Redrawing and Ironing Cylindrical Shells, Part 1

December 27, 2019

Fig. 1—Direct redrawing reduces cup diameter by redrawing in the same direction as the first draw.
Last month’s column defined key parameters for successful redrawing of square and rectangular box shells. This month: redrawing cylindrical shells and wall ironing.

Redrawing and Ironing Operations

The first draw reduction for a cylindrical shell, generally limited to less than 50 percent of the flat blank diameter, 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. Multiple operations reduce the shell diameter 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 blankholder, 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 percent 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 subsequent redraws.

Number of Redraw Operations

The approximate number of drawing operations can be determined from the final product dimensions by dividing the finished height (H) of the cylindrical 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 number of draw/redraw reductions for variety 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 original blank diameter (D), or percent-reduction, as follows:

% reduction = 1 - (d/D) x100

For example, a 6-in.-dia. cup produced 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 material 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 previous experience with the alloy and temper with which you are working.

Wall Ironing

Drawing and redrawing processes cause the sheet metal to compress circumferentially with each reduction, causing the cylindrical walls to increase in thickness. Controlling the finished part’s wall thickness or surface finish often requires ironing.

Fig. 2—In reverse redrawing, the cup inverts in order to reduce its diameter.
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 uniform wall height will result.

Employed to reduce the wall thickness below that of the original blank thickness, heavier ironing also increases cup body height and strengthens the wall. Ideally, the shell should be drawn to its finished inside diameter and the outside diameter ironed in separate dies to achieve the desired wall thickness. Ironing can be combined with one or more redraw operations, but the reduction in diameter for each redraw operation should be about one-half the amount that occurs without ironing.

Except for especially ductile materials, a 50-percent reduction in wall thickness represents a good starting point. When multiple ironing operations are required to achieve the desired wall thickness, annealing often is required to restore ductility to the workpiece between ironing operations. Even though in some cases, 90-percent reduction in wall thickness have been reported for a single ironing operation, these unique cases happen only when the combination and control of material properties, tooling geometry and lubrication are ideal for the process. MF

Next month: Punch-nose geometry, draw-sleeve design and redrawing without a blankholder.

Industry-Related Terms: Alloys, Blank, Cold Worked, Compress, Draw, Drawing, Ductility, Point, Surface, Thickness, Work Hardening
View Glossary of Metalforming Terms

Technologies: Tooling


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