Abstract:
A permanent magnet chuck for holding or lifting workpieces is formed of two overlying/underlying flat surfaced chuck layers having a common center of rotation. In each chuck layer an even number of permanent magnet plates radiate from the center of rotation at equal angles with blocks of magnetically soft material between them. The permanent magnet plates are magnetized so that the interposed soft magnet blocks exhibit alternating N-S magnetic polarities. The organization of the permanent magnet plates and soft magnet bodies is matching and complementary so that relative rotation of the chuck layers magnetically activates or de-activates the chuck.

Description:
TECHNICAL FIELD  
       [0001]     This invention pertains to modular permanent magnet chucks. More specifically, this invention pertains to a magnetic chuck comprising two layers of permanent magnet sheets or plates interposed with larger soft magnet blocks. Each layer contains an even number of similarly shaped permanent and soft magnet bodies, and one chuck layer is rotated with respect to the other to convert the chuck from a magnetically inactive state to an active state for holding workpieces.  
       BACKGROUND OF THE INVENTION  
       [0002]     Manually activated permanent magnet chucks have been in existence for many years. One of the common applications is as a work-holding device used in machining operations on grinders, lathes, and mills. Another application is a mechanical lifter for material handling purposes. These permanent magnet chucks usually have relatively low levels of attractive force and their load carrying capacities are somewhat limited. Consequently, the magnetic chucks are very bulky in their physical dimensions and have large surface contact areas in order to meet their performance targets.  
         [0003]     Typically permanent magnet chucks have a movable magnet portion that must be rotated from an inactive position to a magnetically active position for use of the chuck. In prior designs, activation of the chuck is often accomplished by actuation of a lever on the side of the device. Typically the arrangement of the magnets in the chuck is such that the actuating lever must be moved through a large angle of rotation, typically ranging from 120° to 180°. These conditions pose physical obstructions in many practical workplace applications where there is not room for the sweeping reach of such a side-located lever.  
       SUMMARY OF THE INVENTION  
       [0004]     This permanent magnet chuck design provides a unique architecture of complementary overlying/underlying layers of magnetic members. Each layer comprises a like pattern of alternating thin permanent magnet pieces (sheets or plates) and larger magnetically soft (e.g. iron or low alloy steel) blocks. Rectangular permanent magnet sheets or plates are placed on edge in each chuck layer, and the thin pieces are magnetized through their thickness so that one main face of the sheet is a magnetic north pole (N) and the opposite face is a magnetic south pole (S). Larger soft magnet blocks are placed at each face of the permanent magnet sheet to receive and direct the magnetic flux emanating from the magnetic poles of the interpositioned permanent magnet. Thus, an even number of the soft magnet block pieces are used in each clutch layer.  
         [0005]     In a preferred embodiment of the invention each chuck layer is shaped like a disk with four, six, eight, or more radial permanent magnet plates each separated by identical, pie-shaped magnetically soft steel blocks. One end of each permanent magnet plate lies at the center of the disk and the other end at the circumference. The close lying central ends of the permanent magnet layers are separated by a rod or bar of non-magnetic material such as brass to prevent magnetic flux from passing directly from the end of one permanent magnet to another. The permanent magnet plates are placed with N poles facing N poles and S poles facing S poles. The induced polarities of the intervening soft magnet blocks are, thus, alternating N-S-N-S for a four permanent magnet clutch layer. In other embodiments of the invention, the chuck layers may be shaped as regular polygons for example, square, hexagonal, or octagonal, with the permanent magnet pieces extending from the center of the shape to its periphery.  
         [0006]     A chuck assembly includes two complementary (preferably matching) chuck layers placed in face to face (overlying/underlying) arrangement with the edges of the permanent magnet pieces aligned and the soft magnet blocks aligned. The facing edges of the permanent magnets in the facing chuck layers are separated by strips of non-magnetic material to minimize short circuiting of magnetic flux between permanent magnets.  
         [0007]     When facing soft magnet blocks between the chuck layers are of opposite polarity (N-S) the chuck is in a magnetically inactive state because the magnetic flux lines are directed within the facing chuck layers. The magnetic chuck is then tuned “off” and will not attract a ferromagnetic workpiece to its working face. However, when one chuck layer is rotated respect to the other so that facing soft magnet blocks are of the same polarity (N-N and S-S) then the chuck is magnetically activated and will strongly attract and hold a ferromagnetic workpiece. With inter-chuck layer facing magnet blocks of the same polarity, magnetic flux lines extend well above the working face of the chuck assembly into a ferromagnetic workpiece, and the device is turned “on.” The rotation of a chuck layer is accomplished from the top or bottom of a horizontal layer chuck. In preferred embodiments of the chuck design, the required rotation is no more than 90°. The arrangement of an even number of alternating permanent magnet plates and soft magnet blocks effectively utilizes the magnetic flux of the permanent magnets in the magnetically active alignment of the chuck layers.  
         [0008]     The relatively thin permanent magnet bodies (sheets or plates) are made of any suitable permanent magnet material such as neodymium-iron-boron compositions, rare earth element-cobalt compositions, hard ferrite compositions, or the like. Magnetically soft iron and/or magnetically soft alloys of iron-silicon, nickel-iron and soft ferrites are suitable for use as flux carriers between the magnet blocks. Brass or other non-magnetic materials are placed between the central edges of the permanent magnet bodies in each chuck layer and at facing edges of permanent magnet bodies between chuck layers to minimize “short-circuiting” of magnetic flux between permanent magnets.  
         [0009]     The use of complementary or matching chuck layers of alternating thin permanent pieces and larger soft magnet blocks enables the concentration of a high level of magnetic flux in a workpiece. The flux is concentrated in a clutch face most of which is soft magnet material. The magnitude of attractive magnetic force appears to be maximized with respect to the inherent magnetic properties of the permanent magnet and soft magnet pieces. This chuck architecture permits the use of smaller structures, or lower-in-height structures, for a given chuck application. And, as stated, when the chuck layers and working face of the device are oriented horizontally, a chuck layer can be positioned or rotated for magnetic activation from the top or bottom of the chuck.  
         [0010]     The two chuck layers of permanent magnet plates and soft magnet bodies are enclosed in a suitable chuck frame of non-magnetic material. The chuck layers are enclosed in the frame so that one layer can be rotated with respect to the other for magnetic activation of the chuck. Usually one face of one chuck layer will be the working face of the chuck to hold workpieces, and means will be provided as part of the frame for rotation of the other chuck layer. A single two complementary layer chuck assembly of suitable size and shape can be used as a magnetic chuck for holding workpieces. Alternatively, a plurality of individually-actuated, two layer chuck assemblies of this invention can be incorporated in a frame suitable for handling a workpieces of varying sizes and shapes. In this embodiment of the invention, individual chuck elements in a chuck frame can be adapted for a workpiece by selective activation of the magnet elements forming a footprint of the workpiece.  
         [0011]     Other objects and advantages of the invention will be apparent from a more detailed description of preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an oblique view of one layer of a magnetic chuck embodiment comprising four rectangular permanent magnet plates arranged on their side edges as four arms of a cross and separating four identical square soft magnet blocks. The lower right-hand soft magnet block is pulled aside in an exploded position to better reveal the relatively thin permanent magnet pieces between the other three soft magnet blocks.  
         [0013]      FIG. 2A  is an oblique view of two square chuck layers, each layer containing four permanent magnet plates and four square soft magnet blocks as illustrated in  FIG. 1 . In  FIG. 2A  the layers are positioned so that the chuck is in a magnetically inactive state for holding ferromagnetic workpieces against a working surface of the chuck.  
         [0014]      FIG. 2B  is an oblique view of two chuck layers like that of  FIG. 2A  with the upper chuck layer rotated 90° from its position in  FIG. 2A  so that the two chuck layers are in a magnetically active state for holding ferromagnetic workpieces.  
         [0015]      FIG. 3A  is an oblique view of two round chuck layers (disks) illustrating another embodiment of this invention. Each chuck layer includes six radially positioned permanent magnet plates separated by six arcuately shaped (pie shaped) soft magnet blocks with alternating north-south magnetic poles. In  FIG. 3A  the layers are positioned so that the chuck is in a magnetically inactive state for holding ferromagnetic workpieces against a working surface of the chuck.  
         [0016]      FIG. 3B  is an oblique view of two chuck layers like that of  FIG. 3A  with the upper six permanent magnet plates and six soft magnet block chuck layer rotated 60° from its position in  FIG. 3A  so that the two chuck layers are in a magnetically active state for holding ferromagnetic workpieces.  
         [0017]      FIG. 4A  is a plan view of a round permanent magnet/soft magnet chuck layer assembly in a square non-magnetic frame, the combination formed for compact rotation of the layer in an assembled two layer chuck between magnetically active and magnetically inactive positions.  
         [0018]      FIG. 4B  is a plan view of a round magnet chuck layer assembly in a hexagonal non-magnetic frame, the combination adapted for compact rotation of the layer in an assembled two layer chuck between magnetically active and magnetically inactive positions.  
         [0019]      FIG. 5  is an oblique view, partly broken away and in cross-section, of an assembled six soft magnet block per chuck layer, two layer permanent magnet chuck.  
         [0020]      FIG. 6  is a view of the front face of a magnetic chuck comprising a frame with a planar array of twenty-five modular magnetic chuck members.  
         [0021]      FIG. 7  is a view of the back face of a multi-piece modular magnetic chuck of  FIG. 6  with a few modular chucks removed. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]     This invention provides a design for a compact, modular, two layer permanent magnet chuck. The arrangement of relatively thin permanent magnet pieces between larger soft magnet blocks in cooperatively matching layers of the chuck results in a compact device that provides a high magnetic flux per unit volume of the chuck. In other words, lower or smaller chucks can be used in a workpiece lifting or holding application.  
         [0023]     The modular chuck comprises two geometrically similar or identical layers of identical or interchangeable permanent magnet and soft magnet members.  FIGS. 1 and 2 A and  2 B illustrate an embodiment of the invention in which four identical square soft magnet blocks are used in each layer of the chuck assembly. In  FIG. 1 , one clutch layer  10  comprises four identical square soft magnet blocks  12 ,  14 ,  16  and  18  with flat top  20  and bottom  22  surfaces (indicted only on block  12  for simplicity of illustration). As stated above, soft magnet block  18  is pulled out of its position in clutch layer  10  to better illustrate the permanent magnet and non-magnetic clutch elements between the four soft magnet blocks. Soft magnet blocks  12 ,  14 ,  16 , and  18  are made of soft ferromagnetic material such as compacted iron powder, alloys of iron-silicon, nickel-iron, or soft ferrite material.  
         [0024]     Four rectangular permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″ are positioned like the arms of a cross (or like radii) between soft magnet blocks  12 ,  14 ,  16 , and  18 . For example, permanent magnet plate  28  lies between side face  24  of soft magnet block  12  and side face  26  of soft magnet block  18 . The permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″ are identical in this example, but are separately identified because of the positioning of their magnetic poles in clutch layer  10 . Each permanent magnet plate  28 , etc. is magnetized with N-S poles on its major rectangular faces as illustrated in  FIG. 1  with respect to permanent magnet plate  28 ′″. One end of each plate is positioned toward the center of rotation  32  of clutch layer  10 . A bar (or rod)  34  of non-magnetic material separates the central ends of permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″. Four bars  36  of non-magnetic material are placed respectively at the bottom edges of the four rectangular permanent magnet plates  28 , etc, to space the permanent magnet plates in clutch layer  10  from like permanent magnet plates in a facing clutch layer (not shown in  FIG. 1 ) in an assembled magnetic chuck. Permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″ are made of any suitable permanent magnetic material such as Fe—Nd—B, hard ferrites, rare earth element-cobalt compositions or the like.  
         [0025]     Permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″ are arranged in clutch layer  10  so that their N-S magnetic poles cooperate in magnetizing soft magnet blocks  12 ,  14 ,  16 , and  18  with alternating magnetic polarities (N or S) as indicated in  FIG. 1  (and in each of the drawing figures). Permanent magnet plates  28  and  28 ′″ are positioned with their S poles facing soft magnet block  18 . The N magnetic poles of permanent magnet plates  28  and  28 ′ make soft magnet block  12  of N polarity. The S magnetic poles of permanent magnet plates  28 ′ and  28 ″ make soft magnet block  14  of S polarity. And the N magnetic poles of the faces of permanent magnet plates  28 ″ and  28 ′″ render soft magnet block  16  of N polarity. Thus, in single chuck layer  10 , the direction of magnetic flux lines is as indicated by the four arrows on the upper face of clutch layer  10 . The magnetic flux resulting from the respective polarities of rectangular permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″ is effectively concentrated in soft magnet blocks  12 ,  14 ,  16 , and  18  and results in their alternating magnetic polarities indicated on their upper surfaces in  FIG. 1 .  
         [0026]     The respective sizes of permanent magnet plates  28 ,  28 ′,  28 ″, and  28 ′″ and soft magnet blocks  12 ,  14 ,  16 , and  18  in plan view and height are determined by the designer of the modular chuck based on the required working surface area and magnetic attraction force of the chuck in its intended workpiece-holding application.  
         [0027]      FIGS. 2A and 2B  show two-layer chuck assemblies including upper chuck layer  40  and lower chuck layer  42 . Chuck layers  40 ,  42  each include four relatively thin, rectangular permanent magnet plates  28  extending outwardly from a central bar  34  of non-magnetic material. The four mutually perpendicular permanent magnetic plates separate four square soft magnet blocks in each layer which are identified in  FIGS. 2A and 2B  only by their alternating magnetic polarities (N-S-N-S) for simplicity of illustration. As described in reference to  FIG. 1 , the respective alternating magnetic polarities of the soft magnet blocks are the result of the alternating N-S magnetic polarities through the thicknesses of the permanent magnet plates  28 .  
         [0028]     When four-soft magnet block chuck layers  40 ,  42  are placed on top of each other such that overlying/underlying soft magnet blocks are of opposite N-S magnetic polarity as indicated in  FIG. 2A , the resulting magnetic flux lines are close-circuited among the magnetically soft, iron blocks so that the chuck as a whole (layers  40 ,  42 ) is completely balanced and neutralized magnetically. The close-circuited magnetic flux line pattern  44  is illustrated below chuck layers  40 ,  42  as part of  FIG. 2A . In this orientation of chuck layers  40 ,  42 , the assembled chuck is now in the “magnetically inactive” state and it will not exert significant attractive force on an external ferromagnetic object.  
         [0029]     However, if top chuck layer  40  is rotated 90° such that overlying/underlying soft magnet blocks are of the same polarities, N-N and S-S, of the two layers are now aligned as illustrated in  FIG. 2B , the magnetic circuits of clutch layers  40 ,  42  are incomplete and remain open. The magnetic flux pattern  46  of the rotated chuck layer assembly is illustrated in  FIG. 2B . The magnetic chuck, comprising chuck layers  40 ,  42 , is in the “magnetically active” state and will need to attach an external object to complete the flux loop. Because of the particular arrangement of the permanent magnet plates  28  and the large volume of soft iron layers N-S-N-S packaged in this invention, the magnitude of the attractive magnetic force generated is much higher than anything that has so far been developed. With the present invention, the overall height of the chuck is comparatively much lower than conventional ones.  
         [0030]     As a further illustrative embodiment of the invention, a round (or, optionally, hexagonal) clutch layer arrangement  50 ,  52  for a set of six (6) radially aligned rectangular permanent magnet plates  28  per layer is illustrated in  FIGS. 3A and 3B . For simplicity of illustration, the identical pie shaped soft magnet blocks are designated by the magnetic polarity induced in them by the alternating poles of permanent magnet plates  28 . Each soft magnet block N, S, N, S, N, and S has a center edge located at the center of rotation of the layer. A rod of non-magnetic material  34  separates the central edges of the six rectangular permanent magnet blocks  28 . The radially extending side walls of the soft magnet blocks N, S, N, S, N, and S are separated by permanent magnet plates  28 , and the outer walls of the soft magnet blocks N, S, N, S, N, and S are arcuately shaped, spanning 60° of arc. Radially extending bodies  36  of non-magnetic material separate the lower edges of permanent magnet plates  28  in clutch layer  50  from matching permanent magnet plates  28  in facing clutch layer  52 . The non-magnetic material bodies  34 ,  36  minimize the passage of magnetic flux lines directly between adjacent permanent magnet plates  28 , but force the flux through adjacent soft magnet blocks. As a result of the alternating arrangement of the magnetic poles of the six permanent magnet blocks  28 , the soft iron blocks N, S, N, S, N, and S assume the magnetic polarities imposed by the permanent magnet plates  28  at their radial sides. Thus, the magnetic state of the soft magnet blocks in chuck layers  50 ,  52  is as indicated in  FIG. 3A .  
         [0031]     When two six-magnetic block, round chuck layers  50 ,  52  are placed on top of each other with soft magnet blocks of opposite N-S polarities aligned as indicated in  FIG. 3A , magnetic flux lines are close-circuited among chuck layers  50 ,  52  so that the chuck as a whole is completely balanced and neutralized magnetically. The chuck is now in the “inactive” state and it will not exert attractive force on an external ferromagnetic object. However, if one magnet block layer, for example, the top chuck layer  50  is rotated only 60° such that soft magnet blocks of the same polarities N-N and S-S of the two layers are now aligned with each other ( FIG. 3B ), the magnetic circuits are incomplete and remain open. The magnetic chuck, comprising permanent magnet block chuck layers  50 ,  52 , is in the “active” state and will need to attach an external object to complete the flux loop.  
         [0032]     In principle, the two chuck layer concept can be extended to sets of 8, 10, 12, or more permanent magnet arrangements for very much larger and stronger chucks. The chuck with two sets of four magnets can be designed to form a square base while the chuck with two sets of six magnets can be designed to form a hexagonal base.  
         [0033]      FIGS. 3A and 3B  illustrate a magnetic chuck with round layers which are convenient for rotation of one layer between magnetically active and inactive positions within the plan view of the chuck. However, it may be preferred to have a magnetic chuck including a non-magnetic frame in which the overall plan-view shape of the chuck is not necessarily round. For example,  FIGS. 4A and 4B , respectively, illustrate square and hexagonal chuck frame configurations that are adapted to hold round permanent magnet plate/soft magnet block assemblies. Sometimes it is desirable to have a chuck frame that is not round for placing or handling of the chuck while having a round magnet assembly for ease of rotation between chuck active and inactive positions.  
         [0034]      FIG. 4A  illustrates chuck layer  60  with a square non-magnetic (for example, brass) frame  74  enclosing a round assembly of four soft magnet blocks with interposed permanent magnet plates.  FIG. 4B  illustrates a chuck layer  80  with a hexagonal non-magnetic frame  98  enclosing a round assembly of six soft magnet blocks and interposed permanent magnet plates. Only the round magnet assemblies of the respective chuck layers need be rotated within the stationary non-magnetic frames of any suitable plan view shape.  
         [0035]     In the  FIG. 4A  plan view, square chuck layer  60  includes a round assembly of four rectangular permanent magnet plates  70  between four pie-shaped soft magnet blocks  62 ,  64 ,  66 , and  68 . Permanent magnet plates  70  are placed on their side edges and in the configuration of a cross. The central edges of outwardly extending permanent magnet plates  70  are separated by a rod-shaped body  72  of non-magnetic material. Permanent magnet plates  70  are magnetized in the direction between their major faces and positioned in chuck layer  60  to induce alternating magnetic polarities in soft magnet blocks  62 ,  64 ,  66 , and  68 . Arrows show the direction of magnetic flux between the alternating N-S-N-S polarity of soft magnet blocks  62 ,  64 ,  66 , and  68 .  
         [0036]     The round assembly of permanent magnet plates  70  and soft magnet blocks  62 ,  64 ,  66 , and  68  is enclosed within non-magnetic frame  74  which has a square perimeter for placement or handling in a work environment for chuck layer  60 .  
         [0037]      FIG. 4B  illustrates, in plan view, a hexagonal chuck layer  80  made of six identical pie shaped soft magnet blocks  82 ,  84 ,  86 ,  88 ,  90  and  92 . The respective soft magnet blocks are arranged side-by-side with central edges separated by a body  96  of non-magnetic material. Radially extending, facing sides of soft magnet blocks  82 - 92  are separated by radially extending permanent magnet plates  94 . The positioning of magnetized permanent magnet plates  94  induces alternating S-N-S-N-S-N polarities, respectively in soft magnet blocks  82 ,  84 ,  86 ,  88 ,  90  and  92  as indicated by the arrows in  FIG. 4B . The round assembly of permanent magnet plates  94  and soft magnet blocks  82 - 92  is confined within non-magnetic frame  98 . In this illustration frame  98  has a hexagonal perimeter. Thus, the inner round assembly of six soft magnet blocks and interposed permanent magnet plates is rotated 60° to convert a two layer chuck from an inactive position to a magnetically active position, or vice versa.  
         [0038]      FIG. 5  illustrates an assembled, two-layer, permanent magnet chuck  110  of the six permanent magnet plate, six pie-shaped soft magnet block, circular (disk) chuck layer embodiment as described with respect to  FIGS. 3A, 3B , and  4 B.  FIG. 5  is a sectional view. The assembled permanent magnet chuck  110  has two circular chuck layers, upper chuck layer  112  and lower chuck layer  114 . Chuck layers  112  and  114  are supported in a brass (non-magnetic) chuck frame that includes a hexagonal top  116  and a side wall  118  that has a hexagonal periphery with a round internal surface to receive round chuck layers  112  and  114 . Top  116  is suitably bolted to side wall  118  through bolt holes  120 . Top  116  may also have bolt holes  122  for eye-bolts for lifting of magnet chuck  110 .  
         [0039]     Upper chuck layer  112  is adapted to be rotated, as will be described, between magnetically activated and magnetically in-activated positions of chuck  110 . Lower chuck layer  114  is fixed stationary within side wall  118  of the frame of chuck  110 . Upper chuck layer  112  has six pie-shaped soft magnet (iron or low alloy steel) blocks  124  and six interposed permanent magnet plates  126 , although only a few of the plates and blocks are visible in the  FIG. 5  sectional view. Permanent magnet plates  126  are suitably formed of an iron-neodymium-boron composition and magnetized through the thickness of the plate as described above, to induce alternating magnetic polarities in the six soft magnet blocks  124 . Each pair of one soft magnet block  124  and adjacent permanent magnet plate nominally spans about 60° of the circumference of chuck layer  112 .  
         [0040]     Lower chuck layer  114  also has six pie-shaped soft magnet blocks  128  and six interposed permanent magnet plates  130 . Except for modifications for mechanical attachment in their respective chuck layers  112 ,  114 , the soft magnet blocks  124 ,  128  in the two layers are of matching shape and composition. And the permanent magnet plates  126 ,  130  are likewise matching in shape and performance.  
         [0041]     Upper chuck layer  112  also includes non-magnetic bars or ribs  132  fixed at one end to a non-magnetic, rotatable vertical hub  136 . These non-magnetic components are suitably made of brass. Six nonmagnetic bars  132  extend radially in chuck layer  112  from hub  136  to the circumference of the chuck layer  112  and underlie a permanent magnet plate  126 , separating that plate  126  from a matching permanent magnet plate  130  in the lower chuck layer  114 . Hub  136  contains six vertical slots to receive inward ends of permanent magnet plates  126 . In addition to supporting permanent magnet plates  126 , non-magnetic bars  132  prevent magnetic flux from magnet plates  126  from directly combining with magnetic flux from permanent magnet plates  130 . It is preferred that the flux from the respective permanent magnet plates  126 ,  130  be directed into the flux enhancing soft magnet blocks  124 ,  128 .  
         [0042]     Lower chuck layer  114  also has a central non-magnetic hub  138 , but hub  138  is not adapted for rotation in this example. Fixed to hub  138  and extending radially outwardly at 60° angles are six non-magnetic bars or ribs  140 . Each of bars  140  lies under a permanent magnet block  130 . Bars  140  are secured at their outer ends to frame side wall  118  and prevent rotation of soft magnet bodies  128 .  
         [0043]     As viewed in  FIG. 5 , the lower surface  142  of chuck  110  is the non-rotating working face of the chuck against which workpieces are to be held. Surface  142  is enclosed within frame sidewall  118  and includes the bottom surfaces of non-magnetic bars  140  and the bottom surfaces of the six pie-shaped soft magnet blocks  128  through which the magnetic flux of the chuck is directed.  
         [0044]     Upper chuck layer  112  is adapted to be rotated with respect to lower chuck layer  114  to activate chuck  110 . The relative rotation of round six member permanent magnet plate-soft magnet block chuck layers is as illustrated in  FIGS. 3A, 3B , and  4 B. Hub  134  of upper chuck layer  112  is connected to a round chuck layer rotor  144 . Rotor  144  is received in a suitable cavity formed in the inner surface of chuck frame top  116 . Rotor journal  146  is positioned in a center hole in frame top  116 . Fixed to the top of journal  146  is a hexagonal lug nut head  148  for gripping with a wrench or other tool or means for rotating chuck layer  112  through an angle of 60° with respect to lower chuck layer  114 . Boltholes  150  are provided at the periphery of rotor  144  for attachment to soft magnet blocks  124  in chuck layer  112 . Index holes  152  in frame top  116  may be used with springs and studs (not shown) in holes  154  in rotor  144  for controlling precise positioning of chuck layer  112 . Thus, chuck layer  112  with its central hub  134  and attached rotor  144 , rotor journal  146  and lug nut  148  are movable elements of chuck  110 . As stated, the effect of the relative rotation of the chuck layers is as described with respect to  FIGS. 3A and 3B .  
         [0045]     By way of illustration and not limitation of the invention, the physical characteristics a chuck like that illustrated in  FIG. 5  are described. The dimension across opposing hexagonal sides of the frame was 5.8 inches and the height from working surface to frame top was 2.6 inches. Each chuck layer had six rectangular commercial Fe—Nd—B permanent magnets 2.25 inches long, 0.8 inch high, and 0.2 inch thick. The magnets were magnetized with the N pole on one major rectangular surface and the S pole on the opposite rectangular surface. The outer diameter of each pair of opposing pie-shaped, soft magnet steel blocks was 5.3 inches. When the chuck layers were in their magnetically active positions the holding force of the chuck for a ferromagnetic workpiece was 3,200 pounds force (14.2 kN).  
         [0046]      FIG. 6  shows the front face, the workpiece holding surface, of a magnetic chuck  200  comprising a plurality of modular magnetic chuck members. Magnetic chuck  200  comprises a unitary square frame  202  with four sides  204  and orthogonal vertical and horizontal ribs  206  for holding a 5 by 5 array of twenty-five identical modular magnetic chuck members  208 . Frame  202  is suitably made of steel or other strong material for mechanized handling of the multi-piece chuck  200 .  
         [0047]     In this embodiment of the invention, each modular magnetic chuck member  208  has a square non-magnetic frame  210  that is removably secured within sides  204  and/or ribs  206  of unitary chuck frame  202 . Within each non-magnetic frame  210  are four permanent magnet plates  212  radiating outwardly from a non-magnetic centerpiece  214 . Four identical pie-shaped blocks  216  of soft magnetic material separate permanent magnet plates  212 . Soft magnet blocks  216 , permanent magnet plates  212  and non-magnetic centerpiece  214  form the upper chuck layer of each of the twenty-five modular magnet members  208 . The permanent magnet plates  212  are magnetized through their thickness and arranged as described with respect to  FIG. 4A  to impose alternating N-S-N-S magnetic polarities in soft magnet blocks  216 . In this example, the upper chuck layers are the non-rotating layers of the modular magnet members  208 . The modular magnet members  208  are secured in chuck frame  202  by pins, rods, set screws, or the like, not shown in these drawing figures.  
         [0048]     Magnetic chuck  200  may be adapted for holding a particular workpiece by magnetically activating some or all of its twenty-five individual modular chuck magnet members  208 . Or, as is illustrated in  FIG. 7 , some of the modular chuck magnet members  208  may be removed from chuck fame  202  for receiving the workpiece.  
         [0049]      FIG. 7  shows the rear face of multi-piece modular chuck  200 . In addition to showing the rear face of chuck  200 , the chuck is illustrated with seven of its modular magnet members removed. The remaining modular magnet members  208  each have a non-magnetic frame  210  with the four permanent magnet plates  218  and four pie-shaped soft magnet blocks  220  of their rotatable backside chuck layers. The permanent magnet plates  218  are magnetized through their thickness and arranged as described with respect to  FIG. 4A  to impose alternating N-S-N-S magnetic polarities in soft magnet blocks  220 . The backside chuck layer of each modular chuck magnet member  208  is rotated between chuck magnetically active and inactive positions by mechanical engagement and rotation of center-nut  222  which is attached to a non-magnetic central rod, hidden below center-nut  222  in  FIG. 7 .  
         [0050]     Thus, multi-magnet piece chuck  200  uses many individual magnetic chuck members  208  of this invention to adapt a chuck holding surface to workpieces of different contacting surface shapes and areas. Each magnetic chuck member  208  in the chuck frame  202  may be individually activated or removed to obtain a desired workpiece holding pattern on the upper, working surface of the chuck  200 .  
         [0051]     Practices of this invention of magnetic chucks have been illustrated in descriptions of preferred embodiments. However, the scope of the invention is not limited to the illustrated embodiments.