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The application of laser cutting machines is becoming more and more popular. How to use laser technology to produce products with high quality and efficiency requires laser cutting machine operators to learn relevant knowledge, and more importantly, to constantly summarize the experience in practice. Let’s first understand several commonly used laser cutting technical parameters.

Special Device

To reduce the change in the size of the focal spot caused by the difference in the size of the pre-focus beam, the manufacturers of laser cutting systems at home and abroad provide some special devices for users to choose from:

(1) Parallel light pipe. This is a commonly used method, that is, adding a parallel light tube to the output end of the CO₂ laser for beam expansion processing, the beam diameter after beam expansion becomes larger, and the divergence angle becomes smaller, so that the near and far ends of the cutting working range can be The beam size is nearly uniform before focusing.

(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 (standoff) between the nozzle and the surface of the material. When the machine 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 spot diameter after the beam is focused remains the same in the entire processing area.

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

(4) The compensation optical path system in the x and y directions is added to the flying optical path máquina de corte. That is, when the optical path at the cutting end increases, the compensation optical path is shortened; on the contrary, when the optical path at the near cutting end decreases, the compensation optical path is increased to keep the optical path length consistent.

Cutting And Perforating Technology

Any thermal cutting technique, except in a few cases where it can start from the edge of the board, generally has to make a small hole in the board. Earlier, on the laser stamping compound machine, a punch was used to punch a hole, and then a laser was used to start cutting from the small hole. There are two basic methods of piercing for laser cutting machines without punching devices:

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 flow coaxial with the laser beam to form a hole. Generally, the size of the hole is related to the thickness of the plate. The average diameter of the blasting perforation is half of the plate thickness. Therefore, the diameter of the blasting perforation for thicker plates is larger and not round, so it should not be used on parts with higher requirements, such as oil screen slit pipes can only be used on scrap. In addition, since the oxygen pressure used for perforation is the same as that for cutting, the splash is larger.

Pulse Drilling

Pulse drilling uses a pulsed laser with high peak power to melt or vaporize a small amount of material. Air or nitrogen is often used as auxiliary gas to reduce the expansion of the hole due to exothermic oxidation. The gas pressure is lower than the oxygen pressure during cutting. Each pulsed laser produces only a small jet of particles, progressively deeper, so the perforation time for thick plates takes a few seconds. Once the piercing is complete, immediately change the assist gas to oxygen for cutting. In this way, the diameter of the perforation is smaller, and the perforation quality is better than that of blasting perforation. For this purpose, the laser used should not only have high output power; but more importantly, the time and space characteristics of the beam, so the general cross-flow CO₂ laser cutting technical cannot meet the requirements of laser cutting.

In addition, pulse perforation also needs a more reliable gas circuit control system to realize the switching of gas type, gas pressure, and control of perforation time. In the case of pulse piercing, in order to obtain high-quality cuts, the transition technology from pulse piercing when the workpiece is stationary to the constant-speed continuous cutting of the workpiece should be paid attention to. Theoretically speaking, laser cutting technical is usually possible to change the cutting conditions of the acceleration section: such as focal length, nozzle position, gas pressure, etc., but in practice, it is unlikely to change the above conditions because the time is too short.

Nozzle Design And Airflow Control Technology

When corte a laser steel, oxygen, and a focused laser beam are injected into the material to be cut through a nozzle to form a gas stream. The basic requirements for the airflow are that the airflow entering the incision should be large and the speed should be high so that enough oxidation can make the incision material fully perform an exothermic reaction; at the same time, there is enough momentum to blow 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 for laser cutting adopts a simple structure, that is, a tapered hole with a small round hole at the end. The design laser cutting technical is usually done using an experiment and error method.

Since the nozzle is generally made of red copper, the volume is small, and it is a vulnerable part, which needs to be replaced frequently, so the fluid mechanic’s calculation and analysis are not carried out. When used, gas with a certain pressure Pn (gauge pressure is 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 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. The research work shows that with the increase of Pn, the airflow velocity increases, and the Pc also increases.

The following formula can calculate it: V=8.2d2(Pg+1) V-gas flow rate L/mind-nozzle diameter mmPg-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 rate transitions from subsonic to supersonic. This threshold is related to the ratio of Pn and Pa and the degree of freedom (n) of gas molecules: such as oxygen and air, 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 of the airflow becomes a normal shock wave, the cutting pressure Pc decreases, the airflow velocity decreases, and the Eddy currents are formed on the surface of the workpiece, which weakens the effect of the airflow to remove 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 oxygen is often below 3 bar.

Máquina de corte a laser para venda