Patent Publication Number: US-2020276677-A1

Title: Machine tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national phase patent application of International Patent Application No. PCT/JP2017/042076 filed Nov. 22, 2017, which is hereby incorporated by reference in the present disclosure in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present invention relates to a machine tool comprising two rotary feed axes for controlling the posture of a tool and three linear feed axes for moving the position of a workpiece relative to the tool. 
     BACKGROUND OF THE DISCLOSURE 
     In machine tools comprising two rotary feed axes in addition to orthogonal three-axis linear feed axes, singularities are present in the operating range of the machine. When a feed axis passes through a singularity, even if the tool posture change commanded by the machining program is small, the tool posture may actually change significantly, and the speeds of the rotary feed axes and the linear feed axes may increase rapidly. 
     Patent Literature 1 discloses a numerical control device for five-axis machine tools which can prevent significant movements in the rotational axis speed or acceleration at singularities when it is determined in tool posture control that a singularity is to be encountered. 
     PATENT LITERATURE 
     PTL 1 Japanese Unexamined Patent Publication (Kokai) No. 2010-140312 
     SUMMARY OF THE DISCLOSURE 
     The numerical control device for five-axis machine tools of Patent Literature 1 determines whether the spindle for rotating a tool will pass through a singularity or will pass through the vicinity of a singularity by calculation based on the command at a command block start point and the command at a command block end point in a machining program, and when it is determined that the spindle will pass through a singularity or the vicinity of a singularity, by changing the tool path at the singularity and the vicinity of the singularity, the singularity can be avoided by means of software. However, in the invention of Patent Literature 1, since the tool path is changed at a singularity or in the vicinity of the singularity, there is a problem that the machined surface may have differences in shape from the desired shape. 
     The present invention seeks to solve the problem of the prior art as a technical problem, and aims to provide a machine tool which can solve the problem of singularities by means of hardware rather than solving the problem of singularities by means of software. 
     In order to achieve the object described above, according to the present invention, there is provided a machine tool including two rotary feed axes for controlling the posture of a tool and a linear feed axis for moving the position of a workpiece relative to the tool, the machine tool comprising a spindle on an end of which the tool is mounted, a spindle head for supporting the spindle so as to be capable of rotating about a spindle axis of rotation, a spindle head support unit for supporting the spindle head so as to be rotatable about a first axis extending orthogonal to the spindle axis of rotation, a first rotary feed axis device which is provided on the spindle head support unit and which rotates the spindle head about the first axis, a saddle which supports the spindle head support unit so as to be rotatable about a second axis which is orthogonal to the first axis and which intersects the vertical direction at a predetermined inclination angle, a second rotary feed axis device which is provided on the saddle and which rotatably feeds the spindle head support unit about the second axis of rotation, and restricting means for restricting a rotary feed range of the first rotary feed axis device so that the spindle axis of rotation is not parallel or coincident to the second axis. 
     According to the present invention, since the problem of singularities is solved by means of hardware by providing a restricting means which limits the rotary feed range of the spindle head so that the spindle axis of rotation is neither parallel nor coincident with the second axis, which is orthogonal to the first axis, which is the axis of rotation of the spindle head, and intersects the vertical direction at a predetermined inclination angle, significant changes in tool posture and rapid increases in the speeds of the rotary feed axes and the linear feed axis can be prevented, and in addition to preventing machining marks from being formed on the machined surface, if the machining program does not include commands which pass through a singularity, the rotary feed axis device operates smoothly and a suitable machined surface can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a partially broken side view showing a machine tool according to a preferred embodiment of the present invention. 
         FIG. 2  is a front view of the machine tool of  FIG. 1 . 
         FIG. 3  is a partially enlarged cross-sectional view of A-axis and C-axis rotary feed axes at origin positions. 
         FIG. 4  is a partially enlarged cross-section view similar to  FIG. 3  when the A-axis is in a maximum rotational position. 
         FIG. 5  is a partially enlarged cross-sectional view similar to  FIG. 3  when the A-axis is in a minimum rotational position. 
         FIG. 6  is a schematic view showing a vector representing a tool tip direction when the A-axis and C-axis rotary feed axes are in origin positions. 
         FIG. 7  is a schematic view similar to  FIG. 6  illustrating the movement range of the A-axis rotary feed axis. 
         FIG. 8  is a schematic view similar to  FIG. 6  illustrating the movable ranges of the A-axis and C-axis rotary feed axes. 
         FIG. 9  is a schematic view illustrating the range in which the tip of a tool can be positioned according to present embodiment. 
         FIG. 10  is a partially enlarged cross-sectional view similar to  FIG. 3  illustrating restricting means according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The preferred embodiments of the present invention will be described below. 
       FIGS. 1 and 2  illustrate an example of a machine tool having two rotary feed axes for controlling the posture of a tool and three linear feed axes for moving the position of a workpiece relative to the tool. The machine tool  10  comprises a bed  12  as a base affixed to a floor of a factory, a column  14  which is attached to and stands on an upper surface of the bed  12  on a rear end side (the right side in  FIG. 1 ) of the bed  12 , a table  26  which is provided on an upper surface of a front portion (the left side in  FIG. 1 ) of the bed  12  and which is movable in the frontward and rearward directions or Y-axis directions (left and right directions in  FIG. 1 ) and on which a workpiece W is affixed, a spindle head  22  which is provided above the table  26  and which rotatably supports a spindle  24 , a saddle  16  which is provided on the top of the column  14  so as to be movable in the left and right directions or X-axis directions (the direction orthogonal to the surface of the paper of  FIG. 1 ), and a spindle head support unit  60  which is provided on the saddle  16  and which supports the spindle head  22  so as to be capable of rotating about an axis of rotation Oa. The spindle head support unit  60  comprises a Z-axis slider  18  which is attached to the front surface of the saddle  16  so as to be capable of moving in the upward and downward directions or Z-axis directions, and a C-axis pivot shaft  20  which is supported on the Z-axis slider  18  so as to be capable of rotating in the C-axis direction about an inclination axis Oc as a second axis of rotation. 
     A pair of X-axis guide rails  28  extend in the horizontal X-axis direction (the direction orthogonal to the plane of  FIG. 1 ) on the upper end of the column  14 , and a guide block  30  is attached to the column  14  so as to be capable of sliding on the X-axis guide rails  28 . A ball screw  32  extending in the X-axis direction and an X-axis servomotor  34  which is connected to one end of the ball screw are provided as an X-axis drive device for reciprocally driving the saddle  16  along the X-axis guide rails  28 , and a nut (not illustrated) for engaging with the ball screw  32  is attached to the saddle  16 . Furthermore, the top of the column  14  forms an inclined plane  14   a.  The inclined plane  14   a  extends obliquely upward from the front side to the back side of the machine tool  10  parallel to the X axis. The angle of the inclined plane  14   a  can be, for example, 45° with respect to the horizontal plane (XY plane). 
     A pair of X-axis guide rails  36  extend in the Z-axis direction (the upward and downward directions in  FIGS. 1 and 2 ), which is the vertical direction, on the front surface of the saddle  16 , and a guide block  38  is attached to the Z-axis slider  18  so as to be capable of sliding on the Z-axis guide rails  36 . The saddle  16  is provided with, as a Z-axis drive device for reciprocally driving the Z-axis slider  18  along the Z-axis guide rails  36 , a ball screw  40  extending in the Z-axis directions and a Z-axis servomotor  44  which is connected to one end of the ball screw  40 , and a nut for engaging with the ball screw  40  is attached to the Z-axis slider  18  ( FIG. 1  shows a bracket  42  for attachment of the nut). 
     The C-axis pivot shaft  20  is supported on the lower end of the Z-axis slider  18  so as to be capable of being rotated about the inclination axis Oc by a bearing  50 . The inclination axis Oc extends orthogonally to the X-axis from the front surface side obliquely upward toward the back side of the machine tool  10  and substantially parallel to the inclined plane  14   a.  Though the inclination axis Oc and the inclined plane  14   a  may be inclined in the same direction, it is more effective if they are nearly parallel. The Z-axis slider  18  further incorporates a C-axis servomotor  52  as a C-axis feed device or a second rotary feed axis device for rotationally driving the C-axis pivot shaft  20 . A pair of left and right arms  20   a,    20   b  which extend parallel to the inclination axis Oc are formed on the end on the front side of the C-axis pivot shaft  20 . Note that the C-axis pivot shaft  20  is capable of rotating within a predetermined angle range about the inclination axis Oc, for example, with a rotation angle range of ±60° about the origin of the C-axis feed device. 
     The spindle head  22  supports the spindle  24  so as to be capable of rotating about a spindle axis Os, and a tool T is mounted on the end of the spindle  24 . The spindle head  22  is arranged between the pair of left and right arms  20   a,    20   b  of the C-axis pivot shaft  20 . The spindle head  22  is supported on the arms  20   a,    20   b  of the C-axis pivot shaft  20  of the spindle head support unit  60  so as to be capable of being rotated, in the A-axis direction about the axis of rotation Oa as the first axis of rotation orthogonal to the inclination axis Oc, by A-axis pivot shafts  22   a,    22   b  formed on both sides thereof. An A-axis servomotor  21  connected to one of the A-axis pivot shafts  22   a,    22   b  (A-axis pivot shaft  22   a  in the present embodiment) is incorporated as an A-axis feed device or first rotary feed axis device in one of the arms  20   a,    20   b  (the arm  20   a  in the present embodiment). 
     Note that the inclination axis Oc passes through the vicinity of the intersection of the spindle axis Os of the spindle  24  and the axis of rotation Oa. Furthermore, as shown in  FIG. 2 , the rotary position of the C-axis feed device in which the axis of rotation Oa is parallel to the horizontal X-axis is the origin position of the C-axis. Further, when the C-axis is in the origin position, the rotary position of the A-axis feed device in which the spindle axis Os of the spindle  24  is vertical (parallel to the Z-axis) is the origin position of the A-axis. When the A-axis is in the origin position, the spindle  24  is oriented downward so as to face the table  26 . 
     When the A-axis and the C-axis are in their respective origin positions, the spindle  24  end surface and the tool T mounted on the spindle  24  face the table  26  on which the workpiece (not illustrated) is attached. A pair of Y-axis guide rails  46  extend in the horizontal Y-axis direction (the left and right directions of  FIG. 1 ) on the upper surface of the bed  12 , and a guide block  48  which is capable of sliding on the Y-axis guide rails  46  is attached to the table  26 . The bed  12  is further provided with, as a Y-axis feed device for reciprocally driving the table  26  along the Y-axis guide rails, a ball screw (not illustrated) which extends in the Y-axis directions and a Y-axis servomotor (not illustrated) connected to one end of the ball screw, and a nut (not illustrated) for engaging with the ball screw is attached to the table  26 . Preferably, a recess  16   b  is formed in the front surface of the saddle  16  so that the Z-axis ball screw  40  can be arranged within the recess  16   b.    
     The X-axis servomotor  34 , the Y-axis servomotor, the Z-axis servomotor  44 , the A-axis servomotor  21 , and the C-axis servomotor  52  are connected to an NC device  100  ( FIG. 2 ) of the machine tool  10  and are controlled by the NC device  100 . 
     In the machine tool  10 , by inclining the C-axis pivot shaft  20  on the column  14  side, overhang of the movable body including the two rotary feed axes (the A-axis feed device and the C-axis feed device) and the spindle head  22  with respect to the column  14  can be reduced, and the center of gravity position of the movable body can be shifted to the column  14  side. As a result, deformation of the column  14  and saddle  16  can be reduced. Furthermore, when the movable body moves, deformation of the saddle  16  and the Z-axis slider  18  during acceleration/deceleration is reduced. At least a part of the spindle head support unit  60  can be arranged in the recess  16   b  of the saddle  16 . As a result, it is possible to further reduce the overhang of the movable body with respect to the column  14 . 
     If the spindle axis Os is coincident with the inclination axis Oc, which is the axis of rotation of the C-axis pivot shaft  20 , even if the changes in tool posture commanded by the machining program are small, the tool posture actually changes significantly, whereby the speeds of the rotary feed axes and the linear feed axes can increase rapidly. Such a problem is commonly referred to as a machine tool singularity problem. In the present invention, in order to prevent such a singularity problem by means of hardware, a restricting means for limiting the rotational feed range of the A-axis is provided. In the embodiment described above, the machine tool  10  comprises a stopper  54  provided on the C-axis pivot shaft  20  as restricting means for limiting so that the spindle axis Os and the inclination axis Oc, which is the axis of rotation of the C-axis pivot shaft  20 , do not become coincident. 
     Referring to  FIGS. 3 to 5 , the machine tool  10  further comprises the stopper  54  provided on the C-axis pivot shaft  20  as restricting means for limiting the A-axis pivot range. The stopper  54  has a first abutment surface  54   a  for limiting the positive A-axis direction rotational range and a second abutment surface  54   b  for limiting the negative A-axis direction rotational range. In the example of  FIGS. 3 to 5 , the angle of rotation of the A-axis is defined as α, and when the A-axis is in the rotational position α=α 1  ( FIG. 4 ) in the position direction from the origin (α=0), the first abutment surface  54   a  of the stopper  54  abuts the side surface of the spindle head  22 , which prevents rotational movement of the spindle head  22  and defines the upper limit of the rotational movement of the A-axis, and conversely, when the A-axis is in the rotary position α=−α 2  ( FIG. 5 ) in the negative direction from the origin (α=0), the second abutment surface  54   b  of the stopper  54  abuts the side surface of the spindle head  22 , which prevents rotational movement of the spindle head  22  and defines the lower limit of the rotational movement of the A-axis, and thus, the A-axis can rotationally move the spindle head  22  in the range α 1 ≤α≤α 2 . Thus, the spindle head  22  is rotated and fed in the A-axis direction between a first rotational position (α=α 1 ) and a second rotational position (α=−α 2 ). 
     In the present embodiment, since the axis of rotation Oa as the first axis of rotation, which is the center of rotation of the A-axis, intersects the inclination axis Oc, though the spindle axis Os and the inclination axis Oc are coincident when α 1 =θ, when the axis of rotation Oa does not intersect the inclination axis Oc, the spindle axis Os does not become coincident with the inclination axis Oc. In such a case, the stopper  54  as restricting means is formed so as to prevent the spindle axis Os from becoming parallel to the inclination axis Oc. 
     Note that in the present embodiment, the direction (clockwise direction in  FIGS. 3 to 5 ) in which the tool T rotates and moves upward from the origin of the A-axis at which the end surface of the spindle  24  on which the tool T is mounted is oriented downward to face the table  26 , as shown in  FIG. 3 , toward the front of the machine tool  10  (left in  FIGS. 3 to 5 ), as shown in  FIG. 4 , is defined as the positive A-axis direction. 
     Referring to  FIG. 6 , the axis of rotation Oa as the first axis of rotation, which is the center of rotation of the A-axis, extends parallel to the X-axis, and the inclination axis Oc as the second axis of rotation, which is the center of rotation of the C-axis, is orthogonal to the axis of rotation Oa and is inclined by an inclination angle θ relative to the Z-axis, which is the vertical direction. Specifically, in  FIG. 6 , the left side is the front direction of the machine tool  10  and the upper side is the direction of the machine tool  10  oriented toward the table  26 . The inclination angle of the C-axis can be set within the range of 30°&lt;θ&lt;45°. Note that in  FIGS. 6 to 8 , the arrows represent unit vectors along the spindle axis Os in which the tip direction of the tool T is the positive direction. 
     Referring to  FIG. 7 , the A-axis rotational feed range a becomes α=α 1 +α 2  with the origin (Z-axis, which is the vertical direction) interposed therebetween. Though the maximum rotational angle α 1  in the positive direction from the origin of the A-axis and the maximum rotational angle α 2  in the negative direction can be different values, α 1  may be equal to α 2 . As an example, the following can be satisfied α 1 =α 2 =30°. In this case, the rotational feed range of the A-axis becomes −30°≤α≤30°. 
     Further, referring to  FIG. 8 , vector V 1  represents the tip direction of the tool T when the A-axis is in the α=0 origin position and the C-axis is in the minimum rotational angle position, vector V 2  represents the tip direction of the tool T when the A-axis is in the α=0 origin position and the C-axis is in the maximum rotational angle position, vector V 3  represents the tip direction of the tool T when the A-axis is in the minimum rotational angle position or the α=−α 2  second rotational position and the C-axis is in the minimum rotational angle position, vector V 4  represents the tip direction of the tool T when the A-axis is in the minimum rotational angle position or the α=−α 2  second rotational position and the C-axis is in the maximum rotational angle position, and vector V 5  represents the tip direction of the tool T when the A-axis is in the maximum rotational angle position or the α=α 1  first rotational position and the C-axis is in the maximum rotational angle position. Note that the vector representing the tip direction of the tool T when the A-axis is in the maximum rotational angle position or the α=α 1  first rotational position and the C-axis is in the minimum rotational angle position has been omitted from  FIG. 8  in order to clearly illustrate the drawing. Thus, in the present embodiment, the range in which the tip of the tool T can be arranged is represented by range R in  FIG. 9 . 
     In the embodiment described above, since the table  26  is arranged below the C-axis pivot shaft  20 , the tool T is arranged below the inclination axis Oc. Thus, if the first abutment surface  54   a  of the stopper  54  is formed so that α 1 &lt;θ, the singularity problem will not occur. 
     In the embodiment described above, the restricting means comprises the stopper  54  provided on the C-axis pivot shaft  20 , but the present invention is not limited thereto. The restricting means may comprise, for example, a limit switch  56  ( FIG. 10 ) provided on the C-axis pivot shaft  20  and connected to the NC device  100 . In this case, when the C-axis feed device is in the α=α 1  or α=α 2  rotational position, the NC device  100  of the machine tool  10  can stop the driving of the C-axis servomotor  52  based on a signal from the limit switch  56 . 
     In the embodiment described above, the angle cd defining the upper limit of the rotational feeding of the C-axis feed device or the first rotational position is closer to the inclination angle θ of the inclination axis Oc so that the range R in which the tip of the tool T can be arranged can be widened, and the angle cd can be determined in advance by experimentation or the like within a range in which the singularity problem does not occur. 
     REFERENCE SIGNS LIST 
     
         
           10  Machine Tool 
           12  Bed 
           14  Column 
           14   a  Inclined Plane 
           16  Saddle 
           16   b  Recess 
           18  Z-axis Slider 
           20  C-axis Pivot Shaft 
           20   a  Arm 
           20   b  Arm 
           21  A-axis Servomotor 
           22  Spindle Head 
           22   a  A-axis Pivot Shaft 
           22   b  A-axis Pivot Shaft 
           24  Spindle 
           26  Table 
           28  X-axis Guide Rail 
           34  X-axis Servomotor 
           36  Z-axis Guide Rail 
           42  Bracket 
           44  Z-axis Servomotor 
           46  Y-axis Guide Rail 
           50  Bearing 
           52  C-axis Servomotor 
           54  Stopper 
           56  Limit Switch 
           60  Spindle Head Support Unit