How to Calculate Press Tonnage for Blanking and Piercing (With Example)
Choosing the wrong press is expensive either way. Too small, and the tool stalls or the press is overloaded. Too large, and you pay for capacity and running cost you don't need. For cutting operations, the required force is easy to estimate.
The formula
Cutting force (N) = cut perimeter (mm) × material thickness (mm) × shear strength (N/mm²)
To convert to tonnes, divide by 9,810.
- Cut perimeter is the total length of all edges cut in one stroke: the outer profile plus every hole and notch cut at the same time.
- Shear strength is the stress at which the material shears. If you only know the tensile strength, a common approximation is shear strength ≈ 0.8 × tensile strength.
Typical shear strength values
These are approximate ranges for planning only. Always check the material certificate or supplier data for your exact grade and temper.
| Material | Approx. shear strength (N/mm²) |
|---|---|
| Aluminium (soft grades) | 70 – 110 |
| Copper (soft to half-hard) | 150 – 220 |
| Brass (half-hard) | 220 – 300 |
| Mild steel / CRCA | 280 – 320 |
| Stainless steel 304 | 400 – 500 |
Worked example
A rectangular plate 50 mm × 30 mm with two Ø6 mm holes, in 1.2 mm CRCA. The blank and both holes are cut in the same stroke.
- Outer perimeter: 2 × (50 + 30) = 160 mm
- Hole perimeters: 2 × (π × 6) = 37.7 mm
- Total cut length: 160 + 37.7 = 197.7 mm
- Force: 197.7 × 1.2 × 300 = 71,172 N
- In tonnes: 71,172 ÷ 9,810 ≈ 7.3 tonnes
Add the margins
The cutting force is not the full story:
- Stripping force: pulling the punch back out of the material typically adds around 10–20% of the cutting force.
- Forming stations: in a progressive die, bends and forms add their own force. All stations working in the same stroke must be added together.
- Spring pressure: strippers and pressure pads use springs that the press must also compress.
- Safety margin: presses should not run at their rated limit. Allow a margin of roughly 20–30%.
For our example, 7.3 t plus 15% stripping force is about 8.4 t. With a 25% margin, that gives about 10.5 t. A press of about 12 tonnes or more would be a sensible choice.
Reducing the force you need
- Stagger the cuts across stations so the heaviest cuts don't happen in the same stroke.
- Use different punch heights so the punches enter the material one after another.
- Add shear angles to punches or dies on long cuts.
- Choose the right clearance. Clearance that is too tight increases force and wear. See Controlling Burrs in High Speed Stamping.
Our presses in Manor run from 10T to 60T, and every tool we design is matched to the press it will run on. Learn about our sheet metal stamping, or send your drawing and we'll recommend the tooling and press size.