control accuracy is generally ± (0.05 ~ 0.005) mm.

(4) Mechanical system

The mechanical system includes two parts: cutting head and worktable.

The worktable is fixed. The cutting head is driven by two stepping motors controlled by computer to make plane free movement.

Various graphics are made according to the control program to cut the workpiece with high precision.

2. CNC laser cutting equipment

CNC laser cutting equipment includes high-speed and high-precision laser cutting machine, three-dimensional laser cutting machine, large format thick plate laser cutting machine, special laser cutting machine for special industries, etc.

Three dimensional laser cutting equipment mainly includes 3D laser cutting machine tool and laser cutting robot.

The following mainly introduces the 3D laser cutting machine.

Laser cutting machine is usually composed of computer system, laser control electrical system, laser generator, focusing unit, workbench control unit and three-dimensional workbench.

It also needs external auxiliary gas, as shown in figure 2.

main components of laser cutting equipment

Figure 2 Main components of laser cutting equipment

The computer system is the control center.

It is responsible for generating cutting graphics and setting cutting process parameters such as laser power, laser pulse width and cutting speed;

According to the needs of cutting graphics and cutting speed, the computer system sends control instructions to the laser control electrical unit and worktable control unit.

According to the received instructions, the CNC unit of the worktable controls the running track of the three-dimensional worktable in X and Y directions according to the shape of the graphics, and controls the moving speed of the three-dimensional worktable in X and Y directions according to the set cutting speed and in dynamic cooperation with the laser output, so that any graphics drawn by the computer can be cut.

The computer controls the three-dimensional worktable to move along the Z direction, which can make the workpiece surface at the appropriate position of the focal plane or focal length of the focusing unit.

For the graphics to be cut, the laser control electrical unit controls the laser generator according to the received instruction to make the generator output the laser beam with corresponding laser power, and the laser beam acts on the workpiece after being focused by the laser focusing unit;

After cutting, the laser control electrical unit “turns off” the output of the laser generator according to the received command.

The three-dimensional laser cutting machine tool has the advantages of high rigidity, fast processing speed and high processing accuracy, but the ability of the laser head to approach the processing area is poor and the price is high.

The three-dimensional laser cutting machine tool adopts five-axis linkage, and its structural form is mostly gantry type, as shown in figure 3a, the movement of the laser cutting head is shown in figure 3b, and the schematic diagram of optical path transmission is shown in Fig. 4.

See table 1 for the technical parameters of the three-dimensional laser cutting machine tool.

three dimensional five axis linkage laser cutting machine

Fig. 3 three dimensional five-axis linkage laser cutting machine

a) Three dimensional five axis linkage laser cutting machine tool

b) Schematic diagram of cutting head movement

schematic diagram of optical path transmission

Figure 4 schematic diagram of optical path transmission

Table 1 Technical parameters of 3D laser cutting machine tool

Axis tripMaximum speedpositioning accuracyRepeat positioning accuracy
X
Y
Z
4500mm
2500mm
1000mm
50m/min
50m/min
30m/min
±0.05mm±0.02mm
A
B
n × 360°
±135°
540°/s
540°/s
±0.015°±0.005°

3. Cutting torch for laser cutting

(1) Requirements for cutting torch and structure of cutting torch

During laser gas cutting, the requirements for the cutting torch are as follows:

1) It is required that the cutting torch can spray enough air flow.

2) The optical axis of the mirror is required to be coaxial with the direction of gas injection.

3) It is required that during cutting, metal vapor and metal splash will not damage the mirror.

4) It is required that the focal length can be easily adjusted.

The structure diagram of the coaxial laser gas cutting torch for laser gas cutting is shown in fig. 5.

structure of coaxial laser gas cutting gun

Fig. 5 structure of coaxial laser gas cutting torch

(2) Nozzle structure and air flow control technology

When cutting steel with laser, oxygen and focused laser beam are sprayed to the cut material through the nozzle to form an air flow 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 conduct exothermic reaction;

At the same time, there is enough momentum to spray and blow out the molten material.

Therefore, in addition to the beam quality and its control directly affecting the cutting quality, the nozzle structure and air flow control (such as nozzle pressure, workpiece position in the air flow, etc.) are also very important factors.

The nozzle for the laser cutting machine adopts a simple structure, that is, a conical hole with a small circular hole at the end (see Fig. 6).

The nozzle is generally made of pure copper with small volume.

It is a vulnerable part and needs to be replaced frequently.

When in use, the gas with a certain pressure Pn (gauge pressure Pg) is introduced from the side of the nozzle, which is called the nozzle pressure.

It is ejected from the nozzle outlet and reaches the workpiece surface through a certain distance. Its pressure is called the cutting pressure Pc, and finally the gas expands to the atmospheric pressure Pa.

With the increase of PN, the gas flow rate increases and Pc also increases.

It can be calculated by the following formula:

v=8.2d2(Pg+1)

Where,

  • V – gas flow rate / (L / min);
  • D — nozzle diameter / mm;
  • Pg — nozzle pressure (gauge pressure) / kPa.

There are different pressure thresholds for different gases.

When the nozzle pressure exceeds this value, the flow is a normal oblique shock wave, and the flow velocity transits from subsonic to supersonic.

This threshold is related to the ratio of Pn and Pa and the degree of freedom (n) of gas molecules: for example, n = 5 of oxygen and air, so its threshold Pn = 100KPA × 1.23.5=189kPa。

When the nozzle pressure is higher, Pn / P= (1 + 1 / n) 1 + n / 2 (Pn > 400kPa), the normal oblique shock wave of the air flow becomes a positive shock wave, the cutting pressure Pc decreases, the gas flow rate decreases, and eddy currents are formed on the workpiece surface, which weakens the role of the air flow in removing molten materials and affects the cutting speed.

Therefore, the nozzle with conical hole and small circular hole at the end is adopted, and the nozzle pressure of oxygen is often below 300kPa.

Nozzlea/mm
No. 11.0
No. 21.2
No. 32.0
structure of common nozzles

Fig. 6 structure of common nozzles

Nozzlea/mmb/mmc/mm
No. 41.51.75
No. 51.52.06

Fig. 7 structure of Lava1 nozzle

Whether for laser cutting of metal or non-metal materials, gas injection is very important.

The size of the jet volume and the structure of the nozzle are directly related to the cutting quality and cutting speed.

The nozzle diameter d is related to the spot diameter db, which is generally taken as d = (2 ~ 5) db. It is required to intake air along the tangent direction and make the ejected high-pressure air flow have a certain stiffness, which is related to the nozzle channel length L, which is generally taken as L= (1 / 25 ~ 1 / 35) f.

The blowing pressure is generally 2 ~ 6 × 105Pa (gauge pressure), which is related to the thickness of the material to be cut and the laser power.

Generally speaking, when the thickness of the material increases, the blowing pressure should also increase.

With the increase of laser power, the blowing pressure can be reduced.

The cutting speed and blowing pressure increase in a linear proportional relationship, and the notch width changes with the increase of blowing pressure.

In order to further improve the laser cutting speed, a scaled nozzle – Lava1 nozzle, can be designed and manufactured according to the principle of aerodynamics without generating a positive shock waves on the premise of increasing the nozzle pressure.

The structure shown in figure 7 can be used for manufacturing convenience.

It should be noted that the cutting pressure Pc is a function of the distance between the workpiece and the nozzle.

Due to the multiple reflections of oblique shock waves at the boundary of gas flow, the cutting pressure changes periodically.

The first high cutting pressure area is close to the nozzle outlet.

The distance from the workpiece surface to the nozzle outlet is about 0.5 ~ 1.5mm.

The cutting pressure Pc is large and stable. It is a common cutting process parameter in plate processing.

The second high cutting pressure area is 3 ~ 3.5mm of the nozzle outlet, and the cutting pressure Pc is also large, which can also achieve good results, protect the lens and improve its service life.

The selection and maintenance of the laser nozzle are very important.

Due to the roundness of the nozzle or local blockage caused by hot metal splash, the cutting quality will be significantly reduced. 

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