Patent Application
20110021116
This invention is directed to a method of processing a glass plate, and in particular shaping an edge of the plate.
Glass plate manufacturing comprises three principal steps, melting of the raw material to form a molten glass, forming the molten glass into sheets or plates and finally processing the plate into a final shape satisfactory to purchaser or user. Methods of forming thin glass plates include an overflow downdraw process, or fusion process, wherein a molten glass is supplied to an open-top conduit. The molten glass overflows the conduit and flows down converging surfaces comprising the outer surface of the conduit. At the bottom of the conduit the separate flows rejoin, or fuse, to form a thin glass ribbon. Other methods include the well known float process, where molten glass is floated on a bath of usually tin, slot draw, up draw and others. Generally, all of these processes include a final processing step of separating individual plates of glass from a parent sheet, sizing the plates in a cutting operation and edging the glass to strengthen the piece for subsequent handling operations. The individual plates are edged both to remove flaws that may be formed when individual plates are cut from the parent, and to eliminate sharp edges that are easily damaged during handling.
Thin plate glass edging is typically done using a grinding wheel consisting of formed grooves. These formed grooves will create a shape on the glass that mirrors the groove. An example of this process is documented in U.S. Pat. No. 6,685,541 to Brown, et al. and U.S. Pat. No. 6,325,704 Brown, et al.
As the need for ever thinner plates of glass increases, owing largely to the electronic display industries (computers, cell phones, digital cameras and the like), producing a consistent edge shape in the wheel is becoming increasingly difficult:
the wheel profile becomes misshapen with use, causing inconsistent plate edge shape;
the surface area used by the wheel is limited to the groove, which increases the cost due to poor utilization of material;
the relatively small surface area of the wheel actually contacting the glass necessitates the use of coarser abrasive grain sizes and, ultimately, poorer glass sheet surface finishes;
the lack of chip clearance between the glass and the wheel during grinding increases the potential for defects in the plate as the wheel becomes clogged by glass particles; and
wheel profiles are difficult to make when a small tight radius is required. Formed wheels are typically made using an EDM process. As the tool used to create the form wears, often quickly, it creates an undesirable blunt profile at the bottom of the resultant groove.
The edging process generates particulate (e.g. chips), which is often difficult to remove from the plates.
In one embodiment, a method of shaping the edge of a glass plate is described comprising coupling a glass plate to a holding fixture, a portion of the glass plate extending from the holding fixture a distance L and comprising a first surface, a second surface opposing the first surface and an end surface, and wherein the first surface and the end surface intersect along a first edge and the second surface and the end surface intersect along a second edge, contacting the first edge with a first abrasive cup wheel rotating about a first axis of rotation angled relative to the first surface, wherein the first abrasive cup wheel contacts the first edge with a first force F1 that produces a first displacement δ1 of the extended portion, contacting the second edge of the glass plate with a second abrasive cup wheel rotating about a second axis of rotation angled relative to the second surface and spaced apart from the first abrasive cup wheel axis of rotation by a distance D, the second abrasive cup wheel contacting the second edge with a second force F2 that produces a second displacement δ2 of the extended portion opposite from δ1, and wherein the second abrasive cup wheel contacts the second edge simultaneous with the first abrasive cup wheel contacting the first edge, producing relative motion between the first and second abrasive cup wheels and the glass plate during the contacting of the first and second abrasive cup wheels with the first and second edges, respectively, and wherein the first displacement does not overlap the second displacement. Preferably, there is no relative motion between the first and second abrasive cup wheels during the contacting of the first and second abrasive cup wheels with the first and second edges, respectively. D is preferably equal to or greater than 220 mm, preferably equal to or greater than 250 mm, preferably greater equal to or greater than 275 mm or preferably equal to or greater than 300 mm. L is preferably equal to or greater than 10 mm, preferably equal to or greater than 25 mm, and more preferably L is equal to or greater than 50 mm, although in some instances, such as when the thickness of the glass plate is very small (e.g. less than about 0.3 mm), L may be as small as 5 mm. In some embodiments, the edges produced by the beveling may be further polished.
In certain other embodiments, an edge of the fixturing device may be shaped such that L, the amount of extension of the glass plate, varies relative to the edge of the fixturing device (support). The fixture may, for example, comprise an edge proximate the extended portion that includes a nonlinear shape. The nonlinear shape may be a curve, or the nonlinear shape may be a combination of linear segments.
In some embodiments, a distance between the first abrasive wheel and the first edge is varied, respectively, to maintain a constant bevel width and supplement the compliance of extended portion of the glass plate.
In another embodiment, a method of shaping the edge of a glass plate is disclosed comprising coupling a glass plate having a thickness equal to or less than 2 mm to a holding fixture, a portion of the glass plate extending from the holding fixture a distance L and comprising a first surface, an second surface opposing the first surface and an end surface, wherein the first surface and the end surface intersect along a first edge and the second surface and the end surface intersect along a second edge, contacting the first edge with a first abrasive cup wheel rotating about a first axis of rotation angled relative to the first surface, wherein the first abrasive cup wheel contacts the first edge with a first force F1 that produces a first displacement in the extended portion, contacting the second edge of the glass plate with a second abrasive cup wheel rotating about a second axis of rotation angled relative to the second surface and spaced apart from the first abrasive cup wheel axis of rotation by a distance D, the second abrasive cup wheel contacting the second edge with a second force F2 that produces a second displacement in the extended portion opposite in direction from the first displacement, and wherein the second abrasive cup wheel contacts the second edge simultaneous with the first abrasive cup wheel contacting the first edge, producing relative motion between the first and second abrasive cup wheels and the glass plate during the contacting of the first and second abrasive cup wheels with the first and second edges, respectively and wherein the extended portion extends a distance L equal to or greater than 25 mm from the holding fixture and D is selected such that the first displacement does not overlap the second displacement.
An included angle formed by the intersection of the planes of the bevels is preferably between about 40 and 140 degrees.
In some embodiments, edges formed by the beveling process may subsequently be polished to remove their sharpness and avoid cracking that may occur if the sharp bevel-produced edges are contacted.
To vary the stiffness of the extended portion, and therefore its flexure resulting from contact with the grinding wheels, L may vary as a function of position along the first or second edge. Preferably, L in the range between 5 mm and 50 mm.
D may be selected to be equal to or greater than 220 mm, preferably equal to or greater than 275 mm, and in some cases equal to or greater than about 300 or 320 mm.
In still another embodiment an apparatus for grinding bevels in a glass plate is described, the glass plate comprising substantially parallel major surfaces and at least one end surface intersecting the major surfaces along substantially parallel first and second edges. The apparatus comprises first and second grinding wheels comprising substantially flat grinding surfaces, wherein the grinding surfaces are positioned at angles relative to the end surface of the glass plates to produce a bevel along each of the first and second edges of the glass plate, the first and second grinding wheels configured to rotate about first and second axes of rotation, respectively. The apparatus further comprises a support member (e.g. a vacuum chuck) that supports the glass plate so that a portion of the glass plate extends beyond the support member and allows the glass plate to flex in response to contact with the first and second edges by the first and second grinding surfaces, respectively, the extended portion comprising the first and second edges. The first and second axes of rotation are separated by a distance such that a deflection of the extended portion of the glass plate resulting from contact between the first grinding surface and the first edge does not affect deflection of the extended portion of the glass plate resulting from contact between the second grinding surface and the second edge, and wherein the contact between the first and second grinding surfaces and the first and second edges is concurrent.
The apparatus is preferably supported in a manner such that a stiffness of the extended portion varies as a function of location along a length of the first or second edge. In some embodiments, the apparatus the distance the extended portion extends from the support varies as a function of location along a length of the first or second edge.
The invention will be understood more easily and other objects, characteristics, details and advantages thereof will become more clearly apparent in the course of the following explanatory description, which is given, without in any way implying a limitation, with reference to the attached Figures. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of the present invention. Finally, wherever applicable, like reference numerals refer to like elements.
Thin glass plates supplied to equipment manufacturers such as electronic display manufacturers typically comprise processed edges. That is, the edges are ground and shaped (e.g. beveled) to eliminate sharp edges that are easily damaged and edge flaws (chips, cracks, etc.) resulting from the cutting process that can decrease the strength of the glass. Such plates are typically equal to or less than about 2 mm in thickness between the opposing major surfaces of the plate, and more preferably a thickness equal to or less than about 0.7 mm and in some applications a thickness equal to or less than about 0.5 mm. Very thin plates of glass can be equal to or less than 0.3 mm and still be afforded the benefits of the present invention.
It is known that the fracture of glass can be traced to an initial flaw, for example a small crack, and the fracture extends from this initial flaw. Fracture can occur spontaneously over a very short period of time, or incrementally over an extended period of time depending on the stresses present in the article. Nevertheless, each fracture began at a flaw, and flaws are most typically found along the edge of a glass plate, and most especially an edge that has been previously scored and cut. To eliminate edge flaws, the plate edges may be ground or polished so that only the smallest flaws remain, thereby increasing the strength of the sheet by increasing the stress necessary to propagate a flaw.
On the other hand, grinding of the glass forms glass particulate. This particulate is often difficult to remove from the glass surfaces, even with washing. Thus, there is a competing desire to minimize the amount of material that is removed from the glass (ground away), while still minimizing sharp edges and flaws. Referring to
Additionally, the grinding process itself is rarely uniform, as the abrasive wheel may have a certain play or variation in its position as it traverses the glass edges. That is, the abrasive wheel may move closer or farther from the glass plate so that the force exerted against the plate by the grinding wheel may vary both as a function of time and/or position. This positional variation may lead directly to changes in the amount of material removed from an edge. The variation can result in uneven grinding and changes in the amount of particulate produced. More simply, the bevel width may vary, and this variation is most acute if the plate edge undergoing grinding is rigid.
Shown in
As shown in
Formed wheels, such as that depicted in
Typically, grinding surface 22 comprises diamond particulate as a cutting medium dispersed in a suitable matrix or binder (e.g. resin or metal bond matrixes). Good results have been obtained with 600 mesh diamond particulate, although particulate sizes ranging from 300 mesh through 1000 mesh have also been successfully demonstrated. Other cutting mediums may also be used, such as carbide particulate. Abrasive wheel 18 is mounted to a rotatable shaft 26, such as the shaft of an electric motor, the shaft comprising an axis of rotation 28 about which the abrasive wheel is rotated. Since there is a significant increase in abrasive surface area being applied to the glass plate with an abrasive cup wheel as described above when compared to a formed wheel, the abrasive cup wheels are more cost effective from the perspective of grinding medium used to the amount of glass that is ground. More simply, an abrasive cup wheel makes more efficient use of the grinding medium by applying more of the grinding medium to the task of grinding than a formed wheel design. Also, since a larger surface area is used with abrasive wheels having a flat contact surface, these wheels can last much longer than formed wheels. This may not only decrease yearly abrasive wheel cost but may also reduce production cost since line downtime associated with abrasive cup wheel changes can be significantly less frequent than formed wheel changes.
Referring still to
First abrasive wheel 18a is rotated about axis of rotation 28a and acts on first surface 30 with a force F1. This force F1 in turn produces a deflection δ1 in glass plate 14. That is, glass plate 14 bends in response to the applied force. This can be seen generically with the aid of
where force F divided by deflection δ is also proportional to the elastic modulus of the glass plate multiplied by the moment of inertia and divided by the amount of extension of the glass plate beyond the fixture to the third power.
It can also be shown that the amount of material removed by an abrasive wheel is directly proportional to the applied force. From the above equation it can be seen that a plate fully supported by a rigid support, with no extended portion and no deflection in a plane of the glass plate in the presence of an applied force, the stiffness is infinite. In this instance, an increase in force, such as the force applied by an abrasive wheel on a glass plate, will result in a commensurate increase in the amount of material removed, and therefore an increase in the bevel width. Such a system becomes unattractively sensitive to small variations in the position of the grinding wheel as are often observed in a real life system. This sensitivity can be as high as 1:1, wherein a doubling in the applied force results in a doubling of the material removed.
On the other hand, the relationship above also suggests that if a portion of the plate is extended past the fixture (e.g. beyond support 16), the stiffness of the extended portion is reduced and finite and the plate may flex. For a low, finite stiffness, this compliance results in a reduced bevel width. In other words, the deflection resulting from small positional variations of an abrasive wheel in contact with a plate having low stiffness (exhibiting compliance) can avoid large increases in material removed when compared to the same positional movement relative to a rigid plate (e.g. high stiffness). Additionally, the precision level of the beveling apparatus need not be as high as would be necessary if the glass plate did not exhibit compliance. This may reduce equipment costs, since, for example, bearing precision may be relaxed.
In accordance with embodiments of the present invention, a plurality of abrasive wheels are used to produce a chamfer or bevel on both edges of an end of a glass plate constrained by a fixturing device and wherein the glass plate includes a portion thereof that extends beyond the fixturing device. At least two abrasive wheels are deployed, and arranged so that each of the at least two abrasive wheels engage an end of the glass plate on opposite sides of the glass plate. Each wheel is rotated about an axis of rotation and traversed along the end of the glass plate so that double bevels are formed along the end of the glass plate.
For example, a bevel is formed by abrasive wheel 18a along first edge 38 of glass plate 14. Preferably, angle α of the bevel relative to the plane of end surface 36 is about 60 degrees, although good results have been seen with angles between and inclusive of 20 to 70 degrees (
To isolate the effects of abrasive cup wheels 18a from 18b, the axes of rotation 28a, 28b of first and second cup wheels 18a, 18b, respectively, are spaced apart a pre-determined distance D as depicted in
The amount of material removed, or the bevel width, is used to gage the performance of the grinding operation.
It will be understood by one skilled in the art that a similar set of circumstances described above can be depicted for second abrasive wheel 28b. That is, considering second abrasive wheel 28b in contact with second edge 40 and applying a force F2. However, since F2 is applied in a direction opposite that for F1, displacement of the extended portion of the glass sheet occurs in a direction opposite to the deflection produced by the first abrasive wheel.
While one embodiment comprises beveling first one edge and then the other, the process is less efficient than beveling both edges simultaneously. However, because the force applied to the extended portion by each abrasive cup wheel causes a deflection in the extended portion that is opposite for each of the first and second abrasive cup wheels, it is desirable to separate the cup wheels so that the deflection caused by one cup wheel does not influence the grinding by the other cup wheel. In other words, the axes of rotation of the abrasive cup wheels should be separated by a distance D such that at least a portion of the glass between the cup wheels is substantially undeflected.
In another embodiment the shape of the support may be altered to reflect the fact that the stiffness at the corners of the glass plate is less than the stiffness of the plate in the central region of the plate. This can be easily understood by noting that a point at the corner of the plate has glass only to one side and not the other. The same can be said for the opposite corner at the other end of an edge, except that the lack of glass is one the opposite side than the first corner. The result is that an abrasive wheel set for pre-determined position relative to the glass plate (i.e. set for a pre-determined grinding depth) will remove more material from the central region of the glass plate edge than at the corners of the glass plate edge. This occurs in part because the corner regions flex more, and may in fact exhibit curling. To maintain a constant stiffness and consistent material removal along the length of the edge, it may be necessary to alter one of the variables upon which stiffness depends. One can, if desired, vary the position of the abrasive wheel as the wheel traverses over a given edge. Alternatively, the shape of support member 16 may be varied so that the extension L of the glass plate varies along the edge. In this case, L should be reduced proximate the corners of the glass plate, reducing L at those points, and effectively increasing the stiffness of the plate in those regions. For example,
Additionally, once bevels have been produced on the glass plate, the resulting additional edges (46, 48 and 50, 52) may be further polished to eliminate the sharp corner at those edges and form arcuate edges (see
It should be emphasized that the above-described embodiments of the present invention, particularly any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.
