Patent Publication Number: US-2022219261-A1

Title: Groove processing device and groove processing method

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a groove processing device and a groove processing method that form a groove in an object using a laser. The present application claims priority based on Japanese Patent Application No. 2019-091043 filed on May 14, 2019, the contents of which are incorporated herein by reference. 
     RELATED ART 
     In the related art, a groove processing device is known which irradiate a surface of a steel sheet with a laser beam in a direction (scanning direction) intersecting a sheet travelling direction of the steel sheet, using a polygon mirror, to periodically form a groove in the surface of the steel sheet, thereby improving iron loss characteristics (see, for example, Patent Document 1). 
     PRIOR ART DOCUMENT 
     [Patent Document] 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2002-292484 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     As shown in  FIGS. 1A and 1B , a laser beam LB incident on a polygon mirror  10  of the groove processing device is not a point light source and has a predetermined radius φ. 
     As shown in  FIG. 1A , when the laser beam LB is incident so as to fall within one surface of the polygon mirror  10 , the laser beam LB reflected by the polygon mirror  10  is focused on one spot on the surface of the steel sheet  20  through a condensing lens (hereinafter, simply referred to as a lens)  12 , and a groove is formed at the spot on the surface of the steel sheet  20 . 
     On the other hand, as shown in  FIG. 1B , when the laser beam LB is incident on a corner portion in which two adjacent surfaces of the polygon mirror  10  meet, the laser beam LB is reflected from each of the two adjacent surfaces and is divided into two laser beams LB 1  and LB 2 . The divided laser beams LB 1  and LB 2  are focused on the surface of the steel sheet  20  through the lens  12 . As a result, an end portion of the groove in the scanning direction is processed by the laser beams LB 1  and LB 2  with insufficient energy densities. Therefore, the end portion of the groove is shallow, and it is difficult to form a uniform groove. In addition, the divided laser beams LB 1  and LB 2  are irradiated in a direction different from that of the laser beam LB. Therefore, there is a concern that a position different from the position where a groove is to be formed in the surface of the steel sheet  20  or devices and the like other than the surface of the steel sheet  20  will be erroneously processed. 
     In order to avoid this situation, a configuration is considered in which a shielding plate, such as a mask, is provided such that a portion corresponding to the end portion of the groove in the surface of the steel sheet  20  is not irradiated with the laser beams LB 1  and LB 2 . However, this configuration has a problem that the shielding plate is processed and optical components are contaminated by minute pieces of the shielding plate generated by the processing. 
     The invention has been made in view of the above-mentioned problems, and an object of the invention is to provide a groove processing device and a groove processing method that suppress the contamination of optical components and achieve uniform groove processing and groove depth. 
     Means for Solving the Problem 
     Means for solving the problems include the following aspects. 
     (1) According to an embodiment of the invention, there is provided a groove processing device that forms a groove in a surface of an object using laser beams. The groove processing device includes: a light source device that outputs the laser beams; a polygon mirror that reflects the laser beams output from the light source device; a condensing optical system that is provided on an optical path of the laser beams reflected by the polygon mirror and focuses the laser beams; and a shielding plate that is provided between the condensing optical system and the object at a position which blocks some of the laser beams focused through the condensing optical system and blocks some of the laser beams. Among the laser beams focused through the condensing optical system, some of the laser beams that are not blocked by the shielding plate form the groove in the surface of the object at a focus of the laser beams. The shielding plate is provided closer to the condensing optical system than the focus and is rotated with respect to the surface of the object so as to block the laser beams that do not form the groove. 
     (2) In the groove processing device according to (1), when an angle of the shielding plate with respect to the surface of the object is ψ and a critical angle which is a maximum angle at which the laser beam falls within one plane mirror of the polygon mirror is θc(°), the angle ψ of the shielding plate may be inclined in a range of 2θc&lt;ψ≤90(°). 
     (3) In the groove processing device according to (2), assuming that a position where a perpendicular line is drawn from a rotation axis of the polygon mirror to the plane mirror of the polygon mirror is a reference position, an angle formed between a boundary between two adjacent plane mirrors of the polygon mirror and the reference position is θ0(°), a position where the shielding plate, which is inclined at the angle ψ, is irradiated with the laser beam reflected by the polygon mirror at an angle of 2θ0(°) when a rotation angle of the polygon mirror is θ0(°) is a point P 0 , a position where the shielding plate, which is inclined at the angle ψ, is irradiated with the laser beam reflected by the polygon mirror at an angle of 2θc(°) when the rotation angle of the polygon mirror is θc(°) is a point P, a height difference between the point P and the point P 0  is Lp 0 , and a distance from the condensing optical system to a height of the point P is L 2 , Lp 0 &lt;L 2  may be satisfied. 
     (4) The groove processing device according to any one of (1) to (3) may further include: a position adjustment portion that adjusts a position of the shielding plate in a scanning direction in which scanning is performed with the laser beams by the polygon mirror. 
     (5) The groove processing device according to any one of (1) to (4) may further include: a housing that has the shielding plate disposed in a lower portion. The housing may have an upper opening portion that is located on the optical path of the laser beams focused by the condensing optical system, and a colorless and transparent window plate that transmits the laser beams without absorbing or reflecting the laser beams may be attached to the upper opening portion. 
     (6) According to an embodiment of the invention, there is provided a groove processing method that forms a groove in a surface of an object using laser beams. The groove processing method includes: an output step of outputting the laser beams from a light source device; a reflection step of reflecting the laser beams output from the light source device by a polygon mirror; a condensing step of focusing the laser beams on the surface of the object using a condensing optical system that is provided on an optical path of the laser beams reflected by the polygon mirror; and a shielding step of blocking some of the laser beams using a shielding plate that is provided between the condensing optical system and the object at a position which blocks some of the laser beams focused through the condensing optical system. Among the laser beams focused through the condensing optical system, some of the laser beams that are not blocked by the shielding plate form the groove in the surface of the object at a focus of the laser beams. In the shielding step, the shielding plate is provided closer to the condensing optical system than the focus and is rotated with respect to the surface of the object so as to block the laser beams that do not form the groove. 
     (7) In the groove processing method according to (6), in the shielding step, when an angle of the shielding plate with respect to the surface of the object is ψ and a critical angle which is a maximum angle at which the laser beam falls within one plane mirror of the polygon mirror is θc(°), the angle ψ of the shielding plate may be inclined in a range of 2θc&lt;ψ≤90(°). 
     (8) In the groove processing method according to (7), in the shielding step, assuming that a position where a perpendicular line is drawn from a rotation axis of the polygon mirror to the plane mirror of the polygon mirror is a reference position, an angle formed between a boundary between two adjacent plane mirrors of the polygon mirror and the reference position is θ0(°), a position where the shielding plate, which is inclined at the angle ψ, is irradiated with the laser beam reflected by the polygon mirror at an angle of 2θ0(°) when a rotation angle of the polygon mirror is θ0(°) is a point P 0 , a position where the shielding plate, which is inclined at the angle ψ, is irradiated with the laser beam reflected by the polygon mirror at an angle of 2θc(°) when the rotation angle of the polygon mirror is θc(°) is a point P, a height difference between the point P and the point P 0  is Lp 0 , and a distance from the condensing optical system to a height of the point P is L 2 , Lp 0 &lt;L 2  may be satisfied. 
     (9) The groove processing method according to any one of (6) to (8) may further include: a shielding plate position adjustment step of adjusting a position of the shielding plate in a scanning direction in which scanning is performed with the laser beams by the polygon mirror. 
     (10) The groove processing method according to any one of (6) to (9) may further include: a housing attachment step of attaching a colorless and transparent window plate that transmits the laser beams without absorbing or reflecting the laser beams to an upper opening portion of a housing that has the shielding plate disposed in a lower portion and has the upper opening portion which is located on the optical path of the laser beams focused by the condensing optical system. 
     Effects of the Invention 
     According to the invention, the shielding plate is inclined to reduce the damage of the shielding plate which occurs when the shielding plate blocks the laser beam. Therefore, it is possible to provide a groove processing device and a groove processing method that suppress the contamination of optical components and achieve uniform groove processing and groove depth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram showing a state in which a laser beam reflected from a polygon mirror is focused on a surface of a steel sheet when the laser beam is incident so as to fall within one surface of the polygon mirror. 
         FIG. 1B  is a schematic diagram showing a state in which the laser beam reflected from each of two adjacent surfaces is focused on the surface of the steel sheet when the laser beam is incident across the two adjacent surfaces of the polygon mirror. 
         FIG. 2  is a schematic diagram showing a configuration of a groove processing device according to an embodiment of the invention as viewed from a rolling direction of the steel sheet. 
         FIG. 3  is a schematic diagram showing a rotation angle of the polygon mirror. 
         FIG. 4  is a schematic diagram showing a configuration of a movable shielding plate device. 
         FIG. 5  is a schematic diagram showing the optimum position and angle of a shielding plate. 
         FIG. 6  is a graph showing a relationship between an angle ψ of the shielding plate and a height difference Lp 0 . 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     Hereinafter, an embodiment of the invention will be described with reference to the drawings. In the specification and the drawings, the same components are designated by the same reference numerals. 
       FIG. 2  schematically shows a configuration of a groove processing device  100  according to the embodiment of the invention as viewed from a rolling direction of a steel sheet  20 . The groove processing device  100  is a device that periodically forms a groove in a surface of the steel sheet  20 , which is an object to be processed, using a laser. The steel sheet  20  is made of, for example, a well-known grain-oriented electrical steel sheet material. In the groove processing device  100 , the position of the steel sheet  20  in a width direction is set on the basis of the length and position of the groove formed in the surface of the steel sheet  20 , and the position of the steel sheet  20  in a longitudinal direction is set on the basis of the dimensions of the groove processing device  100 . The width direction of the steel sheet  20  is a scanning direction of the laser and is a left-right direction of the plane of paper in  FIG. 2 . The longitudinal direction of the steel sheet  20  is the rolling direction of the steel sheet  20  and is a depth direction of the plane of paper in  FIG. 2 . 
     As shown in  FIG. 2 , the groove processing device  100  includes a polygon mirror  10 , a light source device  11 , a collimator  11 A, a lens  12 , and a movable shielding plate device  30 . 
     The polygon mirror  10  has, for example, a regular polygonal prism shape, and a plurality of (N) plane mirrors are provided on each of a plurality of side surfaces constituting a regular polygonal prism. A laser beam LB is incident on the polygon mirror  10  from the light source device  11  through the collimator  11 A in one direction (horizontal direction) and is then reflected by the plane mirror (reflection step). 
     The polygon mirror  10  has a configuration in which it can be rotated on a rotation axis O1 by the driving of a motor (not shown), and the incident angle of the laser beam LB on the plane mirror changes sequentially depending on the rotation angle of the polygon mirror  10 . Therefore, the polygon mirror  10  sequentially changes the reflection direction of the laser beam LB such that the steel sheet  20  is scanned with the laser beam LB in the width direction. 
     In addition,  FIGS. 1A, 1B, 2, and 3  show an example in which the polygon mirror  10  has eight plane mirrors. However, the number of plane mirrors constituting the polygon mirror  10  is not particularly limited. 
     The light source device  11  outputs a laser beam using a predetermined irradiation method (for example, a continuous irradiation method or a pulse irradiation method) under the control of a control unit (not shown) (output step). 
     The collimator  11 A is connected to the light source device  11  through an optical fiber cable  15 . The collimator  11 A adjusts the radius of the laser beam output from the light source device  11  and outputs the adjusted laser beam LB to the polygon mirror  10 . The laser beam LB output to the polygon mirror  10  has a laser diameter having a predetermined radius φ, and the laser diameter is that of a circle. However, the laser diameter may be that of an ellipse. In this case, an elliptical condensing shape can be formed by inserting a cylindrical lens or a cylindrical mirror between the collimator  11 A and the polygon mirror  10  to change the radius of the beam along one axis (for example, a scanning direction). 
     The lens  12  is a condensing optical system that is provided on the optical path of the laser beam reflected by the polygon mirror  10  and is manufactured by performing processing, such as grinding and polishing, on a piece of glass. In addition, a mirror may be adopted as the condensing optical system constituting the groove processing device  100  instead of the condensing lens  12 . 
     The lens  12  may have a non-condensing portion (not shown) that is integrally provided outside (in the outer circumference of) the lens  12 . The non-condensing portion is located on the optical paths of laser beams LB 1  and LB 2  that have been divided and reflected from a corner portion in which two adjacent plane mirrors of the polygon mirror  10  meet and transmits the divided laser beams LB 1  and LB 2 . The non-condensing portion is a planar optical system of a donut-shaped plate. The non-condensing portion does not have a focus because the focal length thereof is infinite. Since the laser beams LB 1  and LB 2  that have passed through the non-condensing portion are not focused, they do not have a high energy density. Therefore, even when a shielding plate  35  is irradiated with the laser beams LB 1  and LB 2  that have passed through the non-condensing portion, the damage of the shielding plate  35  is small. In addition, the non-condensing portion may not be the planar optical system and may be, for example, an optical system that diverges the divided laser beams LB 1  and LB 2 . 
     The movable shielding plate device  30  which will be described below is provided between the lens  12  and the steel sheet  20 . The movable shielding plate device  30  is disposed on the optical path of the laser beam LB that is reflected by the polygon mirror  10  and passes through the lens  12 . The laser beam LB reflected by the polygon mirror  10  passes through the lens  12  and the movable shielding plate device  30  and is focused on the surface of the steel sheet  20  (condensing step). Therefore, a groove is formed in the surface of the steel sheet  20 . 
     Further, in a groove processing method which irradiates the surface of the steel sheet  20  with the laser beam LB to form a groove, base steel sheet is melted and removed to form a groove. Therefore, as the groove becomes deeper, the probability that a molten protrusion will occur on the surface becomes higher. Therefore, the groove processing device  100  may be configured to include a supply nozzle (not shown) which injects an assist gas for blowing off a molten material and is provided at a predetermined position. Further, the collimator  11 A, the polygon mirror  10 , the lens  12 , and the movable shielding plate device  30  of the groove processing device  100  may be covered with a housing (not shown), and the inside of the housing may be filled with a clean gas such that the internal pressure of the housing is positive. In this case, it is possible to prevent a molten material and the like from entering the housing and to prevent the optical system of the groove processing device  100  from being contaminated by the molten material and the like. 
     Next, the rotation angle of the polygon mirror  10  will be described with reference to  FIG. 3 . In this embodiment, it is assumed that the rotation angle θ(°) of the polygon mirror  10  is defined by a central angle with respect to a reference position for each of the plane mirrors constituting the polygon mirror  10 . As shown in  FIG. 3 , it is assumed that a position where a perpendicular line PL is drawn from the rotation axis O1 of the polygon mirror  10  to a plane mirror  101  is the reference position (θ=0(°)). The rotation angle θ of the polygon mirror  10  is an angle (central angle) formed between the position of a center LBc of the laser beam LB incident on each plane mirror and the reference position (θ=0(°)). In  FIG. 3 , a counterclockwise angle from the reference position (θ=0(°)); the perpendicular line PL) is defined as a positive angle, and a clockwise angle from the reference position is defined as a negative angle. 
     An angle θ0 formed between the reference position (θ=0(°)) in each plane mirror and a boundary with an adjacent plane mirror is 180(°)/N. The rotation angle θ of one plane mirror is defined in the range of −θ0≤θ≤+θ0. Therefore, in  FIG. 3 , the rotation angle θ=+θ0 of the plane mirror  101  and the rotation angle θ=−θ0 of a plane mirror  102  adjacent to the plane mirror  101  in the counterclockwise direction indicate the same position on the polygon mirror  10 . 
     In this embodiment, the maximum angle at which the incident laser beam LB falls within one surface (one plane mirror) of the polygon mirror  10  is defined as a critical angle θc. That is, when the laser beam LB is totally reflected by one plane mirror without being divided by a corner portion in which two adjacent plane mirrors of the polygon mirror  10  meet, the critical angle θc is the maximum angle at which the center LBc of the laser beam LB is located. Assuming that the radius (circumscribed radius) of a circumscribed circle Cl of the polygon mirror  10  is R and the radius of the laser beam LB incident on the polygon mirror  10  is φ, the critical angle θc is defined by the following Expression (1). 
     
       
         
           
             
               
                 
                   
                     θ 
                     ⁢ 
                     c 
                   
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           ( 
                           
                             
                               R 
                               × 
                               sin 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θ0 
                             
                             - 
                             φ 
                           
                           ) 
                         
                         / 
                         R 
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Next, a specific configuration of the movable shielding plate device  30  will be described with reference to  FIG. 4 . As shown in  FIG. 4 , the movable shielding plate device  30  has a configuration in which it has a box-shaped housing  31  formed of, for example, a metal material and a pair of shielding plates  35  disposed so as to face each other in a scanning direction x of the laser beam LB are disposed in a lower portion of the housing  31 . In addition, the shielding plate  35  is rotated on a rotating portion  37   a  as a fulcrum, which will be described below. In  FIG. 4, 35   a  indicates the shielding plate when the shielding plate  35  is inclined by rotation. 
     In the housing  31 , an upper opening portion  31   a  is formed in an upper portion that is located on the optical path of the laser beam LB focused by the lens  12 , a lower opening portion  31   b  is formed in a lower portion, and a colorless and transparent window plate  33  is attached to the upper opening portion  31   a  (housing attachment step). The window plate  33  is, for example, a glass plate. The window plate  33  transmits the laser beam without absorbing or reflecting the laser beam. For example, the window plate  33  is obtained by coating both surfaces of a synthetic quartz glass plate with antireflection films. Therefore, the upper opening portion  31   a  can be closed by the window plate  33 , and the laser beam LB that has been reflected by the polygon mirror  10  and passed through the lens  12  can pass through the upper portion of the housing  31 . 
     Further, the laser beam LB which has passed through the window plate  33  from the lens  12  passes through the lower opening portion  31   b  of the housing  31 , and the surface of the steel sheet  20  is irradiated with the laser beam LB. When the polygon mirror  10  is rotated, the inclination angle of the laser beam LB changes depending on the rotation angle of the polygon mirror  10 . The irradiation position of the laser beam LB passing through the movable shielding plate device  30  is moved on the surface of the steel sheet  20  in the width direction of the steel sheet  20 . That is, the laser beam LB passing through the movable shielding plate device  30  is moved on the surface of the steel sheet  20  in the width direction of the steel sheet  20  as the scanning direction x. 
     In the lower opening portion  31   b  of the housing  31 , the shielding plates  35  are provided in the vicinity of opening portion ends  31   c  which are disposed so as to face each other in the scanning direction x of the laser beam LB. The shielding plate  35  is provided between the lens  12  and the steel sheet  20 . That is, the shielding plate  35  is provided closer to the lens  12  than the focus of the laser beam LB that has passed through the lens  12 . The shielding plate  35  blocks some of the laser beams focused through the lens  12  (shielding step). The pair of shielding plates  35  disposed so as to face each other in the scanning direction x of the laser beam LB have the same configuration and are formed of, for example, a steel material in a plate shape. Each of the shielding plates  35  is provided with a position adjustment portion  34  that adjusts the position of the shielding plate  35  in the scanning direction x of the laser beam LB and an angle adjustment portion  36  that adjusts the angle of a plate surface of the shielding plate  35  with respect to the surface of the steel sheet  20 . 
     In this embodiment, the position adjustment portion  34  is, for example, a guide groove that is formed in the lower opening portion  31   b  of the housing  31 , and the guide groove is formed along the scanning direction x of the laser beam LB. The position adjustment portion  34  is provided such that the shielding plate  35  can be slid in the guide groove and moves the shielding plate  35  along the guide groove in the scanning direction x of the laser beam LB (shielding plate position adjustment step). 
     As described above, the position adjustment portion  34  adjusts the position of the shielding plate  35  in the scanning direction x of the laser beam LB such that the shielding plate  35  is irradiated with the laser beam LB moved to the end portion of the steel sheet  20  in the width direction when the laser beam LB that has passed through the lens  12  is moved from the center to the end portion of the steel sheet  20  along the scanning direction x. This configuration makes it possible to adjust the range of the scanning direction x (the width direction of the steel sheet  20 ) in which the shielding plate  35  is irradiated with the laser beam LB. 
     The shielding plate  35  blocks some of the laser beams LB that are focused through the lens  12  and moved in the scanning direction x at the end in the scanning direction such that the surface of the steel sheet  20  is not irradiated with the laser beam LB at the end in the scanning direction and no grooves are formed in the surface of the steel sheet  20 . On the other hand, among the laser beams LB which are focused through the lens  12  and moved in the scanning direction x, the remaining laser beams LB which are not blocked by the shielding plate  35  converge into the focus of the laser beams LB on the surface of the steel sheet  20  to form a groove. 
     Therefore, the shielding plate  35  blocks unnecessary beams which are irradiated with positions other than a groove processing position on the steel sheet  20  among the laser beams LB which have been reflected by the plane mirror of the rotating polygon mirror  10  and have a high energy density or the laser beams LB 1  and LB 2  which have been divided in the corner portion of the polygon mirror  10  and have a low energy density. Therefore, the end portion of the groove in the scanning direction x is not shallow, and positions other than the position of the groove on the steel sheet  20  are not processed. As a result, it is possible to achieve uniform groove processing and groove depth in the steel sheet  20 . 
     In this embodiment, the angle adjustment portion  36  includes the rotating portion  37   a  that enables the shielding plate  35  to rotate with respect to the lower portion of the housing  31 , a guide portion  37   b  that defines a trajectory on which the shielding plate  35  is inclined, and a connection portion  37   c  that rotatably connects the shielding plate  35  to the guide portion  37   b . The shielding plates  35  are rotated such that the surfaces of the shielding plates  35  face each other. Therefore, the rotating portion  37   a  rotates the shielding plate  35  on a rotation axis to incline a flat sheet surface of the shielding plate  35  with respect to the surface of the steel sheet  20 . 
     In the case of this embodiment, the rotating portion  37   a  and the guide portion  37   b  are provided so as to be slidable along the scanning direction of the laser beam LB by the position adjustment portion  34  in operative association with the shielding plate  35 . That is, the rotating portion  37   a  has a configuration in which it is provided in, for example, the position adjustment portion  34  and is movable along the scanning direction of the laser beam LB by the sliding of the position adjustment portion  34 . In addition, the guide portion  37   b  is, for example, a guide groove that is provided in a part of the position adjustment portion  34  along the inner wall of the housing  31  and has a configuration in which it is movable along the scanning direction by the sliding of the position adjustment portion  34  and guides the trajectory of the connection portion  37   c  such that the shielding plate  35  having the connection portion  37   c  can be moved along the scanning direction of the laser beam LB. 
     Here, the guide portion  37   b  is, for example, an annular member that is made of a metal material or the like and has a curved elongated hole, and the connection portion  37   c  provided in the shielding plate  35  is moved along the curved elongated hole. In this case, the connection portion  37   c  is, for example, a protrusion member that is disposed in the elongated hole of the guide portion  37   b  and is moved along the elongated hole of the guide portion  37   b . The shielding plate  35  is rotated on the rotating portion  37   a  that is provided in the lower end portion of the shielding plate  35 , and the connection portion  37   c  is moved along the curved elongated hole of the guide portion  37   b  to change the inclination angle of the shielding plate  35  with respect to the surface of the steel sheet  20 . 
     The shielding plate  35  is rotated with respect to the surface of the steel sheet  20  so as to block the laser beam LB that does not form a groove. Therefore, the angle adjustment portion  36  adjusts the angle of the shielding plate  35  with respect to the steel sheet  20  to prevent the shielding plate  35  from being irradiated with the laser beam LB having a high energy density when the shielding plate  35  is irradiated with the laser beams LB. Therefore, it is possible to reduce the damage of the shielding plate  35  caused by irradiation with the laser beam LB. 
     That is, the laser beam LB has a laser diameter having a predetermined radius φ. However, the size of the laser diameter that appears on the shielding plate  35  changes due to a change in the angle of the shielding plate  35  with respect to the laser beam LB, which causes a change in the energy density of the laser beam LB irradiated with the shielding plate  35 . Therefore, it is possible to set the shielding plate  35  to an angle at which the damage of the shielding plate  35  is reduced. 
     Further, the laser beam LB is focused by the lens  12  so as to have the highest energy density on the surface of the steel sheet  20  on which the laser beam LB is focused. Therefore, an object that is at the same distance (that is, a focal length) as the distance from the lens  12  to the steel sheet  20  is irradiated with the laser beam LB having a high energy density. As the distance from the lens increases, the energy density decreases. Therefore, the shielding plate  35  that is provided closer to the lens  12  than the focus is inclined to balance the distance from the lens  12  and the change in the laser diameter, which makes it possible to obtain a suitable energy density and to reduce the damage of the shielding plate  35 . 
     Next, the position of the shielding plate  35  in the scanning direction of the laser beam LB will be described with reference to  FIG. 5 . In addition, in  FIG. 5, 10   a  indicates a portion of the plane mirror when the polygon mirror  10  is rotated. In this case, it is assumed that the focal length of the lens  12  which is the condensing optical system is f. When the polygon mirror  10  is rotated by θ(°), the laser beam LB reflected by the polygon mirror  10  is moved by 2θ(°). Then, when the rotation angle θ(°) of the polygon mirror  10  is from θc(°) to θ0(°), the laser beam LB needs to be blocked by the shielding plate  35 . 
     Here, it is assumed that a distance from the plane mirror of the polygon mirror  10  to the lens  12  which is the condensing optical system is L 1 . Further, it is assumed that a distance from the lens  12  which is the condensing optical system to the height of a point P at the position where the shielding plate  35  is irradiated with the laser beam LB reflected by the polygon mirror  10  at an angle of 2θc(°) when the rotation angle θ of the polygon mirror  10  is θc(°) is L 2 . In addition, in this embodiment, as shown in  FIG. 5 , the lower end portion of the shielding plate  35  in which the rotating portion  37   a  is provided is the point P. 
     Furthermore, it is assumed that a perpendicular line which is drawn from the polygon mirror  10  to the steel sheet  20  and through which the laser beam LB passes when the rotation angle θ of the polygon mirror  10  is 0(°) is PL 1 . Moreover, it is assumed that a straight line which horizontally extends from the point P at the position where the shielding plate  35  is irradiated with the laser beam LB reflected by the polygon mirror  10  when the rotation angle θ of the polygon mirror  10  is θc(°) to the perpendicular line PL 1  is XL 1 . Then, assuming that the point where the perpendicular line PL 1  through which the laser beam LB passes and the straight line XL 1  from the point P intersect each other is P 1 , a distance d between the point P and the point P 1  can be represented by the following Expression (2). In addition, as described above, φ indicates the radius of the laser beam LB incident on the polygon mirror  10  ( FIG. 3 ). 
     
       
         
           
             
               
                 
                   d 
                   = 
                   
                     
                       
                         ( 
                         
                           
                             L 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           + 
                           
                             L 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                         
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                       × 
                       tan 
                       ⁢ 
                       2 
                       ⁢ 
                       θ 
                       ⁢ 
                       c 
                     
                     - 
                     
                       { 
                       
                         
                           φ 
                           / 
                           f 
                         
                         × 
                         
                           
                             ( 
                             
                               f 
                               - 
                               
                                 L 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                             ) 
                           
                           / 
                           cos 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                         ⁢ 
                         θ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         c 
                       
                       } 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In this embodiment, the position adjustment portion  34  moves the shielding plate  35  in the scanning direction of the laser beam LB to adjust the distance d from the perpendicular line PL 1  through which the laser beam LB passes to the position of the shielding plate  35  to the distance d calculated by the above-mentioned Expression (2). 
     Next, an angle ψ of the shielding plate  35  with respect to the surface of the steel sheet  20  will be described with reference to  FIG. 5 . In this case, the angle ψ of the shielding plate  35  is an angle formed between the surface direction of the surface of the steel sheet  20  and the plate surface of the shielding plate  35 . Here, the energy density of the laser beam LB is highest at the point P of the shielding plate  35  when the angle ψ of the shielding plate  35  with respect to the surface of the steel sheet  20  is 2θc(°) at which the laser beam LB is vertically incident on the shielding plate  35 . 
     Therefore, it is desirable to incline the shielding plate  35  as much as possible while avoiding that the angle ψ of the shielding plate  35  is close to 2θc(°) in order to avoid the damage of the surface of the shielding plate  35  by the laser beam LB as much as possible. 
     Assuming that the energy density of the laser beam LB when the angle ψ of the shielding plate  35  is ψ=2θc is Ipc, the energy density Ipc of the laser beam LB can be represented by the following Expression (3). In addition, P indicates the laser power (W) of the laser beam LB. 
     
       
         
           
             
               
                 
                   Ipc 
                   = 
                   
                     P 
                     / 
                     
                       { 
                       
                         π 
                         × 
                         
                           
                             ( 
                             
                               
                                 φ 
                                 / 
                                 f 
                               
                               × 
                               
                                 ( 
                                 
                                   f 
                                   - 
                                   
                                     L 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             ) 
                           
                           2 
                         
                       
                       } 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The energy density Ip of the laser beam LB when the shielding plate  35  is inclined at the angle ψ can be represented by the following Expression (4). 
     
       
         
           
             
               
                 
                   Ip 
                   = 
                   
                     Ipc 
                     × 
                     
                       cos 
                       ⁡ 
                       
                         ( 
                         
                           ψ 
                           - 
                           
                             2 
                             ⁢ 
                             θ 
                             ⁢ 
                             c 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     When the angle ψ of the shielding plate  35  is from 0(°) to 2θc(°), the focusing diameter of the laser beam LB is small, and the energy density Ip of the laser beam LB is high in a portion other than the point P, which is not desirable. In addition, when the angle ψ of the shielding plate  35  is 90(°) or greater, the shielding plate  35  is inclined too much, and the influence of the laser beam LB irradiated with the side surface of an upper portion of the shielding plate  35  is large. Therefore, unexpected processing is performed, which is not desirable. 
     Therefore, it is desirable that the angle ψ of the shielding plate  35  is 2θc&lt;ψ≤90(°). In this embodiment, the angle adjustment portion  36  changes the inclination of the shielding plate  35  using the rotating portion  37   a  as the rotation axis to adjust the angle ψ of the shielding plate  35  with respect to the surface of the steel sheet  20  in the range of 2θc&lt;ψ≤90(°). 
     When the angle ψ of the shielding plate  35  is large, the lens  12  which is the condensing optical system is close to the tip of the shielding plate  35 . For this reason, it is not desirable that the angle ψ of the shielding plate  35  is too large. Therefore, it is more desirable that the angle ψ of the shielding plate  35  is 80(°) or less. 
     Next, constraint conditions will be described. Here, it is assumed that, when the shielding plate  35  is inclined at the angle ψ, the position where the shielding plate  35  is irradiated with the laser beam LB reflected by the polygon mirror  10  at an angle of 2θ0(°) when the rotation angle θ of the polygon mirror  10  is θ0(°) is a point P 0 . 
     Then, similarly, assuming that, when the shielding plate  35  is inclined at the angle ψ, the height difference between the point P at the position where the shielding plate  35  is irradiated with the laser beam LB reflected by the polygon mirror  10  at an angle of 2θc(°) when the rotation angle θ of the polygon mirror  10  is θc(°) and the point P 0  is Lp 0 , Lp 0  can be represented by the following Expression (5). 
     
       
         
           
             
               
                 
                   
                     Lp 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     0 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                         + 
                         
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                       ) 
                     
                     × 
                     tan 
                     ⁢ 
                     2 
                     ⁢ 
                     θ0 
                     × 
                     sin 
                     ⁢ 
                     
                       ψ 
                       / 
                       
                         { 
                         
                           
                             sin 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   9 
                                   ⁢ 
                                   0 
                                 
                                 + 
                                 
                                   2 
                                   ⁢ 
                                   θ0 
                                 
                                 - 
                                 ψ 
                               
                               ) 
                             
                           
                           × 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                           ⁢ 
                           θ0 
                         
                         } 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Here, the height difference Lp 0  needs to be smaller than the distance L 2  from the lens  12  which is the condensing optical system to the height of the point P. Therefore, Lp 0 &lt;L 2  needs to be satisfied. 
     In the above-mentioned configuration, in the groove processing device  100 , the shielding plate  35  provided between the lens  12  and the steel sheet  20  blocks the laser beam LB that has passed through the lens  12 , and the angle adjustment portion  36  inclines the shielding plate  35  at the angle ψ with respect to the surface of the steel sheet  20  when an end portion of a groove is formed in the surface of the steel sheet  20 . As described above, in the groove processing device  100 , the shielding plate  35  is inclined at the angle ψ to reduce the damage of the shielding plate  35  which occurs when the shielding plate  35  blocks the laser beam LB. Therefore, it is possible to achieve uniform groove processing and groove depth, without contaminating optical components, and to produce a product having excellent iron loss characteristics. 
     Further, in the groove processing device  100 , the position adjustment portion  34  adjusts the position of the shielding plate  35  in the scanning direction x in which scanning is performed with the laser beam LB by the polygon mirror  10 . Therefore, in the groove processing device  100 , when the laser beam LB that has passed through the lens  12  is moved from the center to the end portion of the steel sheet  20  along the scanning direction x, the shielding plate  35  is irradiated with the laser beam LB reflected by the plane mirror of the polygon mirror  10 . As a result, a groove that has a uniform groove depth even in an end portion can be formed in the surface of the steel sheet  20 . In addition, it is possible to adjust the range of the scanning direction x (the width direction of the steel sheet  20 ) in which the shielding plate  35  is irradiated with the laser beam LB. 
     Further, in the groove processing device  100 , when the angle iv of the shielding plate  35  is adjusted, it is desirable to set the angle ψ in the range of 2θc&lt;ψ≤90(°) while satisfying Lp 0 &lt;L 2  as a constraint condition. Furthermore, it is desirable that the most desirable angle ψ of the shielding plate  35  is in the range of 2θc&lt;ψ≤90(°) and in the angle range of the central angle ±5(°) satisfying the constraint condition of Lp 0 &lt;L 2 . As described above, the angle ψ of the shielding plate  35  is adjusted to the range of 2θc&lt;ψ≤90(°) and to the angle range of the central angle ±5(°) satisfying the constraint condition of Lp 0 &lt;L 2  to more reliably reduce the damage of the shielding plate  35  by the laser beam LB. 
     In addition, the shielding plate according to the above-described embodiment may be formed of a material that absorbs the laser beam LB. For example, a black alumite treatment or absorption lacquer coating is performed on the surface of the shielding plate in order for the shielding plate to absorb the energy of the laser beam. Further, a water channel may be provided in the shielding plate to perform indirect water cooling in order to cool the shielding plate. 
     Furthermore, in the above-described embodiment, the groove processing device  100  provided with both the position adjustment portion  34  and the angle adjustment portion  36  has been described. However, the invention is not limited thereto, and the groove processing device may be provided with only the angle adjustment portion  36 . 
     Moreover, in the above-described embodiment, the case in which the position adjustment portion  34  which is the guide groove is applied as the position adjustment portion has been described. However, the invention is not limited thereto. For example, various configurations of mechanisms may be applied as long as they can move the shielding plate  35  in the scanning direction of the laser beam LB. 
     Further, in the above-described embodiment, as the angle adjustment portion, the angle adjustment portion  36  is provided which rotates on the rotating portion  37   a  provided at the base end of the shielding plate  35  to move the shielding plate  35  along the guide portion  37   b , thereby inclining the shielding plate  35 . However, the invention is not limited thereto. For example, only the rotating portion  37   a  may be provided to rotate the shielding plate  35  such that the angle can be adjusted, or only the guide portion  37   b  may be provided to incline the shielding plate  35  such that the angle can be adjusted. 
     EXAMPLES 
     Next, examples will be described. Here, first, the laser power of the laser beam LB or the like was defined, and the angle θ0 formed between the reference position (θ=0(°)) in the plane mirror of the polygon mirror  10  and the boundary with an adjacent plane mirror was calculated. In addition, the critical angle θc was calculated from the above-mentioned Expression (1). 
     In this case, when the laser power of the laser beam LB was 1000 (W), the radius φ of the laser beam LB was 6 (mm), the number of plane mirrors N in the polygon mirror  10  was eight, and the circumscribed radius R of the polygon mirror  10  was 140 (mm), the angle θ0 was 22.5(°), and the critical angle θc was 19.9(°). 
     When the distance d between the point P and the point P 1  was calculated from the above-mentioned Expression (2) assuming that the distance L 1  from the plane mirror of the polygon mirror  10  to the lens  12  which was the condensing optical system was 50 (mm), the distance L 2  from the lens  12  to the height of the point P of the shielding plate  35  was 150 (mm), and the focal length f of the lens  12  was 200 (mm), the distance d was 164.7 (mm). 
     From the above, the position adjustment portion  34  can adjust the position of the shielding plate  35  in the scanning direction of the laser beam LB on the basis of the calculation result of the distance d. 
     Then, the height difference Lp 0  between the point P and the point P 0  when the shielding plate  35  was inclined at the angle ψ was calculated, and the relationship between the angle ψ of the shielding plate  35  and the height difference Lp 0  was investigated. The results shown in  FIG. 6  were obtained. In  FIG. 6 , the horizontal axis indicates the angle ψ(°) of the shielding plate  35 , and the vertical axis indicates the height difference Lp 0  (mm) between the point P and the point P 0  when the shielding plate  35  is inclined at the angle ψ. 
     Here, as described in the embodiment, it is desirable that the angle ψ of the shielding plate  35  with respect to the surface of the steel sheet  20  is adjusted in the range of 2θc&lt;ψ≤90(°) and that Lp 0 &lt;L 2  is satisfied as a constraint condition. Therefore, it was confirmed from  FIG. 5  that the minimum angle 2θc(°) of the angle ψ of the shielding plate  35  was about 40(°) and the maximum angle of the angle ψ of the shielding plate  35  was about 70(°) from the constraint condition. 
     In addition, it can be seen that the optimum range of the angle ψ of the shielding plate  35  is from 40(°) to 70(°) and the most desirable angle ψ is 55(°) which is the central angle. From the above, the angle adjustment portion  36  can adjust the angle ψ of the shielding plate  35  on the basis of the above-mentioned calculation results. 
     INDUSTRIAL APPLICABILITY 
     According to the invention, the shielding plate is inclined to reduce the damage of the shielding plate which occurs when the shielding plate blocks the laser beam. Therefore, it is possible to provide a groove processing device and a groove processing method that suppress the contamination of optical components and achieve uniform groove processing and groove depth. Therefore, the invention has extremely high industrial applicability. 
     BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS 
     
         
         
           
               10  Polygon mirror 
               11  Light source device 
               12  Lens 
               20  Steel sheet 
               34  Position adjustment portion 
               35  Shielding plate 
               36  Angle adjustment portion 
               100  Groove processing device 
               101 ,  102  Plane mirror 
             LB Laser beam