Experimental Determination of the Blankholder Forces Needed for Stretch Draw Die Design

Empirical equations are developed to predict the forces necessary to form sheet metal around draw beads and then keep the blankholder closed as material is drawn through the beads. Calculations using current design formulas yield a single value of necessary blankholder force independent of bead radii or bead height (interleaf). The expressions in this paper for the restraining (pulling) and normal (blankholder) forces are derived from experimental data using a Draw Bead Simulator. Five materials with different yield strengths and thicknesses were evaluated under various conditions of bead radii and interleaf. Using these expressions, concepts are suggested that could prove useful in determining press tonnage and the number of nitrogen cylinders and/or nitrogen pressures for a stretch draw die.