Abstract:
A reconfigurable fixture for workpieces is disclosed. It utilizes a flat surface magnetic chuck with modular workpiece supporting, locating and clamping elements that are carefully located on it and held by magnetic attraction to it. These elements are precisely located to hold the workpiece in spaced relation to the chuck surface. Some locating elements are mechanically fixed to the chuck for securing the workpiece from transverse movement on the chuck surface. Preferably, each of the supporting, locating and clamping members are fixed to separate steel bases for magnetic attraction to the chuck. Each base may also carry its own hydraulic actuator for height adjustment of a support element or for height adjustment and closure of a clamping element. These modular elements have locating slots or grooves or the like so that they can be selected from a storage magazine and located on a chuck by numerically controlled mechanisms.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of co-pending U.S. application Ser. No. 10/243,860, filed Sep.  13 ,  2002 . 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention pertains to reconfigurable magnetic workholding fixtures. More specifically, this invention relates to robust and easily re-configured fixtures comprising a flat magnetic chuck surface with magnetically attached modular supporting, locating and clamping elements that are useful for locating and securing workpieces, especially non-magnetic workpieces.  
         BACKGROUND OF THE INVENTION  
         [0003]    There is a need in manufacturing operations for durable and reconfigurable workholding fixtures especially for flexible manufacturing operations. The fixtures must be readily and accurately adaptable to hold different workpiece shapes for machining applications or the like. Sometimes the workpieces are similar or related part shapes such as cast aluminum cylinder heads for different engines. In other manufacturing situations the parts may be of unrelated design but requiring similar manufacturing operations. In these varied applications, the fixture reconfiguration or changeover from one part design to another has to be fast enough to meet the productivity requirements of current manufacturing systems.  
           [0004]    Magnetic chucks have been available for holding some magnetic workpieces. For example, magnetic chucks providing flat holding surfaces are commercially available under the trade designation “Quadsystem” by Technomagnete S.p.a. These chucks are provided with a plurality of magnetic pole pieces often arranged in a rectangular grid on a base plate within peripheral walls. Apart from the coils that energize or de-energize the steel pole pieces, the chuck also includes high energy permanent magnets (neodymium-iron-boron magnets) interspersed among the poles. U.S. Pat. Nos. 4,847,582 and 4,956,625 appear to describe such flat surface chucks. Suitably placed and oriented electrical coils permit the chuck to be magnetized to different levels and demagnetized for locating, securing and removing a magnetizable work piece. However, this chuck has not proven useful for securing and machining non-magnetic workpieces such as, e.g., cast aluminum alloys.  
           [0005]    It is an object of this invention to provide a fixture utilizing a magnetic chuck for workpieces, especially non-magnetic workpieces. It is a more specific object of this invention to provide an easily reconfigured workpiece holding fixture comprising a magnetic base and modular workpiece supporting, locating and clamping elements.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with a preferred embodiment of this invention a workholding fixture basically comprises an electromagnetic chuck and several modular fixture elements for locating, clamping, and supporting a non-magnetic workpiece. Such a fixture is also useful for iron or steel workpieces.  
           [0007]    A suitable magnetic chuck is one that can be energized and de-energized with an electric pulse from a stationary electric power source. Thus, the magnetic chuck provides a fast attach/release capability and a flat surface on which modular workpiece supporting, locating and clamping elements can be securely held by the magnetic force. Preferably, the strength of the field can be varied from a first level for sensitive and accurate modular element placement to a second, higher level for strongly securing the elements. In addition, the energized magnetic chuck can maintain its magnetic attractive force even when the chuck is disconnected from the power source. This is important because it provides the required mobility of the workholding fixture to be transferred from one machining station to another machining station.  
           [0008]    The autonomous integrated modular clamping or supporting elements are designed and constructed to be precisely located on the flat surface of the magnetic chuck and to then receive and securely hold a non-magnetic workpiece for a machining or other manufacturing operation. The work piece may be a nonferrous alloy such as an aluminum or magnesium alloy or the like. The workpiece is held spaced apart from the magnetic chuck by suitably located and cooperating supporting, locating and clamping members.  
           [0009]    The supporting, locating and clamping elements may be individually carried on a suitable base plate. The base plate is preferably made of steel or other magnetizable alloy and sized so that the support or clamp is tightly held on the magnetized chuck plate. Support or clamp posts stand upright from the base plate. In one embodiment, the support is simply a post of predetermined length fixed at one end to the base plate with the other end configured to engage and support a surface of a workpiece. The length of the support post is determined by the shape of the work piece and its intended distance from the chuck surface. The clamp is typically a rotatable horizontal arm attached to the upper end of a post carried by a base plate. Clamp arms of varying shapes may be used to accommodate different workpieces.  
           [0010]    In addition to the magnetically attached locating, supporting and clamping elements, the fixture preferably comprises a rigid guide rail and at least one locator element (which is also a support element) that are precisely located and mechanically fixed to the surface of the magnetic chuck. These mechanically fixed elements constrain the workpiece from transverse movement on the chuck surface. Movable locator elements used in combination with these fixed elements assure that the fixture is suitably reconfigured for different cylinder heads or other workpieces.  
           [0011]    In its simplest terms the fixture comprises a magnetic chuck with a mechanically fixed guide rail and locator element, and at least one support element and one clamp arm element each secured to the surface of the chuck by magnetic force. Generally, a plurality of the magnetically attached elements will be located on the chuck surface to cooperate in holding the workpiece for a machining operation or the like. However, in a preferred embodiment of the invention, the height of the movable support elements and clamping elements are not fixed, they are adjustable to accommodate different workpieces.  
           [0012]    In support elements or clamping elements of adjustable height, the posts can, for example, be fitted into a cylinder or other housing and threaded for screw height adjustment, or adjusted hydraulically. Such support and clamping elements are commercially available. Hydraulic height adjustment of post height may require that the base plate be modified to accommodate suitable hydraulic pressure supplying components.  
           [0013]    The magnetic chuck and modular support and clamp elements allow a fixture to be easily assembled or modified. For example, support and clamp elements can be individually selected from a suitable inventory or storage magazine by a numerically controlled robot and precisely located on a magnetic chuck surface. The element-placing robot arm will have a suitable gripping adapter to grip the support or clamp element and place it base down on the chuck surface. Similarly, support and clamp elements are removed from the chuck when the need for the present fixture assembly is finished. The chuck is partially magnetized for element placement and removal and fully magnetized to secure the elements to the chuck in the working fixture.  
           [0014]    The fixed guide rail and locator post(s) are located for general fixture applications. The magnetically attached locator, support and clamp elements are positioned for each workpiece configuration. They are positioned so that the clamp arms can press against a surface of the workpiece and force it against a suitable surface piece on the top of each support post. The workpiece is hoisted and placed against the support posts and the arms of the clamps rotated into position engaging the workpiece.  
           [0015]    The assembled fixture can be situated horizontally or vertically, or in any desired attitude with the workpiece securely held by the clamping elements against the locator and support elements in a position spaced from the surface of the chuck. The combination of the mechanically fixed and magnetically fixed supporting, locating and clamping elements holds the workpiece for machining or the like, even with heavy stock removal.  
           [0016]    In another embodiment of the invention the roles or positions of the large magnetic chuck plate surface and the support plates for the support or clamp elements are reversed. In this embodiment, the fixture system essentially consists of a large conventional, magnetizable steel fixture plate and a collection of modular fixture elements for locating, clamping, and supporting a workpiece. The modular elements are all mounted on magnetic holding bases of some common dimensions. These magnetic holding bases are smaller versions of the magnetic chuck plates described above. The magnetic holding base for the support or clamp elements can be an electro-permanent magnet construction so that it can be activated or de-activated with an electric switch. Alternatively, it can be a simple lift magnet design activated by a mechanical lever. The magnetic holding base will carry a support and/or clamp element and optionally a hydraulic pressure supplying component. The role of the magnetic holding base is to provide a fast attach/release capability to the flat steel fixture plate and also a platform on which a modular fixture element can be affixed mechanically. In addition, the activated magnetic support or clamp base will maintain its magnetic attractive force when the base is disconnected from the electric source or as long as the mechanical lever stays in the same position. This portability provides the required mobility of the smart workholding fixture plate to be transferred from one machining station to another machining station.  
           [0017]    Other objects and advantages of the invention will be understood from a description of preferred embodiments. Reference will be made to drawing figures that are described in the following section. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is an exploded, generally plan view of an assembled workpiece fixture in accordance with the invention with an overlying aluminum cylinder head.  
         [0019]    [0019]FIG. 2 is an oblique view of a modular, self contained hydraulic clamping element, comprising a hydraulically actuated clamp arm and a screw actuated hydraulic pump both contained in a steel base member, suitable for use in a workpiece fixture of this invention.  
         [0020]    [0020]FIG. 3 is a side view, partially in cross-section of a modular clamp assembly similar to that of FIG. 2.  
         [0021]    [0021]FIG. 4 is a side view, partly in cross-section, of a modular, self contained hydraulic support element and screw actuated hydraulic pump both contained in a steel base member, suitable for use in workpiece fixture of this invention.  
         [0022]    [0022]FIG. 5 is an oblique, fragmentary view of a locator element, clamped to a guide rail for use in a workpiece fixture of this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    The practice of the invention will be illustrated by describing a fixture for machining of an aluminum alloy cylinder head for an automotive vehicle engine. Such a part is typically cast of a silicon containing, aluminum alloy as close to final shape as possible. However an automobile engine cylinder head casting is a very complex structure. The cylinder head shown schematically in FIG. 1 at  10  is for a three cylinder engine. The cylinder head contains openings for four valves in each cylinder, openings for spark plugs, intake passages for air and exhaust passages for exhaust gas. The head also contains oil passages and passages for coolant. It is a complex structure that requires a considerable amount of machining following the casting operation.  
         [0024]    In order to facilitate such machining the cylinder head must be securely held in a fixture so that numerically controlled machine tools can properly remove metal from the casting to arrive at its specified finish dimensions. A reconfigurable, magnetic fixture to support and hold cylinder head  10  is shown generally at  12  in FIG. 1. Fixture  12  includes a magnetic chuck  14  that, in this example, has a complex flat rectangular working surface (indicated generally at  16 ) of many magnetic pole elements  18 . Applied to the surface  16  of chuck  14  are a number of fixture elements that are movable to accommodate varying workpiece shapes or machining positions.  
         [0025]    Fixture  12  also includes a rigid guide rail  20  that is bolted, or otherwise suitably fixed, to surface  18  of the magnetic chuck  14 . Guide rail  20  is illustrated as straight but it may be of any desired length or configuration. Also, clamped to guide rail  20  are two locator post elements  22  that are thus mechanically fixed to the surface  18  of chuck  14  (see FIG. 5 for a suitable clamping arrangement). On the top of each locator posts  22  are pins  23  that are shaped and located for fitting into locator holes (sometimes termed B-C holes not seen in FIG. 1) formed in the bottom of cast cylinder head  10 . The B-C holes serve to facilitate precise location of the workpiece for machining operations. Guide rail  20  and locator posts  22  are important in precisely locating and rigidly supporting a massive work piece such as cylinder head  10 . Furthermore, they assist the magnetic chuck  12  in keeping workpiece  10  from sliding transversely on surface  18  under dynamic or impact machining forces.  
         [0026]    Also attached by magnetic attraction to flat surface  16  of magnetic chuck  14  are several self-contained modular hydraulically adjustable and actuated clamping elements  24  and several modular support elements  26  of fixed height. Both the clamping elements  24  and support elements  26  are shown schematically in FIG. 1. In FIG. 1 the hydraulic clamping elements  24  are identical to each other and the fixed height support elements  26  are identical to each other. It may be preferred to use identical modular clamping and support elements for simplicity of inventory and reconfiguration of the fixture. But these elements don&#39;t have to be of common design. Similarly, the support elements in FIG. 1 are illustrated as simply having a base  29  with a post  27  of fixed length. However, the support elements  26  may also be provided with hydraulic height adjustments like the clamping elements  24 . A representative clamping element is shown in more detail in FIGS. 2 and 3, and an adjustable support element is shown in FIG. 4.  
         [0027]    The support elements  26  are located so as to space cylinder head  10  from chuck surface  16  and to support it from deflection during machining operations. The clamping elements have clamp arms  28  (best seen in FIGS. 2 and 3) that are rotatable from an open position when cylinder head  10  is being placed on the fixture, or removed from it, to a closed position. In the closed position the clamp arms engage a flange or other portion of the cylinder head. Clamps  24  press the cylinder head against the tops of the support members  26 .  
         [0028]    While the guide rail  20  and locator posts  22  are mechanically attached to the magnetic chuck  12 , the modular support elements  26  and clamp elements  24  are held to the chuck surface  16  by its magnetic field.  
         [0029]    As stated above, magnetic chucks are commercially available in a variety of sizes and shapes. Chuck  12  is larger than the footprint of cylinder head  10 . In a representative commercial embodiment, chuck  12  has a steel base, not seen in FIG. 1, with opposing rectangular side walls  30 ,  32 . Supported on the base and confined within side walls  30 ,  32  are many steel cubes that serve to concentrate magnetic flux and as magnetic poles. Only the upper flat surfaces  18  of the cubes are seen in FIG. 1 and there are 126 such pole surfaces in this embodiment. Each polar square (typically 50 mm by 50 mm) is an independent magnet body presenting a magnetic pole at surface  16 . And each such square  18  is surrounded on the other four cube sides by smaller magnets of opposite polarity. Embedded electric coils serve to magnify and de-magnify the magnetic pieces. The construction and assembly of the magnets assures that the magnetic flux produced by the chuck runs flat and horizontally at the surface  16  of chuck  12 . The coils around the reversible permanent magnets can be energized to generate a strong momentary electromagnetic field and they are capable of reversing the polarity of the chuck. Thus a relatively low magnetic field may be initially applied to assist in holding and locating the modular clamping  26  and support  24  members without jerking them to the surface  16  of the chuck. But the magnetic field can be substantially increased to strongly hold the modular pieces to the surface of the chuck. When it is time to reconfigure the workpiece fixture the magnetic field is partially turned off and the pieces removed.  
         [0030]    The external frame of the chuck has the mechanical function of containing the magnetic components. It also serves to convey the magnetic flux lines so as to insulate the active surface of the chuck from other machine or processing elements.  
         [0031]    An illustrative modular clamping element  24  for use in the workholding fixture of this invention is shown in FIGS. 2 and 3. Clamping element  24  comprises a hydraulically actuated and lockable clamping cylinder and arm  40  of a known type. The clamping element  24  also contains a hydraulic screw pump  42  of a known type. However, in the practice of this invention, screw pump  42  and the hydraulic clamping arm  40  are fixed in a specially designed base  44 . Base  44  preferably made of a ferrous metal alloy such as steel. The steel base  44  is large enough to receive a suitable screw pump  42  and hydraulic clamping arm  40 . But it is also sized and constructed so as to concentrate magnetic flux lines from the magnetic chuck  14  to be strongly attracted to surface  16 . One suitable clamp base is rectangular as seen in FIG. 2 with dimensions of 100 mm×125 mm. Base  44  also provides a channel  46  (FIG. 3) for hydraulic fluid between screw pump  42  and clamp  40 .  
         [0032]    Hydraulic clamping arm devices suitable for use in the practice of this invention are commercially available. Such a device comprises a clamp arm  28  attached to the end of a rod  48 , such as by a bolt  50 . Rod  48  extends into one end of a hydraulic cylinder  52  (FIG. 3). The other end of cylinder  52  admits hydraulic fluid that applies force to the rod  48 , or a piston affixed to it. Upon the application of fluid pressure (such as from screw pump  42  through channel  46 ), the mechanism within cylinder rotates rod  48  and arm  28  to a clamping position. When fluid is withdrawn from cylinder  52 , rod  48  is retracted and arm  28  rotated to a clamp release position. Depending upon the specifications of the commercial device, arm  28  may be rotated, e.g., ninety degrees and the stroke of rod  48  may be 11 to 25 mm.  
         [0033]    Various commercial clamps are available depending upon the height required for clamp arm  28  and different clamp arm shapes are available. In FIGS. 2 and 3, arm  28  is illustrated as being straight. However, clamp arms with intermediate bent sections provide higher or lower height adjustments.  
         [0034]    The hydraulic clamp mechanism  40  is fixed within tube  54  and attached upright to base  44 . As illustrated in FIG. 2, a spacer  56  may also be used to alter the height of clamp arm  28 .  
         [0035]    In modular clamp unit  24 , hydraulic clamp  40  is used in combination with a hydraulic screw pump  42 . Such pumps are commercially available. However, in the practice of this invention, the screw pump is inserted upright into a bore  66  in elevated portion  68  of magnetic base  44  (see FIG. 3). Screw pump  42  contains a rod  58  with helical threads  59  along the central portion of its length. One end  60  of rod  58  is inserted in threaded  59  engagement into hydraulic cylinder  62 . The upper end  64  of rod  58  is a hexagonal nut for manual or robotic actuation of the pump  42 . The lower end  60  of rod  58  is attached to piston  70 . The space  72  within cylinder  62  above piston  70  is vented  74  to the atmosphere. Space  76  below piston  70  contains hydraulic fluid which also fills channel  46  and the fluid space of hydraulic clamp  40 . Seal  78  provides a fluid barrier between piston  70  and cylinder  62 .  
         [0036]    The structure of screw pump  42  in FIGS. 2 and 3 is the same. In FIG. 3 the pump  42  is simply shown as inserted into elevated portion  68  of base  44 . In FIG. 2, screw pump  42  in enclosed in a cover piece  71  with slots  73 . This cover piece  71  with slots  73  is to facilitate robotic grasping of the modular clamping element  24  for placing on, and removal from, magnetic chuck  14   
         [0037]    When the screw pump rod  58  is rotated so as to drive hydraulic fluid into the hydraulic clamp, the clamp is initially rotated from a non-clamping position to its clamping position. At the same time the clamp is lowered by a half inch or so until it comes into engagement with a work piece such as a flange surface or other suitable surface on the aluminum cylinder head. So long as the hydraulic pressure is then maintained by the screw pump, the clamp arm remains locked in position. When it is time to release a work piece, the screw pump rod  58  is reversed, piston  70  is lifted and hydraulic fluid flows back from the modular clamp body. Clamp arm  28  is raised and turned from its clamping position.  
         [0038]    [0038]FIG. 4 illustrates a three component, hydraulically adjustable, modular support element  80  suitable for use in an embodiment of this invention. Different support posts have been used in the practice of this invention. Some have been of fixed height and some were adjustable. These supports are the workpiece locators in the direction perpendicular to the base  16  of the fixture  12 . In principle, three support elements could be sufficient to define a locating plane for the workpiece. However, extra supports can be added to bear loads more evenly and to reduce workpiece deflection during machining. In one embodiment a simple flat top post of suitable length was mounted on a 75 mm by 75 mm magnetic steel base. These support posts are illustrated at  26  in FIG. 1. These structures are useful where the support height is fairly constant in different fixture requirements. However, hydraulically adjustable modular units can also be used.  
         [0039]    A three component modular support unit  80  is illustrated in cross section in FIG. 4. Again, hydraulic support units are commercially available in different sizes and capacities. A representative hydraulic support unit  82  is obtained and adapted for support in an upright pedestal portion  86  in steel base  84 . The steel base  84  also receives an upright hydraulic screw pump  42  in pedestal portion  88 . Screw pump  42  in this support module may be the same as the screw pump used in the clamping module  24 . The screw pump was illustrated in cross-section and described in connection with FIG. 3. It is similarly illustrated in FIG. 4 but the identical description will not be repeated. Base  84  also contains a hydraulic fluid channel  90  connecting screw pump  42  and hydraulic support  82 .  
         [0040]    Hydraulic support unit  82  includes a rod  92  inserted within hydraulic cylinder  94 . Cylinder  94  is fixed in base pedestal  86 . Screwed into the upper end of rod  92  is a workpiece support surface body  96 . Support surface  96  is shown to be generally flat but may be shaped as desired for contact with a workpiece. In a suitable commercial support element like  82  the other end of rod  92  engages a compression spring (not shown) carried on the base of the support element. The lower end of cylinder  94  contains an opening (not shown) for hydraulic fluid which is admitted into cylinder  94  from screw pump  42  through channel  90  to lock the desired position of rod  92 . Rod  92  also preferably contains a V-groove  98  for engagement with an adapter used for placing modular support  80  at a predetermined x-y coordinate position on surface  16  of magnetic chuck  12 .  
         [0041]    The height of support surface  96  is adjusted with the module  80  on the surface  16  of chuck  14 . Surface  96  and rod  92  is pushed down against such compression spring to a precise predetermined height. In general, the height of support surface  96  above chuck surface  16  can be controlled to a part of a millimeter. This adjustment may be made manually or with mechanical robotic assistance. Hydraulic fluid pressure is then applied by screw pump  42  in cylinder  94  to hydraulically lock the hydraulic support  82  in position.  
         [0042]    [0042]FIG. 5 is a fragmentary view of the clamping mechanism for securing the locator elements  22  to the guide rail  20 . Each locator element  22  is fixed to a steel base  110 . A pair of clamp arms  112  on base  110  fix the locator element  22  to rail  20 . After the locator element  22  with its workpiece engaging tip  23  has been precisely positioned against rail  20 , clamp arms  112  are locked by screws  114 . As stated above locator tips  23  engages the workpiece in a locator hole such as the B-C holes formed in cylinder heads and like engine castings.  
         [0043]    A principle feature of this workholding fixture system is to reconfigure a part holding fixture quickly and automatically at a suitable fixture setup station using the modular supporting, locating and clamping components described above in this specification. The fixture setup station may, for example, be organized like a state-of-the-art multi-axis machining center in which a numerically controlled robot (CNC) is programmed to select a cutting tool and perform a machining operation on a precisely fixtured workpiece. A state-of-the-art pick and place robotic station may be selected if its accuracy and repeatability lies within the tolerance requirements of the intended machining and fixturing applications. In any case, the special fixture setup station needs CNC servo drives to control its X-, Y-, and Z-motions accurately. For this fixture setup application, the programmable cutting tool storage magazine and the automatic cutting tool changer is redesigned into a fixture element storage magazine and a fixture element changer. The end of the machine spindle nose will have a special adapter so that a modular fixture element can be swiftly attached to or released from it. The adapter is configured, for example, to engage cover  71  with slots  73  on a screw pump  42  of a clamping element as shown in FIG. 2 or to engage V-groove  98  on modular support element  80  (FIG. 4) or V-groove  116 , or the like on a locating element  22  as shown in FIG. 5. Thus, the machine spindle nose selects and places the locating, supporting and clamping elements on a suitably positioned magnetic chuck  14  to configure a workpiece fixture  12  as seen in FIG. 1.  
         [0044]    In the first fixture setup station, the magnetic chuck is brought to the station table, hooked up to a power source, and energized at a reduced magnetization level (for a horizontal spindle machining center, the magnetic chuck will be located on the pallet). A modular fixture element (locating  22  in FIG. 1, clamping  24 , or supporting  26 ) as dictated by the specific part fixture design is gripped by the fixture element changer from the storage magazine and attached to the spindle nose adapter. The XY drives of the fixture setup station will move simultaneously so that a precise X-Y coordinate position on the magnetic chuck  14  is aligned with the spindle nose adapter centerline, i. e., the centerline of the fixture element base plate. The fixture element is then brought very close to the surface  16  of the magnetic chuck  14  by the motion of the Z axis. Once the fixture element base plate (for example, clamp base plate  44  or support base plate  84 ) is in solid contact with the magnetic chuck  14 , the fixture element is released by the spindle nose adapter. The fixture element automatically attaches itself gently to the X-Y coordinate location on the magnetic chuck surface  16  by the magnetic attractive force. The reduced pre-selected magnetization level is to ensure the fixture element will not be slamming violently onto the chuck. If a hydraulic work support cylinder element is to be used as a locating element, then a command is given to the machine to push in and lock up the support cylinder plunger at the desired height. The fixture setup program then initiates the next round of commands to setup the second fixture element, the third element, and so on. After all the modular fixture elements are set up accurately in their respective positions, the magnetic chuck  14  is energized so as to hold the modular elements ( 24 ,  26 ,  80 ) with the designed maximum holding power.  
         [0045]    Then, the part  10  will be laid on the fixture  12  (FIG. 1) by an overhead gantry loader or other material handling equipment. The spindle nose adapter will pick up a wrench type of device (a nut-runner with a torque limiter) from the storage magazine and actuate the screw pumps  42  in the various supporting  80  and clamping elements  24  in a prescribed sequence. Finally, the magnetic chuck  14  together with its fixture elements and the clamped part is transferred to a designated machining station for processing.  
         [0046]    It is a necessary and common practice to orient and fixture the part differently several times in a complex machining line so that various part surfaces and design features can be accessed for machining. Hence, it is envisioned that there will be similar number of fixture setup stations interspersed among the machining stations for the purpose of re-fixturing. When a part locked up by the modular fixture elements and its magnetic chuck is brought to one of these fixture setup stations, the part is unclamped and released from the fixture. It will then be located and clamped onto a different fixture and transferred down the production line. The previous fixture is recycled to its originating setup station upstream to be used again. It is estimated that the total time for unclamping, transferring, and clamping the part at the fixture setup station will be well within the typical cycle time for the machining line and so there will be no loss in the overall line throughput.  
         [0047]    An alternative way of setting up the fixture is to use the machine tool for both machining and fixturing. Most of the modern CNC machine tools or machining centers are accurate enough to carry out precise fixturing. This practice would save capital investment in a dedicated setup station but the production tool would lose a few productive cycles during changeover.  
         [0048]    During the transition phase of a changeover to machine a new part design (in-family or cross-family), the magnetic chuck has to be de-magnetized and the modular fixture elements need to be re-arranged to form new fixturing configurations at all the fixture setup stations. Depending on the cycle time of the machining line, there may or may not be a lower throughput of the line during the transition. For example, if the fixture re-arrangement takes 5 to 6 minutes and the machining cycle time is longer than that, then the changeover for a new part design will be transparent to the production system.  
         [0049]    It is to be understood that the function of the large magnetic chuck plate  14  could be incorporated into the support bases for the support or clamp elements. The large magnetic chuck could be replaced with a steel plate of suitable size and shape, for example, like that of the chuck  14  illustrated in FIG. 1. Smaller versions of the chuck plate would then be adapted as, for example, base  44  for clamp  40  in FIGS. 2 and 3 or base  84  for support  82  as depicted in FIG. 4. The chuck plate bases would be adapted to carry, a hydraulic pressure supplying component like screw pump  42  shown in FIGS.  2 - 4  and hydraulic fluid channels like channel  46  in FIG. 3 and channel  90  in FIG. 4.  
         [0050]    Thus, in this embodiment, the fixture system essentially consists of a large conventional, magnetizable steel fixture plate and a collection of modular fixture elements for locating, clamping, and supporting a workpiece. The modular elements are all mounted on magnetic holding bases of some common dimensions. These magnetic holding bases are smaller versions of the magnetic chuck plates described above. The magnetic holding base for the support or clamp elements can be an electro-permanent magnet construction so that it can be activated or de-activated with an electric switch. Alternatively, it can be a simple lift magnet design activated by a mechanical lever. The magnetic holding base will carry a support and/or clamp element and optionally a hydraulic, pressure supplying component. The role of the magnetic holding base is to provide a fast attach/release capability to the flat steel fixture plate and also a platform on which a modular fixture element can be affixed mechanically. In addition, the activated magnetic support or clamp base will maintain its magnetic attractive force when the base is disconnected from the electric source or as long as the mechanical lever stays in the same position. This portability provides the required mobility of the smart workholding fixture plate to be transferred from one machining station to another machining station.  
         [0051]    While the invention has been described in terms of certain preferred embodiments it is apparent that other embodiments could readily be devised by one skilled in the art. The scope of the invention is to be considered limited only by the following claims.