Patent Publication Number: US-11654519-B2

Title: Machine tool

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority to Japanese Patent Application Serial No. 2018-105205, filed on May 31, 2018, and titled “VEHICLE”, and Japanese Patent Application Serial No. 2018-180738, filed on Sep. 26, 2018 and titled “SYSTEM AND CONTROL METHOD,” which are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The following description relates to a machine tool. 
     BACKGROUND OF THE INVENTION 
     In typical machine tools, a bed is generally supported on an installation surface such as a floor surface at four positions corresponding to the four corners of the bed. This support is generally referred to as four-point support. In such a four-point support, it is difficult to perform height adjustment at the four points such that the bed is supported on the installation surface with a uniform force at every point. 
     To cope with this problem, Japanese Laid-Open Patent Publication No. 2005-131768 discloses a machine tool in which the bed is supported on an installation surface such as a floor surface at three points. In this machine tool, support points on the installation surface are provided at three positions of the lower surface of the bed, which has a simple quadrilateral box shape. 
     Japanese Laid-Open Patent Publication No. 10-138132 discloses a typical grinding machine in which a work table is supported on the bed such that the work table is movable back and forth in a sideward direction and a column is supported on the bed such that the column is movable back and forth in a front-rear direction in the same manner. A tool shaft including a tool is supported by the column such that the tool shaft is movable up and down. Such a grinding machine is generally referred to as columnar type. 
     Conventionally, in some grinding machines, a saddle that moves back and forth in the front-rear direction is supported on the bed, and a work table is supported on the saddle such that the work table is movable back and forth in the sideward direction. Further, a column is arranged at a fixed position, and a tool shaft including a tool is supported on the column such that the tool shaft is movable back and forth. Such a grinding machine is generally referred to as a saddle type. 
     In the machine tool of Japanese Laid-Open Patent Publication No. 2005-131768, while three-point support is employed, the bed has a simple quadrilateral box shape. In order to ensure sufficient strength and stability, the bed needs to be formed through casting. Thus, it is difficult to prevent the bed from becoming heavy. In addition, the rigidity of the entire bed decreases. Decreases in the rigidity result in decreases in the accuracy of machining a workpiece. Further, the quadrilateral box-shaped bed tends to be unstable when supported at three points. Additionally, the bed has a high volume and weight. This reduces the natural frequency and thus reduces the resonant frequency, which adversely affects the machining accuracy. When the resonant frequency of the bed decreases, the amplitude of vibration increases. As a result, the machining accuracy decreases. 
     In the column-type grinding machine of Japanese Laid-Open Patent Publication No. 10-138132, when the column accelerates or decelerates in the front-rear direction, the column tends to be inclined in the front-rear direction due to the inertial force of the column. Further, in the column-type grinding machine, the column has a vertically long shape in which the column is relatively long in the vertical direction. This reduces the thickness of the bed located below the column. That is, since the vertically-long, heavy column moves in the front-rear direction on the thin bed, the bed easily deforms due to the weight of the column. Thus, it is difficult to limit decreases in the position accuracy of the column in the vertical direction. As a result, machining with high accuracy is difficult. 
     In the saddle-type grinding machine, the table is mounted on the saddle that moves back and forth. Thus, the total weight of the saddle and the table may be excessive. While the saddle needs to be reduced in size in order to decrease the weight, the table needs to have a certain length in the sideward direction for machining a workpiece. Thus, the opposite ends of the table protrude from the saddle and are overhung. Accordingly, when the table that moves in the sideward direction particularly reaches the right or left movable end, the overhung part droops greatly. As a result, machining with high accuracy is difficult. 
     SUMMARY OF THE INVENTION 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     It is an objective of the following description to provide a machine tool in which the accuracy of machining a workpiece is improved. 
     According to one general aspect, a machine tool includes a bed including a bottom surface and legs arranged at three positions of the bottom surface. The bed includes two oblique sides arranged such that the bottom surface has a triangular shape. 
     According to another general aspect, a machine tool includes a bed, a first support surface arranged on the bed, the first support surface extending along a substantially horizontal first axis, a work table supported by the first support surface such that the work table moves back and forth along the first axis, a second support surface arranged on the bed, the second support surface extending along a substantially horizontal second axis, which is perpendicular to the first axis, a machining unit supported by the second support surface such that the machining unit moves back and forth along the second axis, a third support surface arranged at the machining unit, the third support surface extending along a substantially vertical third axis, and a machining shaft unit supported by the third support surface such that the machining shaft unit moves in a vertical direction along the third axis, the machining shaft unit including a tool for machining a workpiece on the work table. The second support surface is located at a position higher than the first support surface. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a machine tool according to a first embodiment as viewed from above. 
         FIG.  2    is an exploded perspective view of the machine tool shown in  FIG.  1   . 
         FIG.  3    is a perspective view of the machine tool of  FIG.  1    as viewed from below. 
         FIG.  4    is a front view of the machine tool shown in  FIG.  1   . 
         FIG.  5    is a partially broken front view of the machine tool shown in  FIG.  4   . 
         FIG.  6    is a side view of the machine tool shown in  FIG.  1   . 
         FIG.  7    is a cross-sectional view taken along line  7 - 7  in  FIG.  4   . 
         FIG.  8    is a cross-sectional view taken along line  8 - 8  in  FIG.  6   . 
         FIG.  9    is a partial cross-sectional view of a machine tool according to a second embodiment. 
         FIG.  10    is a partial cross-sectional view of the machine tool shown in  FIG.  9   . 
     
    
    
     Throughout the drawings and the detailed description, the same reference numerals refer to the same elements. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience. 
     DETAILED DESCRIPTION 
     This description provides a comprehensive understanding of the methods, apparatuses, and/or systems described. Modifications and equivalents of the methods, apparatuses, and/or systems described are apparent to one of ordinary skill in the art. Sequences of operations are exemplary, and may be changed as apparent to one of ordinary skill in the art, with the exception of operations necessarily occurring in a certain order. Descriptions of functions and constructions that are well known to one of ordinary skill in the art may be omitted. 
     Exemplary embodiments may have different forms, and are not limited to the examples described. However, the examples described are thorough and complete, and convey the full scope of the disclosure to one of ordinary skill in the art. 
     First Embodiment 
     A first embodiment of the present disclosure will now be described with reference to the drawings. 
     As shown in  FIGS.  1  and  2   , a machine tool according to a first embodiment includes a bed  10  and a table  12 . The bed  10  includes a bed body  11 . The table  12  is supported to be movable along a horizontal X-axis at the front part of the bed body  11 . The X-axis, which is a first axis, extends in the sideward direction of the machine tool. Further, the machine tool includes two support members  14  and a machining unit  15 . The two support members  14  are respectively fixed to the opposite sides of the upper surface of the bed body  11 . The machining unit  15  is supported to be movable along a horizontal Z-axis on the two support members  14 . The Z-axis, which is a second axis, extends in the front-rear direction of the machine tool and is perpendicular to the X-axis. The support members  14  configure part of the bed  10 . The machining unit  15  includes a rotation grindstone  16  movable along a Y-axis. The Y-axis, which is a third axis, extends in the vertical direction of the machine tool and is perpendicular to the X-axis and Z-axis. In a state in which a workpiece is supported on the table  12 , the table  12  moves along the X-axis and the rotation grindstone  16  moves along the Z-axis and the Y-axis. With such movements, the rotation grindstone  16  rotates about an axis parallel to the Z-axis is used to perform a predetermined machining on the workpiece. 
     The structure of each component of the machine tool will now be described in detail. 
     First, the bed body  11  and the components related to the bed body  11  will be described. 
     Referring to  FIGS.  1  to  3   , the bed body  11  is an integrated body entirely formed through casting and includes a substantially triangular bottom plate  20 , two side plates, a front plate  23 , and a rear plate  24 . Each of the two side plates includes a substantially triangular lower side plate  21  and a substantially triangular upper side plate  22 . The lower side plate  21  is one example of an oblique side. The upper edges of the two side plates, that is, the upper edges of the two upper side plates  22 , extend along the Z-axis in parallel to each other. The upper ends of the two upper side plates  22  are provided with horizontal upper plates  25 , and the front surfaces of the two upper side plates  22  are provided with upright plates  26 . As shown in  FIG.  7   , an inner plate  27  is arranged rearward from the front plate  23 . As shown in  FIG.  2   , the bed body  11  includes an open portion  28  that opens upward between the two upper plates  25 . 
     As shown in  FIG.  3   , the bottom plate  20  is triangular because of the arrangement of the lower side plates  21 , which are the oblique sides. That is, the lower side plates  21  are configured and arranged such that the bottom plate  20  has a triangular shape. In other words, the lower side plates  21  are arranged to define the bottom plate  20  in a triangular shape. The lower edges of the two side plates, that is, the lower edges of the two lower side plates  21 , configure two of the three outer edges (sides) of the triangular bottom plate  20 . Thus, the bottom plate  20  includes two triangular tops located at the front part of the bottom plate  20  and the remaining one triangular top located at the rear part of the bottom plate  20 . The three tops are provided with legs. That is, the two front tops are provided with front legs  31 , and the rear top is provided with a rear leg  32 . The two front legs  31  are located on a line parallel to the X-axis, and the rear leg  32  is located on a line that extends through the center of the two front legs  31  and is parallel to the Z-axis. 
     As shown in  FIGS.  6  and  7   , each front leg  31  includes a fixed cam body  34  that has an inclined surface  33  and a leg member  35  that is movable up and down. The upper surface of the leg member  35  is provided with a movable cam body  37  having an inclined surface  36  that is in contact with the inclined surface  33 . The leg member  35  is prevented from being separated from the fixed cam body  34  and the bed body  11  by restriction portions  39  arranged at the front and rear of the leg member  35 . The leg member  35  is arranged on an installation surface such as a floor surface. Screw action based on rotation of an adjustment screw  38  causes the inclined surface  33  of the movable cam body  37  to slide on the inclined surface  36  of the fixed cam body  34 . This moves the leg member  35  up and down. As a result, the height of the front leg  31  is adjusted. The height of the rear leg  32  is fixed. 
     As shown in  FIGS.  3 ,  6 , and  7   , the rear plate  24  is inclined such that the lower part is located frontward from the upper part. A rear end corner of the bottom plate  20  is located rearward from the rear plate  24 , and the rear leg  32  is located at the rear end corner. Two reinforcement ribs  41 , which are reinforcement portions, are provided between the rear end corner of the bottom plate  20  and the outer surface of the rear plate  24 . The two reinforcement ribs  41  have a triangular shape. Further, the two reinforcement ribs  41  are inclined and extended such that the reinforcement ribs  41  are located closer to each other toward the lower side. The reinforcement ribs  41  are integrated with the rear end corner of the bottom plate  20  and the rear plate  24 . Rear ends  29  of the two lower side plates  21  extend to the lower ends of the reinforcement ribs  41 . 
     As shown in  FIGS.  1 ,  2 , and  6   , a gutter-shaped table traveling portion  43  extending along the X-axis is arranged between the front ends of the two upper side plates  22  at the front of the upright plates  26 . The table traveling portion  43  opens upward and has opposite ends that open in the longitudinal direction. The two front legs  31  are located immediately below the opposite ends of the table traveling portion  43 , more specifically, located immediately below the center of the opposite ends in the width direction. The opposite ends of the table traveling portion  43  are closed by end members  44 . First guide rails  45  are fixed to the table traveling portion  43  between the two end members  44  such that the first guide rails  45  are arranged back and forth. The first guide rails  45  are first support surfaces. The table  12  is supported by the first guide rails  45  to be slidable along the X-axis. The table  12  is moved along the X-axis by, for example, a linear motor (not shown). 
     As shown in  FIGS.  1 ,  2 ,  4 , and  5   , the two support members  14  are respectively fixed on the upper surfaces of the two upper plates  25  by bolts  13  such that the support members  14  are located on the opposite sides of the open portion  28 . The upper parts of the front ends of the two support members  14  are provided with overhung parts  51  that protrude frontward. A second guide rail  52  extending along the Z-axis is coupled to the upper surface of each support member  14 . The second guide rail  52 , which is a second support surface, extends over the entire length of the support member  14  including the overhung part  51  in the direction in which the Z-axis extends. 
     As shown in  FIGS.  1  and  2   , a unit frame  61 , which is a support body, arranged between the second guide rails  52  and in the open portion  28 . The unit frame  61  is movable along the Z-axis. The unit frame  61  includes two sliders  62 , which are side members, and quadrilateral support frame  63 . The two sliders  62  are respectively supported by the two second guide rails  52 . The support frame  63  is arranged between the two sliders  62  and fixed to the two slides  62 . Third guide rails  65  extending along the Y-axis are coupled to the inner surface of the support frame  63 . The third guide rails  65  are third support surfaces. When linear motors  66  arranged on the opposite sides of the support frame  63  are driven, the support frame  63  and the sliders  62  move back and forth along the second guide rails  52  in the open portion  28 . A curved portion, that is, a side bulged portion  67  bulged outward in an arcuate manner, is integrated with the outer surface of each slider  62 . Reinforcement ribs  68  are integrated with the inner surface of the support frame  63 . Curved portions, that is, rear bulged portions  69  bulged in an arcuate manner, are integrated with the rear surface of each support frame  63 . 
     As shown in  FIGS.  1 ,  2 , and  7   , a lift frame  71  is supported by the third guide rails  65  to be movable up and down in the support frame  63 . A motor  72  is coupled to the lower end of the support frame  63 . In the support frame  63 , a ball screw  73  is supported to extend along the Y-axis. The ball screw  73  rotates when the motor  72  is driven. The lift frame  71  moves up and down as the ball screw  73  rotates forward and backward. A grindstone shaft unit  74 , which is a machining shaft unit, is supported by the lift frame  71 . A grindstone shaft  76  that is rotated by a motor  75  is supported by the grindstone shaft unit  74 . The rotation grindstone  16 , which is located frontward from the overhung parts  51 , is supported by the front end of the grindstone shaft  76 . 
     In the present embodiment, the machining unit  15  is configured by, for example, the unit frame  61 , the lift frame  71 , and the grindstone shaft unit  74 . As shown in  FIG.  7   , in a state in which the machining unit  15  is supported by the second guide rails  52 , the center of gravity α of the machining unit  15  is located in the inner space of the bed body  11 . The center of gravity α of the machining unit  15  is located at a position lower than the second guide rails  52 . Further, the center of gravity β of the machine tool is located at a position lower than the center of gravity α of the machining unit  15 . 
     As shown in  FIGS.  3 ,  4 , and  8   , the lower surface of the bed body  11  has an arcuate recess  81 . The recess  81  is located immediately below the center of the table traveling portion  43  in the longitudinal direction and the width direction. In the recess  81 , two projections  82  are spaced apart from each other. Receiving members  83  are respectively fixed to the lower surfaces of the two projections  82 . The receiving members  83  respectively include internal threads  84  in which the spirals are wound in different directions. Threaded parts of the opposite ends of an external thread member  85  are coupled to the two internal threads  84 . The receiving members  83 , which are first thread bodies, and the external thread member  85 , which is a second thread body, configure an adjustment member  86  having the form of a turnbuckle. A locknut  87  that resists loosening is coupled to each threaded part of the external thread member  85 . The locknut  87  is configured by, for example, a double nut. An operation part arranged on the center of the external thread member  85  is operated to rotate the external thread member  85 . This causes the two receiving members  83  to receive force that moves the receiving members  83  toward or away from each other. As a result, the bed body  11  is deformed to adjust the straightness of the first guide rail  45 . 
     The two upper side plates  22 , the front plate  23 , the rear plate  24 , the inner plate  27 , and the table traveling portion  43  have through-holes  91 , which are thinned portions. The through-holes  91  have various shapes such as quadrilateral, circle, triangle, trapezoid, and deformed quadrilateral. The lower surface of the bottom plate  20  has recesses  90 . 
     As shown in  FIGS.  7  and  8   , two plate-shaped first reinforcement frames  92  and two plate-shaped second reinforcement frames  93  are arranged between the front plate  23  and the inner plate  27 . The first reinforcement frames  92  and the second reinforcement frames  93 , which are reinforcement portions, are integrated with the front plate  23  and the inner plate  27 . As shown in  FIG.  8   , as viewed in the direction in which the Z-axis extends, the first reinforcement frames  92  and the second reinforcement frames  93  are symmetrical. The first reinforcement frames  92  and the second reinforcement frames  93  cross each other to be integrated with each other. Quadrilateral through-holes  91 , which are thinned portions, extend through the middle parts of the first reinforcement frames  92  and the second reinforcement frames  93  in the vertical direction. The first reinforcement frames  92  extend from the center of the lower surface of the table traveling portion  43  to the intersection parts of the upper side plate  22  and the bottom plate  20 . Thus, the front legs  31  and the surroundings of the front legs  31  are reinforced by the intersection points. The second reinforcement frames  93  extend from the center of the lower surface of the table traveling portion  43  to the part of the bottom plate  20  where the recess  81  is located. The front plate  23  and the inner plate  27  have through-holes  91  at positions located away from the first reinforcement frames  92  and the second reinforcement frames  93 . 
     The operation of the machine tool will now be described. 
     In the machine tool of the present embodiment, a workpiece is supported on the table  12 . In a state in which the rotation grindstone  16  is rotating, the lift frame  71  and the rotation grindstone  16  are moved along the third guide rails  65  in the direction in which the Y-axis extends. This adjusts the height of the rotation grindstone  16 . Further, the table  12  is moved back and forth along the first guide rails  45  in the direction in which the X-axis extends, and the grindstone shaft unit  74  and the rotation grindstone  16  are moved along the second guide rails  52  in the direction in which the Z-axis extends. In this manner, the rotation grindstone  16  is used to grind the workpiece. 
     The machine tool of the present embodiment has the following advantages. 
     (1) The machine tool includes two front legs  31  and one rear leg  32  at three positions of the bottom surface of the bottom plate  20  of the bed  10 . Thus, the machine tool is supported at three points on an installation surface such as the floor surface of a factory. That is, the machine tool can be supported stably regardless of the flatness of the installation surface. As the adjustment screws  38  of the front legs  31  are operated, the inclined surfaces  33  and  36  slide relative to each other. This adjusts the heights of the leg members  35  of the front legs  31 . Thus, the horizontality and height of the machine tool are maintained properly. In contrast, in a bed with four-point support, adjusting the heights of the legs is troublesome. Thus, the accuracy of machining a workpiece may be reduced due to insufficient adjustment. Three-point support allows adjustment to be performed relatively easily and thus reduces such inconvenience. 
     (2) The bottom plate  20  of the bed body  11  has a triangular shape. Thus, as compared to a prior-art machine tool having a quadrilateral bottom plate, the bed body  11  is reduced in size. This reduces the weight of the machine tool. Particularly, in the present embodiment, the bottom plate  20  is substantially triangular, and the lower side plate  21  and the upper side plate  22  are triangular. Further, the rear plate  24  is inclined such that the lower part is located frontward from the upper part. This reduces the volume of the bed body  11  and thus further reduces the size and weight of the bed body  11 . Such reduction of the bed body  11  in size improves the rigidity of the entire bed body  11  and increases the accuracy of machining a workpiece. Additionally, the bed body  11  includes the recesses  90  and the through-holes  91  at multiple positions of the bed body  11  while satisfying the required strength. This reduces limits on installation places of the machine tool and facilitates movement and conveyance of the machine tool. 
     (3) The reduction of the bed body  11  in size, the reduction of the volume of the bed body  11 , and the formation of the recesses  90  and the through-holes  91  allow the bed body  11  to be formed with fewer components and reduced in weight. This increases the natural frequency of the bed body  11 . When the vibration energy is the same, the amplitude of the resonant frequency decreases as the resonant frequency of the bed body  11  increases. Thus, the machining accuracy is improved. 
     (4) The parts of the bed body  11  other than the table traveling portion  43  have a smaller width than the table traveling portion  43  (dimension along the X-axis). Further, the bed body  11  includes the lower side plates  21 , which are triangular, and thus gradually decreases in width toward the rear. Thus, as described above, the bed body  11  can be reduced in size and weight. This allows for machining with high accuracy. Additionally, space exists below each of the lower side plates  21  of the bed body  11 . This space can be employed to install hydraulic pipes and wires. 
     (5) The front legs  31  are arranged at the positions corresponding to the opposite ends of the table traveling portion  43 . Thus, even if the part of the bed body  11  located rearward from the table traveling portion  43  has a smaller width than the table traveling portion  43 , the table traveling portion  43  is prevented from being deformed. This allows for machining with high accuracy. 
     (6) The first and second reinforcement frames  92  and  93 , which are integrated with the table traveling portion  43  and are inclined to extend such that the first and second reinforcement frames  92  and  93  cross each other, are arranged below the table traveling portion  43 . This prevents the table traveling portion  43  from deforming and thus allows for machining with high accuracy. Further, the first and second reinforcement frames  92  and  93  are inclined to extend, and the lower ends of the first reinforcement frames  92  are integrated with the upper side plates  22 , where the front legs  31  are located. In addition, the two second reinforcement frames  93  converge at the lower center of the bed body  11  to be integrated with each other. Thus, machining pressure acting on the table traveling portion  43  is applied to the first and second reinforcement frames  92  and  93  in a direction in which the first and second reinforcement frames  92  and  93  are compressed. This effectively prevents deformation of the table traveling portion  43 . 
     (7) The rear leg  32  is located rearward from the table traveling portion  43  on a line that extends through the center of the table traveling portion  43  in the longitudinal direction and is parallel to the Z-axis. Thus, since the rear leg  32  is located immediately below the movable range of the unit frame  61 , which includes the grindstone shaft unit  74 , the unit frame  61  moves stably. This contributes to the high-accuracy machining of a workpiece. 
     (8) The sliders  62  of the unit frame  61 , which are supported by the second guide rails  52 , are provided with the side bulged portions  67 , which bulge in the horizontal direction. This improves the rigidity for supporting the unit frame  61  and contributes to the high-accuracy machining of a workpiece. 
     (9) The reinforcement ribs  68  are formed on the inner side of the support frame  63  of the unit frame  61 . Further, the rear bulged portions  69 , which bulge rearward, are arranged at the rear part of the support frame  63 . This improves the rigidity for supporting the unit frame  61  and contributes to the high-accuracy machining of a workpiece. 
     (10) The unit frame  61  is supported by the second guide rails  52  via the sliders  62 , which are located on the opposite sides of the unit frame  61 . Further, the machining unit  15  is located in the inner space of the bed body  11  through the open portion  28  of the bed body  11 . This allows the machining unit  15 , which is the uppermost part of the machining unit  15 , to be located at a lower position and thus lowers the height of the entire machining unit  15 . Further, the motor  72 , which drives the ball screw  73  of the machining unit  15 , is located at the lower part of the machining unit  15 . This also allows the center of gravity α of the machining unit  15  to be located below the second guide rail  52 . In this manner, the center of gravity α can be located at a low position in the bed body  11 . This lowers the height of the machining unit. Thus, the machining unit  15  can be supported stably. Further, vibration of the machining unit  15  is limited to improve the accuracy of machining a workpiece. 
     (11) Since the center of gravity β of the machine tool is located at a position lower than the center of gravity α of the machining unit  15 , the machining unit  15  can be installed stably. This reduces vibration of the machine tool and improves the accuracy of machining a workpiece. 
     (12) The front ends of the two support members  14  are provided with the overhung parts  51 , which protrude frontward. Thus, even if the grindstone shaft unit  74  is moved forward to machine a workpiece, the grindstone shaft  76  is rarely cantilevered. Accordingly, even if the grindstone shaft  76  is moved forward to a large extent, decreases in the accuracy of machining a workpiece are limited. 
     (13) The front plate  23  of the bed body  11  is inclined such that the lower part is located rearward from the upper part. This limits interference of the front plate  23  with the lower limbs of a worker such as a toe located on the front side of the bed body  11 . Thus, the worker can perform operations easily. 
     (14) The framework of the machine tool is configured as an assembled unit including multiple members such as the bed body  11 , the support members  14 , the support frame  63 , the sliders  62 , and the unit frame  61 . Further, the movement accuracy of the members that move along the X-axis, the Y-axis, and the Z-axis can be easily adjusted by adjusting the positional relationship of the members that configure the framework of the machine tool. 
     (15) The reinforcement ribs  41  are arranged above the rear leg  32 . Thus, even if the rear plate  24  is inclined, the strength and rigidity of the portion where the rear leg  32  is located are ensured. 
     (16) The adjustment member  86 , which has the form of a turnbuckle, is located at the lower center of the front end of the bed body  11 . The curvedness of the bed body  11  (especially, the lower front part of the bed body  11 ) can be adjusted by loosening the locknuts  87  and rotating the adjustment member  86 . Thus, the adjustment member  86  facilitates fine adjustment of the degree of deformation and the inclination angle relative to the horizontal surface of the table traveling portion  43 . This improves the movement accuracy of the table  12  and consequently improves the accuracy of machining a workpiece. 
     (17) The through-holes  91  are formed at multiple positions of the bed body  11 . Thus, air flows smoothly between the inside and outside of the machine tool through the through-holes  91 . This limits differences in the temperature of the inside and outside of the machine tool, limits the deformation of the machine tool resulting from the temperature, and improves the accuracy of machining a workpiece. Additionally, the inner part of the machine tool can be checked, inspected, and cleaned through the through-holes  91 . This improves the maintainability of the machine tool. 
     (18) The second guide rails  52 , which are the second support surfaces that guide movement of the machining unit  15  including the grindstone shaft unit  74  and the support frame  63  along the Z-axis of the machining unit  15 , are located at a position higher than the first guide rails  45 , which are the first support surfaces. This increases the dimension (height or thickness) of the portion of the bed  10  in the vertical direction located below the second guide rails  52 . Accordingly, the rigidity of the portion that supports the second guide rail  52  is improved to limit the deformation of the bed  10  resulting from load on the machining unit  15 . This improves the movement accuracy of the machining unit  15  and the machining accuracy. 
     (19) As described above, the second guide rails  52  are located at a position higher than the first guide rails  45 . This eliminates the need to arrange a tall, heavy column that supports the grindstone shaft unit  74 . This avoids inconveniences such as inclination of the column that results from acceleration and deceleration of the column. This allows the machining unit  15  to be accurately moved straight along the second guide rails  52 , ensures the position accuracy of the grindstone shaft unit  74 , and allows for high-accuracy machining as described above. 
     Second Embodiment 
     A second embodiment of the present disclosure will now be described, mainly focusing on the differences from the first embodiment. 
     As shown in  FIGS.  9  and  10   , in the second embodiment, the adjustment member  86  of the first embodiment, which has the form of a turnbuckle, is replaced with an adjustment member  186  as described below. That is, the recess  81  located at the lower center of the bed body  11  has two inner surfaces opposed to each other, and fixed wedge members  95  including inclined surfaces  96  are respectively fixed to the inner surfaces. An adjustment screw  97  is supported by the bed body  11  such that the adjustment screw  97  cannot move in the axial direction and cannot rotate. An internal thread  100  of a movable wedge member  99  is coupled with the adjustment screw  97 . The opposite side surfaces of the movable wedge member  99  have inclined surfaces  98 . The inclined surfaces  98  are respectively in close contact with the inclined surfaces  96  of the fixed wedge members  95 . 
     When the adjustment screw  97  is rotated, the movable wedge member  99  moves along the Z-axis in a first direction or in a second direction, which is opposite to the first direction. As the movable wedge member  99  moves, the two fixed wedge members  95  move toward or away from each other. This adjusts the flexing amount of the bed body  11  and adjusts the straightness of the first guide rails  45 . 
     Modifications 
     The first and second embodiments may be modified as described below. 
     The locations of the adjustment members  86  and  186  may be changed to, for example, the rear part of the bed body  11 . 
     The number of the adjustment members  86  and  186  may be changed. For example, the adjustment members  86  and  186  may be arranged at the lower rear part of the bed body  11  and/or the opposite sides of the rear part of the bed body  11  in addition to the center of the lower front part of the bed body  11 . Alternatively, the adjustment members  86  and  186  may be arranged at the middle and/or upper part of the bed body  11  in the vertical direction in addition to the lower part of the bed body  11 . 
     The tool is not limited to a rotation grindstone and may be, for example, a drill, a milling cutter, an endmill, or a hob. 
     The recesses  90  or the through-holes  91 , which are the thinned portions, may be omitted. 
     The lower side plates  21 , which are the oblique sides, may be omitted. The upper side plate  22  may be inclined in the same manner as the lower side plates  21  such that at least part of the upper side plate  22  configures the oblique sides. 
     The bed body  11  and the support members  14  may be formed as an integrated body. 
     The adjustment member that adjusts the deformation amount of the bed body  11  may include two external thread bodies (first thread bodies) fixed to the bed body  11  and arranged coaxially to be opposed to each other and an internal thread body (second thread body) arranged between the two external thread bodies and coupled to the two external thread bodies. The internal thread body includes an operation part and threaded parts that respectively extend from the opposite ends of the operation part. When the internal thread body is rotated, the two external thread bodies move toward or away from each other, thereby adjusting the deformation amount of the bed body  11 . 
     The technical ideas understood from the above-described embodiments and the modifications are as follows. 
     (A) A machine tool comprising:
         a bed;   a table traveling portion arranged at an upper front part of the bed;   a work table configured to travel along the table traveling portion; and   legs arranged at three positions of a bottom surface of the bed, wherein   the legs include two front legs arranged at positions corresponding to opposite ends of the table traveling portion in a longitudinal direction, and   the table traveling portion has a length larger than a width of a part of the bed located rearward from the table traveling portion.       

     (B) The machine tool according to item (A), wherein the legs includes a rear leg arranged rearward from a center of the table traveling portion in the longitudinal direction. 
     (C) A machine tool comprising:
         a bed; and   an adjustment member configured to adjust a deformation degree of the bed, the adjustment member being arranged at the bed,   wherein the adjustment member includes   two first thread bodies fixed to the bed, the two first thread bodies being arranged coaxially to be opposed to each other, and   a second thread body including an operation part and two threaded parts respectively arranged at opposite ends of the operation part and coupled to the first thread bodies.       

     The adjustment member of the machine tool according to item (C) is also applicable to a machine tool other than the above-described embodiments, for example, a machine tool including four legs and supported at four points on an installation surface. 
     (D) A machine tool comprising:
         a bed; and   an adjustment member configured to adjust a deformation degree of the bed, the adjustment member being arranged at the bed,   wherein the adjustment member includes   two fixed wedge members fixed to the bed and opposed to each other, and   a movable wedge member arranged between the fixed wedge members and engaged with the fixed wedge members.       

     The adjustment member of the machine tool according to item (D) is also applicable to a machine tool other than the above-described embodiments, for example, a machine tool including four legs and supported at four points on an installation surface. 
     (E) The machine tool according to item (C) or (D), comprising a table traveling portion arranged at a front part of the bed and extending along a first axis,
         wherein the adjustment member is located at a position corresponding to a center of the table traveling portion in a longitudinal direction.       

     (F) The machine tool according to any one of claims  1  to  11 , wherein the bed includes a reinforcement portion configured to reinforce the legs. 
     (G) The machine tool according to claim  3 , wherein
         the bed includes a reinforcement portion arranged below the table traveling portion, and   the two front legs are arranged at positions corresponding to the reinforcement portion.       

     (H) The machine tool according to any one of claims  1  to  11 , wherein the bed includes a bed body and two support members that support the machining unit, the two support members being respectively arranged on opposite sides of an upper surface of the bed body. 
     (I) The machine tool according to claim  15 , comprising:
         two front legs arranged at positions of a lower part of the bed that correspond to opposite ends of the first support surface; and   a rear leg arranged at one position of the lower part of the bed located between the second support surfaces.