CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent Application Ser. No. 13/221,255, filed Aug. 30, 2011; U.S. patent application Ser. No. 13/221,255 is a division of U.S. patent application Ser. No. 11/829,897, filed Jul. 28, 2007, granted as U.S. Pat. No. 8,052,030, issued on Nov. 8, 2011; U.S. patent application Ser. No. 11/829,897 claims the benefit of Provisional U.S. Patent Application No. 60/849,670, filed Oct. 5, 2006; U.S. patent application Ser. No. 11/829,897 is a continuation-in-part of U.S. patent Application Ser. No. 11/053,630, filed Feb. 7, 2005, granted as U.S. Pat. No. 7,703,654, issued on Apr. 27, 2010; U.S. patent application Ser. No. 11/829,897 is a continuation-in-part of U.S. patent application Ser. No. 11/041,836, filed Jan. 24, 2005, granted as U.S. Pat. No. 7,641,096, issued on Jan. 5, 2010; U.S. patent Application Ser. No. 11/053,630 claims the benefit of Provisional U.S. Patent Application No. 60/628,832, filed Nov. 17, 2004; the entire disclosures of each of the above mentioned applications are hereby incorporated by reference. 
     
    
     BACKGROUND INFORMATION 
       [0002]    1. Field 
         [0003]    This disclosure generally relates to spindle heads used in friction stir welding machines, and deals more particularly with a compact, spindle-in-spindle head having stacked drive motors. 
         [0004]    2. Background 
         [0005]    Friction stir welding (FSW) may be used to join two sheets of metal along a weld line without the use of welding filler materials. The weld is created by a combination of frictional heating of the metal by a rotating tool, and mechanical deformation of the weld using a rotating tool. These tools may comprise a retractable pin tool rotatable within an annually shaped shoulder tool. The pin tool and shoulder tool may be connected to concentric spindles which are respectively driven by separate motors. These motors are typically mounted on or near a spindle head and are laterally offset from the spindle axes. The ends of the concentric spindles are coupled with the motors either through gear sets or drive belts. 
         [0006]    The spindle head construction described above may not be suitable for some applications due to the bulk of the spindle head caused by the laterally offset motors. For example, FSW machines on which the spindle head is mounted may have structural elements that interfere with the laterally offset motors as the spindle head is moved along multiple axes during a welding operation. 
         [0007]    Accordingly, there is a need for a FSW apparatus having a compact spindle head which overcomes the problems discussed above. Embodiments of the disclosure are intended to satisfy this need. 
       SUMMARY 
       [0008]    FSW apparatus may include a spindle head having concentric spindle shafts driven by stacked, coaxial motors contained within a spindle housing. The coaxial arrangement of the motors minimizes lateral projections from the spindle head, resulting in a more compact package that facilitates movement of the spindle head in multiple axes without interfering with other structural elements on the FSW machine. Each of the motors is concentrically arranged around one of the spindles by directly connecting a rotor of the motor to a spindle shaft. The stators of the motors are mounted on the housing and are concentrically arranged around the concentric spindle shafts. 
         [0009]    According to one disclosed embodiment, friction stir welding apparatus is provided, comprising: a shoulder tool; a pin tool rotatable within the shoulder tool; a housing; first and second coaxial spindle shafts respectively connected to the shoulder tool and the pin tool; a first motor connected to the first spindle shaft for rotating the shoulder tool; and, a second motor stacked in end-to-end relationship to the first motor for rotating the pin tool. Each of the first and second motors includes a stator secured to and surrounding a corresponding spindle shaft. The motors each may include a stator mounted on the housing and surrounding the corresponding spindle shaft. Encoders may be employed to generate information related to the rotation of the motor and/or spindle shafts. One of the spindle shafts may pass through the motor that drives the other spindle shaft. 
         [0010]    According to another disclosed embodiment, friction stir welding apparatus is provided, comprising: a shoulder tool; a pin tool coaxial with and rotatable within the shoulder tool; first and second coaxial spindle shafts respectively connected to the shoulder tool and the pin tool; a first motor surrounding and connected to the first spindle shaft for rotating the shoulder tool; and, a second motor surrounding and connected to the second spindle shaft for rotating the pin tool. The apparatus may include a spindle housing within which the first and second motors are contained in end-to-end relationship to each other. 
         [0011]    In accordance with another embodiment, friction stir welding apparatus is provided, comprising: first and second electric motors stacked end-to-end and arranged to drive around a common axis; a shoulder tool, a pin tool rotatable within the shoulder tool around the common axis; a first spindle shaft connecting the shoulder tool with the first motor; and, a second spindle shaft connecting the pin tool with the second motor. The first motor may include a stator, and a rotor secured to the first spindle shaft. The second motor may include a stator and a rotor secured to the second spindle shaft. The first spindle shaft may pass through the second motor. 
         [0012]    In accordance with a method embodiment, friction stir welding a workpiece my comprise the steps of: stacking first and second motors end-to-end along a common axis; rotating a pin tool and a shoulder tool using the first and second motors, respectively; and, producing a friction stir weld in the workpiece using the pin tool and the shoulder tool. The pin tool may be rotated inside the shoulder tool. The motors may be stacked by mounting them inside a common housing. The tools may be rotated by coupling the first and second motors respectively through first and second spindle shafts to the pin tool and the shoulder tool. 
         [0013]    Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The novel features believed characteristic of the advantageous embodiments are set forth in the appended claims. The advantageous embodiments, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an advantageous embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
           [0015]      FIG. 1  is an isometric illustration of a spindle head for performing friction stir welding on a workpiece. 
           [0016]      FIG. 1A  is a functional block diagram illustration of the spindle head shown in  FIG. 1 . 
           [0017]      FIG. 2  is a diagrammatic, sectional illustration of the upper portion of the spindle head shown in  FIG. 1  showing the relative positions of the motors. 
           [0018]      FIG. 3  is a sectional illustration taken along the line  3 - 3  in  FIG. 2 . 
           [0019]      FIG. 4  is an isometric illustration of the area designated as “A” in  FIG. 1 . 
           [0020]      FIG. 5  is a sectional illustration taken along the line  5 - 5  in  FIG. 4 . 
           [0021]      FIG. 6  is an exploded, isometric illustration of the pin tool assembly. 
           [0022]      FIG. 7  is an exploded, isometric illustration of the shoulder tool assembly. 
           [0023]      FIG. 8  is an exploded, isometric illustration of the area designated as “B” in  FIG. 6 . 
           [0024]      FIG. 9  is an exploded, isometric illustration of the area indicated as “C” in  FIG. 7 . 
           [0025]      FIG. 10  is an exploded, sectional illustration taken along the line  10 - 10  in  FIG. 8 . 
           [0026]      FIG. 11  is a sectional illustration taken along the line  11 - 11  in  FIG. 9 . 
           [0027]      FIG. 12  is a longitudinal sectional illustration taken along the line  12 - 12  in  FIG. 1 . 
           [0028]      FIG. 13  is a flow diagram of aircraft production and service methodology. 
           [0029]      FIG. 14  is a block diagram of an aircraft. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Referring first to  FIG. 1 , a spindle head generally indicated by the numeral  20  includes a spindle housing  24  that may be mounted on a machining center or machine tool (not shown) for movement along multiple axes. The spindle head  20  includes a nosepiece  26  having later discussed tools for performing FSW operations on a workpiece  21 . As will be described below, the rotating components of the spindle head  20  are coaxially arranged along a central, longitudinal axis  29 . 
         [0031]    Referring now also to  FIGS. 2-12 , a rotatable and retractable pin tool assembly  46  is coaxially disposed within a shoulder tool assembly  28  ( FIG. 7 ). The pin tool assembly  46  includes a pin spindle shaft  48  connected with a pin tool  54  through a pin adapter  50  and a pin tool holder  52 . The pin tool  54  includes a tip  54   a  for plunging into the workpiece  21  during a FSW operation. 
         [0032]    The shoulder tool assembly  28  may comprise a shoulder spindle shaft  30  connected to a shoulder tool  44  through a spindle-to-adapter interface  32 , a shoulder adapter  34 , a cover  36 , and a shoulder tool holder  38 . The shoulder tool holder  38  may include a collet  42  for releasably holding the shoulder tool  44 . Shoulder tool  44  may include an annular shoulder  44   a  that surrounds the tip  54   a  of the pin tool  54 . The pin spindle shaft  48  is linearly displaceable within the shoulder spindle shaft  30 , allowing the pin tool  54  to be extended or retracted within the shoulder  44 , as may be required by a FSW operation. As previously described, the shoulder  44   a  functions to forge material in the workpiece  21  as the material is stirred by the pin tool tip  54   a.  The shoulder tool  44  may rotate in either the same or the opposite direction of the rotation of the pin tool  54 . 
         [0033]    Referring now particularly to  FIGS. 2 ,  3  and  12 , first and second motors  23 ,  25  may be mounted within the spindle housing  24  in stacked, end-to-end relationship and are coaxially arranged along the longitudinal axis  29 . The first motor  23  may include a stator  23   a  secured to the housing  24  by suitable brackets (not shown), and a rotor  23   b  that may be secured directly to the upper end of the shoulder spindle shaft  30 . The second motor  25  may include a housing  35  mounted in a ball cage  31  for linear sliding movement, as shown by the arrows  37  (see  FIG. 2 ). Motor  25  may include a stator  25   a  secured to the housing  35 , and a rotor  25   b  that may be attached directly to the upper end of the pin tool spindle shaft  48 . 
         [0034]    As previously described, spindle shafts  30 ,  48  are coaxial and rotate independently of each other. The pin tool spindle shaft  48  extends through the center of the rotor  23   b  of motor  23 , and is driven to rotate by motor  25 . An electric motor and ball screw drive combination or a pneumatic cylinder  39  function to linearly displace the motor  25  along with the pin spindle assembly  46  in order to control the linear displacement position of the pin tool  54 . Motor  23  rotates the shoulder tool spindle shaft  30 . 
         [0035]    Encoders  27  may be provided to sense the rotation of either the spindle shafts  30 ,  48  or the motors  23 ,  25  in order to generate signals that may be used by controllers (not shown) to control either the speed of the motors  23 ,  25  or feed rate of the spindle head  20 . The encoders  27  may be incorporated into the construction of the motors  23 ,  25 , if desired. A linear position sensor  33  may be provided to sense the linear position of the pin spindle shaft  48 , and thus the position of the pin tool  54 . 
         [0036]    Referring now to  FIGS. 13 and 14 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  60  as shown in  FIG. 13  and an aircraft  76  as shown in  FIG. 14 . During pre-production, exemplary method  60  may include specification and design  62  of the aircraft  76  and material procurement  64 . During production, component and subassembly manufacturing  66  and system integration  68  of the aircraft  76  takes place. Thereafter, the aircraft  76  may go through certification and delivery  70  in order to be placed in service  72 . While in service by a customer, the aircraft  76  is scheduled for routine maintenance and service  74  (which may also include modification, reconfiguration, refurbishment, and so on). 
         [0037]    Each of the processes of method  60  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of venders, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
         [0038]    As shown in  FIG. 14 , the aircraft  76  produced by exemplary method  60  may include an airframe  78  with a plurality of systems  82  and an interior  80 . Examples of high-level systems  82  include one or more of a propulsion system  84 , an electrical system  86 , a hydraulic system  88 , and an environmental system  90 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
         [0039]    The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  60 . For example, components or subassemblies corresponding to production process  66  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  76  is in service. Also, one or more apparatus embodiments may be utilized during the production stages  66  and  68 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  76 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  76  is in service, for example and without limitation, to maintenance and service  74 . 
         [0040]    Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art. 
         [0041]    The description of the different advantageous embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous embodiments may provide different advantages as compared to other advantageous embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.
A spindle head for performing friction stir welding includes concentric spindle shafts driven by stacked, coaxial motors contained within a spindle housing. The coaxial arrangement of the motors results in a more compact package. Each of the motors is concentrically arranged around one of the spindles by directly connecting a rotor of the motor to a spindle shaft. The stators of the motors are mounted on the housing and are concentrically arranged around the concentric spindle shafts.