Abstract:
In a friction stir welding method of manufacturing a piston for a variable capacity swash plate type compressor, the welder is shifted from the welded portions to a predetermined position while maintaining contact between the welder and one of the piston members. After finishing the friction stir welding, a hole is not formed in the welded portions of the piston members to improve durability of the welded portions. The piston members are elastically supported by support rollers to align the central axial lines of the piston members with the axial line of the chuck, even though the diameters of the piston members differ.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a manufacturing method of a piston for a swash plate type compressor with variable capacity. More particularly, the present invention relates to a manufacturing method of a piston for a swash plate type compressor with variable capacity, which manufactures a piston from two round sections of piston members through friction stir welding without forming a hole in welded portions of the piston members, thereby improving durability of the welded portions and easily manufacturing a piston regardless of the size of the outer diameter of the piston members. 
   2. Background of the Related Art 
   In general, a swash plate type compressor with variable capacity of a cooling apparatus of an automobile includes: front and rear housings forming sealed spaces therein, such as a crank chamber, a suction chamber, a discharge chamber, and so on; a cylinder block installed between the front and rear housings and having a number of cylinder bores arranged in the circumferential direction thereof; a driving shaft rotatably mounted on the center of the cylinder block; a lug plate and a swash plate respectively mounted on the driving shaft; a number of pistons coupled along the circumference of the swash plate via shoes; a valve unit disposed between the cylinder block and the rear housing; a control valve for controlling the movement of the pistons; and a spring elastically mounted on the swash plate for supporting the swash plate at the minimum slope angle when the lug plate does not rotate. 
   In such a swash plate type compressor with variable capacity, weight of the piston acts as an inertia preventing reciprocating motion of the piston during a refrigerant compressing process, thereby deteriorating compression efficiency. To solve the above problem, the piston is manufactured of light-weight material, and recently, the piston has a hollow part formed therein for more light weight. 
   Conventionally, to manufacture the hollow piston, a first piston member and a second piston member are molded separately, wherein the first piston member has a bridge having a shoe pocket for seating shoes connecting the piston and the swash plate therein, and the second piston member has a hollow part. After that, the first and second piston members are temporarily coupled to each other after mechanical processing, and then, the coupled surfaces of the first and second piston members are welded and adhered to each other by electron beam welding under a vacuum condition. 
   However, the conventional hollow piston manufacturing method has several problems in that it is necessary to process the coupled surfaces precisely, that it takes too much time to assemble the first and second piston members, and that it is complex and difficult to process the first and second piston members, so that its productivity is deteriorated, and defective rate becomes too high. Furthermore, the conventional hollow piston manufacturing method has another problem in that durability of the piston is lowered as fine holes are formed inside the piston during the electron beam welding step. The conventional hollow piston manufacturing method has a further problem in that refrigerant and oil are run short because the refrigerant and oil permeate inside the hollow portion through the fine holes. Particularly, the conventional hollow piston manufacturing method has a problem in that maintenance and manufacturing costs are increased as the welding area of the piston must be kept in a high vacuum condition for the electron beam welding. 
   To solve the above problems, European Patent Publication No. 0,959,227 discloses a friction welding method in which after the first and second piston members are molded, parallel joint surfaces are respectively formed on the front surfaces of the first and second piston members, and then, a hollow piston part is formed by frictional heat generated by the first and second piston members to each other under a state where the two joint surfaces face to each other in an atmospheric condition. 
   However, under the above conventional method, the welding temperature is not uniformly distributed on the joint surface due to a minute difference in flatness of the joint surface as the joint surface is flat, and so, it is difficult to uniformly keep the welding strength of the joint surface. Additionally, the temperature and the welding strength are gradually increased toward the outer circumference of the pistons due to a difference in angular velocity on the joint surface of the first and second piston members. However, the welding strength of the remaining portions becomes relatively lower as the outer circumferential layer is removed by mechanical processing after the welding. 
   Meanwhile, U.S. Pat. No. 5,460,317 discloses a friction stir butt welding method, which is different from the above friction welding method. In U.S. Pat. No. 5,460,317, the surface of a continuous or substantially continuous material is provided with a probe of material, which is harder than that of the above material. After the above material and the probe are contacted to each other, the probe is inserted into the material, and at the same time, relatively periodic motion occurs between the probe and the material. A plasticised region is formed in the material around the probe by frictional heat generated by the periodic motion. Then, the material is bonded by coagulating the material. 
   Such a friction stir butt welding method has an advantage in that it has little defect, such as deformation due to thermal distortion, without restriction in kinds of material. However, the friction stir butt welding method has a disadvantage in that a hole is formed in the friction portion between the materials by the probe after the friction welding. 
   As shown in  FIG. 8 , in manufacturing a piston  1 , the friction stir butt welding method has a disadvantage in that it is impossible to form coating film of uniform thickness during a coating process after the friction welding and durability of the welded portion is deteriorated because a hole  5  is formed in a friction portion, i.e., welded portion, between the first piston member  3  having a bridge  2  and the second piston member  4 . 
   That is, as the second piston member  4  of the piston  1  used for the swash plate type compressor with variable capacity is rubbed against the inner surfaces of the cylinder bores during reciprocating motion of the piston  1 , for abrasion resistance and sealability of the outer circumference of the second piston member  4 , a uniform coating film is formed on the outer circumference of the second piston member  4  including the welded portions, and then, an abrasion process is performed as a post-process. Therefore, as described above, if the hole is formed in the welded portion between the piston members, coating liquid penetrates into the hole. When plastic working is performed in this condition, an elevated portion is formed at the position of the hole  5 , and so, the coating film is not uniform. Therefore, an abrasion process can be performed eccentrically, thereby resulting in lowering of durability. 
   The friction stir butt welding method has other disadvantages in that there is possibility to cause bending deformation on the members by pressure when the probe is inserted into the welded portion between the two members, and so, it is difficult to manufacture the piston, which must have a precise outer diameter. 
   To prevent the bending deformation, Japanese Patent laid-open Publication No. Hei 11-156560 discloses a friction stir welding apparatus of preventing bending of piston members, in which one or more support rollers being in contact with the outer circumferences of piston members are installed at the opposite side of the probe, namely, under the two piston members. The support rollers support pressure generated when the probe is inserted, thereby preventing bending of the piston members. 
   However, because the support rollers are installed under a state in which they are fixed to support the lower portions of the two members, in order to rotate the two members on the fixed support rollers, the two members must have precise outer diameter to exactly arrange the two members on the central axis line of rotating means. 
   Therefore, the friction stir welding apparatus of the prior art can manufacture only one kind of piston having the same outer diameter. To manufacture pistons having different outer diameters, it is necessary to change or separately manufacture the welding apparatus according to the outer diameter, thereby increasing the manufacturing costs. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made in view of the above described problems, and an object of the present invention is to provide a manufacturing method of a piston for a swash plate type compressor with variable capacity, which prevents formation of a hole in a welded portion between two piston members, which are coupled integrally to each other by friction stir welding, thereby preventing lowering of durability of the welded portions during a coating process and an abrasion process after the friction stir welding. 
   Another object of the present invention is to provide a manufacturing method of a piston for a swash plate type compressor with variable capacity, which can easily manufacture a piston regardless of the outer diameter size of the piston members. 
   To achieve the above objects, the manufacturing method of a piston according to the present invention comprises: a step of forming the first piston member having a bridge and the first coupling part extending from the bridge; a step of forming the second piston member having the second coupling part coupled with the first coupling part and the hollow part formed by coupling the first piston member and the second piston member with each other; a step of rotatably supporting the first and second piston members to the first and second supporting parts after coupling the first and second piston members temporarily; a step of friction stir welding using friction heat generated by friction contact while the first support part is rotated at predetermined speed after a welder is rotatably advanced downwardly and inserted to a welding portion of the first and second piston members; a step of moving the welder from the welded portions to a predetermined position while maintaining contact between the welder and the first piston member after finishing the friction stir welding of the welding portion and; a step of separating the welder from the first piston member at said position. 
   The manufacturing method of a piston according to the present invention also comprises: a step of forming the first piston member having a bridge and the first coupling part extending from the bridge; a step of forming the second piston member having the second coupling part coupled with the first coupling part and the hollow part formed by coupling the first piston member and the second piston member with each other; a step of position controlling of the central axis line of the first and second piston members to that of the first and second supporting parts after temporarily coupling the first and second piston members and supporting the first and second piston members on the support rollers installed elastically by a elastic spring; a step of rotatably supporting the first and second piston members to the first and second supporting parts after the position controlling step; a step of friction stir welding using friction heat generated by friction contact while the first support part is rotated at predetermined speed after a welding means is rotatably advanced downwardly and inserted to a welding portion of the first and second piston members; a step of moving the welding means inserted to the welded portion to the predetermined position after finishing the friction stir welding of the welding portion and; a step of separating the welding means from the welded portion. 
   The step of friction stir welding can be accomplished by one-time rotation of the first and second piston members using the first supporting part. 
   The step of moving the welding means can be carried out by moving a table within the predetermined distance in the axial direction of the piston. 
   The welding means is moved to the bridge portion of the first piston member from the welded portion by the movement of the table. 
   The support rollers are lifted or lowered by ascent and descent member which is connected to the lower side of supporting member supporting the both support rollers and elastically supported by the elastic spring. 
   The position controlling of the central axis line of the first and second piston members to that of the first and second supporting parts can be accomplished in a state which the upper surface of the second piston member elastically supported by the support rollers is pressurized by a position controlling guide member installed upper side of the second supporting part. 
   After the supporting step of the first and second piston members, the next step can be included; a step of fixing the ascent and descent member by moving the fixing means driven by driving means to the lower side of the ascent and descent member and having the upper surface of the fixing means contacted with the lower side of the ascent and descent member to fix the lower portion of the ascent and descent member which is elastically installed by elastic spring. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a brief sectional view of a swash plate type compressor with variable capacity, to which a piston according to the present invention is adopted; 
       FIG. 2  is a perspective view of the piston of  FIG. 1 ; 
       FIG. 3  is a front view of a friction stir welding apparatus of the piston according to the present invention; 
       FIG. 4  is a side sectional view of a flexible structure of a support roller of  FIG. 3 ; 
       FIG. 5  is a perspective view of a guide member mounted on the upper portion of a second support means of  FIG. 3 ; 
       FIGS. 6   a  to  6   e  are sectional views showing a friction stir welding process of the piston according to the present invention; 
       FIG. 7  is a view showing a state in which a piston is welded according to the present invention; and 
       FIG. 8  is a view showing a state in which a piston is welded by prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 1  briefly shows the structure of a swash plate type compressor with variable capacity, to which a piston according to the present invention is mounted. 
   As shown in  FIG. 1 , the swash plate type compressor includes: front and rear housings  11  and  12  forming sealed spaces therein, such as a crank chamber  11   a , a suction chamber  12   a , and a discharge chamber  12   b ; a cylinder block  13  installed between the front and rear housings  11  and  12  and having a number of cylinder bores  13   a  arranged in the circumferential direction; a driving shaft  14  rotatably mounted on the center of the cylinder block  13  and having a lug plate  15 ; a swash plate  16  hinged with a side of the lug plate  15  and having a slant angle varied according to cooling load; and a number of pistons  20  connected with shoes  17 , which are provided along the circumference of the swash plate  16 , and respectively reciprocating inside each bore  13   a  of the cylinder block  13  according to rotation of the swash plate  16 . 
   The piston used in the swash plate type compressor with variable capacity has a hollow part formed therein to reduce inertia due to the reciprocating motion. 
     FIG. 2  shows the structure of the piston in detail. The piston  20  includes: the first piston member  23  having a bridge  21 , which has a shoe pocket therein for seating the shoe connecting the piston and the swash plate, and the first coupling part  22  extending from the bridge  21  in the longitudinal direction; and the second piston member  26  having the second coupling part  24  extending in the longitudinal direction at a side thereof to couple with the first coupling part  22  of the first piston member  23 , and the hollow part  25  sealed by the coupling of the first and second piston members  23  and  26 . 
   Moreover, stepped support portions  22   a  and  24   a  can be respectively formed at the first coupling part  22  of the first piston member  23  and the second coupling part  24  of the second piston member  26 . 
   The piston  20  is generally manufactured by a friction stir welding method as follows. The first and second piston members  23  and  26  are molded separately, and then, temporarily coupled with each other after mechanical working. Then, after a welding means is in contact with welding portions of the first and second piston members  23  and  26 , the welding portions are welded with each other by friction heat generated during the rotation of the first and second piston members  23  and  26 . 
     FIG. 3  shows a friction stir welding apparatus of the piston for the swash plate type compressor with variable capacity according to the present invention. 
   As shown in  FIG. 3 , the friction stir welding apparatus of the piston includes: a rotation supporting means  31  fixed on the upper surface of a table  36  to rotatably support the first and second piston members  23  and  26 , which are temporarily coupled facing to each other on the coaxial line; and a welding means  40  being in contact with the welding portions of the first and second piston members  23  and  26 , which are supported by the rotation supporting means  31 , to weld the welding portions with each other using friction heat. 
   The rotation supporting means  31  includes: a first supporting part  31   a  for rotatably supporting a side of the first piston member  23 ; a second supporting part  31   b  for rotatably supporting a side of the second piston member  26 ; and a driving part (not shown) for driving one of the first and second supporting parts  31   a  and  31   b . Of course, the driving part can freely control revolution per minute(RPM) of the first and second supporting parts  31   a  and  31   b . Furthermore, one of the first and second supporting parts  31   a  and  31   b , for example, the second supporting part  31   b , is provided with a hydraulic cylinder  32  disposed at an end thereof to move back and forth in an axial direction of the piston. 
   The welding means  40  includes: a servo-transferring device (not shown) for moving vertically to the welding portions of the first and second piston members  23  and  26  temporarily coupled with each other and supported by the rotation supporting means  31 ; and a pin type probe  41  disposed on the front end thereof and inserted into the welding portions while being rotated by a spindle motor (not shown) to generate friction heat for welding. 
   Particularly, support rollers  33  are mounted between the first and second supporting parts  31   a  and  31   b . The support rollers  33  are in contact with the lower portions of the first and second piston members  23  and  26 , which are supported by the first and second supporting parts  31   a  and  31   b , to securely keep the temporarily coupled condition of the first and second piston members  23  and  26 . As shown in  FIGS. 3 and 4 , the two support rollers  33 , which are adjacent to each other to stably support the cylindrical piston  20 , are rotatably mounted on two supporting members  34 , which are mounted on the upper portion of a fixed stand  35  fixed on the upper surface of the table  36 . 
   If the piston  20  having the outer diameter of, for example, about 30 mm, namely, the temporarily coupled first and second piston members  23  and  26 , are loaded on the support rollers  33 , the central axis line of the piston  20  and the central axis line of the rotation supporting means  31  are coincided with each other. In this condition, while the second supporting part  31   b  of the rotation supporting means  31  is moved toward the first supporting part  31   a  by the hydraulic cylinder  32 , a protrusion (not shown), which is formed on the central portion of both ends of the first and second supporting parts  31   a  and  31   b , is exactly inserted into a center hole  27 , which is formed at both ends of the first and second piston members  23  and  26 , and thereby, the piston  20  is supported on the first and second supporting parts  31   a  and  31   b . As described above, in the above condition in which the first and second piston members  23  and  26  are supported on the rotation supporting means  31 , pressure by the probe  41 , which is applied by inserting the probe  41  into the welding portions of the first and second piston members  23  and  26  counteracts to supporting force generated by the support rollers  33 , so that the first and second piston members  23  and  26  can be prevented from bending. 
   However, if the piston  20  having outer diameter larger than the above is loaded on the support rollers  33 , the central axis line of the piston  20  and the central axis line of the rotation supporting means  31  are not coincided, and thereby, the piston  20  cannot be supported on the rotation supporting means  31 . 
   To solve the above problem, the present invention is provided with the support rollers  33  capable of elastic ascent and descent movement. According to the elastic ascent and descent movement of the support rollers  33 , regardless of the outer diameter of the piston  20 , the central axis line of the piston  20  can be exactly coincided with that of the rotation supporting means  31 . 
   That is, as shown in  FIGS. 4   a  and  4   b , the fixed stand  35  disposed on the lower portion of the supporting member  34  for supporting the support rollers  33  has a vertical through-hole  51 . An ascent and descent member  53  is inserted into the vertical through-hole  51 . The ascent and descent member  53  protrudes vertically from the lower surface of the supporting member  34  and is elastically supported by an elastic spring  52 . The ascent and descent member  53  includes the second piston member  53   a  connected to the lower surface of the supporting member  34 , and a supporting rod  53   b  connected to the lower surface of the second piston member  53   a , having the outer diameter smaller than the outer diameter of the second piston member  53   a , and having an inclined end. The lower portion of the supporting rod  53   b  is protruded to the outside through a path narrower than the vertical through-hole  51 . 
   Moreover, the second piston member  53   a  is provided with a concave groove  54  formed in the outer circumference of the lower portion thereof, and the elastic spring  52  is fit on the outer circumference of the supporting rod  53   b . At this time, the front end of the elastic spring  52  is supported on the lower end of the second piston member  53   a , and the rear end of the elastic spring  52  is supported on a latch jaw  55  formed on the border between the vertical through-hole  51  and the path. 
   If the piston  20  of large outer diameter is loaded on the support rollers  33 , load force of the piston  20  is applied to the support rollers  33 , and the supporting member  34  is elastically moved by the ascent and descent member  53  mounted under the supporting member  34 . Therefore, the position of the central axis line can be controlled so that the central axis lines of the piston  20  and the rotation supporting means  31  are coincided with each other. 
   At this time, for reliable position control of the central axis lines, as shown in  FIG. 5 , on the upper portion of one of the supporting parts  31   a  and  31   b , for example, on the upper portion of the second supporting part  31   b , mounted is a guide member  60  pressurizing and holding the upper surface of the piston  20 , which is elastically moved on the support rollers  33 . The guide member  60  includes: a coupling section  60   a  having coupling holes  60   a   1  and  60   a   2  for removably coupling the coupling section  60   a  to the upper portion of the second supporting part  31   b ; an extension portion  60   b  bended and extended from the coupling section  60   a  in a longitudinal direction; an inclined portion  60   c  having a predetermined inclined angle from the extension portion  60   b ; and a holding portion  60   d  bended vertically from the inclined portion  60   c  and having an arc-shaped end for holding the outer surface of the front end of the second piston member  26 . 
   As described above, after the central axis lines of the piston  20  is exactly coincided with that of the rotation supporting means  31  by the guide member  60 , the piston  20  is supported by the rotation supporting means  31 . After the piston  20  is supported on the rotation supporting means  31 , it is necessary to fix the ascent and descent member  53  to prevent further movement of the supporting member  34 . For this purpose, a fixing means  70  is mounted under the supporting rod  53   b  of the ascent and descent member  53  to stop the movement of the ascent and descent member  53 . 
   The fixing means  70  includes: a wedge type fixing member  74  having a horizontal through-hole  71  therein, which is inclined like the end of the supporting rod  53   b , and a movable rod  73  mounted inside the horizontal through-hole  71  and elastically supported by a elastic spring  72 ; and a connection member  76  connected to an end of the movable rod  73 , which protrudes to the outside of the horizontal through-hole  71  of the wedge type fixing member  74 , by a coupling tool  75  such as a bolt. Furthermore, on an end of the connection member  76  connected is an air cylinder  77  for moving the wedge type fixing member  74  by moving the connection member  76  back and forth. Non-explained reference numeral  78  designates the coupling tool such as a bolt. 
   In the fixing means  70 , the connection member  76  is moved back and forth in an arrow direction by pneumatic pressure when the air cylinder  77  is operated, and at the same time, the wedge type fixing member  74  connected to the connection member  76  is moved back and forth. At this time, the movable rod  73  mounted inside the horizontal through-hole  71  of the wedge type fixing member  74  is moved by elasticity of the elastic spring  72 , and then, the wedge type fixing member  74  is also gradually moved. 
     FIG. 4   a  shows a state in which the wedge type fixing member  74  goes back to an end of the supporting rod  53   b  of the ascent and descent member  53 . In this case, the support rollers  33  are elastically lifted or lowered to the extent of ‘H’ by the ascent and descent member  53  because there is a gap between the end of the supporting rod  53   b  and the front end of the wedge type fixing member  74 . 
   In this condition, as shown in  FIG. 4   b , when the wedge type fixing member  74  is advanced, the inclined front end of the wedge type fixing member  74  is in contact with the inclined end of the supporting rod  53   b , and thereby, the ascent and descent member  53  is not moved any more. 
   Meanwhile, as shown in  FIG. 3 , the table  36  mounted on the lower portion of the rotation supporting means  31  is connected to a transferring means  38  so as to be moved laterally within a predetermined distance in an axial direction of the piston by a servo-motor  37 . Thereby, when the servo-motor  37  is operated under a condition in which the probe  41  is inserted into the welding portion of the piston  20  supported on the rotation supporting means  31 , the table  36  is moved within the predetermined distance. The servo-motor  37  for the transferring means can be mounted on the side of table  36  or the welding means  40 . 
   Hereinafter, referring to  FIG. 6 , a manufacturing method of a piston  20  by friction stir welding apparatus will be described. 
   First, the first piston member  23 , which has the bridge  21  and the first coupling part  22  extending from the bridge  21 , is formed. Then, the second piston member  26 , which has the second coupling part  24  coupled with the first coupling part  22  and the hollow part formed by coupling the first piston member  23  and the second piston member  26  with each other, is formed. 
   As described above, the forming of the first and second piston members  23  and  26  can be carried out through a forging process. Moreover, a step for forming a stepped portion can be further included to keep a coupled condition so that the first coupling part  22  is precisely coupled with the second coupling part  24  of cylinder type. 
   When the forming of the first and second piston members is finished, as shown in  FIG. 6   a , the first and second coupling parts  22  and  24  are temporarily coupled with each other. 
   After that, the temporarily coupled first and second piston members  23  and  26  are loaded on the support rollers  33  between the first and second supporting parts  31   a  and  31   b  of the rotation supporting means  31 , and then, the first and second piston members  23  and  26  are supported on the first and second supporting parts  31   a  and  31   b . When the second supporting part  31   b  is advanced toward the second piston member  26  loaded on the support roller  33  by the air cylinder, the protrusion formed on the central portion of the end of the second supporting part  31   b  is inserted into the center hole  27  formed on the center of the second piston member  26 . In this condition, the second supporting part  31   b  is continuously advanced toward the first supporting part  31   a , the center hole  27  of the first piston member  23  temporarily coupled with the second piston member  26  is in contact with the protrusion formed on the end of the first supporting part  31   a , so that the first and second piston members  23  and  26  are firmly supported on the first and second supporting parts  31   a  and  31   b.    
   At this time, if the outer diameter of the first and second piston members  23  and  26 , which are loaded on the support rollers  33  moving upwardly and downwardly, is about 30 mm, the central axis lines of the first and second piston members  23  and  26  and supporting parts  31   a  and  31   b  are coincided with each other, so that the first and second piston members  23  and  26  are almost never influenced by the guide member  60  mounted on the upper portion of the second supporting part  31   b . However, if the outer diameter of the first and second piston members  23  and  26  is larger than the above, the central axis lines are not coincided with each other, and so, the position of the central axis lines must be controlled. In the position controlling step, when the support rollers  33  are moved downwardly under a condition in which the upper portion of the second piston member  26  temporarily coupled with the first piston member  23  is pressed by the guide member  60 , the central axis line of the second piston member  26  is coincided with the central axis line of the second supporting part  31   b . At this time, the guide member  60  can be replaced with a new one according to the outer diameter of the piston  20  to hold the upper portion of the second piston member  26 . 
   When the position controlling of the central axis lines is finished, the wedge type fixing member  74  of the fixing means  70  mounted under the support rollers  33  is advanced by the air cylinder  77 , so that the ascent and descent member  53  is fixed to prevent further movement of the supporting member  34 . 
   After that, as shown in  FIG. 6   b , the welding means  40  is advanced downwardly while being rotated by the spindle motor, and then, as shown in  FIG. 6   c , the probe  41  of the welding means  40  is inserted into the first and second coupling parts  22  and  24 , which are the welding portions of the first and second piston member  23  and  26 . In this condition, by rotating the first supporting part  31   a  once, the probe of the welding means  40  welds the first and second coupling parts  22  and  24  using friction heat generated by friction contact while the prove  41  is moving along the trace of the first and second coupling parts  22  and  24  of the first and second piston members  23  and  26 . 
   When the welding of the first and second coupling parts  22  and  24  by the one-time rotation of the rotation supporting means is finished, the welding means  40 , which is inserted into the welded portions of the first and second coupling parts  22  and  24 , is separated from the welded portions. At this time, when the welding means  40  is separated from the welded portion, as shown in  FIG. 8 , the hole  5  is formed in a portion contacted with the welding means  40  (the portion is included in a coating area), and so, coating fluid is penetrated into the hole  5  during a coating process after the welding. In this condition, when the plastic working and abrasion process are performed, the thickness of the coating film may not be uniform, and durability may be considerably lowered. 
   Therefore, to solve the above problems, before separating the welding means  40  from the welded portion, as shown in  FIG. 6   d , the servo-motor  37  is operated to move the table  36  mounted under the rotation supporting means  31  in the axial direction of the piston, and thereby shifting the position of a hole  28  from the welded portion to a non-coating area to prevent formation of the hole in the welded portion. The position of a hole  28  can be shifted by moving the table  36  or the welding means  40 . 
   By the above moving step, when the position of the hole  28  is moved to the bridge  21  portion of the first piston member  23 , which is the non-coating area, as shown in  FIG. 6   e , the welding means  40  is returned to its original position from the first and second coupling parts  22  and  24  of the first and second piston members  23  and  26  by the spindle motor. 
     FIG. 7  shows the structure of the piston  20  manufactured by the above method. After the friction welding, the hole  28  formed by the probe  41  of the welding means  40  is shifted to the bridge  21  portion of the first piston member  23 . 
   Non-explained reference numeral  29  designates the trace of the probe  41  of the welding means  40  formed on the welded portions of the first and second coupling parts  22  and  24  of the first and second piston members  23  and  26 . 
   As described above, after the welding means is inserted into the welding portions of the two piston members supported by the rotation supporting means and the friction stir welding is performed, the rotation supporting means is moved to a predetermined distance to form the hole in the non-coating area, so that the welding strength of the welded portions is increased and the coating film can be formed uniformly on the coated area of the piston during the coating process after the welding, and thereby, durability of the piston can be increased. 
   Furthermore, according to the present invention, the support rollers are elastically moved upwardly and downwardly to keep the temporarily coupled condition of the two piston member, so that the central axis lines of the two piston members can be easily coincided with the central axis line of the rotation supporting means regardless of the outer diameter of the piston, and thereby, the piston can be freely manufactured size by size. 
   While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.