CA2234761C - Deterministic magnetorheological finishing - Google Patents

Deterministic magnetorheological finishing Download PDF

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Publication number
CA2234761C
CA2234761C CA002234761A CA2234761A CA2234761C CA 2234761 C CA2234761 C CA 2234761C CA 002234761 A CA002234761 A CA 002234761A CA 2234761 A CA2234761 A CA 2234761A CA 2234761 C CA2234761 C CA 2234761C
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Canada
Prior art keywords
magnetorheological fluid
workpiece
fluid
carrier surface
carrier
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CA002234761A
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French (fr)
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CA2234761A1 (en
Inventor
Stephen David Jacobs
William Kordonski
Igor Victorovich Prokhorov
Donald Golini
Gennadii Rafailovich Gorodkin
Tvasta David Strafford
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University of Rochester
QED Technologies International LLC
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Byelocorp Scientific Inc
University of Rochester
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Publication of CA2234761A1 publication Critical patent/CA2234761A1/en
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B31/00—Machines or devices designed for polishing or abrading surfaces on work by means of tumbling apparatus or other apparatus in which the work and/or the abrasive material is loose; Accessories therefor
    • B24B31/10—Machines or devices designed for polishing or abrading surfaces on work by means of tumbling apparatus or other apparatus in which the work and/or the abrasive material is loose; Accessories therefor involving other means for tumbling of work
    • B24B31/112—Machines or devices designed for polishing or abrading surfaces on work by means of tumbling apparatus or other apparatus in which the work and/or the abrasive material is loose; Accessories therefor involving other means for tumbling of work using magnetically consolidated grinding powder, moved relatively to the workpiece under the influence of pressure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B1/00—Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes
    • B24B1/005—Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes using a magnetic polishing agent
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B9/00—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor
    • B24B9/02—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground
    • B24B9/06—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground of non-metallic inorganic material, e.g. stone, ceramics, porcelain
    • B24B9/08—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground of non-metallic inorganic material, e.g. stone, ceramics, porcelain of glass
    • B24B9/14—Machines or devices designed for grinding edges or bevels on work or for removing burrs; Accessories therefor characterised by a special design with respect to properties of materials specific to articles to be ground of non-metallic inorganic material, e.g. stone, ceramics, porcelain of glass of optical work, e.g. lenses, prisms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/44—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of magnetic liquids, e.g. ferrofluids
    • H01F1/447—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of magnetic liquids, e.g. ferrofluids characterised by magnetoviscosity, e.g. magnetorheological, magnetothixotropic, magnetodilatant liquids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y25/00—Nanomagnetism, e.g. magnetoimpedance, anisotropic magnetoresistance, giant magnetoresistance or tunneling magnetoresistance

Abstract

Method and apparatus (10) for finishing a workpiece surface using magnetorheological fluid (16) wherein the workpiece (26) is positioned near a carrier surface (14) such that a converging gap (22) is defined between the workpiece surface and the carrier surface (14); a magnetic field is applied at the gap (22); a flow of magnetorheological fluid (16) is introduced into the gap (22) creatign a work zone in the fluid (16) to form a transient finishing tool for engaging and causing material removal at the workpiece surface.

Description

DETERMINISTIC MAGNETORHEOLOGICAL FINISHING
FIELD OF THE INVENTION
1 This invention relates to a method and 2 apparatus for finishing employing magnetorheological 3 fluids and the fluid compositions used therein.
BACKGROUND OF THE INVENTION
6 Processes for finishing a workpiece such as an 7 optical lens generally comprise removing material at the 8 surface of the workpiece to accomplish three objectives:
9 (1) removal of subsurface damage, (2) surface smoothing, and (3) figure correction. Many known polishing 11 processes can achieve objectives (1) and (2), but have 12 difficulty achieving objective (3). Examples of such 13 processes include full aperture contact polishing on 14 pitch laps or on polyurethane laps. These processes are generally inefficient and often require many iterations 16 to correct the figure of an optical lens. Other 17 techniques such as ion beam milling can achieve objective 18 (3), but are not effective in meeting objectives (1) and 19 (2). Ion beam milling cannot smooth, and has been shown . 20 to introduce subsurface damage if not precisely 21 controlled. _ 22 Finishing of precision optics typically 23 requires the production of a surface that conforms t.o the 24 desired figure to within 0.50 micron peak-to-valley.
Finishing of optics typically requires relatively high > >

1 rates of material removal, even with hard materials such 2 as glass. Finishing of optics also typically requires 3 sufficient material removal to eliminate subsurface 4 damage from previous grinding operations and achieve a microroughness of 20 A rms or less.

6 Conventional finishing processes employ 7 precisely shaped, viscoelastic pitch or polyurethane 8 foam-faced laps to transfer pressure and velocity through 9 an abrasive slurry to the workpiece. The lap is large enough to cover the entireoptically useful.portion of 11 the lens and is therefore termed a full aperture lap.

12 The working surface of the finishing tool must conform to 13 the desired workpiece surface. If the viscoelastic 14 finishing tool is compliant, as would be the case for a tool made from pitch, rosin, or wax, it deforms under the 16 influence of pressure and heat generated during the 17 finishing process. The finishing tool loses the desired 18 surface shape and assumes the surface shape of the actual 19 workpiece, which is not yet corrected. Surface smoothing may continue, but the ability of the tool to further 21 correct the surface figure is severely diminished. The 22 finishing tool must be reshaped against a metaltemplate 23 possessing the desired surface shape before finishing is 24 resumed. This iterative process is unpredictable and time consuming. It requires highly skilled craftsmen or 26 master opticians. It also requires an inventory of metal 27 templates including one for each workpiece shape.

1 Alternately, a viscoelastic finishing tool may 2 be less compliant, as in the case of a tool made from a 3 hard, thin polyurethane pad mounted on a metal backing 4 template. This type of finishing tool is better at maintaining the desired shape during the finishing 6 process, but it wears away with time, causing removal 7 rates to diminish. As the tool's ability to smooth the 8 workpiece surface is degraded it becomes difficult to 9 achieve the required levels of surface smoothness. A

master optician must periodically stop, redress or 11 replace the pad, and then continue the finishing process.

12 All conventional full aperture, viscoelastic 13 finishing tools suffer from the problem of embedded 14 particulate material. Glass shards and/or abrasive polishing grains become embedded in the tool surface with 16 time. The surface may glaze over and become smooth.

17 This reduces removal rates. Alternately, the embedded 18 particulate material may scratch the workpiece surface, 19 damaging the workpiece in the final stages of finishing.

This form of tool degradation is unpredictable. For 21 these reasons, finishing complex surfaces is complicated 22 and difficult to adapt to large-scale production.

. 23 Some finishing processes make use of a sub-24 aperture lap, i.e. a finishing tool that is smaller than the portion of the workpiece that requires finishing.

26 See, e.g., U.S. Patent No. 4,956,944 to Ando et al.

27 However, such processes make use of solid finishing tools 1 and therefore suffer from many of the same problems as 2 processes that use full sized laps.

3 Certain milling processes, including processes 4 that use solid tools and processes that use ion beam S bombardment, may also make use of a sub-aperture lap.

6 While such processes are capable of shaping or figuring a 7 workpiece, they cannot perform surface smoothing and 8 indeed may cause surface roughness by exposing sub-9 surface damage.

It is known to use -fluids containing magnetic 11 particles in polishing applications. U.S. Patent No.

12 4,821,466 to Kato et al. discloses a polishing process in 13 which a "floating pad" immersed in a fluid containing la colloidal magnetic particles is pushed against a workpiece by buoyancy forces caused by the application of 16 a nonuniform magnetic field. This polishing process has 17 a rudimentary capability for figure correction which is 18 similar to that used with full aperture, viscoelastic 19 finishing tools. The shape of the float and the shape of the magnetic field must be custom tailored to achieve a 21 specific desired surface shape. To finish another shape 22 with the same process requires different lapping-motions, 23 as well as the design and fabrication of a different , 24 float and possibly a different magnet configuration.

Substantial process and machine modifications are 26 therefore required in order to change optic shapes.

27 It is also known to polish a workpiece by 28 immersing it in a fluid containing magnetic particles and 1 applying a rotating magnetic field to the fluid. See, 2 e.g., U.S. Patent No. 2,735,232 to Simjian. The rotating 3 field is said to cause the fluid to flow circularly 4 around the workpiece thereby polishing it. This method 5 suffers from the disadvantage that it does not create 6 sufficiently high pressure on the workpiece and therefore 7 does not achieve a satisfactory material removal rate.
8 It is also not possible to substantially correct surface 9 figure errors to optical requirements with this method.

11 OBJECTS AND SITRY OF THE TNVENT~ON

12 In light of the foregoing, it is an object of 13 the invention to provide an improved finishing method and la apparatus employing a magnetorheological fluid. It is a further object of the invention to provide a 16 magnetorheological fluid for use in said method and 17 apparatus.

18 It is a further object of the invention to 19 provide a finishing system that can be used in finishing of optics.

21 It is a further object of the invention to 22 provide a finishing system that provides a high degree of 23 smoothing in that it both creates substantially no 24 surface or subsurface damage (scratches, cracks, or subsurface cracks) and substantially eliminates existing 26 surface and subsurface damage.

WO 97/14532 PCTlUS96/16568 1 It is a further object of the invention to 2 provide a finishing system that provides for both surface 3 smoothing and figure correction.

4 It is a further object of the invention to provide a finishing system that may be easily automated 6 and that is flexible in its applications.

7 It is a further object of the invention to 8 provide a means for automating said system.

9 It is a further object of the invention to provide a finishing system that operates at a relatively 11 high removal rate for a variety of materials.

12 It is a further object of the invention to 13 provide a finishing system that smoothes the surfaces of 14 a wide variety of materials to accepted standards of 1~ precision optics.

lo' It is a further object of the invention to 17 provide a finishing system whose removal rates may be 18 accelerated for hard materials like silicon or especially 19 hard materials like sapphire with the addition of nanocrystalline diamond abrasives to the standard MR

21 fluid composition, or to otherwise optimized MR fluid 22 compositions.

23 It is a further object of the invention to 2a provide a finishing system wherein the finishing tool self-adjusts to any workpiece surface form -- whether 26 convex, concave, or flat -- without requiring any changes 27 to the structure of the finishing machine such as 28 replacement of precisely shaped laps.

1 It is a further object of the invention to ., 2 provide a finishing system wherein the finishing tool is 3 represented by a removal function in the form of a 4 finishing spot.
It is a further object of the invention to 6 provide a finishing system comprising a 7 magnetorheological fluid that is resistant to degradation 8 under operating conditions of heat, abrasion and exposure 9 to air.
These and other objectives are achieved by a MR
11 finishing method and apparatus in accordance with the 12 present invention. The method of finishing a workpiece 13 surface using MR fluid comprises the steps of:
14 position_ng the workpiece near a carrier surface such that a converging gap is defined between a portion of the 16 workpiece surface and the carrier surface; applying a 17 magnetic field substantially at said gap; introducing a 18 flow of magnetic field-stiffened MR fluid through said 19 converging gap such that a work zone is created in the MR
fluid to form a transient finishing tool for engaging and 21 causing material removal at the portion of the workpiece 22 surface; and moving the workpiece or the work zone 23 relative to the other to expose different portions of the 24 workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said 26 workpiece surface to predetermined degrees. The 27 apparatus for finishing a workpiece using MR fluid 28 comprises: a carrier surface adapted to carry the MR

fluid; a workpiece holder for holding the workpiece and positioning a portion of the workpiece surface near the carrier surface to define a converging gap therebetween, such that said carrier surface carries MR fluid through said gap; a magnet for applying a magnetic field at said gap to stiffen the MR fluid flowing through said gap for creating a transient finishing tool for engaging and causing material removal at the portion of the workpiece surface; and means for moving the workpiece or the work zone relative to the other to expose different portions of the workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said workpiece surface in predetermined degrees.
Accordingly, in one aspect of the invention, there is provided a method of finishing a workpiece surface using magnetorheological fluid, comprising: (a) positioning the workpiece near a continuous carrier surface such that a converging gap is defined between a portion of the workpiece surface and the carrier surface, wherein said carrier surface extends along a rim of a vertically oriented wheel;
(b) applying a magnetic field substantially at said gap;
(c) depositing magnetorheological fluid from a magnetorheological fluid source on the carrier surface;
(d) moving the carrier surface past said workpiece by rotating the wheel such that magnetic field-stiffened magnetorheological fluid flows through said converging gap defining a work zone forming a transient finishing tool for engaging and causing material removal at a portion of the workpiece surface; (e) moving the workpiece or the work zone relative to the other to expose different portions of the workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said 8a workpiece surface to predetermined degrees; (f) collecting magnetorheological fluid that has flowed through said gap from said carrier surface; and (g) returning the magnetorheological fluid collected in step (f) to the magnetorheological fluid source.
In a second aspect, there is provided an apparatus for finishing a workpiece surface using magnetorheological fluid, comprising: a continuous movable carrier surface; a nozzle for depositing magnetorheological fluid from a magnetorheological fluid source on the carrier surface; a workpiece holder for holding the workpiece and positioning a portion of the workpiece surface near the carrier surface to define a converging gap therebetween, said carrier surface being movable past said workpiece such that the magnetorheological fluid flows through said gap; a magnet for applying a magnetic field at said gap to stiffen the magnetorheological fluid flowing through said gap for creating a transient finishing tool for engaging and causing material removal at a portion of the workpiece surface;
means for moving the workpiece or the work zone relative to the other to expose different portions of the workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said workpiece surface in predetermined degrees; a collector for collecting magnetorheological fluid having flowed through the gap from the carrier surface; and recirculating means for returning the magnetorheological fluid to the magnetorheological fluid source.
In a third aspect, there is provided a method of finishing a workpiece surface using magnetorheological fluid, comprising: rotating a wheel about a horizontally-oriented axle, said wheel including an outer rim defining a 8b moving carrier surface; positioning the workpiece near the carrier surface such that a gap exists between a portion of the workpiece surface and the carrier surface; applying a magnetic field substantially at said gap; depositing magnetorheological fluid on the moving carrier surface such that field stiffened magnetorheological fluid is carried by the carrier surface and at least some of said magnetorheological fluid flows through said gap defining a work zone forming a transient finishing tool for engaging and causing material removal on a portion of the workpiece surface; and moving the workpiece relative to the work zone to expose different portions of the workpiece surface to the work zone for predetermined time periods to finish said portions of said workpiece surface to predetermined degrees.
In a fourth aspect, there is provided an apparatus for finishing a workpiece surface using magnetorheological fluid, comprising: a vertical wheel rotatable about a horizontally-oriented axle, said wheel including an outer rim defining a carrier surface; a nozzle for depositing magnetorheological fluid from a magnetorheological fluid source on the carrier surface such that the magnetorheological fluid is carried by the carrier surface as the wheel is rotated; a workpiece holder for holding the workpiece and positioning a portion of the workpiece surface near the carrier surface with a gap therebetween, wherein as the wheel is rotated, the carrier surface is moved past the workpiece to carry the magnetorheological fluid through said gap; a magnet for applying a magnetic field at said gap to stiffen the magnetorheological fluid flowing through said gap creating a finishing work zone in the fluid for engaging and causing material removal at the portion of the workpiece surface; means for moving the workpiece relative to the work 8c zone to expose different portions of the workpiece surface to the work zone for predetermined time periods to finish said portions of said workpiece surface in predetermined degrees; and a collector for collecting magnetorheological fluid having flowed through the gap from the carrier surface and returning the magnetorheological fluid to the magnetorheological fluid source.
According to a fifth aspect, there is provided a magnetorheological fluid comprising non-colloidal magnetic particles, an aqueous carrier fluid, and an alkaline salt wherein the pH of the magnetorheological fluid is between 7 and 11.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view of an exemplary finishing apparatus in accordance with the present invention.
Figure 2 is an enlarged view of a portion of the apparatus of Figure 1.
Figure 3A-C are schematic depictions of the carrier wheel and MR fluid ribbon of the present invention in use to finish concave, flat and convex workp.ieces.
Figure 3D is a cross-section view of :figure 3C
with the addition of the magnetic polepieces.

1 Figures 4A and 4B are schematic drawings 2 depicting the angle with which the MR fluid would impinge 3 on the workpiece using circular and flat carrier 4 surfaces, respectively.

Figure 5A is a cross-section view of an 6 exemplary magnetic polepiece in accordance with the 7 present invention.

8 Figure 5B is a front view of the magnetic 9 polepiece.

Figure 5C is a top view of the magnetic 11 polepiece.

12 Figures 6A and 6B are field plots of magnetic 13 field magnitude and direction in and surrounding an 14 exemplary polepiece according to the present invention.

Figure 7 is a schematic depiction of an 16 exemplary fluid circulation system in accordance with the 17 present invention.

18 Figure 8A is a side cross section view of an 19 exemplary fluid delivery nozzle of the present invention in contact with a vertical wheel.

21 Figure 8B is a frontal view of an examplary 22 fluid delivery nozzle.

23 Figure 9A is a perspective view of a workpiece 2a shaped by use of the scraper of figure 9B.

Figure 9B is a perspective view of a scraper 26 used to create a sawtooth pattern in MR fluid.

1 Figure 9C is a profile of the workpiece of 2 figure 9B taken with a Rank Taylor Hobsen Form Talysurf 3 profiler.

4 Figure 9D is a perspective view of a scraper 5 used to create a triangular MR fluid ribbon.

6 Figure 10A is a side cross-section view of an 7 exemplary fluid collector of the present invention.

8 Figure lOB is a frontal cross-sectional view of 9 the collector.
10 Figure 10C is a bottom view of the collector.
11 Figure 11A is a schematic depiction of a 12 reservoir of the present invention.
13 Figure 11B is a schematic depiction of an 14 alternate reservoir for use with the present invention.

Figure 12 is a graph illustrating finishing 15 spot widths and lengths achieved with varying sizes of 17 abrasive grit.

18 Figure 13 is a graph indicating volumetric 19 removal rates, normalized to a starting rate of one unit, measured over periods ofsix or more hours for three MR

21 fluids made with different carrier fluids.

22 Figure 14A is a schematic depiction of the 23 apparatus used to obtain the MRF removal function "spot" , 24 depicted in Figures 14B and 14C.

Figures 14B and 14C are representations of the 26 MRF removal function "spot" on BK7 glass after 5 seconds 27 of finishing.

WO 97/14532 PCT/US96/i6568 1 Figures 15A-15C are representations of the 2 effect of the finishing spot on a spinning workpiece_ 3 Figure 16 is a flow chart of the computer 4 control algorithm of the present invention.

Figure 17A and 17B are representations of the 6 MRF removal function "spot" on fused silica.

7 Figures 17C and 17D are representations of the 8 MRF removal function "spot" on SK7 glass.

9 Figure 18 is a portion of the user interface of software used in an embodiment of the present invention 11 depicting an initial, a predicted, and an actual final 12 interferogram of a workpiece finished in accordance with 13 the present invention.

14 Figures 19A-C are schematic depictions of possible alternate finishing apparatus of the present 16 invention.
17 Figures 20A and B are schematic depictions of 18 two views of a possible alternate finishing apparatus of 19 the present invention. a 3 Magnetorheological ("MR") fluids, which 4 comprise uniformly dispersed, noncolloidal magnetic material in a carrier fluid, are designed to change 6 rheological properties (such as plasticity, elasticity, 7 and apparent viscosity) or other fluid properties when 8 subjected to a magnetic field. Typical uses of known 9 magnetic fluid compositions have included shock absorbers, clutches and actuating modules.

11 The present invention is directed to improved 12 methods and devices for magnetorheological finishing of 13 workpiece surfaces. In accordance with the invention, 14 the workpiece surface is positioned above a carrier surface defining a gap therebetween. MR fluid is 16 deposited on the carrier surface, which then carries MR

17 fluid through the gap. A magnetic field is applied at 18 the gap to substantially stiffen the MR fluid flowing 19 through the gap to form a transient work zone or finishing spot for causing material removal at the 21 workpiece surface. The finishing spot is smaller than 22 the workpiece surface, and by moving the workpiece 23 relative to the carrier surface, the workpiece can be , 24 moved over the finishing spot. By controlling the dwell time of the spot in different locations, the process may 26 achieve surface smoothing, removal of subsurface damage, 27 or figure correction as desired to within high 28 tolerances. In the process of finishing the workpiece, 1 the MR fluid carries away heat, abrasive°particles, and - 2 particles of workpiece material while continuously 3 delivering fresh abrasive particles to the work zone.
4 Figure 1 illustrates an exemplary MR finishing device 10 in accordance with the present invention. The 6 device 10 includes a vertically-oriented carrier wheel 12 7 which comprises an outer rim 14 defining a carrier 8 surface (shown in greater detail in Figure 2). (Although 9 not shown, the carrier surface may also comprise the bottom wall of-the inside of a circular trough, the upper 11 surface of a turntable or belt or any other suitable 12 moving surface. Some possible alternate configurations 13 are disclosed Figures 19 and 20.) The vertical carrier 14 wheel supports a ribbon-shaped volume of MR fluid 16.

The ribbon of MR fluid is deposited on one side of the 16 wheel by a fluid delivery nozzle 18 and is carried by the 17 rotation of the wheel to the far side where it is 18 retrieved by fluid collector 20. The wheel carries the 19 ribbon through gap 22 between the workpiece surface and the carrier surface at which an optimal magnetic field is 21 applied by a magnet. The vertical carrier wheel is 22 preferably made of a non-magnetic material such as 23 aluminum or plastic.

24 Preferably the rim of the carrier wheel is not flat, so as to form a cylinder, but instead is convexly 26 curved across its width.. In a preferred embodiment the 27 wheel is a spheric section, which is to say that the 28 radius of curvature across the width of the rim is equal 1 to the z-adius of the wheel around its circumference. A

2 carrier wheel that presents a convexly curved surface at .

3 the finishing spot has the advantage that it can be used 4 in finishing flat and concave surfaces as well as convex surfaces as shown in Figure 3. As a result, the carrier 6 wheel of the present invention in its vertical 7 orientation can be used in finishing arbitrary shapes, 8 including, for example, toric shapes and cylinders.

9 However, a wheel dedicated to finishing of flat surfaces only may advantageously have a rim that is flat across 11 its width.

12 Another advantage of the use of a vertical 13 carrier wheel is that the wheel brings the MR fluid into la contact with the workpiece at a steeper angle than a flat carrier surface. As shown in Figure 4, in contrast to a 16 flat carrier surface 24, which typically brings the MR

17 fluid ribbon into contact with workpiece 26 at an angle 18 that is nearly tangent to the workpiece, carrier wheel 12 19 causes the MR fluid ribbon to impinge on workpiece 26 at an angle that is more obtuse. As a result, the ribbon is 21 less likely to be obstructed by the outer-edge 28 of the 22 workpiece as it travels to the finishing spot. The 23 inventors have found that this advantage can be obtained , 24 by use of a vertical carrier wheel with no decrease in removal rate.
26 The magnetic field at the gap may be created by 27 any means, including electromagnets and permanent 28 magnets. In the exemplary device of the present 1 invention the magnetic field is created by a DC

- 2 electromagnet equipped with polepieces 30 positioned 3 under the carrier surface for applying the magnetic field 4 to the MR fluid. The volume of space that falls directly 5 between the polepieces can be termed a magnet gap. The 6 magnetic field outside the magnet gap can be termed the 7 fringing field. The magnetic field lines in the fringing 8 field are arcs connecting the poles. The magnet gap 9 between the polepieces of the present invention is 10 positioned under the carrier surface. The polepieces of 11 the present invention may be used with vertical wheel 12 12 of Figure 1 or with any other suitable configuration of a 13 carrier surface. The MR fluid ribbon is carried through 14 the fringing field by the carrier surface. Where the 15 carrier surface is the rim of a vertical carrier wheel, 16 the polepieces are situated on either side of the wheel 17 under the rim.

18 The present invention contemplates polepieces 19 that create an optimal fringing field in the gap between the workpiece and the carrier surface where the finishing 21 spot or work zone is created. The polepieces are also 22 preferably designed to minimize field strength at the 23 fluid collector so as to inhibit stiffening of the MR

24 fluid and assist in fluid pick-up. Both of these goals may be met by polepieces that create an enhanced fringing 26 field above the magnet gap, i.e., in the direction of the 27 finishing spot, and a diminished fringing field below the 28 magnet gap.

WO 9T/14532 PCTlUS96/16568 1 An exemplary polepiece design for use with a 2 vertical carrierwheel that is predicted to have these 3 characteristics is depicted in Figure 5. Figures 6A and 4 6B are cross-sectional profiles of the exemplary polepiece which include field vectors of magnetic field 6 magnitude and direction in and surrounding the polepiece.
7 This design has the additional advantage that it can be 8 made with conventional machining rather than CNC
9 machining, which should substantially reduce the cost of l0 manufacturing.
11 The exemplary apparatus additionally comprises 12 a fluid circulation system. (Figure 7). The fluid 13 circulation system comprises fluid delivery nozzle 18, 14 fluid collector 20, and apparatus to recycle the fluid from the collector back to the delivery nozzle. The 16 fluid circulation system of the present invention may be 17 used with any carrier surface including the carrier wheel 18 depicted in Figure 1.
19 Tn order to reduce evaporation of carrier fluid and degradation of the MR fluid due to air contact, the 21 fluid circulation system preferably reduces exposure of 22 the MR fluid to the atmosphere, excepting the ribbon of 23 fluid transiently present on the carrier surface. Since , 24 the fluid circulation system is external to the finishing apparatus, the MR fluid may be subjected to any number of 26 regulatory functions between the collector and the 27 nozzle, allowing the reproducibility and predictability 28 of the MR fluid ribbon to be greatly enhanced.

1 An exemplary fluid delivery nozzle 18 for use 2 with vertical carrier wheel 12 is depicted in Figure 8.
3 The fluid delivery nozzle is preferably composed of 4 magnetically soft material such as iron. The S magnetically soft nozzle shields the MR fluid from 6 magnetic Melds and thus inhibits the stiffening of fluid 7 before it leaves the nozzle. The nozzle and the tube 8 feeding the nozzle preferably provide laminar flow of the 9 MR fluid. The nozzle may be internally tapered. The nozzle may or may not make direct contact with the 11 carrier discharge end of the surface. If it does, a 12 Teflon~ or similar coating may be advantageous to prevent 13 wear on the carrier surface. The trajectory of the fluid 14 as it exits the nozzle is preferably tangential to the carrier surface.
16 In one embodiment, the MR fluid ribbon is 17 shaped by the fluid delivery nozzle when the MR fluid is 18 deposited on the carrier surface. In an alternate 19 embodiment, the ribbon may be shaped by a scraper brought into contact or near-contact with the carrier surface, 21 wherein the scraper has an opening that shapes the MR
22 fluid deposited on the carrier surface by the nozzle.
23 Since in this embodiment the MR fluid may form a pool 24 behind the scraper, the carrier surface may be equipped with sidewalls (not shown) such as those in a circular 26 trough. The nozzle or scraper is preferably located in a 27 region subjected to some magnetic field strength so that 28 the MR ribbon takes on sufficient plasticity to 1 substantially maintain the shape it receives from the 2 nozzle or scraper.
3 The shape of the nozzle or the scraper is one 4 factor in determining the cross-sectional size and shape of the ribbon. This in turn may influence the size of 6 the finishing spot; a narrower ribbon can create a 7 narrower finishing spot. A narrower spot can provide 8 higher resolution in the finishing process and therefore 9 is especially useful in finishing very small workpieces.
ZO Figure 9D is a perspective view of scraper 48 that was 11 used to create a ribbon with a triangular cross-section 12 and the triangular ribbon it created. This "tapered"
13 ribbon has been used to successfully finish lenses as 14 small as 5 mm in diameter.
Figure 9B is a perspective view of scraper 50 16 that was used to create a sawtooth pattern and the 17 sawtooth-shaped ribbon it created. This scraper was made 18 for the purpose of illustrating the ability to create a 19 ribbon that maintains its shape and transfers that shape to the workpiece. Figure 9A is a perspective view of an 21 originally flat workpiece made of K7 glass that was 22 shaped by contact for 5 minutes with the ribbon formed by 23 the scraper of figure 9B. Figure 9C is a profile of the 24 workpiece of figure 9A taken with a Rank Taylor-Hobsen Form Talysurf° profiler.
26 Figure 7 is a schematic illustration of the 27 fluid circulation system. As shown, the MR fluid may be 28 pressurized by one or more delivery pumps 32. The fluid 1 circulation system preferably delivers the MR fluid to 2 the carrier surface at a linear rate equal to or greater 3 than the linear speed of the carrier surface. When the 4 MR fluid delivery rate is slower that the carrier surface S speed, a discontinuous ribbon may be formed. When the MR

6 fluid delivery rate is faster than the carrier surface 7 speed, a thicker ribbon is created. The thickness of the 8 ribbon may be controlled by means of changing the MR

9 fluid delivery rate. Expressed mathematically, the fluid delivery rate, Q (cm3/sec), is equal to the cross-11 sectional area of the ribbon, S (cm2), times the linear 12 velocity of the carrier surface, V (cm/sec): Q = S x V.

13 Thus for a given carrier surface velocity, an increase in 14 the fluid delivery rate Q results in an increased ribbon cross-section, S. Likewise, for a-given fluid delivery 16 rate Q, a decrease in the carrier surface velocity V

17 results in an increased ribbon cross-section, S.

18 Fluid collector 20 (Figure l0) may include a 19 pickup scraper 52 of rubber, flexible plastic or like material that acts as a squeegee to separate the fluid 21 from the carrier surface. The wheel engaging portion of 22 the pickup scraper should conform to the shape of the 23 carrier surface. The wheel engaging portion preferably 24 forms a cup-like shape, or U-shape, with the MR fluid ribbon entering at the open side. Fluid collector 20 is 26 preferably connected to one or more suction pumps 34 so 27 as to draw in the MR fluid. The fluid collector 28 advantageously comprises or is covered by a magnetic 1 shield of magnetically soft material such as iron. This 2 magnetic shield substantially releases the MR fluid from 3 the effects of surrounding magnetic field so as to allow 4 the fluid to return to a less viscous state. In 5 addition, the collector is advantageously located in a 6 position that is farther from the magnetic polepieces 7 than the nozzle so as to reduce the intensity of its 8 exposure to the magnetic field.

9 The circulation system advantageously uses 10 peristaltic pumps so that contact between the MR fluid, 11 which may contain abrasive particles, with degradable 12 parts, which are difficult to replace, is reduced. In a 13 peristaltic pump, the only part subject to any 14 substantial wear by the MR fluid is a short piece of 15 plastic tubing that lasts through several hundred hours 16 of use and can be replaced cheaply. Peristaltic pumps 17 themselves are relatively inexpensive. It has been found 18 that they can be operated at low flow rates without 19 generating gaps in the ribbon. Two or more pumps may be 20 used in parallel to stagger the pulsations that may be 21 created and thereby decrease their amplitude. In a 22 preferred embodiment, two three-headed delivery pumps 32 23 are used wherein the drive heads are offset by 60 with , 24 respect to each other.

MASTERFLEX~ 6485-82 PharMedo tubing can be used 26 for the suction section~of the fluid circulation system.

27 IMPERIAL-EASTMAN 3/8 tubing can be used for the delivery 28 portion of the fluid circulation system. EASY LOAD

WO 97/14532 PCTlUS96/16568 1 MASTERFLEX~ pumps, mod. no. 7529-00 can be used as 2 delivery pumps. COLE-PALMER MASTERFLEX~ pump, mod. no.

3 7019-25 can be used as suction pumps. Permanent magnet 4 motor mod. no. 2M168C, Dayton, can be used to drive the pumps, with DC Speed Controls mod. no. 5X485C, Dayton.

6 The MR fluid removed from the carrier wheel by 7 the collector may be routed to reservoir 36 as shown in 8 Figure 11A. A PP NALGENE~, 1000 ml Seperatory Funnel can 9 be used as a reservoir. Preferably the MR fluid is delivered to such a reservoir with sufficient force to 11 homogenize the MR fluid by breaking up any remaining 12 magnetic particle structures created by the applied 13 magnetic fields. However, the reservoir may also contain 14 additional agitating equipment for this purpose such as a stirrer 38. A Laboratory stirrer TLINE, mod. no. 102 can 16 be used for this purpose. Alternately, other mixing or 17 homogenizing equipment could be used. The reservoir may 18 be nonmagnetic, wear-resistant material like stainless 19 steel. It may have a conical or some other shape that provides no settling zones where MR fluid could 21 aggregate. Also, it may be configured to allow an 22 agitator that reaches a large volume of the reservoir to 23 fit therein so as to leave no settling zones. (Figure 24 11B) The fluid circulation system may additionally 26 comprise temperature regulating apparatus such as cooling 27 mechanisms to remove the heat generated in the finishing 28 zone and carried away by the MR fluid. The temperature 1 of the MR fluid may also be increased by heat generated 2 by the operation of the MR fluid circulation pumps or the , 3 electromagnet. Unregulated high temperatures may lower 4 MR fluid viscosity and may result in high rates of carrier fluid evaporation. Unregulated high temperatures 6 may also cause thermal expansion of parts of the device, 7 leading to inaccurate positioning of the workpiece in the 8 MR fluid and a resultant loss of figure control. In an 9 exemplary apparatus, the MR fluid is cooled by immersion of a cooling coil 40 in the reservoir. Constant 11 temperature chilled water is supplied to the cooling coil 12 by connecting it to a closed loop water chiller such as a 13 Brinkman Lauda RM6. The temperature of the MR fluid is 14 typically kept at about 21-22 degrees C.

The fluid circulation system may additionally 16 comprise composition regulating apparatus such as an 17 automatic viscosity control system to restore carrier 1F fluid lost to evaporation or other causes from the MR

19 fluid. An automatic viscosity control system 54 may be used to maintain the MR fluid at a constant viscosity by 21 automatically dripping carrier fluid into the reservoir 22 to replace losses. The viscosity control system may 23 comprise a viscosity monitoring apparatus functionally 24 connected to a carrier fluid pump 44 with a reservoir of carrier fluid 56. In an exemplary apparatus shown in 26 Figure 7, viscosity is monitored by use of one or more 27 pressure probes 42 located in the delivery line, since 28 pressure changes in the delivery line between the 1 delivery pumps) and the delivery nozzle are proportional 2 to viscosity changes for a constant flow rate.
3 Preferable pressure probes are diaphragm sensors such as 4 Cooper PFD 102, since they minimize stagnation points in the line which would allow the MR fluid to settle and 6 clog the sensors. The pressure probe signal (or, when 7 multiple probes are used, the difference in the signals 8 from successive pressure probes) is proportional to the 9 viscosity of the MR fluid if the flow rate of the fluid is constant. The pressure probe signal (or differential 11 signal) is compared to a reference value and if the 12 signal exceeds the reference, an error signal is sent to 13 an electrical relay or motor driver, which activates the 14 carrier fluid mini-pump 44 until the signal returns below the maximum level. In some embodiments the constant of 16 proportionality between the pressure probe signal (or 17 differential signal) and the error signal must be chosen 18 to avoid over-correction (resulting in oscillation), or 19 under-correction (resulting in sluggish control of viscosity).
21 Alternatively, the MR fluid magnetic particle 22 concentration may be monitored by use of an inductance 23 probe, such as a coil ofwire wrapped around a tube 24 carrying the flowing MR fluid. A higher inductance reading from the coil indicates a higher magnetic 26 particle concentration and a higher MR fluid viscosity.
27 However, this technique fails to sense significant 28 viscosity changes brought about by changes in temperature " . . -1 or nonmagnetic particle concentration. It should be 2 considered only as a secondary indicator of MR fluid 3 stability. Pressure measurements have proven to be more 4 sensitive than inductance measurements.
Selection of a stable MR
fluid greatly enhances 6 reproducible and predictable finishing.
Many MR
fluid 7 compositions are known in the art, including both oil-8 based and water-based fluids.
The present application 9 contemplates use of an MR
fluid that is preferably based on an aqueous carrier fluid for most applications.

11 However, an aqueous carrier fluid might not be suitable 12 for use on some workpieces such as water soluble 13 workpieces like those comprising KDP
(KH2P0~) crystals.

14 The MR
fluid contains non-colloidal magnetic particles such as carbonyl iron particles.
Table 16 identifies four carbonyl iron powders available from GAF

17 Corp.
that have been found to be useful in MR
fluids.

1 9 Type Description Median Largest Diam. aarticle 2 0 S-1100largest size 5.170pm 22.79~m available 2 1 S-1701silica coated4.667um 15.17~m articles 2 2 N-1370Sk nitrided 3.845um 15.17~m aarticles 2 3 S-3700smallest size3.089y~m 13.24~m available 15 To enhance material removal, the MR fluid may 16 also contain a nonmagnetic abrasive material, such as 17 cerium oxide (Ce02) particles. The choice of nonmagnetic 18 abrasive material is dictated by the physical properties 19 (e. g. hardness)_and chemical properties (e. g. chemical 1 durability) of the workpiece to be finished. Table 2 2 lists MR fluid formulae using a variety of abrasive 3 particles and the removal rates obtained with those 4 formulae when the MR fluid was used to finish a fused 5 silica workpiece using the method of the present 6 invention on an apparatus employing a rotating trough 7 carrier surface. Removal rates were measured by 8 comparison of before and after profiles of the workpiece 9 taken with a Zygo Mark IV xp~ interferometer. The first 10 two formulae comprise no added abrasives; they rely on 11 the abrasive qualities of the carbonyl iron alone. Table Z2 3 lists removal rates for a variety of workpiece 13 materials obtained using a standard MR fluid formula 14 containing a cerium oxide abrasive (Formula D of Table 2) 15 and an enhanced formula containing cerium oxide and 16 nanodiamond abrasives (Formula E of Table 2) using the 17 method of the present invention on an apparatus employing 18 a rotating trough carrier surface. The data demonstrate 19 that the present method is useful for finishing even very 20 hard materials such as sapphire (A1203) .

Z. Table AbrasiveAbrasivolt Volt Volt volt Volt peak Vol.

type ve abrasiCI H,O GlycerNa,CO,removremoval size, ve ine al race ycm rate mm'/min ~m/mi n -none, -- 0.00 44.3 53.2 2.05 0.31 1.28 0.176 just 6 7 CI, ~J A S-3700, 3 ~m none, -- 0.00 46.0 51.5 2.12 0.67 1.8360.1556 just 4 3 CI, B S-1701, 4.5 um small 1 4.84 35.1 57.2 2.38 0.39 2.0640.2808 CeO, 6 3 C CeROx Std. 3.5 5.7 36.0 55.1 2.41 0.74 2.1040.22 CeO, S i D CeRice Std. -- -- -- -- -- -- 2.4 .36 Ce0 CeRite w/

E 15 g nanodiam and mixture:3.5/1.4.2/4.36.0 53.1 2.33 0.00 1.9920.2464 CeO, 0 2 9 8 and F A1,0, 2 5 C A1,0, 2.1 8.67 35.8 53.1 2.26 0.12 1.4 O.1fi04 #1 2 6 H A1203 7.0 8.77 35.2 52.6 2.28 0.1 1.6320.1552 #9 5 2 7 I SiC 4 6.72 36.0 54.9 2.2 0.09 1.1230.1257 2 8 J B,C 7 5.94 35.1 55.2 2.28 0.37 0.5740.0567 WO 97!14532 PCT/US96/I6568 Ta ble 3 Workpiece Hardness Standard Enhanced Standard Enha.-.ced Material H" peak peak v~_. vol.

removal removal removal re.-...~.val rate rate rat rate um/min ~m/min mm'/min -'/min A1,0, 2000 .0313 .524 .00189 .0294 Silicon 1100 1.46 4.03 .23 .6 6 TaFDS 683 1.872 1.776 0,205 0.23 7 fused silica669 2.076 2.; .29 .36 L3K10 650 2.42 2.256 .45 .504 9 SK7 559 6.935 ..108 1 ~ HK7 , 527 4.03 4.62 .48 ,7 1 1 K7 516 4.87 3,792 .53 .636 1 2 KzF6 434 8.592 .88 -3 SF7 405 5.556 .726 1.
SF56 366 9,24 6.Oe4 1.24 .,058 1 J LHGB 338 9.156 23.99 1.43 3.8, 1 o ZnSe 120 2.545 6.4= .1935 .270 18 It is an advantage of the present invention 19 that the finishing spot is relatively insensitive to 20 abrasive particle size.
Figure is a graph of 21 finishing spot widths and lengths achieved with varying 22 sizes of abrasive grit.
The finishing spots were 23 measured with a Zygo Marr.
IV
xp~
interferome~er.
Spot 2-~_~_ size remained relatively constant with particles of 25 microns.
A
further advantage of the present invention is 26 that unwanted, oversized abrasive particles are less 27 troublesome because they cannot become embedded and 28 scratch the workpiece surface as they can with a solid 29 lap.
30 The MR
fluid may also contain a stabilizer such 31 as glycerol.
The stabilizer is used to add viscosity to . 32 the MR
fluid and to create conditions that help to keep 33 the magnetic particles and abrasive articles in p 34 suspension.
However use of an e i , xcess ve amount cf a 32 PCTlUS96/16568 1 stabilizer like glycerol can be detrimental in finishing 2 certain materials such as silicate glasses. It is 3 thought that this result is due to the effect of glycerol 4 in inhibiting the ability of water to hydrate and thereby soften the glass surface.

6 Any form of degradation of the MR fluid can 7 present difficulties in MR finishing since an unstable MR

8 fluid produces a less predictable finishing spot. Rust 9 may cause stability problems with the present type of MR

fluid, since the fluid employs finely divided iron 11 particles in an aqueous slurry. Since iron oxide has 12 different magnetic properties than carbonyl iron, the 13 magnetic properties of an MR fluid that is rusting are 14 continually changing and thus rust is a source of unpredictability. In addition, rust in the MR fluid can 16 stain the workpiece.

17 Since the MR fluid is partially exposed to the 18 atmosphere, it can absorb carbon dioxide, which lowers 19 the pH of the fluid and contributes to the oxidization of the metal. Using deionized water as a carrier fluid 21 slows corrosion but does not entirely solve the problem 22 and it adds to inconvenience and expense.

23 The inventors have found that the addition of 24 alkali sufficient to raise the pH to about. l0 both improves stability and simultaneously increases removal 26 rates. Particularly useful alkalis in this application 27 are buffers such as NaZC03. A further advantage to the 28 use of an alkaline buffer is that the use of deionized WO 97!14532 PCT/US96/I6568 1 water is no longer necessary and tap water may be used . 2 instead. Figure 13 is a graph indicating volumetric 3 removal rates, normalized to a starting rate of one unit, 4 measured over periods of six or more hours for MR fluids made with three different carrier fluids: deionized (DI) 6 water at pH7, DI water at pHlO with NaOH, and tap water 7 at pHlO with Na2C03. (Note that the DI water is assumed 8 to be at pH7 by theory - since it contains no ions, its 9 pH cannot be measured by the use of conventional probes.) The finishing runs were made using the method of the 11 present invention on an apparatus employing a rotating 12 trough carrier surface using identical formulae other 13 than the differing carrier fluid and using identical la workpieces and the removal rates were measured as described above. The removal rate for the pH 10 fluid 16 containing Na2C03 remained high, while the removal rate 17 for the pH 10 fluid containing NaOH fell off to SOo of 18 the initial rate after 7 hours use, and the removal rate 19 for the pH 7 fluid fell off to about 600 of its initial value after two hours and became erratic. Table 4 21 demonstrates an increase of about 39a in volumetric 22 removal rate (volume of material removed per second) and 23 an increase of -about 50o in peak removal rate (depth of 2a material removed per second) which occurs with the use of a pH 10 carrier fluid (Runs 1 and 2) in comparison to a 26 pH7 carrier fluid (Runs 3 and 4). The finishing runs in 27 table 4 were made using the indicated formulae and using 28 the method of the present invention on an apparatus Z employing a rotating trough carrier surface with 2 identical workpieces and the removal rates were measured 3 as described above.
4 Table 4 51710 CI Run 1 Run 2 Run 3 Run 4 Formula 9.4t glycerin4.Ot glycerin4.Ot glycerin4.Ot glycerin tAll use 24.31 Ce02 21.7tCe02 22.21 Ce02 22.21 C~C2 carbonyl 41 CI
iron . 28.9tCI 29.St CI 29.St C.
(CI) and 20 rpm 20 rpm 20 rpm 20 r m deionized pHlO-11 pH9-10 oH7 p .racer) oH7 Peak removal2.4 2.8 1.6 1.84 race Nm/min 2.4 2.8 1.75 2.6 2.6 avg. peak 2.4 2.7 1.6 1.8 removal rate Vol. removal0.15 0.18 0.11 0.124 race mm3/min 0.17 0.15 0.124 0.15 0,16 ~ ~ 0.16 ~ 0.16 ~ 0.1. 0.12 avg, vol.
removal ~

.
race l9 ' 20 The present invention contemplates a MR
fluid 21 that comprises an aqueous carrier fluid that includes an 22 alkaline buffer such as Na2C03 that demonstrates improved 23 stability and resistance to rust, increased removal 2a rates, and which can be formulated with tap water.

25 Workpiece to be finished may be mounted on a 26 workpiece holder comprising a rotatable workpiece spindle 27 46, which is preferably made from a non-magnetic 28 material.
The spindle is lowered and the workpiece is 29 brought into contact with the MR
fluid ribbon so as to 30 create a finishing spot (Figure 14).
As angle a is 31 varied, the spot can be swept from the center to the edge 32 of the lens.
(Figure 15).
For rotationally symmetric 33 workpieces, the spindle rotates the workpiece about the 34 spindle axis.
Since the workpiece is spinning and _ sweeping, the finishing spot removes material in annular 36 rings from center to edge on the workpiece surface and 1 the resulting lens or other workpiece is symmetrical y 2 about the spindle axis. The dwell time at each location 3 on the lens is controlled so that shape errors in the 4 workpiece are corrected. Preferably the determination of dwell times and the control of the spindle motion is done 6 by computer.
7 Angle 8 is measured relative to the vertical.

8 The spindle is pivoted through angle B around a pivot 9 point. The spindle can be pivoted through angle 8 in any direction, but preferably it is pivoted in a direction 11 parallel to or perpendicular to the direction of motion 12 of the MR fluid. In the apparatus of Figure 1, carrier 13 wheel 12 can be rotated about the Z axis to allow the 14 operator to change the direction in which angle 8 pivots relative to the direction of motion of the MR fluid.

16 Normally, the spindle rotation rate would be 17 held constant. A typical rate is 75 rpm. However, to 18 finish non-rotationally symmetric workpieces or to 19 correct non-rotationally symmetric flaws, the spindle velocity may be varied as a function of the spindle's 21 rotational position. For workpieces such as cylinders, 22 the spindle motion may consist of translational and 23 pivotal motions without any spindle rotation. For flat 24 workpieces, the spindle motion may consist of a combination of translational motion and spindle rotation 26 without any pivotal motion or may consist of 27 translational motions in a raster pattern.

WO 97!14532 PCT/US96/16568 1 In one system for moving the finishing spot 2 over the surface of the workpiece, angle 0 is the only , 3 variable. In this system the spindle is lowered until 4 the workpiece is brought into contact with the MR fluid ribbon. The spindle is then rotated through angle a 6 about a mechanical pivot point, the B-axis, comprising 7 the rotating joint which holds the spindle above the 8 carrier surface. The B-axis is parallel to the Y-axis as 9 shown in Figure 1. In this system a constant working gap (i.e., the gap between the workpiece and the carrier 11 surface) is maintained ifthe workpiece is spherical but 12 not when the workpiece is aspheric. However, it is an 13 advantage of the present invention that the finishing 14 spot is tolerant of large variations in gap height. For this reason, aspheric workpieces may be finished even 16 when the spindle is restricted to spheric motion, as 17 demonstrated by Example 2, discussed below.

18 Another system for polishing rotationally 19 symmetric workpieces is shown in Figure 1_ In this embodiment, spindle motion is reduced to three active 21 degrees of freedom, other than the motion of the 22 spindle's rotation of the workpiece. The motion of the 23 spindle axis is restricted to the XZ plane. The spindle 24 may be moved translationally up and down along the Z-axis, translationally left and right along the X-axis, ' 26 and rotationally through-angle a clockwise or counter-27 clockwise around the B-axis.

1 The machine also has two passive degrees of 2 freedom. The carrier wheel and its support base may be 3 rotated manually about the Z-axis so that the wheel is 4 either parallel or perpendicular to the X-axis. The spindle may be moved manually along the Y-axis for fine 6 alignment of the spindle with the wheel during machine 7 set up.

8 By moving the active axes in synchronism, the 9 workpiece may be manipulated such that working gap is maintained constant and the finishing zone is moved from 11 the center of the workpiece to the edge of the workpiece 12 along a diameter. This motion, along with the rotation 13 of the workpiece on its axis, can move the entire surface 14 of lens through the working zone.

Control of the spindle arm motion may be 16 achieved by any convenient mechanical means. The spindle 17 arm controller may advantageously be directed by computer 18 control.

19 Computer controlled finishing of a part may be 2o accomplished by the process depicted by Figure 16. A

21 computer code called the Forbes-Dumas Finishing Algorithm 22 (FDFA) is used. It requires three inputs: A) the shape 23 of and magnitude of the MRF removal function or finishing 24 "spot", B) the initial workpiece surface shape, and C) the processing objectives, e.g. do material removal, 26 figure correction, or both. As output, the FDFA

27 generates a machine control operating program, known as 28 the MCOP. The FDFA may also generate a prediction of the l residual surface figure errors that will remain in the 2 part after processing. The MRF machine is controlled by 3 the MCOP to effect the finishing of the workpiece.
4 The MRF removal function can be obtained by generating a spot on a test piece of the same material 6 type and shape to be finished. An interferogram of the 7 removal "spot", recorded by an interferometer like the 8 Zygo Mark IV xp~, can be acquired and loaded into the 9 computer control code. Alternatively, a previously recorded and stored "spot" profile may be called up from 11 a database for use.
12 The finishing spot is specific to the machine 13 platform, the magnetic field strength, the workpiece 14 geometry, the carrier surface velocity, the MR fluid properties, the spindle/carrier surface geometry, and the 16 properties of the material being finished. Figure 14B
17 shows the removal "spot" (direction of fluid motion 18 indicated with arrows) for a 40 mm diameter, 84 mm radius 19 of curvature BK7 glass lens, resulting from 5 seconds contact with the MR fluid at a height of 1 mm above the 21 carrier surface. An apparatus employing a rotating 22 trough carrier surface was used. For this apparatus, the 23 radius from the center of the trough to the inner edge is 24 23 cm and the radius to the outer edge is 30 cm. The trough was rotated at 20 rpm and the magnetic field 26 strength at the gap was 2-4 kG. The spindle arm was 27 oriented at an angle of 6 = 2° and it was locked to 28 prevent workpiece rotation. As this depth profile shows, 1 the finishing spot has a "D" shape, with a region of peak 2 removal at the point of deepest penetration of the lens 3 surface into the suspension. The peak removal is 4.6 4 ~.m/minute, and the volumetric removal is 0.48 mm3/minute.

S The finishing spot is dependent on the material 6 type. Figure 17 shows interferograms of spots taken on 7 two different glass types: fused silica and SK7. For 8 the fused silica part, the spot was acquired by lowering 9 the part into the suspension to a height of lmm above the 10 carrier surface and at A = 0, turning the magnetic field 11 on for 20 seconds, turning the field off and raising the 12 part up and out of the suspension. Depth profile line 13 scans, taken in directions parallel (~~) and perpendicular 14 (1) to the direction of flow are displayed below the 15 spot. They indicate a peak removal rate of 2.3 /,cm/minute 16 for this glass. For the SK7 part, a spot is acquired by 17 first turning on the magnetic field. The spindle-mounted 18 part is then swept through an angle to the near-normal 19 incidence orientation in the suspension at a height of 20 lmm above the carrier surface. It is kept there for a 21 period of 4 seconds and then swept back out. Because of 22 its composition and physical properties, SK7 finishes 23 faster than fused silica. The measured peak removal rate 24 is 9.4 E,cm/minute. The spot shapes for these glasses are 25 very similar. This is a characteristic of the MR

26 process.

27 The second input to the FDF.A in the initial 28 surface error profile of the surface to be finished, 1 which for a spherical surface is another interferogram 2 showing initial deviation from a best fit sphere. For an , 3 aspheric surface the input could be a surface error 4 profile obtained with a stylus instrument like the Rank S Taylor Hobsen Form Talysurf~.

6 The third input is the processing objective.

7 This could be do removal to eliminate subsurface damage, 8 figure correction, or a combination of the two.

9 The computer code combines the removal function with the initial surface shape to derive an operating 11 program for the spindle arm angular controller on the MRF

12 machine. The code specifies angles and accelerations of 13 the controller, the number of sweeps required between 14 positive and negative angles, and the total estimated processing time. Finally, the code may give a prediction 16 for figure expected from the process cycle.

17 In the embodiment depicted in Figure 1 the 18 operating program for the spindle arm angular controller 19 can be derived by use of a "virtual pivot point". A

virtual pivot point may be determined when finishing any 21 surface. The virtual pivot point coincides with the 22 center of the sphere which includes the surface to be 23 finished. The virtual pivot point is stationary relative 24 to the workpiece for spherical surfaces. In the case of a convex workpiece the virtual pivot point will lie above 26 the workpiece surface, whereas in the case of a concave 27 workpiece the virtual pivot point will lie below the 28 workpiece surface. In the case of an asphere, the 1 virtual pivot point will change appropriately to coincide 2 with the local curvature of the zone being polished.
3 The MCOP for the spindle arm controller causes 4 the spindle arm to move such that the virtual pivot point will remain at a predetermined location. Here, input for 6 MCOP is the radius of curvature of the part being 7 finished. In the case of an asphere, the asphere sag 8 equation must be provided as an input. The virtual pivot 9 point approach enables simulated rotation about an arbitrary point in space with three degrees of freedom.
11 The pivot point can be constant (for spheres) or variable 12 (for aspheres). Without a simulated or virtual pivot 13 point, several dedicated machines would be required to 14 perform the many tasks that can be accomplished in a single machine.

17 Examt~le 1 18 Table 5 presents the results of a three-cycle 19 finishing proces s using the FDFA which illustrates do removal, figure correction, surface smoothing. The and 21 workpiece was spheric convex fused silica part 40 mm a in 22 diameter with 58 mm radius curvature which was a of 23 generated on the Opticam~SX.

1 Table 2 Cycle amount duratiofigure areal removed n error roughness um minutesum p-v d rms 3 initial -- -- 0.31 40 #1: do 3.0 32 0.42 8 removal/smoothin 6 #2: figure correction0.7 6 0.1.1 7 #3: do removal/figure3.0 92 0.09 8 correction 11 The first cycle lasted minutes, removing 3~m 12 uniformly from e and the surfac reducing the areal surface 13 roughness from A rms 40 A to 8 (as measured with an 14 unfiltered, Zygo D optical Maxim 3 profiler).
Symmetric surface wavefront error was held to an increase of 0.11 16 um for 3 /.cm of material removed. A second cycle brought 17 figure error down from 0.42 ~m to 0.14 ~.m. This was 18 accomplished in 6 minutes with the radially selective 19 removal of ~0.7 ~m of material. A third cycle was implemented which removed an additional 3 ~m of material 21 while further reducing symmetric figure error to 0.09 Vim.
22 The final areal roughness was again 8 A rms.
23 A portion of the Forbes/Dumas user interface 24 for cycle #2 is shown in Figure 18. Tnterferograms for the initial, predicted, and actual surface figure errors 26 are shown at the top of the figure. Below each 27 interferogram is a line scan representing a radial , 28 section depicting the symmetric wavefront error compared 29 to a best fit sphere_ Note that this figure correction cycle removed a hole at the center of the surface.

WO 97/x4532 PCT/US96/16568 1 Example 2 2 Table 6 presents the results of a two-cycle 3 finishing process using the FDFA which illustrates do 4 removal and surface smoothing in cycle #1 and figure correction in cycle #2. The workpiece was an aspheric 6 convex BK7 glass part 47 mm in diameter with 140 ~m of 7 aspheric departure from a 70 mm radius of curvature which 8 was generated on the Opticam°SM.
9 Table 1 cycle amount cycle rms p-v removedtime roughne figure ~m min ss ~m A

1 initial --- --- 9400 6.42 ~1: do ssdd 1z loo l0 a.ao removal /

smoothing N2: figure 4 40 10 0.86 correction 19 The first cycle lasted minutes, removing 20 12~m uniformly from the surface, reducing the areal 21 surface roughness from A
to A
rms (as measured 22 with a Zygo New Views 20x Mirau optical profiler), and 23 removing all subsurface damage.
Symmetric surface 24 wavefront error decreased from 6.42 ~.m to 4.40 ~.m.
The 25 second cycle brought figure error down from 4.40 ~m to 26 0.86 ~.~.m.
This was accomplished in minutes with the 27 radially selective removal of ~.m of material.
The 28 final areal roughness remained A
rms.

29 The finishing procedures of examples and 30 were performed on an apparatus having a mechanically 31 fixed pivot point.
However, the workpiece in example 32 was aspheric, having ~Cm of departure from a spheric 1 shape. As a result, the gap between the carrier surface 2 and the workpiece varied during the finishing operation.
3 Since the finishing spot did not vary in effectiveness 4 over this aspheric surface, example 2 illustrates the 5 relative insensitivity of the finishing spot to the 6 height of the gap between the carrier-surface and the 7 workpiece.

Claims (107)

CLAIMS:
1. A method of finishing a workpiece surface using magnetorheological fluid, comprising:
(a) positioning the workpiece near a continuous carrier surface such that a converging gap is defined between a portion of the workpiece surface and the carrier surface, wherein said carrier surface extends along a rim of a vertically oriented wheel;
(b) applying a magnetic field substantially at said gap;
(c) depositing magnetorheological fluid from a magnetorheological fluid source on the carrier surface;
(d) moving the carrier surface past said workpiece by rotating the wheel such that magnetic field-stiffened magnetorheological fluid flows through said converging gap defining a work zone forming a transient finishing tool for engaging and causing material removal at a portion of the workpiece surface;
(e) moving the workpiece or the work zone relative to the other to expose different portions of the workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said workpiece surface to predetermined degrees;
(f) collecting magnetorheological fluid that has flowed through said gap from said carrier surface; and (g) returning the magnetorheological fluid collected in step (f) to the magnetorheological fluid source.
2. The method of claim 1, wherein said wheel is rotatable about a horizontally oriented axle, and wherein said step of rotating the wheel comprises rotating the wheel about the axle.
3. The method of claim 1, wherein said carrier surface comprises an outer surface of a continuous flexible belt.
4. The method of claim 1, wherein said wheel includes an outer surface having a trough such that a center portion of the outer surface has a reduced diameter with respect to edge portions thereof.
5. The method of claim 1, wherein said step of depositing magnetorheological fluid comprises ejecting magnetorheological fluid from a nozzle.
6. The method of claim 5, wherein said nozzle ejects said magnetorheological fluid onto said carrier surface in a direction substantially tangential to said carrier surface and in the direction of motion of the carrier surface.
7. The method of claim 1, further comprising the step of imparting a predetermined geometric shape to said flow of magnetorheological fluid after step (c) to vary the configuration of the work zone.
8. The method of claim 1, wherein said step of applying a magnetic field comprises the step of maximizing a fringing field in the vicinity of the converging gap.
9. The method of claim 1, wherein step (f) comprises using a collector to collect magnetorheological fluid from the carrier surface and further comprising the step of reducing the strength of the magnetic field at the collector.
10. The method of claim 9, wherein said collector is magnetically shielded to reduce the strength of the magnetic field in the collector.
11. The method of claim 1, wherein step (c) comprises ejecting the magnetorheological fluid through a nozzle, the nozzle being magnetically shielded to inhibit application of the magnetic field to magnetorheological fluid within the nozzle.
12. The method of claim 1, wherein step (e) comprises rotating the workpiece relative to the work zone.
13. The method of claim 1, wherein said workpiece is mounted on a pivoting workpiece holder and step (e) comprises pivoting the workpiece holder to sweep the workpiece surface through the work zone.
14. The method of claim 1, wherein step (e) comprises moving the workpiece in a plane.
15. The method of claim 14, wherein said step of moving the workpiece comprises moving the workpiece in a plane in a direction substantially parallel to the direction of motion of the magnetorheological fluid.
16. The method of claim 14, wherein the step of moving the workpiece in a plane comprises moving the workpiece in a direction substantially perpendicular to the direction of motion of the magnetorheological fluid
17. The method of claim 1, further comprising the step of monitoring the viscosity of the magnetorheological fluid collected in step (f).
18. The method of claim 17, wherein said step of monitoring the viscosity of the magnetorheological fluid comprises causing the magnetorheological fluid to flow through a tube at a substantially constant flow rate, measuring a pressure drop at two points along the tube, and comparing the pressure drop to a predetermined value.
19. The method of claim 17, further comprising the step of adjusting the viscosity of the magnetorheological fluid to a predetermined level if said step of monitoring the viscosity of the magnetorheological fluid detects a variation from said predetermined viscosity level.
20. The method of claim 19, wherein said step of adjusting the viscosity of the magnetorheological fluid comprises adding carrier fluid to the magnetorheological fluid.
21. The method of claim 1, wherein said magnetorheological fluid has an initial predetermined pH level, and further comprising the steps of monitoring and maintaining the predetermined pH level of the magnetorheological fluid collected in step (f).
22. The method of claim 21, wherein said predetermined pH level is between 7 and 11.
23. The method of claim 21, wherein said predetermined pH level is between 9 and 11.
24. The method of claim 1, further comprising the step of cooling the magnetorheological fluid collected in step (f).
25. The method of claim 1, further comprising the steps of monitoring the temperature of magnetorheological fluid collected in step (f) and adjusting the temperature of the magnetorheological fluid to a predetermined level if said step of monitoring the temperature detects a variation from said predetermined temperature level.
26. The method of claim 1, further comprising the step of inhibiting the degradation of the magnetorheological fluid by collecting the magnetorheological fluid from the carrier surface and limiting its exposure to ambient air prior to depositing the magnetorheological fluid on the carrier surface.
27. The method of claim 1, further comprising the step of rehomogenizing a portion of the magnetorheological fluid collected in step (f) that has agglomerated in the presence of the magnetic field.
28. The method of claim 27, wherein said step of rehomogenizing the magnetorheological fluid comprises ejecting said magnetorheological fluid into a tank with sufficient force to break up agglomerated particles therein.
29. The method of claim 27, wherein said step of rehomogenizing the magnetorheological fluid comprises stirring said magnetorheological fluid.
30. The method of claim 1, wherein said magnetorheological fluid comprises non-colloidal magnetic particles and an aqueous carrier fluid wherein the pH of the magnetorheological fluid is between 7 and 11.
31. The method of claim 30, wherein the pH of the magnetorheological fluid is between 9 and 11.
32. An apparatus for finishing a workpiece surface using magnetorheological fluid, comprising:
a continuous movable carrier surface;

a nozzle for depositing magnetorheological fluid from a magnetorheological fluid source on the carrier surface;
a workpiece holder for holding the workpiece and positioning a portion of the workpiece surface near the carrier surface to define a converging gap therebetween, said carrier surface being movable past said workpiece such that the magnetorheological fluid flows through said gap;
a magnet for applying a magnetic field at said gap to stiffen the magnetorheological fluid flowing through said gap for creating a transient finishing tool for engaging and causing material removal at a portion of the workpiece surface;
means for moving the workpiece or the work zone relative to the other to expose different portions of the workpiece surface to the work zone for predetermined time periods to selectively finish said portions of said workpiece surface in predetermined degrees;
a collector for collecting magnetorheological fluid having flowed through the gap from the carrier surface; and recirculating means for returning the magnetorheological fluid to the magnetorheological fluid source.
33. The apparatus of claim 32, wherein said carrier surfaces comprises an outer rim of a vertically-oriented wheel.
34. The apparatus of claim 33, wherein said carrier surface comprises an outer rim of a wheel rotatable about a horizontally-oriented axle.
35. The apparatus of claim 34, wherein said wheel comprises a non-magnetic material.
36. The apparatus of claim 34, wherein said carrier surface is convexly curved across the width of the rim.
37. The apparatus of claim 34, wherein said carrier surface comprises a spheric section.
38. The apparatus of claim 34, wherein said carrier surface has a cylindrical configuration.
39. The apparatus of claim 32, further comprising a viscosity monitor for monitoring the viscosity of the magnetorheological fluid collected by the collector.
40. The apparatus of claim 39, wherein said viscosity monitor comprises a tube for transporting the magnetorheological fluid at a substantially constant flow rate, pressure sensors for measuring a pressure drop between two points along the tube, and means for comparing the pressure drop against a predetermined value.
41. The apparatus of claim 39, further comprising a dripper for adding carrier fluid to the magnetorheological fluid collected by the collector to adjust the viscosity of the magnetorheological fluid to a predetermined level if a variation from the predetermined viscosity level is detected by the viscosity monitor.
42. The apparatus of claim 32, further comprising cooling means for cooling magnetorheological fluid collected by the collector.
43. The apparatus of claim 32, further comprising a mixer for rehomogenizing magnetorheological fluid collected by the collector and having been agglomerated in the presence of the magnetic field.
44. The apparatus of claim 43, wherein said mixer comprises stirrer.
45. The apparatus of claim 32, further comprising means for imparting a predetermined geometric shape to the magnetorheological fluid entering the gap to vary the configuration of the work zone.
46. The apparatus of claim 45, wherein said means for imparting a shape comprise a scraper engaging said carrier surface, said scraper having an opening for flow of magnetorheological fluid therethrough, said opening corresponding to said predetermined geometric shape.
47. The apparatus of claim 32, wherein said magnet comprises pole pieces configured for maximizing the fringing field in the vicinity of the converging gap.
48. The apparatus of claim 32, wherein said collector comprises a magnetically soft material for magnetically shielding said collector to inhibit application of the magnetic field to magnetorheological fluid in the collector.
49. The apparatus of claim 48, wherein said collector includes a scraper portion for engaging said carrier wheel to enhance removal of magnetorheological fluid therefrom.
50. The apparatus of claim 49, wherein said scraper portion has a cup-like configuration.
51. The apparatus of claim 32, wherein said nozzle comprises a magnetically soft material for magnetically shielding said nozzle to inhibit application of the magnetic field to magnetorheological fluid within the nozzle.
52. The apparatus of claim 32, wherein said magnet comprises pole pieces and wherein said collector is located at a greater distance away from said pole pieces than said nozzle.
53. The apparatus of claim 32, further comprising means for rotating the workpiece relative to the work zone.
54. The apparatus of claim 32, wherein said workpiece is mounted on a pivoting workpiece holder adapted to sweep the surface of the workpiece through the work zone.
55. The apparatus of claim 32, further comprising means for moving the workpiece in a plane.
56. The apparatus of claim 32, wherein said magnet is mounted on a support base and wherein said support base is rotatable relative to the workpiece.
57. A magnetorheological fluid comprising non-colloidal magnetic particles, an aqueous carrier fluid, and an alkaline salt wherein the pH of the magnetorheological fluid is between 7 and 11.
58. The magnetorheological fluid of claim 57, wherein the pH of the magnetorheological fluid is between 9 and 11.
59. The magnetorheological fluid of claim 57, wherein said alkaline salt is Na2CO3.
60. The magnetorheological fluid of claim 57, wherein said non-colloidal magnetic particles are carbonyl iron.
61. The magnetorheological fluid of claim 57, additionally comprising an abrasive.
62. The magnetorheological fluid of claim 61, wherein said abrasive is CeO2.
63. The magnetorheological fluid of claim 62, wherein said abrasive is CeO2 and nanodiamond particles.
64. The magnetorheological fluid of claim 62, comprising about 5.7 volume percent CeO2 abrasive, about 36.05 volume percent carbonyl iron, about 55.11 volume percent water, about 2.41 volume percent glycerol, and about 0.74 volume percent Na2CO3.
65. A method of finishing a workpiece surface using magnetorheological fluid, comprising:
rotating a wheel about a horizontally-oriented axle, said wheel including an outer rim defining a moving carrier surface;
positioning the workpiece near the carrier surface such that a gap exists between a portion of the workpiece surface and the carrier surface;
applying a magnetic field substantially at said gap;
depositing magnetorheological fluid on the moving carrier surface such that field stiffened magnetorheological fluid is carried by the carrier surface and at least some of said magnetorheological fluid flows through said gap defining a work zone forming a transient finishing tool for engaging and causing material removal on a portion of the workpiece surface; and moving the workpiece relative to the work zone to expose different portions of the workpiece surface to the work zone for predetermined time periods to finish said portions of said workpiece surface to predetermined degrees.
66. The method of claim 65, further comprising the step of collecting magnetorheological fluid that has moved past the workpiece from the carrier surface for reuse in finishing the workpiece.
67. The method of claim 65, wherein said step of depositing magnetorheological fluid comprises ejecting magnetorheological fluid from a nozzle.
68. The method of claim 67, wherein said nozzle ejects said magnetorheological fluid onto said carrier surface in a direction substantially tangential to said carrier surface and in the direction of motion of the carrier surface.
69. The method of claim 65, further comprising the step of imparting a predetermined geometric shape to said magnetorheological fluid upstream of the workpiece surface to vary the configuration of the work zone.
70. The method of claim 65, wherein said step of applying a magnetic field comprises the step of maximizing a fringing field present in the vicinity of the gap.
71. The method of claim 65, further comprising the step of collecting magnetorheological fluid that has flowed past the workpiece from the carrier surface for reuse in finishing the workpiece, and wherein said step of collecting magnetorheological fluid comprises engaging a surface of a cup-like collector against the carrier surface to collect magnetorheological fluid from the carrier surface.
72. The method of claim 71, wherein said collector is magnetically shielded to reduce the intensity of the magnetic field in the collector.
73. The method of claim 65, wherein said step of depositing magnetorheological fluid comprises ejecting the magnetorheological fluid through a nozzle, the nozzle being magnetically shielded to inhibit application of the magnetic field to magnetorheological fluid within the nozzle.
74. The method of claim 65, further comprising the step of rotating the workpiece relative to the work zone.
75. The method of claim 65, wherein said workpiece is mounted on a pivoting workpiece holder and said step of moving the workpiece comprises pivoting the workpiece holder to sweep the surface of the workpiece through the work zone.
76. The method of claim 65, wherein said step of moving the workpiece comprises moving the workpiece in a plane.
77. The method of claim 76, wherein said step of moving the workpiece comprises moving the workpiece in a plane in a direction substantially parallel to the direction of motion of the magnetorheological fluid.
78. The method of claim 76, wherein the step of moving the workpiece in a plane comprises moving the workpiece in a direction substantially perpendicular to the direction of motion of the magnetorheological fluid.
79. The method of claim 65, further comprising the step of collecting magnetorheological fluid that has flowed past the workpiece from the carrier surface for reuse in finishing the workpiece, and further comprising the step of monitoring the viscosity of the collected magnetorheological fluid.
80. The method of claim 79, wherein said step of monitoring the viscosity of the magnetorheological fluid comprises causing the collected magnetorheological fluid to flow through a tube at a substantially constant flow rate, measuring a pressure drop at two points along the tube, and comparing the pressure drop to a predetermined value.
81. The method of claim 79, further comprising the step of adjusting the viscosity of the magnetorheological fluid to a predetermined level if said step of monitoring the viscosity of the magnetorheological fluid detects a variation from said predetermined viscosity level.
82. The method of claim 81, wherein said step of adjusting the viscosity of the magnetorheological fluid comprises using carrier fluid to the magnetorheological fluid.
83. The method of claim 65, further comprising the step of collecting magnetorheological fluid that has flowed past the workpiece from the carrier surface for reuse in finishing the workpiece, and further comprising the steps of monitoring the temperature of the collected magnetorheological fluid and adjusting the temperature of the magnetorheological fluid to a predetermined level if said step of monitoring the temperature detects a variation from said predetermined temperature level.
84. The method of claim 65, further comprising the step of collecting magnetorheological fluid that has flowed past the workpiece from the carrier surface for reuse in finishing the workpiece, and further comprising the step of rehomogenizing a portion of the collected magnetorheological fluid which has become agglomerated in the presence of the magnetic field.
85. The method of claim 84, wherein said step of rehomogenizing the magnetorheological fluid comprises ejecting said fluid into a tank with sufficient force to break up agglomerated particles therein.
86. An apparatus for finishing a workpiece surface using magnetorheological fluid, comprising:
a vertical wheel rotatable about a horizontally-oriented axle, said wheel including an outer rim defining a carrier surface;
a nozzle for depositing magnetorheological fluid from a magnetorheological fluid source on the carrier surface such that the magnetorheological fluid is carried by the carrier surface as the wheel is rotated;
a workpiece holder for holding the workpiece and positioning a portion of the workpiece surface near the carrier surface with a gap therebetween, wherein as the wheel is rotated, the carrier surface is moved past the workpiece to carry the magnetorheological fluid through said gap;
a magnet for applying a magnetic field at said gap to stiffen the magnetorheological fluid flowing through said gap creating a finishing work zone in the fluid for engaging and causing material removal at the portion of the workpiece surface;
means for moving the workpiece relative to the work zone to expose different portions of the workpiece surface to the work zone for predetermined time periods to finish said portions of said workpiece surface in predetermined degrees; and a collector for collecting magnetorheological fluid having flowed through the gap from the carrier surface and returning the magnetorheological fluid to the magnetorheological fluid source.
87. The apparatus of claim 86, wherein said wheel comprises a non-magnetic material.
88. The apparatus of claim 86, wherein said carrier surface is convexly curved across the width of the rim.
89. The apparatus of claim 86, wherein said carrier surface comprises a spheric section.
90. The apparatus of claim 86, wherein said carrier surface has a cylindrical configuration.
91. The apparatus of claim 86, further comprising a viscosity monitor for monitoring the viscosity of the magnetorheological fluid collected by the collector.
92. The apparatus of claim 91, wherein said viscosity monitor comprises a tube for transporting the magnetorheological fluid at a substantially constant flow rate, pressure sensors for measuring a pressure drop between two points along the tube, and means for comparing the pressure drop against a predetermined value.
93. The apparatus of claim 91, further comprising a dripper for adding carrier fluid to the magnetorheological fluid collected by the collector to adjust the viscosity of the magnetorheological fluid to a predetermined level if a variation from the predetermined viscosity level is detected by the viscosity monitor.
94. The apparatus of claim 86, further comprising cooling means for cooling magnetorheological fluid collected by the collector.
95. The apparatus of claim 86, further comprising a mixer for rehomogenizing magnetorheological fluid collected by the collector and agglomerated in the presence of the magnetic field.
96. The apparatus of claim 95, wherein said mixer comprises stirrer.
97. The apparatus of claim 86, further comprising means for imparting a predetermined geometric shape to the magnetorheological fluid entering the gap to vary the configuration of the work zone.
98. The apparatus of claim 97, wherein said means for imparting a shape comprise a scraper having an opening to permit movement of magnetorheological fluid therethrough, said opening corresponding to said predetermined geometric shape.
99. The apparatus of claim 86, wherein said magnet comprises pole pieces configured for maximizing the fringing field in the vicinity of the gap.
100. The apparatus of claim 86, wherein said collector comprises a magnetically soft material for magnetically shielding said collector to inhibit application of the magnetic field to magnetorheological fluid in the collector.
101. The apparatus of claim 86, wherein said collector includes a scraper portion for engaging said carrier wheel to enhance removal of magnetorheological fluid therefrom.
102. The apparatus of claim 101, wherein said scraper portion has a cup-like configuration.
103. The apparatus of claim 86, wherein said nozzle comprises a magnetically soft material for magnetically shielding said nozzle to inhibit application of the magnetic field to magnetorheological fluid within the nozzle.
104. The apparatus of claim 86, further comprising means for rotating the workpiece relative to the work zone.
105. The apparatus of claim 86, wherein said workpiece is mounted on a pivoting workpiece holder to sweep the surface of the workpiece through the work zone.
106. The apparatus of claim 86, further comprising means for moving the workpiece in a plane.
107. The apparatus of claim 86, wherein said magnet is mounted on a support base and wherein said support base is rotatable.
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US08/543,426 US5795212A (en) 1995-10-16 1995-10-16 Deterministic magnetorheological finishing
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