To determine the required press tonnage (nominal pressure), you must first calculate the total process force required for the product and then consider a safety factor along with the press’s “tonnage-stroke” characteristics.
Below are the key steps for calculation and selection:
1. Calculate the Process Force
The calculation formulas vary depending on the nature of the operation:
- Blanking (Shearing): The basic formula is P = ℓ × t × τ × k.
- ℓ: Total length of the sheared contour (mm).
- t: Material thickness (mm).
- τ: Shear strength of the material (kg/mm² or MPa).
- k: Correction factor (usually 1.1 to 1.3 to compensate for tool wear and material fluctuations).
- Bending: The required pressure is proportional to the square of the sheet thickness and is related to the bending length and the material’s tensile strength.
- Deep Drawing: The formula is F = π × d₁ × t × σb × K₁.
- d₁: Drawing diameter.
- σb: Tensile strength of the material.
- K₁: Correction factor.
- Coining and Leveling: These operations involve extremely high pressure per unit area. The calculation is P = A × q (Projected Area × Unit Pressure).
2. Calculate the Total Load (FΣ)
The actual output required from the press is not just the forming force; it must also include the loads from auxiliary mechanisms:
- Stripping Force: The force required to strip the material from the punch, roughly 2% to 10% of the blanking force.
- Ejection Force: The force required to eject the part from the die.
- Blank Holder Force: The force used to prevent wrinkling during deep drawing.
- Total Load (FΣ) = Forming Force + Stripping Force + Ejection Force + Blank Holder Force.
3. Consider the Safety Factor
To protect the machine and extend tool life, the nominal pressure of the selected press must be greater than the total process force:
- General Stamping: The press tonnage should be 1.2 to 1.3 times the calculated total force.
- Deep Drawing: Due to the long stroke and sustained load duration, it is recommended to select a press with a tonnage 1.6 to 2.0 times the total force.
4. Critical Limitation: The Tonnage Curve
This is the most frequently overlooked point when selecting a mechanical press:
- A mechanical press can only output 100% of its nominal pressure when it is near the Bottom Dead Center (BDC).
- If your operation (such as deep drawing or thick plate punching) makes contact with the material while the slide is far from BDC, the available tonnage at that position will drop significantly.
- If the processing position is too far from BDC (e.g., at 100mm or 200mm up), the available tonnage may drop to below 50% of the nominal pressure. In such cases, you must choose a higher tonnage machine or switch to a hydraulic press (which can output full tonnage throughout the entire stroke).
Summary
Required Tonnage = (Process Force + Auxiliary Loads) × 1.3 (Safety Factor). Always cross-reference this with the machine’s tonnage-stroke curve to ensure the available tonnage at the point of impact is sufficient to handle your process force.
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