Robot welding gun unit
This application is a continuation of U.S. Ser. No. 08/257,808, filed Jun. 10, 1994. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of The Invention 
     This invention relates to a robot welding gun unit which is a robot equipped with a welding gun. 
     2. Description of The Prior Art 
     Conventional robot gun units had a problem that when it proceeded to next welding operation without compensating for the wear of electrodes in preceding welding operation, which caused the formation of a gap between an electrode on the side of a fixed arm and a weldment, the gap being formed as the fixed electrode was worn away, and which caused the deformation of the weldment when the same was clamped between the electrodes for welding. In order to solve the problem, the welding gun was equipped with an equalizer unit and the gun body was supported by a spring at the time of welding to prevent the weldment from being deformed. In case of a robot welding gun, however, the welding posture of the weldment varied according to the different positions and shapes of the weldment so that the spring sometimes did not work effectively and it was difficult to adjust the spring. 
     Each of some robot welding guns clamps the weldment between an electrode on the side of a fixed arm and a presser electrode for welding with the fixed arm side electrode being always in contact with a surface of the weldment and with the presser electrode being advanced toward the weldment by a robot instead of the equalizer unit described above (e.g., the Japanese Patent Laid-Open Publication No. 2 179363). 
     The robot welding gun disclosed in the above publication, however, has a problem that it is very troublesome in control since an electrode on the side of the fixed arm is always in contact with a surface of the weldment so that the robot has to be moved variously in accordance with the wearing amount of the electrode although the generation of the gap due to the wear of the electrode can be prevented. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of such a problem of the prior art to eliminate the variation of movement of the welding gun caused by the robot, described above, by adjusting the position of the welding gun on the side of the fixed arm in accordance with the wearing amount of the electrode. Thus allowing the electrode on the side of the fixed arm to be always in contact with a surface of the weldment and provide a robot welding gun capable of safe and accurate welding with easy control and without generating distortion in the weldment. 
     In order to attain the above object, the robot welding gun according to the present invention is characterized by a welding gun mounting bracket provided at, the tip end of a robot arm, a cylinder unit incorporated in the bracket for adjusting the position of the fixed arm, a rod which is rotated or moved forward or backward by the cylinder unit and is connected to the fixed arm, and a gun body which include the arm and is slidably mounted on the mounting bracket. 
     In the robot welding gun unit which clamps a weldment between electrodes for welding, the electrodes are gradually worn away as welding is repeated. Accordingly, the wearing amount of the electrode on the side of the fixed arm is measured by a standard plate or a sensor when the robot is in rest position to compensate the wearing amount of the electrode by driving the fixed arm positional adjusting cylinder unit to advance the gun body toward the mounting bracket by the wearing amount. Therefore, the movement of the robot does not change relative to the wearing amount of the electrode at all. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a robot welding gun unit according to an embodiment of the present invention; and 
     FIG. 2 is a view for explaining the operation of the robot welding gun unit including an auxiliary unit to measure the wearing amount of an electrode. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     An embodiment of the invention will be described hereinafter with reference with FIGS. 1 and 2. 
     FIG. 1 is an embodiment employing a C-type gun. In the figure denoted at 1 is a bracket for mounting a welding gun body 2 on a robot arm and an electric motor 3 which constitutes a cylinder unit is fixed to the bracket 1. 
     The welding gun body 2 is mainly composed of a fixed arm 5 equipped with an electrode 4 at the tip end thereof, a pressure rod 8 which is moved forward or backward by a pressure cylinder 6 and is equipped with an electrode 7 at the tip end thereof and a sliding bracket 9 which is disposed on the upper surface of the pressure cylinder 6 and is slidably supported by the bracket 1. 
     A rod 12 of a ball screw extends between a supporting bracket 10 formed on the fixed arm 5 and the output side of the motor 3, the rod 12 being connected to the motor 3 by way of a coupler 11. 
     An adjusting auxiliary unit 14 having a standard plate or a sensor 13 serving as a yardstick for moving the welding gun body 2 relative to the bracket 1 according to the wearing amount of the electrode 4 on the side of the fixed arm is positioned upright at the rest position of the robot. 
     In case of the robot welding gun unit having the above construction according to the embodiment of the present invention, at first, the electrode 4 on the side of the fixed arm 5 is brought into contact with a surface of a weldment 15 by a robot arm. At this state, the pressure cylinder 6 is driven to advance the pressure rod 8 toward the weldment 15 until the electrode 7 at the tip end thereof is brought into contact with the weldment 15, which is then clamped between the electrodes 4 and 7 for welding. When the welding of the weldment 15 is completed, the pressure cylinder 6 returns the electrode 7 to its standby position. 
     After an operating cycle of welding is completed in this way, the welding gun body 2 is moved to the rest position of the robot by the robot arm, where the adjusting complementary unit 14 checks the wearing amount of the electrode 4. In case there is a gap between the standard plate or a sensor 13 and the electrode 4 due to the wear of the electrode 4, the welding gun body 2 including the electrode 4 is moved upward relative to the mounting bracket 1 in FIG. 2 by driving the motor 3 to rotate the rod 12 of ball screw until the electrode 4 is brought into contact with the sensor 13. Alternatively the gap is measured by the sensor 13 and the motor 3 may be driven to move the welding gun body 2 as far as the measured gap. The robot arm does not move while the welding gun body 2 is moved. Accordingly, the robot can keep the electrode 4 always in contact with the weldment 15 while performing a predetermined movement regardless of the wear of the electrode 4. 
     Although an electric cylinder unit is employed as a cylinder unit for moving the welding gun body 2 relative to the mounting bracket 1 in the above embodiment, a hydraulic or pneumatic power cylinder unit may be employed instead of the electric cylinder unit and the welding gun unit body may be of X-type instead of the C-type set forth above. 
     Since the robot welding gun according to the present invention has a welding gun mounting bracket at the tip end of a robot arm, a cylinder unit incorporated in the bracket for adjusting the position of the fixed arm, a rod which is rotated or moved forward or backward by the cylinder unit and is connected to the fixed arm and a gun body which include the arm and is slidably mounted on the mounting bracket, it is possible to compensate the wearing amount of an electrode when the same is gradually worn away as welding is repeated by driving the fixed arm positional adjusting cylinder unit to advance the gun body toward the mounting bracket, so that the movement of the robot does not change at all relative to the wearing amount of the electrodes, resulting in a robot welding gun unit easy in control and capable of safe and accurate welding without generating distortion in the weldment.