Abstract:
A crude work having a cylindrical boss formed integrally with a base plate is processed to a final product by a press work without using additional machining. An inner diameter of the boss is enlarged by a punch, and at the same time an outer diameter of the boss is shrunk by an outer die, while the work is supported on a cushion ring to which a force against the movement of the punch and the outer die is applied. Some of the material constituting the cylindrical boss is squeezed, and a bulge is formed at a foot of the cylindrical boss out of the material squeezed. Since the bulge is fromed in a confined space in the die structure, no crack or defect develops in or on the bulge. The bulge is further compressed by a press work to form a flat flange out of the bulge. The cylindrical boss having the flange formed integrally therewith is used as a center boss for holding a bearing of a rotary machine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority of Japanese Patent Applications No. Hei-9-307282 filed on Nov. 10, 1997, the content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming a cylindrical boss on a base plate and a die for forming the boss, and more particularly to a method of forming a center boss having a flange for holding a bearing therein and a die for forming such a center boss. 
     2. Description of Related Art 
     As a method of manufacturing a cylindrical boss having a small step therein from a steel plate, a method of controlling a thickness and a shape of a metal plate (FCF method) is generally known. The CFC method utilizes various processes such as fixing in cold-forging, ironing, and extruding in addition to conventional processes such as stamping, drawing, bending and fluing. 
     A method of making a bulge at a foot of a cylindrical boss and a die for use in the method are disclosed in JP-A-7-124657, a part of which is illustrated in FIG. 8 attached hereto for reference purpose. Referring to FIG. 8, the die includes a lower die 35, an upper die 38 and a punch 30. A work having a cylindrical boss 32 is held between the lower die 35 and the upper die 38. The punch 30 is pressed into an inner bore of the cylindrical boss 32 to form a bulge 34 at a base portion 33 of the cylindrical boss 32. The punch 30 is rod-shaped and has a front surface 36 connected to a main rod by a tapered portion 31. A corner having a small radius r is formed at a foot of the cylindrical boss 32 at the same time the bulge 34 is formed by pressing down the punch 30. The bulge 34 is formed along a depressed curve of the punch 30, and the corner with the radius r bulges out into a space 37 between the lower die 35 and the punch 30. At the same time a depression 39 is formed at an outside foot of the cylindrical boss 32. The depression 39 is undesirable because a crack develops at the depression 39 when the bulge 34 is further processed to form a flange. The crack developed at the outer foot of the cylindrical boss 32 is detrimental to a mechanical strength of the cylindrical boss. Because of this, the flange in the cylindrical boss is separately made and assembled into the boss in a conventional process. Accordingly, a production cost for making the cylindrical boss becomes expensive. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method of forming a cylindrical boss having a high and reliable mechanical strength integrally with a base plate in an inexpensive process. Another object of the present invention is to provide a die for forming such a cylindrical boss. 
     According to the present invention, a crude cylindrical boss formed on and integrally with a base plate is processed to a final shape of a cylindrical boss which includes a flat flange formed at an inside foot of the cylindrical boss. First, the crude cylindrical boss is processed to form a bulge from which the flange is formed. A punch having a diameter larger than an inner diameter of the crude cylindrical boss is forcibly inserted into the crude cylindrical boss, and at the same time an ironing die having a bore, the diameter of which is smaller than an outer diameter of the crude cylindrical portion, is forcibly slid along the outer diameter of the crude cylindrical boss. The crude cylindrical boss is supported on a cushion ring which is pressed against the force applied by inserting the punch into the crude cylindrical boss and sliding the ironing die along the outside diameter of the crude cylindrical boss. Some of the material constituting the crude cylindrical boss is squeezed by the punch and the ironing die and swells into a confined space in the die structure. The bulge is formed at the inside foot of the cylindrical boss out of the material swelled into the confined space. At the same time, the inner and outer diameters of the crude cylindrical boss are shaped into final dimensions, so that the finished cylindrical boss is used as a center boss for holding a bearing therein without performing additional machining. 
     Then, the bulge is deformed into a flat flange extending inwardly from the inside foot of the cylindrical boss in the next process. The work having the bulge is held between an upper die and a lower die, and a punch is inserted into the cylindrical boss to deform the bulge into a flat flange. In this manner, the cylindrical boss having a flat flange at its inside foot is formed integrally with the base plate solely in a press work. Since the bulge is formed in a completely confined space in the die structure, no crack or defect is developed in or on the bulge as opposed to the conventional process described above. 
     Preferably, a third process is further performed after the flange is formed in the cylindrical boss to form a corner step at the foot of the flange which supports a bearing and to obtain a further accurate inner diameter of the cylindrical boss. A punch is inserted into the inner diameter of the cylindrical boss to squeeze some material out of the cylindrical boss while firmly holding the work between lower and upper dies. The squeezed material forms a corner step at the foot of the flange. 
     The cylindrical boss having accurate dimensions, a high mechanical strength and a high reliability can be manufactured in a simple process according to the present invention. 
     Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing an ultrasonic motor in which a housing base having a cylindrical boss formed by a method according to the present invention is used; 
     FIG. 2 is a cross-sectional view showing the housing base used in the ultrasonic motor shown in FIG. 1; 
     FIG. 3 is a cross-sectional view showing a die structure for making a bulge at a base portion of the cylindrical boss; 
     FIG. 4 is a perspective view showing a portion of a work to be processed and a portion of the die structure; 
     FIG. 5 is a cross-sectional view showing the die and the work on which the bulge has been formed; 
     FIG. 6 is a cross-sectional view showing a work and a die for forming a flange by deforming the bulge; 
     FIG. 7 is a cross-sectional view showing a work and a die for forming a recess at a foot of the cylindrical boss; and 
     FIG. 8 is a schematic cross-sectional view for explaining a conventional method of forming a bulge on a cylindrical boss. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described referring to FIGS. 1-7, taking a housing base used in an ultrasonic motor as an example. Referring to FIG. 1, an ultrasonic motor 1 is composed of a housing 6 having a base 7 and a cover 8, a stator 3 mounted on the base 7, a rotor 4 having a rotational shaft 5 slidably rotating on the stator 3, and other components. The rotational shaft 5 is rotatably supported in the housing 6 by two bearings. The rotational shaft 5 fixed to the rotor 4 extends through a through-hole 7c formed on a flange 7b of the base 7. A bearing 9 is held in a center boss 7a, an outer race of which abuts with a corner step 7d formed at a foot of the flange 7b. A piezoelectric element 2 is attached to the stator 3 on its bottom surface. When a drive voltage is supplied to the piezoelectric element 2, progressive wave is generated, and thereby the rotor 4 contacting the stator 3 rotates. 
     The base 7 of the housing 6 is shown in FIG. 2. The base 7 has the center boss 7a and the flange 7b, both being formed integrally with a base plate 10. The corner step 7d is formed at a foot of the cylindrical boss 7a. The through-hole 7c is formed at a center portion of the flange 7b. The base 7 is processed from a crude work made of steel plate through a bulge forming process (a first process), a flange forming process (a second process) and a corner step forming process (a third process), all of which will be described below. 
     The bulge forming process (the first process) will be described first, referring to FIGS. 3, 4 and 5. FIG. 3 shows a die 13 for forming a bulge 12b (shown in FIG. 5) on a crude work W1. FIG. 4 shows a perspective view of the work W1 and a cushion ring 16. The crude work W1 made of a steel plate has a base plate 10 and a cylindrical portion 11a which is formed by a fluing process. The die 13 is composed of a holder 14, a first plate 15 placed on the holder 14, a cushion ring 16 which is slidably guided by the first plate 15, an ironing die 17, and a first punch 18. The holder 14 supports the first plate 15 thereon, and has a through-hole 14a through which a fluidic pressure is introduced. The first plate 15 has an upper surface 15a for supporting the work W1 thereon and a lower surface 15b contacting the holder 14. A guide bore 15d and an enlarged bore 15c are formed in the center of the first plate 15. The cushion ring 16 (shown in FIGS. 3 and 4) has a flange 16a and a cylindrical portion 16b. A rounded recess 16c for supporting the work W1 thereon is formed at an upper end of the cylindrical portion 16b. The rounded recess 16c is shaped so that it fits a rounded corner of the work W1. A guide bore 16d for guiding the first punch 18 therein is formed in the center of the cushion ring 16. An outer diameter of the cylindrical portion 16b is the same as a diameter of the guide bore 15d, so that the cushion ring 16 is slidably guided by the first plate 15. A height h2 of the flange 16a is smaller than a height H2 of the enlarged bore 15c, and an outer diameter of the flange 16a is little smaller than the enlarged bore 15c, so that the flange 16a is freely received in the enlarged bore 15c. A height hl of the cushion ring 16 (a distance form the lower surface 16e to a flat surface of the rounded recess 16c) is the same as a height H1 of the first plate 15. Therefore, as shown in FIG. 3, the rounded recess 16c sticks out from the upper surface 15a when a fluidic pressure P is applied to the lower surface 16e and the first punch 18 does not press down the work W1. 
     The work W1 is placed on the cushion ring 16, being received on the rounded recess 16c. The ironing die 17 is placed on the cylindrical boss 11a of the work W1. The first punch 18 is positioned above the cylindrical boss 11a, as shown in FIG. 3. A bore 17a having a diameter of D2 is formed in the ironing die 17. The diameter D2 is smaller than an outer diameter D1 of the cylindrical boss 11a. The first punch 18 is composed of a large rod 18a having a diameter of d2, a small rod 18b having a diameter of d3, and a connecting portion 18c which connects the large rod 18a and the small rod 18b. The connecting portion 18c has two corner radii 18d and 18e. The diameter d2 is larger than an inner diameter d1 of the cylindrical boss 11a, and the diameter d3 is smaller than the diameter d1 and the same as the diameter of the guide bore 16d. 
     The bulge 12b is formed at the base portion 12 of the cylindrical boss 11a by pressing down the first punch 18 and the ironing die 17 from the position shown in FIG. 3 to the position shown in FIG. 5. After the work W1 (a crude work at this time) is positioned in the die 13 as shown in FIG. 3, the first punch 18 and the ironing die 17 are pressed down simultaneously against the work W1, while the pressure P is applied to the cushion ring 16 from its lower surface 16e. The downward movement continues until the die 13 comes to the position shown in FIG. 5. During the die movement, the diameter d1 of the cylindrical boss 11a is enlarged by the diameter d2 of the first punch 18, and the diameter D1 of the cylindrical boss 11a is ironed (squeezed) by the diameter D2 of the ironing die 17. Therefore, the cylindrical boss 11a is deformed, and squeezed material from the cylindrical boss 11a forms the bulge 12b when the die reaches the position shown in FIG. 5. When the die 13 takes the position shown in FIG. 5, the lower surface 16e of the cushion ring 16 abuts against the holder 14, and the flat surface of the rounded recess 16c comes to the level of the upper surface 15a of the first plate 15, because the height H1 of the first plate 15 is equal to the length h1. Therefore, a space for forming the bulge 12b is completely confined by the cushion ring 16, the first punch 18 and the ironing die 17, and accordingly the bulge 12b does not swell beyond the confined space. 
     After the bulge 12b is formed as above, the flange forming process (second process) is carried out. FIG. 6 shows a die 19 for flange forming process and the work W1 with the flange 12c formed in this process. The flange forming die 19 is composed of a second plate 20, a fixing die 21 and a second punch 22. The second plate 20 has an upper surface 20b which supports the work W1 thereon and a guide bore 20a for slidably guiding the second punch 22 therein. The fixing die 21 has a lower surface 21b for pushing down the upper surface 10a of the work W1 and a bore 21a having the diameter D2 (which is same as the outer diameter of the cylindrical boss 11a) for supporting the outer periphery of the cylindrical boss 11a. The second punch 22 is composed of a large rod 22a having a diameter of d5, a small rod 22b having a diameter of d6, and a tapered connecting portion 22c connecting the large rod 22a and the small rod 22b. The diameter d5 is equal to the diameter d2 of the large rod 18a of the first punch 18. The diameter d6 is much smaller than the diameter d3 of the small rod 18b of the first punch 18. 
     The work W1 having the bulge 12b is firmly held between the second plate 20 and the fixing die 21. Then, the second punch 22 is forcibly moved to the position shown in FIG. 6, thereby the bulge 12b is deformed into the shape of the flange 12c in a confined space between the upper surface 20b and the connecting portion 22c, as shown in FIG. 6. 
     Then, the work having the flange 12c is subjected to the corner step forming process (third process). A die 23 used in the corner step forming process is shown in FIG. 7. The die 23 is composed of a third plate 24, a fixing die 25 and a third punch 26. The third plate 24 has an upper surface 24b for supporting the work thereon and a guide bore 24a for guiding the third punch 26 therein. The fixing die 25 has a lower surface 25b for pushing down the base plate 10 of the work and a bore 25a having the diameter D2 which is the same as the outer diameter of the cylindrical boss 11a. The third punch 26 has a large rod 26a having a diameter d7, a small rod 26b having a diameter d8 and a tapered connecting portion 26c which connects the large rod 26a and the small rod 26b. The diameter d7 is a little larger than the diameter d5 of the second punch 22. In other words, the diameter d7 is a little larger than the inner diameter of the cylindrical boss 11a before performing the corner step forming process. The diameter d8 is smaller than the diameter d6 of the second punch 22. A recess 26d is formed at the corner of the large rod 26a for forming a corner step 12d at the foot of the cylindrical boss 11a. 
     The corner step forming process is performed in the following manner. The work having the flange 12c is firmly held between the third plate 24 and the fixing die 25. The third punch 26 is forcibly moved into the cylindrical boss 11a, thereby squeezing down some material of the cylindrical boss 11a and forming the corner step 12d at the inner foot of the cylindrical boss 11a. At the same time, the flange 12c is further pressed down into a final shape. The cylindrical boss 11a is shaped into a final size in this process. The final shape of the cylindrical boss 11a corresponds to the center boss 7a shown in FIG. 1, the final shape of the flange 12c to the flange 7b, and the corner step 12d to the corner step 7d which supports the outer race of the bearing 9. 
     Features and advantages of the method of forming a cylindrical boss according to the present invention will be summarized as follows. (1) In the bulge forming process, the first punch 18 forcibly enlarges the inner bore of the cylindrical boss 11a, and at the same time the ironing die 17 forcibly compresses the outer periphery of the cylindrical boss 11a. Therefore, material constituting the cylindrical boss 11a is squeezed into a confined space between the die components, and thereby the bulge 12b which has no defects or cracks is formed in a simple process. (2) In the bulge forming process, the cushion ring 16 is used to support the work thereon, and the cushion ring 16 is pressurized upwardly against the downward force of the upper dies. The work interposed between the cushion ring 16 and upper dies including the first punch 18 and the ironing die 17 is always pressed from both directions during the whole process. Therefore, the space for forming the bulge 12b is confined by the die components, which results in forming a solid bulge. (3) In the flange forming process, the work having the bulge 12b is firmly held between the second plate 20 and the fixing die 21 during the downward movement of the second punch 22. Therefore, the bulge 12b is easily deformed into the shape of the tapered flange 12c. (4) In the corner step forming process, the third punch 26 having a diameter which is a little larger than the inner bore of the cylindrical boss 11a is used, while the outer periphery of the cylindrical boss 11a is firmly held in the bore of the fixing die 25. Therefore, the inner diameter of the cylindrical boss 11a is ironed into a final dimension so that no additional machining is necessary. At the same time, the corner step 12d which serves as a support of a bearing is formed from material squeezed from the inner bore of the cylindrical boss 11a. Because the corner step 12d is formed integrally with the cylindrical boss 11a, there is no need to use an additional part for supporting a bearing. 
     A product having a cylindrical boss processed in the method described above is intended to be used as a housing base for an ultrasonic motor. However, the product manufactured according to the present invention can be used in various ways. For example, it may be used as a housing for various kinds of DC or AC motors, or as a part which requires a cylindrical boss and/or a flange. 
     The present invention may be modified in various ways. For example, only the first process of forming the bulge may be utilized without performing the other two processes. Alternatively, either one of the second and third processes may be performed after the first process. Though the flange 16a of the cushion ring 16 is disc-shaped in the foregoing embodiment, it may be formed in other shapes. The taper angle of the flange 12c may be variously changed according to needs of applications. The recess 26d of the third punch 26 may be eliminated, and the corner step 12d may be formed by adjusting a downward stroke of the third punch 26. 
     While the present invention has been shown and described with reference to the foregoing preferred embodiment, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.