Grinding wheel

Abstract: A number of pyramidal or truncated-pyramidal projections (50) are formed on a portion of a metal base (1) forming a grinding face, to be homogeneously distributed at substantially uniform heights. Super abrasive grains (8) having grain sizes which are smaller than the heights of the projections are fixed to the surfaces of the projections (50). A coating film (10) consisting of fluororesin is formed to at least partially cover the outer surface of the grinding face (2) including the surfaces of the super abrasive grains (8), for preventing deposition of a workpiece. Since the outer surface of the grinding face (2) is covered with the coating film (10), sharpness of this grinding wheel is improved and clearances between the super abrasive grains are hardly clogged up with chips, while the chips are hardly deposited onto the grinding face.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a grinding wheel, and more particularly, it relates to a grinding wheel, employed for grinding various materials such as a plastic material, a glass material and a ceramics material, comprising super abrasive grains of diamond or CBN (cubic boron nitride) which are fixed to its grinding face. Description of the Background Art In relation to this type of grinding wheel, mainly employed is an electrodeposition grinding wheel, which has a higher degree of concentration of abrasive grains on a grinding face as compared with a grinding wheel comprising abrasive grains which are fixed to a portion, forming a grinding face, of a metal base serving as a base material by a resinoid, metal or vitrified bond material. The electrodeposition grinding wheel has excellent sharpness, due to a large amount of protrusion of abrasive grains concerning its grinding. With abrasion of the abrasive grains following progress of the grinding, however, dischargeability of chips is so deteriorated that clearances between the abrasive grains are clogged up with the chips and the chips are deposited onto the grinding face. In order to solve this problem, Japanese Utility Model Laying-Open No. 57-3561 (1982) proposes a rotary grinding wheel which is provided with a plurality of clearance grooves obliquely extending along its outer peripheral grinding face. When a plastic lens, for example, is ground by the grinding wheel which is proposed in the aforementioned gazette, however, sharpness of the grinding wheel is extremely reduced due to viscosity which is specific to the material of the workpiece. Thus, the aforementioned problem cannot be solved by this grinding wheel. In order to solve the aforementioned problem, the inventors have proposed a rotary grinding wheel which is provided with a knurled grinding face on its outer periphery in Japanese Utility Model Publication No. 7-15725 (1995). The inventors have also proposed a grinding wheel comprising a metal base, having a knurled portion forming a grinding face, which is made of an abradable material in Japanese Patent Laying-Open No. 6-114743 (1994). In every one of the grinding wheels which have hitherto been proposed, however, it is difficult to solve such a problem that clearances between the abrasive grains are clogged up with chips or chips are deposited onto the grinding face, depending on the material of the workpiece. US-A-4511373 discloses a grinding wheel, in which fluororesins are used as bonding material but not as a coating film on abrasive grains already bonded. SUMMARY OF THE INVENTION An object of the present invention is to provide a grinding wheel which can solve such a problem that clearances between abrasive grains are clogged up with chips, with a grinding face which is substantially free from deposition of chips. The present invention provides a grinding wheel having a grinding face being provided with a number of projections being formed on a portion of a base material forming said grinding face, said projections being homogeneously distributed at substantially uniform heights, and abrasive grains having grain sizes being smaller than said heights of said projections, being fixed to the surfaces of said projections, wherein said abrasive grains are fixed to said grinding face of the base material through an electrodeposited layer, a coating film chips for preventing deposition of said coating film being formed to at least partially cover the outer surface of said grinding face and including the surfaces of said abrasive grains, wherein said coating film includes fluororesin. The aforementioned projections are preferably formed by pyramidal projections. More preferably, respective top portions of those of the pyramidal projections which are adjacent to each other along the rotational direction of the grinding wheel are arranged to be displaced from each other perpendicularly to the rotational direction of the grinding wheel. Further, the aforementioned projections are preferably formed by truncated-pyramidal projections. More preferably, respective top faces of those of the truncated-pyramidal projections which are adjacent to each other along the rotational direction of the grinding wheel are arranged to be displaced from each other perpendicularly to the rotational direction of the grinding wheel. The abrasive grains are fixed to the surfaces of the projections by electrodeposition. Further, the abrasive grains are fixed to the surfaces of the projections through nickel layer. According to a preferred embodiment, resin containing polytetrafluoroethylene (PTFE) is employed as the fluororesin forming the coating film. In order to improve wear resistance of the coating film, about 20 to 30 percent by weight of a filler of diamond, metal oxide, metal, SiC, glass, graphite, molybdenum disulfide or mica may be mixed into the coating film. In this case, the filler is preferably mixed into the coating film in the form of particles, powder, fiber or whisker. In order to improve strength of the coating film and adhesion to the base material, a mixture of fluororesin and silicon resin and optionally another resin such as polyamide, polyimide, polystyrene, epoxy, polyester, polyurethane or polyphenylene sulfide resin, for example, may be employed as the material for the coating film. In the grinding wheel according to the present invention, a number of projections are formed on the portion of the base material forming the grinding face to be homogeneously distributed at substantially uniform heights similarly to knurled surfaces, for example. The abrasive grains are fixed to the surfaces of the projections, i.e., crests and troughs of the projections, by a bond material. Therefore, those of the abrasive grains which are fixed to the crests of the projections directly concern the grinding, while the trough portions of the projections serve as chip pockets. Chips fall in the trough portions. Thus, it is possible to prevent the clearances between the abrasive grains from being clogged up with chips, as compared with a grinding wheel comprising abrasive grains which are fixed to a planar grinding face. Further, the grinding wheel according to the present invention is provided with the coating film for preventing deposition of the workpiece, to at least partially cover the outer surface of the grinding face including the surfaces of the abrasive grains. The abrasive grains which are fixed onto the surfaces of the crests of the projections directly concern the grinding. Portions of the coating film covering the surfaces of the abrasive grains are abraded and separated, with progress of the grinding. On the other hand, portions of the coating film covering the abrasive grains which are fixed to the surfaces of the troughs of the projections remain as such with those covering the bond material, regardless of progress of the grinding. The grinding progresses in such a state that these portions of the coating film remain on the trough portions of the projections, whereby the chips falling in the trough portions serving as chip pockets are discharged following rotation of the grinding wheel, without coming into direct contact with the abrasive grains and the bond material, with no adhesion to portions around the chip pockets. If no coating film is provided, the chips falling in the trough portions of the projections serving as chip pockets are pressed against the surfaces of the abrasive grains and the bond material to adhere to the same. Thus, the clearances between the abrasive grains are clogged up with the chips to deteriorate sharpness of the grinding wheel, and the chips are finally deposited onto the grinding face. In the grinding wheel according to the present invention, the material for the coating film for preventing deposition of the workpiece is prepared from fluororesin, preferably from fluororesin containing polytetrafluoroethylene (PTFE) which is called "Teflon" (trade name: by Du Pont Co., Ltd.). Such fluororesin has the maximum specific gravity and the minimum coefficient of friction among the synthetic resin materials, is not bonded to other materials, and has chemically stable properties. The fluororesin having such properties is adapted to prevent the workpiece from being deposited onto the grinding face in grinding. In the grinding wheel according to the present invention, silicon resin is further employed as the material for the coating film for preventing deposition of the workpiece. The silicon resin, which has extremely strong water repellency and low adhesion with other materials, can be employed as the material for the coating film together with the aforementioned fluororesin. Both of these resin materials may conceivably be employed as the materials for the coating film, depending on the position of formation and the shape of the coating film. According to the preferred embodiment, the projections are formed by pyramidal or truncated-pyramidal projections. A homogeneous irregularity pattern is formed on the grinding face through the projections of such shapes, so that a number of clearances are defined in the grinding face to serve as chip pockets. Only the abrasive grains provided around the top portions or top faces of the pyramidal or truncated-pyramidal projections concern the grinding, whereby the clearances between the abrasive grains are not clogged up with chips. Further, the respective top portions or top faces of the pyramidal or truncated-pyramidal projections which are adjacent to each other along the rotational direction of the grinding wheel are arranged to be displaced from each other perpendicularly to the rotational direction of the grinding wheel. Thus, the respective pyramidal or truncated-pyramidal projections bite into the working surface at fine pitches, whereby an accurate machined surface can be obtained. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view schematically showing the outer surface of a grinding wheel as an embodiment of a rotary grinding wheel of the present invention; Fig. 2 is a front elevational view illustrating a part of a grinding face of the grinding wheel shown in Fig. 1 in an enlarged manner; Fig. 3 is a sectional view taken along the line III - III in Fig. 2; Fig. 4 is a front elevational view showing a part of a grinding face of a grinding wheel according to another embodiment of the present invention in an enlarged manner; Fig. 5 is a sectional view taken along the line V - V in Fig. 4; Fig. 6 is a sectional view showing top portions of projections which are deformed into planes by abrasion due to progress of grinding in the grinding face shown in Fig. 3; and Fig. 7 is a model diagram showing the structure of a nickel electroplating tank which is employed manufacturing the grinding wheel according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in Fig. 1, a portion 2 defining a grinding face of an electrodeposition grinding wheel according to an embodiment of the present invention is provided on the outer peripheral edge portion of a metal base 1 which consists of a steel disc of 150 mm in diameter, 50 mm in axial hole diameter and 10 mm in thickness. The portion 2 defining a grinding face is provided with a number of inclined grooves, i.e., trough lines 3 and 4 which extend inclinedly with respect to the circumferential direction at constant pitches from both side surfaces of the metal base 1. Pyramidal projections 50 are formed in quadrilateral regions which are defined by the trough lines 3 and 4 intersecting with each other. As shown in Fig. 2, ridge lines 6 and 7 extend from the intersections between the trough lines 3 and 4. The pyramidal projections 50 have top portions 5 in the regions enclosed with the trough lines 3 and 4. These top portions 5 are formed at intersections between the ridge lines 6 and 7 in the regions which are enclosed with the trough lines 3 and 4. As shown by top portions 5a, 5b, 5c and 5d, the respective top portions 5 of those of the pyramidal projections 50 which are adjacent to each other along the rotational direction Q of the grinding wheel are arranged to be displaced from each other little by little perpendicularly to the rotational direction Q. The trough lines 3 or 4 are inclined with respect to the circumferential direction, i.e., the rotational direction Q, to obliquely extend at constant pitches P. The trough lines 3 are formed to be at an angle A with respect to the axis of rotation. On the other hand, the trough lines 4 are formed to be at an angle B with respect to the axis of rotation. The angles A and B are set in ranges of 0° to 45° and ±10° about 45° respectively, to be different from each other (A ≠ B). Thus, the top portions 5 of the pyramidal projections 50 which are formed in the regions enclosed with the trough lines 3 and 4 are displaced from each other little by little perpendicularly to the circumferential direction of the grinding wheel, i.e., the rotational direction Q of the grinding wheel, along the axis of rotation. As shown in Fig. 3, a number of super abrasive grains 8 consisting of artificial diamond or cubic boron nitride are fixed to the surface of each pyramidal projection 50 by a bond material 9 consisting of a nickel layer. The number of super abrasive grains 8 are fixed to the surface of each projection 50 by electrodeposition. The super abrasive grains 8 have grain sizes which are smaller than the pyramidal projections 50, and are homogeneously distributed along the overall grinding face 2 in a single-layer manner, to define a surface concerning grinding. Further, a coating film 10 of fluororesin is formed by coating the outer surfaces of the super abrasive grains 8 and the bond material 9 with Teflon (trade name: by Du Pont Co., Ltd.). This coating film 10 is formed by applying Teflon, heating the same at a temperature of about 300°C to 400°C for several 10 minutes, and thereafter cooling the same. Such application and heating may be repeated. The thickness of the coating film 10 is preferably within the range of 5 to 30 µm. The grain sizes of the super abrasive grains 8 are preferably within the range of 30 to 120 in U.S. mesh size (100 to 700 µm in grain size). In the electrodeposition grinding wheel according to this embodiment having the aforementioned structure, a number of clearances 60 are homogeneously defined in the overall grinding face 2 by the trough lines 3 and 4 and the ridge lines 6 and 7, to serve as chip pockets. Thus, chips are discharged through the clearances 60, and clearances between the super abrasive grains 8 are prevented from being clogged up with the chips. The grinding face 2 is provided with the projections 50, whereby the distance L between each adjacent pair of super abrasive grains 8 directly concerning grinding is larger than that in a planar grinding face. Thus, the grinding wheel bites into a workpiece in an excellent state with improved sharpness. Further, the positions of the top portions 5 of the pyramidal projections 50 are displaced from each other little by little with respect to the rotational direction Q of the grinding wheel so that rotation loci of the pyramidal projections 50 differ from each other, whereby a working surface of the workpiece is ground by the surfaces of the projections 50 at fine pitches. Thus, an accurate machined surface can be obtained with small irregularity. If all pyramidal projections 50 are on the same rotation locus, on the other hand, the working surface of the workpiece is repeatedly scooped out by the projections 50 and hence deep stripes are formed to deteriorate accuracy of the machined surface. As shown in Figs. 4 and 5, a grinding wheel according to another embodiment of the present invention may be provided with truncated-pyramidal projections 50. In the truncated-pyramidal projections 50, top faces 51 are formed on intersections between ridge lines 6 and 7. Structures and shapes of other portions of the grinding wheel according to this embodiment are similar to those shown in Figs. 2 and 3, and hence redundant description is omitted. In the grinding wheel according to each of the aforementioned embodiments of the present invention, portions of the coating film 10 covering the top portions 5 or the top faces 51 of the projections 50 are abraded and separated to expose the surfaces of the super abrasive grains 8. The super abrasive grains 8 which are positioned on the top portions 5 or the top faces 51 directly concern grinding, while trough portions which are covered with the coating film 10 are adapted to discharge chips. With progress of the grinding, the super abrasive grains 8 which are positioned on the top portions 5 or the top faces 51 are abraded and dropped, while the top portions 5 of the projections 50 are also abraded to define planes, as shown in Fig. 6. In this case, the super abrasive grains 8 which are positioned on the trough portions are successively exposed to concern the grinding. In order to confirm the effect of preventing chips from being deposited onto the grinding face, which is the maximum effect of the grinding wheel according to the present invention, an experiment was made as follows: Trough lines 3 and 4 were formed on the outer peripheral edge portion of a discoidal metal base 1 of 150 mm in outer diameter and 10 mm in width, which was made of S25C, as inclined grooves which were inclined with respect to the circumferential direction at pitches P of 1.5 mm from both side surfaces. Pyramidal projections 50 each having a base of 0.8 mm were formed in regions which were enclosed with these trough lines 3 and 4. The angles A and B of inclination shown in Fig. 2 were set at 11° and 49° respectively. The metal base 1 which was formed in the aforementioned manner was subjected to electrolytic degreasing and hydrochloric acid treatment as pretreatment for plating. Thereafter the metal base 1 was introduced into a nickel electroplating tank 100 containing diamond abrasive grains 80 shown in Fig. 7, and subjected to nickel electrodeposition. In this electrodeposition, the metal base 1 was rotated along arrow R, and the diamond abrasive grains 80 were stirred by a pump 11 and precipitated on the metal base 1, to adhere to its outer peripheral surface. The grain sizes of the diamond abrasive grains 80 were #80/100 in U.S. mesh size. The thickness of the bond material 9 which was formed on the grinding face 2 as a plating layer was set at about 60 % of the grain sizes of the abrasive grains 8. Referring to Fig. 7, a nickel plate 12 and an electric heater 13 were arranged in the plating tank 100. The electrodeposition grinding wheel obtained in the aforementioned manner was heated at a temperature of 380°C for 40 minutes, to be degreased. Thereafter the grinding wheel was sandblasted, and then air was sprayed to this grinding wheel to wash the outer surface. A coating film of 5 µm in thickness was formed on the grinding face 2 of the electrodeposition grinding wheel which was pretreated in the aforementioned manner by primer coating with fluororesin through an air spray. Thereafter the grinding wheel was dried at a temperature of 100 to 120°C for 15 minutes. The grinding wheel was cooled in the air, and thereafter another coating film of 20 µm in thickness was formed on the grinding face 2 of the grinding wheel by enamel coating with fluororesin through an air spray. Then, the coating film was fired at a temperature of 380°C for 20 minutes. Thereafter the grinding wheel was cooled in the air. Thus, a coating film 10 of fluororesin having a thickness of 25 µm was uniformly formed on the outer surface of the grinding face 2. Workpieces of polycarbonate resin of 300 mm in length, 50 mm in height and 5 mm in thickness were surface-ground with the electrodeposition grinding wheel obtained in the aforementioned manner and another electrodeposition grinding wheel which was provided with a grinding face of the same shape as the above and with no coating film 10. The grinding was performed under conditions of a grinding wheel peripheral velocity of 1500 m/min., a depth of cut of 0.2 mm/pass and a workpiece feed rate of 5 m/min., in a down-cut system as dry working. In the electrodeposition grinding wheel which was coated with fluororesin according to the present invention, it was possible to surface-grind the workpieces up to an amount of working of 50,000 mm 3 , while the clearances between the abrasive grains were hardly clogged up with chips and absolutely no chips were deposited onto the grinding face. In the conventional electrodeposition grinding wheel which was not coated with fluororesin, on the other hand, the clearances between the abrasive grains were clogged with chips at an amount of working of about 100 mm 3 and the chips were deposited onto the grinding face to disable continuation of the grinding. On the other hand, a rotary grinding wheel according to the present invention which was formed in the aforementioned manner was employed to cylindrically grind a disc of polycarbonate resin having a diameter of 100 mm and a thickness of 5 mm. This grinding was performed under conditions of a grinding wheel peripheral velocity of 800 m/min. and a depth of cut of 0.02 to 0.03 mm/rev. The cylindrical grinding was carried out in both of wet and dry types, and chips were hardly deposited onto the grinding face and the clearances between the abrasive grains were hardly clogged up with chips in either type. While the projections 50 are formed by knurling in each of the aforementioned embodiments, the same may alternatively be formed by another cutting or grinding. Further, the coating film 10, which is made of Teflon in the above description, may alternatively be made of another fluororesin. The super abrasive grains 8, which are prepared from diamond abrasive grains in the aforementioned embodiments, may alternatively be prepared from CBN abrasive grains. The present invention is not restricted to a discoidal grinding wheel, but is also applicable to another grinding wheel such as a straight cup grinding wheel. If necessary, a plurality of obliquely extending clearance grooves may be provided in the outer peripheral grinding face of the rotary grinding wheel, as described with reference to the background technique. In the structure of the grinding face provided with the pyramidal or truncated-pyramidal projections 50 in the regions which are enclosed with the trough lines 3 and 4 serving as inclined grooves intersecting with each other as shown in Fig. 2 or 4, the number of the abrasive grains acting on grinding can be freely changed by changing the angles A and B of inclination and the pitches P. Thus, the number of working abrasive grains optimizing discharge of chips can be set in response to the properties of the workpiece to be worked, whereby stable working can be carried out in high efficiency. While the grinding face 2 is in the form of a cylinder in each of the aforementioned embodiments, the present invention is also applicable to a deformed grinding wheel having a grinding face in the form of a circular arc which is convexed or concaved in the radial direction, or a grinding wheel consisting of a metal base which is in the form of a bar. In the grinding wheel according to the present invention, as hereinabove described, a number of projections are formed on the grinding face to be homogeneously distributed in substantially uniform heights and the outer surface of the grinding face is covered with the coating film for preventing deposition of the workpiece, whereby sharpness of the grinding wheel is improved and the clearances between the abrasive grains are hardly clogged up with chips, while the chips are hardly deposited onto the grinding face. Thus, it is possible to efficiently provide a work of good quality in high accuracy. In particular, the inventive grinding wheel is optimum for working the outer peripheral surface of a plastic lens for an eyeglass which is made of polycarbonate resin.

Claims: A grinding wheel having a grinding face (2) being provided with a number of projections (50) being formed on a portion of a base material (1) forming said grinding face, said projections being homogeneously distributed at substantially uniform heights, and abrasive grains (8) having grain sizes being smaller than said heights of said projections, being fixed to the surfaces of said projections, wherein said abrasive grains (8) are fixed to said grinding face (2) of the base material (1) through an electrodeposited layer (9), characterized by a coating film (10) for preventing deposition of chips, said coating film (10) being formed to at least partially cover the outer surface of said grinding face (2) and including the surfaces of said abrasive grains (8), wherein said coating film (10) includes fluororesin. The grinding wheel in accordance with claim 1, wherein said projections (50) are formed by pyramidal projections. The grinding wheel in accordance with claim 2, wherein respective top portions (5) of those of said pyramidal projections being adjacent to each other along a rotational direction (Q) of said grinding wheel are arranged to be displaced from each other perpendicularly to said rotational direction. The grinding wheel in accordance with claim 1, wherein said projections (50) are formed by truncated-pyramidal projections. The grinding wheel in accordance with claim 4, wherein respective top faces (51) of those of said truncated-pyramidal projections being adjacent to each other along a rotational direction (Q) of said grinding wheel are arranged to be displaced from each other perpendicularly to said rotational direction. The grinding wheel in accordance with claim 1, wherein said coating film (10) further contains silicon resin. The grinding wheel in accordance with claim 6, wherein said coating film (10) contains a filler consisting of at least one material being selected from the group consisting of diamond, metal oxide, metal, SiC, glass, graphite, molybdenum disulfide and mica. The grinding wheel in accordance with claim 7, wherein said filler is contained in said coating film (10) in the form of at least one selected from the group consisting of particles, powder, fiber and whisker. The grinding wheel in accordance with claim 6, wherein said coating film (10) further contains another resin in addition to said fluororesin and said silicon resin. The grinding wheel in accordance with claim 1, wherein said abrasive grains (8) are fixed to said surfaces of said projections (50) through a nickel layer (9).

Inventors: FUKUSHIMA KENJI, FUKUSHIMA, KENJI, Fukushima, Kenji, c/o Osaka Diamond Ind. Co.
Applicants: OSAKA DIAMOND IND, OSAKA DIAMOND INDUSTRIAL CO.
Application year: 1995
Main classification: B24D3
IPC codes: B24D3, B24D18