Patent Description:
Some rotation angle detection devices for detecting a rotation angle of a rotating body detect the rotation angle by detecting a change in a magnetic field formed by a magnet that rotates integrally with the rotating body using a magnetic sensor, and for example, this type of rotation angle detection device is provided for detecting a rotation angle of an electric motor mounted on a power steering device of an automobile.

The rotation angle detection device is provided with the magnet and the magnet holder for holding the magnet. For example, Patent Literature <NUM> describes a rotation angle detection device having a configuration in which a holder member attached to one end side in an axial direction of the rotating body holds the magnet on a side opposite to a side attached to a rotating shaft in the axial direction. In this device, when the holder member and the magnet rotate integrally with a rotation shaft due to rotation of the rotation shaft, the magnetic sensor provided to face the magnet can detect the change in the magnetic field and detect the rotation angle of the rotation shaft.

In the device as described above, a retaining mechanism is generally provided in order to prevent the magnet from falling off the holder member. For example, in Patent Literature <NUM>, as illustrated in <FIG>, a cylindrical holder member <NUM> is provided, and the magnet member <NUM> is integrally molded with a recess 101a that is provided on an inner peripheral surface side of the holder member <NUM> and opens to one end side in the axial direction. An inner peripheral wall of the holder member <NUM> forming the recess 101a is an undercut portion 101b having a diameter increasing from one side to the other side in the axial direction. The magnet member <NUM> is locked by the undercut portion 101b, and the magnet member <NUM> can be prevented from falling off the opening on the one side in the axial direction of the holder member <NUM>. From <CIT>, a magnetic encoder device is known. The encoder device comprises a base portion to be press fitted onto a rotary shaft, and a magnet fixed on the base portion on the outer periphery thereof directly or via an intermediate member. <CIT> discloses a sintered porous bearing capable of providing a magnet at an optimal bearing portion. The bearing is intended to slidably support a rotary shaft, the magnet being bonded to the inner or outer periphery of a sintered porous member.

<CIT> relates to a rotation angle detection device that detects a rotation angle of a rotating body. This rotation angle detection device comprises a magnet integrally rotating with the rotation shaft, and a magnetic sensor for detecting the magnetic field of the magnet, and detects the rotation angle of the rotation shaft based on the detection output of the magnetic sensor.

It is difficult to integrally mold an undercut shape as in Patent Literature <NUM> when the holder member is molded by die molding. Therefore, for example, Patent Literature <NUM> discloses that the undercut portion 101b is formed by machining such as lathe machining. However, such a post-processing method has a problem that the number of steps increases, leading to an increase in manufacturing cost.

In view of such circumstances, an object of the present invention is to manufacture a magnet unit including the magnet holder having a magnet retaining mechanism at low cost.

The scope of the present invention is defined by independent claim <NUM>, and further embodiments of the invention are specified in dependent claims <NUM>-<NUM>.

The sintered metal is rich in plastic fluidity, so that high surface accuracy can be obtained by sizing. Therefore, as in the above-described configuration of the present invention, by integrally forming the magnet holding portion and the shaft attachment portion with sintered metal and sizing the inner peripheral surface of the shaft attachment portion, the inner peripheral surface with high accuracy can be molded by one shot without requiring a plurality of steps. In addition, by forming the undercut portion by plastic deformation, processing cost can be reduced as compared with the case of machining. As described above, the cost of the magnet holder can be reduced. Furthermore, by increasing the accuracy of the inner peripheral surface of the shaft attachment portion, it is easy to control a press-fitting margin when the shaft is press-fitted into the shaft attachment portion, and it is possible to improve attachment accuracy (coaxiality or the like) of the shaft to the magnet holder.

In the present invention, the manufacturing cost of the magnet holder can be reduced.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings.

As illustrated in <FIG>, a rotation angle detection device <NUM> includes a magnet unit <NUM> and a magnetic sensor <NUM>. The magnet unit <NUM> includes a magnet holder <NUM> and a magnet <NUM>, and is attached to one axial end of a rotating shaft <NUM> that is a shaft.

The magnet holder <NUM> is formed of sintered metal by powder metallurgy, and is particularly formed of a non-magnetic material so as not to affect a magnetic field of the magnet <NUM>. For example, it is preferable to select an austenitic stainless steel material that can be plastically deformed in a sizing step described later. The magnet <NUM> is a bonded magnet molded by injecting a magnetic material into the magnet holder <NUM> and magnetizing the magnetic material.

The magnet holder <NUM> has a substantially cylindrical shape (see <FIG>), and one axial end side (upper side in <FIG>) has a larger diameter than the other axial end side. The magnet holder <NUM> holds the magnet <NUM> on the one axial end side on an inner peripheral surface side thereof.

The magnet <NUM> has a configuration in which N poles and S poles are alternately arranged in a circumferential direction thereof, and changes the magnetic field to be formed by rotating integrally with the rotating shaft <NUM>.

The magnetic sensor <NUM> is disposed to face the magnet <NUM>, and can detect a change in magnitude or direction of the magnetic field formed by the magnet <NUM>. A known magnetic sensor can be appropriately used for the magnetic sensor <NUM>.

The rotating shaft <NUM> is provided to be rotatable about an axis Z. When the rotating shaft <NUM> rotates, the magnet unit <NUM> attached to one end side of the rotating shaft <NUM> rotates integrally with the rotating shaft <NUM>. Thus, the magnitude or the direction of the magnetic field formed by the magnet <NUM> changes. Then, the magnetic sensor <NUM> can detect a rotation angle of the rotating shaft <NUM> by detecting this change. Hereinafter, an extending direction of the axis Z is also simply referred to as an axial direction.

As illustrated in <FIG>, the magnet holder <NUM> has a configuration in which a hollow cylindrical shaft attachment portion <NUM> for press-fitting and fixing the rotating shaft <NUM> and a hollow cylindrical magnet holding portion <NUM> provided on one side in the axial direction of the shaft attachment portion <NUM> and holding a periphery of the magnet <NUM> are integrated. An inner peripheral space of the shaft attachment portion <NUM> and an inner peripheral space of the magnet holding portion <NUM> are continuous in the axial direction. An inner peripheral surface (inner side surface) <NUM> of the magnet holding portion <NUM> faces the periphery of the magnet <NUM>, and an inner diameter dimension of the inner peripheral surface <NUM> is larger than an inner diameter dimension of an inner peripheral surface of the shaft attachment portion <NUM>. A magnet housing portion 4a, which is a space for housing the magnet <NUM>, is formed by a magnet-side end surface <NUM> of the shaft attachment portion <NUM> facing an end surface of the magnet <NUM> and the inner peripheral surface <NUM> of the magnet holding portion <NUM>.

As illustrated in a partially enlarged view of <FIG>, a tapered portion 43a and a reverse tapered portion 43b are continuously provided on the inner peripheral surface <NUM> of the magnet holding portion <NUM> from the one side in the axial direction. The tapered portion 43a is a portion having a diameter decreasing from the one side to the other side in the axial direction. Further, the reverse tapered portion 43b is a portion having a diameter increasing from the one side to the other side in the axial direction, and an undercut portion <NUM> is formed by the tapered portion 43a and the reverse tapered portion 43b.

The end surface <NUM> of the shaft attachment portion <NUM> is provided with a recess 44a for preventing rotation of the magnet <NUM> with respect to the magnet holder <NUM>. At least one recess 44a may be provided, and in the present embodiment, four recesses 44a are provided at equal intervals in the circumferential direction. However, a position where the recess for preventing the rotation of the magnet <NUM> is provided is not limited to the end surface <NUM>. For example, as illustrated in <FIG>, a recess <NUM> may also be provided in the reverse tapered portion <NUM>. At least one recess <NUM> may be provided, and for example, four recesses <NUM> can be provided at equal intervals in the circumferential direction.

A groove portion <NUM> extending in the axial direction is provided on the inner peripheral surface of the shaft attachment portion <NUM>. The groove portion <NUM> is provided in a partial region in the circumferential direction and is open to the other axial side of the shaft attachment portion <NUM>. In the magnet holder <NUM>, the rotating shaft <NUM> is press-fitted into the shaft attachment portion <NUM> in a state where the magnet <NUM> is held by the magnet holding portion <NUM>, so that a sealed space is formed in the magnet holder <NUM>. On the other hand, by providing the groove portion <NUM> open to the other axial side, an inside and an outside of the magnet holder <NUM> communicate with each other via the groove portion <NUM>, so that the air in the magnet holder <NUM> can be released to the outside along with press-fitting. Thus, it is possible to prevent the magnet <NUM> from being detached or the magnet unit <NUM> from being damaged due to increase in internal pressure of the magnet holder <NUM> during the press-fitting of the rotating shaft <NUM>.

Next, a method for producing the magnet unit <NUM> will be described.

In the present embodiment, the magnet holder <NUM> is molded by powder metallurgy. That is, metal powder is compression-molded with a molding die, and then a green compact thus obtained is heated and sintered to obtain a sintered body. Then, dimensions of the sintered body are corrected by sizing to complete the magnet holder <NUM>.

A sintered body <NUM>' of the present embodiment obtained in the above process is illustrated in <FIG>.

A portion <NUM>' corresponding to the shaft attachment portion <NUM> (including the recess 44a and the groove portion <NUM>) and a portion <NUM>' corresponding to the magnet holding portion <NUM> are formed in the sintered body <NUM>'. An outer diameter dimension of the portion (hereinafter, referred to as a "shaft attachment portion-corresponding portion") <NUM>' corresponding to the shaft attachment portion <NUM> and an outer diameter dimension of the portion (hereinafter, referred to as a "magnet holding portion-corresponding portion") <NUM>' corresponding to the magnet holding portion <NUM> are respectively larger than those of the shaft attachment portion <NUM> and the magnet holding portion <NUM> by a sizing margin. A shape of a magnet holding portion-corresponding portion <NUM>' is different from that of the magnet holding portion <NUM>. That is, an inner peripheral surface <NUM>' of the magnet holding portion-corresponding portion <NUM>' is formed in a cylindrical surface shape having a uniform diameter, and the undercut portion <NUM> illustrated in <FIG> does not exist in the inner peripheral surface <NUM>'. In addition, an annular protrusion <NUM> is formed in a region on the one side in the axial direction of an outer peripheral surface of the magnet holding portion-corresponding portion <NUM>'. A sizing margin of an outer peripheral surface of the protrusion <NUM> is larger than that of an outer peripheral surface of the other region of the magnet holder <NUM>.

Subsequently, by sizing the sintered body <NUM>', dimensional accuracy of each part is improved, and the undercut portion <NUM> is formed to obtain the magnet holder <NUM>.

Specifically, first, as illustrated in <FIG>, the sintered body <NUM>' is inserted into an annular cavity X1 formed between an annular core <NUM> and a die <NUM>. Note that although only one cross-section in the circumferential direction of the sintered body <NUM>' is illustrated in <FIG>, the entire sintered body <NUM>' is inserted into the annular cavity X1.

The core <NUM> has a constant outer diameter in the axial direction, and an outer peripheral surface 71a thereof is formed in a cylindrical surface shape. On the other hand, a diameter of an inner peripheral surface 72a of the die <NUM> is gradually reduced from an upper side to a lower side in the drawing, and has a plurality of stepped surfaces. That is, a width of the cavity X1 is reduced from the upper side to the lower side in the drawing, and a sufficient width for inserting the sintered body <NUM>' is provided at an upper end thereof.

The sintered body <NUM>' inserted into the cavity X1 is pushed down by an upper punch <NUM> (<FIG>), pressed by the upper punch <NUM> and a lower punch <NUM> (<FIG>), and then removed from a mold by raising the upper punch <NUM> and the lower punch <NUM> (<FIG>).

In a process from <FIG>, the sintered body <NUM>' is compressed by receiving a pressing force in an inner diameter direction from the die <NUM>. By this pressing force, an inner peripheral surface of the sintered body <NUM>' is reduced in diameter and pressed against the core <NUM>. Therefore, an outer peripheral surface of the sintered body <NUM>' is molded by the inner peripheral surface 72a of the die <NUM>, and an inner peripheral surface of the shaft attachment portion-corresponding portion <NUM>' of the sintered body <NUM>' is molded by the outer peripheral surface 71a of the core <NUM>. Further, both end faces of the sintered body <NUM>' are formed by the upper punch <NUM> and the lower punch <NUM>.

As described above, the sizing margin on the outer peripheral surface of the sintered body <NUM>' is the largest at the protrusion <NUM>. Further, as illustrated in FIGS. 4A and 4B, there is a space between the magnet holding portion-corresponding portion <NUM>' of the sintered body <NUM>' and the core <NUM>, and the magnet holding portion-corresponding portion <NUM>' that receives the pressing force from the die <NUM> toward the inner diameter side is in a state of being freely deformable toward the inner diameter side. Therefore, when the protrusion <NUM> is compressed, an inner peripheral surface of the protrusion <NUM> protrudes toward the inner diameter side by plastic flow in the inner diameter direction. Thus, as illustrated in an enlarged view of FIG. 4B, the undercut portion <NUM> including the tapered portion 43a and the reverse tapered portion 43b is formed on the inner peripheral surface <NUM> of the magnet holding portion <NUM>.

Thus, sizing of the inner peripheral surface of the shaft attachment portion <NUM> and formation of the undercut portion <NUM> can be simultaneously performed in one shot.

In the present invention, by sizing, the outer peripheral surfaces of the shaft attachment portion <NUM> and the magnet holding portion <NUM> are subjected to a compression operation while sliding on the die <NUM>, so that vacancies are crushed on the outer peripheral surfaces. On the other hand, the inner peripheral surface <NUM> of the magnet holding portion <NUM> does not slide on the die, and is not subjected to the compression operation, so that almost no vacancy is crushed on the inner peripheral surface <NUM>. Therefore, a vacancy rate of the inner peripheral surface of the magnet holding portion <NUM> is larger than that of the outer peripheral surfaces of the shaft attachment portion <NUM> and the magnet holding portion <NUM>. The vacancy rate is represented by an area ratio occupied by the vacancies when a micrograph of a surface is subjected to image analysis.

Then, using the removed magnet holder <NUM> as a part of the molding die, the magnet housing portion 4a (see <FIG>) is filled with the magnetic material, and a magnet material is injection-molded. At this time, a surface on the one side in the axial direction of the magnet material is present in a region of the tapered portion 43a. Thereafter, the magnet <NUM> can be obtained by magnetizing the magnet material by an appropriate means. In addition, at the time of injection molding, the recess 44a (see <FIG>) is filled with the magnetic material, so that a protrusion that comes into close contact with the recess 44a and engages with the recess 44a in the circumferential direction is formed in the magnet <NUM>. As described above, by providing the recess 44a in the magnet holder <NUM>, the protrusion can be formed in the magnet <NUM> by subsequent injection molding, and a rotation stopping mechanism of the magnet <NUM> can be provided with a simple configuration.

As described above, as illustrated in <FIG>, the magnet unit <NUM> in which magnet <NUM> and magnet holder <NUM> are integrated is completed.

The magnet <NUM> has a linear expansion coefficient larger than that of the magnet holder <NUM> formed of sintered metal. Therefore, when the injection-molded magnet <NUM> is cooled and contracted, there is a possibility that looseness may occur between the magnet <NUM> and the magnet holder <NUM> or the magnet <NUM> may fall off the magnet holder <NUM>. However, in the present embodiment, the undercut portion <NUM> is provided on the inner peripheral surface <NUM> of the magnet holding portion <NUM>, and the undercut portion <NUM> is formed over the undercut portion <NUM> and an upper part of the undercut portion <NUM> to be in close contact with the magnet <NUM>, so that the magnet <NUM> is restrained by the magnet holder <NUM>, and an axial movement of the magnet <NUM> with respect to the magnet holder <NUM> is restricted. Therefore, it is possible to prevent the magnet <NUM> from falling off the magnet holder <NUM>. In addition, the looseness of the magnet <NUM> in the magnet holder <NUM> mainly in the axial direction can be suppressed, and detection accuracy of the rotation angle by the rotation angle detection device <NUM> can be improved.

In the present embodiment, the magnet holding portion <NUM> and shaft attachment portion <NUM> are integrally formed of sintered metal. Since the sintered metal is rich in plastic fluidity, and high surface accuracy can be obtained by sizing, by sizing the inner peripheral surface of the shaft attachment section <NUM> made of sintered metal, the inner peripheral surface with high accuracy can be formed in one shot without requiring a plurality of machining steps. Further, by forming the undercut portion <NUM> by plastic deformation, processing cost can be reduced. From the above, it is possible to reduce the cost of the magnet holder <NUM>. By increasing the accuracy of the inner peripheral surface of the shaft attachment portion <NUM> by sizing, it is easy to control a press-fitting margin when the rotating shaft <NUM> is press-fitted into the shaft attachment portion <NUM>, and it is possible to improve attachment accuracy (coaxiality or the like) of the rotating shaft <NUM> to the magnet holder <NUM>. In addition, since the undercut portion <NUM> is formed by plastically deforming the magnet holding portion <NUM> by the pressing force in the inner diameter direction acting on the magnet holding portion <NUM> with the sizing, it is possible to simultaneously correct the dimensional accuracy with respect to the inner peripheral surface of the shaft attachment portion <NUM> and form the undercut portion <NUM>, and it is possible to improve productivity of the magnet holder <NUM>.

Further, in the present embodiment, since the protrusion of the magnet <NUM> is fitted into the plurality of recesses 44a provided in the circumferential direction of the magnet holder <NUM>, it is possible to prevent the rotation of the magnet <NUM> in the circumferential direction with respect to the magnet holder <NUM>. Thus, the detection accuracy of the rotation angle by the rotation angle detection device <NUM> can be improved.

Next, the magnet unit having the magnet holder according to another embodiment will be described with reference to <FIG>. Hereinafter, description of configurations similar to those of the above embodiment will be appropriately omitted.

As illustrated in an enlarged view of <FIG>, in the magnet holder <NUM> of the present embodiment, the inner peripheral surface <NUM> forming the magnet holding portion <NUM> is constituted by a projecting portion 43c provided on the one side in the axial direction and projecting in the inner diameter direction, and a flat surface 43d provided on the other side in the axial direction continuously to the projecting portion 43c and having a uniform diameter. This projecting portion 43c forms the undercut portion <NUM> on the inner peripheral surface <NUM> of the magnet holding portion <NUM>.

An end surface <NUM> of the magnet holding portion <NUM> is formed in a stepped flat surface shape. The end surface <NUM> is provided one step lower than an end surface on an outer diameter side thereof (retreated in a right direction in <FIG>), and these end surfaces are connected by a tapered surface 46a.

As illustrated in <FIG>, a groove portion <NUM> extending in the axial direction is provided in a partial region in the circumferential direction of the magnet holder <NUM> in a region from the outer peripheral surface of the magnet holding portion <NUM> to the outer peripheral surface of the shaft attachment portion <NUM>. The groove portion <NUM> identifies a circumferential position of the magnet holder <NUM>, and is used to determine a magnetization direction. However, it is sufficient that a shape of this portion can be determined from the outside, and for example, the groove portion <NUM> can be a cutout or a recess having a specific shape.

Next, a process of producing the magnet holder <NUM> of the present embodiment will be described.

In the same manner as in the above-described embodiment, the metal powder is compression-molded with the molding die, and then the green compact thus obtained is heated and sintered to obtain the sintered body <NUM>'.

As illustrated in <FIG>, in the sintered body <NUM>' of the present embodiment, the inner peripheral surface of the magnet holding portion-corresponding portion <NUM>' is formed in a cylindrical surface shape with a constant diameter. On the inner diameter side of the end surface of the magnet holding portion-corresponding portion <NUM>', as illustrated in an enlarged view of <FIG>, a protrusion <NUM> protruding to the one side in the axial direction is formed. An end surface of an inner diameter end of the protrusion <NUM> is a flat surface 82b, and an inclined surface 82a having an inclination angle (inclination angle with respect to a radial direction) of <NUM>° or less is provided on an outer diameter side thereof. In addition, a flat surface <NUM> is formed on the outer diameter side of the inclined surface 82a, and a tapered surface <NUM> having an inclination angle (same as above) of <NUM>° or less is provided on the outer diameter side of the flat surface <NUM>. When the inclination angle of the inclined surface 82a exceeds <NUM>°, it is difficult to plastically deform the protrusion <NUM> during sizing. In addition, when the inclination angle of the tapered surface <NUM> exceeds <NUM>°, the tapered surface <NUM> is undercut, so that the molding is difficult. Therefore, by setting angles as described above, moldability of the magnet holder <NUM> is secured.

Next, a process of sizing the sintered body <NUM>' and molding the magnet holder <NUM> will be described.

As illustrated in <FIG>, the sintered body <NUM>' is inserted into an annular cavity X2 formed between an annular core <NUM> and a die <NUM>.

The sintered body <NUM>' inserted into the cavity X2 is pushed down by an upper punch <NUM> (<FIG>), pressed by the upper punch <NUM> and a lower punch <NUM> (<FIG>), and then removed from the mold by raising the upper punch <NUM> and the lower punch <NUM> (<FIG>).

The sizing step illustrated in <FIG> is basically common to the sizing step illustrated in <FIG>. Therefore, description of common parts is omitted, and different parts will be described below.

In this sizing step, as illustrated in an enlarged view of <FIG>, an inner diameter side of an end surface 77a of the upper punch <NUM> for molding the end surface of the magnet holding portion <NUM> has a shape protruding downward in the drawing from an outer diameter side thereof. Therefore, on the end surface of the magnet holding portion-corresponding portion <NUM>' of the sintered body <NUM>', the protrusion <NUM> has the maximum sizing margin. An inner diameter side of the protrusion <NUM> is a gap without the mold, so that an inner peripheral surface <NUM>' of the magnet holding portion-corresponding portion <NUM>' can be freely deformed. By pressurizing the protrusion <NUM> in the axial direction in an inner diameter side region of the end surface of the upper punch <NUM>, a thickness of the convex portion <NUM> plastically flows to the inner diameter side, and as illustrated in the enlarged view of FIG. 7B, the projecting portion 43c is formed on the inner peripheral surface <NUM> of the magnet holding portion <NUM>. In addition, a region where the protrusion <NUM> is present becomes a flat surface, and the end surface <NUM> of the magnet holding portion <NUM> is molded.

Also in the present embodiment, the outer peripheral surfaces of the shaft attachment portion <NUM> and the magnet holding portion <NUM> slide on the die <NUM> and are compressed, whereas the inner peripheral surface of the magnet holding portion <NUM> is not in contact with the mold, so that the vacancy rate of the inner peripheral surface <NUM> of the magnet holding portion <NUM> is larger than that of the outer peripheral surfaces of the shaft attachment portion <NUM> and the magnet holding portion <NUM>.

Thereafter, the magnet <NUM> is formed in the magnet housing portion 4a by the same process as the above-described embodiment using the removed magnet holder <NUM> as the molding die. Thus, as illustrated in <FIG>, magnet unit <NUM> in which magnet holder <NUM> and magnet <NUM> are integrated is formed. Note that the surface on the one side in the axial direction of the magnet <NUM> is present in a region of the tapered surface 46a.

In the present embodiment, since the magnet <NUM> is in close contact with the projecting portion 43c serving as the undercut portion, when the magnet <NUM> contracts, the looseness of the magnet <NUM> with respect to the magnet holder <NUM> can be suppressed, and the magnet <NUM> can be prevented from falling off the magnet holder <NUM>.

Next, the magnet unit having the magnet holder according to still another embodiment will be described with reference to <FIG>. Hereinafter, description of configurations similar to those of the above embodiment will be appropriately omitted.

As illustrated in an enlarged view of <FIG>, in the magnet holder <NUM> of the present embodiment, the inner peripheral surface <NUM> forming the magnet holding portion <NUM> is constituted by a flat surface 43e provided on the one side in the axial direction and having a substantially constant diameter, and a reverse tapered portion 43f provided continuously to the flat surface 43e and on the other side in the axial direction. The reverse tapered portion 43f is an inclined surface having a diameter decreasing from the one side to the other side in the axial direction. The undercut portion <NUM> is formed on the inner peripheral surface <NUM> of the magnet holding portion <NUM> by the reverse tapered portion 43f.

A stepped portion 44b is provided on the end surface <NUM> of the shaft attachment portion <NUM>. Due to the stepped portion 44b, a radially inner side of the end surface <NUM> is lower than a radially outer side thereof by one step.

As illustrated in <FIG>, the end surface <NUM> of the shaft attachment portion <NUM> is provided with recesses 44c extending in the circumferential direction. Three recesses 44c are provided at equal intervals in the circumferential direction. Since the protrusion of the magnet is fitted into the recesses 44c, it is possible to prevent the rotation of the magnet in the circumferential direction with respect to the magnet holder. Since a plurality of recesses 44c are formed in the circumferential direction, a pressure applied to the recesses 44c can be dispersed when the rotation of the magnet is prevented. In particular, by arranging the recesses 44c at equal intervals, the pressure applied to the recesses 44c can be uniformly dispersed.

As illustrated in <FIG>, in the sintered body <NUM>' of the present embodiment, the inner peripheral surface of the magnet holding portion-corresponding portion <NUM>' is formed in a cylindrical surface shape with a constant diameter. Further, on an outer peripheral surface side of the magnet holding portion-corresponding portion <NUM>', a protrusion <NUM> protruding to the outer diameter side is provided, and an outer dimension of the magnet holding portion-corresponding portion <NUM>' is increased by the sizing margin. The protrusion <NUM> has an inclined surface having a diameter decreasing from the one side to the other side in the axial direction on an outer peripheral surface side thereof. Further, the end surface <NUM> of the shaft attachment portion-corresponding portion <NUM>' is not provided with a stepped portion and has a flat surface shape.

As illustrated in <FIG>, the sintered body <NUM>' is inserted into an annular cavity X3 formed between an annular core <NUM> and a die <NUM>.

The sintered body <NUM>' inserted into the cavity X3 is pushed down by the upper punch <NUM> (<FIG>), pressed by the upper punch <NUM> and a lower punch <NUM> (<FIG>), and then removed from the mold by raising the upper punch <NUM> and the lower punch <NUM> (<FIG>).

In a process from <FIG>, an inner peripheral surface 92a of the die <NUM> generates a pressing force toward the inner peripheral surface side in the protrusion <NUM> which is the outer peripheral surface side of the magnet holding portion-corresponding portion <NUM>'. This pressing force causes the plastic flow toward the inner diameter side, and as illustrated in an enlarged view of <FIG>, the outer peripheral surface side of the magnet holding portion-corresponding portion <NUM>' is formed in a flat surface along the inner peripheral surface 92a of the die <NUM>, and a material on the protrusion <NUM> side projects toward the inner diameter side. At this time, the one side in the axial direction (upper side in the drawing) having a large thickness projects to a position in contact with an outer peripheral surface of the upper punch <NUM>, and an amount of projection is reduced toward the other side in the axial direction. Therefore, the flat surface 43e and the reverse tapered portion 43f are formed on the inner peripheral surface side of the magnet holding portion-corresponding portion <NUM>'.

Further, in the process from <FIG>, a part of the inner diameter side of the end surface <NUM> of the shaft attachment portion-corresponding portion <NUM>' is pressed by the upper punch <NUM> to be recessed, and the stepped portion 44b is formed on the end surface <NUM>. In addition, since the end surface <NUM> is pressed by the upper punch <NUM> in this manner, the pressed portion can be flattened.

Also in the present invention, the outer peripheral surfaces of the shaft attachment portion <NUM> and the magnet holding portion <NUM> slide on the die <NUM> and are compressed, whereas the inner peripheral surface of the magnet holding portion <NUM> is not in contact with the die, so that the vacancy rate of the inner peripheral surface <NUM> of the magnet holding portion <NUM> is larger than that of the outer peripheral surfaces of the shaft attachment portion <NUM> and the magnet holding portion <NUM>.

Also in the present embodiment, since the magnet <NUM> is in close contact with the reverse tapered portion 43f serving as the undercut portion, when the magnet <NUM> contracts, the looseness of the magnet <NUM> with respect to the magnet holder <NUM> can be suppressed, and the magnet <NUM> can be prevented from falling off the magnet holder <NUM>.

Note that the undercut portions <NUM> (the reverse tapered portion 43b and 43f, and the projecting portion 43c) are formed to prevent the magnet holder <NUM> from coming off as described above, and does not require strict dimensional accuracy. Therefore, they can be formed by the sizing step as described above. Therefore, as illustrated in <FIG>, <FIG>, and <FIG>, the undercut portion <NUM> satisfying required functions can be formed even when the inner diameter side of the magnet holding portion-corresponding portion <NUM>' is not constrained by the mold and is freely plastically deformable.

Claim 1:
A magnet unit(<NUM>):
comprising a magnet holder (<NUM>) and a magnet (<NUM>), wherein the magnet holder (<NUM>) has a shaft attachment portion (<NUM>) and a magnet holding portion (<NUM>),
wherein the shaft attachment portion (<NUM>) is configured to be press-fitted and fixed to a shaft (<NUM>);
and wherein the magnet holding portion (<NUM>) is provided on one side in an axial direction of the shaft attachment portion (<NUM>) and holds a periphery of a magnet (<NUM>),
and wherein the magnet holder (<NUM>) holds the magnet (<NUM>) on one axial end side on an inner peripheral surface side thereof,
and wherein the magnet holder (<NUM>) has an undercut portion (<NUM>) that engages with the magnet (<NUM>) in the axial direction on an inner surface of the magnet holding portion (<NUM>) facing the magnet (<NUM>),
and wherein the magnet holding portion (<NUM>) and the shaft attachment portion (<NUM>) are integrally formed of sintered metal,
and wherein, by sizing an outer peripheral surface and an end surface of the magnet holding portion (<NUM>) and an inner peripheral surface of the shaft attachment portion (<NUM>), during which the outer peripheral surfaces of the shaft attachment portion (<NUM>) and the magnet holding portion (<NUM>) are subjected to a compression operation while sliding on a die (<NUM>), whereas the inner peripheral surface of the magnet holding portion is not in contact with the die,
a vacancy rate of the inner peripheral surface of the magnet holding portion (<NUM>) is larger than that of the outer peripheral surfaces of the shaft attachment portion (<NUM>) and of the magnet holding portion (<NUM>), wherein the vacancy rate is represented by an area ratio occupied by the vacancies when a micrograph of a surface is subjected to image analysis,
and wherein the undercut portion (<NUM>) is formed by plastic deformation of the magnet holding portion (<NUM>),
and wherein a tapered portion (43a) and a reverse tapered portion(43b) are continuously provided on the inner peripheral surface of the magnet holding portion (<NUM>) from the one side in the axial direction,
and wherein, the tapered portion (43a) is a portion having a diameter decreasing from the one side to the other side in the axial direction,
and wherein, the reverse tapered portion (43b) is a portion having a diameter increasing from the one side to the other side in the axial direction,
and an undercut portion (<NUM>) is formed by the tapered portion (43a) and the reverse tapered portion (43b).