Abstract:
A magnetic disc system for grinding or polishing metallographic test specimens on a movable platen includes a barrier that is magnetically attached to a movable platen. A grinding or polishing surface attaches to the barrier. The barrier magnetically attaches to the platen and is also capable of blocking or greatly reducing the magnetic field magnetism from passing through to the grinding or polishing surface and restricting or preventing swarf adhesion resulting from the polishing or grinding operation that could harm the prepared surface.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to grinding or polishing systems. More specifically, this invention relates to systems for grinding or polishing a specimen&#39;s surface on a rotatable or movable platen. 
     Metallographic studies are typically conducted by properly polishing and etching the surface of a specimen so that it may be examined with a microscope. The polishing operation usually involves positioning the surface of the specimen to be polished against a grinding or polishing paper or cloth mounted on a rotating, circular platen. Normally, an abrasive and wetting/cooling agent is applied to the cloth or paper to facilitate the grinding and polishing operation. 
     Historically, metallographers have attached grinding papers or polishing cloths to platens in several ways. One approach was to lay the grinding paper on a “wheel” (i.e., a round, rotating platen surface) wetted with water and then slip a tight fitting ring over the periphery of the paper. In some systems, this ring mated with a beveled, raised portion beyond the edge of the paper on the platen. In other systems, the paper extended to the edge of the flat platen, and the hold-down ring covered the outer edge of the paper and part of the outer edge of the periphery of the platen perpendicular to the platen surface. Use of water enhanced adherence of the grinding paper to the platen. The combination of the effect of water and the retention ring around the paper edge usually provided adequate “adhesion” so that the paper would rotate with the wheel when the metallographic specimen was pressed against it. Initially, there might be some slippage, but this would usually stop during use. 
     Another prior art method utilized a polishing cloth. When using a polishing cloth, the metallographer would provide a cloth of greater diameter than that of the platen by several inches. The cloth was placed over the platen surface, and a round band of steel having a diameter slightly larger than the platen&#39;s diameter, was placed over the cloth around the platen periphery. This band could be tightened by turning a screw which connected the ends of the band. The cloth edges extending down between the band and the platen were pulled to stretch the cloth tightly over the platen surface. Then the band was tightened by turning the screw. If stretching of the cloth was inadequate, the polishing rate would be reduced. Further, the cloth could be gouged or ripped, especially if unmounted specimens were being polished. 
     Another, more modern approach to fasten grinding paper, polishing cloths, and other preparation surfaces to the platen, was to use a pressure-sensitive adhesive (PSA) film on the back of the material—paper, cloth, etc. The PSA backing held the grinding or polishing material affixed to the platen. The strength of the adhesive affected the performance of the polishing material. If the adhesive was too weak, the PSA backed material may come off during use. If the adhesive was too strong, the material may be difficult to remove from the platen when it was to be replaced. Nonetheless, most metallographers prefer the PSA-backed grinding and polishing materials or products, rather than the non-PSA products since they are simple to use. 
     Some products, such as grinding paper, have a short life; whereas polishing cloths can last for many days depending upon the number of specimens polished. To reduce costs, many labs keep a polishing cloth on a platen until the cloth is completely worn. Further, a given polishing cloth is used with only one specific abrasive. Hence, most labs need a number of platens, each one being covered by a different polishing cloth or several having the same polishing cloth for use with different abrasives. 
     Because platens are expensive, rather than having a large inventory of platens covered with the various grinding and polishing materials, alternate approaches have been used. One alternative was to use the same platen in combination with numerous, easily removable sheet steel discs, each disc having a distinctive pressure sensitive adhesive cloth glued to one side. Any of the steel discs could then be attached to the platen using a hold-down ring around the disc-platen periphery to keep the disc in place while permitting easy and quick replacement. This approach has a disadvantage common to all such procedures. Namely, to achieve good flatness over the entire specimen&#39;s polished surface, particularly with large specimens, the specimen must periodically be moved to the extreme edge of the grinding paper or cloth surface (particularly the former). This approach is most easily done using automatic polishing devices but is nearly impossible to do with “hand” (manual) polishing. With certain grinding and polishing machines, it is impossible to move the specimen over the edge of the grinding or polishing surface without accidentally hitting the retention ring thereby damaging the specimen. 
     To reduce the need for multiple platens, others have used magnetic materials to attach the grinding or polishing disc to the platen. Thus, in one such alternative system, a rubberized magnetic disc is permanently attached to a platen using an extremely strong adhesive. The steel disc (with the polishing material thereon) can then be held on the platen by the magnetic field from the magnetic layer of material adhered on the platen. However, because the magnetic material will eventually wear, diminish or degrade or the magnetic field will become reduced in strength during use, this approach requires the user to eventually purchase a new platen with a magnetic material surface layer. 
     If grinding with silicon carbide (SiC) paper is to be performed, thin steel discs with the paper adhered thereto can be used to magnetically attach the SiC paper to the platen. Typically, after two minutes or less of grinding, SiC paper will be worn and must be removed to allow another sheet of SiC grinding paper to be attached to the disc. Thus even though the steel disc remains attached magnetically to the platen, a system using SiC paper for grinding or polishing requires constant replacement of the SiC paper. 
     Another magnetic attachment system uses a steel-backed disc with the surface of the disc partially covered by hexagon-shaped areas of a resin containing iron, copper or other metal particles used for grinding with added diamond abrasive. This plate is placed on and retained by the magnetic material disc attached to the platen. This disc is used after the first grinding step (for example, 120, 180, or 240 grit SiC paper depending on the material being prepared) to replace the subsequent SiC paper steps (320, 400, and 600 grit for example) and 9 or 15 μm diamond is sprayed onto the surface for grinding. After use of the hexagon-shaped resin coated steel disc, a variety of grinding and polishing cloths can be used on the specimen. The cloths are typically permanently glued onto a thin, flexible sheet of steel. The cloth covered steel disc is then held magnetically on the magnetic material-covered platen. When the cloth is worn, the cloth with its steel back is discarded. Discarding the steel plate having the attached worn cloth presents an undesirable situation from the standpoint of waste disposal. It is undesirable to discard steel in this manner because steel with a permanently attached cloth is difficult to properly recycle. 
     Another magnetic-disc system uses a series of wire-mesh discs with diamond abrasive firmly attached to the top surface of the wire mesh or screen. Three disc grades are available: fine grind (FG), rough polish (RP), and medium polish (MP). First, the specimens may need to be ground flat using SiC paper. Then a platen with a piece of PSA-backed magnetic, rubberized material is used in combination with a disc. Thus, a FG disc is magnetically attached to the magnetic rubberized material on the platen, and the sample is ground. This is repeated using the RP and MP discs each magnetically attached to the platen. A standard final cloth polishing step is usually required to complete the preparation. 
     All the described magnetic disc systems generate a magnetic field that surrounds the test sample during the grinding or polishing step. In addition, for specimens that are ferromagnetic materials, including iron, cobalt, nickel, gadolinium, and Huesler alloys, grinding and polishing produce fine particles, “swarf.” This swarf may adhere to the preparation surface due to the magnetic field and degrade the surface quality of the specimens being prepared. 
     Thus, all of the previously described magnetic attachment systems are susceptible to magnetically attracted swarf when preparing ferromagnetic materials, that can degrade the prepared surface. In addition, the previously described magnetic attachment systems do not avoid the problems of PSA product which may detach prematurely or may be extremely difficult to remove. Thus, there is a need for a system to overcome the problems associated with a magnetic field penetrating the polishing or grinding surface and the problems associated with attaching polishing or grinding materials directly to a platen using PSA backing. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention recognizes and provides a solution to the problems associated with using a magnetic attachment system for coupling grinding or polishing surfaces and materials to a platen. Briefly the present invention comprises a system for grinding or polishing materials in which the grinding or polishing surface material is affixed to a platen using a magnetic field in a manner which precludes the field from acting upon the grinding or polishing swarf. The invention thus comprises a combination of elements including a movable platen utilized to move the grinding or polishing element to prepare the specimen surface. The movable platen has a top side that magnetically attaches to a barrier element. The barrier element inhibits the magnetic field from crossing or passing through the barrier element and interfering with the grinding or polishing process. The barrier element has two sides or layers, a first side or layer and a second side or layer. The first side is magnetically attached to the top side of the movable platen. The second side is impervious or nearly impervious to a magnetic field and is attached to a grinding or polishing element, for example, by a PSA material. The grinding or polishing element has a preparation side, to prepare specimens, and a platen attachment side that is attached to the second side of the barrier element. 
     The barrier element shields the grinding or polishing surface from the magnetic field and thus the resulting grinding or polishing surface of this system is not susceptible to the magnetic field caused by the magnetized material. Because the system prevents residual magnetism from interfering with the preparation of materials, problems with swarf adhering to the grinding or polishing surface are eliminated. Furthermore, because only the grinding paper or polishing cloth is disposed of when the grinding or polishing surface wears out, cost is reduced and waste disposal is improved because the barrier and the platen in this magnetic system are reused. 
     Accordingly, an object of the present invention is to provide a grinding or polishing surface that has a reduced magnitude of magnetic field. Thus, this invention reduces the magnetic field to which the preparation side of the grinding or polishing element is exposed. 
     Another object of this invention is to provide a system that is easier to maintain. One prior art system includes a magnetic fabric permanently attached to the platen. When the magnetic material becomes worn, the entire platen must be replaced. This invention provides for a magnetic material that is not permanently bonded to the platen. This allows removal of the magnetic material and replacement of the magnetic material rather than the entire platen. 
     A further object of this invention is to provide an economical specimen grinding or polishing system. The system allows reduced time to change the grinding or polishing element on the platen. Changing the grinding or polishing surface entails merely breaking the magnetic bond between the barrier element and the platen and placing a different barrier element with a different grinding or polishing surface on the platen. 
     Yet another object of this invention is to provide a system that is better for the environment from a waste disposal perspective. This invention allows easier recycling of products during waste disposal. Because the polishing surface is removable from the metal barrier, the cloth can be separately disposed by recycling. 
     An additional object of this invention is to provide for ease of removing the barrier element. Providing a space, sized for a tool such as a screw driver, between the barrier element and the platen allows one to wedge a tool between the platen and the barrier element to break the magnetic field for easy removal of the barrier element. 
     The full range of objects, advantages, and features of the invention are only appreciated by a full reading of this specification and a full understanding of the invention. Therefore, to complete this specification, a detailed description of the invention and the preferred embodiments follow, after a brief description of the drawing wherein additional objects, advantages and features of the invention are disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described in relation to the accompanying drawings comprised of the following figures: 
     FIG. 1 is an exploded, isometric view of a first preferred embodiment of the magnetic preparation disc system of the present invention having a magnetic interface and magnetic barrier positioned between a grinding or polishing surface and a platen. 
     FIG. 2 is an isometric view of a platen, magnetic element and barrier element depicting the slot in the magnetic element to facilitate removal of the element. 
     FIG. 3 is an isometric view of the polishing element including a removal tab. 
     FIG. 4 is a schematic view of the layers of the construction of a preferred embodiment. 
     FIG. 5 is an isometric view of an alternate embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a magnetic element  30  is attached to a movable platen  10  on a top side  20  thereof. The magnetic element  30  can be attached to such a movable platen  10  by means of a pressure sensitive adhesive  15 . The periphery of the magnetic element  30  may be congruent with the periphery of the movable platen  10 . 
     A separate barrier element  80 , includes a non-magnetically responsive portion  50  and a magnetically responsive portion  40  that is attracted to the magnetic element  30 . Thus portion  40  of barrier element  80  is placed against the magnetic element  30 . Magnetically responsive portion  40  is interposed between non-magnetically responsive portion  50  and magnetically responsive portion  40 . Barrier element  80  can be composed of bimetallic discs made from  18 -gauge stainless steel. The magnetic portion  40  can be ferromagnetic (type  430 ) ferritic stainless steel which is attracted to the magnetic element  30  attached to the movable platen  10 . The non-magnetic portion  50  of the barrier element  80  can be non-magnetic (type  304  or  316 ) austenitic stainless steel which is not magnetically attracted and which shields or prevents a magnetic field from completely penetrating the barrier element  80 . The non-magnetic portion  50  and the magnetic portion  40  are bonded together preferably with a bonding material  60  such as an unsupported acrylic pressure sensitive transfer film like Unifilm U261 that is not degraded by alcohol, water or acetone. A grinding or polishing preparation element  70  is attached to the non-magnetic portion  50  using a pressure sensitive adhesive. 
     Referring to FIG. 2, the movable platen  10  can extend beyond the periphery of the magnetic element  30 . A notch  35  can be put into the magnetic element  30  to facilitate removal of the barrier element  80 . An object, such as a screwdriver, can then easily be wedged between the movable platen  10  and the barrier element  80  at the notch  35  to facilitate removal. Alternatively, it is also possible to remove a notch  35  of material from the magnetic portion  40  of the barrier element  80  to thereby create a notch  35  which will facilitate removal. 
     The described platen system for grinding or polishing specimens utilizes a means for magnetically attaching the grinding or polishing preparation element  70  to the movable platen  10  and a means for attenuating or shielding the element  70  from the magnetic field, created by the magnetic attachment means, interposed between the means for magnetically attaching the grinding or polishing preparation element  70  to the movable platen  10  and the grinding or polishing preparation element  70 . In addition to the methods known to those skilled in the art, the means for magnetically attaching the grinding or polishing preparation element  70  consists of a magnetic element  30  attached to the movable platen  10  and a barrier element  80  having two sides, layers, or portions  40 ,  50 , with the first side  40  magnetically attached to the magnetic element  30 . The grinding or polishing preparation element  70  is then bonded to the second side  50  of the barrier element  80  with an adhesive. The means for attenuating the magnetic field, in addition to the methods known to those skilled in the art, include a barrier element  80  composed of two layers  40 ,  50 . One layer  40  is magnetically responsive and magnetically attaches to the magnetic element  30 . The other layer  50  is non-magnetically responsive and screens the magnetic field from the grinding or polishing preparation element  70 . Various means for magnetically attaching the grinding or polishing preparation element and the means for attenuating the magnetic field in addition to the means described herein may be utilized. 
     Many non-magnetic metal, alloy, ceramic, etc. materials may be used for the non-magnetic portion  50  of the barrier element  80 . An austenitic stainless steel is a preferred choice due to its relative low cost, availability in sheet form (with a polished surface), and corrosion resistance (types  304  and  316  are the most widely available). Likewise, the magnetic portion  40  of the barrier element  80 , may include various ferromagnetic materials such as, Fe, Ni, or Co metal and alloys, and a few nonferrous alloys. However, a ferritic stainless steel (of which type  430  is the most commonly made in sheet form) is a preferred choice due to cost, availability and corrosion resistance. The thickness of the respective portions  40 ,  50  ( 430  and  304  or  316 ) must preferably be sufficient to provide rigidity (so that they are not bent when the grinding or polishing preparation element  70  or other surface is removed) and to shield the working surface from the magnetic field of the underlying magnetic element  30 . 
     The strength of the magnetic field of the magnetic element  30  should preferably prevent the barrier element  80  from rotating due to the force of a test sample against a grinding or polishing preparation element  70 . As depicted in FIG. 5, an alternative method of preventing the rotation of the barrier element  80  and the grinding or polishing preparation element  70  during use is to utilize the magnetic force of the magnetic element  30  in combination with an antirotation device. The antirotation device can consist of one or more pins  25  that extend between the barrier element  80  and the movable platen  10 . 
     The optimal thickness required to shield the working surface can be determined empirically. The thickness is influenced by the magnetic strength (permeability) of the magnetic element  30 , by the thickness of the magnetic portion  40  and by the thickness of the non-magnetic portion  50 . The preferred embodiment includes a 0.060 inch thick magnetic element  30  with a magnetic field of  120  pounds per foot squared where the non-magnetic portion  50  is 0.025 to 0.045 inches thick and the magnetic portion  40  is 0.020 to 0.045 inches thick. 
     The grinding or polishing preparation element  70  can be attached to the barrier element  80  using a pressure sensitive adhesive  65  such as a hot melt thermal plastic such as Henkel Adhesives #6476. The adhesive used should be strong enough to prevent the grinding or polishing element  70  from moving but still allow relatively easy removal. Referring to FIG. 3, to aid removal of the grinding or polishing preparation element  70  from the non-magnetic portion  50 , an extra portion of paper or cloth, that can be in the shape of a semi-circular or thumb-nail shaped tab  75 , extends beyond the normal outer diameter of the grinding or polishing preparation element  70 . The tab  75  sticks out beyond the outer edge of the barrier element  80 . After the grinding or polishing preparation element  70  is worn, the tab  75  is grasped and pulled to remove the abrasive surface from the barrier element  80 . 
     As depicted in FIG. 4, the preferred embodiment is comprised of a series of layers. The first layer is the movable platen  10 . The next layer is an adhesive  15  such as an adhesive tape with the side adjacent to the magnetic material  20  being an acrylic adhesive while the side adjacent to the platen  10  is a rubber-based adhesive. The magnetic element  30  is attached to the platen  10  using the adhesive  15 . The magnetically attracted portion  40  is magnetically attached to the magnetic element  30 . A bonding material  60 , such as an unsupported acrylic pressure sensitive transfer film like Unifilm U261, attaches the magnetic portion  40  to the non-magnetic portion  50 . A pressure sensitive adhesive  65  is applied to the grinding or polishing preparation element  70  and this element is affixed to the non-magnetic portion  50 . 
     Some alternative structures include, but are not limited to, the following variations. The magnetic portion  40  and the non-magnetic portion  50  of the barrier element  80  can be mechanically connected. The magnetic portion  50  and the non-magnetic portion  40  may, for example, be riveted together using counter bored holes to ensure flat surfaces. Also, the barrier element  80  may be composed of various unspecified materials that have similar characteristics to those defined. In addition, the thickness of the barrier element  80  may vary depending on the type of material and the strength of the magnetic element  30 . The various dimensions of thickness of materials to maintain both connection between the movable platen  10  and the barrier element  80  and to block or reduce the magnetic field are considered equivalent to this invention. 
     Therefore, the description of the apparatus of this invention is not intended to be limiting but is merely illustrative of the preferred embodiment of this invention. Other apparatus which incorporate modifications or changes to that which has been described herein are equally included within the scope of the following claims and equivalents thereof.