XT Laser-fiber laser cutting machine

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

1. Special equipment.

In order to reduce the change of focus spot size caused by the change of beam size before focusing, domestic and foreign laser cutting system manufacturers provide some special devices for users to choose:

（1） Collimator. This is a common method, that is, add a collimator at the output end of the CO2 laser to expand the beam. The beam size is almost uniform before focusing.

(2) The lower axis of an independent movable lens is added to the cutting head. It and the Z axis controlling the distance (distance) from the nozzle to the material surface are two independent parts. When the worktable moves or the optical axis moves, the F axis of the beam moves from the near end to the far end at the same time, so the diameter of the spot after the beam is focused remains the same in the whole processing area.

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

(4) A compensation optical path system in the x and y directions is added to the flying optical path cutter. In other words, when the optical path at the far end of the cutting increases, the compensation optical path decreases. On the contrary, when the optical path at the near end of the cutting decreases, the compensation optical path increases to maintain the same length of the optical path.

2. Cutting and perforating technology.

Any kind of thermal cutting technology, except in a few cases, can start from the edge of the plate. Generally, a small hole must be made on the plate. In the early days, on the laser punch compound machine, a punch was used to punch holes, and then a laser was used to cut holes. There are two basic drilling methods for laser cutting machine without drilling device:

（1） Blasting and perforation: (blasting and drilling), the material is irradiated by a continuous laser to form a hole in the center, and then the molten material is rapidly removed by the oxygen flow coaxial with the laser beam to form a hole. Generally speaking, the size of the hole is related to the thickness of the plate. The average diameter of blasting holes is half of the plate thickness. Therefore, the diameter of the blasting hole on the thicker plate is large and not round, so it is not suitable for use on parts with higher requirements (such as oil screen slot tube), and can only be used on scrap steel. In addition, because the oxygen pressure used during perforation is the same as that used during cutting, the spatter is large.

(2) Pulse drilling: (pulse drilling) use peak power pulse laser to melt or evaporate a small amount of material. Air or nitrogen is often used as auxiliary gas to reduce pore expansion caused by exothermic oxidation. The gas pressure in the cutting process is lower than the oxygen pressure. Each laser pulse only produces a gradually penetrating small particle jet, so it takes several seconds to penetrate a thick plate. Once the puncture is completed, immediately switch the auxiliary gas to oxygen for cutting. This kind of perforating diameter is small, and its perforating quality is better than blasting perforating. The laser used for this purpose should not only have high output power. More important is the temporal and spatial characteristics of the beam, so the general transverse 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 switch of gas type and gas pressure and the control of perforation time. In the case of pulse piercing, in order to obtain high-quality cutting, attention should be paid to the transition technology from pulse piercing when the workpiece is stationary to constant speed continuous cutting of the workpiece. Theoretically, the cutting conditions of the acceleration section can usually be changed, such as focal length, nozzle position, gas pressure, etc., but in actual operation, due to the short time, it is unlikely to change the above conditions.

3. Nozzle design and airflow control technology.

When laser cutting steel, oxygen and focused laser beam are directed to the material to be cut through the nozzle, thus forming an air jet. The basic requirement for air flow is that the air flow into the incision should be large and high in order to fully oxidize and make the incision material fully conduct exothermic reaction. At the same time, there is enough momentum to eject the molten material. Therefore, in addition to the quality of the beam and its control, which directly affect the cutting quality, the design of the nozzle and the control of the air flow (such as the nozzle pressure, the position of the workpiece in the air flow, etc.) are also very important factors. The nozzle used for laser cutting adopts a simple structure, that is, a conical hole with a small round hole at the end. The design usually adopts trial and error method.

Because the nozzle is generally made of red copper, its volume is small, it is a vulnerable part, and it needs to be replaced frequently, so the calculation and analysis of hydrodynamics is not carried out. When in use, Pn (gauge pressure is Pg) with a certain pressure is sprayed from the side of the nozzle, which is called nozzle pressure. Spray from the nozzle outlet and reach the workpiece surface after a certain distance. The pressure is called cutting pressure Pc, and finally the gas expands to atmospheric pressure. Dad’s research shows that with the increase of Pn, the air flow increases, and Pc also increases.

It can be calculated by the following formula: V=8.2d2 (Pg+1).

V-type gas flow L/min

D – Nozzle diameter mm.

PG – nozzle pressure (gauge pressure) rod.

There are different pressure thresholds for different gases. When the nozzle pressure exceeds this value, the air flow is a positive oblique shock wave, and the air flow transits from subsonic to supersonic. This threshold is related to the ratio of Pn to Pa and the degree of freedom (n) of gas molecules: for example, the n of oxygen and air is 5, so the threshold Pn=1bar × （1.2）3.5=1.89 bar。 When the nozzle pressure is high, Pn/Pa=(1+1/n) 1+n/2 (Pn. 4bar), the positive oblique shock wave of the air flow becomes positive shock wave, the cutting pressure Pc decreases, the air flow speed decreases, and the eddy current forms on the surface of the workpiece, which weakens the effect of the air flow to remove the molten material and affects the cutting speed. Therefore, when using nozzles with conical holes and small round holes at the end, the nozzle pressure of oxygen is often lower than 3 bar.