Patent Publication Number: US-2022226928-A1

Title: Stir pin, friction stir welding tool, and machine tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Application No. PCT/JP2019/039587, filed Oct. 8, 2019. The contents of this application are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a stir pin, a friction stir welding tool, and a machine tool. 
     Discussion of the Background 
     Friction stir welding (Friction Stir Welding) is known. In friction stir welding, a tool pressed against two workpieces, which are joined objects, is rotated to weld the two workpieces together. More specifically, metal workpieces are softened by friction heat generated by the rotation of the tool. The softened metals are guided by the rotation of the tool to flow around the tool. Then, the metals flowing around the tool are solidified, so that the two workpieces are welded together. 
     As a related technique, JP 2018-1178A discloses a friction stir tool. The friction stir tool recited in JP 2018-1178A includes a stir probe and a shoulder. The stir probe has a flow-prevention surface that changes the flow direction of a flowing object from the axial direction of the stir probe to its radial direction. Flowing metal is prevented by the flow-prevention surface from flowing farther and accumulates in an annular flowing-object container. In the friction stir tool recited in JP 2018-1178A, the stir probe is directly connected to a rotational driving axis. 
     JP 6512727B1 discloses a friction stir welding tool. The friction stir welding tool recited in JP 6512727B1 includes a stir pin and a housing having a first surface. The housing has a discharge hole connecting a container hole formed on the first surface to the outside of the housing (through the discharge hole, excess metal is discharged). 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a stir pin includes a base end portion configured to be held in a pin holder to be rotatable about a first axis, a stirring portion provided to project from a shoulder member to be rotatable with respect to the shoulder member about the first axis together with the base end portion, and an intermediate portion including a first portion and a second portion. The second portion is connected to the stirring portion to be rotatable about the first axis together with the stirring portion and has a second diameter passing through the first axis. The first portion is provided between and connected to the base end portion and the second portion to be rotatable about the first axis together with the base end portion and the second portion. The first portion has an end surface to which the second portion is connected coaxially with the first portion and which has a maximum diameter passing through the first axis larger than the second diameter. The end surface has a ring-shaped receiving surface configured to receive a material waste generated when the stirring portion performs a friction stirring. 
     According to another aspect of the present invention, a friction stir welding tool includes a shoulder member having a shoulder surface configured to press a workpiece, and a stir pin provided in the shoulder member rotatable about a first axis with respect to the shoulder member. The stir pin includes a base end portion configured to be held in a pin holder to be rotatable about the first axis, a stirring portion provided to project from the shoulder member to be rotatable with respect to the shoulder member about the first axis together with the base end portion, and an intermediate portion. The intermediate portion includes a second portion connected to the stirring portion to be rotatable about the first axis together with the stirring portion and having a second diameter passing through the first axis, and a first portion provided between and connected to the base end portion and the second portion to be rotatable about the first axis together with the base end portion and the second portion. The first portion has an end surface to which the second portion is connected coaxially with the first portion and which has a maximum diameter passing through the first axis larger than the second diameter. The end surface has a ring-shaped receiving surface configured to receive a material waste generated when the stirring portion performs a friction stirring. 
     According to the other aspect of the present invention, a machine tool includes a friction stir welding tool, a workpiece support member configured to support a workpiece, a tool support member supporting the friction stir welding tool, a first driver configured to move the tool support member relative to the workpiece support member, a second driver configured to rotate a stir pin, and a controller configured to control the first driver and the second driver. The friction stir welding tool includes a shoulder member having a shoulder surface configured to press a workpiece, a pin holder, and a stir pin provided in the shoulder member rotatable about a first axis with respect to the shoulder member. The stir pin includes a base end portion configured to be held in the pin holder to be rotatable about the first axis, a stirring portion provided to project from the shoulder member to be rotatable with respect to the shoulder member about the first axis together with the base end portion, and an intermediate portion. The intermediate portion includes a second portion connected to the stirring portion to be rotatable about the first axis together with the stirring portion and having a second diameter passing through the first axis, and a first portion provided between and connected to the base end portion and the second portion to be rotatable about the first axis together with the base end portion and the second portion. The first portion has an end surface to which the second portion is connected coaxially with the first portion and which has a maximum diameter passing through the first axis larger than the second diameter. The end surface has a ring-shaped receiving surface configured to receive a material waste generated when the stirring portion performs a friction stirring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic cross-sectional view of a friction stir welding tool according to a first embodiment, schematically illustrating this friction stir welding tool; 
         FIG. 2  is a schematic enlarged cross-sectional view of the friction stir welding tool according to the first embodiment, with part of the friction stir welding tool enlarged; 
         FIG. 3  is a schematic cross-sectional view of a stir pin according to the first embodiment, schematically illustrating this stir pin; 
         FIG. 4  is a schematic cross-sectional view of a friction stir welding tool according to a second embodiment, schematically illustrating this friction stir welding tool; 
         FIG. 5  is a schematic enlarged cross-sectional view of the friction stir welding tool according to the second embodiment, with part of the friction stir welding tool enlarged; 
         FIG. 6  is a schematic cross-sectional view of a stir pin according to the second embodiment, schematically illustrating this stir pin; 
         FIG. 7  is a schematic perspective view of the stir pin according to the second embodiment, schematically illustrating this stir pin; 
         FIG. 8  is a schematic perspective view of a stir pin according to a first modification of the second embodiment, schematically illustrating this stir pin; 
         FIG. 9  is a schematic perspective view of a stir pin according to a second modification of the second embodiment, schematically illustrating this stir pin; 
         FIG. 10  is a schematic perspective view of a stir pin according to a third modification of the second embodiment, schematically illustrating this stir pin; 
         FIG. 11  is a schematic enlarged cross-sectional view of a friction stir welding tool according to a third embodiment, with part of the friction stir welding tool enlarged; 
         FIG. 12  is a cross-sectional view of the friction stir welding tool cut along a plane indicated by the arrow A-A illustrated in  FIG. 11 ; 
         FIG. 13  is a schematic partial cross-sectional view of a machine tool according to a fourth embodiment, schematically illustrating this machine tool; and 
         FIG. 14  is a schematic cross-sectional view of a friction stir welding tool according to a modification of an embodiment, schematically illustrating this friction stir welding tool. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     By referring to the accompanying drawings, description will be made with regard to a stir pin  3 , a friction stir welding tool  100 , and a machine tool  200  according to some embodiments of the present invention. It is noted that in the following description of the embodiments, identical reference numerals are used to denote identical portions, members, or components having identical functions, and redundant description of identical portions, members, or components will be eliminated or minimized. 
     Definitions of Directions and Terms 
     As used herein, the term “first direction DR 1 ” is defined as a direction from a base end portion  30  of the stir pin  3  (or a base end portion of the shoulder member  7 ) toward a stirring portion  37  of the stir pin  3  (or a shoulder surface  72   s  of the shoulder member  7 ). Also as used herein, the term “second direction DR 2 ” is defined as a direction opposite to the first direction DR 1 . For example, when a leading end portion of the stir pin  3  is pointed downward, the first direction DR 1  corresponds to downward direction, and the second direction DR 2  corresponds to upward direction. 
     As used herein, the term “machine tool” is intended to mean any machine to which a friction stir welding tool is attachable. The machine tool may be a combined multi-functional machine tool, which is capable of performing a plurality of different kinds of machining (an example is a machining center). An example of the machine tool is a machine capable of cutting, machining, grinding, or joining metal. 
     As used herein, the term “material waste” is intended to mean a piece of material detached from a workpiece as a result of a friction stirring. When the friction stir welding tool  100  joins metal workpieces together, the term “workpiece”, as used herein, can be rephrased as “metal workpiece”, and the term “material waste”, as used herein, can be rephrased as “metal waste”. This, however, does not exclude any configuration in which the friction stir welding tool  100  is used to join non-metal workpieces (for example, resin workpieces) together. 
     First Embodiment 
     By referring to  FIGS. 1 to 3 , a friction stir welding tool  100 A and a stir pin  3 A according to the first embodiment will be described.  FIG. 1  is a schematic cross-sectional view of the friction stir welding tool  100 A according to the first embodiment, schematically illustrating the friction stir welding tool  100 A.  FIG. 2  is a schematic enlarged cross-sectional view of the friction stir welding tool  100 A according to the first embodiment, with part of the friction stir welding tool  100 A enlarged.  FIG. 3  is a schematic cross-sectional view of the stir pin  3 A according to the first embodiment, schematically illustrating this stir pin  3 A. 
     The friction stir welding tool  100 A according to the first embodiment includes the shoulder member  7  and the stir pin  3 A. 
     In the example illustrated in  FIG. 1 , the shoulder member  7  has the shoulder surface  72   s.  The shoulder surface  72   s  is for pressing workpieces, which are to-be-joined objects. The shoulder surface  72   s  is the end face of the shoulder member  7  on the first direction DR 1  side. The shoulder surface  72   s  flattens material (of which the workpiece is made) softened by the rotation of the stir pin  3 A. In the example illustrated in  FIG. 1 , the shoulder surface  72   s  is a surface perpendicular to the first direction DR 1  (in other words, perpendicular to a first axis AX, described later). 
     In the example illustrated in  FIG. 1 , the shoulder member  7  includes: a first member  70 , which has the shoulder surface  72   s;  and a second member  76 . The first member  70  is mounted on the second member  76 . In the example illustrated in  FIG. 1 , the first member  70  and the second member  76  are threaded to each other. The configuration in which the shoulder member  7  includes the first member  70  and the second member  76  makes the frequency by which the second member  76  is replaced lower than the frequency by which the first member  70  is replaced. As a result, the running cost involved in the use of the friction stir welding tool  100 A is reduced. 
     In the example illustrated in  FIG. 1 , the shoulder member  7  is an assembly of two piece-parts ( 70 ,  76 ). Alternatively, the shoulder member  7  may be made up of a single piece-part or may be an assembly of three or more piece-parts. 
     In the example illustrated in  FIG. 1 , the shoulder member  7  has an internal space SP, which extends along the first direction DR 1 . In the internal space SP, at least a part of the stir pin  3 A is provided. That is, the shoulder member  7  also functions as a housing member for at least a part of the stir pin  3 A. In the example illustrated in  FIG. 1 , the entirety of the stir pin  3 A is provided in the internal space SP of the shoulder member  7 . 
     The stirring portion  37  of the stir pin  3 A is inserted into workpieces with the stirring portion  37  in rotating state. The stir pin  3 A may also be referred to as a probe. Then, frictional heat generated by the friction between the stir pin  3 A and the workpieces makes the material of the workpieces (more specifically, metal material) softened. The softened material is stirred by the rotating stir pin  3 A. Then, the stirred material is solidified, and thus the workpieces are joined together. 
     In the example illustrated in  FIG. 1 , the stir pin  3 A is rotatable about the first axis AX relative to the shoulder member  7 . 
     In the example illustrated in  FIG. 1 , the stir pin  3 A rotates relative to the workpieces, whereas the shoulder surface  72   s  does not rotate relative to the workpieces. In the configuration in which the shoulder surface  72   s  does not rotate relative to the workpieces, the area over which frictional heat is generated is smaller than in the configuration in which the shoulder surface  72   s  rotates relative to the workpieces. This reduces the occurrence of workpiece deformation and burrs, resulting in a more satisfactory joint surface. Also in the configuration in which the shoulder surface  72   s  does not rotate relative to the workpieces, the area of contact between the workpieces and the rotatable member (more specifically, the stir pin  3 A) is smaller. This requires a smaller level of force in pressing the rotatable member (more specifically, the stir pin  3 A) against the workpieces in a friction stir welding. 
     The stir pin  3 A includes the base end portion  30 , an intermediate portion  34 , and the stirring portion  37  (in other words, a leading end portion). 
     The base end portion  30  is a portion holdable by a pin holder  8 . Upon rotation of the pin holder  8  about the first axis AX, the base end portion  30  held by the pin holder  8  rotates about the first axis AX relative to the shoulder member  7 . 
     When the stir pin  3 A is mounted on the pin holder  8 , the base end portion  30  is provided further in the second direction DR 2  than a leading end portion  8   e  of the pin holder  8 . 
     The stirring portion  37  is a portion to be inserted into the workpieces and to stir the material of the workpieces. The stirring portion  37  protrudes beyond the shoulder surface  72   s  in the first direction DR 1 . The stirring portion  37  is rotatable about the first axis AX relative to the shoulder member  7 . By rotating the stirring portion  37  about the first axis AX, the workpieces contacting the stirring portion  37  are stirred due to the friction between the workpieces and the stirring portion  37 . 
     The intermediate portion  34  is a portion provided between the base end portion  30  and the stirring portion  37 . In the example illustrated in  FIG. 2 , the portion of the stir pin  3 A that is provided further in the first direction DR 1  than the leading end portion  8   e  of the pin holder  8  and that is provided further in the second direction DR 2  than the shoulder surface  72   s  is the intermediate portion  34 . 
     The intermediate portion  34  is rotatable about the first axis AX together with the stirring portion  37 . The intermediate portion  34  is also rotatable about the first axis AX together with the base end portion  30 . For example, the stirring portion  37 , the intermediate portion  34 , and the base end portion  30  make up a single, integrally formed piece-part. 
     In the example illustrated in  FIG. 2 , the intermediate portion  34  includes a first portion  340  and a second portion  344 , which is provided further in the first direction DR 1  than the first portion  340 . The first portion  340  has a first diameter which passes through the first axis AX. The second portion  344  has a second diameter which passes through the first axis AX. 
     The first portion  340  includes a protrusion  341 , which protrudes beyond an outer surface  344   t  of the second portion  344  in a direction away from the first axis AX. Also, the surface (an example of “an end surface” recited in claims) of the protrusion  341  pointed in the first direction DR 1  includes a ring-shaped receiving surface  341   s,  which receives a material waste generated as a result of a friction stirring. The maximum diameter of the ring-shaped receiving surface  341   s  is equal to the first diameter of the first portion  340 . The maximum diameter of the ring-shaped receiving surface  341   s  is larger than the second diameter of the second portion  344 . 
     Functions of the ring-shaped receiving surface  341   s  will be described. 
     The following description assumes a case in which the stirring portion  37  illustrated in the example illustrated in  FIG. 2  rotates about the first axis AX with the workpieces and the stirring portion  37  in mutually contacting state. In this case, part of the material (in other words, material waste) detached from the workpieces as a result of a friction stirring enters the shoulder member  7  through a first through hole  72   h,  which is provided at a leading end portion  72  of the shoulder member  7 . The material waste entering the shoulder member  7  is received by the ring-shaped receiving surface  341   s.  The material waste received by the ring-shaped receiving surface  341   s  is moved in a direction away from the first axis AX by centrifugal force generated by the ring-shaped receiving surface  341   s  rotating about the first axis AX. 
     If the material waste attached to the pin holder  8  is cooled and solidified, it is possible that the pin holder  8  and the stir pin  3 A are fixed to each other via the material waste. In contrast, in the stir pin  3 A and the friction stir welding tool  100 A according to the first embodiment, the material waste is moved in a direction away from the first axis AX. Thus, the material waste is prevented from attaching to the pin holder  8  (in particular, to the boundary, BR, between the pin holder  8  and the stir pin  3 A). 
     In the stir pin  3 A and the friction stir welding tool  100 A according to the first embodiment, the pin holder  8  and the stir pin  3 A are prevented from being fixed to each other via the material waste. This makes the stir pin  3 A easily detachable from the pin holder  8  (in other words, the stir pin  3 A is made easily replaceable). Also, since no material waste enters the gap between the pin holder  8  and the stir pin  3 A, the pin holder  8  is prevented from being deformed (more specifically, a first hole  81   h,  which receives the stir pin  3 A, is prevented from being deformed). 
     Next, by referring to  FIGS. 1 to 3 , an optional configuration employable in the first embodiment will be described. 
     Ring-Shaped Receiving Surface  341   s    
     In the example illustrated in  FIG. 3 , the outer diameter of the ring-shaped receiving surface  341   s  is equal to the outer diameter of the base end portion  30  of the stir pin  3 A. In other words, a first imaginary circle is defined by an outermost edge P 1  of the ring-shaped receiving surface  341   s  (that is, a point that is among the points on the ring-shaped receiving surface  341   s  and that is farthest from the first axis AX) rotating about the first axis AX, and the radius of the first imaginary circle is defined as a first radius D 1  (an example of a half of “a maximum diameter”); and a second imaginary circle is defined by an outermost edge P 2  of the base end portion  30  (that is, a point that is among the points on the outer circumferential surface of the base end portion  30  and that is farthest from the first axis AX) rotating about the first axis AX, and the radius of the second imaginary circle is defined as a second radius D 2 . In this case, the first radius D 1  is equal to the second radius D 2 . Also in the example illustrated in  FIG. 3 , the distance between the outermost edge P 1  of the ring-shaped receiving surface  341   s  and the first axis AX is equal to the maximum distance between an arbitrary point on the outer circumferential surface of the stir pin  3 A and the first axis AX. 
     As exemplified in  FIG. 2 , in the configuration in which the base end portion  30  of the stir pin  3 A is held by the pin holder  8 , the position of the outer surface of the base end portion  30  of the stir pin  3 A is approximately equal to the position of the inner surface of the pin holder  8 . With this configuration, the material waste received by the ring-shaped receiving surface  341   s  is moved by centrifugal force in a direction away from the first axis AX beyond the outermost edge P 1  of the ring-shaped receiving surface  341   s.  Thus, the material waste is prevented from entering the gap between the outer surface of the base end portion  30  of the stir pin  3 A and the inner surface of the pin holder  8 . 
     It is to be noted that from the viewpoint of more reliably preventing the material waste from entering the gap between the outer surface of the base end portion  30  of the stir pin  3 A and the inner surface of the pin holder  8 , the outer diameter of the ring-shaped receiving surface  341   s  is preferably larger than the outer diameter of the base end portion  30  of the stir pin  3 A (see  FIG. 6 ). In other words, the first radius D 1  is preferably larger than the second radius D 2 . 
     In the example illustrated in  FIG. 2 , the ring-shaped receiving surface  341   s  includes a protruding curved surface (in other words, a curved surface protruding in the first direction DR 1 ). In this case, the gap between the ring-shaped receiving surface  341   s  and the shoulder member  7  (more specifically, a first inner surface  72   b,  described later) becomes smaller. This prevents the material waste from entering the gap between the ring-shaped receiving surface  341   s  and the first inner surface  72   b.    
     In the example illustrated in  FIG. 2 , an outer portion  3410   s  of the ring-shaped receiving surface  341   s  has an inclined surface TS, which is inclined in a direction toward the first axis AX as the inclined surface TS is closer to the stirring portion  37 . In this case, an inner portion  3411   s  of the ring-shaped receiving surface  341   s  protrudes toward the first direction DR 1  side beyond the outer portion  3410   s  of the ring-shaped receiving surface  341   s.  This diminishes the gap between the inner portion  3411   s  of the ring-shaped receiving surface  341   s  and the shoulder member  7  (more specifically, the first inner surface  72   b ). Thus, the material waste is prevented from entering the gap between the ring-shaped receiving surface  341   s  and the first inner surface  72   b.  The shape of the cross-section of the inclined surface TS (a cross-section cut along a plane including the first axis AX) may be a curvilinear shape, as exemplified in  FIG. 2 , or may be a linear shape. 
     First Portion  340  and Second Portion  344   
     In the example illustrated in  FIG. 3 , the first portion  340  of the intermediate portion  34  is a portion provided further in the second direction DR 2  than an end P 3  of the ring-shaped receiving surface  341   s  on the first direction DR 1  side. As exemplified in  FIG. 2 , when the stir pin  3 A is mounted on the pin holder  8 , the first portion  340  of the intermediate portion  34  is provided further in the first direction DR 1  than the leading end portion  8   e  of the pin holder  8 . Thus, the ring-shaped receiving surface  341   s,  which is provided at the first portion  340 , prevents the material waste from attaching to the pin holder  8 . Also, the first portion  340  is provided further in the second direction DR 2  than the first inner surface  72   b  of the shoulder member  7 . This enables the ring-shaped receiving surface  341   s,  which is provided at the first portion  340 , to receive the material waste entering the shoulder member  7 . 
     In the example illustrated in  FIG. 3 , the second portion  344  of the intermediate portion  34  is a portion provided further in the first direction DR 1  than the end P 3  of the ring-shaped receiving surface  341   s  on the first direction DR 1  side. Also, the second portion  344  of the intermediate portion  34  is a portion provided further in the second direction DR 2  than the stirring portion  37 . As exemplified in  FIG. 2 , in the configuration in which the shoulder member  7  is provided outside the stir pin  3 A, the second portion  344  of the intermediate portion  34  is provided between the shoulder surface  72   s  and the ring-shaped receiving surface  341   s.  In the example illustrated in  FIG. 2 , part of the second portion  344  is provided in the first through hole  72   h  of the shoulder member  7 . 
     When material waste has entered the shoulder member  7  through the first through hole  72   h  of the shoulder member  7 , the material waste is guided by the outer surface  344   t  of the second portion  344  in a direction toward the ring-shaped receiving surface  341   s.  In a cross-section perpendicular to the first axis AX, an example shape of the outer surface  344   t  of the second portion  344  is a circular shape. 
     Stirring Portion  37   
     In the example illustrated in  FIG. 3 , the stirring portion  37  has a tapered shape. The stirring portion  37  may be threaded on at least a part of the side surface. The shape of the cross-section of the stirring portion  37  cut along a plane perpendicular to the first axis AX may be a circular shape, an approximately polygonal shape (for example, a round-cornered triangle), or any other shape. 
     Base End Portion  30   
     In the example illustrated in  FIG. 3 , the base end portion  30  of the stir pin  3 A has an outer circumferential surface  30   t  and a base end surface  30   s.  The outer circumferential surface  30   t  may have a circular first surface  31   t  and a planar second surface  32   t.    
     In the example illustrated in  FIG. 2 , the first surface  31   t  has a shape complementary to the inner circumferential surface of the pin holder  8 , which defines the first hole  81   h  of the pin holder  8 . In a cross-section perpendicular to the first axis AX, the first surface  31   t  has a circular shape. 
     The base end portion  30  has a pressed surface pressable by a first fixing member  83 . In the example illustrated in  FIG. 2 , the pressed surface is the above-described second surface  32   t.  In a cross-section perpendicular to the first axis AX, the second surface  32   t  has a linear shape. 
     By pressing the pressed surface (more specifically, the second surface  32   t ), the first fixing member  83  fixes the base end portion  30  of the stir pin  3 A to the pin holder  8  unmovably relative to the pin holder  8 . 
     The base end portion  30  has a position adjustable surface GS, whose position in a direction along the first direction DR 1  is adjustable by a stopper member  85 , described later. In the example illustrated in  FIG. 1 , the position adjustable surface GS is the base end surface  30   s.  An example of the base end surface  30   s  is a flat surface. A mechanism for adjusting the position of the position adjustable surface GS using the stopper member  85  will be described later. 
     Shoulder Member  7   
     In the example illustrated in  FIG. 2 , the shoulder member  7  has the shoulder surface  72   s  and the first inner surface  72   b.  The shoulder surface  72   s  is a surface of the shoulder member  7  pointed in the first direction DR 1 , and the first inner surface  72   b  is a surface of the shoulder member  7  facing the ring-shaped receiving surface  341   s.    
     In the example illustrated in  FIG. 2 , the position of the stir pin  3 A is adjustable so that the gap between the first inner surface  72   b  of the shoulder member  7  and the ring-shaped receiving surface  341   s  is a minimal gap. In this case, it is difficult for the material waste to enter the gap between the first inner surface  72   b  and the ring-shaped receiving surface  341   s.    
     As used herein, the term “minimal gap” is intended to mean a gap so small that material waste is substantially prevented from entering the gap. The size of the minimal gap (in other words, the gap between the first inner surface  72   b  and the ring-shaped receiving surface  341   s ) is, for example, 2 mm or less, 1 mm or less, or 0.5 mm or less. 
     In the example illustrated in  FIG. 2 , the shoulder member  7  has a second through hole  74   h,  in addition to the first through hole  72   h.  Through the second through hole  74   h,  the material waste entering the shoulder member  7  is discharged to the outside of the shoulder member  7 . 
     In the example illustrated in  FIG. 2 , the first through hole  72   h  is provided on the shoulder surface  72   s  of the shoulder member  7 . The first through hole  72   h  extends along the first direction DR 1 . 
     In the example illustrated in  FIG. 2 , the second through hole  74   h  is provided on the side surface of the shoulder member  7 . The second through hole  74   h  extends in a direction crossing the first direction DR 1  (for example, a direction orthogonal to the first direction DR 1 ). In the example illustrated in  FIG. 2 , the number of second through holes  74   h  (in other words, through holes through which to discharge the material waste) that the shoulder member  7  has is one. Alternatively, the number of second through holes  74   h  that the shoulder member  7  has may be two, or three or more. 
     In the configuration in which the shoulder member  7  includes the second through hole  74   h,  the material waste is prevented from staying in the shoulder member  7 . This, as a result, prevents the wear of the stir pin  3 A and/or the pin holder  8  that may otherwise occur due to the friction between the stir pin  3 A and/or the pin holder  8  and the material waste (more specifically, metal waste). Also in the configuration in which the material waste is discharged to the outside of the shoulder member  7 , the material waste (more specifically, metal waste) is prevented from being stirred continuously in the shoulder member  7 . This, as a result, prevents overheating of the stir pin  3 A and/or the pin holder  8 . 
     Thus, in the configuration in which the shoulder member  7  includes the second through hole  74   h,  the stir pin  3 A and/or the pin holder  8  are kept in good condition. Thus, the lifetime (durability) of the stir pin  3 A and/or the pin holder  8  is elongated. Also, as a result of the stir pin  3 A and/or the pin holder  8  being kept in good condition, the joint surfaces of the workpieces are kept in good quality. 
     In the example illustrated in  FIG. 2 , from the viewpoint of efficiently discharging the material waste to the outside of the shoulder member  7 , the position of the stir pin  3 A is preferably adjustable so that a cross-section CS 1  crosses the second through hole  74   h.  The cross-section CS 1  passes through the outermost edge P 1  of the ring-shaped receiving surface  341   s  and is perpendicular to the first axis AX. In this case, the material waste receives centrifugal force from the ring-shaped receiving surface  341   s,  and using this centrifugal force, the material waste can be guided to the second through hole  74   h.    
     In the example illustrated in  FIG. 2 , an opening OP 1  of the second through hole  74   h  on the second direction DR 2  side is substantially covered by the ring-shaped receiving surface  341   s;  and an opening OP 2  of the second through hole  74   h  on the first axis AX side is substantially covered by the outer circumferential surface ( 344   t ) of the stir pin  3 A. In this case, the material waste entering the shoulder member  7  through the first through hole  72   h  is smoothly guided to the second through hole  74   h.    
     In the example illustrated in  FIG. 2 , the outermost edge P 1  of the ring-shaped receiving surface  341   s  is provided further in the first direction DR 1  than an end P 4  of the second through hole  74   h  on the second direction DR 2  side. In this case, the material waste entering the shoulder member  7  is effectively prevented from moving in the second direction DR 2  beyond the second through hole  74   h.  Thus, the material waste is prevented from staying in the shoulder member  7  for the elongated period of time that the material waste may otherwise take if the material waste came farther in the shoulder member  7  beyond the second through hole  74   h.    
     The friction stir welding tool  100 A (more specifically, the pin holder  8 ) preferably includes an adjustment mechanism G (see  FIG. 1 ), which adjusts the position of the ring-shaped receiving surface  341   s  in the first direction DR 1 . This is from the viewpoint of adjusting the position of the ring-shaped receiving surface  341   s  so that the gap between the first inner surface  72   b  of the shoulder member  7  and the ring-shaped receiving surface  341   s  is a minimal gap; and/or the viewpoint of adjusting the position of the ring-shaped receiving surface  341   s  so that the outermost edge P 1  of the ring-shaped receiving surface  341   s  is positioned further in the first direction DR 1  than the end P 4  of the second through hole  74   h  on the second direction DR 2  side. Details of the adjustment mechanism G will be described later. 
     Pin Holder  8   
     In the example illustrated in  FIG. 1 , the friction stir welding tool  100 A includes the pin holder  8 , which is rotatable about the first axis AX together with the stir pin  3 A. 
     The pin holder  8  includes: a holder body  80 ; the first hole  81   h,  which is for receiving the base end portion  30  of the stir pin  3 A; and the first fixing member  83 , which is for fixing the base end portion  30  of the stir pin  3 A to the holder body  80 . 
     The first hole  81   h  is provided in the holder body  80  and extends along the first direction DR 1 . In the example illustrated in  FIG. 1 , the first hole  81   h  is capable of receiving the base end portion  30  of the stir pin  3 A, and is capable of receiving a leading end portion of the stopper member  85 , described later. 
     In the example illustrated in  FIG. 1 , the holder body  80  has a second hole  82   h . The second hole  82   h  is formed on the side wall of the holder body  80 . The second hole  82   h  is connected to the first hole  81   h.    
     In the second hole  82   h,  the first fixing member  83  is inserted. The first fixing member  83  is for fixing the base end portion  30  of the stir pin  3 A. An example of the first fixing member  83  is a set screw. In the example illustrated in  FIG. 2 , the first fixing member  83  has an external screw portion  83   a,  which is screwable with an internal screw portion  82   a , which is formed on the second hole  82   h.    
     Adjustment Mechanism G 
     In the example illustrated in  FIG. 1 , the pin holder  8  includes the adjustment mechanism G, which adjusts the position of the ring-shaped receiving surface  341   s.  The adjustment mechanism G includes: the stopper member  85 , which is contactable with the position adjustable surface GS (more specifically, the base end surface  30   s ) of the stir pin  3 A; and a guide  86 , which guides the movement of the stopper member  85 . 
     The guide  86  is implemented by, for example, a third hole  86   h,  which is formed on the side wall of the pin holder  8  (more specifically, the holder body  80 ). In the example illustrated in  FIG. 1 , the third hole  86   h  is connected to the first hole  81   h.  The third hole  86   h  is inclined relative to the first hole  81   h.    
     In the example illustrated in  FIG. 1 , the position of the stopper member  85  is adjustable along the guide  86  (more specifically, the third hole  86   h ). In this manner, the position of the stopper member  85  (more specifically, a stopper surface  85   s,  which is contactable with the position adjustable surface GS of the stir pin  3 A) in a direction along the first direction DR 1  is adjustable. By adjusting the position of the stopper surface  85   s  in a direction along the first direction DR 1 , the position of the position adjustable surface GS, which is provided to contact the stopper surface  85   s,  is adjusted. Also, by adjusting the position of the position adjustable surface GS, the position of the ring-shaped receiving surface  341   s  in the first direction DR 1  is adjusted. 
     In the example illustrated in  FIG. 1 , the stopper member  85  is a set screw having an external screw portion that is screwabale with an internal screw portion  86   a,  which is formed on the third hole  86   h.    
     In the example illustrated in  FIG. 1 , after the position of the stopper member  85  is adjusted, the stir pin  3 A is inserted into the first hole  81   h  such that the stir pin  3 A contacts the stopper member  85 . Then, the stir pin  3 A is fixed to the holder body  80  using the first fixing member  83 . Thus, the stir pin  3 A is positioned approximately relative to the pin holder  8  and is fixed to the pin holder  8 . 
     Second Embodiment 
     By referring to  FIGS. 4 to 10 , a friction stir welding tool  100 B and a stir pin  3 B according to the second embodiment will be described.  FIG. 4  is a schematic cross-sectional view of the friction stir welding tool  100 B according to the second embodiment, schematically illustrating this friction stir welding tool  100 B.  FIG. 5  is a schematic enlarged cross-sectional view of the friction stir welding tool  100 B according to the second embodiment, with part of the friction stir welding tool  100 B enlarged.  FIG. 6  is a schematic cross-sectional view of the stir pin  3 B according to the second embodiment, schematically illustrating this stir pin  3 B.  FIG. 7  is a schematic perspective view of the stir pin  3 B according to the second embodiment, schematically illustrating this stir pin  3 B.  FIG. 8  is a schematic perspective view of a stir pin  3 B according to a first modification of the second embodiment, schematically illustrating this stir pin  3 B.  FIG. 9  is a schematic perspective view of a stir pin  3 B according to a second modification of the second embodiment, schematically illustrating this stir pin  3 B.  FIG. 10  is a schematic perspective view of a stir pin  3 B according to a third modification of the second embodiment, schematically illustrating this stir pin  3 B. 
     The stir pin  3 B (or the friction stir welding tool  100 B) according to the second embodiment is different from the stir pin  3 A (or the friction stir welding tool  100 A) according to the first embodiment in that the ring-shaped receiving surface  341   s  has a form of flange F. Otherwise, the stir pin  3 B (or the friction stir welding tool  100 B) according to the second embodiment is similar to the stir pin  3 A (or the friction stir welding tool  100 A) according to the first embodiment. 
     The following description of the second embodiment will primarily focus on the flange F and omit those descriptions already provided in the first embodiment, to avoid redundancy. Thus, it will be readily appreciated that those respects that are not explicitly described in the second embodiment but are described in the first embodiment apply in the second embodiment. 
     The stir pin  3 B according to the second embodiment includes: a base end portion  30 , which is holdable by the pin holder  8 ; an intermediate portion  34 ; and a stirring portion  37  (in other words, leading end portion). The structure and the shape of the base end portion  30  described in the first embodiment may be employed as the structure and the shape of the base end portion  30  of the stir pin  3 B. The structure and the shape of the stirring portion  37  described in the first embodiment may be employed as the structure and the shape of the stirring portion  37  of the stir pin  3 B. 
     As illustrated in  FIG. 5 , the intermediate portion  34  includes a first portion  340  and a second portion  344 . The structure and the shape of the second portion  344  described in the first embodiment may be employed as the structure and the shape of the second portion  344  according to the second embodiment. 
     The first portion  340  is a portion provided further in the second direction DR 2  than the end P 3  of the ring-shaped receiving surface  341   s  on the first direction DR 1  side. The second portion  344  is a portion provided further in the first direction DR 1  than the end P 3  of the ring-shaped receiving surface  341   s  on the first direction DR 1  side. 
     In the example illustrated in  FIG. 5 , the first portion  340  includes a protrusion  341 , which protrudes in a direction away from the first axis AX beyond the outer surface  344   t  of the second portion  344 . In the example illustrated in  FIG. 5 , the protrusion  341  includes the flange F, which protrudes in a direction away from the first axis AX. The flange F may also be referred to as a brim or a disc. 
     In the example illustrated in  FIG. 5 , the surface (an example of “an end surface” recited in claims) of the flange F pointed in the first direction DR 1  includes the ring-shaped receiving surface  341   s.  The shape of the ring-shaped receiving surface  341   s  described in the first embodiment may be employed as the shape of the ring-shaped receiving surface  341   s  according to the second embodiment. The position of the ring-shaped receiving surface  341   s  relative to the shoulder member  7  described in the first embodiment may be employed as the position of the ring-shaped receiving surface  341   s  relative to the shoulder member  7  (more specifically, the second through hole  74   h ) according to the second embodiment. 
     The stir pin  3 B (or the friction stir welding tool  100 B) according to the second embodiment has a ring-shaped receiving surface  341   s.  With this configuration, the stir pin  3 B (or the friction stir welding tool  100 B) according to the second embodiment provides effects similar to the effects provided by the stir pin  3 A (or the friction stir welding tool  100 A) according to the first embodiment. 
     Also, in the configuration in which the protrusion  341  includes the flange F, the surface of the flange F pointed in the second direction DR 2  (in other words, a rear surface  342  of the flange F) is pointed to the leading end portion  8   e  (or a face  80   e ) of the pin holder  8  on the first direction DR 1  side. This configuration enables the flange F to cover (in other words, protect) at least a part of the leading end portion  8   e  of the pin holder  8  on the first direction DR 1  side. The above configuration also enables the flange F to prevent material waste from attaching to the leading end portion  8   e  of the pin holder  8  on the first direction DR 1  side. 
     In the example illustrated in  FIG. 5 , as seen from the direction from the stirring portion  37  toward the base end portion  30  (in other words, as seen from the direction along the second direction DR 2 ), the boundary BR between the outer surface of the base end portion  30  of the stir pin  3 B and the inner surface of the pin holder  8  is covered by the flange F. In this case, the material waste is prevented from entering the gap between the outer surface of the base end portion  30  of the stir pin  3 B and the inner surface of the pin holder  8 . As a result, the pin holder  8  and the stir pin  3 B are prevented from being fixed to each other via the material waste. This makes the stir pin  3 B easily detachable from the pin holder  8  (in other words, the stir pin  3 B is made easily replaceable). Also, since no material waste enters the gap between the pin holder  8  and the stir pin  3 B, the pin holder  8  is prevented from being deformed (more specifically, the first hole  81   h,  which receives the stir pin  3 B, is prevented from being deformed). 
     In the example illustrated in  FIG. 6 , the outer diameter of the ring-shaped receiving surface  341   s  is larger than the outer diameter of the base end portion  30  of the stir pin  3 B. In other words, a first imaginary circle is defined by an outermost edge P 1  of the ring-shaped receiving surface  341   s  (that is, a point that is among the points on the ring-shaped receiving surface  341   s  and that is farthest from the first axis AX) rotating about the first axis AX, and the radius of the first imaginary circle is defined as a first radius D 1  (an example of a half of “a maximum diameter”); and a second imaginary circle is defined by an outermost edge P 2  of the base end portion  30  (that is, a point that is among the points on the outer circumferential surface of the base end portion  30  and that is farthest from the first axis AX) rotating about the first axis AX, and the radius of the second imaginary circle is defined as a second radius D 2 . In this case, the first radius D 1  is larger the second radius D 2 . Also in the example illustrated in  FIG. 6 , the distance between the outermost edge P 1  of the ring-shaped receiving surface  341   s  and the first axis AX is equal to the maximum distance between an arbitrary point on the outer circumferential surface of the stir pin  3 B and the first axis AX. 
     In the configuration in which the first radius D 1  is larger than the second radius D 2 , the boundary BR between the outer surface of the base end portion  30  of the stir pin  3 B and the inner surface of the pin holder  8  is hidden behind the ring-shaped receiving surface  341   s.  This reliably prevents the material waste from entering the gap between the outer surface of the base end portion  30  of the stir pin  3 B and the inner surface of the pin holder  8 . 
     Stir Pin  3 B According to First Modification 
     In the example illustrated in  FIG. 7 , the outer circumstantial shape of the flange F is a circular shape. Alternatively, as exemplified in  FIG. 8 , the outer circumstantial shape of the flange F may be a non-circular shape (for example, a polygonal shape such as a hexagonal shape). 
     Stir Pin  3 B According to Second Modification 
     In the examples illustrated in  FIGS. 7 and 8 , the ring-shaped receiving surface  341   s  is a smooth surface. Alternatively, a protrusion, a depression, or a groove may be formed on the ring-shaped receiving surface  341   s.    
     In the example illustrated in  FIG. 9 , a plurality of protrusions  341   d  are formed on the ring-shaped receiving surface  341   s.  In the configuration in which the plurality of protrusions  341   d  are formed on the ring-shaped receiving surface  341   s,  the material waste entering the gap between the ring-shaped receiving surface  341   s  and the shoulder member  7  is raked out by the plurality of protrusions  341   d.  In the example illustrated in  FIG. 9 , the plurality of protrusions  341   d  are arranged at equal angular intervals around the first axis AX. It is to be noted that the arrangement of the plurality of protrusions  341   d  will not be limited to the arrangement illustrated in  FIG. 9 ; any other arrangement is possible. 
     Each of the protrusions  341   d,  which are formed on the ring-shaped receiving surface  341   s,  may have such a shape that a width W of the protrusion  341   d  in a radial direction of the protrusion  341   d  (in other words, a direction perpendicular to the first axis AX) gradually increases along a circumferential direction of the protrusion  341   d  (more specifically, a direction opposite to the rotation direction R 1  of the stir pin  3 B). Alternatively or additionally, each of the protrusions  341   d,  which are formed on the ring-shaped receiving surface  341   s,  may have such a shape that the height of the protrusion  341   d  (in other words, the height over which the protrusion  341   d  protrudes in the first direction DR 1 ) gradually increases along the circumferential direction of the protrusion  341   d  (more specifically, the direction opposite to the rotation direction R 1  of the stir pin  3 B). 
     Stir Pin  3 B According to Third Modification 
     In the example illustrated in  FIG. 10 , a groove (more specifically, a spiral groove  341   m ) is formed on the ring-shaped receiving surface  341   s.  The spiral groove  341   m  preferably has such a shape that the distance from the first axis AX gradually increases along the circumferential direction (more specifically, the direction opposite to the rotation direction R 1  of the stir pin  3 B). In this case, the spiral groove  341   m  rotates about the first axis AX together with the stir pin  3 B, causing the material waste contacting the spiral groove  341   m  to move in a radially outward direction. 
     Third Embodiment 
     By referring to  FIGS. 11 and 12 , a shoulder member  7 C and a friction stir welding tool  100 C according to the third embodiment will be described.  FIG. 11  is a schematic enlarged cross-sectional view of the friction stir welding tool  100 C according to the third embodiment, with part of the friction stir welding tool  100 C enlarged.  FIG. 12  is a cross-sectional view of the friction stir welding tool  100 C cut along a plane indicated by the arrow A-A illustrated in  FIG. 11 . In  FIGS. 11 and 12 , the stir pin  3  and the pin holder  8  are indicated by broken lines. 
     The shoulder member  7 C (or the friction stir welding tool  100 C) according to the third embodiment is characterized in the shape and the structure of the leading end portion  72  of the shoulder member  7 . Otherwise, the shoulder member  7 C (or the friction stir welding tool  100 C) according to the third embodiment is similar to the shoulder member  7  (or the friction stir welding tool  100 A) according to the first embodiment or to the shoulder member  7  (or the friction stir welding tool  100 B) according to the second embodiment. 
     The following description of the third embodiment will primarily focus on the shoulder member  7 C and omit those descriptions already provided in the first embodiment or the second embodiment, to avoid redundancy. Thus, it will be readily appreciated that those respects that are not explicitly described in the third embodiment but are described in the first embodiment or the second embodiment apply in the third embodiment. 
     The shoulder member  7  (more specifically, the leading end portion  72  of the shoulder member  7 ) has a first through hole  72   h,  through which the stir pin  3  is passed. The stir pin  3  according to the third embodiment may be the stir pin  3 A according to the first embodiment, the stir pin  3 B according to the second embodiment, or any other stir pin. 
     In the example illustrated in  FIG. 11 , the shoulder member  7  (more specifically, the leading end portion  72  of the shoulder member  7 ) includes a material waste receiving portion  720 . The material waste receiving portion  720  is closer to the base end portion than the first through hole  72   h  is to the base end portion (in other words, the material waste receiving portion  720  is provided further in the second direction DR 2  than the first through hole  72   h ). 
     In the example illustrated in  FIG. 11 , a pocket PC, which is for receiving the material waste, is defined by: a wall surface  720   s  (more specifically, the inner circumferential surface of the material waste receiving portion  720 ), which defines the material waste receiving portion  720 ; and the outer circumferential surface of the stir pin  3 . 
     In the example illustrated in  FIG. 11 , the shoulder member  7  (more specifically, the leading end portion  72  of the shoulder member  7 ) has a second through hole  74   h,  through which the material waste is discharged. Also in the example illustrated in  FIG. 11 , the pocket PC is connected to the second through hole  74   h.  In this case, the material waste temporarily stored in the pocket PC is discharged to the outside of the shoulder member  7  through the second through hole  74   h.    
     In the example illustrated in  FIG. 11 , the number of second through holes  74   h  (in other words, discharge holes) that the shoulder member  7  has is one. Alternatively, the number of second through holes  74   h  that the shoulder member  7  has may be two or more. 
     In the example illustrated in  FIG. 11 , the end of the pocket PC on the second direction DR 2  side is covered by the ring-shaped receiving surface  341   s.  In this case, the material waste entering the pocket PC is smoothly guided to the second through hole  74   h  by the ring-shaped receiving surface  341   s.  Also in the example illustrated in  FIG. 11 , the end of the pocket PC on the first direction DR 1  side is substantially covered by an inward protrusion  72   r,  which is provided at the leading end portion  72  of the shoulder member  7 . This ensures that the inward protrusion  72   r  reduces the intrusion of the material waste into the pocket PC. 
     In the example illustrated in  FIG. 11 , the material waste receiving portion  720  is provided such that a cross-section CS 2  crosses the second through hole  74   h.  The cross-section CS 2  passes through the material waste receiving portion  720  and is perpendicular to the first axis AX. In this case, the material waste received in the material waste receiving portion  720  is smoothly moved to the second through hole  74   h  using centrifugal force. 
     In the example illustrated in  FIG. 12 , the inner diameter, D 3 , of the material waste receiving portion  720  is larger than the inner diameter, D 4 , of the first through hole  72   h.  In this case, the material waste entering the shoulder member  7  through the first through hole  72   h  is smoothly received in the material waste receiving portion  720 . 
     In the example illustrated in  FIG. 12 , the first inner surface  72   b  of the shoulder member  7  (in other words, the surface pointed to the ring-shaped receiving surface  341   s  or the surface pointed in the second direction DR 2 ) has a first area  721  and a second area  722 , which is depressed in the first direction DR 1  beyond the first area  721 . The first area  721  and the second area  722  may be connected to each other via an inclined surface  723 . Alternatively or additionally, the first area  721  and the second area  722  may be connected to each other via a step  724 . In the example illustrated in  FIG. 12 , the inclined surface  723  is provided at one side portion of the second area  722 , and the step  724  is provided at an opposite side portion of the second area  722 . 
     In the example illustrated in  FIG. 12 , the first inner surface  72   b  of the shoulder member  7  has the first area  721  and the second area  722 , which is depressed in the first direction DR 1  beyond the first area  721 . In this case, the material waste entering the internal region (more specifically, the pocket PC) of the material waste receiving portion  720  is smoothly guided to the region of a back portion of the second area  722  (when the stirring portion  37  is pointed downward, the region of the back portion of the second area  722  corresponds to the region immediately over the second area  722 ). 
     In the configuration in which the first inner surface  72   b  of the shoulder member  7  includes the inclined surface  723 , the material waste entering the minimal gap between the first area  721  and the ring-shaped receiving surface  341   s  is smoothly guided to the region of the back portion of the second area  722  through the inclined surface  723 . In the configuration in which the step  724  is provided between the first area  721  and the second area  722 , it is difficult for the material waste guided to the region of the back portion of the second area  722  to enter the minimal gap between the first area  721  and the ring-shaped receiving surface  341   s.    
     In the example illustrated in  FIG. 12 , as seen from a direction along the first axis AX, the second area  722  is provided in an angle range AR 1 , which extends from the first axis AX toward a discharge opening EP of the second through hole  74   h.  In this case, the material waste guided to the region of the back portion of the second area  722  is moved smoothly toward the discharge opening EP of the second through hole  74   h.    
     In the example illustrated in  FIG. 12 , as seen from the direction along the first axis AX, the inclined surface  723  may be provided in the angle range AR 1 , which extends from the first axis AX toward a discharge opening EP of the second through hole  74   h.    
     Fourth Embodiment 
     By referring to  FIG. 13 , a machine tool  200  according to the fourth embodiment will be described.  FIG. 13  is a schematic partial cross-sectional view of the machine tool  200  according to the fourth embodiment, schematically illustrating this machine tool  200 . 
     The machine tool  200  includes: a friction stir welding tool  100 D; a workpiece support member  201 ; a tool support member  203 , which supports the friction stir welding tool  100 D; a first driver  205 , which moves the tool support member  203  relative to the workpiece support member  201 ; a second driver  207 , which drives the stir pin  3  into rotation; and a controller  209 . 
     An example of the friction stir welding tool  100 D according to the fourth embodiment is the friction stir welding tool ( 100 A,  100 B,  100 C) according to any one of the above-described embodiments. The friction stir welding tool has already been described in the first to third embodiments, and a redundant description of the friction stir welding tool is omitted. 
     The workpiece support member  201  supports workpieces W, which are to-be-worked objects (more specifically, to-be-joined objects). An example of the workpiece support member  201  is a support table to which the workpieces W are fixed. In the example illustrated in  FIG. 13 , the workpiece support member  201  (support table) supports a first workpiece W 1  and a second workpiece W 2 . The first workpiece W 1  and the second workpiece W 2  are joined together using the friction stir welding tool  100 D. The first workpiece W 1  and the second workpiece W 2  may be joined together continuously (in other words, linearly) using the friction stir welding tool  100 D. Alternatively, the first workpiece W 1  and the second workpiece W 2  may be spot-welded (in other words, point-welded) together using the friction stir welding tool  100 D. 
     In the example illustrated in  FIG. 13 , the machine tool  200  includes a base  202  and a driver  205   a  (for example, a support table driver), which moves the workpiece support member  201  relative to the base  202 . The driver  205   a  is one example of the first driver  205 , which moves the tool support member  203  relative to the workpiece support member  201 . 
     The tool support member  203  supports the friction stir welding tool  100 D. In the example illustrated in  FIG. 13 , the tool support member  203  includes: a frame  203   a,  to which the shoulder member  7  is fixed; and a shaft  203   b,  which transmits rotational force to the pin holder  8 . The tool support member  203  may also be referred to as a headstock. The shaft  203   b  may also be referred to as a rotation spindle. 
     In the example illustrated in  FIG. 13 , the machine tool  200  includes a second base  204  and a driver  205   b,  which moves the tool support member  203  relative to the second base  204 . The driver  205   b  is another example of the first driver  205 , which moves the tool support member  203  relative to the workpiece support member  201 . 
     The first driver  205  is a device that moves the tool support member  203  relative to the workpiece support member  201 . In the example illustrated in  FIG. 13 , the first driver  205  includes: the driver  205   a,  which moves the workpiece support member  201  relative to the base  202 ; and the driver  205   b,  which moves the tool support member  203  relative to the second base  204 . Alternatively, the first driver  205  may include only one of the driver  205   a  and the driver  205   b.    
     In the example illustrated in  FIG. 13 , the driver  205   a  is a device that moves the workpiece support member  201  in a direction along a horizontal plane (in other words, in a direction along the X-Y plane). 
     In the example illustrated in  FIG. 13 , the driver  205   b  is a device that moves the tool support member  203  three-dimensionally. In other words, the driver  205   b  is capable of moving the tool support member  203  in a direction along the X axis, capable of moving the tool support member  203  in a direction along the Y axis, and capable of moving the tool support member  203  in a direction along the Z axis. In the example illustrated in  FIG. 13 , the Z axis is a direction along a vertical direction and is a direction parallel to the first direction DR 1 . 
     The second driver  207  drives the stir pin  3  into rotation. More specifically, the second driver  207  is connected to the shaft  203   b  in such a manner that motive power can be transmitted to the shaft  203   b.  With this configuration, the second driver  207  drives the stir pin  3  into rotation via the shaft  203   b  and the pin holder  8 . 
     The controller  209  controls the first driver  205  and the second driver  207 . In the example illustrated in  FIG. 13 , the controller  209  includes: first driver controlling means  209   a,  which controls the first driver  205 ; and second driver controlling means  209   b,  which controls the second driver  207 . 
     Upon receipt of a control signal from the controller  209  (more specifically, the first driver controlling means  209   a ), the first driver  205  moves the workpiece support member  201  and/or the tool support member  203 . In other words, upon receipt of a control signal from the controller  209 , the first driver  205  moves the tool support member  203  relative to the workpiece support member  201 . 
     Upon receipt of a control signal from the controller  209  (more specifically, the second driver controlling means  209   b ), the second driver  207  rotates the stir pin  3  about the first axis AX. More specifically, upon receipt of a control signal from the controller  209 , the second driver  207  rotates the shaft  203   b.  The rotation of the shaft  203   b  is transmitted to the stir pin  3  via the pin holder  8 . Thus, the stir pin  3  rotates about the first axis AX. 
     In the example illustrated in  FIG. 13 , the controller  209  includes a storage device  2091  (in other words, a memory), which stores programs and data. By executing a program stored in the storage device  2091 , the controller  209  serves as the first driver controlling means  209   a  and/or the second driver controlling means  209   b.    
     In the example illustrated in  FIG. 13 , the machine tool  200  includes an input device  208 , through which parameters such as a control parameter are input into the controller  209 . 
     The machine tool  200  includes the friction stir welding tool  100  according to any one of the above-described embodiments. The friction stir welding tool  100  that the machine tool  200  according to the fourth embodiment includes provides effects similar to the effects provided by the friction stir welding tools  100  according to the first to third embodiments. The machine tool  200  according to the fourth embodiment also includes the first driver  205 , the second driver  207 , and the controller  209 . With this configuration, the machine tool  200  is capable of joining the first workpiece W 1  and the second workpiece W 2  together in any desired form by moving the tool support member  203  relative to the workpiece support member  201  and by rotating the stir pin  3 . 
     Also in the example illustrated in  FIG. 13 , the shoulder member  7  is mounted on the frame  203   a,  which is not rotationally drivable. In this case, the force that the shoulder member  7  (more specifically, the shoulder surface  72   s ) receives from the workpieces W is supported by the frame  203   a.  This ensures that not a large amount of axial load acts on the bearing provided between the tool support member  203  and the shaft  203   b.  With this configuration, the machine tool  200  according to the fourth embodiment is capable of rotating the shaft  203   b  at high speed. In view of this capability, the machine tool  200  according to the fourth embodiment may not necessarily be a machine dedicated to friction stir welding. In other words, the machine tool  200  according to the fourth embodiment may be a multi-tasking machine capable of performing both friction stir welding and machining. In this case, in order to perform friction stir welding, the friction stir welding tool  100  may be attached to the tool support member  203 ; and in order to perform machining, a machining tool may be attached to the tool support member  203 . 
     The present invention will not be limited to the above-described embodiments; it will be appreciated that the embodiments may be modified or changed in any manner deemed convenient within the technical spirit and scope of the present invention. It will also be appreciated that the various kinds of technology and/or technique used in one embodiment are applicable to the other embodiments unless a technical contradiction occurs. Further, it will be appreciated that the optional configurations in each embodiment may be omitted if deemed necessary. 
     For example, in the above-described embodiments, the shoulder member  7  is described as including the second through hole  74   h  (in other words, a discharge hole). Alternatively, as exemplified in  FIG. 14 , the shoulder member  7  may not necessarily be provided with the second through hole  74   h.    
     As used herein, the term “comprise” and its variations are intended to mean open-ended terms, not excluding any other elements and/or components that are not recited herein. The same applies to the terms “include”, “have”, and their variations. 
     As used herein, a component suffixed with a term such as “member”, “portion”, “part”, “element”, “body”, and “structure” is intended to mean that there is a single such component or a plurality of such components. 
     As used herein, ordinal terms such as “first” and “second” are merely used for distinguishing purposes and there is no other intention (such as to connote a particular order) in using ordinal terms. For example, the mere use of “first element” does not connote the existence of “second element”; otherwise, the mere use of “second element” does not connote the existence of “first element”. 
     As used herein, approximating language such as “approximately”, “about”, and “substantially” may be applied to modify any quantitative representation that could permissibly vary without a significant change in the final result obtained. All of the quantitative representations recited in the present application shall be construed to be modified by approximating language such as “approximately”, “about”, and “substantially”. 
     As used herein, the phrase “at least one of A and B” is intended to be interpreted as “only A”, “only B”, or “both A and B”. 
     Obviously, numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present disclosure may be practiced otherwise than as specifically described herein.