Patent Publication Number: US-2016221138-A1

Title: Feed apparatus and machine tool

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2015-019031 filed on Feb. 3, 2015 including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a feed apparatus used for a machine tool. 
     2. Description of Related Art 
     A feed apparatus for a machine tool is conventionally available which has a moving member, a screw shaft screwed in the moving member, and a motor that rotates the screw shaft as depicted in FIG. 1 of Japanese Patent Application Publication No. 2003-25178 (JP 2003-25178 A). A work and a tool used to machine the work (hereinafter referred to as simply referred to as an object as needed) are mounted on the moving member. The center of gravity of a combination of the moving member and the object is located higher than a position where the screw shaft is crewed in the moving member. Consequently, when the motor rotates the screw shaft to move the moving member in an axial direction in conjunction with rotation of the screw shaft, the moving member and the object mounted on the moving member tend to remain at the position of the center of gravity thereof. This causes the moving member to rotate about, as a center of rotation, an axis in a lateral direction obtained when a direction in which the moving member travels at the screwed position is defined as a forward direction. Accordingly, the moving member is thus pitched. When the moving body in motion is pitched, the object mounted on the moving member performs unstable behavior, and the work and the tool mounted on the moving body slightly move upward or downward. The work and the tool mounted on the moving body are displaced from regular positions thereof, which reduces machining accuracy of the machine tool. 
     When the screw shaft screwed in the moving member rotates, frictional heat generated at a portion of the screw shaft that is screwed in the moving member increases the temperature of the screw shaft to thermally expand the screw shaft. To suppress the thermal expansion of the screw shaft, a structure may be employed in which cooling water is allowed to flow through a cooling water passage formed along the axis of the screw shaft so as to cool the screw shaft. However, in this structure, formation of the cooling water passage leads to an increased dimension of the screw shaft in a radial direction and an increased size of the feed apparatus. Furthermore, since a mechanism that allows the cooling water to flow along the axis of the screw shaft is provided, the structure of the feed apparatus becomes complicated and manufacturing costs of the feed apparatus increases. Thus, instead of the structure in which cooling water is allowed to flow along the axis of the screw shaft, a structure is commonly used in which one end of the screw shaft is fixed in the axial direction, while the other end is enabled to move freely in the axial direction. However, in such a structure, the effect of elastic deformation of the screw shaft increasers consistently with a distance from the fixed end of the screw shaft to the position of the moving member. This reduces the rigidity of the screw shaft that is required to feed the moving member. Consequently, the amount of machining achieved by the tool machine becomes unstable, which reduces the machining accuracy of the machine tool. 
     A feed apparatus is available in which two motors and two screw shafts are provided in parallel in order to deal with an inertia moment in a rotating direction of the screw shaft and an inertia moment in a traveling direction of the moving member and the object. See FIG. 1 of JP 2003-25178 A. In such a feed apparatus, when the position of the center of gravity of the combination of the moving member and the object in the horizontal direction is shifted from an intermediate position between the pair of screw shafts provided in parallel toward one of the screw shafts, radial loads of the moving member and the object acting on the respective screw shafts are unbalanced. Thus, a larger radial load is imposed on the screw shaft closer to the position of the center of gravity of the combination of the moving member and the object as compared to the other screw shaft. Consequently, the life of the screw shaft closer to the position of the center of gravity is shortened. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a feed apparatus that is used for a machine tool and improves machining accuracy of the machine tool. 
     According to an embodiment of the present invention, a feed apparatus includes: 
     a base, 
     a moving member that is movable in a predetermined axial direction with respect to the base, 
     a first screw shaft provided on the base so as to be parallel to the axial direction and rotatably supported by the base, 
     a second screw shaft provided separately from the first screw shaft on the base so as to be parallel to the axial direction and rotatably supported by the base, 
     a first driving source mounted on the base to rotate the first screw shaft, 
     a second driving source mounted on the base to rotate the second screw shaft, 
     a first nut provided on the moving member and screwed over the first screw shaft to move in the axial direction in conjunction with rotation of the first screw shaft, and 
     a second nut provided on the moving member and screwed over the second screw shaft to move in the axial direction in conjunction with rotation of the second screw shaft. 
     The first nut and the second nut are disposed at different positions in the axial direction. 
     In this configuration, when the first screw shaft and the second screw shaft are rotated to move the moving member, the magnitude of pitching occurring in the moving member as a result of relative rotation between the first screw shaft and the first nut is different from the magnitude of pitching occurring in the moving member as a result of relative rotation between the second screw shaft and the second nut. In this case, pitching is rotation in a direction orthogonal to the axial direction and to a direction in which the moving member is disposed with respect to the base. Therefore, unlike a single nut disposed at one position in the axial direction, the first nut and the second nut disposed at different positions in the axial direction can suppress the pitching occurring in the moving member. Thus, the object such as a work and a tool mounted on the moving member can be suppressed from being displaced from the regular position. This improves the machining accuracy of the machine tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a top view of a feed apparatus of the present embodiment; 
         FIG. 2  is a side sectional view of the feed apparatus of the present embodiment, taken along line A-A in  FIG. 1 ; 
         FIG. 3  is an enlarged view of a B portion in  FIG. 2 ; 
         FIG. 4  is a top view of a feed apparatus of a comparative example; and 
         FIG. 5  is a side sectional view of the feed apparatus of the comparative example, taken along line C-C in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Configurations of a feed apparatus  100  and a machine tool  1000  of the present embodiment will be described based on  FIGS. 1 to 3 . The feed apparatus  100  is a mechanical element of the machine tool  1000  such as a lathe or a grinding machine and moves an object  80  mounted on a moving member  2 . The moving member  2  is, for example, a table on which a work is mounted and which moves linearly with respect to a bed, a saddle on which a main spindle apparatus holding a tool such as an end mill is mounted and which moves linearly with respect to a column or the bed, a column on which the saddle is mounted and which moves linearly to the bed, or a wheel spindle stock table that holds a grinding wheel and moves linearly with respect to the bed. As the moving member  2 , a table on which a tool as the object  80  is mounted is taken as an example. 
     As depicted in  FIGS. 1 to 3 , the feed apparatus  100  has a base  1 , the moving member  2 , guide members  3 , a first screw shaft  11 , a second screw shaft  12 , a first motor  21 , a second motor  22 , first bearings  31 , a second bearing  32 , third bearings  33 , and a fourth bearing  34 , control section  50 , a first nut  51 , and a second nut  52 . In  FIGS. 1 to 3 , a lateral direction in the drawing plane is defined as an axial direction, a right side of the drawing plane is defined as the front (one side), and a left side of the drawing plane is defined as the rear (the other side). In  FIG. 1 , an up-down direction of the drawing plane, in other words, a direction in the drawing plane that is orthogonal to the axial direction, is defined as a width direction. In  FIGS. 2 and 3 , an up-down direction in the drawing sheet is defined as an up-down direction. 
     As depicted in  FIG. 1  and  FIG. 2 , the base  1  includes a base portion  1   a  and a first support portion  1   b , a second support portion  1   c , a third support portion  1   d , and a fourth support portion  1   e  all of which are integrated together. The base portion  1   a  is shaped like a block. The first to fourth support portions  1   b  to  1   e  are formed to protrude upward from an upper surface of the base portion  1   a . The first to fourth support portions  1   b  to  1   e  are disposed from a front side (one side) toward a rear side (the other side) of the base portion  1   a  in the axial direction in this order. The first support portion  1   b  is formed at a front end (one end) of the base portion  1   a . The fourth support portion  1   e  is formed at a rear end (the other end) of the base portion  1   a . The second support portion  1   c  and the third support portion  1   d  are provided in a central portion of the base portion  1   a  in the axial direction so as to be adjacent each other. 
     As depicted in  FIG. 1 , a plurality of (in the present embodiment, two) guide members  3  is mounted on the base  1  such that the guide members  3  are in parallel in the width direction and extend along the axial direction. The guide members  3  are engaged with the moving member  2 . The guide members  3  are, for example, linear motion guides. In this structure, the moving member  2  is mounted on an upper surface of the base  1  and mounted on the base  1  so as to be movable in the axial direction with respect to the base  1 . 
     As depicted in  FIG. 1 , the first screw shaft  11  and the second screw shaft  12  are provided parallel to each other in the axial direction. The first screw shaft  11  and the second screw shaft  12  are separate from each other. Both ends of each of the first and second screw shafts  11  and  12  are rotatably supported by the base  1 . An axis of the first screw shaft  11  and an axis of the second screw shaft  12  are coaxially arranged. In the present embodiment, the axis of the first screw shaft  11  and the axis of the second screw shaft  12  are provided on one straight line (a long dashed short dashed line depicted in  FIG. 1 ) in a central portion of the base  1  in the width direction. The first screw shaft  11  is provided on a front side (one side) of the base  1 . The second screw shaft  12  is provided on a rear side (the other side) of the base  1 . The screw shafts  11  and  12  may be well-known ball screws or trapezoidal screws. 
     The first screw shaft  11  is formed of a base-end bearing portion  11   a , a first threaded portion  11   b , and a distal-end bearing portion  11   c  that are formed in this order from a base end (front end) toward a distal end (rear end) as depicted in  FIG. 2  and  FIG. 3 . The base-end bearing portion  11   a  and the distal-end bearing portion  11   c  are shaped like columns. A groove is formed in the outer peripheral surface of the first threaded portion  11   b .    
     The base-end bearing portion  11   a , which is a base end (one end or front end) of the first screw shaft  11 , is rotatably supported by a first bearing  31  fixed to the first support portion  1   b . As depicted in  FIG. 3 , the first bearing  31  is a ball bearing and includes an inner ring  31   a , an outer ring  31   b  disposed at an outer periphery of the inner ring  31   a , and a plurality of balls  31   c  provided between the inner ring  31   a  and the outer ring  31   b . The inner ring  31   a  and the outer ring  31   b  are immovable in the axial direction. In the present embodiment, two first bearings  31  are provided in parallel in the axial direction. The two first bearings  31  are mounted by being fitted into a first fixation hole if formed in the first support portion  1   b  so as to penetrate the first support portion  1   b  in the axial direction. A rear end of the outer ring  31   b  of the rear first bearing  31  is in contact with a step  1   g  formed on a rear side of the first fixation hole  1   f . This inhibits the two first bearings  31  from moving rearward with respect to the first support portion  1   b . A front end of the outer ring  31   b  of the front first bearing  31  is in contact with a C ring  61  installed in a ring groove  1   h  formed in the first fixation hole if over the entire a circumference of the first fixation hole  1   f . This inhibits the two first bearings  31  from moving forward with respect to the first support portion  1   b . In this structure, the two first bearings  31  are fixed to the first support portion  1   b  so as to be immovable in the axial direction. 
     The base-end bearing portion  11   a  is inserted through the inner rings  31   a  of the two first bearings  31 . The first threaded portion  11   b  has an outside diameter larger than an outside diameter of the base-end bearing portion  11   a . A step  11   g  is formed between the first threaded portion  11   b  and the base-end bearing portion  11   a . A rear end of the inner ring  31   a  of the rear first bearing  31  is in contact with the step  11   g  to inhibit the first bearing  31  from moving rearward with respect to the first screw shaft  11 . A ring groove  11   d  is formed in the outer peripheral surface of the base-end bearing portion  11   a  over the entire circumference of the base-end bearing portion  11   a . A C ring  11   h  is installed in the ring groove  11   d . The C ring  11   h  is in contact with a front end of the inner ring  31   a  of the front first bearing  31  to inhibit the first bearing  31  from moving forward with respect to the first screw shaft  11 . In this structure, the base-end bearing portion  11   a , which is the base end (one end) of the first screw shaft  11 , is rotatably supported by the first bearings  31  so as to be immovable in the axial direction of the first screw shaft  11 . 
     The distal-end bearing portion  11   c , which is the distal end (the other end or rear end) of the first screw shaft  11 , is rotatably supported by a second bearing  32  fixed to the second support portion  1   c . The second bearing  32  is a ball bearing and includes an inner ring  32   a , an outer ring  32   b , and a plurality of balls  32   c . The outer ring  32   b  is provided at an outer periphery of the inner ring  32   a . The balls  32   c  are provided between the inner ring  32   a  and the outer ring  32   b . A ball groove  32   d  is formed in an outer peripheral surface of the inner ring  32   a  over the entire circumference of the inner ring  32   a , and the balls  32   c  engage with the ball groove  32   d . On the other hand, no ball groove with which the balls  32   c  engage is formed in an inner peripheral surface of the outer ring  32   b . Thus, the inner ring  32   a  and the outer ring  32   b  are movable in the axial direction. An embodiment is possible in which a ball groove with which the balls  32   c  engage is formed in the inner peripheral surface of the outer ring  32   b  and no ball groove is formed in the outer peripheral surface of the inner ring  32   a.    
     The second bearing  32  is mounted by being fitted into a second fixation hole  1   i  formed in the second support portion  1   c  so as to penetrate the second support portion  1   c  in the axial direction. A front end of the outer ring  32   b  of the second bearing  32  is in abutting contact with a step  1   j  formed on a front side of the second fixation hole  1   i . This inhibits the outer ring  32   b  of the second bearing  32  from moving forward with respect to the second support portion  1   c . A rear end of the outer ring  32   b  of the second bearing  32  is in abutting contact with a C ring  62  installed in a ring groove  1   k  formed in the second fixation hole  1   i . This inhibits the outer ring  32   b  of the second bearing  32  from moving rearward with respect to the second support portion  1   c.    
     The distal-end bearing portion  11   c  is inserted through the inner ring  32   a  of the second bearing  32 . The first threaded portion  11   b  has the outside diameter larger than an outside diameter of the distal-end bearing portion  11   c . A step  11   k  is formed between the first threaded portion  11   b  and the distal-end bearing portion  11   c . A front end of the inner ring  32   a  of the second bearing  32  is in contact with the step  11   k . This inhibits the inner ring  32   a  of the second bearing  32  from moving forward with respect to the distal-end bearing portion  11   c . A ring groove  11   e  is formed in the outer peripheral surface of the distal-end bearing portion  11   c  over the entire circumference of the distal-end bearing portion  11   c . A C ring  11   f  is installed in the ring groove  11   e . The C ring  11   f  is in contact with a rear end of the inner ring  32   a  of the second bearing  32 . This inhibits the inner ring  32   a  of the second bearing  32  from moving rearward with respect to the distal-end bearing portion  11   c . As described above, the inner ring  32   a  and the outer ring  32   b  are movable in a moving direction. Thus, the distal-end bearing portion  11   c  is movable in the axial direction. As a result, when the first screw shaft  11  is thermally expanded, the distal-end bearing portion  11   c  moves rearward (toward the distal end) (m 1  in  FIG. 3 ) to prevent deformation such as curving of the first screw shaft  11 . When the first screw shaft  11  is thermally contracted, the distal -end bearing portion  11   c  moves forward (toward the base end) (m 2  in  FIG. 3 ) to prevent stress that acts in a direction in which the first screw shaft  11  is pulled. 
     The first motor  21  (first driving source) rotates the first screw shaft  11 . As depicted in  FIG. 2  and  FIG. 3 , the first motor  21  (first driving source) is attached to a front end surface of the first support portion  1   b  of the base  1 . A rotating shaft  21   a  of the first motor  21  is inserted through the first fixation hole if and coupled to the base-end bearing portion  11   a  of the first screw shaft  11  via a first coupling member  41  such as a coupling. 
     As depicted in  FIG. 2 , the second screw shaft  12  is formed of a base-end bearing portion  12   a , a second threaded portion  12   b , and a distal-end bearing portion  12   c  that are formed in this order from a base end (rear end) toward a distal end (front end) as depicted in  FIG. 2 . The base-end bearing portion  12   a  and the distal-end bearing portion  12   c  are shaped like columns. A groove is formed in an outer peripheral surface of the second threaded portion  12   b . The groove in the first threaded portion  11   b  and the groove in the second threaded portion  12   b  are formed in the same direction. 
     The base-end bearing portion  12   a , which is a base end (rear end or the other end in the axial direction) of the second screw shaft  12 , is rotatably supported by the third bearings  33  fixed to the fourth support portion  1   e  so as to be immovable in the axial direction. A structure in which the base-end bearing portion  12   a  is rotatably supported by the third bearings  33  fixed to the fourth support portion  1   e  is similar to the structure in which the base-end bearing portion  11   a  of the first screw shaft  11  is rotatably supported by the first bearings  31  fixed to the first support portion  1   b  as described above. 
     The distal-end bearing portion  12   c , which is a distal end (front end or one end in the axial direction) of the second screw shaft  12 , is rotatably supported by the fourth bearing  34  fixed to the third support portion  1   d  so as to be movable in the axial direction. Thus, even when the second screw shaft  12  is thermally expanded and extended in the axial direction, the distal end of the second screw shaft  12  moves forward to prevent deformation such as curving of the second screw shaft  12 . When the second screw shaft  12  is thermally contracted, the distal-end bearing portion  12   c  moves rearward (toward the base end) to prevent stress that acts in a direction in which the second screw shaft  12  is pulled. A structure in which the distal-end bearing portion  12   c  is rotatably supported by the fourth bearing  34  fixed to the third support portion  1   d  is similar to the structure in which the distal-end bearing portion  11   c  of the first screw shaft  11  is rotatably supported by the second bearing  32  fixed to the second support portion  1   c  as described above. 
     The second motor  22  (second driving source) rotates the second screw shaft  12 . As depicted in  FIG. 2 , the second motor  22  (second driving source) is attached to a rear end surface of the fourth support portion  1   e  of the base  1 . A rotating shaft  22   a  of the second motor  22  is inserted through the third fixation hole  1   m  formed in the fourth support portion  1   e  and coupled to the base-end bearing portion  12   a  of the second screw shaft  12  via a second coupling member such as a coupling. A rotating direction of the first motor  21  is different from a rotating direction of the second motor  22 . In the present embodiment, the first motor  21  and the second motor  22  are servo motors. 
     As depicted in  FIG. 2 , the rear end (first end) of the first screw shaft  11  faces the front end (second end) of the second screw shaft  12 . A separation distance a (depicted in  FIG. 2 ) between the rear end (first end) of the first screw shaft  11  and the front end (second end) of the second screw shaft  12 ) is set to such a dimension that the rear end (first end) of the first screw shaft  11  does not contact the front end (second end) of the second screw shaft  12  when the first screw shaft  11  and the second screw shaft  12  are maximally thermally expanded. 
     The moving member  2  includes a base portion  2   a , a first protruding portion  2   b , and a second protruding portion  2   c . The base portion  2   a  is shaped like a block, and the object  80  such as a tool and a work (including a mounting portion used to attach the tool and the work to the base portion  2   a ) is mounted on the base portion  2   a . In the embodiment depicted in  FIG. 2 , the object  80  is a tool rest  81  and a tool  82  mounted on the tool rest  81 .  FIG. 2  illustrates a chuck  501  forming the machine tool  1000  and a work  502  mounted on the chuck  501 . The machine tool  1000  for which the feed apparatus  100  is used is a lathe. 
     The first protruding portion  2   b  extends downward from a lower portion of a front end portion (one end portion) of the base portion  2   a . A first nut  51  is provided on the first protruding portion  2   b . The first nut  51  is provided on a front side (one side) of the moving member  2  with respect to the center thereof in the axial direction. The first threaded portion  11   b  of the first screw shaft  11  is screwed in the first nut  51 . In conjunction with rotation of the first screw shaft  11 , the first nut  51  moves in the axial direction to move the moving member  2  in the axial direction. 
     The second protruding portion  2   c  extends downward from a lower portion of a rear end portion (the other end portion) of the base portion  2   a . A second nut  52  is provided on the second protruding portion  2   c . The second nut  52  is provided on a rear side (the other side) of the moving member  2  with respect to the center thereof in the axial direction. The second threaded portion  12   b  of the second screw shaft  12  is screwed in the second nut  52 . In conjunction with rotation of the second screw shaft  12 , the second nut  52  moves in the axial direction to move the moving member  2  in the axial direction. 
     As depicted in  FIG. 1  and  FIG. 2 , the first nut  51  and the second nut  52  are disposed at different positions in the axial direction. When the screw shafts  11  and  12  are ball screws, a plurality of balls is provided between the threaded portions  11   b  and  12   b  of the screw shafts  11  and  12  and the respective nuts  51  and  52 , and each of the nuts  51  and  52  is provided with a circulation mechanism that circulates the balls. 
     As depicted in  FIG. 2 , the first nut  51  is provided on the front side (one side) of the moving member  2  with respect to the position of the center of gravity  99  of the combination of the moving member  2  and the object  80  (hereinafter simply referred to as the position of the center of gravity  99 ) in the axial direction. The second nut  52  is provided on the rear side (the other side) of the moving member  2  with respect to the position of the center of gravity  99  in the axial direction. In other words, the position of the center of gravity  99  of the combination of the moving member  2  and the object  80  attached to the moving member  2  is located between a first screwed portion  91  in which the first screw shaft  11  is screwed in the first nut  51  of the moving member  2  and a second screwed portion  92  in which the second screw shaft  12  is screwed in the second nut  52  of the moving member  2 . 
     The control section  50  supplies a driving current to the first motor  21  and the second motor  22  to rotate the first motor  21  and the second motor  22  in different directions. The first screw shaft  11  and the second screw shaft  12  rotate in the same direction. When the screw shafts  11  and  12  are rotated by the motors  21  and  22 , respectively, the screw shafts  11  and  12  and the nuts  51  and  52  rotate relative to one another. The moving member  2  moves forward or rearward by a moving distance corresponding to the amounts of rotation of the screw shaft  11  and  12 . 
     As is apparent from the above description, as depicted in  FIG. 1  and  FIG. 2 , the feed apparatus  100  includes the base  1 , the moving member  2  that is movable in the predetermined axial direction with respect to the base  1 , the first screw shaft  11  provided on the base  1  parallel to the axial direction and rotatably supported by the base  1 , the second screw shaft  12  provided separately from the first screw shaft  11  on the base  1  parallel to the axial direction and rotatably supported by the base  1 , the first motor  21  (first driving source) mounted on the base  1  to rotate the first screw shaft  11 , the second motor  22  (second driving source) mounted on the base  1  to rotate the second screw shaft  12 , the first nut  51  provided on the moving member  2  and screwed over the first screw shaft  11  to move in the axial direction in conjunction with rotation of the first screw shaft  11 , and the second nut  52  provided on the moving member  2  and screwed over the second screw shaft  12  to move in the axial direction in conjunction with rotation of the second screw shaft  12 . The first nut  51  and the second nut  52  are disposed at different positions in the axial direction. 
     In this structure, when the first screw shaft  11  and the second screw shaft  12  are rotated to move the moving member  2 , the magnitude of pitching occurring in the moving member  2  as a result of relative rotation between the first screw shaft  11  and the first nut  51  is different from the magnitude of pitching occurring in the moving member  2  as a result of relative rotation between the second screw shaft  12  and the second nut  52 . In this case, pitching is rotation in a direction orthogonal to the axial direction and to a direction orthogonal to a direction in which the moving member  2  is disposed with respect to the base  1 . Therefore, unlike a single nut disposed at one position in the axial direction, the first nut  51  and the second nut  52  disposed at different positions in the axial direction can suppress the pitching occurring in the moving member  2 . Thus, the object  80  such as the work and the tool mounted on the moving member  2  can be suppressed from being displaced from a regular position. This improves the machining accuracy of the machine tool  1000 . 
     The first nut  51  is provided on the front side (one side) of the moving member  2  with respect to the center thereof in the axial direction. The second nut  52  is provided on the rear side (the other side) of the moving member  2  with respect to the center thereof in the axial direction. This allows the pitching occurring in the moving member  2  as a result of the relative rotation between the first screw shaft  11  and the first nut  51  and the pitching occurring in the moving member  2  as a result of the relative rotation between the second screw shaft  12  and the second nut  52  to act in opposite directions. Therefore, the pitching occurring in the moving member  2  can be more reliably suppressed. 
     As depicted in  FIG. 2 , the moving member  2  is loaded on the upper surface of the base  1 , and the object  80  is mounted on the moving member  2 . The first nut  51  is provided on the front side (one side) of the moving member  2  with respect to the position of the center of gravity  99  of the combination of the moving member  2  and the object  80  in the axial direction. The second nut  52  is provided on the rear side (the other side) of the moving member  2  with respect to the position of the center of gravity  99  of the combination of the moving member  2  and the object  80  in the axial direction. Effects of this configuration will be described below. 
     When the moving member  2  moves from the front side (one side) toward the rear side (the other side) with respect to the base  1 , the relative rotation between the first screw shaft  11  and the first nut  51  causes the moving member  2  to be pushed rearward (toward the other side). The center of gravity  99  is positioned rearward (on the other side) of the first nut  51 . Thus, when the moving member  2  is pushed rearward (toward the other side) by the first screw shaft  11 , the moving member  2  and the object  80  tend to remain at the position of the center of gravity  99  thereof. Consequently, the moving member  2  is caused to rotate clockwise about the first nut  51  (first screwed portion  91 ), and the moving member  2  and the object  80  is caused to move upward (a blank arrow depicted in  FIG. 2 ). 
     When the moving member  2  moves from the front side (one side) toward the rear side (the other side) with respect to the base  1 , the relative rotation between the second screw shaft  12  and the second nut  52  causes the moving member  2  to be pulled rearward (toward the other side). The center of gravity  99  is positioned forward (on the one side) of the second nut  52  (second screwed portion  92 ). Thus, when the moving member  2  is pulled rearward (toward the other side) by the second screw shaft  12 , the moving member  2  and the object  80  tend to remain at the position of the center of gravity  99  thereof. Consequently, the moving member  2  is caused to rotate clockwise about the second nut  52  (second screwed portion  92 ), and the moving member  2  and the object  80  are caused to move downward (a filled arrow depicted in  FIG. 2 ). 
     In this manner, while the moving member  2  and the object  80  are caused to move upward in conjunction with rotation of the first screw shaft  11 , they are caused to move downward in conjunction with rotation of the second screw shaft  12 . Thus, the pitching that causes the moving member  2  and the object  80  to move upward due to the rotation of the first screw shaft  11  and the pitching that causes the moving member  2  and the object  80  to move downward due to the rotation of the second screw shaft  12  offset each other. In contrast, when the moving member  2  moves from the rear side (the other side) toward the front side (one side), the pitching that causes the moving member  2  and the object  80  to move downward due to the rotation of the first screw shaft  11  similarly and the pitching that causes the moving member  2  and the object  80  to move upward due to the rotation of the second screw shaft  12  offset each other. 
     As described above, the pitching occurring in the moving member  2  and the object  80  as a result of the relative rotation between the first screw shaft  11  and the first nut  51  and the pitching occurring in the moving member  2  and the object  80  as a result of the relative rotation between the second screw shaft  12  and the second nut  52  act in opposite directions. Therefore, the pitching occurring in the moving member  2  can be reliably suppressed. As a result, the behavior of the object  80  mounted on the moving member  2  in motion is stabilized. Since the object  80  such as the work and the tool mounted on the moving member  2  is not displaced from the regular position, the machining accuracy of the machine tool  1000  is not reduced. 
     As depicted in  FIG. 1 , the axis of the first screw shaft  11  and the axis of the second screw shaft  12  are coaxially disposed. Consequently, even when the position of the center of gravity  99  of the combination of the moving member  2  and the object  80  in the width direction is shifted from the axis of the first screw shaft  11  or the second screw shaft  12  as depicted in  FIG. 1 , radial loads imposed on the first screw shaft  11  and the second screw shaft  12  by the moving member  2  and the object  80  are not unbalanced. Thus, the life of one of the first and second screw shafts  11  and  12  is prevented from being shortened. The radial load is load acting in the direction orthogonal to the axial direction of the first screw shaft  11  and the second screw shaft  12 . In the present embodiment, the radial load is a load in the vertical direction in  FIG. 2 . 
     As depicted in  FIG. 2 , the first screw shaft  11  and the second screw shaft  12  each are rotatably supported at the both ends thereof by the base  1 . Consequently, the both ends of each of the first and second screw shafts  11  and  12  are constrained in the radial direction orthogonal to the axial direction. Thus, as compared to a structure in which one end of each of the first and second screw shafts  11  and  12  is released in the radial direction, the present structure suppresses the first screw shaft  11  and the second screw shaft  12  from being destroyed by self-excited vibration (divergent vibration) when the first screw shaft  11  and the second screw shaft  12  move at high speed. 
     As depicted in  FIG. 2 , the first bearings  31  rotatably support one end (front end) of the two ends of the first screw shaft  11  such that the end is immovable in the axial direction. The second bearing  32  rotatably supports the other end (rear end) of the two ends of the first screw shaft  11  such that the end is movable in the axial direction. The third bearings  33  rotatably support one end (rear end) of the two ends of the second screw shaft  12  such that the end is immovable in the axial direction. The fourth bearing  34  rotatably supports the other end (front end) of the two ends of the second screw shaft  12  such that the end is movable in the axial direction. 
     As described above, one of the two ends of each of the screw shafts  11  and  12  is immovable in the axial direction, whereas the other of the two ends of each of the screw shafts  11  and  12  is movable in the axial direction. Consequently, when the screw shafts  11  and  12  are thermally expanded by friction between the screw shafts  11  and  12  and the nuts  51  and  52 , deformation such as curving of the screw shafts  11  and  12  is prevented. Thus, when the screw shafts  11  and  12  are thermally expanded, deformation such as curving of the screw shafts  11  and  12  is prevented without the need for a mechanism that allows cooling water to flow along the axes of the screw shafts  11  and  12 . This prevents increased dimensions of the screw shafts  11  and  12  in the radial direction as a result of formation of the cooling water passage, and precludes an increase in manufacturing costs of the feed apparatus  100 . When the screw shafts  11  and  12  are thermally contracted, stress is prevented from being generated which acts in the direction in which the screw shafts  11  and  12  are pulled. 
     As depicted in  FIG. 2 , the first bearings  31  rotatably support the front end (one end) of the two ends of the first screw shaft  11  in the axial direction such that the end is immovable in the axial direction. The second bearing  32  rotatably supports the rear end (the other end) of the two ends of the first screw shaft  11  in the axial direction such that the end is movable in the axial direction. The third bearings  33  rotatably support the rear end (the other end) of the two ends of the second screw shaft  12  in the axial direction such that the end is immovable in the axial direction. The fourth bearing  34  rotatably supports the front end (one end) of the two ends of the second screw shaft  12  in the axial direction such that the end is movable in the axial direction. In this configuration, while the first nut  51  of the moving member  2  is close to the first bearings  31 , the first bearings  31  rotatably supporting the first screw shaft  11  such that the first screw shaft  11  is immovable in the axial direction secure feeding rigidity of the first screw shaft  11 . On the other hand, although the feeding rigidity of the first screw shaft  11  decreases as the first nut  51  of the moving member  2  moves away from the first bearings  31 , the second nut  52  of the moving member  2  approaches the third bearings  33 , which rotatably support the second screw shaft  12  such that the second screw shaft  12  is immovable in the axial direction, and thus the feeding rigidity of the second screw shaft  12  increases. In this manner, regardless of the position of the moving member  2  in the axial direction, the feeding rigidity of the combination of the first screw shaft  11  and the second screw shaft  12  is not reduced. Accordingly, the amount of machining performed by the machine tool  1000  does not become unstable, which does not reduce the machining accuracy of the machine tool  1000 . 
     As described above, since the groove in the first threaded portion lib of the first screw shaft  11  and the groove in the second threaded portion  12   b  of the second screw shaft  12  are formed in the same direction, the same rolling die may be used to form the groove in the first screw shaft  11  and the groove in the second screw shaft  12 . Thus, the groove in the first screw shaft  11  and the groove in the second screw shaft  12  are formed so as to have the same pitch in the axial direction. As a result, when the first screw shaft  11  and the second screw shaft  12  are rotated by the same amount, the distance that the moving member  2  fed by the first screw shaft  11  is the same as the distance that the moving member  2  is fed by the second screw shaft  12 . The moving member  2  can be stably moved in the axial direction. 
     As depicted in  FIG. 2 , the distal-end bearing portion  11   c  (first end) of the ends of the first screw shaft  11 , which is an end of the first screw shaft  11  opposite to the end thereof connected to the first motor  21  (first driving source), faces the distal-end bearing portion  12   c  (second end) of the ends of the second screw shaft  12 , which is an end of the second screw shaft  12  opposite to the end thereof connected to the second motor  22  (second driving source). The separation distance a between the distal-end bearing portion  11   c  (first end) and the distal-end bearing portion  12   c  (second end) is set to such a dimension that the distal-end bearing portion  11   c  (first end) does not contact the distal-end bearing portion  12   c  (second end) when the first screw shaft  11  and the second screw shaft  12  are maximally thermally expanded. Thus, even when the screw shafts  11  and  12  rotate and are thermally expanded by frictional heat generated between the screw shafts  11  and  12  and the nuts  51  and  52 , the distal-end bearing portion  11   c  (first end) does not contact the distal-end bearing portion  12   c  (second end). This prevents the screw shafts  11  and  12  from being curved by the contact between the distal-end bearing portion  11   c  (first end) and the distal-end bearing portion  12   c  (second end), and thus the machining accuracy of the machine tool  100  is not reduced. The maximum amounts of thermal expansion of the first screw shaft  11  and the second screw shaft  12  are set as needed based on the maximum amount of change in temperature of the feed apparatus  100 , which is estimated as specifications when the feed apparatus  100  is designed, simulation results for the amount of thermal expansion resulting from a change in the temperature of the feed apparatus  100 , actually measured values of the amount of thermal expansion resulting from a change in the temperature of the feed apparatus  100 , or the like. 
     A feed apparatus  200  in a comparative example will be described below using  FIG. 4  and  FIG. 5 . As depicted in  FIG. 4  and  FIG. 5 , in the feed apparatus  200  in the comparative example, a base  101  is provided with two screw shafts  111  and  112  and two motors  121  and  122  that rotate the screw shafts  111  and  112 . The screw shafts  111  and  112  are screwed in threaded holes  102   a  and  102   b , respectively, formed at the same position in the axial direction. In an example illustrated in  FIG. 5 , the position of the center of gravity  199  of a combination of a moving member  102  and an object  180  mounted on the moving member  102  (hereinafter referred to as the position of the center of gravity  199 ) is located on the rear side of screwed portions  191  and  192  between the moving member  102  and the screw shafts  111  and  112 . 
     In the feed apparatus  200  configured as described above, when the screw shafts  111  and  112  rotate, the moving member  102  behaves as follows. When the moving member  102  moves from the front side toward the rear side with respect to the base  101 , the moving member  102  is pushed rearward by the screw shafts  111  and  112 . Since the moving member  102  and the object  180  tend to remain at the position of the center of gravity  199  thereof, the moving member  102  will rotate clockwise about screwed portions  191  and  192  between the moving member  102  and the screw shafts  111  and  112  and the object  180  mounted on the moving member  102  will move upward (a blank arrow depicted in  FIG. 5 ). On the other hand, when the moving member  102  moves from the rear side toward the front side with respect to the base  101 , the moving member  102  is pulled forward by the screw shafts  111  and  112 . Since the moving member  102  and the object  180  tend to remain at the position of the center of gravity  199  thereof, the moving member  102  will rotate counterclockwise about the screwed portions  191  and  192  and the object  180  mounted on the moving member  102  will move downward (a filled arrow depicted in  FIG. 5 ). 
     When the moving member  102  is caused to rotate using the screwed portions  191  and  192  as centers of rotation as described above, a work and a tool that are the object  180  mounted on the moving member  102  in motion exhibit unstable behavior. The work and the tool that are the object  180  mounted on the moving member  102  slightly move upward or downward and are displaced from regular positions thereof. Thus, the machining accuracy of the machine tool including the feed apparatus  200  is reduced. 
     As depicted in  FIG. 4  and  FIG. 5 , in the feed apparatus  200  of the comparative example, one end of each of the screw shafts  111  and  112  is rotatably supported by corresponding bearings  131  and  132  provided on the base  101  so that the screw shafts  111  and  112  are immovable in the axial direction. The other end of each of the screw shafts  111  and  112  is rotatably supported by a corresponding one of bearings  133  and  134  provided on the base  101  so that the screw shafts  111  and  112  are movable in the axial direction. In the feed apparatus  200 , the effect of elastic deformation of the screw shafts  111  and  112  increasers as threaded holes  102   a  and  102   b  in the moving member  102  moves away from the bearings  131  and  132  provided on the base  101  so that the screw shafts  111  and  112  are immovable in the axial direction to. This reduces the feeding rigidity of the screw shafts  111  and  112 . 
     As depicted in  FIG. 4 , when the position of the center of gravity  199 , in the width direction, of the combination of the moving member  102  and the object  180  mounted on the moving member  102  is shifted from an intermediate position between the pair of screw shafts  111  and  112  provided in parallel toward the screw shaft  111  on one side, radial loads acting on the respective screw shafts  111  and  112  are unbalanced. Thus, a heavier radial load is imposed on the screw shaft  111 , which is closer to the position of the center of gravity  199 , than on the other screw shaft  112 . Consequently, the life of the screw shaft  111 , which is closer to the position of the center of gravity  199 , is shortened. 
     In the above-described embodiment, the first screw shaft  11  and the second screw shaft  12  are provided on a straight line as depicted in  FIG. 1 . However, an embodiment is possible in which the first screw shaft  11  and the second screw shaft  12  are not provided on a straight line but are offset from each other in the width direction. Even in this embodiment, the position of the center of gravity  99  of the combination of the moving member  2  (moving member) and the object  80  mounted on the moving member  2  is located between the first screwed portion  91  and the second screwed portion  92 . Thus, as described above, a force exerted by the first screw shaft  11  to move the object  80  upward or downward and a force exerted by the second screw shaft  12  to move the object  80  downward or upward offset each other. The object  80  mounted on the moving member  2  thus does not move in the up-down direction. Accordingly, the machining accuracy of the machine tool  1000  is not reduced. 
     An embodiment having configurations (1) to (4) is also possible instead of the above-described embodiment. 
     (1) The first bearings  31  rotatably support the rear end (the other end) of the two ends of the first screw shaft  11  in the axial direction such that the end is immovable in the axial direction. 
     (2) The second bearing  32  rotatably supports the front end (one end) of the two ends of the first screw shaft  11  in the axial direction such that the end is movable in the axial direction. 
     (3) The third bearings  33  rotatably support the front end (one end) of the two ends of the second screw shaft  12  in the axial direction such that the end is immovable in the axial direction. 
     (4) The fourth bearing  34 rotatably supports the rear end (the other end) of the two ends of the second screw shaft  12  in the axial direction such that the end is movable in the axial direction. 
     Even in such an embodiment, regardless of the position of the moving member  2  in the axial direction, the feeding rigidity of the combination of the first screw shaft  11  and the second screw shaft  12  does not decrease. Furthermore, the amount of machining performed by the machine tool  1000  does not become unstable, which does not reduce the machining accuracy of the machine tool  1000 . 
     An embodiment having configurations (5) to (8) is also possible instead of the above-described embodiment. 
     (5) The first bearings  31  rotatably support the front end (one end) of the two ends of the first screw shaft  11  in the axial direction such that the end is immovable in the axial direction. 
     (6) The second bearing  32  rotatably supports the rear end (the other end) of the two ends of the first screw shaft  11  in the axial direction such that the end is movable in the axial direction. 
     (7) The third bearings  33 rotatably support the front end (one end) of the two ends of the second screw shaft  12  in the axial direction such that the end is immovable in the axial direction. 
     (8) The fourth bearing  34  rotatably supports the rear end (the other end) of the two ends of the second screw shaft  12  in the axial direction such that the end is movable in the axial direction. 
     An embodiment having configurations (9) to (12) is also possible instead of the above-described embodiment. 
     (9) The first bearings  31  rotatably support the rear end (the other end) of the two ends of the first screw shaft  11  in the axial direction such that the end is immovable in the axial direction. 
     (10) The second bearing  32  rotatably supports the front end (one end) of the two ends of the first screw shaft  11  in the axial direction such that the end is movable in the axial direction. 
     (11) The third bearings  33  rotatably support the rear end (the other end) of the two ends of the second screw shaft  12  in the axial direction such that the end is immovable in the axial direction. 
     (12) The fourth bearing  34  rotatably supports the front end (one end) of the two ends of the second screw shaft  12  in the axial direction such that the end is movable in the axial direction. 
     Even in the embodiment having the configurations (5) to (8) or the embodiment having the configurations (9) to (12), the force exerted by the first screw shaft  11  to move the object  80  upward or downward and the force exerted by the second screw shaft  12  to move the object  80  downward or upward offset each other. The object  80  mounted on the moving member  2  thus does not move in the up-down direction. Accordingly, the machining accuracy of the machine tool  1000  is not reduced. 
     In the above-described embodiment, the groove in the first threaded portion  11   b  of the first screw shaft  11  and the groove in the second threaded portion  12   b  of the second screw shaft  12  are formed in the same direction. However, an embodiment is possible in which the groove in the first threaded portion  11   b  of the first screw shaft  11  and the groove in the second threaded portion  12   b  of the second screw shaft  12  are formed in different directions. 
     In the above-described embodiment, the first motor  21  is provided on the first support portion  1   b  formed on the front side (one side) of the base  1 , and the second motor  22  is provided on the fourth support portion  1   e  formed on the rear side (the other side) of the base  1 . However, an embodiment is possible in which the first motor  21  is provided on the second support portion  1   c  formed in the central portion of the base  1  and the second motor  22  is provided on the third support portion  1   d  formed in the central portion of the base  1 . In this embodiment, the first motor  21  and the second motor  22  rotate in the opposite directions. 
     An embodiment is possible in which the first motor  21  is provided on the first support portion  1   b  formed on the front side (one side) of the base  1  and the second motor  22  is provided on the third support portion  1   d  formed in the central portion of the base  1 . Alternatively, an embodiment is possible in which the first motor  21  is provided on the second support portion  1   c  formed in the central portion of the base  1  and the second motor  22  is provided on the fourth support portion  1   e  formed on the rear side (the other side) of the base  1 . In these embodiments, the first motor  21  and the second motor  22  rotate in the same direction. 
     In the above-described embodiment, the object  80  is mounted on the moving member  2 . However, an embodiment is possible in which the object  80  is not mounted on the moving member  2 . In this embodiment, the first nut  51  is provided on the front side (one side) of the moving member  2  in the axial direction with respect to the position of the center of gravity of the moving member  2 . The second nut  52  is provided on the rear side (the other side) of the moving member  2  in the axial direction with respect to the position of the center of gravity of the moving member  2 . Such a configuration allows the pitching occurring in the moving member  2  as a result of the relative rotation between the first screw shaft  11  and the first nut  51  and the pitching occurring in the moving member  2  as a result of the relative rotation between the second screw shaft  12  and the second nut  52  to act in the opposite directions. Therefore, the pitching occurring in the moving member  2  can be reliably suppressed. 
     In the above-described embodiment, the first nut  51  and the second nut  52  are provided on the moving member  2 . However, an embodiment is also possible in which a first nut screwed over the first screw shaft  11  is formed in the moving member  2  and a second nut screwed over the second screw shaft  12  is formed in the moving member  2 . 
     An embodiment is also possible in which a driving source such as an air actuator or an engine is used instead of the motors  21  and  22  that rotate the screw shafts  11  and  12 .