Page 14 - MetalForming March 2020

Tooling by Design
By Peter Ulintz
Redrawing and Ironing Cylindrical Shells, Part 3
Part 1 of this series examined the differences between cylindrical redrawing and ironing processes. Part 2 provided guidelines for punch- nose geometry and draw-sleeve design. This month: A practical application for combining drawing and two redrawing operations in a single die process.
Product Design
Manufacturing processes most often are designed based on the product design. Occasionally, a product can be designed or redesigned based on the process.
For example, in 1997 a customer sought to produce an internal compo- nent for an air-dryer unit used on a truck braking system. The design com- prised two deep-drawn stampings seam-welded together (Fig. 1). Since this component creates two discrete cham- bers inside of the air dryer, the seam welds could not leak. Such a design requires a significant amount of tooling (two different parts), weld fixturing and 100-percent leak testing of the welds. As a result, an alternative design (Fig. 2) was proposed.
Process Design
The proposed process required a
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Fig. 1—This two- piece, seam- welded canister represents the original design of an internal component for an air-dryer unit used on a truck braking system.
Outer Shell
Inner Shell
Seam- Welded Joint
pendently of speed and oil temper- ature. These attributes provide opportunities to control material flow in ways not possible with a tra-
ditional hydraulic press.
Tool Geometry
To preserve as much formability as possible, large radii that blended completely and smoothly into the
die walls were incorporated to reduce high strains caused by bending and unbending. These features were made to print in subsequent restrike
operations.
During initial tool tryout, the draw
and first reverse-redraw performed as expected but the canister fractured immediately upon start of the second reverse-draw. This was due to highly localized tensile forces acting at the punch-nose radius (Fig. 4a) and the high forces required to cause the shell wall’s edge to flow toward the die cavity.
A hemispherical dome was incor- porated into the geometry of the final redraw punch (Fig. 4b) for two reasons: 1) reduce the localized tensile strains that caused breakage during the initial tool tryout and, 2) provide a predeter- mined amount of biaxial stretching of material into the die cavity to control material flow throughout the second reverse draw.
draw, reverse-draw and another reverse-draw (or double-reverse draw) performed in a single die and single press cycle.
Providing the needed volume between the inner and outer walls of the chamber required relatively large and unbalanced draw reductions. Specifically, a 1.14-mm-thick blank was reduced by 47 percent in the first draw, 30 percent in the second draw to estab- lish the outer wall (Fig. 2) and 35 per- cent in the final draw to create the inner wall. The third draw reduction presented the greatest challenge, as that reduction typically would be lim- ited to 25 to 27 percent.
A computer-controlled four-cylinder hydraulic press was selected for this process. The press and tooling (Fig. 3) consisted of four main components: (A) main cylinder to which the outer slide (or ram) is mounted,
(B) inner cylinder, (C) bolster plate with a hydraulic cushion and (D) an additional cylinder through the center of the bolster plate.
The press (cylinder) actions
are programmable for force, position, speed, time (delay) and direction. A closed servo system controlled the press, allowing pro- grammable press force acting inde-
Inner wall Outer wall
Fig. 2—As the design shown in Fig. 1 would require a signifi- cant amount of tooling, weld fix- turing and 100-
percent leak test- ing of welds, this
one-piece alternate design was proposed.
12 MetalForming/March 2020
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