What punching force is
Punching a hole in sheet metal shears the material all around the edge of the punch. The force needed depends on the length of that cut edge, the thickness of the sheet and how strong the material is in shear. Knowing it tells you whether a hand punch, a hydraulic punch or a press brake tool can do the job.
The formula
Force = cut perimeter × thickness × shear strength
The perimeter is π × diameter for a round hole, 4 × side for a square and 2 × (length + width) for a rectangle or slot. Shear strength is roughly 75% to 80% of the tensile strength of the metal.
Example
A 1/2 in round hole in 1/8 in mild steel (shear strength about 50,000 psi):
- perimeter: π × 0.5 ≈ 1.571 in;
- force: 1.571 × 0.125 × 50,000 ≈ 9,800 lbf, about 4.9 US tons.
The shop rule
For mild steel, many punch makers quote a shortcut:
Tons = diameter (in) × thickness (in) × 80
For the example, 0.5 × 0.125 × 80 = 5 tons. For other materials, multiply by a factor: about 1.5 for stainless steel, 0.6 for 6061 aluminum and 0.4 for soft aluminum.
Typical shear strengths
| Material | Shear strength |
|---|---|
| Mild steel | about 50 ksi (345 MPa) |
| Stainless steel 304 | about 75 ksi (520 MPa) |
| Aluminum 6061-T6 | about 30 ksi (207 MPa) |
| Aluminum 5052-H32 | about 20 ksi (138 MPa) |
| Brass | about 35 ksi (240 MPa) |
| Soft copper | about 22 ksi (150 MPa) |
Choosing the press
- Add the stripping force, needed to pull the punch back out, often 5% to 15% of the punching force.
- Keep a margin of at least 20% over the calculated value.
- Shear-angled punches spread the cut over the stroke and can lower the peak force by up to half on thick material.
Limits of punching
As a rule, the hole diameter should be at least equal to the sheet thickness; smaller punches in thick or hard sheet tend to break. Leave at least one to two thicknesses between the hole and the edge of the sheet, and between holes, to avoid tearing and distortion.