Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1998-06-12
2001-04-17
Mills, Gregory (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S056220, C219S117100
Reexamination Certificate
active
06218638
ABSTRACT:
The present invention relates to a method for inspecting the quality of a resistance weld formed, for example, when a strand composed of twisted copper wires of one electric cable and a strand composed of twisted copper wires of the other electric cable are put on one another and pressed between a pair of electrodes, and a large current is passed through the strands for a short time to utilize resistance heating caused by the current conduction to thereby perform resistance welding.
BACKGROUND OF THE INVENTION
A conventional resistance welding apparatus for welding the strands composed of twisted copper wires of a pair of electric cables, for example, comprises an apparatus using an AC thyristor system (a system in which thyristors are used as electric source switches for performing welding current regulation continuously by changing the firing phase of the thyristors) as shown in FIG.
8
(
a
). The resistance welding apparatus
1
is designed to perform resistance welding on a stack of strands
31
and
31
′ composed of twisted copper wires of a pair of electrically insulating coated electric cables
30
and
30
′ (hereinafter simply referred to as “cables”) as illustrated in FIG.
9
. The resistance welding apparatus
1
has a box-like apparatus body
2
which is substantially U-shaped in side view. A cable-setting jig
3
is disposed in the center of the apparatus body
2
. An air cylinder
4
is attached to the upper front of the apparatus body
2
. A pair of upper and lower electrodes
5
A and
5
B pass a welding current therebetween and are provided below the air cylinder
4
, and the cable-setting jig
3
respectively. These thus serves to pass the current through a welding portion of the strands
31
and
31
′ and also serve to apply a predetermined amount of pressure to the welding portion.
The upper electrode
5
A is connected to a piston rod
4
a
of the air cylinder
4
through an electrode holder
6
so as to move vertically. Further, the upper electrode
5
A is also connected, through an ounce copper plate
8
, to a welding transformer (electric source)
7
, which serves to supply a welding current. Further, the lower electrode
5
B is fixed to the center portion of the apparatus body
2
and is connected to the welding transformer
7
. As shown in FIG.
8
(
b
), the welding transformer
7
is connected to a welding timer
9
, which serves to set the current value and current-conduction time of the welding current. An electromagnetic valve
4
A of the air cylinder
4
is opened/closed on the basis of conduction-start and conduction-end signals obtained from the welding timer
9
. As shown in
FIG. 9
, each of the electrodes
5
A and
5
B is constituted by a columnar chromium-copper matter
5
a
and a rectangular tungsten tip
5
b.
The step of performing resistance welding of the overlapping strands
31
and
31
′ of the pair of cables
30
and
30
′, by means of the AC thyristor system resistance welding apparatus
1
as shown in
FIG. 9
, will be described with reference to a flow chart shown in FIG.
10
. First, after the exposed strands
31
and
31
′ of the pair of cables
30
and
30
′ are put in between the pair of electrodes
5
A and
5
B through the cable-setting jig
3
, a start input switch
9
A is turned on so that the welding timer
9
starts (step S
1
). As a result, the electromagnetic valve
4
A, that is connected to a (not-shown) compression air source, is opened and the upper electrode
5
A is moved down by the air cylinder
4
. After completion of initial pressure application to the strands
31
and
31
′ between the pair of electrodes
5
A and
5
B (step S
2
), a welding current is passed between the pair of electrodes
5
A and
5
B alternately upward and downward by the welding transformer
7
(step S
3
). The welding current is passed for the current-conduction time which is set (fixed) in advance. Resistance heating caused by the conduction of the welding current is utilized so that the strands
31
and
31
′ are subjected to thermo-compression bonding. After resistance welding, the current conduction is stopped (step S
4
). Then, cooling is performed while the pressure application state between the pair of electrodes
5
A and
5
B is held for a predetermined time (step S
5
). The operation for the steps S
2
to S
5
is carried out automatically under the sequence control of the welding timer
9
. Then, when the pressurized state is canceled, the resistance welding is completed (step S
6
). Such technique is disclosed in Japanese Patent Unexamined Publication Nos. Hei-1-278973 and Hei-5-329661.
The inter-tip resistance (calculated on the basis of an inter-tip voltage value and a current value under current conduction) between the pair of electrodes
5
A and
5
B, which shows a correlation with the adhesive force, is used in a non-destructive inspection method for inspecting the characteristics of the weld, such as the welding strength (inter-wire adhesive force), or the like, that bond the twisted copper wires in strands
31
and
31
′. As shown in
FIG. 11
, in the case where the twisted copper wire in strands
31
and
31
′ are welded, these are two noteable effects. First, the inter-tip resistance value in an initial stage of current conduction (up to about 2 cycles in the case of an AC system) is high compared with that in copper plates, or the like, because a space is generated between the strands
31
and
31
′. Second, variations occur in contact resistance because the state of the alignment of the strands
31
and
31
′ is not uniform. Accordingly, when the inter-tip resistance is measured during the entire period of current conduction and averaged to evaluate the state of the weld, the inter-tip resistance is greatly affected by the contact resistance between the strands
31
and
31
′ in the initial stage of current conduction. As a result, the state of heating (increase of specific resistance) of the pair of electrodes
5
A and
5
B made from tungsten, or the like, cannot be determined accurately, so that the welding strength (adhesive force) cannot be predicted.
Therefore, the present invention is designed to solve the above problem. Accordingly, it is an object of the present invention to provide a method for inspecting the quality of a resistance weld, in which the adhesive force between the twisted wires of the strands as materials to be welded can be predicted on the basis of the inter-tip resistance between electrodes so that the welding quality can be evaluated accurately.
SUMMARY OF THE INVENTION
The present invention concerns a method for inspecting the quality of a resistance weld formed by passing a welding current between a pair of electrodes and through a welding portion of a material to be welded and applying pressure between the electrodes to the welding portion of twisted wires to be joined for thermo-compression bonding of the welding portion to thereby perform resistance welding of the welding portion under pressure applied between the pair of electrodes.
In a first embodiment, the present method comprises the steps of: measuring a welding width of the welding portion; calculating a reference welding height of the welding portion from the measured welding width on the basis of a predetermined reference welding sectional area of the welding portion; passing the welding current between the electrodes and through the welding portion until the height of the welding portion reaches the calculated reference welding height; and inspecting the state of the weld joining the twisted wires on the basis of substantially lowest values of inter-tip resistance of the pair of electrodes in an initial stage of conduction of the welding current.
In the first embodiment of the present method for inspecting the quality of resistance welding, the bottom value of inter-tip resistance in the initial stage of the conduction of a welding current is determined on the basis of the state of the twisted wires and the welding current value. Accordingly, the
Mills Gregory
Morgan & Lewis & Bockius, LLP
Yazaki -Corporation
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