Patent Application
20200135393
The present invention relates to a solid or hollow cylindrical ferrite core having a taper portion in an end portion, a method and an apparatus for efficiently producing it with high precision, and an inductance device comprising it.
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 JP H08-050535 A, 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.
A small ferrite core used in an electronic pen as described in JP H08-050535 A has a thin, elongated, hollow, cylindrical shape having, for example, an outer diameter of 5 mm or less, a thickness of 1 mm or less and a length of 10 mm 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.
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.
Thus, the tapered ferrite core of the present invention having a solid or 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 preferably substantially free from defects due to granule boundaries.
The tapered ferrite core of the present invention is preferably 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.
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 solid or hollow cylindrical ferrite core is preferably produced by sintering a solid or hollow cylindrical ferrite green body free from granule boundaries.
The method of the present invention for producing a tapered ferrite core preferably comprises
using a centerless grinding apparatus comprising a rotatable work-indexing wheel having a circular outer peripheral surface, and a work-pushing member opposing the circular outer peripheral surface of the work-indexing wheel;
rotatably supporting the ferrite core between the rotating work-indexing wheel and the work-pushing member; and
rotating the ferrite core around its center axis by rotation speed difference between the work-indexing wheel and the work-pushing member.
It is preferable that in the method of the present invention for producing a tapered ferrite core,
the grinder has a circular outer peripheral surface concaved in an axial center portion;
the rotation axis of the grinder and the rotation axis of the work-indexing wheel are substantially perpendicular to each other;
each ferrite core rotating around its center axis is centerless-ground by sliding contact with the concaved circular outer peripheral surface of the grinder, to form the taper portion.
It is preferable that in the method of the present invention for producing a tapered ferrite core,
an annular carrier guide having pluralities of axial slits is arranged around the work-indexing wheel; and
each ferrite core is received in each groove constituted by each slit of the carrier guide and the outer peripheral surface of the work-indexing wheel.
It is preferable that in the method of the present invention for producing a tapered ferrite core,
the work-indexing wheel has pluralities of axial grooves on the outer peripheral surface; and
each ferrite core is received in each groove.
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.
It is preferable that in the method of the present invention for producing a tapered ferrite core,
the groove is provided at its longitudinal rear end with a work stopper for limiting the longitudinal movement of the ferrite core; and
the work stopper acts as a longitudinal reference surface for centerless grinding.
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.
The first apparatus of the present invention for producing the above tapered ferrite core comprises
a rotatable work-indexing wheel having a circular outer peripheral surface;
a work-pushing member opposing the circular outer peripheral surface of the work-indexing wheel;
a rotatable, cylindrical carrier guide having pluralities of slits extending in the direction of the rotation axis of the work-indexing wheel, and arranged around the work-indexing wheel; and
a grinder having a circular outer peripheral surface and rotating substantially in the longitudinal direction of the slit;
a solid or hollow cylindrical ferrite core being received in each groove constituted by the circular outer peripheral surface of the work-indexing wheel and each slit of the cylindrical carrier guide;
each ferrite core rotating around its center axis by rotation speed difference between the work-indexing wheel and the work-pushing member, and revolving along the work-indexing wheel by the rotation of the cylindrical carrier guide, so that each ferrite core moves to a position at which it comes into sliding contact with the grinder; and
at least one end portion of each ferrite core rotating around its center axis being centerless-ground by the grinder, to form a taper portion having ground streaks extending in the longitudinal direction of the ferrite core.
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.
The second apparatus of the present invention for producing the above tapered ferrite core comprises
a rotatable work-indexing wheel having pluralities of axial grooves on the circular outer peripheral surface;
a work-pushing member opposing the circular outer peripheral surface of the work-indexing wheel; and
a grinder having a circular outer peripheral surface and rotating substantially in the longitudinal direction of the groove of the work-indexing wheel;
a solid or hollow cylindrical ferrite core being received in each groove of the work-indexing wheel;
each ferrite core rotating around its center axis by rotation speed difference between the work-indexing wheel and the work-pushing member, and revolving by the rotation of the work-indexing wheel, so that each ferrite core moves to a position at which it comes into sliding contact with the grinder; and
at least one end portion of each ferrite core rotating around its center axis being centerless-ground by the grinder, to form a taper portion having ground streaks extending in the longitudinal direction of the ferrite core.
In the above second apparatus, the work-pushing member is preferably an annular belt rotating around the work-indexing wheel.
The inductance device of the present invention comprises a conductor wire wound around the above tapered ferrite core.
According to the present invention, a solid or 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 its technical idea. In the attached drawings, only important portions are shown for easiness of understanding the present invention, with their details omitted.
[1] Production Method of Ferrite Core
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 (1) 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; (2) 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 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
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
(1) Preparation of Moldable Material
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 750-1000° C. 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 1000° C. 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 0.8-5 μm, more preferably 1-3 μm.
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 3-10 parts by mass per 100 parts by mass of the soft ferrite powder. The amount of pure water added is preferably 10-20 parts by mass per 100 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 40° C., more preferably 10° C. or lower, to prevent gelation during blending which starts at about 40-50° C. 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 5° C. 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.
(2) Extrusion
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.
(3) Drying
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 50-200° C. Though variable depending on the size of the green body, the drying time is preferably 2-10 minutes for a size of 5 mm or less.
(4) Preliminary Cutting
The hollow or solid 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.
(5) Sintering
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 900-1300° C. for 4-24 hours, though variable depending on the composition and particle size of soft ferrite powder.
(6) Final Cutting
Both ends of the resultant sintered body are cut by a cutter to form a hollow or solid 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.
(7) Centerless Grinding
The centerless grinding of end portions of the hollow or solid cylindrical ferrite core provides a ferrite core having a high-precision taper portion.
The work-indexing mechanism 210 is mounted to the base 250 via a movable bed 230 comprising pluralities of sliding members, such that it is slidable in an X-Z plane in
The rotation axis C2 of the grinder 100 is positioned under the rotation axis C1 of the disc-shaped work-indexing wheel 101 for rotating ferrite cores 10. The grinder 100 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 C2 of the grinder 100 is perpendicular to the rotation axis C1 of the work-indexing wheel 101. The term “perpendicular” is not restricted to geometrically strict perpendicular, but permits the inclination of about 2-3°.
Arranged around the work-indexing wheel 101 is a cylindrical carrier guide 104 having longitudinally aligned comb-like slits 109 open toward the grinder 100 at predetermined pitches.
Arranged under the work-indexing wheel 101 is a work-pushing member 102 opposing the circular outer peripheral surface of the work-indexing wheel 101. In the depicted example, the work-pushing member 102 is fixed, and has a circular inner surface concentric with the circular outer peripheral surface of the work-indexing wheel 101. The gap between the work-indexing wheel 101 and the work-pushing member 102 is substantially equal to the outer diameter of a ferrite core 10 received in each groove 16 of the work-indexing mechanism 210.
The work-pushing member 102 preferably has a wear-resistant layer 108 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 101 coming into contact with a ferrite core 10 is preferably made of an elastic material such as urethane rubber having proper elasticity and friction resistance, etc.
The grinder 100 is rotated substantially in the longitudinal direction of the ferrite core 10, such that the outer peripheral surface of the grinder 100 moves along a taper portion 13a formed in an end portion of the ferrite core 10. Because the grinder 100 is rotated in the arrow direction R5 (direction toward a rear end of the ferrite core 10) shown in
A ferrite core 10 supplied one by one from a supply apparatus (not shown) to the groove 16 passes through a gap between the circular outer peripheral surface of the work-indexing wheel 101 and the circular inner surface of the work-pushing member 102 opposing each other, while being gripped by them, as shown in
The rotation speed of the ferrite core 10 around its center axis is generally determined by the rotation speed difference between the work-indexing wheel 101 and the work-pushing member 102. To rotate the ferrite core 10 at a desired speed, the rotation speed V1 of the work-indexing wheel 101 and the rotation speed V2 of the work-pushing member 102 are properly set. Because the rotation speed V2 of the work-pushing member 102 is zero in the depicted example, the rotation speed V1 of the work-indexing wheel 101 per se corresponds to the “rotation speed difference.” However, with the work-pushing member 102 rotating as described later, the “rotation speed difference” is the difference of their rotation speeds V1 and V2 when the work-indexing wheel 101 and the work-pushing member 102 rotate in the same direction, and the sum of their rotation speeds V1 and V2 when they rotate in opposite directions.
The ferrite core 10 rotating around its center axis while being pushed by the work-indexing wheel 101 to the work-pushing member 102 moves at a speed corresponding to the rotation speed between the circular outer peripheral surface of the work-indexing wheel 101 and the work-pushing member 102. This movement is hereinafter called “revolution.” However, a sufficient rotation speed V4 leads to too high a revolution speed V5, resulting in too short a sliding contact time of the ferrite core 10 with the grinder 100. To secure a sufficient sliding contact time of the ferrite core 10 with the grinder 100, the rotation speed V3 of the carrier guide 104 is preferably sufficiently lower than the rotation speed V1 of the work-indexing wheel 101. A ratio of the rotation speed V3 of the carrier guide 104 to the rotation speed V1 of the work-indexing wheel 101 is preferably 0.4-0.7.
The ferrite core 10 received in the groove 16 with its tip portion projecting from the open end of the groove 16 and its rear end surface in contact with the work stopper 103 rotates around its center axis in the groove 16 at a speed V4 determined by the rotation speed V1 of the work-indexing wheel 101, while revolving at the same speed V5 as the rotation speed V3 of the carrier guide 104 in an annular space between the work-indexing wheel 101 and the work-pushing member 102, so that the tip portion of the ferrite core 10 comes into sliding contact with the outer peripheral surface of the grinder 100 for a sufficient period of time as shown in
As shown in
Because the grinder 100 has a sufficiently larger diameter than the outer diameter of the ferrite core 10, the inclination angle α of the taper portion 13 (angle between the worked surface of the taper portion 13 and the center axis C3 of the ferrite core 10 in
As shown in
A ferrite core 10 received in each groove 116 on the outer peripheral surface of the work-indexing wheel 101 rotates around its center axis by opposite rotation directions between the work-indexing wheel 101 and the belt 105. In this centerless grinding apparatus, too, the end portion of the ferrite core 10 is brought into contact with the grinder 100 to form a taper portion 13, thereby producing a ferrite core having a high-precision taper portion. Incidentally, for the revolution of the ferrite core 10, the same carrier guide and work-indexing wheel as in the centerless grinding apparatus shown in
[2] Tapered Ferrite Core
Ground streaks (tool marks or wheel marks) remain on the centerless-ground surface of the taper portion 13. Because the rotation speed of the grinder 100 is sufficiently larger than the rotation speed of the ferrite core 10 around its center axis, ground streaks on the worked surface of the taper portion 13 extend substantially linearly in the longitudinal direction of the hollow cylindrical ferrite core 10. Such ground streaks radially isotropically extending from the center axis C3 of the ferrite core 10 can make up for the reduced mechanical strength of the taper portion 13 of the ferrite core 10, securing cracking resistance, breakage resistance, impact resistance, etc.
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 13 can be formed with high precision, with less breakage and cracking even if the ferrite core is as small as 3 mm or less in outer diameter or as thin as 0.5 mm or less. Because the taper portion 13 is formed by centerless grinding, there are no needs of chucking the ferrite core 10 and centering the ferrite core 10 for fixing, resulting in high productivity.
[3] Inductance Device
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 103 Ω·m. Inductance devices using the ferrite core of the present invention can be used for electronic pens, LF antennas, choke coils, etc.
In the present invention, a taper portion is highly efficiently formed by centerless grinding in an end portion of a solid or hollow cylindrical ferrite core, with suppressed breakage and cracking, and with no likelihood of human error without needing a special skill.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-111413 | Jun 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/021531 | 6/5/2018 | WO | 00 |
