US3817076A

US3817076A - Apparatus for and method of drawing a cup-shaped article

Info

Publication number: US3817076A
Application number: US00356964A
Authority: US
Inventors: J Wallace
Original assignee: Reynolds Metals Co
Current assignee: Reynolds Metals Co
Priority date: 1973-05-03
Filing date: 1973-05-03
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18
Also published as: FR2227910A1; DE2421218A1; CA1003702A; JPS5315826B2; JPS5015174A; AU6822474A; FR2227910B1; GB1442066A; AU463165B2

Abstract

An apparatus for and method of drawing a cup-shaped article from a flat sheet blank wherein the apparatus comprises a first die structure having a die cavity therein and having a first drawing surface and a second die structure having a die center adapted to be received within the die cavity and having a pressure pad assembly provided as a part of the second die structure and such assembly comprises a plurality of concentric drawing surfaces which include an outer drawing surface and an inner drawing surface, with the outer and inner drawing surfaces being relatively axially movable. The apparatus has means causing relative movement in an axial direction between the die structures to draw the sheet blank between the first drawing surface and the concentric drawing surfaces and fluid cushion means assures the entire blank may be drawn without pinching and the forming of knife-like edges in its peripheral edge portion.

Description

United States Patent [191 Wallace June 18, 1974 APPARATUS FOR AND METHOD OF DRAWING A CUP-SHAPED ARTICLE [75] Inventor: Joseph W. Wallace, Richmond, Va.

[73] Assignee: Reynolds Metals Company,

Richmond, Va.

[22] Filed: May 3, 1973 [21] Appl. No.: 356,964

Primary Examiner-Richard J. Herbst 5 7] ABSTRACT An apparatus for and method of drawing a cup shaped article from a flat sheet blank wherein the apparatus comprises a first die structure having a die cavity therein and having a first drawing surface and a second die structure having a die center adapted to be received within the die cavity and having a pressure pad assembly provided as a part of the second die structure and such assembly comprises a plurality of concentric drawing surfaces which include an outer drawing surface and an inner drawing surface, with the outer and inner drawing surfaces being relatively axially movable. The apparatus has means causing relative movement in an axial direction between the die structures to draw the sheet blank between the first drawing surface and the concentric drawing surfaces and fluid cushion means assures the entire blank may be drawn without pinching and the forming of knifelike edges in its peripheral edge portion.

14 Claims, 6 Drawing Figures fl 4 1e PAIENIEDJul 18 m4 FIG. 4 f

sum 2 or 2 APPARATUS FOR ANDMETHOI) OF DRAWING A CUP-SHAPED ARTICLE BACKGROUND OF THE INVENTION In the process of drawing a cup-shaped article, or the like, from a flat blank, the outer area of such blank is usually confined between a punch and an associated pressure pad. The pressure exerted by the pressure pad on the blank is usually controlled by external fluid means which may be in the formof either air or oil under pressure and such pressure is necessary to keep the blank flat and to keep wrinkles from forming therein as the blank is drawn.

During the routine drawing of a flat blank, the drawing action continues until only a comparatively small annular peripheral surface area remains to resist the pressure exerted by the action of the punch and pressure pad and this pressure on a small area often is such that the yield point of the metal being drawn is exceeded whereby the metal flows plastically and is pinched or flattened out into knife-like edges, or the like In order to minimize knife edges of this type, it has been proposed previously 'to provide a stop on either the pressure pad or the punch at such a location that at the beginning of the drawing action it engages the edge of the blank being drawn; and, once the blank is drawn inwardly of the stop it controls the spacing between the punch and pressure pad and prevents pinchingand knife-like edges in theperipheral portion of the blank during the later stages of drawing each blank. However, when drawing blanks made of a comparatively thin gage material and/or a material which has a comparatively high temper, this technique is unsatisfactory because at the start of the drawing operation when themetal is on top of the stop there is an undesirable wrinkling which is usually not eliminated as the drawing action continues whereby the completed cuplike article has flaws and is generally unsatisfactory.

SUMMARY This invention provides an improved apparatus for and method of drawing a cup-shaped article from a flat sheet metal blank which avoids the above-mentioned problems of prior art apparatus and methods and results in a cup-shaped article of comparatively high quality.

In particular, the apparatus comprises a first die structure having a die cavity therein and having a first drawing surface and a second die structure having a die center adapted to be received within the die cavity and having a pressure (pad assembly provided as a part of the second die structure and such assembly comprises a plurality of concentric drawing surfaces which include an outer drawing surface and an inner drawing surface, with the outer and inner drawing surfaces being relatively axially movable. The apparatus has means causing relative movement in an axial direction between the die structures to draw the sheet blank between the first drawing surface and the concentric drawing surfaces and fluid cushion means for the pressure pad assembly assures the drawn blank is free of knife-like edges in its peripheral edge portion.

Other details, uses, and advantages of this invention will be readily apparent from the embodiment thereof presented in the following specification, claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings show present preferred embodiments of this invention, in which FIG. 1 is a view with parts in cross section, parts in elevation, and parts broken away illustrating an exemplary apparatus and method for drawing cup-shaped articles from a sheet blank and illustrating sheet stock 'moved in position prior to blanking or cutting a circular blank from such sheet stock;

FIG. 2 is a perspective view of a cup-shaped article made by the apparatus and method of FIG. 1;

FIG. 3 is a fragmentary cross-sectional view drawn to a greatly enlarged scale particularly illustrating inner and outer draw rings comprising the pressure pad of the apparatus of FIG. I at the beginning of a drawing apparatus;

FIG: 4 is a view with parts in cross section, parts in elevation, and parts broken away drawn to an enlarged scale of a first die structure and a. second die structure of the apparatus of FIG. 1 and particularly illustrating the positions of inner and outer dlraw rings-comprising the pressure pad of thesecond die structure at the beginning of a drawing operation;

FIG.5 is a view similar to FIG. 4 particularly illustrating the relative axial positions of the inner and outer draw rings once the peripheral edge portion of a blank being drawn is moved out of engagement with the drawing surface of the outer draw ring; and

FIG. 6 is a view similar to FIG. 4 particularly illustrating the drawing operation with only the peripheral-edge portion of the blank remaining to be drawn.

DESCRIPTION OF ILLUSTRATED EMBODIMENT Reference is now made to FIG. 1 of the drawings which illustrates one exemplary embodiment of the apparatus and method of this invention which is designated generally by the reference numeral 10 and is particularly adapted to draw a cup-shaped article in the form of a cup 11 which is illustrated in FIG. 2. The cup 11 has a substantially planar bottom wall 12 adjoined by a right circular cylindrical side wall 13 definedas an integral part thereof and the cylindrical side wall 13 has a substantially planar top circular edge 14 which has a substantially full thickness and is free of knife-like edges, or the like.

The apparatus 10 may be supplied with metal sheet stock 15 from any suitable supply source and the stock 15 may 'be suitably moved in position preferably in an indexing manner as illustrated at 16 between a first or upper die structure 20 of the apparatus 10 and a lower die structure 21 of such apparatus and cut to define a flat circular blank or workpiece which is designated generally by the reference numeral 22 in FIGS. 4, 5, and 6 of the drawings. In particular, to provide the cutting action the die structure 20 has a circular cutting edge 23 which is particularly adapted to cooperate with an inner cutting edge 24provided on a cutting ring 25 which is fixed to the die structure 21 and the cutting edges 23 and 24 cooperate once the die structures20 and 21 are relatively moved together to seve'rthe circular blank 22.The die structure20 may be provided with a stripper ring (not shown) for stripping the sheet stock 15 from around the die structure 20 in the event such sheet stock clings thereto. The workpiece 22 is drawn in accordance with the teachings of this invention utilizing the apparatus and method of this invention to define the previously described cup shaped article or cup 11.

The die structure 20 has a die cavity 26 provided therein and an annular planar drawing surface 27 which is adjoined by an annular surface of arcuate configuration 30 leading into the die cavity 26. The die structure 21 is suitably supported at a fixed position by a supporting structure or support 31 and the die structure 21 has a die center 32 which is particularly adapted to be received within the die cavity 26 during the process of drawing the blank 22 between the die

structures

20 and 21.

The die structure 21 also has a pressure pad assembly which is designated generally by the reference numeral 35 and such pressure pad assembly comprises an outer draw ring 36 and an inner draw ring 37. The outer draw ring 36 has an outer annular substantially planar drawing surface 41, see FIG. 3, and the inner draw ring 37 has an inner substantially planar drawing surface 42 whereby the

drawing surfaces

41 and 42 are in essence concentric drawing surfaces which are arranged substantially coplanar. The

draw rings

36 and 37 are relatively axially movable and in this example are movable along a vertical central axis 43 for the entire apparatus 10, see FIG. 4, whereby the outer and

inner drawing surfaces

41 and 42 respectively are also relatively axially movable.

The apparatus has suitable means, or a device 44, causing or providing relative movement in an axial direction between the die structures and 21 and such means or device 44 is shown schematically as a double arrow in FIG. 1. Because in this example of the invention the die structure 21 is shown as being supported in a fixed position by the support 31 such relative movement is provided by movement of only the die structure 20 using device 44 which may be in the form of an actuator, the ram of a press of known construction, or the like. Thus, upon relatively moving the die

structures

20 and 21 along the axis 43 by utilizing the moving means 44 to move the die structure 20, the blank 22 is drawn between the

die structures

20 and 21 to define the cup 11 and as will be readily apparent from the following description presented in connection with FIGS. 3-6.

The apparatus 10 has fluid cushion means which is designated generally by the reference numeral 45 and is preferably in the form of fluid cushion means for the pressure pad assembly 35. The cushion means 45 operates to hold the

rings

36 and 37 and hence their respective

concentric drawing surfaces

41 and 42 against the bottom surface of the blank 22 at the beginning of the drawing action on the blank and as illustrated particularly in FIG. 4. The fluid cushion means 45 then allows relative movement in an axial direction between the inner and outer

concentric drawing surfaces

42 and 41 respectively once the peripheral edge portion 47 of the blank 22 has moved out of engagement with the outer drawing surface 41. In particular, the fluid cushion means 45 allows relative axial movement of

drawing surfaces

41 and 42 so that the distance indicated at 46, in FIG. 4, between the

draw rings

36 and 37 is eliminated once the peripheral edge portion 47 moves out of engagement with the outer annular drawing surface 41 and such edge portion 47 is positioned in the manner illustrated at 50 in FIG. 5. The fluid cushion means also controls the inner drawing ring 37 and in particular the drawing surface 42 thereof when only the peripheral edge portion 47 of blank 22 remains to be drawn and as illustrated at 51 in FIG. 6 to thereby prevent pinching and forming of a knife-like edge in the peripheral edge portion 47. 5 As will be apparent particularly from FIG. 3 of the drawings, the outer draw ring 36 comprises a substantially L-shaped ring having a vertical leg portion 52 and a horizontal leg portion 53 and the outer annular drawing surface 41 defines the top horizontal surface of the vertical leg portion 52. The horizontal leg portion 53 has a stepped surface which includes a raised substantially horizontally extending surface portion 58.

The inner draw ring 37 has a main portion 55 provided with a bottom surface 56 which is adapted to be supported on the raised surface portion 58 of the stepped surface 54 and the ring 37 has a downwardly extending vertical extension 57 defined as an integral part of the main body portion 55. The portion 57 has a vertical surface 60 which is arranged adjacent a vertical surface 61 which adjoins the raised surface portion 58 of the horizontal leg portion 53 and surface 60 is adapted to move or slide vertically relative to surface 61. The main body portion 55 of the inner ring 37 also has a vertical surface 62 which is adapted to slide vertically relative to an inside surface 63 of the vertically ex tending leg portion 52 of the outer ring 36.

Fluid seal means is provided between the

rings

36 and 37 so as to define a fluid pressure chamber 70 therebetween. In particular, such seal means comprises a fluid tight annular seal 64 provided between the surfaces 60 and 61 and a fluid-tight annular seal 65 provided between the

surfaces

62 and 63 and the arrangement of the

rings

36 and 37 and the

seals

64 and 65 is such that the fluid pressure chamber 70 is provided be tween the

draw rings

36 and 37 and the purpose of the chamber 70 will be explained subsequently.

As seen particularly in FIG. 1, the fluid cushion means 45 includes a fluid operated tubular cylinder 71 which supports the outer draw ring 36 and hence in essence supports both

draw rings

36 and 37. A fluid under pressure is provided in a chamber 72 in the lower portion of the die structure 21 and such pressurized fluid operatively associates with the cylinder 71 to provide the desired movement of the

rings

36 and 37 during the drawing operation.

The fluid cushion means 45 has orifice means indicated in the form of an orifice 74, see FIGS. 3 and 4, which extends through the horizontal leg portion 53 of the draw ring 36 and is used to introduce fluid under pressure into the pressure chamber 70 and to remove fluid under pressure from within such chamber. The tubular cylinder 71 also has a passage 76 extending axially along its length and the passage 76 has an upper portion in the form of a bore 77 in the wall of the cylinder and a passage portion 78 defined by a groove in the lower portion of the cylinder and an adjoining sleeve 79. Thus, the fluid in chamber 70 is in flow communication with the fluid in chamber 72 via the passage 76.

The die structure 21 also has other suitable seal means or seals, shown in FIG. 1 yet not described in detail, to assure satisfactory operation of the fluid cushion means including the cylinder 71.

During the drawing of a particular blank 22, the orifice 74 allows fluid in the chamber to be expelled therethrough and through the passage 76 to allow relative movement between the

rings

36 and 37 from the position of FIG. 4 to the position of FIG. 5. Once only the peripheral edge portion 47 of the blank remains to be drawn as illustrated at 51 in FIG. 6, the oriflce 74 serves to delay the rapid introduction of the fluid into the chamber 70. This delay is sufficient to allow portion 47 to be drawn and the cup 11 completed whereby there is no tendancy to pinch and thus form knife-like edges in the peripheral edge portion 47. Accordingly, the cup 11 has a planar substantially full thickness circular edge 14 defining the top of its side wall 13.

The chamber 72 may be provided with a suitable fluid which may be in the form of a gas such as air under pressure or in the form of a liquid such as oil, or the like, in accordance with techniques known in the art. The fluid in the chamber 72 may be supplied thereto and removed therefrom through

appropriate fittings

80 and 81 as shown in FIG. 1 and in accordance with techniques known in the art. Further, such fluid may be provided from any suitable supply source for such fluid and any means known in the art may be employed to keep such fluid under a predetermined controlled pressure.

Having described the detailed construction of the apparatus 10, the description will now proceed with a brief presentation of the method of this invention which enables the drawing of a cup-shaped article or cup 11 and for simplicity of presentation the apparatus will be employed to describe this method; however, it will be appreciated that any suitable apparatus may be employed for this purpose.

Accordingly, a workpiece or blank 22 is placed between associated die structures and 21 of apparatus 10 and the die structure 20 has the die cavity 26 therein and a planar annular drawing surface 27 while the die structure 21 has a plurality of concentric drawing surfaces including an outer drawing surface 41. and an inner drawing surface 42. The drawing surfaces 41 and 42 are relatively axially movable along axis 43 of apparatus 10 due to the fact that the associated pressure pad rings 36 and 37 with draw surfaces 41 and 42 respectively thereon are relatively axially movable along such axis.

The

die structures

20 and 21 are then relatively axially moved; and, in this example because the die structure 21 is held in a stationary position by the support 31 moving means in the form of a moving device 44 is employed to move the die structure 20 relative to the die structure 21. During this step of relatively axially moving the die structures the pressure pad draw rings 36 and 37 and hence their respective drawing surfaces 41 and 42 are supported by fluid cushion means 45. The fluid cushion means 45 allows relative movement in the axial direction between the outer and

inner surfaces

41 and 42 respectively once the peripheral edge portion of the blank has moved out of engagement with the outer drawing surface 41 and in particular, it will be seen that the axial movement takes place so that the inner draw ring 37 moves a distance 46 illustrated in FIG. 3.

During this axial movement, fluid present in the fluid pressure chamber 70 is subjected to full drawing pressure whereby it is forced out of the chamber 70 through orifice 74 to reduce the volume thereof from the volume illustrated in FIG. 4 to the comparatively small volume illustrated in FIGS. 5 and 6. The fluid from the pressure chamber 70 is expelled through the orifice 74, passage 76, and into chamber 72. Once the edge of the blank is positioned inwardly of surface 41, the outer draw ring helps limit the magnitude of the drawing force on the blank 22. Further as the drawing continues so that only the small annular area of peripheral portion 47 remains to resist the drawing force or pressure, there would normally be a tendancy for the inner ring 37 to spring back and pinch the peripheral portion 47.

However, the inner ring 37 is, in essence, a floating ring and in order for it to be urged outwardly, fluid must travel through the passage 76 and orifice 74 into the chamber causing relative movement of the inner ring 37 from the position of FIG. 6 to the position of FIG. 4. The orifice 74 serves to control any tendancy for a rapid entry of fluid under pressure into the chamber 70 and such orifice serves as a builtin delay in the movement of ring 37. The delay is sufficient that with continued drawing movement of the die structure 20 the peripheral portion 47 is drawn and the cup 11 is completed so that it has its cylindrical side wall 13 free of knife-like edges, or the like.

It will be appreciated that any suitable valve means may be provided instead of the orifice 74 for introduc ing a delay in the build up of pressure in chamber 70. However, when using orifice 74, the area of such orifice is precisely controlled depending upon the operating characteristics of the overall apparatus 10 to introduce the desired delay into the application of substantial pressure against the inner ring 37 and hence prevent the above described pinching action.

Thus, it is seen that the apparatus and method of this invention employs a pressure pad which has a plurality of concentric drawing surfaces including an outer drawing surface and an inner drawing surface and the outer and inner drawing surfaces are relatively axially movable in a controlled manner utilizing fluid cushion means so that once a cup-shaped article is drawn using a flat sheet blank made of metal, or the like, the peripheral edge portion of the blank may be easily drawn without pinching and hence without the forming of knife-like edges therein in a manner heretofore unknown.

Although in this example of the invention, a plurality of only two concentric drawing surfaces are shown in the pressure pad, it will be appreciated that more than two drawing surfaces may be provided if desired and a corresponding number of cylinders associated therewith together with suitable orifice means to take into account the different number of concentric drawing surfaces so that varying pressures may be provided at different points in the drawing operation. Further, it will be appreciated that suitable controls for the cylinders associated with each of the drawing surfaces may be provided in accordance with techniques known in the art.

The apparatus and method of this invention may be utilized to draw cup-shaped articles from a sheet blank made of any suitable metal; however, the apparatus and method of this invention has been especially effective in drawing sheet-like blanks of metallic materials containing aluminum.

While present exemplary embodiments of this invention, and methods of practicing the same, have been illustrated and described, it will be recognized that this invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is: i

1. An apparatus for drawing a cup-shaped article from a sheet blank comprising; a first die structure having a die cavity therein and having a first drawing surface; a second die structure having a die center adapted to be received within said die cavity; a pressure pad comprising said second die structure; said pressure pad having a plurality of concentric drawing surfaces and including an outer drawing surface and an inner drawing surface; said outer and inner drawing surfaces being relatively axially movable; means causing relative movement in an axial direction between said die structures to draw said sheet blank between said first drawing surface and said concentric drawing surfaces; and fluid cushion means for said pressure pad; said fluid cushion means holding said plurality of concentric drawing surfaces against said blank at the beginning of the drawing action on said blank, allowing relative movement in said axial direction between said inner and outer surfaces once the peripheral edge portion of said blank has moved out of engagement with said outer drawing surface, and controlling said inner drawing surface once the peripheral edge portion of said blank remains to be drawn to prevent pinching and forming of a knife edge in said peripheral edge portion; said pressure pad comprising an outer draw ring having said outer drawing surface defined thereon and an inner draw ring having said inner drawing surface defined thereon; said fluid cushion means comprising a fluid pressure chamber between said draw rings and means for introducing a fluid under pressure into said chamber and removing said fluid therefrom, said means for introducing and removing allowing fluid in said chamber to be expelled therethrough to allow said relative movement between said rings and hence said inner and outer surfaces and once said peripheral edge portion of said blank remains to be drawn serves to delay introduction of fluid in said chamber to prevent said pinching.

2. An apparatus as set forth in claim 1 in which said means for introducing and removing said fluid comprises orifice means.

3. An apparatus as set forth in claim 2 in which said outer draw ring comprises a substantially L-shaped ring having a vertical leg portion and a horizontal leg portion and said outer drawing surface comprises a top horizontal surface of said vertical leg portion.

4. An apparatus as set forth in claim 3 in which said horizontal leg portion of said outer draw ring has a substantially horizontally extending surface and said inner draw ring is supported concentrically within said vertical leg portion and has a bottom surface which is adapted to be supported on said horizontally extending surface of said horizontal leg portion.

5. An apparatus as set forth inclaim 4 and further comprising seal means between said rings defining said pressure chamber.

6. An apparatus as set forth in claim 5 in which said means for introducing and removing said fluid comprises an orifice extending through said horizontal leg portion of said outer draw ring and communicating with said fluid under pressure.

7. An apparatus as set forth in claim 6 in which said fluid cushion means further comprises a fluid operated cylinder supporting said outer draw ring and hence both of said draw rings, said fluid operated cylinder having passage means therein, said passage means having one end communicating with said fluid under pressure and its opposite end communicating with said orifice.

8. An apparatus as set forth in claim 1 and further comprising a support supporting said second die structure at a fixed position and said means causing relative movement comprises means for moving said first die structure toward and away from said second die structure.

9. An apparatus as set forth in claim 8 in which said means for moving said first die structure is in the form of the ram of a press.

10. A method of drawing a cup-shaped article comprising the steps of; placing a sheet blank between a first die structure having a die cavity therein and having a first drawing surface and a second die structure having a die center adapted to be received within said cavity; said second die structure comprising a pressure pad which has an outer draw ring provided with an outer drawing surface and an inner draw ring provided with an inner drawing surface with said outer and inner rings and their drawing surfaces being relatively axially movable; relatively moving said die structures in an axial direction to draw said sheet blank between said drawing surfaces; supporting said pressure pad with fluid cushion means during said step of relatively moving; said fluid cushion means holding said inner and outer drawing surfaces against said blank at the beginning of the drawing action on said blank, allowing relative movement in said axial direction between said inner and outer surfaces once the peripheral edge portion of said blank has moved out of engagement with said outer drawing surface, and controlling said inner drawing surface once the peripheral edge portion of said blank remains to be drawn to prevent pinching and forming of a knife edge in said peripheral edge portion; said step of supporting said pressure pad with fluid cushion means comprising providing a fluid pressure chamber between said draw rings and introducing a fluid under pressure into said pressure chamber and removing said fluid therefrom through associated means, said associated means allowingfluid in said pressure chamber to be expelled therethrough to allow said relative movement between said rings and hence said inner and outer surfaces at the beginning of the drawing of said blank and once said peripheral edge portion of said blank remains to be drawn serves to delay introduction of fluid in said pressure chamber to prevent said pinchmg.

11. A method as set forth in claim 10 in which said placing step comprises placing a circular sheet blank between said die structures.

12. A method as set forth in claim 10 in which said supporting step comprises supporting said draw rings with said fluid cushion means including a fluid operated cylinder which supports said outer draw ring and hence both of said draw rings and with means providing said fluid under pressure so that it operatively associates with said cylinder.

13. A method as set forth in claim 10 in which said second die structure is supported at a fixed position and said relatively moving step comprises moving said first die structure with associated actuating means.

14. A method as set forth in claim 13 in which said step of moving said first die structure with actuating means comprises moving said first die structure with the ram of a press.

Claims

5. An apparatus as set forth in claim 4 and further comprising seal means between said rings defining said pressure chamber.

10. A method of drawing a cup-shaped article comprising the steps of; placing a sheet blank between a first die structure having a die cavity therein and having a first drawing surface and a second die structure having a die center adapted to be received within said cavity; said second die structure comprising a pressure pad which has an outer draw ring provided with an outer drawing surface and an inner draw ring provided with an inner drawing surface with said outer and inner rings and their drawing surfaces being relatively axially movable; relatively moving said die structures in an axial direction to draw said sheet blank between said drawing surfaces; supporting said pressure pad with fluid cushion means during said step of relatively moving; said fluid cushion means holding said inner and outer drawing surfaces against said blank at the beginning of the drawing action on said blank, allowing relative movement in said axial direction between said inner and outer surfaces once the peripheral edge portion of said blank has moved out of engagement with said outer drawing surface, and controlling said inner drawing surface once the peripheral edge portion of said blank remains to be drawn to prevent pinching and forming of a knife edge in said peripheral edge portion; said step of supporting said pressure pad with fluid cushion means comprising providing a fluid pressure chamber between said draw rings and introducing a fluid under pressure into said pressure chamber and removing said fluid therefrom through associated means, said associated means allowing fluid in said pressure chamber to be expelled therethrough to allow said relative movement between said rings and hence said inner and outer surfaces at the beginning of the drawing of said blank and once said peripheral edge portion of said blank remains to be drawn serves to delay introduction of fluid in said pressure chamber to prevent said pinching.