Abstract:
A NC machine tool having a constitution in which a table ( 15, 115 ) for placing a work thereon and a spindle ( 14, 114 ) for setting a tool are relatively moved by three feed mechanisms ( 18-20, 118-120 ), for X-axis, for Y-axis, and for Z-axis, to carry out machining. A preload is applied to all the three feed mechanisms ( 18-20, 118-120 ), for X-axis, for Y-axis, and for Z-axis, using a rack-pinion ( 21, 22, 24, 25, 27, 28, 121, 122, 124, 125, 127, 128 ), and using two motors ( 16, 23, 29 ) for each axis to prevent backlash in the rack-pinion.

Description:
FIELD OF THE INVENTION 
     This invention relates to a NC machine tool having a constitution in which a table for placing a work thereon and a spindle for setting a tool are relatively moved by feed mechanisms to carry out machining. 
     RELATED ART 
     In the conventional machine tool, a feed mechanism using a ball screw is general, by which a table or a spindle is moved. 
     With the recent progress of higher speed of a machine tool, it has been required to feed a table or a spindle at high speed. 
     For achieving the higher speed, a feed mechanism using a linear motor as a drive source has been produced by way of trial, but the sufficient result has not been obtained. 
     The conventional feed mechanism of a ball screw type is able to realize a highly accurate and smooth feeding. However, in the ball screw type, it has been difficult to correspond to the higher speed since a feed speed has a limitation. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a NC machine tool capable of making a feed speed higher, and being provided with a feed mechanism enabling smooth and highly accurate feeding. 
     Solving means of the present invention are illustrated as follows: 
     (1) A NC machine tool having a constitution in which a table for placing a work thereon and a spindle for setting a tool are relatively moved by three feed mechanisms, for X-axis, for Y-axis, and for Z-axis, to carry out machining, wherein a preload is applied to all the three feed mechanisms, for X-axis, for Y-axis, and for Z-axis, using a rack-pinion, and using two motors for each axis to prevent backlash in the rack-pinion. 
     (2) A horizontally arranged rack has teeth directed downward. 
     (3) As a rack-pinion, a helical rack-pinion is used. 
     (4) As a rack-pinion, a double-helical rack-pinion is used. 
     In the NC machine tool according to the present invention, a table for placing a work thereon and a spindle for setting a tool are relatively moved by three feed mechanisms, for X-axis, for Y-axis, and for Z-axis, to carry out machining. In all the three feed mechanisms, for X-axis, for Y-axis, and for Z-axis, a rack-pinion is used. 
     Two motors are used for each axis to double the driving force, and backlash of a rack and a pinion is removed by preloading. 
     Moreover, a horizontally arranged rack has teeth directed downward or directed laterally. Thereby, it is possible to prevent the rack tooth face from staying of foreign matter thereon. 
     As a rack-pinion, a helical rack-pinion or a double-helical rack-pinion can be used. By using them, smoother feeding can be realized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of the present invention will be described hereinafter with reference to the drawings: 
     FIG. 1 is a conceptual view conceptually showing a first embodiment of a NC machine tool according to the present invention; 
     FIG. 2 is a conceptual view conceptually showing a second embodiment of a NC machine tool according to the present invention; and 
     FIG. 3A is a perspective view showing a helical rack used for the NC machine tool, and FIG. 3B is a perspective view showing a double-helical rack and a double-helical pinion. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a conceptual view conceptually showing a first embodiment of a NC machine tool according to the present invention. 
     A vertical type machine tool  10  has the constitution similar to that of a normal machine tool except a feed mechanism for a table and a spindle. 
     The machine tool  10  has a base  11 , on which a table  15  is arranged. The table is guided movably along the Y-axis direction by guide means (not shown) like rails, and is driven by a feed mechanism  20  for Y-axis. 
     The feed mechanism  20  for Y-axis is constituted by one rack  27 , two pinions  28 , and two servo motors  29 . The one rack  27  is secured to the lower surface of the table along the Y-axis direction. The two servo motors  29  are installed on the base  11 . The two pinions  28  are mounted on output shafts of the two motors  29 , respectively, and are engaged with the rack  27 . For the rack  27 , a helical type as shown in FIG. 3A can be used. 
     As the rack-pinion, double-helical rack-pinions  21   a ,  22   a  can be also used. 
     A column  12  having a gate-shaped leg is formed above the base  11 . 
     A spindle head  13  is arranged on the column  12 . 
     The spindle head  13  is guided movably along the X-axis by guide means (not shown) like rails, and is driven by a feed mechanism  19  for X-axis. 
     The feed mechanism  19  for X-axis is constituted by one rack  24 , two pinions  25 , and two servo motors  16 . The one rack  24  is arranged along the X-direction to the lower surface of the spindle head  13 . The two servo motors  16  are installed on the column  12 . The two pinions  25  are connected to output shafts of the two motors, respectively, and are engaged with one rack  24 . 
     A spindle  14  is arranged on the spindle  13  in a longitudinal direction. 
     The spindle  14  is guided movably in a vertical direction (Z-axis direction) relatively to the spindle head  13 , and is driven by a feed mechanism  18  for Z-axis. 
     The feed mechanism  18  for Z-axis is constituted by one rack  21 , two pinions  22 , and servo motors  23 . The one rack  21  is secured in parallel with a diametrically central part of the spindle head  14 . The two servo motors  23  are installed on the spindle head  13 . The two pinions  22  are connected to output shafts of the two motors  23 , respectively, and are engaged with one rack  21 . 
     The spindle  14  is able to automatically detachably mount a tool by an automatic tool exchanging arm  17 . 
     A control unit  16  for carrying out controlling of a tool magazine and the whole machine tool is arranged adjacent to the automatic tool exchanging arm  17 . 
     The second embodiment of the present invention will now be briefly described with reference to FIG.  2 . 
     In a lateral type machine tool  110 , a spindle  114  is arranged in a horizontal direction, and is movable in a Y-axis direction by a feed mechanism  20  for Y-axis. 
     A table  115  is constituted to be movable along the X-axis and the Z-axis. 
     A saddle  130  is arranged below the table  115 . This saddle  130  is guided movably in a Z-direction by guide means like rails, and is driven by feed means for Z-axis. 
     The table  115  is guided movably in an X-axis direction on the upper surface of the saddle  130 , and is driven by feed means  119  for X-axis. 
     The feed means  119 ,  120  and  118  for X-Z axes are respectively constituted using one rack, two pinions, and two servo motors, similarly to the feed means in the above-described embodiment. 
     Also in any of the above-described embodiments, a preload function is provided for the torque controlled by a position (speed) feed back by combination of two motors, an offset is applied and torques reverse to each other are applied to two shafts (a master shaft and a slave shaft) connected to two motors to assume a state that two pinions are always competed to one rack. By the preload function as described, the influence caused by backlash on the rack-pinion is reduced. 
     Further, the racks  24 ,  27 ,  121  and  124  arranged in the horizontal direction have their tooth directed downward. That is, there presents that the pinion supports the lower side of the rack. 
     In the NC machine tool according to the present invention, since the feed mechanism for each shaft is constituted using one rack, two pinions and two motors, the driving force is doubled, and the backlash of the rack-pinion can be prevented by the preload function of two motors. As a result, the feed speed can be made higher in a three-dimensional manner, and the smooth and highly accurate feed can be realized. Since there are less contact parts like the ball screw, no thermal displacement caused by heat generation occurs. Therefore, this can sufficiently correspond to the higher speed of processing. 
     Further, the racks arranged in the horizontal direction have their teeth directed downward. This obtains the effect capable of preventing foreign matter from staying on the tooth surface of the rack. 
     Particularly, the helical rack-pinion and the double-helical rack-pinion are used to thereby enable the smooth feeding. 
     The present invention is not limited to the aforementioned embodiments. The present invention can be applied to all vertical and lateral type machining centers, a gate type machining center, a NC milling machine, a drill, a mold processing machine and so on.