Plasma arc cutting is a processing method that uses the heat of a high-temperature plasma arc to locally melt (and evaporate) the metal at the incision of the workpiece and use the momentum of the high-speed plasma to remove the molten metal to form an incision. Plasma cutting machines are widely used in automobiles, locomotives, pressure vessels, chemical machinery, nuclear industry, general machinery, engineering machinery, steel structures, ships, and other industries. The factors that affect the quality and efficiency of plasma cutting are many and interact. Similarly, various methods of improvement and optimization must be used comprehensively to achieve better results.
Overview of plasma arc cutting
Plasma arc cutting is a processing method that uses the heat of a high-temperature plasma arc to locally melt (and evaporate) the metal at the incision of the workpiece and use the momentum of the high-speed plasma to remove the molten metal to form an incision. Plasma cutting with different working gases can cut all kinds of metals that are difficult to cut by oxygen, especially for non-ferrous metals (stainless steel, aluminum, copper, titanium, nickel). The cutting effect is better; its main advantage is that the thickness of the metal is not large. At this time, plasma cutting speed is fast, especially when cutting the ordinary carbon steel sheet; the speed can reach 5-6 times that of the oxygen cutting method. The cutting surface is smooth, the thermal deformation is small, and the heat-affected zone is less.
Plasma cutting machines are widely used in automobiles, locomotives, pressure vessels, chemical machinery, nuclear industry, general machinery, engineering machinery, steel structures, ships, and other industries. There are many influencing factors of plasma cutting, including cutting parameters, working gas type and purity, operator's technical ability, and understanding of the equipment. This article discusses several methods that can improve the surface quality and production efficiency of plasma cutting products from cutting program settings and discharging.
Reserved craft small boss
To ensure the integrity of the parts to be cut, the start and endpoints of the cutting are not set on the contour of the part during cutting. For outer contour parts, cutting starts and ends outside the part contour; for inner contour parts, cutting starts and ends inside the contour. We respectively call the extra cutting lines from the starting point and ending point to the part outline as the lead-in line and the lead-out line.
Generally, when designing the lead-in line, the intersection of the two is still on the contour of the part. Take the arc lead-in and lead-out line as an example, the lead-in line is a semi-circular arc lead-in, and the lead-out line is a quarter arc. Although the cutting start and endpoints avoid the contour of the part, the lead-in and lead lines meet from the actual cutting situation. There will still be defects such as pits, which will affect the appearance quality of the parts. For the parts with subsequent welding procedures they need to be repaired and then polished. For this reason, by modifying the program, a small craft boss is added to the outside of the part, and the intersection of the lead-in and lead-out lines is "moved" outside the part outline.
To reduce the subsequent polishing workload, the thickness of the plate and the width of the cutting seam should be considered when designing the small boss to ensure that the protruding amount of the small boss is within 5mm, which effectively solves the quality problems such as depression and scar at the intersection. Through observation and statistics, for plasma cutting parts with no machining allowance, the method of reserved craft small bosses, combined with the normal pre-welding grinding process, can be easily removed. The workload is not a large quality improvement effect is obvious. For plasma cutting parts with high precision requirements and no machining allowance, the application of small process bosses solves the defects such as pits at the cutting lead-in and lead-out lines. Still, for parts with low precision requirements and machining allowances, There is no need to reserve small craft bosses.
Lead wire processing
For plasma cutting, programmers will habitually set the lead-in and lead-out lines at the same time. However, when cutting the inner contour of the part of the outer contour of the small part, the author found that the torch would automatically break the arc and stop. After observation and analysis, the following reasons are derived: the sheet material is deformed during the cutting process, and the gap between the support plate at the bottom of the cutting platform of the business department is 90mm, and the set introduction and lead line make the cutting form a full-profile complete cutting process, so the cutting progresses to When near the lead-out line, the sheet material tilts or falls to the bottom of the cutting platform, causing a sudden huge change in the arc height, and the cutting torch automatically cuts off the arc and stops. For this reason, the lead wire is removed, and even 0.5～1.5mm is reserved without cutting. In this way, even if the part is tilted because the cutting program has ended, the arc has been broken and automatically idling to another cutting line according to the program, so the equipment will not automatically stop. This method has been applied to the plasma cutting production of various locomotive parts, avoiding abnormal arc interruption during the cutting process and greatly improving the cutting efficiency.
Co-edge continuous cutting cutting
The CNC plasma cutting program usually ignites and cuts one part at a time, so the accuracy of the processed parts is relatively high, but the cutting path is long, the life of the equipment is short, and the cost is high. In the pursuit of efficiency today, co-edge continuous cutting is a good choice. Common edge cutting means that there is no partition wall between two parts. A common edge is used to reduce the cutting path and cutting time and improve material utilization; continuous cutting is to cut a few steel plates without lifting the cutting nozzle. Even dozens of parts reduce the number of perforations, which is good for improving the service life of wearing parts. As the length of the cutting path and the number of perforations decrease, the life of wearing parts and reducing production will be increased. A continuous cutting time should not be too long. It is necessary to set the cutting time or the total path length reasonably according to the cutting consumables. Otherwise, the continuous cutting time is too long, and the quality of the final cut parts will be affected. And for the parts with no machining allowance, the slit width must be controlled well to prevent the compensation from not being in place.
The suitable starting point for cutting
Generally speaking, during plasma cutting, the more times of perforation and cutting, the greater the loss of wearing parts. For the same material, the thickness of plasma cutting is larger than the thickness of perforation, and some parts must be pre-pierced and then re-pierced before cutting.
For example, a certain model of HySpeedH t2000 plasma cutting machine in the United States can cut carbon steel with a maximum thickness of about 50mm, and the punching thickness can only reach about 38mm. Therefore, under normal circumstances, when using a CNC cutting machine, try to minimize the use of "perforation cutting," but set the starting point of the cutting point at the edge of the steel plate, and aim the nozzle directly at the edge of the workpiece before starting the plasma arc, and cutting into the part The position is on the maximum outer contour size of the part. Simultaneously, since most of the current plasma cutting machine slits are of good quality along the right side of the cutting direction, after the starting point of the cutting is determined, the cutting direction is basically fixed. A good cutting direction selection should keep the last cutting edge mostly separated from the motherboard. If it is separated from the motherboard prematurely, the surrounding corner frames may not be enough to withstand the thermal deformation stress during the cutting process, resulting in cutting. The part shifted during the cutting process, and the size was out of tolerance. To improve the quality of the cutting surface of the part, when the edge is used as the starting point of the cutting, an appropriate length of lead is added to improve the quality of the cut.
The lead wire avoids the problem of an automatic shutdown during the plasma cutting process, which improves the cutting efficiency and reduces the operation intensity. Preserving small bosses solves the problem of notch recession and has certain advantages for machining parts with no margin. The use of common edge continuous cutting can reduce the length of the cutting path and greatly reduce the number of perforations. Choose a suitable starting point for parts of different shapes, thereby reducing the number of perforations, which is beneficial to improve the life of wearing parts and improve the ability to cut thick plates in certain procedures.