Abstract:
An apparatus for compensating position error of a spindle head, comprises: Y-axis control parts  32   a,    32   b  for controlling first and second servomotors  24   a,    24   b  on the basis of a spindle head moving command fed from a numerical control unit  30;  a correction data storage part  36  for storing correction data determining the relationship of a feed distance of a ram  18  to a compensation value for correcting the vertical deviation of the ram  18  in the feed distance of the ram  18;  and a correction means  44  for being introduced a ram feed instruction, which is fed from the numerical control unit, and for reading out compensation value corresponding to the feed distance of the ram from the correction data storage unit  36,  to correct the spindlehead feed instruction by adding or subtracting the compensation value to or from the spindlehead feed instruction.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of The Invention  
           [0002]    The present invention generally relates to a numerically controlled machine tool, such as a horizontal boring machine. More specifically, the invention relates to an apparatus for compensating position errors resulting from fluctuation of the center of gravity caused by the horizontal movement of a ram carried by a spindle head.  
           [0003]    2. Description of The Prior Art  
           [0004]    In a typical horizontal boring machine, a spindle head moves in the vertical direction along a guided on a column, and a ram carried by the spindle head moves in the horizontal directions. If the ram moves in the horizontal direction, there is caused a phenomenon that the spindle head is inclined by the variation of the center of gravity. In the case of a large-sized horizontal boring machine, the ram itself carries heavy weight, so that the position of the center of gravity in the spindle head when the ram does not move is greatly different from that when the ram is fed to the end. By this position fluctuation of the center of gravity, the ram can not keep the straightness during the horizontal movement, so that position errors of the spindle at the front end of the ram in vertical directions are arose. Therefore, it is required to carry out any compensation in order to insure machining precision.  
           [0005]    As a conventional method for compensating the position error due to changes of center of gravity, there is known a method for automatically hydraulically changing a rate of lifting forces of right and left two wires for supporting the weight of a spindle head, to hold the straightness of a ram during the horizontal movement.  
           [0006]    However, the hydraulic compensation for position fluctuation of the center of gravity is restricted by a hydraulic system which has a response lag and a bad follow-up performance, so that the actual position of the front end of the spindle is different from the corrected position thereof.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an apparatus for compensating position errors of the spindle head resulting from the position fluctuation of the center of gravity in a spindle head, which is capable of responsively and precisely keep the straightness of a movable member such as a ram during the horizontal movement even if the position of the center of gravity is changed with the feeding of the movable member.  
           [0008]    In order to accomplish the aforementioned and other objects, there is provided a compensator for compensating a vertical deviation of the movable member resulting from the fluctuation of the center of gravity of the spindle head caused by the feed motion of the movable member, said compensator comprising a Y-axis control means for controlling the first and second Y-axis servomotors on the basis of spindlehead feed instructions provided from the numerical control unit, a Z-axis control means for controlling the Z-axis servomotors on the basis of a movable member feed instruction provided from the numerical control unit; correction data storage means for storing correction data determining the relationship of a feed distance of the movable member to a compensation value for correcting a vertical deviation of the movable member; and correction means for being introduced the movable member feed instruction provided from the numerical control unit, and for reading out the compensation value corresponding to the feed distance of the movable member from the correction data storage means, to correct the spindlehead feed instruction by adding or subtracting the compensation value to or from the spindlehead feed instruction.  
           [0009]    According to the present invention, the first servomotor rotates the ball screw by a larger amount than that corresponding to the spindle moving command before correction, to move the spindle head upward, and the second servomotor rotates the ball screw by a smaller amount than that corresponding to the spindle moving command before correction, to move the spindle head downwards, so that the attitude of the spindle head is maintained to be horizontal as a whole. Thus, the guide face of the ram is held so as to be horizontal, so that it is possible to prevent errors from being caused at the position of the tip of the ram (main spindle). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention. However, the drawings are not intended to imply limitation of the invention to a specific embodiment, but are for explanation and understanding only.  
         [0011]    In the drawings:  
         [0012]    [0012]FIG. 1 is a schematic diagram for explaining the construction of a horizontal boring machine to which a compensator according to the present invention is applied;  
         [0013]    [0013]FIG. 2 is a block diagram of the compensator according to the present invention;  
         [0014]    [0014]FIG. 3 is a schematic diagram for explaining the measurement of correction data in the horizontal boring machine of FIG. 1; and  
         [0015]    [0015]FIG. 4 is a diagram showing an example of a correction data table forming the basis for the correction of the center of gravity of a spindle head. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring now to the accompanying drawings, a preferred embodiment of an apparatus for compensating position errors of a spindle head according to the present invention will be described below.  
         [0017]    [0017]FIG. 1 shows a numerically controlled (NC) horizontal boring machine to which the present invention is applied. Reference number  10  denotes a floor. On the face of the floor  10 , a bed  11  is fixed. On the bed  11 , a column base  12  capable of moving along a guide in directions perpendicular to the plane of the figure is mounted. On the column base  12 , a column  14  is mounted fixedly.  
         [0018]    On the side face of the column  14 , a guide face (not shown) for guiding the vertical movement of the spindle head  16  is provided. The spindle head  16  is mounted on the column  14  so as to be movable along the guide face in the vertical direction. In the spindle head  16 , a ram  18  is carried so as to be movable in the horizontal direction. A spindle unit is built in the ram  18 . In this preferred embodiment, a closed type guide face for guiding four faces of the box-shaped ram  18  is adopted as the guide face of the spindle head  16 . Thus, structural rigidity of the spindle head  16  is increased.  
         [0019]    On the top of the column  14 , a pulley  19  for a counterweight is installed. One end portion of a wire  20  wound onto the pulley  19  is connected to the spindle head  16 , and the other end portion of the wire  20  is connected to a counterweight (not shown) so as to balance against the weight of the spindle head  16 .  
         [0020]    In this horizontal boring machine, the vertical movement of the spindle head  16  is carried out by first and second screw mechanisms comprising two ball screws  22   a  and  22   b,  and first and second Y-axis servomotors  24   a  and  24   b  for driving the screw mechanisms, respectively. The feeding of the ram  18  is carried out by driving a ball screw mechanism  26  by a ram feed (Z-axis) servomotor  28 . Furthermore, reference number  29  denotes a motor for rotating the spindle. When this horizontal boring machine is numerically controlled by the NC unit, an axis for controlling the feed motion of the spindle head  16  is Y-axis, and an axis for controlling the feed motion of the ram  18  is Z-axis.  
         [0021]    In the spindle head  16 , when the ram  18  is fed forward and backward, the position of the center of gravity in the spindle head  16  is changed according to the position of the ram  18 , so that the guide face of the ram  18  is slightly inclined from the horizontal plane together with spindle head  16 . By this inclination of the guide face, the position of the front end of the ram  18  (main spindle) is deviated in a vertical direction from the position when the guide face of the ram  18  is maintained to be horizontal. FIG. 2 is a block diagram of a control unit for compensating such deviation of the ram.  
         [0022]    In FIG. 2, reference number  30  denotes an NC unit for numerically controlling a horizontal boring machine. This NC unit  30  has an processing unit  32  for analyzing inputted NC data to generate instructions. A spindlehead feed instruction (Y-axis feed instruction) indicative of the position of the spindle head  16  and ram feed instruction (Z-axis feed instruction ) indicative of the position of the ram  18  outputted from the NC unit  30  are provided to Y-axis and Z-axis servo amplifiers  32   a  and  32   b  of a servo control unit  31 , respectively. Furthermore, control loops for feedback controlling positions and servo amplifiers belonging to other axes are omitted since they are not related directly to the correction of the center of gravity.  
         [0023]    The spindlehead feed instruction outputted from the NC unit  30  is introduced to each of the Y-axis serve amplifiers  32   a  and  32   b.  On the basis of this spindlehead feed instructions, the Y-axis servo amplifiers  32   a  and  32   b  give command signal to and control the position of the servomotors  24   a  and  24   b  for driving the spindle head  16  in the vertical direction. Similarly, the ram feed instruction outputted from the NC unit  30  is introduced to a Z-axis servo amplifier  34 . On the basis of this ram feed instruction, the Z-axis servo amplifier  34  give command signal to the ram feed servomotor  28  and controls its rotation. Reference number  36  denotes a storage unit in which a correction data table required to carry out compensation is stored.  
         [0024]    The correction data table required to carry out the compensation for the position errors will be described below.  
         [0025]    This correction data table comprises correction data which determine the relationship of a given feed distance of the ram  18  to a compensation value for correcting the vertical deviation of the ram  18  in the feed distance of the ram  18 .  
         [0026]    [0026]FIG. 3 is a schematic diagram for explaining the measurement of the vertical deviation of the ram  18  when the ram  18  is moved forward. Reference number  40  denotes a reference block used as a reference of measurement. In this case, a pair of indicators  42   a  and  42   b  are used. The indicator  42   a  is attached to the front end portion of the ram  18 , and the other indicator  42   b  is mounted on the reference block  40 . As shown in FIG. 3, when the ram  18  is moved forward by a predetermined distance Z 1 , the center of gravity is changed by a weight corresponding to the movement of the ram  18 , to apply a moment to the spindle head  16 , so that the ram  18  is inclined downwards toward front end. At this time, assuming that the vertically upward direction is the positive direction of Y-axis using the top face of the reference block  40  as a horizontal reference, the ram  18  is displaced in the negative direction of Y-axis at the position of the indicator  42   a  and in the positive direction of Y-axis at the position of the indicator  42   b.    
         [0027]    When the ram feeding distance is Z 1 , driving the first and second Y-axis servomotors  24   a  and  24   b  count the number of pulses corresponding to an angle of rotation of the first Y-axis servomotor  24   a  immediately before the indicator  42   a  having contacted with the top surface of the reference block  42  detached from it, and the number of pulses corresponding to an angle of rotation of the second Y-axis servomotor  24   b  immediately before the indicator  42   b  contacts the ram  18 , it is possible to measure the compensation value in the ram feed distance Z 1 .  
         [0028]    [0028]FIG. 4 is a graph showing the results of such measurements of the compensation value with respect to many different feed distances Z. The correction data used as the correction data table of FIG. 2 is prepared by using data as shown in FIG. 4. Given any feed distance Z of the ram  18  is given, the relationship of the feed distance Z to a compensation value Äy 1  with respect to the first Y-axis servomotor  24   a  and a compensation value Äy 2  with respect to the second Y-axis servomotor  24   b  is determined by one to one. In a case where an attachment is attached to the ram  18 , it is required to prepare a correction data table beforehand by measuring a renewed compensation value of the ram  18  to which the attachment is attached.  
         [0029]    In FIG. 2, reference number  44  denotes a correction processing unit. This correction processing init  44  is designed to introduce a ram feed instruction and read out a compensation value, which corresponds to the feed distance of the ram  18 , from the storage unit  36  in which the correction data table is stored. And the correction processing unit is designed to add or subtract the compensation value to or from the spindlehead feed instruction.  
         [0030]    That is, if the feed distance of the ram  18  is z, the ram feed instruction is introduced to the correction processing unit  44 . Then the correction processing unit  44  read out the compensation values Äy 1  and Äy 2  corresponding to the feed distance z from the correction data table  36 . Furthermore, the correction processing unit  44  add the compensation value Äy 1  to the spindlehead feed instruction with respect to the first Y-axis servo amplifier  32   a  and subtract the compensation value Äy 2  from the spindlehead feed instruction with respect to the second Y-axis servo amplifier  32   b.    
         [0031]    As a result, the first Y-axis servomotor  22   a  rotates the ball screw  22   a  by a larger amount than that corresponding to the spindlehead feed instruction compared with the state where no correction is executed, to move the spindle head  16  upward. And the second Y-axis servomotor  22   b  rotates the ball screw  22   b  by a smaller amount than that corresponding to the spindlehead feed instruction command compared with the no correction is executed, to move the spindle head  16  downwards, so that the horizontal position of the spindle head  16  is maintained as a whole. Therefore, the guide face of the ram  18  is held so as to be horizontal, so that it is possible to prevent errors from being caused at the position of the front end of the ram (main spindle).  
         [0032]    In addition, the correction value is added or subtract to or from the instruction, which is fed from the NC unit  30 , to cause the amounts of rotation of the servomotors  22   a  and  22   b  for feeding the spindle head to be different from each other, to correct the fluctuation of the center of gravity of the spindle head  16 . Therefore, it is possible to far greatly enhance response than conventional hydraulic systems, so that it is possible to enhance the precision of correction.  
         [0033]    As can be clearly seen from the above descriptions, according to the present invention, it is possible to responsively and precisely hold the spindle head in the horizontal position to hold the straightness of the horizontal movement of the ram even if the position of the center of gravity is changed with the feed motion of the ram.  
         [0034]    While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.