Abstract:
The design of permanent magnet chucks of desired specified attractive force at the base or working face of the chuck is expedited by the discovery of a general correlation between two specific engineering parameters. The attractive force intensity expressed as force per unit chuck working face contact area correlates with the ratio of area of the iron block surfaces at the working face to the area of the permanent magnet surfaces contacting the iron blocks and providing the magnetic flux path to them.

Description:
This is a continuation-in-part of co-pending application Ser. No. 11/106,333, filed Apr. 14, 2005, entitled “Modular Permanent Magnet Chuck” and assigned to the assignee of this invention. 
    
    
     TECHNICAL FIELD 
     This invention pertains to the design of permanent magnet chucks of the type having permanent magnet plates interposed with magnetically soft ferrous blocks. Magnetic chucks are designed with a base or working face for attracting workpieces with a specified magnetic force. This invention provides a fast and accurate method for estimating the attractive force obtainable by a design for a chuck before constructing the chuck. 
     BACKGROUND OF THE INVENTION 
     Strong permanent magnets are useful in manufacturing operations requiring reconfigurable holding and transporting devices for workpieces. For example, manufacturing facilities for high volume vehicle engine production require many pallets for moving engine components and assemblies between machining stations. Magnetic chucks can be used to reconfigure the pallets for different engines. Of course, chucks are also used for holding workpieces in a fixed position during machining. Compact permanent magnet chucks that can be conveniently turned on and off provide a means of providing reconfigurable fixtures for making different engines or other workpieces on the same manufacturing line. 
     The text and drawings of the specification of the parent application describe a very compact, efficient, and useful arrangement of permanent magnet plates and magnetically soft bodies for a magnetic chuck. The magnetic chuck has two substantially identical layers of an even number of alternating permanent magnet plates and interposed soft magnet bodies. The top surfaces of the two layers are sufficiently alike in the plan views of the permanent and soft magnet elements that one layer can be rotated through a small angle (usually no more than 90°, depending on the number of magnets) from an magnetically inactive position with like overlying shapes to an active position with like overlying shapes. In the active position of the chuck the magnetic field extends above the working face of the chuck and a strong attracting force is exerted on a ferromagnetic workpiece(s) to secure it against the chuck base or working surface. 
     A specific chuck or group of chucks must be designed to have a working face of specified shape and area and to exert a specified attractive force on a workpiece. It has been difficult to estimate the attractive force obtainable from a new design without building the chuck. 
     A commercial computer simulation software package for three-dimensional magnetostatics design is available. This software contains a CAD three-dimensional modeler, a material database, a Solver for computational algorithms of the magnetostatics field, and a graphical display capable of presenting the computational results in various parametric forms and solid modeling views. The primary goal of using such magnetostatics software is to shorten the time in the development and validation of any new magnetic chuck concept by not having to actually to build the unit. Still, it is found that the total time required in modeling the various sub-components and their engineering features in a proposed chuck design and in running the Solver to compute the expected magnetic attractive force takes one to three days. This invention provides a method for accurately estimating the strength of the magnetic attraction force given the magnetic properties of the permanent magnet plates and the soft magnet blocks, the clutch face area of the soft magnet blocks, and the contacting area of the sides of the interposed permanent magnet bodies that induce the magnetic field in the enclosing soft magnets. 
     SUMMARY OF THE INVENTION 
     In many magnetic chuck designs, including the designs disclosed in the parent of this application, one or more permanent magnet bodies are positioned to direct magnetic flux into a one or more magnetically soft bodies (of, for example, steel or iron). Combinations of permanent magnet bodies and soft magnet bodies are arranged to form a closed magnetic flux loop and to obtain a specified holding force. The soft magnet bodies confine, concentrate, and direct the magnetic flux lines produced by the permanent magnet bodies. In the activated position of these magnet chuck elements, a strong magnetic flux field extends from a working surface of the soft magnet material, enters the workpiece(s), and returns through a suitable flux return path to the permanent magnet source(s). The loop of magnetic flux exerts an attractive force on the workpiece(s) holding it against the face of the chuck 
     The permanent magnets may, for example, be suitably shaped blocks or plates of magnetized iron-neodymium-boron composition. The soft magnet bodies are typically formed of ferromagnetic ferrous compositions such as soft irons, or alloys of iron-silicon, nickel-iron, and soft ferrites. 
     An examination of many such magnet chucks has revealed to this inventor that the force due to magnetic attraction at a working face area of a soft magnet block is related to the total surface area of permanent magnet bodies introducing magnetic flux into the soft magnet body. A plot of magnetic force intensity in newtons per square millimeter (N/mm 2 ) of working face area versus the dimensionless ratio of cross sections of iron and Fe—Nd—B permanent magnets is like that of  FIG. 5  of this specification. A typical 4-parameter algebraic expression for the  FIG. 5  curve is: 
             F   =       a   +   bx       1   +   cx   +     dx   2               
where F is force per unit area, x is the ratio of cross sections of iron and permanent magnets, a=4.57, b=−1.45, c=4.37, and d=−0.19.
 
     This relationship of force intensity and magnet areas is used as follows. Once the general arrangement of the iron and permanent magnets and their rough dimensions have been described, the cross sections of the iron and the permanent magnets can be quickly established. Using  FIG. 5  of this specification or the above mathmatical expression, the force intensity is determined in, for example, N/mm 2 . The total attractive force created by the magnetic chuck is obtained by multiplying the force intensity by the chuck base area. The whole calculation takes only a few minutes and the proposed chuck design can be decided as favorable to move into the detailed design phase if the force magnitude is deemed large enough. Otherwise, design changes in either the geometric configuration or the iron/magnet dimensions can be made and another force estimate is performed with the invention methodology. Hence, the design iterations can be accomplished in a very short time period as opposed to weeks and months using just the computer modeling and simulation software. 
     The data summarized in graphical form in  FIG. 5  was obtained from an analysis of a number of different magnet chucks in which the permanent magnets were iron-neodymium-boron magnets of about the same magnetic properties and the magnetically soft materials were low carbon steel blocks. The shapes and arrangements of the permanent and soft magnet bodies differed substantially. But a correlation like that formulated in  FIG. 5 , and converted by regression analysis to the above equation or the like, can be used as described in this specification in determining the magnetic force of a chuck of a different arrangement of permanent and soft magnet bodies when the respective bodies have the same magnetic properties. 
     Other objects and advantages of the invention will become apparent from a detailed description of preferred embodiments which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an oblique view of two round chuck layers (disks) for illustrating an 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. 1  the two chuck layers are positioned so that the chuck is in a magnetically active state for holding ferromagnetic workpieces against a working surface of the chuck. 
         FIG. 2  is an exploded view of isolated elements of the  FIG. 1  hexagonal chuck arrangement illustrating how magnetic flux emanating from the magnetically north pole (N) sides of four permanent magnet plates enters two pie shaped magnetically soft steel blocks and is directed to and concentrated at the upper surface of the upper steel block at the working surface of the chuck. 
         FIG. 3  is a plan view of the round magnet chuck layer assembly of  FIG. 1  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. 
         FIG. 4  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. 
         FIG. 5  is a graph of magnetic force intensity in newtons per square millimeter (N/mm 2 ) of magnet chuck face area versus the ratio of iron cross-sectional area to permanent magnet cross-sectional area. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A practice of the invention will be illustrated with respect to magnetic chuck designs described in the above identified parent application of this case. The text and drawings of the parent application are incorporated by reference into this specification for their disclosures of their magnetic chuck constructions. However, the invention may be practiced with other permanent magnet chuck designs using a combination of permanent magnet bodies and magnetically soft bodies. 
     Further, the practice of the invention uses data obtained by taking apart existing magnet chucks containing permanent magnet parts (in this case Fe—Nd—B magnets) and magnetically soft iron (steel) parts. The sizes, shapes, and arrangements of the magnetically soft and hard magnets in the various chucks differed. However, it was observed that a relationship existed between the areas of specific surfaces of the permanent magnets and the iron magnets that could be used to significantly shorten the time required in estimating the attractive force to be realized in a magnet chuck design that has not yet been constructed. Exemplary data is summarized in the curve of  FIG. 5  and in an algebraic expression that will be used in the practice of this invention. 
     A preferred permanent magnet chuck employs two identical sets of permanent magnets; one set is positioned directly on top of the other set. The permanent magnets can be made from any magnetically hard ferromagnetic materials but the rare earth-containing neodymium-iron-boron (Nd—Fe—B) material is the preferred choice because of its high magnetic property values. For example, commercially available Nd—Fe—B peermanent magnets display values of residual induction (Br) in the range of 13,800 to 14,700 Gauss, values of coercivity (iHc) in the range of 11,000 to 14,000 Oersteds, and values of coercivity (bHc) in the range of 10,300 to 13,100. 
     Inserted between the permanent magnet bodies are magnetically soft iron blocks or other soft magnetic materials. Brass or other non-magnetic materials are used as spacer and container materials in the chuck design to permit the formation of a closed loop of magnetic flux for the function of the chuck. 
       FIG. 1  shows permanent magnet chuck  10  formed of two round disc-like layers  12 ,  14 . Magnet chuck layers  12 ,  14  each have flat upper and lower surfaces and are preferably of the same size, shape and construction. Each of magnet chuck layers  12  and  14  includes a non-magnetic brass center rod  16 , six generally rectangular permanent magnet plates  18  disposed vertically and radially at sixty degree angles of arc from brass center rod  16 , and six pie-shaped magnetically soft iron or steel blocks  20 . The opposing major rectangular faces of each permanent magnet plate  18  lies aganst a radial face of an adjacent iron block  20 . The permanent magnet plates  18  are magnetized with north (N) and south (S) magnetic poles on their opposing large area rectangular faces so that their N-S magnetic axes extend through their relatively small thicknesses. The six permanent magnet plates  18  are arranged so that the radial side of each magnetically soft iron block  20  is contacted with a permanent magnet surface of the same polarity, N or S. Thus, the N or S designations on the upper side of each iron block  20  refers to the like polarity of the adjacent faces of the permanent magnet plates  18  lying against its radial faces. 
     Magnet chuck layers  12  and  14  may be oriented by rotation of one of the layers about the common central axis so that soft iron blocks (designated N or S) may be aligned N opposite N or N opposite S with respect to the opposing layer. When a magnetically soft iron block designated N of one chuck layer  12  lies opposite a block designated S of the other layer  14  the magnetic flux loop lies within the chuck layers  12 ,  14  and the chuck  10  is in its inactive state. However, as viewed in  FIG. 1 , the magnet chuck layers  12  and  14  are aligned so that iron blocks  20  designated N in the top layer  12  overlie an iron block of like designation N in the lower layer  14  and the chuck  10  is then magnetically activated. As seen in the lower portion of  FIG. 1  the magnetic flux (illustrated symbolically with six loops  26 ) extends above the upper surface of chuck layer  12  for attraction of a ferromagnetic workpiece, not shown. 
       FIG. 3  illustrates a plan view of magnetic chuck  10  showing the chuck layers confined in a hexagonal brass frame  22 . At least one of the chuck layers  12  and  14  are rotatable within frame  22  to switch the chuck  10  between magnetically inactive and magnetically active positions. In this illustration, the visible surface of upper chuck layer  12  in  FIG. 3  is the working face  24  of the chuck  10  for holding a workpiece. Working face  24  includes the upper surfaces of brass center rod  16 , six rectangular permanent magnet plates  18 , and six pie-shaped magnetically soft iron blocks  20 . But center rod  16  is non-magnetic. And, in this example, the polarity of permanent magnet plates  18  is oriented parallel to the working surface  24  so that the magnetic flux contribution of plates  18  to the working surface  24  is largely through the visible surfaces of iron blocks  20 . The three pairs of directional arrows seen in  FIG. 3  indicate the orientations of the magnetically north faces of permanent magnets  18  which result in alternate iron blocks  20  being designated as N. Of course, the opposite sides of the permanent magnets  18  are magnetically south faces, and the intevening pie-shaped iron blocks are designated as S. 
     As an extension of the chuck design concept illustrated in  FIG. 1 , a similar arrangement for a set of four (4) magnets (sometimes called Quad arrangement) is presented in the parent application. The working principle for the Quad arrangement is basically the same as is illustrated in  FIGS. 1 and 3 , but the angle of rotation to activate (or deactivate) a Quad chuck is then 90°. The concept can be extended to sets of 8 (Octa), 10 (Deca), 12 (Dodeca), etc. . . . magnets arrangements for very much larger and stronger chucks. The chuck with two sets of 4 magnets can be designed to form a square base while the chucks with two sets of 8, 10, or 12 magnets can be designed to form the respective polygonal base or a round base. However, when such a chuck design is contemplated, it is usually necssary to predict or estimate the attractive force that the chuck can exert given the properties of the permanent magnet materials selected and the soft magnet materials selected. The procedure is summarized as follows with illustrative reference to the hexagonal (Hexa) permanent magnet chuck  10  illustrated in  FIGS. 1 ,  2 , and  3 . 
     Procedure for Establishing Design Parameters of a Magnetic Chuck 
     Specifics of the following procedure are directed to chuck designs of the parent application. 
     1. Assume a radius for the center brass cylinder and a thickness of the permanent magnets. 
     2. Using the following mathematical expression and for a matrix of permanent magnet plate (PM) lengths and heights, compute the different quantities: total soft iron chuck surface area, Fe/PM cross-section areas ratio, force intensity, and total force for quadragonal, hexagonal, or octagonal chuck arrangements. 
     
       
         
           
             F 
             = 
             
               
                 a 
                 + 
                 bx 
               
               
                 1 
                 + 
                 cx 
                 + 
                 
                   dx 
                   2 
                 
               
             
           
         
       
     
     where F is force per unit area, x is the ratio of cross sections of iron and permanent magnets, a=4.57, b=−1.45, c=4.37, and d=−0.19. These values are determined by a regression analysis of the data obtained by inspection and analysis of several Nd—Fe—B permanent magnet/magnetically soft iron magnetic chucks and summarized in  FIG. 5 . 
     3. Determine the range of the PM dimensions (height H &amp; length L) which would bracket the level of magnetic force to be produced. If need be, finer subdivisions of the H or L parameters can be assumed and their corresponding forces computed. 
     4. When a set of H&amp;L dimensions has been established, then a 3D CAD drawing including details of the various chuck design features such as outside housing, cover plate, bolt holes, lock pins, etc. can be modeled on a magnetostatics finite element software to calculate at an even higher accuracy the expected magnetic holding force. 
     In order to determine the attractive force of magnetic chuck  10  in accordance with this invention, it is necessary as one step to determine the effective cross-sectional area of permanent magnet plates  18  through which magnetic flux is being introduced to each working surface of magnetically soft iron blocks  20 . This determination is illustrated by referance to  FIG. 2 .  FIG. 2  shows two pie-shaped magnetically soft iron blocks  20 ′ and  20 ″ broken out from the view of chuck  10  in  FIG. 1 . So this illustrative analysis represents about one-sixth of the effective working surface of chuck  10 . 
     Top surface  30  of upper iron block  20 ′ is the surface through which magnetic flux (represented schematically by dashed lines  38 ) pass into a workpiece, not shown. The north magnetic poles of two permanent magnet plates  18  lie against radial side surfaces  32  of pie shaped upper iron block  20 ′ and two more permanent magnet plates  18  (N sides) lie against identical side surfaces  34  of lower iron block  20 ″. Thus, in the chuck embodiment of  FIG. 2 , magnetic flux (lines  38 , drawn from only two permanent magnet plates to simplify the illustration in  FIG. 2 ) from the N-pole sides of four permanent magnet plates  18  enters through their contacting sides  36  into the radial sides  32 ,  34  of two soft iron blocks  20 ′,  20 ″. But all of this magnetic flux passes through the upper surface  30  of upper soft iron block  20 ′. The total flux transmitting area for the working surface ( 24  in  FIG. 3 ) of chuck  10  is the sum of the areas of six identical soft iron block  20 ′ upper surfaces  30 . Thus, in this example, the value of x (which is a ratio obtainable by analysis of any of the six identical PM-soft iron combinations) is the area of the pie shaped surface  30  of block  20 ′ divided by four times the area of a face  36  of a permanent magnet  18 . The value of x is then used to determine the attractive force of a region of working surface  24  or the total surface. 
     Sample calculations for hexagonal chuck constructions as illustrated in  FIGS. 1–3  and their quad and octagonal variations are illustrated in the following computational table. In these calculations the radius of the brass center rod was 0.5000 inch, the thickness of each PM plate was 0.2500 inch, and the values of the regression coefficients were; a=4.5666, b=−1.4533, c=4.3664, and d=−0.1939. The values of the height (h) and length (L) of the PM were varied. In these embodiments, the length and height of the PM are also the facing dimensions of the soft iron body. The working areas of the soft iron bodies were determined from their shape and did not include the area of the brass center rod or of the upper surfaces of the PM plates. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Chuck 
                 PM height 
                 PM length 
                 Total Fe Area, 
                 Fe/PM 
                 F intensity, 
                 Total Force, 
               
               
                 Design 
                 (h), inch 
                 (L), inch 
                 sq. inch 
                 Ratio 
                 N/sq. mm. 
                 N 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Quad 
                 0.8 
                 1 
                 5.28 
                 0.41 
                 1.43 
                 4,883 
               
               
                   
                 0.8 
                 2 
                 16.85 
                 0.66 
                 0.95 
                 10,355 
               
               
                   
                 0.8 
                 3 
                 34.70 
                 0.90 
                 0.68 
                 15,214 
               
               
                   
                 0.8 
                 4 
                 58.83 
                 1.15 
                 0.50 
                 19,084 
               
               
                   
                 0.7 
                 1 
                 5.28 
                 0.47 
                 1.29 
                 4,385 
               
               
                   
                 0.7 
                 2 
                 16.85 
                 0.75 
                 0.83 
                 9,045 
               
               
                   
                 0.7 
                 3 
                 34.70 
                 1.03 
                 0.58 
                 12,944 
               
               
                   
                 0.7 
                 4 
                 58.83 
                 1.31 
                 0.42 
                 15,766 
               
               
                 Hexa 
                 0.8 
                 1 
                 4.78 
                 0.25 
                 2.03 
                 6,251 
               
               
                   
                 0.8 
                 2 
                 15.85 
                 0.41 
                 1.43 
                 14,648 
               
               
                   
                 0.8 
                 3 
                 33.20 
                 0.58 
                 1.08 
                 23,136 
               
               
                   
                 0.8 
                 4 
                 56.83 
                 0.74 
                 0.85 
                 31,032 
               
               
                   
                 0.7 
                 1 
                 4.78 
                 0.28 
                 1.86 
                 5,753 
               
               
                   
                 0.7 
                 2 
                 15.85 
                 0.47 
                 1.29 
                 13,156 
               
               
                   
                 0.7 
                 3 
                 33.20 
                 0.66 
                 0.95 
                 20,387 
               
               
                   
                 0.7 
                 4 
                 56.83 
                 0.85 
                 0.73 
                 26,872 
               
               
                 Octa 
                 0.8 
                 1 
                 4.28 
                 0.17 
                 2.51 
                 6,925 
               
               
                   
                 0.8 
                 2 
                 14.85 
                 0.29 
                 1.84 
                 17,649 
               
               
                   
                 0.8 
                 3 
                 31.70 
                 0.41 
                 1.43 
                 29,295 
               
               
                   
                 0.8 
                 4 
                 54.83 
                 0.54 
                 1.15 
                 40,828 
               
               
                   
                 0.7 
                 1 
                 4.28 
                 0.19 
                 2.35 
                 6,484 
               
               
                   
                 0.7 
                 2 
                 14.85 
                 0.33 
                 1.68 
                 16,131 
               
               
                   
                 0.7 
                 3 
                 31.70 
                 0.47 
                 1.29 
                 26,312 
               
               
                   
                 0.7 
                 4 
                 54.83 
                 0.61 
                 1.02 
                 36,140 
               
               
                   
               
             
          
         
       
     
     It is seen that the total force in newtons increases with the size and number of permanent magnets. These calculations based on the ratio of iron surface area to PM surface area, when correctly determined, provides a realtively easy and accurate way of estimating the attractive force of a permanent magnet based chuck. 
     While this estimation process enables the design of the magnetic components of the chuck, an illustration of a more complete chuck is provided in  FIG. 4 . 
       FIG. 4  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. 1–3 .  FIG. 4  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 . 
     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. 4  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 . 
     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. 
     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  134  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  134  contains six vertical slots  136  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 . 
     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 . 
     As viewed in  FIG. 4 , 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. 
     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 described with respect to  FIGS. 1 and 3 . 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 . Bolt holes  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  FIG. 3 . 
     By way of illustration and not limitation of the invention, the physical characteristics a chuck like that illustrated in  FIG. 4  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). 
     Variations of chuck  110  have been made without the non-ferrous bars  132  between the two magnetic layers  112  and  114 . This change simplifies the fabrication of soft magnet blocks  124  and does not affect the magnetic performance of the chuck  110 . In a further modification of chuck  110 , a thin round steel disc (5.3″ diameter x 0.010″ thick) was inserted between magnetic layers  112  and  114 . The disc did not affect the magnetic performance of the chuck, but the disc did serve as a separator between layers  112 ,  114  and facilitated their rotation. 
     While the practice of the invention has been described with respect to specific magnetic chuck structures the method of determining the attractive force of a chuck in not limited to the specific examples.