Method of and apparatus for manufacturing a top plate for a metallic drum container

Abstract: Method of and apparatus for forming an opening (2) in a plate (1), the opening being defined by a tubular flange (3) projecting from the plate (1). The method comprises the steps of: forming a hole (20) in the plate (1), the hole (20) being of smaller diameter than the said opening (2); deforming the material of the plate (1), in a region immediately surrounding the hole, out of the plane of the plate (1); subsequently deforming the material of the plate (1) in the region surrounding the hole (20) into a frustoconical configuration of which the larger diameter is substantially equal to the intended diameter 1₃ of the tubular flange (3); and subsequently forming the frustoconical configuration into the said tubular flange (3).

Description: The present invention relates to a method of and an apparatus for manufacturing a top plate for a metallic drum container, the top plate having an opening defined therein for introducing a material into or removing a material out of the metal drum container. Generally, metallic drum containers comprise a cylindrical drum, a disk-shaped bottom plate closing the bottom of the cylindrical drum, and a disk-shaped top plate closing the top of the cylindrical drum. FIG. 6 of the accompanying drawings shows one conventional top plate 50 for a metallic drum container, the top plate 50 having an opening 51 defined therein for introducing a material into or removing a material out of the metal drum container. The top plate 50 has a tubular flange 52 disposed around the peripheral edge of the opening 51 and extending upwardly therefrom. A cylindrical attachment 53 is inserted in the tubular flange 52, and has an internally threaded inner wall surface 54. The cylindrical attachment 53 serves to receive therein a plug (not shown) for closing the opening 51. The plug has an externally threaded outer wall surface, which is threaded in the internally threaded inner wall surface 54 of the cylindrical attachment 53, thereby closing the opening 51. To prevent the contents from leaking out of the metallic drum container, it is necessary that the cylindrical attachment 53 inserted in the tubular flange 52 be securely fixed to the flange 52. If the cylindrical attachment 53 and the tubular flange 52 were not securely fixed to each other, then the contents would leak through the gap between the cylindrical attachment 53 and the tubular flange 52. When the plug is removed from the opening 51, the cylindrical attachment 53 and the plug may possibly turn together, and the plug may not be detached from the cylindrical attachment 53. To alleviate this drawback, a seal member 55 is inserted between the tubular flange 52 and the cylindrical attachment 53 to allow the cylindrical attachment 53 to be reliably secured to the tubular flange 52 in close contact therewith. However, the process of inserting the seal member 55 is complex and costly. The cylindrical attachment 53 that is inserted in the tubular flange 52 has a radially outwardly extending peripheral edge 56 projecting on the inner surface of the top plate 50, defining a stepped recess 57 thereon. When the stored material is removed from the metallic drum container, some material tends to be trapped in the recess 57, and the metallic drum container cannot fully be emptied. The trapped material cannot easily be cleared out of the recess 57 when the interior of the metallic drum container is cleaned for reuse. U.S. patent No. 4,852,238 (Japanese Laid-Open Patent Publication No. 1-313119) discloses a method of making a top plate which has an opening but does not have any separate attachment. According to the disclosed method, a region of the top plate where an opening is to be defined is raised upwardly to form a flat disk on its top, and the top plate stock is drawn from the center of the disk toward the peripheral edge thereof. Then, a circular hole that is smaller in diameter than the opening to be eventually formed is defined in the disk. The top plate stock that has been flowed toward the peripheral edge of the disk is raised upwardly into a tubular flange. At the same time, the circular hole is enlarged in diameter. Thereafter, the entire peripheral edge of the tip end of the flange is vertically compressed to increase the thickness of the flange. The inner wall surface of the flange is then internally threaded. The increased thickness of the flange increases the mechanical strength of the flange. A plug is directly inserted in the tubular flange in threaded engagement with its internally threaded inner wall surface. Since no separate attachment is employed, the cost is lowered, and the interior of the drum container can easily be cleaned. However, the integral formation of the flange and the top plate poses the following problems: Top plates for use on drum containers are generally made of thin sheet steel having a thickness in the range of from 1.0 mm to 1.2 mm, specifically SPHC for general use according to JIS of Hot-Rolled Mild Steel Sheet, Strip and Plate, or SPCC for general use according to JIS of Cold-Rolled Carbon Steel Sheet and Strip. The top plates are required to have an opening for threaded engagement with a 2-inch (50.8 mm) plug and an opening for threaded engagement with a 3/4-inch (19.05 mm) plug according to international standards. The flanges around the openings must have a height of about 8 mm so that the inner wall surfaces of the flanges are internally threaded over a length of 6 mm or more for threaded engagement with the plugs. The SPHC, referred to above, whose wall thickness is 1.6 mm or less has an elongation percentage of 27 %, which is smaller than the elongation percentage of 30 % of SPHD that is to be machined by drawing and the elongation percentage of 31 % of SPHE that is to be machined by deep drawing. Likewise, the SPCC whose wall thickness is 1.6 mm or less has an elongation percentage of 37 %, which is smaller than the elongation percentage of 39 % of SPCD that is to be machined by drawing and the elongation percentage of 41 % of SPCE that is to be machined by deep drawing. When a flange that is 8 mm high is formed on a top plate of SPHC or SPCC whose wall thickness ranges from 1.0 mm to 1.2 mm to define a ¾-inch (19.1 mm) opening according to the above conventional process, the flange may crack under stresses because of the limited elongation percentage. The inner wall surface of the flange should preferably be internally threaded by roll threading rather than cutting because cut threads would reduce the mechanical strength of the flange. If a small crack were formed in the tip end of the flange at the time it is drawn, it might develop into a larger crack when the flange is internally threaded, and the top plate could not be offered for sale as a finished product. Even with no crack formed in the flange, if the flange were progressively thinner toward its upper edge, then the flange might crack when it is internally threaded. To avoid this shortcoming, after the flange is formed by deep drawing, it is downwardly compressed to prevent the upper edge of the flange from being thinner, according to the conventional method described above. However, the step of downwardly compressing the flange in addition to the step of forming the flange makes the manufacturing apparatus complex. US-A-2271762 describes a method of making bung openings. The method involves an initial cup drawing operation in which the material in the region of the opening to be formed is pushed upwardly out of the plane of a sheet metal blank. Thereafter, a combined drawing and punching operation is performed on the cup blank to provide an integral neck defining a bung opening. US-A-2455311 discloses a method of forming conduit connections for tanks. The method involves forming an opening in the tank and forcing a first circular dye through the opening to form a flange. Thereafter a second dye is forced from the opposite direction through the opening and forces the walls of the flange outwardly into a curved configuration. Thereafter, an internally threaded annular body is welded to the rim of the flange to form the required threaded opening. In view of the aforesaid problems of the conventional method and apparatus for manufacturing top plates for metallic drum containers, the present invention seeks to provide a method of and an apparatus for manufacturing a top plate for a metallic drum container of thin sheet steel, the top plate having an opening with a mechanically strong flange extending therearound. According a first aspect of the present invention, there is provided a method of forming an opening in a plate, the opening being defined by a tubular flange projecting from the plate, the method comprising the successive steps of: (a) forming a hole in the plate, the hole being of smaller diameter than the opening; (b) deforming the material of the plate, in a region immediately surrounding the hole, out of the plane of the plate; (c) deforming the material of the plate in the region surrounding the hole into a frustoconical configuration of which the larger diameter is substantially equal to the intended diameter of the tubular flange; and (d) forming the frustoconical configuration into the tubular flange. Preferably, the step (b) comprises raising and radially outwardly expanding a peripheral edge of the circular hole into a substantially frustoconical first flange blank, the first flange blank having a circular hole whose diameter is smaller than the inside diameter of the opening and a height smaller than the intended height of the tubular flange. Preferably, also the step (c) comprises raising and radially outwardly expanding a peripheral edge of a proximal portion of the first flange blank into a substantially frustoconical second flange blank contiguous to the first flange blank, the second flange blank having a lower portion beneath the peripheral edge of the proximal portion, the lower portion having a diameter which is substantially equal to the inside diameter of the tubular flange, the first and second flange blanks jointly serving as a substantially frustoconical third flange blank. The third flange blank maybe formed by a die which comprises an upper portion complementary in shape to an inner wall surface of the first flange blank and a lower portion contiguous to the upper portion and complementary in shape to an inner wall surface of the second flange blank. The third flange blank may be formed by raising the second flange blank progressively upwardly, forming the first flange blank progressively upwardly into the shape of the second flange blank when the second flange blank is raised, and raising the first flange blank as it is formed into the shape of the second flange blank, progressively upwardly. According to a second aspect of the present invention, there is provided an apparatus for forming an opening in a plate, the opening being defined by a tubular flange projecting from the plate, the apparatus comprising: punching means for defining a circular hole in a top plate stock of thin sheet material, the circular hole having a diameter smaller than a desired inside diameter of the opening; and a die member comprising a substantially cylindrical lower portion, and an upper head portion having a flat top surface and first and second substantially frustoconical forming surfaces, the first forming surface comprising a rounded convex surface disposed beneath the flat top surface and projecting radially outwardly therefrom, and the second forming surface comprising a rounded convex surface disposed beneath the first forming surface and projecting radially outwardly therefrom, and further comprising a concave rounded surface disposed between the first and second forming surfaces. A second die having a circular hole may be provided, the base portion of the die member being insertable into the circular hole of the second die. The punching means may comprise a cylindrical third die, and a hole opening in the head portion of the die member, the third die being insertable in the hole. In an embodiment in accordance with the present invention, the peripheral edge of the circular hole is raised and radially outwardly expanded into the first flange blank which is smaller in diameter than the tubular flange to be finally formed. The extent to which the first flange blank is formed is therefore relatively small, and the first flange blank is prevented from cracking when it is formed. The proximal portion of the first flange blank is then raised and radially outwardly expanded into the second flange blank. The third flange blank that is composed of the first and second flange blanks is then raised into the tubular blank while increasing the diameter of the circular hole. Since the tubular blank is successively formed from the top plate stock by raising and radially outwardly expanding the flange blanks successively, the tubular blank is prevented from being greatly reduced in thickness at local regions. Therefore, the top end of the tubular flange is prevented from cracking when it is formed. The tubular flange thus formed around the opening in the top plate has a relatively high degree of mechanical strength. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- FIG. 1 is a fragmentary plan view of a top plate manufactured according to the present invention; FIG. 2 is a cross-sectional view taken along line II - II of FIG. 1; FIG. 3 is a vertical cross-sectional view of an apparatus according to the present invention; FIG. 4 is an enlarged fragmentary cross-sectional view of an inner die in the apparatus shown in FIG 3; FIGS. 5(a) through 5(d) are vertical cross-sectional views showing successive steps of the method according to the present invention; and FIG. 6 is a cross-sectional view of a conventional top plate with an opening and an attachment. As shown in FIGS. 1 and 2, a disk-shaped top plate 1 for use on a metallic drum container (not shown) has a circular opening 2 defined therein. The top plate 1 also has a tubular flange 3 projecting upwardly and having an internally threaded inner wall surface 4. A plug (not shown) with an externally threaded outer wall surface can detachably be threaded into the opening 2. The top plate 1 may be made of SPCC or SPHC and has a wall thickness ℓ₁ of 1.2 mm. The flange 3 has a height ℓ₂ of 8 mm from the top plate 1, and has an inside diameter ℓ₃ of 25.1 mm so that an ordinary 3/4 inch (19.1mm) plug can be threaded in the flange 3. The internally threaded inner wall surface 4 of the flange 3 has an axial length ℓ₄ of 6 mm, with the threads on the internally threaded inner wall surface 4 having a pitch of 1/14 inch (1.8mm). The opening 2 is defined in the top plate 1 by an apparatus 5 shown in FIG. 3. The apparatus 5 has a lower die 7 with a tubular through hole 6 defined centrally therein, the lower die 7 being mounted on a lower base 9. The lower die 7 is normally urged to move upwardly by a spring 8 disposed between the lower base 9 and the lower die 7. A substantially vertically disposed cylindrical inner die 10 is fixed to the center of the lower base 9, and is inserted in the through hole 6 in the lower die 7. The inner die 10 can extend upwardly through the hole 6 and projects upwardly of the lower die 7 when the lower die 7 is lowered. The inner die 10 has a shoulder 11 on its upper end portion, and a top 12 of reduced diameter which is positioned upwardly of the shoulder 11. The inner die 10 and the lower base 9 have a through hole 13 extending centrally therethrough in the vertical direction. The inner die 10 has a main portion beneath the shoulder 10, the main portion having an outside diameter that is equal to the inside diameter ℓ₃ of the flange 3. The apparatus 5 also includes an upper die 15 disposed above the lower die 7 in confronting relationship thereto, the upper die 15 having a tubular forming region 14 which has a diameter corresponding to the diameter of the opening 2. The upper die 15 is mounted on a guide member 16 mounted on an upper base 18 and is normally urged to move downwardly by a spring 17 between the upper die 15 and the guide member 16 while being guided by the guide member 16. The guide member 16 has a punch rod 19 fixed thereto and projecting downwardly from the center of the lower end thereof. The punch rod 19 has a diameter corresponding to the inside diameter of the through hole 13 in the inner die 10, such that the punch rod 19 can be inserted into the through hole 13. The upper base 18 can be moved downwardly by an actuator (not shown). The inner die 10 and the punch rod 19 will be described in detail with reference to FIG. 4. The inside diameter, denoted at ℓ₅, of the through hole 13 governs the height of the flange 3 that has been formed. The smaller the inside diameter ℓ₅, the greater the height of the flange 3. If the inside diameter ℓ₅ were too small, the flange 3 might crack when it is formed. Therefore, the inside diameter ℓ₅ should appropriately be selected depending on the desired height of the flange 3. If the desired height of the flange 3 is 8 mm, then the inside diameter ℓ₅ should preferably be 10.5 mm. The top 12 of the inner die 10 has a flat surface 12a having a width ℓ₆. The flat surface 12a has an edge 12b around the upper open end of the hole 13, the edge 12b serving as a cutting edge which cooperates with the punch rod 19 in punching the stock of the top plate 1. If the width ℓ₆ were too small, then the top 12 of the inner die 10 would be damaged due to the load imposed on the top 12. If the width ℓ₆ were too large, the tip end of the flange 3 would crack when the flange is formed. Accordingly, the width ℓ₆ should appropriately be selected to avoid the damage to the top 12 and the crack of the flange 3. If the inner die 10 is made of SKD11 according to JIS or D2 according to AISI ASTM, then the width ℓ₆ should be in the range of from 1.0 mm to 1.5 mm, preferably from 1.2 to 1.3 mm to meet the conditions for forming the flange 3. The shoulder 11 of the inner die 10 is composed of first, second, and third round sections R₁, R₂, R₃ extending round the entire circumferential surface thereof. The first round section R₁, which is the uppermost round section, is contiguous to the flat surface 12a of the top 12 and has a radially outwardly convex curved surface. The second round section R₂, which is the lowermost round section, is contiguous to the cylindrical section of the inner die 10 beneath the shoulder 11 and has a radially outwardly convex curved surface. The third round section R₃, which is positioned between the first and second round section R₁, R₂, has a radially inwardly concave curved surface. The first and second round sections R₁, R₂ basically serve to draw upwardly the peripheral edge of a circular hole 20 that has been formed in the top plate stock by the punch rod 19 whose diameter is smaller than the outside diameters of the first and second round sections R₁, R₂ while raising the peripheral edge of the circular hole 20 into a substantially frustoconical shape and pressing the peripheral edge radially outwardly. More specifically, as indicated by the imaginary lines in FIG. 4, the peripheral edge of the circular hole 20 is formed substantially along the first and second round sections R₁, R₂. At first, the top plate stock does not contact the third round section R₃. When the peripheral edge of the circular hole 20 moves past the third round section R₃, the peripheral edge springs back into contact with the third round section R₃. At the time the peripheral edge of the circular hole 20 moves past the third round section R₃, the peripheral edge follows the third round section R₃, which releases strains that have been quickly accumulated in the peripheral edge when it has been formed by the first round section R₁. Thereafter, the peripheral edge of the circular hole 20 is raised upwardly into a substantially frustoconical shape and expanded radially outwardly to a desired diameter by the second round section R₂. When the peripheral edge of the circular hole 20 is expanded radially outwardly by the first and second round sections R₁, R₂ the extent to which the peripheral edge is expanded radially outwardly is reduced at lower regions of the first and second round sections R₁, R₂. In order that the first, second, and third round sections R₁, R₂, R₃ form the flange 3 without developing cracks therein and the upper edge of the flange 3 has substantially the same thickness as that of the top plate stock, it is necessary to satisfy the following conditions: The extent to which the peripheral edge of the circular hole 20 is formed by a region A which extends from the flat surface 12a through the first round section R₁ to an intermediate position of the third round section R₃, and the extent to which the peripheral edge of the circular hole 20 is formed by a region B which extends from the intermediate position of the third round section R₃ to the lower end of the second round section R₂, are related to each other as follows: The extent to which the peripheral edge is formed by the region A in the radially outward direction is greater than the extent to which the peripheral edge is formed by the region B in the radially outward direction. More specifically, the extent to which the peripheral edge is radially outwardly formed by the region A is 55 to 65 %, preferably 60 %, of the entire extent to which the peripheral edge is formed, and the extent to which the peripheral edge is radially outwardly formed by the region B is 35 to 45 %, preferably 40 %, of the entire extent to which the peripheral edge is formed. If the extent to which the peripheral edge is radially outwardly formed by the region A were greater than 65 % of the entire extent, then the flange would tend to crack when it is formed. If the extent to which the peripheral edge is radially outwardly formed by the region A were smaller than 55 %, then the formed flange would not have a desired height. The extent to which the peripheral edge is formed by the region A in the direction of the height of the flange, i.e., in the axial direction, is substantially equalto or smaller than the extent to which the peripheral edge is formed by the region B in the direction of the height of the flange, i.e., in the axial direction. More specifically, the extent to which the peripheral edge is axially formed by the region A is 40 to 50 %, preferably 45 to 49 %, of the entire extent to which the peripheral edge is axially formed, and the extent to which the peripheral edge is axially formed by the region B is 50 to 60 %, preferably 51 to 55 %, of the entire extent to which the peripheral edge is formed. If the extent to which the peripheral edge is axially formed by the region A were smaller than 40 % of the entire extent, then the formed flange would not have a desired height when it is formed. If the extent to which the peripheral edge is axially formed by the region A were greater than 50 %, then the flange would tend to crack when it is formed. To meet the conditions for drawing the flange 3, the region A has a radial length ℓ₇ ranging from 4.0 mm to 4.8 mm, the region B has a radial length ℓ₈ ranging from 2.6 mm to 3.3 mm, the region A has an axial length ℓ₉ ranging from 3.8 mm to 4.8 mm, and the region A has an axial length ℓ₁₀ ranging from 4.8 mm to 5.7 mm. The curvature r₁ of the first round section R₁ should preferably be smaller than the curvature r₂ of the second round section R₂. The curvature r₃ of the third round section R₃ should preferably be greater than the curvatures r₁, r₂. Under the conditions for drawing the flange 3, the radius of the curvature r₁ ranges from 5.0 mm to 7.0 mm, the radius of the curvature r₂ ranges from 6.0 mm to 8.0 mm, and the radius of the curvature r₃ ranges from 8.0 mm to 12.0 mm. In order to form the flange 3 without cracks, it is preferable to minimize a gap ℓ₁₁ between the punch rod 19 and the edge of the through hole 13 in the inner die 10 when the circular hole 20 is defined in the top plate stock. Preferably, the gap ℓ₁₁ should be in the range of from 0.03 mm to 0.05 mm. If the gap ℓ₁₁ were too large, the peripheral edge of the circular hole 20 which has been cut by the punch rod 19 and the edge of the through hole 13 would be forced into the through hole 13 by the punch rod 19, presenting an obstacle to the enlargement of the peripheral edge of the circular hole 20 in the radially outward direction. The process of forming the flange 2 with the apparatus 5 will be described below with reference to FIGS. 3, 4(a) through 4(d), and 5. As shown in FIG. 3, a top plate stock 1 is placed on the upper surface of the lower die 7 with a region where the opening 2 is to be defined being aligned with the through hole 6. Then, as shown in FIG. 5(a), the upper base 18 is moved downwardly until the top plate stock 1 is gripped between the upper die 15 and the lower die 7. The upper base 18 is further moved downwardly to cause the punch rod 19 to thrust through the top plate stock 1 into the through hole 13, thus defining a circular hole 20 in the top plate stock 1. The diameter of the circular hole 20 thus defined is smaller than the outside diameter of the inner die 10 which corresponds to the inside diameter of the opening 2 (FIG. 2). As shown in FIG. 5(b), the upper base 18 is moved downwardly to depress the lower die 7 so that the upper portion of the inner die 10 projects above the lower die 7. The peripheral edge of the circular hole 20 is raised by the region A of the shoulder 11 of the inner die 10, and while at the same time, is radially outwardly expanded thereby into a first flange blank 21 of a substantially frustoconical shape. The first flange blank 21 is smaller in diameter and width than the flange 3 that is to be formed around the opening 2 in the top plate 1. Then, as shown in FIG. 5(c), the upper base 18 is further moved downwardly to depress the lower die 7 so that the upper portion of the inner die 10 further projects above the lower die 7. The peripheral edge of a proximal portion of the first flange blank 21 is raised and radially outwardly expanded by the region B of the shoulder 11, thereby forming a substantially frustoconical second flange blank 22 that is downwardly contiguous to the first flange blank 21. The first and second flange blanks 21, 22 now jointly form a substantially frustoconical third flange blank 23. A lower portion of the third flange blank 23 has an inside diameter close to that of the flange 3 to be eventually formed. Subsequently, as shown in FIG. 5(d), the upper base 18 is further moved downwardly to depress the lower die 7 so that the upper portion of the inner die 10 further projects above the lower die 7. At this time, the third flange blank 23 is drawn into the flange 3 by an outer wall surface 23 of the inner die 10 below the shoulder 11 and the forming region 14 of the upper die 10. As described above, the flange 3 is formed successively by the top 12, the shoulder 11, and the outer wall surface 23 of the inner die 10. In this manner, the flange 3 is prevented from cracking, and has substantially the same wall thickness as that of the top plate stock. Thereafter, the inner wall surface 4 (see FIG. 2) of the flange 3 is internally threaded by roll threading, using grooved rolls (not shown). The opening 2 surrounded by the flange 3 is thus defined in the top plate 1. Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.

Claims: Method of forming an opening (2) in a plate (1), the opening being defined by a tubular flange (3) projecting from the plate (1), the method comprising the successive steps of: (a) forming a hole in the plate (1), the hole being of smaller diameter than the opening (2); (b) deforming the material of the plate (1), in a region immediately surrounding the hole, out of the plane of the plate (1); (c) deforming the material of the plate (1) in the region surrounding the hole into a frustoconical configuration of which the larger diameter is substantially equal to the intended diameter of the tubular flange (3); and (d) forming the frustoconical configuration into the tubular flange (3). A method as claimed in claim 1, wherein the step (b) comprises raising and radially outwardly expanding a peripheral edge of the circular hole into a substantially frustoconical first flange blank, the first flange blank having a circular hole whose diameter is smaller than the inside diameter of the opening (2) and a height smaller than the intended height of the tubular flange (3). A method as claimed in claim 2, wherein the step (c) comprises raising and radially outwardly expanding a peripheral edge of a proximal portion of the first flange blank into a substantially frustoconical second flange blank contiguous to the first flange blank, the second flange blank having a lower portion beneath the peripheral edge of the proximal portion, the lower portion having a diameter which is substantially equal to the inside diameter of the tubular flange (3), the first and second flange blanks jointly serving as a substantially frustoconical third flange blank. A method as claimed in any one of the preceding claims, further comprising the step (e) of internally threading an inner wall surface of the tubular flange (3) after the step (d) has been completed. A method as claimed in any one of the preceding claims, wherein the plate (1) comprises either a sheet of hot-rolled mild steel or a sheet of cold-rolled carbon steel. A method as claimed in claim 5, wherein the sheet of hot-rolled mild steel has a thickness ranging from 1.0 mm to 1.6 mm and an elongation percentage of 27% or less, and the sheet of cold-rolled mild steel has a thickness ranging from 1.0 to 1.6 mm and an elongation percentage of 37% or less. A method as claimed in any one of the preceding claims when appendant to claim 3, wherein the step (c) comprises the steps of raising the second flange blank progressively upwardly, and then raising the first flange blank progressively upwardly. A method as claimed in any one of the preceding claims when appendant to claim 3, wherein the step (d) comprises the steps of raising the second flange blank progressively upwardly, forming the first flange blank progressively upwardly into the shape of the second flange blank when the second flange blank is raised, and raising the first flange blank as it is formed into the shape of the second flange blank, progressively upwardly. A method as claimed in claim 8, further including the step of releasing strains produced when the peripheral edge of the circular hole is radially outwardly expanded, at the time the first flange blank is formed progressively upwardly into the shape of the second flange blank in the step (d). A method as claimed in any one of the preceding claims when appendant to claim 3, wherein the extent to which the first flange blank is radially outwardly formed is 55 to 65% of the extent to which the tubular flange (3) is finally radially outwardly formed. A method as claimed in any one of the preceding claims when appendant to claim 3, wherein the extent to which the first flange blank is axially formed is 55 to 65% of the extent to which the tubular flange (3) is finally axially formed. Apparatus for forming an opening (2) in a plate (1), the opening (2) being defined by a tubular flange (3) projecting from the plate (1), the apparatus comprising: punching means (10, 19) for defining a circular hole in a top plate stock (1) of thin sheet material, the circular hole having a diameter smaller than a desired inside diameter of the opening (2); and a die member (10) comprising a substantially cylindrical lower portion (23), and an upper head portion having a flat top surface (12) and first and second substantially frusto-conical forming surfaces, the first forming surface comprising a rounded convex surface (R₁) disposed beneath the flat top surface (12) and projecting radially outwardly therefrom, and the second forming surface comprising a rounded convex surface (R₂) disposed beneath the first forming surface and projecting radially outwardly therefrom, and further comprising a concave rounded surface (R₃) disposed between the first and second forming surfaces. Apparatus as claimed in claim 12, wherein the flat top surface (12) of the die member (10) has a diameter greater than the diameter of the circular hole. Apparatus as claimed in claim 12 or 13, wherein the radius of curvature (r₁) of the first forming surface is smaller than the radius of curvature (r₂) of the second forming surface. Apparatus as claimed in any one of claims 12 to 14, wherein the radius of curvature (r₃) of the third forming surface is greater than the radii of curvature (r₁, r₂) of the first and second forming surfaces. Apparatus as claimed in any one of claims 12 to 15, further comprising a second die member (15) having a circular hole (14), the cylindrical lower portion (23) of the first die member being insertable into the circular hole (14) in the second die member (15). Apparatus as claimed in claim 16, wherein the punching means comprises a cylindrical third die member (19), and a hole (13) opening in the head portion of the first die member (10), the third die member (19) being insertable into the hole (13).

Inventors: TSUZUKI TOSHIO C O TANNAN CO L, TSUZUKI, TOSHIO, C/O TANNAN CO., LTD.
Applicants: TANNAN IND CO LTD, TANNAN INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: B21D51
IPC codes: B21D51