Punching force calculation
Punching force calculation is a core component of blanking process design. Its purpose is to determine the total force required to complete the blanking process, providing a basis for stamping equipment selection, die strength design, and production process control. Punching force calculations must include multiple components, including shear force, discharge force, and push force. Each component must be calculated based on material properties, part structure, and die type. A combination of theoretical formulas and empirical coefficients is used to ensure the accuracy and practicality of the results.
Shear force, the primary component of punching force, is calculated based on the material’s shear strength and the part’s geometric parameters. The basic calculation formula is Fshear = K × L × t × τ. The meaning and values of each parameter must be strictly standardized: 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. A larger value is used for thick sheet metal or high-strength materials. L is the perimeter of the part being punched. For special-shaped parts, the calculation must be segmented and summed. For example, the perimeter of a rectangular part is L = 2 × (length + width). Parts with holes must include the perimeter of the inner hole. t is the sheet thickness, which should be taken based on the maximum thickness actually measured to ensure a safe calculation result. τ is the material’s shear strength, which can be found in the material manual or determined experimentally. For materials with limited data, an estimate of τ ≈ 0.6-0.8σb (σb is the tensile strength) can be used. For example, when punching a low-alloy steel circular part (diameter 100mm) with a thickness of 3mm and a tensile strength of 500MPa, the shear force Fshear = 1.2×(π×100)×3×(0.7×500)=1.2×314×3×350≈395640N (about 396kN).
The calculation of the unloading force requires selecting an appropriate coefficient based on the unloading method and material thickness. The calculation formula is Funload = Kunload × Fshear. The value of Kunload ranges from 0.03 to 0.08. Specifically, for elastic unloading (spring or rubber), the value is 0.03-0.05 for thin materials (t ≤ 1mm) and 0.05-0.07 for thick materials (t > 1mm). For rigid unloading, the value is 0.06-0.08. For example, if the 3mm-thick low-alloy steel part mentioned above is unloaded using elastic unloading and Kunload is 0.06, the unloading force Funload = 0.06 × 396 ≈ 23.8kN. The magnitude of the unloading force also depends on the surface roughness of the blank and the surface quality of the punch. The rougher the surface, the greater the unloading force. If necessary, the unloading force can be reduced by increasing the wrap angle of the unloading plate or using lubricants.
The calculation of push and ejection forces is closely related to the die structure. The formula for push force is Fpush = n × Kpush × Fshear, where n is the amount of scrap stuck in the die and is equal to the ratio of the die edge wall height to the sheet thickness (generally ranged from 1 to 5). Kpush is the push force coefficient, ranging from 0.05 to 0.15, with lower values for thinner materials and higher values for thicker materials. The formula for ejection force is Ftop = Ktop × Fshear, with Ktop ranging from 0.03 to 0.08, similar to the ejection force coefficient. For example, if the die edge wall height is 15mm and the sheet thickness is 3mm, then n = 15/3 = 5, Kpush is 0.1, and the push force Fpush = 5 × 0.1 × 396 ≈ 198 kN. If the ejection force Ktop is 0.05, then Ftop = 0.05 × 396 ≈ 19.8 kN. In actual calculation, the pushing force and the ejecting force do not exist at the same time. The pushing force is calculated for the lower ejection mold, and the ejecting force is calculated for the upper ejection mold.
Calculating the total punch force requires determining the combination of various force components based on the mold structure: For molds with elastic discharge and bottom ejection, the total punch force, Ftotal, = Fshear + Fdump + Fpush; for molds with rigid discharge and top ejection, Ftotal = Fshear + Ftop. If a progressive die uses stepped punches, the total punch force is calculated as the sum of the maximum shear force of each punch and the corresponding auxiliary force, rather than simply adding them. For example, in the above example, the total punch force for the elastic discharge and bottom ejection mold, Ftotal, = 396 + 23.8 + 198 ≈ 617.8 kN. When selecting equipment, choose a press with a tonnage greater than this (e.g., 800 kN), and allow a 20%-30% margin to accommodate unexpected loads.
Correction and verification of punching force calculations are crucial for ensuring reliable results. For beveled-edge dies, the flat-edge shear force should be multiplied by a correction factor, Kskew (0.5-0.7). The larger the bevel angle, the smaller Kskew. For heated blanking, recalculate based on the shear strength at the actual temperature. For fine blanking, a counterpressure (generally 10%-20% of Fshear) should be added. During test punching, the actual punching force should be measured and compared with the calculated value. If the deviation exceeds 10%, the material properties or coefficient values should be checked for rationality and corrections should be made. Through precise calculation and field verification, the accuracy of punching force data can be ensured, ensuring safe and efficient stamping production.
