In the processing of copper busbars in electrical systems, punching is a crucial process for connecting components. However, the burrs on the surface of the copper busbar after punching remain a persistent “hidden killer” affecting electrical safety. These seemingly tiny metal protrusions can not only lead to poor contact in bolt connections and increased conductive losses, but may also trigger corona discharge under high current conditions, or even break down the insulation layer, causing short circuits. In-depth investigation reveals a direct and close correlation between the generation of burrs in copper busbar punching and the die clearance—both excessively large and small die clearances disrupt the shearing and separation process of the copper busbar, ultimately resulting in burrs of different shapes. Understanding this intrinsic relationship is key to controlling burr formation and ensuring the quality of copper busbar processing.
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I. The Essence of Burr Generation: The Shearing and Separation Process Dominated by Die Clearance
The essence of copper busbar punching is the application of shearing force to the copper busbar through the relative movement of the punch and die, causing it to separate at a predetermined position to form a hole. In this process, the die clearance of the busbar processing machine (i.e., the distance between the punch and the die cutting edge) determines the stress distribution and fracture path of the copper busbar. When the punch presses downwards into the copper busbar, elastic deformation first occurs within the material. As the pressure increases, the deformation enters the plastic stage, and stress concentration begins at the cutting edge. When the stress exceeds the shear strength of the copper busbar, the material fractures at the cutting edge, and the upper and lower fracture surfaces eventually meet, completing the punching operation.
If the die clearance is appropriate, the fracture lines of the upper and lower cutting edges of the copper busbar can precisely align, resulting in a smooth fracture surface with almost no burrs. If the clearance is inappropriate, the fracture lines will be misaligned, and incompletely separated metal will form burrs. From a microscopic perspective, copper, as a metal with excellent ductility, produces a large number of metal fibers during fracture. Deviations in the die clearance can prevent these fibers from breaking in time, ultimately leaving burrs at the edge of the hole.
II. Insufficient Die Clearance: “Tear-Type Burrs” Caused by Extrusion Deformation
When the die clearance is less than a reasonable value, the extrusion force of the punch and die on the copper busbar is significantly enhanced. At this time, the copper busbar not only bears shear force during shearing but also experiences strong extrusion from the die cutting edge, leading to excessive plastic deformation within the material. Due to the insufficient clearance, the fracture lines of the upper and lower cutting edges cannot converge properly, resulting in a “tearing” phenomenon when the copper busbar separates—the incompletely fractured metal is forcibly peeled off from the material body under the push of the punch, ultimately forming a “tear-type burr” with a thick root and irregular shape.
These burrs are characterized by their short length but high hardness and strong connection strength to the surface of the copper busbar, making them difficult to remove through conventional grinding. More seriously, an insufficient die clearance exacerbates the wear of the punch and die, making the cutting edge prone to chipping or deformation, further deteriorating the punching quality. Furthermore, extrusion deformation also leads to a decrease in the dimensional accuracy of the hole, with a diameter deviation exceeding 0.08mm, affecting the tightness of the bolt connection.
III. Excessive Clearance: “Traction Burrs” Caused by Stretch Deformation
Conversely, when the clearance is too small, the copper busbar will exhibit significant stretch deformation during shearing when the die clearance is larger than the reasonable value. When the punch presses down, due to the excessive clearance, the die edge does not adequately constrain the copper busbar, causing the material to be excessively stretched into the die hole, forming a “flared” deformation zone. At this time, the fracture lines of the upper and lower cutting edges will be severely misaligned. After the upper cutting edge breaks, the material at the lower cutting edge must continue to bear tensile force until the stress exceeds the limit before breaking. This process generates a large number of fine, long metal fibers, forming “traction burrs” with thinner roots and longer lengths.
Although these burrs are easily broken off, their length typically exceeds 0.2 mm, posing a serious safety hazard in electrical system assembly. For example, during the processing of copper busbars at a new energy battery factory, the excessively large mold clearance (1.8 times the reasonable value) resulted in 0.25-0.3mm burrs appearing on the edge of the punched holes. During battery pack assembly, some of these burrs detached and became stuck between the cells, causing a decrease in insulation resistance and ultimately triggering a battery thermal runaway warning. Simultaneously, the excessive clearance also increases the perpendicularity deviation of the holes. When the copper busbar thickness exceeds 8mm, the perpendicularity error of the holes can exceed 0.5°, failing to meet the requirements for high-precision installation. Furthermore, tensile deformation increases the surface roughness of the hole walls, with Ra values reaching over 3.2μm, easily leading to impurity accumulation and increased contact resistance.
IV. Selection of Reasonable Clearance: Precise Matching of Copper Busbar Thickness and Performance
To control burrs during copper busbar punching, the key lies in determining a reasonable die clearance range based on parameters such as the copper busbar’s thickness and hardness. Generally, the reasonable die clearance value is positively correlated with the copper busbar thickness and needs to be adjusted in conjunction with the tensile strength of the copper busbar—for copper busbars of the same thickness, the higher the tensile strength, the larger the required reasonable clearance.
From industry practice, when the copper busbar thickness is 1-3mm (commonly used in low-voltage electrical equipment), the reasonable die clearance is typically 8%-12% of the copper busbar thickness. For example, when processing 2mm thick T2 copper busbar (tensile strength approximately 220MPa), the die clearance of the busbar punching machine should be controlled within 0.16-0.24mm. At this setting, the burr length can be controlled within 0.03mm, and the hole dimensional accuracy can reach ±0.02mm. When the copper busbar thickness is 4-10mm (mostly used in high-voltage switchgear), the appropriate clearance should be increased to 10%-15% of the busbar thickness. For example, for a 6mm thick T2 copper busbar, the clearance should be set to 0.6-0.9mm to effectively prevent tensile or compressive deformation, with a burr rate of less than 3%.
For high-strength copper alloys (such as Cu-Cr-Zr alloys, with tensile strength exceeding 400MPa), due to the material’s stronger shear resistance, the appropriate clearance needs to be further expanded to 12%-18% of the busbar thickness. A special electrical equipment factory, when processing 8mm thick Cu-Cr-Zr copper busbars, successfully reduced the burr length from 0.12mm to 0.04mm by adjusting the die clearance from the conventional 0.8mm (10% thickness) to 1.2mm (15% thickness), while also extending the punch life from 20,000 cycles to 35,000 cycles. Furthermore, the selection of a reasonable clearance also needs to consider the punching diameter—when the hole diameter is smaller than the copper busbar thickness, the clearance should be appropriately reduced to prevent excessive deformation of the hole wall; when the hole diameter is more than three times the copper busbar thickness, the clearance can be slightly increased to improve shearing efficiency.
V. Technical Means and Maintenance Points for Clearance Control
After determining a reasonable clearance, precise machining and maintenance methods are needed to ensure that the die clearance is always within the design range. In the die manufacturing process, a high-precision wire EDM machine (accuracy up to ±0.005mm) should be used to process the punch and die, and a coordinate measuring machine should be used for clearance detection to ensure that the uniformity error of the clearance in the circumferential direction of the cutting edge does not exceed 0.01mm. Uneven clearance will lead to excessively large or small clearances in some areas during punching, forming “local burrs” and affecting the overall quality.
In daily use, the changes in the die clearance need to be checked regularly. Due to wear on the punch and die over long-term use, the cutting edge size gradually increases, leading to a larger clearance. It is generally recommended to check the die clearance every 10,000 copper busbars processed. If the clearance increases by more than 20% of the reasonable value, the die should be repaired or replaced promptly. One distribution cabinet manufacturer extended the average lifespan of its dies by 40% and kept the burr rate consistently below 5% by establishing a die maintenance log and regularly checking and adjusting the clearance.
Furthermore, the impact of clearance can be further controlled by optimizing punching process parameters. For example, appropriately increasing the punch’s downward speed (within the equipment’s allowable range) can shorten the shearing and separation time of the copper busbar, reducing deformation. Using a die structure with a flexible stripper plate can apply uniform clamping force to the copper busbar during punching, preventing material movement and further reducing the probability of burr formation.
In copper busbar processing, the die clearance acts like an “invisible ruler,” directly determining the shape and size of the punched burrs. Only by precisely matching the characteristics of the copper busbar with the die clearance, coupled with scientific manufacturing and maintenance methods, can burr formation be controlled at its source, ensuring the conductivity and installation accuracy of the copper busbar, and laying a solid foundation for the safe and stable operation of the electrical system. With the development of intelligent manufacturing technology, the introduction of online clearance monitoring systems and real-time adjustment of die parameters will enable intelligent and zero-burr control of copper busbar punching, propelling the processing quality of electrical equipment to a new level.
