Optimization of CNC plasma cutting process path and deformation control

Optimization of CNC plasma cutting process path and deformation control

CNC plasma cutting machine has many advantages such as simple operation, high accuracy, high work efficiency, low labor intensity, etc., and it is widely used in many kinds of processed parts. CNC plasma cutting machines are often used in many industries, such as chemical machinery, automotive industry, general engineering machinery, etc. For those materials that are difficult to cut by traditional methods, it can be completed by CNC plasma cutting. In addition, in the cutting speed, when cutting ordinary carbon steel sheets with small thickness, the CNC plasma cutting speed is several times faster than the traditional oxygen cutting speed. At the same time, the cutting surface remains smooth, and the thermal deformation is better. The quality of cutting parts of the CNC plasma cutting machine has a critical influence on product quality, and scientific and reasonable optimization of the cutting path is of great value.

Analysis of the causes of thermal deformation of cutting parts

In the process of plasma cutting operation, the probability of thermal deformation is small. However, it is difficult to avoid residual internal stress in the rolling and cooling of sheet metal. This is because during the cutting process, due to the influence of local high-temperature heat source factors, the metal sheet will expand along the cutting direction, and the motherboard around the expanded metal sheet will be subject to certain restrictions, causing the metal sheet to cut Larger stress occurs at the position of the edge. When the stress is greater than the yield strength of the metal, compression plastic deformation may occur, and shrinkage tends to occur during the subsequent cooling process. When it is cooled to room temperature, due to the limitation of the surrounding base metal, it may appear along the cutting direction. Shorten the deformation problem, and the residual tensile stress appears inside, which is also the source of the cutting deformation.

cnc plasma cutters CNC system

The choice of the process in the cutting process

In order to minimize the deformation caused by cutting, it is necessary to accurately position the metal sheet before cutting the sheet. When conditions permit, we can use a multi-point contact electromagnetic platform to level the metal sheet so that the internal residual stress of the sheet is completely eliminated and the flatness of the metal sheet is improved. CNC plasma cutting machine is a combination of high-speed, high-temperature, high-energy plasma cutting machines, and computer-controlled sheet metal cutting equipment. The work is automatically controlled according to pre-programming, so before determining the processing program, the cutting process of the processing-the starting point, direction, sequence of cutting, and the speed of the processing quality of the piece is decisive.

1.Starting point selection

In general, the arc of the cutting piece is mostly located at the edge of the metal plate cutting or the middle part of the slit. In the case of relatively large distances, there will often be problems of broken arcs and no arcs, resulting in some impenetrable cutting problems caused by the cutting of the workpiece. If the broken arc problem is more serious, it may even cause serious situations in the scrapping of the cutting parts. In the case that the distance between the arc starting point is very small, there are some short circuits between the nozzle and the workpiece and the danger of burning nozzles, which may also cause some damage to the cutting process. Practical construction shows that it is more appropriate to maintain a 5-7mm gap between the nozzle and the workpiece, and the distance between the air plasma cutting and the water nozzle and the compressed plasma arc cutting height can be slightly lower than the distance of 5-7mm.

2.Cutting direction selection

When cutting, to ensure the correct cutting direction, to ensure that the last cutting edge is separated from the cutting motherboard. If the cutting part and the motherboard are separated too early, it may often cause the surrounding corner frames to be unable to resist the thermal deformation stress that occurs during cutting and the cutting displacement phenomenon that occurs during the cutting process, which will eventually cause the size of the device to be out of tolerance.

plasma cutting and oxyfuel cutting machine

3.Influence of cutting sequence

The cutting sequence also has a greater influence on the cutting piece. Usually, the nested parts on the steel plate should be cut according to the order determined. The currently recognized principle is "inside first and then out:, from small to large, the line first and hole first." The first step is to cut the inner contour of the workpiece; the second step is to cut the outer contour of the workpiece; third, The first step is to cut parts with a smaller area; the fourth step is to cut large-size parts. If it is not carried out in accordance with the specified order, the cutting of the inner contour on the metal plate and the small parts will often cause deformation problems during the processing, and in severe cases, it may even cause a scrap of the workpiece; when performing common edge cutting, ensure that the workpiece The common edge and contour line are separated and cut, and the two cases of dividing into line and forming hole are considered separately, and the order is still carried out in the order of line first and hole.

4.Cutting speed selection

Cutting speed refers to the relative movement speed between the cutting torch and the workpiece during the cutting process. Proper cutting speed is a key factor in ensuring a straight cut surface. It needs to be pointed out that the current used for cutting, the gas flow rate and type, the thickness of the workpiece material, the nozzle structure, and the drag amount have a great influence on the cutting speed. If the power is constant, increasing the cutting speed will often lead to the phenomenon of skew. This requires us to make the cutting torch perpendicular to the surface of the workpiece when we are doing cutting work. In order to facilitate the removal of slag in actual operation, it can have a back inclination angle within 3°, which can maintain the best cutting on the basis of ensuring the penetration speed.

Deformation analysis and control of cutting parts

Generally speaking, the processed plate is easy to expand and contract with heat. Therefore, during the cutting process, the relative movement will appear between the processed part and the remaining material to a large extent. We compare the weight of the processed part with the weight of the remaining material, find the difference, and summarize the following three possible relative displacements.
(1) When the weight of the processed part is much higher than the weight of the remaining material, during processing, ensure that the processed part does not move, and the remaining material moves relative to the platform. The purpose of this is to not affect the size of the workpiece.
(2) When the weight of the processed part is much lower than the weight of the remaining material, during processing, the remaining material remains stationary, and the processed part moves relative to the platform. This is very easy to cause deviations in the processing process.
(3) When the weight of the processed part is basically the same as the weight of the remaining material, the processed part and the remaining material may move relative to the platform, which has a significant impact on the size of the processed part. The practice has proved that, under normal circumstances, the size error of the processed part of the remaining material relative to the platform will be between 0.3 and 3mm.

1.Deformation control of one side of the workpiece

Cutting process path optimization 1

In the CNC plasma cutting process, choosing different cutting processes will also produce differences in deformation. In the type of sheet material shown in cutting figure 1, if point A is selected as the arc starting point, the selected cutting direction and order are: A→D→C→B→A (shown in Figure 1a), after completing the AD cutting, Processing the DC section, because the residual material in the DC section is relatively narrow, during the cutting process, the high temperature will cause the residual material in the DC section to appear
With the linear extension, the CB segment will deflect outward. After the cutting is completed, the size of the DC segment will be reduced to a certain extent by δ (shown in Figure 1b), and the size of δ is proportional to the size of the DC segment. If the cutting sequence of A→B→C→D→A is selected, the workpiece will be separated from the mother through the DA path at the end, which can effectively reduce the cutting deformation.

Cutting process path optimization 2

2.Deformation control of slender parts

For the cutting of slender parts such as Figure 2, if the cutting sequence and route of A→B→C→D→A are followed, during the cutting of the DA segment, the expansion of the BC segment can further prevent the CD segment from appearing and continuing. Expansion, after the overall cutting is completed and cooled, the shrinkage of the DA segment is greater than the shrinkage of the BC segment, which will cause the workpiece to bend to the DA side. The amount of side bend δ is related to the aspect ratio of the workpiece. The greater the disparity of the aspect ratio, the greater the amount of side bend δ will become. If two pieces are paired for cutting (as shown in Figure 3), if we choose to point A as the starting point, the cutting direction and order of the workpiece are: A→B→C→D→E→A→F. When the DE section is completed, the aspect ratio of the workpiece separated from the motherboard will be reduced by half, and the side bending amount δ will also be reduced accordingly. If the shrinkage and expansion on both sides of the workpiece are roughly the same during the cutting of the AF section, Then the side bending deformation of the elongated piece can be further reduced.

3.Cutting process of special-shaped parts

Cutting process path optimization 3

For the special parts shown in Figure 4, the above-mentioned methods can be comprehensively integrated to select the cutting route.

Cutting process path optimization 4

(1) For concave parts, we can adopt two paired cutting methods, first cut the inner shape, then cut the outer shape, and finally cut the two pieces separately from the outside to the inside. The specific cutting sequence can be as shown in Figure 5, the inner order is: A1→ B1→ C1→ D1→ A1, the outer order is: A→B→C→D→A The cutting order is: E→F, H→G.

Cutting process path optimization 5

(2) For offset hollow parts, two paired cuttings can be adopted, and finally, the separation is completed. The cutting sequence can be carried out as shown in Figure 6; the inner order is: A1→B1→C1→D1 → A1, A2→ B2→ C2, → D2→ A2, the outer order is: A→B→C→D→A; finally, perform E→F cutting and separation.

In the processing industry, compared with flame cutting, CNC plasma cutting has more prominent advantages in cutting quality and efficiency. It can cut a variety of metals through different working gases, and the effect is remarkable, especially for the cutting of non-ferrous metals. In-depth research and exploration of the cutting occurrence of the CNC plasma cutting machine have a very good effect on the leveling treatment before cutting and the fixing treatment of the plate, which can effectively prevent the occurrence of displacement phenomenon during cutting; programming process, the cutting process we have a reasonable choice to ensure maximum largest dimension of the end face is separated from the mother cut; when cutting shaped pieces, we pay attention to two control modes paired cutting, cutting member preventing deformation the occurrence of state.

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