Draw Ring For Cans

Abstract

An improved die means for sheet metal drawing is provided which permits lower pad pressures and mitigates the problem of earring by the provisions of concentric depressions in at least one of the draw rings.

Description

BACKGROUND OF THE INVENTION

This invention relates to sheet metal drawing. More particularly, this invention relates to improved die means for sheet metal drawing.

The drawing of shaped objects, such as cylinders, from sheet metal stock involves a controlled flowing of the metal stock into the drawing dies using clamping pressure applied peripherally about the dies by draw rings. Problems of wrinkling and earring may occur due to uneven or excessive pressure applied to the sheet metal by the draw rings.

It has been proposed to lower the total force applied to the sheet metal by providing vertically walled grooves in the face of the draw ring. However, the sharp corners on such grooves can, when coated metal stock is used, cause coatings on the sheet metal stock to be scraped from the metal as well as cause buildup of bare metal on the draw surface requiring cleaning of the grooves to remove such foreign matter.

It has also been proposed to use draw beads or undulations on the surfaces of the draw rings to remove wrinkles and maintain even pressure on the sheet metal stock by bending or working the sheet metal during flow. For example, pp. 10-3, 10-4 and 12-6 to 12-9 of the Die Design Handbook (McGraw-Hill Book Company) and Cvacho U. S. Pat. No. 3,664,174 shows intermeshing grooves and projections on the draw ring surfaces.

Such approaches, however, require costly tooling and precise tolerances to provide controlled bending or working of the sheet metal while maintaining even pressure.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide improved die means for the drawing of either coated or uncoated sheet metal.

It is another object of the invention to provide an improved draw ring means wherein one or more of the draw rings is provided with a plurality of symmetrically spaced depressions to reduce the total force applied by the draw rings to the sheet metal.

It is a further object of the invention to provide improved draw ring means wherein the sheet metal may flow therethrough in planar fashion while mitigating the formation of wrinkles or earring.

It is yet another object of the invention to provide improved draw ring means having a plurality of depressions therein of predetermined width and forming a predetermined angle with the planar surface of the draw ring.

These and other objects of the invention will be apparent from the drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross sectional view of sheet metal drawing apparatus.

FIG. 2 is a fragmentary cross sectional view of a portion of FIG. 1 illustrating, in exaggeration the profile of the draw ring of the invention.

FIG. 3 is a top view of the draw ring of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a sheet metal drawing apparatus constructed in accordance with the invention is generally illustrated at 2. The apparatus, which can be used with any drawable metal including aluminum or steel in either a coated or non-coated form, comprises an upper die 10 and a lower die 30. Lower die 30 comprises a center post 32 which is detachably mounted by means not shown to the bed of the press. A draw ring 34 surrounds post 32. Draw ring 34 is urged upwardly by pressure pins 36 which exert a variable pressure against draw ring 34 via air pressure means not shown. Surrounding draw ring 34 is a stationary collar 38 which is mounted via support 40 to the bed of the press. Collar 38 has a front cutting edge 38a which (as will be described below) assists in cutting the sheet metal blank from the sheet metal stock.

Die means 10 are attached to the ram of the press and comprise a knockout 12 which is, in turn, surrounded by upper draw ring assembly 14 which comprises draw ring 16 and a cylindrical supporting member 18. A stripper plate 20 comprises a cylindrical washer of approximately the same diameter as collar 38. Upon the lowering of the ram the sheet metal stock is first engaged between washer 20 and collar 38 and clamped therebetween. The downward travel of draw ring 16 provides a cutting action between edge 16a on draw ring 16 and edge 38a on collar 38 to sever the blank from the sheet metal stock.

As the ram continues to close, draw ring 16 forces the sheet metal blank against draw ring 34 which in turn is biased against the underside of the sheet metal blank via the air pressure applied against pressure pins 36. As the ram continues to lower, the pressure exerted by draw ring 16 against the sheet metal stock and draw ring 34 overcomes the biasing pressure against draw ring 34 via pressure pins 36 and the sheet metal blank is drawn over center post 32 as shown in the drawings. As the sheet metal is drawn over post 32 it flows between the draw rings 16 and 34 and, if the pressure exerted between draw rings against the blank is correct, a shaped object is formed over center post 32.

The ram of the press is then opened thereby releasing the pressure exerted against lower draw ring 34 by upper draw ring 16. The air pressure exerted against pins 36 urges draw ring 34 upward providing therefore a shearing action against any ear or lip remaining on the drawn sheet metal. The drawn object can then be ejected from center post 32 by well known means such as a central piston or central air orifice or the like.

Referring now to FIGS. 2 and 3 the improved draw ring structure of the invention is illustrated. The top surface 50 of lower draw ring 34 comprises a series of concentric depressions 52-58 which are, in turn, separated by raised flat portions 60 which define a common plane.

As best seen in FIG. 2 each of the depressions 52, 54, 56 and 58 comprises, in a preferred embodiment, arcuate shaped depressions defining small segments of sufficiently large radius to provide an angle with the adjacent flat portions 60 of about 7.degree.-12.degree. and preferably about 9.degree.-10.degree.. This has been found to provide a corner between the flat portion 60 and the arcuate depressions 52 et al of small enough angle to prevent stripping or mutilating of the coated surface of the sheet metal. Depressions 52-58 have been exaggerated in FIG. 2 for illustrative purposes only.

Preferably, the depth of the depressions should not exceed about one-third of the thickness of the sheet metal to minimize wrinkling. It has also been found that the total width of the depressions should not exceed about 0.09 to 0.15 inches. Apparently, larger or deeper grooves support an insufficient area of the sheet metal to prevent wrinkling.

It will be noted in the illustrations that the width of each of the depressions exceeds the flat portion 60 separating the depressions. It has been found that a ratio of about 2 to 1 is satisfactory provided that the overall width and depth of the depressions does not exceed the values previously discussed. Depressions of smaller overall width as well as smaller radius of width of the depressions to width of the flat portion are, of course, satisfactory. However, if the width of flat portion 60 greatly exceeds the width of the depressions the purpose of decreasing the overall pressure exerted by draw ring 34 against the sheet metal blank is defeated. Preferably, therefore, the overall width of the depression and the ratio between the flat and the depressed portions should be maintained.

Referring specifically to FIG. 3 which is a top view of draw ring 34 it will be noted that four concentric depressions have been placed in the face of the draw ring. A larger or smaller number of depressions can, of course, be used depending upon the overall dimensions of draw ring 34.

It should be further noted that the illustrated embodiment is directed toward the formation of a cylindrical object, such as, for example, a can or the like. While cylindrical shapes are most widely used it is deemed to be within the scope of the invention to utilize the draw ring for rectangular shaped objects as well.

Referring again to FIG. 2 it will be noted that draw ring 16 has a planar surface which is approximately parallel to the flat portion 60 of draw ring 34. Thus a sheet metal blank when clamped between draw ring 16 and the flat portion 60 of draw ring 34 is maintained in a plane without deformation of the sheet metal blank. Thus the flow of the sheet metal blank during the drawing operation is lateral providing, therefore, an even drawing of the sheet metal blank through the draw rings.

To further illustrate the invention, a number of cylinders were drawn using respectively the draw rings of the invention and a pair of smooth face draw rings. The effect of the lower pad pressure on the sheet metal blanks drawn using the draw ring of the invention was noted as well as the reduced amount of earring produced on the drawn object both during a first or initial drawing and during redrawing operation as shown by the tables below.

TABLE I

Smooth First Grooved First % Pad Draw (pad Draw (pad Pressure Item pressure pressure Reduced in lb.) in lb. A 7920 7040 11.1 B 7040 6160 12.5 C 7040 5280 25 D 7040 6160 12.5 E 4400 3520 20

TABLE II

Smooth Grooved Item First Draw Redraw First Draw Redraw A 3.6% 6.6% 2.8% 3.3% C 4.0% 6.0% 2.4% 2.7% D 3.1% 4.6% 3.5% 3.8% E 3.0% 4.5% 1.7% --

the draw rings were used against the inside coating, that is, the inner, coated, side of the sheet metal and, to evaluate the effect on the coating, the cylinders formed on both the smooth and grooved draw rings were processed in steam and water for 90 minutes at 240.degree. F. At the end of the drawing process, the cans were checked by enamel rater, scotch tape and copper sulfate. The cans produced using both types of tooling passed the copper sulfate and scotch tape. At the enamel rater the cans processed in water had readings ranging from 0-5 and those processed in steam had readings from 0-2.5. These readings are within accepted values and illustrate that the use of the draw rings having the depressions of the invention therein did not significantly affect the coating on the surface of the sheet metal.

While the invention has been defined with respect to a preferred embodiment, minor modifications may be apparent to those skilled in the art and should be considered to be within the scope of the invention.

Claims

What is claimed is:

1. In a drawing die apparatus for drawing shaped members from sheet material wherein the sheet is peripherally engaged respectively by opposing surfaces of a pair of dies while a central post member on one of the dies draws the sheet material into a corresponding cylinder on the other die, the improvement wherein: at least one of said opposing surfaces is provided with a plurality of concentric depressions each having a width of about 0.09-0.15 inch, and forming an angle with the planar portion of the die not exceeding about 12.degree. separated by planar portions which are opposed by planar portions of the opposed die surface defining the extremities of the opposed surfaces to engage said sheet material in a common plane without deformity of said sheet material.

2. The improvement of claim 1 wherein said width of each of said depressions is greater than the width of the planar portion between adjacent depressions to provide a substantial reduction in contact area between the opposing die surfaces and the sheet material being drawn therebetween.

3. The improvement of claim 1 wherein the depressions are arcuate in cross section and the radius of the arc defined by each of said depressions is about 4.25 times the width of the depression.

4. The improvement of claim 1 wherein the depth of each of said depressions does not exceed about one-third of the thickness of sheet material.

5. Improved draw ring members for peripheral engagement of sheet metal stock during a drawing operation comprising opposing draw ring surfaces wherein at least one of said surfaces contains a plurality of concentric concave channels therein, each of said concave channels having a width of approximately 0.09-0.15 inch, a depth not exceeding about one-third of the thickness of the sheet metal to be engaged by said opposing surfaces, and an angle of intersection with the planar portions of the surface therebetween not exceeding about 12.degree., each of said planar portions having a width not exceeding the width of said channel to provide a reduction in contact area between the opposing die surfaces and the sheet metal being drawn therebetween.