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To calculate the required press tonnage (nominal pressure), you must determine the total process force required for the specific operation, add any auxiliary loads, and apply a safety factor. Crucially, for mechanical presses, you must also verify the tonnage curve to ensure the machine can deliver the necessary force at the point in the stroke where processing begins.

1. Calculate the Process Force (F)

The calculation method depends on the nature of the stamping operation:

• Blanking/Shearing: The force required to cut the material is calculated using the formula F=L×t×τ×K.
  • L: Total perimeter of the sheared edge (mm).
  • t: Material thickness (mm).
  • τ: Shear strength of the material (kg/mm² or MPa).
  • K: Correction factor (typically 1.1 to 1.3) to account for tool wear and material variations.
• Bending: For V-bending, the force is proportional to the width (b), the square of the thickness (t2), and the tensile strength (σb​), and is inversely proportional to the die opening width (l).
• Drawing: The maximum drawing force is calculated as Pmax​=π×d1​×t×σb​×K1​.
  • d1​: Punch diameter (mm).
  • σb​: Tensile strength of the material.
  • K1​: Drawing coefficient based on the drawing ratio.
• Coining/Leveling: These operations require very high unit pressures. The formula is P=A×q.
  • A: Projected area of the part (mm²).
  • q: Unit pressure (MPa), which varies by material and precision requirements (e.g., 50–200 MPa for leveling).

2. Calculate the Total Load (FΣ​)

The actual output required from the press must include the force needed for auxiliary die components:

• Stripping Force (F卸​): The force to remove scrap or the workpiece from the punch, typically 2% to 10% of the blanking force.
• Ejection/Push-back Force (F推​): The force required to push parts or scrap out of the die cavity.
• Blank Holder Force (FY​): In drawing, the force used to prevent wrinkles.
• Total Load: FΣ​=Process Force+Stripping Force+Ejection Force+….

3. Apply a Safety Factor and Select Tonnage

To protect the press frame and drive mechanism, the nominal tonnage of the press should be higher than the calculated total load:

• General Stamping: Press Tonnage ≥FΣ​×(∗∗1.2∼1.3∗∗).
• Deep Drawing: Due to the long stroke and sustained high pressure, a higher safety factor is recommended, typically 1.6 to 2.0 times the total load.

4. Verify the Tonnage Curve (Critical for Mechanical Presses)

A mechanical press can only output its full rated tonnage when the ram is very close to the Bottom Dead Center (BDC).

• If your operation starts high in the stroke (e.g., drawing a deep cup or punching very thick plate), the available tonnage at that point may be less than 50% of the machine’s nominal capacity.
• Always compare your required force at the specific contact height against the machine’s pressure-stroke curve to prevent overloading or “sticking” the press.