Abstract:
The present invention discloses an automatic polishing device and the method of using the device for the surface finishing of a workpiece such as complex-curved-profile parts. Automatic polishing is performed by mounting the workpiece on an apparatus which enables different orientations along multiple axes and immerging the workpiece in a controlled flow of abrasive slurry. The device of the present invention provides a uniform finish on free form surface. It is also precise, low cost, non-destructive, and non-polluting.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from the U.S. provisional application Ser. No. 61/686,852 filed on Apr. 13, 2012, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to an automatic polishing device, in particular, to a flow-based polishing device for the surface finishing of complex-curved-profile parts. 
       TECHNICAL BACKGROUND 
       [0003]    Polishing is an essential process of creating a smooth and shiny surface by using mechanical or chemical reaction and it has been employed by manufacturing industry for centuries. Comparing with hand-polishing, the process using automated polishing machines, such as those used for polishing semiconductor wafers and for polishing fine optics, are more efficient. However, the use of these automated polishing machines is restricted to those parts with surface profiles in regular shape. Therefore, hand-polishing is still widely used in a number of industries (e.g., watch, jewelry, mould and die, etc.) as in many circumstances, the surface profiles of the polishing parts are not in regular shape as those of semiconductor wafers or optical lenses. Most of the existing automated polishing machines just could not be applied to the complex-curved-profile parts, such as small watch parts and jewelry parts that are made of metal or metal alloy, jade stone, ivory, and ceramics, etc. There are considerable needs to develop an efficient automated polishing technique yet with the flexibility as skilled hands. Moreover, the machine-based polishing technique would yield a more uniform and reliable polishing effect comparing to those done by hand-polishing. It is believed that hand-polishing can reach the best surface roughness of R&gt;0.1 μm, while machine-based polishing can reduce the surface roughness by one order of magnitude. This is essential, for example, for the mould-and-die industry which requires a precise surface treatment of molding-tool-steel. 
         [0004]    Many prior arts have attempted to overcome the difficulties of surface finishing of complex-curved-profile parts. U.S. Pat. No. 6,171,175 has disclosed a hand polishing method using conventional grinding, lapping, honing techniques, or an automatic polishing method with improved adaptive mechanical tools but none of these methods can give enough precision at acceptable efficiency. Computer programmed robotic polishing (European Pat. No. EP1935564), magnetorheological polishing (U.S. Pat. No. 5,449,313), or ion beam machining (US Patent Application Pub. No. 20070227878) possess high precision but the capital cost is too high. The flow-based machining, e.g., abrasive flow machining (U.S. Pat. No. 4,936,057), magnetic abrasive finishing (U.S. Pat. No. 4,175,930), fluid jet polishing (U.S. Pat. No. 7,455,573), can balance the requirements of precision and cost but only working for hard workpieces with strength against the flow force. The chemical polishing (U.S. Pat. No. 4,826,563), electrochemical polishing (U.S. Pat. No. 4,740,280), and plasma-electrolytic polishing (US Patent Application Pub. No. 20100200424) provide a solution of precise, gentle and low-cost surface finishing. However, the treatment and disposal of used chemicals and waste water are troublesome. Such wet-polishing processes involving chemical reactions are restricted due to environmental regulations in many circumstances. The disclosure of these cited references and their entirety are incorporated herein by reference. 
         [0005]    There is a need for a new polishing means which is low-cost, environmental-friendly, precise, gentle and suitable for various types of workpiece including complex-curved-profile parts. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the present invention is to provide an alternate polishing device. The device includes a bath of controlled flow of abrasive slurry, at least one apparatus for mounting at least one workpiece with which its orientation varies along multi-axes, and at least one container containing the abrasive slurry. The device of the present invention provides a uniform finish on a free form surface. It is also precise, low cost, non-destructive, and non-polluting. The at least one apparatus for mounting the at least one workpiece includes a first rotary stage which can be a one-axis, two-axis, or three-axis rotary stage and is immerged inside flow of abrasive slurry during operation of the device. The first rotary stage is automatically adjustable such that the orientation of the mounted workpiece varies in a controlled manner. The abrasive slurry comprises a plurality of abrasive particles and a carrying liquid which is driven by a liquid pump. Optionally, the abrasive slurry includes a plurality of abrasive particles which is pre-mixed with a plurality of elastic polymer particles and the carrying liquid. Alternatively, a plurality of relatively smaller abrasive particles can be pre-coated on the surface of a relatively larger elastic polymer particle. The relatively larger elastic polymer particle may further include a magnetic core to facilitate the flow of the abrasive slurry driven by a plurality of magnetic bars situated under the container. Depending on the shape of the workpiece, the device of the present invention may be further modified to include a second rotary stage for driving the rotation of the first rotary stage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of one embodiment of the device having a circular container for holding an abrasive slurry and a three-axis rotary stage for mounting a workpiece to be polished. Abrasive flow is driven by a liquid pump. 
           [0008]      FIG. 2  is a schematic diagram of another embodiment of the device having a circular container for holding an abrasive slurry and a three-axis rotary stage for mounting a workpiece to be polished. The abrasive slurry contains magnetic particles and abrasive flow is driven by electromagnetic force. The inset shows the structure of the magnetic abrasive slurry. 
           [0009]      FIG. 3  is a schematic diagram of a further embodiment of the device having a cylindrical container for holding an abrasive slurry and a two-axis rotary stage for mounting a workpiece to be polished. The relative movement between the abrasive slurry and the workpiece is achieved by rotating the cylindrical container. 
           [0010]      FIG. 4  is a schematic diagram of other embodiment of the device having a fixed (non-rotatable) cylindrical container for holding an abrasive slurry and a first one-axis rotary stage for mounting a workpiece. The relative movement between the abrasive slurry and the workpiece is achieved by rotating the first rotary stage inside the cylindrical container with a second rotary stage outside of the cylindrical container. 
           [0011]      FIG. 5  shows the surface roughness of a steel work piece before and after polishing using the method in one of the embodiments illustrated in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  illustrates an exemplary embodiment of the present invention. The automatic polishing device in this embodiment comprises a circular container  11  containing an abrasive slurry  12  which comprises a plurality of abrasive particles and a carrying liquid driven by a liquid pump  13 , wherein the liquid pump  13  drives the abrasive slurry to form abrasive slurry flow  12  in the circular container  11 . A workpiece  15  to be polished is mounted in a three-axis rotary stage  16  immerged inside the abrasive slurry flow  12 . The orientation of the workpiece  15  varies in a controlled manner by the three-axis rotary stage  16  capable of automatic adjustment to facilitate a gentle and uniform surface finish of the workpiece  15 . In one example, the workpiece  15  is a complex-curved-profile aluminum article at dimension of less than 50(L)×50(W)×50(H)mm 3 . It is immerged in the abrasive slurry  12  containing 600 mesh silicon carbide (SiC) abrasive particles, other polymers such as silicone-based elastic polymer, and a carrying liquid. The abrasive slurry flow  12  is driven by the liquid pump  13  inside the circular container  11 . The circular container  11  is made of plastic and the inner diameter of the container  11  is 200 mm and the outer diameter is 400 mm and the height is 400 mm. Comparing with containers of other shapes (e.g., the cylindrical container), the circular container  11  confines the flow of the abrasive slurry  12  in a circular track at relatively constant speed ranging from 0.3-3.0 m/s. The relative movement between the abrasive slurry  12  and the workpiece  15  yields an adaptive polishing of a curved surface. In the meantime, with the automatic three-axis rotary stage  16 , the orientation of the workpiece  15  slowly varies along three axes periodically at the rate of ˜one cycle per minute, so that a uniform surface polishing can be achieved. In such operation, the as-cut workpiece  15  can reach a mirror-like shinning surface in 30-60 min. 
         [0013]      FIG. 2  illustrates another embodiment of the present invention having a circular container  21  for holding a magnetic abrasive slurry  22  and a three-axis rotary stage  23  for mounting a workpiece  24  to be polished. The magnetic abrasive slurry  22  in this embodiment comprises a plurality of elastic polymer particles of about 1 mm in size. Each of the elastic polymer particles has a magnetic core inside the particle and a plurality of abrasive particles of about 1-100 μm in size which are pre-coated on the surface of the elastic polymer particle. The magnetic abrasive slurry  22  in this embodiment flowing inside the circular container  21  is driven by a plurality of magnets on a magnetic board  25  driven by a motor  26  for rotating under the circular container  21 . The workpiece  24  to be polished is mounted in the three-axis rotary stage  23  immerged inside the magnetic abrasive slurry flow  22 . The orientation of the workpiece  24  varies in a controlled manner by the three-axis rotary stage  23  capable of automatic adjustment to facilitate a gentle and uniform surface finish of the workpiece  24 . In one example, the workpiece  24  is a complex-curved-profile steel article at the dimension of less than 50(L)×50(W)×50(H)mm 3 . It is immerged in the magnetic abrasive slurry  22  comprising silicone-based elastic polymer particles of about 1 mm in size. Each of the elastic polymer particles has a magnetic core of specially-treated magnetic stainless steel of 0.3 mm(D)×1 mm(L) pin inside the particle. A plurality of SiC abrasive particles of about 1-100 μm in size which are pre-coated or glue-mixed on the surface of the elastic polymer particle. The magnetic abrasive slurry  22  in such is flowing inside the circular container  21 , driven by a plurality of magnets rotating under the circular container  21  or a plurality of electromagnets under the circular container  21  alternating the electromagnetic field inside the circular container  21 . The circular container  21  is made of plastic and the inner diameter of the container  21  is 200 mm and the outer diameter is 400 mm and the height is 400 mm. The circular container  21  confines the flow of the magnetic abrasive slurry  22  in a circular track and the magnetic abrasive slurry  22  flow driven by the magnetic force runs as fast as 30 Hz. Moreover, it automatically switches direction between clockwise and counter-clockwise once a minute with pre-setting. The relative movement between such magnetic abrasive flow and the workpiece  24  yields an efficient polishing of curved surface. In the meantime, with the automatic three-axis rotary stage  23 , the orientation of the workpiece  24  slowly varies along three axes periodically at the rate of ˜one cycle per minute, so that a uniform surface polishing can be achieved. In such operation, the use of carrying liquid in abrasive slurry is optional. That is, the polishing process can be achieved with a ‘dry flow’ of the magnetic-abrasive-polymer particles. In certain embodiments, this is convenient that the workpiece  24  needs not to be in contacted with water or oil. Depending on the size and surface complexity, the as-cut workpiece  24  can reach a mirror-like shinning surface in 30-60 min with this method.  FIG. 5  shows the surface profile of a steel workpiece sample treated as such measured with Alpha Step from Tencor Instruments. As a direct readout from the Instrument, the surface roughness (Ra) improves from 3.7 μm to 0.1 μm after the said polishing treatment. 
         [0014]      FIG. 3  illustrates a further embodiment of the present invention. An abrasive slurry  31  in this embodiment comprises a plurality of abrasive particles which are pre-mixed with a plurality of elastic polymer particles and a carrying liquid. The abrasive slurry  31  is held in a cylindrical container  32  which is about 400 mm in diameter and rotatable, as such, the abrasive slurry  31  rotates together with the rotating cylindrical container  32  driven by a rotating stage  33  connected to the bottom of the cylindrical container  32 . The rotating stage  33  is further driven by a motor  34 . The workpiece  35  to be polished is mounted in a two-axis rotary stage  36  immerged inside the abrasive slurry flow and about 100-150 mm away from the rotating axis at the centre of the cylindrical container  32 . The orientation of the workpiece  35  varies in a controlled manner by the two-axis rotary stage  36  capable of automatic adjustment to facilitate a gentle and uniform surface finish of the workpiece  35 . In one example, the workpiece  35  is a mammoth task figure at the dimension of less than 50(L)×50(W)×50(H)mm 3 .The abrasive slurry  31  contains 1200 mesh Al 2 O 3  abrasive particles and silicone-based elastic polymer particles and an oil-based carrying liquid. The abrasive slurry  31  is held in the stainless steel cylindrical container  32  at the diameter of 400 mm and the height of 400 mm. The abrasive slurry  31  rotates together with the rotating cylindrical container  32  driven by a rotating stage  33  connected to the bottom of the cylindrical container  32  at the rate of 60-600 rpm. The workpiece  35  is mounted in the two-axis rotary stage  36  immerged inside the abrasive slurry flow at about 100-150 mm away from the rotating axis at the centre of the cylindrical container  32 . Such arrangement results in a relative movement between the abrasive slurry  31  and the workpiece  35  at the linear speed of 1-10 m/s. With the relatively fine and soft abrasive slurry  31  used, the rotation provides a gentle polishing on a complex-curved surface of relatively soft material. For the workpiece  35  such as a mammoth task figure, only the outline along the vertical axis of the figure needs to be polished. The bottom surface to support the figure is flat and treated with other means. In this special case, the multi-axis rotary stage that mounts the workpiece  35  can be simplified as the two-axis rotary stage  36 , and the figure to be polished is mounted with vertical axis along with one of the rotary axis as shown in the  FIG. 3 . With the automatic two-axis rotary stage  36 , the orientation of the workpiece  35  slowly varies along two axes periodically at the rate of ˜one cycle per minute, so that a uniform surface polishing can be achieved. Depending on the finest or color effect required, the polishing process of such a delicate workpiece  35  takes about 1-10 hours. 
         [0015]      FIG. 4  illustrates other embodiment of the present invention. An abrasive slurry  41  in this embodiment comprises a plurality of abrasive particles of about 1-100 μm in size which are pre-coated on the surface of each of the elastic polymer particles of about 100-1000 μm in size. The abrasive slurry  41  is held in a fixed (non-rotatable) cylindrical container  42  of about 400 mm in diameter. A workpiece  43  (e.g., a patterned stainless steel sheet) to be polished is mounted in a one-axis rotary stage  44  immerged inside the abrasive slurry  41  flow and about 100-150 mm away from the central axis of the fixed cylindrical container  42 . The first rotary stage  44  is further mounted on a second rotary stage  45  rotating along the central axis of the fixed cylindrical container  42 . The second rotary stage  45  is driven by a motor  46 . While the orientation of the workpiece  43  varies in a controlled manner by the first rotary stage  44  capable of automatic adjustment, the relative movement between the abrasive slurry  41  and the workpiece  43  is driven by the second rotary stage  45 , and as such a gentle and uniform surface finish of the workpiece is achieved. In one example, the abrasive slurry  41  comprised silicone-based elastic polymer particles of about 100-1000 μm in size. A plurality of SiC abrasive particles of about 1-100 μm in size are pre-coated or glue-mixed on the surface of the elastic polymer particle. The abrasive slurry  41  is held in the fixed (non-rotatable) stainless steel cylindrical container  42  at the diameter of 400 mm and the height of 400 mm. The workpiece  43  is a patterned stainless steel sheet of the size of 100(L)×50(W)×0.5(H)mm 3 . The typical dimensions of the through-patterns (holes and lines) on the sheet are ranging from 0.5-5 mm, and it requires smooth lines and edges at-finish on both sides of the sheet. In order to polish the through-patterns of the stainless steel sheet, the abrasive slurry  41  must flow across the sheet, passing through the patterns at appropriate force that does not deform the 0.5 mm-thick sheet. In this specific embodiment, the rotary stage that mounts the sample can be simplified as the one-axis rotary stage  44 . In the setup, the workpiece  43  is mounted in the first one-axis rotary stage  44  so that the center line along longitude direction of the sheet is aligned with the rotary axis of the one-axis rotary stage  44 . It rotates slowly along this axis at the rate of ˜one cycle per minute to provide a uniform operation. On the other hand, the first rotary stage  44  is further mounted on the second rotary stage  45 , which is driven by the motor  46  and suspends on top of the container  42 . The second rotary stage  45  rotates along the central axis of the cylindrical container such that the workpiece  43  is immerged inside the abrasive slurry  41  at about 100-150 mm away from the central axis. While the second rotary stage  45  rotates at the rate of 60-600 rpm, the workpiece  43  further rotates in the abrasive slurry  41  along the central axis at the linear speed of 1-10 m/s, in addition to the foresaid slow rotation driven by the first rotary stage  44 . While the relative movement between the abrasive slurry  41  and the workpiece  43  is governed by the fast rotation driven by the second rotary stage  45 , the orientation of the workpiece  43  is controlled by the slow rotation driven by the first rotary stage  44 . Even though stainless steel is a hard material, the deburing process of the pattern edges can be achieved in this operation within one hour duration. 
         [0016]    If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions/embodiments may be optional or may be combined. 
         [0017]    For example, the abrasive slurry might be composed of:
       (a) abrasive particles and carrying liquid; or   (b) abrasive particles and elastic polymer particles; or   (c) abrasive particles and elastic polymer particles and carrying liquid; or   (d) abrasive particles and magnetic particles; or   (e) abrasive particles and magnetic particles and carrying liquid; or   (f) abrasive particles and magnetic particles and elastic polymer particles; or   (g) abrasive particle and magnetic particles and elastic polymer particles and carrying liquid; and the abrasive particles are not limited to SiC or Al 2 O 3  particles as mentioned in the above descriptions.       
 
         [0025]    In terms of industrial applications, the container and the rotary stage can be readily scaled up to meet the needs in various applications. Multiple containers engaged with single or multiple driving forces and multiple rotary stages for mounting multiple workpieces in each container can be applied. 
         [0026]    Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. 
         [0027]    It is also noted herein that while the above describes exemplary embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims. 
       INDUSTRIAL APPLICABILITY 
       [0028]    The present invention is useful for polishing both regular and irregular shaped objects in a fully automated polishing process to replace the conventional manual polishing or partially automated polishing process. The setup of the automatic polishing device is flexible and the composition of the abrasive slurry flow could also be modified to suit different shapes and size of the workpiece. The multi-axis rotary stage of the mounting apparatus in the present invention also enables different orientations of the movement of the workpiece along different axes during the automatic adjustment.