What factors affect the accuracy of CNC flame cutting?

CNC flame straight cutting machine

With the continuous development of the steel structure industry, the component form has gradually developed from a single H-shaped beam-column to diversified and complicated. At the same time, the precision requirements for the processing of complex components by the manufacturer are also getting higher and higher, especially the lower parts of special-shaped parts. The material accuracy determines the final production accuracy of the component to a large extent, so it is necessary to study and analyze the factors that affect the cutting accuracy during the production process and take effective control measures.

Cutting gas selection

cnc oxyfuel torch

For flame cutting machines, the selection of cutting gas has a great influence on cutting quality. The currently available gases are acetylene, propylene, propane, and MPS. Different gases have different combustion characteristics, so the cutting scenes are different. Choosing the right gas can give full play to the advantages of gas cutting and realize high-efficiency and low-cost cutting operations.
Acetylene flame has the characteristics of concentrated heat, high temperature, short preheating time, low oxygen consumption, high cutting efficiency, and small cutting deformation. It is suitable for cutting thin plates and introducing shorter parts. The heat of propane flame is dispersed, the temperature is lower, the preheating time is longer than that of acetylene, the cutting edge is smooth and flat, and the cutting cost is also low. Therefore, when cutting large parts of thick plates, the cutting cost is lower than that of acetylene. The flame temperature of propylene is higher, and the preheating time is shorter than that of propane and slightly longer than that of acetylene. Due to the high heat content of the outer flame of propylene flame, it is suitable for cutting thick plates and large parts.

Oxygen concentration, speed, cutting nozzle height selection

In addition to choosing the right gas, the cutting oxygen pressure, cutting speed, and cutting nozzle height are also important factors that affect the quality of flame cutting. Studies have shown that under the same oxygen pressure, as the oxygen concentration increases, the cutting time is shortened and the oxygen demand decreases.
The choice of cutting speed is also particularly important. If the speed is too high, although the output is high, it is prone to quality defects and rough cuts. The cutting speed is too slow, the output is low, the oxidation slag adheres, and the incision surface is uneven. After a lot of practical experience, it is concluded that the best cutting speed should be in the middle of the rated speed of the cutting nozzle. If using No. 5 nozzles to cut 40mm steel plate, the rated speed range is 250~380 mm/min, and the medium speed is 315 mm/min. This range is divided into 10 levels. The cutting speed is between 336.6~358mm/min, and the cutting effect is the best.
The choice of cutting nozzle height also affects the cutting quality. If the height of the cutting nozzle is too low, the flame core may touch the surface of the part, causing the cutting seam to collapse, slag splashing to block the cutting nozzle, and even backfire. If the height of the cutting nozzle is too high, the flame cannot fully heat the incision, resulting in a decrease in cutting ability and difficulty in discharging slag. Generally speaking, it is best to keep the flame core of the flame and the working surface at 3~5mm.

Cutting sequence and lead application

A reasonable cutting sequence is conducive to uniform heating of the steel plate, and internal stresses cancel each other, thereby reducing thermal deformation of the parts. Therefore, when cutting the contour of the part, you should follow the principle of first inside and then outside, first small and then big, first-round and back, cross jumping, and complex first and simplified. To avoid displacement, deformation, and size deviation when cutting parts. Reasonable lead position and form are conducive to the complete cut of the part and stable cutting to ensure the contour quality. According to the actual production situation, the introduction position of the outer contour is generally placed at the bottom right of the contour, and the lead form of the inner contour is suitable for the art form.

The influence of the CNC program on the cutting accuracy of parts

The influence of drawing on part cutting

Generally speaking, after the CNC cutting machine is adjusted, the cutting head strictly follows the program instructions. The program instructions are compiled according to the part drawing combined with the set parameters, so the drawing of the part drawing is a direct factor that affects the CNC cutting of the part.

The influence of cutting speed

When using a flame to cut steel plates, it is necessary to select the cutting speed corresponding to the plate thickness. If the cutting speed is too fast, there will be a large amount of back drag, which will cause the steel plate to be difficult to cut through; if the cutting speed is too slow, the edges and corners on both sides of the steel plate will melt seriously, and the collapse angle will be too large, which will cause the thin plate to produce larger Deformation (Figure 1).

Drag chart at the same cutting speed

In the actual cutting process, with the appropriate increase of the cutting speed, the slit becomes narrower and the finish improves. Therefore, under the premise of ensuring that the steel plate of the part is cut through, the cutting speed should be set as high as possible.

Impact of "Bridge" set

Due to the local heating and rapid cooling of the steel plate during cutting, the cutting area will produce three-dimensional deformation, especially the thin plate parts will produce larger warping deformation.
Due to the thermal expansion and contraction of the steel plate, the cut part and the uncut part of the steel plate will be laterally displaced, resulting in errors or even errors during the program cutting of subsequent parts. In the case of many parts, it is necessary to set up a "bridge" between the parts and the base material, and between the parts and the parts, so that the base material steel plate provides certain constraints on the cut parts, thereby reducing the dimensional deviation of the parts and ensuring the cutting accuracy.

Part bridge setting diagram

According to the shape of the part, keep a small section (the length of 10~15mm) in the appropriate part without cutting temporarily (Figure 2), so that the base material will restrain the cut part and restrain its deformation and displacement. After the CNC cutting of all parts is completed, use the manual cutting torch to cut the "bridging" section. As shown in Figure 2, two 15mm long "bridges" are set at the adjacent parallel cutting lines of Part 1 and Part 2, which can significantly improve the dimensional accuracy of the cut parts. When “Bridge” is not set, cut in order from bottom to top. Due to heat input, after part 2 is cut, the uncut steel plate of part 1 will shift upwards, resulting in a deviation of the cutting width of part 1 by 4~8mm. After setting "Bridge", the width deviation of part 2 is reduced to 0, and the effect is good.

The effect of cutting order

When programming nesting materials, due to the improper cutting sequence specified, the scrap edges and remaining materials cannot be kept closed to limit the deformation and displacement of internal parts, resulting in dimensional deviation. The following principles should be paid attention to when specifying the cutting order at will:
(1) Cut the parts with small size, complex shape, and high precision requirements from the inside of the sheet, and then cut the parts with large size and simple shape.
(2) First cut the opening inside the part, and then cut the shape.
(3) After cutting the nesting parts on the steel plate, cut the surrounding waste edges.
Figure 3 shows a reasonable cutting sequence

Diagram of reasonable cutting sequence

Impact of kerf compensation

Because the theoretically given flame cutting radius does not match the actual cutting kerf, the size of the cut part deviates. At this time, it is necessary to consider the kerf compensation reasonably. If the kerf compensation is set too small, the size of the cut part will be smaller than the size of the drawing part, otherwise, the size of the cut part will be larger than the drawing part size.

The influence of cutting equipment on the cutting accuracy of parts

After research, the quality of the cutting nozzle airline directly affects the cutting quality and accuracy of the parts. The cutting speed and cutting gas pressure used for different thickness plates are different, and the selected cutting nozzle models are also different. Due to the heat variability of the sheet thickness of 6-20mm, when starting to cut the parts, it is not necessary to cut in from the edge of the steel plate, but use the perforation method to make the edge of the steel plate in a closed state, and there is a certain distance between the edge of the steel plate and the edge of the steel plate. Part size deviation caused by denaturation; for thick plates of 20 to 100 mm, since deformation is not easy to occur during cutting and the cutting nozzle is easily turned over during piercing operations, cutting can be carried out from the edge of the steel plate.
Due to the frequent movement of the torch up and down, it is very easy to cause it to be non-perpendicular to the cut steel plate, causing the cutting surface of the part to tilt and the size of the deviation. Therefore, always pay attention to adjust the verticality of the cutting gun. Before cutting the thick plate, cut out two cuts in the horizontal and vertical directions. Use a square ruler to detect and adjust the perpendicularity of the cutting gun in two directions so that the cutting surface is perpendicular to the surface of the steel plate.

The influence of operation on the cutting accuracy of parts

Oxygen-propane gas cutting surface diagram

During the cutting process, the cutting nozzle should keep a certain distance from the steel plate. If the cutting nozzle is too high, the preheating flame wind line will be scattered, and the steel plate will not be easy to cut through; if the cutting nozzle is too low, the flame for cutting the steel plate is an internal flame, which cannot reach the cutting temperature. , Thus affecting the cutting accuracy. Therefore, during the cutting process, the distance between the cutting nozzle and the steel plate should be adjusted in time due to the unevenness of the steel plate itself and other reasons. Generally, the distance between the cutting nozzle and the steel plate should be kept at 6-8mm.
In the choice of cutting fuel, oxygen, and propane gas should also be selected. By comparing the cutting quality of oxygen, propane gas, and oxygen and acetylene gas (Figure 4), it can be seen that there is very little slag on the lower edge of the cutting edge of oxygen and propane gas cutting parts. The roughness of the cut surface is low (Figure 5).

Oxygen-acetylene gas cutting surface diagram

In summary, the following conclusions can be drawn:
(1) The main factor that affects the accuracy of CNC flame cutting parts is the heat input to the steel plate.
(2) The reasonable setting of cutting speed and "bridge" through the numerical control program can effectively reduce the part error caused by numerical control flame cutting, and improve the processing efficiency and processing quality.
(3) When the CNC flame cutting equipment is in use, it should be inspected and maintained frequently; it should be standardized during operation.
(4) Our factory has begun to implement standard parameters during CNC flame cutting, and the cutting precision control effect of parts is good.

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