Magnetorheological fluids workpiece holding apparatus and method

A fixturing or workpiece holding and clamping apparatus or device, as well as method, utilizing the viscosity increase or solidification of a magnetorheological fluid work contacting medium to secure both regular and irregular shaped workpieces for precision machining or measuring operations. The apparatus or device comprises a perforated fixture into which the workpiece is placed in a desired position and orientation. The perforated fixture and positioned workpiece are placed in an open cell containing a magnetorheological fluid which conforms to a portion of the surface of the workpiece. A magnet then applies a magnetic field to the magnetorheological fluid to increase the viscosity thereof and to solidify the fluid around the workpiece with a uniform clamping pressure, securing the workpiece in the desired position and orientation for machining or measuring operations. A clamp configured to apply a compressive force to the solidified magnetorheological fluid optionally increases the uniform clamping pressure applied to the workpiece by compressing the solidified magnetorheological fluid to further increase viscosity and solidification thereof. The solidified magnetorheological fluid attenuates vibrations in the workpiece, and reverts to a liquid state for removal of the perforated fixture and workpiece upon removal of the compressive force and the magnetic field.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 Not Applicable.
 BACKGROUND OF THE INVENTION
 The present invention relates generally to a fixturing or workpiece holding
 and clamping device and method, and in particular, to a fixturing or
 workpiece holding and clamping device utilizing a viscosity increase or
 solidification of a magnetorheological fluid work contacting medium as a
 method to secure both regular and irregular shaped workpieces for
 precision machining or measuring operations.
 The securing of irregularly shaped workpieces, such as jet engine or
 turbine blades having an enlarged end projecting from an elongated and
 considerably thinner airfoil section, for machining operations such that
 it does not result in damage to the workpiece has been found to be
 difficult. Typical methods of clamping through clamps or fixtures are not
 practical since they can cause permanent damage to the workpiece. As a
 result, the traditional solution to prevent damage to these and other
 irregularly shaped workpieces include encapsulation by the casting of a
 low melting point molten matrix material such as lead or zinc around the
 thin irregularly shaped portion of the workpiece, such as the airfoil
 section, after which the machining or measuring of the enlarged end
 portion is performed, as is seen in U.S. Pat. No. 5,947,662 to Becker et
 al. for "System For Holding Thin-walled Workpiece During Machining."
 Generally, this procedure involves inserting the elongated thin portion of
 the workpiece into a cast iron block having a cavity which is
 significantly larger than the workpiece itself. The molten matrix material
 is then poured into the cavity, surrounding and encapsulating the
 workpiece. After the matrix material cools and solidifies, the workpiece
 is secured in a fixed position for the machining or measuring operations.
 Upon completion of the machining or measuring operation, the matrix
 material is melted away from the thin irregularly shaped portion of the
 workpiece, leaving a finished product. This procedure, however, adds
 considerably to the expense of producing such workpieces, increases health
 and environmental risks associated with the vapors released from the
 molten matrix material, and fails to adequately protect the workpiece
 against deformation damage during the machining operations. Furthermore,
 such solutions cannot be applied to workpieces which are vulnerable to
 damage from the heating and cooling cycles associated with the addition
 and removal of the matrix material, or which have finished or treated
 surfaces which may become contaminated by residue from the molten matrix
 material.
 Other solutions to the problem of securing irregularly shaped workpieces
 include the use of complex single-purpose hydraulic clamping devices such
 as is shown in U.S. Pat. No. 4,033,569 to Dunn for "Deformation-Preventing
 Workpiece-Holding Fixture for Machine Tools." These devices are typically
 suitable for holding only a limited range of irregularly shaped objects,
 and operate by applying a plurality of clamping members to a number of
 locations on the surface of the workpiece. Application of a clamping force
 to only a limited number of locations along the surface of a workpiece
 while retaining it during the machining operations can result in the
 buildup of stress or damage in the workpiece from the non-uniform
 application of the clamping forces.
 A similar solution is exemplified by U.S. Pat. No. 3,818,646 to Peterson
 for "Fixture For Holding Precisely Shaped Parts" wherein an irregularly
 shaped workpieces, such as the thin elongated airfoil portion of a jet
 engine turbine blade, is secured for machining operations by a plurality
 of individual movable pins extending from the side wall of a clamping
 fixture to engage both the convex and concave surfaces of the workpiece.
 While increasing the number of individual movable pins extending from the
 side wall of the clamping fixture results in a more uniform application of
 clamping force to the irregularly shaped workpiece, this solution still
 fails to provide a completely uniform application of clamping force, and
 is limited to operation on workpieces having exterior surfaces with
 generally smooth curvature.
 An alternative solution which applies a more uniform clamping pressure to
 the surfaces of a regular or irregular workpiece involves the use of a dry
 particulate material fluidized by a pressure of gas injection means for
 insertion of a workpiece, which is then substantially solidified by the
 application of a vacuum force or magnetic field to the dry particulate
 material. Examples of these types of fixturing devices may be found in
 U.S. Pat. No. 3,953,013 to Griffith et aL for "Method and Apparatus for
 Clamping A Workpiece In A Quasi-liquid Medium" and U.S. Pat. No. 3,660,949
 to Cose, Jr., for "Work Holder For Irregular Shaped Workpieces." However,
 the use of a dry particulate fluidizable material or quasi-liquid requires
 a complicated variety of associated gas injection and vacuum generating
 elements, as well as containment for the dry particulate fluidizable
 material, since an excess of fluidizing pressure can easily expel the
 particulate material from the device.
 A second alternative solution for applying a more uniform clamping pressure
 to the surfaces of a regular or irregular workpiece involves the use of
 electrofluids which respond to the presence of either alternating electric
 fields or a voltage difference by manifesting an apparent change in bulk
 viscosity. It is known that if these fluids are applied as a film over a
 dielectric surface, and an alternating electric field is applied to the
 fluid from beneath the surface, a workpiece placed on or in the
 electrofluid film causes the electrofluid to be energized by the electric
 field to secure the workpiece firmly in place. These devices, exemplified
 by U.S. Pat. No. 3,197,682 to Klass et al. require the application of a
 high voltage and potentially dangerous, three-phase current to the device,
 and do not permit workpieces to be immersed in the electrofluid film to
 any great depth, thereby limiting the clamping pressure of the device.
 Furthermore, electrorheological fluids are temperature sensitive, and
 typically have an inability to withstand water contamination, rendering
 them useless in machining applications wherein a machining tool is cooled
 by the application of water or other water-based liquid coolant to an
 exposed cutting surface.
 Accordingly, there is a need in the industry for a self-contained fixturing
 or workpiece holding and clamping apparatus or device and method capable
 of securing both regular and irregularly shaped workpieces, such as jet
 engine turbine blades, for machining operations with a uniform clamping
 force so as to reduce the stresses associated with the machining
 operations on the workpiece, while also being easy to use, simple to
 construct, and which also eliminates the risk of environmental and
 workpiece contamination, as well as the risk to an operator's health from
 electric shock or the inhalation of harmful vapors or particles.
 It is believed that an apparatus and method for immobilizing and securing
 both regular and irregularly shaped workpieces through the solidification
 or viscosity increase of a magnetorheological fluid subjected to a
 magnetic field will solve many of the problems associated with traditional
 work holding fixtures. It is known that in the presence of an appropriate
 magnetic field, solid magnetizable particles in fluids such as mineral
 oil, silicone oil, or other suitable organic liquid move into alignment,
 forming fibrous structures parallel to the applied field, significantly
 increasing the viscosity of the fluids and substantially decreasing the
 ability of the fluid to flow or be sheared.
 A magnetizable carrier fluid or ferrofluid may be substituted for the
 mineral oil, silicone oil, or other fluid used as a carrier for the solid
 magnetizable particles in traditional magnetorheological fluids. While
 ferrofluids themselves do not solidify when subjected to an applied
 magnetic field, they similarly exhibit magnetic field-induced viscosity
 increases, and may be utilized to achieve yield stress levels
 significantly in excess of traditional magnetorheological fluids, as is
 taught by U.S. Pat. No. 5,549,837 to Ginder et al. for "Magnetic
 Fluid-Based Magnetorheological Fluids."
 The basis for the magnetorheological effect can be explained by the
 inter-particle forces induced by the applied magnetic field. When an
 external magnetic field is applied to an initially random arrangement of
 magnetizable particles, a magnetic moment which is approximately parallel
 to the applied field is induced in each particle. The force between two
 particles whose moments are aligned head-to-tail is attractive, promoting
 the formation of chains or more complicated networks of nearly contacting
 particles aligned along the direction of the field, significantly
 increasing the viscosity and essentially solidifying the fluid. The
 strength of this solidified magnetorheological fluid can be characterized
 by the yield sheer stress at which the network of aligned particles is
 disrupted and the particles flow. Fluids having a high yield stress can
 sustain larger mechanical forces when solidified in the presence of a
 magnetic field before flowing. Magnetorheological fluids easily obtain
 yield stress values in excess of 5 psi in the presence of a magnetic
 field, and may be prepared to achieve yield stresses on the order of 20
 psi as taught by U.S. Pat. No. 5,667,715 to Foister for
 "Magnetorheological Fluids." In general, for a magnetorheological fluid,
 it is known that an increase in the flux density of the magnetic field to
 which it is subjected will result in an increase in the yield stress, i.e.
 an increase in viscosity which in this context is understood to mean
 solidification.
 BRIEF SUMMARY OF THE INVENTION
 Among the several objects and advantages of the present invention are:
 The provision of a work holding apparatus or device utilizing an increase
 in viscosity or solidification of a magnetorheological fluid work
 contacting medium to secure a workpiece;
 The provision of the aforementioned work holding apparatus or device
 wherein the viscosity increase or solidification of the magnetorheological
 fluid work contacting medium is achieved by the application of a magnetic
 field to the magnetorheological fluid work contacting medium;
 The provision of the aforementioned work holding apparatus or device
 wherein the workpiece is further secured by the application of a clamping
 force to the solidified magnetorheological fluid work medium, further
 increasing the viscosity of the magnetorheological fluid;
 The provision of the aforementioned work holding apparatus or device
 wherein a decrease in viscosity of the magnetorheological fluid work
 contacting medium is achieved by the removal of the magnetic field;
 The provision of the aforementioned work holding apparatus or device
 wherein the workpiece may have either a regular or irregular shape;
 The provision of the aforementioned work holding apparatus or device
 wherein the magnetorheological fluid work contacting medium is contained
 within an open-faced container;
 The provision of the aforementioned work holding apparatus or device
 wherein the magnetorheological fluid work contacting medium is
 encapsulated in a deformable packet;
 The provision of the aforementioned work holding apparatus or device
 wherein the open-faced container is configured to absorb peak vibrational
 forces, preventing movement or climbing of the workpiece inside the work
 holding device;
 The provision of the aforementioned work holding apparatus or device
 wherein the workpiece is secured for measuring or machining operations by
 the solidification of the magnetorheological fluid work medium;
 The provision of the aforementioned work holding apparatus or device
 wherein the magnetorheological fluid work contacting medium attenuates
 vibrations induced in the workpiece by the machining operations;
 The provision of the aforementioned work holding apparatus or device
 wherein the magnetorheological fluid work contacting medium applies a
 uniform clamping force to the workpiece upon solidification;
 The provision of the aforementioned work holding apparatus or device
 wherein the apparatus or device is suited for use in securing heat
 sensitive and non-magnetic materials;
 The provision of the aforementioned work holding apparatus or device
 wherein the apparatus or device is suited for use in securing both
 metallic and non-metallic workpieces;
 The provision of the aforementioned work holding apparatus or device
 wherein the workpiece is not subjected to a heating and cooling cycle;
 The provision of the aforementioned work holding apparatus or device
 wherein the emission of harmful and environmentally damaging vapors or
 particulate matter is significantly reduced or eliminated;
 The provision of the aforementioned work holding apparatus or device
 wherein the apparatus or device requires no external power source;
 The provision of the aforementioned work holding apparatus or device
 wherein the apparatus or device requires no associated fluid pressure or
 vacuum delivery systems;
 The provision of the aforementioned work holding apparatus or device
 wherein the apparatus or device is readily adaptable to operate as a
 component in an assembly line manufacturing process; and
 The provision of the aforementioned work holding apparatus or device
 wherein the device is easy to assemble, simple to operate, and may be
 manufactured for a low cost.
 Briefly stated, the preferred embodiment of the work holding apparatus or
 device of the present invention utilizes a work contacting medium
 comprising a magnetorheological fluid and a specifically configured work
 holding container or fixture to secure a workpiece of either a regular or
 irregular shape for machining or measuring operations without damage to
 the workpiece. The work holding container or holding fixture comprises a
 open-faced container perforated by threaded holes within which a workpiece
 of either a regular or irregular shape may be placed. The workpiece is
 secured within the container by means of screws threaded through the
 threaded holes to contact the surface of the workpiece with minimal force.
 The perforated container or holding fixture is then positioned within a
 open cell containing either a liquid magnetorheological fluid work
 contacting medium which flows around the portion of the workpiece placed
 within the perforated container or deformable packets of encapsulated
 magnetorheological fluid which conform to the surfaces of the workpiece
 and the open cell. The cell is located in an adjustable gap of a magnet
 such that a magnetic field generated by either a permanent magnet or an
 electromagnet will pass through the cell. The cell is constructed from two
 walls and a centerpiece, with each wall further constructed from two
 parts. The first part is made of a non-magnetic material which secures the
 second part made of a magnetic material in contact with the poles of the
 magnet. The centerpiece of the cell forms a hollow center, and holds the
 perforated container or holding fixture within which the workpiece is
 placed, in a fixed position in the cell. On either side of the
 centerpiece, a "U" shaped groove contains a compressible sealing material
 to retain the magnetorheological fluid within the cell and to permit
 compression of the hollow center.
 Once the workpiece is secured within the container, and placed within the
 magnetorheological fluid in the cell, a magnetic field is applied to the
 magnetorheological fluid, solidifying it to apply a uniform clamping
 pressure to the surfaces of the workpiece immersed within the
 magnetorheological fluid or contacting the deformable packets. The
 clamping pressure may be further increased by decreasing the gap of the
 magnet within which the cell is placed, compressing the compressible
 sealing material and squeezing the solidified magnetorheological fluid
 within the cell. Under compression, the magnetic particles comprising the
 magnetorheological fluid form thick columnar structures, further
 increasing the viscosity or solidifying of the magnetorheological fluid.
 The solidified magnetorheological fluid work contacting medium supplies a
 uniform holding force to the workpiece, and allows the perforated
 container or holding fixture within which the workpiece is placed to
 absorb any peak forces applied to the workpiece, preventing displacement
 thereof during a machining or measuring operation. The solidified
 magnetorheological fluid further serves to attenuate any vibrations
 generated in the workpiece during the machining or measuring operations.
 Upon completion of the machining or measuring operation, the clamping
 pressure is withdrawn from the cell, and the magnetic field removed,
 thereby allowing the magnetorheological fluid to revert to a liquid state,
 after which the workpiece may be removed from the perforated container or
 holding fixture and the device reset for a subsequent use.
 In addition, the present invention also relates generally to a method for
 immobilizing or securing a workpiece having either a regular or irregular
 shape wherein a portion of the workpiece is immersed in a
 magnetorheological fluid at a desired position and orientation or placed
 between deformable packets containing the magnetorheological fluid. A
 magnetic field is applied to the magnetorheological fluid to cause the
 viscosity of the fluid to substantially increase, resulting in the
 solidification of the magnetorheological fluid about the immersed
 workpiece. The increase in viscosity results in the application of a
 uniform holding force to the surface immersed workpiece or to any surfaces
 to which the deformable packets have conformed against. A clamping
 pressure applied to the solidified magnetorheological fluid results in an
 additional increase in the viscosity of the magnetorheological fluid,
 thereby increasing the uniform holding force on the surface of the
 immersed workpiece, immobilizing or securing the workpiece in place. Once
 immobilized or secured, the workpiece is machined or measured as desired.
 To remove the finished workpiece, the process is reversed. First, any
 clamping force applied to the solidified magnetorheological fluid is
 removed. Next, the magnetic field is removed, resulting in a decrease in
 the viscosity of the magnetorheological fluid and a reversion to a liquid
 state. Finally, the finished workpiece is removed from the
 magnetorheological fluid or from between the deformable packets.
 The foregoing and other objects, features, and advantages of the invention
 as well as presently preferred embodiments thereof will become more
 apparent from the reading of the following description in connection with
 the accompanying drawings.

Corresponding reference numerals indicate corresponding parts throughout
 the several figures of the drawings.
 DESCRIPTION OF THE PREFERRED EMBODIMENT
 The following detailed description illustrates the invention by way of
 example and not by way of limitation. The description clearly enables one
 skilled in the art to make and use the invention, describes several
 embodiments, adaptations, variations, alternatives, and uses of the
 invention, including what is presently believe to be the best mode of
 carrying out the invention.
 Turning to FIGS. 1 and 2, a preferred embodiment of the workholding device
 10 of the present invention is illustrated. The workholding device 10
 includes a magnetorheological (MR) fluid cell 12, and a magnetic field
 assembly 14. The magnetic field assembly 14 comprises a permanent magnet
 16, preferably composed of rare earth alloys, as a high-strength magnetic
 field source secured into a square shaped arrangement of magnetic arms
 18A, 18B, and 18C which are composed of a soft iron or other magnetic
 material having a high permeability and low residual magnetization, and
 which define a gap region 20. The MR fluid cell 12 is detachably secured
 within the gap region 20, forming a closed loop magnetic circuit with the
 permanent magnet 16 and the magnetic arms 18a, 18b, and 18c. A frame 22
 secured to the magnetic arm 18A and provides a solid structure for
 attachment of the workholding device 10 to a workbench (not shown) or
 other suitable location.
 As shown in FIG. 2, magnetic arm 18a comprises an elongated rectangular
 base portion 24, a first upright extension 26 at one end of the base
 portion 24, and a second upright extension 28 at the opposite end of the
 base portion 24. Both the first and second extensions 26, 28 are arrayed
 perpendicular to the base portion 24 in the same direction, defining a
 generally U-shaped member, with the first extension 26 having a greater
 length than the second extension 28. An upper surface of the first
 extension 26 includes a tongue 30 configured to engage a groove 32 on the
 underside of magnetic arm 18b, thereby permitting magnetic arm 18b to
 slide parallel to the base portion 24 of magnetic arm 18a while
 maintaining contact with the first extension 26.
 As also best seen in FIG. 2, the permanent magnet 16 is preferably
 rectangular in shape, and enclosed on two sides by solid arch-shaped
 magnet shoes 34A and 34B composed of a soft iron or other good magnetic
 material having a high permeability and low residual magnetization. The
 permanent magnet 16 and the arch-shaped magnet shoes 34A and 34B are
 secured within a magnet receiving slot 36 passing radially through a
 cylindrical magnet holder 38 composed of a non-magnetic material, such
 that an outer surface 40 of each magnet shoe 34A, 34B is flush with, and
 has the same curvature as, the exterior surface of the magnet holder 38. A
 first support shaft 42 extends axially from an anterior surface of the
 cylindrical magnet holder 38, and is surrounded by a bearing bushing 44. A
 second support shaft 46 extends axially from a posterior surface of the
 cylindrical magnet holder 38. A bushing frame 48 secured to the upper
 surface of the magnetic arm 18A, adjacent the second extension 28 receives
 the first support shaft 42 and bearing bushing 44 in a receiving bore 49.
 The second support shaft 46 passes through a second bearing bushing 50
 seated in a second receiving bore 52 in a upright connection plate 54
 secured perpendicular to said frame 22 adjacent the second extension 28 of
 the magnetic arm 18A. The permanent magnet 16 secured within the
 cylindrical magnet holder 38 is thereby positioned adjacent a
 cylindrically concave upper surface 56 of the second extension 28, and is
 free to rotate through a full revolution.
 Magnetic arm 18C is secured to the upright connection plate 54 above the
 permanent magnet 16 and cylindrical magnet holder 38. Generally L-shaped
 magnetic arm 18C includes a cylindrically convex surface 58 adjacent the
 cylindrical magnet holder 38, such that magnet holder 38 and the permanent
 magnet 16 are partially enclosed between surfaces 56 and 58. Magnetic arm
 18C extends parallel to the elongated base portion 24 of magnetic arm 18A,
 towards magnetic arm 18b. The combined lengths of magnetic arms 18B and
 18C are shorter than the length of the elongated base portion 24, thereby
 defining the gap region 20 into which the MR fluid cell 12 is secured,
 closing the magnetic circuit.
 The second support shaft 46 passing through the second bearing bushing 50
 extends axially though an elongated bushing 60 seated in an axial bore 62
 of a horseshoe magnet 64 fitted around the upright connection plate 54
 perpendicular to the plane defined by the magnetic arms 18A, 18B, and 18C.
 The horseshoe magnet 64 includes two cylindrical convex surfaces 66A and
 66B which lie adjacent cylindrical convex surfaces 56 and 58, thereby
 defining a generally cylindrical chamber within which the cylindrical
 magnet holder 38 and permanent magnet 16 are positioned.
 The distal end of the second support shaft 46 extends beyond the exterior
 surface of the horseshoe magnet 64, and is fitted with a perpendicular
 turning lever 66. Rotation of the turning lever 66 about the longitudinal
 axis of the second support shaft 46 causes rotation of the cylindrical
 magnet holder 38 and the permanent magnet 16, thereby opening the closed
 magnetic circuit through magnetic arms 18A, 18B, 18C, and the MR fluid
 cell 12. Horseshoe magnet 64 provides a second closed magnetic circuit
 when the magnetic field is not supplied to the MR fluid cell 12, thereby
 reducing energy loss in the permanent magnet 16. Rotation of the
 cylindrical magnet holder 38 and permanent magnet 16 by 90 degrees allows
 the magnetic field flowing through magnetic arms 18A, 18B, 18C, and the MR
 fluid cell 12 to be selectively switched on or off. In the off position,
 the magnetic field flows through the horseshoe magnet 64.
 Turning next to FIGS. 3 through 6C, the magnetorheological fluid cell 12 is
 preferably constructed from three adjacent U-shaped frame sections 100A,
 100B, and 100C composed of a non-magnetic material such as aluminum,
 brass, or stainless steel. The outermost frame sections 100A and 100C each
 encase a cell wall 102A, 102B on three sides. The cell walls 102A, 102B
 are composed of a magnetic material such as soft iron, cast iron, or other
 magnetic alloys having high permeability and low residual magnetization,
 and are secured to the frame sections by means of countersunk threaded
 bolts 104. When the MR cell 12 is secured between magnetic arms 18B and
 18C, cell wall 102A contacts magnetic arm 18B, and cell wall 102B contacts
 magnetic arm 18C, allowing the magnetic field to extend into the MR cell
 12. While shown in a square configuration in FIGS. 3 through 6C, it will
 be recognized that the cell walls 102A and 102B may be configured in any
 manner which will increase the strength of the magnetic field extending
 into the MR cell 12 by directing or focusing the magnetic flux between
 magnetic arms 18B and 18C into a region having an narrower cross sectional
 area than that of the magnetic arms 18B and 18C.
 The outermost frame sections 100A and 100C includes recessed grooves 101 in
 the faces adjacent center frame section 100B, into which compressible
 seals 106 are placed to form a fluid barrier between each of said U-shaped
 frame sections 100A, 100B, and 100C. Countersunk threaded bolts 108 secure
 frame sections 100A, 100B, and 100C together, defining an open-faced
 volume 110 within which a magnetorheological fluid 112 is contained. The
 magnetorheological fluid 112 is prevented from seeping between the frame
 sections 100A, 100B, and 100C by the fluid barrier of compressible seals
 106. The center frame section 100b further includes a pair of recessed
 regions 114A, 114B on an inner surface 116 each sized to receive a portion
 of workpiece holding fixture 118.
 The preferred embodiment of the workpiece holding fixture 118 is shown in
 FIG. 5, and is composed of either a magnetic or non-magnetic material. The
 holding fixture 118 is preferably a hollow rectangular container having an
 open end 120, and an interior volume 121, but may be of any shape such as
 cylindrical, triangular, or irregular, depending upon the size and shape
 of workpieces with which it is to be utilized. Opposite sides of the
 preferred holding fixture 118 each includes a plurality of threaded bores
 122 which are axially aligned. Holding setscrews or threaded bolts 124 are
 seated within a number of the threaded bores 122, while a number of the
 bores 122 are left empty. The exterior surface of the workpiece holding
 fixture 118 includes a pair of hemi-cylindrical protrusions 126A and 126B
 configured to seat loosely within the recessed portions 114A, 114B on the
 inner surface 116 of the center frame section 100B.
 During use, a workpiece 130 to be immobilized is placed in the open end 120
 of the holding fixture 118, as seen in FIG. 7, and secured in the desired
 position and orientation by a plurality of workpiece holding elements such
 as holding setscrews or threaded bolts 124 threaded in through threaded
 bores 122. The holding screws or threaded bolts 124 contact the surface of
 the workpiece 130 with a minimum force necessary to hold the workpiece 130
 in the desired position and orientation, and are preferably utilized in
 pairs from opposite sides of the holding fixture 118, thereby absorbing
 peak forces and minimizing distortion of the workpiece 130. It is
 preferred that the holding screws or threaded bolts 124 be composed of a
 soft material, such as Teflon.TM., to avoid damage to the surface of the
 workpiece 130. In any case, the hardness of the holding setscrews or
 threaded bolts 124 is less than the harness of the workpiece 130 to avoid
 workpiece damage. The number of setscrews or threaded bolts 124 utilized
 depends upon the size and geometry of the workpiece 130. The remaining
 threaded bores 122 are left empty. It will be readily apparent to one of
 ordinary skill in the art that a variety of workpiece holding elements
 other than holding setscrews or threaded bolts 124 may be utilized to
 secure the workpiece 130 at the desired position and orientation. For
 example, shims, wedges or cams may be utilized separately or together with
 holding setscrews or threaded bolts 124, as well as other commonly known
 holding elements. Correspondingly, various thread-locking fluids or
 materials may be employed to secure the holding setscrews or threaded
 bolts 124 in position, preventing accidental unthreading thereof.
 Next, as best seen in FIG. 6B, the open-faced volume 110 in the
 magnetorheological fluid cell 12 is partially filled with the
 magnetorheological fluid 112 to a level at or below the upper surface of
 the volume 110. It is preferred that the magnetorheological fluid utilized
 with the present invention be a mixture of carbonyl iron powder in silicon
 oil with a volume percentage of powder being 20% or more, and with the
 powder particles being generally spherical in shape and having a mean size
 of approximately 5 .mu.m. However, any magnetorheological fluid such as is
 described in U.S. Pat. No. 5,549,837 to Ginder et al. for "Magnetic
 Fluid-Based Magnetorheological Fluids" which will alter viscosity to a
 solid or near solid state upon application of a magnetic field may be
 used. An alternative class of magnetorheological fluids is disclosed in
 U.S. Pat. No. 5,667,715 to Foister for "Magnetorheological Fluids" and
 utilizes powdered magnetizable solids of at least two different sizes
 dispersed in a base carrier liquid to substantially increase the yield
 stress of the magnetorheological fluid in the presence of a magnetic
 field.
 Once the open-faced volume 110 is partially filled with the
 magnetorheological fluid 112, the holding fixture 118 and secured
 workpiece 130 are immersed within the magnetorheological fluid 112 until
 the protrusions 126A, 126B of the holding fixture seat within the recessed
 regions 114A, 114B on the inner surface 116 of the center frame section
 100B. The magnetorheological fluid is free to flow through the unused
 threaded bores 122 and surround or immerse a portion of the workpiece 130
 and holding fixture 118. Retaining bolts 132 may be passed through bores
 134 in the holding fixture 118 to threaded receiving bores 136 in the
 center frame 100B, thereby securing the holding fixture 118 into the
 magnetorheological fluid cell 12.
 If the magnetorheological fluid cell 12 is not already secured into the gap
 region 20 between magnetic arms 18B and 18C, it is secured therein such
 that the cell walls 102A and 102B are in contact with the respective
 magnetic arms.
 To solidify the magnetorheological fluid, a magnetic field is applied to
 the magnetorheological fluid by closing the magnetic circuit defined by
 the magnetic arms 18A, 18B, 18C, the MR cell 12, and the permanent magnet
 16. The magnetic circuit is closed when the permanent magnet 16 of the
 preferred embodiment is rotated to a first position bringing the poles of
 the permanent magnet 16 into alignment with magnetic arms 18A and 18C, and
 opened when the permanent magnet 16 is rotated 90 degrees to a second
 position, bringing the poles of the permanent magnet 16 into alignment
 with the cylindrical convex surfaces 66A and 66B of horseshoe magnet 64.
 When in the closed position, the magnetic field significantly increases
 the viscosity of the magnetorheological fluid to a solid or near solid
 state, applying a uniform holding force between surfaces of the workpiece
 130, the holding fixture 118 immersed therein, and the MR cell 12,
 immobilizing the workpiece 130 for machining or measuring operations. The
 solidified magnetorheological fluid further serves to attenuate vibrations
 in the workpiece 130 during machining or measuring operations, while the
 holding fixture 118 absorbs or attenuates peak vibration forces
 transmitted through the workpiece 130. For measurement and some simple
 machining operations, solidifying the magnetorheological fluid 112 may be
 all that is necessary. However, for most machining operations, the use of
 the holding fixture 118 and further compression of the solidified
 magnetorheological fluid 112, as described further below is typically
 required.
 Upon completion of the machining or measuring operations, the magnetic
 circuit is opened, by rotating the permanent magnet of the preferred
 embodiment to the open position, diverting the magnetic field away from
 the magnetorheological fluid 112. The holding fixture 118 and workpiece
 130 are removed by reversing the insertion operations.
 In the preferred embodiment, the uniform holding force applied to the
 workpiece 130 immersed in the magnetorheological fluid 112 is further
 increased by the application of a compressive force to the solidified
 magnetorheological fluid 112. Applying a force to the magnetic arm 18B in
 the direction of the MR fluid cell 12 and in the direction of the magnetic
 field causes movement of the magnetic arm 18B along the tongue and groove
 connection with magnetic arm 18A as the compressible seals 106 between the
 frames 100A, 100B, and 100C of the MR fluid cell 12 are compressed,
 decreasing the volume defined by the interior of the MR fluid cell 12.
 Compression of the seals 106 in turn applies a compressive force on the
 solidified magnetorheological fluid in the direction of the magnetic
 field, further increasing the viscosity of the magnetorheological fluid by
 causing the magnetic particles suspended in the magnetorheological fluid
 to form thick columnar structures, correspondingly increasing the uniform
 holding force immobilizing the workpiece 130 as is illustrated graphically
 in FIGS. 8A-8C. Once a desired level of compression is reached, a lockbolt
 138 in magnetic arm 18B may be tightened, securing the magnetic arm 18B in
 the altered position to maintain the force on the solidified
 magnetorheological fluid, and the compressive force removed. To release
 the force, the lockbolt 138 is loosened and the magnetic arm 18B withdrawn
 from the altered position prior to the removal of the magnetic field from
 the magnetorheological fluid.
 Those of ordinary skill in the art will recognize that any suitable
 magnetic field source and magnetorheological fluid may be utilized in the
 workholding device of the present invention, provided that the magnetic
 field through the magnetorheological fluid work holding material may be
 selectively introduced and removed. For example, FIG. 9 illustrates an
 alternate embodiment of the apparatus or device of the present invention
 utilizing an switchable electromagnet 140 in place of the permanent magnet
 16. Applying an electrical current to the electromagnet 140 results in the
 generation of an electromagnetic field, and the closure of the magnetic
 circuit defined by the magnetic arms 142A, 142B, 144A, 144B, the MR fluid
 cell 12, and the magnetorheological fluid contained therein. In such an
 alternate embodiment, mechanical components associated with the rotation
 of the permanent magnet 16 and the second magnetic circuit defined by 16,
 64 are not necessary, as removal of the electrical current supplied to the
 electromagnet 140 will result in removal of the electromagnetic field from
 the MR fluid cell 12.
 Additionally shown in FIG. 9 is an alternative arrangement for applying a
 compressive force to the magnetorheological fluid cell 12. The tongue 30
 and groove 32 interface between magnetic arms 18A and 18B of the preferred
 embodiment is replicated between magnetic arms 144A and 144B, and is
 actuated by a threaded piston 146. Rotation of the threaded piston 146 by
 means of a handle 148 advances or withdraws the face of the magnetic arm
 144B to and from contact with cell wall 102a of the magnetorheological
 fluid cell 12, while maintaining contact between magnetic arms 144A and
 144B, correspondingly applying or removing a compressive force on the
 magnetorheological fluid cell 12.
 One of ordinary skill in the art will recognize that numerous mechanical
 configurations of the present invention are possible. For example, FIG. 9
 illustrates the use of an alternative support base 150. Similarly,
 numerous configurations utilizing either permanent magnets or
 electromagnets to selectively apply a magnetic field to the volume of
 magnetorheological fluid 112 contained within an open cell are possible,
 resulting in the solidification of the magnetorheological fluid about a
 workpiece immersed therein. Similarly, a variety of well known mechanical
 and hydraulically actuated configurations for applying a compressive force
 to the solidified magnetorheological fluid contained within the open cell
 are possible. For example, an external clamping force may be applied
 in-line with the magnetic field flowing between the cell walls 102A, 102B,
 or such external clamping force may be applied to the magnetorheological
 fluid cell 12 parallel to, but external to the magnetic field through the
 frame members comprising the magnetorheological fluid cell 12.
 Turning to FIGS. 10A through 12, an alternative embodiment suited for use
 in mass production or assembly line manufacturing applications is shown in
 which the magnetorheological fluid cell 12 is modified to accept and
 utilize deformable packets 200 encapsulating the magnetorheological fluid
 112 in a thin flexible membrane 202 such as latex or other flexible
 material in place of filling the volume 110 of the MR fluid cell 12. It
 will be recognized that the exact size and shape of the deformable packets
 200 may be configured to conform closely to the surface of a workpiece
 130, or may be of a generic rectangular shape suitable for use with a
 variety of different workpieces 130 having different configurations or
 shapes. Referring to FIGS. 11 and 12, modified cell walls 204A, 204B
 replace cell walls 102A, 102B in the MR fluid cell 12, and a workpiece
 holding fixture 206 configured for use with at least two packets 200
 replaces workpiece holding fixture 118. Each cell wall 204A, 204B includes
 a recessed cavity 208 on an interior face configure to receive a portion
 of a packet 200. Workpiece holding fixture 206 includes packet receiving
 openings 210A, 210B on opposite faces, opening to an interior volume 212.
 The dimensions of each packet receiving opening 210A, 210B are preferably
 smaller than the dimensions of the corresponding face defining the
 interior volume 212 of the work holding fixture 206, such that a number of
 threaded perforations 122 passing through the work holding fixture 206
 into the interior volume 212 are located both above and below each packet
 receiving opening 210A, 210B.
 During use of the alternate embodiment of the magnetorheological fluid cell
 12, a workpiece 130 is secured into the interior volume 212 of the
 workpiece holding fixture 206 at a desired position and orientation with
 setscrews 124 as before. Next a packet 200 is placed in each packet
 receiving opening 210A, 210B, such that the flexible membrane 202 contacts
 and conforms to the surface of the workpiece 130 secured within the
 interior volume 212 of the workpiece holding fixture 206. The combination
 of the workpiece holding fixture 206, secured workpiece 130, and packets
 200 is placed into the MR fluid cell 12. The flexible membrane 202 of each
 packet 200 contacts and conforms to the surface of cell walls 204A, 204B,
 expanding into recessed cavities 208, filling all or most of the remaining
 volume of the MR fluid cell 12. Prior to the application of the magnetic
 field, it may be desirable to initially compress the flexible membrane 202
 of each packet 200 to eliminate all voids and/or air pockets. Application
 of a magnetic field results in an increase in the viscosity of the
 encapsulated magnetorheological fluid, exerting a uniform clamping force
 on the portion of workpiece 130 in contact with the flexible membrane 202
 of each packet 200. Those of ordinary skill in the art will recognize that
 an equal and opposite force is exerted on the cell walls by the
 encapsulated magnetorheological fluid 112. Applying a compression force to
 the MR fluid cell 12 and accordingly to the magnetorheological fluid 112
 encapsulated in the deformable packets 200 results in an additional
 increase in the viscosity of the magnetorheological fluid, further
 increasing the uniform clamping force applied to the workpiece 130 secured
 in the workpiece holding fixture 206.
 As a method, the present invention preferably incorporates the steps of (1)
 immersing a portion of a workpiece in a magnetorheological fluid at a
 desired position and orientation, and (2) applying a magnetic field to the
 magnetorheological fluid to increase the viscosity of, or solidify, the
 magnetorheological fluid, thereby applying a uniform holding force to the
 workpiece and immobilizing or securing it at the desired position and
 orientation during the application of the magnetic field. The
 magnetorheological fluid may further serve to attenuate vibrations in the
 workpiece.
 In a first alternative method, the workpiece may be secured in the desired
 position and orientation in a holding fixture with a minimum of force, and
 the combination of the holding fixture and a portion of the workpiece
 immersed in the magnetorheological fluid prior to the application of the
 magnetic field. Upon the increase in viscosity or solidification of the
 magnetorheological fluid responsive to the application of the magnetic
 field, the magnetorheological fluid will apply a uniform holding force to
 the fixture and to the workpiece while the fixture absorbs peak vibration
 forces applied to the workpiece, thereby immobilizing or securing the
 workpiece at the desired position and orientation during the application
 of the magnetic field.
 In a second alternative method, the workpiece may be located in the desired
 position and orientation in an open cell, either directly or by means of a
 holding fixture, with a minimum of force in an open cell, and a deformable
 packet encapsulating a magnetorheological fluid conformed between a
 surface of said workpiece and said open cell. A magnetic field is then
 applied to said encapsulated magnetorheological fluid, resulting in an
 increase in viscosity or solidification of the magnetorheological fluid
 and the exertion of a uniform holding force between the surface of said
 workpiece and said open cell, holding the workpiece at the desired
 position and orientation.
 An additional step may be applied to the methods of the present invention
 to further increase the uniform holding force applied to the workpiece or
 to the fixture and workpiece combination by the solidified
 magnetorheological fluid by incorporating the application of a first
 compressive force to the magnetorheological fluid during or after the
 application of the magnetic field. It is preferred that the first
 compressive force act on the magnetorheological fluid in the general
 direction of the magnetic field, thereby resulting in an additional
 increase in the viscosity of the magnetorheological fluid by altering the
 physical arrangement of the magnetized particles suspended in the fluid
 carrier. Increasing the viscosity of the magnetorheological fluid results
 in an increase in the uniform holding force applied to the immersed
 portion of the workpiece, as well as the combination of the fixture and
 workpiece, thereby further securing the workpiece at the desired position
 and orientation. For applications utilizing a magnetorheological fluid
 encapsulated in a deformable packet, the application of the compressive
 force has the same effect on the magnetorheological fluid, and results in
 an increase in the uniform holding force exerted on the portion of the
 workpiece to which the deformable packet has conformed. A second
 compressive force may be applied perpendicular to the first compressive
 force to achieve further increases in the viscosity of the
 magnetorheological fluid.
 In view of the above, it will be seen that the several objects of the
 invention are achieved and other advantageous results are obtained. As
 various changes could be made in the above constructions without departing
 from the scope of the invention, it is intended that all matter contained
 in the above description or shown in the accompanying drawings shall be
 interpreted as illustrative and not in a limiting sense.