Abstract:
Plural spaced actuators mounted along a first axis are used to control machine tool motion along a first axis. The plural actuators are used in synchronism to create linear motion of the machine tool and the tool tip mounted on the machine tool along the first axis. The plural actuators are used differentially to create angular motion of the tool tip about an axis which is orthogonal to the first axis in order to correct angular errors in the axis of the tool.

Description:
This application is a371 of PCT/US 99/18634 filed Aug. 17, 1999 which claims benefit of Provisional No. 60/096,948 filed Aug. 18, 1998. 
    
    
     This invention is made with United States Government Support under Cooperative Agreement No. 70NANB5H1158 awarded by NIST. The United States Government has certain rights in this invention. 
    
    
     The invention relates to the use of plural actuators to position a machine tool along a single axis whereby the machine tool may be more accurately located at a desired location. 
     BACKGROUND OF THE INVENTION 
     During any machining process, relative motion between the cutting tool and part must occur. In the ideal working condition, the machine tool moves to the position commanded by the machine tool controller and the machining operation commences. 
     The machining operation is associated with several sources of error. First, the platen carrying the cutting tool may not move to the desired position in the direction of motion due to a difference between the actual and commanded position. This difference is called linear displacement error (LDE). Second, the machine surfaces may not be completely flat, resulting in linear error motions in the two lateral directions; such errors are called horizontal and vertical flatness or straightness errors. In addition, inaccuracies in the manufacture and assembly of the components may cause unintended rotary motions about each machine axis; such rotary motions are called roll, pitch, and yaw. 
     More often than not, effects of these errors do not completely cancel each other out, and their net effect will generate errors in machined features. If sufficient degrees of freedom are available, all the errors can be minimized or eliminated. However, in most machine tools, the available degrees of freedom are usually limited to three. For example, in a single axis machine tool, there is only one degree of freedom in the feed direction. Therefore, only linear displacement error motions in the direction of feed can be corrected. 
     Pitch and yaw are the major sources of error at the cutting insert when using long tools. The pitch error can be caused by deformation of the machine structure due to gravity, geometric errors in the components and assembly of the machine tool, and thermally induced strains due to ambient temperature changes. It is not possible to compensate for pitch and yaw errors on traditional three axis machine tools unless additional rotary axes are added to the machine. 
     Because geometric errors are a function of the mechanical components of the machine tool, they can usually be altered by mechanical intervention. Various techniques exist for reducing the angular errors associated with a machine tool; however, they are time consuming to execute and very laborious. In the case of errors due to gravity, there is no easy method to correct for such errors on hee axis machine tools that have only one actuator per axis. Gravity induced errors are predominantly in the Y direction, and such “droop” errors have a large effect on the pitch of the Z-axis in the YZ plane. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     Machine tool accuracy can be compromised by errors induced by gravity or by geometric inaccuracies in the structure of the machine tool itself. Compensation for such errors can be generated by using multiple drives to actuate the tool rather than a single drive. Differential control of the multiple drives can be used to introduce an intended variance in tool position, which is opposite to, and therefore cancels out, any gravity, or geometric structure related errors. 
     It is accordingly an object of the invention to generate more degrees of freedom in a single axis machine tool to compensate for errors by employing multiple linear actuators in place of a single drive. 
     It is another object of the invention to generate an extra degree of freedom in a single axis machine tool to compensate for errors by employing two ballscrew actuators. 
     It is another object of the invention to generate an extra degree of rotational motion in order to compensate for errors by creating differential linear motion between two ballscrews on the Y axis of a machine tool. 
     It is yet another object of the invention to use two actuators on the same axis of a machine tool to generate both linear and rotary motion in order to compensate for positional errors of the tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical machine tool and worktable setup. 
     FIGS. 2A-2G show a machine tool coordinate system and the six basic errors which exist for a single axis machine tool. 
     FIG. 3 shows the use of two linear actuators to control the motion of a machine tool platen. 
     FIG. 4 shows a machine tool spindle mounted on a platen actuated by two ballscrews. 
     FIG. 5 shows the angular error which can be created by a mismatch in ballscrew length. 
     FIG. 6 shows a machine tool spindle mounted on a platen in which a linear encoder and an electronic level are used as position feedback devices. 
     FIG. 7 shows a machine tool spindle mounted on a platen in which two linear encoders are used as position feedback devices. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the typical elements of a machine tool  14  which is set up to perform a boring operation. The machine tool comprises a spindle  15  which supports a cutting tool  16 . The spindle  15  is mounted on a column  17  by a vertical slide and the base  18  of the column is mounted for axial movement relative to a support  19 . The support  19  is mounted on a lateral slide  21 . The machine tool includes a worktable  22  which normally supports a workpiece (not shown). The X-axis  23  defines lateral motion of the cutting tool, the Y-axis  24  defines vertical motion of the cutting tool, and the Z-axis  26  defines axial motion of the cutting tool  16  in the feed direction. 
     FIG. 2A defines the X axis  23 , Y axis  24 , and Z axis  26  of a typical machine tool where the direction of feed of the spindle  15  and the tool  16  is along the Z axis. FIGS. 2B-2G show the six error terms for the Z-axis motion of a single axis machine tool. Specifically, FIG. 2B shows Linear Displacement Error (LDE)  31  as an error ΔZ along the Z-axis  26 . FIG. 2C shows Roll as rotational error  32  about the Z-axis  26 . FIG. 2D shows Pitch  33  as rotational error about the X axis  23 . FIG. 2E shows Yaw  34  as rotational error about the Y-axis  24 . FIG. 2F shows Horizontal Straightness error  35  as error motion along the X axis  23 . FIG. 2G shows Vertical Straightness error  36  as error motion along the Y axis  24 . 
     Error measurement of the complete machine tool is rather complex since for a three axis machine, twenty-one error terms exist. These twenty errors are comprised of six error terms for each linear axis as illustrated in FIG. 2, plus three error terms relating to the squareness of the three axes with respect to each other (XY), (XZ), and (YZ). As a general manufacturing practice, if the function of the platen is to carry the workpiece, these errors are measured with respect to a nominal cutting tool position. If the function of the platen is to carry the cutting tool, measurements are made with respect to a nominal workpiece position. 
     FIG. 3 shows a platen  40  mounted on a column  41  by two ways  42 . A first end  44  of the platen is coupled to a first ballscrew actuator  46  comprising a first ballscrew  47 , a first motor  48 , and a first encoder  49 . The second end  54  of the platen is coupled to a second ballscrew actuator  56  comprising a second ballscrew  57 , a second motor  58 , and a second encoder  59 . The two ballscrews  47  and  57 may be driven in unison to provide equal displacement of the first and second ends  44  and  54  of the platen  40 , or may be driven differentially to create an angular tilt β in the platen as shown. 
     FIG. 4 shows the platen  40  of FIG. 3 with a spindle  60  and a rotary tool  61  having a cutting insert  62  mounted thereon. The spindle  60  is mounted on a pair of ways  63  for motion along the Z axis  26 . A ballscrew actuator  64  comprising a motor  66 , an encoder  67 , and a ballscrew  68  drive the spindle  60  to the desired position along the Z axis  26 . The dual ballscrew drives of FIG. 3 are represented schematically in FIG. 4 by the reference letters B1 and B2, and are separated from one another by the distance S. The variables B1 and B2 represent the ball screw lengths. When B1 is not equal to B2, an angular error, β, is introduced as shown in FIGS. 3 and 5. This angular error translates to a linear error, ΔY, at the cutting tip. 
     This dual drive system can be effectively used for correcting error due to pitch, as well as linear errors in the Y direction. The pitch error results in a magnified linear error ΔY in the Y direction at the tool tip  62  due to the amplification through the boring bar length. If an angular pitch error β is present, the platen  40  carrying the tool  61  can be rotated in the opposite direction through this angle by creating a differential motion between the two ballscrews B1 and B2. If a linear error is ΔY is due to an error in vertical straightness, the two ends  44  and  54  of the platen  40  can be displaced equal amounts by the ballscrews  47  and  57  to correct the linear error. 
     Ballscrews are typically manufactured with a constant pitch p. When installed on the machine tool, each ballscrew is rotated by a servomotor with an attached rotary encoder that has a resolution e. The encoder functions to provide closed loop feedback of position to the servomotor controller in a manner which is well known in the art. 
     The linear motion d generated by a ballscrew subjected to n turns is equal to: 
     
       
         d=np  (Eqn. 1) 
       
     
     where: 
     d=the resultant linear motion 
     n=the number of turns applied to the screw 
     p=the pitch of the ballscrew. 
     The degree of resolution which can be obtained depends on the type of encoder which is used on the ballscrew. Commercially available digital encoders have a resolution in excess of one million divisions per revolution, while analog encoders typically have a resolution of 64,000 divisions per revolution. The minimum amount of linear motion d min  (resolution) which can be generated by a servomotor actuated ballscrew is equal to: 
      d min =p/e  (Eqn 2) 
     For example, a 20 mm pitch ballscrew with an analog encoder that has 64,000 divisions has a resolution e of 20 mm divided by 64,000 or 0.0003125 mm. 
     When two ballscrews are used to move a machine tool along a common axis, as in FIGS. 3-5, incremental differences in ballscrew motion produce an angular motion in the moving platen. For the purposes of the instant invention, it is assumed that all incremental differences in ballscrew length will be small and the resultant angle generated will be very small. In terms of the known geometry of the machine tool, the value of the angular error β is:              β   =     a                   tan        (       B2   -   B1     S     )                 (     Eqn                 3     )                                
     The resultant motion ΔY at the tool tip is equal to:                Δ                 Y     =         (     B2   -   B1     )          (     S   +   O   +   L     )       S             (     Eqn                 4     )                                
     The resolution of a boring machine using dual ballscrews as shown in FIG. 4 can be computed as follows. In this example, the distance, S, between the two baliscrews B1 and B2 is 1600.0 mm, the distance 0 between B1 and the end gage line of the spindle 60 is 100.0 mm, and the length L of the boring bar is 1016.0 mm. The ballscrews have a 20 mm pitch and the servomotors have an analog encoder that contains 64,000 divisions. The angular resolution, ΔO, can be found using Equation 5 in which B2−B1 is computed for the least difference in length between B1 and B2 that can be generated by keeping one ballscrew fixed and rotating the other ballscrew 1 increment as measured by the encoder.                Δ                 β     =       a                   tan        (       B2   -   B1     S     )         =       a                   tan        (       0.0003125                 mm       1600                 mm       )         =     2.30822                 arc        -          sec   .                   (     Eqn                 5     )                                
     With this resolution, the minimum linear error ΔY which can be corrected at the tool tip, can be found using Equation 4:                Δ                 Y     =           0.0003125                   mm        (       1600.0                 mm     +                           100.0                 mm     +     1016.0                 mm       )                        1600.0                 mm       =     0.0005                 3                 mm               (     Eqn                 6     )                                
     To increase the precision of error compensation, two different ball screw pitches may be used, and the resolution of pitch compensation may be magnified. For example, the pitch p 1  of one ballscrew may be chosen to be 20 mm, and the pitch p 2  of the other may be 15 mm. Both baliscrews are coupled to an analog encoder with 64,000 divisions. The difference in ballscrew lengths B2−B1, which can be generated is:              d   =         p1   e     -     p2   e       =           20                 mm       64        ,        000       -       15                 mm       64        ,        000         =     0.000078125                 mm                 (     Eqn                 7     )                                
     Using the same machine parameters as in the previous example, the resolution of pitch compensation can be re-computed:                Δ                 β     =       a                   tan        (         p1   e     -     p2   e       S     )         =       a                   tan        (       0.00007812                 5      mm       1600                 mm       )         =     0.577                 arc        -          sec   .                   (     Eqn                 8     )                                
     With this increased resolution, the minimum linear error which can be corrected at the tool tip is:                Δ                 Y     =         0.000078125                   mm        (       1600.0                 mm     +                           100.0                 mm     +     1016.0                 mm       )           1600.0                 mm       =     0.0001326                 mm               (     Eqn                 9     )                                
     The accuracy of resolution using two linear actuators as described herein is inversely proportional to the difference in the pitches of the ballscrews. Thus, the minimum error which can be corrected at the tool tip using two ballscrew pitches which differ by 25% is one fourth the minimum error which can be corrected using two ballscrews with the same pitch. 
     This technique could also be used with linear encoders and electronic levels as feedback devices. These feedback devices minimize the difference obtained due to temperature differences in the two ballscrews which would otherwise affect the accuracy of the system. 
     FIG. 6 shows an embodiment of the invention in which an electronic level  70  is mounted on the platen  40 , and a linear encoder  71  is mounted on the column  41 . A movable sensor  72  on the linear encoder  71  is attached to the platen  40  so that movement of the platen  40  relative to the column  41  produces a signal in the linear encoder  71  which can be coupled by lead  73  to suitable processing equipment (not shown). The signal on lead  73  together with a signal on lead  74  from the electronic level  70  can be processed to develop position and error signals in a manner known in the art for the machine tool as shown in FIGS. 4 and 5 mounted on the platen. 
     FIG. 7 shows an embodiment of the invention in which two liner encoders  76  are mounted on the column  41 . Each linear encoder  76  has a movable sensor  77  which is attached to the platen  40  so that movement of the platen relative to the column  41  produces a signal in the respective encoders  76  which can be coupled by leads  78  to suitable processing equipment (not shown). The signals on the two leads  78  can be processed to develop position and error signals in a manner known in the art for the machine tool as shown in FIGS. 4 and 5 mounted on the platen. 
     Having thus described the invention, various alteration and modification will occur to those skilled in the art, which alterations and modifications are intended to be within the scope of the invention as defined by the appended claims.