Abstract:
A method for measuring errors in the linear feed shafts of a multi-spindle machine tool having two or more rotating feed shafts in addition to three linear feed shafts, wherein: at least first through third reflecting minors are attached to a table of the machine tool; a laser length-measuring machine is attached to the tip of a principal shaft of the machine tool; the linear feed shafts are driven, and the laser length-measuring machine is moved to prescribed measuring points; the two or more rotating feed shafts are driven at each of the measuring points; the coordinates at each measuring point are calculated by measuring the distances between the first through third reflecting minors and the laser length-measuring machine; and errors in the linear feed shafts of the machine tool are obtained by comparing the machine coordinates of the machine tool.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase patent application of PCT/JP2011/077682, filed on Nov. 30, 2011, the entirety of which is hereby incorporated by reference in the present disclosure. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of measuring an error in linear feed axes of a machine tool having at least two rotational feed axes of A-axis and C-axis in addition to three orthogonal axes of X-, Y- and Z-axes, and a machine tool. 
       BACKGROUND OF THE INVENTION 
       [0003]    A machine tool processes a workpiece by relatively moving, in three orthogonal axes directions of X-, Y- and Z-axes, a workpiece attached to a table and a tool attached to the end of a spindle. In the field of machine tools, in order to increase accuracy, error measurement methods using a laser beam have been developed. 
         [0004]    For example, Patent Document 1 describes a laser tracking-type measuring device for tracking the travel of a reflector attached to a tool mounting shaft in order to detect the coordinate of a tool based on the displacement of the tracked reflector. 
       Patent Publications 
       [0005]    Patent Document 1: Japanese Unexamined Patent Publication No. H07-246547 
       SUMMARY OF THE INVENTION 
       [0006]    The invention described in patent document 1 must have four laser tracking devices, each of which includes a rotational supporting mechanism in order to track a reflector, and thus is very large and expensive. 
         [0007]    The present invention is directed to solve the problems in the prior art, and the objective of the invention is to provide a simple and low cost method and machine tool for measuring errors in the linear feed axes. 
         [0008]    According to the invention, there is provided a method of measuring errors of linear feed axes of a multi feed axis machine tool having at least two rotational feed axes as well as three orthogonal linear feed axes, i.e., X-, Y- and Z-axes, the method comprising the steps of attaching at least first to third reflector mirrors to a table of the machine tool, attaching a laser length measuring device to an end of a spindle of the machine tool, moving the laser length measuring device to predetermined measuring points by driving the three linear feed axes, orienting the laser length measuring device to the reflector mirrors by driving the at least two rotational feed axes to measure the lengths between the first to third reflector mirrors and the laser length measuring device whereby the coordinates of the respective measuring points are calculated, and comparing the machine coordinates of the machine tool at the measuring points with the calculated coordinates of the measuring points whereby errors of the linear feed axes of the machine tool are obtained. 
         [0009]    Further, according to another feature of the invention, there is provide a machine tool with a table to which a workpiece is attached, a spindle, supported for rotation, for holding a tool, linear feed axes for relatively moving the table and the spindle in three orthogonal X-, Y-, and Z-axes directions and at least two rotational feed axes, the machine tool comprising a laser length measuring device attached to an end of a spindle, and first to third reflector mirrors attached to a table of the machine tool, wherein the laser length measuring device is moved to predetermined measuring points by driving the three linear feed axes, and oriented to the reflector mirrors by driving the at least two rotational feed axes to measure the lengths between the first to third reflector mirrors and the laser length measuring device whereby the coordinates of the respective measuring points are calculated, the calculated coordinates of the measuring points being compared with the machine coordinates of the machine tool at the measuring points whereby errors of the linear feed axes of the machine tool are measured is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side view of a machine tool according to a preferred embodiment of the invention. 
           [0011]      FIG. 2  is a front view of a column of the machine tool of  FIG. 1 . 
           [0012]      FIG. 3  is a block diagram of an embodiment of a numerical control device for controlling the feed axes of the machine tool of  FIG. 1 . 
           [0013]      FIG. 4  is a flowchart showing an example of an error measurement method. 
           [0014]      FIG. 5  is a schematic illustration of the error measurement apparatus, explaining the error measurement method. 
           [0015]      FIG. 6  is a schematic illustration of the error measurement apparatus, explaining the error measurement method. 
           [0016]      FIG. 7  is a schematic illustration of the error measurement apparatus, explaining the error measurement method. 
           [0017]      FIG. 8  is a schematic illustration of the error measurement apparatus, explaining the error measurement method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    With reference to the drawings, a preferred embodiment of the invention will be described below. A numerically controlled machine tool according to the invention is provided with a numerical control device for operating the machine toll in accordance with a processing program. With reference to  FIGS. 1 and 2 , a horizontal type machine tool  10  having five feed axes wherein A- and C-axes are provided on a spindle is shown. The machine tool  10  comprises a rear bed  12  fixed on a floor, a column  14  mounted on a top face of the rear bed  12  for linear motion in a horizontal direction, i.e., X-axis direction, a head stock  15  mounted to the column  14  for linear motion in both vertical and horizontal directions, i.e., Y- and Z-axis directions, a bracket  18  mounted to a front face of the head stock  16  for rotation around C-axis parallel to the Z-axis, a spindle head  20 , mounted to the bracket  18  for rotation about A-axis parallel to the X-axis, for rotatably supporting a spindle  22 , a front bed  24  placed side by side in the Z-axis direction relative to the rear bed  12  and a table  26  mounted to the front bed  24  so as to face the spindle  22 . The Z-axis extends horizontally perpendicular to both the X- and Y-axes. Further, the machine tool  10  comprises a measurement apparatus  52 . 
         [0019]    With reference to  FIG. 3 , a numerical control device  30 , for controlling the position of the feed axes of the machine tool  10 , comprises a read and interpretation unit  34  for reading and interpreting a processing program  32 , and for calculating command speeds and positions for the respective feed axes, an interpolation unit  36  for calculating command pulses based on the command positions and speeds in order to linearly or circularly interpolating the feed rates of the respective feed axes, position command recognizing means  38  for receiving the command pulses and recognizing the position commands to the respective feed axes, error calculating and storing means  48  for calculating errors based on measurement data which is measured by a measurement apparatus  52  and the machine coordinates which are obtained by reading the digital scales of the respective X-, Y- and Z-axes of the machine tool  10  and for storing the obtained errors, correction data calculating means  40  for calculating correction data for correcting the position commands based on the position commands and the error data stored in the error calculating and storing means  48 , correction pulse calculating means  42  for obtaining correction pulses for correcting the position commands based on the correction data, a servo unit  46  for controlling the motors  50  of the respective feed axes and adder means  44  for outputting pulses obtained by adding the command pulses and the correction pulses. 
         [0020]    The measurement apparatus  52  will be described in detail. In this embodiment, the measurement apparatus  52  comprises a laser length measuring device  54  fitted into a tool fitting hole (not shown) defined in the spindle  22  and a plurality of reflector mirrors  56  attached to the table  26 . In this embodiment, a laser interferometer is used as the laser length measuring device  54 . Laser interferometers include, for example, a laser source for emitting a frequency stabilized helium-neon laser, a beam splitter for dividing the laser beam from the laser source into two beams, and a counter composed of, for example, a photodiode array for counting the number of interference of fringes generated by the interference between one of the two split beams and the other of the split beams reflected from the reflector mirrors  56  whereby changes in the optical pass length is measured based on the changes in the number of the interference of fringes. 
         [0021]    The reflector mirrors  56  comprises a retroreflector which reflects the laser beam in the original direction regardless of changes in the incident angle of the laser beam into the reflector mirrors  56 . In this embodiment, the reflector mirrors  56  comprises first to fourth reflector mirrors  56   a - 56   d  secured to the four corners of a pallet  28  detachably secured to the table  26 . 
         [0022]    The measurement principal of the measurement apparatus  52  according to this embodiment will be described below. 
         [0023]    As described above, in this embodiment, the laser length measuring device  54  is a laser interferometer which measures, based on the changes in the number of the interference of fringes, the difference in the length (optical pass length) between one of the first to fourth reflector mirrors  56   a - 56   d  and a measuring point and between the same reflector mirror and a current measuring point base on the following equation. 
         [0000]        ΔL ( i, j )=( L ( Pi, Hj )− L ( P 0 , Hj ))
 
       Where: 
       [0000]    
       
         ΔL: optical pass length difference 
         Pi: ith measuring point 
       
     
         [0026]    P 0 : first measuring point providing a reference measuring point
   Hj: jth reflector mirror   L(P 0 , Hj): the length (optical pass length) between the first measuring point and the jth reflector mirror   
 
         [0029]    L(Pi, Hj): the length (optical pass length) between the ith measuring point and the jth reflector mirror 
         [0030]    The length between the ith measuring point and the jth reflector mirror is generally expressed by the following equation. 
         [0000]        L ( Pi, Hj )=(( Xi−Xhj ) 2 +( Yi−Yhj ) 2 +( Zi−Zhj ) 2 )1/2   (1)
 
       Where: 
       [0000]    
       
         Xi: X-coordinate of the ith measuring point (Pi) 
         Xhj: X-coordinate of the jth reflector mirror (Hj) 
         Yi: Y-coordinate of the ith measuring point (Pi) 
         Yhj: Y-coordinate of the jth reflector mirror (Hj) 
         Zi: Z-coordinate of the ith measuring point (Pi) 
         Zhj: Z-coordinate of the jth reflector mirror (Hj) 
       
     
         [0037]    When m is the number of the measurement points, i.e., i=1 to m, if the reflector mirrors  56  comprises four reflector mirrors, then the number of the unknown is 12+3×m (the coordinates of the four reflector mirrors provide  12  unknowns, and the coordinates of the measurement points provide 3×m unknowns). Therefore, if m=12, equation (1) can be solved by simultaneous equations. If m is larger than 12, then the number of the simultaneous equations is larger than the number of the unknowns, whereby the solution becomes redundant (different combinations of solution are provided depending on the combination of the equations). The solutions are averaged by for example least-squares method. The measured errors are stored in the error calculating and storing means  48  in the form of an error map in a processing space defined by the three orthogonal X-, Y- and Z-axes. 
         [0038]    With reference to  FIGS. 4-8 , an error measurement method according to the present embodiment will be described below. 
         [0039]    When the error measurement process is started, the parameters I and J are reset to 0 (step S 10 ), followed by inputting 1 into J (step S 12 ). The parameter I denotes the measurement point. The parameter J relates to the first to fourth reflector mirrors  56   a - 56   d,  i.e., J=1, J=2, J=3 and J=4 denote the first reflector mirror  56   a , the second reflector mirror  56   b,  the third reflector mirror  56   c  and the fourth reflector mirror  56   d , respectively. 
         [0040]    As shown in  FIG. 4 , by driving the three orthogonal linear feed axes, i.e., X-, Y- and Z-axes and rotational feed axes, i.e., A- and C-axes of the machine tool  10 , the laser length measuring device  54 , in particular the working section of its counter for counting the number of the interference of fringes is moved to the reference measuring point P 0  so that a laser beam is emitted to the reflector mirror (J=1), i.e., the first reflector mirror  56   a  whereby the number of the interference of fringes is measured at the reference measuring point P 0  (step S 14 ). Then, 1 is added to the parameter I (step S 16 ), followed by moving the laser length measuring device  54  to the measuring point (P 1 ) along the predetermined pass  58 , as shown in  FIG. 5 , by driving the X-, Y- and Z-feed axes (step S 18 ), with the laser beam continuously oriented to the first reflector mirror  56   a.  At the measuring point (P 1 ), the counted number of the interference of fringes is stored in relation to the reflector mirror (J) and the measuring point (P 1 ) (step S 20 ). 
         [0041]    Then, I is compared with a predetermined integer II in order to determine whether or not the present measuring point (Pi) is the last one of the measuring points (step S 22 ). II=4, when the program has, for example, five measuring points, as shown in  FIGS. 6 and 8 , for each of the first to fourth reflector mirrors  56   a - 56   d.  When the measurement is not completed for all of the measuring points (No at step S 22 ), the flowchart goes back to step S 16  whereby 1 is added to I, then steps S 18  and S 20  are carried out again. 
         [0042]    After the measurement is completed for all of the measuring points (Yes at step S 22 ), J is compared with a predetermined integer JJ in order to determine whether or not the currently measured reflector mirror (J) is the last one of the reflector mirrors (step S 24 ). If the measurement is not completed for all of the reflector mirrors (No at step S 24 ), the flowchart goes back to step 
         [0043]    S 12  whereby  1  is added to J whereby steps S 14  to S 22  of the measuring process are carried out for the next reflector mirror, for example, the second reflector mirror as shown in  FIG. 7  ( FIG. 8 ). The measurement is completed for all of the reflector mirrors (Yes at step S 24 ), the measuring process is completed. 
         [0044]    The error calculating and storing means  48  compares the respective coordinates of Pi obtained by solving equation (1) with the machine coordinates obtained by reading the respective digital scales of the X-, Y- and Z-feed axes, whereby the errors of the respective X-, Y- and Z-axes are obtained. 
         [0045]    According to the present embodiment, the laser length measuring device  54  is attached to the tool mounting hole at the end of the spindle  22 , and therefore the laser length measuring device  54  can be continuously oriented to the first to fourth reflector mirrors  56   a - 56   d  by using the three orthogonal linear feed axes, i.e., X-, Y- and Z-axes and the rotational feed axes, i.e., A- and C-axes of the machine tool  10  without providing a special tracking device, and therefore a simple and low cost error measuring apparatus is provided. Further, the laser length measuring device  54  can be automatically attached by using a tool changer incorporated with the machine tool  10 . 
         [0046]    Although the reflector mirrors are attached to the table through the pallet  28  in the aforementioned embodiment, the reflector mirrors may be directly attached to the table. Further, provision of the pallet  28  with the reflector mirrors  56  precedingly attached thereto enables an automatic error measurement by using a pallet changer (not shown) incorporated with the machine tool  10  and the aforementioned automatic laser length measuring device  54 . This enables programed automatic daily or seasonal error measurements. 
         [0047]    Although the laser length measuring device  54  comprising a laser interferometer is used in the aforementioned embodiment, the present invention is not limited to this configuration, and a laser length measuring device which can measure the absolute lengths between the reflector mirrors  56  and the laser length measuring device  56  may be used. In this case, error measurement can be carried out with the reflector mirrors  56  including three reflector mirrors, instead four, if the positions of the mirrors are precedingly known. 
         [0048]    If there are errors in the rotational positioning or the inclination of the rotational axis of the rotational feed axes (A- and C-axes), the errors adversely affect the measurement results. Accordingly, the machine tool is assembled while measuring so that the errors are minimized in a reference region. After the machine tool is assembled, the errors of the rotational feed axes (A- and C-axes) within the reference region is stored in order to enable correction during measurement of the liner feed axes (X-, Y- and Z-axes). 
         [0049]    In particular, the errors in position and inclination are stored in relation to the rotational angles of the rotational feed axes (A- and C-axes), whereby the feed axes (X-, Y- and Z-axes) are corrected depending on the position errors, and the rotational feed axes are corrected depending on the inclination errors. If the inclination errors of the rotational feed axes (A- and C-axes) are small, declinations of the referent point of the laser length measuring device are stored in relation to the rotational angles of the rotational feed axes (A- and C-axes), whereby only the liner feed axes (X-, Y- and Z-axes) can be corrected so as to position a point of reference of the laser length measuring device at a desired coordinate value. 
         [0050]    When measuring the liner feed axes (X-, Y- and Z-axes), by correcting the errors of the rotational axes (A- and C-axes) depending on the rotational positions of the rotational feed axes (A- and C-axes), the errors of the liner feed axes (X-, Y- and Z-axes) can be efficiently measured. 
       EXPLANATION OF REFERENCE NUMBERS 
       [0000]    
       
           10  Machine Tool 
           12  Rear Bed 
           14  Column 
           16  Head Stock 
           18  Bracket 
           22  Spindle 
           24  Front Bed 
           26  Table 
           28  Pallet 
           30  Numerical Control Device 
           32  Processing Program 
           34  Read and Interpretation Unit 
           36  Interpolation Unit 
           46  Servo Unit 
           50  Feed Motor 
           52  Measurement Apparatus 
           54  Laser Length Device 
           56  Reflector Mirror 
           56   a  First Reflector Mirror 
           56   b  Second Reflector Mirror 
           56   c  Third Reflector Mirror 
           56   d  Fourth Reflector Mirror