KR20220052366A - High Removal Rate Magnetorheological Finish Head - Google Patents

Abstract

Magnetorheological finish head with pole piece, nozzle geometry and wheel geometry tailored to maximize volume removal rates. The carrier wheel for the ribbon of magnetorheological fluid is an aspherical surface, preferably a toroid with a minor radius perpendicular to the axis of rotation and a major radius parallel to the axis of rotation, but the shape of the wheel is aspheric or freeform, such as a toroid, parallel to the axis of rotation of the wheel. Or it may be cylindrical. By shaping the pole pieces to produce a substantially uniform magnetic field over a defined gap therebetween, the magnetic field is created such that the magnetic field strength in the region of the fluid ribbon is uniform. The nozzle has a non-circular opening to provide a fluid stream having a width that covers a range of widths of the magnetic field. It is the combination of these three features that allows for the new MRF cancellation capability.

Description

High Removal Rate Magnetorheological Finish Head

The present invention relates to a system for magnetorheological finishing of a substrate surface; More specifically, a magneto-rheological fluid (MR fluid) disposed between opposing pole pieces and working on the surface of a wheel and between the wheel and the working surface of a substrate being finished (“workpiece”). A magnetorheological finish head comprising a rotatable working surface (the outer surface of the equatorial portion of a "wheel") capable of passing through a zone (a "spot")—the MR fluid is transferred by means of a pole piece, which may be an electromagnet or a permanent magnet. "stiffened" by receiving an applied magnetic field, and material is removed from the substrate surface through abrasion by the hardened MR fluid; Most specifically, such a magnetorheological finish head with a rotatable working surface that is non-spherical—the pole piece generates a substantially uniform magnetic field, and the MR fluid is applied to a working area on the rotatable working surface of the fluid material. Supplied as a broad ribbon.

The use of magnetically curing magnetorheological fluids for abrasive finishing and polishing of substrates is well known. Such a fluid comprising magnetically soft abrasive particles dispersed in a liquid carrier exhibits magnetically induced plastic behavior in the presence of a magnetic field. Since the apparent viscosity of the MR fluid can be magnetically increased by several orders of magnitude, the consistency of the MR fluid varies from an almost water-like state to a very hard abrasive paste. When such abrasive pastes are properly directed against the substrate surface to be shaped or polished (eg optical elements), very high levels of finish quality, accuracy and control can be achieved.

U.S. Patent No. 5,795,212 to Jacobs et al., issued August 18, 1998, "Deterministic magnetorheological finishing," discloses a method and apparatus for finishing a workpiece surface using an MR fluid, comprising: A workpiece is positioned near the carrier surface such that a converging gap is defined between a portion of the workpiece surface and the carrier surface. A magnetic field is applied substantially to the gap and a flow of hardened MR fluid is introduced into the converging gap, such that a working zone is created in the MR fluid, such that a transient sub-opening below the opening to engage and effect removal of material from a portion of the workpiece surface. Form a sub-aperture transient finishing tool. The workpiece or work zone is moved relative to each other to expose various portions of the workpiece surface to the work zone for a predetermined period of time to selectively finish portions of the workpiece surface to a predetermined degree.

US Patent No. 5,839,944 to Jacobs et al., issued Nov. 24, 1998, "Apparatus for deterministic finishing of workpieces," is a method for finishing a workpiece surface using MR fluid. and an apparatus, wherein the workpiece is positioned proximate a carrier surface such that a converging gap is defined between a portion of the workpiece surface and the carrier surface. A magnetic field is applied substantially to the gap and a flow of hardened MR fluid is introduced into the converging gap, such that a working zone is created in the MR fluid, such that a transient sub-opening below the opening to engage and effect removal of material from a portion of the workpiece surface. Form a finishing tool. The workpiece or work zone is moved relative to each other to expose various portions of the workpiece surface to the work zone for a predetermined period of time to selectively finish portions of the workpiece surface to a predetermined degree.

U.S. Patent No. 5,951,369 to Kordonski et al., issued September 14, 1999, "System for magnetorheological finishing of substrates," is an improved system for increasing the effectiveness of magnetorheological finishing of substrates. to start The inline flow meter is closed loop linked to the rotational speed of the pressurization pump to ensure a constant flow of the magnetorheological fluid to the work area. A simplified capillary viscometer is placed on the wheel surface at the outlet of the fluid delivery system. The output signals from the flowmeter and viscometer pressure sensors are sent to a computer, which calculates the viscosity of the MRF delivered to the work area, replenishes the MR fluid concentrated on the workpiece to obtain the viscosity by supplementing the carrier fluid with a constant concentration of magnetic solid. Return to ensure that the work area is provided. The asymmetrical pole piece to the field magnet in the working zone extends the magnetic field along the wheel surface upstream of the working zone, sufficiently magnetically hardening the MRF before it engages the workpiece, while the fringing magnetic field near the viscometer. It minimizes fringing field and also shortens the magnetic field along the wheel surface downstream of the working area.

US Patent No. 6,106,380 to Jacobs et al., issued Aug. 22, 2000, "Deterministic magnetorheological finishing," discloses a method and apparatus for finishing a workpiece surface using MR fluid, wherein The workpiece is positioned near the carrier surface such that a converging gap is defined between a portion of the workpiece surface and the carrier surface. A magnetic field is applied substantially to the gap and a flow of hardened MR fluid is introduced into the converging gap, such that a working zone is created in the MR fluid, such that a transient sub-opening below the opening to engage and effect removal of material from a portion of the workpiece surface. Form a finishing tool. The workpiece or work zone is moved relative to each other to expose various portions of the workpiece surface to the work zone for a predetermined period of time to selectively finish portions of the workpiece surface to a predetermined degree.

U.S. Patent No. 6,506,102 to Kordonski et al., issued Jan. 14, 2003, "System for magnetorheological finishing of substrates," describes a substrate comprising a vertically oriented bowl-type carrier wheel having a horizontal axis. Disclosed is an improved system for the magnetic rheological finish of The carrier wheel is preferably the equatorial part of the sphere such that the carrier surface is spherical. The carrier wheel comprises a radially circular plate connected to the rotational drive means, the radially circular plate supporting a spherical surface extending laterally from the plate. Electromagnets with planar N-pole and S-pole pole pieces are arranged within the wheel, within the envelope of a sphere, preferably within the envelope of the spherical part defined by the wheel. The magnets extend over a center wheel angle of about 120 degrees so that the magnetorheological fluid remains partially cured before and after the work zone. A magnetic scraper removes the MR fluid from the wheel as the hardening eases and returns it to the existing fluid delivery system for conditioning and re-extrusion into the wheel. This system is useful for finishing large concave substrates that must extend beyond the edge of the wheel, as well as finishing very large substrates in the working area at the bottom dead center of the wheel.

U.S. Patent No. 8,944,883, "System for magnetorheological finishing of a substrate," to Kordonski, issued February 3, 2015, discloses a spherical wheel for carrying magnetorheological finishing fluids, This spherical wheel houses a variable magnetic field permanent magnet system with iron pole pieces of north and south poles separated by primary and secondary gaps with a cylindrical cavity passing through the center. A cylindrical permanent magnet magnetized perpendicular to the cylindrical axis is rotatably disposed within the cavity. The actuator may cause the permanent magnet to rotate at an arbitrary angle, and the rotation changes the magnetic flux distribution of the magnetic circuit through the pole piece. Thus, the magnetic field strength within the gap can be controlled by positioning the permanent magnet at an angle that provides the required magnetic field strength. Because the magnetic field also passes over the pole pieces and confines the fringing magnetic field outside the wheel surface, the variable magnetic field extends through the MR fluid layer on the wheel, changing the hardness of the MR fluid that may be required for finish control.

In all of these prior art references, the disclosed wheel is a spherical equatorial part; A method for adjusting the shape of a magnetic field by shaping the tip of a pole piece is not disclosed; A method and apparatus for shaping the cross-sectional area of a magnetorheological fluid ribbon on a wheel by means of a non-circular nozzle outlet is not disclosed.

The prior art magneto-rheological finishing head has a limitation in material removal rate caused by two main factors. The first factor is the fluid used in the process, which leads to the maximum removal rate. The second factor is the device making up the finishing head, which causes both the maximum removal rate and the volumetric removal rate.

The device geometry limits the physical size of the removal tool and in many cases limits the crystalline magneto-rheological finishing process. In particular, prior art processes focused on finishing large optical elements and/or inducing aspherical shapes to spherical surfaces may take hours or even days to finish to reach desired final dimensions. In this case, a larger removal function can greatly reduce the cycle time of the polishing operation.

In the prior art, greater removal capabilities have been created simply by increasing the diameter of spherical magnetorheological finish wheels, but in many cases large wheels are not feasible or practical and are also very expensive to manufacture with the required precision. As the wheels get larger, the same precision for wheel runout is still required, and achieving desired tolerances becomes much more difficult and expensive.

Considering the factor controlling the removal rate, the working area should be enlarged both in the direction of wheel movement and transverse to the direction of wheel movement in order to increase the removal rate. It is known that increasing the wheel radius can lengthen the spot and making the spot between the wheel and the substrate workpiece deeper can increase the maximum removal rate. What is unknown in the prior art is a method of increasing the volume removal rate by making the spot wider, preferably without substantially increasing the radius or other geometry of the wheel.

What is needed in the art is a magnetorheological finish head with a custom magnetic field, nozzle shape and wheel shape that maximizes the volume removal rate of the substrate material.

The magnetorheological finish head comprises: a pole piece having opposing tips of a specific shape to maximize volume removal rate; non-circular nozzle shape; and non-spherical wheel shapes and surfaces. The carrier wheel for the MR fluid ribbon is non-spherical, preferably the equatorial portion of the toroid with a minor radius about an axis parallel to the axis of rotation of the wheel and a major radius about an axis perpendicular to the axis of rotation of the wheel, but The shape may be any aspherical or free form shape with an axis of rotation parallel to the axis of rotation of the wheel, such as toroidal or cylindrical. A tailored magnetic field is created by shaping the tips of the pole pieces to create a fringing magnetic field over a larger gap defined between them than the prior art gap so that the magnetic field strength is maintained at a useful intensity across the width of the ribbon. The magnet may be either an electromagnet or a permanent magnet, but an electromagnet is generally used as in the prior art. The nozzle has a non-circular opening to provide a fluid stream with a width that covers a range of magnetic fields. It is the combination of these three features that allows for the maximum increase in MRF cancellation capability, however, taking these features alone or in pairs can significantly increase the MRF cancellation capability compared to the prior art.

The present invention creates opportunities for material removal rates that are at least four times higher than prior art systems.

This system is particularly useful for performing low order figure correction on large substrates and introducing shape changes such as aspheric surface creation into optical surfaces.

Other features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description and drawings of preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described by way of example with reference to the accompanying drawings, in which:
1 is an elevational cross-sectional view of a portion of a prior art magnetorheological finishing head;
2A is an elevational cross-sectional view of a portion of a magnetorheological finishing head in accordance with the present invention;
FIG. 2B is an elevation view of a portion of the magnetorheological finish head shown in FIG. 2A , further showing the MR fluid ribbon on the wheel surface, the workpiece in position for material removal, and a working zone or “spot” therebetween;
Fig. 3 is a perspective view from above of the magneto-rheological finishing head shown in Figs. 2a and 2b;
4 is an elevation view of a first embodiment of a nozzle according to the invention;
Fig. 5 is a cross-sectional pan view of the nozzle shown in Fig. 4;
6 is an elevational cross-sectional view of elements of a magnetorheological work zone;
7 is an elevational cross-sectional view of a portion of an MR finishing head showing the dimensions of an MR fluid ribbon in accordance with the present invention;
Fig. 8a is a diagram showing the relationship of a toroid formed in accordance with the present invention and a finish wheel taking the equatorial portion of the toroid;
Fig. 8b is a view similar to that shown in Fig. 8a, showing vertically intersecting arcs on the surface of a toroid formed in accordance with the present invention, the arcs having respective radii R ₁ and R ₂ , where R ₁ ≠ R ₂ ;
9 is an isometric view of a first embodiment of a pole piece according to the present invention;
Fig. 10 is an isometric view of a second preferred embodiment of a pole piece according to the invention;
11 is a cross-sectional view of a magnetic field caused by simply spacing prior art parallel magnetic pole planes in an effort to widen the magnetic field and thus the working zone;
12 is a cross-sectional view of a magnetic field caused by forming opposing magnetic pole surfaces into conical cross-sections in an effort to widen the magnetic field and thus the working zone;
13 is a cross-sectional view of a magnetic field caused by forming opposing magnetic pole surfaces into toroidal cross-sections in an effort to widen the magnetic field and thus the working zone;
Fig. 14 is a graph showing the ideal magnetic field lines from Figs. 11 to 13;
15 is a top view of material removal rates in a typical prior art work zone;
16 is a plan view of a typical material removal rate in a work zone produced by a magneto-rheological finish head formed in accordance with the present invention.

1 and 6 , a portion of a prior art magnetorheological finishing head 10 has a disk-shaped central portion supporting an equatorial spherical finishing portion 16 having a finished surface 18 ( 14) with a finish wheel (12). The wheel 12 is mounted for rotation on an axle 20 supported on precision bearings 22a, 22b. The axle 20 is driven about an axis of rotation 30 by an electric motor system (not shown). There are first and second pole pieces 24a, 24b, preferably the same but opposite polarity, i.e. N and S poles, on both sides of the disc-shaped central part 14 as well as below and adjacent to the closure part 16 . . These pole pieces generally have planar opposing surfaces 26a, 26b set at a first predetermined distance from each other. The pole pieces 24a and 24b may be electromagnets or permanent magnets.

When the electromagnet is energized, a fringing magnetic field (not shown) is formed through and over the finishing portion 16 , and the ribbon of MR fluid 17 carried on the surface 18 cures to a paste consistency. do. A substrate 21 to be finished, such as a lens as shown in FIG. 6 , is positioned to rotate about its own axis 23 generally above the wheel surface at a distance from the wheel that is less than the thickness of the incoming MR fluid ribbon, so that it converges. An abrasive finish of the substrate 21 disposed in the working zone 19 is performed, creating a gap and forming a working zone or “spot” 19 . The dimensions of the converging gap may vary depending on the requirements of the particular finishing application. 6 , the height of the ribbon entering the working zone is RH, the plunge depth of the workpiece into the ribbon is D, and the final gap G between the workpiece and the wheel surface is the thickness of the working zone 19 . am.

Referring now to FIGS. 2A, 2B, 6, 8, and 9-13, an improved magnetorheological finishing head 110 for forming a wider and longer working area includes pole pieces 124a, 124b. It is substantially the same as the prior art magneto-rheological finishing head 10 shown in FIG. 1 except that the upper corner of the is modified as shown. Preferably, the upper corner is either rounded 128a, 128b as shown in FIGS. 2A, 10 and 13 or beveled 126a, 126b and 226a as shown in FIGS. 9 and 12 . , 226b), may be any desired shape, such as conical, radially curved, or freeform. The actual values of the spacing and radius between the pole pieces 124a and 124b may be chosen as desired to form a specific sized working area for any particular application. Providing a round or bevel-like shape would produce a fringing magnetic field 40, 240 having lateral uniformity over a substantially wider width than that formed by the prior art pole piece arrangement shown in FIG. It turned out that it can Preferably, the curved shape 128a, 128b is a torus, in particular the distance from the center of the tube to the center of the torus is greater than the radius of the tube, according to the equation shown below with respect to the shape of the finish wheel surface. It is formed as part of a ring torus.

Still referring to the improved magnetorheological finishing head 110 , as described above, the finished portion 116 having the finished surface 118 is non-spherical, preferably having a minor radius perpendicular to the axis of rotation 130 and an axis of rotation 130 . ), but the shape of the wheel may be any aspherical or freeform parallel to the axis of rotation 130 of the wheel, such as toroidal or cylindrical (toroid with infinite major radius). The advantage of this geometry is that it allows for greater removal capabilities without significantly increasing the overall tool size, ie the diameter of the wheel. Another advantage is that the toroidal wheel allows a wider removal capability without requiring the increase in the volume of fluid required by older prior art wheels. These features help reduce the need for higher flow rates and larger pumping systems to achieve equivalent results.

2A , 8A and 8B , a finish wheel 116 having a finish surface 118 may be more generally defined as a non-spherical rotating surface. 8A and 8B show an ideal shape 142 having a first radius R ₁ rotated about a second radius R ₂ to form a three-dimensional shape 144 , where R ₁ and R ₂ create respective vertically intersecting arcs A ₁ , A ₂ on the wheel surface 118 (note that when R ₁ = R ₂ the wheel surface is spherical as in the prior art) . In its simplest form, shape 142 is a circle and shape 144 is a torus, but higher-order polynomials or other equations may be used to define a surface that can be rotated about R ₂ . For a higher removal rate, R ₁ should be much larger than R ₂ . These values can be selected based on two factors: 1) the shape of the optics (especially concave optics) to prevent the wheel geometry from interfering with the geometry of the workpiece, and 2) the radius (R ₂ ) ), the wider (and therefore larger) the removal capability for a given MRF flow rate.

In explicit form, the geometry of the wheel can be expressed as

Z = f(x, y) = R _y ±√[(R _y -g(x)) ² -y ² ]

where g(x) is the creation curve and Z is the logarithmic shape of the wheel.

For Taurus:

g(x) = R _x {l-√[l-(x/R _x ² ]}

where g(x) is a circle with radius R _x .

3-5 , the present invention requires a shape change for the MRF ribbon formed on the finished surface 118 . Prior art MRF ribbons are produced using a circular nozzle outlet of a specified inner diameter. A typical ribbon shape when extruded from a prior art outlet port with a diameter of 3 mm and a cross-sectional area of 7.3 mm ² is circular.

In order to increase the size of the removal function (work area), the width must be increased. The wider ablation function is a laterally spreading MRF stream injected onto the wheel across an area covering the width of the ablation function before the MRF ribbon 150 reaches the working zone, typically at the top dead center position of the wheel. need. Thus, where the nozzle exit is non-circular and preferably slot-shaped, the MR fluid will spread before landing on the wheel, allowing for a wider removal function.

The nozzle assembly 132 includes a supply tube 134 that enters into a housing block 136 and terminates at a distributor 138 within the housing block 136 , the distributor 138 being formed to a desired width of the MRF ribbon to be produced, and It is discharged to the inner slot terminating in the outlet slot (140). In the presently preferred embodiment, the exit slot 140 is about 19 mm wide and about 0.9 mm high, resulting in an aspect ratio greater than 20. The cross-sectional area of this design is 17.8 mm ² , allowing nearly 2.5 times the flow rate of prior art nozzles when operated at the same delivery pressure. Increased flow is required to fill the larger area between the wheel and the substrate to create a wider removal function. Preferably, the ends of the slots are rounded to avoid stagnant areas at corners and unwanted fluid build-up.

Obviously, other slot shapes and dimensions may be selected as may be needed for a particular finishing application. For example, a "slot" may be formed of a series of discharge holes rather than a continuous slot, or the height of the slots may not be uniform.

The height and width of the ribbon may be manipulated on the wheel after extrusion. The angle of incidence of the fluid jet on the wheel can affect the ribbon width, with the ribbon tending to spread laterally on the wheel as the nozzle extrusion angle increases from tangential to vertical. Increasing the wheel speed above a “flow matching” value where the fluid jet speed matches the tangential speed of the wheel, the fluid spreads, resulting in a lower cross-sectional area of the ribbon. The benefits of unfolding the ribbon allow the operator to manage the overall height of the ribbon and the dimensions shown in FIG. 6 to achieve a wide removal capability. When the fluid ribbon is activated by a magnetic field, an abrasive boundary layer 19 (working zone) is created across the width of the ribbon.

Preferably, the height of the magnetorheological fluid ribbon on the finish wheel when entering the work zone is between 1.20 mm and 1.56 mm, and the depth of plunge into the magnetorheological fluid ribbon by the workpiece finished by the magnetorheological finish head is 0.60 mm. to 0.81 mm, and the clearance between the workpiece and the finish wheel is 0.60 mm to 0.75 mm.

3 and 7 show a ribbon 150 of width W and thickness RH disposed on wheel surface 118 .

Referring now to Fig. 11, simply moving the prior art planar opposing pole pieces 26a, 26b further than the standard spacing shown in Fig. 1 produces a laterally non-uniform and rather weak magnetic field 140 in the center of the working zone It can be seen that this results in an undesirable bimodal removal function. Alternatively ( FIGS. 9 , 12 and 14 ), beveling the pole pieces as in conical surfaces 226a and 226b creates a substantially uniform overall magnetic field 240 with slightly lower magnetic field strength. Referring now to FIGS. 10 , 13 and 14 , having a radius on the pole pieces 124a , 124b creates a substantially uniform overall magnetic field 40 having a higher magnetic field strength.

Referring now to FIG. 14 , ideal magnetic fields just above the wheel surface are shown for the conditions disclosed above in FIGS. 11-13 .

Referring now to FIGS. 15 and 16 , a prior art working zone spot 55 ( FIG. 15 ) as compared to a working zone spot 155 achievable by a magnetorheological finishing apparatus according to the present invention as shown in FIG. 2A . ) is shown. A typical prior art spot 55 from a spherical 150 mm diameter wheel has a width 60 of about 4.0 mm and a length 70 of about 10.0 mm with a working area of about 40.0 mm ² , whereas the improved spot 155 ) can have a working area of about 378.0 mm ² with a width 160 of about 18.0 mm and a length 170 of about 21.0 mm, thus providing removal rates several times greater than prior art spots.

Accordingly, the present invention comprises three novel elements: a) a pole piece having a rounded upper corner, b) an aspherical, preferably toroidal wheel finish surface, and c) an MRF application nozzle having a non-circular outlet. It is the combination of these three features that allows for a maximum increase in the MRF cancellation capability, but taking these features alone or in pairs can significantly increase the MRF cancellation capability compared to the prior art.

Various changes may be made in the structure and method of implementing the principles of the present invention. The foregoing embodiments are described in an illustrative rather than a restrictive sense. The scope of the invention is defined by the claims.

Claims (11)

In a magnetorheological finishing head,
a) a rotatable finishing wheel having a non-spherical finishing surface;
b) first and second pole pieces of opposite polarity having opposite faces and having corners disposed within the finish wheel, the corners of the opposing face closest to the finish surface being conical, beveled, toroidal ( having a shape selected from the group consisting of toroidal, radial and freeform; and
c) a nozzle assembly terminating at a non-circular outlet;
Magnetic rheological finish head. A magnetorheological finish head, comprising:
a) a rotatable finish wheel with a non-spherical finish surface;
b) first and second pole pieces of opposite polarity disposed within the finish wheel and having opposite faces, the corners of the opposing face closest to the finish surface being conical, beveled, toroidal, radial and freeform having a shape selected from -; and
c) a nozzle assembly terminating at a non-circular outlet
containing any one of the three elements of
Magnetic rheological finish head. A magnetorheological finish head, comprising:
a) a rotatable finish wheel with a non-spherical finish surface;
b) first and second pole pieces of opposite polarity having opposite faces and having corners disposed within the finish wheel, the corners of the opposing face closest to the finish surface being conical, beveled, toroidal, radial and freeform having a shape selected from the group consisting of -; and
c) a nozzle assembly terminating at a non-circular outlet
containing any two of the three elements of
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
The shape of the non-spherical finished surface is selected from the group consisting of toroidal, cylindrical and freeform.
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
The pole piece is a component of a magnetic system selected from the group consisting of electromagnets and permanent magnets.
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
The magnetic field formed over the finished surface is substantially uniform from edge to edge of the magnetic field.
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
The non-circular outlet is a slot
Magnetic rheological finish head. The method of claim 1,
The rotatable finish wheel is formed according to the following formula,
Z = f(x, y) = R _y ±√[(R _y -g(x)) ² -y ² ]
where g(x) is the generating curve and Z is the algebraic definition of the rotatable finish wheel
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
wherein the first and second pole-pieces are formed such that a uniform magnetic fringing field is formed over a desired width on the rotatable finish wheel when these pole-pieces are energized.
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
The nozzle assembly is formed so that the ribbon of magnetorheological fluid extruded therefrom has a uniform thickness from edge to edge of the ribbon.
Magnetic rheological finish head. 4. The method according to any one of claims 1 to 3,
wherein the non-circular outlet of the nozzle assembly is selected from the group consisting of a slot, a slot having a rounded end, and a plurality of apertures.
Magnetic rheological finish head.

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