Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF Tl-IE INVENTION
The present invention relates to a method for
producing a magnet plate for electroacoustic transducers and
more particularly relates to operational improvement in the
method for producing a magnet plate for electroacoustic
transducers through compression of magnet powders between a
pair of pressure surfaces.
In one type of electroacoustic transducer, a pair
of mutually perforated magnet plates are used in combination
with a diaphragm placed between the two and the perforations
are adapted for allowing passage of air therethrough when
the latter is driven by vibration of the diaphragm. ._
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The magnet plate of the above-described type is
conventionally produced by compressing magnet powders between
a pair of cooperating pressure surfaces to the relatively
thin thickness of the magnet plate to be finally obtained.
Concurrently-with this compression, numerous perforations
are formed in the thickness direction through the compressed
plate configuration. After this compression and peTforating,
the plate configuration is subjected to sintering.
The above-described producing system is inevitably-
accompanied by two serious drawbacks. In the first place,
- the magnet powders are abruptly compressed to the final
thickness at once and this abrupt compression tends to cause
uneven compaction of the powders leading to the uneven dis-
tribution of the density. Such uneven density distribution
naturally degrades the electromagnetic characteristics of
the magnet plate obtained. In addition, it may cause unbalanced
sintering effect over the entire configuration. Finally~
the perforated configuration of the plate is likely to cause
undesirable destruction of the plate during the sintering
process.
It is the principal object of the present invention
` to provide a method for producing a magnet plate for electroacoustic
transducers having uniform and improved electromagnetic
characteristics.
It is another object of the present invention to
provide a method for producing a magnet plate for electro-
acoustic transducers with reduced destruction of the plate
conflguration during the sintering.
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In accordance with the present invention, con-
currently with formation of perforations, the magnet powders
are compressed to form a plate configuration of a thickness
by far larger than the final thickness of the end product.
After the sintering, one side surface of the plate configur-
ation is removed, i.e. by mechanical cutting and grinding, to
the thickness of the end product. In a preferred embodiment,
the perforations open in one side surface of the plate con-
figuration only after the compression and in both side sur-
faces after the surface removal.
Further features and advantages of the presentinvention will be made clearer from the following descrip-
tion, reference being made to the embodiment shown in the
accompanying drawings, in which;
Figure 1 is a sectional side plan view of one
embodiment of the arrangement for carrying out the present
invention.
Figure 2 is a sectional side plan view of an ex-
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ample of the plate configuration in accordance with the
present invention.
Figure 3a is a sectional side plan view of anexample of the magnet plate produced in accordance with the
present invention.
Figure 3b is a top plan view of the magnet plate
shown in Figure 3a.
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Figure 4 is a sectional side plan view of an-
other example of the magnet plate produced in accordance
with. the present invention~ and
Figure 5 is a perspecti.ve plan ~iew of the oth.er
example of the magnet plate produced in accordance with the
present ;nvention.
A typical arrangement for produci`ng a magnet
. plate in accordance with.the present invention is shown in
Figure 1, in which a stationary die head 1 and a movable
punching h.ead 2 are encased w~thin a compressive cylinder 3
in a face-to-face relationship in such.an arrangement that
the punching head 2 is movable towards and away from the die
head 1. On the operating surface 21 fac;`ng the die head 1,
the punching head 2 is provided with.a plurality of pro-
jeCtions 22 wh.ich converge towards the free ends.
- . Generally, the projections 22 are either pyramid
or conical shaped. They may ~e given i~ the form somewhat
modified from these basic shapes. Further, they may be given
in the form of round or cub~c columns.
- The space between the two heads 1 and 2 is fi.lled
. with magnet powders 4 and compression is carried out ~y mov-
. ing the punching head 2 towards the die head 1. ~ompressing
the magnet powders gradually into a plate configuration,
movement of the punching head 2 is continued until the dis-
tance hetween the punching h.ead 2 and the die head 1 comes
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to a preset value. This preset distance is equal to th.e
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thickness Tp of the plate configuration 5a obtained and shown
in Figure 2. As later descri~ed this thickness Tp of the
plate configuration 50 is by far larger than the thickness Tf
of the end product~ i.e. the magnet plate lOa shown in
Figures 3a and 3~.
Thus, the thickness Tp o~ the plate configur-
ation 50 will hereinafter be referred to as "the provisional
- thickness" whereas the thickness Tf of the magnet plate 100
will hereinafter be referred to as "the final thickness".
In the case of the illustrated embodiment, the
` plate configuration 50. is provided with a plurality of
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perforations 51 opening in one side surface of the configura-
tion 50 only. This is because the height of the projections 22
provided on the punc~ing head 2 is designed smaller than the
above-described provisional thickness Tp in the case of the
present embodiment. When the height of the projections 22 is
equal to the provisional thickness Tp, the peforations 51
may open in both side surfaces of the plate configuration 50.
However, by designing the projections' height shorter than
the provisional thickness Tp, one can successfully avoid
damaging the punching heads 2, especially the projections
22, through collision against the die head 1 during repeated
use.
As mentioned previously, there is no special
limitation to the shape o-f the projections 22 provided on
the punching head 2. However, in the case of the projections
of round or cubic column shape, the stress applied to the
magnetic powder mass by the head 2 via the projections 22
works on portions of the mass positioned just under the pro-
jections 22 only. ~see Fig. 1). Thus, the stress does not
2Q work effectively on portions of the magnet powder mass
positioned between the projections 22. This naturally leads
to uneveness in the compaction, i.e. uneven distribution of
density of the obtained plate configuration 50. In contrast
to this, the projections 22 of the illustrated embodiment
converge towards their free ends. Thus, the stress should
have a component working in the direction perpendicular to the
outer surfaces of the projections 22. This means that the
stress partly acts on the portions of the magnet powder mass
positioned between the projections 22. Thus, the variation
in position of the density of the obtained plate configuration
50 is considerably mitigated, resulting in uniform electro-
magnetic characteristics of the end product.
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After application of sintering, a portion of the
thickness of the sur-face of the plate con-figuration 50 is
removed mechanically, for example, by mechanica] cutting and
grinding, so that the reaminder has the final thickness Tf.
In the case of the illustrated embodiment, this surface removal
is applied to the plate configuration 50 from the unrecessed
side to the line L in Figure 2, thereby the perforations 51
being rendered to open in both side surfaces of the end product.
Thus, the obtained magnet plate 100 shown in Figures
3a and 3b is provided with a number of perforations 101
opening in both side surfaces and having the final thickness Tf.
The distribution and dimension of the perforations
101 over the magnet plate 100 can be designed quite freely
in accordance with requirement in the use of the magnet
plate 100. The thickness of the magnet plate 100 is selected
in a similar sense.
In one practical example of the present invention,
the provisional thickness Tp of the plate configuration 50
after the compaction is in the range of 5.7 to 5.8 mm., the
~ 20 thickness was reduced to about 5.0 mm. after the sintering
- and the removal is carried out to the final thickness Tf of
about 2.5 mm. Compression is carried out at a pressure of
60 tons per the entire surface area of the plate configura-
tion, i.e. the entire internal surface area of the compression
cylinder 3 shown in Figure 1. The magnet plate of 46 mm.
diameter is provided with 4~ perforations of frustum shape
distributed in an approximate square matrix arrangement. The
` width at the larger side opening of the perforations is 2.7 mm.
~; and, that at the smaller side opening is 1.7 mm. and the
distance between neighboring perforations is 5 mm.
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In another practical example of the present in-
vention, the magnet plate of 55 mm. diameter is provided with
44 perforations of a straight round column shape distributed
in an approximate square matrix arrangement. The diameter
of the perforations is about 3 mm. and the distance betweèn
perforations is about 6 mm.
In the case of the embodiment shown in Figure 4,
the magnet plate lOQ is provided with perforations 102 of a
pseudo-conical or pseudo-frustum shape whereas the magnet
lU plate 10~ shown in Figure 5 is provi`ded with perforations
of a frustum shape.
In accordance with the present invention, the
intermediate product, i.e. the plate configuration, having a
provisional thickness larger than the final thickness is
su~jected to the sinter;ng and this relatively large pro-
visional thickness effectiYely prevents destruction of the
I plate configuration during the sintering. Further, the
relatively thick construction of the plate configuration is
accompanied with enhanced uniformity in the density thereof,
2Q assuring improved and uniform electromagnetic characteristics
of the end prbduct.
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