Force comes from perimeter, thickness and shear strength
You are shearing a cylinder of metal whose surface is the hole perimeter times the sheet thickness. Multiply that sheared area by the material's shear strength to get force, then divide by 2,000 to read it in US tons. All three inputs are linear: double any one and you double the tonnage.
Shear strength sets the tonnage for the same hole
A 1 inch round hole (perimeter 3.14 in) in 1/8 in (0.125 in) sheet needs very different presses depending only on the metal.
Identical hole and thickness. Switching from aluminum to stainless nearly triples the tonnage, which is why the metal must be picked before the press.
Shear strength of common sheet metals
| Material | Shear strength (psi) | Shear strength (MPa) |
|---|---|---|
| Aluminum | 10,000–22,000 | 70–150 |
| Copper | 22,000–29,000 | 150–200 |
| Mild steel | 29,000–36,000 | 200–250 |
| Brass | 26,000–32,000 | 180–220 |
| Stainless 304 | 44,000–65,000 | 300–450 |
| Titanium | 58,000–87,000 | 400–600 |
In metric the same formula gives kilonewtons: F(kN) = perimeter(mm) × thickness(mm) × shear(MPa) ÷ 1000. One metric ton-force is 9.81 kN; one US ton-force is 8.9 kN, a 10 percent gap.
What the plain formula leaves out
- Safety margin. Add about 30 percent to the theoretical force so tool wear and material variation do not overload the press.
- Die clearance. Too little clearance (under 5 percent of thickness) makes the metal tear rather than shear cleanly, raising force by 20 to 50 percent.
- Multiple holes at once. Punching several holes in one stroke adds the forces together; size the press for the total.
- Shear, not tensile. Use shear strength, not tensile strength. Shear is only about 60 to 70 percent of tensile, so the tensile figure overstates the tonnage.
Common questions
What is the formula for punch press tonnage?
Force equals the hole perimeter times the material thickness times the shear strength of the material. Divide the result by 2,000 to read it in US tons: tons = perimeter x thickness x shear strength divided by 2,000.
How do I find the perimeter of a round hole?
Multiply the diameter by pi (3.1416). A 1 inch round hole has a perimeter of about 3.14 inches. For a rectangle it is twice the length plus width; for a square, four times the side.
Why does stainless steel need so much more tonnage than aluminum?
Tonnage scales directly with shear strength. Stainless runs around 55,000 psi while aluminum is near 20,000 psi, so the same hole in stainless needs roughly three times the force.
