Process Enhancements for Precision U-Shaped Connector Production
Using a pure copper U-shaped contact as an example, we implemented process improvements to address the shortcomings of the mechanical processing that occurs after bending. By analyzing the part’s structure and designing an appropriate mold, we replaced the traditional drilling and milling methods with a stamping process. This change allows us to efficiently complete the processing of the arc, notch, and hole in the U-shaped contact. As a result, we simplified the process flow, reduced errors associated with multiple clamping, and significantly enhanced both product quality and production efficiency.
1. Introduction
The processing quality and precision of the pure copper U-shaped contact are crucial to the performance and reliability of the disconnector. The structure of the U-shaped contact is illustrated in Figure 1. In traditional processing methods, the workpiece must first be bent into a U shape and then undergo multiple clamping and processing steps. This includes milling circular holes, keyway holes, end chamfers, and bottom notches in three-dimensional directions.
This process is complicated and time-consuming. The need for multiple clamping can lead to the accumulation of processing errors, which negatively impacts both the dimensional accuracy and surface quality of the final product. Additionally, the numerous processing steps increase production costs and decrease efficiency, making it challenging to meet modern industrial demands for high efficiency, low costs, and high-quality output.
To address the limitations of traditional processing technology and enhance the efficiency and quality of U-shaped contacts, we propose a die stamping process. This approach aims to simplify the processing workflow, reduce the number of steps involved, and eliminate errors caused by multiple clamping. As a result, it improves both the accuracy and quality of the final product. Furthermore, the die stamping process offers benefits such as high production efficiency and low costs, which can significantly lower production expenses and increase economic efficiency.
2. Process analysis and improvement
2.1 Product structure
The contact of the isolating switch is made from pure copper, which boasts excellent conductivity, ductility, and corrosion resistance. Depending on the current levels, these contacts are manufactured in various thicknesses and sizes. The contact is designed in a U-shape, allowing it to create a larger surface area when it connects with the corresponding contact. This design reduces contact resistance and enhances conductivity. Additionally, it minimizes the accumulation of dust and dirt, improves self-cleaning capabilities, and decreases the likelihood of poor connections due to pollution.
For instance, the U-shaped contact in the isolating switch model GW10-363DW/J5000 has specific structural dimensions detailed in Figure 2. The contact is made from a 5mm thick pure copper plate (T2Y), with a hardness of ≥80 HBW, conductivity of ≥97% IACS, and copper content (mass fraction) of ≥99.9%.
The U-shaped contact features two through holes and keyway holes on each side, along with notches at both ends. Additionally, it has threaded holes and through holes located on the bottom. The through holes on both sides and the bottom are used for precise positioning, which must meet strict dimensional requirements. The two side holes must be through holes with a high precision of only 0.022 mm. This presents a significant challenge for mechanical processing.
2.2 Existing process flow
The current process flow for producing the U-shaped contact includes the following steps: cutting the sheet material, pressing and bending, correcting, milling edges, removing burrs from all sides, milling a 20mm radius arc, making side holes and keyway holes on a vertical machining center, drilling the bottom hole, milling notches on the bottom surface, drilling additional holes, tapping, reaming, and finally, repairing and polishing with pliers.
In this existing process, the through holes, keyway holes, bottom holes, and notches at both ends of the bottom surface of the U-shaped contact are created through milling, drilling, and other mechanical processing methods. This requires multiple clamping setups and increases the likelihood of processing errors. A random sample of ten U-shaped contacts was taken for dimensional measurement after machining, revealing a product qualification rate of only approximately 66% (see Table 1).
2.3 Process Improvement
After conducting a 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 efficiently manufactured through stamping. This represents an improvement over the existing process.
The improved process flow includes cutting the sheet material, pressing and bending, correcting the shape, milling the edges, removing burrs from all four sides, stamping the R20mm arc, side holes, and keyway holes, stamping the bottom holes, stamping the notches on the bottom surface, tapping, and finally repairing and polishing with pliers.
Additionally, the original process, which involved milling the R20mm arc, side holes, and keyway holes on a vertical machining center, has been modified to stamping. Similarly, the previous drilling of five bottom holes has been replaced with a one-time stamping operation, and the milling of notches has also been changed to stamping.
3. Mold Design
Using molds for stamping involves a thorough consideration of several factors, including the properties of the product material, the parameters of the stamping process, the design of the mold, and overall production efficiency. For the U-shaped contact structure, it is necessary to design three sets of molds. These molds will be used to punch through holes, keyway holes, and R20mm arcs on both sides. Additionally, they will facilitate the one-time punching of five bottom holes and the notching of both ends of the bottom.
3.1 Process parameters
The calculation formula of 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 material shear strength (N/mm2), pure copper τ=240N/mm2.
The blanking force of the U-shaped contact is calculated as follows: \( P = \frac{1.3 \times 170 \times 5 \times 240}{1000} = 265.2 \, \text{kN} \). Therefore, you can select a press with a capacity of either 350 kN or 400 kN.
For pure copper, the recommended blanking gap is typically 13% of the material thickness. For a 5 mm thick plate, this double-sided blanking gap is approximately 0.65 mm.
It is important to adjust the stamping speed based on actual production conditions to ensure both the stability of the stamping process and the quality of the final product.
3.2 Die structure design and working principle
The three sets of dies required for stamping U-shaped contacts all utilize a straight-mounted die structure, which consists of an upper die and a lower die. Figure 3 illustrates the stamping die for the two-sided through-hole, keyway hole, and R20mm arc. Figure 4 shows the one-time stamping die for the five bottom holes, while Figure 5 displays the bottom notch stamping die.
The working principle of the side hole, keyway hole, and R20mm arc stamping die is as follows: The workpiece is placed on the die and positioned according to the thickness of the contact. The workpiece is held in place by the limit block, which includes the side limit block and the rear stop block. The upper part of the die is firmly connected to the punch through the die handle, while the lower part is attached through the lower die seat.
When the punch is activated, the upper die moves downward, clamping the workpiece between the unloading plate and the die. After the punching machine completes the process, the through-holes, keyway holes, and R20mm arcs on both sides of the workpiece are finished. The press then stops at its lowest position, compressing the elastic body to its maximum extent. Subsequently, the unloading plate moves downward due to the elastic force of the body, and the upper die returns to its original position, completing one stamping cycle.
Since multiple holes are punched simultaneously, the punch is designed in a stepped configuration to reduce the pressure on the punch and delay its impact.
The structural design and working principle of the stamping die illustrated in Figures 4 and 5 are similar to those of the stamping die shown in Figure 3.
In Figure 4, the bottom hole of the die is positioned by the inner shape of the U-shaped contact piece. Therefore, the U-shaped groove must be uniform, and its bottom surface should be smooth and flat to ensure the accuracy of the size of the bottom hole and the positioning of the holes after punching.
In Figure 5, the bottom hole created in the previous process serves as a reference point for positioning the notch, ensuring the accuracy of the notch size.
3.3 Selection of die material
Since the stamping die is subject to significant pressure, impact, and vibration during operation, it is prone to damage, breakage, and deformation over time due to wear. Therefore, when selecting die materials, it is advisable to choose Cr12Mo1V1 die steel. This steel offers high hardness, excellent wear resistance, and high strength. The elevated levels of metals such as molybdenum and vanadium in Cr12Mo1V1 die steel contribute to its refined grain structure and improved carbide distribution. As a result, this type of steel exhibits outstanding bending strength and toughness, ensuring an exceptionally long service life and wear resistance. This provides a solid foundation for the long-term operation of stamping dies.
4. Process verification and analysis
4.1 Processing size
After verification on the machine, 10 U-shaped contact pieces 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.
4.2 Comparison of tolerance bands
The comparison of tolerance bands between machining and stamping is presented in Table 3. It is evident that the tolerance band after machining exhibits significant fluctuations and instability. This instability results in large dimensional variations. The tolerance band for machined parts exceeds the standard tolerance band, indicating a noticeable size difference within the same batch. This discrepancy directly impacts the interchangeability and assembly accuracy of the parts, contributing to a low product qualification rate of only 66%, which results in high processing costs.
In contrast, after stamping, the size tolerance band shows minimal variation, with a narrower tolerance band that is below the standard tolerance band. This indicates high stability and precision, reflecting a smaller size difference within the same batch of custom CNC aluminum parts. Consequently, the product qualification rate reaches 100%. The enhanced stability not only improves the interchangeability of the products but also simplifies the subsequent assembly process, thereby increasing production efficiency.
4.3 Work efficiency
The stamping process saves 48 minutes of processing time per piece compared to mechanical processing, increasing work efficiency by three times. This reduction simplifies the overall processing steps.
4.4 Economic benefits
Stamping saves an average of 57.53 yuan per piece compared to mechanical processing, which greatly reduces the processing cost and enhances market competitiveness.
5. Conclusion
This paper presents improvements to the processing method for U-shaped contact pieces based on their structural characteristics. It proposes using a die-stamping process instead of traditional drilling and milling techniques, thereby simplifying the workflow. The following conclusions were drawn after verifying the process:
1) By changing the mechanical processing of U-shaped contact pieces to a stamping process, we can enhance dimensional accuracy and stability, reduce complexity, improve processing efficiency and product quality, and lower costs.
2) The stamping process effectively addresses the issue of perforation accuracy that is difficult to achieve with traditional mechanical processing. It overcomes the bottleneck related to the precise positioning of three-dimensional directional holes and keyway holes, which often suffers from errors due to multiple clamping.
3) This stamping process can be widely applied to the production and processing of all U-shaped contacts used in disconnectors, and it serves as a valuable reference for the manufacturing of U-shaped parts in various other industries.
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