Patent Description:
It is known that a sheared edge of a punched hole of a metal sheet subjected to blanking has lower fatigue strength than an end surface of a hole subjected to machining by a drill or the like because tensile residual stress in the circumferential direction of the punched hole is generated by blanking, the end surface is rough, and the like (see Non Patent Literature <NUM>), which causes fatigue fracture in an automotive part and the like. Therefore, it is desired to improve the properties of the sheared edge of the punched hole of the metal sheet subjected to blanking to improve the fatigue strength.

In order to form a punched hole with improved properties of the sheared edge, for example, the following technique has been proposed. Patent Literature <NUM> and Patent Literature <NUM> propose a method of subjecting a metal sheet to plastic deformation to form an indentation in advance and then punching out the metal sheet. Meanwhile, Patent Literature <NUM> proposes a method of rubbing a sheared edge of a punched hole of a metal sheet by providing, in a punch at a position on a further base-end side than a shearing part located on a distal end side of the punch, a large-size part having a diameter larger than that of the shearing part, and moving the punch further in a punching direction from a state where the shearing part penetrates the punched hole to let the large-size part pass through the punched hole. Meanwhile, Patent Literature <NUM> proposes a method using a combination of a punch, a die, and a blank holder of subjecting a metal sheet to blanking while pressing the metal sheet with the blank holder in conjunction with movement of the punch.

Patent Literature <NUM> (<CIT>) discloses the features of the preamble of the appended claim <NUM>.

However, in the methods disclosed in Patent Literature <NUM> and Patent Literature <NUM>, since another mold is needed in addition to a tool for forming a punched hole in a metal sheet, there is a problem that the number of steps increases and productivity is low. In addition, the method disclosed in Patent Literature <NUM> has a problem that the metal sheet around the punched hole is deformed when the large-size part passes through the punched hole.

Further, the method disclosed in Patent Literature <NUM> is described as a method in which the fatigue strength can be improved by increasing the ratio of a shear surface in the sheared edge. However, even if the above method is employed, a fracture surface remains in the sheared edge, and thus the occurrence of cracks in the fracture surface cannot be sufficiently suppressed. Furthermore, since the blank holder having a special shape is needed, there is a problem that an applicable place is limited.

The present invention has been made in view of the above problems, and aims to provide a metal sheet punching device capable of forming a punched hole having improved properties of a sheared edge in one step without requiring another mold in addition to a mold for forming a punched hole, inhibiting a crack from being generated in the sheared edge, and improving fatigue strength.

A metal sheet punching device according to the present invention includes: a die including a circular opening and configured to support a metal sheet; and a punch configured to form a circular punched hole in the metal sheet supported by the die, wherein the punch includes: a main body part; a cylindrical punching part provided at a distal end; rotating part disposed between the main body part and the punching part so as to be rotatable about a central axis of the punching part as a rotation axis; a coated abrasive provided on an outer circumferential surface of the rotating part and configured to polish a sheared edge of the punched hole; spring disposed between the main body part and the punching part so as to be capable of contracting and extending in a punching direction, the spring being configured to contract and accumulate a part of a punching load as elastic energy until the punching part comes into contact with and punches the metal sheet, release the elastic energy, and extend after the punching part punches the metal sheet; and a rotational motion conversion device configured to convert a linear motion in the punching direction of the punching part due to extension of the spring into a rotary motion of the rotating part.

The rotational motion conversion device may include: a plate-shaped rack provided to extend in the punching direction from a distal end of the main body part; first gear disposed so as to be rotatable about an axis orthogonal to the punching direction and including a first spur gear part meshing with the plate-shaped rack, and a first bevel gear part provided coaxially with the first spur gear part; a second gear disposed so as to be rotatable about an axis parallel to the punching direction and including a second bevel gear part meshing with the first bevel gear part, and a second spur gear part provided coaxially with the second bevel gear part; a gear support provided in the punching part to rotatably support each of the first gear and the second gear; and a cylindrical rack provided on an inner circumferential surface side of the rotating part and configured to mesh with the second spur gear part.

In the present invention, by forming the punched hole and polishing the sheared edge of the punched hole, it is possible to reduce unevenness of the sheared edge and prevent the direction of a polishing mark on the sheared edge from matching the direction of a crack generated in the sheared edge when a load is repeatedly applied to the metal sheet in which the punched hole is formed, and also possible to form the punched hole of the sheared edge having excellent fatigue strength in one step without requiring a power source other than the power source for punching the metal sheet. In addition, according to the present invention, it is possible to expect improvement in formability by preventing ductile fracture of the sheared edge when press forming is performed after punching, and improvement in delayed fracture characteristics in which brittleness-like fracture occurs when a predetermined time elapses in a state where a metal sheet having a punched hole is subjected to a static load. Further, by reducing the unevenness of the sheared edge to reduce the surface area, improvement in coating properties and corrosion resistance of the coating material can also be expected.

In order to solve the above problems, the inventors of the present invention first conducted intensive studies on the properties and fatigue strength of a sheared edge of a punched hole obtained by blanking.

<FIG> illustrates a cross-sectional view (side surface) of a punched hole <NUM> formed by subjecting a metal sheet <NUM> to blanking. A sheared edge <NUM> of the punched hole <NUM> is divided into a shear surface 5a and a fracture surface 5b. When a load is repeatedly applied to this metal sheet <NUM> in which the punched hole <NUM> is formed, as illustrated in the top view of <FIG>, a crack <NUM> is likely to occur in the fracture surface 5b of the sheared edge <NUM>, which results in fatigue fracture starting from the crack <NUM>.

In addition, in an unevenness of the fracture surface 5b, with a portion where recesses are continuous in a punching direction by the punch serving as a starting point, the crack <NUM> develops due to a tensile stress in the circumferential direction of the punched hole applied due to blanking or a stress such as bending of the metal sheet. Further, it has become clear that the generation of the crack <NUM> is accelerated even if the direction of the polishing mark remaining on the sheared edge <NUM> is in the punching direction.

Therefore, the inventor has obtained a finding that, by using a punch having a cylindrical punching part, a rotating part rotatable about a central axis of the punching part as a rotation axis, and a coated abrasive provided on an outer circumferential surface of the rotating part, subjecting a metal sheet to blanking to form a punched hole, rotating the rotating part in a state where the rotating part is located inside the punched hole, and polishing a sheared edge of the punched hole with the coated abrasive, it is possible to form the punched hole and polish the sheared edge in one step so that the direction of a polishing mark and the direction of a crack do not match each other, and thus solve the above problem.

Furthermore, the inventor has conceived of an idea that, by providing a spring between a main body part and the punching part at a distal end of the punch and providing a device for accumulating a part of a punching load as elastic energy of the spring and releasing the elastic energy accumulated in the spring after punching to rotate the rotating part, it is possible to polish the sheared edge without requiring a power source for rotating the rotating part other than a power source for blanking.

A metal sheet punching device according to an embodiment of the present invention is described below. Note that, in this specification and the drawings, elements having substantially the same function and configuration are denoted by the same reference signs, and redundant description is omitted. In addition, in the drawings used in the following description, in order to facilitate understanding of features, features may be illustrated in an enlarged manner for convenience, but dimensions, ratios, and the like of each component are not necessarily the same as actual dimensions, ratios, and the like.

As illustrated in <FIG> as an example, a metal sheet punching device <NUM> (Hereinafter, referred to as a "punching device <NUM>". ) according to the embodiment of the present invention forms the punched hole <NUM> in the metal sheet <NUM> using a die <NUM> and a punch <NUM>.

The die <NUM> has a circular opening 13a and is designed to support the metal sheet <NUM>. The punch <NUM> includes a main body part <NUM>, a cylindrical punching part <NUM> provided at its distal end in a punching direction, a rotating part <NUM>, a coated abrasive <NUM>, a spring <NUM>, and a rotational motion conversion device <NUM>.

The rotating part <NUM> is disposed between the main body part <NUM> and the punching part <NUM> so as to be rotatable about the central axis of the punching part <NUM> as a rotation axis.

The coated abrasive <NUM> is provided on the outer circumferential surface of the rotating part <NUM>, and is designed to polish the sheared edge <NUM> of the punched hole <NUM> by the rotation of the rotating part <NUM>.

The spring <NUM> is disposed between the main body part <NUM> and the punching part <NUM> so as to be able to contract and extend in the punching direction. Then, in the process of punching the metal sheet <NUM> by moving the punch <NUM> in the punching direction, the spring <NUM> contracts and accumulates a part of a punching load as elastic energy until the punching part <NUM> comes into contact with and punches the metal sheet <NUM>, and after the punching part <NUM> punches the metal sheet <NUM>, the spring <NUM> releases the elastic energy accumulated in the spring <NUM> and extends. Note that the punching direction is a direction in which the punch <NUM> is relatively moved toward the die <NUM> in order to form the punched hole <NUM> in the metal sheet <NUM>.

The rotational motion conversion device <NUM> is configured to convert a linear motion in the punching direction of the punching part <NUM> due to extension of the spring <NUM> into a rotary motion of the rotating part <NUM>.

An example of a specific configuration of the rotational motion conversion device <NUM> is illustrated in <FIG>.

As illustrated in <FIG>, the rotational motion conversion device <NUM> includes a plate-shaped rack <NUM>, a first gear <NUM>, a second gear <NUM>, and a gear support <NUM>.

The plate-shaped rack <NUM> is provided to extend in the punching direction from a distal end of the main body part <NUM>, and moves together with the main body part <NUM> when the punch <NUM> is moved in the punching direction.

As illustrated in <FIG>, the first gear <NUM> includes a first spur gear part 31a meshing with the plate-shaped rack <NUM> and a first bevel gear part 31b provided coaxially with the rotation axis of the first spur gear part 31a, and is disposed rotatably about an axis orthogonal to the punching direction (about an axis C<NUM> in <FIG>). Here, the first spur gear part 31a and the first bevel gear part 31b are connected by a first gear shaft part 31c so that they can rotate coaxially.

As illustrated in <FIG>, the second gear <NUM> includes a second bevel gear part 33a meshing with the first bevel gear part 31b and a second spur gear part 33b provided coaxially with the rotation axis of the second bevel gear part 33a, and is disposed rotatably about an axis parallel to the punching direction (about an axis C<NUM> in <FIG>). Here, the second bevel gear part 33a and the second spur gear part 33b are connected by a second gear shaft part 33c so that they can rotate coaxially.

As illustrated in <FIG> and <FIG>, the gear support <NUM> is provided in the punching part <NUM> to rotatably support each of the first gear <NUM> and the second gear <NUM>.

A cylindrical rack <NUM> is provided on the inner circumferential surface side of the rotating part <NUM> and meshes with the second spur gear part 33b. Here, as illustrated in <FIG>, the cylindrical rack <NUM> is the same as the rotating part <NUM>, and the coated abrasive <NUM> is directly attached to the outer circumferential surface of the cylindrical rack <NUM>. However, the cylindrical rack <NUM> is not limited to one which is the same as the rotating part <NUM>, and may be separately provided on the inner circumferential surface side of the rotating part <NUM>, for example.

Next, the operation of the punching device <NUM> in the process of forming the punched hole <NUM> in the metal sheet <NUM> using the punching device <NUM> is described with reference to <FIG>.

First, the metal sheet <NUM> is placed so as to straddle the opening 13a of the die <NUM>, the metal sheet <NUM> is supported at its both end sides, and the punch <NUM> is installed above the metal sheet <NUM> (<FIG> and <FIG>).

Next, the punch <NUM> is moved in the punching direction, and the spring <NUM> is contracted until the punch <NUM> comes into contact with the metal sheet <NUM> and punches the metal sheet <NUM>. As a result, a part of a punching load is accumulated in the spring <NUM> as elastic energy (<FIG> and <FIG>).

Subsequently, the punch <NUM> is further applied with a punching load to cause the punching part <NUM> to punch the metal sheet <NUM>, so that the elastic energy accumulated in the spring <NUM> is released and the spring <NUM> extends. As a result, the punching part <NUM> linearly moves in the punching direction toward the opening 13a of the die <NUM>, and the rotating part <NUM> is located inside the punched hole <NUM> (<FIG> and <FIG>).

Along with the linear motion of the punching part <NUM>, as illustrated in <FIG>, the plate-shaped rack <NUM> relatively moves in the direction opposite to the punching direction, and the first gear <NUM> rotates via the first spur gear part 31a meshing with the plate-shaped rack <NUM>. Then, the rotation of the first gear <NUM> is transmitted to the second bevel gear part 33a meshing with the first bevel gear part 31b, and the second gear <NUM> rotates.

Thus, the rotation of the second gear <NUM> is transmitted to the rotating part <NUM> (the cylindrical rack <NUM> in <FIG>) meshing with the second spur gear part 33b, and the rotating part <NUM> rotates. As a result, the sheared edge <NUM> of the punched hole <NUM> is polished by the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM> (<FIG> and <FIG>).

After the spring <NUM> has fully extended and the rotation of the rotating part <NUM> has stopped, the punch <NUM> is moved in the direction opposite to the punching direction to pull out the punch <NUM> from the punched hole <NUM> (<FIG>).

As described above, according to the metal sheet punching device <NUM> according to the embodiment of the present invention, a part of the punching load of the metal sheet <NUM> by the punch <NUM> is accumulated in the spring <NUM> as elastic energy, and after the metal sheet <NUM> is punched to form the punched hole <NUM>, the elastic energy accumulated in the spring <NUM> is released and thus the rotating part <NUM> rotates. Then, the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM> polishes the sheared edge <NUM> of the punched hole <NUM>, so that the rotating part <NUM> is rotated without requiring a power source other than the power source for punching the metal sheet <NUM>, and the sheared edge <NUM> is polished in one step to form the punched hole <NUM> with reduced unevenness.

Further, it is possible to prevent the direction of a polishing mark on the sheared edge <NUM> polished by the coated abrasive <NUM> from matching the direction of a crack generated in the sheared edge <NUM> when a load is repeatedly applied to the metal sheet <NUM> in which the punched hole <NUM> is formed. As a result, it is possible to inhibit a crack from being generated in the sheared edge <NUM> when a repeated load is applied, and to form the punched hole <NUM> with improved fatigue strength.

Furthermore, according to the metal sheet punching device <NUM> according to this embodiment, improvement in formability by preventing ductile fracture of the sheared edge <NUM> when press forming is performed after punching, improvement in delayed fracture characteristics of the punched hole <NUM>, and improvement in coating properties and corrosion resistance of a coating material by reducing the unevenness of the sheared edge <NUM> to reduce the surface area can also be expected.

Note that the strength of the spring <NUM> may be any strength as long as the strength is enough to punch the metal sheet <NUM> in a state where the spring is contracted when the punching part <NUM> comes into contact with the metal sheet <NUM>.

Meanwhile, the rotating part <NUM> is preferably of a cylindrical shape. The coated abrasive <NUM> is not limited to one provided so as to cover the entire outer circumferential surface of the rotating part <NUM>, and may be one provided on a part of the outer circumferential surface of the rotating part <NUM>.

Meanwhile, in order to sufficiently polish the sheared edge <NUM> of the punched hole <NUM> by the rotation of the rotating part <NUM>, the coated abrasive <NUM> is preferably set so that the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM> extends outward of the outer circumferential surface of the punching part <NUM>, that is, the outer diameter of the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM> is equal to or larger than the outer diameter of the punching part <NUM>.

However, when the outer diameter of the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM> is too larger than the outer diameter of the punching part <NUM>, after the rotating part <NUM> provided with the coated abrasive <NUM> on its outer circumferential surface is inserted into the punched hole <NUM>, the coated abrasive <NUM> extends outward of the sheared edge <NUM> of the punched hole <NUM>. As a result, even if the rotating part <NUM> can be inserted into the punched hole <NUM>, when the rotating part <NUM> is rotated, the coated abrasive <NUM> polishes not only the sheared edge <NUM> but also the opening 13a of the die <NUM>, so that the life of the coated abrasive <NUM> may be reduced. Accordingly, the outer diameter of the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM> is preferably about the same as the inner diameter of the opening 13a of the die <NUM>.

Meanwhile, the punch <NUM> used in the above description is one in which the surface of the coated abrasive <NUM> is parallel to the punching direction, that is, an angle θ (see <FIG>) formed by the surface of the coated abrasive <NUM> and a cross-section 19a orthogonal to the central axis of the punching part <NUM> is <NUM>°.

However, the angle θ formed by the surface of the coated abrasive <NUM> and the cross-section 19a orthogonal to the central axis of the punching part <NUM> is not limited to <NUM>°. For example, as a preliminary test, the punched hole <NUM> may be formed in the metal sheet <NUM> without providing the coated abrasive <NUM> to the punch <NUM> and an angle θ' (see <FIG>) formed by the fracture surface 5b at the sheared edge <NUM> and the surface 1a of the metal sheet <NUM> may be measured, and then the angle θ may be set within a predetermined error range from the angle θ' measured by the preliminary test. The predetermined error range is ± <NUM>° or less, for example.

As a result, the fracture surface 5b on which the crack <NUM> is likely to occur when a repeated load is applied can be intensively polished. Note that, for installing the coated abrasive <NUM> so that the angle θ formed by the surface of the coated abrasive <NUM> and the cross-section 19a orthogonal to the central axis of the punching part <NUM> becomes a predetermined angle, for example, the shape of the outer circumferential surface of the rotating part <NUM> may be appropriately set.

Meanwhile, it is preferable to use a general buff for the coated abrasive <NUM>. However, if the coated abrasive <NUM> does not have stretching properties in its thickness direction, there is a possibility that the coated abrasive cannot be removed from the punched hole <NUM>. Therefore, it is preferable to appropriately select the type and material of the coated abrasive <NUM>.

Further, the yarn count (grain size) of the coated abrasive <NUM> is not particularly limited, but is preferably determined according to the hardness and the like of the metal sheet <NUM>, and is preferably about #<NUM> to #<NUM> for a general steel sheet as the metal sheet <NUM>.

Note that, in the punching device <NUM> having the rotational motion conversion device <NUM> described above, even when the spring <NUM> is contracted until the metal sheet <NUM> is punched out, the first spur gear part 31a meshing with the plate-shaped rack <NUM> rotates, so that the rotating part <NUM> rotates. At this time, the rotating part <NUM> rotates in a direction opposite to the rotation of the rotating part <NUM> after the metal sheet <NUM> is punched out. However, the rotation of the rotating part <NUM> rotating until the metal sheet <NUM> is punched out does not contribute to the polishing of the sheared edge <NUM> of the punched hole <NUM>, and conversely, it may shorten the life of the gear and the like of the punching device <NUM>.

To deal with this, for example, a ratchet mechanism (not illustrated) for preventing reverse rotation of the rotating part <NUM> may be provided in the first spur gear part 31a of the first gear <NUM> in the rotational motion conversion device <NUM> to suppress reverse rotation of the rotating part <NUM> until the metal sheet <NUM> is punched out.

Furthermore, the punching device <NUM> according to this embodiment preferably includes a drop preventing mechanism <NUM> as illustrated in <FIG> and <FIG>, for example. The drop preventing mechanism <NUM> includes a hole part <NUM> formed in a direction opposite to the punching direction from the distal end of the main body part <NUM>, and a drop preventing rod <NUM> provided to extend from the rear end of the punching part <NUM> toward the main body part <NUM> and inserted into the hole part <NUM>. A stopper 43a for preventing the drop preventing rod <NUM> from coming off from the hole part <NUM> is provided at the rear end of the drop preventing rod <NUM>.

As described above, according to the punching device <NUM> having the drop preventing mechanism <NUM>, in the process of punching the metal sheet <NUM>, polishing the sheared edge <NUM>, and then pulling out the punch <NUM> from the punched hole <NUM>, the stopper 43a is caught by the inlet of the hole part <NUM>, so that the punching part <NUM> can be prevented from dropping from the punch <NUM>.

An experiment having been conducted for confirming the operation and effect of the metal sheet punching device according to the present invention is described below.

In the experiment, first, a <NUM> MPa class hot rolled steel sheet (sheet thickness: <NUM>) was used as the metal sheet <NUM>, and the punched hole <NUM> was formed in the metal sheet <NUM> by the punching device <NUM> illustrated in <FIG>.

The outer diameter of the punching part <NUM> of the punching device <NUM> was set to <NUM>, and the clearance between the outer diameter of the punching part <NUM> and the inner diameter of the opening 13a of the die <NUM> was set to <NUM>%.

As the coated abrasive <NUM> provided on the outer circumferential surface of the rotating part <NUM>, coated abrasive with a yarn count of #<NUM> was used, the outer diameter of the coated abrasive <NUM> provided to the rotating part <NUM> was set to <NUM>, and the angle θ (see <FIG>) formed by the coated abrasive <NUM> and the cross-section 19a orthogonal to the central axis of the punching part <NUM> was set to <NUM>°.

Subsequently, a fatigue specimen <NUM> having the punched hole <NUM> as illustrated in <FIG> was produced from the metal sheet <NUM> having the punched hole <NUM> formed using the punching device <NUM>. Then, using a Shenck plane bending fatigue-testing device, a fatigue test was performed in which a load was repeatedly applied to the fatigue specimen <NUM> by double swing.

In the fatigue test, the time point when the torque decreased by <NUM>% at a normal stress of <NUM> MPa was determined as fatigue fracture, and the number of repetitions of load until fracture was measured. In addition, the load was set to be applied until <NUM> million times, and the fatigue test was terminated.

In the experiment, an example using the fatigue specimen <NUM> having the punched hole <NUM> formed using the punching device <NUM> was set as an invention example. Further, as a comparison target, an example in which the fatigue specimen <NUM> having the punched hole <NUM> formed using an integrated punch having the same diameter as the punching part <NUM> of the punching device <NUM> was prepared and subjected to the same fatigue test as described above was set as a conventional example. Table <NUM> illustrates the results of the fatigue test.

From Table <NUM>, in the conventional example, the fatigue specimen <NUM> was fractured at the number of repetitions of <NUM>,<NUM> times. On the other hand, in the invention example, the fatigue specimen was not fractured even after the number of repetitions of <NUM> million times, and the fatigue life was improved by <NUM> times or more as compared with the conventional example.

Claim 1:
A metal sheet punching device (<NUM>) comprising:
a die (<NUM>) including a circular opening (13a) and configured to support a metal sheet (<NUM>); and
a punch (<NUM>) configured to form a circular punched hole (<NUM>) in the metal sheet supported by the die, wherein
the punch includes:
a main body part (<NUM>);
a cylindrical punching part (<NUM>) provided at a distal end;
a rotating part (<NUM>) disposed between the main body part and the punching part so as to be rotatable about a central axis of the punching part as a rotation axis; the metal sheet punching device being characterized in that the punch further comprises:
a coated abrasive (<NUM>) provided on an outer circumferential surface of the rotating part and configured to polish a sheared edge of the punched hole;
a spring (<NUM>) disposed between the main body part and the punching part so as to be capable of contracting and extending in a punching direction, the spring being configured to
contract and accumulate a part of a punching load as elastic energy until the punching part comes into contact with and punches the metal sheet,
release the elastic energy, and
extend after the punching part punches the metal sheet; and
a rotational motion conversion device (<NUM>) configured to convert a linear motion in the punching direction of the punching part due to extension of the spring into a rotary motion of the rotating part.