FIELD OF INVENTION The invention relates generally to a surface polishing apparatus.

BACKGROUND

One challenge of using magneto-rheological (MR) fluid to perform surface polishing of a workpiece is the tendency for a gap to form between the workpiece and the MR fluid. It is therefore difficult to use MR fluid to perform surface finishing of workpieces having irregular surfaces or surfaces with concave features.

To bring the workpiece into contact with the MR fluid, it has been suggested to first calculate material removal (based on assumptions of spot polishing) and subsequent constant repositioning of the workpiece by computer numerical control (CNC). Furthermore, in conventional systems, it is generally desired to increase material removal rate by providing a strong magnetic field. However, this leads to increased media viscosity which again aggravates the problem of a gap forming between the MR fluid and the workpiece because the MR fluid does not recover its shape. It is therefore difficult to achieve both a desired finishing pressure and a high material removal rate.

Clearly, there is a need for a more effective and efficient way to perform MR finishing that can be used on surfaces of any form or shape. SUMMARY

According to one aspect of the invention, there is provided a surface polishing apparatus comprising a base; a first magnet coupled to the base to have the first magnet be rotatable about a longitudinal axis of the first magnet; a second magnet coupled to the base to have the second magnet be rotatable about a longitudinal axis of the second magnet, wherein the first magnet and the second magnet are orientated to be in mutual attraction and wherein at least a portion of the first magnet is positioned adjacent to a portion of the second magnet with a gap between the adjacent portions; and a magnetic fluid reservoir containing magnetic fluid for performing surface polishing, the magnetic fluid reservoir positioned to allow the magnetic fluid to be in fluid communication with the gap. BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention, in which:

Figure 1 is a schematic cross sectional view and bottom view of a first embodiment of a surface polishing apparatus.

Figures 2(a) and 2(b) show magnetic fields generated by the magnets of the surface polishing apparatus of Figure 1.

Figure 3 shows accumulation of magnetic fluid at a finishing area of the surface polishing apparatus of Figure 1 ,

Figure 4 illustrates a surface polishing apparatus in accordance to a second embodiment of the invention.

Figure 5 shows MRF media flow at the finishing area of the surface polishing apparatus of Figure 1.

Figure 6 shows cluster reformation of magnetic particles within the magnetic fluid used by the surface polishing apparatus of Figure 1 after one complete rotation of the magnets of the surface polishing apparatus.

Figure 7 shows segregation of magnetised material and non-magnetised material that occurs within the magnetic fluid used by the surface polishing apparatus of Figure 1.

Figure 8 shows a schematic cross sectional view of a surface polishing apparatus in accordance to a third embodiment of the invention.

Figure 9 shows a graph plotting the variation of surface roughness against the number of passes made by the surface polishing apparatus of Figure 1 at different areas on a workpiece with a curved surface.

Figure 10 respectively show photos of a workpiece before and after finishing by rapid MRF (magneto-rheological finishing).

DEFINITIONS

The following provides sample, but not exhaustive, definitions for expressions used throughout various embodiments disclosed herein. The term "longitudinal axis" may refer to an axis along which the first or the second magnets are coupled to the base or an axis along which the base is coupled to the platform. DETAILED DESCRIPTION

In the following description, various embodiments are described with reference to the drawings, where like reference characters generally refer to the same parts throughout the different views.

Figure 1 is a schematic cross sectional view 140 and bottom view 180 of a first embodiment of a surface polishing apparatus 100 for rapid magneto- rheological finishing (rapid MRF), suitable for use with workpieces having irregular, freeform, or intricate surface features.

The surface polishing apparatus 100 comprises a base 102. A first magnet 104 is coupled to the base 102 to have the first magnet 104 be rotatable about a longitudinal axis 106 of the first magnet 104. A second magnet 108 is coupled to the base 102 to have the second magnet 108 be rotatable about a longitudinal axis 110 of the second magnet 108. The first magnet 104 and the second magnet 108 are orientated to be in mutual attraction, i.e. it is ensured that immediately adjacent facing surfaces of the first magnet 104 and the second magnet 108 are of opposite magnetic polarities before the magnets 104 and 108 are coupled to the base 102. For instance, if the first magnet 104 is coupled to the base 102 to have its south pole facing the base 102, then the second magnet 108 is coupled to the base 102 to have its north pole facing the base 102. At least a portion of the first magnet 104 is positioned adjacent to a portion of the second magnet 108 with a gap 112 between the adjacent portions.

The surface polishing apparatus 100 includes a magnetic fluid reservoir (not shown) containing magnetic fluid for performing surface polishing. The magnetic fluid reservoir is positioned to allow the magnetic fluid to be in fluid communication with the gap 1 2 between the first magnet 104 and the second magnet 108. In one embodiment, the magnetic fluid reservoir is coupled to the base 102, whereby a channel is provided in the base 102 to guide magnetic fluid within the magnetic fluid reservoir to the gap 112. In another embodiment, the magnetic fluid reservoir is provided on a separate structure where a channel is established between the magnetic fluid reservoir and the gap 112 for the magnetic fluid flow. For the purposes of simplicity, the magnetic fluid is not shown in Figure 1. In the embodiment shown in Figure 1 , the first magnet 104 and the second magnet 108 of the surface polishing apparatus 100 are ring shaped. However, it is also possible for the magnets 104 and 108 to be cylindrical shaped. For instance, the magnets 104 and 108 may be realised by two sub-assemblies of a 5 ring magnet each having a shaft 114 and each providing a tool to facilitate

perspective, the end of one ring magnet has a magnetic polarity opposite to that of a corresponding end of the other ring magnet. From the bottom view 180 perspective, the north pole of the first magnet 104 and the south pole of the second

15 magnet 108 can be seen. Commercially available Nd-Fe-B magnets can be used as ring shaped magnets for the first magnet 104 and the second magnet 108.

During operation of the surface polishing apparatus 100, the first magnet 104 and the second magnet rotate 108 in opposite directions. The rotation of the magnets 104 and 108 in opposite directions creates a dragging force that causes

20 magnetic fluid circulation. For instance, the second magnet 108 may rotate in a clockwise direction and the first magnet 104 may rotate in a counter clockwise direction. A workpiece (not shown) to be polished is located beneath the first magnet 104 and the second magnet 108.

The gap 112 distance is such that a magnetic field is established between

25 both magnets 104 and 108, which is greater than the ambient magnetic field. An exemplary gap 112 distance range is 0.5mm to 1.0mm.

Figures 2(a) and 2(b) show an electromagnetic analysis simulation, by ANSYS Multiphysics, of the magnetic field generated by the magnets 104 and 108 of Figure 1. At the center of the bottom surfaces between the two magnets

30 104 and 108, magnetic field lines connect the two magnets 104 and 108 to each other and generate a closed magnetic circuit, minimising the leakage of magnetic flux (see Figure 2(a)). As a result, a high magnetic flux density is generated to provide a finishing area 220 (see Figure 2(b)) for locating a workpiece (not shown) that is to be polished. The finishing area 220 is located beneath gap 112

35 and is proximate to the first magnet 104 and the second magnet 108.

When magnetic fluid (such as magneto-rheological fluid) is introduced, it is held on both the first magnet 104 and the second magnet 108 surfaces by magnetic force. The form of the MRF (magneto-rheological fluid) media is automatically shaped, with media concentrating at areas with higher magnetic flux density. With a high magnetic flux density being generated at the finishing area 220, the magnetic fluid accumulates at the finishing area 220.

This accumulation at the finishing area 220 can be seen in Figure 3, which shows a schematic cross sectional view of the surface polishing apparatus 100 of Figure 1 , having magnetic media 322 on the surfaces of the magnets 104 and 108. Magnetic field lines occurring around the gap 112 region are denoted using the dotted lines labeled with the reference numeral 326. The rotation of the magnets 104 and 108 in their respective directions facilitate accumulation of the magnetic media 322 on portions of the magnets 104 and 108 adjacent to the gap 112. When the two magnets 104 and 108 approach a workpiece 324 and the workpiece 324 surface penetrates into the media 322, the media 322 conforms to the workpiece surface and pressure is generated due to the media 322 deformation. This pressure generated allows the media 322 to perform finishing actions on the workpiece 324 surface.

Figure 4 illustrates a surface polishing apparatus 400 in accordance to a second embodiment of the invention. Reference numeral 450 shows a perspective view of the surface polishing apparatus 400; reference numeral 440 shows a side view 140 and reference numeral 480 shows a bottom view.

The surface polishing apparatus 400 comprises a pair of rare earth permanent magnets 404 and 408 fabricated from, for example, Nd-Fe-B. The magnets 404 and 408 are performing rapid MRF on a workpiece 424 using MRF media 422 that is coated on the magnets 404 and 408 surfaces.

The shape of the MRF media 422 automatically transforms depending on the workpiece 424 shape. Abrasive present in the MRF media 422 is automatically pressed out from within the MFR media 422 to the finishing area 420 of the surface polishing apparatus 400.

The mechanism or principles of rapid MRF will be detailed with reference to Figures 5 to 7. The MRF may be magnetic fluid comprising magnetic particles dispersed in a medium. The magnetic fluid may further comprise abrasive particles. The magnetic particles may comprise ferromagnetic material such as any or more of the following: carbonyl iron powder, nickel or cobalt. The medium may comprise any one or more the following: oil or water.

Figure 5 shows MRF media flow at the finishing area 220, where the MRF media comprises carbonyl iron powder (CIP), abrasive particles and a disperse medium (oil or water). The media flow 524 is in a direction substantially perpendicular to the magnetic field lines 526. The CIP, which are a few micrometers in mean diameter, form chain-like clusters 528 (see Figure 6) along the magnetic field line 526. Due to the formation of these clusters 528, the viscosity of the media greatly increases. During a rapid MRF process, it is believed that a reaction force from a workpiece surface breaks the clusters 528 in the media. This reaction force also pushes the media away from the finishing area 220 along the magnet 104 and 108 surface. With cross-reference to Figure 6, the broken clusters 626 return to the finishing area 220 after one complete rotation of the single ring magnet 104 and 108. After that, the cluster is restored (indicated by reference numeral 628) by the magnetic force and the media thereby maintains its shape (see the reformed clusters indicated using reference numeral 630) during or throughout the finishing process. Thus, counter rotation of the two magnets 104 and 108 (i.e. the magnets 104 and 108 rotating in opposite directions during operation) automatically recovers the shape of the magnetic fluid used to perform surface polishing of a workpiece.

In addition, since the CIP in the media is magnetised material and the abrasive particles are non-magnetised material, segregation occurs where a CIP rich layer 702 and an abrasive rich layer 704 are created naturally, as shown in Figure 7. This arrangement of layers causes abrasive particles accumulation on the surface of the CIP rich layer 702, resulting in high concentration of abrasive particles on the media 322 surface, which consequently increases contact between the abrasive particles and a workpiece surface. Chips 706 of material removed from the workpiece surface automatically adhere to the disperse medium of the MRF media 322 by the effect of magnetic buoyant force and centrifugal force.

Figure 8 shows a schematic cross sectional view of a surface polishing apparatus 800 in accordance with a third embodiment of the invention. The surface polishing apparatus 800 of Figure 8 shares several similarities with the surface polishing apparatus 100 of Figure 1 as follows.

The surface polishing apparatus 800 comprises a base 802. A first magnet

804 is coupled to the base 802 to have the first magnet 804 be rotatable about a longitudinal axis 806 of the first magnet 804. A second magnet 808 is coupled to the base 802 to have the second magnet 808 be rotatable about a longitudinal axis 810 of the second magnet 808. The first magnet 804 and the second magnet 808 are orientated to be in mutual attraction. At least a portion of the first magnet 804 is positioned adjacent to a portion of the second magnet 808 with a gap 812 between the adjacent portions. An exemplary gap 812 distance range is 0.5mm to 1.0mm. The surface polishing apparatus 800 includes a magnetic fluid reservoir (not shown) containing magnetic fluid 822 for performing surface polishing. The magnetic fluid reservoir is positioned to allow the magnetic fluid 822 to be in fluid communication with the gap 812 between the first magnet 804 and the second magnet 808. During operation of the surface polishing apparatus 800, the first magnet 804 and the second magnet 808 rotate in opposite directions. The rotation of the magnets 804 and 808 in opposite directions creates a dragging force that causes the magnetic fluid 822 circulation.

In contrast to the surface polishing apparatus 100 of Figure 1 , the surface polishing apparatus 800 of Figure 8 has its first magnet 804 and its second magnet 808 both coupled to the base 802 to have their respective longitudinal axis 806 and 810 be inclined to the base 802 surface. The surface polishing apparatus 800 further comprises a platform 840 to which the base 802 is coupled to have the base 802 be rotatable about a longitudinal axis 830 of the base 802. Thus, the surface polishing apparatus 800 advantageously includes an additional axis of rotation 830. This improves the final surface roughness and the evenness of finished surfaces because this additional axis rotation 830 crosses the abrasive particles path. Both the first magnet 804 and the second magnet 808 are concave shaped. This further advantageously allows the tilted magnet configuration of Figure 8 to polish more free form surfaces, especially concave surfaces.

Figure 9 shows a graph plotting the variation of surface roughness against the number of passes made by the surface polishing apparatus 100 of Figure 1 at different areas on a curved surface 902. At portions of the curved surface 902 located at horizontal distances of 0 mm to 3 mm away from the top of the curved surface 902, the respective plots obtained (i.e. the four plots collectively indicated by reference numeral 904) show that improvements of surface roughness are almost the same. For the case of the horizontal distance of 4 mm, the difference of improvement becomes more significant (see the plot indicated by reference numeral 906). Generally, material removal of a polishing process is proportionate to the pressure, velocity and time. Thus the experimental results of Figure 9 indicate that near-uniform pressure is generated at different areas with different magnet-to-workpiece gap sizes up to a certain or predetermined limit.

A further experiment was also conducted to verify the efficiency of the surface polishing apparatus described herein, especially for non magnetic or weakly magnetic workpieces. In this further experiment, rapid MRF facilitated by the surface polishing apparatus 100 of Figure 1 is applied to micro bumps made by laser machining. The material of the workpiece in this case is austenitic stainless steel 304. The experiment parameters and conditions are shown in Tables 1 and 2 below:

Table 1. Experimental conditions. Table 1. Properties of media.

Workpiece Stainless steel 304 MR fluid Waterbase MR fluid

Nd-Fe-B permanent magnet AI ₂ Q ₃

Abrasive

030 x 018 x 11 mm Size: 0.3, 1 :0, 12 μπι

Tool type

Magnetic flux density on MR fluid :90 wt%

Mixed ratio

magnet surface: 0.4 T Abrasive: 10wt%

Tool revolution ω 1 ,000 /min

Feed speed V _f 100 mm/min

Feed amplitude 2.5 mm

Reference numerals 1002 and 1004 in Figure 10 respectively show photos of the workpiece before and after finishing by rapid MRF. It is evident from the photos indicated by reference numeral 1004 that the surface that has undergone rapid MRF is finished smooth without a significant or observable deformation of the micro bumps. This means that the whole surface including the micro bump is finished uniformly. Since MRF media can transform its shape depending on the workpiece form and the media form automatically recovers during polishing, uniform pressure acts on the workpiece surface. As a result, the surface can be finished uniformly even if there are some convex structures on the surface. It will be appreciate that surface polishing apparatus described herein will work as well on workpieces fabricated from other materials.

Alternatively described, there are provided methods and apparatus for rapid MRF involving automatic media form recovery via counter-rotation of two ring, annular or circular magnets, assembled with magnetic poles opposing each other and resulting in attractive magnetic forces about their own axes (see Figure 1 ). Advantageously, there is generation of uniform pressure on freeform and intricate surfaces of workpieces due to the aforementioned automatic media form recovery. Additionally, there is superior material rate removal compared to conventional MRF processes due to the augmentation of magnetic field intensity by arranging two magnets in the configuration described with respect to Figure 1 , at the same time maintaining perpendicularity between the magnetic field and a normal of the finished surface. Furthermore, localised rapid MRF can be achieved by tilting of the two magnets axes of revolution as shown in Figure 8. This advantageously extends rapid MRF to concave surfaces or features previously out of reach by conventional MRF methods. The provision of a third axis of rotation, as show in Figure 8, further introduces isotropic rapid MRF of convex surfaces. Polishing of convex surfaces pose a problem as they may have certain areas shielded from the media, giving rise to non-uniform pressure generation. The _v surface polishing apparatus described herein therefore addresses this problem and provides an effective and automated finishing of convex surfaces.

Applications of rapid MRF are wide, including mould and micro-mould surface finishing, localised surface finishing of remanufactured aerospace components with freeform or complex surfaces such as fan blades and turbine blades, as well as finishing of brittle and ductile materials to remove sub-surface damage layer, etc.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the embodiments without departing from a spirit or scope of the invention as broadly described. The embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.