Abstract:
An office chair is provided having a contoured back assembly comprising a back frame and a suspension fabric. The fabric is attached to the back frame through a spline wherein the back frame has a three-dimensional contoured shape formed by overlapped frame rings with the fabric being joined thereto by a spline. The fabric is secured to the frame by a machine wherein the fabric is first stretched from the outer edge of the fabric and then is clamped to a frame surface in the pre-tensioned or pre-streched condition. The fabric edges are allowed to hang loose and then fixed in the frame by a spline. Once secured, the fabric is unclamped from the frame.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/689 761, filed Jun. 10, 2005, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to an office chair with an improved chair back and more particularly, to a chair back with a peripheral frame and a suspension fabric which is secured to the frame by an improved attachment apparatus and method.  
       BACKGROUND OF THE INVENTION  
       [0003]     Preferably, conventional office chairs are designed to provide significant levels of comfort and adjustability. Such chairs typically include a base which supports a tilt control assembly to which a seat assembly and back assembly are movably interconnected. The tilt control mechanism includes a back upright which extends rearwardly and upwardly and supports the back assembly rearwardly adjacent to the seat assembly. The tilt control mechanism serves to interconnect the seat and back assembly so that they may tilt rearwardly together in response to movements by the chair occupant and possibly to permit limited forward tilting of the seat and back. Further, such chairs typically permit the back to also move relative to the seat during such rearward tilting.  
         [0004]     The back assembly of such office chairs may have a variety of constructions wherein one type of construction includes an annular back frame which defines an open interior in which the chair occupant&#39;s back is supported. This central open area is enclosed by a suspension fabric which spans the opening and has the outer peripheral edge thereof affixed to the annular frame.  
         [0005]     The back frame further has a section thereof rigidly connected to an upright of the chair. The upright is connected to a tilt control mechanism, which mechanism supports the seat assembly and governs rearward tilting of the upright. As such, the back assembly moves in combination with the upright when the occupant reclines within the chair.  
         [0006]     It is an object of the invention to provide an office chair having a chair back arrangement which is improved relative to prior chair back constructions and specifically is improved relative to the attachment of a suspension fabric to the rigid back frame.  
         [0007]     Therefore, the invention relates to an office chair and more particularly, to the construction of a chair back assembly and the process for stretching and attaching the suspension fabric to the back frame in a stretched condition.  
         [0008]     The back assembly comprises an annular frame having a suspension fabric which spans the central opening of the back and has the periphery of the suspension fabric connected to the frame. The suspension fabric comprises a single layer of suspension material which connects to the frame by a plastic spline that is press fitted into an annular spline channel formed about the periphery of the back frame.  
         [0009]     The back frame is defined by side frame rails, which extend vertically, and transverse cross rails, which extend horizontally between the side rails at the top and bottom thereof. The back frame has a contoured shape wherein the side frame rails curve rearwardly and outwardly in the side-to-side direction, while the cross rails also curve in the front-to-back direction to provide a contoured shape for the back frame that comfortably conforms to the chair back of the chair occupant.  
         [0010]     To form the spline channel, the back frame is constructed of molded front and back ring sections which are grooved about their respective peripheries to define front and rear channel portions. The front channel portion defines the front wall of the spline channel while the rear channel portion defines the rear channel wall. The front and rear ring sections overlie each other in opposing relation and are rigidly secured together by suitable fastening means such as ultrasonic welding, adhesives or even threaded fasteners. When the two frame sections are rigidly joined together, the grooved portions align with each other and define the spline channel. In this manner, complex contours may be formed in a molded back frame while still permitting the formation of a spline channel about the periphery thereof.  
         [0011]     Once the frame is assembled, the suspension fabric is fastened to the frame by a spline which is press fitted into the spline channel and traps the edge portions of the suspension fabric within this channel.  
         [0012]     The attachment structure thereby permits the securement of the suspension fabric to the frame using an elastomeric spline. This fabric is pre-stretched during assembly and then secured to the frame in this pre-stretched condition by the spline.  
         [0013]     The process involves securing or assembling the two frame sections together to define a frame unit, and then attaching the fabric thereto. Attachment is accomplished by positioning the frame in the bottom nest of an attachment machine, laying a fabric blank over the frame and stretching the fabric in place by a plurality of stretcher clamps placed about the peripheral edge of the fabric. These peripheral stretcher clamps pull or tension the fabric longitudinally to pretension the fabric.  
         [0014]     The fabric is then clamped on the upper surface of the frame by a top nest or clamp head which drops over the bottom nest and has a downward facing clamp surface that clamps the fabric surface onto the opposing frame face. This clamping holds the fabric in a stretched condition. Once clamped by the top nest, the peripheral stretcher clamps are released so that the peripheral edge of the fabric blank hangs loose. Thereafter, a spline tool is run around the spline groove of the frame to simultaneously drive a spline and the fabric edge into the groove about the frame periphery which removes the slack in the peripheral fabric edge and secures the fabric to the frame. When the top nest is removed, the fabric remains in the stretched condition due to the securement by the spline. This provides an improved method for securing a fabric to a contoured chair frame.  
         [0015]     The foregoing features provide an improved back frame arrangement, wherein other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a front elevational view of an office chair having an inventive back assembly arrangement.  
         [0017]      FIG. 2  is a side elevational view of the office chair.  
         [0018]      FIG. 3  is a rear isometric view thereof.  
         [0019]      FIG. 4  is a front isometric view thereof.  
         [0020]      FIG. 5  is a side view of a back assembly.  
         [0021]      FIG. 6  is a front view thereof.  
         [0022]      FIG. 7  is an exploded isometric view of the back assembly with a fastener for mounting the back frame on the tilt control mechanism of the chair.  
         [0023]      FIG. 8  is an enlarged exploded view of the mounting section of the back frame.  
         [0024]      FIG. 9  is an exploded isometric view of the back components forming the back assembly.  
         [0025]      FIG. 10  is a side cross-sectional view of the back assembly as taken along the centerline of the back assembly.  
         [0026]      FIG. 11  is a rear view of the back frame.  
         [0027]      FIG. 12  is a front view of the back frame.  
         [0028]      FIG. 13  is a diagrammatic view illustrating the manufacture of sheets of suspension fabric from a stock material.  
         [0029]      FIG. 14  is a top cross-sectional view of a side frame member as taken along line  14 - 14  of  FIG. 6 .  
         [0030]      FIG. 15  is a side cross-sectional view of the top frame member as taken along line  15 - 15  of  FIG. 6 .  
         [0031]      FIG. 16  is a side cross-sectional view of the bottom frame member as taken along line  16 - 16  of  FIG. 6 .  
         [0032]      FIG. 17  is an isometric view of an attachment machine for securing the suspension fabric to the back frame.  
         [0033]      FIG. 18  is a plan view of the attachment machine illustrating a bottom nest in a use position with a top nest removed therefrom.  
         [0034]      FIG. 19  is an end view of the attachment machine.  
         [0035]      FIG. 20  is a front isometric view of the attachment machine with the top nest disposed in position.  
         [0036]      FIG. 21  is a side view of the attachment machine.  
         [0037]      FIG. 22  is an isometric view of the bottom nest.  
         [0038]      FIG. 23  is a side view of the bottom nest.  
         [0039]      FIG. 24  is an end view of the bottom nest.  
         [0040]      FIG. 25  is an end view thereof.  
         [0041]      FIG. 26  is a bottom isometric view of the top nest.  
         [0042]      FIG. 27  is a bottom view of the top nest.  
         [0043]      FIG. 28  is a side view thereof.  
         [0044]      FIG. 29  is an end view thereof.  
         [0045]      FIG. 30  is a further end view thereof.  
         [0046]      FIG. 31  is an isometric view of a robotic-driven spline tool.  
         [0047]      FIG. 32  is an isometric view of the spline tool with a guide cap removed therefrom and illustrating an elastomeric spline in phantom outline.  
         [0048]      FIG. 33  is a flowchart of the attachment process.  
         [0049]      FIG. 34  illustrates the step of positioning the back frame in the bottom nest.  
         [0050]      FIG. 35  illustrates the step of positioning a fabric thereon.  
         [0051]      FIG. 36  illustrates the step of stretching the fabric.  
         [0052]      FIG. 37  illustrates the step of clamping the fabric to the frame.  
         [0053]      FIG. 38  illustrates the step of securing the fabric to the frame by the spline tool. 
     
    
       [0054]     Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.  
       DETAILED DESCRIPTION  
       [0055]     Referring to  FIGS. 1-4 , the invention generally relates to an office chair  10  which includes various inventive features therein that accommodate the different physical characteristics and comfort preferences of a chair occupant and also improve the assembly of the chair  10 .  
         [0056]     Generally, this chair  10  includes improved height-adjustable arm assemblies  12  which are readily adjustable. The structure of each arm assembly  12  is disclosed in U.S. Patent Application Ser. No. 60/657,632, entitled ARM ASSEMBLY FOR A CHAIR, which is owned by Haworth, Inc., the common assignee of this present invention. The disclosure of this patent application is incorporated herein in its entirety by reference.  
         [0057]     The chair  10  is supported on a base  13  having radiating legs  14  which are supported on the floor by casters  15 . The base  12  further includes an upright pedestal  16  which projects vertically and supports a tilt control mechanism  18  on the upper end thereof. The pedestal  16  has a pneumatic cylinder therein which permits adjustment of the height or elevation of the tilt control mechanism  18  relative to a floor.  
         [0058]     The tilt control mechanism  18  includes a control body  19  on which a pair of generally L-shaped uprights  20  are pivotally supported by their front ends. The uprights  19  converge rearwardly together to define a connector hub  22  on which is supported the back frame  23  of a back assembly  24  the structure of which is disclosed in U.S. Patent Application No. 60/657,313 entitled CHAIR BACK which is owned by Haworth, Inc. The disclosure of this patent application is incorporated herein in its entirety by reference. The structure of this tilt control mechanism is disclosed in U.S. Patent Application Ser. No. 60/657,541, entitled TILT CONTROL MECHANISM FOR A CHAIR, and U.S. Patent Application Ser. No. 60/657,524, entitled TENSION ADJUSTMENT MECHANISM FOR A CHAIR, which applications are owned by Haworth, Inc. The disclosure of each of these patent applications is incorporated herein in its entirety by reference.  
         [0059]     The back assembly has a suspension fabric  25  supported about its periphery on the corresponding periphery of the frame  23  to define a suspension surface  26  against which the back of a chair occupant is supported.  
         [0060]     To provide additional support to the occupant, the back assembly  24  also includes a lumbar support assembly  28  which is configured to support the lumbar region of the occupant&#39;s back and is adjustable to improve the comfort of this support. The structure of this lumbar support assembly  28  and pelvic support structure is disclosed in U.S. Patent Application Ser. No. 60/657,312, entitled CHAIR BACK WITH LUMBAR AND PELVIC SUPPORTS, which is also owned by Haworth, Inc. The disclosure of this patent application is incorporated herein in its entirety by reference.  
         [0061]     Additionally, the chair  10  includes a seat assembly  30  that defines an upward facing support surface  31  on which the seat of the occupant is supported.  
         [0062]     More particularly as to the back assembly  24 , the back assembly  24  is generally illustrated in  FIGS. 5-7 . The back frame  23  comprises a pair of vertical side frame rails  35 , a top frame rail  36 , and a bottom frame rail  37  which are joined together at the upper corners  38  of the back assembly  24  as well as the lower corners  39  to define an annular or endless frame having a central opening  40 .  
         [0063]     As can be seen in  FIGS. 5-7 , the back frame  23  has a contoured shape which ergonomically supports the back of the user. In particular, the side rails  35  curve backwardly as seen in  FIGS. 5 and 7  as well as outwardly ( FIG. 6 ) relative to the bottom portions of the side rails  35 . Further, the top rail  36  and bottom rail  37  each have a respective curvature to closely conform to the curvature of a typical chair occupant.  
         [0064]     To support the occupant, the back assembly  24  includes the suspension fabric  25  which is secured taughtly on the frame. Specifically, the back frame  23  includes a peripheral spline channel  42 , in which is fixed the peripheral edge  25 A of the suspension fabric  25  as will be discussed in further detail herein.  
         [0065]     The back frame  23  also generally includes a support structure  43  to which the side rails  35  and bottom rail  37  are rigidly interconnected. This support structure  43  comprises an upright support column  44  which extends along the chair centerline  41  ( FIG. 7 ) to an elevation located just below the middle of the side rails  35 . The upper end of the support column  44  includes a pair of support arms  45  which extend sidewardly and have each respective outer end connected rigidly to one of the side rails  35 .  
         [0066]     The lower end of the support column  43  includes a generally L-shaped connector flange  46  ( FIGS. 5 and 7 ) which projects forwardly and then downwardly into fixed engagement with the lower cross rail  37 . Still further, this lower column end includes a bayonet connector  49  which projects downwardly for rigid connection to the uprights  20  by fastener bolt  50  and nut  51  as will be described in further detail hereinafter.  
         [0067]     Referring more particularly to the components of the back assembly  24 ,  FIG. 9  illustrates these components in an exploded view thereof. In particular, the frame  23  comprises a molded rear frame unit  55  that includes the support structure  43  described above as well as a rear frame ring  56  which is supported on the support arms  45  of the support structure  44 . The back frame  23  further includes a molded front frame ring  57  which is adapted to be mounted to the rear frame ring  56  in overlying relation to define the spline channel  42  about the periphery thereof. Further, the back assembly  24  includes the above-described suspension fabric  25  and an elastomeric spline  58 .  
         [0068]     Referring to  FIGS. 11 and 12 , the rear frame unit  55  comprises the support structure  43  and the rear frame ring  56 . Both the support structure  43  and the rear frame ring  56  are molded simultaneously together in a one-piece monolithic construction having the contoured shape described above. To facilitate molding of this contoured shape while still possessing the spline channel  42  mentioned above, the rear frame ring  56  and front frame ring  57  are molded separate from each other and then affixed together.  
         [0069]     Turning to the support structure  43 , the support column  44  thereof is located centrally within the lower half of the central frame opening  40 . The support column  44  has a base end  59  and a pair of column halves  60  and  61  which are separated from each other by a vertically elongate column slot  62 . The column  44  therefore is formed as a split column by the slot  62  which extends along a substantial portion of the length of the column  44  with the column halves  60  and  61  being joined together by the solid base section  59 . As such, the column halves  60  and  61  are supported in cantilevered relation by the base section  59 . The rear frame unit  55  and front frame ring  57  are formed from a glass filled nylon material that is molded into the desired shapes wherein this material has limited flexure so as to permit flexing of the various areas of the frame when placed under load by a chair occupant. Since the column halves  60  and  61  are separated from each other by the slot  62 , these column halves  60  and  61  may articulate independently of each other to facilitate flexing movement of the various frame corners  38  and  39 .  
         [0070]     The upper ends of the frame halves  60  and  61  join integrally to the transverse arms  45 . The outer ends of the arms  45  extend outwardly and are molded integral with the vertical sides of the rear frame ring  56 . As seen in  FIG. 12 , the inside faces of the column halves  60  and  61  and the support arms  45  have an appropriate pattern of ribbing  64  to selectively rigidify the support structure  43  while still permitting flexure thereof.  
         [0071]     In the column base  59 , this column base  59  terminates at a bottom wall  65  ( FIGS. 9, 10  and  12 ), which is formed with a bore  66  extending vertically therethrough. The bottom wall  65  further is formed integral with the bayonet connector  49  wherein the bore  66  extends vertically through this bottom wall  65  and the bayonet  49  as seen in  FIG. 10 . When joining the back frame  23  to the chair uprights  20 , the fastener  50  extends upwardly from the uprights  20  as will be described in further detail herein and extends through the fastener bore  66  so that it projects vertically above the bottom column wall  65 . The upper end of the fastener  50  is engaged by the threaded nut  51  as seen in  FIG. 10  to thereby secure the back frame  23  to the uprights  20 . This interconnection between these components will be described in further detail hereinafter.  
         [0072]     Further as to the bottom wall  65 , this wall extends forwardly to define a horizontal leg  68  of the L-shaped flange  46 , which flange  46  then turns downwardly to define a vertical leg  69  ( FIGS. 5, 7  and  10 ). The bottom column section  59  serves to rigidly support the bottom cross rail  37  of the back frame  23 . As such, the bottom frame rail  37  is more rigidly supported and has less relative movement under occupant loads than the middle frame areas supported by the support arms  45  or even the upper frame corners  38  which have the greatest amount of displaceability. In this manner, the rear frame unit  55  provides for controlled flexing of the entire back frame  23 .  
         [0073]     Referring to  FIGS. 11 and 12 , the rear frame ring  56  comprises top and bottom ring sections  71  and  72  and left and right ring sections  73  which extend vertically. As seen in  FIG. 14 , each side ring section  73  includes raised connector ribs  74  and  75  as well as an additional rib  76  which defines the inside end of the spline channel  42  and is also located directly adjacent to a spline groove  77  that is adapted to accommodate the thickness of the spline  58 . The outer edge of the ring side section  73  includes a thinner portion  78  and essentially defines the rear side portion of the spline channel  42 .  
         [0074]     As to the horizontal rail section  71 , this rail section  71  includes an interior rib  80 , a channel rib  81  and an intermediate connector face  82 . Adjacent to the channel rib  81 , a spline groove  83  is provided to accommodate the thickness of the spline  58  while the outer edge portion  73  continues across the top ring section  71 .  
         [0075]     Referring to  FIG. 16 , the lower rail section  72  includes a spline channel  85  along its entire lateral width so as to accommodate the spline  58 . Here again, the outer edge portion  78  continues through this region to define the back portion of the spline channel  42 . In the middle of the lower ring section  72 , a recessed pocket  87  is defined which opens upwardly and is located vertically adjacent to a locking post  88 , the function of which will be described in further detail hereinafter.  
         [0076]     The pocket  87  is defined by side walls  89  which side walls  89  include notches  90  at the bottom end thereof directly adjacent to the spline groove  85 . It is noted that all of the spline grooves  77 ,  83  and  85  are arranged in end-to-end relation so as to define a continuous groove for continuously accommodating the spline  58  therein. Further, the channel ribs disposed directly adjacent to these spline grooves  77 ,  83  and  85  are formed continuously in end-to-end relation.  
         [0077]     Turning next to the front frame ring  57 , this frame ring  57  has a front face  92  which faces forwardly and a rear face  93  which faces rearwardly towards the rear frame ring  56  and is adapted to abut thereagainst and be fixedly secured thereto. This frame ring  57  is defined by vertical ring sections  94  and a top ring section  95  and a bottom ring section  96 .  
         [0078]     As to the side ring sections  94  ( FIG. 14 ), these ring sections  94  include a rearwardly projecting connector rib  98  and an interior edge portion  99  which are adapted to abut against and be fixedly secured to the connector ribs  75  on the rear frame ring  56 . Preferably, the front and rear frame rings  57  and  56  respectively are joined together by ultrasonic welding of these components with the faces disposed in contact being welded together. The front ring sections  94  are molded with a channel rib  99  disposed directly adjacent to a spline groove  100 , which spline groove  100  is disposed in opposing relation with the other spline groove  77  to define an enlarged interior portion in which the spline  58  may be received. The outer edge portion  101  of the front ring  57  is disposed adjacent to but spaced apart from the other edge portion  78  to thereby define the entry portion of the spline channel  42 .  
         [0079]     As to the top ring section  95  ( FIG. 15 ), this is formed similar in that it includes a channel rib  102  and a spline groove  103  which is disposed directly adjacent to the outer edge portion  101  to thereby form the spline channel  42  in cooperation with the spline groove  83  and outer edge portion  78  of the rear frame ring  56 . The outer edge portion  101  of the front frame ring  57  furthermore extends downwardly along the other vertical ring section  93  and then across the bottom ring section  96 .  
         [0080]     The bottom ring section  96  ( FIG. 16 ) is formed with a spline groove  105  extending thereacross in alignment with the opposing rear spline groove  85 . In this manner, the rear frame ring  56  defines a rear portion of the spline groove  42  while the front frame ring  57  defines a front portion thereof which said frame rings  56  and  57  when disposed in opposing relation define the channel  42  so that it opens radially outwardly and has an enlarged interior groove in which the spline  58  and associated fabric material may be received in non-removable engagement. As best seen in  FIG. 14 , the fabric  25  is wrapped around the spline  58  to essentially define a folded hem which folded hem is then pressed into the spline channel  42  by the tooling described herein relative to  FIGS. 17-38 .  
         [0081]     In this manner, the suspension fabric  25  is tightly fitted onto the back frame  23  so that the fabric material  25  is stretched taut as generally illustrated in  FIG. 10  and then angles away from the back frame  23  and spans the central frame opening  40 .  
         [0082]     The suspension fabric  25  is formed of any suitable suspension material which preferably is elastomeric and preferably has an open weave that provides for breathability.  FIG. 13  generally illustrates the formation of the pieces of suspension fabric  25  wherein  FIG. 13  illustrates an initial stock material  110  from which is cut multiple pieces of the suspension fabric  25 . The suspension fabric  25  preferably is formed as a single layer but may have multiple overlying layers and may also include cushioning included therein.  
         [0083]     Generally, the back frame  24  is assembled by first joining the rear frame ring  56  to the front frame ring  57  by ultrasonic welding. Thereafter, the suspension fabric  25  is resiliently stretched over the back frame  23  with the peripheral edges of the fabric  25  being affixed into the spline groove  42  by the spline  58 .  
         [0084]     Referring to  FIGS. 17 and 20 , an attachment machine  120  is illustrated for mounting the fabric  25  to the frame  23  by the spline  58 . The attachment machine  120  comprises a lower nest  121  ( FIG. 17 ) which is adapted to support the above-described frame  23  thereon, and further comprises a clamping body formed as an upper nest  122  ( FIG. 20 ) which is adapted to press downwardly on the lower nest  121  and against the frame  23  therebetween.  
         [0085]     More particularly as to  FIGS. 17-19 , the attachment machine  120  includes a base assembly  124  having a base plate  125  on which the bottom nest  121  is supported. This base plate  125  supports a plurality of stretchers or clamp assemblies  126  thereon which clamp assemblies include a base column  127  that supports a pneumatic cylinder and includes an upstanding cylinder rod  128  thereon. On the upper end of the cylinder rod  128 , a jaw assembly  129  is supported wherein actuation of the pneumatic cylinder  127  selectively raises and lowers the jaw assembly  129 .  
         [0086]     The lower nest  121  is disposed in the central region of the base plate  124  with a plurality of individual clamp assemblies  126  provided along each edge of the lower nest  121 . In particular as seen in  FIG. 18 , six clamp assemblies  126  are provided along each opposite side edge of the lower nest  121  while four clamp assemblies  126  extend along each of the upper and lower end edges of the lower nest  121 . The upper and lower end edges are identified relative to the upper and lower ends of the chair back  24  referenced above.  
         [0087]     Referring to  FIG. 19 , each jaw assembly  129  generally is formed the same as each other and comprises a first lower fixed jaw  131  and a movable upper jaw  132  which define a clamping region or interface  133  therebetween. The upper jaw  132  is connected to a lever actuated cam mechanism  134  which allows the upper jaw  132  to be pivoted upwardly (such as at location  135  illustrated in  FIGS. 17-9 ) and downwardly. It is noted that the contour of each jaw interface  133  of each of the various jaw assemblies  129  has a variable and preferably arcuate shape which conforms generally to the adjacent contour of the lower nest  121 . Thus, when the jaw assemblies  129  are in the fully raised position generally illustrated in  FIG. 20 , the jaw interfaces  133  generally extend parallel to and directly align with the contour of the bottom nest  121 . From the end as illustrated in  FIG. 19 , the jaw assemblies  129  at the opposite ends of the mechanism  120  curve downwardly in a generally concave shape. When viewed from the side as seen in  FIG. 21 , the jaw interfaces  133  generally extend or curve upwardly in a convex configuration.  
         [0088]     Referring to  FIGS. 20 and 21 , the upper nest  122  is adapted to be pressed downwardly onto the top of the bottom nest  121  wherein a frame receiving peripheral gap  140  is defined at the interface therebetween. This peripheral gap  140  is adapted to accommodate the thickness of the back frame  55  therein as generally depicted in  FIGS. 34-38 .  
         [0089]     These jaw assemblies  129  therefore are normally positioned directly adjacent to the peripheral gap  140  when in the raised position illustrated in  FIGS. 20 and 21 . In this raised position, the jaw interfaces  133  are disposed directly adjacent to and at the same vertical elevation as the gap  140 .  
         [0090]     Since the jaw assemblies  129  are also drivingly interconnected to the cylinder shaft  128 , the jaw assemblies  129  may be selectively lowered wherein the jaw assembly  129  at position  141  ( FIG. 17 ) is illustrated in its lowermost limit of the down stroke thereof. Thus the jaw assemblies  129  may travel vertically from the lower stroke limit illustrated at position  141  ( FIG. 17 ) to the uppermost position such as that illustrated at position  135  ( FIG. 17 ) wherein the jaw assembly  129  may be opened by the cam actuator  134 .  
         [0091]     These jaw assemblies  129  therefore serve as peripheral stretcher clamps for stretching the suspension fabric  25  ( FIGS. 35-38 ) lengthwise across the face of the fabric  25  as will be described in further detail herein. All of the stretcher clamps or jaw assemblies  129  are interconnected to a pneumatic control system which is configured so that these jaw assemblies  129  all move in unison downwardly during the stretching step of the method of the invention and the magnitude of the vertical stroke may be varied.  
         [0092]     More particularly as to the bottom nest  121 , this component is illustrated in  FIGS. 22-25 , wherein the lower nest  121  is formed out of a monolithic block of a suitable material such as plastic which will align and locate the frame  23  in position without marring the surface thereof.  
         [0093]     The bottom nest is formed with a peripheral frame receiving space  145  which is shaped so as to have the same contour as the back frame  23 . The interior edge  146  of this space  145  is disposed closely adjacent to the frame  23  and locates the frame  23  relative thereto. To accommodate the upright frame column  44  and the transverse support arms  45 , the lower nest also includes a vertically elongate clearance pocket  147 , which generally extends vertically, and includes transverse clearance pockets  148 , wherein the pockets  147  and  148  respectively support the frame structures  44  and  45  therein so that the frame  23  seats uniformly. In this regard, the frame  23  has the back surface thereof disposed directly on the nest surface  149  to provide continuous support to the frame  23  about substantially its entire periphery.  
         [0094]     As seen in  FIG. 23 , the outer terminal edge  150  of the frame space  145  has a convex contour along most of its length with a slight concave contour approximate the upper end area thereof which is near the upper end of the frame  23 . Next as seen in  FIG. 24 , the end region near the lower frame end has a concave contour and a relatively narrow lateral width.  
         [0095]     Referring to  FIG. 25 , the end portion of the lower nest  121  near the upper frame end also has a concave contour but with a wider lateral width than the lower end portion illustrated in  FIG. 24 . As such, when the upper nest  122  is elevated, the lower nest  121  is exposed as in  FIG. 17  and the frame  23  is fitted therein to the seated position illustrated in  FIG. 34 , preferably manually although it also would be possible to automate this process. In this position, the upper surface  152  ( FIG. 34 ) of the frame  23  faces upwardly and is exposed.  
         [0096]     Referring to  FIGS. 26-30 , the upper nest  122  is defined by a peripheral wall  160  and an upper wall  161 . The upper wall  60  includes a pair of attachment bores  162  therein which attach to a lift mechanism  163  ( FIG. 20 ) which is adapted to automatically raise and lower the upper nest  122  by hydraulics to permit insertion and removal of the frame  55  from the frame space  145 . When viewed from below as seen in  FIG. 27 , the upper nest  122  has the same geometric shape as the lower nest  121  illustrated in  FIG. 18 .  
         [0097]     The side wall  160  projects downwardly and the lower surface  163  thereof defines a downward facing clamping surface  164  which is adapted to press downwardly against the upper frame surface  149  as seen in  FIG. 37 . The thickness of the wall  160  is generally less than the width of the frame space  145  to provide clearance therebetween as the upper nest  122  is displaced downwardly during the attachment process. Referring to  FIG. 28 , the side contour of the clamping surface  164  generally conforms to the mirror image side contour of the lower nest  121  as seen in  FIG. 23 . Also, this clamping surface  164  has a generally arcuate shape as illustrated at location  165  which surface  165  at least partially conforms to the contoured frame face  149 .  
         [0098]     Referring to  FIG. 29 , the clamping surface  164  at the narrow end portion has an arcuate contour which conforms to the opposing mirror image end contour of the lower nest  121  as illustrated in  FIG. 24 . Similarly as seen in  FIG. 30 , the wider end contour conforms to the respective end contour of the bottom nest  121  ( FIG. 25 ). As such, the top nest  122  presses relatively evenly about the entire peripheral frame surface  149 . This top nest  122  thereby serves as a clamp head which squeezes downwardly on the upper frame surface  49  to temporarily clamp and maintain the suspension fabric  25  thereon as will be described hereinafter.  
         [0099]     The attachment machine  120  further includes a conventional robot unit  166  ( FIG. 20 ) with a programmable articulating arm  167  that has a distal tool end  168  which is adapted to travel around the periphery of the frame  23  and in particular, travel around the frame space  140 . This robot unit  168  is adapted to mechanically insert the spline  58  into the spline channel  42  during securement of the suspension fabric  25  onto the frame  23 .  
         [0100]     Referring to  FIGS. 31 and 32 , the robot unit  166  comprises a spline installation tool  170  on the movable distal end  168  thereof which is adapted to press the spline  58  into the spline groove  42  as diagrammatically depicted in  FIG. 38 . More particularly, the tool  170  ( FIGS. 31-32 ) includes a base plate  171  wherein the tool  170  is rigidly affixed to the free end  168  of the robot arm  167  and is moved about the frame space  140  during installation of the spline  58 . To effect insertion of the spline  58 , the tool base plate  171  includes a linear drive unit  172  that is connected to a drive plate  173  to effect reciprocating linear movement of the drive plate  173 . The drive plate  173  includes a slide block  174  that slidably engages a slide rail  175 .  
         [0101]     The outer end of the base plate  171  includes a guide housing  177  defined by a housing base  178  and a housing cover  179 . As seen in  FIG. 32  with the housing cover  179  removed, the housing base  178  includes an elongate guide channel  181  extending longitudinally therethrough as well as a spline guide  182  through which the free end of the spline  58  projects outwardly therefrom as seen in  FIG. 32 . This outer free end  183  of the spline  58  is the end which is fitted into the spline channel  42  when the frame is disposed within the machine  120 .  
         [0102]     Additionally, an installation blade  185  is mounted to the block  173  and projects horizontally through the guide channel  181 . Therefore during linear movement of the slide block  174 , the blade  185  is adapted to be either retracted as seen in  FIG. 32  or extended as illustrated in phantom outline in  FIG. 32 . When the blade  185  is extended in the direction of arrow  186 , this pushes the free end  183  of the spline  58  into the spline channel  42 . The blade end  185  remains inside of the spline channel  42  during installation such that as the spline tool  170  is moved in parallel relation along the frame space  140 , the blade  185  causes the spline  58  to be pressed between a fold of the suspension fabric  25  which is retained within the channel  42  as seen in the above-described  FIGS. 14-16 .  
         [0103]     Referring more particularly to the process of the invention,  FIG. 33  illustrates the various process steps in a preferred arrangement. First, the frame  23  is assembled together in step  190  by placing the front and back frame section  56  and  57  in opposing relation and then ultrasonically welding these two frame sections together as described above. Thereafter, in step  191 , the frame is positioned into the bottom nest  121  as illustrated in  FIG. 34 . The frame unit  55  seats within the various grooves as described above with the frame surface  149  facing upwardly and the top nest  122  being elevated to a raised position by the lifting mechanism  163 . This therefore exposes the upper surface of the bottom nest  121 .  
         [0104]     In step  192  the suspension fabric  25  is laid over the upper surface of the bottom nest  121  in a loose unstretched condition. The material blank for the suspension fabric  25  has a dimension which is slightly larger than the overall dimension of the frame unit  55  so that the outer peripheral edge  25 A hangs outwardly from the outer peripheral edge of the frame  55  as seen in  FIG. 35 .  
         [0105]     Thereafter, this peripheral edge  25 A is slid into each of the various clamp assemblies  126  and the upper and lower jaws  132  and  131  of each jaw assembly  129  are pressed together by the respective cam mechanism  134 . During this initial seating of the fabric edge  25 A into the clamp assemblies  126 , the jaw assemblies  129  are in the raised position illustrated in  FIG. 19 . To insert the fabric edge  25 A in position, the cam actuator  134  is actuated to lift the upper jaw  132  as indicated in position  135  of  FIG. 19 . Thereafter, the lever  134  is articulated to lower the jaw  132  and clamp the fabric  25  therebetween in the interface  133 . While the illustrated stretchers  129  are manually operated to grippingly engage the fabric edge  25 A, this procedure also could be automated if desired.  
         [0106]     In step  193 , the fabric  25  is then stretched by displacing the jaw assemblies  129  downwardly as indicated by reference arrow  129 A ( FIG. 36 ) which thereby stretches the fabric  25  laterally and tensions same in the direction of reference arrow  129 A ( FIG. 36 ). The amount of downward displacement of the jaw assemblies  129  thereby governs the amount of tension in the fabric  25  and the individual jaw assemblies  129  may be displaced vertically downwardly in different amounts to vary the overall tension in different areas of the fabric  25 . It is noted the jaw assemblies  129  are provided in aligned pairs to pull away from each other and tension the fabric therebetween.  
         [0107]     Once the fabric  25  is stretched, the fabric  25  is clamped to the frame  55  in step  194  ( FIGS. 33 and 37 ). In step  194 , the top nest  122  is lowered downwardly until the nest surface  164  presses snuggly against the opposing frame face  149  which thereby clamps the fabric  25  downwardly therebetween in a direction transverse to the fabric face. This clamping force applied by the top nest  122  is sufficient to maintain the fabric  25  in a stretched condition. Thereafter, the jaw assemblies  129  are disengaged by separating the upper and lower jaws  132  and  133  and allowing the fabric edge  25 A to hang loosely. Preferably, the jaw assemblies  129  are displaced downwardly to the lower position as seen in position  141  of  FIG. 17  so as to provide an unobstructed space extending around the entire frame space  140 .  
         [0108]     In step  195  ( FIGS. 33 and 38 ), the fabric edge  25 A is then secured to the frame  55  by insertion or press fitting of the spline  58  into the spline groove  42  which thereby folds the fabric edge  25 A about the spline  58  and secures the fabric  25  about its entire periphery to the frame  55 . Once the fabric  25  is fully secured in position, the clamping force provided by top nest  122  is released. Therefore, the next step is step  196  ( FIGS. 33 and 38 ) wherein the top nest  122  is raised upwardly in the direction of reference arrow  200 . This thereby discontinues the clamping action of the top nest  122  and since the edge  25 A of the suspension fabric  25  is secured to the frame  55 , the fabric  25  remains in its stretched condition and the entire back frame assembly  24  is assembled.  
         [0109]     With this arrangement, selective stretching may be provided to the fabric  25  depending upon the different amounts of vertical travel or stroke of the various jaw assemblies  129  while still using a spline type connection between the fabric  25  and the frame  55 .  
         [0110]     Although particular preferred embodiments of the invention have been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.