Abstract:
A machine tool has a spindle head and ball screws on a saddle. The spindle head is provided via sliding or rolling guide members so as to be linearly movable. The ball screws move the spindle head along the guide members. Concave sliding or rolling guide portions parallel to the guide members are provided on both sides of the guide members. Counterweights linearly movable on the concave guide portions in a direction opposite to the spindle head are provided. Racks are provided on opposed surfaces of the counterweights and the spindle head along the directions of movement of the spindle head and the counterweights. Pinions meshing with the racks are rotatably supported in the saddle. The machine tool can effectively suppress vibrations of a structure (such as the saddle) generated during movement of a moving object (such as the spindle head) relative to the structure by a drive line (such as the ball screws), and always keep the center of gravity of an entire relatively moving portion constant.

Description:
[0001]    The entire disclosure of Japanese Patent Application No. 2001-349812 filed on Nov. 15, 2001 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a machine tool, such as a machining center, which performs machining while moving a work table and a main spindle for a tool relative to each other.  
           [0004]    2. Description of Related Art  
           [0005]    In a machine tool, such as a machining center, a moving object, such as a table or a spindle head, is supported linearly movably on a structure, such as a bed or a frame, via a sliding or rolling guide surface, and is driven (relatively moved) by a drive line which causes a relative movement in the direction of the guide surface.  
           [0006]    In this type of machine tool, when the moving object begins or stops relative movement by the action of the drive line, a flexural vibrating force, which deforms the structure, is increased by the inertial force of the moving object and a reaction force generated in the structure by the drive line. As a result, vibrations are liable to occur. If machining is started before settlement of the vibrations, the surface roughness of a workpiece increases (namely, its machining precision decreases). Thus, it has been customary practice to slow down acceleration or deceleration when the moving object begins or stops movement; or to start machining after the vibrations settle. Hence, it has been impossible to shorten the machining time and thereby increase productivity.  
           [0007]    When the structure moves on another structure via the sliding or rolling guide surface, the balance of the structure changes depending on the position of the moving object, posing difficulty in exercising control when moving the structure on the different structure.  
           [0008]    Techniques for achieving fast acceleration and deceleration in machine tools have been proposed, for example, by Japanese Unexamined Patent Publication No. 1996-318445 and Japanese Unexamined Patent Publication No. 1999-235631. The former technique uses a linear motor as a drive source in a box frame type machine tool. The latter technique gives movement, in a direction perpendicular to each of the moving directions of first and second slides of a main spindle, as movement of a work table, whereby a uniform feeding load is exerted on a pair of drive mechanisms for driving the first slide of the main spindle, no matter what position is taken by the main spindle for a tool which moves toward and away from a workpiece on the first slide. However, neither of these techniques takes into consideration the positional relationship between a pair of guide surfaces and the center of gravity of a moving object. Hence, the aforementioned problems remain unsolved.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has been accomplished in light of the above-mentioned problems with the conventional machine tool. Its object is to provide a machine tool which can effectively suppress vibrations of a structure generated during movement of a moving object relative to the structure by a drive line, and always keep the center of gravity of an entire relatively moving portion constant.  
           [0010]    According to the present invention, there is provided a machine tool comprising:  
           [0011]    a moving object movable along a first guide surface provided in a structure; and  
           [0012]    a drive line for moving the moving object along the first guide surface, and further including:  
           [0013]    a second guide surface provided parallel to the first guide surface;  
           [0014]    a counterweight movable along the second guide surface and having a mass equal to a mass of the moving object; and  
           [0015]    means for moving the counterweight in a direction opposite to movement of the moving object and in an amount equal to an amount of movement of the moving object.  
           [0016]    According to the machine tool of the present invention, vibrations of the structure, which occur when the moving object is moved relative to the structure by the drive line, can be effectively suppressed, the machining time can be shortened to increase productivity, and it becomes easy to exercise control, such as control for keeping the center of gravity constant when the structure housing the moving object is moved in another structure.  
           [0017]    The machine tool may have another aspect in a special case. That is, the mass of the counterweight may be a half of the mass of the moving object, and the centers of gravity of the counterweights may be located in the same plane in the direction of movement and at equal distances from the center of gravity of the moving object. In this case, the method of manufacturing is simplified.  
           [0018]    In the machine tool, the racks and a gear box for nullifying backlash may be employed. In this case, smooth motions with few vibrations are made. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0020]    [0020]FIG. 1 is a schematic perspective view of a horizontal machining center according to an embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a longitudinal sectional side view of the horizontal machining center;  
         [0022]    [0022]FIG. 3 is a sectional view taken on line III-III of FIG. 2;  
         [0023]    [0023]FIG. 4 is a sectional view taken on line IV-IV of FIG. 3;  
         [0024]    [0024]FIG. 5 is a sectional view taken on line V-V of FIG. 2;  
         [0025]    [0025]FIG. 6 is an enlarged sectional view of a rack and pinion portion;  
         [0026]    FIGS.  7 ( a ) and  7 ( b ) are various schematic views of the present invention; and  
         [0027]    [0027]FIG. 8 is an operational explanation drawing of X-axis counterweights for a spindle head. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    A machine tool according to the present invention will now be described in detail by an embodiment with reference to the accompanying drawings, which in no way limit the invention.  
         [0029]    [Embodiment] 
         [0030]    [0030]FIG. 1 is a schematic perspective view of a horizontal machining center according to an embodiment of the present invention. FIG. 2 is a longitudinal sectional side view of the horizontal machining center. FIG. 3 is a sectional view taken on line III-III of FIG. 2. FIG. 4 is a sectional view taken on line IV-IV of FIG. 3. FIG. 5 is a sectional view taken on line V-V of FIG. 2. FIG. 6 is an enlarged sectional view of a rack and pinion portion. FIGS.  7 ( a ) and  7 ( b ) are various schematic views of the present invention. FIG. 8 is an operational explanation drawing of X-axis counterweights for a spindle head.  
         [0031]    As shown in FIGS.  1  to  5 , a gate-shaped column  2  is integrally erected at a rear portion of a bed (as a structure)  1  to constitute a horizontal machining center. On the bed  1 , a table body (as a moving object)  3  is disposed via right and left guide members  4  as a pair, each of which has a sliding or rolling guide surface, so as to be movable in a fore-and-aft direction (a Z-axis direction) by right and left ball screws  5  as a pair. In the drawings, the numeral  6  denotes a servo motor for the ball screw  5 . A tilt table  8 , which rotatably supports a rotary table  7  bearing a workpiece (not shown), is tiltably assembled to and supported on the table body  3 .  
         [0032]    In the present embodiment, the right and left guide members  4  as a pair, the right and left ball screws  5  as a pair, and the table body  3  are provided such that the guide members  4 , the ball screws  5  and the center of gravity of the table body  3  are in the same plane parallel to the direction of movement of the table body  3 , and that a midpoint between the right and left guide members  4  as a pair, and a midpoint between the right and left ball screws  5  as a pair are located at the position of the center of gravity of the table body  3 .  
         [0033]    That is, as shown in FIG. 7( b ), the guide surfaces of the right and left guide members  4  as a pair on the bed  1  are provided so as to be in a first plane A, which includes the position of the center of gravity, G, of the table body  3  and which is parallel to the direction of movement of the table body  3 . The guide surfaces are also provided so as to be close to both sides of the table body  3 , and such that the distances from the center of gravity, G, of the table body  3  to the respective guide surfaces are equal. Moreover, nut portions  9  of the ball screws  5  beside the table body  3  are installed at a pair of positions in a second plane B which includes the position of the center of gravity, G, of the table body  3  and which is perpendicular to the axis of the table body  3  in the direction of movement, the midpoint between the pair of positions being located at the position of the center of gravity, G, of the table body  3 .  
         [0034]    In the illustrated embodiment, the nut portions  9  of the right and left ball screws  5  as a pair are also provided in the first plane A together with the guide members  4  and the center of gravity, G, of the table body  3 . However, the nut portions  9  of the ball screws  5  may be provided at any positions in point symmetry about the position of the center of gravity, G, of the table body  3  in the second plane B.  
         [0035]    As shown in FIG. 7( a ), moreover, if the table body  3  can be driven with a single ball screw  5 , the nut portion  9  of the ball screw  5  beside the table body  3  may be installed on a straight line C including the position of the center of gravity, G, of the table body  3  and having a direction vector in the direction of movement of the table body  3 .  
         [0036]    In the gate-shaped column (as a structure)  2 , a box-shaped saddle (as a moving object)  10  is disposed via right and left, front and rear guide members (having sliding or rolling guide surfaces)  11  as two pairs so as to be movable in an up-and-down direction (Y-axis direction) by right and left ball screws  12  as a pair.  
         [0037]    In a manner similar to the manner explained in connection with FIG. 7( b ), nut portions (not shown) of the right and left ball screws  12  as a pair are installed at a pair of positions in a plane which includes the position of the center of gravity of the saddle  10  and which is perpendicular to the axis of the saddle  10  in the direction of its movement, the midpoint between the pair of positions being located at the position of the center of gravity of the saddle  10 .  
         [0038]    The four guide members  11 , when viewed in plan, are provided at positions which are on diagonals intersecting at the position of the center of gravity of the saddle  10 , and which are at equal distances from the position of the center of gravity of the saddle  10 .  
         [0039]    In the box-shaped saddle  10  (as a structure), a spindle head (as a moving object)  15  is disposed via upper and lower, front and rear guide members (having sliding or rolling guide surfaces)  16  as two pairs so as to be movable in a right-and-left direction (X-axis direction) by front and rear ball screws  17  as a pair.  
         [0040]    In a manner similar to the manner explained in connection with FIG. 7( b ), nut portions (not shown) of the front and rear ball screws  17  as a pair are installed at a pair of positions in a plane which includes the position of the center of gravity of the spindle head  15  and which is perpendicular to the axis of the spindle head  15  in the direction of its movement, the midpoint between the pair of positions being located at the position of the center of gravity of the spindle head  15 .  
         [0041]    The four guide members  16 , when viewed from the side (see FIG. 2), are provided at positions which are on diagonals intersecting at the position of the center of gravity of the spindle head  15 , and which are at equal distances from the position of the center of gravity of the spindle head  15 .  
         [0042]    In the drawings, the numeral  18  denotes a tool magazine installed on an upper surface portion of the column  2 , and the numeral  19  denotes a tool replacement arm installed on a front surface portion of the column  2 .  
         [0043]    In the present embodiment, as shown in FIG. 3, Y-axis counterweights  20  are provided on outer surfaces of right and left side walls of the column  2  via right and left, front and rear guide members  21  (see FIG. 1) as two pairs so as to be movable in a Y-axis direction. The Y-axis counterweights  20  can be moved in a direction opposite to the direction of movement of the saddle  10  by the engagement of pinions  22   a  and  22   b  rotatably supported by the right and left side walls with racks  23   a  and  23   b  engraved on the outer surfaces of right and left side walls of the saddle  10  and the inner surfaces of the Y-axis counterweights  20 . That is, the Y-axis counterweights  20  achieve weight balance during movement of the saddle  10  in the Y-axis direction. The achievement of weight balance in the direction of gravity has hitherto been performed with the use of ropes or chains.  
         [0044]    As shown in FIGS. 2 and 3, moreover, X-axis counterweights  25  are provided on outer surfaces of upper and lower walls of the saddle  10  via concave guide portions (having second sliding or rolling guide surfaces)  26 , which are elongated in a right-and-left direction, so as to be movable in an X-axis direction. The X-axis counterweights  25  can be moved in a direction opposite to the direction of movement of the spindle head  15  by the engagement of pinions  27   a  and  27   b  rotatably supported by the upper and lower walls with racks  28   a  and  28   b  engraved on the upper and lower surfaces of the spindle head  15  and the inner surfaces of the X-axis counterweights  25 .  
         [0045]    In the illustrated embodiment, the pinions  27   a  and  27   b  are provided in a central portion in the right-and-left direction of the saddle  10 . When central portions in the right-and-left direction of the racks  28   a  beside the spindle head  15  engage the pinions  27   a  and  27   b , central portions in the right-and-left direction of the racks  28   b  beside the X-axis counterweights  25  also engage the pinions  27   a  and  27   b.    
         [0046]    Thus, as shown in FIG. 8, the mass of the X-axis counterweight  25  on each side of the spindle head  15  is set at a half of the mass of the spindle head  15 , whereby when the spindle head  15  is moved in the X-axis direction (the right-and-left direction in the drawing) by a servo motor  29  via the ball screw  17 , the passive negative phase motions of the X-axis counterweights  25  are obtained, giving the damper action that the vibrating force is canceled out. Besides, as described earlier, the position of the engagement of the racks  28   a  on the spindle head  15  and the racks  28   b  on the X-axis counterweights  25  with the pinions  27   a  and  27   b  is set, whereby the position of the center of gravity of the spindle head  15  in the X-axis direction is unchanged (constant), even when the spindle head  15  is moved in the X-axis direction.  
         [0047]    Of the pinions  22   a ,  22   b  and the pinions  27   a ,  27   b , at least the pinions  27   a ,  27   b  and the racks  28   a ,  28   b  meshing with them are formed of helical gears and helical racks, as shown in FIG. 6. The pinion  27   a  or  27   b  is divided into two parts in the direction of its axis of rotation, and one of the parts is always urged inwardly by a coned disc spring  33  to be capable of eliminating a backlash due to the engagement of the gears. Methods of eliminating a backlash include not only the helical racks and single helical gear box as described above, but also parallel racks and a double pinion system.  
         [0048]    Because of the above configuration, the positional control of the spindle head  15  during machining is exercised by the movement of the spindle head  15  per se in the X-axis direction and the movement of the saddle  10  in the Y-axis direction.  
         [0049]    At this time, the spindle head  15  and the saddle  10 , as stated earlier, are driven via the pair of ball screws  17  and the pair of ball screws  12  disposed at the positions in point symmetry about their centers of gravity, while the spindle head  15  and the saddle  10  are being guided by the two pairs of guide members  16  and  11  disposed at the positions in point symmetry about their centers of gravity. These actions decrease the inertial forces of the spindle head  15  and the saddle  10  at the start or stop of movement, and the reaction forces caused to the saddle  10  and column  2  in support of them due to the driving of the ball screws  17  and  12 .  
         [0050]    In the spindle head  15 , moreover, the vibrating force is canceled out by the negative phase motions of the X-axis counterweights  25 , and the position of the center of gravity in the X-axis direction is unchanged, as stated earlier. In the saddle  10 , on the other hand, weight balance is achieved by the negative phase motions of the Y-axis counterweights  20 , as described above.  
         [0051]    Hence, the flexural vibrating force, which deforms the saddle  10  and the column  2 , is markedly decreased, and vibrations during movement of the spindle head  15  in the X-axis direction and during movement of the saddle  10  in the Y-axis direction are effectively suppressed. Incidentally, when the spindle head  15  moves in the X-axis direction, the position of its center of gravity is unchanged, so that during movement of the saddle  10  in the Y-axis direction, load imposed is always equal for the right and left ball screws  12  as a pair.  
         [0052]    The positional control of the workpiece on the rotary table  7  during machining is performed by the movement (feeding) of the table body  3  in the Z-axis direction, in addition to the rotation of the rotary table  7  and the tilting of the tilt table  8 .  
         [0053]    At this time, the table body  3  is driven by the pair of guide members  4  and the pair of ball screws  5  disposed in the same plane as the center of gravity, G, of the table body  3  and at the positions spaced laterally equally from the center of gravity, G, as stated earlier. As shown in FIG. 5, the table body  3  transmits its inertial force to racks  36   b  of Z-axis counterweights  30  via pinions  35   a ,  35   b  meshing with racks  36   a  disposed in the table body  3 , thereby canceling out the inertial force. Thus, the vibrating force of the table body  3  at the start or stop of movement is decreased.  
         [0054]    Hence, the flexural vibrating force, which deforms the bed  1 , is markedly decreased, and vibrations during movement of the table body  3  in the Z-axis direction are effectively suppressed.  
         [0055]    Consequently, with the present machining center, the machining time can be shorted to increase productivity. Furthermore, it is easy to exercise various types of control when the saddle  10  housing the spindle head  15  is moved within the column  2 .  
         [0056]    While the present invention has been described by the foregoing embodiment, it is to be understood that the invention is not limited thereby, but may be varied in many other ways. For example, the drive line is not limited to the ball screws and servo motors, but may be other means. The guide members may be linear guides, etc. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.