Electric wire joining method
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric wire joining method for joining two electric wires together or joining an electric wire and a connecting terminal in an electrical wiring circuit, for example in a wiring harness, for industrial machinery and vehicles such as automotive vehicles, ships, and aircraft. 
     2. Description of the Prior Art 
     In the past, a mechanical crimping method has generally been used for joining electric wires together or joining an electric wire and a connecting terminal in a wiring harness. 
     For joining an electric wire 1 and an electric wire 2, for example, as shown in FIGS. 8 and 9, the electric wires 1 and 2 are stripped of their insulative coating portions 1a and 2a at the joint to expose their core wire portions 1b and 2b, respectively (coating stripping process step). The exposed core wire portions 1b and 2b are held together in an embraced manner by a crimp terminal 4 made of sheet metal such as copper or copper alloy and pressurized to be joined together by crimping (crimping process step). The electric wires 1 and 2 arc electrically connected to each other in this manner. 
     Likewise, for joining an electric wire and a connecting terminal, the electric wire is stripped of its insulative coating portion at one end to expose its core wire portion (coating stripping process step). The exposed core wire portion is embraced by a crimp portion of the connecting terminal made of copper or copper alloy. Pressure is applied to the joint to join the crimp portion and the core wire portion by crimping (crimping process step). The electric wire and the connecting terminal are electrically connected to each other in this manner. 
     However, recent pronounced increase in the number of circuits for a wiring harness has added to the complexity of the stripping of the insulative coating portion from the electric wire in the assembling step of the wiring harness, resulting in troublesome operation. In particular, it is troublesome and difficult to strip the insulative coating portion from an intermediate portion of the electric wire. 
     SUMMARY OF THE INVENTION 
     The present invention is intended for an electric wire joining method for crimping a crimp terminal to mechanically and electrically connect a plurality of electric wires to each other with the crimp terminal, each of the electric wires having a core wire portion and an insulative coating portion covering the core wire portion, the crimp terminal having a plurality of invading projections breaking through the insulative coating portions of the electric wires. According to the present invention, the method comprises the steps of: crimping the crimp terminal to connect the crimp terminal to the electric wires such that the invading projections of the crimp terminal break through the insulating coating portions of the electric wires into the core wire portions of the electric wires, respectively; and ultrasonically welding the invading projections and the core wire portions of the electric wires to each other after the crimping step. 
     According to the present invention, the crimp terminal is crimped and connected to the electric wires in such a manner that the invading projections of the crimp terminal break through the insulative coating portions of the electric wires into the core wire portions of the electric wires. Then the invading projections and the core wire portions of the electric wires arc ultrasonically welded to each other to provide diffusion bonding. 
     There is no need to strip the insulative coating portions from the electric wires, thereby to improve workability. The diffusion bonding between the invading projections and the core wire portions provides stable electrical connection. 
     The present invention, according to another aspect, is also intended for an electric wire joining method for crimping a crimp portion formed in a connecting terminal to mechanically and electrically connect the connecting terminal to an electric wire with the crimp portion of the connecting terminal, the electric wire having a core wire portion and an insulative coating portion covering the core wire portion, the crimp portion having an invading projection breaking through the insulative coating portion of the electric wire. According to the second aspect of the present invention, the method comprises the steps of: crimping the crimp portion to connect the crimp portion to the electric wire such that the invading projection breaks through the insulative coating portion of the electric wire into the core wire portion of the electric wire; and ultrasonically welding the invading projection and the core wire portion of the electric wire to each other after the crimping step. 
     According to the second aspect of the present invention, the crimp portion of the connecting terminal is crimped and connected to the electric wire in such a manner that the invading projection of the crimp portion breaks through the insulative coating portion of the electric wire into the core wire portion of the electric wire. Then the invading projection and the core wire portion of the electric wire are ultrasonically welded to each other to provide diffusion bonding. 
     There is no need to strip the insulative coating portion from the electric wire, to improve workability. The diffusion bonding between the invading projection and the core wire portion provides stable electrical connection. 
     An object of the present invention is to provide an electric wire joining method which necessitates no stripping process of an insulative coating from an electric wire and provides stable electrical connection. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a major part of a first preferred embodiment according to the present invention; 
     FIG. 2 is a plan view of a crimp terminal of the first preferred embodiment; 
     FIG. 3 illustrates a setting step of electric wires of the first preferred embodiment; 
     FIG. 4 illustrates the electric wires joined together of the first preferred embodiment; 
     FIG. 5 is a flow chart of the processing of first and second preferred embodiments according to the present invention; 
     FIG. 6 is a plan view of a connecting terminal of the second preferred embodiment; 
     FIG. 7 is a perspective view of the connecting terminal and electric wire joined together of the second preferred embodiment; 
     FIG. 8 illustrates a process step of electric wires of the prior art; and 
     FIG. 9 is a plan view of the electric wires joined together of the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 to 5, a first preferred embodiment according to the present invention will now be described. FIG. 2 is a plan view of a crimp terminal 10 made of fiat sheet metal such as copper or copper alloy. With reference to FIG. 2, the crimp terminal 10 includes pairs of crimp pieces 10a protruding outwardly, i.e. leftwardly and rightwardly, at its upper and lower ends. Two types of, long and short, spike-like invading projections 10b and 10c suitably spaced in the vertical direction and projecting leftwardly and rightwardly are alternately formed between the upper and lower opposite crimp pieces 10a. Each pair of laterally aligned invading projections 10b and 10c are adapted so that one invading projection of each pair is long and the other invading projection is short. 
     Electric wires 11 and 12 are set such that portions of the electric wires 11 and 12 to be joined are placed on the crimp terminal 10, as shown in FIG. 3 (setting step S1 shown in FIG. 5). The electric wires 11 and 12 are not stripped of their insulative coating portions 11a and 12a when placed. 
     In a crimping step S2 of FIG. 5, the crimp pieces 10along invading projections 10b and short invading projections 10c of the crimp terminal 10 arc bent by the conventional mechanical crimping process. The crimp pieces 10a arc crimped to hold the electric wires 11 and 12 together in an embracing manner as shown in FIG. 4. Each of the long invading projections 10b breaks through one of the insulative coating portions 11a and 12a of the electric wires 11 and 12 which is the farther therefrom into a corresponding one of the core wire portions 11b and 12b of the electric wires 11 and 12 to contact the corresponding core wire portion. Each of the short invading projections 10c breaks through one of the insulative coating portions 11a and 12a of the electric wires 11 and 12 which is the closer thereto into a corresponding one of the core wire portions 11b and 12b of the electric wires 11 and 12 to contact the corresponding core wire portion. This allows electric conduction between the electric wires 11 and 12 through the crimp terminal 10. 
     In an ultrasonic welding step of FIG. 5, the long and short invading projections 10b, 10c and the core wire portions 11b, 12b of the electric wires 11, 12 are ultrasonically welded to each other in the conducting state. The ultrasonic welding provides diffusion bonding between the long and short invading projections 10b, 10c and the core wire portions 11b, 12b. 
     Junction of the electric wires 11 and 12 provided in this manner according to the first preferred embodiment necessitates no troublesome process of stripping the insulative coating portions 11a and 12a from the electric wires 11 and 12 to improve workability. The diffusion bonding between the invading projections 10b, 10c and the core wire portions 11b, 12b provides stable electrical connection with low contact resistance. 
     Referring to FIGS. 5 to 7, a second preferred embodiment according to the present invention will be described hereinafter. FIG. 6 is a plan view of a connecting terminal 15 made of flat sheet metal such as copper or copper alloy. The connecting terminal 15 includes a connecting portion 15a formed in its upper part and a crimp portion 15b formed in its lower part. Pairs of crimp pieces 15c protruding outwardly, i.e. leftwardly and rightwardly, are formed at upper and lower ends of the crimp portion 15b. Spike-like invading projections 15d projecting rightwardly and leftwardly arc formed in vertically spaced relation between the upper and lower opposite crimp pieces 15c. 
     An electric wire 16 is set such that one end of the electric wire 16 to be joined is placed on the crimp portion 15b of the connecting terminal 15 as shown in FIG. 7 (the setting step S1 of FIG. 5). The electric wire 16 is not stripped of its insulative coating portion 16a when placed. 
     In the crimping step S2 of FIG. 5, the crimp pieces 15c and invading projections 15d of the crimp portion 15b are bent by the conventional mechanical crimping process. The crimp pieces 15c are crimped to hold the electric wire 16 in an embracing manner. The invading projections 15d break the insulative coating portion 16a of the electric wire 16 into a core wire portion 16b of the electric wire 16 to contact the core wire portion 16b. This allows electric conduction between the electric wire 16 and the connecting terminal 15 through the crimp portion 15b. 
     In the ultrasonic welding step S3 of FIG. 5, the invading projections 15d and the core wire portion 16b of the electric wire 16 are ultrasonically welded to each other in the conducting state to provide diffusion bonding between the invading projections 15d and the core wire portion 16b. 
     Junction of the electric wire 16 and the connecting terminal 15 provided in this manner according to the second preferred embodiment necessitates no troublesome process of stripping the insulative coating portion 16a from the electric wire 16 to improve workability. The diffusion bonding between the invading projections 15d and the core wire portion 16b provides stable electrical connection with low contact resistance. 
     It should be noted that the configuration of the crimp terminal 10 and connecting terminal 15 is not limited to that of the first and second preferred embodiments but the crimp terminal 10 and connecting terminal 15 may be made in an appropriate configuration in accordance with the electric wires 11, 12 and 16 joined thereto. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.