Minimum spacing between continuous punched holes on busbar

In the stamping process of busbars, the minimum hole spacing design between two adjacent holes is crucial. If the hole spacing is too small, the strong shearing force of the die during stamping will cause the metal between the two holes to be squeezed and deformed, severely thinned, or even cracked directly; at the same time, the lower die core (die wall thickness) of the punch press is also prone to chipping or breaking due to excessive force.

For common copper and aluminum busbars, the minimum hole spacing is usually calculated based on the busbar thickness t:

Standard safety value:

The distance between the edges of the two holes should be greater than or equal to twice the generatrix thickness. This is the optimal state to ensure both die life and stamped part quality. So：

S_min≥2t

After completing the production of a 3 in 1 busbar processing equipment for its customer, busbar machine manufacturer SUNSHINE conducted the following experiments on the punching function of the busbar machine:

Using the punching unit of SUNSHINE’s SS-30-3CNC PRO busbar processing machine, two 12mm diameter round holes were punched on a T2 copper busbar with a thickness of t = 8mm. During continuous punching, a width of 2t = 16mm was left between the edges of the two holes. The surface of the copper busbar was very flat, with almost no visible warping or deformation, and the dimensions of the two holes perfectly met the tolerances.

Extreme operating conditions:

When space is extremely limited and the mold has a good blanking device and high-precision guidance, the edge distance can barely reach 1.5 times the plate thickness. However, it is strongly discouraged to be less than 1.5 tons, otherwise the material in the middle is very prone to “sickle bending” or bulging deformation. So:

S_min≥1.5t

SUNSHINE also conducted an experiment on the 1.5t limit margin value, using a T2 copper busbar with a thickness of t = 8mm to punch two 12mm diameter holes. During continuous punching, a width of 1.2t = 9.6mm < 1.5t = 12mm was left in the middle. After punching the first hole, the 1.5t range around the hole wall was filled with work hardening and internal stress. When the punch pressed down on the second hole, it was accompanied by a piercing noise. The “copper skin” between the two holes was visibly bulging and thinning outwards, the busbar side was stretched, and the entire copper busbar instantly bent into a “sickle” shape.

When the distance between the two holes is exactly 1.5t=12mm, the copper busbar does not crack completely as it does at 1.2t, but the busbar will have slight bulging and slight collapse on the back.

Therefore, 1.5t is a critical threshold determined by physical laws. When the hole edge distance is less than 1.5t, the material cannot provide the necessary structural support to absorb the stamping stress, nor can it provide a symmetrical reaction force for the punch. The experimental consequences are material defects, structural instability, and die chipping. Therefore, no matter how the process is optimized, this “boundary” is insurmountable at the physical level.

Standard safety recommendation values

To cope with complex working conditions, the standard recommends that the safe centerline spacing no longer solely depend on the busbar thickness t, but instead raises the safe centerline spacing to a level related to large apertures to ensure the stiffness of the connection:

Lmin≥（d1+d2）/2+2t

When both holes are bolt holes of the same specification (d1 = d2 = d):

Lmin≥d+2t

Limit processing boundary value

In special cases where switchgear space is extremely limited and a compact design is required, if the busbar material is relatively soft (such as T2 soft copper busbar) and the stamping die has extremely high precision, the following limits can be challenged:

Lmin≥（d1+d2）/2+1.5t

• d1,d2：Diameter of the two holes
• 2t：This is the safest recommendation for minimum hole edge distance.
• 1.5t：Limit edge value
• t：busbar thickness

To ensure the accuracy of the formula, busbar machine manufacturer SUNSHINE used a CNC busbar punching and shearing machine MX602K-8C to continuously punch three circular holes with a diameter of d=14mm on a T2 copper busbar with a thickness of t = 10mm.

• Strictly adhering to safety recommendations, the center-to-center distance was set at 34mm (i.e., hole diameter 14mm + 2 × thickness 10mm). Continuous high-speed punching produced a crisp punching sound. The copper busbar was evenly stressed longitudinally, maintaining perfect flatness throughout the entire busbar. The connecting bridge between the two holes was dense and crack-free, and the die showed no abnormal wear.

• Based on a side distance of less than 1.5t, the centerline spacing was set at 25mm (calculating the internal side distance to be only 11mm≈1.1t). When punching the third hole, the copper sheet between the first two holes underwent cumulative plastic rheology due to continuous strong compression. The copper busbar instantly exhibited a severe “sickle bend” and overall lateral twisting, and the material between the two holes partially sank, rendering the product unusable.

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