Calculation of punching force
Calculating punching force is a crucial step in blanking process design, directly impacting the selection of stamping equipment, die structure design, and production safety. Punching force refers to the total force required during the blanking process, including shear force, unloading force, push force, and ejection force. Shear force is the core component, accounting for the vast majority of the total punching force. Accurately calculating punching force requires comprehensive consideration of the material’s mechanical properties, the part’s geometric parameters, and process conditions. Scientific formula derivation and empirical corrections ensure the reliability of the calculated results.
Calculating shear force is fundamental to punching force calculations. Its magnitude depends on the material’s shear strength, the part’s circumference, and the sheet thickness. The classic formula for calculating shear force is: F = K × L × t × τ, where F is the shear force (N), K is the safety factor (generally set between 1.1 and 1.3 to account for factors such as material property fluctuations and cutting edge wear), L is the part’s circumference (mm), t is the sheet thickness (mm), and τ is the material’s shear strength (MPa). For example, for a circular part with a diameter of 50 mm and a thickness of 2 mm made of mild steel (τ = 300 MPa), its circumference L = π × 50 ≈ 157 mm, and the shear force F = 1.2 × 157 × 2 × 300 ≈ 113,040 N (approximately 11.3 kN). This formula is applicable to the punching process using flat-edge dies. It is simple to calculate and its accuracy meets most production requirements.
The calculation of discharge force, push force, and ejection force should be determined based on the die structure and the characteristics of the part being punched. The discharge force is the force required to remove the part or scrap from the punch. It is calculated as: Fun = Kun × F, where Kun is the discharge force coefficient and ranges from 0.03-0.08, depending on the material thickness and discharge method (lower values for elastic discharge and higher values for rigid discharge). The ejection force is the force required to push out scrap or workpieces stuck in the die. It is calculated as: Fpush = n × Kpush × F, where n is the number of scraps stuck in the die at once (generally 1-5), and Kpush is the ejection force coefficient, ranging from 0.05-0.15. The ejection force, similar to the push force, is used to eject the workpiece from underneath the die. It is calculated as: Ftop = Ktop × F, with Ktop ranging from 0.03-0.08. The total punching force is the algebraic sum of shear force, discharge force, push force and ejection force. The specific combination needs to be determined according to the mold structure (for example, the elastic discharge mold needs to calculate the shear force + discharge force, and the bottom ejection mold needs to calculate the shear force + push force).
The mechanical properties of the material have a significant impact on the shear force calculation. The shear strength of different materials varies greatly, so accurate values must be found through experiments or manuals. For example, the shear strength of low carbon steel is about 250-350 MPa, high carbon steel is about 400-600 MPa, aluminum alloy is about 80-150 MPa, and stainless steel is about 300-500 MPa. The shear strength of the same material may vary due to heat treatment conditions and rolling processes, so the calculation should be based on the actual material conditions. In addition, the uniformity of sheet thickness will also affect the punching force. When the thickness deviation is large, it is necessary to calculate according to the maximum thickness to ensure that the equipment has sufficient load-bearing capacity.
Punching force calculations also need to consider the effects of the die’s cutting edge condition and blanking speed. Reduced cutting edge sharpness leads to increased shear force, so the cutting edge needs to be regularly sharpened during mass production, and the safety factor K should be appropriately increased during calculations. The effect of blanking speed on shear force is more complex. At low-speed punching (≤30 times/min), shear force changes little. At high-speed punching (≥100 times/min), shear force may increase by 5%-10% due to material deformation lag. For precision punching or punching of special materials, additional forces such as back pressure and blank holder force must also be considered. These forces are typically taken as 10%-30% of the shear force based on experience. By comprehensively considering these factors, punching force calculations can more closely reflect actual production needs, providing a reliable basis for equipment selection and mold design.
