Page 12 - MetalForming February 2020

Tooling by Design
By Peter Ulintz
Redrawing and Ironing Cylindrical Shells, Part 2
A
12345
•
B
•
r1 • •
r2 •
•
C
•
r3 •
r4 •
Last month, we examined the differences between cylindrical redrawing and ironing, and guidelines for choosing diameter and thickness reduction percent- ages. This month we turn our attention to punch-nose geom- etry and draw-sleeve design.
Punch-Nose Geometry
One fundamental rule exists when it comes to the size of the punch nose radius for successive redrawing of cylindrical shells: Each should be proportionally smaller than the pre- ceding operation.
In general, the punch-nose radius for drawing will be four to 10 times the material thickness in order to prevent thinning in the bottom of the shell. Very thin materials require much larger radii, as much as 20 times material thickness.
When employed in the first draw operation, sharp radii may cause thinning in the wall of successive redrawing operations, showing as a line or depression. This impression moves higher up the wall with each additional reduction.
A specific relationship between the redraw shell radius and the die-entry radius exists to minimize metal thinning in each redraw operation, as shown in Fig. 1.
• The center of each redraw radius, approximately 0.125 in. outside of the previous cup wall (point A), depends on material thickness and the diameter of the preceding shell. The objective: Have the outside radius of the shell contact the die entry radius at approximately 45 deg. of tangency.
• The center on the punch-nose radius should be slightly inside of the following shell diameter (point B).
• The center of the punch-nose radius for the final two operations, on about the same line (point C), maintains the flat on the bottom of the shell.
Shells also may be drawn with angular corners. These beveled edges, often employed to minimize the severity of plastic flow, help align the shell in the next redraw station. However, because wrinkling is more likely to occur with
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Fig. 1—Relationship between shell radii (blue) and die radii (black). Adapted from Die Design Handbook, 3rd Ed.
angular corners, they generally require smaller reduction ratios, especially in the absence of an internal blankholder.
The angled corner profiles for the redrawing operations are developed from the finished shell, as shown in Fig. 2.
• The angle in the bottom of the preceding shell should start at a point equal to approximately one-fourth of the bottom radius in the finished shell (A).
• Angles for addition redraws should start midway between the draw reduction diameter ( Y ) of the previous shell ( Y/2), • Angle (Z) will vary, depending on material thickness: 30 deg. for materials less than 0.030 in. thick; 40 deg. for materials 0.030 to 0.060 in. thick; and 45 deg. for materials
Y 2
A = 4r
r´
YY
r
Z
10 MetalForming/February 2020
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Fig. 2—Angular-corner relationships. Adapted from Die Design Handbook, 3rd Ed.