Method for connecting film-coated wire to subject member

A film-coated wire is wound around a terminal at least one turn and pressed by an upper electrode of a resistance welding machine. A current is passed through the upper electrode when the wire is pressed, and heat is produced. The heat melts a film on the wire. The current is passed between the upper electrode and the lower electrode via the wire and the terminal to connect the wire with the terminal by resistance welding. A contact area between the wire and the terminal is increased by winding the wire around the terminal. Therefore, the wire is less likely to shift its position when a force is applied by the upper electrode. This increases reliability of the connection. Moreover, the terminal is reduced in size because it is not required to be formed in a U-shape.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-141062 filed on May 16, 2002.

FIELD OF THE INVENTION

The present invention relates to a method for connecting a film-coated electric wire to a subject member by resistance welding.

BACKGROUND OF THE INVENTION

A method for connecting a film-coated electric wire to a subject member is disclosed in JP-A-5-38583. A terminal is provided by shaping a conductive plate made of copper or copper alloy into a U-shape. A film-coated electric wire is inserted in a curved portion of the U-shaped conductive member without removing the film. The curved portion is held between upper and lower electrodes. A force is applied and a current is passed between the electrodes. Heat is produced by a resistance between the electrodes and it removes the film on the wire. As a result, a core wire of the wire is exposed. The core wire is connected to the curved portion of the terminal by resistance welding.

Since the terminal is formed into the U-shape in this method, the size of the terminal cannot be reduced in size and weight. To reduce the size of the terminal, another method is disclosed in JP-A-2001-275224. In this method, a terminal is not formed into a U-shape. A film-coated wire is placed on the terminal. The terminal and the wire are held between electrodes. The electrodes produce heat that melts and removes the film on the wire. A current is passed through the core wire and the terminal to connect the core wire and the terminal together by resistance welding. With this method, the terminal and the wire are connected together without shaping the terminal into a U-shape. Therefore, the terminal can be reduced in size.

To heat and apply a force to the film-coated wire via the electrodes, the film-coated wire should be accurately arranged at a predetermined position on the terminal. If not, a contacting area between the electrodes and the wire may vary each time they are brought into contact. As a result, reliability of the connection between the terminal and the wire is reduced.

SUMMARY OF THE INVENTION

The present invention therefore has an objective to provide a method for connecting a film-coated wire to a subject member without increasing a size of the subject member and reducing reliability of the connection between the wire and the subject member. The method includes a winding step, a force applying step, a first current passing step, a film removing step, a second current passing step, and a connecting step.

In the winding step, the film-coated wire is wound around the subject member at least one turn. In the force applying step, a force is applied to the subject member via at least one of first and second electrodes of a resistance welding machine. In the first current passing step, a current is passed between the first and the second electrodes to produce heat. In the film removing step, a film on the film-coated wire is removed by the heat. In the second current passing step, a current is passed between the first and the second electrodes via the film-coated wire and the subject member. In the connecting step, the film-coated wire is connected to the subject member by welding.

With this method, a large contact area is provided between the film-coated wire and the subject member because the film-coated wire is wound around the subject member at least one turn. Therefore, a friction resistance between the film-coated wire and the subject member increases. This reduces a contact area variation that may occur during the application of force via the electrode. As a result, the reliability of the connection between the wire and the subject member is ensured. Furthermore, the subject member can be reduced in size since it is not required to be formed in a U-shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention will be explained with reference to the accompanying drawings. In the drawings, the same numerals are used for the same components and devices.

Referring toFIG. 1, a resistance welding machine includes a terminal1, a film-coated wire2, an upper electrode3, and a lower electrode4. The film-coated wire2is wound around the terminal1. The upper electrode3and the lower electrode4, both are made of tungsten, face each other on opposed sides of the terminal1. The lower electrode4has contact surfaces5on which the terminal1is arranged. A force is applied to the upper electrode3in the direction toward the terminal1so that a pressing surface of the upper electrode3is pressed against the film-coated wire2. The force is generated by a force application device6that has an air cylinder.

Wires are routed from a power source7to both ends of the upper electrode3. When a current is passed between the ends of the upper electrode3, heat is produced. A relay10is connected in one of paths between the upper electrode3and the power source7. Power supply to the upper electrode3is controlled by the relay10. The lower electrode4has a dent portion11between the contact surfaces5for housing the film-coated wire2. Therefore, the film-coated wire2is not placed between the terminal1and the contact surfaces5when they are brought into contact.

The lower electrode4is also connected to the power source7via connecting wires12. A current detecting coil9is connected in a path between the lower electrode4and the power source7. The current detecting coil9is used to detect a current starting to flow through the upper electrode3and the lower electrode4via the terminal1and the film-coated wire2. This occurs when the film coated around the film-coated wire2is removed by the heat produced by the upper electrode3.

A signal detected by the current detecting coil9is inputted to a control device8. When the control device8determines that a current has started to flow through the lower electrode4based on the detected signal, it turns off the relay10to stop the current flow through the upper electrode3.

The terminal1that is made of electric conductive material, such as copper alloy and copper, has a circular or a square cross section. A diameter of the circular cross section or a crossline of the square cross section is approximately 0.3 mm–5 mm. The film-coated wire2is constructed of an electric wire (core wire) and a film made of an insulating material, such as polyimide, polyamide-imide, polyamide-polyester-imide, polyester, polyurethane, enamel, and vinyl. The wire2is coated with the film and wound around the terminal1at least one turn, preferably multiple turns (two to five turns) before resistance welding. The multiple turns are made in a spiral at intervals so that the removed film does not touch the adjacent wire.

In the film removing step, the upper electrode3is pressed against the wire2with a force sufficient for the film removal. When the relay10is turned on and a current is passed through the upper electrode3, heat is produced. The heat melts the film on the wire2. The melted film is removed by the pressing force applied by the upper electrode3, and the core wire of the wire2is exposed.

After the core wire is exposed, the welding step is performed. More specifically, a current starts flowing from the upper electrode3to the lower electrode4via the core wire and the terminal1after the core wire is exposed. The current flowing through the lower electrode4is detected by the current detecting coil9. The control device8turns the relay10off based on the detected signal. The current only flows between the upper electrode3and the lower electrode4via the core wire and the terminal1after the relay10is turned off. Because of this current flow, the core wire is connected to the terminal1by the resistance welding.

In the film removing step, the pressing force and the current are adjusted to acquire a connecting strength. The adjustment is made according to the kind of the material used for the film or the thickness of the film. In the welding step, the pressing power and the current are controlled according to the thickness and the kind of the material of the core wire, or the kind of the material of the terminal1.

In the case that the wire2is placed on the terminal1, the contact area between the wire2and the terminal1is small. Thus, the wire2easily shifts its position when the force is applied. This causes a contact area variation between the upper electrode3and the wire2, which result in an unreliable connection.

In this embodiment, the wire2is wound around the terminal1multiple turns, which increases the contact area between the wire2and the terminal1. The friction resistance between the wire2and the terminal1increases and the position shifting of the wire2is less likely to happen. Therefore, the wire2can be connected to the terminal1by resistance welding with the connecting strength.

Furthermore, the dent portion11is provided in this embodiment to house the part of the wire2adjacent to the lower electrode4. If the terminal1and the wire2are pressed by the upper electrode3, the current cannot flow between the upper electrode3and the lower electrode4due to the film. By housing the wire2in the dent portion11, a current path between the upper electrode3and the lower electrode4is provided. The contact surfaces5directly contact with the terminal1. Therefore, the current can flow between the upper electrode3and the lower electrode4when the film is removed by the upper electrode3.

Referring toFIG. 2, a resistance welding machine has a pressure bearing jig13and an electrode14. The jig13is provided to stably support a terminal1around which a film-coated wire2is wound. The jig13has a dent portion for housing the film-coated wire2and supports the terminal1on contact surfaces.

The electrode14is arranged on the terminal1in a portion that the film-coated wire2is not wound around and horizontally in line with an upper electrode3. With this configuration, the pressing force is not applied to the electrode14when the upper electrode3is pressed against the terminal1. The electrode14is used in pair with the upper electrode3. Furthermore, the electrode14can be arranged any positions as long as it has contact with the terminal1. Therefore, the flexibility of the arrangement of the electrode14increases.

Referring toFIG. 3, a resistance welding machine has a lower electrode24and a film melting function. The upper electrode3and the lower electrode24are connected in series with the power source7when the relay10is turned on. When the upper electrode3and the lower electrode24are electrically connected in series, they produce heat and the heat melts the film on the film-coated wire2. The relay10is connected in a path between the upper electrode3and the lower electrode24.

When the film on the wire2is removed, a current flows between the electrodes3and24via the core wire of the wire2and the terminal1. Therefore, an amount of the current flowing through the wire that connects the upper electrode3with the lower electrode24decreases.

The current detecting coil9is connected in a path between the upper electrode3and the lower electrode24to detect a decrease in current due to the film removal. When the decrease is detected, the control device8turns off the relay10. As a result, the current path between the upper electrode3and the lower electrode24is formed having only the terminal1and the film-coated wire2in between. With this configuration, the core wire of the film-coated wire2is connected to the terminal1by resistance welding.

The present invention should not be limited to the embodiment previously discussed and shown in the figures, but may be implemented in various ways without departing from the spirit of the invention.