Taking the pure copper U-shaped contact as an example, the process improvement is implemented to address the shortcomings of the mechanical processing process after bending. Through part structure analysis and mold design, the stamping process is used instead of drilling and milling to complete the processing of the arc, notch and hole of the U-shaped contact, simplifying the process flow, avoiding the processing errors caused by multiple clamping, and greatly improving the product quality and production efficiency.
1. Introduction
As a key component of the disconnector, the processing quality and precision of the pure copper U-shaped contact directly affect the performance and reliability of the whole machine. The structure of the U-shaped contact is shown in Figure 1. In the traditional processing technology, the workpiece must first be bent into a U shape, and then clamped and processed multiple times, including milling through circular holes, keyway holes, end chamfers, milling bottom notches and other processes in the three-dimensional direction. The process flow is complicated and the processing time is long. Due to multiple clamping, it is easy to cause the accumulation of processing errors, affecting the dimensional accuracy and surface quality of the product. In addition, multiple processing also increases production costs and reduces production efficiency, making it difficult to meet the needs of modern industrial production for high efficiency, low cost and high quality.
Figure 1 U-shaped contact structure
In order to overcome the drawbacks of traditional processing technology and improve the processing efficiency and product quality of U-shaped contacts, a process based on die stamping is proposed to simplify the processing flow, reduce the processing steps, and avoid the processing errors caused by multiple clamping, so as to achieve the purpose of improving product processing accuracy and quality. In addition, since the die stamping process has the advantages of high production efficiency and low cost, it can significantly reduce production costs and improve economic efficiency.
2. Process analysis and improvement
2.1 Product structure
The contact of the isolating switch is made of pure copper material, which has good conductivity, ductility and corrosion resistance [1]. According to different current levels, it is processed into various thicknesses and sizes of contacts. The contact is designed to be U-shaped, so that it can form a larger contact area when it contacts the corresponding contact, thereby reducing contact resistance and improving conductivity. It also helps to reduce the accumulation of dust and dirt, improve self-cleaning ability, and reduce problems such as poor contact caused by pollution.
Taking the U-shaped contact on the disconnector GW10-363DW/J5000 as an example, its structural dimensions are shown in Figure 2. The workpiece material is 5mm thick pure copper plate T2Y, hardness ≥80HBW, conductivity percentage %IACS ≥97, Cu content (mass fraction) ≥99.9%[1].
Figure 2 Structural dimensions of the U-shaped contact on the disconnector GW10-363DW/J5000
The U-shaped contact has through-holes and keyway holes on both sides, notches on both ends, and threaded holes and through holes on the bottom. The through holes on both sides and the through holes on the bottom are used for positioning, and the positioning dimensions are strictly required. The holes on both sides are required to be through-hole, and the precision requirement is high, only 0.022mm. This is a great challenge for machining.
2.2 Existing process flow
The existing process flow of U-shaped contact is: sheet material cutting → pressing bending → correction → milling around → removing burrs on four sides → milling R20mm arc, side hole and keyway hole on vertical machining center → drilling bottom hole → milling bottom surface notch → drilling → tapping → reaming → pliers repair and polishing.
In the existing process flow, the circular holes, keyway holes, bottom holes and notches on both sides of the U-shaped contact are completed by milling, drilling and other mechanical processing methods, which require multiple clamping and are prone to processing errors. Ten U-shaped contact pieces after machining were randomly selected for size measurement, and the product qualification rate was only about 66% (see Table 1).
Table 1 Machining dimensions and qualified rate of U-shaped contact
2.3 Process improvement
After structural analysis of the U-shaped contact, it is believed that the through holes, keyway holes, bottom holes and notches on both sides can be completed by stamping. The existing process is improved. The improved process flow is: sheet material cutting → pressing bending → correction → milling around → removing burrs on four sides → punching R20mm arc, side hole and keyway hole → punching bottom hole → punching bottom notch → tapping → pliers repair and polishing.
The original vertical machining center milling R20mm arc, side hole and keyway hole is changed to be completed by stamping; the original drilling of 5 bottom holes is changed to one-time stamping; the original milling notch is changed to be completed by stamping.
III. Mold design
The use of molds for stamping requires comprehensive consideration of product material properties, stamping process parameters, mold structure and production efficiency. According to the U-shaped contact structure, three sets of molds need to be designed to realize the punching of the through holes, keyway holes and R20mm arcs on both sides, the one-time punching of the five bottom holes, and the punching of the notches at both ends of the bottom.
3.1 Process parameters
The calculation formula of the blanking force is
P＝KLtτ/1000 (1)
Wherein, P is the blanking force (kN); K is the safety factor, generally 1.3; L is the blanking circumference (mm), that is, the circumference of the blanking part; t is the material thickness (mm); τ is the shear strength of the material (N/mm2), pure copper τ=240N/mm2.
The blanking force of the U-shaped contact is P＝1.3×170×5×240/1000＝265.2 (kN), and a 350kN or 400kN press can be selected. The blanking clearance of pure copper is generally 13% of the material thickness, and the double-sided blanking clearance of a 5mm thick plate is about 0.65mm. The stamping speed is adjusted according to the actual production situation to ensure the stability of the stamping process and product quality [2].
3.2 Die structure design and working principle
The three sets of dies required for the stamping of U-shaped contacts all adopt a straight-mounted die structure, consisting of an upper die and a lower die. Figure 3 shows the two-side through-hole, keyway hole and R20mm arc stamping die, Figure 4 shows the five bottom hole one-time stamping die, and Figure 5 shows the bottom notch stamping die. The working principle of the side hole, keyway hole and R20mm arc stamping die is as follows: put the workpiece 3 on the die 6, and position it based on the thickness of the contact. The workpiece 3 is positioned under the clamping of the limit block (side limit block 4 and rear stop block 7). The upper die part of the die is fixedly connected to the stamping machine through the die handle 17, and the lower die part is fixedly connected to the stamping machine through the lower die seat 1. When the stamping machine is started, the upper die part moves downward, and the workpiece is clamped between the unloading plate 8 and the die 6. After punching by the punching machine, the processing of the through-holes, keyway holes and R20mm arcs on both sides is completed. Then the press stops at the lowest position, the elastic body 11 is compressed to the maximum position, and then the discharge plate moves downward under the elastic force of the elastic body, and the upper die part returns to its original state, completing a punching cycle.
Figure 3 Side hole, keyway hole and R20mm arc stamping die 1-lower die seat 2-lower pad 3-workpiece 4-side limit block 5-guide column 6-die 7-rear stopper 8-unloading plate 9-positioning guide column sleeve 10-punch 1 11-elastic body 12-guide sleeve 13-punch fixing plate 14-upper pad 15-upper die seat 16-punch 17-die handle 18-punch 2 19-punch 3
Figure 4 Bottom hole one-time stamping die
Figure 5 Bottom notch stamping die
Since it is a multi-hole simultaneous punching, the punch is made into a stepped type to delay or reduce the pressure on the punch.
The structural design and working principle of the stamping die shown in Figures 4 and 5 are similar to those of the stamping die shown in Figure 3.
The bottom hole of the die shown in Figure 4 is positioned by the inner shape of the U-shaped contact piece, so the U-shaped groove should be uniform and the bottom surface should be smooth and flat to ensure the accuracy of the bottom hole size and hole positioning size after stamping.
The die shown in Figure 5 uses the bottom hole punched in the previous process as a reference to position the notch to ensure the accuracy of the notch size.
3.3 Selection of die material
Since the stamping die needs to withstand great pressure, impact and vibration during operation, the die is very likely to be damaged, broken and deformed due to long-term wear. Therefore, when selecting the die material, Cr12Mo1V1 die steel with high hardness, good wear resistance and high strength should be preferred. Because the content of metal elements such as molybdenum and vanadium in Cr12Mo1V1 die steel is relatively high, and the grains are fully refined and the distribution of carbides is fully improved, this type of steel has good bending strength and toughness, and has a very high service life and wear resistance, which can provide a good basic guarantee for the long-term operation of the stamping die [2]. IV. Process verification and analysis
4.1 Processing size
After verification on the machine, 10 U-shaped contacts were randomly selected for stamping size measurement, and the measured sizes were all within the tolerance range. The measured average size and product qualification rate are shown in Table 2.
Table 2 U-shaped contact stamping size and product qualification rate
4.2 Tolerance band comparison
The comparison of mechanical processing and stamping size tolerance bands is shown in Table 3. It can be seen that the size tolerance band after mechanical processing shows large volatility and instability. This instability leads to large size jumps. The tolerance band is larger than the standard tolerance band, indicating that the size difference of the same batch of parts is relatively obvious, which will directly affect the interchangeability and assembly accuracy of parts during assembly. The product qualification rate is only 66%, which means that the processing cost is very high. After stamping, the size tolerance band is small, narrow, smaller than the standard tolerance band, stable in size, and high in precision, indicating that the size difference of parts in the same batch is small, and the product qualification rate reaches 100%. The high stability of size not only improves the interchangeability of products, but also simplifies the subsequent assembly process and improves production efficiency [3].
Table 3 Comparison of size tolerance bands between machining and stamping (unit: mm)
4.3 Work efficiency
The stamping process saves 48 minutes of processing time per piece compared with the machining process, and the work efficiency is increased by 3 times, which reduces the number of processes and reduces the processing difficulty.
4.4 Economic benefits
Stamping saves an average of 57.53 yuan per piece compared with machining, greatly reducing the processing cost and enhancing market competitiveness.
V. Conclusion
This paper improves the process based on the structural characteristics of the U-shaped contact piece, adopts the die stamping process to replace the traditional drilling and milling process, simplifies the process flow, and draws the following conclusions after process verification.
1) The U-shaped contact piece machining process is changed to a stamping process, which can improve dimensional accuracy, enhance dimensional stability, reduce processes, reduce processing difficulty, improve processing efficiency and product quality, and reduce costs.
2) The stamping process solves the problem that the machining process cannot guarantee the perforation accuracy, and breaks through the bottleneck that the relative position of the three-dimensional direction hole and the keyway hole cannot be guaranteed due to multiple clamping and error accumulation.
3) This stamping process can be promoted and applied to the production and processing of all U-shaped contacts of disconnectors, and also provides an important reference for the processing of U-shaped parts in other industries.
