Abstract:
This invention relates to a table feed mechanism of the machine tool. The table feed mechanism of this invention includes a bed; a table slidably mounted on the bed for mounting a work thereon and for defining a plurality of screw holes therein; a plurality of feed screw shafts engageable with the screw holes respectively; and a drive mechanism for simultaneously driving the feed screw shafts to move the table.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a table feed mechanism for a machine tool. In the conventional machine tools where a relative position of work to a tool can be determined along a plural directions (i.e., a X-dir.; a Y-dir.; and a Z-dir.), there are several types of machine tools; one of the types is to drive a tool along the three directions (XYZ-dirs.) and another type is to drive the tool along the two of the directions (for instance YZ-dirs.) and to drive a table for mounting the work along the rest of the direction (for instance X-dir.). In the machine tool of the latter one, it is commonly the case that a nut is fixedly mounted on the table and a feed screw shaft is provided to engage with the inner thread of the nut and then the table is fed by driving the feed screw shaft. 
     With the aforementioned type of table feed mechanism to feed the table and the work mounted thereon, it is likely the case that the screw shaft is subjected to a great magnitude of tension or compression load due to the inertia force caused by the weight of the object to be fed (including the table and the work) at the time of the acceleration or deceleration operation of the feeding operation. This tension or compression force generated on the feed screw shaft causes the deflection of the shaft that generates an undesirable error in the work path in a non-negligible order. In addition, the inertia force generated on the shaft especially when the shaft being subjected to the acute acceleration and deceleration could also cause the torsional deflection on the shaft. And this torsional deflection of the shaft in turn produces the error in the actual position of the table because of the angular phase position of the drive shaft of the motor will be affected by the torsional deflection. 
     In order to solve the aforementioned problem of the conventional feeding mechanism of the machine tool, it is natural for the person with ordinary skill in the art to think that increasing the rigidity (including rigidity against the torsional deflection and the rigidity against the bending deflection) of the feed screw shaft to basically strengthen the feed shaft. Furthermore, it is quite natural to think that to gain more rigidity, increasing the diameter of the screw shaft is the simple way. However, the moment of inertia is proportional to the fourth power of the diameter of the torsional shaft, thus slight increase of the diameter of the shaft may significantly increase the moment of inertia. As a result, significantly increased drive power would be required for the motor to drive the thickened screw shaft especially when at accelerating and decelerating procedures. Thus it may necessitate a procurement of a non-standard type larger motor for this purpose, leading a cost increase for the machine tool as a whole and cost reflection upon the products made by the machine tool. 
     SUMMARY OF THE INVENTION 
     In view thereof, an object of this invention is to provide the feeding mechanism suppressing a significant increase of the required drive load of the screw shaft while maintaining the accuracy in feeding the relatively heavy table and the work mounted thereon. 
     In order to meet the above object, the table feed mechanism of the machine tool, according to this invention, comprises: a bed; a table slidably mounted on the bed for mounting a work thereon and for forming a plurality of screw holes therein; a plurality of feed screw shafts engageable with the screw holes respectively; a drive means for simultaneously driving the feed screw shafts to move the table. 
     With the thus constructed table feed mechanism, the tension or compression force, generated by the inertia of the table and the work both combined when being accelerated or decelerated, is distributed to act upon each feed screw shaft. Thus the tension or compression force that each feed screw shaft must bear is greatly lowered as compared to the case when the single feed screw shaft is equipped as for the conventional machine tool. As a result, a feed motion of the table and work mounted thereon is accurately controlled. Furthermore, a momentum load subjected to the drive member such as a motor is not increased as much as the case when increasing the diameter of the feed screw shaft, thus it enhances an availability of employment of the ordinary motor of smaller size to drive each feed screw shaft. 
     These and other objects, features and advantages of the present invention will become more apparent upon reading the following detailed description with reference to accompanied drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional front view of a machine tool of this invention; 
     FIG. 2 is a plan view of the machine tool; 
     FIG. 3 is a cross sectional side view showing an essential portion of the machine tool; 
     FIG. 4 is a schematic diagram showing an internal structure of a gear box mounted to the machine tool; 
     FIG. 5 is a diagram showing one of the examples of feed instruction to the table and work; and 
     FIG. 6 is a cross sectional front view of the drive transmission mechanism employing three feed screw shafts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter the preferred embodiment according to this invention will be described with reference to the drawings. 
     With reference to FIGS. 1,  2 , and  3 , a machine tool  10  has a bed  14  supported on a base  12 . A right and left pair of guide rails  15  are disposed on the bed  14 . A table  16  for supporting a work is horizontally slidably (in a X-dir. in FIG. 2) supported on the guide rails  15 . 
     A frontal end of the table  16  (left end in FIGS. 2 and 3) has a nut mounting portion  17  projecting downward and a right and left pair of nuts  18 A,  18 B are fixedly mounted on the side surface of the nut mounting portion  17 . Two of feed screw shafts (preferably a ball screw shaft)  20 A,  20 B in horizontally extending posture are set to engage with the respective nuts  18 A,  18 B. The left feed screw shaft and the right feed screw shaft in FIG. 1 may be identical with each other or substantially the same configuration. The screw shafts are parallely extending along the X-direction as shown in FIG.  2 . 
     As shown in FIG. 2, these feed screw shafts  20 A,  20 B are rotatably supported by a bearing portion  22  provided on a right end of the bed  14 . The other ends of the feed screw shafts  20 A,  20 B are coupled to a X-direction drive motor  26  that is to drive the feed screw shafts via a gear box  24 . 
     An inner structure of the gear box  24  is shown in FIG.  4 . The both screw shafts  20 A,  20 B are rotatably supported by a housing of the gear box  24  and the substantially the same gears  28 A,  28 B are fixedly mounted on the ends of the both shafts. In the gear box  24 , there is provided a double gear  30  having a pinion gear  30   a  on the one end and a gear  30   b  on the other end. The pinion gear  30   a  is disposed between the gears  28 A,  28 B to mesh with both gears  28 A,  28 B. In addition, the gear  30   b  is set to mesh with a pinion gear  31  mounted on an output shaft of the X-direction motor  26 . 
     Accordingly when the X-direction motor  26  is actuated, then a drive of the motor  26  is transmitted to the pinion gear  30   a  through the pinion gear  31  and the gear  30   b,  then to the gears  28 A &amp;  28 B. Then the feed screw shafts  20 A,  20 B are to be driven by the gears  28 A,  28 B respectively. In the case of identical pinion gears selected for gears  28 A and  28 B, the same rotational speed is achieved for the feed screw shafts  20 A and  20 B. 
     As shown in FIG. 1, there are a pair of upstanding pillars  32 ,  32  on the left and right sides of the bed  14  and a cross beam  34  extending horizontally in the Y-direction is supported on the pillars. A saddle  36  for a main shaft head is supported on this cross beam  34  and the saddle  36  is slidable along the longitudinal direction (i.e., the Y-dir.) of the cross beam  34 . Opposite sides of the cross beam  34  along the longitudinal direction have a gear box  38  on the left and a bracket  39  on the right respectively. Opposite ends of a parallely extending feed screw shaft  40  are rotatably supported by the gear box  38  and the bracket  39  respectively. The feed screw shaft  40  extends through a nut (not shown ) fixed on a rear surface of the main shaft head saddle  36  and is set to engage with the nut. Accordingly the feed screw shaft  40  is set to be driven by a Y-direction motor  42  provided on the side of the gear box  38  to feed the main shaft head saddle  36  along the longitudinal direction of the cross beam  34 . 
     A main shaft header  44  is supported by the main shaft head saddle  36  in such a manner that the main shaft header  44  is vertically displaceable (i.e., movable along the Z-dir.) with respect to the main shaft head saddle  36 . This main shaft header  44  is set to be driven in the vertical direction (Z-dir.) by a Z-direction motor  46  and a feed screw mechanism (not shown). The main shaft header  44  has a chucking mechanism at its bottom for chucking a tool such as ball end mill  48  and the like. As a result, the tool  48  of this machine tool is adjustable its position with respect to the work placed on the table  16  along the X, Y, and Z directions in a three dimensional coordinate by activation of each of the X-direction motor  26 , the Y-direction motor  42 , and the Z-direction motor  46  respectively. 
     As described in the above passages, the machine tool  10  can halve a magnitude of a tension or a compression force acting on each feed screw shaft  20 A or  20 B that is generated by the inertia force due to the weight of the work and table  16  that are subjected to the velocity changes such as when they are accelerated or decelerated as compared to the case in the conventional mechanism where there is a single feed screw shaft. Thereby the deflection amount in terms of bending and torsional forces of each feed screw shaft  20 A or  20 B can be greatly reduced. With the thus described machine tool  10 , the accurate positioning of the tool with respect to the table in turn precision machining can be realized even when the total weight of the object to be fed (including the work and the table  16 ) by the feed screw shafts is heavy, .i.e., the inertia force thereof is large. 
     Take the following case as an example to show how the table of the machine tool  10  is to be positioned with reference to FIG.  2 . Let us assume the case when a programming instruction, linearly moving the main header  44  and the tool  48  from the present point P 0  to a point P 1 , is given. Then with the conventional machine tool with a single feed screw shaft for the table, the feed screw shaft is deflected because of the inertia force of the object to be fed (incl. table and work) and the velocity changes at the initiation of the feed operation and at the end of the feed operation (i.e., acceleration at the former case and the deceleration at the latter case); therefore, the accurate positioning of the table  16  along the X-direction can not be realized, causing to move the table along the curved path (shown in dotted lines  50 ,  51  in FIG. 5) instead of following the linear path indicated at A 1 . With the feed mechanism with the two or more than two feed screw shafts according to this invention, the deflection of the screw shafts for each is effectively suppressed under the circumstance when the table is subjected to the velocity changes at the beginning and at the end of the feed motion, thus accurate feed control operation in response to the given program instruction is realized, resulting in the precise machining operation. 
     Furthermore, in the case where the diameter of the feed screw shaft is increased in an attempt to suppress the deflection amount of the feed screw shaft when encountering velocity change; however, the fact of increasing the diameter of the feed screw shaft significantly affects the magnitude of the inertia force as the change in the inertia force is proportional to the fourth power of the radius (diameter) of the shaft, i.e., when the radius is doubled, then the inertia force becomes 16 times greater than its original. Thus merely increasing the smaller amount in radius may greatly increase the magnitude of the inertia force that may in turn subject the motor to overcome the greater load at the beginning and end of the motion when the velocity change takes place. In this case, it is highly likely that the present motor of its smaller conventional size is not suffice thereby it requires a use of higher powered motor that may not be readily available on the market. Thus use of the customized motor for driving the table will result in cost increase of the tool machine as a whole and cost reflection upon each of machined products produced by the machine tool. 
     Another Embodiments 
     This invention is not limited to the structure described in the above passages, in fact, the followings are selected examples of modifications of this invention. 
     (1) It may be possible to equip a motor to each feed screw shaft thus the same number of the motors as the number of feed screw shafts are to be equipped. In this way, even if the total moment load subjected to the motor increases the certain level, say beyond the maximum load handled by a single standardized motor readily available on the market, the same motor can be equipped to assist the excess load thus it will not necessitate the customized motor (maybe more expensive than the two or three standardized smaller motors all together). However, it is obvious that providing the same gears (in size and pitch) on the both ends of the feed screw shafts  20 A,  20 B and disposing a pinion gear  30   a  the is connected to a drive shaft of the single X-direction motor  26  for engageable with both gears  28 A,  28 B to drive the both shafts  20 A,  20 B is more desirable in terms of cost and simplicity of the structure. Furthermore, synchronizing operation of the two shafts with a single driver (in this case single X-direction motor and its gear trains) is much easier and eliminate all those parts that would have been needed to synchronize the motion of the shafts when two motors were used. Thus it would be advantageous to use a single motor to drive the plurality of feed screws shafts. 
     (2) Furthermore orientations of the two feed screw shafts  20 A,  20 B may be such that they are in parallel and are vertically apart from one to the other. However this orientation may cause the table  16  to be much thicker in a vertical direction to accommodate the two feed screw shafts vertically therein. A height increase of the machine tool may result. Whereas the horizontally parallel orientations of two shafts  20 A,  20 B do not usually necessitate the increase a size of the table in widthwise direction as the table has usually a wider dimension in widthwise direction. Thus it is more advantageous to place the two shafts in horizontally parallel posture. 
     (3) Another alternative design is to have three or more than three feed screw shafts depending upon the weight of the table  16  and work both combined. FIG. 6 shows the case where three feed screw shafts  20 A,  20 B,  20 C are had; in this case it would be desirable to arrange the axis of these three shafts along the circle and to mount the identical gears  28 A,  28 B,  28 C on the ends of the shafts respectively and to have a pinion gear  30   a,  that is set to mesh with all three gears  28 A,  28 B,  28 C, connected to the output shaft of the X-direction motor. By this configuration, the synchronize rotational motion of the three screw shafts  20 A,  20 B,  20 C with the single driver (the X-direction motor) can be easily realized. 
     Comparison Results 
     In Table 1, there are three test samples to be compared with each other; (a) a first comparison sample; a conventional feed mechanism with a single feed screw shaft of 90 mm in dia; (b) a second comparison sample; a feed mechanism with a single feed screw shaft of 125 mm in dia such that the deflection amount can be roughly halved under the subjection of the same load and (c) a feed mechanism of this invention; a feed mechanism with two feed screw shafts each of 90 mm in dia. With these three test samples, a moment load of a motor for driving each of the items is obtained by calculating the moment of inertia for each test samples needed to be driven and is tabulated in the table as follows: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Comparison 
                 Comparison 
                   
               
               
                   
                 Sample 1 
                 Sample 2 
                 This Invention 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Dia (in mm) of 
                 90 
                 125 
                 90 
               
               
                 A Feed Screw 
               
               
                 Shaft 
               
               
                 The number of 
                 1 
                 1 
                 2 
               
               
                 Feed Screw 
               
               
                 Shaft(s) 
               
               
                 Motor load 
               
               
                 corresponding to 
               
               
                 a moment of 
               
               
                 inertia of the feed 
                 0.0934 
                 0.3408 
                 0.1867 
               
               
                 screw shaft(s). 
               
               
                 (kgf-cm-s 2 ) 
               
               
                 Motor load 
               
               
                 corresponding to 
               
               
                 a moment of 
               
               
                 inertia of the 
                 0.0713 
                 0.0713 
                 0.0782 
               
               
                 gear(s). 
               
               
                 (kgf-cm-s 2 ) 
               
               
                 Total Moment 
               
               
                 Load 
                 0.1647 
                 0.4121 
                 0.2649 
               
               
                 (kgf-cm-s 2 ) 
               
               
                 Ratio of total 
                 100%  
                 250%  
                 161%  
               
               
                 Moment load 
               
               
                   
               
             
          
         
       
     
     Note that: 
     A length corresponding to a threaded portion of the feed screw shaft is 6,397 mm: 
     A diameter of each bearing housing is 65 mm; 
     A depth of each bearing housing is 627 mm; 
     A deflection amount in the longitudinal direction of the feed screw shaft of comparison example 2 is substantially the same as that of each feed screw shaft of this invention. 
     From the Table 1, it can be observed that with the use of feed shaft in comparison example 2, the deflection amount of the feed screw shaft can be reduced whereas a sharp increase of the total moment load subjected to the motor can not be avoided (2.5 times of the comparison example 1), thus it may no longer be possible to use the motor that is readily available on the market, resulting in procurement of the customized motor with a greater power that is usually much more expensive than the conventional one on the market. 
     With the use of inventive structure, it can suppress the deflection amount of the feed screw shaft to the level achieved with the use of the comparison example 2 while maintaining the total moment load subjected to the motor at a certain low level (in this example 1.6 times of the comparison example 1). Thus the moment load does not necessitate the procurement of the larger sized motor. 
     Although preferred embodiments of the present invention have been fully described with reference to the accompanied drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the spirit and scope of the invention, they should be construed as being included therein.