Two way punching process without concave die for steel pipe
In order to meet the assembly requirements of punched steel pipes, the deformation characteristics and influencing factors of steel pipe punching without concave die were analyzed, and it was concluded that the steel pipe material and the structure form of the grinding tool were the main factors affecting the quality of steel pipe punching. Through material selection and die structure design, a concave die punching process was improved to guarantee the dimensional accuracy and production efficiency of the punched steel pipe. The results show that this process overcomes the problems of concave and flattening deformation of steel pipe without concave die punching, achieves bi-directional punching of steel pipe without concave die, and improves the precision and productivity of steel pipe punching.
1. Overview
Punched steel pipe is mainly used to fix and connect other workpieces in the overall structure. In order to meet the assembly requirements, the steel tube must maintain a good straightness after processing, while the outer diameter must be kept within a reasonable range. The actual production process of a perforated steel pipe as shown in Figure 1, in the diameter of 26.5mm, wall thickness of 2.75mm on the welded steel pipe, along the length of the processing of a number of Φ10mm × 20mm drum-shaped through-hole. The traditional processing method is to drill a Φ10mm bottom hole first, and then mill the long hole. This method is a tedious process with many production steps, long cycle time and low processing efficiency, which cannot meet the production demand.
In order to improve the production efficiency, this study changed to the punching and cutting processing method. Punching on a circular surface, especially on the surface of a hollow steel pipe, is different from punching on a flat steel plate, as there is no bottom die support, the surface of the steel pipe can easily be depressed and deformed or even flattened. Therefore, solving the deformation problem of steel pipe in punching is the key to the whole processing. By optimizing the design of the die structure, the problem of local deformation of the steel pipe is overcome and the steel pipe is punched without concave die in both directions, which greatly improves the production efficiency.
Figure 1 Punching cross pipe
2. Analysis of steel pipe punching process without concave die
2.1 No concave die punching deformation mechanism
The usual punching die is composed of convex and concave die. Under the action of impact force, the material between the convex and concave die produces 3 stages of changes: elastic deformation, plastic deformation and fracture, and finally separates from the parent body. The punching of steel pipe is carried out without concave die, and the study shows that it also has 4 stages of elastic deformation, plastic deformation, fracture and separation. In the elastic deformation stage, the pipe wall is supported by its own stiffness when the convex die exerts pressure on the pipe wall; in the plastic deformation stage, when the pressure transmitted by the convex die exceeds the force that the pipe wall can withstand, the pipe wall material is compressed and bent at the edge, while the material around the edge and the bottom material is stretched and plastic deformation occurs, gradually forming a crater; in the fracture and separation stages, the convex die In the fracture and separation stage, the convex die is squeezed into the wall of the tube, the plastic deformation reaches its limit and a micro-crack is produced, the material near the edge forms a bright section due to the squeezing of the convex die, and the material at the edge of the convex die and the material at the bottom are necked by the tensile action and plastic fracture is produced.
Fig. 2 Punching process without concave die
2.2 Deformation characteristics and influencing factors of steel pipe punching without concave die
2.2.1 Deformation characteristics of steel pipe punching without concave die
Since the hollow part of the steel pipe cannot be supported by the concave die, under the action of punching force, the wall material at the edge is compressed and bent, and the steel pipe cross-section gradually becomes oval; the material around the edge and at the bottom is stretched and plastic deformation occurs, and the material around the convex die gradually forms a crater due to bending and stretching. Practice shows that the deformation of steel pipe punching is characterized by compression and concave deformation, as shown in Figure 3.
Figure 3 Example of steel pipe punching deformation without concave die
2.2.2 Influencing factors of steel pipe punching deformation without concave die
The dynamic modal solution of steel pipe flattening is shown in Fig. 4, from which the deformation of steel pipe under external forces can be further understood.
The deformation of the tube under biaxially symmetric load (without lateral restraint) is a four-plastic hinge deformation (Figure 4(a)), in which the circular tube is flattened under the load, and eventually the two semicircles of the circular tube plastic yield until the tube wall is in complete contact; and in the case of lateral restraint (Figure 4(b)), the deformation process of the circular tube is under the joint action of load and restraint, the steel tube wall changes from line contact to surface contact, and the tube wall firstly becomes open to the nearby Area plastic expansion, round tube gradually into a rectangular tube.
Figure 4 Illustration of plastic deformation of steel pipe under pressure
From the above analysis, it can be seen that there are two factors affecting the deformation of steel pipe. First, the impact of the punching die. The size of the deformation of the punching tube depends on the structural form of the die, and the radial support of the die can significantly improve the stiffness of the steel pipe. It can be seen that in the punching process, the steel pipe radial restraint, so that the steel pipe circumference to maintain a state of pressure to prevent the expansion of the plastic area of the wall, so that when the punch to the wall pressure, this part of the wall is punched outward expansion is inhibited, the wall can only stretch along the punching direction, thereby greatly reducing the deformation of the steel pipe, so that the punching process can be carried out smoothly.
Secondly, the impact of steel pipe material. Practice has shown that different materials of raw materials with different carbon and alloy content, the mechanical properties of steel pipe is also different. Low carbon content of the material, plasticity and toughness is better, but its strength is lower, the wall bending and tensile deformation before fracture is larger, the steel pipe collapse serious or even flattened, there is a punching piece and the steel pipe can not be separated from the phenomenon of punching impervious; high carbon content of the material, higher strength, flattening and collapse deformation is small, shallow crater, easy to meet the process requirements. In this study, punching tests were conducted on steel pipes made of Q195, Q215B and Q235B respectively, and the results showed that the steel pipes made of Q195 and Q215B have more serious problems of flattening and punching impermeability, and the steel pipes made of Q235B can meet the deformation requirements of the steel pipes and meet the dimensional requirements of the workpiece after punching.
3. Steel pipe punching die design without concave die
From the above analysis, it can be seen that the steel pipe is always under radial pressure, and the wall can only be stretched in the direction of the punch without expanding the deformation in other directions, which is the key to guarantee the quality of steel pipe punching. For the steel pipe without concave die bi-directional punching, the steel pipe is extruded by two external forces from the vertical direction at the same time, to keep the steel pipe ellipticity not to exceed the difference, the structure design of the punching die is the key.
3.1 Die structure and the role of each component
The structure of the punching die for a certain specification is shown in Figure 5. The key of this die is that the punch applies pressure to the steel pipe from opposite directions at the same time. The lower punching die in Fig. 5 consists of fixing plate 1, fixing plate 2, sleeve, rubber ring, punch, pipe holder, gland and base, and the whole is fixed on the working table of the punching machine. The composition of the upper punching die is basically the same as that of the lower punching die. When working, the lower punching die is stationary and the upper punching die moves reciprocally with the slide to complete the punching of the steel pipe.
Figure 5 Structure of punching die without concave die for steel pipe
The upper and lower pipe supports have a semi-circular device with the same diameter as the steel pipe, which plays a supporting role for the steel pipe; the rubber ring is placed in the lower part of the pipe supports, and under the static condition, the rubber ring makes the pipe supports at the top of the set under the action of elasticity. At the same time, the rubber ring also plays the role of mold release. After the tube wall is punched through, the punch is embedded in the tube wall. When the upper die slider moves up, the rebound force of the rubber ring pushes the tube holder and releases the workpiece from the punch; the punch is the main part of the die and is fixed on the connecting plate.
3.2 Working principle of the die
The workpiece is first placed on the lower die tray, with the slider going down the upper die tray touches the workpiece, at this time, the upper and lower die tray holds the steel pipe, the slider continues to go down, compressing the rubber ring, while the rubber ring forms a reaction force on the pipe tray to compress the steel pipe, the steel pipe circumference is subjected to radial pressure, which plays a supporting role for the steel pipe; as the upper and lower dies approach, the punch edge touches the pipe wall to bend the material from elastic to plastic deformation, when When the slider goes down to the lowest point, the wall material is fractured and separated, and the wall material is pushed into the inside of the steel tube, and the punch is embedded in the wall of the tube. When the slider moves up, the force applied on the rubber ring is released and the pipe holder is gradually reset under the action of the rubber ring elasticity, meanwhile, the pipe holder pushes the steel pipe out of the punch, completing a cycle of punching process.
Since the upper and lower dies have the same structure, the rubber ring is compressed to the same extent during the punching process, so the upper and lower punches basically complete the punching process at the same time. During the punching process, the upper and lower brackets are always compressed by the rubber ring, which increases the strength of the steel pipe against impact deformation. The arc on the brackets restricts the space for the wall material to stretch around the deformation, therefore, the deformation of the steel pipe can only be stretched and bent along the punching direction, which eventually forms a crater in the steel pipe punching area, while the shape and size of the steel pipe remain unchanged in the direction perpendicular to the punch.
4. Adjustment of the die and precautions
The upper and lower punches are punched into the tube from opposite directions at the same time. The smaller the stroke of the slider, the better to ensure the punching through, so as to avoid the upper and lower dies from colliding together during the working process, so the die adjustment is very important.
The purpose of die position adjustment is to ensure that the punching position is consistent and there is no deviation. The upper and lower dies are assembled together according to the drawing requirements, the upper die is mounted on the slider of the punching machine and the die orientation is adjusted according to the workpiece requirements, the slider is at the top of the stroke, and the lower die is placed on the punching machine table. Manually rotate the inertia wheel to make the slider fall slowly, so that the upper and lower dies’ pipe rests just touch, adjust the lower die’s front, back, left and right positions to align with the upper die, and finally fix the lower die’s base on the punching table, which completes the die’s position adjustment.
When the die stroke is adjusted, when the slider is at the bottom of the stroke, adjust the slider trimmer screw to control the distance between the upper and lower punches in a reasonable range. This reasonable range is necessary to ensure that punching is carried out normally. A large size may result in impervious punching or unclear separation of the punching piece from the mother body; a small size may cause the punch to be embedded too deeply in the tube wall, resulting in difficulties in release or even the top and bottom punches colliding and damaging the die. The best size for stroke adjustment is to ensure the separation of the punching piece from the mother, and the less the punch is embedded in the tube, the better.
It is worth noting that the pipe holder, punch and rubber ring are all wearing parts and should be replaced during use according to the degree of deformation of the steel pipe, the size of the punch burr and the ease of release to ensure the quality of punching and production efficiency.
5. Conclusion
- (1) Steel pipe without concave die punching. Since the hollow part of the steel pipe cannot be supported by the concave die, the deformation of the steel pipe is characterized by flattening and depression deformation under the action of punching force.
- (2) The material of the steel pipe itself affects the size of the deformation of the punching tube. Low carbon content material, its strength is low, plastic deformation ability is strong, easy to flatten and collapse deep crater; high carbon content material, higher strength, deformation resistance, flattening and collapse deformation is small, shallow crater. This product selects the steel pipe made of Q235B, which can meet the deformation requirements of the steel pipe.
- (3) The size of the deformation of the punching tube depends on the structural form of the die. The die imposes radial restraint on the steel tube to keep the steel tube circumference under pressure, which can significantly improve the stiffness of the steel tube, stop the expansion of the plastic area of the tube wall and reduce the degree of deformation of the steel tube, and the structure of the punching die described in this paper can meet the deformation requirements of the workpiece.
Author: Jinqi Wang
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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thank you for publishing this report.
what could one expect from a 25,4mm diameter pipe with 2mm wall thickness of 306 grade stainless steel?
do you think a press is necessary, could the operation be done with a clamped on screw press?
thanks again!
Thanks for explaining how it’s important for steel tubes to maintain their straightness after the punching process. Because of this, I think it would be beneficial to hire a professional punching service. That way, you can be sure that they’ll have the right equipment and expertise to ensure the straightness of the tubes.