CNC Busbar Punching Tonnage: Formula and Machine Sizing

Punching tonnage is determined by the length of material being cut in one stroke, the busbar thickness, and the material’s resistance to shearing. Busbar width alone does not determine the force. A 120 × 12 mm copper bar may need a modest load for one small hole or a much larger load for a long slot or several holes punched simultaneously.

The calculation below is suitable for initial machine sizing. It does not replace the allowable-load diagram for the press, the rated capacity of the punch and die, or a trial using the buyer’s actual copper or aluminum grade.

Use one unit system from start to finish

The clearest engineering form is:

Punching force (kN) = cutting perimeter (mm) × thickness (mm) × shear resistance (N/mm²) ÷ 1,000

For a round hole, the cutting perimeter is π × diameter. For a rectangular opening without corner-radius correction, it is 2 × (length + width). If several punches cut at the same instant, add their perimeters before calculating force.

The CONIC tooling guide publishes this kN method and provides reference shear-resistance values of 300 N/mm² for copper and 200 N/mm² for aluminum. Those values are useful for screening, but temper, alloy and supplier data can change the result. Do not label 300 N/mm² as 300 kg/mm²; mixing those units produces an incorrect conversion.

Worked example: 13 mm hole through 10 mm copper

1. Perimeter: π × 13 = 40.84 mm
2. Sheared area: 40.84 × 10 = 408.4 mm²
3. Force: 408.4 × 300 ÷ 1,000 = 122.5 kN
4. Tonne-force equivalent: 122.5 ÷ 9.80665 = 12.5 tf

If the mill certificate or tool supplier specifies 250 N/mm² for the actual copper temper, the same geometry calculates to 102.1 kN, or about 10.4 tf. The difference is why a buyer should preserve the material assumption beside every result.

Thickness and perimeter scale the load linearly

The following screening table uses a 13 mm round hole, 300 N/mm² copper and 200 N/mm² aluminum. Values are calculated cutting loads, not recommended press ratings.

Thickness	Copper	Aluminum
5 mm	61.3 kN / 6.2 tf	40.8 kN / 4.2 tf
8 mm	98.0 kN / 10.0 tf	65.3 kN / 6.7 tf
10 mm	122.5 kN / 12.5 tf	81.7 kN / 8.3 tf
12 mm	147.0 kN / 15.0 tf	98.0 kN / 10.0 tf
15 mm	183.8 kN / 18.7 tf	122.5 kN / 12.5 tf

Doubling thickness doubles the calculated load. Doubling the diameter of a round hole also doubles it because circumference is proportional to diameter. A long slot can require more force than a larger-looking round hole, so the controlling RFQ input is maximum cutting perimeter per stroke, not simply maximum hole diameter.

Convert the calculation into a machine requirement

Start with the worst intended stroke, then check conditions the simple formula does not capture:

• the number of holes that cut simultaneously;
• punch and die capacity at the selected station;
• off-centre loading and the press maker’s load diagram;
• stripper force and slug-removal behaviour;
• copper alloy and temper variation;
• punch wear and sharpening interval;
• continuous duty cycle and hydraulic temperature;
• any derating applied to special shapes or thick material.

An arbitrary 30–50% multiplier is not a substitute for these checks. A project may use an engineering allowance, but its value should come from the machine maker, tooling maker and the buyer’s material range. Record both calculated load and approved press setting in the process sheet.

The DHCNC-BP-60 punching and shearing workstation is published with 600 kN nominal punching force, a maximum workpiece envelope of 6,000 × 200 × 15 mm, and hole-pitch accuracy of ±0.20 mm/m. Those specifications describe the machine envelope; each proposed hole pattern still needs the perimeter calculation and tooling confirmation above.

Die clearance is a tooling decision, not a fixed busbar rule

Clearance affects cutting load, burr, rollover, fracture surface, slug release and tool life. It must also be stated unambiguously as total clearance or clearance per side.

CONIC defines total clearance as die opening minus punch size, with per-side clearance equal to half that value. Its cutting-die clearance guide gives copper reference factors for specific servo/hydraulic and mechanical punch-press tooling. It also advises multiplying the reference for plate thicker than 3.2 mm by about 1.4 and checking the machine specification.

That table is a starting point, not a universal busbar setting. A 15% total clearance and a 15% per-side clearance are two very different tools. Put these items on the tooling drawing:

• material grade and temper;
• nominal thickness and allowed variation;
• punch dimension;
• die dimension;
• total and per-side clearance;
• acceptable burr height and cut-face condition;
• lubrication and sharpening instructions.

Hole spacing and edge distance come from the part design

Rules such as “edge distance equals two hole diameters” can be useful shop heuristics, but they are not universal requirements of IEC 61439. Edge tearing, joint bearing stress, contact area, washer dimensions, insulation clearance and the verified assembly design all influence the acceptable geometry.

The machine should reproduce the approved drawing; it should not invent the drawing’s structural limits. For switchgear work, separate three questions:

1. Does the busbar and joint design satisfy the assembly designer’s electrical and mechanical verification?
2. Can the punch and die make the feature without excessive distortion or tool risk?
3. Can the machine hold the drawing tolerance across the full bar length and batch?

IEC 61439 governs low-voltage switchgear and controlgear assemblies. It does not prescribe a general ±0.1 mm hole-position requirement for every busbar. The IEC 61439-1 scope should therefore be cited as an assembly framework, not as a machining-tolerance table.

Build a repeatable shop calculation sheet

For each recurring part family, store the hole or slot geometry, simultaneous feature count, material grade, thickness, shear-resistance source, calculated kN, tool station, clearance, approved machine setting and inspection result. Recalculate whenever material, thickness, tool layout or simultaneous-hole count changes.

Buyers comparing machine classes can use the broader CNC busbar punching machine selection guide for turret, nesting and maintenance questions. The tonnage worksheet should remain the source of truth for the load decision.