Patent Description:
Electronic appliances such as smartphones and tablet PCs are provided with position-detecting devices composed of electronic pens for instructing position and sensor boards for detecting the position, as means for enabling users to easily input operation information and word information. For example, in the position-detecting device disclosed in <CIT>, pulse signals are sent from a coil of an electronic pen to sensor coils disposed in X-Y directions on a sensor board, to generate an electromotive force by electromagnetic induction in the coils, thereby obtaining position information in X-Y coordinates. In the electronic appliance, a sensor board is disposed under a display panel, such that information displayed on the display panel by various pieces of software, etc. is combined with the position information to make information input to the electronic appliance easy.

In an electronic pen used in such position-detecting device, a hollow cylindrical magnetic core is disposed in a hollow portion of a coil, to increase coupling with coils on a sensor board for higher accuracy of position information. <FIG> shows the internal structure of the electronic pen disclosed in <CIT>. In this electronic pen, a casing <NUM> contains a hollow cylindrical ferrite core <NUM> around which a coil <NUM> is wound. The hollow cylindrical ferrite core <NUM> has a tapered tip portion <NUM> having a diameter reducing according to the internal structure of the casing <NUM>, a switch rod <NUM> with a cover <NUM> having a cap-shaped tip, and a hollow portion <NUM> in which the switch rod <NUM> is slidable. A rear end of the ferrite core <NUM> is fixed to a support <NUM> in the casing <NUM>. A rear end of the switch rod <NUM> is connected to an operation switch <NUM> fixed to the circuit board <NUM>.

A small ferrite core used in an electronic pen as described in <CIT> has a thin, elongated, hollow, cylindrical shape having, for example, an outer diameter of <NUM> or less, a thickness of <NUM> or less and a length of <NUM> or more, such that it is contained in a thin, elongated casing. In such a small hollow cylindrical ferrite core, it may be considered to form a taper portion by grinding its end portion while being chucked by a hollow cylindrical grinding machine, but it needs a complicated operation of centering a ferrite core fixed to a spindle (rotation axis) of the grinding machine with desired precision, not suitable for working large numbers of ferrite cores. Also, because the ferrite core is easily broken due to brittleness, breakage and cracking likely occur during chucking.

Though even long, small, hollow, cylindrical ferrite cores can be dry-molded, it is difficult to densely charge ferrite granules into a die. Particularly tapered end portions tend to have insufficient molding densities. Defects such as deformation, pores, etc. are generated in portions having low molding densities in the sintering step. It is thus difficult to form long, small, hollow, cylindrical ferrite cores by dry molding, with high precision, near-net shape and efficiency. Document <CIT> discloses a method of carrying out chamfer processing of the edge of a cylinder-shaped object by centerless grinding, in which a grinding wheel in contact with the edge revolves while the object also revolves. Document <CIT> discloses an apparatus comprising a rotatable wheel having axial grooves, and a grinder having an outer peripheral surface and rotating in the longitudinal direction of said grooves. Document <CIT> discloses a preparation method of soft magnetic ferrites including a step of using a vacuum extruder to extrude the material so as to obtain a MnZn ferrite green body, and a step of cutting the MnZn ferrite green body into multiple segments of green bodies with a certain length, followed by drying and sintering the multiple segments of green bodies.

Accordingly, an object of the present invention is to provide a solid or hollow cylindrical ferrite core having a taper portion formed in its end portion with high precision, and a method for efficiently producing such a tapered ferrite core by centerless grinding while suppressing breakage and cracking, and an inductance device comprising such a tapered ferrite core.

Claim <NUM> is directed to a tapered ferrite core.

Thus, the tapered ferrite core of the present invention having a hollow cylindrical shape with larger length than outer diameter comprises a ground taper portion at least in one end portion;
the taper portion having ground streaks extending in the longitudinal direction of the ferrite core.

The tapered ferrite core of the present invention is substantially free from defects due to granule boundaries.

The tapered ferrite core of the present invention is substantially as-sintered in surface portions excluding the taper portion.

The taper portion is preferably constituted by pluralities of worked surfaces having different tapering ratios.

The tapered ferrite core of the present invention may have taper portions at both ends.

Claim <NUM> is directed to a method for producing the tapered ferrite core.

The method of the present invention for producing the above tapered ferrite core comprises
centerless-grinding at least one end portion of a solid or hollow cylindrical ferrite core by a rotating grinder while rotating the ferrite core around its center axis as a rotation axis, to form a taper portion having ground streaks extending in the longitudinal direction of the ferrite core.

The hollow cylindrical ferrite core is produced by sintering a hollow cylindrical ferrite green body free from granule boundaries.

The method of the present invention for producing a tapered ferrite core preferably comprises.

It is preferable that in the method of the present invention for producing a tapered ferrite core,.

The work-pushing member is preferably (a) a fixed member having a circular inner surface concentric with the circular outer peripheral surface of the work-indexing wheel, or (b) an annular belt rotating around the work-indexing wheel.

The fixed member preferably comprises a wear-resistant layer whose inner surface comes into contact with the ferrite core.

The wear-resistant layer is preferably made of cemented carbide.

In the method of the present invention for producing a tapered ferrite core, the grinder preferably rotates in a direction pushing the ferrite core toward the work stopper in centerless grinding.

In the method of the present invention for producing a tapered ferrite core, the groove is preferably inclined by a predetermined angle relative to the direction of the rotation axis of the work-indexing wheel, thereby pushing the ferrite core in the groove to the work stopper.

In the method of the present invention for producing a tapered ferrite core, the solid or hollow cylindrical ferrite green body free from granule boundaries is preferably formed by extrusion molding.

Claim <NUM> is directed to the use of a first apparatus for producing the tapered ferrite core as defined in claim <NUM>.

The first apparatus of the present invention for producing the above tapered ferrite core comprises.

In the above first apparatus, the work-pushing member is preferably a fixed member having a wear-resistant layer whose inner surface comes into contact with the ferrite core.

Claim <NUM> is directed to the use of a second apparatus for producing the tapered ferrite core as defined in claim <NUM>.

The second apparatus of the present invention for producing the above tapered ferrite core comprises.

In the above second apparatus, the work-pushing member is preferably an annular belt rotating around the work-indexing wheel.

Claim <NUM> is directed to an inductance device comprising the tapered ferrite core as defined in claim <NUM>.

The inductance device of the present invention comprises a conductor wire wound around the above tapered ferrite core.

According to the present invention, a hollow cylindrical ferrite core having a taper portion having longitudinal ground streaks in at least one end portion can be produced with high efficiency while suppressing breakage and cracking, because at least one end portion of the ferrite core is centerless-ground by a rotating grinder.

The embodiments of the present invention will be explained in detail below referring to the attached drawings, without intention of restricting the present invention thereto. The present invention may be properly modified within the scope of the appended claims.

In the attached drawings, only important portions are shown for easiness of understanding the present invention, with their details omitted.

<FIG> is a flow chart showing an example of methods for producing the tapered ferrite core of the present invention. This method comprises a molding step S1 for forming soft ferrite powder into a ferrite green body free from granule boundaries, a step S2 for sintering the ferrite green body under predetermined temperature and conditions to form a solid (not claimed) or hollow cylindrical ferrite core having a substantially as-sintered surface, and a step S3 for centerless-grinding an end portion of the ferrite core to a taper shape. A coil can be wound around the ferrite core having a taper portion to provide an inductance device (coil-winding step S4).

The ferrite green body free from granule boundaries is a ferrite green body obtained by molding soft ferrite powder without granulation. The methods for forming a ferrite green body free from granule boundaries include (<NUM>) a method of adding a water-soluble binder such as methyl cellulose, etc. to soft ferrite powder, blending the resultant mixture by a high-shear blender such as a Banbury mixer, a mixing roll, etc. to form a clay-like, moldable material, and extrusion-molding it; (<NUM>) a method of mixing soft ferrite powder with a thermoplastic resin or wax as a binder, heating the resultant slurry, and injection-molding it, etc. Particularly to obtain a long, solid (not claimed) or hollow cylindrical ferrite green body free from granule boundaries, extrusion molding is suitable from the aspect of productivity.

Before explaining methods for forming a ferrite green body free from granule boundaries, a dry-molding method using ferrite granules will be explained. Dry molding is a method of granulating ferrite powder to granules having proper sizes for molding, and compressing ferrite granules charged into a die cavity having a predetermined shape to form a ferrite green body having a predetermined shape. A surface of a ferrite green body obtained by dry molding is schematically shown in <FIG>. Because the ferrite green body is constituted by relatively large granules <NUM>, large pores <NUM> are likely to remain in boundaries (granule boundaries) <NUM> of granules <NUM>. Pores <NUM> in the granule boundaries are likely to remain as defects (defects due to granule boundaries) in a ferrite core obtained by sintering such green body.

On the other hand, extrusion molding or injection molding, which does not use granules, provides a ferrite green body with no granule boundaries. Accordingly, the sintered ferrite core has high mechanical strength without defects due to granule boundaries. As an example of the molding steps S1, the extrusion molding method shown in <FIG> will be explained in detail below.

Used in the extrusion molding is a clay-like, moldable material comprising a predetermined percentage of a binder added to soft ferrite powder. Considering the magnetic characteristics of a ferrite core depending on its applications, the soft ferrite powder may be selected from general Mn ferrite, Ni ferrite, etc. The soft ferrite powder can be obtained, for example, by wet-mixing oxides of Fe, Zn, Cu, Ni, etc. at predetermined proportions, drying the resultant mixture, calcining it at <NUM>-<NUM> to form a substantially entirely spinelized calcined body, disintegrating it by a pulverizer, introducing the calcined body together with ion-exchanged water into a ball mill, etc. to pulverize it to predetermined particle sizes, and drying the resultant soft ferrite powder slurry. Though the drying of the slurry by a spray drier after a binder such as polyvinyl alcohol (PVA), etc. is added provides soft ferrite powder granules, agglomerated soft ferrite powder can be disintegrated by blending (described later) to obtain a ferrite green body free from granule boundaries. In this case, the binder is preferably removed before blending.

Soft ferrite powder having smaller particle sizes has higher reactivity to each other, resulting in accelerated sintering densification from a low sintering temperature, so that a dense ferrite core having small and uniform crystal grain sizes can be obtained even at a sintering temperature of <NUM> or lower. The low-temperature sintering can shorten the sintering time and reduce energy consumption. On the other hand, soft ferrite powder having smaller particle sizes has a larger specific surface area, so that a larger amount of a binder is needed for molding. In view of the above, the average particle size of the pulverized soft ferrite powder measured by an air permeability method is preferably <NUM>-<NUM>, more preferably <NUM>-<NUM>.

The preferable binders are water-soluble binders such as cellulose resins (methylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, etc.), water-soluble acrylic resins, etc. The soft ferrite powder is mixed with an aqueous solution of a binder in pure water containing, if necessary, a dispersant, a lubricant, etc., and blended to form a material for extrusion molding (moldable material). If the amount of the binder is too small, blending cannot provide a uniform moldable material, and an extruded green body undergoes an excessive load and does not have desired strength. A larger amount of the binder results in a lower green body density, leading to increased sintering shrinkage and more deformation of the ferrite core. The amount of the binder added is preferably <NUM>-<NUM> parts by mass per <NUM> parts by mass of the soft ferrite powder. The amount of pure water added is preferably <NUM>-<NUM> parts by mass per <NUM> parts by mass of the soft ferrite powder, though variable depending on the kind and amount of the binder, and the desired hardness of the moldable material.

Blending can be conducted by a blending apparatus such as a Banbury mixer, a super mixer, a Henschel mixer, a three-roll mixer, a compression kneader, etc. Blending is conducted preferably in a cooled state, to suppress the evaporation of water. In the case of a cellulose binder, the blending temperature of the moldable material is preferably lower than <NUM>, more preferably <NUM> or lower, to prevent gelation during blending which starts at about <NUM>-<NUM>. On the other hand, too low a blending temperature generates dew, which is added to the moldable material to have an uneven water content, or leaves the moldable material too hard for blending. To prevent this, the blending temperature of the moldable material is preferably <NUM> or higher. To adjust the temperature of the moldable material, temperature-adjusted cooling water is preferably circulated through the blending apparatus itself, or through a water path in a water jacket covering the blending apparatus.

The blended moldable material is molded to a hollow or solid cylindrical shape by an extruder having a cooling mechanism. Cooling is conducted to suppress the heat generation of the moldable material as in blending. Extrusion may be conducted by using a plunger, but the use of a screw is preferable to add further blending to the moldable material. A ferrite green body extruded from a die of the extrusion-molding machine is free from granule boundaries. The ferrite green body is quickly and continuously sent by a conveyer to a drying step.

The ferrite green body is continuously dried at a temperature of the gelation temperature of the binder in the green body or higher and lower than its thermal decomposition temperature, by a belt drier, etc. The drying temperature is preferably <NUM>-<NUM>. Though variable depending on the size of the green body, the drying time is preferably <NUM>-<NUM> minutes for a size of <NUM> or less.

The hollow or solid (not claimed) cylindrical ferrite green body having mechanical strength increased by drying solidification is preliminarily cut to a desired length. A rotating grinder is preferably used for cutting, but blade may be used. Because the dried ferrite green body has higher deformation resistance than before drying, deformation such as dent and elongation by cutting can be suppressed.

The cut ferrite green body is degreased to remove the binder, and sintered to form a sintered body. A sintering ceramic jig (setter), on which ferrite green bodies are arranged, preferably has dents for preventing the rolling of the ferrite green bodies. In the sintering step, a continuous sintering furnace such as a roller hearth kiln, etc., and a batch-type sintering furnace may be used. The sintering is preferably conducted at <NUM>-<NUM> for <NUM>-<NUM> hours, though variable depending on the composition and particle size of soft ferrite powder.

Both ends of the resultant sintered body are cut by a cutter to form a hollow or solid (not claimed) cylindrical ferrite core having a predetermined length. It is preferable to use a rotating grinder to cut end portions of the ferrite core perpendicularly to its center axis. The resultant ferrite core is free from pores due to granule boundaries, etc., with less deformation and excellent dimension precision.

The centerless grinding of end portions of the hollow or solid (not claimed) cylindrical ferrite core provides a ferrite core having a high-precision taper portion.

<FIG> shows an example of centerless grinding apparatuses used for producing the ferrite core of the present invention, and <FIG> shows its important portion. As shown in <FIG>, the centerless grinding apparatus <NUM> comprises as main parts a work-indexing mechanism <NUM> mounted onto a base <NUM> and a work-grinding member <NUM>. The work-indexing mechanism <NUM> comprises a cylindrical carrier guide <NUM>, a disc-shaped work-indexing wheel <NUM> having a circular outer peripheral surface, which is arranged inside the carrier guide <NUM>, and a work-pushing member <NUM> for supporting a work (ferrite core) <NUM>, which is opposing the work-indexing wheel <NUM>. The work-indexing wheel <NUM> is arranged with its rotation axis C<NUM> aligned in the X direction in <FIG>, and connected to a driving means <NUM> including a servomotor, etc. The work-grinding member <NUM>, which is arranged with its rotation axis C<NUM> aligned in the Z direction in <FIG>, comprises a grinder <NUM> connected to a driving means (not shown) such as a servomotor, etc..

The work-indexing mechanism <NUM> is mounted to the base <NUM> via a movable bed <NUM> comprising pluralities of sliding members, such that it is slidable in an X-Z plane in <FIG> to enable its positional adjustment relative to the grinder <NUM>.

The rotation axis C<NUM> of the grinder <NUM> is positioned under the rotation axis C<NUM> of the disc-shaped work-indexing wheel <NUM> for rotating ferrite cores <NUM>. The grinder <NUM> preferably comprises, for example, abrasive diamond particles, abrasive CBN (cubic boron nitride) particles, etc. fixed by a binder such as a metal bond, etc. In the depicted example, the rotation axis C<NUM> of the grinder <NUM> is perpendicular to the rotation axis C<NUM> of the work-indexing wheel <NUM>. The term "perpendicular" is not restricted to geometrically strict perpendicular, but permits the inclination of about <NUM>-<NUM>°.

Arranged around the work-indexing wheel <NUM> is a cylindrical carrier guide <NUM> having longitudinally aligned comb-like slits <NUM> open toward the grinder <NUM> at predetermined pitches. <FIG> shows a combination of the carrier guide <NUM> and the work-indexing wheel <NUM>. Each slit <NUM> and the circular outer peripheral surface of the work-indexing wheel <NUM> constitutes each groove <NUM> receiving each ferrite core <NUM>. In the depicted example (<FIG>), the carrier guide <NUM> rotates in the same direction R<NUM> as the rotation direction R<NUM> of the work-indexing wheel <NUM>.

Arranged under the work-indexing wheel <NUM> is a work-pushing member <NUM> opposing the circular outer peripheral surface of the work-indexing wheel <NUM>. In the depicted example, the work-pushing member <NUM> is fixed, and has a circular inner surface concentric with the circular outer peripheral surface of the work-indexing wheel <NUM>. The gap between the work-indexing wheel <NUM> and the work-pushing member <NUM> is substantially equal to the outer diameter of a ferrite core <NUM> received in each groove <NUM> of the work-indexing mechanism <NUM>.

The work-pushing member <NUM> preferably has a wear-resistant layer <NUM> made of cemented carbide having excellent rigidity and wear resistance, etc. on the side of contacting with a ferrite core. A circular outer peripheral surface of the work-indexing wheel <NUM> coming into contact with a ferrite core <NUM> is preferably made of an elastic material such as urethane rubber having proper elasticity and friction resistance, etc..

The grinder <NUM> is rotated substantially in the longitudinal direction of the ferrite core <NUM>, such that the outer peripheral surface of the grinder <NUM> moves along a taper portion 13a formed in an end portion of the ferrite core <NUM>. Because the grinder <NUM> is rotated in the arrow direction R<NUM> (direction toward a rear end of the ferrite core <NUM>) shown in <FIG>, the ferrite core <NUM> is pushed toward a rear end of the work-indexing wheel <NUM> (opposite side of an open end of the slit <NUM>) by the grinding force of the grinder <NUM>. Accordingly, a longitudinal rear end of the slit <NUM> is provided with a work stopper <NUM> abutting the rear end surface (end surface not subjected to centerless grinding) of the ferrite core <NUM>. Because the work stopper <NUM> is always pushed by the ferrite core <NUM> during centerless grinding, the ferrite core <NUM> is precisely longitudinally positioned during centerless grinding.

A ferrite core <NUM> supplied one by one from a supply apparatus (not shown) to the groove <NUM> passes through a gap between the circular outer peripheral surface of the work-indexing wheel <NUM> and the circular inner surface of the work-pushing member <NUM> opposing each other, while being gripped by them, as shown in <FIG>. Because the ferrite core <NUM> is pushed toward the work-pushing member <NUM> by the work-indexing wheel <NUM>, the rotation of the work-indexing wheel <NUM> is transmitted to the ferrite core <NUM>, so that the ferrite core <NUM> rotates around its center axis in a direction R<NUM> opposite to the rotation direction R<NUM> of the work-indexing wheel <NUM>.

The rotation speed of the ferrite core <NUM> around its center axis is generally determined by the rotation speed difference between the work-indexing wheel <NUM> and the work-pushing member <NUM>. To rotate the ferrite core <NUM> at a desired speed, the rotation speed V<NUM> of the work-indexing wheel <NUM> and the rotation speed V<NUM> of the work-pushing member <NUM> are properly set. Because the rotation speed V<NUM> of the work-pushing member <NUM> is zero in the depicted example, the rotation speed V<NUM> of the work-indexing wheel <NUM> per se corresponds to the "rotation speed difference. " However, with the work-pushing member <NUM> rotating as described later, the "rotation speed difference" is the difference of their rotation speeds V<NUM> and V<NUM> when the work-indexing wheel <NUM> and the work-pushing member <NUM> rotate in the same direction, and the sum of their rotation speeds V<NUM> and V<NUM> when they rotate in opposite directions.

The ferrite core <NUM> rotating around its center axis while being pushed by the work-indexing wheel <NUM> to the work-pushing member <NUM> moves at a speed corresponding to the rotation speed between the circular outer peripheral surface of the work-indexing wheel <NUM> and the work-pushing member <NUM>. This movement is hereinafter called "revolution. " However, a sufficient rotation speed V<NUM> leads to too high a revolution speed V<NUM>, resulting in too short a sliding contact time of the ferrite core <NUM> with the grinder <NUM>. To secure a sufficient sliding contact time of the ferrite core <NUM> with the grinder <NUM>, the rotation speed V<NUM> of the carrier guide <NUM> is preferably sufficiently lower than the rotation speed V<NUM> of the work-indexing wheel <NUM>. A ratio of the rotation speed V<NUM> of the carrier guide <NUM> to the rotation speed V<NUM> of the work-indexing wheel <NUM> is preferably <NUM>-<NUM>.

The ferrite core <NUM> received in the groove <NUM> with its tip portion projecting from the open end of the groove <NUM> and its rear end surface in contact with the work stopper <NUM> rotates around its center axis in the groove <NUM> at a speed V<NUM> determined by the rotation speed V<NUM> of the work-indexing wheel <NUM>, while revolving at the same speed V<NUM> as the rotation speed V<NUM> of the carrier guide <NUM> in an annular space between the work-indexing wheel <NUM> and the work-pushing member <NUM>, so that the tip portion of the ferrite core <NUM> comes into sliding contact with the outer peripheral surface of the grinder <NUM> for a sufficient period of time as shown in <FIG>.

As shown in <FIG>, the outer peripheral surface of the grinder <NUM> is preferably in a circular shape whose axial center portion is concaved concentrically with the work-indexing wheel <NUM>. While the ferrite core <NUM> is ground during revolution around the work-indexing wheel <NUM>, the tip portion of the ferrite core <NUM> projecting from the groove <NUM> is ground by substantially uniform sliding contact with the grinder <NUM>, resulting in the formation of the taper portion <NUM>.

Because the grinder <NUM> has a sufficiently larger diameter than the outer diameter of the ferrite core <NUM>, the inclination angle α of the taper portion <NUM> (angle between the worked surface of the taper portion <NUM> and the center axis C<NUM> of the ferrite core <NUM> in <FIG>) is substantially equal to an angle θ between a line extending perpendicularly (in a Y direction) from a center point on the center axis C<NUM> of the grinder <NUM>, and a line connecting a contact point of the ferrite core <NUM> with the outer peripheral surface of the grinder <NUM> and the above center point.

As shown in <FIG>, the groove <NUM> of the work-indexing mechanism <NUM> is desirably inclined from the rotation axis C<NUM> of the work-indexing wheel <NUM> by a predetermined angle β. The work-indexing wheel <NUM> and the carrier guide <NUM> rotate in the same direction (rightward in <FIG>) with a predetermined rotation speed difference (V<NUM> - V<NUM>). The grinder <NUM> is positioned on the front side in <FIG>. For example when the groove <NUM> is inclined with its portion on the side of the grinder <NUM> lagging in the rotation direction, the outer peripheral surface of the ferrite core <NUM> comes into contact with a side surface (left side in <FIG>) of the slit <NUM> of the carrier guide <NUM> due to the rotation speed difference (V<NUM> - V<NUM>) between the work-indexing wheel <NUM> and the carrier guide <NUM>. When the tip portion of the ferrite core <NUM> is centerless-ground by the grinder <NUM> in this state, a rear end surface of the ferrite core <NUM> is likely pushed to the work stopper <NUM> (lower side in <FIG>) by a reaction force from the side surface of the slit <NUM>. As a result, the position of the ferrite core <NUM> is longitudinally precisely set by the work stopper <NUM>. To prevent the breakage and cracking of an end edge of the ferrite core <NUM> in contact with the work stopper <NUM>, the inclination angle β of the groove <NUM> is desirably set to <NUM>° or less to reduce a force component applied to the work stopper <NUM>.

<FIG> shows another centerless grinding apparatus used in the present invention. This centerless grinding apparatus comprises a rotatable work-indexing wheel <NUM> having pluralities of axial grooves <NUM> on a circular outer peripheral surface, in place of the rotatable, flat work-indexing wheel having a circular outer peripheral surface, which is shown in <FIG>; and a belt <NUM> moving along the outer peripheral surface of the work-indexing wheel <NUM> in an opposite direction R<NUM>, in place of the fixed work-pushing member shown in <FIG>. A grinder <NUM> having a circular outer peripheral surface rotates substantially in the longitudinal direction of the groove <NUM> of the work-indexing wheel <NUM>, such that the outer peripheral surface of the grinder <NUM> moves along a taper portion <NUM> formed in an end portion of the ferrite core <NUM>.

A ferrite core <NUM> received in each groove <NUM> on the outer peripheral surface of the work-indexing wheel <NUM> rotates around its center axis by opposite rotation directions between the work-indexing wheel <NUM> and the belt <NUM>. In this centerless grinding apparatus, too, the end portion of the ferrite core <NUM> is brought into contact with the grinder <NUM> to form a taper portion <NUM>, thereby producing a ferrite core having a high-precision taper portion. Incidentally, for the revolution of the ferrite core <NUM>, the same carrier guide and work-indexing wheel as in the centerless grinding apparatus shown in <FIG> may be used.

<FIG> shows the appearance of a hollow cylindrical ferrite core whose end portion is centerless-ground, <FIG> shows its longitudinal cross section, and <FIG> shows a taper portion of the ferrite core. The ferrite core <NUM> has an outer peripheral surface <NUM>, an inner surface <NUM>, both end surfaces 14a, 14b cut perpendicularly to the center axis C<NUM>, a taper portion <NUM> formed on the side of an end surface 14a, and an opening <NUM> communicating with the inner surface <NUM>. The outer peripheral surface <NUM> except for the taper portion <NUM> and the inner surface <NUM> are in an as-sintered state ("as-sintered surface"). The depicted ferrite core <NUM> is as long as about <NUM> times the outer diameter of the outer peripheral surface <NUM>.

Ground streaks (tool marks or wheel marks) remain on the centerless-ground surface of the taper portion <NUM>. Because the rotation speed of the grinder <NUM> is sufficiently larger than the rotation speed of the ferrite core <NUM> around its center axis, ground streaks on the worked surface of the taper portion <NUM> extend substantially linearly in the longitudinal direction of the hollow cylindrical ferrite core <NUM>. Such ground streaks radially isotropically extending from the center axis C<NUM> of the ferrite core <NUM> can make up for the reduced mechanical strength of the taper portion <NUM> of the ferrite core <NUM>, securing cracking resistance, breakage resistance, impact resistance, etc..

<FIG> shows another example of the tapered ferrite cores. This tapered ferrite core <NUM> has chamfered portions 13b, 13c formed on the tip taper portion <NUM> and the rear end surface 14b. The chamfered portions 13b, 13c can be formed by centerless grinding using the apparatus of the present invention, like the taper portion <NUM>. Of course, the inclination angle θ of the ferrite core <NUM> to the outer peripheral surface of the grinder <NUM> is properly changed to form the chamfered portions 13b, 13c.

When a ferrite core having excellent roundness, concentricity, cylindricity and straightness without pores due to granule boundaries is subjected to centerless grinding, the taper portion <NUM> can be formed with high precision, with less breakage and cracking even if the ferrite core is as small as <NUM> or less in outer diameter or as thin as <NUM> or less. Because the taper portion <NUM> is formed by centerless grinding, there are no needs of chucking the ferrite core <NUM> and centering the ferrite core <NUM> for fixing, resulting in high productivity.

A coil is wound around the ferrite core in the coil-winding step S4, to obtain an inductance device. Though not restrictive, a wound conductor wire may be, for example, a stranded wire such as an enameled wire (copper wire coated with polyamideimide), a Litz wire, etc., to increase the Q factor of the inductance device at high frequencies. The number of winding a conductor wire can be properly set depending on the inductance required, and the diameter of the conductor wire can be properly set depending on current. Though a coil may be wound around the ferrite core directly, it is preferable to use a bobbin made of resins such as polyphenylene sulfide, liquid polymers, polyethylene terephthalate, polybutylene terephthalate, etc., when the specific resistance of the ferrite core is as low as, for example, less than <NUM><NUM> Ω·m. Inductance devices using the ferrite core of the present invention can be used for electronic pens, LF antennas, choke coils, etc..

Claim 1:
A tapered ferrite core having a hollow cylindrical shape with larger length than outer diameter; wherein
said ferrite core (<NUM>) has a ground taper portion (<NUM>) in at least one end portion; said taper portion (<NUM>) has ground streaks extending in the longitudinal direction of the ferrite core (<NUM>);
said ground streaks radially isotropically extend from the center axis (C3) of the ferrite core (<NUM>);
surface portions excluding said taper portion (<NUM>) have substantially as-sintered surfaces;
said ferrite core (<NUM>) has substantially no defects due to granule boundaries (<NUM>); and
said ferrite core (<NUM>) is made by a sintered body of Mn ferrite or Ni ferrite.