Abstract:
A grinding machine has an abrasive wheel mounted on a spindle carried by a wheelhead which provides for movement in a direction perpendicular to the spindle. A pair of workheads are separately mounted on workslides and cross slides so that the workheads may be moved along an X-axis or a Y-axis as required to bring chucks of the workheads into grinding position. Sliding movement of the workslides and cross slides is produced by ball screws powered by servo motors and mounted on bearing housings, with the ball screws propelling ball nuts connected to the slides. The machine uses CNC computerized controls, with each of the workheads having a separate and independent control. This enables attainment of a high degree of precision.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of applicant&#39;s Provisional Application Serial No. 60/153,777, filed Sep. 14, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to grinding machines and more particularly to high-precision grinders using CNC controls. 
     BACKGROUND OF THE INVENTION 
     Definitions: “CNC” refers to computer numerical control systems as employed for operation of machine tools. 
     “Chucker” refers to a workhead for a grinding or turning machine having a clamping device or chuck capable of holding only a part short enough not to require a tailstock at the opposite end of the part during grinding or turning. No part support other than the chuck is provided at the driven end of the part. 
     Grinding machines using a single workhead and a CNC control system have been employed previously for high-precision applications. Prior grinders of this type have generally coupled a CNC control system to a grinding wheel spindle for movement along an in-feed axis, which brings the wheel into contact with a supported workpiece being held in position on a single workhead by a chuck or other clamping device. The workhead in this approach is not fed into the wheel for grinding, but is held in place once adjusted. 
     It would be advantageous to provide improved CNC chucker grinders capable of providing increased production rates without sacrificing any of the high degree of precision obtained. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a grinding machine comprising a grinding wheel and a pair of workheads mounted on supporting structure enabling independent movement of the workheads toward and away from the grinding wheel, with operation of the workheads and chucks attached thereto being carried out under separate and independent CNC controls. 
     Supporting structure for the workheads includes a horizontally extending base having a generally flat bed surface carrying laterally extending linear rails or V and flat linear motion guides adapted for receiving mating elements placed underneath a pair of aligned workslides. Movement of the workslides in a lateral direction, generally parallel to the grinding wheel spindle, may be carried out by linear ball screws driven by servo motors. The ball screws typically are rotatably mounted in bearing blocks or housing and are connected to ball nuts which are fixedly attached to the workslides. 
     Each of the workslides carries a cross slide and connecting elements necessary for support and movement of an upper platform of the cross slide in a direction perpendicular to the wheel spindle. The platform rests on a lower block portion of the workslide, with linear rails attached to the lower portion and mating elements provided underneath the upper platform. Movement of the cross slide is enabled by use of a ball screw, bearing blocks or housings and a driving servo motor in a manner. similar to those used for work slide movement. 
     A spindle case aligned for driving a workhead chuck is provided of the upper platform of each cross slide. A spindle carried in the case extends from the innermost side of the. case and has a spindle nose which serves as a plate for attachment of a rotatable chuck. The chuck is driven by a servo motor placed alongside and propelling a belt engaged with pulleys on the motor and spindle. 
     The machine may thus provide for controlled linear movement of both workheads and a wheelhead along five or more linear axes and for controlled rotary movement of each of two chucks around rotary axes. Each of the movements involved is controlled by the CNC System of the machine. In order to obtain high precision between workpieces being simultaneously ground-while supported on dual workheads, each of the workheads is controlled independent. of the other, but in coordination with control of the grinding wheel. 
     The CNC control system may make use of computer software programmed to include parameters such as extent and timing of movements, grinding speeds, starting and final dimensions of workpieces and the like. Additional data entries and modifications may be inserted by a keyboard operater using conventional techniques. Information derived from in-process gauges may also be fed into the control system to provide precision timing of grinding shutoffs. By using independent CNC controls for the two workheads, separate workpieces may be ground to meet the same or different requirements. Grinding of one workpiece may be completed and terminated by retracting its workhead while the other is still being ground. 
     Grinding machines of this invention provide a high degree of precision such as 20 millionths of an inch. This result is enabled by various factors, including use of servo motors having a resolver or encoder capable of providing a high number of incremental counts per revolution such as 4,000,000. This level of precision would not be available if a single CNC control were used in propelling a grinding wheel against a pair of separately supported workpieces. 
     An increased production rate, twice the rate for single workhead machines once set-up is performed, may be obtained by this invention without any compromise of precision. 
     It is, therefore, an object of this invention to provide a high-precision grinding machine capable of operating at an increased production rate. 
     Another object is to provide a grinding machine wherein a pair of independently controlled workpieces may be ground simultaneously on a single grinding wheel. 
     Other objects and advantages of the invention will be apparent from the following detailed description and claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall isometric view of a grinding machine embodying the invention. 
     FIG. 2 is an isometric view showing underlying support structure for the movable workheads and a wheelhead of the machine of FIG.  1 . 
     FIG. 3 is an isometric view showing movable support structure for a cross slide of the grinding machine. 
     FIG. 4 is an end view of a workhead assembly taken from an outside location at a corner of the machine. 
     FIG. 5 is a schematic view showing operation of a control system for the machine. 
     FIG. 6 is a schematic side view of an embodiment wherein dual grinding wheels for internal grinding are provided. 
     FIG. 7 is a schematic top view of the machine of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings, there is shown a grinding machine  10  on which workpieces supported on dual workheads,  12 ,  13  may be ground by being brought into contact with a grinding wheel  14  carried on spindle  16 . Right workhead  13 , designated only by dotted lines, is a mirror image of left workhead  12 , and it is to be understood that all description of the left workhead applies equally to the right one. 
     Workhead  12  is supported on carriage assembly  18  structured to provide linear movements in X and Y directions as required for positioning of workpieces, while wheelhead assembly  20  provides for movement only in a direction perpendicular to the wheel spindle. 
     Wheelhead assembly  20  has a fixed block  22  secured to horizontal base  24 , with a V and flat way  25 , including a V groove  26  defined adjacent one edge and a flat surface  28  defined along the other edge. A channel  30  of rectangular cross section, in which bearing blocks  32  are located, extends along the length of the block. 
     Wheel slide  34  is arranged for linear movement-along block  22  with movement being guided by a V-shaped projection  35  extending along the bottom of the slide on one side and conforming to V groove  26 . A flat surface  36  is provided along the other side of the slide in position to come in to sliding contact with flat surface  28  defined along the other edge. A channel  30  of rectangular cross section, in which bearing blocks  32  are located, extends along the length of the block. 
     Wheel slide  34  is arranged for linear movement along block  22  with movement being guided by a V-shaped projection  35  extending along the bottom of the slide on one side and conforming to V-groove  26 . A flat surface  36  is provided along the other side of the slide in position to come into sliding contact with flat surface  28  along the top of block  22 . 
     Servo motor  38 , located at the outside end of wheelhead block  22 , drives a ball screw  40  (FIG.  2 ), which propels a ball nut  42  connected to wheelslide  34 , thus moving the grinding wheel inward toward the workheads and outward away from them. 
     In operation, movement of the wheelslide toward and away from the workheads may be controlled by the CNC system in accordance with programmed or keyboard-inputted instructions. For high-precision applications inward movement at startup of grind would be limited to bringing the wheelhead into a desired starting position and holding such position fixed while independently controlled movements of the dual workheads provide movements during grinding. 
     Workhead carriage assembly  18  has a workslide  44  with a bottom structure adapted to fit over linear rails  46  disposed on base  24  and aligned parallel to spindle  16  of the wheelhead. Movement of the workslide in a direction parallel to the spindle is provided by ball screw  48  (FIG. 2) driven by servo motor  50 . The balls crew is mounted on bearing blocks  52 ,  54  fixed to the base, and ball nut  56 , bolted to the slide, propels the slide upon turning of the ball screw. Bearing packs  58  include bearings which come into moving contact with sides of linear rails  46 . 
     Cross slide  60 , which enables controlled movement perpendicular to the workslide linear rails and to the grinding wheel spindle, is located on top of workslide  44 . Cross slide  60  has a base plate  62  slidably mounted on top of supporting plate  64  (FIG. 3) attached to the work slide. Linear rails  66 ,  68  connected to base plate  64  provide a track for movement of the cross slide. In a manner similar to structure of the workslide, movement of the cross slide is produced by a ball screw  70  driven by a servo motor  72 , with a movable ball nut  73  being connected to base plate  62 . Bearing packs  74  (FIG. 4) disposed along the lengths of linear rails  66 ,  68  to facilitate movement of the cross slide. 
     A spindle case  76 , carrying a spindle  78  for operation of a rotating chuck  80 , is connected to plate  62  with a driving motor  82  placed alongside. Chuck  80  is mounted on spindle nose  84  and includes clamping jaws  86 . 
     As shown in FIG. 4, chuck  80  is powered for rotation by motor  82  connected to the chuck through belt  88  which engages pulleys  90 ,  92 . The workhead may also carry a gauge  94  in position to obtain a real-time indication of completion of programmed grinding. The gauge shown in FIG. 1 has cantilevered sensors  96 ,  98 , one provided for outer diameter monitoring of a cylindrical workpiece and the other monitoring other parameters, as required. The sensor may provide a signal to the control system which initiates immediate shutdown. 
     In operation of the machine, workpieces to be ground are secured in position by jaws or other holders of the at chucks and are placed in proper alignment for coming into contact with the grinding wheel upon being moved. Movements of the workheads which support the workpieces are controlled by a CNC system which includes dual workhead controls, each one independent of the other. 
     FIG. 5 schematically shows a CNC control system for implementing the invention. Inputs to the system may come from prepared computer programs, keyboard-inputted modifications to the programs or gauges or other sensors monitoring the grinding process. Data processing outputs are converted to signals actuating the dual workheads, chucks and wheelhead. The wheelhead is programmed to provide movement of the grinding wheel along a single input axis to a desired location, after which movement of the wheelhead ceases, and relative movement of wheel and workpieces is obtained only by workhead movements. Inputs to each of the workheads provides signals implementing programmed movements along X and Y axis through movements of the work slide and cross slide. Movements of workpiece supporting chucks around programmed rotary axes are also implemented. 
     A typical sequence of events in operation of the machine is illustrated as follows: 
     1. Part is loaded into work holding device in both workheads. 
     2. Cycle start is actuated. 
     3. Grinding wheel axis (main process) initiates command for workheads to run their respective programs. Left workhead and its related axis is designated as process  1  and the right workhead (and related axis) is designated as process  2 . 
     4. Grinding Wheel axis moves into grind position, which is designated by its program. 
     5. Left and right workheads position themselves into their respective grind positions. 
     6. Left and right workheads begin running the axis cross feeds at the programmed feed rate to the desired position to achieve final part size: 
     a. should either process ( 1  or  2 ) achieve size first, that respective workhead is retracted from the grinding wheel and commanded to return to its “home” or programmed load position 
     b. the remaining process continues program execution until final size is reached, irrespective of the other process 
     c. when process in b. achieves size, the workhead retracts from grinding wheel and returns to “home” or its commanded position. 
     7. Processes  1  and  2  signal main process that they have completed their tasks. 
     8. Main process completes its program execution. 
     9. Repeat steps  1 - 8 , as required. 
     The dual workhead chucker grinding machine as described above for external grinding, may be adapted, by modifications made at the manufacturing level to provide for simultaneous internal grinding of bores in a pair of workpieces. 
     In order to obtain this result the grinding wheel, spindle case and driving motor would be removed from the wheelhead assembly and replaced with a pair of grinding wheels, a double-shaft motor and driving belts carried on supporting structure. 
     As shown schematically in FIG. 6, fixed block  22  and slide  34  of the wheelhead are retained. A dual internal grinding assembly  100  is secured to the moving slide. Assembly  100  includes a double-shaft motor  102  aligned as shown in FIG. 7, with the shafts  104 ,  106  driving a pair of internal grinding wheels  108 ,  110  supported on arbors  112 ,  114  and. extending in position for workpieces supported by the workheads to be brought into grinding contact. The wheels are driven by belts connected to pulleys  116 ,  118  on shafts  104 ,  106 . 
     In operation, movement of the dual workheads is controlled by separate and independent controls using the CNC control system as for external grinding, as described above. 
     Although the invention is illustrated by a specific described embodiment including numerous details, it is not to be understood as so limited, but is limited only as indicated by the appended claims.