Patent Publication Number: US-2012031880-A1

Title: Electrode unit of spot welding machine

Description:
TECHNICAL FIELD 
     The present invention relates to a structure of an electrode unit provided in a spot welding machine. 
     BACKGROUND ART 
     The spot welding is a conventional welding technique extensively employed to weld metal members, an example of which is disclosed in the Patent Document 1. Describing the spot welding, electric current is applied to the metal members while they are being pressed to be bonded to each other so that the metal members are melted to be welded by resulting electric resistance heat. As illustrated in  FIG. 5 , a spot welding machine used to perform the spot welding has an electrode unit  90  including a pair of cylindrical shanks  91  that can be attached to and detached from each other, and a pair of cap chips  95  respectively attached to edges of the pair of shanks  91 . When the spot welding machine is used to perform the spot welding, electric current is applied to the metal members while they are being pressed to be bonded to each other by such pair of cap chips  95 . A fitting portion  91   a  taperedly reduced in diameter is formed in each of the edges of the shanks  90 . A recessed fitting portion  95   a  taperedly reduced in diameter toward the far end is formed in each of the cap chips  95 . The fitting portions  91   a  are fitted in the recessed fitting portions  95   a  so that the cap chips  95  are securely attached to the shanks  91 . A water filling pipe  92  closely facing a bottom part of the recessed fitting portion  95   a  is provided inside the shanks  91 . When cooling water is fed in the water feed pipe  92 , the cap chip  95  is cooled down. Thus, the cap chip  95  can be protected from overheat. 
     In recent years, high-tensile steel is increasingly often used as a material of automobiles for weight reduction and better safety. To perform the spot welding to members made of high-tensile steel which is very resistant to deformation, it is necessary to apply a high pressure to press the members to be bonded to each other. Such a high pressure makes the tapered fitting portion  91   a  widen the recessed fitting portion  95   a  as the spot welding advances. Another problem is that a reaction generated when the edge of the fitting portion  91   a  abuts the bottom part of the recessed fitting portion  95   a  often makes the cap chip  95  drop from the fitting portion  91   a  of the shank  91 . To put the cap chip  95  that dropped back to the fitting portion  91   a , the cap chip  95  is let through the bore of a dies to be narrowed in diameter so that the widened recessed fitting portion  95   a  is back to its original shape. This is a very time-consuming work. 
     In the event of any interference of the shank or cap chip with members to be welded or a tool because of a very small welding part, the spot welding is performed by an electrode unit  190  having a substantially columnar mini chip  195  attached to a surface of a plate-shape projecting portion  191   a  formed in an electrode holder  191  as illustrated in  FIG. 6 . A tapered fitting hole  191   b  having an inner diameter gradually reduced toward the far end is formed in the projection portion  191   a . A tapered fitting portion  195   a  fittable to the fitting hole  191   b  is formed in a base part of the mini chip  195 . The fitting portion  195   a  is fitted in the fitting hole  191   b  so that the mini chip  195  is attached to the projecting portion  191   a . An edge of the mini chip  195 , which is an abutting portion  195   b  to contact the members to be welded, protrudes from the surface of the projecting portion  191   a . A welding current supplied to the electrode holder  191  is supplied to the mini chip  195  through contact surfaces of the fitting hole  191   b  and the fitting portion  195   a.    
     In the electrode unit  190  of the spot welding machine structured as illustrated in  FIG. 6 , the fitting portion  195   a  widens the fitting hole  191   b  gradually as the spot welding advances, inviting the mini chip  195  into the fitting hole  191   b . To ensure an enough contact area between the fitting portion  195   a  and the fitting hole  191   b  and maintain a good fitting state between the fitting portion  195   a  and the fitting hole  191   b  so that the mini chip  195  is prevented from falling off, it is necessary to form the fitting portion  195   a  at another position exposed from the fitting hole  191   b  as illustrated in  FIG. 6 . As a result, the mini chip  195  overly protrudes from the projecting portion  191   a . It is not allowed to increase thickness dimensions of the mini chip  195  and the projecting portion  191   a  in total because of the need to avoid any interference, meaning that the projecting portion  191   a  cannot be increased in thickness. Therefore, it is not possible to form a cooling water passage in the projecting portion  191   a.    
     As illustrated in  FIG. 6 , in the electrode holder  191 , a cooling water passage  191   c  is formed only to extend to a position near the base part of the projecting portion  191   a . Therefore, the projecting portion  191   a  thus lacking the cooling water passage  191   c  is not cooled down enough. As a result, the projecting portion  191   a  is softened and deflected by heat during the welding, resulting in a poor welding accuracy. There are other problems. The heat further oxidizes the fitting portion  195   a  and the fitting hole  191   b , resulting in a poor electrical conduction between the electrode holder  191  and the mini chip  195 . This leads to a poor welding result. Because of the projecting portion  191   a  not cooled down enough, heat is not conducted well from the mini chip  195  to the projecting portion  191   a . As a result, the mini chip  195  thereby overheated is easily softened and defaulted, therefore, had to be replaced frequently to attain a good welding quality.
     [PATENT DOCUMENT 1] Japanese Unexamined Patent Application Publication No. 2001-87864   

     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     To solve the conventional technical problems, the present invention provides an electrode unit of a spot welding machine structurally advantageous in that an electrode can be prevented from falling off from an electrode holder and protected from overheat. 
     Means for Solving the Problem 
     In order to solve the conventional technical problems, the invention recited in Claim  1  provides an electrode unit of a spot welding machine, comprising: 
     an electrode holder provided with a projecting portion formed in a plate shape and having a welding current fed thereto; and 
     an electrode attached to a surface of the projecting portion so as to protrude therefrom to press members to be welded using an abutting portion formed in an edge thereof, wherein: 
     a recessed fitting portion having a circular shape in section is formed in the projecting portion; 
     a first O ring groove is formed in an inner peripheral surface of the recessed fitting portion; 
     a cooling water passage continuous to the recessed fitting portion where a cooling water is supplied is formed in the electrode holder; 
     a planar power-feed face is formed around an opening of the recessed fitting portion; 
     a columnar fitting portion is formed in a base end of the abutting portion of the electrode; 
     a second O ring groove is formed in an outer peripheral surface of the fitting portion; 
     a planar power-to-be-fed face is formed around the fitting portion in the base end of the abutting portion; 
     the fitting portion is inserted in the recessed fitting portion, an O ring is fitted in the first and second O ring grooves, and the power-feed face and the power-to-be-fed face are brought into contact with each other, to attach the electrode to the projecting portion. 
     The invention recited in Claim  2  is characterized in that an end face of the fitting portion of the electrode and a bottom part of the recessed fitting portion are situated away from each other in the invention recited in Claim  1 . 
     The invention recited in Claim  3  is characterized in that a water feed pipe into which the cooling water is fed is provided in the water cooling passage, and an opening of the water feed pipe closely faces the fitting portion of the electrode in the invention recited in Claim  1 . 
     The invention recited in Claim  4  provides an electrode unit of a spot welding machine, comprising: 
     a cylindrical electrode holder provided with a cooling water passage therein and having a welding current supplied thereto; and 
     an electrode attached to an edge of the electrode holder to press members to be welded using an abutting portion formed in an edge thereof, wherein: 
     a recessed fitting portion having a circular shape in section and continuous to the cooling water passage is formed in the edge of the electrode holder; 
     a first O ring groove is formed in an inner peripheral surface of the recessed fitting portion; 
     a planar power-feed face is formed around an opening of the recessed fitting portion in the edge of the electrode holder; 
     a columnar fitting portion is formed in a base end of the abutting portion of the electrode; 
     a second O ring groove is formed in an outer peripheral surface of the fitting portion; 
     a planar power-to-be-fed face is formed around the fitting portion in the base end of the abutting portion; 
     the fitting portion is inserted in the recessed fitting portion; 
     an O ring is fitted in the first and second O ring grooves, the power-feed face and the power-to-be-fed face are brought into contact with each other to attach the electrode to the electrode holder; 
     a water feed pipe into which the cooling water is fed is provided in the cooling water passage, and an opening of the water feed pipe closely faces the fitting portion of the electrode. 
     Effect of the Invention 
     The invention recited in Claim  1  provides an electrode unit of a spot welding machine, comprising an electrode holder provided with a projecting portion formed in a plate shape and having a welding current supplied thereto; and an electrode attached to a surface of the projecting portion so as to protrude therefrom to press members to be welded using an abutting portion formed in an edge thereof, wherein: a recessed fitting portion having a circular shape in section is formed in the projecting portion; a first O ring groove is formed in an inner peripheral surface of the recessed fitting portion; a cooling water passage continuous to the recessed fitting portion where a cooling water is fed is formed in the electrode holder; a planar power-feed face is formed around an opening of the recessed fitting portion; a columnar fitting portion is formed in a base end of the abutting portion of the electrode; a second O ring groove is formed in an outer peripheral surface of the fitting portion; a planar power-to-be-fed face is formed around the fitting portion in the base end of the abutting portion; the fitting portion is inserted in the recessed fitting portion; an O ring is fitted in the first and second O ring grooves; the power-feed face and the power-to-be-fed face are brought into contact with each other; and the electrode is attached to the projecting portion. 
     Accordingly, the electrode and the projecting portion of the electrode holder are cooled down by the cooling water supplied through the cooling water passage so that the electrode and the electrode holder can be protected from overheat. 
     The invention recited in Claim  2  is characterized in that an end face of the fitting portion of the electrode and a bottom part of the recessed fitting portion are situated away from each other in the invention recited in Claim  1 . 
     Accordingly, the cooling water can reach and contact the end face of the fitting portion of the electrode as well, effectively cooling down the electrode. 
     The invention recited in Claim  3  is characterized in that a water feed pipe into which the cooling water is fed is provided in the water cooling passage, and an opening of the water feed pipe closely faces the fitting portion of the electrode in the invention recited in Claim  1 . 
     Accordingly, the cooling water supplied from the edge of the water feed pipe directly contacts the fitting portion of the electrode, effectively cooling down the mini chip. 
     The invention recited in Claim  4  provides an electrode unit of a spot welding machine, comprising: a cylindrical electrode holder provided with a cooling water passage therein and having a welding current supplied thereto; and an electrode attached to an edge of the electrode holder to press members to be welded using an abutting portion formed in an edge thereof, wherein: a recessed fitting portion having a circular shape in section and continuous to the cooling water passage is formed in the edge of the electrode holder; a first O ring groove is formed in an inner peripheral surface of the recessed fitting portion; a planar power-feed face is formed around an opening of the recessed fitting portion in the edge of the electrode holder; a columnar fitting portion is formed in a base end of the abutting portion of the electrode; a second O ring groove is formed in an outer peripheral surface of the fitting portion; a planar power-to-be-fed face is formed around the fitting portion in the base end of the abutting portion; the fitting portion is inserted in the recessed fitting portion, an O ring is fitted in the first and second O ring grooves, the power-feed face and the power-to-be-fed face are brought into contact with each other, to attach the electrode to the electrode holder; a water feed pipe into which the cooling water is fed is provided in the cooling water passage, and an opening of the water feed pipe closely faces the fitting portion of the electrode. 
     Accordingly, the power-to-be-fed face abuts the power-feed face during the welding, and a pressing force thereby exerted on the electrode is supported by the power-feed face. This avoids the conventional problem that the recessed fitting portions of the paired cap chips pressing the members to be welded are widened, which possibly causes the cap chips to fall off from the shanks. Further, the O ring is fitted in the first and second O ring grooves when the electrode is attached to the electrode holder. This prevents the electrode from falling off from the electrode holder. 
     Further, the electrode and the projecting portion of the electrode holder are cooled down by the cooling water supplied through the cooling water passage so that the electrode and the electrode holder can be protected from overheat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an electrode unit provided in a spot welding machine according to a first embodiment of the present invention. 
         FIG. 2  is a sectional view cut along A-A in  FIG. 1 . 
         FIG. 3  is a descriptive drawing of the electrode unit provided in the spot welding machine in use. 
         FIG. 4  is a sectional view of an electrode unit provided in a spot welding machine according to a second embodiment of the present invention. 
         FIG. 5  is a descriptive drawing of an electrode unit provided in a conventional spot welding machine. 
         FIG. 6  is a descriptive drawing of an electrode holder and a mini chip conventionally used. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     Hereinafter, preferable embodiments of the present invention are described referring to the accompanied drawings. An electrode holder  10  is attached to a power feed unit of a spot welding machine so that the electrode holder  10  is supplied with a welding current from the power feed unit. The electrode holder  10  is made of an copper alloy such as copper-chromium alloy or beryllium copper having a remarkable strength and a good electrical conductivity. The electrode holder  10  is provided with a plate-shape projecting portion  10   a  formed so as to protrude therefrom. A recessed fitting portion  10   b  is formed in a surface of the projecting portion  10   a . The recessed fitting portion  10   b  has a circular shape in section. The recessed fitting portion  10   b  is formed orthogonal to the surface of the projecting portion  10   a  with no tilt toward either side of the projecting portion  10   a . A first O ring groove  10   c  is formed in an inner peripheral surface of the recessed fitting portion  10   b  along an entire circumference thereof. A planar power-feed face  10   d  is formed on the surface of the projecting portion  10   a  around an opening of the recessed fitting portion  10   b.    
     An open hole  10   e  is formed from a side surface of the electrode holder  10  to a base part of the projecting portion  10   a . A threaded groove is formed in an opening  10   f  of the open hole  10   e  so that a cylindrical inflow port member  41  is threaded into the opening  10   f . A cooling water pipe from which a cooling water is fed is connected to the inflow port member  41 . A cooling water passage  10   g  continuous to the recessed fitting portion  10   b  is formed from an edge of the open hole  10   e . A water feed member  42  is attached to an edge of the inflow port member  41 . A water feed pipe  42   a  is formed in an edge of the water feed member  42 . A flow path  42   b  where the cooling water is circulated is formed. The flow path  42   b  is continuous from a base end of the water feed member  42  to an edge of the water feed pipe  42   a . The water feed pipe  42   a  is inserted in the cooling water passage  10   g , and an edge of the water feed pipe  42   a  closely faces the recessed fitting portion  10   b . The water feed pipe  42   a  has an outer diameter smaller than the inner diameter of the cooling water passage  10   g . As illustrated in  FIG. 2 , a discharge channel  10   h  having an opening in a side surface of the electrode holder  10  is formed continuous from the edge of the open hole  10   e . A threaded groove is formed in an opening  101  of the discharge channel  10   h  so that a cylindrical discharge port member  43  is threaded into the threaded groove. A discharge pipe is connected to the discharge port member  43 . 
     An electrode  20  includes an abutting portion  20   a  and a fitting portion  20   b  which are integrally formed. The abutting portion  20   a  abuts members to be welded, and the fitting portion  20   b  is formed in a base end of the abutting portion  20   a . The electrode  20  is made of an copper alloy such as copper-chromium alloy or beryllium copper having a remarkable strength and a good electrical conductivity. The fitting portion  20   b  has a columnar shape. The fitting portion  20   b  has an outer diameter slightly smaller than the inner diameter of the recessed fitting portion  10   b . A second O ring groove  20   c  is formed in an outer peripheral surface of the fitting portion  20   b  along an entire circumference thereof. The abutting portion  20   a  has a width dimension larger than that of the fitting portion  20   b . A planar power-to-be-fed face  20   d  is formed in the base end of the abutting portion  20   a  around the fitting portion  20   b.    
     An O ring  30  is fitted in the first O ring groove  10   c . The materials used to form the O ring  30  include nitrile rubber, hydrogenated nitrile rubber, fluororubber, silicon rubber, and urethane rubber. The fitting portion  20   b  of the electrode  20  is inserted in the recessed fitting portion  10   b , and the O ring  30  is fitted in a second O ring groove  20   c , so that the electrode  20  is attached to the projecting portion  10   a  of the electrode holder  10 . In such a state, the power-to-be-fed face  20   d  is in contact with the power-feed face  10   d.    
     The cooling water supplied from the inflow port member  41  is circulated in the flow path  42   b  of the water feed member  42 , and then supplied from the edge of the water feed pipe  42   a  to the cooling water passage  10   g  and the recessed fitting portion  10   b  to cool down the fitting portion  20   b  of the electrode  20 . The opening of the water feed pipe  42   a  is closely facing the fitting portion  20   b  of the electrode  20 . Therefore, the cooling water supplied from the edge of the water feed pipe  42   a  directly contacts the fitting portion  20   b  to effectively cool down the fitting portion  20   b . Further, for effective cooling down of the fitting portion  20   b , an end face  20   e  of the fitting portion  20   b  and a bottom part  10   k  of the recessed fitting portion  10   b  are situated away from each other so that the cooling water contacts the end face  20   e  of the fitting portion  20   b . According to the present invention, the O ring  30  provided between the recessed fitting portion  10   b  and the fitting portion  20   b  eliminates the possibility that the cooling water supplied from the water feed pipe  42   a  of the water feed member  42  leaks from between the recessed fitting portion  10   b  and the fitting portion  20   b . The cooling water used to cool down the fitting portion  20   b  of the electrode  20  is circulated in a flow path  10   j  between the inner side of the cooling water passage  10   g  and the outer side of the water feed pipe  42   a , circulated through the open hole be and the discharge channel  10   h , and then finally discharged into a discharge pipe from the discharge port member  43 . 
     In the case where the flow path  10   j  formed between the inner side of the cooling water passage  10   g  and the outer side of the water feed pipe  42   a  fails to have a sectional area adequately large for a flow rate of the cooling water circulated in the flow path  42   b  of the water feed member  42 , a water pressure in the recessed fitting portion  10   b  thereby increased pushes the electrode  20  upward, making the power-to-be-fed face  20   d  lose the contact with the power-feed face  10   d  as illustrated in  FIG. 3(A) . The present invention, the second O ring groove  20   c  in which the O ring  30  is fitted is formed in the outer peripheral surface of the fitting portion  20   b  of the electrode  20 . In the presence of the O ring  30 , the electrode  20 , even if pushed upward by the water pressure, does not fall off from the recessed fitting portion  10   b . Besides that, the first O ring groove  10   c  is formed in the inner peripheral surface of the recessed fitting portion  10   b  to ensure that the O ring  30  does not fall off from the recessed fitting portion  10   b.    
     When members to be welded  98  and  99  are pressed to be bonded to each other by means of the electrode  20  and an electrode  50  facing the electrode  20  as illustrated in  FIG. 3(B) , the power-to-be-fed face  20   d  contacts the power-feed face  10   d . Then, the welding current is flown to between the electrode  20  and the electrode  50 . As a result of the electrical conduction therebetween, the members to be welded  98  and  99  are melted by resulting electric resistance heat and finally welded to each other. The power-to-be-fed face  20   d  and the power-feed face  10   d  are both planar faces. This ensures the electrical conduction between the power-to-be-fed face  20   d  and the power-feed face  10   d  during the welding. The electrical resistance heat generated in the members to be welded  98  and  99  is transmitted to the abutting portion  20   a  of the electrode  20  during welding. When the welding is successfully completed, however, the water pressure in the recessed fitting portion  10   b  pushes the electrode  20  upward, and the power-to-be-fed face  20   d  is drawn away from the power-feed face  10   d  as illustrated in  FIG. 3(A) , so that heat is hardly transmitted from the electrode  20  toward the projecting portion  10   a  of the electrode holder  10 . Further, the cooling water circulated in the cooling water passage  10   g  cools down the projecting portion  10   a  so that the projecting portion  10   a  is not softened or deformed by heat. 
     When the welding is not performed, there is no contact between the power-to-be-fed face  20   d  and the power-feed face  10   d , letting the power-to-be-fed face  20   d  exposed to air to be cooled down. During the welding, the power-to-be-fed face  20   d  and the power-feed face  10   d  are brought into contact with each other, and the air which took the heat away from the power-to-be-fed face  20   d  is discharged from between the power-to-be-fed face  20   d  and the power-feed face  10   d . Since the power-feed face  10   d  is cooled down by the air as well, neither of the power-to-be-fed face  20   d  nor the power-feed face  10   d  is oxidized. This further ensures the electrical conduction between the power-to-be-fed face  20   d  and the power-feed face  10   d . Though the power-to-be-fed face  20   d  is away from the power-feed face  10   d , hardly enabling the heat transmission from the electrode  20  toward the projecting portion  10   a  of the electrode holder  10 , the fitting portion  20   b  of the electrode  20  is constantly cooled down by the cooling water supplied from the water feed pipe  42   a . This avoids softening the electrode  20  due to overheat. Therefore, the abutting portion  20   a  of the electrode  20  is less likely to be deformed by the welding. As a result, the replacement interval of the electrode  20  can be extended to three times longer than the prior art. 
     The present invention defines the flow rate of the cooling water circulated in the flow path  42   b  of the water feed member  42  and the sectional area of the flow path  10   j  formed between the inner side of the cooling water passage  10   g  and the outer side of the water feed pipe  42   a  so that the water pressure in the recessed fitting portion  10   b  effectively serves to draw the power-to-be-fed face  20   d  away from the power-feed face  10   d.    
     The present invention is characterized in that the power-to-be-fed face  20   d  of the electrode  20  is brought into contact with the power-feed face  10   d  of the projecting portion  10   a  so that the electrode holder  10  and the electrode  20  are electrically conducted therebetween. Therefore, it is neither necessary to form a tapered fitting hole  90   b  in a projecting portion  90   a  of an electrode holder  90  nor form a tapered fitting portion  95   a  in a chip  95  as so far conventionally done. Accordingly, the mini chip  95  is prevented from overly protruding from the fitting hole  90   b . As a result of this, a measure of protrusion of the abutting portion  20   a  from the projecting portion  10   a  can be reduced, but the projecting portion  10   a  can be increased in thickness instead. Now that the projecting proportion  10   a  is thus increased in thickness, the cooling water passage  10   g  can be formed in the projecting proportion  10   a  to cool down the electrode  20 . The projecting portion  10   a  larger in thickness is more rigid and more difficult to be deformed during the welding, which leads to maintain a better welding quality. 
     Second Embodiment 
     Hereinafter are described differences of an electrode unit provided in a pot welding machine according to a second embodiment of the present invention as compared to the first embodiment. An electrode holder  110  according to the second embodiment has a cylindrical shape. Such an electrode holder  110  is conventionally called a shank. A cooling water passage  110   d  is formed inside the electrode holder  110 . A recessed fitting portion  110   a  continuous to the cooling water passage  110   d  is formed in an edge of the electrode holder  110 . The recessed fitting portion  110   a  has a circular shape in section. The recessed fitting portion  110   a  has no tilt relative to the longitudinal direction of the electrode holder  110 . A first O ring groove  110   b  is formed in an inner peripheral surface of the recessed fitting portion  110   a  along an entire circumference thereof. A planar power-feed face  110   c  is formed in the edge of the electrode holder  110  around an opening of the recessed fitting portion  110   a . The power-feed face  110   c  is orthogonal to the inner peripheral surface of the recessed fitting portion  110   a.    
     An electrode  120  according to the second embodiment is structurally similar to the electrode  20  according to the first embodiment. An O ring  130  is fitted in the first O ring groove  110   b . The fitting portion  120   b  of the electrode  120  is inserted in the recessed fitting portion  110   a , and the O ring  130  is fitted in a second O ring groove  120   c , so that the electrode  120  is attached to the electrode holder  110 . In this state, a power-to-be-fed face  120   d  is in contact with the power-feed face  110   c . A water feed pipe  140  which supplies a cooling water is provided in the cooling water passage  110   d  of the electrode holder  110 . An opening of the water feed pipe  140  is closely facing an end face  120   e  of the fitting portion of the electrode  120 . 
     Similarly to the first embodiment, the second embodiment is characterized in that the cooling water supplied from the water feed pipe  140  increases a water pressure in the recessed fitting portion  110   a , making the power-to-be-fed face  120   d  lose the contact with the power-feed face  110   c . An effect exerted by such a characteristic is similar to that of the first embodiment. 
     During the welding, the power-to-be-fed face  120   d  abuts the power-feed face  110   c , and a pressure loading thereby applied to the electrode  120  is supported by the power-feed face  110   c . This eliminates the conventional problem of widening the recessed fitting portion of the cap chip when the members to be welded are pressed to be bonded to each other by the paired cap chips, causing the cap chip to fall off from the shank. The O ring  130  is fitted in the first O ring groove  110   b  and the second O ring groove  120   c  when the electrode  120  is attached to the electrode holder  110 . As a result, the electrode  20  does not fall off from the electrode holder  110 . 
     Thus far, the present invention was described based on what is at present considered to be the best and most effective embodiments of the invention. However, the present invention is not necessarily limited to the embodiments described in the description of the present application. The present invention can be suitably modified unless such modifications go beyond the scope and the idea of the invention that can be read from the claims and the description. It is intended to cover in the technical scope such modifications of the electrode unit of the spot welding machine. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
           10  electrode holder 
           10   a  projecting portion 
           10   b  recessed fitting portion 
           10   c  first O ring groove 
           10   d  power-feed face 
           10   e  open hole 
           10   f  opening of open hole 
           10   g  cooling water passage 
           10   h  discharge channel 
           10   i  opening of discharge channel 
           10   j  flow path 
           10   k  bottom part of recessed fitting portion 
           20  electrode 
           20   a  abutting portion 
           20   b  fitting portion 
           20   c  second O ring groove 
           20   d  power-to-be-fed face 
           20   e  end face of fitting portion 
           41  inflow port member 
           42  water feed member 
           42   a  water feed pipe 
           42   b  flow path 
           43  discharge port member 
           50  electrode unit 
           90  electrode unit of conventional spot welding machine 
           91  shank 
           91   a  fitting portion 
           92  water feed pipe 
           95  cap chip 
           95   a  recessed fitting portion 
           110  electrode holder 
           110   a  recessed fitting portion 
           110   b  first O ring groove 
           110   c  power-feed face 
           110   d  cooling water passage 
           120  electrode 
           120   a  abutting portion 
           120   b  fitting portion 
           120   c  second O ring groove 
           120   d  power-to-be-fed face 
           120   e  end face of fitting portion 
           140  water feed pipe 
           190  electrode unit of conventional spot welding machine 
           191  electrode holder 
           191   a  projecting portion 
           191   b  fitting hole 
           191   c  cooling water passage 
           195  mini chip 
           195   a  fitting portion 
           195   b  abutting portion 
           198  member to be welded 
           199  member to be welded