Abstract
A new method of applying blank holding force approximately proportional to the punch force in cup drawing is presented. An annular urethane pad compressed between two parts of a punch expands and applies pressure against the tubular part of a blank holder. As the punch moves to draw the blank, friction between the urethane pad and the blank holder applies automatically a blank holding force approximately proportional to the punch force. Experimental investigations were carried out with aluminum blanks to determine the optimum conditions to obtain wrinkle-free cups. Variations of the punch force and the blank holding force with punch stroke for aluminum blanks under different degrees of wrinkling are presented. Strains on the cup wall measured with this method are presented and compared with those obtained under constant blank holding force conditions. Theoretical critical blank holding pressure variation with punch stroke is compared with the pressure variation that obtains with the present method. Using the deep drawing theory an expression to predict the punch force with this method is also presented.
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