Patent Description:
Shearing is a process for manufacturing (e.g. cutting, blanking, punching, shaving, and trimming) metal components used for, for instance, automobiles, rail cars, building components, ships, and home electric appliances. The shearing is usually performed by pushing an upper blade toward a lower blade that is in contact with the component. At this time, the component is plastically deformed between the upper blade and the lower blade to be eventually cut. It is known that a part affected by work-hardening (work-hardened part) caused by the plastic deformation during the shearing remains on an end face of the component after being cut. When the component is, for instance, flanged in a subsequent step, the work-hardened part may sometimes be cracked.

Various proposals have thus been made in order to restrain the work-hardening on a component during the shearing process to provide a sheared surface excellent in stretch-flangeability. For instance, Patent Literature <NUM> discloses a technique for providing a sheared surface excellent in stretch-flangeability by appropriately setting a slant angle of a punching blade based on a numerical simulation. Patent Literature <NUM> discloses a technique for providing a sheared surface excellent in stretch-flangeability by gradually increasing a clearance with an increase in a distance from a dangerous region determined based on a simulation on stretch-flange cracks in subsequent step(s).

A machine according to the preamble of claim <NUM> is known from <CIT>.

An equipment according to the preamble of claim <NUM> is known from <CIT>.

However, the shearing, which inherently plastically deforms the components, inevitably entails work-hardening. Further, a fractured surface of the component, which extends along a plane connecting the upper blade and the lower blade, intersects a region in which the work-hardening is concentrated. Accordingly, even when the techniques disclosed in, for instance, the above Patent Literatures <NUM> and <NUM> are employed, the work-hardened parts still remain on the end face of the component after being cut, leaving a margin for improvement in the property of the components (e.g. stretch-flangeability).

In view of the above, an object of the invention is to provide a novel and improved shearing method, shearing machine, and shearing equipment capable of reducing influence of work-hardening on an end face of a component after being cut.

According to the above aspects of the invention, the clearance is changed to be increased during a shearing process, so that the influence of work-hardening on an end face of a component after being cut can be reduced.

An exemplary embodiment of the invention will be described below in detail with reference to the attached drawings. It should be noted that the same reference numerals will be attached to components having substantially the same structures and functions to omit duplicated explanations therefor in the specification and drawings.

<FIG> is a schematic cross section showing a shearing machine <NUM> according to an exemplary embodiment of the invention. Referring to <FIG>, the shearing machine <NUM> according to the exemplary embodiment includes a die <NUM>, a punch <NUM>, a holder <NUM>, and an actuator 5a. The die <NUM> is provided with a lower blade <NUM> to be in contact with a lower side of a plate-shaped workpiece <NUM>. The punch <NUM> is provided with an upper blade <NUM>. The punch <NUM> is driven by a motor or a hydraulic device (not shown) to be capable of movement relative to the die <NUM> in a thickness direction (i.e. approaching/separating direction of the upper blade <NUM> and the lower blade <NUM>) of the workpiece <NUM>. In accordance with the movement of the punch <NUM>, the upper blade <NUM>, which is initially located above the workpiece <NUM>, is brought into contact with an upper side of the workpiece <NUM> as illustrated, and pushed against the workpiece <NUM>. In the process for the upper blade <NUM> to be pushed against the workpiece <NUM>, a fractured surface is created between the lower blade <NUM> and the upper blade <NUM>, so that the workpiece <NUM> is cut off at a portion against which the punch <NUM> is pushed. The holder <NUM> is configured to be brought into contact with the upper side of the workpiece <NUM> so that the workpiece <NUM> is held between the holder <NUM> and the die <NUM>. The actuator 5a is, for instance, a motor or a hydraulic device. The actuator 5a is connected to the die <NUM> and configured to move the die <NUM> in a surface direction of the workpiece <NUM> (i.e. in a direction orthogonal to the thickness direction of the workpiece <NUM>).

Though it is described in the above that the upper blade <NUM> moves relative to the lower blade <NUM> in the approaching/separating direction, the lower blade <NUM> may alternatively be configured to move relative to the fixed upper blade <NUM>, or the upper blade <NUM> and the lower blade <NUM> may be configured to move relative to each other.

The upper blade <NUM> faces the lower blade <NUM> at a clearance C in the surface direction (a direction orthogonal to the approaching/separating direction of the upper blade <NUM>) of the workpiece <NUM>. The movement of the die <NUM> in the surface direction of the workpiece <NUM> caused by the actuator 5a displaces the lower blade <NUM> toward or away from the upper blade <NUM>. The clearance C is reduced when the lower blade <NUM> moves toward the upper blade <NUM>, and is enlarged when the lower blade <NUM> moves away from the upper blade <NUM>. The actuator 5a increases the clearance C depending on the movement distance of the upper blade <NUM> after the upper blade <NUM> is in contact with the upper side of the workpiece <NUM> until the fractured surface is created in the workpiece <NUM>. Specifically, the actuator 5a moves the lower blade <NUM> away from the upper blade <NUM> depending on the movement distance of the upper blade <NUM> in the thickness direction of the workpiece <NUM>. The actuator 5a thus functions as the clearance adjuster in the exemplary embodiment. It should be noted that the actuator 5a may be configured to continuously move the lower blade <NUM> away from the upper blade <NUM> depending on the movement distance of the upper blade <NUM>, or, alternatively, may be configured to move the lower blade <NUM> away from the upper blade <NUM> in a stepwise manner depending on the movement distance of the upper blade <NUM>. Further, the actuator 5a may be configured to move the lower blade <NUM> away from the upper blade <NUM> by a predetermined distance at a single timing determined in accordance with the movement distance of the upper blade <NUM> in the thickness direction.

<FIG> are schematic illustrations showing the operation of the shearing machine <NUM> shown in <FIG> and the behavior of the workpiece <NUM>.

<FIG> shows the upper blade <NUM> being in contact with the upper side of the workpiece <NUM>. At this time, the clearance C is set at c<NUM>. In the description below, a movement distance H (movement distance in the thickness direction of the workpiece <NUM>) of the upper blade <NUM> at this time is defined as <NUM>. When the upper blade <NUM> is further moved from the state shown in <FIG> to start being pushed against the workpiece <NUM>, the plastic deformation of the material and work-hardening in accordance therewith start inside the workpiece <NUM>. At this time, the work-hardened parts of the material concentrate in a region R1A along a plane connecting the lower blade <NUM> and the upper blade <NUM>.

The state shown in <FIG> corresponds to a (shear start) step of starting applying a shear force on the workpiece <NUM> with a clearance between action points in the surface direction orthogonal to the thickness direction of the workpiece <NUM> in the exemplary embodiment.

<FIG> shows the upper blade <NUM> being further moved from the state shown in <FIG> to be pushed against the workpiece <NUM>. At this time, the movement distance H of the upper blade <NUM> is defined as h<NUM>. The actuator 5a moves the die <NUM> after the state shown in <FIG>, so that the lower blade <NUM> is moved away from the upper blade <NUM> with an increase in the clearance C (from c<NUM> to c<NUM>). The work-hardening of the material inside the workpiece <NUM> occurs in a region R1B. As compared with the region R1A, the region R1B is enlarged at the lower side of the workpiece <NUM>.

<FIG> shows the upper blade <NUM> being further moved from the state shown in <FIG> to be further deeply pushed against the workpiece <NUM>. The movement distance H of the upper blade <NUM> is h<NUM>, which is larger than h<NUM>. At this time, a fractured surface <NUM> is created in the workpiece <NUM>. The actuator 5a further moves the die <NUM> after the state shown in <FIG>, so that the lower blade <NUM> is moved further away from the upper blade <NUM> with an increase in the clearance C (from c<NUM> to c<NUM>). The work-hardening of the material inside the workpiece <NUM> occurs in a region R1C. As compared with the region R1A, the region R1C is further enlarged at the lower side of the workpiece <NUM>.

The states shown in <FIG> correspond to steps subsequent to the shear start, which are a (shear end) step for applying the shear force until the fractured surface is created in the workpiece <NUM>, and a step for increasing the clearance depending on a deformation of the workpiece <NUM> in the thickness direction until the fractured surface <NUM> is created in the workpiece <NUM> in the exemplary embodiment, respectively.

As shown in <FIG>, the fractured surface <NUM> of the workpiece <NUM> intersects the region R1C in which the work-hardening occurs. Accordingly, the region affected by the work-hardening remains in the end face of the workpiece <NUM> after being cut. However, as described below, due to the increase in the clearance C during the shearing depending on the movement distance H of the upper blade <NUM> in the exemplary embodiment, the work-hardening occurs in a dispersed manner and in a wider region than in a typical arrangement. Accordingly, the end face of the workpiece <NUM> after being cut is less affected by the work-hardening in the exemplary embodiment than in a typical shearing machine.

<FIG> are schematic illustrations comparable to <FIG>, showing an operation of a typical shearing machine and a behavior of the workpiece <NUM>. In the illustrated typical shearing machine, the clearance C is fixed at c<NUM> (the same as the clearance C in <FIG>) throughout the shearing process.

In the state shown in <FIG>, the work-hardening of the material inside the workpiece <NUM> occurs in a region R2A. As in the region R1A shown in <FIG>, the region R2A is a region along a plane connecting the lower blade <NUM> and the upper blade <NUM>.

In the state shown in <FIG>, the work-hardening of the material inside the workpiece <NUM> occurs in a region R2B. It should be noted that, though the region R1B shown in <FIG> is enlarged at the lower side of the workpiece <NUM>, there is no such enlargement in the region R2B shown in <FIG>.

In the state shown in <FIG>, the work-hardening of the material inside the workpiece <NUM> occurs in a region R2C. It should be noted that, though the region R1C shown in <FIG> is further enlarged at the lower side of the workpiece <NUM>, there is no such enlargement in the region R2C shown in <FIG>. In other words, the region R2C is a relatively narrow region extending along the plane connecting the lower blade <NUM> and the upper blade <NUM>.

There is no significant difference between the end profiles of the workpiece <NUM> after being cut in the exemplary embodiment shown in <FIG> and the typical shearing machine shown in <FIG> because of the same clearance C (c2) at the creation of the fractured surface <NUM> in the workpiece <NUM>. The region affected by the work-hardening remains in the end face of the workpiece <NUM> after being cut in both of the shearing machines. However, the region R1C with the work-hardening being developed at the time of cutting in the exemplary embodiment is larger than the region R2C in the typical shearing machine. In other words, the work-hardening occurs in a wider region in a dispersed manner in the exemplary embodiment in a larger region than in a typical shearing machine. Accordingly, the end face of the workpiece <NUM> after being cut is less affected by the work-hardening in the exemplary embodiment than in a typical shearing machine.

<FIG> is a schematic cross section showing another example of a clearance adjuster according to an exemplary embodiment of the invention. Referring to <FIG>, the shearing machine <NUM> in this example includes a clearance adjuster in a form of a linear cam mechanism 5b. The linear cam mechanism 5b includes a first slant surface <NUM> formed on the punch <NUM> and a second slant surface <NUM> formed on the die <NUM>. The first slant surface <NUM> is movable in the thickness direction of the workpiece <NUM> integrally with the upper blade <NUM> formed on the punch <NUM>. The second slant surface <NUM>, which is configured to be in slidable contact with the first slant surface <NUM>, is movable in the surface direction of the workpiece <NUM> integrally with the lower blade <NUM> formed on the die <NUM>. The shearing machine <NUM> further includes a spring <NUM> configured to bias the die <NUM> toward the punch <NUM> in the surface direction of the workpiece <NUM>. The spring <NUM> serves as a clearance retainer configured to keep the clearance C when the first slant surface <NUM> is not in slidable contact with the second slant surface <NUM>.

In the above example, when the first slant surface <NUM> is brought into contact with the second slant surface <NUM> in accordance with the movement of the punch <NUM> in the thickness direction of the workpiece <NUM>, the die <NUM> moves in the surface direction of the workpiece <NUM>, so that the lower blade <NUM> starts moving away from the upper blade <NUM>. Subsequently, while the first slant surface <NUM> and the second slant surface are in slidable contact with each other, the punch <NUM> continues movement in the thickness direction of the workpiece <NUM>, in accordance with which the lower blade <NUM> continuously moves away from the upper blade <NUM>. Thus, the linear cam mechanism 5b continuously moves the lower blade <NUM> away from the upper blade <NUM> depending on the movement distance of the upper blade <NUM> in the above example.

The use of the clearance adjuster in a form of the linear cam mechanism 5b, which can change the clearance C using a drive force of the punch <NUM>, allows, for instance, simplification of the equipment and increase in the process speed. In contrast, the use of the clearance adjuster in a form of the above-described actuator 5a, which can change the clearance C independently of the drive of the punch <NUM>, allows, for instance, adjustment of the change in the clearance C (e.g. change amount, change rate, start and end points of the change) as desired.

<FIG> are schematic cross sections showing still another example of the clearance adjuster according to an exemplary embodiment of the invention.

In the example explained with reference to <FIG>, the clearance C is changed by forcibly moving the highly rigid die <NUM> provided with the lower blade <NUM> in a direction for changing the clearance by the linear cam mechanism 5b.

In contrast, as illustrated in <FIG>, the die <NUM> of the shearing machine <NUM> according to this example includes a die body 2a and an elastic body 5c, the lower blade <NUM> of the die body 2a with low rigidity being supported by the elastic body 5c to control a flexure of the lower blade <NUM> in a direction for changing the clearance ("clearance-changing direction").

The elastic body 5c supports the die body 2a in a manner allowing a movement of the die body 2a in the clearance-changing direction.

The die body 2a has a thin portion at an upper part (including the lower blade <NUM>) whose thickness in the clearance-changing direction is smaller than other portions in the top-bottom direction.

The elastic body 5c at least supports the thin portion of the die body 2a when the punch <NUM> is in contact with the workpiece <NUM>.

The die body 2a and the elastic body 5c are not necessarily bonded but may be separated when the shear force is not applied. The thickness of the thin portion may be different in the top-bottom direction.

It is only necessary for the elastic body 5c to be continuously extended in a direction for the lower blade <NUM> to be extended.

The dimensions in the top-bottom direction and in the clearance-changing direction of the elastic body 5c are determined based on, for instance, the location of the holder <NUM> and an elastic modulus of the elastic body 5c.

The elastic body 5c is attached to a wall (not shown) or the like at a side opposite the die body 2a. The upper part of the lower blade <NUM>, which receives a pushing force on the workpiece <NUM> in the surface direction, is elastically deformed in the clearance-changing direction. The elastic body 5c applies an elastic force, which resists the pushing force received by the lower blade <NUM> in a direction orthogonal to the surface direction of the workpiece <NUM>, to the die body 2a including the lower blade <NUM>. Thus, in accordance with the gradual increase in the pushing force in the surface direction of the workpiece <NUM> applied on the lower blade <NUM>, the elastically deformed upper part of the die body 2a gradually moves in the direction of the pushing force in the surface direction of the workpiece <NUM> while being supported by the elastically deformed elastic body 5c.

It should be noted that the die body 2a does not necessarily have the thin portion. In this case, an entirety of the die body 2a is attached in a manner movable along the surface direction of the workpiece <NUM> and the elastic body 5c supports the die <NUM> in a manner allowing the movement of the die <NUM>. In accordance with the gradual increase in the pushing force in the surface direction of the workpiece <NUM> applied on the lower blade <NUM>, the entirety of the die body 2a moves in the surface direction of the workpiece <NUM>, so that the lower blade <NUM> gradually moves in the direction of the pushing force in the surface direction of the workpiece <NUM>.

In the shearing machine <NUM>, the pushing force in the surface direction of the workpiece <NUM> (specifically, the pushing force in a direction for the lower blade <NUM> to be away from the upper blade <NUM>) is sometimes applied to the die <NUM> including the lower blade <NUM> in accordance with the movement of the punch <NUM> including the upper blade <NUM> in the thickness direction of the workpiece <NUM> after the punch <NUM> contacts the upper side of the workpiece <NUM>. The pushing force gradually increases in accordance with the movement of the upper blade <NUM>. In this case, the above-described elastic force applied by the elastic body 5c to the die body 2a for resisting the pushing force allows the clearance C to be continuously increased depending on the movement distance of the upper blade <NUM>.

It should be noted that, in this example, an initial thickness of the elastic body 5c (i.e. a thickness without the workpiece <NUM> placed thereon or a thickness when the upper blade <NUM> is not in contact with the workpiece <NUM>) and the properties of the elastic body 5c are determined based on an initial value c<NUM> of the clearance C. Further, the properties (e.g. elastic modulus (Young's modulus)) of the elastic body 5c are determined in accordance with an appropriate increase rate of the clearance C to the movement distance H of the upper blade <NUM>.

A mechanical elastic member using a coil spring or an air cushion may be used in place of the elastic body 5c. For instance, the mechanical elastic member may include a support provided on a back side of the lower blade <NUM> and configured to support the lower blade <NUM>, and a cam mechanism and/or a link mechanism driven in conjunction with the movement of the support, where the cam mechanism and/or the link mechanism is configured to, when being driven by a predetermined amount, apply a compression force on the coil spring or the air cushion to apply the elastic force resisting the pushing force of the lower blade <NUM> to the die body 2a.

<FIG> is a schematic cross section showing a further example of the clearance adjuster according to an exemplary embodiment of the invention. Referring to <FIG>, the punch <NUM> in this example stands by at a position shown in solid lines in <FIG>. When the punch <NUM> is at the position represented by the solid lines in <FIG>, the upper blade <NUM> of the punch <NUM> overlaps the region of the die <NUM> as viewed in a normal direction of the workpiece <NUM>.

The punch <NUM> is configured to retract with respect to the die <NUM> at any one of timings of: prior to a start of descending; simultaneously with descending; or after a start of descending, to be positioned as shown in chain double-dashed lines in <FIG> when the punch <NUM> reaches the surface of the workpiece <NUM>. At the position represented by the chain double-dashed lines in <FIG>, the punch <NUM> is in contact with the upper side of the workpiece <NUM> at the clearance C = c<NUM>.

In other words, it is only necessary for the punch <NUM>, which may be at any standby position, to move from the standby position to a position capable of retaining the clearance C = c<NUM> between the punch <NUM> and the die <NUM> before or simultaneously with the timing for the punch <NUM> to reach the surface of the workpiece <NUM>.

It should be noted that, though the punch <NUM> is illustrated to be moved relative to the die <NUM> in <FIG>, the die <NUM> and the punch <NUM> may be moved in any manner as long as there occurs a relative movement between the die <NUM> and the punch <NUM>. For instance, the die <NUM> may be moved relative to the punch <NUM>, or both of the die <NUM> and the punch <NUM> may be moved.

<FIG> is a schematic cross section showing a further example of the clearance adjustment according to an exemplary embodiment of the invention. Referring to <FIG>, shearing equipment <NUM> in this example includes a transfer device <NUM> and shearing machines 1A to 1C, which are arranged along the transfer device <NUM> and configured to share the shearing process of the workpiece <NUM>. It should be noted that each of the shearing machines 1A, 1B, and 1C does not have the clearance adjuster in this exemplary embodiment.

The transfer device <NUM>, which includes a robot arm, a belt conveyor and the like, is configured to transfer the workpiece <NUM> from an upstream to a downstream (i.e. from the shearing machine 1A to the shearing machine 1C in this order).

The clearance C in the shearing machine 1A is set at c<NUM> (C =c<NUM>). The clearance C in the shearing machine 1B is set at c<NUM> (C = c<NUM>), which is larger than the clearance in the shearing machine 1A. The clearance C in the shearing machine 1C is set at c<NUM> (C = c<NUM>), which is larger than the clearance in the shearing machine 1B.

The shearing machine 1A applies a pushing force on the workpiece <NUM> with the clearance C = c<NUM> and a pushing amount H (i.e. the movement distance of the upper blade <NUM> and/or the lower blade <NUM> in the thickness direction of the workpiece <NUM>) = <NUM>. When a work-hardened region R1A as shown in <FIG> is developed in this state, the workpiece <NUM> is transferred to the shearing machine 1B using the transfer device <NUM>, the shearing machine 1B shearing the workpiece <NUM> with the clearance C = c<NUM> and pushing amount H = h<NUM> as shown in <FIG>. Finally, the workpiece <NUM> is transferred to the shearing machine 1C using the transfer device <NUM>, the shearing machine 1C shearing the workpiece <NUM> with the clearance C = c<NUM> and pushing amount H = h<NUM> as shown in <FIG>. In other words, the plurality of shearing machines 1A, 1B, 1C in this example are arranged in an ascending order of the clearances from the upstream to the downstream of the transfer path.

Though the shearing equipment in the above example includes the three shearing machines 1A, 1B, 1C, the shearing equipment may alternatively include two shearing machines or four or more shearing machines. When there are three or more shearing machines, other part(s) of the workpiece <NUM> may be sheared while the clearance C of the shearing machine 1A is kept at C=c<NUM>, the clearance C of the shearing machine 1B is kept at C=c<NUM>, and the clearance C of the shearing machine 1C is kept at C=c<NUM>.

With the use of the above-described plurality of shearing machines 1A to 1C, the step for starting applying the shear force on the workpiece <NUM> with a clearance between the action points in the surface direction orthogonal to the thickness direction of the workpiece <NUM>, the step for applying the shear force after the start of applying the shear force until the fractured surface <NUM> is created in the workpiece <NUM>, and the step for increasing the clearance depending on a deformation of the workpiece <NUM> in the thickness direction until the fractured surface <NUM> is created in the workpiece <NUM> can be performed. Accordingly, the same effect(s) and advantage(s) as described above can be obtained.

It should be noted that the clearance adjuster according to an exemplary embodiment of the invention is not limited to the above examples. For instance, the clearance adjuster may be multiple dies <NUM> exchangeable during the shearing process. In this case, the multiple dies <NUM>, which correspond to different clearances C, are sequentially exchanged depending on the movement distance H of the upper blade <NUM> to change the clearance C in a stepwise manner. Alternatively, two types of the dies <NUM> each corresponding to an initial value of the clearance C (the clearance c<NUM> shown in <FIG>) and an end value (the clearance c<NUM> shown in <FIG>) may be prepared, which may be exchanged at one timing determined depending on the movement distance H of the upper blade <NUM> to increase the clearance C.

The exemplary embodiment(s) of the invention has been described above. It should be noted that the structure of the shearing machine shown in the cross section of <FIG> is not necessarily common to all of the shearing machines. Specifically, the shearing machine according to the exemplary embodiment may be provided with the above-described clearance adjuster (e.g. the actuator 5a, the linear cam mechanism 5b) only at a part of a shearing portion and the other part of the shearing portion may be provided with no clearance adjuster (and, consequently, with fixed clearance). More specifically, the clearance adjuster may be provided, for instance, only at a curved portion at which the stretch-flange cracks are likely.

Next, Examples of the invention will be described below. It should be noted that the workpiece was a steel sheet with a tensile strength of <NUM> MPa and a plate thickness of <NUM> in all of Examples described below. The shearing process was punching using a <NUM>-mm-diameter punch. Twelve dies with varying hole inner diameter from <NUM> to <NUM> in <NUM> increments were prepared and sequentially exchanged for shearing in accordance with the procedures explained below with reference to <FIG> and <FIG>.

<FIG> is a graph for explaining Examples of the invention in which the clearance is continuously increased. The graph in <FIG> shows a relationship between the clearance C and the movement distance H of the upper blade in Examples <NUM> to <NUM>. It should be noted that the clearance C is described with reference to a ratio to the plate thickness t (C/t) in the description of Examples below. In Examples, the increase in the clearance C from <NUM> to <NUM> in <NUM> increments by exchanging the dies results in the increase in C/t from <NUM>% to <NUM>% in <NUM>% increments. It should also be noted that the movement distance H is described with reference to a ratio to a reference movement distance H_ref (H/H_ref) in the description of Examples below. The reference movement distance H_ref herein refers to the movement distance H at which the fractured surface is created when the same workpiece as used in Examples is sheared with the clearance being fixed at a maximum value (C/t = <NUM>%). The reference movement distance H_ref, which is measured in a test conducted in advance, is used as an index common to Examples for controlling the clearance C.

In the illustrated Examples, the twelve dies were sequentially exchanged while the H/H_ref varied from <NUM> to a predetermined value (<NUM>% in Example <NUM>, <NUM>% in Example <NUM>, <NUM>% in Example <NUM>, <NUM>% in Example <NUM>, <NUM>% in Example <NUM>, <NUM>% in Example <NUM>, <NUM>% in Example <NUM>), thereby quasi-continuously increasing the C/t from <NUM>% to <NUM>%. After H/H_ref reached the predetermined value, the movement of the upper blade was continued with C/t being kept at <NUM>% until the fractured surface was created on the workpiece.

<FIG> is a graph for explaining Examples of the invention in which the clearance is increased in a stepwise manner. The graph in <FIG> shows a relationship between the clearance C (C/t) and the movement distance H (H/H_ref) of the upper blade in each of Examples <NUM> and <NUM>. It should be noted that the graph of Example <NUM> is the same as that shown in <FIG>. In Example <NUM>, the dies were sequentially exchanged in the substantially same range of the movement distance H as in Example <NUM> to increase C/t from <NUM>% to <NUM>%. However, Example <NUM> used only four of the twelve dies to change the clearance C in <NUM> increments (i.e. in <NUM>% increments of C/t) in a stepwise manner.

<FIG> are photographs showing end profiles of the workpiece after being cut in Example and Comparative of the invention, respectively. <FIG> shows the end profile of the workpiece after being cut in Example <NUM> shown in <FIG> above. <FIG> shows an end profile of the workpiece after being cut in Comparative (Comparative <NUM>), in which the same workpiece as in Examples was sheared with the clearance fixed at the maximum value (C/t = <NUM>%). As shown in these photographs, there is no significant difference between the end profiles of the workpiece after being cut in Example <NUM> and that in Comparative <NUM>. There is also no significant difference between end profiles of the workpiece after being cut in Examples <NUM> to <NUM> and <NUM> and that in Comparative <NUM>. Example <NUM> only shows slight change in the end profile as compared with Comparative <NUM>, which will be described later.

<FIG> is a graph showing an average Vicker's hardness of an end face of the workpiece after being cut in Examples and Comparative of the invention. The hardness in each of Examples and Comparative was measured by: cutting the cut workpiece in a direction intersecting the end face; and conducting Vicker's hardness test (JIS Z <NUM>) at <NUM> points arranged in the thickness direction of the workpiece and remote from the end face by <NUM> microns. It should be noted that the points near the end face of the workpiece observed in <FIG> are the measured points in the Vicker's hardness test. The graph in <FIG> shows an average (Hv_ave) of values at all the measured points of the measurements in the Vicker's hardness test in each of Examples and Comparative.

As shown in the graph, it can be understood that the values Hv_ave in all of Examples <NUM> to <NUM> and <NUM> are lower than that in Comparative <NUM>, showing the reduction in influence of work-hardening at the end face of the workpiece after being cut in each of Examples. Meanwhile, when comparing Examples, the value of Hv_ave becomes especially small when the clearance C continuously increases and the increase in the clearance C occurs when the movement distance H is in a range from <NUM>% to <NUM>% of H_ref (Examples <NUM> to <NUM>). It is believed that the hardness in Example <NUM> is slightly greater than other Examples because there is a little difference in the end profile between Example <NUM>, and Comparative <NUM> and other Examples, as described above.

<FIG> is a graph showing hole expandability of the workpiece after being cut in Examples and Comparative of the invention. In Examples and Comparative, the hole expandability was measured in a hole-expansion test (JIS Z <NUM>) conducted on the workpiece punched using a <NUM>-mm-diameter punch as described above. The graph in <FIG> shows the hole expandability (λ) measured in the hole-expansion test in each of Examples and Comparative.

As shown in the graph, it can be understood that the values λ in all of Examples <NUM> to <NUM> and <NUM> are larger than in Comparative <NUM>, showing the improvement in the hole expandability of the workpiece after being cut in each of Examples. Meanwhile, when comparing Examples, the value of λ becomes especially large when the clearance C continuously increases and the increase in the clearance C occurs when the movement distance H is in a range from <NUM>% to <NUM>% of H_ref (Examples <NUM> to <NUM>). It is believed that the hole expandability in Example <NUM> is slightly lower than other Examples because there is a little difference in the end profile between Example <NUM>, and Comparative <NUM> and other Examples, as described above.

<FIG> is a graph for explaining other Examples of the invention in which the clearance is increased in a stepwise manner. The graph in <FIG> shows a relationship between the clearance C (C/t) and the movement distance H (H/Href) of the upper blade in Examples <NUM> and <NUM>. The clearance C was increased in a stepwise manner in Example <NUM>, where the dies were exchanged so that C/t increased from the initial value of <NUM>% in increments of <NUM>% when H/Href was <NUM>%, <NUM>%, and <NUM>%. Thus, C/t increases in four stages from the initial value of <NUM>% to the maximum value of <NUM>%. The clearance C was increased in a stepwise manner in Example <NUM> as in Example <NUM>, where the dies were exchanged when H/Href was <NUM>%, <NUM>%, and <NUM>%, thereby more gently increasing C/t.

<FIG> is a graph showing an opening ratio measured in a side bend test in Examples and Comparative shown in <FIG>. The side bend test is described in detail in "<NPL>. <FIG> shows opening ratios measured in the side bend test in Examples <NUM> and <NUM> shown in <FIG>, and Comparative <NUM>, in which the workpiece was sheared with the clearance fixed at the maximum value (C/t = <NUM>%). As shown in the graph, it can be understood that the opening ratios in both of Examples <NUM> to <NUM> are larger than that in Comparative <NUM>, showing the improvement in the stretch-flangeability in each of Examples. Meanwhile, comparison between Examples <NUM> and <NUM> shows that the opening ratio is larger in Example <NUM>, in which C/t was more gently increased.

Among the above-described Examples <NUM> to <NUM>, though Examples <NUM> and <NUM>, in which the clearance C was increased in the movement distance H range exceeding <NUM>% of H_ref, showed improvement in the measurements in the Vicker's hardness test and hole expandability, the improvement was slightly less than in Examples <NUM> to <NUM>. In contrast, comparison between Examples <NUM> and <NUM> shows that Example <NUM>, in which the increase in the clearance C was stopped at H/H_ref = <NUM>%, exhibited larger improvement in the opening ratio than in Example <NUM>, in which the increase in the clearance C was stopped at H/H_ref = <NUM>%. The above results show that, depending on the properties desired for the workpiece after shearing, the step of increasing the clearance C is not necessarily performed in the range of the movement distance H equal to or less than <NUM>% of H_ref, but the step of increasing the clearance C sometimes should be performed in the range the movement distance H exceeding <NUM>% of H_ref.

<FIG> is a graph for explaining Examples of the invention in which the clearance is increased at a single timing determined depending on the relative movement distance. The graph in <FIG> shows a relationship between the clearance C (C/t) and the movement distance H (H/H_ref) of the upper blade in Comparative <NUM> and Examples <NUM>, <NUM>, and <NUM>.

In Comparative <NUM>, the die was fixed so that C/t was increased to the maximum value (<NUM>%) at an initial stage.

In Example <NUM>, the dies were exchanged so that C/t was increased to the maximum value (<NUM>%) when H/Href = <NUM>%.

In Example <NUM>, the dies were exchanged so that C/t was increased from <NUM>% to <NUM>% when H/Href = <NUM>%.

Further, in Example <NUM>, the dies were exchanged so that C/t was increased from <NUM>% to <NUM>% when H/H_ref = <NUM>%.

<FIG> is a graph showing an opening ratio measured in a side bend test in Examples <NUM> to <NUM> and Comparative <NUM> shown in <FIG>. <FIG> shows an opening ratio measured in a side bend test in Examples <NUM> to <NUM> and Comparative <NUM> shown in <FIG>. As shown in <FIG>, the opening ratio, which is as low as <NUM>% in Comparative <NUM>, gradually increases as H/H_ref (marking the die exchange timing) is increased, and significantly improves in Example <NUM>.

It is speculated that the opening ratio is improved in the side bend test shown in Examples <NUM> to <NUM> for the following reasons.

Initially, as shown in <FIG>, the shearing on the workpiece <NUM> is started using the upper blade <NUM> of the punch <NUM> at the clearance C = c<NUM>, where a work-hardened region R3A is developed between the upper blade <NUM> and the lower blade <NUM>. When the upper blade <NUM> is lowered in this state, a work-hardened region R3B in the workpiece <NUM> enlarges as shown in <FIG>.

When the upper blade <NUM> is further lowered, the most work-hardened region R3C appears as shown in <FIG> (immediately before the creation of the fractured surface). At this time, when the lower blade <NUM> of the die <NUM> is retracted, the fractured surface <NUM> connecting the upper blade <NUM> and the lower blade <NUM> reaches a non-work-hardened region beyond the work-hardened region R3C as shown in <FIG>.

Thus, since the work-hardened region is hardly present at the end of the workpiece <NUM>, the worked workpiece <NUM> has a region without being influenced by the work-hardening on the fractured surface, thereby enlarging the opening ratio.

The above results show that, in order to increase the clearance C depending on the movement distance H to reduce the influence of the work-hardening on an end face of a component after being cut, it is not only effective to increase the clearance C continuously or in a stepwise manner depending on the movement distance H, but also effective to increase the clearance C at a single timing determined depending on the movement distance H. However, since sufficient effect as in Examples <NUM> and <NUM> sometimes cannot be obtained depending on the timing for increasing the clearance C, the timing suitable for the movement distance H should be determined in advance through test or the like.

It should be noted that, as can be understood from <FIG>, the opening ratio is increased as the die exchanging timing H/H_ref is as close as the timing immediately before the fractured surface <NUM> is created. Accordingly, it is speculated that the clearance C should most preferably be increased at a timing immediately before the fractured surface <NUM> is created.

The above-described Examples show that the invention is effective in reducing the influence of the work-hardening at the end face of the workpiece after being cut and in improving processability (e.g. hole expandability) in subsequent steps.

Though the step for increasing the clearance C is started when the movement distance H is <NUM> (i.e. immediately after the upper blade is brought into contact with the upper side of the workpiece) in the above-described Examples, the step may be started when the movement distance H reaches a predetermined value that is larger than <NUM> (i.e. when the upper blade is pushed against the workpiece to some degree).

Though the workpiece in a form of a steel sheet with a tensile strength of <NUM> MPa is sheared in the above Examples, the inventors have found that the invention is more effectively appliable on a steel sheet with a relatively high strength (i.e. having a tensile strength of <NUM> MPa or more). This is because the behavior of the workpiece described in the above exemplary embodiment of the invention, which is on the premise that ductile fracture cracks occur during the shearing process, is not likely to occur in a steel sheet with low strength because of its excellent local deformability. The inventors have found that the invention is effectively applicable to a high-strength steel sheet with, for instance, a tensile strength of <NUM> MPa or more, in which the ductile fracture cracks stably occur during the shearing process.

Though the workpiece in a form of a <NUM>-mm thick steel sheet is sheared in the above Examples, the inventors have found that the invention is more effectively applied on a workpiece in a form of a steel sheet with a thickness ranging from <NUM> to <NUM>. This is because, when the plate thickness is excessively small, the clearance C becomes small in accordance therewith, so that it becomes difficult to stably control the clearance C by the clearance adjuster. Meanwhile, when the plate thickness is excessively large, the end profile often changes in accordance with the change in the clearance C, so that the effect for reducing the influence of work-hardening at the end surface of the workpiece is not easily obtained. It should be noted that the invention is effectively applicable to a workpiece whose plate thickness ranges from <NUM> to <NUM>, or from <NUM> to <NUM> depending on the material of the workpiece and the shape of the sheared portion. Further, the workpiece of the invention is not necessarily a steel sheet but may be other metal plate (e.g. aluminum alloy plate).

Claim 1:
A shearing method for applying a shear force on a plate-shaped workpiece (<NUM>) in a single direction in a thickness direction to cut the workpiece (<NUM>), the method comprising:
starting application of the shear force on the workpiece (<NUM>) with a clearance (C) between action points in a surface direction orthogonal to the thickness direction of the workpiece (<NUM>);
applying the shear force after starting applying the shear force until a fractured surface (<NUM>) is created on the workpiece (<NUM>); and
increasing the clearance (C) depending on the relative movement distance of the action points in the thickness direction of the workpiece (<NUM>) after starting applying the shear force until the fractured surface (<NUM>) is created on the workpiece (<NUM>),
wherein C/t, a ratio of the clearance (C) to a thickness (t) of the workpiece (<NUM>), is equal to or greater than <NUM>% and is equal to or less than <NUM>%.