Page 15 - MetalForming March 2020

Fig. 3 —A computer- controlled
four-cylinder hydraulic press, as
diagrammed here, was selected to produce the redesigned part shown in Fig. 2.
Computer Assistance
The press was programmed to incor- porate a regressive kinematic motion for the redraw die ring, keeping the upper die ring stationary while the ascending lower punch stretches the dome until the material is tangent with points A and B (Fig. 4b). At the instant of tangency, the die ring regresses (rais- es) at a relative rate while the punch continues its upward travel and draws additional material around the die- entry radius and into the cavity. A trac- tive force on the die ring still is required at the sheet and radius interface to pre- vent wrinkling, which makes program- ming both challenging and critical.
As the process proceeds, the edge of the open end of the shell remains in the same position as at the conclu- sion of the first reverse-draw step. Material in the wall of the outer shell never moves during the second reverse draw. This reduces the force required to deform the metal and allows a larger diameter reduction. The process does not ‘know’ that material located beyond the die radius even exists. It only experiences, in terms of applied force and friction, the material being ‘freed’ by the regressing die ring.
Though remarkable, the final process was not revolutionary. The draw-reduction ratios, tool geometry and metal flow, although enhanced by the regressing die ring, follow long established metal forming principles. The application of computer controls provided specific die kinematics and material restraint, making the process possible.
Something to consider: The process
described was developed more than 20 yr. ago and continues in production to this day. Over the past two decades, computer controls, hydraulic technol- ogy, die steels, tool surface treatment and lubricants all have advanced con- siderably. What’s possible today is lim-
The preceding column was adapted from a technical paper written by the author, Tractive Kinematic Regression— A Computer Assisted Deep Drawing Method, and presented at the 2000 ASME International Mechanical Engi- neering Conference and Expo (MED- Vol.11 p.523-530).
Tooling by Design
ited only by one’s imagination.
MF
Fig. 4—Fracturing of material immediately upon the start of a second reverse-draw (a) operation necessitated a change in tool geometry (b).
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