This patent application is a U.S. National Phase Application under 35 U.S.C. §371 of International Application No. PCT/SG2011/000405, filed Nov. 15, 2011, entitled APPARATUS AND METHOD FOR POLISHING AN EDGE OF AN ARTICLE USING MAGNETORHEOLOGICAL (MR) FLUID, which claims priority to Singapore Patent Application No. 201008438-2, filed Nov. 15, 2010.
This invention relates to an apparatus and method for polishing an edge of an article using magnetorheological fluid, more particularly but not exclusively, for polishing the edges of large glass panels.
It is known to use magnetorheological finishing (MRF) uses magnetorheological (MR) fluid to polish or remove materials from surfaces of optical lens. The MR fluids include suspensions of ferro-magnetic particles carried by a carrier fluid. Under influence of a magnetic field, the ferro-magnetic particles are magnetized by the magnetic field and viscosity of the MR fluid changes almost instantaneously from a liquid state to a semi-solid state which is still sufficiently pliant to conform to a surface of a workpiece being polished. However, for certain applications, such as removal of sub-surface damage including sub-surface micro-crack, current MRF techniques do not yield sufficiently useful material removal rates for required production yields. Commercially available glass polishing disks are also not suitable for removing sub-surface micro-cracks.
There is therefore a need to provide an apparatus and method for polishing an edge of an article using magnetorheological fluid to address at least one of the disadvantages of the prior art and/or to provide the public with a useful choice.
In accordance with a first aspect, there is provided an apparatus for polishing an edge of an article using a magnetorheological (MR) fluid, the apparatus including at least one carrier including first and second opposing surfaces defining a groove, the first and second opposing surfaces being spaced apart along a first direction to receive the edge; and a magnetic field generator configured to provide a magnetic field in the groove, wherein in operation the MR fluid is disposed in the groove and stiffens in response to the magnetic field to provide at least one polishing zone; and a driver configured to provide relative motion between the at least one carrier and the edge of the article in a second direction substantially transverse to the first direction for polishing the edge of the article in the at least one polishing zone.
The magnetic field generator may further include first and second permanent magnets providing the first and second opposing surfaces respectively. The magnetic field generator can be configured to provide the magnetic field throughout the groove such that the MR fluid is stiffened throughout the groove.
The groove can be configured to retain substantially all of the MR fluid disposed therein. The groove can be annular, the groove being characterised by an axis of rotational symmetry parallel to the first direction. The groove may extend substantially parallel to the second direction.
The apparatus can be configured such that the relative motion further includes a reciprocating motion. The relative motion may include rotational motion of the at least one carrier.
The apparatus may include a plurality of the carriers aligned substantially parallel to the second direction to simultaneously provide at least one polishing zone. The apparatus may be configured such that each one of the at least one carrier is rotatable about an axis parallel to the first direction. Immediately adjacent ones of the carriers are rotatable in different directions.
Optionally, the apparatus may include a restoring tool configured to shape the MR fluid.
The at least one carrier may further include a conveyor configured to provide the first and second opposing surfaces, the conveyor being operable by the driver. The magnetic generator may include a plurality of magnets arranged equidistantly along the conveyor. The apparatus can further include a moisturizing device configured to moisturize the MR fluid.
In another aspect, there is provided a method for polishing an edge of an article, the method comprising: providing at least one carrier including: first and second opposing surfaces defining a groove, the first and second opposing surfaces being spaced apart in a first direction to receive the edge; and a magnetic field generator configured to provide a magnetic field in the groove to stiffen magnetorheological (MR) fluid disposed in the groove to provide at least one polishing zone; receiving the edge in the polishing zone; and driving relative motion between the at least one carrier and the edge in a second direction substantially transverse to the first direction.
The method may include stiffening the MR fluid throughout the groove. The method may further include retaining substantially all of the MR fluid disposed in the groove. The method may include simultaneously receiving different parts of the edge in the groove. The method may further include rotating immediately adjacent ones of the carriers in different directions.
An advantage of the described embodiment is that with the plurality of polishing zones to polish different portions of the linear surface simultaneously, polishing time may be reduced and material removal rate may also be increased.
With an elongate polishing zone, a much quicker polishing time of the linear surface may be achieved.
With the pair of magnetic field generators, the generators generate respective magnetic fields which compliment each other to generate a combined magnetic field of greater intensity. In this way, a much quicker grinding or polishing is achieved.
Examples of the invention will now be described with reference to the accompanying drawings, in which:
a is a schematic diagram of a top view of an apparatus according to another embodiment;
b is a cross-sectional view of the apparatus of
a is a simulation result of magnetic flux distribution of a single magnet similar to what would be generated by the apparatus of
b is a simulation result of magnetic flux distribution of a pair of magnets similar to what would be generated by the apparatus of
a(i) to (iii) and 5b(i) to (iii) show a series of surface roughness profiles and magnified photographs respectively to compare between unpolished and polished surfaces using the arrangements of
a shows the apparatus according to one embodiment of the apparatus;
b shows the apparatus of
a and 17b illustrate a restoring tool for restoring the shape of the MR fluid of
To appreciate advantages of the embodiments, it would be useful to begin with an explanation of various parameters which may affect material removal rate of a magnetorheological finishing (MRF) process.
It has been found that tribologically, the MRF process is a combination of two and three body abrasive wear. Hence, the following process equations discussed are applicable to the MRF process:
where
Ra is surface roughness achieved in a polishing time, t from an initial surface roughness, Ri of a surface to be polished;
v is the sliding velocity or tangential contact velocity between a MRF abrasive media and the surface being polished;
R∞ is a limiting surface roughness, or the lowest surface roughness that can be achieved;
pa is defined as normal force per unit area acting on the surface being polished;
H is hardness of the surface being polished;
kT and kS are wear coefficients;
h is wear depth, and
a is a geometrical constant.
To obtain the wear coefficients kT and kS, for the purpose of predicting surface roughness and geometrical change distribution, experiments are carried out on a test strip made of glass.
Estimation of kT
If the area of the test strip I coupon is Ac and force acting on the test strip as measured by a force sensor is Fc′ the granular pressure, pg may be estimated as:
With a known tangential velocity, vs, and referring to equation (1), it is possible to estimate kT from a plot of
versus time. Of course, Ra is the surface roughness of the exposed surface of the test strip or coupon as explained earlier.
Estimation of kS
Now for large t, equation (2) reduces to:
from which ks may be estimated from a plot of h versus time. Of course, h is the wear depth or change in thickness of the test strip or, coupon
Estimation of a
From equations (1) and (2), it follows immediately that:
Thus, a may be estimated from a plot of
provided data for small t are available.
From equation (2), it can be appreciated that material removal, which is based on wear depth, h, is intertwined with surface roughness evolution. Hence, to increase material removal rate (MMR) and also polishing rate, a consideration is to increase, {dot over (p)}a, v or the wear coefficients, kT and kS or combinations or permutations of these factors. The following descriptions of various embodiments will teach those skilled in the art how to achieve this.
In one embodiment, the apparatus 100 includes a carrier 103 that includes a rotatable central shaft 102, a cylindrical housing 104 coupled to the central shaft 102, and a ring permanent magnet 106 with two poles (N-pole and S-pole) oriented as shown. The shaft 102 is connected to a driver or spindle (not shown) to spin the shaft 102 and the cylindrical housing 104.
The carrier 103 includes a cylindrical housing 104 which houses or encases the ring permanent magnet 106. The cylindrical housing 104 includes a first surface 105 and a second surface 107 with both surfaces opposing each other to define a groove 108. The housing 104 further houses the ring permanent magnet 106 such that the magnetic field extends through the groove 108. The groove 108 may be a circumferential channel or side groove 108 between the top and bottom surfaces 105,107 arranged to contain a MR fluid 110. In this embodiment, the MR fluid 110 comprises ferro-magnetic particles of between 1 and 10 microns suspended in water, as carrier fluid. The concentration of the ferro-magnetic particles is 20-40% by volume. The MR fluid 110 also includes minute quantities of abrasive of about 0.3-1% by volume in the form of Silicon Carbide (SiC) to increase material removal rates of a linear glass surface or edge 202 of the glass 200 to be polished by the MRF apparatus 100. It should be appreciated that other abrasives may be used, for example, Aluminum Oxide, Cerium Oxide or diamonds.
The ring permanent magnet 106 used in this setup is Nd(neodymium)-Fe (ferrite)-B(boron) rare earth permanent magnet to create sufficiently strong magnetic fields to produce an almost instantaneous change of the MR fluid from a liquid state to a semi-solid state throughout the entire side groove 108 and which is still sufficiently pliant to conform to the edge 202 of the article to be polished.
To polish the straight glass edge 202, the spindle is rotated to spin the central shaft 102 and the carrier 103 about a central axis of the central shaft 102 and from
According to another embodiment of the apparatus, instead of having one movable carrier 103 arranged to house the ring permanent magnet 106, the apparatus 100 further includes a pair of permanent magnets 112,119 arranged to a rotatable central shaft 121 as shown in
a illustrates magnetic flux distribution generated by a single ring magnet based on FEM analysis and
With the increase magnetic flux density, this thus increases “media pressure” which is the pressure acting on the MR fluid 110 and the normal stress is increased. In other words, pa, the normal force per unit area is increased. Consequently, the material removal rate is increased.
b)(i) is a magnified photograph of a surface to be polished with the corresponding surface roughness profile of Ra: 0.51 μm illustrated in
In a second embodiment, the MRF apparatus 100 of
v=ω·R=2πθ·R (3)
where
It can be appreciated that the tangential contact velocity, v, may be increased by either increasing the speed of rotation and/or the radius R of the cylindrical housing.
In yet another embodiment, the apparatus 100 of
It should be appreciated that the leftmost carrier 302a of
v=2d0πf·cos(2πft) (4)
where,
v is contact velocity;
f is oscillation or reciprocating frequency of the movable carrier;
do is displacement amplitude of the movable carrier; and
t is variable time.
In other words, increasing the oscillation frequency, f, increases the contact velocity, v, and thus, the material removal rate. It should be appreciated that increasing the contact velocity, v, may also be applicable for the embodiments of
It has been found that reciprocating displacement of the carrier 502, d, is related to the displacement amplitude based on the following equation:
d=d0·sin(2πft) (5)
Displacement amplitude is defined as a maximum distance that the movable carrier moves from a starting (or zero) position about which the carrier reciprocates or oscillates. As it can be appreciated from Equation (5), to reduce the polishing time, a longer permanent magnet may be used for the movable carrier 502 which results in a greater contact length since a greater polishing zone is created.
Another embodiment of the apparatus employs a plurality of the rectangular carrier 502 and this is shown in
With the plurality of carriers 502 polishing the edge 572 simultaneously at the respective polishing zones 514, the polishing time may be drastically reduced to obtain a required finish. Further, the reciprocating frequency, f, of the carriers may be selected to further increase the material removal rate as suggested earlier.
a shows a top view of an apparatus 600 according to yet another embodiment of the apparatus. The apparatus 600 includes carrier 601 having opposing surfaces (not shown) which define a groove suitable for receiving the edge 652 of the article 650 to be polished. The carrier 606 is in the form of an endless conveyor for carrying MR fluid 614 and the conveyor 606 is driven by a driver that may be a gear arrangement comprising first and second gears 602,604 spaced apart from each other. The carrier 606 includes an inner channel 608 for storing a plurality of permanent magnets 610 arranged equidistantly throughout the conveyor 606. The plurality of permanent magnets 610 are arranged to stiffen the MR fluid 614 magnetically at their corresponding positions and indeed, throughout the entire length of the conveyor 606. The distance between the first and second gears 602,604 creates an elongate polishing zone 616 for polishing a linear portion of an edge 652 of an article 650. As the first and second gears 602,604 rotate in a same direction, this drives the conveyor 606 in an endless loop carrying the MR fluid 614 to the elongate polishing zone 616 to polish the edge 652 and then away from the elongate polishing zone 616. The continuous movement of the conveyor 606 thus allows the MR fluid 614 to polish the edge 652 continuously and over a large distance or area.
It should be apparent that the elongate polishing zone 616 may be adjusted depending on the configuration of the endless conveyor 606 so that the elongate polishing zone 616 covers the entire length of the edge 652 to be polished.
Optionally, the apparatus 600 further includes a moisturizing device 680 for maintaining moisture content of the MR fluid 614 during the polishing process, and this is illustrated in
As it can be appreciated from the above, increasing the contact length may reduce the polishing time and to increase the material removal rate, the contact velocity, v, may be increased by increasing rate of rotation, w, of the conveyor 606, and/or radius, R, of the first and/or second gears 602,604.
The described embodiments enhance or accelerate MRF material removal rate and reduce the MRF time of profiling or polishing edges or surfaces of materials such as sheets or panels of glass or a non-magnetic material. This may be used to finish profiled glass edges to achieve super-polishing quality surfaces, and finishing of brittle materials to removal of sub-surface damage. In particular, the embodiments are particularly useful for polishing generally straight edges or sides of articles. Specifically, the polishing zones of the described embodiments are substantially where the MR fluid in the groove would interface with the part of the article received in the groove.
Since the magnetized MR fluid conforms to the surface or edge to be cleaned or polished, over time, the MR fluid may retain the profile of an edge or surface on which the MR fluid is polishing. This effect is illustrated in
Alternatively described, the apparatus 100, 500, 300, 600 is configured for polishing an to edge 202, 402, 552, 652 of an article 200, 400, 550, 570, 650 using a magnetorheological (MR) fluid 110 in which the apparatus includes at least one carrier 103 including first and second opposing surfaces 105, 501, 107, 503 defining a groove 108, 504. While particularly suitable for addressing the unmet need for efficient method and apparatus for polishing glass edges, it would be apparent that the proposed apparatus and method are not limited to the polishing of glass articles. The first and second opposing surfaces are spaced apart along a first direction 109 to receive the edge. It should be appreciated that the edge can be a side surface or a minor surface of the article, where the width of the edge is narrower than the spacing between the first and second opposing surfaces.
The apparatus includes a magnetic field generator 106 configured to provide a magnetic field in the groove, wherein in operation the MR fluid is disposed in the groove and stiffens in response to the magnetic field to provide at least one polishing zone 111. As can be understood from the figures, the polishing zone is where the MR fluid interfaces with the article, which would include the edge to be polished. The shape and size of the polishing zone depends therefore on the shape of the groove and the article, and can substantially be found in the groove. The groove may be characterised by an axis of rotational symmetry parallel to the first direction, or it may extend substantially parallel to the second direction.
The apparatus as described above may include one or, more than one of the carriers, such as shown in
The apparatus 100 includes a driver configured to provide relative motion between the at least one carrier and the edge of the article in a second direction substantially transverse to the first direction for polishing the edge of the article in the at least one polishing zone. The relative motion can be contributed by the driver providing a rotational motion B of the carrier about an axis in the first direction 109 that contributes to a tangential velocity at the groove relative to article. Alternatively, the relative motion can be contributed by the driver providing a translational relative motion C between the carrier 103 and the article 200 in a direction substantially transverse to the first direction 109. Yet alternatively, the relative motion may be a combination of both a rotational motion and a translation motion provided by the driver. The relative motion may further be a reciprocating motion, that is, alternating between two opposite directions substantially transverse to the first direction 109.
The magnetic field generator may be one magnet as shown in
Also disclosed is a method for polishing an edge of an article, the method involving providing at least one carrier including first and second opposing surfaces defining a groove, the first and second opposing surfaces being spaced apart in a first direction to receive the edge; and a magnetic field generator configured to provide a magnetic field in the groove to stiffen MR fluid disposed in the groove to provide at least one polishing zone; receiving the edge in the polishing zone; and driving relative motion between the at least one carrier and the edge in a second direction substantially transverse to the first direction. The method may further include stiffening the MR fluid throughout the groove. The method may also include retaining substantially all of the MR fluid disposed in the groove. The method may inviolve simultaneously receiving different parts of the edge in the groove. The method may further include rotating immediately adjacent ones of the carriers in different directions.
The described embodiments should not be construed as limitative. For example, instead of water as the carrier fluid, other types of carrier fluids such as oil may be used. Further, other suitable magnets may be employed, not just the Nd—Fe—B permanent magnet. Indeed, any type of permanent magnets such as rare earth permanent magnets and above may be used to produce relatively strong magnetic fields to produce sufficient stiffness in the MR fluid 110 for rapid removal of materials. Although certain features are explained in relation to one embodiment, it should be appreciated that those features may also be applicable to the other embodiments.
Having now fully described various embodiments of the proposed method and apparatus, it should be apparent to one of ordinary skill in the art that many modifications can be made hereto without departing from the scope as claimed.
Number | Date | Country | Kind |
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201008438-2 | Nov 2010 | SG | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/SG2011/000405 | 11/15/2011 | WO | 00 | 5/15/2013 |
Publishing Document | Publishing Date | Country | Kind |
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WO2012/067587 | 5/24/2012 | WO | A |
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Number | Date | Country | |
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20130260651 A1 | Oct 2013 | US |