Method and tool for redrawing

Metal deforming – By use of closed-die and coacting work-forcer – Cup or shell drawing

Patent

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Details

72349, 72350, B21D 2200

Patent

active

044257783

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The invention relates to a method for redrawing a predrawn cup, particularly a cup of thin metal in a process for making a can body, in a tool including a punch and a die; the invention relates equally to a said tool for performing such a redrawing operation.


BACKGROUND ART

Redrawing is performed in the so-called drawing-redrawing method (DRD method) which is hereinafter described in more detail. The redrawing step used in this method has some inherent disadvantages which will also be explained.


DISCUSSION OF THE INVENTION

The invention provides a method and tooling for imparting substantial wall thickness reduction during the redrawing operation of the cup drawn from slightly wax lubricated material, without applying the extreme compression usually necessary in a typical wall-ironing operation, by achieving wall thickness reduction subjecting the material to simultaneous bending and tension. This is followed, if desired, by slight sizing (i.e. corrective ironing) to ensure for the consistency of final wall thickness, whilst some additional lubricant may be used, the method of the invention does not call for essential provision of such lubricant, and therefore can avoid the requirement to wash the finished article to remove the residual lubricant.
The invention in a first aspect provides a method for redrawing a predrawn cup in a tool comprising an annular die and an annular blank holder or guide ring having a common axis and a punch movable forwardly along the axis through first the blank holder and then the die, wherein the wall of the predrawn cup, being of predetermined initial thickness, is drawn by the punch through a path which is delimited by parts of the tool and is substantially S-shaped in radial section, and the path includes (a) a first curved portion defined by a radiused leading outer edge of the blank holder, the first curved portion being in contact with the interior of the cup and the wall being bent convexly in the first curved portion so that the diameter of the cup wall is reduced; (b) an optional transitional portion between the blank holder and the die, in which the convexly bent wall is straightened; and (c) a second curved portion defined by a radiused rear inner edge of the die, the second curved portion being in contact with the exterior of the cup and the straightened wall of the cup being bent concavely thereon, the wall, after leaving the second curved portion, passing through a convergent part of the die which leads forwardly from the second curved portion and in which the wall is in contact with the die but out of contact with the punch, the radii of the two curved portions being three to four times the initial thickness of the wall. The wall thickness is reduced mainly in the second curved portion and the converging portion of the path.
In a preferred embodiment the method includes a sizing step. The method of the invention, irrespective whether or not it includes sizing, represents a single step. However several such steps may be repeated discretely in succession. A method according to the invention will be referred to herein as a "bend stretching" method, since the wall thickness is reduced by stretching due to back tension induced mainly by bending the material of the wall through the S-shaped path.
The invention in a second aspect also provides a tool for the redrawing of a predrawn cup having a predetermined wall thickness, the tool comprising an annular die and an annular blank holder having a common axis, and a punch movable along the axis through first the blank holder and then the die, the leading outer edge of the blank holder and the rear inner edge of the die being radiused and the bore of the die, leading forward from its said inner edge, being convergent, the die and blank holder optionally having opposed radial rear and leading faces respectively which are substantially planar, so that when the blank holder and the die are in position for redrawing, the cup wall can be drawn between them by the punch in a substantially S-shaped path del

REFERENCES:
patent: 344457 (1886-06-01), Paull et al.
patent: 1935854 (1933-11-01), McCreary
patent: 3793871 (1974-02-01), Kinghorn
patent: 4193279 (1980-03-01), Maeder
Eary & Reed, Techniques of Press Working Sheet Metal, 1974, pp. 112 and 113.

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