Abstract:
A method of laser machining a small hole with high machining precision in a machined object. The method includes the steps of emitting a laser beam with a fixed optical axis onto a machined object while the machined object is rotated. When the optical axis of the laser beam is fixed in place, the edges of the small hole to be machined are irradiated and the small hole becomes essentially circular even if the cross-sectional shape at the focus of the laser beam is not circular. When the small hole is formed completely through the machined object, a plume is suctioned for removal from a portion of the machined object on a side opposite from the machined hole.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an improvement in a method of laser machining or forming a small hole in a machined object. 
       BACKGROUND OF THE INVENTION 
       [0002]    A method of machining small holes in which pilot holes are formed in a workpiece using a laser beam and are then finished by electrical discharge machining is disclosed in Japanese Patent Application Laying-Open Publication No. 2001-150248 (JP 2001-150248 A). Referring to  FIG. 7  hereof, the disclosed machining method will be described below. 
         [0003]    As shown in  FIG. 7 , a laser machining head  101  moves directly above a position for machining a hole in a workpiece  102 , a laser beam  103  is emitted from the laser machining head  101  onto the workpiece  102 , and a pilot hole is formed in the workpiece  102  in a machining fluid. 
         [0004]    When small holes are opened using the laser beam  103 , the cross-section of the laser beam  103  in focal position may not be circular, and the precision of the internal diameter and the roundness of the opened small holes may be poor even if the laser beam  103  is focused. There is a method of rotating the laser beam  103  using a beam rotator, a Galvano mirror, or the like in order to increase machining accuracy, but this affects the shape of the focus cross-section described above, making it difficult to greatly increase machining accuracy. 
         [0005]    In particular, a thermal modification layer in which the structure in the base material is changed is readily formed in the periphery of the hole which has been machined in a state in which the laser beam  103  is stationary because the energy density of the laser beam  103  is high. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore an object of the present invention to increase the accuracy of machining of a small hole and to make a thermal modification layer less likely to occur. 
         [0007]    According to the present invention, there is provided a method of laser machining a small hole in a machined object by emitting a laser beam onto the object, which method comprising the steps of: rotating the machined object; emitting the laser beam onto the rotated machined object with an optical axis of the laser beam fixed in place; and suctioning, after the small hole is formed, a plume from a portion of the machined object on a side opposite from a machined part of the object. 
         [0008]    In this arrangement, since the same portion of the cross-sectional shape stationary focus constantly strikes the edge of the small hole to be opened in the rotating machined object, the shape of the small holes will be circular or nearly circular when the optical axis of the laser beam is fixed while the machined object is rotated, even if the cross section at the focus of the laser beam is not circular. For this reason, circular machining accuracy is substantially achieved without being affected by the cross-sectional shape at the focus of the laser beam. 
         [0009]    Since plumes are removed from the reverse side of the machined object after a small hole has been formed, laser light is not obstructed, absorbed, or scattered by plumes, and the laser beam makes constant contact with the rotating machined object. 
         [0010]    Preferably, the machined object is irradiated with the laser beam in an inert gas atmosphere. 
         [0011]    In a preferred form, the plume generated during machining of the machined object is removed by a suction tube disposed in the vicinity of an opening of a hole being machined. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a side elevational view showing a fuel injection valve to be machined by a machining method according to the present invention; 
           [0014]      FIG. 2  is an enlarged cross-sectional view showing an injection port formed at a distal end of a nozzle of the fuel injection valve shown in  FIG. 1 ; 
           [0015]      FIG. 3  is a cross-sectional view showing a laser machining apparatus for implementing the laser machining method according to the present invention; 
           [0016]      FIGS. 4A to 4C  are schematic views illustrating laser machining performed, in accordance with the present invention, with an optical axis of a laser beam fixed in place and the nozzle body being rotated; 
           [0017]      FIG. 5  is a cross-sectional view showing removal suctioning of plumes generated during the laser machining; 
           [0018]      FIGS. 6A and 6B  are views illustrating a relationship between the machined hole and the laser beam in conventional laser machining and in the inventive laser machining; and 
           [0019]      FIG. 7  is a schematic view illustrating a conventional small hole laser machining method. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Reference is now made to  FIGS. 1 and 2  showing a fuel injection valve and nozzle with a small hole opened using the laser machining method according to the present invention. 
         [0021]    The fuel injection valve  10  shown in  FIG. 1  is composed of a nozzle holder  11  and a nozzle  12  that is held at the distal end of the nozzle holder  11 . Fuel is taken in through the inlet  15 . 
         [0022]    As shown in  FIG. 2 , the nozzle  12  is a hole-type nozzle and is composed of a nozzle body  21  and a nozzle needle  22  that opens and closes the fuel channel of the nozzle body  21 . 
         [0023]    The nozzle body  21  has a distal end portion  21   a  that protrudes downward. The tip  21   a  has a plurality of injection ports  25  that inject fuel. These injection ports  25  are opened using the method of laser machining a small hole according to the present invention. 
         [0024]      FIG. 3  shows a laser machining apparatus  30  used when the method of the present invention is implemented. 
         [0025]    The laser machining apparatus  30  comprises a laser oscillator  31 , a machining head  32  provided on the lower portion of the laser oscillator  31  for emitting a laser beam, and a workpiece support part  33  disposed below the machining head  32 . 
         [0026]    The workpiece support part  33  is composed of a base portion  35 , a rotating portion  37  rotatably supported by bearings  36  and  36  on the base portion  35 , a workpiece holding portion  38  provided on an upper portion of the rotating portion  37 , and a drive motor  45  for rotatingly driving the rotating portion  37  via a belt  44 . 
         [0027]    The rotating portion  37  has a rotating cylindrical member  53  that is supported by the bearings  36  and  36 . The rotating cylindrical member  53  is mounted on the bearings  36  and  36  by using a collar  56  and a nut  57 . 
         [0028]    A drive pulley  61  is mounted on a rotating shaft  45   a  of the drive motor  45 . A driven pulley  62  is mounted on the lower portion of the rotating cylindrical member  53 . The belt  44  is wound around the drive pulley  61  and the driven pulley  62 . 
         [0029]    The workpiece holding member  38  includes a holding base  41  mounted on the rotating cylindrical member  53 . The workpiece-holding main body  43  is rotatably mounted on the holding base  41  by way of bearings  42  and  42 . A collar  47  and a nut  48  prevent the bearings  42  and  42  from separating from the workpiece-holding main body  43 . An annular member  41 A is mounted on the holding base  41  in order to support one of seals  49  and  49  positioned on the two sides of the bearings  42  and  42 . 
         [0030]    The workpiece holding member  38  also includes an extension member  51  that extends from the holding base  41  to the distal end side of the workpiece-holding main body  43  in order to support one end of the workpiece-holding main body  43 . Driven gear  52  which receives driving power from the driving apparatus (not shown) that rotates the workpiece-holding main body  43  is mounted on the distal end of the workpiece holding device  43 . A workpiece rotation angle indexing mechanism  58  positions the workpiece holding device  43  at each prescribed rotational angle. 
         [0031]    The positioning pin  59  stops the rotation of the nozzle body  21  with respect to the workpiece-holding main body  43  when the nozzle body  21  as the workpiece is supported by the workpiece-holding main body  43 . 
         [0032]    The workpiece-holding main body  43  has a pathway  43   a  that passes through to a pathway  21   b  inside the nozzle body  21 , and also has a pathway  43   b  that is orthogonal to the pathway  43   a . Pathway  43   b  is in communication with a pathway  51   a  formed inside the extension member  51 . This pathway  51  is in communication with a hollow portion  53   a  formed in the rotating cylindrical member  53  by way of the pathway  41   a  formed in the holding base  41 . 
         [0033]    The pathways  21   b ,  43   a ,  43   b ,  51   a , and  41   a , and the hollow portion  53   a  constitute a plume suction pathway  65  for suctioning plumes generated during laser machining (i.e., the ionized mixed gas or metallic vapors produced when the nozzle body  21  evaporates due to the heat). 
         [0034]    A workpiece rotation angle indexing mechanism  58  is composed of a plurality of concavities  43   d  formed at prescribed angles in the circumferential direction on the external peripheral surface of a large-diameter portion  43   c  disposed on the workpiece-holding main body  43 ; a case  66  provided to the holding base  41  so as to face the external peripheral surface of the large-diameter portion  43   c ; a plurality of balls  67  that are disposed inside the case  66  and that can be fitted into the plurality of concavities  43   d , respectively; and springs  68  disposed inside the case  66  in order to press each of the balls  67  into each concavity  43   d.    
         [0035]    The circumferential interval (angle) of adjacent concavities  43   d  and  43   d  conforms to the angle in the circumferential direction in which the plurality of injection ports  25  ( FIG. 2 ) opened in the distal end portion  21   a  of the nozzle body  21  are adjacent to each other. 
         [0036]      FIGS. 4A through 4C  show a state in which the laser beam  71  is fixed in place while the nozzle body  21  is rotated as machining is performed. 
         [0037]    In  FIG. 4 , the laser beam  71  is emitted from the machining head  32  in an inert gas atmosphere and is irradiated on the distal end portion  21   a  of the nozzle body  21 . Injection ports  25  to be formed are shown using alternate long and two short dashes lines. 
         [0038]    In  FIG. 4B , the optical axis of the laser beam  71  shown in  FIG. 3A  is fixed in place during laser machining, and the nozzle body  21  is rotated in the direction of the arrow by the workpiece support part  33  ( FIG. 2 ) while laser machining is performed 
         [0039]    In  FIG. 4C , the nozzle body  21  is furthermore rotated in the direction of the arrow. The nozzle body  21  is rotated at a constant speed in a fixed direction during laser machining. 
         [0040]      FIG. 5  shows a state in which plumes  73  generated during machining are removed. 
         [0041]    Since plumes  73  generated during laser machining may obstruct, absorb, or scatter the laser beam  71 , plumes  73  are suctioned and removed through a suction tube  76  disposed in the vicinity of an opening of a hole  75  being machined. 
         [0042]    When the hole  75  is formed completely through a workpiece, plumes are suctioned and removed from the nozzle body  21  through the plume suction pathway  65  shown in  FIG. 2 . 
         [0043]    Suctioning off the plumes  73  in this manner prevents the machining performed by the laser beam  71  from being intermittent, continuous constant machining to be performed is a steady manner, and machining precision to be improved because the laser beam  71  is not obstructed, absorbed, or scattered by the plumes  73 . 
         [0044]      FIGS. 6A and 6B  show the relationship between the hole and the laser beam in conventional laser machining and in the present invention. 
         [0045]    In the conventional example shown in  FIG. 6A , the nozzle body  21  is fixed in place and the laser beam  71  is rotated by a beam rotator or the like. 
         [0046]    Since the cross-sectional shape at the focus of laser beam  71  is not circular, the shape of the laser beam  71  that makes contact with the edge of the hole  75  constantly varies when the laser beam  71  is rotated. For this reason, the shape of the hole  75  will not be circular. In other words, the shape of the hole  75  is affected by the cross-sectional shape at the focus of the laser beam  71 . In the example shown in the diagram, the hole  75  has a shape that is nearly elliptical. 
         [0047]    In the embodiment of  FIG. 6 , the optical axis of the laser beam  71  is fixed in place and the nozzle body  21  rotates in the direction of the arrow. 
         [0048]    Fixing the optical axis of the laser beam  71  in place and rotating the nozzle body  21  in this manner allows the hole  75  to be circular or nearly circular because the cross-sectional shape at the focus of the laser beam  71  that makes contact with the edge of the hole  75  is the same all the time. Therefore, machining accuracy is improved so that the injection ports  25  ( FIG. 2 ) are essentially circular in shape. 
         [0049]    Obviously, various minor changes and modifications of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.