Abstract:
A flexible manufacturing line and manufacturing process employs an overhead gantry system to transfer fixtured workpieces between machining stations served by the gantry system. The machining stations include CNC machines, turret cells, and/or dedicated machines which receive the fixtured workpieces through top-entry openings. The invention provides flexibility and adaptability to variation, improved maintenance of reference position on the fixture during manufacture, improved accessibility to machining stations during operation, and increased safety.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates in general to subjecting a workpiece to a series of manufacturing operations, and, more specifically, to the machining of parts or workpieces by a plurality of machining stations using fixtures transported by a gantry system. 
     In the manufacturing of certain cast metal products, it is not possible to achieve the final part shape and tolerances by casting alone. Part manufacture usually includes several machining operations that must be applied to the original casting, such as drilling, boring, milling, cutting, and tapping. Products of this type include engine manifolds, wheels, brake rotors, and suspension components such as steering knuckles, control arms, and cross members. 
     In mass production, it is necessary to organize a manufacturing process with high reliability, short cycle times, easy maintenance, and worker safety while minimizing cost and space requirements. It is also highly desirable to achieve flexibility of the manufacturing process and the manufacturing equipment so that the process and equipment can be adapted at low cost to changes in part design, part mix, or part quantities. 
     Manufacturing systems utilizing automation are preferred because of increased consistency and reduced time and cost. Automated machining stations typically receive a workpiece (e.g., an unfinished casting) mounted on a fixture (also called a pallet) and automatically perform the desired machining operations on the workpiece. The workpieces may be delivered to and removed from the machining station either manually or automatically by a conveyance. 
     Various types of machining stations are known. Dedicated stations are constructed to perform a fixed set of operations and cannot be easily adapted to perform other tasks. Dedicated stations usually have a cost advantage when a large volume of parts is to be made and no significant design changes are to occur during a long production run. Another type of station is the computer-numerically-controlled (CNC) machine, which is programmable to perform a variety of machining operations and has advantages of being able to produce parts with a lower number of machining stations and therefore requiring less relocation of a workpiece during manufacture. In addition, CNC machines are more easily adapted to new products or processes and can reduce overall capital investment for a changeover. A typical CNC machine has programmable multidimensional movement of both the tool head and the table that receives the fixture and workpiece. 
     An especially adaptable type of CNC machine is the flex turret cell which employs a multi-spindle head that automatically reconfigures itself to use selected ones of several tools contained on the multi-spindle head. The multi-spindle head usually comprises a gearbox with multiple output shafts (a different tool on each shaft) driven by a common input shaft. The head is indexed between separate operations while a workpieces remains at the machining station, which improves cycle time and accuracy. 
     Nevertheless, previous automated systems have suffered from various drawbacks. For example, transfer of workpieces between work stations has remained labor intensive, slow, and/or inflexible (i.e., not easily adaptable to process changes or substitutions). A single workpiece may need to be swapped between various fixtures corresponding to different machining stations when the particular set of machining operations to be performed on the workpiece occurs at several different machining stations. Overall accuracy suffers due to a loss of an exact registration in a reference position between fixtures. Another disadvantage has been the inaccessibility of the CNC machines during operation, making observation and maintenance more difficult. 
     These disadvantages are overcome by the present invention. 
     SUMMARY OF THE INVENTION 
     It is an object and advantage of the present invention to provide a flexible manufacturing line and manufacturing process providing flexibility and adaptability to variation, improved maintenance of reference position on the fixture during manufacture, improved accessibility to machining stations during operation, and increased safety. 
     In one aspect, the present invention provides apparatus of applying a series of machining operations to a workpiece. The apparatus comprises an operator station for affixing workpieces to respective fixtures, the-fixtures registering and retaining the workpieces during the series of machining operations. There are a plurality of machining stations, each machining station being configured to receive the fixtures and performing respective machining operations. A gantry system includes a plurality of movable carriages running proximate to the machining stations and the operator station. The gantry system receives the fixtures for conveyance between and among the operator station and the machining stations. A main controller communicates with the operator station, the plurality of machining stations, and the gantry system. The main controller monitors status of the workpiece and is programmed with a sequence for applying predetermined ones of the machining operations to the workpiece. The main controller commands the gantry system to convey the fixture to at least one of the machining stations, commands the one machining station to perform a selected one of the machining operations, and commands the gantry system to return the fixture to the operator station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front, plan view of a gantry system used in a preferred embodiment. 
     FIG. 2 is a side, plan view of the gantry system of FIG.  1 . 
     FIG. 3 is an aerial layout view of a manufacturing line according to the present invention and using a monorail loop. 
     FIG. 4 is a perspective view of a portion of the manufacturing line of FIG.  3 . 
     FIG. 5 is a perspective view of a portion of the operator station of FIG.  3 . 
     FIG. 6 is a front view of a CNC machine of the present invention with a front guard panel removed. 
     FIG. 7 is a top view of the CNC machine of FIG.  6 . 
     FIG. 8 is a top view of the CNC machine of FIG. 6 with a top panel removed. 
     FIG. 9 is a front view of a flex turret machine used in a preferred embodiment. 
     FIG. 10 is a partially exploded, perspective view of a fixture holder of a CNC machine and a fixture and workpiece. 
     FIG. 11 is an aerial layout view of a manufacturing line according to the present invention and using a monorail line. 
     FIG. 12 is a flowchart of a preferred method for performing a sequence of machining operations on a workpiece. 
     FIG. 13 is a flowchart showing decisions made by a main controller of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a gantry system  10  is used in the present invention to transfer fixtures and the workpieces mounted thereon between machining stations. Preferably, gantry system  10  is constructed to primarily convey the fixtures and workpieces in an overhead manner. Gantry system  10  may for example be comprised of a roboLoop system sold by Güdel AG of Langenthal, Switzerland. The roboLoop system is a gantry, transfer, and carrier system that may be laid out with straight lines and curves. It is modular so that sections can be easily added or removed when expanding or reconfiguring a manufacturing operation based on changed needs. 
     Gantry system  10  includes a support structure  11  which employs a free-standing I-beam frame to locate a mounting beam  12  in a desired overhead position. Mounting beam  12  could alternatively be ceiling mounted. A guideway and rack  13  is suspended from mounting beam  12  and receives at least one carriage  14 . In a preferred embodiment, guideway and rack  13  forms a continuous loop and a plurality of carriages  14  are provided. 
     Carriage  14  includes a carrier  15  having rollers engaged with guideway and rack  13  for movement along the loop. A rack-and-pinion encoder in carrier  15  engages guideway and rack  13  so that carriage  14  can determine its exact location at all times. Carrier  15  supports and controls a pair of “H” arms  16  and  17 . A servomotor  18  provides a linear axis drive to create motion in three axes (x, y, and z dimensions). Arms  16  and  17  have end-of-arm clamps  20  and  21 , respectively, with each containing a servo-controlled mechanism for grasping fixtures. “H” arms  16  and  17  may have a lifting capacity of about 120 kilograms, for example. 
     Carriage  14  also includes a carriage controller  22  which stores and implements all commands necessary to perform each of its assigned tasks, such as presenting a fixture to a particular machining station, removing a fixture from a particular machining station, conveying a fixture between machining stations, etc. Carriage controller  22  includes an RF transceiver  23  for communicating with a main system controller located remotely from gantry system  10 . RF communication includes sending positional and status information from carriage controller  22  to the main controller and sending task commands from the main controller to carriage controller  22 . 
     Gantry system  10  also includes an insulated conductor rail (not shown) for distributing electrical power to carriage  14  via a current collector (not shown). Other rails such as the guideway or mounting beam can be used to provide a ground return for the electric power. 
     FIG. 3 shows two complete manufacturing loops according to the invention. A first manufacturing loop  25  is arranged to perform a sequence of predetermined machining operations in order to manufacture a workpiece into a desired product. A second manufacturing loop  26  is deployed adjacent to loop  25 , there being an operator station  27  disposed between the two loops. Loops  25  and  26  are shown as being substantially identical and could be used to manufacture identical products; however, the loops could also manufacture completely different products using similar or very different machining stations. Furthermore, each loop could be designed to manufacture more than one particular product simultaneously. Due to the similarities of loops  25  and  26 , only loop  25  will be described in further detail. 
     Loop  25  includes a monorail  28  with a mounting beam, guideway and rack, and provision for electric power as discussed with reference to FIGS. 1 and 2. A pair of carriages  30  and  31  are mounted to monorail  28  for movement around the loop to operator station  27 , CNC machines  32 ,  33 , and  34 , and a flex turret cell  35 . Operator station  27  includes a loading/unloading bay  36  located proximate to monorail loop  28  where an operator loads and unloads workpieces on and off of fixtures that are delivered to bay  36  by carriages  30  and  31 . Operator station  27  may also include a central platform  37  and stairway  38  for easy operator access to the bays and to a part delivery and removal system  40  which may include a conveyor system, overhead wire transfer, fork lifts, etc. Unfinished workpieces are delivered to and finished workpieces removed from operator station  27  as shown by arrow  41 . 
     In a preferred embodiment, the layout of FIG. 3 employs a predetermined fixture design for carrying workpieces and a corresponding fixture-holder design in each machining station (i.e., CNC machines  32 - 34  and flex turret cell  35 ) so that each single workpiece remains affixed to the same fixture throughout the entire sequence of machining operations. Thus, a workpiece is loaded onto a fixture at bay  36  and automatically proceeds to the appropriate machining stations to accomplish the desired machining operations for the particular workpiece and is then automatically returned to bay  36  for removal from the fixture by the operator. 
     FIG. 4 is a perspective view along the sight line indicated by arrow  39  in FIG.  3 . CNC machine  33  includes a table portion  42  and a tool portion  43 . Table portion  42  is proximate to (e.g., beneath) the gantry system and has a top entry opening to receive fixtures from carriages  30  and  31 . Fixtures a vertically delivered to a removed from a fixture table within the side enclosure walls of table portion  42 . Tool portion  43  contains any conventional type of CNC machine. Control box  44  includes a microcontroller for controlling all actions of table portion  42  and tool portion  43  and for communicating (e.g., receiving commands and sending status information) with a main controller by direct wiring (not shown). 
     A preferred embodiment of the present invention employs a raised platform  45 , preferably at about the height of the top entry openings of the machining stations (e.g., about 8 feet). Platform  45  increases the safety of persons moving about in the corresponding floor area and facilitates maintenance by allowing 360° access around all machines, even during normal production operation. A guard rail  46  may also be provided at the periphery of platform  45  to form a barrier around the area where the fixtures are conveyed between machining stations. As a consequence of these self-contained guarding features, the present invention can avoid the cost, loss of floor space, and inconvenience of perimeter fencing that is required by typical robotic cells. 
     Loading/unloading bay  36  of the operator station is shown in greater detail in FIG. 5 with the operator platform removed. An enclosure  47  has front sliding doors  48  through which an operator loads and unloads workpieces. A fixture holder  50  securably receives a fixture  51  during the loading or unloading of a workpiece. End-of-arm clamp  20  is shown released from fixture  51  although it would preferably remain connected during the unloading/loading of workpieces. Depending upon the particular manufacturing process being implemented, end-of-arm clamp  21  may remain empty at the operator station so that it can first retrieve a fixture at the first machining station in sequence. 
     Loading/unloading bay  36  includes a control box  53  connected to a main controller via a wiring conduit  54 . Control box  53  provides an interface between a human operator and the main controller allowing the operator to send status information and operational requests or commands. 
     CNC machine  33  is shown in greater detail in FIGS. 6-8. FIG. 6 is a front, partially cutaway view revealing a fixture table  55  with means for securably receiving fixture  51 . Once secured, fixture  51  is manipulated into a desired position for machining by automatic slewing of table  55 . A CNC tool  56  then performs a commanded machining operation on a workpiece secured to fixture  51 . A top entry opening  57  permits vertical access to table  55  by the gantry arm. FIG. 7 is a top view showing top entry opening  57  cut within a machine top plate  58 . FIG. 8 is a top view with top plate  58  removed. 
     Flex turret cell  35  is shown in greater detail by the front, partially cutaway view of FIG.  9 . Cell  35  includes a fixture table  60  for receiving fixtures and presenting them to an indexing turret  61 . Turret  61  preferable comprises a multi-faceted spindle nose wherein indexing of the spindle allows several different tools to be utilized on a workpiece without removal or reacquisition of a reference data point. Cell  35  has a top plate  62  with a top entry opening  63  for vertically receiving fixtures carried by the gantry arms. 
     Fixture table  55  is shown in greater detail in FIG.  10 . Table  55  can be based on a conventional table such as a Nikken NC. Depending upon the desired machining operations, table  55  can comprise a four-axis or five axis mechanized table. This conventional aspect of table  55  will not be described in detail. 
     In a preferred embodiment of the present invention, table  55  is specially adapted to receive fixtures vertically. Table  55  may also automatically secure the fixtures using a quick connect/disconnect mechanism as shown in FIG.  10  and more specifically described in co-pending U.S. application Ser. No. 10/060,703, which is hereby incorporated by reference. 
     Table  55  includes a fixture holder  64  having a central taper socket  65  for receiving a taper shank  66  projecting from the bottom of fixture  51 . The taper of socket  65  and shank  66  guide fixture Si into position as fixture  51  is lowered by the gantry arm. As shank  66  enters socket  65 , rough alignment pins  67  and  68  approach and enter matching holes in fixture  51  (not shown), and then fine alignment pin  69  approaches and enters fine alignment hole  70  in fixture  51  to accurately position fixture  51  on table  55 . Fixture  51  is held in place by a drawbar actuated clamp mechanism (not shown) inside table  55  which locks onto locking tip  71 . 
     A workpiece  73 , such as a steering knuckle being machined, is held to fixture  51  by mechanical clamps  72 . A pair of U-bars  74  are provided on fixture  51  for grasping by the gantry end-of-arm clamps. Fixture  51  also includes a set of fittings  75  which may include electrical fittings or pneumatic fittings for receiving electric power or compressed air for various robotic functions that might be performed within fixture  51 . Connection to and disconnection from fittings  75  can be done in any known manner. 
     Overall system operation of the present invention, including coordination by a main controller, will now be discussed together with an alternative embodiment shown in FIG. 11. A monorail line  76  is supported overhead in a straight line proximate to a series of machining stations including turret cells  77 , CNC machines  78 , and a dedicated machining station  80 . Dedicated machining station  80  lacks computer numeric control and is constructed to perform a specific set of machining operations only. 
     An operator station  81  is proximate monorail line  76  for loading and unloading workpieces onto fixtures as in the previous embodiment. 
     A main controller  83  is coupled to operator station  81  by a communication bus  84  and to each of the machining stations by a communication bus  85 . Carriages  86  and  87  and main controller  83  each include respective radio-frequency transceivers for establishing an RF communication link using technology well known in the art. Main controller  83  can be any commercially available control system. 
     Main controller  83  coordinates machining operations and overall functioning of each element of the manufacturing system. Major functions within the control strategy include 1) registering each workpiece type, 2) determining the appropriate manufacturing steps to be performed for the workpieces, 3) commanding and coordinating carriages as they deliver workpieces from point to point, 4) monitoring current location and in-process progress of the workpieces, and 5) commanding machining stations to perform the appropriate tasks for workpieces being delivered to them. 
     More specifically, a preferred method of the invention is shown in FIG.  12 . In step  90 , an operator affixes a workpiece to a fixture at the operator station. In step  91 , the operator activates a pushbutton or keypad on a control panel at the operator station to send a signal to the main controller to signify that a part is loaded on the fixture and is ready for the machining operations. If the manufacturing line is set up to have the ability to produce more than one specific finished part, then the operator might also generate a signal identifying which kind of workpiece and finished part are desired. Alternatively, the fixture may have automatic means for identifying the type of workpiece and then that information may be transmitted by the carriage to the main controller using the RF communication link. 
     In step  92 , the main controller determines what machining operations to be applied to the workpiece to produce the finished part. It also determines which machining stations should be used and in what sequence to achieve the desired machining of the workpiece. The sequence of machining operations and the stations utilized may be a static, predetermined sequence. Alternatively, if the finished part is capable of being machined using various orders of operations and/or if there are more than one machining stations that could perform certain of the operations, then the main controller may also use an optimization strategy to determine a sequence for any particular workpiece on the fly. 
     Once the sequence of operations and stations is determined, the main controller commands the carriages to convey the workpiece to the selected machining stations in step  93 . In step  94 , the main controller detects the arrival of the fixture at a selected machining station and then commands the machining station to perform its corresponding machining operations. The machining station is preferably preprogrammed with each of the steps necessary to accomplish its assigned machining operations (e.g., grasping the fixture, moving the fixture table to present the workpiece to a tool, changing the tool if necessary, operating the tool, moving the table into an unloading position, and releasing the fixture once it is recovered by the carriage arm). 
     In step  9 S, the main controller detects that the machining operations to be performed at one machining station for a particular workpiece are finished (e.g., in response to a completion signal from the machining station) and determines the next machining station and corresponding machining operations that need to be performed on the particular workpiece. In step  96 , the main controller commands a carriage to pick up and convey the workpiece to the next machining station, or if the machining operations are completed then back to the operator station. When the workpiece is returned to the operation station, it is removed by the operator in step  97 . The operator returns the finished part to the inventory handling system and obtains a new unfinished workpiece for loading onto the fixture just vacated by the finished part. 
     The decision-making process used in the main controller corresponding to the progress of one particular fixture is shown in greater detail in FIG.  13 . After starting at step  100 , the main controller detects that a part exchange has occurred at the operator station in step  101 . In step  102 , the main controller checks whether the particular manufacturing line or loop has been set up to handle multiple parts of different designs. If yes, then the proper machines for processing this particular part are determined in step  103 . 
     In step  104 , the main controller checks whether there are multiple machines in the manufacturing line or loop that can be used for the processing of the particular part. If more than one are available, then a check is made in step  105  to determine which machine will be available next (i.e., first). In step  106 , a check is made to determine whether the identified machine is ready to accept the fixture with the current part. If not, then the identified machine is rechecked until it is ready. 
     Once the identified machine is ready, the fixture is exchanged with (i.e., loaded onto) the machine in step  107  and then machining can begin. In step  108 , the main controller checks to determine whether the machine is done with the part, and it continues to recheck until it determines that the part is done. Then the fixture is exchanged (i.e., removed) in step  109 . 
     In step  110 , the main controller determined whether more machining operations are needed for the particular part. If there are more operations to be done, then a return is made to step  104  to decide if more than one machine is available for the next processing operation. Otherwise, the part is returned to the operator station in step  111  and the process returns to step  101  for tracking the next part loaded onto the fixture at the operator station.