Sintered ring magnet and method of manufacturing the same
A ring magnet manufacturing method includes the steps of stacking a plurality of radially oriented ring-shaped powder compacts (102) in an axial direction thereof to produce a ring-shaped powder compact rod, sintering the ring-shaped powder compact rod to produce a sintered ring-shaped powder compact rod (300) in which the ring-shaped powder compacts (102) are joined together, and dividing the sintered ring-shaped powder compact rod (300). In this ring magnet manufacturing method, protruding parts (103) are formed on upper end surfaces of the ring-shaped powder compacts (102) which will be located in uppermost layers of individual sintered ring magnets (100), for example, such that the ring-shaped powder compacts (102) are joined with a reduced joint strength at specific boundary regions of the sintered ring-shaped powder compact rod (300) where the protruding parts (103) are located than at the other boundary regions. The sintered ring magnets (100) are obtained by dividing the sintered ring-shaped powder compact rod (300) at the specific boundary regions having the reduced joint strength.
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1. Field of the Invention
The present invention relates to a radially oriented sintered ring magnet used in compact motors, for instance, and a method of manufacturing the radially oriented sintered ring magnet.
2. Description of the Background Art
A radially oriented anisotropic ring magnet is often used in permanent magnet motors. It is common practice to use an axially elongate ring magnet when manufacturing a compact high-power motor having a small inertia.
A problem encountered when pressing magnetic powder in a magnetic field for producing an axially elongate ring magnet is that a sufficient field strength of alignment field is not obtained, resulting in a reduction in alignment coefficient of the magnetic powder and an inability to achieve a high magnetic property.
When a ring magnet is radially oriented magnetically, a magnetic flux passing through a core of a metal die unit for pressing magnetic powder into a ring shape becomes equal to a magnetic flux passing inside a curved inner surface of a die. Therefore, expressing the inside diameter of the ring magnet (the core diameter of the metal die unit) as Di, the outside diameter of the ring magnet (the inside diameter of the die of the metal die unit) as Dd, the height of the ring magnet (the height of the die) as H, magnetic flux density within the core of the metal die unit as Bc, and magnetic flux density inside the curved inner surface of the die as Bd, there is a relationship given by equation (1) below:
2×p/4×Di2×Bc=p×Dd×H×Bd (1)
A steel product, such as S45C, if used for making the core of the metal die unit, has a saturation flux density of approximately 1.5 T. Thus, substituting Bc=1.5 in equation (1) above and assuming that a magnetic field necessary for magnetic alignment is equal to or larger than 1.0 T which translates to Bd=1.0 T, the height H of the ring magnet which can be pressed with magnetic alignment is given by equation (2) below:
H=3Di2/4Dd (2)
It is commonly known that a problem occurs due to deterioration of a property of magnetic alignment when pressing a ring magnet in a magnetic field if the ring magnet has an axial length larger than a value of H given by equation (2) above. In the context of this Specification, an “axially elongate ring magnet” refers to a ring magnet of which height (axial length) is larger than the value of H given by equation (2) above.
Accordingly, conventional practice has been to produce multiple ring magnet pieces, each having a short axial length which is within a range well allowing execution of pressing operation in a magnetic field, and join these ring magnet pieces with a bonding agent, for instance, to manufacture a ring magnet having a desired axial length.
Japanese Patent Application Publication No. 1990-281721 proposes a method of manufacturing a ring magnet having a desired axial length. According to the method of this Publication, an axially elongate ring magnet is produced in a step-by-step fashion by pressing magnetic powder in multiple layers as if by stacking powder compacts one on top of another in a metal die, each layer of the powder compacts having an axial length falling within a range which enables magnetic alignment.
Generally, a powder compact becomes reduced in size by 20% to 30% when sintered. A chronic problem encountered in the manufacture of a sintered ring magnet is that the powder compact does not shrink uniformly but becomes deformed as a result of shrinkage during sintering operation. This deformation of the powder compact occurs due to the influence of a difference in density within the powder compact, a difference in shrinkage ratio in a direction of magnetic alignment and other directions, and friction between the powder compact and a sintering tray, for example.
To overcome this deformation problem, Japanese Patent Application Publication No. 2001-335808 proposes a method of reducing deformation occurring in sintering operation by use of a compact restraining jig. According to the method of this Publication, a ring-shaped powder compact is placed to surround the compact restraining jig and sintered together with the restraining jig. The compact restraining jig serves to prevent deformation of the ring-shaped powder compact when the ring-shaped powder compact shrinks during the sintering operation.
The aforementioned method of Japanese Patent Application Publication No. 1990-281721 includes the steps of setting a specific metal die in a powder shaping press (pressing machine for pressing the magnetic powder in a magnetic field) incorporating an electromagnetic coil, filling magnetic powder into the metal die, forming each successive layer of a powder compact by pressing the magnetic powder in a magnetic field followed by demagnetizing operation, releasing the powder compact from the metal die, removing the powder compact from the pressing machine, and placing the powder compact on a vessel. Since a ring magnet is produced by making multiple layers of powder compacts by carrying out these sequential steps, the method of Japanese Patent Application Publication No. 1990-281721 leads to low manufacturing productivity. Also, since all these steps are performed in the pressing machine, there exist various limitations in operation in the individual steps. Furthermore, the powder compacts in lower layers are pressurized more times than those in upper layers, magnetic alignment is disturbed in the lower-layer powder compacts, inevitably causing deterioration of magnetic properties. Moreover, this ring magnet manufacturing method is associated with a problem that the magnetic properties deteriorate at each boundary region between the adjacent powder compacts due to the influence of the powder compact in a lower layer.
The aforementioned method of Japanese Patent Application Publication No. 2001-335808 has a problem that the compact restraining jig is a consumable and it is necessary to set the compact restraining jig with a central axis thereof precisely aligned with a central axis of the ring-shaped powder compact, resulting in low manufacturing productivity. Furthermore, as the ring-shaped powder compact is sintered on a sintering jig (setter), a lower part of the ring-shaped powder compact does not normally shrink in the sintering operation due to friction with the setter. For this reason, greater deformation is likely to occur in the lower part of the ring-shaped powder compact.
SUMMARY OF THE INVENTIONIn light of the aforementioned problems of the prior art, it is an object of the invention to provide a sintered ring magnet featuring a high degree of magnet shape accuracy and a method of manufacturing such a ring magnet with high productivity.
In one principal aspect of the invention, a method of manufacturing sintered ring magnets includes the steps of stacking a plurality of radially oriented ring-shaped powder compacts in an axial direction thereof to produce a ring-shaped powder compact rod, sintering the ring-shaped powder compact rod to produce a sintered ring-shaped powder compact rod in which the ring-shaped powder compacts are joined together as a result of sintering operation, and dividing the sintered ring-shaped powder compact rod. In this ring magnet manufacturing method, the ring-shaped powder compacts are joined to one another with a reduced joint strength at specific boundary regions than at the other boundary regions when sintered, and the sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at the specific boundary regions having the reduced joint strength.
According to this ring magnet manufacturing method of the invention, several ring magnets joined into a single structure, or the sintered ring-shaped powder compact rod, by the sintering operation are handled together in subsequent processes. This approach of the invention produces the following advantageous effects. Specifically, since several sintered ring magnets are handled as a single structure, the sintered ring magnets can be transferred together from one process to next with high efficiency. Since curved inner surfaces and outer surfaces of several sintered ring magnets can be machined together in the form of the sintered ring-shaped powder compact rod, it is possible to achieve improved machining efficiency. Since several sintered ring magnets can be subjected together to an anticorrosion surface treatment in the form of the sintered ring-shaped powder compact rod, it is possible to achieve improved treatment efficiency. Additionally, it is possible to produce the sintered ring-shaped powder compact rod with a high degree of shape accuracy.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description along with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.
First Embodiment(1) Structure of the Sintered Ring Magnet of the First Embodiment
First, the structure of a sintered ring magnet 100 according to a first embodiment of the invention is described with reference to
Referring to
(2) Method of Manufacturing the Sintered Ring Magnet of the First Embodiment
Next, a method of manufacturing the sintered ring magnet 100 of the first embodiment is described.
Generally, in the manufacture of a radially oriented ring magnet, there are limitations on the axial length of a ring-shaped powder compact which can be magnetically oriented by applying an alignment magnetic field in a single operation. Therefore, common practice for manufacturing a radially oriented ring magnet having a large axial length is to produce a plurality of ring-shaped powder compacts by pressing magnetic powder in a metal die, each of the ring-shaped powder compacts having an axial length which can be radially oriented in a magnetic field, stack the multiple ring-shaped powder compacts removed from the metal die in an axial direction, and join the ring-shaped powder compacts to form a single structure by performing a sintering process.
According to the method of manufacturing the sintered ring magnet 100 of the first embodiment, a larger number of ring-shaped powder compacts 102 than producing a single sintered ring magnet 100 are stacked as shown in
A curved inner surface and a curved outer surface of the sintered ring-shaped powder compact rod 300, in which the ring-shaped powder compacts 102 are joined into a single structure by sintering, are finished by machining, and the sintered ring-shaped powder compact rod 300 is subjected to an anticorrosion surface treatment. Then, a mechanical bending stress is applied to boundary regions of the sintered ring-shaped powder compact rod 300 and, as a result, the sintered ring-shaped powder compact rod 300 breaks at the boundary regions where the protruding parts 103 are formed and multiple sintered ring magnets 100 are obtained as shown in
Instead of finishing the curved inner surface of the sintered ring-shaped powder compact rod 300 by machining operation as stated above, curved inner surfaces of the individual sintered ring magnets 100 obtained by dividing the sintered ring-shaped powder compact rod 300 at the boundary regions may be machined according to the invention. Also, because the sintered ring-shaped powder compact rod 300, and thus the sintered ring magnets 100 obtained by dividing the sintered ring-shaped powder compact rod 300, have a high degree of shape accuracy, each of the sintered ring magnets 100 may be fitted on and bonded to a rotor shaft without performing any finishing operation on the curved inner surface.
Generally, when a powder compact is sintered, the powder compact is placed on a tray in which parting powder is dusted and input into a sintering furnace. The parting powder is used for preventing the powder compact from sticking to the tray as a result of sintering operation. The parting powder also serves to decrease friction between the powder compact (sintered powder compact) and the tray when sintering shrinkage occurs during the sintering operation to thereby prevent sintering deformation. When a ring-shaped powder compact is sintered, a disturbance to normal sintering shrinkage due to friction between the powder compact and the tray can not be avoided by use of the parting powder alone, and this results in the occurrence of sintering deformation. Typically, a lower part of the ring-shaped powder compact which is in contact with the tray deforms into an oval shape or increases in size.
Because the sintered ring-shaped powder compact rod 300 of the present embodiment is formed by stacking a plurality of ring-shaped powder compacts 102 in the axial direction and sintering the stacked ring-shaped powder compacts 102, deformation due to the sintering shrinkage occurs only in the ring-shaped powder compact 102 in a lowermost layer and the ring-shaped powder compacts 102 in upper layers maintain their ring shape with high shape accuracy even after the sintering operation. Therefore, it is preferable that the ring-shaped powder compact 102 to be used in the lowermost layer of the sintered ring-shaped powder compact rod 300 be shaped to have a smaller inside diameter so that the curved inner surface of that ring-shaped powder compact 102 can be finished with a sufficient machining allowance even if the sintering deformation occurs. Although the ring-shaped powder compact 102 in only the lowermost layer must have greater machining allowance, the sintered ring-shaped powder compact rod 300 thus manufactured has a high degree of shape accuracy as a whole.
According to the above-described ring magnet manufacturing method, several ring magnets 100 joined into a single structure, or the sintered ring-shaped powder compact rod 300, in the sintering process are handled together in subsequent processes. This approach of the invention produces the following advantageous effects:
(a) Since several sintered ring magnets 100 are handled as a single structure, the sintered ring magnets 100 can be transferred together from one process to next with high efficiency.
(b) Since the curved inner surfaces and outer surfaces of several sintered ring magnets 100 can be machined together in the form of the sintered ring-shaped powder compact rod 300, it is possible to achieve improved machining efficiency.
(c) Since several sintered ring magnets 100 can be subjected together to the anticorrosion surface treatment in the form of the sintered ring-shaped powder compact rod 300, it is possible to achieve improved treatment efficiency.
(d) It is possible to produce the sintered ring-shaped powder compact rod 300 with a high degree of shape accuracy.
According to the ring magnet manufacturing method of the present embodiment, the protruding parts 103 formed on the upper end surfaces of the specific ring-shaped powder compacts 102 need not necessarily be ring-shaped as illustrated in
(3) Procedure for Manufacturing the Sintered Ring Magnet of the First Embodiment
Now, a procedure for manufacturing the sintered ring magnet 100 of the first embodiment is specifically described.
The sintered ring magnet 100 is a permanent magnet manufactured by using as a raw material neodymium magnetic alloy containing neodymium (Nd), dysprosium (Dy), iron (Fe) and boron (B). The neodymium magnetic alloy is subjected to a hydrogen occlusion treatment and finely pulverized by use of a jet mill to obtain fine alloy powder (magnetic powder) having an average particle size of about 5 micrometers. The ring-shaped powder compacts 102 stacked in multiple layers of each sintered ring magnet 100 are made by pressing this powder.
As shown in
First, the transferable metal die unit 10 is transported to the powder feeding unit 3 by the belt conveyor 2.
Shown in
Shown in
The transferable metal die unit 10 of which cavity 10h has been charged with the magnetic powder 11 is then transported to the punch setup unit 4 by the belt conveyor 2.
As shown in these Figures, the punch setup unit 4 includes a tong-grip lifter 4a for catching the upper punch 10g and a transfer mechanism (not shown) for raising and lowering the tong-grip lifter 4a and moving the upper punch log caught by the tong-grip lifter 4a. With this punch setup unit 4, it is possible to set the transferable metal die unit 10 in a condition where the magnetic powder 11 in the cavity 10h can be pressurized by the upper punch log.
First, the palette 10a loaded with the transferable metal die unit 10 is transported onto a stage of the punch setup unit 4 and set at a prescribed position as shown in
The transferable metal die unit 10 fitted with the upper punch 10g is advanced to a pressing stage at the pressing unit 5 by the belt conveyor 2.
As shown in
Referring to
Referring to
When the transferable metal die unit 10 has been advanced from the punch setup unit 4 to the pressing unit 5 by the belt conveyor 2, a metal die portion of the transferable metal die unit 10 is transferred from the palette 10a to a pressing portion of the pressing unit 5 together with the first holder 10b by the transfer mechanism 5h as shown in
Next, as the up/down drive mechanism is actuated, the electromagnetic coil 5a and the pressing device 5c descend, the upper and lower frames are fixed to each other by a clamping function thereof and the die 10f is secured in position by the ring-shaped elastic member 5j which is attached to a bottom part of the upper frame as shown in
As shown in
The radially oriented ring-shaped powder compact 102 is returned onto the palette 10a together with the transferable metal die unit 10 by the transfer mechanism 5h.
In the above-described pressing stage, the intensity of the aligning magnetic field applied in the pressing operation can be controlled by regulating the amount of current to be flowed through each electromagnetic coil 5a. It is possible to adjust the alignment coefficient of the ring-shaped powder compact 102 to be used in each layer by regulating the current to be flowed through each electromagnetic coil 5a. This makes it possible to ring-shaped powder compacts to be used in individual layers that have specific magnetic properties described with reference to the first to third embodiments.
Next, the transferable metal die unit 10 carrying the ring-shaped powder compact 102 is advanced to a die release stage at the die release unit 6 by the belt conveyor 2.
As shown in these Figures, the die release unit 6 is provided with a pressing mechanism including an air cylinder 6a for pressurizing the ring-shaped powder compact 102 and an upper punch stopper 6d, and a die lifting mechanism including a table 6c upward and air cylinders 6b for lifting the die 10f upward.
Next, as shown in
Subsequently, the air cylinder 6a retracts and the palette 10a lies on the belt conveyor 2 as shown in
In the die release process for drawing out the ring-shaped powder compact 102 from the transferable metal die unit 10, there is a difference in internal stress between an upper portion of the ring-shaped powder compact 102 drawn out of the transferable metal die unit 10 and a lower portion of the ring-shaped powder compact 102 still remaining in the transferable metal die unit 10. Generally, cracks are likely to develop in a boundary region between the upper portion of the ring-shaped powder compact 102 drawn out of the transferable metal die unit 10 and the lower portion of the ring-shaped powder compact 102 remaining in the transferable metal die unit 10 due to the difference in internal stress. In the die release unit 6 of this embodiment, however, the ring-shaped powder compact 102 is drawn out of the die 10f under conditions where the ring-shaped powder compact 102 is pressurized and, therefore, the difference in internal stress between the upper and lower portions of the ring-shaped powder compact 102 is so small that the occurrence of cracks is avoided.
The transferable metal die unit 10 from which the ring-shaped powder compact 102 has been drawn out is advanced to the powder removal unit 7 by the belt conveyor 2.
As shown in these Figures, the powder removal unit 7 for performing a powder removal process is provided with a raise/lower mechanism including a table 7a and air cylinders 7b for raising and lowering the table 7a, a nozzle 7c for spewing out nitrogen gas and a dust collecting duct 7d for drawing and collecting excess magnetic powder and iron powder into a dust collector. As the excess magnetic powder adhering to the ring-shaped powder compact 102 is removed at the powder removal unit 7, it is possible to prevent the ring-shaped powder compact 102 from listing or shifting away from a normal position in a stacking process at a succeeding stage.
The palette 10a loaded with the transferable metal die unit 10 from which the ring-shaped powder compact 102 has been drawn out is transported by the belt conveyor 2 and set at a prescribed position in the powder removal unit 7, and then the air cylinders 7b are actuated, causing the table 7a to ascend as shown in
When the upper end surface of the ring-shaped powder compact 102 has slightly protruded above the core 10d in the aforementioned process of drawing out the ring-shaped powder compact 102 from the core 10d, the upper punch 10g is removed, nitrogen gas is spewed out from the nozzle 7c to blow off the magnetic powder adhering to surfaces of the ring-shaped powder compact 102, and the magnetic powder is sucked up by the dust collecting duct 7d as shown in
First, the tong-grip lifter 8a of the gripping mechanism is moved to a position just above the ring-shaped powder compact 102 drawn from the core 10d as shown in
To obtain the sintered ring magnet 100 shown in
It is impossible to produce a stack of multiple ring-shaped powder compacts by a conventional pressing method in which individual ring-shaped powder compacts are pressed by use of a metal die fixed to a pressing machine. According to the present embodiment, a ring-shaped powder compact rod having the same shape as the sintered ring-shaped powder compact rod 300 of
If variations in height occur among the individual ring-shaped powder compacts 102 (when the stacked ring-shaped powder compacts 102 become high), undesirable pressure will be exerted on the ring-shaped powder compacts 102 during the stacking process, potentially causing the ring-shaped powder compacts 102 to crush, or the tong-grip lifter 8a may accidentally release the ring-shaped powder compact 102 in the air, potentially causing breakage of the ring-shaped powder compact 102 as a result of an impact of fall. In the present embodiment, however, the weight of the magnetic powder 11 to be pressed for making the ring-shaped powder compact 102 in each cycle of pressing process is measured to a fixed amount in the magnetic powder weighing process carried out by the powder feeding unit 3, so that the height of each ring-shaped powder compact 102 is made constant and there will not arise such a problem that an undesirable force or an impact force is exerted on the ring-shaped powder compact 102 during the stacking process.
Upon completion of the stacking process, the first holder 10b, the core 10d and the lower punch 10e of the transferable metal die unit 10 are returned onto the palette 10a by the transfer mechanism 12 and the transferable metal die unit 10 is conveyed to the powder removal/die setup unit 9 where a next process is performed. The powder removal/die setup unit 9 is provided with a powder removal mechanism for removing magnetic powder adhering to the transferable metal die unit 10 and a setup mechanism for setting individual parts of the transferable metal die unit 10 to an initial condition in which the powder feeding unit 3 can feed the magnetic powder 11 again.
The powder removal mechanism has a nozzle for blowing nitrogen gas against the individual parts of the transferable metal die unit 10 and a vacuum mechanism for drawing and collecting the magnetic powder blown off by nitrogen gas. The setup mechanism is a mechanism for lifting the die 10f placed on the second holder 10j and moving the die 10f onto the lower punch 10e placed on the first holder 10b upon completion of the stacking process. With the provision of the powder removal mechanism and the setup mechanism, it is possible to smoothly carry out a next cycle of the pressing to staking processes.
The ring-shaped powder compact rod obtained by stacking the ring-shaped powder compacts 102 is transferred to a sintering/heat treatment furnace, in which the ring-shaped powder compact rod is sintered and subjected to heat treatment at a specified temperature. As a result, a sintered ring-shaped powder compact rod like the one shown in
As illustrated in
Generally, in the manufacture of sintered ring magnets, a substantial proportion of total manufacturing time is devoted to surface finishing operation performed in a later stage. Thus, even if the ring-shaped powder compact 102B which serves as a dummy powder compact is placed in the lowermost layer of the sintered ring-shaped powder compact rod 301 before the sintering operation, the total manufacturing time is shortened due to a reduction in sintering deformation and a consequent reduction in time required for machining. Accordingly, the method of manufacturing the sintered ring magnets of the second embodiment allows for overall cost reduction. Additionally, because a large number of sintered ring magnets 100 can be obtained by using one ring-shaped powder compact 102B serving as a dummy powder compact according to the manufacturing method of the second embodiment, a loss of cost and time needed for the preparation of the dummy ring-shaped powder compact 102B is almost negligible as a whole. It is therefore appreciated that the manufacturing method of the second embodiment contributes to eventual cost savings.
Third Embodiment
The ring-shaped powder compacts 102 used for producing the sintered ring-shaped powder compact rod 302 of the third embodiment are made by substantially the same manufacturing method as explained earlier with reference to the first embodiment. For example, to produce the sintered ring-shaped powder compact rod 302 of
According to the above-described manufacturing method of the third embodiment, it is possible to easily produce the sintered ring-shaped powder compact rod 302, in which the ring-shaped powder compacts 102 are joined with a reduced joint strength at the specific boundary regions, by a simplified manufacturing process without the need to form protruding parts or recessed parts on the end surfaces of the ring-shaped powder compacts 102 unlike the foregoing embodiments.
It is to be noted that powder of magnesia or other ceramic material may be used instead of the alumina powder 104 used in this embodiment.
Fourth Embodiment
The ring-shaped powder compacts 102 used for producing the sintered ring-shaped powder compact rod 303 of the fourth embodiment are made by substantially the same manufacturing method as explained earlier with reference to the first embodiment except that the ring-shaped powder compact 102 to be placed immediately below each boundary region 106 is advanced from the die release process to the stacking process, bypassing the powder removal process shown in
According to the above-described manufacturing method of the fourth embodiment, it is possible to produce the sintered ring-shaped powder compact rod 303, in which the ring-shaped powder compacts 102 are joined with a reduced joint strength at the specific boundary regions 106, without the need to form protruding parts or recessed parts or dust ceramic powder on the end surfaces of the ring-shaped powder compacts 102 unlike the foregoing embodiments.
Claims
1. A sintered ring magnet comprising a plurality of radially oriented ring-shaped powder compacts which are stacked one on top of another in an axial direction thereof and joined together by sintering, wherein one of a protruding part and a recessed part is formed on at least one of longitudinal end surfaces of said sintered ring magnet.
2. A method of manufacturing sintered ring magnets comprising the steps of:
- stacking a plurality of radially oriented ring-shaped powder compacts thereof in an axial direction to produce a ring-shaped powder compact rod;
- sintering the ring-shaped powder compact rod to produce a sintered ring-shaped powder compact rod in which the ring-shaped powder compacts are joined together as a result of sintering operation; and
- dividing the sintered ring-shaped powder compact rod;
- wherein the ring-shaped powder compacts are joined to one another with a reduced joint strength at specific boundary regions than at the other boundary regions when sintered, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at said specific boundary regions having the reduced joint strength.
3. The method of manufacturing the sintered ring magnets according to claim 2, wherein the ring-shaped powder compacts are stacked on top of a dummy ring-shaped powder compact such that the dummy ring-shaped powder compact is located in a lowermost layer of the ring-shaped powder compact rod, wherein the dummy ring-shaped powder compact is joined to the ring-shaped powder compact stacked immediately above with a reduced joint strength at a boundary region therebetween as a result of the sintering operation, and wherein said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod after the sintering operation at said specific boundary regions having the reduced joint strength.
4. The method of manufacturing the sintered ring magnets according to claim 2, wherein one of a protruding part and a recessed part is formed on at least one of longitudinal end surfaces of each ring-shaped powder compact facing one of said specific boundary regions, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at each of said specific boundary regions where one of the protruding part and the recessed part is formed after the sintering operation.
5. The method of manufacturing the sintered ring magnets according to claim 2, wherein ceramic powder having a particle size between 1 micrometer and 100 micrometers is dusted to a thickness between 1 micrometer and 100 micrometers at each of said specific boundary regions, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at said specific boundary regions after the sintering operation.
6. The method of manufacturing the sintered ring magnets according to claim 2, wherein burrs formed on surfaces of the ring-shaped powder compacts in a pressing process or excess magnetic powder which adheres to the surfaces of the ring-shaped powder compacts in a die release process is left unremoved from said specific boundary regions of the ring-shaped powder compacts, the ring-shaped powder compact rod is produced by using the ring-shaped powder compacts thus prepared, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at said specific boundary regions after the sintering operation.
7. The method of manufacturing the sintered ring magnets according to claim 2, wherein the sintered ring-shaped powder compact rod is divided at said specific boundary regions having the reduced joint strength after at least one of a curved inner surface and a curved outer surface of the sintered ring-shaped powder compact rod, in which the ring-shaped powder compacts are joined into a single structure by sintering, is finished by machining.
8. The method of manufacturing the sintered ring magnets according to claim 7, wherein the sintered ring-shaped powder compact rod is divided at said specific boundary regions having the reduced joint strength after the sintered ring-shaped powder compact rod of which at least one of the curved inner surface and the curved outer surface has been machined is subjected to an anticorrosion surface treatment.
Type: Application
Filed: Sep 19, 2005
Publication Date: Mar 9, 2006
Applicant: MITSUBISHI DENKI KABUSHIKI KAISHA (Tokyo)
Inventors: Yoshikazu Ugai (Tokyo), Taizo Iwami (Tokyo), Yuji Nakahara (Tokyo), Satoshi Yamashiro (Tokyo)
Application Number: 11/228,422
International Classification: B22F 7/02 (20060101);