Page 13 - MetalForming January 2020

Tooling by Design
By Peter Ulintz
Redrawing and Ironing Cylindrical Shells, Part 1
Table 1—Draw/Redraw Reductions
H/D ratio
Probable Number of Draw/Redraw Operations
< 0.75
1 (Draw)
0.75 to < 1.5
2 (Draw + 1 Redraw)
1.5 to < 3.0
3 (Draw + 2 Redraw)
3.0 to < 4.7
4 (Draw + 3 Redraw)
Draw punch Draw sleeve
Last month’s column defined key parameters for successful redraw- ing of square and rectangular box shells. This month: redrawing cylin- drical shells and wall ironing.
Redrawing and Ironing Operations
The first draw reduction for a cylin- drical shell, generally limited to less than 50 percent of the flat blank diam- eter, often is referred to as the first draw or cup draw. When the required product diameter is smaller than that achievable in a single draw operation, redrawing operations reduce the cup diameter and lengthen the wall. Mul- tiple operations reduce the shell diam- eter and lengthen the wall further.
Two basic redrawing methods exist for cylindrical shells: direct redrawing and reverse redrawing. Direct redrawing (Fig. 1) reduces the cup diameter by redrawing in the same direction as the first draw. In reverse redrawing (Fig. 2), the cup inverts inside out in order to reduce its diameter. Redrawing, often performed with an internal blankhold- er, as depicted in both figures, also can be accomplished without a draw sleeve (internal blankholder) when using smaller-diameter reductions.
Regardless of the method, the per-
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—Direct redrawing reduces cup diameter by redrawing in the same direc- tion as the first draw.
cent reduction in the redraw operation always will be less than the first draw because sheet metal hardens as it deforms or is cold worked. Subsequent redrawing operations induce more work hardening, thereby reducing the percent reduction achievable for sub- sequent redraws.
Number of Redraw Operations
The approximate number of draw- ing operations can be determined from the final product dimensions by divid- ing the finished height (H) of the cylin- drical part by its final diameter (D). When H/D exceeds 0.75, then more than one draw reduction is required. Table 1 provides an approximate num- ber of draw/redraw reductions for vari- ety of H/D ratios.
Percent Reduction
When drawn from a flat blank, a cylindrical shell diameter (d) can be expressed as a percentage of the orig- inal blank diameter (D), or percent- reduction, as follows:
% reduction = 1 - (d/D) x100
For example, a 6-in.-dia. cup pro- duced from a 10-in.-dia. blank equals a 40-percent reduction in diameter: 1 - (6/10) x100 = 40 percent. Likewise,
if this 6-in. cup is redrawn to 4.5 in., its diameter decreases by 25 percent.
Because thick materials can stretch farther than thin materials, drawing reductions usually get reported as a function of material thickness. Due to differences in work hardening behavior, surface topography and other factors, draw reduction tables often are mate- rial type-, grade- and temper-specific. Table 2, adapted from a variety of sources, illustrates the differences between select grades and thickness ranges. In general, be conservative with aluminum alloys unless you have pre- vious experience with the alloy and temper with which you are working.
Wall Ironing
Drawing and redrawing processes cause the sheet metal to compress cir- cumferentially with each reduction, causing the cylindrical walls to increase in thickness. Controlling the finished part’s wall thickness or surface finish often requires ironing.
Besides producing a thinner and uniform wall thickness, ironing also can decrease the degree of earing in cups produced from sheet materials with high planar anisotropy. Since the wall thickness in the valleys is thicker than the eared portions, the valley heights increase when the cup wall is ironed slightly. This technique cannot totally remove earing; however, a more
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