Metal deforming – By use of closed-die and coacting work-forcer – Cup or shell drawing
Reexamination Certificate
2000-09-14
2002-11-05
Larson, Lowell A. (Department: 3725)
Metal deforming
By use of closed-die and coacting work-forcer
Cup or shell drawing
C072S347000
Reexamination Certificate
active
06474126
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to processes, apparatus, and articles relating to presswork, stamping, and cold forming of metal. More specifically, this invention relates to deep drawing of metal plate into a drawn part for an automotive suspension system.
2. Description of the Related Art
Successful drawing is a process of forming a flat blank of metal, or workpiece, to convert the blank into a hollow shell without unacceptable defects such as cold-shutting, wrinkling, puckering, tearing, thinning, or fracture. The process involves placing the blank on top of a lower die section over an opening die cavity thereof and using an upper die punch to force the blank into the die cavity under forces sufficient to draw the blank past an edge of the opening and into the die cavity, so as to form the hollow shell. Additionally, a ring-like blank holder is usually employed to hold the periphery of the blank in place. The blank is thereby confined between the blank holder and the top of the lower die portion, with the amount of pressure exerted on the blank being sufficient to prevent the part from incurring the above-mentioned defects. Where heavier gage blanks are drawn, however, the thickness of the metal may be sufficiently internally resistant to wrinkling and, therefore, may not require a hold-down device.
Typical metals used for drawing include carbon and alloy steel, and aluminum. Parts that may be drawn include those having simple cylindrical shapes, or more complex geometries having square or rectangular cross sections. For cylindrical shapes, when the depth of the drawn part exceeds the diameter of the part, the process is generally considered to be deep drawing and usually requires multiple progressive drawing operations; otherwise, the process is generally referred to as shallow drawing and can be drawn in one press operation. For rectangular shapes, it is generally known that such parts can be successfully drawn to a maximum depth equal to approximately six times the size of the corner radius on the part without incurring the above-mentioned defects.
Parts that cannot be successfully drawn in one step must, if at all possible, be redrawn in subsequent progressive steps, and often require an annealing operation between such steps to restore the ductility of the metal for further cold working thereof. Such parts tend to be heavy gage sheet metal greater than 0.100″ in thickness. Heavier gage parts exceeding 0.250″ are characterized as plate steel rather than sheet steel and attempts to successfully deep draw such thicknesses have been limited due to the resistance of such heavy gage materials against entering the draw section of the lower die portion. Defects are often experienced, such as thinning of the wall sections, punching through the bottom of the part, or cracks and splits at the edges or radii of the part. Often times, wrinkling, or cold-shutting, occurs wherein the metal folds over upon itself. Much worse, however, such “experiments” with heavy gage steel often lead to catastrophic damage to the extremely expensive tools and stamping press equipment involved. Therefore, many manufacturers are understandably reluctant to attempt single stroke, single operation deep drawing on such heavy gage plate steel.
Instead, manufacturers typically avoid drawing such parts altogether and use expensive superplastic forming or hydro-forming techniques, or attempt to draw the parts in multi-step progressive drawing operations using stamping presses with multiple stations. For example, U.S. Pat. No. 4,147,049, to Book et al. teaches a method and machine for drawing heavy-walled parts. Book et al. disclose that the method is carried out on a multiple plunger machine including no less than six separate press stations. Book et al. further disclose that each press station uses a supplemental sleeve that circumscribes a die punch. The sleeve assists the die punch in drawing a cup into a die cavity by contacting an annular edge at an open end of the cup to reduce the tensile stress in the cylindrical portion of the cup.
In another example, U.S. Pat. No. 4,509,356, to Budrean et al. also teaches a method and apparatus for drawing heavy walled shells and points out shortcomings in the teachings of Book et al. Specifically, Budrean et al. assert that the annular edge at the open end of the cup does not always remain perfectly square with the axis of the cup, so that the sleeve cannot uniformly reduce the tensile stress in the cup. Instead, Budrean et al. disclose a method for use on a forming machine that includes no less than seven separate press stations. Each station uses a die punch having a reduced diameter for engaging an inside diameter and bottom portion of the cup. Each die punch also includes an enlarged diameter that defines an annular step. At the first station, the annular step of the die punch forms a complementary annular step at the upper inside diameter of the cup that remains square with the axis of the cup. At subsequent stations, the steps in the subsequent die punches locate on the step in the cup to uniformly reduce tensile stress in the cylindrical portion of the cup.
Unfortunately, both Book et al. and Budrean et al. disclose extremely expensive die arrangements involving multiple stations. Additionally, such processes involve significantly longer process time than a single station process, and necessarily involve more failure modes and downtime than a single station process.
Turning now to a more specific problem in the prior art, forming of leaf spring seats for automotive suspension systems has not been, heretofore, conducive to deep drawing. In particular, 4×4 trucks usually incorporate relatively tall spring seat and spacer block assemblies to provide the appropriate height between the body and the chassis. Such spring seats are generally made from ⅜″ thick plate and, therefore, have been impractical to deep draw effectively in one operation to the required height of approximately 3½″. As is well known in the art, such a part is not possible to produce in a one-step operation, due to the aforementioned defects that are inherent with a one-step stamping operation, on such a deeply drawn heavy gage part made of steel. For example, it is recognized by those skilled in the art that the maximum overall depth that can be successfully drawn on a rectangular shell having a radius of approximately ¼″, as with the present invention, is only approximately 1½″.
Before the present invention, attempts to exceed these drawing guidelines typically resulted in thinning at the corners, tearing out in the bottom of the stamped cavity, and wrinkling or cold-shutting at the ends of the leaf spring seat. Therefore, such a part can typically be produced in multiple steps using a progressive die process, as exemplified above by Book et al. and Budrean et al. More typically however, a much shorter leaf spring seat is used in conjunction with a spacer block to achieve the desired height of the parts, as is very well known in the art and exemplified by U.S. Pat. No. 2,678,819 to Douglass. For example, as taught by Douglass, such prior art solutions typically use a relatively short spring seat
6
having a height of approximately 2″ in combination with a 1″ or taller riser block
8
.
Any of the above-mentioned solutions as taught by the prior art are relatively expensive and complex. A multiple-operation progressive die process involves very expensive machinery and tooling, excessive process time, and work-in-process between stations of the stamping press. Similarly, use of a leaf spring bracket and spacer block assembly involves a less robust design, assembly that is more complicated, and a higher overall cost and weight. The spacer block solution is less robust since the spring seat and spacer block inevitably wear against one another, inclusion of extraneous parts tends to increase failure modes within a system, and less precise location of the
Larson Lowell A.
Vanophem & Vanophem, P.C.
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