Patent Publication Number: US-2021187665-A1

Title: Laser machining apparatus and laser machining method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a laser machining apparatus and a laser machining method that machine sheet metal made of stainless steel by a laser beam. 
     BACKGROUND ART 
     Laser machining apparatuses that cut sheet metal by laser beams emitted from laser oscillators, and produce products having predetermined shapes are widely used. In recent years, as the laser oscillator that emits a laser beam used in a laser machining apparatus, a fiber laser oscillator or a direct diode laser oscillator (DDL oscillator) that is compact and low cost has been more widely used as compared with a CO 2  laser oscillator that is large and high cost. 
     The wavelength of the laser beam emitted by a CO 2  laser oscillator is approximately 10 μm, while the wavelength of the laser beam emitted by a fiber laser oscillator or a DDL oscillator is approximately 1 μm. Accordingly, a beam waist of the laser beam emitted by the fiber laser oscillator or a DDL oscillator is small, and a kerf width of a groove formed in a periphery of the product by irradiation of the laser beam is narrow. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2005-279730 
       
    
     SUMMARY 
     The conventional laser machining apparatuses each using the laser oscillator that emits a laser beam in a 1 μm band cut sheet metal made of stainless steel in a defocused state where the focus point of the laser beams is located above or below the top surface of the sheet metal. In this way, it is possible to cut the sheet metal even if the plate thickness is 3 mm or more by widening the kerf width. 
     However, cutting by the conventional laser machining apparatuses and laser machining methods has the problem that the surface roughness of the cut surface is poor, much dross adheres to the cut surface, and the cut surface quality is low. A laser machining apparatus and a laser machining method that can cut sheet metal made of stainless steel by a laser beam while improving quality of a cut surface are required. 
     According to a first aspect of one or more embodiments, a laser machining apparatus is provided, the laser machining apparatus including a machining head configured to emit a laser beam for cutting sheet metal of stainless steel, a moving mechanism configured to relatively move the machining head with respect to a surface of the sheet metal, a beam vibrating mechanism configured to vibrate the laser beam in both a parallel direction with a cutting advancing direction of the sheet metal and an orthogonal direction orthogonal to the cutting advancing direction, when the sheet metal is cut by the machining head being relatively moved by the moving mechanism, and a control device configured to control the moving mechanism and the beam vibrating mechanism, wherein the control device controls the beam vibrating mechanism to vibrate the laser beam in a C-shaped vibration pattern, as a vibration pattern of the laser beam by only the beam vibrating mechanism, the vibration pattern including no movement of the laser beam by relative movement of the machining head, in which a beam spot on the surface of the sheet metal is moved from a first irradiation position at a front end in the cutting advancing direction to a second irradiation position at a rear side in the cutting advancing direction and displaced in an orthogonal direction to the cutting advancing direction, and is moved from the second irradiation position to a third irradiation position at a front end in the cutting advancing direction and displaced in an orthogonal direction to the cutting advancing direction, and movement from the first irradiation position to the third irradiation position via the second irradiation position, and movement from the third irradiation position to the first irradiation position via the second irradiation position are repeated, and performs control to cut the sheet metal by causing a beam spot in the first irradiation position, a beam spot in the second irradiation position, and a beam spot in the third irradiation position to overlap one another, when the machining head is relatively moved by the moving mechanism while the laser beam is vibrated in the C-shaped vibration pattern. 
     According to a second aspect of one or more embodiments, a laser machining method is provided, the laser machining method including emitting a laser beam for cutting sheet metal of stainless steel from a machining head, and irradiating the sheet metal, cutting the sheet metal by relatively moving the machining head by a moving mechanism with respect to a surface of the sheet metal, vibrating, by a beam vibrating mechanism, the laser beam in both a parallel direction with a cutting advancing direction of the sheet metal and an orthogonal direction orthogonal to the cutting advancing direction when the sheet metal is cut, controlling the beam vibrating mechanism to vibrate the laser beam in a C-shaped vibration pattern, as a vibration pattern of the laser beam by only the beam vibrating mechanism, the vibration pattern including no movement of the laser beam by relative movement of the machining head, in which a beam spot on the surface of the sheet metal is moved from a first irradiation position at a front end in the cutting advancing direction to a second irradiation position at a rear side in the cutting advancing direction and displaced in an orthogonal direction to the cutting advancing direction, and is moved from the second irradiation position to a third irradiation position at a front end in the cutting advancing direction and displaced in an orthogonal direction to the cutting advancing direction, and movement from the first irradiation position to the third irradiation position via the second irradiation position, and movement from the third irradiation position to the first irradiation position via the second irradiation position are repeated, and performing control to cut the sheet metal by causing a beam spot in the first irradiation position, a beam spot in the second irradiation position, and a beam spot in the third irradiation position to overlap one another, when the machining head is relatively moved by the moving mechanism while the laser beam is vibrated in the C-shaped vibration pattern. 
     According to the laser machining apparatus and the laser machining method of one or more embodiments, it is possible to cut the sheet metal made of stainless steel with good quality of a cut surface by the laser beam. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an entire configuration example of a laser machining apparatus of one or more embodiments. 
         FIG. 2  is a perspective view illustrating detailed configuration examples of a collimator unit and a machining head in the laser machining apparatus of one or more embodiments. 
         FIG. 3  is a diagram for explaining a displacement of an irradiation position of a laser beam to sheet metal by a beam vibrating mechanism. 
         FIG. 4  is a diagram illustrating a C-shaped vibration pattern of the laser beam by only the beam vibrating mechanism. 
         FIG. 5  is a diagram illustrating an actual C-shaped vibration pattern at a time of moving the machining head in a cutting advancing direction. 
         FIG. 6  is a diagram illustrating phases and frequencies of vibrations in a parallel direction and an orthogonal direction of the C-shaped vibration pattern illustrated in  FIG. 4 . 
         FIG. 7  is a characteristic diagram illustrating a preferable range of an overlap rate of beam spots in a first or third irradiation position and a second irradiation position, when a frequency in a cutting advancing direction of the C-shaped vibration pattern is 1500 Hz. 
         FIG. 8  is a characteristic diagram illustrating a preferable range of an overlap rate of beam spots in the first irradiation position and the third irradiation position, when the frequency in the cutting advancing direction of the C-shaped vibration pattern is 1500 Hz. 
         FIG. 9  is a characteristic diagram illustrating a preferable range of an overlap rate of beam spots in the first or third irradiation position and the second irradiation position, when the frequency in the cutting advancing direction of the C-shaped vibration pattern is 2000 Hz. 
         FIG. 10  is characteristic diagram illustrating a preferable range of an overlap rate of beam spots in the first irradiation position and the third irradiation position, when the frequency in the cutting advancing direction of the C-shaped vibration pattern is 2000 Hz. 
         FIG. 11  is a diagram illustrating a state where beam spots overlap in irradiation positions of two cycles that are adjacent. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, a laser machining apparatus and a laser machining method of one or more embodiments will be described with reference to the accompanying drawings. In  FIG. 1 , a laser machining apparatus  100  includes a laser oscillator  10  that generates and emits a laser beam, a laser machining unit  20 , a process fiber  12  that transmits the laser beam emitted by the laser oscillator  10  to the laser machining unit  20 . 
     Further, the laser machining apparatus  100  includes an operation section  40 , a NC device  50 , a machining program database  60 , a machining condition database  70 , and an assist gas supply device  80 . The NC device  50  is an example of a control device that controls respective parts of the laser machining apparatus  100 . 
     As the laser oscillator  10 , a laser oscillator that amplifies an excitation beam emitted from a laser diode to emit a laser beam of a predetermined wavelength, or a laser oscillator that directly uses a laser beam emitted by a laser diode is preferable. The laser oscillator  10  is, for example, a solid laser oscillator, a fiber laser oscillator, a disk laser oscillator, or a direct diode laser oscillator (DDL oscillator). 
     The laser oscillator  10  emits a laser beam in a band of 1 μm with a wavelength of 900 nm to 1100 nm. Taking a fiber laser oscillator and a DDL oscillator as examples, the fiber laser oscillator emits a laser beam with a wavelength of 1060 nm to 1080 nm, and the DDL oscillator emits a laser beam with a wavelength of 910 nm to 950 nm. 
     The laser machining unit  20  has a machining table  21  where sheet metal W to be machined is placed, a gate-type X-axis carriage  22 , a Y-axis carriage  23 , a collimator unit  30  fixed to the Y-axis carriage  23 , and a machining head  35 . The sheet metal W is made of a stainless steel, and a plate thickness is 3 mm to 15 mm, for example. 
     The X-axis carriage  22  is configured to be movable in an X-axis direction on the machining table  21 . The Y-axis carriage  23  is configured to be movable in a Y-axis direction perpendicular to the X-axis on the X-axis carriage  22 . The X-axis carriage  22  and the Y-axis carriage  23  function as a moving mechanism that moves the machining head  35  in the X-axis direction, the Y-axis direction, or an arbitrary composition direction of an X-axis and a Y-axis, along a surface of the sheet metal W. 
     Instead of moving the machining head  35  along the surface of the sheet metal W, a position of the machining head  35  may be fixed, and the sheet metal W may be configured to move. The laser machining apparatus  100  can include the moving mechanism that moves the machining head  35  relatively to the surface of the sheet metal W. 
     To the machining head  35 , a nozzle  36  that has a circular opening  36   a  at a tip end portion, and emits a laser beam from the opening  36   a  is attached. The sheet metal W is irradiated with the laser beam emitted from the opening  36   a  of the nozzle  36 . The assist gas supply device  80  supplies nitrogen to the machining head  35  as assist gas. At a time of machining the sheet metal W, the assist gas is blown to the sheet metal W from the opening  36   a.    
     As illustrated in  FIG. 2 , the collimator unit  30  includes a collimation lens  31  that converts a divergent laser beam emitted from the process fiber  12  into a parallel laser beam (collimated laser beam). Further, the collimator unit  30  includes a galvano scanner unit  32 , and a bend mirror  33  that reflects a laser beam emitted from the galvano scanner unit  32  toward a lower part in a Z-axis direction perpendicular to the X-axis and the Y-axis. The machining head  35  includes a focusing lens  34  that focuses the laser beam reflected by the bend mirror  33 , and irradiates the sheet metal W. 
     The laser machining apparatus  100  is centered so that the laser beam emitted from the opening  36   a  of the nozzle  36  is located at a center of the opening  36   a . In a regular state, the laser beam is emitted from the center of the opening  36   a . The galvano scanner unit  32  functions as a beam vibrating mechanism that vibrates the laser beam that advances in the machining head  35  and is emitted from the opening  36   a , in the opening  36   a . How the galvano scanner unit  32  vibrates the laser beam will be described later. 
     The galvano scanner unit  32  has a scanning mirror  321  that reflects the laser beam emitted from the collimation lens  31 , and a drive section  322  that rotates the scanning mirror  321  to a predetermined angle. Further, the galvano scanner unit  32  has a scanning mirror  323  that reflects the laser beam emitted from the scanning mirror  321 , and a drive section  324  that rotates the scanning mirror  323  to a predetermined angle. 
     The drive sections  322  and  324  can reciprocally vibrate the scanning mirrors  321  and  323  within a predetermined angle range respectively based on control by the NC device  50 . By reciprocally vibrating either one or both of the scanning mirror  321  and scanning mirror  323 , the galvano scanner unit  32  vibrates the laser beam with which the sheet metal W is irradiated. 
     The galvano scanner unit  32  is one example of the beam vibrating mechanism, and the beam vibrating mechanism is not limited to the galvano scanner unit  32  having a pair of scanning mirrors. 
       FIG. 3  illustrates a state where either one or both of the scanning mirror  321  and the scanning mirror  323  is or are tilted, and a position of the laser beam with which the sheet metal W is irradiated is displaced. In  FIG. 3 , a fine solid line that is bent by the bend mirror  33  and passes through the focusing lens  34  shows an optical axis of the laser beam at the time of the regular state of the laser machining apparatus  100 . 
     Note that, in detail, an angle of the optical axis of the laser beam that is incident on the bend mirror  33  changes by an operation of the galvano scanner unit  32  located in front of the bend mirror  33 , and the optical axis deviates from a center of the bend mirror  33 . In  FIG. 3 , for simplification, incident positions of the laser beams onto the bend mirror  33  are assumed to be same positions before and after the operation of the galvano scanner unit  32 . 
     The optical axis shown by the fine solid line is assumed to be displaced to a position shown by a thick solid line by the action by the galvano scanner unit  32 . When the laser beam reflected by the bend mirror  33  is assumed to incline at an angle θ, an irradiation position of the laser beam on the sheet metal W is displaced by a distance Δs. When a focal length of the focusing lens  34  is EFL (Effective Focal Length), the distance Δs is calculated by EFL×sin θ. 
     If the galvano scanner unit  32  inclines the laser beam at the angle θ in an opposite direction to a direction shown in  FIG. 3 , the irradiation position of the laser beam on the sheet metal W can be displaced by the distance Δs in an opposite direction to the direction shown in  FIG. 3 . The distance Δs is a distance less than a radius of the opening  36   a , and is preferably a distance less than or equal to a maximum distance when the maximum distance is a distance obtained by subtracting a predetermined margin from the radius of the opening  36   a.    
     The NC device  50  can vibrate the laser beam in a predetermined direction within a plane of the sheet metal W by controlling the drive sections  322  and  324  of the galvano scanner unit  32 . By vibrating the laser beam, it is possible to vibrate a beam spot formed on a surface of the sheet metal W. 
     In the laser machining apparatus  100  configured as above, the NC device  50  reads a machining program from the machining program database  60 , and selects any of a plurality of machining conditions stored in the machining condition database  70 . The NC device  50  controls the laser machining apparatus  100  to machine the sheet metal W based on the read machining program and the selected machining condition. The laser machining apparatus  100  cuts the sheet metal W by the laser beam emitted from the laser oscillator  10  and produces a product having a predetermined shape. 
     In one or more embodiments, the galvano scanner unit  32  vibrates the laser beam as illustrated in  FIG. 4 . A parallel direction with the cutting advancing direction of the sheet metal W is referred to as an x-direction, and an orthogonal direction orthogonal to the x-direction within a plane of the sheet metal W is referred to as a y-direction. A right direction in  FIG. 4  is a +x-direction, a left direction is a −x-direction, an upward direction in  FIG. 4  is a +y-direction, and a downward direction is a −y-direction.  FIG. 4  illustrates a vibration pattern in a state where the machining head  35  is not moved in the x-direction, to make it easier to understand the vibration pattern. 
     In  FIG. 4 , explaining the vibration pattern in the state where the machining head  35  is not moved in the x-direction (namely, a state that does not include movement of the laser beam by movement of the position of the machining head  35 ), the NC device  50  controls the galvano scanner unit  32  to vibrate a beam spot Bs as follows. 
     When the beam spot Bs is assumed to be located at an irradiation position P 1  (first irradiation position), the galvano scanner unit  32  moves the beam spot Bs located in the irradiation position P 1  in the −x-direction and the −y-direction to locate the beam spot Bs in an irradiation position P 2  (second irradiation position). The galvano scanner unit  32  subsequently moves the beam spot Bs located in the irradiation position P 2  in the +x-direction and the −y-direction to locate the beam spot Bs in an irradiation position P 3  (third irradiation position). 
     Further, the galvano scanner unit  32  moves the beam spot Bs located in the irradiation position P 3  in the −x-direction and the +y-direction to locate the beam spot Bs in the irradiation position P 2 , and subsequently moves the beam spot Bs in the +x-direction and the +y-direction to locate the beam spot Bs in the irradiation position P 1 . 
     In this way, the galvano scanner unit  32  alternately locates the beam spot Bs in the two irradiation positions P 1  and P 3  that are same in the x-direction but are different in the y-direction. The irradiation positions P 1  and P 3  are front end positions in the cutting advancing direction of the beam spot Bs, and are the front end positions in the vibration pattern. 
     Further, when the galvano scanner unit  32  moves the beam spot Bs from the irradiation position P 1  to the irradiation position P 3 , and moves the beam spot Bs from the irradiation position P 3  to the irradiation position P 1 , the galvano scanner unit  32  causes the beam spot Bs to pass through the irradiation position P 2  that is a position at a rear side in the cutting advancing direction from the positions in the x-direction of the irradiation positions P 1  and P 3 , and is a middle position of the positions in the y-direction of the irradiation positions P 1  and P 3 . The irradiation position P 2  is a rear end position in the vibration pattern. 
     A vibration pattern in which the laser beam is vibrated in both the x-direction and the y-direction illustrated in  FIG. 4  will be referred to as a C-shaped vibration pattern. The C-shaped vibration pattern is a vibration pattern in which the laser beam is vibrated in both the x-direction and the y-direction. 
     In reality, the laser beam is vibrated in the C-shaped vibration pattern while the machining head  35  is moving in the cutting advancing direction, and therefore, as illustrated in  FIG. 5 , the vibration pattern is a vibration pattern in which a displacement in the cutting advancing direction (x-direction) is added to the vibration pattern illustrated in  FIG. 4 . 
     In  FIG. 5 , the beam spot Bs located in the irradiation position P 1  moves in the −x-direction and the −y-direction by displacement of the irradiation position by the galvano scanner unit  32 , and moves to the irradiation position P 2  that is not shown by a two-dot chain line but is shown by a solid line because the machining head  35  moves in the −x-direction. The beam spot Bs located in the irradiation position P 2  moves in the +x-direction and the −y-direction by displacement of the irradiation position by the galvano scanner unit  32 , and moves to the irradiation position P 3  that is not shown by a two-dot chain line but is shown by a solid line because the machining head  35  moves in the −x-direction. 
     As illustrated in  FIG. 4 , when the machining head  35  is moved in the +x-direction while the laser beam is vibrated in the C-shaped vibration pattern, a groove Wk of a kerf width K 1  corresponding to a width in the y-direction of the irradiation positions P 1  and P 3  is formed in the sheet metal W. By vibrating the laser beam in the C-shaped vibration pattern, a wider kerf width can be made as compared with a case where the laser beam is not vibrated in the y-direction. 
     When the galvano scanner unit  32  moves the beam spot Bs from the irradiation position P 1  to the irradiation position P 2 , when the galvano scanner unit  32  moves the beam spot Bs from the irradiation position P 2  to the irradiation position P 3 , and when the galvano scanner unit  32  moves the beam spot Bs in an opposite direction to this, the beam spot Bs is preferably moved in a parabolic fashion. The galvano scanner unit  32  may move the beam spot Bs substantially rectilinearly between the irradiation position P 1  and the irradiation position P 2 , and between the irradiation position P 2  and the irradiation position P 3 . 
     Phases and frequencies of vibrations in the x-direction and the y-direction of the C-shaped vibration pattern illustrated in  FIG. 4  can be expressed as in  FIG. 6 . When a frequency at which the laser beam is vibrated in the x-direction is Fx, and a frequency at which the laser beam is vibrated in the y-direction is Fy, the frequency Fy is ½ of the frequency Fx as is known from  FIG. 6 . The C-shaped vibration pattern is a vibration pattern in which Fx:Fy is 2:1. The frequency Fx is preferably set at 1000 Hz or more. 
     By verification of the present inventor, it has been obvious that in order to improve the quality of the cut surface of the sheet metal W, the laser beam is preferably vibrated as follows in a state where the laser beam is displaced with movement of the machining head  35 . First, the beam spot Bs located at the front end in the cutting advancing direction in the C-shaped vibration pattern, and the beam spot Bs located at the rear side overlap each other. In other words, the beam spots Bs overlap each other in the irradiation position P 1  or P 3  and the irradiation position P 2  shown in  FIG. 5 . Second, the two beam spots Bs located at the front end in the cutting advancing direction in the C-shaped vibration pattern overlap each other. In other words, the beam spots Bs overlap each other in the irradiation position P 1  and the irradiation position P 3  shown in  FIG. 5 . 
     Although Patent Literature 1 describes vibrating the laser beam in the C-shaped vibration pattern, the beam spot Bs located at the front end and the beam spot Bs located at the rear side do not overlap each other, and the two beam spots Bs located at the front end also do not overlap each other. 
     A preferable overlap rate of the beam spots Bs will be described by using  FIG. 7  to  FIG. 10 . The overlap rate is a percentage of overlap in a distance of a diameter of the beam spot Bs in the two irradiation positions of the irradiation positions P 1  to P 3 . 
       FIG. 7  illustrates a preferable range of an overlap rate of the beam spots Bs in the irradiation position P 1  or P 3  and the irradiation position P 2  when the frequency Fx is 1500 Hz.  FIG. 8  illustrates a preferable range of an overlap rate of the two beam spots Bs located in the irradiation positions P 1  and P 3  at the front end in the cutting advancing direction when the frequency Fx is 1500 Hz. Here, the plate thickness of the sheet metal W is set at 3 mm, 4 mm, 6 mm, and 8 mm. 
       FIG. 9  illustrates a preferable range of an overlap rate of the beam spots Bs in the irradiation position P 1  or P 3  and the irradiation position P 2  when the frequency Fx is 2000 Hz.  FIG. 10  illustrates a preferable range of the overlap rate of the two beam spots Bs located in the irradiation positions P 1  and P 3  at the front end in the cutting advancing direction when the frequency Fx is 2000 Hz. Here, the plate thickness of the sheet metal W is set at 3 mm, 4 mm, and 8 mm. 
     In  FIG. 7  to  FIG. 10 , a state where a dross height of dross adhering to the cut surface of the sheet metal W is low is considered to be good quality of the cut surface, and upper limit values and lower limit values of the overlap rates at which good quality of the cut surface is obtained are shown. Specifically, the quality of the cut surface is considered to be good when average dross heights at a time of measuring dross heights at three spots in the cut surface are 20 μm or less, 40 μm or less, 100 μm or less, and 200 μm or less respectively for the plate thicknesses of 3 mm, 4 mm, 6 mm, and 8 mm. 
     When the overlap rate deviates from a range of the upper limit value and the lower limit value, the quality of the cut surface deteriorates, or the sheet metal W cannot be cut. 
     In  FIG. 7 , upper limit values (first upper limit values) of the overlap rates at the time of the plate thickness being 3 mm, 4 mm, 6 mm, and 8 mm are respectively 90%, 94%, 95%, and 95%, and lower limit values (first lower limit values) are respectively 79%, 70%, 50%, and 40%. In  FIG. 8 , upper limit values (second upper limit values) of the overlap rates at the time of the plate thickness being 3 mm, 4 mm, 6 mm, and 8 mm are respectively 96%, 95%, 96%, and 96%, and lower limit values (second lower limit values) are respectively 66%, 65%, 50%, and 40%. 
     In  FIG. 9 , upper limit values (first upper limit values) of the overlap rates at a time of the plate thickness being 3 mm, 4 mm, and 8 mm are respectively 90%, 95%, and 95%, and lower limit values (first lower limit values) are respectively 57%, 52%, and 30%. In  FIG. 10 , upper limit values (second upper limit values) of the overlap rates at a time of the plate thickness being 3 mm, 4 mm, and 8 mm are respectively 96%, 96%, and 96%, and lower limit values (second lower limit values) are respectively 66%, 62%, and 40%. 
     As is known from  FIG. 7  to  FIG. 10 , by causing the beam spots Bs to overlap in the irradiation positions P 1  to P 3 , it is possible to cut the sheet metal W of stainless steel with good quality of the cut surface by a laser beam in a band of 1 μm. 
     As is known from  FIG. 7  to  FIG. 10 , the lower limit value of the overlap rate tends to be larger as the plate thickness of the sheet metal W becomes smaller, and the lower limit value of the overlap rate tends to be smaller as the plate thickness becomes larger. The NC device  50  may control the vibration of the laser beam by the galvano scanner unit  32 , and movement of the machining head  35  by the moving mechanism so that the overlap rate is between the lower limit value and the upper limit value according to the plate thickness of the sheet metal W. 
     The machining condition database  70  preferably stores, according to each of the frequencies Fx, the first upper limit value and the first lower limit value, and the second upper limit value and the second lower limit value set for each of the plate thicknesses of the sheet metal W. The NC device  50  may determine the overlap rate by referring to the machining condition database  70 . 
     Specifically, the NC device  50  reads the first upper limit value and the first lower limit value, and the second upper limit value and the second lower limit value, according to the frequency Fx and the plate thickness of the sheet metal W to be cut, from the machining condition database  70 . The NC device  50  may select the overlap rate between the first upper limit value and the first lower limit value, select the overlap rate between the second upper limit value and the second lower limit value, and vibrate and move the laser beam at the respective selected overlap rates. 
     The first upper limit value and the first lower limit value, and the second upper limit value and the second lower limit value are preferably set for each of the plate thicknesses of the sheet metal W, according to each of the frequencies Fx. For simplification, the first upper limit value and the first lower limit value, and the second upper limit value and the second lower limit value for each of the plate thicknesses of the sheet metal W that can be used in common in each group composed of a plurality of frequencies Fx may be set. For further simplification, the first upper limit value and the first lower limit value, and the second upper limit value and the second lower limit value for each of the plate thicknesses of the sheet metal W that can be used in common at all the frequencies Fx may be set, though a selectable overlap rate is limited. 
     When the first upper limit value and the first lower limit value, and the second upper limit value and the second lower limit value are set for each of the frequencies Fx or each group of the frequencies Fx, the NC device  50  may control the vibration of the laser beam and movement of the machining head  35  as follows. The NC device  50  causes the beam spots Bs in the irradiation positions P 1  to P 3  to overlap one another at the overlap rate between the first upper limit and the first lower limit set in advance and the overlap rate between the second upper limit value and the second lower limit value set in advance, according to the frequency Fx and the plate thickness of the sheet metal W to be cut. 
     In  FIG. 5 , the beam spot Bs located in the irradiation position P 3  is located in the next irradiation position P 2  at the rear side in the cutting advancing direction by vibration of a next cycle, and thereafter is located in the next irradiation position P 1  at the front end in the cutting advancing direction. Further, by vibration of a next cycle, the beam spot Bs located in the irradiation position P 1  is located in the next irradiation position P 2  at the rear side in the cutting advancing direction, and thereafter is located in the next irradiation position P 3  at the front end in the cutting advancing direction. The beam spot Bs repeats reciprocal vibration like this. 
     In  FIG. 11 , the irradiation position P 1  at a cycle directly before the present irradiation position P 1  is set as an irradiation position P 1  (−1), and the beam spot Bs located in the irradiation position P 1  (−1) is shown by a two-dot chain line. In two consecutive cycles, the beam spots Bs in the irradiation positions P 1  to P 3  overlap. The overlap rate of the beam spots Bs in the two consecutive cycles is preferably 80% or more, and more preferably 90% or more. Here, the overlap rate is also a percentage of overlap in the distance of the diameter of the beam spot Bs. 
     As above, according to the laser machining apparatus and the laser machining method of one or more embodiments, the sheet metal W made of stainless steel can be cut with good quality of the cut surface by the laser beam. 
     The present invention is not limited to the one or more embodiments described above, and can be variously changed within the range without departing from the summary of the present invention. 
     The disclosure of this application relates to the subject described in Japanese Patent Application No. 2018-143651 filed on Jul. 31, 2018, the entire disclosed contents of which are incorporated herein by reference.