Method for manufacturing rocker arm

A method for manufacturing according to the present invention includes a step of obtaining a bent workpiece 13 with prepared apertures 21 and 21, wherein the bent work piece 13 includes a pair of side walls 20 and 20 disposed in parallel with each other and connecting walls 30 and 40 for connecting the side walls 20 and 20, the pair of side walls 20 an d20 each having a prepared aperture 21, and a step of forming a roller supporting shaft fixing aperture 25 by inserting a sizing punch 60 into the prepared aperture 21, wherein the sizing punch 60 has a tapered push enlarging portion 61 at a tip end portion, a basal end side of the push enlarging portion being larger than the prepared aperture 21 in diameter, and a tip end side of the push enlarging portion 61 being smaller than the prepared aperture 21 in diameter, whereby the push enlarging portion 61 causes plastic flow of an inner peripheral portion of the prepared aperture to finish the inner peripheral surface of the prepared aperture to thereby obtain a roller supporting shaft fixing aperture 25. By this structure, sufficient shaft holding ability can be given to the roller supporting shaft fixing aperture, and a rocker arm can be manufactured at low costs.

This application claims priority to Japanese Patent Application Nos. 2004-366359 filed on Dec. 17, 2004 and 2004-366376 filed on Dec. 17, 2004, the entire disclosures of which are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a rocker arm for opening and closing a valve in a valve operating system for use in an internal combustion engine for vehicles for example.

2. Description of the Related Art

FIG. 10is a cross-sectional view showing a valve operating system for use in an internal combustion engine for vehicles. As shown in this figure, provided in this valve operating system is a rocker arm1for converting a rotary motion of a cam3which rotates in conjunction with a crankshaft2into a reciprocating motion of a valve stem5for a valve such as an intake valve or an exhaust valve.

As shown inFIGS. 10 and 11, this rocker arm1is provided with a pair of side walls1aand1adisposed in parallel with each other, connecting walls1band1cconnecting one end of the side walls and the other end thereof respectively, a valve stem contacting portion1dwhich is attached to the one end side wall1band is to be brought into contact with the valve stem5, and a pivot engaging portion1ewhich is attached to the other end side wall1cand is to be engaged with a pivot portion6fixed to a cylinder head. Both the side walls1aand1aof this rocker arm1are provided with shaft fixing apertures1fand1fon the same axis line into which a supporting shaft8of a roller7is fixedly inserted, so that the roller7and the cam3are brought into a rolling contact with each other.

This kind of rocker arm has been conventionally manufactured by forging or precision casting (lost-wax processing). However, in accordance with the recent year's movement toward high revolution and high power of an internal combustion engine, it is required to attain lightweighting of a rocker arm to reduce the inertia-weight, and therefore there are a number of proposals on press working techniques for manufacturing a rocker arm.

In a method for manufacturing a rocker arm by press working, as a method for forming a shaft fixing aperture1f, cutting using a drill or press blanking using a punch is generally employed.

However, cutting work causes deteriorated production efficiency and an increased production cost due to the lower efficiency as compared with press working, and also may exert a harmful influence by chips generated during the cutting work.

On the other hand, in a press blanking method, although the production efficiency can be improved, sufficient flatness of the inner peripheral surface of the punched aperture cannot be secured, which makes it difficult to secure, e.g., sufficiently wide sheared surface in the inner peripheral surface. This causes a difficulty in assuredly bringing the inner peripheral surface into contact with the supporting shaft8, which in turn may cause deteriorated shaft holding force.

Under the circumstances, the assignee of the present invention has developed a method for forming a shaft fixing aperture1fas disclosed by Japanese Patent No. 3,497,368 (hereinafter referred to as “Patent Document 1”) and Japanese Patent No. 3,582,977(hereinafter referred to as “Patent Document 2”). In the method, a prepared aperture is initially formed by blanking, and then a shaving punch is driven into the prepared aperture to thereby obtain a smoothed inner peripheral surface of the aperture.

With this method, high productivity can be attained since the shaft fixing aperture1fis formed only by press working using an aperture forming punch and a shaving punch, and a smoothed inner peripheral surface of the aperture by shaving can be obtained, resulting in sufficient shaft holding ability.

In the method for manufacturing a rocker arm disclosed by the aforementioned Patent Documents 1 and 2, a shaving punch used as a tool having a sharp cutting edge is used. However, a large load is intensively applied to the cutting edge during the processing, which causes an early abrasion of the cutting edge. Using such punch with an abraded cutting edge deteriorates the cutting workability, causing generation of burrs, which in turn causes deteriorated quality. As will be understood from the above, a shaving punch is short in life, and therefore it is required to replace with a new one in a short period of time. Thus, the method should be improved in terms of running costs.

Furthermore, in the method as disclosed in Patent Documents 1 and 2, since an inner peripheral portion of the prepared aperture is shaved off by shaving, chips will be generated, which in turn cause defects such as littering of chips and/or damages due to the chips. Accordingly, it is also required to take sufficient measures to remove such chips.

The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.

SUMMARY OF THE INVENTION

The preferred embodiments of the present invention have been developed in view of the above-mentioned and/or other problems in the related art. The preferred embodiments of the present invention can significantly improve upon existing methods and/or apparatuses.

Among other potential advantages, some embodiments can provide a method for efficiently manufacturing a rocker arm with a shaft fixing aperture having sufficient shaft holding ability at row costs without generating any chips.

According to a first aspect of a preferred embodiment of the present invention, a method for manufacturing a rocker arm, comprising the steps of:obtaining a bent workpiece with prepared apertures, wherein the bent work piece includes a pair of side walls disposed in parallel with each other, and connecting wallsfor connecting the side walls, the pair of side walls each having a prepared aperture; andforming a roller supporting shaft fixing aperture by inserting a sizing punch into the prepared aperture, wherein the sizing punch has a tapered push enlarging portion at a tip end portion, a basal end side of the push enlarging portion being larger than the prepared aperture in diameter, and a tip end side of the push enlarging portion being smaller than the prepared aperture in diameter, whereby the push enlarging portion causes plastic flow of an inner peripheral portion of the prepared aperture to finish the inner peripheral surface of the prepared aperture to thereby obtain a roller supporting shaft fixing aperture.

According to a second aspect of a preferred embodiment of the present invention, the method for manufacturing a rocker arm as recited in claim1, wherein an external peripheral surface of the push enlarging portion of the sizing punch is formed as a tapered working surface, and wherein an angle of inclination with respect to an axis of the working surface is set to 10 to 50°.

According to a third aspect of a preferred embodiment of the present invention, a method for manufacturing a rocker arm, comprising the steps of:obtaining a flat workpiece with prepared apertures to be formed into roller supporting shaft fixing apertures;obtaining a bent workpiece with prepared apertures by bending the flat workpiece with prepared apertures, the bent work piece including a pair of side walls disposed in parallel with each other, and a connecting wall for connecting the side walls, the pair of side walls each having a prepared aperture; andforming a roller supporting shaft fixing aperture by inserting a sizing punch into the prepared aperture, wherein the sizing punch has a tapered push enlarging portion at a tip end portion, a basal end side of the push enlarging portion being larger than the prepared aperture in diameter, and a tip end side of the push enlarging portion being smaller than the prepared aperture in diameter, whereby the push enlarging portion causes plastic flow of an inner peripheral portion of the prepared aperture to finish the inner peripheral surface of the prepared aperture to thereby obtain a roller supporting shaft fixing aperture.

According to a fourth aspect of a preferred embodiment of the present invention, the method for manufacturing a rocker arm as recited in claim1, wherein the step of forming the roller supporting shaft fixing aperture includes:disposing the sizing punches at the outsides of both the side walls of the bent workpiece with the prepared apertures so as to oppose each other with the punches aligned each other;simultaneously inserting both the sizing punches into the prepared apertures of the side walls to simultaneously finish the prepared apertures.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [1], since the rocker arm can be manufacture only by press working such as blanking, bending and sizing, high productivity can be attained and smooth finishing of the inner peripheral surface of the aperture can be attained by the sizing. Also, sufficient shaft supporting ability can be secured.

Furthermore, since the sizing punch is inserted into the prepared aperture to cause plastic flow of the inner peripheral portion of the prepared aperture to thereby finish the inner peripheral surface of the prepared aperture, in contract to a method in which a shaving punch is inserted, a load to be applied to the punch can be decreased, and it becomes possible to prevent early abrasion/wear of the punch. Furthermore, the burden of maintenance work can be reduced, resulting in reduced running costs.

Furthermore, since this method causes plastic flow of the inner peripheral surface portion of the prepared aperture, it is possible to assuredly prevent generation of chips or the like during the processing. Accordingly, defects such as generation of scratches due to adhesion of chips or the like and/or generating/scattering of chips or the like can be prevented assuredly.

At the time of, e.g., bending the blank workpiece12after forming the prepared apertures, even if the central axes L of both the prepared apertures21may sometimes be aligned, the penetrations of the sizing punches from both sides will correct the central axes of the apertures simultaneously with finishing of the prepared apertures. Thus, high positional accuracy can be maintained.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [2], the plastic flow of the inner peripheral portion of the prepared aperture can be performed more smoothly, and the aforementioned effects can be secured more assuredly.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [3], since the rocker arm can be manufacture only by press working such as blanking, bending and sizing, high productivity can be attained and smooth finishing of the inner peripheral surface of the aperture can be attained by the sizing. Also, sufficient shaft supporting ability can be secured.

In the present invention, since the processing for forming the prepared aperture is executed against a flat blank workpiece, the removing processing for the punched portions can be performed by normal press operations without requiring special operations. Furthermore, the insertion of the sizing punch into the prepared aperture causes plastic flow of the prepared aperture to thereby finish the aperture, and therefore generation of chips can be prevented. As mentioned above, in the aperture opening processing and/or the prepared aperture finishing processing, no special operation for removing chips is required, resulting in smooth processing, which in turn can improve the productivity. Especially, in the present invention, no special operation will be required in any steps including the step of forming apertures and finishing them, and the method can be performed by normal press working. Therefore, it enables an employment of a transfer press machine executing all of press working continuously, which in turn can attain automation of a sequence of press working from blanking to finishing. This improves the productivity dramatically.

Furthermore, since the sizing punch is inserted into the prepared aperture to cause plastic flow of the inner peripheral portion of the prepared aperture to thereby finish the inner peripheral surface of the prepared aperture, in contract to a method in which a shaving punch is inserted, a load to be applied to the punch can be decreased, and it becomes possible to prevent early abrasion/wear of the punch. Furthermore, the burden of maintenance work can be reduced, resulting in reduced running costs.

Furthermore, at the time of finishing the prepared aperture, no chip or the like generates, and therefore defects such as generation of scratches due to adhesion of chips or the like and/or generating/scattering of chips or the like can be prevented assuredly.

In the present invention, when the flat workpiece is bent after forming the prepared apertures, the positional misalignment of the prepared apertures may occur. However, even if such positional misalignment occurs, the insertion of the sizing punch in the prepared apertures can simultaneously correct the aperture positions simultaneously with the aperture finishing, resulting in high positional accuracy.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [4], the positional misalignment between the prepared apertures can be corrected, and therefore the phase difference can be eliminated to further improve the positional accuracy of the prepared apertures. Furthermore, since the pressures for inserting the sizing punches can be applied evenly in a balanced manner from both sides of the bent workpiece, it is possible to assuredly prevent generation of harmful deformation due to the punch inserting pressures, which in turn makes it possible to maintain high quality.

In the present invention, the following structures can be employed.

[5] The method for manufacturing a rocker arm as recited in the aforementioned Item [1] or [2], wherein the step of obtaining the bent workpiece with the prepared apertures is performed by forming a flat blank workpiece having the prepared apertures by punching and then bending the flat blank workpiece.

[6] The method for manufacturing a rocker arm as recited in the aforementioned Item [1] or [2], wherein the step of obtaining the bent workpiece with the prepared apertures is performed by bending a flat blank workpiece and then forming the prepared apertures in the side walls of the bent blank workpiece by punching.

[7] The method for manufacturing a rocker arm as recited in the aforementioned Item [6], wherein the step of forming the prepared apertures is performed by disposing aperture opening punches for forming the prepared apertures so as to oppose with each other at the outsides of the side walls of the bent workpiece with the axis aligned each other, and simultaneously advancing both the aperture opening punches from the outsides of the side walls to thereby simultaneously form the prepared apertures in the side walls.

[8] The method for manufacturing a rocker arm as recited in the aforementioned Item [1] or [2], wherein the step of forming the roller supporting shaft fixing aperture is performed by disposing the sizing punches so as to oppose with each other at the outsides of the side walls of the bent workpiece with the axis aligned each other, and simultaneously inserting both the sizing punches into the prepared apertures of the side walls to thereby simultaneously finish the prepared apertures.

[9] The method for manufacturing a rocker arm as recited in the aforementioned Item [1] or [2], wherein the step of forming the roller support shaft fixing aperture is performed by causing plastic flow of the material of the inner peripheral portion of the prepared aperture from a sheared region in the prepared aperture to a fractured region.

[10] A rocker arm manufactured by the manufacturing method as recited in the aforementioned Item [1] or [2].

[11] The rocker arm as recited in the aforementioned Item [10], wherein the inner peripheral surface of the shaft fixing aperture is provided with a flat surface larger than a sheared surface of the prepared aperture.

[12] The method for manufacturing a rocker arm as recited in the aforementioned Item [3] or [4], wherein an external peripheral surface of the push enlarging portion of the sizing punch is formed as a tapered working surface, and wherein an angle of inclination of the working surface with respect to an axis of the sizing punch is set to 10 to 50°.

[13] The method for manufacturing a rocker arm as recited in the aforementioned Item [1] or [2], wherein the step of forming the roller supporting shaft fixing aperture is performed by causing plastic flow of the material of the inner peripheral portion of the prepared aperture from a sheared region in the prepared aperture to a fractured region.

[14] A rocker arm manufactured by the manufacturing method as recited in the aforementioned Item [1] or [2].

[15] The rocker arm as recited in the aforementioned Item [14], wherein the inner peripheral surface of the shaft fixing aperture is provided with a flat surface larger than a sheared surface of the prepared aperture.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [5], since the prepared aperture opening processing is executed against the plate workpiece, chip removing processing for removing chips generated by the aperture opening processing can be performed smoothly, which in turn can improve the productivity.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [6], the prepared aperture can be formed with a high degree of positional accuracy.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [7], since the pressures for inserting the aperture opening punches can be applied evenly in a balanced manner from both sides of the bent workpiece, it is possible to assuredly prevent generation of harmful deformation due to the punch applying pressures, which in turn makes it possible to maintain high quality.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [8], the positional misalignment between the prepared apertures can be corrected, and therefore the phase difference can be eliminated to further improve the positional accuracy of the prepared apertures. Furthermore, since the pressures for inserting the sizing punches can be applied evenly in a balanced manner from both sides of the bent workpiece, it is possible to assuredly prevent generation of harmful deformation due to the punch inserting pressures, which in turn makes it possible to maintain high quality.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [9], a wider smooth surface can be secured in the inner peripheral surface of the shaft fixing aperture, which further increases the shaft holding ability.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [10] and [11], the rocker arm having the aforementioned effects can be provided.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [12], the plastic flow of the inner peripheral portion of the prepared aperture can be performed more smoothly, to thereby obtain the aforementioned effects more assuredly.

According to the method for manufacturing a rocker arm of the invention as recited in the aforementioned Item [13], a wider smooth surface can be secured in the inner peripheral surface of the shaft fixing aperture, which further increases the shaft holding ability.

According to the invention as recited in the aforementioned Items [14] and [15], the rocker arm having the aforementioned effects can be provided.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following paragraphs, some preferred embodiments of the invention will be described by way of example and not limitation. It should be understood based on this disclosure that various other modifications can be made by those in the art based on these illustrated embodiments.

Hereinafter, a method for manufacturing a rocker arm according to an embodiment of the present invention will be explained with reference to drawings.

In this method, as shown inFIG. 1A, predetermined areas of a steel plate as an original plate are punched to obtain a flat blank workpiece11with both sides each protruded sideways in an arc-shape.

Then, as shown inFIG. 1B, an aperture opening punch is applied to the flat blank workpiece11so as to form a central aperture15of a drum shape at a central area and prepared apertures21and21at side areas to thereby obtain a flat blank workpiece12with prepared apertures21and21.

Furthermore, the end central portion of the flat blank workpiece11is subjected to bulging to form a pivot engaging dented portion41of a hemisphere shape.

As to the aforementioned blanking processing for punching out the original plate, the aperture opening processing for forming the central aperture15, the aperture opening processing for forming the prepared apertures21and21and the bulging processing for forming the pivot engaging dented portion41, the sequence of these processing is not specifically limited. For example, all of these processing can be performed simultaneously, or two or more processing can be performed simultaneously, or each processing can be performed one by one.

In the case of forming a chamfer at the peripheral edge of the open end portion of the prepared aperture21, it is preferable to employ an aperture opening punch having a chamfer forming portion at the external peripheral surface of the basal end portion of the punch so that the chamfer forming portion forms a chamfer by pressing the peripheral edge of the open end portion of the prepared aperture21when the punch is driven.

In the present invention, as will be explained later, the aperture forming processing for the prepared aperture21can be executed after bending the flat blank workpiece.

Subsequently, as shown inFIG. 1C, the flat blank workpiece12with prepared apertures21and21is subjected to bending processing to obtain a bent workpiece13of a U-shape (inverted U-shape) with prepared apertures21and21.

This bent workpiece13is provided with a pair of side walls20and20disposed in parallel, an one end side connecting wall30as a valve stem contacting portion connecting the one end side upper ends of the side walls20and20, and the other end side connecting wall40having a pivot engaging dented portion41connecting the other end side upper ends of the side walls20and20.

The prepared aperture21of each side wall20of the obtained bent workpiece13is finished with a sizing apparatus50which will be detailed later to obtain a roller supporting shaft fixing aperture25.

As shown inFIG. 2, the sizing apparatus50is provided with a base51. The base51has at its upper portion a sizing die52with a width corresponding to the space between both side walls20and20of the bent workpiece13. This sizing die52is configured so that both side walls20and20of the bent workpiece13can be outwardly fitted thereto.

The sizing die52has a punch passing aperture53extending in the widthwise direction (prepared aperture axial direction) and corresponding to the prepared apertures21and21formed in the side walls21of the bent workpiece13.

The sizing apparatus50further includes punch supporting bases55and55disposed at both sides of the base51. Each punch supporting base55supports a sizing punch60slidable along the axial direction via a punch supporting member57. Both punches60and60are disposed such that the axis of each punch coincides with the axis of the punch passing aperture53of the sizing die52.

At both sides of the punch supporting bases55and55, lifting and lowering members58are disposed in a state in which the cam surface58aformed at the lower end side of the lifting and lowering member58is in slidable contact with the cam surface57aformed at the basal end side of the punch supporting member57. Thus, when both the lifting and lowering members58are lowered, both the sizing punches60advance along the axial direction to be inserted in the punch passing aperture53of the sizing die52from both sides thereof.

Referring toFIG. 3, the sizing punch60used in this embodiment has, at its tip end portion, a push enlarging portion61tapered down to the tip. In this push enlarging portion61, the tip62is formed to have a diameter smaller than the diameter of the prepared aperture21and that the basal end63is formed to have a diameter larger than the diameter of the prepared aperture21. Thus, the external periphery of the push enlarging portion61is formed to function as a tapered working surface61a.

The basal end side portion of the punch60behind the push enlarging portion61is formed as an aperture finishing portion65having the same external diameter as an external diameter of a shaft fixing aperture25which will be explained later.

The finishing processing of the prepared aperture21of the bent workpiece13using the aforementioned sizing apparatus50can be performed as follows. As explained above, the bent workpiece13is mounted on the sizing die52, and then both the punches60and60are advanced by simultaneously lowering both the lifting and lowering members58and58. By this, both the punches60and60run through both the prepared apertures21and21to finish the inner peripheral surface of each aperture21, thereby forming a shaft fixing aperture25.

Now, the behavior of the inner periphery of the prepared aperture21when the sizing punch60is driven into the prepared aperture21will be detailed. Before executing the sizing finish processing of the inner peripheral surface of the prepared aperture21, as shown inFIGS. 3 and 5, the aperture21has a flat and smooth sheared region22aat one side from which the punch60is to be inserted and a rough fractured region22bexpanding outwardly from the sheared region22aat the other side from which the punch60is to be come out. At the opening peripheral edge portion of the prepared aperture21, a chamfer21ais formed.

When the aforementioned sizing punch60is advanced through the aforementioned prepared aperture21, as shown inFIG. 6A, the punch60is inserted into the prepared aperture21, so that the portion (material) of the sheared region22aof the inner peripheral portion is pushed forward with the working surface61aof the punch60. In accordance with the pushing, as shown inFIG. 6B, the portion (inner peripheral portion) of the sheared region22acauses plastic flow, and the plastic flow portion23moves to the fractured region22bof the inner peripheral surface of the prepared aperture as shown inFIGS. 6C and 4. Furthermore, when the finishing portion65of the punch60passes through the prepared aperture21, the wide portion covering from the one end of the inner peripheral surface of the prepared aperture to the other end thereof will be finished into a smooth surface to thereby form a shaft fixing aperture25.

By finishing the prepared aperture21by the plastic flow of the material forming the inner periphery of the prepared aperture21, a wide smooth surface is formed in the inner peripheral surface of the shaft fixing aperture25.

Even in cases where the central axes L of both the prepared apertures21and21of the bent workpiece13do not coincide with each other, i.e., there is a misalignment therebetween, before executing the sizing processing, the insertions of the sizing punches60and60from both sides will correct the positions of the apertures as shown inFIG. 8. Thus, the shaft fixing apertures25and25of both side walls20and20will be disposed with the central axes L aligned correctly.

After completion of the sizing processing, certain processing such as grooving for forming a valve stem contacting groove will be performed. Thus, a rocker arm is manufactured.

With the method for manufacturing a rocker arm according to the embodiment, only press working such as blanking for forming the external configuration and apertures, bending and sizing is employed. Therefore, it is not required to execute machine work such as reaming or cutting which is lower in efficiency as compared with pressing for example, which results in an improved production efficiency and reduced costs.

Furthermore, since the shaft fixing aperture25is formed by subjecting the prepared aperture21to sizing, a large flat surface can be formed on an inner peripheral surface of the shaft fixing aperture25. Accordingly, it becomes possible to bring the inner peripheral surface of the aperture25into face contact with a supporting shaft, resulting in sufficient shaft holding ability.

Furthermore, in this embodiment, the sizing punch60is inserted into the prepared aperture21to cause plastic flow of the inner peripheral portion of the prepared aperture to thereby obtain a finished shaft fixing aperture25. In other words, the inner peripheral surface of the prepared aperture is plastically worked by the working surface61aof the punch60. Therefore, in contract to a method in which the inner peripheral portion of the prepared aperture is cut with a sharp edge portion such as a shaving punch, no large load would be intensively applied to the working surface61aof the sizing punch60, which can extend the use-life dramatically. Accordingly, the burden of punch exchanging work can be reduced, the running cost can also be reduced greatly and the burden of maintenance work can also be reduced, resulting in easy maintenance.

Now, with reference to this embodiment, the following explanation will be directed to the correction amount (plastic flow amount) of the prepared aperture21. The correction amount of the prepared aperture21corresponds to the diameter difference between the diameter of the sizing punch60and that of the prepared aperture forming punch, and is two times the one side expansion amount X as shown inFIG. 5.

In this embodiment, it is preferable that this one side expansion amount X is 1 to 10%, more preferably 5 to 10%, of the thickness T of the side wall20of the rocker arm10. In cases where this expansion amount is set so as to fall within the aforementioned range, the plastic flow amount of the inner peripheral amount of the aperture can be adjusted appropriately at the time of the sizing processing to form a desired large finished inner peripheral surface. This ensures sufficient shaft holding ability.

In other words, if the expansion amount X is insufficient, the plastic flow amount in the inner peripheral portion of the aperture becomes insufficient. This causes the greater part of the fractured region to be remained, resulting in a failure of large smooth inner peripheral surface in the aperture, which in turn may make it difficult to secure sufficient shaft holding ability. To the contrary, if the expansion amount X is excessive, the plastic flow amount becomes excessive, resulting in a buildup at the inner surface side of the side wall20of the rocker arm10, which may cause such defect that the buildup comes into contact with the roller to be disposed between both side walls21.

Furthermore, in this embodiment, it is preferable that the angle θ of inclination of the working surface61aof the push enlarging portion61with respect to the central angle is set to 10 to 50°, more preferably 12 to 40°, still more preferably 15 to 30°.

When the angle θ of inclination is set so as to fall within the aforementioned range, the plastic flow of the inner peripheral portion can be attained assuredly and smoothly, which makes it possible to attain a desired aperture finishing.

In other words, when the angle θ of inclination is set excessively large, it becomes difficult to attain the plastic flow of the inner peripheral portion assuredly. This may cause shearing or fracturing of the inner peripheral portion, resulting in a failure of desired finishing. When the angle θ of inclination is set excessively small, the axial length of the working surface61abecomes long excessively, which in turn requires a longer punch stroke.

In the manufacturing method of this embodiment, in finishing the shaft fixing aperture25, the sizing punch60is inserted into the prepared aperture21to cause the plastic flow of the inner peripheral portion of the aperture. Therefore, in contrast to the case in which the inner peripheral surface of the aperture is shaved by inserting a shaving punch, no chip will generate. Accordingly, after the insertion of the sizing punch60, it is not required to remove chips in the punch passing aperture53of the sizing die52, enabling elimination of a chip removing step. This further improves the workability.

In the case of press working which requires to remove chips caused by a shaving punch or aperture opening punch from both sides of a work as disclosed in the aforementioned Patent Documents 1 and 2 (JP 3,497,368 B and JP 3,582,977 B), since the press working is low in productivity, it is difficult to improve the production efficiency. In detail, in press working which requires a chip removing step, in order to remove chips remaining in a die, special operations such as relatively moving a punch and a die to open the die aperture and removing chips are inevitable. Such special operations for moving the die for example as mentioned above requires a lot of time, resulting in a long processing time. Accordingly, when pressing works accompanied by special operations are executed subsequent to normal pressing works such as bending pressing or punching pressing, works will be pilled up at the special pressing work. Therefore, it is difficult to execute both the press working uninterruptedly. Under the circumstances, it is hard to form a transfer press machine by combining press working accompanied by special operations and normal press working, which may cause a deterioration of productive efficiency. Furthermore, it also required to employ special structure due to special operations, causing increased costs.

To the contrary, in the manufacturing method of this embodiment, the sizing can be completed by merely inserting the sizing punch60into the prepared aperture21of the bent workpiece13, and therefore no chip will generate. As a result, there is no need to remove such chips. Accordingly, in this embodiment, it can be configured only by press working of normal operation which requires no special operation such as die moving operation for removing chips, enabling an employment of a transfer press machine executing all of press working continuously, which in turn can attain automation of a sequence of press working from blanking to finishing. This improves the productivity dramatically. Furthermore, no special operation is required and no special structure for executing special operations is require, which in turn can also reduce production costs.

For reference, in a shaving operation in which punches are inserted from both sides as disclosed in the Patent Documents 1 and 2 mentioned above, the machining ability (working speed) is 10 pieces per minute. To the contrary, in the sizing operation of the embodiment, it is possible to attain the machining ability of about 60 pieces per minute, which is the same machining ability as a normal pressing work not requiring special operations such as a chip removing operation.

Furthermore, in the sizing operation, both sizing punches60and60disposed concentrically are simultaneously advanced to finish both the prepared apertures21simultaneously. Accordingly, both the finished apertures (shaft fixing apertures25) can be precisely disposed concentrically, enabling to further improve the quality. Especially like in this embodiment, in cases where a flat blank workpiece12is bent after forming the prepared apertures, the central axes L of both the prepared apertures21do not always coincide with each other as shown inFIG. 7. Even in cases where there is such positional misalignment, the penetrations of both the sizing punches60and60from both sides correct the central axes of the apertures simultaneously with the plastic working as shown inFIG. 8. Thus, both the shaft fixing apertures25can be coaxially disposed with both the central axes aligned accurately, resulting in high positional accuracy.

Furthermore, in this embodiment, since the pressures for inserting the sizing punches60and60can be applied evenly in a balanced manner from both sides of the bent workpiece13, it is possible to assuredly prevent generation of harmful deformation due to the punch inserting pressures, which in turn makes it possible to maintain high quality.

In the aforementioned embodiment, the sizing punch60with a working surface61aof a chamfer face (C face) shape flat in cross-section is employed. However, the present invention is not limited to the above, and allows the use of a sizing punch60with an R shaped working surface61acircular arc in cross-section as shown inFIG. 9Aor a sizing punch with a working surface which is a combination of an R face and a C face.

In this invention, for the purpose of decreasing the contact pressure between the punch and the inner peripheral surface of the aperture to be generated when the sizing punch is inserted, a sizing punch60having a basal end side portion (rearward side portion) with a diameter smaller than the diameter of the finishing portion65as shown inFIG. 9Bcan be employed.

Furthermore, in the aforementioned embodiment, after forming the prepared aperture21in the flat blank workpiece, the blank work piece with the prepared aperture21is subjected to bending processing. However, the present invention does not limit to the above. In the present invention, it can be configured such that a blank workpiece with no aperture is subjected to bending processing and thereafter prepared apertures are formed in both the side walls of the bent workpiece. In cases where prepared apertures are to be formed in both side walls of a bent workpiece, it is recommended to employ a punching machine similar to the sizing machine in the aforementioned embodiment. In detail, in place of the sizing punch60and the die52of the sizing machine shown inFIG. 2, an apparatus provided with aperture opening punches and dies for forming prepared apertures can be used, so that both the punches are simultaneously driven from both sides of the bent workpiece to form prepared apertures in the side walls simultaneously. Furthermore, in such punching machine, it is preferable to use an aperture opening punch with a shoulder portion on the external periphery of the basal end side of the punch so that the shoulder portion can come into contact with the opening edge portion of the prepared aperture simultaneously with the advance movement of the punch to form a chamfer portion.

In cases where prepared apertures are to be simultaneously formed in both side walls of the bent workpiece, as compared to the case in which prepared apertures are formed before the bending processing, both the prepared apertures can be formed coaxially with a high degree of precision. Furthermore, in cases where the aperture opening punches are driven from both sides, the pressures will be given to the bent workpiece equally from both sides. Therefore, it is possible to prevent harmful deformation from being generated by the punching pressures.

EXAMPLES

In accordance with the method for manufacturing a rocker arm of the aforementioned embodiment, a bent workpiece13with prepared apertures was manufactured. The thickness of the side wall of this bent workpiece13was 3.2 mm, and the diameter of the prepared aperture21(external diameter of the aperture opening punch) was 8.40 mm. In this bent workpiece13, among the inner peripheral surface of the prepared aperture21, the area of 50% along the axis thereof was a sheared region and the remaining area was a fractured region.I

Then, as to this bent workpiece13, the inner peripheral surface of the prepared aperture13was finished using a sizing machine50similar to that of the aforementioned embodiment to thereby form a shaft fixing aperture25. At this time, the sizing punch60(the external diameter (diameter of shaft fixing aperture) of the finishing portion65: 8.70 mm, the one side expansion amount with respect to the thickness of the side wall: 9%, and the angle θ of inclination of the external periphery (working surface61a) of the push enlarging portion61: 46°) was used.

The size of the smooth region of the inner peripheral surface of the shaft fixing aperture of the obtained rocker arm10was measured. The results revealed that an 80% of the inner peripheral region along the center of axis was secured as a smooth region.

From a bent workpiece13with prepared apertures (the wall thickness: 4.0 mm, the prepared aperture diameter: 9.0 mm, the percentage of the sheared region of the inner peripheral surface of the prepared aperture: 40%), a rocker arm was manufactured in the same manner as in Example 1 except that a sizing punch60(one side expansion amount: 5%, the angle θ of inclination of the external periphery (working surface61a) of the push enlarging portion61: 38°) was used.

In this rocker arm, it was confirmed that 75% of the inner peripheral surface area of the shaft fixing aperture was secured as a smooth region.

From a bent workpiece13with prepared apertures (the wall thickness: 2.3 mm, the prepared aperture diameter: 7.1 mm, the percentage of the sheared region of the inner peripheral surface of the prepared aperture: 50%), a rocker arm was manufactured in the same manner as in Example 1 except that a sizing punch60(one side expansion amount: 2%, the angle θ of inclination of the external periphery (working surface61a) of the push enlarging portion61: 30°) was used.

In this rocker arm, it was confirmed that 70% of the inner peripheral surface area of the shaft fixing aperture was secured as a smooth region.

The present invention can be used to manufacture a rocker arm for opening and closing a valve in a valve operating system for use in an internal combustion engine for vehicles for example.