Abstract:
A fluting method for forming a flute in a workpiece while the workpiece is moved relative to a rotating tool by the combination of a rotating feed shaft with other feed shafts, the method comprising a machining process of relatively moving the rotating tool on a tool path that runs along the direction in which the flute extends and machining the workpiece using a rotating tool with a diameter that is smaller than the width of the flute. The machining process changes the relative position of the rotating tool with respect to the workpiece and performs machining multiple times so that the rotating tool makes internal contact with a circle that has the width of the flute being formed in the workpiece as the diameter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase patent application of PCT/JP2012/066776, filed on Jun. 29, 2012, which is hereby incorporated by reference in the present disclosure in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a groove-forming method, a control device of a machine tool and a tool path generating device. 
       BACKGROUND OF THE INVENTION 
       [0003]    Known in the prior art have been machine tools which make a rotary tool rotate to cut or otherwise machine a workpiece. Further, in such machine tools, there has been known a numerical control type of machine tool which designates a path of a rotary tool by coordinates of predetermined axes etc. and machines the workpiece while making the rotary tool automatically move. 
         [0004]    In a machine tool, it is possible to perform a groove-forming operation which forms a groove part in the surface of a workpiece. In a groove-forming operation, it is possible to use an end mill or other rotary tool to cut a workpiece while changing the relative positions of the workpiece and the rotary tool so as to form a groove part which extends in a desired direction. 
         [0005]    Japanese Patent Publication No. 11-347823A (PLT 1) discloses a cutting method which forms a deep groove by reciprocating motion of an end mill wherein the direction and magnitude of the cutting resistance are made constant in the tool feed direction. In this formation of a groove, it is possible to make the cutting resistance equal in the outbound path and the return path by setting the depths of cut different from each other. It is disclosed that by this machining method, it is possible to prevent bending or tilting of the groove or other deterioration in the shape precision. 
         [0006]    PLT 1: Japanese Patent Publication No. 11-347823A 
       SUMMARY OF THE INVENTION 
       [0007]    When producing a cylindrical cam etc. comprised of a columnar shaped member in the peripheral direction surface of which a groove part is formed, it is possible to make the rotary tool rotate in position relative to the workpiece so as to form the groove part. That is, it is possible to perform relative motion for rotation about a predetermined axis of rotation to thereby form a groove part in the circumferential direction surface. 
         [0008]    In this regard, when forming a large width groove part in the surface of a workpiece, it is possible to use a large-sized rotary tool which has the same diameter as the groove width. By making the large-sized rotary tool move along the direction of extension of the groove part, it is possible to form a groove part of a desired width. However, in this case, there is the problem that when the rotary tool becomes worn, fine adjustment of the groove width is not possible. Further, there is the problem that to change the groove width, it is necessary to prepare a rotary tool which has the same diameter as the desired groove width. 
         [0009]    On the other hand, when forming a large width groove part, it is possible to use a dedicated tool head (eccentric holder) for making the rotary tool engage in planetary rotation motion. However, a dedicated tool head which performs planetary rotation motion is smaller in rigidity, so cannot increase the depth of cut. For this reason, it is necessary to set the feed rate of the tool head small. Further, the rotary tool revolves while rotating, so the time when the surface being cut is not contacted becomes longer. For these reasons, when using a tool head which engages in planetary rotation motion, there is the problem that the machining time becomes longer. Further, when forming a groove part of cylindrical cam or other groove part, there was the problem that the machining precision was low and precise machining by the desired groove width was not possible 
         [0010]    The groove-forming method of the present invention is a groove-forming method which combines a rotational feed axis and other feed axes to make a rotary tool and a workpiece relatively move while forming a groove part in the workpiece, comprising a machining step of using a rotary tool which has a diameter smaller than the width of the groove part and making the rotary tool relatively move by a tool path along a direction of extension of the groove part to machine the workpiece. The machining step performs machining a plurality of times while changing a relative position of the rotary tool with respect to the workpiece so that the rotary tool is inscribed inside a circle which has the same diameter as the width of the groove part which is formed at the workpiece. 
         [0011]    In the above invention, the machining step sets an inclination of the rotary tool so that a center axis of the circle which has the same diameter as the width of the groove part and a center axis of the rotary tool become parallel. 
         [0012]    In the above invention, the machining step can include an outbound and return movement step which makes the rotary tool reciprocate along a direction of extension of the groove part, one of the side surfaces of the groove part can be machined at an outbound path of the outbound and return movement step and the other side surface of the groove part can be machined while maintaining a rotation direction of rotation of the rotary tool the same as the rotation direction of the outbound path at a return path of the outbound and return movement step. 
         [0013]    In the above invention, the range of inscription inside the circle which has the same diameter as the width of the groove part can include a range of machining the side surface of the groove part and a range of not machining the side surface of the groove part, and the machining step can perform machining a plurality of times while changing the relative position of the rotary tool in the range of machining the side surface of the groove part. 
         [0014]    In the above invention, when making a direction vertical to the direction of extension of the groove part a reference direction, the method can perform machining a plurality of times while changing the relative position of the rotary tool with respect to the workpiece at the inside of a range of a predetermined angle with respect to the reference direction. 
         [0015]    The control device of a machine tool of the present invention is a control device of a machine tool which combines a rotational feed axis and other feed axes to make a rotary tool which has a diameter smaller than a width of the groove part and a workpiece relatively move while forming a groove part in the workpiece, comprising an input information reading part which reads input information for forming the groove part by a virtual rotary tool which has the same diameter as the width of the groove part and a path setting part which uses the input information as the basis to set a tool path for making the rotary tool relatively move in a direction of extension of the groove part. The path setting part sets a tool path for performing machining a plurality of times while changing a relative position of the rotary tool with respect to the workpiece so that the rotary tool is inscribed inside a circle which has the same diameter as the width of the groove part which is formed at the workpiece. 
         [0016]    In the above invention, the path setting part can include a virtual advance direction setting part which uses the input information as the basis to set a virtual direction of advance when using the virtual rotary tool to form the groove part, a range setting part which uses the virtual direction of advance to set a range of placement of the rotary tool, and a position setting part which sets a plurality of positions for placement of the rotary tool at the inside of the range of placement of the rotary tool. 
         [0017]    In the above invention, the range of inscription inside the circle which has the same diameter as the width of the groove part can include a range of machining the side surface of the groove part and a range of not machining the side surface of the groove part, and the position setting part can set the position of the rotary tool in the range of machining the side surface of the groove part. 
         [0018]    In the above invention, the range setting part can set a direction vertical to the virtual direction of advance as a reference direction and can set a range of a predetermined angle with respect to the reference direction to set a range of placement of the rotary tool. 
         [0019]    The tool path generating device of the present invention is a tool path generating device which combines a rotational feed axis and other feed axes to generate a tool path for making a rotary tool which has a diameter smaller than the width of the groove part and a workpiece relatively move while forming a groove part in the workpiece, comprising a shape data reading part which reads shape data of the workpiece and a path setting part which uses the shape data as the basis to set a tool path for making the rotary tool relatively move in a direction of extension of the groove part. The path setting part sets a tool path for performing machining a plurality of times while changing a relative position of the rotary tool with respect to the workpiece so that the rotary tool is inscribed inside a circle which has the same diameter as the width of the groove part which is formed at the workpiece. 
         [0020]    According to the present invention, it is possible to provide a groove-forming method, a control device of a machine tool, and a tool path generating device which enables precise machining in a short time when forming a groove part in a workpiece. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic view of a numerical control type of machine tool in an embodiment. 
           [0022]      FIG. 2  is a schematic view of a workpiece on which a groove is formed and a groove part in an embodiment. 
           [0023]      FIG. 3  is a schematic view of a workpiece and a groove part when using a rotary tool of a comparative example to form a groove. 
           [0024]      FIG. 4  is a view which explains the track of the center axis of the rotary tool when using a rotary tool of a comparative example to form a groove. 
           [0025]      FIG. 5  is a schematic view which explains a virtual direction of advance and a part which forms the side surfaces of the groove part when using a rotary tool of a comparative example to form a groove. 
           [0026]      FIG. 6  is a schematic cross-sectional view when using a rotary tool of an embodiment to form a groove. 
           [0027]      FIG. 7  is a schematic view which explains machining of a first outbound path and machining of a first return path in the groove-forming method of an embodiment. 
           [0028]      FIG. 8  is a schematic view which explains machining of a second outbound path and machining of a second return path in the groove-forming method of an embodiment. 
           [0029]      FIG. 9  is a schematic view which explains machining of a third outbound path and machining of a third return path in the groove-forming method of an embodiment. 
           [0030]      FIG. 10  is a schematic cross-sectional view which explains machining of a first outbound path in the groove-forming method of an embodiment. 
           [0031]      FIG. 11  is a schematic cross-sectional view which explains machining of a first return path in the groove-forming method of an embodiment. 
           [0032]      FIG. 12  machining a schematic cross-sectional view which explains machining of a second outbound path in the groove-forming method of an embodiment. 
           [0033]      FIG. 13  is a schematic cross-sectional view which explains machining of a second return path in the groove-forming method of an embodiment. 
           [0034]      FIG. 14  is a schematic cross-sectional view which explains machining of a third outbound path in the groove-forming method of an embodiment. 
           [0035]      FIG. 15  is a schematic cross-sectional view which explains machining of a third return path in the groove-forming method of an embodiment. 
           [0036]      FIG. 16  is a schematic view of a machining system which forms a workpiece in an embodiment. 
           [0037]      FIG. 17  is a flow chart which explains control by a control device of a machine tool in an embodiment. 
           [0038]      FIG. 18  is a schematic cross-sectional view which explains a virtual direction of advance of a rotary tool of a comparative example. 
           [0039]      FIG. 19  is a schematic view which explains a virtual direction of advance of a rotary tool and a range of placement of a rotary tool in control by a control device of a machine tool of an embodiment. 
           [0040]      FIG. 20  is a schematic view which explains a scallop height. 
           [0041]      FIG. 21  is a schematic view of another machining system in an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    Referring to  FIG. 1  to  FIG. 21 , a groove-forming method and a control device of a machine tool and tool path generating device in an embodiment will be explained. As the machine tool of the present embodiment, a horizontal type machining center with a spindle which extends in the horizontal direction will be explained as an example. 
         [0043]      FIG. 1  is a schematic view of a numerical control type of machine tool in the present embodiment. A machine tool  10  is provided with a bed  12  which is set on the floor of a factory etc. On the top surface of the bed  12 , a Z-axis guide rail  28  is fastened. The Z-axis in the present embodiment extends in the horizontal direction. The Z-axis guide rail  28  is arranged so as to extend in the Z-axis direction (left-right direction in  FIG. 1 ). 
         [0044]    The machine tool  10  in the present embodiment is provided with a movement device which makes the rotary tool  22  and the workpiece  1  move relatively. At the top surface of the Z-axis guide rail  28 , a table  14  is arranged. The table  14  is arranged to be able to slide with respect to the Z-axis guide rail  28 . The table  14  therefore moves along the Z-axis. On the top surface of the table  14 , a numerical control type of rotary table  42  is arranged for making the workpiece  1  rotate in the B-axis direction. On the top surface of the rotary table  42 , a workpiece  1  is fastened through a workpiece-use holding member  40 . 
         [0045]    At the top surface of the bed  12 , an X-axis guide rail  36  is fastened. The X-axis in the present embodiment perpendicularly intersects the Z-axis and further extends in the horizontal direction (direction vertical to paper surface of  FIG. 1 ). The X-axis guide rail  36  is formed so as to extend along the X-axis. At the X-axis guide rail  36 , a column  16  is arranged to be able to slide. The column  16  moves along the X-axis. 
         [0046]    At the column  16 , a Y-axis guide rail  34  is fastened to the front surface facing the workpiece  1 . The Y-axis in the present embodiment extends along a direction perpendicular to the X-axis and Z-axis. The Y-axis guide rail  34  extends along the Y-axis. On the Y-axis guide rail  34 , a spindle head  18  is arranged. The spindle head  18  is formed to be able to slide on the Y-axis guide rail  34 . The spindle head  18  moves along the Y-axis. The spindle head  18  is formed so as to rotatably support a spindle  20 . 
         [0047]    The movement device of the present embodiment includes a Z-axis movement device which makes the rotary tool  22  move relative to the workpiece  1  in the Z-axis direction. In the present embodiment, at the inside of the bed  12 , at the downside of the table  14 , a Z-axis feed screw  24  is arranged. The Z-axis feed screw  24  extends in the Z-axis direction. At the bottom surface of the table  14 , a nut  26  is fastened. The nut  26  screws over the Z-axis feed screw  24 . At one end part of the Z-axis feed screw  24 , a Z-axis servo motor  25  is connected. By driving the Z-axis servo motor  25  to make the Z-axis feed screw  24  rotate, the nut  26  moves in the Z-axis direction. The table  14  moves together with movement of the nut  26  along the Z-axis guide rail  28 . As a result, the workpiece  1  moves in the Z-axis direction. 
         [0048]    The machine tool  10  of the present embodiment is provided with an X-axis movement device which makes the rotary tool  22  move relative to the workpiece  1  in the X-axis direction. The X-axis movement device, in the same way as the Z-axis movement device, includes an X-axis feed screw which is arranged at the inside of the bed  12  at the downside of the column  16 . The X-axis feed screw is formed so as to extend in the X-axis direction. At the bottom surface of the column  16 , a nut  37  which is screwed over the X-axis feed screw is fastened. At one end of the X-axis feed screw, an X-axis servo motor  38  is coupled. By driving the X-axis servo motor  38  and making the X-axis feed screw rotate, the nut  37  moves in the X-axis direction. The column  16  moves together with movement of the nut  37  along the X-axis guide rail  36 . As a result, the rotary tool  22  moves in the X-axis direction. 
         [0049]    The machine tool  10  of the present embodiment is provided with a Y-axis movement device which makes the rotary tool  22  move relative to the workpiece  1  in the Y-axis direction. At the inside of the column  16 , a Y-axis feed screw  32  is arranged. The Y-axis feed screw  32  is formed so as to extend in the Y-axis direction. At the back surface of the spindle head  18 , a nut  30  which is screwed over the Y-axis feed screw  32  is fastened. At the top end of the Y-axis feed screw  32 , a Y-axis servo motor  31  is connected. By driving the Y-axis servo motor  31  and making the Y-axis feed screw  32  rotate, the nut  30  moves in the Y-axis direction. The spindle head  18  moves together with the movement of the nut  30  along the Y-axis guide rail  34 . As a result, the rotary tool  22  moves in the Y-axis direction. 
         [0050]    The machine tool  10  of the present embodiment is provided with a B-axis movement device which makes the rotary tool  22  move relative to the workpiece  1  in the B-axis direction. The rotary table  42  includes a B-axis servo motor  43  for making the workpiece  1  rotate. By the drive operation of the B-axis servo motor  43 , the workpiece  1  rotates in the B-axis direction. 
         [0051]    The rotary tool  22  is arranged at the front end of the spindle  20 . In the present embodiment, as the rotary tool  22 , an end mill is attached. At the spindle  20 , a motor  23  is connected for making the rotary tool  22  rotate. By the drive operation of the motor  23 , the rotary tool  22  rotates about the center axis of the spindle as the axis of rotation. 
         [0052]    The machine tool  10  in the present embodiment has linear feed axes (X-axis, Y-axis, and Z-axis) and a rotational feed axis (B-axis). It is possible to make the rotary tool  22  rotate while making the column  16 , spindle head  18 , and table  14  operate in the X-axis, Y-axis, and Z-axis directions so as to cut the workpiece  1  which is fastened to the table  14  to a desired shape. Furthermore, it is possible to drive the rotary table  42  so as to make the workpiece  1  rotate about the B-axis. The machine tool  10  in the present embodiment functions as a four-axis machine tool which has a B-axis. 
         [0053]    In the present embodiment, the machine tool  10  which is shown in  FIG. 1  is used to perform the groove-forming method to form a groove part in the surface of the workpiece  1 . 
         [0054]      FIG. 2  is a schematic plan view of a workpiece  1  in the present embodiment. In the present embodiment, a groove part  66  is formed at the peripheral direction surface of a columnar shaped workpiece  1 . The groove part  66  extends in a spiral shape at the surface of the workpiece  1 . By a groove-forming operation which forms such a groove part  66 , for example, it is possible to produce a cylindrical cam etc. 
         [0055]    Referring to  FIG. 1  and  FIG. 2 , when forming a groove part  66  in the workpiece  1 , the workpiece  1  is fastened to the rotary table  42  so that the center axis  1   a  of the workpiece  1  becomes parallel to the Y-axis. Further, the workpiece  1  is fastened so that the center axis  1   a  matches the axis of rotation of the rotary table  42 . In the machining of the workpiece  1 , the workpiece  1  is made to rotate in the B-axis direction in addition to movement in the X-axis direction and the Y-axis direction. 
         [0056]    In the groove-forming method of the present embodiment, the rotary tool  22  is used as a first rotary tool which has a diameter smaller than the width of the groove part  66  for cutting. As the rotary tool  22 , for example, an end mill can be used. The groove part  66  in the present embodiment has a substantially square cross-sectional shape and has one side surface  66   a  and the other side surface  66   b.    
         [0057]    The groove part  66  in the present embodiment is formed so that the depth becomes constant. Further, the groove part  66  is formed so that the width also becomes constant. When forming such a groove part  66 , control is performed to change the X-axis position, Y-axis position, and B-axis position of the rotary tool  22  along the surface of the workpiece  1  without changing the relative position in the depth direction (Z-axis position) at the time of cutting. 
         [0058]    The groove-forming method of the present embodiment includes an outbound and return movement step which makes the rotary tool  22  move reciprocating along the shape of the groove part  66 . At the outbound path of the outbound and return movement step, one side surface  66   a  of the groove part  66  is processed. As shown by the arrow  95 , the rotary tool  22  is made to move relatively in the direction of extension of the groove part  66  so as to machine one side surface  66   a  of the groove part  66 . In the machine tool  10  of the present embodiment, as shown by the arrow  92 , the workpiece  1  is made to move in the Y-axis direction while, as shown by the arrow  91 , the workpiece  1  is made to rotate about the center axis  1   a  to thereby make the workpiece  1  and the rotary tool  22  move relative to each other. When the rotary tool  22  reaches the predetermined end part of the groove part  66 , the rotary tool  22  is moved in the X-axis direction and the Y-axis direction to arrange it at the position of the return path. After this, the direction of relative movement is changed to perform machining along the return path. 
         [0059]    At the return path of the outbound and return movement step, the other side surface  66   b  of the groove part  66  is machined. The rotary tool  22  is made to move relatively in the direction of extension of the groove part  66  to form the groove part  66 . In the present embodiment, as shown by the arrow  94 , the workpiece  1  is made to move in the X-axis direction and the Y-axis direction while the workpiece  1  is made to rotate as shown by the arrow  93 , whereby relative movement of the workpiece  1  and the rotary tool  22  is performed. 
         [0060]    In this regard, if using a rotary tool  22  which has a diameter smaller than the width of the groove part  66  to machine the side surfaces  66   a ,  66   b  of the groove part  66  one time, it is not possible to form a groove part  66  of the desired shape. Uncut parts will remain at parts of the side surfaces  66   a ,  66   b  of the groove part  66  in the depth direction. Here, the remaining uncut parts in the case of using a rotary tool  22  which has a diameter smaller than the width of the groove part  66  to machine the side surfaces of the groove part  66  will be explained. 
         [0061]      FIG. 3  is a schematic plan view of a workpiece  1  when using a rotary tool of a comparative example. A rotary tool  81  of the comparative example used as a second rotary tool has a diameter the same as the width of the groove part  66 . When using the rotary tool  81 , as shown by the arrow  95 , it is possible to make the rotary tool  81  move relatively one time along the direction of extension of the groove part  66  so as to form the groove part  66 . In the machine tool  10  of the present embodiment, it is possible to make the workpiece  1  move in the Y-axis direction as shown by the arrow  92  while making the workpiece  1  rotate about the center axis  1   a  as shown by the arrow  91  to thereby form the groove part  66 . When using the rotary tool  81  of the comparative example, it is possible to form one side surface  66   a  and the other side surface  66   b  by a single machining operation. 
         [0062]      FIG. 4  is a schematic view which explains the locus of the center axis  81   a  of the rotary tool  81  when using the rotary tool  81  of the comparative example. In the example which is shown in  FIG. 4 , the rotary tool  81  is arranged so that the center axis  81   a  of the rotary tool  81  becomes parallel to the radial direction of the workpiece  1 . That is, the center axis  1   a  of the workpiece  1  is arranged on an extension of the center axis  81   a.    
         [0063]    The rotary tool  81 , as shown by the arrow  98 , rotates about the center axis  81   a . The rotary tool  81  is inserted at one end part at the inside of the workpiece  1 . The development view  71  is a view which lays open the peripheral direction surface of the workpiece  1  as shown by the arrow  96 . At the peripheral direction surface of the workpiece  1 , a locus  71   a  through which the center axis  81   a  passed is shown. Further, the rotary tool  81  has a point on the center axis  81   a  at its front end, that is, the tool front end point. The development view  72  is a view which lays open the circumferential direction surface of the workpiece  1  through which the tool front end point passes as shown by the arrow  97 . At the development view  72 , the locus  72   a  of the tool front end point is shown. 
         [0064]    If comparing the locus  71   a  of the point at the surface of the workpiece  1  on the tool center axis  81   a  and the locus  72   a  of the tool front end point, it will be understood that the locuses differ in shape from each other. The radii of rotation when the workpiece  1  rotates with respect to the rotary tool  81  differ from each other, so the locuses of these points differ. For this reason, the relative advancing directions of the rotary tool  81  with respect to the workpiece  1  differ in the depth direction of the groove part  66 . 
         [0065]      FIG. 5  is a schematic view which explains a direction of movement of the center axis  81   a  when making a rotary tool  81  of a comparative example move with respect to a workpiece  1 . The arrow  101  shows the virtual direction of advance at a predetermined point of the rotary tool  81  in the depth direction. The “virtual direction of advance” is the imaginary direction of advance when assuming the workpiece  1  is made to stop and the rotary tool moves. It is learned that the virtual direction of advance changes in the direction of extension of the center axis  81   a . That is, it is learned that the virtual direction of advance changes in the depth direction of the groove part  66 . 
         [0066]    The arrow  102  shows the direction vertical to the directions of the arrows  101 . The points of intersection of the arrow  102  and the surface of the rotary tool  81  become contact locations  81   b . The contact locations  81   b  are parts which contact the workpiece  1  and form the side surfaces  66   a ,  66   b  of the groove part  66 . In this case, there is the characteristic that the line of the contact locations  81   b  does not become parallel with the center axis  81   a  of the rotary tool  81 . Note that, in the example which is shown in  FIG. 5 , the contact locations  81   b  are linear in shape, but sometimes the contact locations  81   b  are curved in shape. 
         [0067]    When using a rotary tool  22  which has a diameter smaller than the width of the groove part  66 , the rotary toll  22  can be arranged at an inclination whereby the center axis  22   a  becomes parallel with the center axis  81   a  of the virtual rotary tool  81 . That is, it is possible to arrange the rotary toll  22  so that center axis  22   a  becomes substantially parallel to the center axis of the circle  84  which has a diameter the same as the width of the groove part. Further, it is possible to arrange it so that the surface of the rotary tool  22  contacts positions on the surface of the rotary tool  81  in the case of using the rotary tool  81 . At this time, the center axis  22   a  of the rotary tool  22  becomes positioned offset from the center axis  81   a  of the rotary tool  81 . That is, the center axis  22   a  is positioned offset from the diameter of the workpiece  1 . Here, no matter how the position of the center axis  22   a  is selected, it is not possible for the surface of the rotary tool  22  to pass over all of the contact locations  81   b  by a single machining operation. For this reason, if arranging the rotary tool  22  for cutting, uncut parts remain at parts of the region in the depth direction of the groove part  66 . There is the characteristic that even if using the rotary tool  22  to try to form the side surface  66   a  or the side surface  66   b  of the groove part  66  by one machining operation, it is not possible to obtain the desired shape of the side surface. 
         [0068]    Therefore, in the groove-forming method in the present embodiment, to form one side surface of the groove part  66 , the machining is performed a plurality of times while changing the relative position of the rotary tool  22  with respect to the workpiece  1 . 
         [0069]      FIG. 6  is a schematic cross-sectional view which explains the groove-forming method in the present embodiment. The groove-forming method in the present embodiment uses a rotary tool  22  which has a diameter smaller than the width of the groove part  66  which is shown by the arrow  99 .  FIG. 6  illustrates machining of one side surface  66   a  of the groove part  66 . 
         [0070]    In the groove-forming method of the present embodiment, the inclination of the rotary tool  22  is set so that the center axis  22   a  of the rotary tool  22  becomes parallel to the depth direction of the groove part  66 . That is, the inclination of the rotary tool  22  is set so that the center axis of the circle  84  for placement of the rotary tool  22  becomes parallel to the diameter of the workpiece  1 . 
         [0071]    In the groove-forming method of the present embodiment, one side surface  66   a  of the groove part  66  is machined a plurality of times while changing the positions of the rotary tool  21  a bit at a time. That is, the path of the rotary tool  22  is changed a bit at a time to form one side surface  66   a . In the example which is shown in  FIG. 6 , in the first machining operation of the side surfaces  66   a , the rotary tool  22  is placed at the position  85   a . Further, as shown in the arrow  103 , the rotary tool  22  is made to move relatively along the direction of extension of the groove part  66 . In the second machining operation of the side surface  66   a , the rotary tool  22  is arranged at the position  85   b  to make the rotary tool  22  move relatively along the direction of extension of the groove part  66 . Further, at the third machining operation of the side surface  66   a , the rotary tool  22  is arranged at the position  85   c  to make the rotary tool  22  move relatively along the direction of extension of the groove part  66 . These positions  85   a ,  85   b , and  85   c  are set so as to contact inside the circle  84  having a diameter the same as the width of the groove part  66 . 
         [0072]    In the machining of the return path as well, in the same way as the machining of the outbound path, to form the other side surface  66   b  of the groove part  66 , the machining is performed a plurality of times while changing the position of the rotary tool  22 . In the present embodiment, at the outbound path and the return path, the machining is performed three times while changing the position of the rotary tool  22 . As the number of times of machining for forming one side surface, any number can be selected. By increasing the number of machining operations, it is possible to improve the precision of the shape of the groove part. 
         [0073]      FIG. 7  is a schematic cross-sectional view which explains the position of the outbound path and the position of the return path of a rotary tool  22  in first machining. The arrow  100  shows the direction of rotation of the rotary tool  22 . The same rotation direction is employed in the outbound path and the return path. In the machining of the first outbound path, the rotary tool  22  is arranged at the position  85   a . By making the rotary tool  22  move as shown by the arrow  103  along the shape of extension of the groove part  66 , one side surface  66   a  is machined. In the processing of the first return path, the rotary tool  22  is arranged at the position  86   a . By making the rotary tool  22 , as shown by the arrow  104 , move along the shape of extension of the groove part  66 , the other side surface  66   b  is machined. 
         [0074]      FIG. 8  is a schematic cross-sectional view which explains the position of the outbound path and the position of the return path of a rotary tool  22  in second machining. In second machining, the rotary tool  22  is arranged at the position  85   b  in the outbound path and made to move in the direction which is shown by the arrow  103  so as to machine one side surface  66   a . By arranging the rotary tool  22  at the position  86   b  at the return path and making it move in the direction which is shown by the arrow  104 , the other side surface  66   b  is machined. 
         [0075]      FIG. 9  is a schematic cross-sectional view which explains the position of the outbound path and the position of the return path of the rotary tool  22  at the third processing. In the third processing as well, in the same way as first machining and the second machining, the rotary tool  22  is arranged at the position  85   c  at the outbound path and the one side surface  66   a  is machined. The rotary tool  22  is arranged at the position  86   c  at the return path to machine the other side surface  66   b.    
         [0076]    In the groove-forming method in the present embodiment, the positions  85   a ,  85   b ,  85   c  of the rotary tool  22  at the outbound path and the positions  86   a ,  86   b ,  86   c  of the rotary tool  22  at the return path become mutually symmetric positions. For example, in the first machining operation, the position  85   a  of the outbound path and the position  86   a  of the return path become symmetric positions with respect to the center point  84   a  of the circle  84 . That is, the tool front end point of the position  85   a  and the tool front end point of the position  86   a  are set at positions symmetric with each other across the center point  84   a  of the circle  84 . 
         [0077]      FIG. 10  is a schematic cross-sectional view when machining one side surface  66   a  in an outbound path in first machining.  FIG. 11  is a schematic cross-sectional view when machining the other side surface  66   b  in a return path in first machining. In these figures, the final machined surface  67  which is finally formed is shown. By performing machining a plurality of times, machining is performed so that the side surfaces  66   a ,  66   b  of the groove part  66  match the final machined surface  67 . 
         [0078]    In the outbound path of the first machining, it is possible to cut the top part of one side surface  66   a  to become substantially the same as the final machined surface  67 . In this regard, at the bottom part of the one side surface  66   a , it is not possible to perform the machining to the final machined surface  67  and uncut parts remain. In the return path of the first machining operation, it is possible to cut the bottom part of the other side surface  66   b  to the final machined surface  67 . In this regard, it is not possible to cut the top part of the other side surface  66   b  to the final machined surface  67  and uncut parts remain. 
         [0079]      FIG. 12  is a schematic cross-sectional view when machining a second outbound path.  FIG. 13  is a schematic cross-sectional view when machining a second return path. In the outbound path of the second machining, it is possible to machine the bottom part of one side surface  66   a  so as to approach the final machined surface  67 . Further, in the second return path, it is possible to machine the top part of the other side surface  66   b  so as to approach the final machined surface  67 . 
         [0080]      FIG. 14  is a schematic cross-sectional view when machining a third outbound path.  FIG. 15  is a schematic cross-sectional view when machining a third return path. In the third outbound path and the return path, it is possible to cut the uncut parts of the one side surface  66   a  and the other side surface  66   b . As a result, the groove part  66  can be made to match the shape of the final machined surface  67 . 
         [0081]    In this way, the groove-forming method in the present embodiment includes a machining step of machining a workpiece  1  by making a rotary tool  22  move relative to the workpiece  1  by a tool path along a direction of extension of the groove part  66 . The machining step arranges the rotary tool  22  so that the rotary tool  22  is inscribed inside the circle  84  which has a diameter the same as the width of the groove part  66  which is formed in the workpiece  1 . Further, it performs machining a plurality of times while changing the relative position of the rotary tool  22  with respect to the workpiece  1 . By employing this method, it is possible to form a groove part with a width larger than the diameter of the rotary tool in a short time. Further, even without using a rotary tool which has the same diameter as the groove width, it is possible to form a groove part with a high precision. Further, even if the width of the groove part which is formed changes, there is no need to change the tool head etc. It is possible to form a groove part by a rotary tool which has a diameter smaller than the groove width. 
         [0082]    For example, it is possible to perform machining in a shorter time than the case of using a dedicated tool head for making a rotary tool engage in planetary rotation motion. Referring to  FIG. 6 , in the dedicated tool head for making the rotary tool engage in planetary rotation motion, the rotary tool is made to be inscribed inside the circle  84  for orbital motion. In this regard, due to the planetary rotation motion, the tool head cannot be raised in rigidity and the cutting rate ends up being limited small. As opposed to this, in the groove-forming method of the present embodiment, it is possible to increase the cutting rate of the workpiece and form a groove in a short time. 
         [0083]    Further, when making the rotary tool engage in planetary rotation motion, there are many regions in which the rotary tool does not contact the workpiece, so time is taken. As opposed to this, in the groove-forming method in the present embodiment, it is possible to set the path of the rotary tool at only the regions which are required for forming the groove part  66  and therefore possible to form the groove in a short time. 
         [0084]    For example, as shown in  FIG. 6 , the range of inscription inside the circle  84  which has the same diameter as the width of the groove part  66  includes the range of machining the side surfaces of the groove part  66  and the range of not machining the side surfaces of the groove part  66 . In the range approaching the side surfaces of the groove part, the side surfaces of the groove part  66  are machined, but in the range which is shown by the arrow  105  other than this range, the operation does not contribute to machining of the side surfaces of the groove part  66 . For this reason, it is possible to arrange the rotary tool  22  while avoiding the range which is shown by the arrow  105  so as to shorten the machining time. By performing machining a plurality of times inside of the range of machining the side surfaces  66   a ,  66   b  of the groove part  66  while changing the relative position of the rotary tool  22 , it is possible to shorten the machining time. 
         [0085]    In the groove-forming method of the present embodiment, one side surface  66   a  is machined at the outbound path and the other side surface  66   b  is machined at the return path. By employing this groove-forming method, it is possible to form the groove part  66  at the outbound path and the return path and possible to form the groove part of the desired shape in a short time. Note that, in the groove-forming operation, it is also possible to machine one side surface consecutively a plurality of times, then machine the other side surface consecutively a plurality of times. 
         [0086]    Furthermore, in the present embodiment, while maintaining the rotation direction of rotation of the rotary tool  22 , at the outbound path, one side surface  66   a  of the groove part  66  is machined, while at the return path, the other side surface  66   b  of the groove part  66  is machined. That is, at the outbound path and the return path, the rotation direction of the rotary tool  22  is maintained the same. By employing this method, it is possible to make the cutting conditions the same at the outbound path and the return path. That is, at the outbound path and the return path, the groove part can be formed by either the machining method of a down cut or an up cut. For this reason, it is possible to make the machining precision of one side surface and the other side surface substantially the same. 
         [0087]    Next, the control device of a machine tool and tool path generating device which are used for the groove-forming method of the present embodiment will be explained. 
         [0088]      FIG. 16  is a schematic view of a of a machining system provided with a machine tool  10  and a device which generates input numerical data  54  to be input to the machine tool in the present embodiment. In the present embodiment, a CAD (computer aided design) device  51  is used to design the shape of the workpiece  1 . The CAD device  51  supplies shape data  52  of the workpiece  1  to a CAM (computer aided manufacturing) device  53 . The shape data  52  includes data on the shape of the groove part  66  which is formed at the workpiece  1 . 
         [0089]    In the CAM device  53 , the shape data  52  is used as the basis to generate input numerical data  54  as input information for input to the control device  55  of the machine tool  10 . The input numerical data  54  in the present embodiment is numerical data when using a rotary tool  81  of the comparative example which has a diameter the same as the width of the groove part  66  so as to form the groove part. 
         [0090]    The numerical control type of machine tool  10  in the present embodiment is provided with a control device  55 . The control device  55  in the present embodiment includes an arithmetic processing device. The arithmetic processing device has a microprocessor (CPU) for performing arithmetic processing etc., a ROM (read only memory) and a RAM (random access memory) served as storage devices, and other peripheral circuits. 
         [0091]    The control device  55  uses the input numerical data  54  to generate the output numerical data  62 . The output numerical data  62  includes instructions for the machine when using a rotary tool  22  which has a diameter smaller than the width of the groove part  66 . The output numerical data  62  includes information of the tool path for performing machining a plurality of times for forming the groove part  66 . In the present embodiment, the output numerical data  62  includes numerical data which makes the rotary tool  22  move relative to the workpiece  1 . 
         [0092]    The control device  55  in the present embodiment includes a numerical data reading part  56  which functions as an input information reading part and a path setting part  57 . The numerical data reading part  56  has the function of reading the input numerical data  54 . The path setting part  57  uses the read input numerical data  54  as the basis to generate the output numerical data  62 . The path setting part  57  in the present embodiment includes a virtual advance direction setting part  58 , a range setting part  59 , and a position setting part  60 . The output numerical data  62  is input to the numerical control unit  63 . The numerical control unit  63  uses the output numerical data  62  as the basis to drive the servo motors  64  of respective axes. The servo motors  64  of respective axes include the X-axis servo motor  38 , Y-axis servo motor  31 , Z-axis servo motor  25 , and B-axis servo motor  43 . 
         [0093]      FIG. 17  is a flow chart of the control in a control device of a machine tool of the present embodiment. Referring to  FIG. 16  and  FIG. 17 , the numerical data reading part  56  of the control device  55  receives as input the input numerical data  54  which was generated by the CAM device  53 . The input numerical data  54  in the present embodiment includes data which shows the path of the tool front end point when using a rotary tool  81  which has a diameter the same as the width of the groove part  66 . The input numerical data  54 , for example, is configured by the coordinate values of the XYZ axes and the rotational angle of the ABC axes. The input information which is input to the control device  55  is not limited to the above numerical data. It is possible to employ input information which shows the path of any part of the rotary tool when the diameter of the rotary tool is the same as the width of the groove part  66 . For example, the input information may also be prepared by macro codes. 
         [0094]    In the control device  55 , first, at step  121 , the numerical data reading part  56  reads the input numerical data  54 . At step  122 , it outputs a string of coordinate values. The “string of coordinate values” here is configured by the coordinate values of the XYZ axes and the rotational angles of the ABC axes. 
         [0095]    Next, the path setting part  57  sets a tool path for performing machining using a rotary tool  22  which has a diameter smaller than the width of the groove part  66 . At step  123 , the virtual advance direction setting part  58  of the path setting part  57  reads the data of the machine tool  10 . The data of the machine tool  10  includes the axial configuration and coordinate systems etc. of the machine tool  10 . Next, at step  124 , the virtual advance direction setting part  58  calculates the virtual direction of advance. 
         [0096]      FIG. 18  is a schematic cross-sectional view which explains the virtual direction of advance. The virtual direction of advance is the direction of advance of the rotary tool  81  with respect to the workpiece  1  when assuming that the workpiece  1  has stopped. As the virtual direction of advance, for example, it is possible to employ the direction in which the tool front end point  81   c  of the rotary tool  81  proceeds. In the example which is shown in  FIG. 18 , the position of the rotary tool  81  is not changed and the workpiece  1  is made to rotate in the direction which is shown by the arrow  91 . That is, the center axis  81   a  of the rotary tool  81  is in a stopped state while the workpiece  1  is made to rotate. At this time, the virtual direction of advance of the rotary tool  81  assuming the workpiece  1  has stopped becomes the direction which is shown by the arrow  101 . The virtual direction of advance can, for example, be set by a vector of a unit length in the XYZ axes. 
         [0097]      FIG. 19  is a schematic plan view of a rotary tool and a groove part when forming a groove in the present embodiment.  FIG. 19  shows an example of use of a rotary tool  22  with a small diameter. The virtual direction of advance is the direction which is shown by the arrow  101 . That is, the virtual direction of advance shows the direction at the time when the virtual rotary tool  81  which has the same diameter as the circle  84  advances along the direction of extension of the groove part  66 . 
         [0098]    Referring to  FIG. 16  and  FIG. 17 , the case of using a rotary tool  81  of the comparative example having a diameter the same as the groove width is assumed up to step  124 . Next, the virtual direction of advance of the rotary tool  81  is used as the basis to set the tool path of a rotary tool  22  which has a diameter smaller than the groove width. The range setting part  59  of the control device  55  sets the range of placement of the rotary tool  22 . 
         [0099]    The range setting part  59  reads the processing setting data at step  125 . The processing setting data includes the width of the groove part  66 , the depth of the groove part  66 , the diameter of the rotary tool  22 , etc. At step  126 , the processing setting data and the virtual direction of advance are used to set a range of placement of the rotary tool  22 . 
         [0100]      FIG. 19  shows a range  89  for placement of the rotary tool  22 . The rotary tool  22  is arranged in the range  89  so as to be inscribed inside the circle  84  which has a diameter the same as the groove width. In the present embodiment, a reference direction vertical to the virtual direction of advance which is shown in the arrow  101  is set. The direction which is shown by the arrow  102  is the reference direction. The range of the predetermined angle  88  with respect to the reference direction is set as the range  89  of placement of the rotary tool  22 . Note that, as the predetermined angle  88 , it is preferable to set a range where the rotary tool  22  does not turn idly so as to obtain a position where the rotary tool  22  machines the side surfaces of the groove part  66 . That is, the range of the predetermined angle  88  is preferably set to a range where there is no tool path for only an air cut where the workpiece is not machined. 
         [0101]    Referring to  FIG. 16  and  FIG. 17 , next, the position setting part  60  sets the position of placement of the rotary tool  22 . The position setting part  60  reads the machining setting data at step  127 . As the machining setting data, the scallop height and tool diameter etc. are included. 
         [0102]      FIG. 20  is a schematic view which explains a scallop height. In the present embodiment, positions  85   a ,  85   b , and  85   c  of the rotary tool  22  are set so as to be inscribed inside the circle  84 . For this reason, the scallop height which is shown by the arrow  106  is determined in accordance with the position of the rotary tool  22 . Further, the scallop height also depends on the diameter of the rotary tool  22 . For example, at the inside of the range  89  of placement of the rotary tool  22 , the number of positions of placement of the rotary tool  22  is increased. Further, by making the interval between positions of placement of the rotary tool  22  smaller, it is possible to reduce the scallop height. That is, it is possible to increase the number of times of machining the side surfaces of the groove part so as to reduce the scallop height. 
         [0103]    Referring to  FIG. 16  and  FIG. 17 , at step  128 , the machining setting data can be used as the basis to set the position of the rotary tool  22 . That is, a plurality of positions of the rotary tool  22  are set inside of the range of placement of the rotary tool  22 . The positions of the rotary tool  22  can, for example, be expressed by the coordinate values of the XYZ axes. Alternatively, for example, it is possible to output a difference from the string of coordinate values which are output at step  122 . 
         [0104]    Next, the position setting part  60  outputs the output numerical data  62  at step  129 . The output numerical data  62 , for example, can be set by the coordinates of the XYZ axes and the relative angles between the rotary tool  22  and workpiece  1  at the ABC axes. 
         [0105]    In this way, the control device  55  in the present embodiment generates output numerical data  62 . The numerical control unit  63  uses the output numerical data  62  as the basis to drive the servo motors  64  of respective axes. Due to this, it is possible to adjust the relative positions of the workpiece  1  and the rotary tool  22 . 
         [0106]    The path setting part  57  in the present embodiment sets a plurality of tool paths while changing the relative position of the rotary tool  22  with respect to the workpiece  1  so that the rotary tool  22  is inscribed inside the circle  84  which has the same diameter as the width of a groove part  66  which is formed in the workpiece  1 . Further, the set tool paths are used for performing the machining. Due to this constitution, in the same way as the groove-forming method in the present embodiment, it is possible to form a groove part precisely in a short time. 
         [0107]    Further, the path setting part  57  in the present embodiment includes a virtual advance direction setting part  58  which uses the input numerical data  54  as the basis to set a virtual direction of advance when using the rotary tool  81  of the comparative example to form the groove part  66 , a range setting part  59  which uses the virtual direction of advance to set a range of placement of the rotary tool  22 , and a position setting part  60  which sets a plurality of positions of the rotary tool  22  for the range of placement of the rotary tool  22 . By employing this constitution, it is possible to use a simple constitution to set a plurality of tool paths of the rotary tool  22 . 
         [0108]    Further, the range setting part  59  in the present embodiment sets the direction vertical to the virtual direction of advance as the reference direction and sets a range of a predetermined angle with respect to the reference direction so as to set the range of placement of the rotary tool  22 . Due to this constitution, it is possible to easily set the range of placement of the rotary tool  22  so as to follow the direction of extension of the groove part  66 . 
         [0109]    In setting the range of placement of the rotary tool  22  in the present embodiment, a single predetermined angle with respect to the reference direction is used for the entire groove part  66 . In setting the range of placement of the rotary tool  22 , the invention is not limited to this. It is also possible to change the angle with respect to the reference direction depending on the location in the groove part. For example, when viewing the groove part by a plan view, it is also possible to change the range of placement of the rotary tool by the curved shaped part and the straight shaped part. 
         [0110]    Further, the range of inscription inside the circle  84  which has the same diameter as the width of the groove part includes a range of machining the side surfaces  66   a ,  66   b  of the groove part  66  and a range of not machining the side surfaces of the groove part  66 . The position setting part  60  preferably sets the relative position of the rotary tool  22  with respect to the workpiece  1  at the inside of the range of machining of the side surfaces  66   a ,  66   b  of the groove part  66 . Due to this constitution, it is possible to shorten the machining time in a groove-forming operation. 
         [0111]    In the above explanation of the present embodiment, the shape data which the CAD device outputs is used as the basis to generate the input numerical data in the CAM device. This input numerical data is used in the control device of the machine tool to prepare output numerical data which forms the path of the rotary tool, but the invention is not limited to this. Shape data which is output from the CAD device may also be used inside of the CAM device to prepare output numerical data. 
         [0112]      FIG. 21  is a schematic view of a machining system provided with a CAD device and a tool path generating device in the present embodiment. The generation of shape data  52  by the CAD device  51  is similar to that in the machining system which is shown in  FIG. 16 . The tool path generating device  75  in the present embodiment has the function of a CAM device. Further, the tool path generating device  75  has the function of using shape data  52  as the basis to generate output numerical data  62  of a tool path for machining a plurality of times. 
         [0113]    The tool path generating device  75  in the present embodiment is provided with a shape data reading part  76  and a path setting part  77 . The shape data reading part  76  in the present embodiment reads the shape data  52  after machining of the workpiece  1 . The path setting part  77  uses the shape data  52  of the workpiece  1  as the basis to set a tool path for making a rotary tool  22  with a diameter smaller than the width of the groove part  66  move relatively in the direction of extension of the groove part  66 . 
         [0114]    The path setting part  77 , for example, has the functions of the CAM device  53 , numerical data reading part  56 , and path setting part  57  of the machining system according to  FIG. 16 . The path setting part  77 , for example, uses the shape data  52  as the basis to generate the input numerical data  54  which shows the path of the rotary tool  81  of a comparative example. The input numerical data  54  is used to generate output numerical data  62  which shows the path of the rotary tool  22 . At this time, it is also possible set a plurality of tool paths which is changed in relative position of the rotary tool  22  to the workpiece  1  so as to be inscribed inside the circle  84  having the same diameter as the width of the groove part  66  which forms the workpiece  1 . The path setting part is not limited to this. It is also possible to not generate the input numerical data. That is, the path setting part may also use the shape data to calculate the virtual direction of advance, the range of placement and the position of placement of the rotary tool, etc. and generate output numerical data. 
         [0115]    The output numerical data  62  is input to the machine tool  10 . The control device  55  of the machine tool  10  uses the output numerical data  62  to drive the servo motors  64  of respective axes. It is therefore possible to make the rotary tool  22  move relative to the workpiece  1 . 
         [0116]    In the tool path generating device in the present embodiment as well, it is possible to generate a tool path which can form a groove part precisely in a short time. 
         [0117]    The movement device for making the rotary tool  22  and the workpiece  1  relatively move in the present embodiment is formed to make the rotary tool  22  move with respect to the workpiece  1  in the X-axis and Y-axis and to move the workpiece  1  with respect to the rotary tool  22  in the Z-axis and B-axis, but the invention is not limited to this. The movement device may also be formed so that at least one of the workpiece and rotary tool can move relative to these axes. 
         [0118]    In the present embodiment, the virtual direction of advance was calculated and the virtual direction of advance was used as the basis to set the range of placement of the rotary tool and the position of the rotary tool, but the invention is not limited to this. The position of the rotary tool may also be set without using the virtual direction of advance. For example, the position of the rotary tool may also be set based on the direction of extension of the groove part. 
         [0119]    The numerical control type of machine tool in the present embodiment has a single rotational feed axis and a plurality of linear feed axes, but the invention is not limited to this. The present invention can be applied to a numerical control type of machine tool which has a rotational feed axis and other feed axes. For example, the present invention can also be applied to a numerical control type of machine tool which has a rotational feed axis and a single linear feed axis and a numerical control type of machine tool which has two or more rotational feed axes etc. 
         [0120]    In the present embodiment, the explanation was given with reference to the example where the depth of the groove part and the width of the groove part became constant, but the invention is not limited to this. The present invention can also be applied to machining where the depth of the groove part is changed and machining where the width of the groove part is changed. 
         [0121]    The above embodiments can be suitably combined. In the above figures, the same or equivalent parts are assigned the same reference signs. Note that, the above embodiments are illustrative and do not limit the invention. Further, in the embodiments, the changes shown in the claims are included. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  workpiece 
               1   a  center axis 
               10  machine tool 
               20  spindle 
               22  rotary tool 
               22   a  center axis 
               25  Z-axis servo motor 
               31  Y-axis servo motor 
               38  X-axis servo motor 
               42  rotary table 
               43  B-axis servo motor 
               51  CAD device 
               52  shape data 
               53  CAM device 
               54  input numerical data 
               55  control device 
               56  numerical data reading part 
               57  path setting part 
               58  virtual advance direction setting part 
               59  range setting part 
               60  position setting part 
               62  output numerical data 
               66  groove part 
               66   a ,  66   b  side surfaces 
               75  tool path generating device 
               76  shape data reading part 
               77  path setting part 
               81  rotary tool 
               84  circle 
               85   a ,  85   b ,  85   c  positions 
               86   a ,  86   b ,  86   c  positions 
               88  angle 
               89  range