Abstract:
Portable friction stir welding apparatus having a crawler which supports a motor for rotating a Friction Stir Welding tool and is coupled to a drive mechanism for urging the crawler along a predefined weld path on work pieces. The crawler can comprise one member or a set of two or three physically separated members each rotatably coupled to the friction stir welding tool. The friction stir welding tool for making the weld is rotatably coupled to each member and restricts axial displacement of the physically separate members relative to each other. The members are adapted to engage rails on the work pieces which provide at least one of guiding, alignment, fixturing, torque compensation for the members as the friction stir welding tool is rotated and the crawler is moved along a joint.

Description:
FIELD OF THE INVENTION 
   One embodiment of the present invention is directed generally toward friction stir welding and surface processing and, more particularly, toward portable apparatus for use in friction stir welding and surface processing processes. 
   BACKGROUND OF THE INVENTION 
   Friction Stir Welding machines currently in use are normally designed for stationary installation in a building where the housing for holding the tool and the motor for spinning the tool are located. The type of weld joint being made such as butt, lap, etc., and the number of welds being made such as single double, etc., is determined by the tool being used. 
   SUMMARY OF THE INVENTION 
   The portable friction stir welding machine disclosed herein can be configured as a crawler having a top member which supports a motor, such as a torque motor, having a rotary shaft for rotating a Friction Stir Welding tool and a drive mechanism for pulling the crawler along a predefined weld path of work pieces. The crawler can include at least two physically separated members where one member is adapted to be located above the work pieces and the other member is adapted to be located below the work pieces. The friction stir welding tool for effecting the weld is rotatably coupled to each member of the crawler and effectively restricts axial displacement of, for example, the two physically separate members relative to each other. Thus, it is the friction stir welding tool which is rotatably coupled to each of the two members of the crawler and prevents the at least two members from being displaced axially relative to each other. 
   Rotational displacement of the at least two members relative to each other and/or relative to the work pieces can be provided by rails or guides located on one or both of the work pieces. Depending on the method used to prevent rotational displacement of the crawler relative to the work pieces, the type of rotating tool that is being used, and whether the weld being made starts and ends at the edges of the work pieces or in from the edges of the work pieces, one or more of the following functions may be required while making a weld: the speed that the crawler advances along the work piece, the rotational speed of the tool, steering the crawler, etc. 
   One embodiment of the present invention is a structure which is both easy to transport and can be moved to a work site rather than moving the work pieces to a building where a stationary Friction Stir Welding machine is located. 
   The foregoing has outlined, rather broadly, an embodiment of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject matter of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention and that such other structures do not depart from the spirit and scope of the invention in its broadest form. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claim, and the accompanying drawings in which similar parts have similar reference numerals where: 
       FIG. 1  is a perspective view of one embodiment of a friction stir welding assemblage of the present invention having a housing with three separate members, where each member has two sections, coupled to a common tool; 
       FIG. 1A  is a side sectional view of the embodiment of  FIG. 1  showing two work pieces being welded. 
       FIG. 1B  is a side sectional view of clamping preloading member in one of the members of the crawler. 
       FIG. 2  is a side sectional view of another embodiment of a friction stir welding assemblage having a housing with two separate members, where each member has two sections, coupled to a common tool; 
       FIG. 3  is a front sectional view of the embodiment of  FIG. 2 ; 
       FIG. 4  is a side sectional view of still another embodiment of a friction stir welding assemblage having a housing with one member having two sections coupled to a tool; 
       FIG. 5  is a perspective view of a friction stir welding tool coupled to a guide on a work piece to prevent rotation of the housing; 
       FIGS. 6-8  show various friction stir welding tools which can be used to make welds; and 
       FIGS. 9-20  show various work piece configurations and types of welds which can be made with the apparatus here disclosed. 
   

   DETAILED DESCRIPTION 
   Friction Stir Welding (FSW) is a process which can be used to weld together a wide variety of materials and their alloys such as aluminum, copper, iron, steel, stainless steel, etc. The weld is formed by plunging a rotating, non-consumable pin type tool into work pieces that are to be welded together with a butt or lap weld. During the welding process, as the pin type tool initially plunges into the work pieces at the weld line, the material is frictionally heated and plasticized at a temperature below that of the melting temperature and typically within the material&#39;s forging temperature range. When the metal becomes sufficiently soft and plastic, and the tool reaches the appropriate penetration depth, the tool is advanced along a weld line. As the tool is moved along the work pieces, metal flows to the back of the pin type tool where it is mixed behind the tool and consolidates while it cools to create a sound metallurgical bond. Friction stir welding, in addition to being used to join together similar metals, can also be used to join dissimilar metals or plastics or other materials that will soften and consolidate under frictional heating to become integrally connected. Friction stir welding can be used to make butt joints, corner joints, lap joints and other types of joints, as well as being used to repair cracks in a given material and for forming a hollow section, such as a round, square or rectangular tube. 
   The friction stir welding process is implemented with a FSW pin type tool which can have a single pin and a shoulder which contacts a top surface of the material being joined. An anvil which contacts the bottom surface of the work pieces opposite the FSW pin type tool can be used to prevent plasticized material from flowing out of the bottom of the weld during the welding process to provide a smooth weld surface. In some applications, the pin type tool may not include the anvil. 
   Referring to  FIG. 1 , there is shown a perspective view of one embodiment of the present invention having a housing of three separate members where each member is composed of two half sections coupled to a friction stir welding tool for making friction stir type of welds. Notwithstanding the illustrations in  FIGS. 1-5  and the disclosure of the embodiment that describes in detail three separate members, it is within the scope and contemplation of this invention that the number of members, along with associated cooperating features, can be as many or as few as necessary to weld two or more work pieces together per design or customer requirements. Therefore, nothing disclosed herein is intended to limit the claimed invention. 
   In the present invention, the housing, hereinafter referred to as crawler  200 , when making a weld, engages rail members at or near the edges of the work pieces (discussed in detail below). The rail members can be raised member or channels which slidably engage a receiving member, such as a slot or a side wall, in the crawler to provide guiding and torque compensation for the crawler during the welding process. The crawler can include, for example, upper member  201 , intermediate member  202 , and lower member  203 . These members can be composed of a semi-yieldable material such as a plastic, or a rigid material such as cast iron or other equivalent material where each of the three members of the crawler is split to provide a front section and a back section. Upper member  201  of the crawler consists of two substantially similar sections, front section  201 A and back section  201 B. Back section  201 B can have, for example, four clearance openings  204  for receiving threaded fasteners (not shown) that pass through clearance openings  204  and thread into threaded openings  205  in front section  201 A to lock or secure the two sections  201 A,  201 B together to form member  201 . Each section has a semi circular shaped cut out  206  for receiving a split sleeve bearing  207  or a race and ball bearings. Each member also includes cut out sections  500  having side walls  501  for receiving raised rails or channels (see  FIGS. 9-20 ) which may be formed by extrusion on the work pieces such as, for example, aluminum sheet work pieces, where the rails are adapted to slidably engage the members to provide guiding, alignment and torque compensation for the members during the making of a weld. In those instances where the rails  701 , see  FIG. 9 , in the work pieces are channels  712 , see  FIG. 15 , then the crawler may have wheels (not shown) or rigid projections (not shown) which fit within the channels  712  to provide guiding and torque compensation for the crawler while a weld is being made. Clamping of the crawler to the work pieces can be provided by rails or channels which are sized to have a close fit with the engaging parts in the members or with spring members located in the crawler and both above and below the work pieces being joined. 
   Referring to  FIG. 1B , member  530  which is located in cutout section  500  is positioned to engage rail  701  on the work piece and is urged by spring  532  to move the work piece toward the right to butt against the work piece to which it is to be joined. Also, a second spring  534  is coupled to urge member  530  to move down to contact the work piece. The member  534  can be provided to contact only the top surface of each work piece or a member  534  can be provided for both the top and bottom surfaces of each work piece to both urge the work pieces toward each other to preload the work pieces and to clamp the crawler to the work pieces. In another embodiment, the springs are eliminated and the members are either sized or adjusted to provide a crawler that has a close fit with the work pieces such that the crawler holds the work pieces securely near each other with sufficient force to prevent the tool, during the welding process, from pushing the work pieces apart and prevent the counter torque from rotating the crawler. It is to be noted that the structure disclosed to urge the work pieces together and to clamp the crawler to the work pieces can compensate for work pieces that have uneven edges and/or varying thicknesses. 
   Thus, by the action of the member  530 , the crawler, by being both above and below the work pieces, clamps itself to the work pieces as a weld joint is being made.  FIGS. 9-20  show pre-weld and post weld work pieces which can be of a desired material such as aluminum and which have rails or channels at or near their edges to provide guiding and torque compensation for the crawler. 
   As is here described for the embodiment shown in  FIG. 1 , it is to be understood that for each embodiment which is disclosed hereinafter, the spacing between adjacent upper and intermediate, and intermediate and lower members of the crawler can be varied to receive work pieces that have different thicknesses. In addition, the members of the crawler have a slot or a rigid projection positioned to slidably engage a rail or a channel on a work piece during the welding process to provide guiding, alignment, fixturing and torque compensation for the crawler in addition to positioning and/or preloading the work pieces by urging the edges of the work pieces toward each other. Also, clamping of the crawler to the work pieces is provided by close fitting or spring loaded members located within the crawler which slidably engage and press against the surfaces of the work pieces during the welding process. Further, for each embodiment disclosed herein, the rail on each work piece can be a raised member or it can be a channel, and the rail can trace a path which is straight, curved or a combination of both. During the welding process, the crawler engages the rails on the work pieces to urge the work pieces to butt against each other to provide a desired preload force to the work pieces during and after the welding process. 
   Additionally, the work pieces can be of aluminum and the rail member can be formed by extrusion or other type of metal working process. It is here noted that, to avoid having duplicate and repetitive paragraphs in the description, this paragraph is not being repeated for each embodiment hereinafter disclosed, and it is understood that this paragraph of the invention is a part of the description of each embodiment the same as it would be if it were actually included in the description of each embodiment. 
   The top surface  201 T of the front  201 A and back  201 B sections of the upper member  201  has four threaded openings  300  for receiving four threaded fasteners (not shown) for securing a motor, such as a torque motor  308  (servo, hydraulic, pneumatic or electric), to the upper member  201  when the front  201 A and back  201 B sections are secured together with threaded fasteners as disclosed above. The front and back surfaces of each member are similar. Therefore, a view of the front surfaces of members  201 ,  202 ,  203  of  FIG. 1  is not disclosed. The front surface of the front section  201 A and the back surface  301  of the back section  201 B of the upper member  201  can have four threaded openings (not shown) for receiving four threaded fasteners (not shown) for securing a pull member  210  to the front  201 A and back  201 B sections of upper member  201 . Pull member  210  is provided to receive a pull cable (shown attached to front section  202 A) to pull the crawler  200  at a predetermined speed along work pieces being welded. 
   Intermediate member  202  consists of two substantially similar sections, front section  202 A and back section  202 B. Back section  202 B has four clearance openings  208  for receiving threaded fasteners (not shown) which pass through clearance openings  208  and thread into threaded openings  209  in front section  202 A to lock or secure the two sections  202 A,  202 B together to form member  202 . Each section  202 A,  202 B has a semi-circular shaped cut out  206  for receiving a split sleeve bearing  207  or race with ball bearings. Each member also has a cutout  500  adapted to engage rails or projecting members (not shown) to engage channels on the work pieces. The front and rear surfaces of the intermediate member  202  has four threaded openings (not shown) for receiving threaded fasteners (not shown) for securing pull members  210  to the intermediate member  202 . The pull members  210  are provided to receive a pull cable  202 P used to pull the crawler  200  at a predetermined speed along work pieces being welded. 
   Lower member  203  consists of two substantially similar sections, front section  203 A and back section  203 B. Back section  203 B has four clearance openings  211  for receiving threaded fasteners (not shown) which pass through the clearance openings  211  and thread into threaded openings  212  in front section  203 A to lock or secure the two sections  203 A,  203 B together to form member  203 . Each section has a semi circular shaped cut out  206  for receiving a split sleeve bearing  207  or a race with ball or roller bearings and cut outs  500  or projecting members (not shown) for engaging rails or channels on the work pieces. The front surface (not shown) and rear surface of the lower member  203  can have four threaded openings (not shown) for receiving threaded fasteners (not shown) for securing pull members  210  to the lower member The pull members  210  are provided to receive a pull cable similar to pull cable  201 P to pull the crawler  200  at a predetermined speed along work pieces being welded. 
   Continuing with  FIG. 1 , prior to joining the front  201 A,  202 A,  203 A and back  201 B,  202 B,  203 B sections of the upper  201 , intermediate  202  and lower  203  members together, and subsequent to inserting the split sleeve bearings  207  into respective cutouts  206  in the upper  201 , intermediate  202  and lower  203  members, a replaceable Friction Stir Welding (FSW) tool  213  is fitted in place in the three members. As described in detail below, the FSW tool  213  disclosed herein can be configured to make a single weld or two welds simultaneously. However, an embodiment with more than two members is contemplated to be within the scope of the invention as discussed herein. The tool  213  can be a single member (as shown) without any removable sections, or it can be made of two or more separate sections (not shown) that can be coupled together with, for example, a threaded pin which engages a threaded opening to form a continuous tool. When the tool is made of two or more sections, the threads on the threaded pin of one section are designed to thread into a threaded opening in the other section as the tool is being rotated. 
   One embodiment of the present invention is shown for use with a FSW tool having three bearing surfaces adapted to rotatably engage the three split sleeve bearings  207  in the front and back sections of the three members  201 ,  202 ,  203 . When the FSW tool  213  is located in the upper  201 , intermediate  202 , and lower members  203 , an upper FSW bobbin  216  is aligned with the space  201 X between the upper  201  and intermediate  202  members; and a lower FSW bobbin  217  is aligned with the space  201 Y between the intermediate  202  and lower  203  members. The spaces  201 Y,  201 X, as explained below, are determined by the relationship of various dimensions relative to each other such as the spacing between the bearings on the FSW tool, the spacing between the semi circular shaped cut outs  206  in the member  201 ,  202 ,  203 , and the spacing between the semi circular cut outs  206  in each member and the physical end of each member at the space. However, in practice, where the members of the crawler are available for use, it is the tool which determines the spacing between the members. Thus, different tools can be used to provide the proper spacing between crawler members to allow the crawler to be used with work pieces of different thicknesses. 
   After the front and back sections of the three members  201 ,  202 ,  203  are joined together around the replaceable FSW tool to entrain or encapsulate the FSW tool  213  within crawler  200 , the torque motor  308  is placed on the top surface  201 T of the upper member  201  to engage a splined engagement pin  214  located at the end of the FSW tool  213 , which projects above the top surface  201 T of the upper member  201 . The torque motor  308  is then attached to threaded openings  300  in the upper member  201  with threaded fasteners (not shown). At this time, the friction stir welding tool is ready to join at least two work pieces together. 
   In operation, where two welds are to be made simultaneously, see  FIG. 1A , a first pair of work pieces  400  that are to be joined are positioned edge to edge in the space  201 Y between the front sections of the lower  203  and intermediate  202  members respectively. A second pair of work pieces  402  that are to be joined are positioned edge to edge in the space  201 X between the front sections of the intermediate  202  and upper  201  members respectively. Prior to starting the welding process, replaceable FSW tool  213  was inserted into the split sleeve bearings  207  located in the front sections  201 A,  202 A,  203 A, and the back sections  201 B,  202 B,  203 B of the three members  201 ,  202 ,  203  were attached to the front members  201 A,  202 A,  203 A. Immediately prior to making the weld, the lower bobbin  217  of the FSW tool  213  is located at the weld seam (defined by the interface/contact of the two work pieces) of the first pair of work pieces  400 , and the upper bobbin  216  of the FSW tool  213  is located at the weld seam of the second pair of work pieces  402 , and generally along the center line (CL). A pull cable (not shown) is attached to pull member (not shown) on, for example, the intermediate member  202  and is used to pull the crawler  200  along the rail  425  on the work pieces  400 ,  402  within cutouts  500  having side walls  501  that slidably engage rails  425  as the FSW tool is rotating and traversing along the weld seam. 
   An alternate embodiment not shown includes work pieces having channels and slidably engaging projections formed within or in place of cutouts  500 . It is here noted that the rails on the work pieces provide guiding, alignment and torque compensation for the crawler  200 . In another embodiment, pull cables (not shown) can be attached to each of the three members  201 ,  202 ,  203  to advance the crawler along the work pieces. 
   Now referring to  FIG. 2  there is shown a side sectional view of another embodiment of the present invention having a crawler  200 A with two separate members  201 ,  203  coupled to a FSW tool  213 A. The embodiment shown in  FIG. 2  is similar to the embodiment of  FIG. 1 , except the embodiment of  FIG. 2  does not include an intermediate member  202  as shown in  FIG. 1 . The various parts of  FIG. 2  which are similar to those parts of  FIG. 1  have been given the same reference numerals and some of the parts of  FIG. 1  may not be shown in  FIG. 2 . Upper member  201  and lower member  203  can be composed of a semi-flexible material such as a yieldable plastic or a rigid material such as cast Iron or other similar material and each member is composed of two substantially similar half sections to provide a front section  201 A,  203 A and a back section  201 B,  203 B: Back section  201 B has four clearance openings for receiving threaded fasteners which pass through the clearance openings and thread into threaded openings in front section  201  A to join the two sections together as described above for the embodiment of  FIG. 1 . Each section has a semi circular shaped cut out  206  for receiving a split sleeve bearing  207  or race with ball bearings and cutouts  219  for engaging rails on the work pieces. The top surface of the front and back sections of the upper member  201  has four threaded openings (not shown) for receiving threaded fasteners (not shown) for securing a motor, such as a torque motor  308  to the upper member  201  when the front and back sections are secured together. The front surface and rear surface of the upper member  201  can have four threaded openings (not shown) for receiving threaded fasteners (not shown) which secure pull members  210  to the upper member  201 . The pull members are provided to receive a pull cable  201 P to pull the crawler  200  at a predetermined speed along work pieces being welded. 
   Lower member  203  consists of two substantially similar sections, front section  203 A and back section  203 B. Back section  203 B has four clearance openings (not shown) for receiving threaded fasteners (not shown) which pass through the clearance openings and thread into threaded openings (not shown) in front section  203 A to join the two sections together. Each section has a semi circular shaped cut out  206  for receiving a split sleeve bearing  207  or a race with ball bearings and cutouts for engaging rails or channels on the work pieces. The front and back sections of the lower member have four threaded openings (not shown) for receiving threaded fasteners (not shown) for securing pull members  210 . The pull members are provided to receive a pull cable  201 P used to pull the crawler at a predetermined speed along work pieces being welded. 
   Prior to joining the front and back sections of the upper  201  and lower  203  members together, and subsequent to inserting the split sleeve bearings  207  into respective cutouts  206  in the upper  201  and lower  203  members, a Friction Stir Welding (FSW) tool  213  is first fitted in place in either the front or back section. The front and back sections of the two members  201 ,  203  are then secured together to envelop or entrap the FSW tool  213 . At this time a gap  201 X is formed between members  201 ,  203  which is slightly larger than the thickness of the work pieces  402 . Thus, when the dimensions of the crawler are fixed to receive and operate with various tool, the spacing  201 X between the members  201 ,  203 , is determined by the spacing between the bearings on the tool. 
   The FSW tool  213 A in this embodiment is configured to make one weld. The tool  213  can be a single member without any removable sections, or it can be made of two or more separate sections which can be coupled together with at least one threaded pin which engages a threaded opening in another section to form a continuous tool. When the tool is made of two or more sections, the threads on the threaded pin of one section are designed to thread into the threaded opening in the other section as the tool is being rotated. 
   The FSW tool  213 A shown in the embodiment of  FIG. 2  has two separate bearing surfaces  206 A adapted to rotatably engage two split sleeve bearings  207  or races with ball or roller bearings in the front  201 A,  203 A and back  201 B,  203 B sections of the two members  201 ,  203 . When the FSW tool  213 A is located in the upper and lower members, a single FSW bobbin  218  is aligned with the space  201 X between the upper  201  and lower  203  members as described above. 
   In operation, when a weld is to be made, a pair of work pieces  402 , which can have rails  701  as shown in  FIG. 9  which are to be joined are positioned edge to edge between the front sections of the upper  201  and lower  203  members. At this time the FSW bobbin  218  of the tool  213  is located at the beginning of the weld joint of the work pieces and a pull cable  201 P is attached to the upper member and is used to pull the crawler  200  along the rails  701  on the work pieces  700  as the FSW tool is rotating. In another embodiment, a pull cable  202 P can be attached to each of the two members  201 ,  203 , instead of to only one member to advance the crawler along the work pieces. In another embodiment, a threaded shaft connected to the crawler can be used to advance the crawler along the work pieces as a weld is being made. In each embodiment, sensors can be provided to control the speed of advance of the crawler and other parameters which may be required as a weld is being made. 
     FIG. 3  is a front partial sectional view of the embodiment of  FIG. 2 . As noted above, in the various embodiments, the space  201 X for the work pieces which is located between adjacent members  201 ,  203  is determined by design of the tool when the crawler is made with dimensions which have been standardized. 
     FIG. 4 , is a side partial sectional view of a friction stir welding crawler  200 B having one member  201  coupled to a friction stir welding tool  213 B. See the description above of  FIG. 1  for a detailed description of the various features of  FIG. 4  which are common with those of  FIG. 1 . The member  201  can be composed of a semi-flexible material such as a plastic or a rigid material such as cast iron or other equivalent material and is split to provide two substantially similar sections, a front section  201 A and a back section  201 B. Back section  201 B has four clearance openings (not shown) for receiving threaded fasteners (not shown) which pass through the clearance openings and thread into threaded openings (not shown) in front section  201  A to join the two sections together. Each section has a semi circular shaped cut out  206  for receiving a race with ball bearings or a split sleeve bearing  207  and having cutouts  219  for engaging rails or projections (not shown) for engaging channels on work pieces that are to be joined by welding. The top surface of the front and back sections of the member  201  has four threaded openings (not shown) for receiving threaded fasteners (not shown) for securing a motor, such as a torque motor  308  to the member  201  when the front and back sections are joined together. The front surface and rear surface of member  201  can have four threaded openings for receiving threaded fasteners for securing pull members  210  to the crawler. The pull members are provided to receive a pull cable  201 P used to pull the crawler  200 B along the rails  701  on the work pieces at a predetermined speed during the welding process. 
   Prior to attaching the front  201 A and back  201 B sections of the member  201  together, and subsequent to inserting the split sleeve bearings  207  into respective cutouts  206  in the member, a Friction Stir Welding (FSW) tool  213 B is fitted in place in the split sleeves in the front section or back section and the two sections are then joined together to form the member  200 A. The FSW tool  213 B is configured to make one weld. The tool  213  can be a single member without any removable sections. 
   Continuing with  FIG. 4 , the FSW tool  213 B has a single bearing surface  206 A adapted to rotatably engage a split sleeve bearing  207  or a race having ball bearings (not shown) in the front  201 A and back  201 B sections of member  201 . After the front and back sections of the member are joined together and encapsulate the replaceable FSW tool  213 B, a torque motor  308  is placed on the top surface  201 T of member  201  to engage a splined engagement pin  214  located at the end of the FSW tool  213 B, which projects above the top surface  201 T of member  201 . The torque motor  308  is then attached to the member  201  with threaded fasteners as described above. 
   In operation, when a weld is to be made, the bobbin  218  of the tool  213 B is located at the edge of two work pieces and in line with a weld joint that is to be made. A pull cable  201 P or other drive structure such as a screw drive (not shown) is coupled to the member  201  and is used to advance the crawler  200 B along the rails or channels on the work pieces  402  as the FSW tool  213 B is rotated and traverses along the weld seam. 
     FIG. 5  is a perspective view of a friction stir welding tool coupled to rails on work pieces where the members  530  located in the crawler  200 , in combination with the rails, urge the edges of the work pieces  600 A,  600 B and  601 A,  601 B) (as described above) toward each other to provide preloading of the work piece pairs and also provide guiding, alignment and torque compensation for the crawler. More specifically, in one embodiment, work pieces  600 ,  601  can be of aluminum with enlarged edges  602 ,  603  butted together with a predefined force by the crawler  200  as explained above. As the crawler is advanced by a driven take up reel  610 , or any other drive structure such as a screw drive (not shown) toward the left, the rotating FSW tool in the crawler  200  forms a weld joint  604 . It is to be noted that, as mentioned above, the members  201 ,  202 ,  203  of the crawler  200  are positioned both above and below the work pieces  600 ,  601  and, as noted previously, the crawler is slidably clamped to work pieces  600 A,  600 B,  601 A,  601 B. Additionally, the crawler  200 , by urging the work pieces  600 A,  600 B and  601 A,  601 B toward each other, preloads the work pieces. The rails on the work pieces function as guides for the crawler  200  and also provides guiding, alignment and torque compensation for the crawler  200 . 
   In those instances where it is not possible to place a back up anvil on the back side of a weld being made, a bobbin-type tool may be used. Such tools include two shoulders and a pin located between them. The bobbin-type FSW tool  20  shown in  FIG. 6  includes a pin  21  located between a pair of shoulders  22  which include work piece engaging surfaces  23 . The shoulders  22  can have a taper angle  24  and can be integral with pin  21 . To insure that the tool contacts and frictionally engages work pieces  111 , which may vary slightly in thickness, the work piece engaging surfaces  23  of the tool are tapered at an angle  24  shown in  FIG. 6 . 
   The taper angle  24  enable work pieces having slightly different and/or somewhat variable thicknesses to be welded together and also ensures that the FSW tool is pressed against the work pieces with the force needed to both plasticize and confine the plasticized material in the weld area to produce smooth surfaces on the upper and lower surfaces of the weld. 
     FIG. 7  illustrates bobbin type tool  30  which can be used with the crawler here disclosed to weld one joint  113  of a pair of joints  113  and  114  to produce a tube from a pair of elongate members. The tube shown in  FIG. 7  has a substantially square cross section, it being understood that the tube can have a cross section which is rectangular, circular, oval etc. In  FIG. 7 , each elongate member  112  has a square C cross-sectional shape, and each elongate member corresponds to one half of the cross-section of the rectangular tube. It is here noted that the tool  30  is capable of welding only one joint at a time. 
     FIG. 8  shows a sectional view of a pin type tool  50  having an integral shank-pin with a shoulder  54  threaded onto the shank-pin to provide compression loading of the pin of a pin type tool. The tool  50  has a close fit  57  between the shank and the inside of shoulder  54  near the base of pin  52 , and has a positive stop  59  between the inside of shoulder  54  and the shank  53 . It is contemplated that this tool can be used with the crawler here disclosed which has only one member such as  201  which is located on the top surface of two work pieces and is pulled along the weld seam as the tool is rotating. 
   The tools referred to above can be used with a crawler here disclosed to make welds on many different types of structures, one such structure being parts for automobiles. 
     FIGS. 9-20  show various work piece configurations and types of welds that can be made with the FSW crawler here disclosed. 
   Referring to  FIG. 9 , there is shown work pieces  700 , prior to being welded, which were extruded to have rails  701  at their edges. The rails  701  are substantially rectangular in shape and are partially consumed during the welding process. Note that the outboard edges  703  of the rails are intact after the weld is completed.  FIG. 10  shows the plates after they have been welded together with the outboard edges  703  of the rails  701  still intact.  FIG. 12  shows another type of weld that can be formed with work pieces of  FIG. 11  having extruded rails  705  which are shorter in length. In the embodiment of  FIG. 11 , the rectangular rails  705  of the work pieces which are shorter in length than the rails  701  are almost fully consumed during the welding process. In  FIG. 12 , it can be seen that only the outboard ends  706  of the guide rails are not consumed during the welding process.  FIG. 14  shows still another type of weld that can be formed with the work piece of  FIG. 13  having extruded ends. The rails  708  of  FIG. 13  are similar to the rails of  FIG. 11  except the inboard ends  709  of the rails are undercut. 
     FIG. 16  shows another type of weld that can be formed with the work pieces  710  of  FIG. 15  having channels  712  located near their ends. Note, in  FIG. 16  the channels are completely consumed during the welding process.  FIG. 18  shows another type of weld that can be formed with work pieces  713  of  FIG. 17  having ends  715  entrapped with a snap guide  716  adapted to function as a guide rail for engagement by the crawler. The snap guide  716  and ends  715  are consumed during the welding process.  FIG. 20  shows another type of weld that can be formed with the work pieces  718  of  FIG. 19  having channels  719  located inboard from their ends  720 . With this embodiment, the channels  719  remain intact after the welding process. 
   While there has been described herein the principles of the invention, it is to be clearly understood to those skilled in the art that this description is made only by way of example and not as a limitation to the scope of the invention. Accordingly, it is intended to cover all modifications of the invention which fall within the true spirit and scope of the invention.