Recent developments in friction-assisted sheet metal forming processes

Three friction-aided sheet metal forming techniques have been introduced recently and are being developed. Friction-actuated blank holding technique was introduced to generate the blank holding force automatically via friction between a urethane pad and the blank holder, thus enabling deep cups to be drawn in a single-action press unaided by a cushion. The technique also provides blank holding force approximately proportional to the punch force, and as a consequence the frictional resistance at the flange of the cup is kept to a minimum necessary to suppress wrinkling. In one of the three variations of this technique, additionally, frictional traction is employed to produce hemispherical and conical cups with improved properties. Direct and inverted redrawing of cups at a draw ratio of 3 was accomplished by repeated squeezing of a urethane pad placed on top of the blank seated on a die—frictional traction helps redraw the first-stage deep-drawn cup into a very deep cup. Collar drawing is the most recent development in which friction enhances the forming of a collar from annular sheet metal blanks. Using this method collars can be drawn with desirable flange diameters or tubular lengths. Results from investigations into the above techniques are presented in this paper.