Abstract:
The present invention is embodied in an automated system for the positioning and support of a machine tool within a workpiece supporting assembly, comprising a pair of generally parallel, planar longitudinal translation modules affixed to the assembly and having longitudinal sliding pads and a longitudinal movement device, a transverse translation module affixed to the longitudinal sliding pads in a generally perpendicular orientation to the longitudinal modules and having transverse sliding pads and a transverse movement device. The system also includes a vertical translation module affixed to the transverse sliding pads in a generally perpendicular orientation to the longitudinal and transverse translation modules and having vertical sliding pads and a vertical movement device, the vertical translation module further comprising a mounting device for the machine tool and a device to rotate the machine tool about a vertical axis and a device to pivot the machine tool about any axis orthogonal to the vertical axis, and a control device.

Description:
This application is a continuation of Ser. No. 08/540,525 filed Oct. 10, 1995, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a numerical control (“NC”) system for a five-axis precision positioning and support of a machine tool with respect to a workpiece surface in a work envelope. 
     2. Background Art 
     The precision machining of large workpieces requires the use of a wide array of expensive machine tools such as full size models and gauges, templates, fixtures, and drill-sets. These tools have a substantial acquisition and maintenance costs, as well as costs related to their storage, property management, inspection, reinspection, and accountability. In addition, the manufacturing tolerances and repeatability achievable with these tools is limited. 
     For example in the aerospace industry, large airframe components such as fuselage sections can be precision machined only with the use of very costly full size models and gauges. A typical series of models needed to drill precision holes is shown in FIGS. 1A-1B. As shown in FIG. 1A, the first step in this process is to fabricate a male master model  100  of a fuselage section, which model is made of metal or plaster and has projections  105  of the size and at the locations required for the holes to be drilled in the fuselage section. A female plaster cast  110  is formed over the model  100 , which cast has apertures  115  formed over the projections  105 . As shown in FIG. 1B, a male cast back  120  is formed from the plaster cast  110 , which cast back is also made from plaster. Again, projections  125  are formed by the plaster flowing into the apertures  115  in the cast  110 . Finally, a drill bonnet  130  made of a composite material, such as fiberglass or graphite composite, is formed over the cast back  120 . The bonnet  130  has apertures  135  of the correct size and at the correct locations where holes are required to be drilled. 
     The first step in using the bonnet  130  is to fasten a fuselage section into an assembly jig using bracing means, or “details”, and locator pins to provide a reference position for the fuselage. The bonnet  130  is then secured adjacent the fuselage section and aligned with the section using the locator pins. The bonnet  130  then serves as a drilling template through which holes are drilled into the fuselage section. 
     The cost to fabricate a typical drill bonnet  130  can average $1 million and take from one to 12 weeks. For the F-18 aircraft, 900 bonnets are needed to drill all the fuselage holes. Thus, the total cost for the drill bonnet tool family for the F-18 is approximately $1 billion. Full scale interior models, called master gages, are also required to precisely locate and drill holes in details which are attached to interior structures of the assembly jig. These details are used to locate the bulkheads, frames and ribs of the aircraft. Such master gages can cost between $5-10 million each and the F-18 requires 33 such master gages, for a total master gage tool family cost of approximately $250 million. 
     One object of the invention is to eliminate the need for these costly tool families and replace them with a machine tool locating system made from standardized parts to reduce cost and fabrication time. Another object of the invention is to improve the accuracy of hole location by eliminating the cumulative tolerance resulting from the use of multiple master models and gages, and related molds. 
     Another object of the invention is to increase the speed with which an assembly jig can be prepared to machine a new workpiece, or implement engineering changes to an existing workpiece design. Previously, new master models and gages would have to be fabricated for either a new aircraft component or changes to an existing one, requiring from four to 24 weeks to prepare. A positioning system of invention can locate machine tools directly from machine design software, reducing this aircraft change time to one or two days. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in an automated system for the positioning and support of a machine tool within a workpiece supporting assembly, comprising a pair of generally parallel, planar longitudinal translation modules affixed to the assembly and having sliding pads and a movement means, a transverse translation module affixed to the longitudinal sliding pads in a generally perpendicular orientation to the longitudinal modules and having sliding pads and a movement means. The system also includes a vertical translation module affixed to the transverse sliding pads in a generally perpendicular orientation to the longitudinal and transverse translation modules and having sliding pads and a movement means, the vertical translation module further comprising a mounting means for the machine tool and a means to rotate the machine tool about a vertical axis and a means to pivot the machine tool about any axis orthogonal to the vertical axis, and a control means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are perspective views prior art molds used to fabricate a drill positioning bonnet. 
     FIG. 2 is a perspective view of a positioning system of the invention. 
     FIG. 3 is a perspective view of a portion of the system of FIG. 2 showing a translation module. 
     FIG. 4 is a block diagram of a control means for the system of FIG.  2 . 
     FIG. 5 is a perspective view of a portion of a second embodiment of the system of FIG. 2 showing a ballrail and pad assembly. 
     FIG. 6 is a perspective view of the positioning system of the present invention within a jig assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 2, a system  200  of the invention uses two longitudinal translation modules  201  positioned parallel to an x axis. The function and construction of these modules  201  is similar to other translation modules used in the invention for transverse and vertical movement as explained below. The modules  201  may be temporarily or permanently attached to a jig frame  202  having a workpiece within the jig frame  202 , as shown in FIG.  6  and located with conventional locator pins in reference positions  203  of the frame  202 , as shown in FIG.  6 . 
     Sliding pads  205  translate along each module  201  in response to synchronized servo motors  210 , by means described below. The sliding pads  205  are similar to sliding pads used on other translation modules used in the invention. The pads  205  will be of an appropriate size depending on the size of the structure being translated and the distance of travel. The modules  201  also include linear sensors  212  along the length of the module. The sensors  212  are of a conventional design such as glass scales or digital strips. Again, the sensors  212  are similar to sensors used on other translation modules and will generally have a length of approximately the same length as the translation module on which it is mounted. 
     Removable mounting bases  215  are fastened to the pads  205  and support bridge members  220 . Members  220  support a transverse translation module  225 , parallel to the y axis and driven by a servo motor  230 , which combined structure forms a bridge  231  over the work envelope with modules  201  on either side of the bridge. The motor  230  may be connected to the module  225  either by a belt reduction drive  232 , gear drive, or a direct drive. The sliding pads  205  support and translate a z axis structure  240  along the y axis and the sensor  212  is mounted along the length of the module  225 . 
     The z axis structure  240  includes two vertical translation modules  245  and sliding pads  205  driven by a single servo motor  250 . Two vertical translation modules  245  provide additional strength to support the weight of the structure  240  and prevent the back pressure from a machining operation from displacing the structure, which could cause machining errors. The modules  245  also include sensors  212  along their length. Again, the motor  250  may be connected to modules  245  either by a belt reduction drive  280 , gear drive, or a direct drive. The belt reduction drives  232 ,  280  or gear drives provide increased accuracy in translational movement of the sliding pads  205 . 
     The modules  245  translate a carriage  255  along the z axis, on which a rotation motor  260  is mounted in order to rotate a machine tool  265  about the z axis. In accordance with one preferred embodiment of the invention, the machine tool  265  will be an electric drill for forming apertures in the workpiece. A pivot motor  270  is also mounted on the carriage  255  and the pivot motor rotates the machine tool  265  about all axes perpendicular to the z axis, depending on the position of the rotation motor  260 . Rotational sensors  272  are mounted on each of the rotational motor  260  and pivot motor  270  to measure the angular rotation of the motors. 
     The translation modules  201 ,  225  and  245  use conventional ballscrew drive construction, which provides accurate control at a minimum cost. As shown in FIG. 3, each module  201 ,  225  and  245  consists of guide rails  300  and a ball lead screw  310  mounted in a parallel position between the rails. The ball lead screw  310  is supported at both ends of the module by bearings  315 , which are mounted on a support plate  305  that also supports the rails  300 . The pad  205  includes a threaded guide  320  which is positioned adjacent between the rails  300  and engages the screw  310 . As the screw  310  turns, the sliding pad  205  translates along the direction of the rails  300 . The screw  310  can be coupled directly to a servo motor, such as the motor  210  in FIG. 2, or by means of the belt reduction drives  232 ,  280  or gear drives, to servo motors  230  and  250 , respectively (also in FIG.  2 ). 
     The positioning system  200  of FIG. 1 is controlled by the NC devices illustrated in FIG. 4. A conventional servo control module  350 , such as a UMH Series, High-Frequency Type, DC Servo Control, made by Baldor of Berne, Switzerland, sends translation signals  355  to the motors  210 ,  230  and  250  (shown in FIG.  2 ), rotation signals  360  to the motors  260  and  270  (shown in FIG. 2) and operation signals  365  to the machine tool  265  (shown in FIG.  1 ). The module  350  receives sensor signals  370  from the linear sensors  212  mounted on each of the modules  201 ,  225 , and  245  and rotational sensors  272  (shown in FIG.  2 ). The sensor signals  370  measure the proximity of (a) the initial machining part of the machine tool  265  (e.g. the tip of a drill) to a desired set of x, y and z coordinates (referred to as the “vector”), and (b) the orientation of the tool path (e.g. the drill centerline) to the contour of the workpiece surface (referred to as the “normal”) as defined by rotation and pivot angles. The module also receives task signals  375  from a conventional industrial controller  380 , such as a Delta Tau Controller (made by Data Systems Inc., of Northridge, Calif.) and sends task completion signals  385  to the controller  380 . The controller  380  generates the task signals  375  from a workpiece database  390  that is sent to the controller  380 . The workpiece database  390  comprises a set of task signals  375  and defines the work to be performed on workpiece, such as the location, orientation and depth of holes. 
     The operation of the system  200  begins by the mounting of the translation modules  201 , as shown in FIG. 2, in a parallel relation on a jig frame  202 , as shown in FIG. 6. A bridge  231  of a suitable height and length to access those portions of the workpiece on which the work is to be performed is attached by the bases  215  to the pads  205 . A conventional laser alignment tool is used to locate the machine tool  165  with respect to a reference datum of the workpiece. 
     As shown in FIG. 4., each task signal  375  defines a task to be performed on the workpiece and is generated by the controller  380 . For example if the task is to drill a hole in the workpiece, a basic data item in the task signal  375  would be the location of the drill tip, i.e. the vector, and is defined by x, y and z coordinates in relation to the workpiece reference datum used to locate the modules  201  (as shown in FIG.  2 ). Another data item is the normal, which is defined by angles about the rotation and pivot axes at a selected vector. Other data to be defined could include the speed of the drill, the feed rate at which the drill moves with respect to the workpiece, and the distance that the drill is to travel (which determines the depth of the hole). 
     The controller  380  holds in memory each task signal  375  in the workpiece database  390 . This workpiece database  390  could be provided by a computer aided design (“CAD”) program defining a finished workpiece and could be entered in the controller  380  by manual or magnetic means. 
     In addition, the controller  380  determines when a task signal  375  (e.g. comprising the vector, normal, drill rates and distance) is sent to the control module  350 . For example, the controller  380  could be programmed to send the task signal  375  to the module  350  only after a hole drilled pursuant to a previous task signal has been finished, i.e., a “when done” command. 
     When a task signal  375  is sent to the control module  350 , it sends translation signals  355  and rotation signals  360  to move the machine tool  265  (shown in FIG. 2) to the desired vector and normal. If the desired vector or normal of the task signal  375  is not reached by means of the translation signals  355  or rotation signals  360 , one or more sensor signals  370  proportional to the error in coordinates or angles will be sent to the module  350 . The module  350  then generates appropriate revised translation signals  355  or rotation signals  360  in order to make the correction in vector or normal. The translation signals  355  and rotation signals  360  also include a velocity command that directs the speed of the motors  210 ,  230  and  250  (shown in FIG. 2) in order to control the time at which the desired vector will be reached. 
     After the desired position is reached, the module  350  sends the operation signal  365  (i.e. the remaining information from the task signal  375 ) to accomplish the desired work. For example when a drill reaches a desired vector and normal, the module  350  sends to a drill the operation signal  365 , comprising a drill speed, drill feed rate, and a drill distance. After this operation signal  365  has been sent, module  350  sends the completion signal  285  to the controller  355 , which then sends a subsequent task signal  375  to the module  350  and the operation is repeated until all the tasks in the workpiece database  390  have been completed. 
     In a second preferred embodiment, the cost and expense of the linear sensors  212  and rotational sensors  272  (shown in FIG. 2) may be eliminated without adversely affecting the performance of the system  200 . This result can be a significant savings because sensors such as digital strips can cost as much as 20 percent of the cost of the system  200 . 
     This embodiment is achieved by using conventional laser measuring means to measure the vector of the machine tool  265  at maximum travel positions of each translation module  201 ,  225  and  245  (shown in FIG.  2 ), and at several commanded intermediate positions. These vectors are compared with the location signals  355  (shown in FIG. 4) sent to reach each of the measured positions, and vector errors are determined for each module. This set of vector errors is programmed into the memory of the controller  380 . After this calibration procedure, when the workpiece database  390  requires movement to a set of coordinates, the controller  380  corrects the task signal  375  by the amount of the vector errors. A similar calibration procedure is used to measure normal errors and to eliminate the need for rotational sensors  272 . 
     In another preferred embodiment of the invention, a ballrail  400  is mounted on the bridge member  220  and parallel to the transverse module  225 . Further, the ballrail  400  is positioned on the opposite side of the module  225  from the z axis structure  240  and is connected to the z axis structure by a modified sliding pad  405 , which translates along the module  225  (i.e. y axis) in a manner identical to sliding pad  205  (shown in FIG.  2 ). The pad  405  is operatively connected to the ballrail  400  at a semicircle  410  whose ballrail facing surface is covered with ball bearings  415 . The ballrail  400  and pad  405  assembly (a “ballrail and pad assembly”) allows translation along the y axis, but prevents motion of the pad  405  is the z direction. The advantage of the ballrail and pad assembly is to offset the lever arm produced by the z axis structure about the module  225 , thus improving stability of the machine tool  265  (shown in FIG. 2) during machine operations. For example during a drilling operation, a resistance force (“drill-back”) may develop that can displace the drill and reduce the hole accuracy. The effect of drill-back is substantially reduced by the ballrail and pad assembly. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.