US3893326A

US3893326A - Apparatus for reduction drawing of hollow bodies of stainless steel

Info

Publication number: US3893326A
Application number: US415312A
Authority: US
Inventors: Karl-Oskar Oberlander; Christian Domscha; Werner Schurr
Original assignee: WMF Group GmbH
Current assignee: WMF Group GmbH
Priority date: 1972-11-16
Filing date: 1973-11-13
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08
Also published as: ES420577A1; FR2213114A1; IT1004642B; AT324079B; JPS49135876A; FR2213114B1

Abstract

A cup shaped stainless steel blank is deep drawn into a closed end hollow shape having a thickened bottom and a thickened rim at its open end. A ram for forcing the blank through a drawing die has a forward plunger portion and a conical body portion merging with and tapering rearwardly from the plunger portion. Axial forward thrust of the ram forces its plunger portion into the blank and pushes the latter through the drawing die, thereby transforming the blank into an elongated hollow shape having a uniform outside diameter from end to end and an inwardly projecting rim at its open end. Stripping of the elongated hollow shaped from the ram radially distends the inwardly projecting rim so that the stripped hollow shape will have the same inside diameter from end to end.

Description

United States Patent Oberl'ainder et a1.

(75] Inventors: Karl-0skar Oberl'a'nder; Christian Domscha; Werner Schurr, all of Geislingen/Steige, Germany [73] Assignee: Wurttembergische Metallwarenfabrlk, Geislingen/Steige, Germany [22] Filed: Nov. 13, 1973 [21] Appl. No.: 415,312

[30] Foreign Application Priority Data Nov. 16, 1972 Germany 2256334 Mar. 29, 1973 Germany 2315832 [52] U.S. C1 72/347; 72/467 [51] Int. Cl 321d 22/20 [58] Field of Search 72/347, 348, 349, 467;

[56] References Cited UNITED STATES PATENTS 1,944,527 1/1934 Pfaendler 72/347 2,877,546 3/1959 Wilcox 1 72/347 2,882,759 4/1959 Altwicker 76/107 A July 8, 1975 3,203,218 8/1965 Bolt et a1. 72 349 3,423,985 1/1969 3,466,922 9/1969 3,785,311 1 1974 Yoshikawa 113/120 H FOREIGN PATENTS OR APPLICATIONS 392,192 3/1924 Germany 72/347 Primary Examiner-Richard .1. Herbst Attorney, Agent, or Firm-James E. Nilles [57] ABSTRACT A cup shaped stainless steel blank is deep drawn into a closed end hollow shape having a thickened bottom and a thickened rim at its open end. A ram for forcing the blank through a drawing die has a forward plunger portion and a conical body portion merging with and tapering rearwardly from the plunger portion Axial forward thrust of the ram forces its plunger portion into the blank and pushes the latter through the drawing die, thereby transforming the blank into an elongated hollow shape having a uniform outside diameter from end to end and an inwardly projecting rim at its open end. Stripping of the elongated hollow shaped from the ram radially distends the inwardly projecting rim so that the stripped hollow shape will have the same inside diameter from end to end.

1 Claim, 7 Drawing Figures mum JUL 81% SHEET 1 APPARATUS FOR REDUCTION DRAWING OF HOLLOW BODIES OF STAINLESS STEEL BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for reducing drawing of hollow bodies, in particular for the production of cooking utensils, in particular pressure cookers, comprising a reinforced bootom and edge from a cup formed by drawing.

Cooking utensils with a thick bottom a thinner cylindrical wall and a reinforced top edge have approximately the same stiffness in both the wall and the bottom. The may, therefore, be subjected to a certain internal pressure as occurs, for example, during pressure cooking. The reinforced edge is desirable for securing a cover in place as required for pressure cooking.

According to a heretofore known process of producing hollow shapes, a cup is formed by deep drawing and further developed by pressure rolling. In this process, the cup is positioned on a rotating pressure casing. The cup is engaged from the outside by at least one pressure roller which is moved in the axial direction of the mandrel-like pressure casing. During this production process, it is possible to produce a reinforced edge for a cooking utensil. This process, however, is expensive, since deep drawing and pressure rolling each require a working operation and special machines. Furthermore, pressure rolling is time-consuming and limited to hollow circular-cylindrical bodies.

SUMMARY OF THE INVENTION The principal object of the present invention is to provide an improved process and apparatus for producing deep drawn hollow shapes of stainless steel in a simple and economical manner.

This object is accomplished in accordance with the invention by positioning the cup on a ram and pushing it by means of said ram through at least one reduction drawing die in a manner known as reduction drawing, said reduction drawing die forming together with said ram an annular gap whose width is smaller than the thickness of the cup wall and by subsequently stripping the elongated blank from said ram. While the wall material of the cup adjacent its free edge region passes through the reduction drawing die said wall material is pushed into a tapered zone of the ram during the reduction drawing operation before the free cup edge passes through the smallest diameter of said drawing die and the edge region, which has been pressed into said tapered zone is then expanded to the diameter of the untapered ram as said hollow body is stripped off said ram.

The reduction drawing technique has heretofore been used for the production of hollow bodies comprising a reinforced bottom and a thinner wall portion, but in the bodies as heretofore produced by said technique the wall thickness of the hollow body was uniform over its entire length. By pressing the cup material into the tapered zone of the ram in accordance with the present invention, an edge region is produced which has a reduced internal diameter and which is thickened toward the inside of the hollow body. During subsequent stripping of the hollow body from the ram, the non-tapered section of the ram moves outwardly through this constricted region thus expanding it to the normal internal dimension of the hollow body. This results in a thickening of the edge region in an outward direction. Thus,

during the stripping operation, a material deforming operation occurs in contrast to previously known processes. This however, does not necessitate any additional expenditure of time. The savings in time achieved with the inventive process as compared to the pressure rolling process is considerable. It, of course, depends on the size, in particular the height, of the cooking utensil and generally lies in a range between 30 and Another advantage of the process according to the invention is in the apparatus required which necessitates much less expenditure than separate deep drawing and pressure rolling means. In addition, the inventive process also lends itself, in addition to the production of cooking utensils with circular-cylindrical shape, to the production of hollow bodies having another cross section, for example, of oval or rectangular shape.

The invention also provides an apparatus for performing the process, and which apparatus comprises an axially movable ram and at least one reduction drawing die which, together with said ram, forms an annular gap whose width is smaller than the thickness of the wall of the cup forming the starting workpiece. The apparatus further comprises a stripping means, and the ram has a tapered zone in the region in which the cup edge comes to rest before it is passed through the smallest diameter of said reduction drawing die.

The cup edge material is pressured through the reduction drawing die into this tapered zone. As soon as the tapered zone has reached the smallest diameter of the reduction drawing die, the annular gap between the drawing die and the ram becomes gradually wider so that the edge region, which has been thickened in an inward direction toward the ram, can pass this position. Another advantage of this arrangement is that the forces of pressure present in the reduction drawing ring abate slowly and not suddenly when the cooking utensil emerges from this position. In this way, the material of the reduction drawing die is not subjected to abrupt great differences in pressure.

In order to be able to employ the inventive process in the case of hollow bodies of stainless steel, the reduction drawing die in a further development of the inventive apparatus consists of a ceramic material for the reduction drawing of cups of stainless steel. In previously known processes for reduction drawing with a ram which has a uniform diameter along the entire length of a region coming into contact with the material to be reduction-drawn, the result in the case of stainless steels, in particular austenitic or ferritic steels, was that cold fusion tended to occur between the steel and the drawing die in Conjunction with the tool steel or with the sintered metal of the reduction drawing die, thereby causing the tension forces to increase considerably after a few draws and the resultant hollow bodies were just as useless as the reduction drawing die due to surface damage and grooves. The use of a special lubricant has reduced the occurrence of cold fusion between the blank and drawing die, but has nevertheless not produced any satisfactory surface quality. In addition, it was difficult to remove this lubricant after manufacture which, however, was absolutely necessary for subsequent annealing which was required. Tests were also made with reduction drawing dies consisting of ceramic material, but these have hitherto failed due to the fact that the sudden release or drop in pressure as the workpiece emerges from the reduction drawing die during reduction drawing led to fissures in the ceramic material. In the inventive apparatus, the fact that the cup material is pressed into a tapered zone of the ram before the free cup edge is drawn through the smallest diameter of the drawing die prevents a sudden release or drop in pressure in the material of the drawing die. On the contrary, the pressure is reduced gradually so that no fissures occur in the drawing die when a ceramic material is used and this drawing die achieves a long service life which satisfies economical demands. Reduction drawing dies of ceramic material are advantageous in that no cold fusion between the blank and die occurs and the resultant hollow bodies of stainless steel have a perfect surface. Lubrication and simultaneous cooling may be accomplished with the help of simple lubricants which can be readily removed subsequently in a washing machine.

The depth of the tapered zone is advantageously dimensioned such that the annular gap is expanded to an amount. at least approximately 80%, preferably 100-12096 of the original cup wall thickness as said tapered zone passes through the smallest diameter of the reduction drawing die. If the depth of the tapered zone produces an expansion of the annular gap to an amount somewhat below the original cup wall thickness, a certain amount of residual pressure tension remains which is suddenly reduced by passing the cup edge through the smallest diameter of the drawing die. If this tension only amounts to a fraction of the maximum tension which occurs, however, this does not result in destruction of the ceramic material. It is more favorable, however, if the annular gap is expanded by the tapered zone of the ram to an amount which is equal to or somewhat greater than the original cup wall thickness. The cup wall thickness then normally increases somewhat toward the free edge of the cup because the cups used as the starting workpiece in reduction drawing are previously deep-drawn. Dimensioning the tapered zone in this manner results in a complete, but gradual reduction of pressure tension as the tapered zone passes through the smallest diameter of the drawing die.

The tapered zone is advantageously formed by an approximately conical shoulder on the ram. The resultant reduction in pressure tension, which progresses approximately linearly, protects the device. Furthermore, a gradual transition between the wall and the thickened edge region is also produced on the manufactured cooking utensil on the outer side. This is more advantageous than a sharp transition, both with respect to further manufacturing processes, for example, during bending over of the edge and during enameling, as well as during later use, in particular cleaning of the cooking utensil.

The entrance angle of the shoulder should amount to between approximately 6 to 30. The size of the entrance angle is decisive for the height of the thickened edge region on the finished cooking utensil. The smaller the entrance angle, the higher the edge region. A smaller entrance angle, however, also leads to smaller forces present when the cooking utensil is stripped from the ram because the material, which has been pressed into the tapered zone, does not have to be repushed outwardly as far. On the other hand, a greater entrance angle produces an edge thickening of the finished hollow body which is not as high as is normally the case. Greater stripping forces, however, must be exerted in this case. Depending on which of these points 4 predominates, the entrance angle can be selected toward the upper or lower limit.

In known apparatus for deep drawing hollow shapes, stripping means are normally used which are designed like fingers. In the improved apparatus according to the present invention, the stripping means are designed as asplit ring whose internal diameter corresponds to the external diameter of the untapered ram. The stripping means thus have a considerably greater abutment surface on the cooking utensil which is to be stripped off the ram. This enlargement in the abutment surface is favorable because relatively great forces must still be overcome due to the deformation of the edge region when the hollow body is stripped off the ram.

In yet another embodiment of the present invention. the stripping means include an ejector which can be moved axially with respect to the ram. This ejector can engage practically the entire inner bottom surface of the cooking utensil to be stripped off the ram. The distribution of force and thus the tension ratio occurring in the cooking utensil are especially favorable in this case.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and advantages of the invention will become more fully apparent as this disclosure proceeds with reference to the accompanying drawings in which:

FIG. 1 is a somewhat schematic vertical section of an apparatus for deep drawing hollow shapes, embodying the invention;

FIGS. 2-6 are views similar to FIG. 1 illustrating various steps of a manufacturing process incorporating the invention, FIG. 6 also illustrating a modified stripping device which may be used in the apparatus shown in FIG. I; and

FIG. 7 is a sectional elevation of the body of a cooking utensil produced in accordance with the invention.

DETAILED DESCRIPTION In the drawings, I indicates a stationary reduction drawing die which is mounted in a receiving ring 2 with a shrink fit under pressure. The reduction drawing die consists of a ceramic material, preferably high purity M 0 sintered corundum.

The drawing die 1 has an inlet surface la (FIG. 6) which tapers conically in the direction of drawing and whose entrance angle may lie between 8 and 0. A cylindrical section lb whose height is approximately 3-5 mm follows the inlet surface 1a. This cylindrical section lb is itself followed by an outlet surface lc which flares conically in the direction of drawing and whose entrance angle may amount to approximately 345.

The cylindrical section lb (FIG. 6) defines the working edgediameter d of the reduction drawing die I (FIG. 1).

A ram which is generally designated as 3 cooperates with the drawing die 1 and includes a cylindrical for- ,ward plunger portion 3a, a stem portion 3b which is ,should have an entrance angle of approximately 6 to 30. The ram 3 is movable in the direction of the double headed arrow P relative to the drawing die 1. During this movement through said drawing die, an annular gap is formed between the plunger portion 3a and the drawing die 1 with the smallest width calculated from the difference of the diameter d and the diameter s of the plunger portion 3a of the ram 3. The width of this annular gap is designated as r in FIG. 1.

A stripping means is disposed at the outlet side of the drawing ring 1. In FIGS. 1 to 5 and in the right half of FIG. 6, the stripping means consists of a ring assembly 4 comprising two or more arc segments which are urged by a circular spring 5 into the innermost position shown in FIGS. 1, 5 and 6. In this position, the inside diameter of the ring assembly 4 corresponds substantially to the outside diameter 5 of the forward plunger portion 3a of the ram. The ring assembly 4 has a conically crowned inlet surface 4a flaring rearward toward the drawing ring 1.

The left hand of FIG. 6 shows an embodiment of the invention with another stripping means. An ejector '7 is reciprocably mounted in the ram 3 by means of bearing elements 8 so as to be axially displaceable. FIG. 6 shows this ejector in the discharge position, i.e., abutting the bottom of the cup 6.

The mode of operation of the apparatus shown in FIG. 1 is as follows: A cup-shaped blank 6 of stainless steel is slipped over the ram 3. This may be done by positioning the cup on the inlet surface 10 of the reduction drawing die 1 so as to be centered thereon The ram 3 may then be pushed into said cup 6.

The cup shaped blank 6 may conventionally be manufactured from a circular disc of sheet metal by backward extrusion, the blank having substantially the same thickness at its side wall and base. The wall thickness may be somewhat expanded towards the free edge 60 as a result of the deep drawing operation. The starting position with the cup 6 seated on the forward cylindrical plunger portion 3a of the ram is shown in FIG. 1.

By thrusting the cylindrical plunger portion 3a of the ram 3 through the reduction drawing die 1, the material of the cup 6 initially comes to rest on the inlet surface la and is then pushed into the cylindrical section lb (FIG. 2) of the drawing die 1. The wall material of the cup 6 is then reduction-drawn to the gap width r (FIG. 1) between the cylindrical plunger portion 3a of the ram 3 and the cylindrical section lb of the drawing die 1. In this manner, reduction drawing up to 40% is readily possible within the scope of the invention. The reduction drawing of the wall material from the original wall thickness of the cup 6 down to the thickness r is best illustrated by FIG. 2.

During the reduction drawing operation, a simple lubricant, for example, soap water, which cools as well as lubricates the drawing die 1, may be sprayed out of a nozzle indicated at 7 in FIG. 2.

As the ram 3 continues to push through the drawing die I, the wall material of the cup 6 is extended along the ram. In FIG. 2, the free cup edge 60 has already reached the tapered shoulder presented by the conical body portion 3c. This is disposed at such a location on the ram 3 that the free cup edge 60 still has a clear axially spaced relation with respect to the smallest diameter of the drawing die I when it arrives at the shoulder 30.

The conical body portion 3c defines a tapered zone V around the ram 3 (FIG. 3). As soon as this zone V, i.e., the conical ram portion 3c, reaches the area of the smallest diameter d of the drawing die 1, the gap between the ram 3 and the drawing die 1 enlarges radially from an amount r (FIG. 1) to an amount which corresponds to the enlargement of the tapered zone V. The tapered zone V is dimensioned such that the width of the gap between the cylindrical section of t he drawing die 1 and the rear cylindrical section 3b of the lam at the trailing end of the conical body portion 30, approaches the wall thickness of the original cup 6 and preferably exceeds it by 10-20%.

FIGS. 3 and 4 show that the cylindrical section lb of the drawing die 1 urges the wall material of the cup 6 against the conical body portion 30 of the ram so that a compacted constricted edge region 6b of the cup is formed which still has a completely cylindrical external wall surface at this time.

It is an essential feature of the invention that the ma-' terial of the edge region of the cup 6 be pressed into the tapered zone around the ram 3 before the free edge 6a passes through the smallest diamater d of the drawing die 1. This condition is met in the embodiment shown in the drawing. One can see in FIG. 4 that the compacted edge region 612 fills out the tapered zone around the ram.3 whereas the free edge 6a of the cup is just moving into the cylindrical section lb of the drawing die 1.

The ram 3 then pushes the reduction-drawn cup 6' against the inlet surface 4a of the stripper ring assembly 4 so that this assembly is clistending radially. As soon as the free edge of the reductiondrawn cup 6" comes to lie below the stripper 4 (FIG. 5), the stripper sectors snap back in an inward direction due to tlfe action of the spring 5 and they come to lie above the free edge of the drawn cup 6.

When the ram 3 is withdrawn in an upward direction, the cup 6 cannot be pulled along because its free edge 6a is stopped by the underside of the stripper 4. The cup 6' therefore slides off the ram. In so doing, the compacted reinforced edge region 612, which abuts the conical ram portion 3c and the rear cylindrical ram portion 3b, is radially expanded by tlte forward cylindrical plunger portion 3a of the ram. This is shown in the right half of FIG. 6.

The mode of operation of the ejector 7 may be seen in the left half of FIG. 6. The ejector pushes against the bottom of the cup 6' while the ram 3' is withdrawn. The edge region of the cup 6' is expanded in exactly the same manner as before, but is subjected to pull whereas it is subjected to push in the embodiment according to the right side of FIG. 6.

The final result of the reduction drawing process is a cylindrical hollow body with a reinforced wall region 6'b and a base 6'6 of approximately the same thickness. The inner wall is completely cylindrical (FIG. 7). The cylindrical wall 6d has a uniform thickness corresponding to the gap width r. In the next operational step in the production of a cooking utensil, the rein forced edge is bent outwardly. It is nickel,and chlzome.

tapered body portion 3c and the entrance angles of the drawing die 1 shown in the drawing may be varied within the limits stated hereinbefore.

Two or more drawing dies may be coaxially disposed relative to one another and may be brought successively into action instead of a single drawing die I. The first drawing die may be disposed downstream in relation to a drawing ring in which the cup 6 forming the starting blank is drawn from a round disc of sheet metal.

The drawing die may consist of cemented carbide or tool steel when producing hollow bodies from normal deep-drawing steels or non-ferrous metals.

The operation of forcing the ram 3 axially into the cup shaped blank 6 so as to advance the latter through the darwing die assembly transforms the blank into an elongated body as sohwn in FIG. which has a bottom portion of substantially the same thickness as the bottom thickness of the blank, a peripheral wall portion of less thickness than the peripheral wall of the blank, and an inwardly projecting edge portion adjacent the open end of the hollow body. Stripping of the hollow body from the ram causes radial expansion or distension of its inwardly projecting edge portion as illustrated by FIG. 6, and may be accomplished either by pulling the ram through the open end of the hollow body, or by subjecting the hollow body to an internal axial expanding force between its closed end and the ram. In the latter case. the axial expanding force may be produced by an ejector element. such as the ejector 7 shown in FIG. 6, which is reciprocably movable on the ram in front of its forward plunger portion.

We claim:

I. In an apparatus for producing hollow bodies of stainless steel. the combination of an axially reciproca ble ram and a surrounding die assembly cooperable with said ram so as to reduction deep draw a cupshaped stainless steel blank. said die assembly comprising a stationary support and an annular die member of ceramic material fixedly mounted thereon and said ram having a forward plunger portion and an attenuated axial portion tapering rearwardly from said forward portion, the peripheral exterior surface of said ram being cooperable with the peripheral interior surface of said blank. and the peripheral exterior surface of said blank being cooperable with the inner edge of said ceramic die member so that by an initial forward movement of said ram a peripheral wall portion of said blank at its closed end will be drawn to an axially extended length of radially reduced wall thickness and so that continued forward movement of said ram will deform the free end of said blank into a rearwardly extending annular rim of inwardly increasing radial thickness, thereby gradually reducing the radial outward load upon said ceramic die member; and stripping means for axially withdrawing said extended and deformed blank forwardly from said ram.

Claims

1. In an apparatus for producing hollow bodies of stainless steel, the combination of an axially reciprocable ram and a surrounding die assembly cooperable with said ram so as to reduction deep draw a cup-shaped stainless steel blank, said die assembly comprising a stationary support and an annular die member of ceramic material fixedly mounted thereon, and said ram having a forward plunger portion and an attenuated axial portion tapering rearwardly from said forward portion, the peripheral exterior surface of said ram being cooperable with the peripheral interior surface of said blank, and the peripheral exterior surface of said blank being cooperable with the inner edge of said ceramic die member so that by an initial forward movement of said ram a peripheral wall portion of said blank at its closed end will be drawn to an axially extended length of radially reduced wall thickness and so that continued forward movement of said ram will deform the free end of said blank into a rearwardly extending annular rim of inwardly increasing radial thickness, thereby gradually reducing the radial outward load upon said ceramic die member; and stripping means for axially withdrawing said extended and deformed blank forwardly from said ram.