Abstract:
A method for abrasive material removal that includes the steps of establishing an optimum force profile relating to the force or contact pressure applied by a processing tool on a workpiece. The actual force generated during the metal removal operation is monitored and compared to the optimum force profile. Based on the comparison of the actual force with the optimum force profile machine parameters are adjusted such that the actual force generated follows the established optimum force profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present invention relates generally to a method and apparatus for finishing a workpiece. More specifically, the method and apparatus measures or monitors various operating parameters occurring during the finishing operation of a workpiece and varies different operating parameters to maintain optimum predetermined or established values. 
         [0006]    2. Description of Related Art 
         [0007]    Microfinishing is a unique process that removes surface defects caused by previous operations to produce a high quality finish. The process involves utilizing an abrasive fed against the workpiece under a low or constant force. As is known, the abrasive determines the rate or duration of the feed. After the abrasive removes the initial roughness and reaches the solid, base material, material removal rate is reduced and the abrasive becomes dull. This completes the geometry portion of the microfinishing process, as the abrasive no longer removes a measurable amount of the workpiece material. Continued application of the abrasive to the workpiece functions to create the required surface finish. 
         [0008]    One of the problems associated with a microfinishing process is maintaining the effectiveness of the abrasive such that it removes the initial roughness and reaches the solid, base material of the workpiece. Depending upon the coarseness of the workpiece and the force applied on the abrasive, for example, abrasive particles located on a microfinishing film, the abrasive may fracture thus reducing the overall effectiveness of the abrasive, in this case the microfinishing film. The fracture rate of the abrasive is a function of the amount of speed and pressure put on the abrasive in relation to the surface texture of the workpiece. If the surface texture of the workpiece is coarse and too much pressure is applied to the abrasive, the abrasive will fracture which correspondingly reduces its ability to cut efficiently during the normal microfinishing cycle. 
         [0009]    Accordingly, too much pressure causes the abrasive to fracture and too little pressure increases the overall cycle time of the microfinishing process. Typically, in order to reduce the risk of fracturing and maintaining the effectiveness of the abrasive, the microfinishing operation is based on a fixed cycle time of increased duration. In short, the abrasive is fed slowly against the workpiece at a reduced rate to correspondingly reduce or prevent fracturing of the abrasive. 
         [0010]    Various methods for finishing a workpiece are known, see for example U.S. Pat. No. 6,782,760, that discloses a method for finishing a workpiece by controlling the feed of the processing tool based on the contact pressure. Specifically, a processing tool attached to a tool spindle advances at a pre-selected feed rate. A force measuring device measures the contact pressure applied by the processing tool on the workpiece and upon recognition of the initial cut and corresponding initial force, stops the feeding or advancing movement. Upon making initial contact, a controller fixes the rate at which the processing tool advances against the workpiece based on preset or predetermined value. If the measured value of the contact pressure or force is greater than the preset value, advancement of the feeding device used to move the processing tool varies in steps or incrementally. In addition, the initial or nominal force value may be reduced during the finishing process with the feed rate values adjusted by a controller subject to a damping function. 
         [0011]    While controlling the feed rate to control the force applied to the processing tool can be very effective in achieving a high quality finish it typically requires starting with a low feed rate and a low force or contact pressure between the processing tool and the workpiece to prevent fracturing of the abrasive on the processing tool due to the condition of the workpiece. This process takes into account the worst-case scenario of the surface texture of the workpiece and builds into the microfinishing operation an increased cycle time to address the worst-case scenario. This equates to a fixed cycle time of somewhat longer duration than is necessary, in that a certain amount of time is used in advancing the processing tool slowly against the workpiece to reduce any undesired premature fracturing of the abrasive particles and consequently reducing their useful life. 
         [0012]    From the above, it can be appreciated that a method and apparatus for microfinishing a workpiece that monitors and controls additional variables in the finishing process in addition to the force applied by the processing tool on the workpiece is needed. Such a method could be used to control the processing parameters and thus reduce potential failure or fracturing of the abrasive thereby increasing the useful life of the processing tool and producing a microfinishing apparatus and method that processes the workpiece in the most economical time and efficient manner. 
       SUMMARY OF THE INVENTION 
       [0013]    According to the preferred embodiment of the present invention, the method includes establishing an optimum force profile used during a material removal operation. The actual force generated during the material removal operation is monitored and compared to the established optimum force profile. Based on the comparison of the actual or monitored force with the establish optimum force profile, parameters of the material removal apparatus are adjusted to bring the actual force generated to more closely approach the optimum force profile. 
         [0014]    In one embodiment of the invention, the torque of various servomotors used in the material removal apparatus is monitored and compared to a known predetermined value. If the torque of the servomotors exceeds a predetermined level, the torque is reduced to a level at or below the predetermined level to reduce potential loss of processing tool efficiency. 
         [0015]    In a further embodiment of the present invention, tool spindle and work spindle speeds are adjusted to maintain the predetermined force profile. In addition, the tool spindle is arranged to swivel about the center of the workpiece resulting in an oscillation motion which improves the rate of stock removal. 
         [0016]    Further, areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0018]      FIG. 1  is a schematic view of a material removal apparatus according to the present invention; 
           [0019]      FIG. 2  is a top view of a material removal apparatus of the invention, specifically showing the base member along which the oscillation motion takes place; 
           [0020]      FIG. 3  is a stock cycle length/force diagram illustrating the changes in the force profile or curve based on the position along the stock cycle or length in accordance with the present invention; 
           [0021]      FIG. 4  is a stock cycle length/force diagram illustrating an alternative embodiment of a force profile or curve according to the present invention; and 
           [0022]      FIG. 5  is a stock cycle length/force diagram illustrating a further embodiment of a force profile or curve according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Turning now to  FIG. 1 , there is shown a microfinishing apparatus, seen generally at  10 , for use in finishing a workpiece  12  which could be a ceramic, metal, carbon, graphite, or other material. The microfinishing apparatus  10  includes a tool spindle  14  supporting a processing tool  16  used to finish the workpiece  12 . While shown herein as a finishing stone, the processing tool  16  may also include a tape or film having an abrasive material located thereon. A tool spindle servomotor  18  connects to and drives the tool spindle  14  through a pulley and timing belt arrangement  20 . The tool spindle  14  is mounted for reciprocal movement on a tool slide  22 . As illustrated herein, the tool spindle  14  is mounted on a non-preloaded ball screw  24 . A tool slide servomotor  26  connected to the ball screw  24  operates to rotate the ball screw  24  and correspondingly move the tool spindle  14  and processing tool  16  into engagement with the workpiece  12 . The tool slide  22  and related pulley and timing belt arrangement  20  is further mounted to a base member  35  to provide a swivel motion to the tool slide  22  through the use of an oscillation servomotor (not shown) so that the complete slide and components may swivel so as to provide an oscillation motion A, along base member  35  with respect to the workpiece  12  as clearly shown in  FIG. 2 . 
         [0024]    The microfinishing apparatus  10  further includes a work spindle  28  including a workpiece support member  30  that supports the workpiece  12  during the microfinishing operation. A work spindle servomotor  32  connects to and drives the work spindle  28  through a drive belt  34 . As is known in the microfinishing art, the work spindle  28  operates to move or rotate the workpiece  12  during the microfinishing operation. 
         [0025]    The tool spindle servomotor  18 , tool slide servomotor  26 , work spindle servomotor  32  and oscillation servomotor (not shown) are each connected to a servo control mechanism  36 . The servo control mechanism  36  connects to a control unit  38 . The control unit  38  functions to drive and monitor the parameters of the various servomotors,  18 ,  26 ,  32  and the oscillation servomotor (not shown) during the microfinishing operation. In addition, a user interface such as a personal computer is used to input specific programming and operation logic into the control unit  38  depending upon the particular requirements for finishing the workpiece  12 . 
         [0026]    A gage assembly  40  including a pair of gage probes  42  is used to monitor the size and shape of the workpiece  12 . Input from the gage probes  42  is sent to the control unit  38  that controls operation of the various servomotors  18 ,  26 ,  32  and the oscillation servomotor (not shown), in accordance with input feedback received from the gage assembly  40  regarding the size and finish of the workpiece  12 . 
         [0027]    A force measuring device or sensor  44  located on the tool slide  22  measures the contact force applied by the processing tool  16  against the workpiece  12 . The force measuring device  44  may be a load cell or other type of measurement mechanism that monitors the force applied on the workpiece  12  by the tool spindle  14 . The force applied to the tool spindle  14  correlates to the force applied on the workpiece  12  by the processing tool  16 . As set forth more fully below, the present invention monitors and controls the force applied by the processing tool  16  on the workpiece  12  during the microfinishing operation. 
         [0028]    In accordance with the present invention, the processing tool  16  exerts a predetermined and variable pressure or force on the workpiece  12  during the microfinishing operation. Initially, the force on the workpiece  12  is determined from empirical data as different workpieces  12  will require a different initial contact force. At the start of the microfinishing operation, the processing tool  16 , containing non-renewable abrasives in either a film or tool (stone) format, is positioned against the workpiece  12  at a predetermined force or contact pressure. With the processing tool  16  in contact with the workpiece  12  at the predetermined pressure, the tool spindle  14  drives the processing tool  16  and the work spindle  28  operates to rotate the workpiece  12 . The oscillation servomotor (not shown) is also used to swivel the tool slide  22  relative to the base member  35  so as to create an oscillation by the processing tool  16 . Since the processing tool  16  is located against the workpiece  12  at start up, if the workpiece  12  has a rough surface texture, it is possible, based upon the contact pressure applied to the processing tool  16  to cause fracturing of the abrasive and thus reduce the overall effectiveness of the processing tool  16 . 
         [0029]    In order to reduce the opportunity for such abrasive fracturing, the present invention utilizes the control unit  38  to monitor the amount of starting torque supplied by the tool spindle servomotor  18  to the tool spindle  14  and that supplied by the work spindle servo motor  32  to the work spindle  28  at startup. The control unit  38  compares the starting torque of both the tool spindle servomotor  18  and the work spindle servomotor  32  with pre-established limits. When the starting torque exceeds the predetermined or pre-established limits, the control unit  38  reacts to the high starting torque by sending a signal to the tool slide servomotor  26  to reduce the initial pressure on the processing tool  16 . Reducing the initial pressure on the processing tool  16  reduces fracturing of the abrasive on the processing tool  16  when the workpiece  12  has an unexpected coarse or rough surface texture. 
         [0030]    As set forth above, the starting torque of the work spindle  28  corresponding to the oscillation of the workpiece  12  is also measured. Once again, the torque generated by the work spindle servomotor  32  is monitored and compared to predetermined or pre-established limits. In some instances, it may be desirable to reduce the speed of rotation and correspondingly the torque generated by the work spindle  28  rather than reduce the force or contact pressure applied by the processing tool  16  on the workpiece  12 . Accordingly, the present invention contemplates controlling the torque generated by the tool spindle servomotor  18  and that generated by the workpiece spindle servomotor  32  so as to enable adjusting the force or contact pressure applied by the processing tool  16  against the workpiece  12 . 
         [0031]    Thus, the present invention contemplates reading or obtaining feedback information pertaining to the torque of the tool spindle servomotor  18 , comparing it to preset limits and adjusting the torque as necessary, including reducing the force or contact pressure applied by the processing tool  16 . In addition, the invention also contemplates reading or obtaining feedback information pertaining to the torque of the work spindle servomotor  32  and adjusting the torque of the work spindle servomotor  32 . Monitoring and adjusting the torque output of the respective tool spindle servomotor  18  and work spindle servomotor  32  in response to variable workpiece  12  surface textures will reduce potential fracture of the abrasive and help maintain a uniform abrasive life cycle. Reacting to the starting torque in this manner creates a cycle based on incoming surface texture conditions rather than a range of conditions. As opposed to starting with a reduced starting pressure and slowly controlling or increasing the pressure to maintain a desired torque which would increase the overall cycle time. 
         [0032]    In addition to monitoring the starting torque and adjusting the initial parameters based thereon, the present invention also contemplates controlling the force or contact pressure on the workpiece  12  during and at the end of the microfinishing cycle or operation. In accordance with known microfinishing processes, the processing tool  16  is advanced against the workpiece  12  at a constant force or contact pressure by varying the feed rate to maintain the force. Once the initial cutting operation is completed, finishing operation continues until at the end thereof the force on the workpiece  12  is gradually reduced until it reaches zero. One method is to stop the tool slide  22  whereby the processing tool  16  remains stationary, by maintaining the processing tool  16  in a stationary position continued operation of the processing tool  16  will gradually reduce the force or contact pressure. 
         [0033]    Turning to  FIG. 2 , there is shown another aspect of the present invention wherein the force or contact pressure applied by the processing tool  16  against the workpiece  12  is controlled throughout and to the end of the microfinishing cycle. As illustrated in  FIG. 3 , the Y-coordinate represents the force or contact pressure applied by the processing tool  16  during the microfinishing operation, with Y 1  being the initial force, converted to a 0-1 factor, set at the control unit  38  and applied during the microfinishing operation. The X-coordinate, also converted to a 0-1 factor, represents the microfinishing cycle length, which can be defined in several ways such as gage distance, time or distance traveled by the tool slide  22 . The force (Y) is determined based on the X-coordinate, that is, the force (Y) is the force or contact pressure for a particular X-coordinate. 
         [0034]    The dotted line  50  in  FIG. 2  represents a linear force to microfinishing cycle length when the feed rate is gradually slowed. For example, as the feed rate slows, the force (Y) gradually decreases or reduces in a linear manner as illustrated by the dotted line  50 . It is desirable, however, to vary the force (y) in a non-linear manner according to various factors such as gage points, time or distance traveled by the tool spindle  14  and correspondingly the processing tool  16 . 
         [0035]    Accordingly, the present invention utilizes a nonlinear force curve or path while maintaining a certain feed profile. The force curve illustrated in  FIG. 3  is calculated according to the following formula: 
         [0000]    
       
         
           
             Y 
             = 
             
               
                 ( 
                 
                   1 
                   - 
                   
                     cos 
                      
                     
                         
                     
                      
                     
                       ( 
                       
                         180 
                          
                         
                             
                         
                          
                         
                           x 
                           α 
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
               2 
             
           
         
       
     
         [0036]    Y=the force applied by the processing tool; 
         [0037]    X=is the position along the X-coordinate; and 
         [0038]    α=a predetermined value used to increase or decrease the force curve relative to the standard or linear force based on feed rate. 
         [0039]    Accordingly, depending upon the workpiece  12 , a particular force profile or curve can be developed which results in optimum finishing. 
         [0040]    Accordingly, the present invention allows for an optimum force profile while maintaining an established feed rate to reduce processing time. The present invention contemplates maintaining the actual force profile by varying the tool spindle  14  speed and the work spindle  28  speed. For example, if the measured force; i.e., the output of the force sensor  44 , falls below the optimum force profile or curve, the tool spindle  14  speed can be decreased and the work spindle  28  speed held constant, increased or decreased depending upon the amount of adjustment needed to increase the overall force and bring the measured actual force up to the optimum force profile or curve. If, however, the measured actual force is greater than the optimum force profile or curve, the tool spindle  14  speed can be increased and the work spindle  28  speed held constant, increased or decreased depending upon the amount of adjustment needed to decrease the measured force. Typically, an increase in tool spindle  14  speed will decrease the force, while an increase in work spindle  28  speed will increase the force. Accordingly, to decrease the overall actual force it is desirable to increase the tool spindle  14  speed and decrease the work spindle  28  speed. Conversely, to increase the overall actual force it is desirable to decrease the tool spindle  14  speed and increase the work spindle  28  speed. Thus, adjustments to the tool spindle  14  speed and the work spindle  28  speed enable the controller to attempt to follow within limits of the optimum predetermined force profile used in connection with microfinishing a workpiece  12 . 
         [0041]      FIGS. 4-5  illustrate various force profiles developed based on the selection of the exponent α. For example,  FIG. 3  illustrates a force profile using 1 as exponent α, while  FIG. 4  illustrates a force profile using for the exponent α, a value less than 1 and  FIG. 5  illustrates a force profile using for the α exponent a value greater than 1. 
         [0042]    As set forth above, the X-coordinate can be set based on a variety of factors. For example, using the gage assembly  40  illustrated in  FIG. 1 , the force profile changes or varies relative to various gage positions. As illustrated in  FIG. 5 , the force profile reduces from gage point X ultimately to zero as the gage reaches zero, which represents the preset size of the finished workpiece  12 . As set forth above, the force profile can be based on time/length of the finishing operation or cycle, or the distance traveled by the processing tool  16 . 
         [0043]    Accordingly, the present invention provides the control unit  38  with the ability to determine a predefined force profile whereby the control unit  38  monitors the force applied to the workpiece  12  throughout the entire process. Because it is the force that is being monitored, the processing time may vary for each part, rather than going through a preset or predetermined finishing cycle based on time or feed amount. 
         [0044]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.