Forming method and forming tools

Abstract: A forming method of forming a plate-shaped workpiece (4) using a female tool (3) having a concave section (3b) of a desired shape and a male tool which matches with the female tool, includes the following steps: providing an elastic punch (6) in a cylindrical holder (5) provided in the male tool; pressurizing and compressing the elastic punch (6) relatively by a pressuring member (7); pressing the holder (5) against the workpiece (4) by means of one portion (6b) of the elastic punch (6); and projecting one portion of the elastic punch from the holder so as to deform a portion of the workpiece to be processed according to the shape of the concave section (3b) of the female tool (3) so that the workpiece is formed in a state such that the workpiece (4) is nipped pressingly between the female tool (3) and the holder (5).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to forming tools comprising an elastic punch made of an elastic material as well as to a forming method using said tools for drawing a flat workpiece like a metal plate into a predetermined shape. Description of the Related Art Conventionally, a punch press suitable for carrying out a forming method for drawing a plate-shaped metal workpiece, comprises as forming tools a punch acting as a male tool and a die acting as a female tool and having a concave section, whereby the workpiece to be drawn is positioned between the die and the punch. When the punch is moved towards the die, the workpiece is formed into the predetermined shape thanks to the cooperation of the die and the punch. When the shape of workpieces formed in a conventional manner is to be changed, special tools are required for each of the various shapes to be drawn. Even though the workpieces to be formed can keep the same drawn shape, a variation of the thickness of the workpiece plates may occur and needs a troublesome adjustment of the clearance between punch and die in accordance with the various plate thicknesses. Conventionally, a special blank holder is to be used for maintaining the border of the workpiece fixed between the punch and the die. The adjustment of the pinch force applied to the workpiece is quite difficult and leads often either to a reduction of the thickness of the workpiece border or to the appearance of fine creases on the surface of the latter. GB-A-1 248 553 reveals a method and an apparatus according to the pre-characterising part of claim 1 for forming an object into a shape determined by the cavity of a die. The known apparatus comprises a die disposed on a support platen and a pressing head movable vertically relative to said platen. The die receives a blank preformed to a hollow shape which is then formed into the desired object, the blank normally being a sheet metal. An elastomeric former is disposed within the die so that the preformed blank is arranged between the former and the die. The apparatus according to GB-A-1 248 553 further comprises a cylindrical housing and a plunger attached to the press head whereas the housing is slidably supported from the presshead by means of lugs which engage a flange on the housing. Within the housing which at its lower end has a nozzle of a shape similar to the one of the top surface of the die, a quantity of elastomeric material is provided beneath the plunger. In operation, the press head is lowered until the end of the housing abuts the top of the die, slightly compressing the elastomeric former, which, when not subjected to pressure, projects only slightly beyond the top of the die. The press head is then further lowered and since the plunger and the housing are relatively moveable, the elastomeric material contained in the housing is compressed and extruded through the nozzle into contact with the former which is deformed under the forming force of the elastomeric material leaving said nozzle and which applies the hollow blank against the die. After the forming operation, the press head retracts the plunger and the housing and the elastomeric material extruded through the nozzle retracts back into the housing. Then, the former is removed from the formed object and the latter is removed from the die. A device for press forming a sheet blank is known from US-A-5 599 565. This known device comprises a retaining box containing a pad of elastomeric material and a vertically movable piston constituting the bottom of the retaining box and disposed on a compression spring which in turn is mounted on a fixed support plate. A lower annular blank holder is disposed on the upper faces of the pad and the box and projects towards the axis of said box and partially over said elastomeric pad. A spring surrounds the piston and is disposed between the box and the support plate. Above the lower blank holder, the device comprises a slidably mounted hollow body whose lower face constitutes an upper peripheral blank holder and whose central cylindrical hole accommodates a plunger presenting a lower profiled surface corresponding to the finished object drawn into the cylindrical hole which together with the plunger constitutes a die. The present invention aims at the elimination of drawbacks of the prior art and at the reduction of elements or components needed in the known devices and apparatuses. These and other aims are attained by forming tools comprising the features of claim 1. Thanks to this embodiment, the number of elements and components necessary in the known pressure forming devices is reduced, blank sheets of different thicknesses can be drawn without changing the tools and/or adjusting the clearance between the male and the female tools, the structure of the latter is simplified and the generation of creases on the drawn objects is prevented. The present invention also aims at a method of forming a hollow object as defined in claim 13. Thanks to this method the pinching or clamping action which the border zone of the sheet shaped workpiece undergoes starts with the application of the pressure force onto the elastic punch and is controlled by said pressure force during the drawing operation thereby preventing any displacement of said border zone towards the center of the female tool and any generation of creases on the sheet shaped workpiece. A further advantage of the method consists in the fact that the forming or drawing action of the elastic punch material starts only after the completion of the pinching or clamping of the workpiece between the female tool and the blank holder. Further advantages of the invention can be deduced from the following description of several embodiments of the invention whose scope of protection is defined by the annexed claims. Brief description of the accompanying drawings The following description of several embodiments is made with reference to the accompanying drawings, in which: FIGs. 1 to 4 are sectional views showing forming tools and a forming method according to a first embodiment of the present invention. FIG. 5 is a detailed drawing showing an enlarged presentation of portion A of Fig. 2. FIGs. 6 to 9 are partial perspective views showing different shapes of products processed by the forming method according to the present invention. FIG. 10 is a sectional view showing forming tools according to a second embodiment of the present invention. FIGs. 11A, 11B and 11C are sectional views showing various embodiments of the blank holding section of a holder. FIG. 12 is a sectional view showing forming tools according to a third embodiment of the present invention. FIG. 13 is a sectional view showing forming tools according to a fourth embodiment of the present invention. FIG. 14 is a sectional view showing forming tools according to a fifth embodiment of the present invention. FIG. 15 is a sectional view showing a modified detail of the forming tools according to the fifth embodiment of the present invention. FIGs. 16A and 17B are sectional views of two different shapes of an elastic punch according to the present invention. FIGs. 17A, 17B, 17C and 17D are explanatory drawings showing steps of the forming process using the forming tools according to the fifth embodiment of the present invention. FIG. 18 is a sectional schematic view showing forming tools according to a sixth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS There will be detailed below the preferred embodiments of the present invention with reference to the accompanying drawings. Like members are designated by like reference characters. FIGs. 1 through 4 are sectional views showing the forming method according to the present invention in an order of the steps, as first embodiment. FIG. 5 is a detailed drawing showing an enlarged A portion of FIG. 2, and FIG. 6 is a partial perspective view showing a product processed by the forming method according to the present invention. First, the description will be given as for a basic structure of tools used in the forming process according to the present invention with reference to FIG. 1. In FIG. 1, a reference numeral 1 is a platen-type bolster in a press, namely, a bottom holder corresponding to a bottom turret or the like in a turret punching press. A metal plate 2 Is set on the bottom holder, and a laminated tool 3 as one example of a female tool is placed on the metal plate 2. The laminated tool 3 is formed by a plurality of laminated metal plates 3a (four plates in the drawing), and shows a concave section 3b having a desired shape (inverted truncated cone in the present embodiment). A metal plate 4, which is a product or workpiece to be processed is set on the laminated tool 3, and a male tool, which matches with the female tool, is provided above the metal plate 4. Two stage holes 5a and 5b having different diameters are vertically bored into a center portion of an approximately cylindrical holder 5 of the male tool. The holder 5 is made of steel. The circular hole 5b having a small diameter opens into the bottom surface of the holder 5, and its height h is set so as to be sufficiently smaller than the overall height H of the holder 5 (h=2 mm in the present embodiment). Further, a tapered surface 5c, which is downwardly constricted, is located between the circular holes 5a and 5b having different diameters in the holder 5. As detailed in FIG. 5, a tilt angle α of the tapered surface 5c with respect to plate 4 is set for 3° to 45° (α = 12° in the present invention). The holder portion having the tapered surface forms a pressure force receiving section 5F which receives a pressure force from an elastic punch 6 mounted in the holder 5. In the set state shown in FIG. 1, the holder 5 is located so that the circular hole 5b having small diameter, which opens into the bottom surface of the holder 5, comes above the concave section 3b of the laminated tool 3. The elastic punch 6, which is formed by an elastic material such as urethane or rubber into en approximately cylindrical shape, is housed inside the holder 5 within the circular hole 5a having a large diameter. A cylindrical slide punch 7 is brought into contact with the upper surface of the elastic punch 6. The slide punch 7 is one example of a pressure member, which slides up and down relatively in the circular hole 5a so as to compress the elastic punch 6. Here, in the set state shown in FIG. 1, the outer diameter d of the elastic punch 6 is smaller than the inner diameter D of the circular hole 5a of the holder 5 (d<D), and a predetermined ring-shaped gap ô is provided between the elastic punch 6 and the holder 5. Moreover, tapered surfaces 6a and 6b, which are constricted upwards and downwards respectively (diameter is reduced), are arranged at the upper and lower edges of the outer periphery of the elastic punch 6. The slide punch 7 is moved up and down in the holder 5 by a hydraulic press mechanism (not shown), and forces are applied thereon by said press mechanism. Consequently, the laminated tool (female tool) 3, the holder (male tool) 5, the elastic punch 6 and the slide punch 7 acting as pressure members compose the tools used in the forming process according to the present invention. The forming process which is performed by using the tools will be described below with reference to FIGs. 1 through 6. In the set state shown in FIG. 1, the hydraulic press (not shown) is driven, and predetermined press force P is applied onto the slide punch 7 by a ram or the like. Then, as shown in FIG. 2, the elastic punch 6 housed in the holder 5 is compressed by the slide punch 7 so as to be elastically deformed (compressively deformed). The diameter of the elastic punch 6 is increased, and its outer peripheral surface closely contacts the inner peripheral surface of the circular hole 5a of the holder 5. Moreover, the tapered surface 6b provided at the lower end periphery of the elastic punch 6 moves smoothly along the tapered surface 5c of the holder 5 into the circular hole 5b having a small diameter and the lower portion of the elastic punch 6 is pushed into the circular hole 5b of the holder 5 so as to get into contact with the upper surface of the metal plate or workpiece 4, as shown in the drawing. At this time, since the tapered surface 5c having the predetermined tilt angle α is provided at the pressure force receiving section 5F of the holder 5, a pressure force F is applied from the elastic punch 6 to the tapered surface 5c as shown in FIG. 5. Therefore, the holder 5 presses the metal plate 4 by means of a vertical component F · cos α of the pressure force F. As a result, the holder 5 serves also as a blank holder. Here, the holder 5 receives the pressure force from the elastic punch 6 by means of the pressure force receiving section 5F, and thus the workpiece 4 is nipped under pressure between the holder 5 and the female tool 3. The nipping pressure becomes stronger when the pressure force of the slide punch 7 becomes stronger and the elastic punch 6 is strongly compressed. Therefore, when the pressure force applied to the elastic punch 6 by the slide punch 7 becomes stronger, the nipping pressure becomes also stronger. When the tilt angle of the tapered surface 6a formed on the upper edge periphery of the elastic punch 6 and the area of the upper surface of the elastic punch 6 (pressure receiving area) are changed, the pressure applied to the upper surface of the elastic punch 6 in the initial process can be adjusted. In the case where the pressure is increased, for example, the tilt angle of the tapered surface 6a is increased, and the area of the upper surface of the elastic punch 6 (pressure receiving area) is decreased. When the pressure force P is continued to be applied onto the elastic punch 6, as shown in FIG. 3, the elastic punch 6 is furthermore elastically deformed, and a portion thereof as well as the metal plate 4 are pushed into the concave section 3b of the laminated female tool 3. For this reason, the elastic punch 6 serves as a punch (rigid body) having a convex shape which matches with the concave section 3b of the laminated female tool 3, and one portion of the metal plate 4 is plastically deformed by drawing according to the shape of the concave section 3b of the laminated female tool 3. At this time, since the pressure force F (see FIG. 5) is applied from the elastic punch 6 to the tapered surface 5c of the holder 5, as mentioned above, the holder 5 serves also as a blank holder so as to prevent crease from occurring on the metal plate 4. When the elastic punch 6 is compressed as mentioned above, one portion of the elastic punch 6 penetrates into the space between the inner peripheral surface of the holder 5 and the outer peripheral surface of the slide punch 7, and thus dragging may occur. However, in the present embodiment, since the tapered surface 6a suitably chamfered is formed on the elastic punch 6, the penetration of a portion of the elastic punch 6 into the space between the holder 5 and the slide punch 7 can be suppressed, and the dragging can be prevented. After the metal plate 4 is drawn into a desired shape, the pressure force P is removed therefrom. As a result, as shown in FIG. 4, the elastic punch 6 is returned to its original state (shown in FIG. 1). Therefore, when the same steps are repeated, other concave sections 4a as the one shown in FIG. 6 can be formed successively on the metal plate 4 by the drawing process. Thanks to the forming method according to the present embodiment, flaws or the like do not occur on the metal plate 4 when a film, a tape or the like is provided between the metal plate 4 and the holder 5. Consequently, any damage of the metal plate 4 due to the pressure force applied by the holder 5 can be prevented securely. As mentioned above, in the present embodiment, the metal plate or workpiece 4 is provided between the laminated tool 3 and the elastic punch 6, and the elastic punch 6 is compressed so as to be elastically deformed. Moreover, one portion of the elastic punch 6 as well as a portion of the metal plate 4 are pushed into the concave section 3b of the laminated tool 3, and as a result, the drawing or forming process is performed on the metal plate 4 according to the shape of the concave section 3b of the laminated tool 3. For this reason, the elastic punch 6 serves as the punch (male tool) in the pressing process, and thus in manufacturing a small quantity and various types of products, the process for drawing the products into a desired shape can be easily performed for a short time at low costs without using expensive tools usually used in drawing processes. Therefore, even if the same punch is used as male tool for various types of products, for example, as shown in FIG. 7 according to which a rectangular convex section 8a is formed on a metal plate 8, or as shown in FIG. 8, according to which a rectangular concave section 9a and a machine screw washer 9b having a small diameter can be formed on a metal plate 9 by varying the shape of the concave section in the die acting as the female tool. Further, as shown in FIG. 9, a hemispheric convex section 10a can be formed on a metal plate 10. In other words, a single punch (male tool) can match with a plurality of dies (female tools). In addition, since in the present embodiment the tapered surface 5c having the predetermined tilt angle α is formed on the pressure force receiving section 5F of the holder 5, the holder 5 applies the metal plate 4 against the female tool so that the metal plate 4 is nipped by the vertical component F . cos α of the pressure force F which is applied from the elastic punch 6 to the tapered surface 5c. As a result, the holder 5 serves also as a blank holder, a special blank holder being not required. Therefore, the structure of the forming tools is simplified, and the cost can be reduced easily. The following will describe the forming tools according to a second embodiment of the present invention with reference to FIG. 10. FIG. 10 is a sectional view of the forming tools according to the present embodiment. The forming tools comprise a die 13 as female tool and a punch as male tool provided above the die 13. The punch includes an approximately cylindrical holder 15, a cylindrical elastic punch 16 and a slide punch 17 as a pressure applying member. A concave section 13b having a desired shape (inverted truncated cone in the present embodiment) is formed on the upper surface of the die 13. The holder 15 is divided into three parts: a slide holder section 15A, a cylindrical main holder body 15B and a blank holder section 15C. The slide holder section 15A is detachably mounted onto the upper end portion of the main holder body 158, and the blank holder section 15C is detachably mounted to its lower end portion. Namely, the slide holder section 15A, which shows a ring shape, is mounted onto the upper portion of the main holder body 15E by bolts 21 (only one is shown in FIG. 10). A ring shoulder 15a provided at the lower inner periphery of the slide holder section 15A is fitted into the inner periphery and is located at the upper end of the main holder body 15B. Moreover, the blank holder section 15C is mounted to the lower end of the main holder body 15B by bolts 22 (only one is shown in FIG. 10) and shows at its upper outer periphery a ring-shaped shoulder 15b projecting upwardly from the blank holder section 15C so as to be fitted and located on a ring-shaped concave portion 15d provided at the lower outer periphery of the main holder body 15B. A pressure force receiving section 15F having a tapered surface 15c, which is constricted downwardly, is formed in the blank holder section 15C of the holder 15. In addition, similarly to the first embodiment, the elastic punch 16 is made of an elastic material such as urethane or rubber. The slide punch 17 is formed so as to have two staggered cylindrical portions showing respectively a large and a small diameter. The large-diameter section 17a constitutes the lower half portion of the slide punch and is fitted into the holder 15. The lower surface of the slide punch 17 contacts with the upper surface of the elastic punch 16, and an annular surface 17b between the two portions of the slide punch 17 contacts with the slide holder section 15A of the holder 15. Therefore, the slide punch 17, the holder 15 and the elastic punch 16 move up and down together until the blank holder section 15C is brought into contact with the workpiece 4. At its upper end portion the slide punch 17 is mounted onto a vertically movable ram 25 by a shank 23 and a bolt 24. As shown in the drawing, the metal plate 4 used as work material to be processed is placed on the die 13 or female tool, the ram 25 is driven up and down so that the slide punch 17, the holder 15 and the elastic punch 16 in the punch or male tool move up and down. As a result, the lower end surface of the holder 15 (blank holder section 15C) contacts with the metal plate 4 and is stopped, and thereafter the slide punch 17 further moves downwardly with respect to the holder 15. Then, the elastic punch 16 is compressed by the slide punch 17 so as to be elastically deformed and similarly to the first embodiment, the elastic punch 16 serves as a punch having a rigid body and a convex shape matching with the concave section 13b of the die 13. One portion of the metal plate 4 is plastically deformed by a portion of punch 16 according to the shape of the concave section 13b of the die 13 so as to be drawn into the latter. Therefore, a concave section 4a shown in FIG. 6 is formed on the metal plate 4. During the above compressing step, the blank holder section 15C of the holder 15 is pressed against the metal plate 4 by the vertical component F-cos α (see FIG. 5) of force P applied from the elastic punch 16 to the tapered surface 15c having the predetermined tilt angle α in the pressure force receiving section 15F so as to prevent crease from occurring on the metal plate 4. The force (pressure force), which is generated when the blank holder section 15C of the holder 15 presses the metal plate 4, varies according to the thickness and the material of which is made the metal plate 4, but the pressure force can be adjusted by changing the tilt angle α of the tapered surface 15c of the blank holder section 15C. More specifically, when the tilt angle α of the tapered surface 15c is set to greater values, the vertical component F · cos α of the pressure force is decreased. In the present embodiment, since the holder 15 is divided into several sections and the blank holder section 15C is detachably mounted onto the main holder body 15B, the blank holder section 15C can be easily exchanged. For example, FIG. 11 shows three types of blank holder sections 15C, 15C' and 15C" whose tapered surfaces 15c have different tilt angles α. A suitable blank holder is selected among the different types of blank holders according to the thicknesses and material compositions of the metal plate 4, and the selected blank holder section is then mounted to the holder main body 15B. Here, FIG 11(a) shows a blank holder section 15C for low-pressure-transmission in which the tilt angle α of the tapered surface 15c has a value between 45° to 89°. FIG 11(b) shows the blank holder section 15C' for intermediate pressure transmission where the tilt angle α of the tapered surface 15c is chosen between 0° to 45°, and FIG. 11(c) shows a blank holder section 15C" for high-pressure-transmission where the tilt angle α of the tapered surface 15c is 0°. Here, the pressure force applied to the metal plate 4 can be adjusted by modifying besides the tilt angle α of the tapered surface 15c the radial or annular width A of the horizontal projection of said tapered surface 15c. As mentioned above, among the forming tools according to the present embodiment, the holder 15 is divided and the blank holder section 15C is exchangeably mounted onto the main holder body 15B. As a result, the pressure force applied onto the metal plate 4 can be adjusted so that its value is suitable for the thickness and the material composition of the metal plate 4 by exchanging only the blank holder section 15C without exchanging the whole holder 15. Moreover, since the holder 15 without the blank holder section 15C, namely, the slide holder section 15A and the main holder body 15B can be used in common, the cost can be reduced. Needless to say, that an effect similar to that obtained with the first embodiment can be produced also with the present second embodiment. The forming tools according to a third embodiment of the present invention will be described with reference to FIG. 12, which is a sectional view of the forming tools. In this FIG. 12, the components which are identical or similar to those shown in FIG. 10 are given the same reference numerals, and thus the description thereof is omitted. The present embodiment shows an example according to which the forming tools are mounted onto an NC turret punching press so that the forming process is automated, and the basic structure of the forming tools is the same as the one of the second embodiment. In the present embodiment, the holder 15 is part of the punch which is arranged as a male tool. The holder 15 in which are disposed the elastic punch 16 and partly the slide punch 17, is mounted onto an upper turret 16 of the NC turret punching press so as to be able to move up and down. The outer periphery of the slide holder section 15A of holder 15 projects outwardly over the main holder body 15B so as to form brim sections, and lifter springs 27 are compressively mounted between the brim sections and the upper turret 26 so as to urge the holder 15 upwardly and away from said upper turret 26 together with said elastic punch 16 and said slide punch 17. As shown in FIG. 12, the metal plate 4 to be processed is placed on the die 13, and the slide punch 17 is pushed downwardly a striker 28 which moves up and down in the NC turret punch press. As a result, the slide punch 17 as well as the holder 15 and the elastic punch 16 move downwardly against the force of the lifter springs 27, and the lower end surface of the holder 15 (blank holder section 15C) is brought into contact with the metal plast 4 where it is stopped. Thereafter, when the slide punch 17 is further pushed and moves downwardly with respect to holder 15 and the elastic punch 16 is compressed by the slide punch 17 so as to be elastically deformed. Similarly to the first embodiment, the elastic punch 16 serves as the punch (rigid body) having a convex shape matching with the concave section 13b of the die 13, and one portion of the metal plate 4 is plastically deformed and drawn into said die 13 by a portion of said punch 16 according to the shape of the concave section 13b of the die 13. As a result, the concave section 4a shown in FIG. 6 is formed on the metal plate 4. After the metal plate 4 is drawn into a desired shape, the striker 28 of the NC turret punching press is moved upward so that pressure force is removed. As a result, the holder 15, the elastic punch 16 and the slide punch 17 move upwards under the antagonistic force of the lifter springs 27 and return to the original position shown in FIG. 12, and thereafter another drawing step of the metal plate 4 can be performed automatically in the same manner. With the present embodiment, similar effects to the ones of the first and second embodiments can be obtained. A further advantage consists in the fact that the forming process can be performed automatically and efficiently. FIG. 13 is an explanatory drawing showing a cross section of the forming tools according to a fourth embodiment of the present invention. In the present fourth embodiment, a difference with respect to the third embodiment shown in FIG. 12 consists in the fact that powerful elastic members 31 such as a coil spring or an urethane rubber tube are provided between a punch head 29 and the holder 15. Head 29 forms the upper end of the slide punch 17 and acts as a pressure transmitting member in the male tool. When the striker 28 is lowered, the slide punch 17 as well as the pressure transmitting member 29 are lowered so as to compress the elastic punch 16 as well as the elastic members 31. Therefore, the force, which is generated when the holder 15 applies the workpiece 4 against the die 31, has a value equal to the sum of the pressure force applied by the elastic punch 16 onto the holder 15 and of the pressure force applied by the elastic members 31 to the holder 15 whereby the total application force is increased. Namely, in the above-mentioned structure, when the pressure force applied by the elastic punch 16 to the holder 15 is weak, the elastic members 31 assist in applying the holder 15 onto the workpiece 4. Accordingly, this structure is effective in the case where the pressure force applied by the elastic punch 16 to the holder 15 cannot be increased. FIG. 14 is an explanatory drawing showing a cross section of the forming tools according to a fifth embodiment of the present invention. In the present embodiment, a female tool 33 having a concave section 33b of a desired shape is provided with a punch, and a male tool which cooperates with the female tool 33 is provided at the location of a die. Namely, the male tool has a casing 35 which is supported by a lower turret punch press (not shown), for example, and a cylindrical holder 37, which can apply the workpiece 4 against the female tool 33, is mounted into the casing 35 so as to be able to move up and down with respect to said casing. An elastic punch 39, which is made of an elastic body such as urethane rubber or rubber, is mounted into the holder 37, and a pressure member 41, which can compress the elastic punch 39 when the holder 37 moves downwardly with respect to said elastic punch 39 and the casing 35, is provided between the lower surface of the elastic punch 39 and the bottom portion of said casing 35. The holder 37, the elastic punch 39 and the pressure member 41 constitute a unit which is mounted in the casing 35 and is exchangeable as a whole. Therefore, as shown in FIG. 15, the unit shown in FIG. 14 and comprising the holder 37, the elastic punch 39 and the pressure member 41 can be replaced by another unit (see for example FIG. 15) suitable to the thickness, the material composition of the workpiece 4 and the shape to be given to the latter by the forming process. The elastic punch 39 has a large diameter section 39D whose diameter is approximately equal with the inner diameter of the holder 37, and has a small diameter section 39d, whose diameter is approximately equal with the inner diameter of the outlet opening of a pressure force receiving section 37F provided on the holder 37, so as to project away from the large diameter section 39D. Moreover, the large diameter section 39D of the elastic punch 39 has a tapered surface which acts as a pressure force transmission section 39T and which contacts with a tapered surface 37T of the pressure force receiving section 37F of the holder 37. Further, a suitable number of longitudinal grooves 39G are formed in the outer peripheral surface of the large diameter section 39D of elastic punch 39, said grooves 39G extending between the one end of the large diameter section 39D and the pressure force transmission section 39T of the elastic punch 39. The grooves 39G are used for leading air to a contact portion between the tapered surface 37T of the holder 37 and the pressing force transmission section 39T of the elastic punch 39 so as to prevent dose contact in said contact portion. The small diameter section 39d of the elastic punch 39 shows a flat end surface 39F. It may be desirable that the border line of the end surface 39F matches with the one of the concave section 33b of the female tool 33 so that the workpiece 4 is deformed accurately according to the shape of said concave section 33b. However, as shown in FIGs. 16(A) and 16(B), for example, a non-flat end surface shape such as a convex spherical surface or a concave spherical surface may also be used. In these cases, the curvature of the spherical surfaces may be defined by a large diameter so that the angle comprised between a horizontal line and a tangent to the spherical surface has only a few degrees. In addition, it is desirable that the elastic punch 39 contains a lubricant such as molybdenum in order to reduce the friction between the holder 37 and the peripheral surface of the elastic punch 39. In this case, since the contact area of the outer peripheral surface of the large diameter section 39D of the elastic punch 39 with the inner peripheral surface of the holder 37 is large, it is desirable that the lubricant is contained only in the outer peripheral portion of the large diameter section 39D in order to reduce the friction of the contact portion. Further, it is desirable that the hardness of the large diameter punch section 39D is different with respect to the one of the small diameter section 39d in the elastic punch 39, and that the pressure force can be transmitted sufficiently from the elastic punch 39 to the holder 37. Moreover, it is desirable that the small diameter section 39d is projected out from the holder 37 so as to press the workpiece 4 sufficiently along the concave section 33b of the female tool 33. The punch, which matches with its die (concave section 33b) with the male tool, has a female punch holder 43 which is supported to the upper turret UT in the turret punch press (not shown) via lifter springs SP so as to be able to move up and down with respect to the upper turret UT. The female tool 33 is exchangeably mounted onto the lower surface of the female tool holder 43 via a plurality of bolts (not shown). Here, a suitable number of air bleeding holes 33H having a small diameter are provided in the bottom of concave section 33b of the female tool 33. A shank 47 is fitted with its upper end into the punch head 45 so that its vertical position can be adjusted. This shank 47 is provided in the female tool holder 43 and is movable up and down with respect to the upper turret UT. The lower end of the shank 47 is provided with a shear plate 49 located in a ring-shaped concave section 43C formed in the female tool holder 43, said plate 49 being exchangeably mounted onto the lower end of the shank section 47 via a mounting bolts (not shown). A ring-shaped die plate 51 is provided between the shear plate 49 and the bottom portion of the concave section 43C. When the punch head 45 is pushed downwardly and overload is generated on said punch head 45, the shear plate 49 is drawn out by the die plate 51 and the shank 47 so that the occurrence of further overload is prevented. The shear plate 49, the die plate 51 and the like compose an overload safety device. The structure of the overload safety device is not limited to the structure composed of the aforementioned shear plate 49 and the die plate 51 or the like. Therefore, the overload safety apparatus may have a structure in which the concave section 43C is sealed and contains a non compressible fluid, for example. In this structure, fluid in the concave section 43C is compressed by the shank 47 at the time of generation of overload so that the pressure becomes high, and when the pressure exceeds a prescribed value, the fluid is discharges outside via a relief valve. In the aforementioned structure, as shown in FIG. 17(A), after the workpiece 4 is placed and located on the upper surface of the holder 37 in the male tool, the ram or the striker (not shown) of the punch press is lowered so that the punch head 45 is lowered by the striker. As a result, the female tool holder 43 is lowered by the down movement of the shank 47 against the action of the lifter springs SP, the stripper plate 49 and the die plate 51, and the female tool 33 is brought into contact with the workpiece 4 as shown in FIG. 17(B). After the lower surface of the female tool 33 is brought into contact with the upper surface of the workpiece 4, the punch head 45 is further lowered by the striker. As a result, while the workpiece 4 is nipped between the upper surface of the holder 37 and the female tool 33, the holder 37 compresses the elastic punch 39 while being lowered simultaneously with respect to casing 35. When the holder 37 is lowered in the above manner, it simultaneously compresses the elastic punch 39 which as a result of the down movement of the holder 37 is also compressed by the pressure member 41 so as to be projected upwards with respect to the holder 37 and partly protrudes from the latter as shown in FIG. 17(C). Thereby, the elastic punch 39 preses the workpiece 4 into the concave section 33b of the female tool 33, and finally the workpiece 4 is formed according to the shape of the concave section 33b as shown in FIG. 17(D). Thereafter, when the striker is returned to its initial position, the punch is raised to the initial position by the action of the lifter springs SP. When the elastic punch 39 in the die is returned to its initial state, the die also returns to its original state. Thanks to the air bleeding holes 33H having a small diameter being provided in the concave section 33b, and thanks to the air bleeding holes 35H provided in the bottom portion of the casing 35, the workpiece is formed in the aforementioned manner, the holder 37 is lowered smoothly, and the workpiece 4 is moved into the concave section 33b wherein it is deformed smoothly. Since the elastic punch 39 contains lubricant, the friction between the holder 37 and the elastic punch 39 is weak, so that the latter moves smoothly within the holder 37. As a result, the elastic punch 39 moves smoothly towards the concave section 33b of the female tool 33 and functions effectively in the process of forming the workpiece 4. According to the present embodiment, the workpiece 4 can be easily formed into a convex shape in the upper direction. FIG. 18 is an explanatory drawing showing a sectional view of the forming tools according to a sixth embodiment of the present invention. In the present embodiment, the difference with respect to the structure of the male tool shown in FIG. 14 consists in the provision of elastic members 55, such as urethane rubber, a belleville spring or a coil spring, located between the lower portion of the holder 37 and the bottom portion of the casing 35. In the above structure, the force, generated when the holder 37 is applied against the workpiece 4 and moves downwards with respect to the elastic punch 39, is equal to the sum of the pressure force applied by the elastic punch 39 to the holder 37 and the pressure force applied from the elastic members 55 to the holder 37. Thanks to this arrangement, pressure force is available.

Claims: Forming tools comprising: a female tool (3) having a concave section (3b) of a desired shape; a male tool including a vertically slidable hollow body (5, 15, 37) which is applicable onto said female tool (3, 13, 33) and presents vertical holes (5a, 5b) of different diameters and a tapered inner surface (5c, 15c, 37T) connecting the hole portion (5a) having a larger diameter to the lower hole portion (5b) having a smaller diameter, an elastic punch (6, 16, 39) provided in said slidable hollow body and having a tapered outer surface portion (6b, 39T) in front of and applicable onto said tapered inner surface (5c, 15c, 37T) of the hollow body, a pressure member (7, 17, 41) partly located inside said hollow body (5, 15, 37) and able to apply a pressure force onto the said elastic punch (6, 16, 39) and to urge the latter towards said female tool (3, 13, 33), characterised in that the elastic punch (6, 16, 39) is made of a single elastic body directly applicable onto a blank constituted by a flat sheet shaped workpiece (4) and located between the female tool (3) and the hollow body (5, 15, 37) and onto the tapered inner surface (5c, 15c, 37T) of said hollow body (5, 15, 37), said tapered surface acting as a pressure receiving section (5F, 15F, 37F) which transmits a part of the pressure force to the flat sheet shaped workpiece (4) via the frontal face of the hollow body (5, 15, 37) thereby transforming the latter into a blank holder. Forming tools according to claim 1, wherein the holder (5, 15) comprises a main holder body (15B) and a blank holder section (15C) including the tapered surface (5c, 15c) of the pressure force receiving section (15F) and wherein the blank holder section (15C) is detachably mounted onto the lower end portion of the main holder body (15B). Forming tools according to claim 2, wherein the main holder body (15B) is associated with one of three detachable blank holder sections (15C, 15C' ,15C") in which the tilt angle α of the tapered surface (15c) thereof comprises respectively values from 45° to 89° for a low pressure transmission to the workpiece (4), values between 0° to 45° for intermediate pressure transmission and a value of 0° for high pressure transmission. Forming tools according to claim 1, wherein the elastic punch (6, 39) comprises a large diameter section (39C), a small diameter section (39d) and a tapered surface acting as force transmission section (39T) wherein the large diameter section (39D) joins said tapered surface (39T) of the holder (37) while the end surface (39F) of the small diameter section (39d) of the elastic punch (39) has a shape which matches with the one of the concave section (33b) of the female tool (33). Forming tools according to any one of claims 1 to 4, wherein the large diameter section (39D) of the elastic punch (39) has a peripheral surface provided with longitudinal grooves (39G) extending between the one end of the large diameter section (39D) and the pressure force transmission section (39T) of said elastic punch (39). Forming tools according to any one of claims 1 to 5, wherein the female tool (33) is exchangably mounted onto the lower surface of a female tool holder (43) which in turn is fixed to a vertically movable punch shaft (47) and wherein an overload safety device (49, 51) is interposed between said female tool holder (43) and said punch shaft (47). Forming tools according to any one of claims 1 to 6 which present a male tool comprising a holder (37), an elastic punch (39) mounted into said holder (37) and a pressure member (41) for compressing said elastic punch (39), and a female tool (33) provided with a concave die section (33b), which is applicable onto a flat sheet shaped workpiece (4) wherein said holder (37), elastic punch (39) and pressure member (41) constitute a unit mounted in a casing (35) in which the pressure member (41) is provided between the bottom of said casing (35) and the elastic punch (39) and wherein said holder (37) is vertically movable with respect to said casing (35) under the pressure action of the female tool (33). Forming tools according to claim 7, wherein elastic spring members (55) are located between the end of the casing (35) and an end portion of the holder (37). Forming tools according to claim 7 or 8, wherein the holder (37), the elastic punch (39) and the pressure member (41) mounted in the casing (35) constitute with the latter an exchangeable unit. Forming tools according to any one of the claims 1 to 9, wherein the large diameter section of the elastic punch (6, 16, 39) contains a lubricant. Forming tools according to any one of claims 1 to 10, wherein air bleeding holes (33H) having small diameters are provided in the concave die section (33b) of the female tool (3). Forming tools according to any one of claims 7 to 11, wherein bleeding holes (35H) are provided in the bottom portion of the casing (35). A method for forming a flat sheet shaped workpiece by drawing the latter into a die of desired shape, whereby said method makes use of the forming tools according to any one of claims 1 to 12 and comprises the following steps: inserting a blank (4) between said female tool (3) and said male tool, applying the female tool (3) and the male tool against each other, applying a pressure force generated by the pressure member (7, 17, 41) onto the elastic punch (6, 16, 39) so that a lower part of the latter leaves the said hollow body (5, 15, 37) and causes the transformation of the blank (4) into the hollow object defined by the shape of the die (3b, 13B, 33b), thereby transmitting a part of this pressure force to urge the hollow body (5, 15, 37) towards the female tool (3) and afterwards releasing the pressure force applied onto the elastic punch (6, 16, 39) so that the lower part thereof can retract into the hollow body (5, 15, 37) and separating the female tool (3) and the male tool from each other, wherein said method further comprises the following steps : using as a blank a flat sheet shaped workpiece (4) using the tapered inner surface (5c, 15c, 37T) of the hollow body (5, 15, 37) as a pressure receiving section (5F, 15F, 37F) which transmits a part of the pressure force to the flat sheet shaped workpiece (4) via the frontal face of the hollow body (5, 15, 37) thereby transforming the latter into a blank holder applying said part of pressure onto the blank (4) whereby the latter is pinched between said blank holder (5, 15, 37) and said female tool (3) and afterwards applying the single elastic body of the elastic punch (6, 16, 39) directly onto the unpinched part of the sheet shaped workpiece (4) until this part is completely drawn into the concave die section (3b, 13b, 33b) of the female tool. A method according to claim 13, wherein a lubricant is used in order to reduce the friction between the holder (5, 15, 37) and the elastic punch (6, 16, 39). A method according to claim 13 or 14, wherein air is lead to a contact portion between the tapered inner surface (37T) of the holder (37) and the pressing force transmission section (39T) of the elastic punch (39). A method according to any one of claims 13 to 15, wherein the pressure force applied via the blank holder (5, 15, 37) onto the sheet shaped workpiece (4) is adjusted by modifying the tilt angle (α) of the tapered inner surface (5c, 15c, 37T) of said blank holder and/or the width (A) of said tapered inner surface.

Inventors: HARAGA YOSHIO, IWAMOTO MASAMI, HARAGA, YOSHIO, IWAMOTO, MASAMI
Applicants: AMADA CO LTD, AMADA COMPANY LIMITED
Application year: 1998
Main classification: B21D22
IPC codes: B21D22