Flame cutting is a common method for rough processing of steel plates. It is a relatively mature thermal cutting method. The cutting metal thickness ranges from 1 mm to 1 meter and is suitable for cutting steel plates above 20 mm. Flame cutting uses the high temperature generated during the combustion of iron oxide to cut carbon steel. The design of the cutting torch provides sufficient oxygen for burning iron oxide to ensure a good cutting effect. The cost of cutting equipment is low and it is the only economical and effective means to cut thick metal plates, but it has its shortcomings in thin plate cutting. Compared with plasma, the heat-affected zone of flame cutting is much larger and the thermal deformation is larger. To cut accurately and effectively, the operator needs to have superb skills to avoid the thermal deformation of the metal plate in time during the cutting process.
Regarding the energy methods used in the cutting process, there are many types in the world. Among them, the common oxygen-acetylene, oxygen-propane, oxygen-natural gas, oxygen-hydrogen, and so on. Each cutting method has its characteristics. Oxygen-acetylene cutting was the first used, and it is still used in large quantities now. However, with the continuous emergence of other new energy sources (such as propane, etc.), the scope of use has been continuously reduced. Compared with propane, oxy-acetylene cutting has the advantages of short preheating time and fast cutting speed, but the disadvantage is that the acetylene used is high in cost, poor in safety, and average quality. The advantages of oxygen-propane cutting are low cost and high safety. The disadvantages are long preheating time, slow cutting speed, and more quality problems. Oxy-acetylene cutting has a long history and has a relatively mature experience. However, the history of oxygen-propane cutting is short, and various parameters are not very mature. Especially in enterprises, the cutting of different thicknesses is based on experience, and there are obvious shortcomings in theory and practice. Regarding the quality of the cutting surface, the laser is the best, plasma is the second, and flame is the worst, which is also an object that needs to be controlled. The quality defects formed in the flame cutting process include more than a dozen kinds of concave cores, rough cutting surfaces, difficult to remove sticky slag, upper edge meltdown, inclined cutting surfaces, and incomplete cuts.
| Quality defects and reasons formed during flame cutting | |
|---|---|
| Type of defect | Causes |
| Concave cutting surface | (1) The cutting speed is too fast (2) The cutting pressure is too high |
| There are gaps in the cutting surface | (1) There is thick rust or oxide scale on the cutting surface (2) The cutting process is interrupted and the connection is not good when restarting (3) The walking of the cutting machine is not stable |
| Rough cutting surface | (1) The pressure of cutting oxygen is too high (2) Cutting speed is too fast (3) Inappropriate selection of cutting nozzle (4) The preheated flame energy is too large |
Regarding the size and quality of the parts after cutting, it is also the worst in flame cutting. The size error of 1000mm can reach ±1mm. It can be seen that the error is very large. The larger the size of the part, the more obvious the error accumulation. The main reason is the influence of thermal expansion and contraction. Especially cutting with multiple torches, the performance is more obvious.
Regarding the shape quality of the parts after cutting, flame cutting is also the worst. Common side bends, out-of-round holes, and twisting of strip parts are common. For the side-bending problem of strip parts, if a straight strip cutting machine is used, the quality can be effectively controlled. However, if the part is slightly irregular and must be processed by a CNC cutting machine, the performance is more obvious. When the part is longer and the steel plate is thinner, the side bending phenomenon becomes more serious.
The factors that affect the cutting quality of CNC flame cutting machines include oxygen purity, preheating flame energy rate and time, oxygen pressure, cutting speed, distance from cutting nozzle to workpiece, cutting nozzle quality, and cutting nozzle selection. In the existing literature, the selection of various cutting parameters generally has a large range and has little reference significance. For example, what pressure, speed, distance, and cutting nozzle are used to cut steel plates with a thickness of 25mm can ensure the best quality.
