Dentistry – Prosthodontics – Holding or positioning denture in mouth
Reexamination Certificate
1999-10-27
2001-10-09
Wilson, John J. (Department: 3732)
Dentistry
Prosthodontics
Holding or positioning denture in mouth
C335S302000
Reexamination Certificate
active
06299450
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin-type dental magnetic attachment which is embedded in a pit of a denture to hold the denture on teeth roots in the oral cavity using magnetic attractive force.
2. Description of the Related Art
Presently, a dental magnetic attachment which strongly holds and stabilizes the denture on teeth roots by magnetic attractive force is well known. As shown in
FIG. 8
, its force acts between a dental magnetic attachment
9
in a denture
80
and a keeper
10
made from a soft magnetic material embedded in a root cap
85
inserted into a tooth root
86
. The denture
80
has artificial teeth
84
, a resin base
83
and a dental magnetic attachment
9
. Japanese Patent application Laid open (Kokai) No. 4-227253 discloses a round shaped dental magnetic attachment shown in FIG.
9
.
This dental magnetic attachment
9
is composed from a cylindrical shaped magnet
91
, a cylindrical shaped yoke
92
having a pit
99
for holding the magnet, and a sealing plate
98
for covering the bottom of the magnet. Here the sealing plate consists of a sealing disk
96
and a sealing ring
97
. The yoke, the sealing disk and the keeper are made from soft magnetic materials. The magnet is magnetized to the normal direction to the magnetic attached plane between the dental magnetic attachment and the keeper. When the dental magnetic attachment is set on the keeper, a closed magnetic circuit composed from the magnet, the yoke and keeper is formed. The magnetic flux flows from the top of the magnet, through the yoke, the keeper and the sealing disk back to the bottom of the magnet. The sealing ring made from nonmagnetic material is placed between the yoke and the sealing disk to prevent occurrence of a magnetic short circuit. All of the parts except the magnet are made from corrosion resistant materials.
The sealing disk
96
and the sealing ring
97
are joined to the yoke by laser welding. The laser welding lines are two boundaries between the yoke and the outer side of the sealing ring and between the sealing disk and the inner side of the sealing ring. That means that laser welding is done two times.
However the above invention has some serious disadvantages concerning the sealing method. In the case that the sealing ring has a comparatively large width, the laser welding can be done easily, but the area of the disk becomes so small that the magnetic attractive force decreases. On the other hand, in the case that the sealing ring is narrow, it is very difficult to laser-weld the sealing ring to both the yoke and the sealing disk. Large strain or stress produced by the heat in welding is apt to make some gap or step among the sealing ring, the yoke and the sealing disk. Sometimes cracks occur in the welded parts.
To solve the above disadvantages, another sealing method as shown in
FIG. 10
is proposed in the proceedings of the annual meeting of the Japanese Metal Society, at p408 (1996). This dental magnetic attachment is produced by a laser-welding technique of a single application instead of the above laser-welding technique of two times. Its small welded part works as a nonmagnetic sealing ring to contribute an increase in the magnetic attractive force.
In this method, a sealing disk
106
is plated or coated by Ni. It means that Ni coat
105
which is a soft magnetic material is substituted for the nonmagnetic sealing ring. The Ni coated disk is placed on the magnet
101
inserted into the pit
108
of the yoke
102
and then it is laser-welded to the yoke. At this time, Ni coat is melt together with the yoke
102
and the sealing disk
106
made from soft magnetic stainless steel and the nonmagnetic welded part is formed. However this method has some inevitable defects.
It is required that the depth of the welded part is controlled to be just equal to the thickness of the sealing disk. If the depth is not enough as shown by a dotted line (F) in FIG.
10
(B) and Ni coat remains, a magnetic short circuit is formed because Ni metal is of soft magnetic materials. Consequently, the magnetic attractive force decreases remarkably. If the depth is too large as shown by a dotted line (E) shown in FIG.
10
(B), the heat of laser-welding process damages the magnet so as to cause a decrease of the magnetic attractive force. When a magnet with a small diameter is applied, the magnet is free from this damage, but it is natural that the magnetic attractive force becomes small.
Moreover, the welded part is apt to become an imperfect austenite phase to contain a small volume of ferrite phase. This means that it is difficult to make the welded part nonmagnetic. The reason of the occurrence of the ferrite phase is considered with the Schaeffler diagram which gives the phase constitution after welding stainless steels. A soft magnetic stainless steel of SUS447J1 type with 30% Cr, 2% Mo and residual of iron is used as the yoke material in the above paper. The welded part shows a chemical composition having more than 15% Ni but it has a large part of ferrite phase. This results in a weak magnetic short circuit between the yoke
102
and the sealing disk
106
to make an undesirable decrease in the magnetic attractive force.
If another soft magnetic stainless steel of SUS430 type having 17% Cr and residual of iron is used instead of SUS447J1 type, nonmagnetic welded part would be obtained to give high magnetic attractive force. But the welded part has less than 17% Cr to become very corrosive in the oral cavity.
It is noted that the thickness of the sealing disk is so thin around 0.2 mm, that a laser-welding process with the yoke and the disk is very difficult normally, and yet in the case of the Ni coated sealing disk the laser-welding process is more difficult. Consequently, this dental magnetic attachment has been not yet produced.
SUMMARY OF THE INVENTION
The present invention intends to solve the difficulty of the laser-welding process and offer a dental magnetic attachment with a round shape, which has a strong magnetic attractive force.
A dental magnetic attachment of the present invention is characterized by nonmagnetic welded part between the yoke and the sealing disk.
The basic structure of the present magnetic assembly is similar to ones disclosed by Japanese Patent application Laid open (Kokai) No. 4-227253, but the nonmagnetic welded part is very different. The dental magnetic attachment is composed from a cylindrical shaped magnet, a cylindrical shaped yoke having a pit for holding the magnet, and a sealing plate for covering the bottom of the magnet which plate consists of a sealing disk and a sealing ring. Here the inner surface of the sealing ring is plated or coated by Ni. The yoke, the sealing disk and the keeper are made from soft magnetic material. The sealing ring is made from nonmagnetic stainless steel. The sealing disk and the sealing ring are joined to the yoke by laser welding with only one welding line. The welded part becomes nonmagnetic and keeps the space between the yoke and the sealing disk nonmagnetic to make a magnetic isolation barrier. After being laser-welded, the welded surface is polished to make flat.
The first point of the present invention is that the welded part of the dental magnetic attachment becomes nonmagnetic by melting the sealing ring coated by Ni together with the yoke and the sealing disk. The second is that there is only one welded line. The third is that the corner of the magnet is polished round. And the fourth is that the welded surface is polished to be made flat and the sealing disk is made thinner.
The effects of the present invention are as follows.
The present invention can make the space between the yoke and the sealing disk nonmagnetic certainly by means of the laser welding technique to form a nonmagnetic welded part and can improve the magnetic attractive force. The welded part is formed by melting four parts which are the yoke, the sealing ring, Ni coat and the sealing disk. But the chemical composition of the welded part is near to that of nonmagnetic stainless
Arai Kazuo
Honkura Yoshinobu
Kimura Kazunari
Tian Lei
Aichi Steel Corporation
Jacobson & Holman PLLC
Wilson John J.
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