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Selecting the correct tonnage is critical. Here is a systematic approach.
## Basic Blanking Force Formula
**F = L × t × τ**
Where: F = force (kN), L = cut length (mm), t = material thickness (mm), τ = shear strength (MPa)
Shear strength is typically 60–80% of tensile strength. For mild steel (UTS 400 MPa): τ ≈ 280 MPa.
## Example Calculation
Blanking a 100mm diameter circle from 2mm mild steel:
- L = π × 100 = 314mm
- t = 2mm
- τ = 280 MPa
- F = 314 × 2 × 280 = 175,840 N ≈ **18 tonnes**
## Drawing Force
For deep drawing: **F = π × d × t × UTS × C**
Where d = punch diameter, C = drawing ratio factor (typically 0.6–0.7 for first draw).
## Safety Factor
Always apply a safety factor of **1.25–1.5** to account for:
- Material property variations (±10% on tensile strength)
- Tool wear increasing cutting forces over time
- Multiple operations in progressive dies
- Dynamic effects at high stroke rates
## Progressive Die Considerations
Sum all station forces. Stagger cutting stations by 1–2mm to reduce peak force by 20–30%.
## Practical Rule of Thumb
Required tonnage = (sum of all station forces) × 1.3 safety factor, then select the next standard press size up.