Ferrite core for deflecting yoke, surface polishing apparatus and grindstone therefor

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface polishing apparatus which, when manufacturing a ferrite core for a deflecting yoke for deflecting the electronic beam of a cathode-ray tube, polishes the surface of the ferrite core to thereby finish it into a given shape, and a ferrite core polished by such a surface polishing apparatus.

[0003] 2. Description of the Related Art

[0004] A cathode-ray tube, which is used in a color television or a display monitor, is required to show high-grade image quality and, in order to be able to obtain high resolving power, it is also required to enhance the position accuracy of an electronic beam which is deflected. Such high-grade image quality and electronic beam position accuracy greatly depend on the characteristic of a core for a deflecting yoke and, as the material of such core, there is conventionally used ferrite. The ferrite core is manufactured by firing ferrite material powder after it is molded; and, in the case where the ferrite material is fired after it is molded, due to the thermal shrinkage thereof, the volume thereof is reduced by approximately 60%. For this reason, in order to make uniform the dimensions of the ferrite cores, the ferrite cores must be ground and polished after they are fired.

[0005] As the technique for polishing the surface of the ferrite core for a deflecting core after it is fired, there have been developed the invention disclosed in JP-A-1-283740 in which the small-diameter portion of a core is fixed by a jig and the inner wall surface of a center hole formed in the core is polished. Further, the invention disclosed in JP-A-1-319226 in which a jig is inserted into a center hole formed in a ferrite core and the outside diameter portion of the ferrite core is polished.

[0006] In both of these inventions, the inner wall surface of the core center hole and the outer wall surface of the core are polished in separate steps. Now, description will be given below of a conventional step of polishing a ferrite core with reference to FIGS. 9(A), (B) and 10. Here, FIG. 9(A) is a section view of a ferrite core 1 for a deflecting yoke, showing a state in which a grindstone 3 is inserted into a center hole 1a formed in the ferrite core 1 and the inner wall surface 1b of the center hole 1a is polished by the grindstone 3. Although not shown here, the ferrite core 1 and the grindstone 3 can be respectively driven and rotated by electric motors which are disposed on the respective rotary shafts of the ferrite core 1 and the grindstone 3. These two rotary shafts are arranged in parallel to each other; and, an operation to polish the inner wall surface 1b of the ferrite core 1 is executed in such a manner that, the ferrite core 1 and the grindstone 3 are both rotated, the distance between the two rotary shafts are caused to vary while maintaining their parallel relationship, and the center hole la is polished while the outside of the grindstone 3 is contacted with the inner wall surface 1b.

[0007] Next, in the case where the outer wall surface 1c of the ferrite core 1 is polished in such a manner as shown in FIG. 9(B), similarly to the above, the ferrite core 1 and grindstone 4 are both rotated by the motors (not shown) to thereby polish the outer wall of the ferrite core 1 in such a manner that the outside of the grindstone 4 is contacted with the outer wall surface 1c of the ferrite core 1, while varying the distance between the two rotary shafts but maintaining their parallel relationship. However, it takes time to polish the inner and outer surfaces of the ferrite core 1 individually in this manner. In order to polish the two surfaces at the same time, the polishing operations must be synchronized with each other, which results in the complicated polishing operations.

[0008] Also, as the technique for carrying out the above two polishing operations simultaneously, there is available a technique shown in FIG. 10. In this technique, a grindstone 3′ for grinding or polishing the center hole 1a of the ferrite core 1 can be driven or rotated by an electric motor 5, whereas a grindstone 4′ for polishing the outer wall surface 1c of the ferrite core 1 can be driven or rotated by an electric motor 6. In this structure, since the outer wall surface 1c and the inner wall surface 1b of the ferrite core 1 can be polished simultaneously, the polishing operation time can be shortened. However, because the number of driving parts remains unchanged, the complication of the polishing operation cannot be relieved but skill is required to carry out such polishing operation.

[0009] As described above, in the conventional process for manufacturing a ferrite core for a deflecting yoke, in the case where the inner wall surface and outer wall surface of the ferrite core are polished in separate steps, the center axis of the center hole of the ferrite core and the center axis of the outside diameter of the ferrite core are shifted with respect to each other. Also, because the polishing operations of the inner and outer wall surfaces are executed in separate steps, it takes time to work the ferrite core, which degrades the production efficiency of the ferrite core.

SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a surface polishing apparatus which, in a process for manufacturing a ferrite core for a deflecting yoke, by using a grindstone formed as an integral body so as to be simultaneously contactable with the inner and outer wall surfaces of the ferrite core for a deflecting yoke, can enhance the working accuracy of the ferrite core without making the polishing operation complicated, can realize reduction in the time necessary for polishing the ferrite core, and can finish accurately the outer wall surface of the ferrite core for a deflecting yoke and the inner wall surface of the central hole of the ferrite core into their given shapes.

[0011] In attaining the above object, according to a first aspect of the invention, there is provided a surface polishing apparatus for a ferrite core for a deflecting yoke for polishing an inner wall surface of a hole portion and an outer wall surface of a lower end portion of a substantially trumpet-shaped ferrite core having a diameter increasing from an upper end portion thereof to the lower end portion thereof, wherein an inner wall surface of the hole portion and an outer wall surface of the lower end portion of the ferrite core are polished simultaneously using a grindstone whose grinding surfaces to be respectively contacted with the inner wall surface of the hole portion and the outer wall surface of the lower end portion of the ferrite core are formed integral with each other.

[0012] According to a second aspect of the invention, in a surface polishing apparatus for polishing a ferrite core for a deflecting yoke as set forth in the first aspect of the invention, there is further included securing means for securing the upper end portion of the ferrite core, holding means for holding the grindstone, and means for contacting the grindstone with the lower end portion of the ferrite core and for driving and rotating the securing means and/or holding means.

[0013] According to a third aspect of the invention, there is provided a grindstone for polishing the inner wall surface of the hole portion and the outer wall surface of the lower end portion of a substantially trumpet-shaped ferrite core having a diameter increasing from the upper end portion thereof to the lower end portion thereof, wherein the grinding surface of the grindstone for the inner wall surface to be contacted with the inner wall surface of the ferrite core having a similar shape to the hole portion of the ferrite core is formed integral with the grinding surface of the grindstone for the outer wall surface to be contacted with the bottom wall surface and the outer wall surface of the lower end portion of the ferrite core.

[0014] According to a fourth aspect of the invention, in a grindstone as set forth in the third aspect of the invention, the pillar-shaped projecting portion of the grindstone increasing in diameter from the upper end portion thereof to the lower end portion thereof, the disk-shaped seat surface portion of the grindstone connecting together the lower end portions of the pillar-shaped projecting portion, and the outer periphery projecting portion of the grindstone provided on the outer peripheral surface of the disk-shaped seat surface portion and projected therefrom in the periphery of the pillar-shaped projecting portion are formed integral with one another.

[0015] According to a fifth aspect of the invention, in a grindstone as set forth in the third and fourth aspects of the invention, between the inner and outer wall surfaces of the outer periphery projecting portion provided on the disk-shaped seat surface portion, there are formed a plurality of grooves.

[0016] According to a sixth aspect of the invention, in a grindstone as set forth in the third to fifth aspects of the invention, there are formed a plurality of spiral-shaped groove portions in the side surface of the pillar-shaped projecting portion.

[0017] According to a seventh aspect of the invention, there is provided a substantially trumpet-shaped ferrite core for a deflecting yoke, having a diameter increasing from the upper end portion thereof to the lower end portion thereof, wherein the inner wall surface of the hole portion and the outer wall surface of the lower end portion of the ferrite core are polished simultaneously using a grindstone whose grinding surfaces to be respectively contacted with the inner wall surface of the hole portion and the outer wall surface of the lower end portion of the ferrite core are formed integral with each other.

[0018] According to an eighth aspect of the invention, in a substantially trumpet-shaped ferrite core for a deflecting yoke as set forth in the seventh aspect of the invention, the inner wall surface of the hole portion and the outer wall surface of the lower end portion of the ferrite core have a coaxiality deviation of 0.03 mm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]
FIG. 1 is a perspective view of a ferrite core for a deflecting yoke to be polished by a surface polishing apparatus according to the invention.

[0020]
FIG. 2 is a perspective view of a grindstone according to a first embodiment of the invention, showing the shape of the grindstone.

[0021] FIGS. 3(A) and 3(B) are schematic views of the shape of the grindstone according to the first embodiment.

[0022]
FIG. 4 is a schematic view of the structure of a surface polishing apparatus according to the first embodiment of the invention.

[0023] FIGS. 5(A) and 5(B) are section views of the surface polishing apparatus according to the first embodiment, showing the polishing state thereof.

[0024]
FIG. 6 is a section view of the surface polishing apparatus according to the first embodiment, showing the polishing state thereof.

[0025]
FIG. 7 is a perspective view of a grindstone according to a second embodiment of the invention.

[0026] FIGS. 8(A) and 8(B) are schematic views of a grindstone according to a third embodiment of the invention.

[0027] FIGS. 9(A) and 9(B) are explanatory views of a polishing operation to be executed when a conventional polishing apparatus is used.

[0028]
FIG. 10 is an explanatory view of a polishing operation to be executed when a conventional polishing apparatus is used.

[0029]
FIG. 11 is a table to show the coaxiality deviation of a ferrite core polished by a grindstone according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] (First Embodiment)

[0031] Now, FIG. 1 is a perspective view of a ferrite core 1 for a deflecting yoke. The ferrite core 1 includes a small-diameter portion 1d in one end portion (upper end portion) thereof and a large-diameter portion 1e in the other end portion (lower end portion) thereof. Also, the ferrite core 1 further includes a connecting portion if the outer wall of which gradually increases in size from the small-diameter portion 1d to the large-diameter portion 1e and also which connects together the small-diameter portion 1d and the large-diameter portion 1e smoothly; and, the small-diameter portion 1d, large-diameter portion 1e and connecting portion if are formed of ferrite material as an integral body. In the ferrite core 1, there is formed a center hole la so as to penetrate through the small-diameter portion 1d and large-diameter portion 1e. The inside diameter of the center hole 1a gradually increases from the small-diameter portion 1d toward the large-diameter portion 1e; and thus, the center hole 1a is formed as a smooth three-dimensional curved surface substantially like a trumpet shape. As the ferrite material, there are used Mg—Mn—Zn system material and Mn—Zn system material. Specifically, a ferrite core is manufactured of the powder of such ferrite material through the following steps: that is, (1) the step of measuring the weight of the ferrite material powder; (2) the step of mixing the ferrite material powder; (3) the step of granulating the mixture; (4) the step of calcining the granulated ferrite material; (5) the step of grinding the calcined ferrite material; (6) the step of granulating the ground ferrite material; (7) the step of molding the granulated ferrite material; (8) the step of firing the molded ferrite material; (9) the step of finishing the fired ferrite material into a finish ferrite core; and, the step of warehousing the finish ferrite core. The ferrite cores, the volumes of which are shrunken in the firing step (8), vary from one another in dimension and shape, so that the deflecting characteristics of cathode-ray tubes are caused to vary greatly from one another. To make uniform the dimensions and shapes of the ferrite cores, as the finishing step (9), a grinding operation and a polishing operation must be executed. In the surface polishing apparatus according to the invention, the outer wall surface 1c (and bottom wall surface 1g) of the large-diameter portion 1e (lower end portion) of the ferrite core 1 and the inner wall surface 1b of the center hole la are ground and polished simultaneously. Also, by making uniform the outside diameters of the small-diameter portions 1d of the ferrite cores 1, there are formed linear portions in the side surface of the ferrite cores 1, which is makes it possible to facilitate the securing of the ferrite cores 1 to the surface polishing apparatus. In order to secure a securing strength (for example, 5 mm or more) or in order not to have a bad influence on the characteristic of the ferrite core 1, the length of this linear portion is set in the range between {fraction (1/10)} and ½ of the height of the ferrite core 1 (the distance from the upper end portion, that is, small-diameter portion id to the lower end portion, that is, large-diameter portion 1e).

[0032] Now, FIGS. 2 and 3 are explanatory views of the shape of a grindstone 2 used in the invention. Specifically, FIG. 2 is a perspective view of the grindstone 2; FIG. 3(A) is a plan view of the grindstone 2; and, FIG. 3(B) is a longitudinal section view of the grindstone 2. The grindstone 2 comprises a disk-shaped bottom end portion 2c forming a disk-shaped seat surface, an outer periphery projecting portion 2d formed on the outer periphery of the bottom end portion 2c, and a pillar-shaped projecting portion 2e provided on and projected long upwardly from the central portion of the bottom end portion 2c; and, these portions 2c, 2d and 2e are formed as an integral body. The inner wall surface of the outer periphery projecting portion 2d provides a grinding surface 2a for grinding or polishing the outer wall surface 1c of the ferrite core 1, whereas the outer periphery of the pillar-shaped projecting portion 2e (the portion corresponding to the pillar side surface) provides a grinding surface 2b for grinding or polishing the inner wall surface 1b of the ferrite core 1. As shown in FIG. 3 (A), the bottom end portion 2c, pillar-shaped projecting portion 2e and outer periphery projecting portion 2d are concentric with one another, while the outer periphery projecting portion 2d encloses the periphery of the pillar-shaped projecting portion 2e. Also, on the surface of the bottom end portion 2c as well, there is formed a grinding surface for grinding or polishing the bottom wall surface 1g of the ferrite core 1.

[0033] Referring in detail to the shape of the pillar-shaped projecting portion 2e, the outside diameter of the upper end face 2f is set smaller than the diameter of the center hole 1a on the small-diameter portion 1d side of the ferrite core 1, while the outside diameter of the pillar-shaped projecting portion 2e increases gradually toward the bottom end portion 2c. As shown in FIG. 3 (B), the section surface of the grinding surface 2b of the pillar-shaped projecting portion 2e provides a smooth curved line; and, this curved line is similar in shape to the diameter of the center hole 1a which gradually increases from the small-diameter portion 1d of the ferrite core 1 toward the large-diameter portion 1e thereof. Also, the inside diameter of the grinding surface 2a, which is the inner wall surface of the outer periphery projecting portion 2d is set larger than the outside diameter of the large-diameter portion 1e of the ferrite core 1.

[0034] Now, FIG. 4 is a schematic view of the first embodiment of a surface polishing apparatus according to the invention, including a section view thereof, in which the small-diameter portion 1d of the ferrite core 1 for a deflecting yoke is secured to securing means 10, while the grindstone 2 is contacted with the ferrite core 1 so that the former is be able to polish the latter. Also, in FIG. 4, the ferrite core 1, grindstone 2 and securing means 10 are shown sectionally. The grindstone 2 is mounted on the rotary shaft 5a of an electric motor 5, while the electric motor 5 drives and rotates the grindstone 2 in such a manner that the grindstone 2 is prevented from being eccentric with respect to the rotary shaft 5a. The securing means 10 is mounted on the rotary shaft 11a of an electric motor 11, while the electric motor 11 drives and rotates the securing means 10 in such a manner that the securing means 11 is prevented from being eccentric with respect to the rotary shaft 11a. Here, XX′ designates the rotation axis of the ferrite core 1 secured to the securing means 10 and YY′ stands for the rotation axis of the grindstone 2. Both or one of the ferrite core 1 and grindstone 2 are or is rotated and thus they are contacted with each other to thereby polish the ferrite core 1 in such a manner that the distance between the two rotational axes XX′, YY′ are caused to vary while maintaining them in parallel to each other.

[0035] The grindstone 2 is held by holding means 12 together with the electric motor 5 or in such a manner that it is connected with the electric motor 5 by transmission means (not shown) for transmission of the rotational driving motion of the electric motor 5. The holding means 12, while holding the grindstone 2, can be moved from a position, where the grindstone 2 can be contacted with the ferrite core 1 secured to the securing means 10, to a position where the grindstone 2 can be separated from the ferrite core 1. Also, the holding means 12 includes moving means 12a which moves the grindstone 2 to thereby adjust its distance and contact positions with respect to the ferrite core 1 according to the progress of the grinding operation. The moving means 12a includes a dial meter for grasping the moving distance of the grindstone 2 that is moved according to the progress of the grinding operation. Holding by the holding means 12 is to keep the parallel relationship between the rotation axis YY′ of the grindstone 2 and the rotation axis XX′ of the ferrite core 1; and, in the fine adjustment of the grindstone 2 by the moving means 12a as well, the parallel relationship between these two rotation axes are maintained. The holding means 12 and 13 are respectively disposed on a base 14 in such a manner that the rotation axis XX′ and rotation axis YY′ can be aligned with each other or can be adjusted into alignment with each other.

[0036] The rotation axis XX′ is the rotation axis of the ferrite core 1 secured to the securing means 10 and the rotation axis YY′ is the rotation axis of the grindstone 2. In the embodiment shown in FIG. 4, the rotation axes (rotation centers) of the electric motors 5 and 11 are respectively the same as the rotation axes XX′ and YY′. However, in the case where there is used transmission means (not shown) for transmission of the rotational driving motion of the electric motors 5 and 11, the electric motors 5 and 11 may also be held at other positions and other types of holding means may also be employed. The securing means 10 secures the ferrite core 1 in such a manner that the outer periphery of the small-diameter portion 1d of the ferrite core 1 is held from a plurality of directions by the securing portion 10b.

[0037] The polishing operation is carried out by rotating both or one of the ferrite core 1 and grindstone 2. In a case where both of them are rotated, in order not only to shorten the time for the polishing operation but also to produce a highly polished surface, the rotation speed of the ferrite core 1 should be slower than the rotation speed of the grindstone 2. Because the ferrite core 1 must be ground and polished so as to have given shape and dimensions, the ferrite core 1 is rotated in such a manner that it is always fixed at a given position; and, the grindstone 2 is moved with respect to the ferrite core 1. Before polishing of the ferrite core 1 is started, in the moving means 12a, there is previously set a reference position in which, for example, the rotation axes XX′ and YY′ can be aligned with each other; and, when the grindstone 2 is moved as the polishing operation progresses, the movement quantities of the grindstone 2 from the reference position is electrically or mechanically read by the dial meter of the moving means 12a respectively as the movement quantity in the rotation axis direction and the movement quantity in the vertical direction. In accordance with the thus obtained movement quantities, in the case of a manual operation, the moving means 12a is controlled by an operator to move the grindstone 2, or in the case of an automatic operation (such as a microcomputer-controlled operation), the moving means 12a is controlled by an electric signal representing the movement quantities to move the grindstone 2, thereby grinding and polishing the ferrite core 1 so as to produce given shape and dimensions, with the result that there can be produced a core for a deflecting yoke.

[0038] The polishing operation of the ferrite core 1 is executed in the following manner. In the case where the holding means 12 and 13 are set in their respective given positions of the base 14 shown in FIG. 4, the grinding surface 2b of the grindstone 2 is contacted with the inner wall surface 1b of the ferrite core 1 and, at the same time, the grinding surface 2a of the outer peripheral projecting portion 2d is contacted with the outer wall surface 1c of the ferrite core 1; that is, the polishing operation is executed in such contact manner. In this polishing operation, there is a possibility that the grindstone 2 cannot be contacted with the ferrite core 1 depending on the shape and/or dimensions of the ferrite core 1 before it is polished. For this reason, in some cases, at the time of the start of the polishing operation, the mutual distance between the rotation axis XX′ of the ferrite core 1 and the rotation axis YY′ of the grindstone 2 is adjusted in the vertical direction and, with the progress of the grinding and polishing operation, the grindstone 2 is moved so that the mutual distance can provide a given distance.

[0039] Now, FIGS. 5(A) and 5(B) are section views of the ferrite core 1 and grindstone 2 at the time when the polishing operation is started. As shown in FIG. 5(A), the rotation axis XX′ of the ferrite core 1 and the rotation axis YY′ of the grindstone 2 are aligned with each other and, after then, the grindstone 2 is gradually moved to approach the ferrite core 1 from the large-diameter portion 1e side of the ferrite core 1, before the polishing operation is started. In this manner, in the case where the two rotation axes are aligned with each other and the pillar-shaped projecting portion 2e of the grindstone 2 is inserted into the center hole 1a of the ferrite core 1, as shown in FIG. 5 (B), the bottom wall surface 1g of the ferrite core 1 is contacted with the grinding surface of the surface of the bottom end portion 2c of the grindstone 2 and thus, firstly, the bottom wall surface 1g of the ferrite core 1 is polished.

[0040] In the case where the polishing operation of the bottom wall surface 1g of the ferrite core 1 progresses and the grindstone 2 is made to approach the ferrite core 1 further and is thus moved up to a given position, the movement of the grindstone 2 is stopped; and, as shown in FIG. 6 which is a section view of the ferrite core 1 and grindstone 2, while maintaining the parallel relationship between the rotation axis XX′ of the ferrite core 1 and the rotation axis YY′ of the grindstone 2, the grindstone 2 is gradually moved downwardly (or upwardly) in the vertical direction. Thus, the grinding surface 2a of the grindstone 2 is contacted with the outer wall surface 1c of the ferrite core 1 and also the grinding surface 2b of the pillar-shaped projecting portion 2e of the grindstone 2 is contacted with the inner wall surface 1b of the ferrite core 1, so that the grindstone 2 polishes the inner wall surface 1b and outer wall surface 1c of the ferrite core 1 simultaneously. By controlling the downward (or upward) movement distance of the grindstone 2, the center hole 1a and large-diameter portion 1e of the ferrite core 1 can be polished to provide given shapes respectively.

[0041] Also, as shown in FIG. 6 which a section view of a surface polishing apparatus according to the first embodiment, the curved surface of the pillar-shaped projecting portion 2e is polished in such a manner that the diameter of the center hole 1a increases gradually from the small-diameter portion 1d toward the large-diameter portion 1e; and, therefore, as described above, the inside diameter of the center hole 1a can be finished into a smooth three-dimensional curved surface.

[0042] (Second Embodiment)

[0043] Next, description will be given below of a second embodiment of a surface polishing apparatus according to the invention. Here, FIG. 7 is a perspective view of a grindstone 20 which is employed in the second embodiment of the invention. The second embodiment is similar to the previously described first embodiment except that the grindstone 20 is different in shape from the grindstone 2; that is, the structure of the second embodiment and the polishing operation of the ferrite core 1 are similar to those previously discussed with reference to FIGS. 2 to 6. In the case of the grindstone 20 shown in FIG. 7, in the outer periphery projecting portion 2d of the grindstone 2 shown in FIG. 2 discussed in the previously described first embodiment, there are formed a plurality of grooves 20g which extend radially from the rotation center of the rotation axis YY′. Each groove 20g extends from a grinding surface 20a, which is the inner wall surface of the outer periphery projecting portion 20d of the grindstone 20, and penetrates through the outer wall surface 20h of the grindstone 20; and also, the groove 20g has a depth substantially equivalent to the distance from the upper surface 20i of the outer periphery projecting portion 20d to the upper surface of the bottom end portion 20c on which a pillar-shaped projecting portion 20e is provided.

[0044] In the case where the grindstone 20 shown in FIG. 7 is rotated in such a manner as shown in FIG. 4 to thereby polish the ferrite core 1, supply of an abrasive (such as cleaning water) through the grooves 20g to the respective grinding surfaces can be facilitated. Owing to this, the load to be applied to the grindstone 20 can be decreased to thereby reduce the wear of the grindstone 20, and the polishing dust and abrasive of the center hole 1a and outer wall surface 1c of the ferrite core 1 can be discharged from the grooves 20 due to a centrifugal force, so that the ferrite core 1 can be finished into a good surface condition. However, in realizing smooth discharge of the polishing dust and abrasive, the invention is not limited to the structure shown in FIG. 7 but other structures are also possible; for example, the grooves 20g may be formed in a spiral shape, or the number, shape and depth of grooves may be varied.

[0045] (Third Embodiment)

[0046] As described above, in the second embodiment, in the outer periphery projecting portion 20d, there are formed grooves from which the polished dust and abrasive (cleaning water) can be discharged. Now, description will be given below of a third embodiment of a surface polishing apparatus according to the invention. Here, FIGS. 8 (A) and 8 (B) are schematic views of a grindstone 30 employed in the third embodiment. In the case of the grindstone 30 shown in FIGS. 8 (A) and 8 (B), in the grinding surface of the pillar-shaped projecting portion of the grindstone 30, there are formed spiral-shaped grooves 30g. Specifically, FIG. 8(A) is a plan view of the grindstone 30, extending at right angles to the rotation axis YY′, and FIG. 8(B) is a perspective view of the grindstone 30. The grindstone 30 is the same in shape as the grindstone 2 shown in FIG. 2 and previously discussed in the first embodiment, except that it includes a plurality of grooves 30g. The third embodiment employs the same structure and polishing operation of the core 1 as those previously described with reference to FIGS. 2 to 7.

[0047] Each of the grooves 30g formed in the grindstone 30 shown in FIGS. 8 (A) and (B) starts at the outer periphery of the upper end face 30f of the grindstone 30, extends through the grinding surface 30b of the pillar-shaped projecting portion 30e thereof and the surface of the bottom end portion 30c thereof, and penetrates through the outer wall surface 30n thereof from the grinding surface 30a of the outer periphery projecting portion 30d thereof; that is, the groove 30g extends in a spiral shape from the outer periphery of the upper end face 30f to the outer wall surface 30n.

[0048] In the case where the ferrite core 1 is polished using the grindstone 30 under the structure and arrangement shown in FIG. 4, supply of the abrasive (such as cleaning water) to the respective grinding surfaces can be facilitated and, at the same time, the polished dust and abrasive (such as cleaning water) of the inner wall surface 1b and outer wall surface 1c of the ferrite core 1 are collected together into the recessed portions of the grooves 30g due to the rotational centrifugal force of the grindstone 30 during the polishing operation, are moved toward the outer wall surface 30n, and are then discharged externally of the grindstone 30.

[0049] In the case where the grooves 30g are formed in such a manner as described above, not only the abrasive (such as cleaning water) can be supplied sufficiently to the contact portion between the pillar-shaped projecting portion 30e of the groove 30 and the inner wall surface 1b of the ferrite core 1 and the contact portion between the outer periphery projecting portion 30d of the groove 30 and the outer wall surface 1c of the ferrite core 1 to thereby reduce the polishing loads, but also the polishing operation can be executed without leaving the polished dust and surplus abrasive (such as cleaning water). Therefore, the wear of the grindstone 30 can be reduced and a polishing finish can be improved more.

[0050] In FIGS. 8 (A) and 8 (B), there is shown the grindstone 30 which includes the four grooves 30g respectively formed in the four portions thereof. However, the number of grooves, the shape of grooves, and the extension direction of the grooves 30g depending on their relationship with the rotation direction of the grindstone 30 are not limited to those shown in FIG. 8 (A) and 8 (B) but can be varied, provided that such variations can provide similar operation effects to the above-described operation effects.

[0051] As has been described above, the inner wall surface 1b (inside diameter) and outer wall surface 1c (outer wall) of the center hole 1a of the ferrite core 1 can be polished simultaneously using the grindstone. Also, since the movement quantity of the pillar-shaped projecting portion by the moving means 12a is equal to that of the outer periphery projecting portion by the moving means 12a, there is no possibility that one of the inner wall surface 1b and the outer wall surface 1c of the ferrite core 1 can be polished excessively or insufficiently. Owing to this, the polishing quantities of the inner wall surface 1b and the outer wall surface 1c of the ferrite core 1 as well as the shapes thereof after polished can be set simultaneously according to the shape and dimensions of the grindstone and the moving amount of the holding position of the grindstone in the polishing operation.

[0052] Also, FIG. 11 is a table which shows the measured values of the coaxiality deviation of the center hole and outside diameter of seven ferrite core samples of which the center hole and outside diameter were polished simultaneously using a grindstone according to the invention. In the case of the coaxiality deviations of the seven samples, the minimum value was 0.01 mm, the maximum value was 0.03 mm, and the mean value was 0.019 mm. For comparison, when the center holes and outside diameters of nine conventional ferrite core samples were polished using different grindstones, the coaxiality deviation of the nine samples, the minimum value was 0.05 mm, the maximum value was 0.12 mm, and the mean value was 0.083 mm. As can be seen from comparison between the maximum value 0.03 mm (in the case of the worst coaxiality deviation) using the grindstone according to the invention and the minimum value 0.05 mm (in the case of the best coaxiality deviation) according to the comparative examples, in case where the ferrite core is polished using the grindstone according to the invention, the coaxiality deviation of the ferrite core can be improved outstandingly. In this manner, the invention can enhance the working accuracy of a ferrite core for a deflecting yoke outstandingly and also can polish the two portions of the ferrite core simultaneously to thereby cut the polishing time by half.

[0053] In the case where a single grindstone is rotated to polish the inner and outer wall surfaces of the hole of a ferrite core for a deflecting yoke simultaneously, the center of the hole of the ferrite core can be made coincident with the center of the outer wall of the ferrite core and thus the two portions of the ferrite core can be polished while maintaining the coaxiality deviation thereof at a desired level.

[0054] Also, since the inner wall surface (inside diameter) and outer wall surface (outer wall) of a ferrite core for a deflecting yoke are polished simultaneously using a single grindstone, simply by adjusting the polishing quantity of one of the inner and outer wall surfaces, the inner and outer wall surfaces of the ferrite core can be ground and polished in such a manner that the ferrite core can provide the above-mentioned given finishing dimensions.

[0055] Further, since a single grindstone polishes a bottom wall surface of the ferrite core, the bottom wall surface can be polished without unevenness compared with a case where a conventional grindstone is used.

[0056] Still Further, since two or more portions of a ferrite core are polished using a single grindstone, the polishing operation can be executed by a single electric motor for driving and rotating the grindstone.

Ferrite core for deflecting yoke, surface polishing apparatus and grindstone therefor

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Priority Claims (1)