Abstract:
An apparatus and method are provided to improve a cutting reliability, to facilitate automation of the connection of wires and to allow for a reliable detection of wire joints. Ends of wires (a, a′) to be connected along their longitudinal directions are linearly placed, and a tape (t) is wound over and around the both wire ends. An adhesive layer of the tape is faced up, and the tape (t) is wound as the wire ends are rolled, thereby connecting the wires (a, a′). Since the tape displays a strong tear resistance in its longitudinal direction, the wires are strongly connected. The tape may contain reinforced fibers to display a stronger tear resistance. The wires (a, a′) are rolled by being held between plates ( 11, 33 ) and moving the plate ( 33 ) with respect to the plate ( 11 ). The tape (t) is placed on an auxiliary base plate ( 12 ) and is wound by the rolling movement of the wires (a, a′). The plate ( 33 ) is moved back after winding of the tape (t) to untwist the wires (a, a′).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for connecting a wire from an existing wire supply to a wire from a new wire supply during manufacturing of a wiring harness. The invention also relates to a method and apparatus for detecting a wire joint. 
     2. Description of the Related Art 
     A prior art wiring harness requires a plurality of wires of a specified lengths. The wires are drawn from a plurality of wire supplies to a cutting/mounting apparatus where the respective wires are cut to the specified lengths. Terminals then are crimped or otherwise connected to the cut ends of the wires. It is often necessary to change the wire that is being processed. To make this change, the wire drawn from a first wire supply is cut. A wire from a second wire supply then is connected with the cut end to enable a continued production of the wiring harness. 
     Several wire connection methods have been known and/or used in the prior art. One prior art wire connection method inserts the ends of the wires into a sleeve and compresses the sleeve to connect the wire ends. However, the compression force or the crimping force of the sleeve sometimes is insufficient. Accordingly the reliability of this connection method is low. Additionally, the sleeve is expensive. 
     A second prior art wire connection method welds the wire ends together, as disclosed in Japanese Unexamined Patent Publication No. 8-138824. However, this prior art method is expensive, and the finishing of the joint portion is unstable. More particularly, burrs and/or flash may be formed. Flash, in particular, may disadvantageously clog the nozzle through which the wire is fed. 
     A third prior art wire connection method successively feeds crimping pieces by a strip, positions the wire ends to be connected on the crimping piece along a straight line, crimps the crimping piece into a tubular shape, and cuts the crimping piece from a carrier piece, thereby connecting the wire ends by crimping. This prior art method requires the difficult step of positioning and abutting the ends of the wires against each other. Additionally burrs are likely to be formed if the wire ends overlap, and the burrs are likely to catch the nozzle. 
     A fourth prior art wire connection method winds an aluminum tape over and around the both wire ends to connect the wires. However the aluminum tape is expensive, and typically must be wound by hand, thereby causing poor operability and low connection reliability. 
     A fifth prior art wire connection method strips sheaths of the wire ends to expose strands and twists or braids the exposed strands to connect the wire ends. The twisting or braiding of the strands at both wire ends can be difficult to achieve. This operation is difficult to automate, and hence generally is done by hand. Manual operations of this type cause a poor operability and low connection reliability. 
     The manufacturing of a wiring harness requires a specified length of wire to be drawn from a wire supply and further requires terminals to be mounted on both ends of the cut wire by crimping, insulation displacement, etc. A prior art wire cutting/crimping apparatus is used for this purpose. A wire joint detecting apparatus is provided in a wire running path that extends from the wire supply to the wire cutting/crimping apparatus. The wire joint detecting apparatus detects the presence of a wire joint and cuts off the section of wire with a wire joint so that a product includes no wire joint. 
     The wire joint generally is formed by twisting exposed conductors. As a result, the wire joint typically is detected by applying a voltage to a pair of opposite untwisting rollers  1  ( 1   a ,  1   b  in FIG. 15) that are provided along a running path of a wire “a”. More particularly, the rollers  1   a ,  1   b  are connected electrically via the wire joint when the exposed conductors of the wire joint pass between the rollers  1   a ,  1   b . Thus the electrical connection of the rollers  1   a ,  1   b  detects the wire joint. A detection error can occur if there is a defective contact between the wire “a” and the rollers  1   a  and  1   b . Additionally the wire joint cannot be detected unless the conductors are exposed. 
     Other prior art apparatus has detected the wire joint by a color sensor that detects the tape used in the prior art for joining wires. The detection by the color sensor does not require the conductors to be exposed. However, the sensor itself is expensive, it is difficult to adjust the sensor for detection and an erroneous detection frequently is made. 
     In view of the above, an object of the present invention is to provide a wire connecting method and apparatus, which have high connection reliability and which can be automated easily. 
     It is also an object of the invention to provide a wire joint detection method and apparatus for reliably allowing a detection of a wire joint regardless of whether the conductors are exposed at the wire joint. 
     SUMMARY OF THE INVENTION 
     The subject invention relates to a wire connecting method for connecting wires. The method commences by placing the ends of the wires to be connected on a base plate. The wires may be aligned along substantially the same axis with the ends of the wires facing each other. The method proceeds by placing a tape near the wire ends with the adhesive surface of the tape facing towards the wires. The method continues by placing a movable plate on the wire ends, and then moving the movable plate to roll the wire ends onto the tape between the movable plate and the base plate. This movement causes the tape to be wound at least partly over and around the wire ends to connect the wires. 
     The tape displays a strong resistance to tearing forces that act in the longitudinal direction of the tape. Thus, the wires are connected strongly. The tape may contain reinforced fibers to provide even a stronger tear resistance. 
     A specific embodiment of the method for connecting wires comprises placing the ends of the wires on a base plate such that portions of the wires on the base plate lie along the same axis and such that the wire ends face each other. The method then includes placing the tape parallel to the portions of the wires on the base plate with the adhesive surface of the tape faced up, and then placing a movable plate for rolling the wires on the wire ends. The method then comprises moving the wire ends toward the tape to roll the wire ends onto the tape between the movable plate and the base plate for winding the tape over and around the wire ends to connect the wires. 
     The above-described method can be employed while guiding a wire from a wire supply to a wire processing apparatus. In particular, the method is employed to replace a first wire drawn from a first wire supply with a second wire drawn from a second wire supply. The replacement is achieved by connecting the trailing end of the first wire with the leading end of the second wire. In this embodiment, the first wire is cut on the base plate to create the trailing end. The leading end of the second wire then is positioned on the same axis as the trailing end of the first wire, and both wire ends are rolled to wind the tape over and around the wires. The connected wire ends then may be rolled in the opposite direction to return the connected wires to the path along which the wire is drawn and guided. Thus, twists created in the wire during the winding of the tape can be untwisted, thereby enabling the wire to run smoothly thereafter. 
     The connected wire ends may be clamped and pulled in directions away from each other to check the adequacy of the connection. The adequacy of the connection may be judged based on any movement of the wires in response to the pulling forces. The wire processing apparatus can be controlled in accordance with a connection judgment signal to prevent possible problems caused by a breakage of the wire and the like resulting from a defective connection. 
     A wire joint makes a step or cross sectional dimensional change on the outer surface of a wire regardless of which connecting method is employed. For example, a joint formed by twisting conductors of wires exposed by stripping sheaths at ends of the wires provides a step because the twisted portion of the conductors has a larger diameter than the sheath-coated portion of the wire. Wires connected by mounting a sleeve or by welding have a step formed by the sleeve or a filling, respectively. 
     The invention may comprise a rotating or pivoting detector that is positioned adjacent to a running wire to detect a step in the wire. The detector will be caught by the step at the joint and will be rotated or pivoted as the joint runs past the detector. This rotating or pivoting movement actuates a switch to detect the joint. The switch may be a micro switch that detects the joint of the wire by projection and retraction of an actuator resulting from the rotation of the detector. The switch also may be a photoelectric switch that detects the joint by a change in an amount of light detected due to the rotation of the detector. 
     A wire connecting apparatus in accordance with the invention comprises a frame with base plates. The apparatus also has a movable plate which is movable toward and away from the base plates and which is capable of reciprocating along a direction parallel to the extension of wires and a tape. A cutting device is provided for cutting the first wire, and a transfer plate is provided for transferring an end of the second wire toward the first wire while supporting the second wire. Jigs are provided for feeding and positioning the tape. More particularly, the jigs may be operative for drawing the tape from a roll of tape, aligning the tape parallel to the ends of the wires and cutting the tape to a specified length. The apparatus further includes driving means for respectively driving the movable plate, the cutting device, the transferring plate and the parallel cutting jigs. 
     The apparatus may further comprise jigs for clamping the wires on a wire drawing/guiding path. The jigs are provided on the apparatus frame on opposed sides of the wire or above and below the base plate. One jig may be movable away from the other jig, or upward along the wire drawing/guiding path. A connection judgment can be made by moving the one jig after the wires are clamped by both jigs. In other words, the connection is satisfactory if the movement of the one jig stops at such a position where the wires are just straightened, whereas the connection is not satisfactory if the one jig moves beyond such a position. 
     The movable plate may be split into two sections for the one wire and the other wire, respectively, and the split sections of the movable plate may be elastically movable toward and away from the wires. Then, even if diameters of the wires to be connected differ, the two split sections of the movable plate take up a diameter difference by elastically adjusting their distances to the wire ends, thereby enabling a smooth connection. 
     According to the invention, there is further provided a wire joint detecting apparatus, in particular for use with a wire connecting apparatus. The wire joint detecting apparatus comprises a rotating or pivoting detector for sliding contact with a running wire. The detector is operative to be rotated or pivoted by a cross sectional dimensional change indicative of a joint of the wire. The wire joint detecting apparatus further comprises a detection switch for detecting the rotation or pivotal movement of the detector to detect the joint. 
     The wire joint detecting apparatus may further comprise an operable plate. The operable plate and the detector switch may be mounted on a frame, such that pivotal movement of the operable plate may actuate the detection switch. The detector that contacts the running wire may be mounted on the operable plate for pivotal or rotational movement. More particularly, the detector is rotated or pivoted by a step formed on the wire by the joint. The operable plate then is pivoted by the rotation of the detector to actuate the detection switch. Thus the joint of the wire is detected by a signal of the detection switch. 
     Preferably, the detection switch is a photoelectric switch, and the joint of the wire is detected by a change in an amount of light detected by the photoelectric switch resulting from the rotation of the detector. 
     The portion of the detector that is in sliding contact with the wire may be a substantially V-shaped groove in which the wire is at least partly fit. 
     The detector for pivoting the operable plate preferably has a shape such that a distance between a point of sliding contact with the wire and a center of rotation changes. For example, the shape may be a polygon such as a rectangle or triangle. The center of the polygon serves as an axis of rotation. If surfaces of a V-shaped groove are employed as contact surfaces of the detector with the wire to increase points of contact (see e.g. FIG.  14 ), a joint can be detected even if the joint forms a small step, thereby improving detection accuracy. Further, the switch is actuated, for example, by having its actuator or the like pushed by the pivotal movement of the operable plate. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings. 
     BREIF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of one embodiment of the invention. 
     FIGS. 2 to  4  are front views showing an action of the embodiment. 
     FIG. 5 is a side view showing an essential portion of the embodiment. 
     FIG. 6 is a perspective view showing how a tape is clamped according to the embodiment. 
     FIGS.  7 ( a ) to  7 ( g ) are schematic front views showing the action of the embodiment. 
     FIG.  8 ( a ) is a schematic plan view showing the action of the embodiment. 
     FIGS.  8 ( b ) to  8 ( g ) are schematic plan views showing the action of the embodiment. 
     FIG. 9 is a diagram showing a connected state of wires. 
     FIGS.  10 ( a ) and  10 ( b ) are a perspective view of one embodiment of the invention and a front view of a detecting section of the embodiment. 
     FIGS.  11 ( a ) and  11 ( b ) are diagrams showing how the embodiment works. 
     FIGS.  12 ( a ) and  12 ( b ) are a perspective view of another embodiment of the invention and a front view of a detecting section of this embodiment. 
     FIGS.  13 ( a ) and  13 ( b ) are diagrams showing how the embodiment of FIGS.  12 ( a ) and  12 ( b ) works. 
     FIGS.  14 ( a ) and  14 ( b ) are a perspective view of an essential portion of another embodiment and a front view of a detecting section of the another embodiment. 
     FIG. 15 is a plan view of prior art wire joint detection. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus in accordance with one embodiment of the invention is illustrated in FIGS. 1-8. As shown most clearly in FIG. 1, the apparatus includes a frame F with a base plate  11  that is used for rolling wire ends so that the wires or wire ends pivot or rotate around their axes or around a direction of their longitudinal extension. A movable plate  33  is provided for rolling the wire ends. The apparatus further includes a cutting device  18  for cutting a first wire “a”, a transferring plate  19  onto which a wire to be connected can be transferred, and a tape drawing device  16  for drawing a tape “t”. A forcible wire withdrawing or dropping mechanism may be provided for forcibly withdrawing the portion of the first wire “a” which is no longer needed, i.e. the portion of the first wire “a” different from that portion of the first wire “a” to be connected to the second wire “a” by means of the tape “t”. The forcible wire withdrawing mechanism may include a withdrawing roller that can be brought into contact with the portion of the first wire “a” to be withdrawn. 
     The base plate  11  is secured to the frame F, and an auxiliary base plate  12  is provided at one side of the base plate  11  for movement toward and away from the frame F by an unillustrated air cylinder or other moving or actuating means, such as a step motor. The auxiliary base plate  12  is retracted toward the frame F when the tape drawing device  16  draws the tape t, and is moved forward to be substantially flush with the base plate  11  when the tape t is wound around the wire “a”. 
     A roll of tape  13  is provided above the base plate  11 , and the tape t can be guided from the tape roll  13  to a position above the auxiliary base plate  12  via guide rollers  14   a ,  14   a . The tape t has an adhesive layer formed on its inner surface in its rolled state, and hangs down slightly while having its adhesive layer adhered to the lower guide roller  14   a . The tape t is positioned on the auxiliary base plate  12  by clamping and lowering the hanging piece of the tape t with a clamping jig  16   a  of the vertically movable drawing device  16 . As shown in FIG. 6, the clamping jig  16   a  has clamping pieces  16   b ,  16   c  that substantially face each other. The clamping piece  16   c  is supported via an insulating plate  16   d , such as a Bakelite plate. Thus voltage can be applied to the clamping pieces  16   b ,  16   c  while providing an electrical insulation therebetween. At least one of the clamping pieces  16   b ,  16   c  is moved toward the other to clamp the tape t. If, for some reason, the tape t is not present, the clamping pieces  16   b ,  16   c  are brought into contact with each other to create an electrical connection, thereby enabling detection of the erroneous clamping. Succeeding operations will not be performed if an erroneous clamping signal is produced. An operator then resets the tape t, and a tape pulling-down operation follows. The drawing device  16  is moved vertically by an unillustrated driving means provided in the guide  15 . 
     A cutting device  17  cuts the tape t after the specified length has been pulled down. The cut end of the tape t then is pressed and supported by a probe provided in the cutting device  17  (at point S in FIG.  7 ( d )). A movable plate  33  moves the cutting device  17  forward when the wire “a” is brought to an end position of the tape t, and the tape t is released at that time. Simultaneously, the drawing device  16  is retracted forward. Accordingly, the movement of the movable plate  33  is neither interfered with nor hindered. The cutting device  17  is moved forward and backward by an air cylinder  17   a , and the drawing device  16  is caused to clamp the tape t and retract by opening and closing movements and by a retracting movement (forward movement) of the clamping jig  16   a  at its leading end. An unillustrated air cylinder preferably causes these movements. 
     The base plate  11  is split into first or upper and second or lower sections, between which the cutting device  18  is provided. The cutting device  18  is actuated by an air cylinder  18   a  to cut the substantially vertically guided wire “a” and to clamp the upper part of the cut wire “a”. The cutting device  18  releases the wire “a” at a moment when or after the wire “a” is clamped between the movable plate  33  and the base plate  11 , and is retracted into a clearance between the upper and lower sections of the base plate  11 . The movable plate  33  also preferably is.split into first or upper and second or lower sections. The cutting device  18  is located between the upper and lower sections of the movable plate  33 , and hence the cutting device  18  neither interferes with nor hinders the movement of the movable plate  33 . 
     The transferring plate  19  is transversely movable by a guide  20 . A wire “a” to be connected is inserted laterally into clips  21  e.g. from the right side to be clamped and supported. The transferring plate  19  is moved transversely to the direction of extension of the wires a, a′ by an unillustrated driving means provided in the guide  20 . A U-shaped arm  22  is provided below the transferring plate  19 , and the wire “a” is inserted into a clip  23  provided in an intermediate position of the arm  22  e.g. from the right side to be clamped. A leading end  22   a  of the arm  22  also drives away the lower part of the cut wire “a”, so that this lower part of the wire “a” does not stand as a hindrance. 
     Guiding jigs  24 ,  25  are provided on the frame F at substantially opposed sides of the base plate  11  or above and below the base plate  11 , respectively. The guiding jigs  24 ,  25  each are provided with a chuck. The upper guiding jig  24  is driven by an air cylinder  24   a  and moves upward while clamping the wire “a” by means of the chuck. The lower guiding jig  25  is actuated by an air cylinder  25   a  to clamp, fix or position the wire “a”. Thus, the upper and lower guiding jigs  24 ,  25  clamp the wires “a”, “a” while the upper guiding jig  24  moves upward, thereby straightening the wires “a”, “a” to judge whether the tape t has been wound satisfactorily, as described later. If the connection of the wires “a”, “a” is satisfactory, the upward movement of the upper guiding jig  24  stops at a specified position. If the guiding jig  24  moves further upward, a defective connection is judged to exist, and the feeding of the wires “a”, “a” is stopped. 
     An auxiliary frame  30  is provided on the front surface of the base plate  11  of the frame F and is pivotal along forward and backward directions about its left end. A movable plate  32  is provided on the frame  30  via a slider  32   a  for transverse movement by an air cylinder  31 . The split movable plate  33  is provided at an inner side of the movable plate  32 . More particularly, the movable plate  33  is supported on the movable plate  32  for movement toward and away from the movable plate  32 , and preferably is biased away from the movable plate  32  by springs  34 . Therefore, as shown in FIG. 5, the movable plate  33  smoothly holds the wires “a”, “a” in cooperation with the base plate  11  by the elastic force of the springs  34  even if the diameters of the wires “a”, “a” differ when the movable plate  33  is pressed into contact with the wires “a”, “a”. 
     Stoppers  35  are provided at the leading end of the frame  30 . The length of the stoppers  35  is adjustable, and a contact degree (degree of proximity or distance) of the movable plate  33  with respect to the base plates  11 ,  12  can be adjusted by adjusting the length of the stoppers  35 . This adjustment is effective to prevent abrasion of the outer surfaces of the plates  33 ,  11 ,  12 . Specifically, a non-slip layer to which e.g. sand-like particles are adhered is formed on the contact surface of each of the plates  11 ,  12 ,  19  and  33  so that the wires “a”, “a” can be rolled smoothly. Adjusting the contact degree can prevent abrasion of the non-slip layers. 
     An air cylinder  37  is fixed to the frame F and has a piston rod  37   a  coupled by a pin to a hook  36  at the leading end of the frame  30 . The frame  30  is pulled toward the base plate  11  by the actuation of the air cylinder  37 , thereby bringing the movable plate  33  substantially into contact (or closer to) the base plate  11  and the wires “a”, “a”. In this state, the movable plate  33  is moved to the right by actuating the air cylinder  31  to wind the tape t over and around the wires “a”, “a” by rolling the wires “a”, “a”. 
     The apparatus illustrated in FIG. 1 can be used to replace the first wire “a” from the first wire supply S 1  by the second wire “a” from the second wire supply S 2  where the first wire “a” is being guided to a wire processing apparatus (not shown) through the upper and lower guiding jigs  24 ,  25 . More particularly, the clips  21 ,  23  of the plate  19  and the arm  22  clamp the wire “a” beforehand. If a wire exchange signal (e.g. by turning a lamp on) is inputted from the wire processing apparatus in this state, the drawing of the wire “a” is stopped e.g. upon pressing an unillustrated start switch, thereby proceeding to the operation of connecting the wires “a”, “a”. The clips  21 ,  23  are provided with clamp completion switches. At this stage, unless these switches are on, the connecting operation preferably is not started even if the start switch is turned on. This is because a connection error is likely to occur if the wires “a”, “a” are clamped insufficiently by the clips  21 ,  23 . 
     The connection operation is performed by first moving the tape drawing device  16  upward from the state shown in FIGS. 2,  7 ( a ) and  8 ( a ) to clamp the tape t drawn by means of the clamping jig  16   a  (see FIGS.  7 ( b ) and  8 ( b )). The tape t then is drawn to extend at least partly substantially along the auxiliary base plate  12  (see FIGS.  7 ( c ) and  8 ( c )). Simultaneously with the completion of this operation, the tape t is cut and the transferring plate  19  is moved to the right to abut against the lower base plate  11  (see FIGS.  7 ( c ) and  8 ( c )). In this state, the frame  30  is moved closer to the frame F so that the movable plate  33  contacts or comes closer to the base plate  11  with the wires “a”, “a” located therebetween (see FIGS.  7 ( d ) and  8 ( d )). The movable plate  33  then is moved to the right (see FIGS.  7 ( e ) and  8 ( e )). As the movable plate  33  is moved, the wires “a”, “a” are rolled between the base plate  11  and the movable plate  33  and, consequently, the wires “a” “a” are rolled on the tape t on the auxiliary base plate  12 . This rolling movement causes the tape t to be wound over and around the wires “a”, “a” to connect or join the wires “a”, “a”, as shown in FIG. 9 (see FIGS.  7 ( e ) and  8 ( e )). 
     The movable plate  33  then is moved to the. left to untwist the wires “a”, “a” (see FIGS.  7 ( f ) and  8 ( f )), and the frame  30  is moved away from the frame F to return the movable plate  33  to its initial position. As the movable plate  33  is returned, the upper and lower guiding jigs  24 ,  25  clamp the wires “a”, “a” and the- upper guiding jig  24  is moved upward to judge whether the connection is satisfactory (see FIGS.  7 ( g ) and  8 ( g )). The above operations are performed automatically to connect the wires “a”, “a” when the wires “a”, “a” are exchanged. 
     The inventive method and apparatus thus constructed achieve high connection reliability and easily enable automation of the wire connection. 
     A second aspect of the invention is described with reference to FIGS. 10 and 11. More particularly, FIGS. 10 and 11 show a location along a wire running path to a wire processing apparatus, such as a wire cutting/crimping apparatus. An operable plate  111  is provided on a frame F for pivotal or rotational movement, and a micro switch  112  is provided above or in proximity to the operable plate  111  along the direction of movement. Thus, if the operable plate  111  is pivoted away from the wire “a” as indicated by an arrow in FIG.  11 ( b ), an actuator  112   a  is pushed up to actuate the micro switch  112 . Altematively or additionally any other detector sensing a pivotal movement, such as a proximity sensor, a light/diode sensor or the like may be used. 
     A wire-contacting jig  113  and a guide  114  are provided on the frame F below or at an opposite side of the operable plate  111 , and the wire “a” runs through or on the wire-contacting jig  113  and the guide  114 . A substantially rectangular detector  115  is provided at an end of the operable plate  111  substantially facing the wire-contacting jig  113 , and is rotatable about its center axis. One side of the detector  115  is constantly held in sliding contact with the wire “a” running on the wire-contacting jig  113 . 
     Thus, while the wire “a” is running, the operable plate  111  is not pivoted or rotated away from the wire “a”, and the micro switch  112  is not turned on as shown in FIG.  11 ( a ). 
     When a joint formed e.g. by winding a tape t (as described above with reference to FIGS. 1 to  9 ) reaches the wire contacting jig  113 , the detector  115  contacts the step or cross sectional dimensional change formed by the wound tape t, as shown in FIG.  11 ( b ), and rotates upward. This rotation causes the operable plate  111  to pivot upward and actuate the micro switch  112 . The joint t is detected based on a signal representing the operation of the micro switch  112 , and the detection is notified to the wire processing apparatus. 
     The succeeding side of the rotated detector  115  comes into sliding contact with the wire “a” beyond the tape t, and the operable plate  111  is pivoted downward to return substantially to its normal state for detecting a next joint tape t. The joint tapes t of the wire “a” are detected successively by repeating the above operation. 
     FIGS. 12 and 13 show an embodiment in which a photoelectric switch  122  is used instead of the micro switch  112 . In this embodiment, a spring  123  is provided between the operable plate  111  and the wire-contacting jig  113 , so that the detector  115  can be held securely in sliding contact with the wire “a” by a biasing force of the spring  123 . Further, the photoelectric switch  122  is comprised of a light detector  122   a  and a light emitter  122 b mounted on the operable plate  111  with the detector  115  located therebetween. 
     Thus, while the wire “a” is running, the operable plate  111  is not pivoted or rotated upward as shown in FIG. 13 and, accordingly, light between the light detector  122   a  and the light emitter  122   b  of the photoelectric switch  122  is not blocked by the detector  115 . If a joint formed e.g. by a tape t reaches the wire contacting jig  113  in this state, the detector  115  is rotated or pivoted as shown in FIG.  13 ( b ) and in phantom in FIG.  12 ( b ), thereby blocking the light between the light detector  122   a  and the light emitter  122   b , as indicated by hatching in FIG.  13 ( b ). As a result, the photoelectric switch  122  is actuated in response to a change in an amount of light detected thereby to detect the joint “t” of the wire “a”. 
     Since a one-point contact is substantially established between the detector  115  and the wire “a” in cross section in the embodiment shown in FIG.  10 ( b ), the step cannot be detected unless passing this point of contact. If the step is small, it may not be detected. Contrary to this, if a sliding-contact surface is provided by a V-shaped groove  115 a, as shown in FIGS.  14 ( a ) and  14 ( b ), it comes into sliding contact with the step substantially at two positions in cross section, thereby improving a detection accuracy. 
     The operable plate  111  (detector  115 ) and the jig  113  may be electrically insulated from each other by making the frame F of an insulating plate such as a Bakelite plate. A voltage then may be applied between the plate  111  and the jig  113  to detect a joint formed by twisting conductors of the wires as described above. In other words, the joint can be detected by an electrical connection established between the detector  115  and the jig  113  by the joint t. 
     Although the joint is detected by the wound tape t in this embodiment, other known joints formed by twisting the conductors or mounting a sleeve can also be detected if there is a step. 
     Since the joint is detected by its step, as described above, it can be detected without any restriction factor, such as the exposure of the conductors.