Abstract: this paper introduces the basic principle of thermal cutting, and describes, analyzes and compares the characteristics of flame cutting, plasma cutting and laser cutting.

Keywords: China Steel plate thermal cutting  Flame cutting  Plasma cutting  laser cutting

There are three kinds of thermal cutting methods for steel plate: flame cutting, plasma cutting and laser cutting. Decades ago, flame cutting dominated the cutting of mild steel, which accounted for the majority of steel structural parts. However, the transition from flame cutting to plasma cutting began in the early 1980s when oxygen plasma cutting machines with several times the speed of flame cutting came on the market. In addition, in the past, laser cutting has been limited to the precision cutting of thin plates, but since 1990, due to the advent of large-scale CNC gantry laser cutting machine equipped with high-power CO2 gas laser, due to its good cutting accuracy, it has expanded its application in the field of thick plate soft steel cutting.

1.Principle of thermal cutting

Principle of thermal cutting:

The principle of thermal cutting is simply to melt the material to be cut locally with heat energy, and then blow off the melted part with air flow, so as to form a cutting method of slit.

In order to achieve cutting out any complex shape parts we need, the equipment needs complex mechanical device and control system (CNC control). Thermal cutting is a cutting method that places the cutting torch head that produces heat source (flame, plasma, laser) at the appropriate height above the steel plate to be cut, and makes it move in the non-contact state, so as to carry out free shape cutting. Unlike mechanical shearing and cutting, there is no machining reaction force, so there is no need to fix the steel plate or parts being cut, and the gas, current and laser that generate heat source are transmitted to the cutting torch head by hose, cable, mirror and optical fiber. The cutting head is driven to move at a certain speed to perform the cutting. Hot melt cutting is almost not limited by the size and shape of cutting components, and can cut relatively large parts. In addition to the two-position cutting of the steel plate, the cutting torch is equipped on the NC controlled XY slide and multi-joint robot, and 3d cutting can be performed on the sections, steel components and stamping parts.

1.1   Principle of flame cutting machine

Flame cutting, also known as gas cutting, is the process of burning gas and oxygen to melt metal.. First, the temperature of the steel plate must rise to the melting point, and then the oxygen flow oxidizes the metal in the long and narrow area.

Oxygen gas cutting can be used for cutting carbon steel and low alloy steel plate, cutting thickness up to several hundred millimeters. The cutting quality depends on the material surface, cutting speed and material thickness.

1.2   Principle of plasma cutting machine

The basic composition of plasma cutting machine is shown below, which is composed of plasma cutting torch, plasma arc power supply equipment and rf generator.

Principle of plasma generation

（1）The high frequency electron flow generates a guide arc between the nozzle and the electrode

（2）The nozzle squirts ionized gas towards the workpiece, creating a strong main arc between the electrode and the workpiece.

（3）The temperature of the main arc is over 10000 ℃. Through intense and high temperature molecular movement, gas atoms are ionized into negative electrons (-) and positive ions (+), which is a highly conductive "plasma" state.

（4）Plasma is concentrated and stabilized in a narrow cutting mouth, providing continuous heat for high-speed cutting.

1.3   Principle of laser cutting

Laser cutting is to use focused high-power density laser beam to illuminate the workpiece, so that the irradiated material quickly melts, vaporizes, ablates or ignites, and at the same time with the help of high-speed airflow coaxial with the beam to blow away the molten material. At the same time, the laser beam makes relative motion on the material to be cut along a certain trajectory, thus forming a certain shape of cutting seam. Laser cutting is one of the hot cutting methods.

 

2. Analysis and comparison of economy

For the steel plate with a thickness of 6 ~ 25mm, flame cutting, plasma cutting and laser cutting can be used. As to which cutting method should be adopted, the decision should be made after comprehensive consideration of the required cutting quality, cutting accuracy, as well as the original cost and operating cost of the device.

Due to the influence of air pressure, cutting nozzle height, preheating time and other factors, the overall cutting material deformation is large, which is not suitable for high-precision cutting, and the cutting speed is low. At the same time, it takes time to warm up before cutting, so it is difficult to meet the needs of unmanned operation.

Plasma cutting has the characteristics of fast cutting speed and wide range of cutting materials. It is suitable for cutting low thickness metal plates and a variety of non-metallic materials. The maximum cutting speed can reach 10 m / min, which is 10 times of flame cutting. Underwater cutting can eliminate the noise, dust, harmful gas and arc generated during cutting, which is conducive to environmental requirements. At present, with the maturity of high-power plasma cutting technology, the cutting thickness can reach 130mm. The cutting quality of high power plasma cutting machine with water jet technology is close to the lower limit of laser cutting accuracy (± 0.2mm). Due to the high price of laser cutting machine, it is only suitable for cutting thin plates (usually the drilling time of thick plates is long), while the cutting accuracy of fine plasma cutting machine can reach the lower limit of laser cutting, the cutting surface quality is similar, and the cutting cost is far lower than that of laser cutting, about 1 / 3 of which, and the maximum cutting thickness can reach 12mm. Therefore, the use of fine plasma cutting machine to replace the expensive laser cutting machine is conducive to the most economic way to the large amount of medium and thin plate to implement high-speed fine cutting.

Laser cutting equipment has a large investment. At present, it is mostly used for cutting sheet steel and some non-metallic materials, with the advantages of fast cutting speed and high precision. However, laser cutting is not ideal for some materials, such as aluminum, copper and other non-ferrous metals, alloys, especially for the cutting of thick metal plates, the cutting surface is not ideal, or even unable to cut. At present, the research of high-power laser generator is to solve the cutting limitation of thick steel plate, but the cost of equipment investment, maintenance and operation consumption is also high.

3. Characteristics of three cutting methods

3.1 Flame cutting

Cutting quality: good inclination, large heat affected area, large amount of slag, rework required.

Production capacity: slow cutting speed, preheating time will increase the perforation speed.

Operation cost: low production efficiency and rework, resulting in higher cost of single operation than plasma.

Maintenance mode: usually simple maintenance can be carried out by the in plant maintenance team.

3.2 Plasma cutting

Cutting quality: large inclination angle, small heat affected area, basically no slag, good or even excellent fine cutting effect.

Production capacity: for various thickness of metal materials, the speed is extremely fast, and the perforation speed is extremely fast.

Operation cost: the wearing parts have long service life, good production efficiency and excellent cutting quality, resulting in lower cost of single operation than other technologies.

Maintenance mode: many components can be properly maintained by in plant maintenance team.

3.3 laser cutting

Cutting quality: the angle is very small, the heat affected area is very small, and there is basically no slag. Under the minimum corner, good and excellent fine cutting effect can be achieved.

Production capacity: the cutting torch can be quickly detached, which improves the production efficiency. When cutting metal materials with thickness less than 6mm, the speed is extremely fast. The thicker the metal is, the slower the speed is. The thicker the metal is, the longer the perforation time is.

Operation cost: when cutting thick materials, the consumption of electricity and gas, high maintenance cost and relatively low cutting speed lead to high cost of single operation.

Maintenance mode: need professional and technical personnel to complete complex maintenance work.

4. Comparison among three cutting methods

- Comparison of flame cutting and plasma cutting: The temperature of flame cutting is lower than that of plasma cutting, which directly results in its cutting speed is lower than that of plasma cutting, and it is unable to cut stainless steel and many other non-ferrous metals. Its advantage is that it can cut very thick plates (China has mastered the flame cutting technology of cutting 2000mm thickness), in addition, the cutting equipment and cutting cost are relatively low, and the pollution is smaller than the plasma cutting machine.

- Comparison between flame cutting and laser cutting: Flame cutting machine is far less than laser cutting machine in cutting speed and accuracy and cutting material types, but it is better than laser cutting machine in cutting thickness and cutting cost.

Conclusion after comprehensive comparison: The flame cutting machine has the advantages of small investment in facilities, large thickness of cutting materials, and suitable for cutting parts with low precision or requiring further processing. High speed cutting performance of plasma cutting is better, but its cutting quality is poorer than laser cutting. Compared with laser cutting and gas cutting, the biggest problem of plasma cutting is the cutting quality, that is, the cutting surface is inclined, cutting surface has slope angle. When the high-temperature and high-speed plasma flow is ejected from the nozzle to the steel plate to form the slit, and along the thickness direction from the surface to the bottom, although the plasma flow is of high temperature, but the heat capacity is small, so it rapidly decays in the steel plate, and the cutting ability decreases. When the cutting surface is melted in the form of balanced cutting ability, the shape of the cutting surface appears the state of wide surface side and narrow bottom side, which is inclined wedge-shaped. As the cutting surface is inclined, the size from the surface to the bottom gradually increases, and plasma cutting slope is the bottleneck of cutting quality.

Conclusion after comprehensive comparison: Plasma cutting equipment investment is moderate, processing thickness is medium, suitable for the processing industry with low precision requirements. Plasma cutting machine facilities investment is moderate, processing thickness is medium, suitable for precision requirements are not high and have further processing parts cutting.

- Compared with flame cutting and plasma cutting, laser cutting has obvious advantages:

（1）Good cutting quality

The width of the cutting seam is narrow (generally 0.1-0.5m m), the accuracy is high (generally the hole center distance tolerance is 0.1-0.4mm, the contour dimension tolerance is 0.1-0.5mm), the surface roughness of the cutting surface is good (generally RA is 12.5-25 μ m), and the cutting surface can be welded or used without further processing. No contact cutting, little thermal effect of cutting edge, basically no thermal deformation of the workpiece, completely avoiding the edge collapse caused by punching and shearing, generally, the cutting surface does not need secondary processing. The laser cutting head will not contact with the material surface to ensure that the workpiece is not scratched. The cutting surface is vertical, unlike plasma cutting, the cutting surface has bevel.

（2）High material utilization

Because the slit is narrow and the cutting surface has no bevel, the raw materials used for cutting the same parts are less. Generally, laser cutting can improve material utilization by 7% - 18% compared with flame and plasma cutting, depending on different nesting software. 

（3）Wide range of materials

Laser cutting can cut all kinds of metal and nonmetal, including carbon steel, stainless steel, aluminum alloy, copper, wood cloth, plexiglass and other materials

（4）Low operating cost

Laser cutting consumables are mainly laser gas and auxiliary gas, but the use of laser gas is almost negligible. Laser cutting machine, unlike plasma cutting machine, need to change the cutting nozzle frequently. Laser processing does not need mold, no mold consumption, no need to repair mold, save time to replace mold, thus saving processing costs, reducing production costs, especially suitable for the processing of large products. With the help of computer programming, the products of different shapes can be nested to cut the whole sheet materials, so as to maximize the utilization rate of materials.

（5）Safety and environmental protection

There is no such photoelectric and noise pollution like flame and plasma cutting machine. The working environment of operators is very good, which can meet the national requirements for environmental protection

（6）Improve the speed of new product development

After the product drawing is completed, the laser processing can be carried out immediately to get the new product in the shortest time

-        The relative disadvantages of laser cutting are:

(a)  Due to the limitation of laser power and equipment volume, laser cutting can only cut medium and small thickness plates and pipes, and with the increase of workpiece thickness, the cutting speed decreases obviously.

(b)  Cutting thickness can not compare with flame and plasma cutting. At present, the maximum thickness is only 50 mm.

(c)  Cutting speed is not as fast as plasma. Even if some cantilever flying light path, at the expense of cutting quality to obtain cutting speed, it can not compare with the plasma cutting speed.

(d)  Laser cutting equipment is very expensive, one-time investment is large

After comprehensive comparison, the conclusion is that the laser cutting machine has high equipment investment and thin material thickness, which is suitable for sheet metal manufacturing industry with high precision requirements.

The choice of cutting method depends on the material, using environment, quality requirements and cost. When we pursue efficiency, we can choose fast cutting methods; when we pursue quality, we can consider cutting methods with high cost but good quality. All in all, we should maximize cutting efficiency and reduce cutting costs.

5. Latest cutting technology trend

With the development of modern mechanical processing industry, the requirements for cutting quality and accuracy are constantly improving, and the requirements for improving production efficiency, reducing production costs and having high intelligent automatic cutting function are also increasing. 

5.1 From the China current situation of several general cutting machines, the function and performance of the flame cutting machine has been relatively mature, its material cutting limitations (only cutting carbon steel plate), low cutting speed and low production efficiency, so its scope of application is gradually shrinking, and the market share is unlikely to have a big increase.

Plasma cutting machine has the characteristics of wide range of cutting materials (all metal materials can be cut), fast cutting speed and high working efficiency. The future development direction lies in the improvement of plasma power technology, reduction of cutting surface tilt and other aspects, such as the improvement of power supply can cut thicker plates.The improvement of fine plasma technology can improve cutting speed, cutting quality and cutting accuracy.

Laser cutting machine has the characteristics of fast cutting speed, high precision and good cutting quality. Laser cutting technology has always been a key industry supported and promoted by the China government, especially the government's emphasis on revitalizing the manufacturing industry, which brings development opportunities to the application of laser cutting technology. In the long and medium term development plan, laser cutting is also listed as the key supporting technology, because it involves national security, national defense construction, industrialization of high and new technology and the development of science and technology frontier, which raises laser cutting to a high degree of attention, and will also bring great business opportunities to the manufacturing and upgrading of laser cutting machine. In the past few years, most of the laser cutting machines sold in China are imported from abroad, and the share of domestic products is very small. With the user's in-depth understanding of the characteristics of laser cutting technology and its demonstrative application, the development and manufacturing of laser cutting machine in domestic enterprises are promoted.

5.2 Development of special cutting machine. CNC pipe cutting machine is suitable for cutting cylinder orthogonal, skew, eccentric intersection and other inertia lines on various pipes. It can cut round hole, square hole, elliptical hole, etc., and cut the intersection inertia lines at the end of pipes. This type of equipment is widely used in the production of metal structures, power equipment, boiler industry, petroleum, chemical industry and other industrial sectors. CNC groove cutting machine is one of the high-end products in the industry. The rotary groove cutting function of this type of equipment can meet the requirements of different plate thickness and different angles of welding groove in the welding process. With the development of technology, Chinese shipyards are equipped with CNC plasma cutting machines with rotary groove cutting function to meet the construction requirements of high-tech and high added value ships.

5.3 Latest development trend: The new development direction of CNC cutting technology is to realize "full-time cutting, automatic cutting, efficient cutting, high-quality cutting and high nesting rate cutting" of CNC cutting machine. The core idea and guiding ideology is that under the condition that the hardware structure design and manufacturing of CNC cutting machine tend to be perfect and the same or similar, through software technology, both CNC cutting optimization nesting programming software, CNC system cutting control software, as well as automatic perforation and automatic cutting technology. Further improve the cutting efficiency and cutting quality of CNC cutting machine, and realize "full-time, automatic, efficient, high-quality and high nesting rate cutting of CNC cutting machine".