Patent Publication Number: US-8967927-B2

Title: Machine tool

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority from Japanese application JP2009-233977 filed on Oct. 8, 2009, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for working a workpiece by relatively moving a rotating cutting tool and a table with the workpiece put thereon, and more particular, to an apparatus for working a workpiece wherein vibrations of the table are suppressed, which vibrations bring about a disadvantage such as degradation in machining accuracy and breakage of the cutting tool, in case of working a large-sized or lengthy workpiece. 
     In cutting work of a workpiece by the use of a rotating cutting tool, a part or parts, which constitute a machine tool, suffers self-excited vibrations in the case where the machine tool and the workpiece are small in dynamic stiffness (stiffness in motion), or in the case where a cutting force acting between the cutting tool and the workpiece is large. Self-excited vibrations as generated are transmitted to the cutting tool with the result that a worked surface of the workpiece becomes wavy and increases in surface roughness, thus making it impossible to obtain a desired machining accuracy. Further, when the self-excited vibrations increase, there is brought about a disadvantage that a cutting blade of the cutting tool itself is nicked, or increased in abrasion, and the cutting tool itself is eventually shortened in service life. 
     In the case where a workpiece is a large-sized or lengthy member, a structure of a machine tool used, for example, in cutting work of a railway rail over 20 meters, working of a high-accuracy guide rail (5 to 8 meters) of a conveyance stage used in manufacture of liquid crystal panels, or the like becomes necessarily large, so that it is desired to improve a constituent member or members in dynamic stiffness not to bring about self-excited vibrations at the time of working. 
     Generally, a large-sized, cutting machine tool described above frequently employs a ball screw system, which is combined with a rotating machine for traveling of a table with a workpiece put thereon. Of course, since the ball screw itself is increased in length, the ball screw itself is liable to suffer self-excited vibrations when the workpiece is worked, and the vibrations are transmitted to the table to make high-accuracy working of the workpiece difficult. 
     As one of countermeasures in such case, a part or parts, which constitute a machine tool, are increased in thickness to improve the machine tool in dynamic stiffness. Since such countermeasure makes the machine tool large-sized and heavy, however, not only a large amount of energy is required, for example, to move a heavy table but also much cost is needed in running of the apparatus. 
     As another countermeasure, a method described in JP-A-2007-203435 has been proposed. That is, in the case where a working head with a cutting tool mounted thereon is driven by a ball screw, an opposite side part of the ball screw with respect to the working head is formed with inverse threads and a weight is moved in an opposite direction with respect to the working head to cancel reaction generated when the working head is driven, thus suppressing vibrations acting on the ball screw. 
     Also, JP-A-10-202461 proposes a method of suppressing vibrations acting on a table by virtue of movements of a damper, which comprises a weight and a spring and is mounted on the table with a workpiece put thereon. 
     JP-A-02-243265 discloses a construction, in which a table travels on rails, that is, a stationary member (base) with the use of a ball screw and a damper provided in the table is used to absorb vibrations at that time. 
     According to the disclosure of the JP-A-2007-203435, variation in moving speed of the working head driven by the ball screw is cancelled but there is not provided a mechanism for suppressing vibrations suffered by a workpiece in working. Also, according to the disclosure of the JP-A-10-202461, a vibration absorbing body, that is, the damper, which comprises a weight and an elastic body (spring), mounted on the table is used to suppress vibrations accompanying movements of a ball screw. At this time, the heavier the weight, which constitutes the damper, the more effective the vibration damping effect, but it cannot but be said that since a member, which supports the table itself, is the ball screw itself for realization of feed of the table, it is difficult to suppress vibrations at the time of working large-sized or lengthy and heavy goods being an object of working in the invention. 
     On the other hand, JP-A-02-243265 discloses a structure for solving matters of bottlenecks in the JP-A-10-202461. That is, the table travels on the rail grooves provided on the stationary member so as to cope with working of heavy goods. At this time, the damper (comprising a weight and a spring) mounted in the table serves to suppress vibrations transmitted to the table. Since the weight is needed to move without suffering an unnecessary resistance in a vibratory direction, however, it must be arranged in no contact with a table inner wall and so there is involved a disadvantage that it is very difficult to make the weight heavy to improve a vibration damping effect. 
     As described above, in case of cutting work while using a ball screw system to move a table on which a large-sized workpiece or lengthy and heavy goods are put, there is involved a serious problem that the ball screw itself becomes necessarily lengthy and spatial displacement of the ball screw itself in operation is transmitted as vibrations to the table to bring about degradation in machining accuracy, or the like. 
     SUMMARY OF THE INVENTION 
     In order to solve the above-mentioned problem, a structure is provided in which a table travels on guide rails provided independently so that the weight of the table, on which a workpiece is put, has no influence on motions of a ball screw part for conveyance of the table, and in which a damper mechanism part travels on the guide rails, is connected to the table in order to effectively damp vibrations acting on the table. More specifically, a machine tool constructed in a manner described hereinafter is provided. 
     The machine tool makes use of a rotating cutting tool to work a workpiece put on a cutting work table traveling on guide rails with the use of torque of a ball screw part and comprises a damper mechanism part mounted in a cavity provided on an opposite side of the table to that surface thereof on which a workpiece is put, and comprising a weight, a spring and a damper, one ends of the weight and the damper being connected to an inner wall of the table with a vibratory direction of the spring agreeing with a moving direction of the table, the other ends of the weight and the damper being connected to the weight and the damper, and the weight being arranged in a manner to travel on the guide rails. 
     In order to efficiently damp vibrations, which a workpiece being heavy goods suffers at the time of working, it is necessary to make the weight heavy and the guide rails are arranged to surely bear and freely move the weight but the guide rails may serve as traveling guide rails for the cutting work table, or may be provided separately. 
     Also, the damper mechanism part may be received in the table, or may be connected thereto to agree with a moving direction of the table. Specifically, the weight is connected through the spring and the damper to that surface of the table, which is perpendicular to a traveling direction of the table, the weight being arranged in a manner to travel on the guide rails. 
     As described above, by using a construction, in which the weight constituting the damper mechanism part is made to independently travel on the guide rails, the weight being heavy can be freely moved without having any influence on driving of the ball screw part. Thereby, it is possible to extremely effectively damp vibrations transmitted to the table and to accurately work a workpiece being heavy goods. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a schematic view illustrating vibrations generated when a lengthy ball screw system is used; 
         FIG. 2  is a schematic view illustrating a first embodiment of the invention; 
         FIG. 3  is a perspective view illustrating a machine tool, according to the first embodiment of the invention, as viewed slantwise from above; 
         FIG. 4  is a perspective view illustrating the machine tool, according to the first embodiment of the invention, as viewed slantwise from under (bask side); 
         FIG. 5  is an equivalent network diagram illustrating a vibration system model in a damper mechanism part used in the first embodiment of the invention; 
         FIG. 6  is a view illustrating a frequency characteristic of vibrations in a feed direction of a table to show a comparative example in the case where a damper mechanism part is not provided; 
         FIG. 7  is a view illustrating a frequency characteristic of vibrations in a feed direction of a table to show an example in the case where a damper mechanism part is provided; 
         FIG. 8  is a schematic view illustrating a cutting depth by a tool in a radial direction and a cutting depth by a tool in an axial direction in the first embodiment of the invention; 
         FIG. 9  is a schematic view illustrating an example of cutting work in the first embodiment of the invention and representing the relationship between conditions of cutting work and presence of vibrations generated on a table in the case where a damper mechanism part is not provided; 
         FIG. 10  is a schematic view illustrating an example of cutting work in the first embodiment of the invention and representing the relationship between conditions of cutting work and presence of vibrations generated on a table in the case where a damper mechanism part is provided; 
         FIG. 11  is a schematic view illustrating a second embodiment of the invention; and 
         FIG. 12  is a schematic view illustrating a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the invention will be described hereinafter with reference to the drawings. 
     First Embodiment 
     Generally, when a table for cutting work is moved and a workpiece put on the table is worked over a whole length thereof, a distance, over which the table is moved, becomes necessarily large and a ball screw part for feeding the table must be made lengthy. For example,  FIG. 1  is a schematic view illustrating vibrations generated on a machine tool when a lengthy ball screw system is used. A workpiece  61  is put on a table  11  for cutting work and the table  11  travels on guide rails  12  moving relative to a cutting tool  51 , which is provided separately, to achieve a cutting work. A nut  21  is mounted on an underside of the table  11  and a ball screw  22  fitted into the nut  21  rotates whereby the table  11  is moved by a distance corresponding to a rotating angle and a lead of the ball screw. In addition, the ball screw  22  is connected to a servomotor  24  and caused to rotate in a forward direction or a backward direction whereby the table  11  advances or retreats. 
     As apparent from the above-mentioned example, the longer in length the ball screw  22 , the lower in stiffness the ball screw  22 , so that the ball screw  22  is liable to flex in a vertical direction as shown in the  FIG. 1 . Due to this, when the cutting tool  51  is used to work the workpiece  61 , exertion of a force on the table  11  causes the ball screw  22  to flex thus generating vibrations in a feed (advancing and retreating) direction of the table  11 . 
       FIG. 2  shows a first embodiment of the invention as a method of suppressing vibrations of the table  11 . 
     The same elements as those in  FIG. 1  are denoted by the same reference numerals as those in the latter and an explanation therefor is omitted. A difference from the machine tool shown in  FIG. 1  resides in damper mechanism parts  300 ,  400  mounted on the table  11 . The damper mechanism part  300 ,  400 , comprises a weight  31 ,  41 , a spring  32 ,  42 , and a damper  33 ,  43 . The table  11  is internally hollow and the weight  31 ,  41  is connected to an inner wall, which extends in a traveling direction of the table  11 , through the spring  32 ,  42  and the damper  33 ,  43 . 
     Wheels  101 ,  102 , which travel on the guide rails  12 , are mounted on an underside of the table  11  and likewise wheels  111 ,  121  and  131 ,  141  are mounted on the weights  31 ,  41 . The guide rails  12  may serve for the table  11  and the weights  31 ,  41 , or the guide rails  12  may be separately provided for the table  11  and for the weights  31 ,  41 . Also, parts for traveling on the guide rails are not limited to wheels provided that they serve for the purpose of the invention. 
     Owing to the structure, in which the table  11 , on which the workpiece  61  is put, and the weights  31 ,  41  are supported by the guide rails  12 , the weights  31 ,  41  being large in weight corresponding to the workpiece  61 , which is large or lengthy and large in weight, can be used whereby it is possible to extremely effectively damp vibrations generated on the table  11 . 
       FIG. 3  is a perspective view showing the machine tool, according to the first embodiment, as viewed slantwise from above. In  FIG. 3 , a tool spindle  52  for driving the rotating cutting tool  51  is used to work the workpiece  61  put on the table  11 . Also, the table  11  travels on the guide rails  12  mounted on upper parts (edge parts) of a concavely-shaped base  10  and the ball screw part  22  is connected at one end thereof to the servomotor  24  and is rotatably supported at the other end thereof by a bearing part  23 , and is housed in a grooved part of the base  10 . 
     While an interior of the table  11  is hidden and unseen, it is hollow as illustrated in  FIG. 2  to internally receive therein the damper mechanism parts  300 ,  400 . In addition, the weights  31 ,  41  are mounted in the interior so as not to come into contact with the inner walls of the table  11 . 
       FIG. 4  is a perspective view showing the machine tool, according to the first embodiment, as viewed slantwise from under. The ball screw part  22  is arranged in a manner to be interposed by the two guide rails  12 , and torque of the ball screw part  22  moves the table  11  through the nut  21  mounted to the underside of the table  11 . As illustrated in  FIG. 2 , the wheels  100 ,  101 ,  102  mounted to the underside of the table  11 , the wheels  111 ,  121  and  131 ,  141  mounted to the weights  31 ,  41  are arranged in a manner to individually travel on the guide rails  12 . A positional relationship of the guide rails  12  and the ball screw part  22  is taken into consideration such that loading by the workpiece  61  put on the table  11  or the weights  31 ,  41  has no influence on movements of the ball screw part  22  through the nut  21  mounted to the underside of the table  11 . 
     The weights  31 ,  41  are arranged in a manner not to come into contact with the table  11 , and they are displaced relative to the table  11  owing to expansion and contraction of the springs ( 32 ,  42 ) and the dampers ( 33 ,  43 ) when the table  11  suffers vibrations. Relative vibrations of the table  11  and the weights  31 ,  41  are damped by the dampers  33 ,  43  and by friction between the guide rails  12  and the wheels. If contact between the inner walls of the table and the weights occurs, generation of abrasion and abraded powder due to sliding causes foreign matters to get onto the guide rail surfaces, thus bringing about shortening of the service life of the guide rails. Also, since friction coefficients between solids vary according to an ambient atmosphere such as temperature, humidity, or the like, it is not possible to obtain a stable damping coefficient and it becomes difficult to stably suppress vibrations of the table. Therefore, it is necessary to pay a careful attention to the positional relationships among the table and the springs, dampers, weights, etc. 
     Subsequently, an explanation will be given to characteristics of vibrations generated on the table  11  in the case where two damper mechanism parts ( 300 ,  400 ) are used. 
       FIG. 5  is an equivalent network diagram for analysis of characteristics of vibrations. The table  11  constitutes a mass point having a mass m0 and is supported by a spring having a spring constant k0 and a damper having a damping coefficient c0. The weight  31  constitutes a mass point having a mass m1 and is connected to the table  11  by a spring having a spring constant k1 and a damper having a damping coefficient c1. The weight  41  constitutes a mass point having a mass m2 and is connected to the table  11  by a spring having a spring constant k2 and a damper having a damping coefficient c2. 
     Excluding displacements of the table  11  given by traveling and assuming that x0 indicates displacements of the table  11  caused by vibrations, x1 indicates displacements of the weight  31  caused by vibrations, and x2 indicates displacements of the weight  41  caused by vibrations, an equation of motion when an external force F0 acts on the table  11  is represented by the following formula (1).
 
 k   0   x   0   +c   0   {dot over (x)}   0   +m   0   {umlaut over (x)}   0   =F   0   +F   10   +F   20   (1)
 
     Here, F10 indicates a force exerted on the table  11  by the weight  31  and F20 indicates a force exerted on the table  11  by the weight  41 . 
     Subsequently, for the weight  31  and the weight  41 , the following formula (4) is obtained by substituting the following formulae (2) and (3) into the formula (1). 
     
       
         
           
             
               
                 
                   
                       
                   
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     Subsequently,  FIGS. 6 and 7  show results obtained by solving the formula (4) to find the frequency response characteristic x0/F0 of vibration displacements x0 relative to the external force F0 exerted on the table  11 . In addition,  FIG. 6  show results in the case where the damper mechanism parts  300 ,  400  in  FIG. 2  are not provided and  FIG. 7  show results in the case where the damper mechanism parts are provided. 
     Here, the following data were used. That is, the table  11  was a rectangular parallelepiped having a width of 2 m, a length of 6 m, and a height of 1 m and being internally hollow. Also, the mass m0 of the table  11  was 20000 (kg), the spring constant k0 of the spring supporting the table  11  was 2.0×10 8  (N/m), and the damping coefficient c0 of the damper supporting the table  11  was 200000 (Ns/m). Also, the mass m1 of the weight  31  and the mass m2 of the weight  41 , respectively, were 5000 (kg). Also, the spring constant k1 of the spring  32  and the spring constant k2 of the spring  42 , respectively, were 2.2×10 7  (N/m) and the damping coefficient c1 of the damper  33  and the damping coefficient c2 of the damper  43 , respectively, were 199000 (Ns/m). 
     As apparent from  FIG. 6 , in the case where the damper mechanism parts were not provided in the table  11 , a large amplitude was observed at the natural frequency of about 16 (Hz). In other words, it is indicated that in the course of cutting a workpiece, the table itself experienced vibrations having the above natural frequency thereby involving the danger of remarkable inhibition in machining accuracy, etc. On the other hand, the results in  FIG. 7  show that the damper mechanism parts were received in the table  11  to thereby enable effectively damping vibrations of the table in any frequency zone. Comparing with the results in  FIG. 6 , vibrations of the table are restricted to about ⅕. 
     As described above, in case of cutting a large-sized or lengthy workpiece, a shaft of a ball screw part for feeding a table cannot but necessarily be made lengthy. In some cases, self-excited vibrations are generated in cutting work as the shaft is made lengthy. Liability of generation of such self-excited vibrations is determined by the dynamic stiffness of that structure, which constitutes a machine tool, that is, a frequency response of vibrations thus obtained, the number of blades of a cutting tool, conditions of cutting work, and so on. Hereupon, with the use of the method disclosed by Altintas: Manufacturing Automation, Cambridge University Press, p. 104 (2000), an examination has been made for stability evaluation of self-excited vibrations when a workpiece is worked by a cutting tool, in relation to the presence of the damper mechanism parts proposed in the present invention. 
       FIG. 8  is a schematic view illustrating the working condition when the workpiece  61  was worked by the use of the cutting tool  51 .  FIG. 8  shows the case where the cutting tool  51  was rotated clockwise to cut the workpiece  61  starting from a left end in the drawing. At that time, the working conditions were a cutting depth Rd (mm) in a radial direction, a cutting depth Ad (mm) in an axial direction, and a number S (rpm) of revolutions of the cutting tool, and specifically, sixteen rotating tools having a diameter of 250 mm were used with the cutting depth Rd in a radial direction being 200 mm. Also, a constant used in calculation of a cutting force was given assuming that for an amount removed by cutting per unit area, a force of 3000 (N/mm 2 ) in a direction tangent to rotation of the tool and a force of 2000 (N/mm 2 ) in a radial direction of rotation of the tool acted. 
       FIGS. 9 and 10 , respectively, show results of stability in self-excited vibrations of a machine tool in the respective cases where the damper mechanism parts were provided and not provided. In both drawings, symbols of white circles indicate a stable cutting work not accompanied by self-excited vibrations and symbols of black circles indicate an unstable cutting work in the relationship of cutting depth Ad (mm) in an axial direction with the number S of revolutions of the cutting tool. 
     As a result, it is seen from  FIG. 9  that when the damper mechanism parts were not used, self-excited vibrations were readily generated and a stable cutting work could not be performed under the working conditions of the number S of 100 (rpm) of revolutions of the cutting tool and the cutting depth Ad of at least 1.0 (mm) in an axial direction. On the other hand, as apparent from the results of analysis in  FIG. 10 , a zone (working conditions) of cutting work in a stable state is increased by adopting the damper mechanism parts. Specifically, it is shown that in case of the number S of 100 (rpm) of revolutions of the tool, the cutting depth Ad in an axial direction was increased to 2.5 (mm) from 1.0 (mm) to enable cutting work in a correspondingly short period of time. 
     The results shown in  FIGS. 9 and 10  show that working of even a large-sized or lengthy and heavy workpiece can be achieved generally in an increased working efficiency and besides stably by mounting the damper mechanism part shown in  FIG. 2  on a table for cutting work and realizing a state of enabling weights, which constitute the damper mechanism part, and the table, respectively, to freely travel on guide rails. 
     Second Embodiment 
       FIG. 11  is a schematic view showing a cutting machine tool, according to a second embodiment, in which a ball screw system is used. A difference of the second embodiment from the first embodiment (see  FIG. 2 ) resides in that a table  200  for cutting work with a workpiece  61  put thereon is a flat plate and the flat-plate table  200  and struts  201 ,  202  are used to form a cavity instead of the cavity formed in the interior of the table  11  as shown in  FIG. 2 . 
     A highly efficient and stable cutting work is possible in the same manner as in the first embodiment. Further, the arrangement enables freely designing the cavity part in the table and is suited to the case where a further large-sized weight must be used. 
     Third Embodiment 
       FIG. 12  is a schematic view showing a cutting machine tool, according to a third embodiment, in which a ball screw system is used. A difference of the third embodiment from the first embodiment (see  FIG. 2 ) resides in a manner of connecting a damper mechanism part  500  to a table  11  instead of providing the damper mechanism part in an interior of the table  11 . It goes without saying that a weight  31 , which constitutes the damper mechanism part  500 , travels on guide rails  12  in the same manner as in the first embodiment. 
     The same cutting effect as that in the first embodiment is produced in this embodiment. In comparison with the first embodiment, it is possible to freely adjust the weight  31  in order to optimize the damping characteristic in response to the weight of the workpiece  61 . 
     The invention provides a machine tool, which performs an efficient and stable cutting work of a large-sized or lengthy and heavy workpiece, and consequently enables contributing to reduction in material cost and to industrial use. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.