Abstract:
Apparatus and methods for use in the manufacture of a spring unit for incorporation into an upholstered article, for example, a mattress, cushion or the like. Coil formation apparatus includes a drive shaft used to control movement of a coil pitch guide member and a link member comprising a connecting rod connected to a radius arm of the drive shaft by means of an adjustable connection. A coil interlinking process comprises compressing a first coil to define a clearance extending a second coil passed the first coil via the clearance, allowing the first coil to extend across the clearance, and contracting the second coil such that the second coil engages the first coil thereby interlinking the first and second coils. Spring unit manufacturing apparatus comprises a plurality of jaw pairs each comprising a first fixed jaw and a pivotal second jaw, the pivotal second jaw being pivoted by a cam and linkage assembly that is operated by a rotary drive shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Patent Application No. PCT/GB2006/001529, filed Apr. 26, 2006, which claims priority to GB patent application No. 0508393.6, filed Apr. 26, 2005, both of which are herein incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to apparatus and method for the manufacture of a spring unit for use in an upholstered article, for example, a mattress, cushion or the like. 
     BACKGROUND OF THE INVENTION 
     A spring unit for an upholstered article comprises an array of interconnected helical coil springs formed from metal wire. 
     The production of such a spring unit conventionally comprises three principal steps that are described below with reference to  FIG. 1 . 
     First the wire is coiled to form the springs. In order to do this, wire  1  from a reel  2  is fed in the direction of arrow A to a coiling machine  3  to form a coiled wire  4  consisting of a continuous series of alternating left and right-handed helical coils  5 ,  6  interposed with substantially straight sections of wire  7 . The coiled wire  4  is folded at appropriate intervals as it emerges from the coiling machine so that the straight sections of wire  7  are parallel to one another and adjacent left and right-handed coils  5 ,  6  are arranged so that their central longitudinal axes are approximately disposed in parallel. 
     The folded coils  4  are fed to a linking table  8  where the adjacent right and left-handed coils are interlinked. The strings of coils  9  are periodically cut into predetermined lengths and each string  9  fed on to a storage reel  10  ready for use in the final step of the process. To form the complete spring unit, the strings of coiled wire  9  are fed from a plurality of such storage reels  10  via channels  11  defined between dividers  12  to a spring unit assembly machine  13  where the strings  9  are interconnected to form the finished spring unit. In an alternative embodiment, sets of folded coils  9  exiting a plurality of folding tables  8  may be fed directly to the spring unit assembly machine  13  via channels  11 . 
     The assembly machine  13  advances the strings  9  in parallel such that the coils  14  are aligned. The strings  9  are indexed by one coil width at a time to a set of transversely extending jaws  15  between which they are clamped. Successive coils  14  in the adjacent strings  9  are clamped with their longitudinal axes substantially upright. The jaws  15  effectively form a continuous helical channel into which a helical binding wire  16  is advanced. The binding wire is formed by passing uncoiled wire  17  from a reel  18  to a coiling passage  19  located to the side of the jaws  15  of the assembly machine  13 . It is rotated and axially advanced in the transverse direction of arrow B through the jaws  15  such that is passes around the wire of the adjacent strings  9  and so as to form a row  20  of bound coils  14 . The jaws  15  are then opened and the joined strings of coils  9  indexed forward in the direction of arrow A so as to locate the next coil of each string  9  within the jaws  15  whereupon the above cycle is repeated to bind the next row of coils together. The binding cycle is repeated a sufficient number of times to bind a suitable number of rows of coils together to produce a spring unit of the desired size. 
     One example of a method for manufacturing the strings of coils prior to the assembly machine is described in U.S. Pat. No. 5,105,642. This method is unduly complex particularly as it includes an additional folding station between the coiler and a coil interlock station. There is no detailed description of interlocking method. A problem with a coiler of this kind is that adjustment of the coil pitch is not possible without significant changes to the relative positions of the machine components. 
     An example of a conventional process for interlinking adjacent left and right handed coils comprises passing the coiled wire to a linking table whereupon a straight section of the wire interposed between the coiled sections is presented to a pivotable butterfly clamp which is located centrally with respect to the table. The straight section of the wire is then held in place by the butterfly clamp with the left and right handed coiled sections to either side. One of the coiled sections is then engaged by a ‘pecker arm’ which moves transverse to the longitudinal axis of the table to engage the coil and hold it in place relative to the linking table. A folding arm mounted above the table surface is then operated to pivot about a substantially upright support member and engage the free coiled section of wire on the opposite side of the butterfly clamp. Pivoting of the folding arm draws the free coiled section in an arc around the butterfly clamp towards the other coiled section which is held by the ‘pecker arm’ to interlink the two coiled sections of wire. 
     The process is unduly complex and requires extremely accurate control of a number of different simultaneous actions. Due to the complicated manner in which adjacent coils are interlinked, the operational efficiency of the process is severely restricted. For example, a process of this kind could typically interlink only 30 to 35 coils per minute. The apparatus required to carry out the process incorporates a number of different cammed surfaces to accurately control the movement of the various components. A problem with linking tables of this kind is that adjustment of the various components to accommodate coils of different sizes is not possible without significant changes to the relative positions of the machine components and the complicated nature of the apparatus results in reliability problems. 
     An example of an assembly machine is described in EP0248661. The disadvantage of this machine is that each of the pairs of jaws are opened and closed by a respective double acting pneumatic piston. Such a piston has at least one sensor so that the opening and closing of the jaws can be monitored. In operation it has been found that the machine operation is often interrupted through the malfunction of at least one sensor. The use of so many sensors increases the scope for interruption of the machine operation. Moreover, since the piston stroke time (and therefore the time required to open and close a pair of jaws) varies between pistons a sufficient time window has to be built into the timing cycle of the assembly operation in order to be sure that all of the jaws have opened or closed. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to the first stage of the above manufacturing process, that is the formation of the coil springs from continuous wire. 
     Further aspects of the present invention relate to the second stage of the above manufacturing process, that is linking of adjacent coils of the coiled wire  4  on the coil linking table  8  to ensure that adjacent left and right-handed coils  5 ,  6  are linked together in the correct orientation for the final assembly stage. 
     A further aspect of the present invention is directed to an assembly machine for use in the third stage of the above process. 
     It is an object of the various aspects of the present invention to obviate or mitigate the aforesaid, and other, disadvantages. 
     According to a first aspect of the present invention there is provided coil formation apparatus for manufacturing spring coils from continuous wire, the coils being arranged to be of alternating hands along the wire, the apparatus comprising a coil forming device and means for feeding the wire to the device, the device comprising a pivotally disposed body providing support for a coil radius forming wheel against which the wire bears to form an arcuate shape and a guide member defining an opening from which the coiled wire emerges, the guide member being pivotally disposed relative to the body such that it can pivot between a first position where the opening is aligned with the wire emerging from the roller so that it passes therethrough without further deformation and at least one second position where it is misaligned and bears against the wire thus imparting the deformation to the wire that gives the coil its axial pitch, the angle of pivotal movement of the guide member being controlled by an adjustable drive mechanism that comprises a rotary drive shaft driven by a servomotor in response to instructions sent by a controller, the drive shaft being connected to the guide member by a transmission linkage that converts rotary movement of the drive shaft into translational movement of a link member and converts the translational movement of the link member to pivotal movement of the guide member as the main body is pivoted, the link member comprising a connecting rod connected to a radius arm of the drive shaft by means of an adjustable connection. 
     Preferably the guide member is pivotal between two second positions, one to each side of the first position. 
     It is preferred that the adjustable connection comprises an arm to which an end of the connecting rod is pivotally connected, the position of the end of the connecting rod being adjustable by an adjustment element. The adjustment element may be a screw or the like that is rotatable in one direction to bear against the end of the connecting rod and move it radially closer to the centre of rotation of the drive shaft. Conveniently, the arm has a slot, and a fixing member passes through the end of the connecting rod and the slot so as to connect the connecting rod to the arm, the adjustment element being adapted to move the end of the rod along the slot. Preferably the adjustment element bears against the fixing member. 
     In a preferred embodiment the transmission linkage comprises a sliding yoke that is connected to the connecting rod and slides along a shaft on which the body is mounted for pivotal movement. 
     It is particularly preferred that the translational movement of the link member is converted into pivotal movement of the guide member by a cam and cam follower comprising a bar with a spiral cam groove in which a pin is received, the axial movement of the bar being restrained such that movement of the pin relative to the bar along the cam groove causes rotation of the bar and therefore pivoting movement of the guide member. 
     According to a second aspect of the present invention there is provided a coil interlinking process for interlinking first and second wire coils defining respective first and second coil axes, the process comprises providing the first and second coils on a supporting surface such that the first and second coil axes are orientated substantially perpendicular to a longitudinal axis of the supporting surface, actuating a first compression member to compress the first coil substantially parallel to said first coil axis to define a first clearance between the first coil and a first edge of the supporting surface, actuating a first indexing member to extend the second coil substantially parallel to said longitudinal axis passed the first coil via said first clearance, retracting the first compression member to allow the first coil to extend substantially parallel to the first coil axis across said first clearance, and retracting the first indexing member to allow the second coil to contract substantially parallel to said longitudinal axis such that the second coil engages the first coil thereby interlinking the first and second coils. 
     A significant advantage provided by this process is that the various steps required to interlink adjacent coils can be achieved in a stepwise fashion using simple sequential linear movements of the compression member and the indexing member. It is therefore no longer necessary to coordinate simultaneously a number of different more complex movements to interlink a pair of spring coils. The timing of the various steps involved in the inventive process is consequently much easier to control than in prior art systems. This fact, together with the removal of the need to pivot one coil with respect to the other coil to interlink them significantly increases the throughput of the interlinking operation. It has been observed that the operational efficiency of the interlinking operation can be doubled by use of the inventive process. 
     Preferably prior to actuation of the compression member a retaining pin is extended substantially perpendicular to the supporting surface to engage a portion of the first coil and retain the first coil in a substantially fixed longitudinal position in relation the supporting surface during compression of the first coil with the first compression member. 
     It is preferred that after interlinking of the first and second coils said retaining pin is retracted so as to no longer engage said portion of the first coil and indexing apparatus subsequently actuated to advance the interlinked first and second coils a predetermined distance substantially parallel to said longitudinal axis. 
     Conveniently the process further comprises actuating a second compression member to compress the first coil substantially parallel to said first coil axis to define a second clearance between the first coil and a second edge of the supporting surface which is opposite to said first edge, the second compression member being actuated sequentially or simultaneously with the first compression member. 
     After interlinking the first and second coils, the interlinked first and second coils may be heat treated. Preferably said heat treatment is carried out by passing an electric current through the first and second interlinked coils. 
     In a preferred embodiment of this aspect of the present invention said first and second coils are formed in a single piece of wire and most preferably said first coil is a right handed coil and said second coil is a left handed coil. 
     A third aspect of the present invention provides coil interlinking apparatus for interlinking first and second wire coils defining respective first and second coil axes, the apparatus comprising a supporting surface, a first compression member and a first indexing member, the supporting surface being arranged to enable the first and second coils to be provided on the supporting surface such that their first and second coil axes are orientated substantially perpendicular to a longitudinal axis of the supporting surface, the first compression member being operable to compress the first coil substantially parallel to said first coil axis to define a first clearance, the first indexing member being operable to extend the second coil substantially parallel to said longitudinal axis passed the first coil via said first clearance, the first compression member being operable to retract to allow the first coil to extend substantially parallel to the first coil axis across said first clearance, and the first indexing member being operable to allow the second coil to contract substantially parallel to said longitudinal axis such that, in use, the second coil engages the first coil thereby interlinking the first and second coils. 
     Preferably the supporting surface additionally comprises a second edge opposite to said first edge, and first and second side walls are provided at said first and second edges respectively, the side walls and the supporting surface together defining a channel. 
     In a preferred embodiment the first side wall defines a first slot extending substantially parallel to said longitudinal axis of the supporting surface, the slot being configured for receipt of a base portion of the first indexing member. 
     The first indexing member may comprise a coil engaging portion connected to said base portion, said coil engaging portion projecting into said channel. Conveniently the coil engaging portion of the first indexing member has an arcuate leading surface. Preferably the coil engaging portion of the first indexing member has a ramped trailing surface. 
     In a further preferred embodiment the support surface defines a first guide slot extending substantially perpendicular to said longitudinal axis of the supporting surface for receipt of the first compression member. The first compression member preferably has an inclined leading edge. 
     It is preferred that the apparatus further comprises a retaining pin which is operable to extend substantially perpendicular to the supporting surface to engage a portion of the first coil and retain the first coil in a substantially fixed longitudinal position in relation the supporting surface during compression of the first coil with the first compression member. 
     The apparatus may further comprise indexing apparatus operable to advance the interlinked first and second coils a predetermined distance substantially parallel to said longitudinal axis. 
     Conveniently heat treatment means may be provided to heat treat the interlinked first and second coils and said heat treatment means preferably comprises a pair of electrodes configured to pass an electric current through the first and second interlinked coils. 
     A fourth aspect of the present invention provides apparatus for manufacturing a spring unit for a mattress or the like, the spring unit comprising a plurality of strings of spring coils, each string arranged so that the coils are disposed in a row in a side by side relationship, the apparatus comprising an inlet unit to which the strings of coils are fed, an indexing device and a binding station by which the plurality of strings are bound together by a helical binding wire, the binding station comprising at least one pair of jaws movable between open and closed positions, the jaws combining in said closed position to define a helical passage through which the helical binding wire is direction so as to bind adjacent strings of coils together, the jaw pairs each comprising a first fixed jaw and a pivotal second jaw, the pivotal second jaw being pivoted by a cam and linkage assembly that is operated by a rotary drive shaft. 
     Preferably the cam is an eccentric cam. 
     Preferably there are a plurality of jaw pairs arranged side by side, each pair having its own eccentric cam and linkage assembly, the assemblies being operated by a common rotary drive shaft. 
     In a preferred embodiment of this aspect of the present invention the linkage assembly comprises a lever arm that is pivotally mounted in a support and is pivotally moveable by the eccentric cam, the lever arm being connected to the pivotal second jaw. The lever arm may be connected to a pivoting arm via a link member, the pivotal second jaw being mounted on the pivoting arm. Conveniently, the jaws may be mounted in a body, the lever arm and pivoting arm being pivotally mounted to the body. The lever arm and pivoting arm are preferably pivotally mounted on shafts supported by the body, and it is preferred that the body has a pair of spaced side walls and the lever arm is pivotally disposed between the side walls. 
     The rotary drive shaft is preferably driven by a servomotor, which may be connected to the drive shaft via a torque limiter device. Conveniently, the torque limiter device is provided in a gearbox. 
     It is particularly preferred that the jaw pairs are arranged into two sets to enable simultaneous binding of opposite sides of the spring unit. 
     The jaws may be mounted in the apparatus on a support that is moveable by an actuator. 
     It will be appreciated that the various apparatus and methods described in this summary section, as well as elsewhere in this application, can be expressed as a large number of different combinations and subcombinations. All such useful, novel, and inventive combinations and subcombinations are contemplated herein, it being recognized that the explicit expression of each of these myriad combinations is excessive and unnecessary. 
     These and other features and aspects of different embodiments of the present invention will be apparent from the claims, specification, and drawings. Although various specific quantities (spatial dimensions, material, temperatures, times, force, resistance, etc.), such specific quantities are presented as examples only, and are not to be construed as limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation in plan view of a conventional spring unit production process showing the manufacturing stages that are also adopted in the present invention. 
         FIG. 2  is a perspective view from one side of a coiling machine in accordance with one aspect of the present invention. 
         FIG. 3  is a perspective view from the side of an upper part of the coiling machine. 
         FIG. 4  is an inset view of part of the coiling machine showing a coil pitch adjustment feature in accordance with one aspect of the present invention. 
         FIG. 5  is a perspective schematic overview of a linking table in accordance with an aspect of the present invention shown with a partly linked helical wire coil at a first step in a linking operation. 
         FIG. 6  is a perspective schematic view of a pair of indexing fingers used to index the helical wire coil of  FIG. 5  across the linking table. 
         FIG. 7  is a perspective schematic overview of the linking table and the partly linked helical wire coil of  FIG. 5  shown at a second step in the linking operation. 
         FIG. 8  is a perspective schematic overview of the linking table and the partly linked helical wire coil of  FIG. 5  shown at a third step in the linking operation. 
         FIG. 9  is a perspective schematic overview of the linking table and the partly linked helical wire coil of  FIG. 5  shown at a fourth step in the linking operation. 
         FIG. 10  is a perspective schematic overview of the linking table and the partly linked helical wire coil of  FIG. 5  shown at a fifth step in the linking operation. 
         FIG. 11  is a photograph taken from a downstream position of the linking table of the present invention with a partly linked helical wire coil. 
         FIG. 12  is a perspective schematic overview of a spring unit assembly machine in accordance with an aspect of the present invention. 
         FIG. 13  is a perspective schematic view of an inlet unit of the spring unit assembly machine shown in  FIG. 12 . 
         FIG. 14  is a perspective schematic view of a detailed section of the inlet unit shown in  FIG. 13 . 
         FIG. 15  is a perspective schematic view of a jaw pair forming part of the spring unit assembly machine of  FIG. 12 , the jaw pair is shown in an open position with a helical binding wire held in an upper jaw of the jaw pair. 
         FIG. 16  is a perspective schematic view of the jaw pair of  FIG. 15  in a closed position with a helical binding wire held between the upper and lower jaws of the jaw pair. 
         FIG. 17  is a perspective schematic view of the lower jaw and main body of the jaw pair of  FIGS. 15 and 16 . 
         FIG. 18  is a perspective schematic view of the lower jaw of the jaw pair of  FIGS. 15 and 16  shown with the main body removed. 
         FIG. 19  is a perspective schematic view of a pair of servomotors which are used to drive a pair of drive shafts operably connected to upper and lower pairs of jaws. 
         FIG. 20  is a perspective schematic view of a motor used to drive a shaft which is used to raise and lower the upper jaw of each jaw pair for servicing and maintenance. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring now to  FIGS. 1 to 4 , for the sake of simplicity only one spring coiling machine is shown in the figures. However, it is to be understood that two or more machines may be arranged in parallel. In such an arrangement all the coiling machines are identical and driven by a common drive mechanism such that they operate synchronously. 
     Each coiling machine  3  comprises an inlet wire feeder (hidden) that takes wire  1  continuously from the reel  2  and advances it in a direction along the longitudinal axis of the wire to a coiling head  30  that forms the wire into the helical coils  5 ,  6 . The radius of the coils  5 ,  6  and their pitch (i.e. the axial distance between identical points on adjacent loops of a coil) is governed by the operation of the coiling head  30 . 
     The head  30  comprises a main body  31  of generally rectangular outline that is fixed on a vertical rotary shaft  32  and supports a forming roller  33  that is disposed in the path of the incoming wire  1  (not shown in  FIGS. 2 to 4 ). The roller has a peripheral groove  34  in which the wire is received and serves to deflect the wire, as it egresses from the main body  31 , into an arcuate form. The main body has a cut out recess  35  that pivotally supports a pair of parallel spaced guide plates  36  between which the arcuate wire passes. The recess  35  is sized in a vertical direction so as to prevent the plates  36  from moving vertically relative to the main body  31 . The axial dimension of the spring coils  5 ,  6  is imparted by pivoting movement of the guide plates  36  relative to the main body  35 . The angle that the guide plates  36  subtend to the plane occupied by the main body  35  determines the pitch of the coil  5 , 6  and therefore the height h of each spring coil. When the guide plates  36  are substantially aligned with the plane of the main body  35  this represents the datum position and the wire is not deflected in axial direction (of the coils). If the plates  36  are disposed at a negative angle to the datum position the wire is deformed into a left hand coil, whereas if they are at a positive angle the wire is deformed into a right hand coil. In operation the plates  36 , are driven to pivot according to a complex algorithm so as to define the pitch of the coil  5 , 6  at any one time. At the same time the position of the roller  33  relative to the wire  1  can be varied by a known mechanism so as to set the radius of the emerging coil of the wire at any point in time. For example, in between the left and right hand coils  5 ,  6  the straight length of wire  7  is produced by virtue of the roller  33  being spaced from the wire and therefore not imparting any deflecting force thereon. It will thus be appreciated that the shape of any given coil  5 ,  6  is determined by the relative movement of the guide plates  36  and the roller  33  with respect to the main body  31  of the coiling head  30 . 
     The various movements of the components of the coiling head  30  are controlled by linkages that are driven by rotary drive shafts  37   38 , which, in turn, are driven by computer-controlled servomotors (not shown). A control computer or processor (not shown) executes a software instruction set to govern the rotation of the output shafts of the servomotors and this is translated into the fine control of the movements of the drive shafts  37 ,  38  by reduction gearboxes (not shown). 
     A known drive mechanism operates to rotate the rotary vertical shaft  32  and the main body  31  through a limited angle of typically 180 degrees or less between first and second limit positions. This arrangement is known and is designed to prevent entanglement of the continuous string of coils as the coiler head  30  produces alternate left hand and right hand coils  5 ,  6 . 
     The rotation of a first drive shaft  37  common to both the coiling heads is used to control the position of the roller  33  so as to control the size of radius applied to the wire  1  in a known manner. 
     The pivoting movement of the guide plates  36  relative to the main body  31  of the coiling head  30  is governed by rotation of a second drive shaft  38  by a servomotor (via a reduction gearbox) operating in accordance with a software program executed on the control computer or processor. 
     The present invention is concerned with the linkage between the second drive shaft  38  and the guide plates  36  and, in particular, its adjustable nature. 
     Referring to  FIG. 2 , a collar  39  is fixed to one end of the second drive shaft  38  and has a radially extending crank arm  40  that supports a first end  41  of a connecting rod  42 . The other end  43  of the connecting rod  42  is fixed to a yoke  44  that is slidably mounted on the vertical shaft  32  on which the main body  31  of the coiling head  30  is supported. The connecting rod  42  is pivotally connected to the crank arm  40  by means of a captive screw  45 . The crank arm  40  has an elongate slot  46  defined along its length and the first end  41  of the connecting rod  42  has an eyelet  47  whose centre is aligned with the slot  46  so that the captive screw  45  passes through both. The arrangement is such that the eyelet  47  is free to rotate on the shank of the captive screw  45 . An adjustment screw  48  is disposed in a threaded bore extending from the free end of the crank arm  40  and projects into the slot  46  so as to contact the shank of the captive screw  45 , the longitudinal axis of the adjustment screw  48  extending substantially perpendicularly to the corresponding axis of the captive screw  45 . The arms  49  of the yoke  44  embrace a sleeve  50  that is slidably supported on the vertical shaft  32  such that it can move up and down the shaft with the yoke  44 . The sleeve  50  has a radially extending arm  51  on which a cylindrical socket  52  is supported such that its longitudinal axis extends substantially parallel to the axis of the rotary vertical shaft  32 . The socket  32  has a main wall with an internally threaded boss  53  that extends in a direction substantially perpendicular to the longitudinal axis of the socket and supports a threaded bolt  54 . A cylindrical barrel cam  55  with a spiral cam groove  56  defined in its outer surface is received in the socket  32  with the bolt  54 , which serves as the cam follower, extending into the spiral cam groove  56 . The barrel cam  55  has an extension  57  that extends into the main body  31  of the coiling head  30  and its end distal to the socket  32  is connected to the bottom of the guide plates  36 . The cam extension  57  is rotatably disposed in the main body  31  and, in use, effects rotation of the guide plates  36  in response to rotational movement of the drive shaft  38  as will now be explained. 
     The reduction gearbox ensures that the extent of angular rotation of the drive shaft  38  is limited to less than around 90 degrees. The rotational movement of the drive shaft  38  is converted into translational vertical movement of the yoke  44  and sleeve  50  by virtue of the crank arm  40  and connecting rod  42 . The crank arm  40  rotates with the drive shaft and  38  carries with it the pivoting end  41  of the connecting rod  42 . The position of the end  41  of the connecting rod  32  along the length of the slot  46  defines the effective radius of the crank arm  40  that governs the length of travel of the yoke  44 . This translational movement is passed to the socket  52  and cam follower bolt  54  and is converted into rotation of the guide plates  36  by virtue of the engagement of the bolt  54  with the walls of the spiral groove  56  defined in the surface of the barrel cam  55  and the fact that the guide plates  36  and cam  56  are prevented from vertical movement relative to the main body  31  of the coiling head  30 . 
     Adjustment to the coil pitch is achieved by loosening the captive screw  45  and turning the adjustment screw  48 . If the screw  48  is turned counterclockwise it pushes the captive screw  45  to the left (as shown in  FIG. 4 ) so as move the connection point and shorten the effective length of the crank arm  40 . This reduces the radius which the connecting rod  42  is orbits the drive shaft  38  and thus shortens the extent of its vertical travel and therefore the distance through which the yoke  44 , sleeve  50  and socket  32  travel. The effect of this is that the relative movement of the cam follower  54  in the spiral cam groove  56  is restricted so as to limit the amount of rotation of the barrel cam  55  and the guide plates  56 . If the adjustment screw  48  is turned in the opposite direction the crank arm  40  of the connecting rod  42  is increased so as to increase the angle of sweep of the guide plates  36  and thus increase the pitch of the coils. This adjustment feature provides for a quick and easy means for changing screw pitch rather than having to make changes to data used by the software. 
     Referring now to  FIG. 5 , the coil linking table  8  comprises a supporting surface  101  and a pair of upwardly extending side walls  102  which together with the surface  101  define a linking channel  103  along which the wire coil  4  is fed during a linking operation in the direction of arrow A. The continuous wire coil  4  has been processed using the coiling machine  3  (shown in  FIGS. 1 to 4 ) to provide the coil  4  with alternately left and right handed coiled sections  5 ,  6 , each coiled section defining a respective central longitudinal coil axis  104 ,  105  along which each coil is designed to be compressed in normal use. The coiling machine  3  is located an adequate distance upstream of the linking table  8  to ensure the wire coil  4  has relaxed to a sufficient degree to enable the linking operation to be carried out. The coils  5 ,  6  are interposed by longer straight (i.e. uncoiled) sections of wire  7 . Each coiled section  5 ,  6  is connected to adjacent longer straight sections  7  by two shorter straight sections of wire  106 ,  107 , one of which is provided at each end of the coiled section  5 ,  6 . The shorter straight sections of wire  106 ,  107  are orientated at approximately 90° to the neighbouring longer straight sections of wire  7  to which they are connected. 
     The linking apparatus further comprises a pair of compression fingers  108 ,  109  which are pneumatically actuated so as to be linearly moveable along a transverse axis  110  with respect to the longitudinal axis  111  of the linking channel  103 . A pair of slots  112 ,  113  extending along transverse axis  110  are defined in the supporting table  101  and connect with a pair of upwardly extending slots  114 ,  115  defined in the side walls  102 . The slots in the table  112 ,  113  and side walls  114 ,  115  are provided to facilitate movement of the compression fingers  108 ,  109  along transverse axis  110  between a rest position outside of the linking channel  103  (as shown in  FIG. 5 ) and an innermost clamping position within the linking channel  103  (as described below with reference to  FIGS. 6 and 7 ). Each compression finger  108 ,  109  is provided with an upwardly sloping leading edge  116 ,  117  so that as each finger  108 ,  109  moves inwardly along transverse axis  110 , the edge  116 ,  117  securely engages and inwardly compresses the longer straight section of wire  7  interposed between adjacent coils  5 ,  6 . 
     A further feature of the linking table  8  is the provision of a longitudinally extending guide slot  118 ,  119  defined by each side wall  102 . A pneumatically actuated indexing hook  120 ,  121  is slidably received in each guide  118 ,  119  and comprises an arcuate leading surface  122 ,  123  and a ramped trailing surface  124 ,  125  (only one of the two hooks  120 ,  121  can be seen in  FIG. 5 ). Each arcuate leading surface  122 ,  123  is of slightly smaller height than the length of each shorter section of wire  106 ,  107  such that, when the wire coil  4  is properly arranged within the linking channel  103 , downstream movement of each hook  120 ,  121  along its guide  118 ,  119  securely engages the next available shorter straight section of wire  106 ,  107  and advances the coil  4  in a downstream direction. Each hook  120 ,  121  is provided with a ramped trailing surface  124 ,  125  so that when each hook  120 ,  121  moves in an upstream direction the next upstream shorter straight section of wire  106 ,  107  passes up and over the ramped surface  124 ,  125  of each hook  120 ,  121  without being appreciably compressed or moved upstream. 
     Another feature of the linking table  8  is a pair of pneumatically actuated retaining pins  126 ,  127  which are alternately moveable in an upright direction into and out of the linking channel  103  via an aperture  128  defined by the linking table  8 . Each pin  126 ,  127  is of greater height when fully extended upwards than the height of the coils  5 ,  6  when lying on the table surface  101 . The purpose of the pins  126 ,  127  is to ensure that the sections of the wire coil  4  to be linked (as described below) are retained in the correct position to be engaged and compressed by the fingers  108 ,  109 . 
     The linking table  8  further comprises a pneumatically actuated ratchet indexer  129  shown in  FIG. 6  together with a section of linked wire coil  4 . The ratchet indexer  129  is received in a longitudinally extending guide channel  130  (described in more detail in relation to  FIG. 11 ) so as to be slidably moveable along the longitudinal axis  111  of the linking channel  103 . The indexer is located downstream of the retaining pins  126 ,  127  shown in  FIG. 5  and is provided to engage and index the wire coil  4  in a downstream direction along the linking channel  103 . 
     The indexer  129  comprises a support  131  which defines a transverse aperture  132  for receipt of a pivot pin  133  upon which is rotatably mounted a pair of indexing fingers  134 ,  135 . The fingers  134 ,  135  are mounted on the pin  133  such that they can only pivot between a retracted position in which the distal ends  136 ,  137  of the fingers  134 ,  135  are positioned adjacent to the support  131  (not shown in  FIG. 6 ) and an extended position in which the distal ends  136 ,  137  of the fingers  134 ,  135  are furthest from the support  131  and the fingers  134 ,  135  extend downwardly (as shown in  FIG. 6 ). In this way, when the indexer  129  is moved in an upstream direction and the fingers  134 ,  135  engage a section of the wire coil  4 , the fingers  134 ,  135  pivot upwardly towards the support  131  and pass over that section of the wire coil  4 . After passing over that section of the wire coil  4  the fingers  134 ,  135  then pivot downwardly to the extended position shown in  FIG. 6 . Subsequent downstream movement of indexer  129  then causes the fingers  134 ,  135  to engage a section of the wire coil  4  and, by virtue of the fingers  134 ,  135  being unable to rotate passed the downward direction shown in  FIG. 6 , the fingers  134 ,  135  advance the wire coil  4  in a downstream direction along the linking channel  103 . 
     A funnel (not shown) is provided at the upstream end of the linking table  8  to direct the moving wire coil  4  into the linking channel  103  in the correct orientation for linking. Furthermore, a set of electrodes (not shown) is attached to the upright side walls  102  at the downstream end of the linking table  8  to heat treat the linked wire coil  4  as it exits the linking table  8 . Heat treatment of coiled wire is known to enhance the resilience of the coils to compression. Two pairs of electrodes are provided with a pair of anodes on one side wall  102  and a pair of cathodes on the opposite side wall  102 . Each electrode is provided with a conducting metal projection which is directed into the linking channel  103  so as to be contactable by coils as they pass the electrode. The electrodes are appropriately arranged to ensure that passage of a coil completes an electric circuit between an anode and a cathode which thereby heats the coil forming part of the circuit. 
     The overall aim of the linking operation is to interlink each coiled section of wire  5 ,  6  to the adjacent upstream and downstream coiled sections  5 ,  6  in such a way that the intervening longer straight sections of wire  7  are essentially parallel to one another, which correctly orientates the various coiled and uncoiled sections of wire  6  for binding to other separate strings of coiled wire in the final step of the spring unit assembly process. References to components of the linking table  8  and portions of the wire coil  4  as being on the left hand side or the right hand side are to be considered as if the table  8  is being viewed from its downstream end. 
     In the following example, a right hand portion  6   a  (shown shaded) of a right handed coil  6  is interlinked with a right hand portion  5   a  (shown shaded) of downstream left handed coil  5 . To complete the linking operation, a left hand portion  6   b  (shown shaded) of the right handed coil  6  would then be interlinked to a left hand portion  5 ′ b  of an upstream left handed coil  5 ′ by repeating the process described below but in the opposite fashion, i.e. by operating the opposite member of each pair of components (e.g. compression fingers  108 ,  110 , retaining pins  126 ,  127 , etc). 
     After the wire coil  4  exits the coiling machine  3  it is passed to the surface  101  of the linking table  8  whereupon it enters the linking channel  103 . The wire coil  4  is then advanced in a downstream direction along the linking channel  103 . In  FIG. 5 , a left hand section  5   b  of the wire  5  has already been linked to a left hand section of the next upstream coil  6 ′ and the section  5   a  is about to be linked. The linking operation will be described beginning at this point. 
     In  FIG. 5  both compression fingers  108 ,  109  are at the rest position clear of the linking channel  103  to enable the coil portion  5   a  to be advanced downstream into the correct starting position as shown. The left hand retaining pin  127  is currently extended and the right hand retaining pin  126  is retracted. The next step, shown in  FIG. 7 , is to actuate the right hand compression member  108  to slide inwardly through the slots  112  and  114  such that its sloping leading edge  116  engages a longer straight section  7   a  of wire interposed between coil portion  5   a  and a right hand portion  6 ′ a  of a downstream right handed coil  6 ′. Inward movement of the compression finger  108  towards its innermost clamping position compresses the straight section  7   a  inwardly away from the side wall  102  which in turn draws the coil portion  5   a  inwards and slightly downwards towards the linking table surface  101 . In an alternative embodiment not shown in the accompanying figures, both compression fingers  108 ,  109  can be actuated to slide inwards simultaneously to engage and compress longer straight sections  7  of the wire  4  located to both the right and left hand sides of the wire  4  at the same time. Regardless of whether the compression fingers  108 ,  109  are actuated sequentially or simultaneously, the remaining steps in the interlinking operation are the same. 
     As shown in  FIG. 8 , the compression finger  108  is actuated to slide a sufficient distance inwards so that when at its innermost clamping position, a clearance c is defined between a rear end  138  of the compression member  108  and the side wall  102 . The hook  120  is then actuated to slide along the guide  118  in a downstream direction such that its arcuate leading surface  122  engages the shorter straight section of the wire  106   a  which is connected to the coil portion  6   a . The clearance c defined between the rear end  138  of the compression finger  108  and the side wall  102  is sufficiently large to enable the hook  120  carrying the straight wire section  106   a  to pass through the clearance c such that coil portion  6   a  is extended and finally located downstream of coil portion  5   a  (not shown). 
     With reference to  FIG. 9 , the compression finger  108  is then actuated to slide outwards and return to its rest position. In doing so, the straight section  7   a  extends outwardly towards the side wall  102  and the coil portion  5   a  extends outwards across the clearance c and upwards back to its initial position as in  FIG. 5 . The right hand hook  120  is then actuated to slide upstream along the guide  118  thereby gradually releasing the coil portion  6   a  and allowing it to contract and move back upstream until it engages the coil portion  5   a  whereupon the coil portions  5   a  and  6   a  become interlinked with the coil portion  6   a  lying to the downstream side of the coil portion  5   a . Continued upstream movement of the hook  120  returns it to its initial starting position as shown in  FIG. 8 . 
     In  FIG. 10 , the left hand retaining pin  127  retracts downwardly out of the linking channel  103  and the right hand retaining pin  126  extends upwardly into the linking channel  103 . The ratchet indexer  129  (shown in  FIG. 6 ) is then actuated to slide downstream along the guide channel  130  such that the downwardly extending indexing fingers  134 ,  135  engage the wire coil  4  and advance it a predetermined distance downstream so as to correctly position the left hand portion  6   b  of the right handed coil  6  for interlinking with the left hand portion  5 ′ b  of the next upstream left handed coil  5 ′. As mentioned above, to complete a linking operation, the above process should then be repeated but by operating the opposite member of each pair of components, e.g. the process will begin by actuation of left hand compression finger  109  and left hand hook  121 . 
       FIG. 11  illustrates the assembly  1  as shown schematically in  FIG. 5  together with the indexer  129  as shown in  FIG. 6 . As can be seen from  FIG. 7 , the indexer  129  is slidably received in the guide channel  130  which is defined in a lid  139  which is hingedly connected to the side wall  102 .  FIG. 11  also illustrates the interlinking of adjacent coils  5 ,  6 . As can clearly be seen, coil  140  has been linked to adjacent upstream and downstream coils  141 ,  142 . A right hand portion  143  of coil  140  overlaps a right hand portion  144  of downstream coil  142  and a left hand portion  145  of upstream coil  141  overlaps a left hand portion  146  of coil  140 , with all adjacent longer straight sections of wire  147 ,  148 ,  149 ,  150  lying approximately parallel to one another. 
     It will be understood that numerous modifications can be made to the embodiment of the invention described above without departing from the underlying inventive concept and that these modifications are intended to be included within the scope of the invention. For example, the compression fingers can be operated alternately as described above or can be operated together. Moreover, the dimensions and relative locations of the various components can be varied to suit a given coil size and number of helical repeats in each coil. It is envisaged that the hooks, retaining pins, compression fingers and indexing fingers may be of any suitable size and shape provided each can still perform its designated function as described above. The above example employs pneumatically actuated linearly moving components which are cheap and reliable, although, any convenient actuating means can be used for any of the various components. The provision of the hinged lid carrying the indexer is optional but may be preferable in view of ensuring the safety of workers operating the machine. The heat treatment step may be carried out using any appropriate number and arrangement of electrode or, alternatively, may be carried out in an oven as in conventional processes of this kind. 
     The spring coil assembly machine  13  is shown in detail in  FIGS. 12 to 20  and receives the strings of coils  10  from storage reels  11  ( FIG. 1 ). The machine has two floor-standing side frames  200  each with a pair of feet  201  that are fixed to the floor. The frames  200  carry an inlet unit  202  in the form of a plurality of guide channels  203  defined between spaced parallel upright plates  204 , a coil string  10  being received in each channel  203 . This inlet unit  202  is shown in more detail in  FIGS. 13 and 14 . The coil strings are drawn through the inlet by an indexing device (not shown) that indexes the strings by one coil width at a time to a binding station  205 . The indexing device is of conventional construction and will not be described in detail here. The binding station  205  comprises upper and lower sets of transversely extending jaw pairs  15  that serve to clamp the coil strings  10  with their longitudinal axes substantially upright whilst the adjacent strings  10  are bound together. The jaws are described in more detail below with reference to  FIGS. 15 to 18 . 
     The upright guide plates  204  of inlet unit  202  are slidably supported on three parallel rods  206  that extend between the side frames  200  and through apertures in the plates  203 . The position of the plates  204  on the rods  206  is slidably adjustable so that the number and size of channels  203  can be varied according to the application and size of the spring unit being produced. When the size and number of channels  203  is finalised the position of each plate  204  is fixed relative to the rods  206  by locking collars  207  disposed on each side of the plate  204  around the apertures. The collars  207  are locked in place on the rods  206  by worm screws or the like. At the base of each channel  203  the strings of coils  10  are supported for forward movement on cylindrical rollers  208 . Three such spaced rollers  208  are shown in  FIG. 13 , each extending in parallel to the support rods  206  and between the side frames  200 . The outermost of the plates  204  are bent out of their parallel planes towards the side frames  200  so as to define channels  203  that flare outwardly with increasing amounts towards the side frames  200 . This allows the strings of coils  10  to be received from storage reels  11  that are laterally spaced by a distance greater than that of the inlet unit  202 . It will be appreciated that the inlet unit design is fully adjustable to accommodate the manufacture of different sizes of spring units. 
     The upper and lower sets of jaw pairs  15  are arranged in two lines along the width of the assembly machine  13  and each pair combine, when closed, to form a continuous helical channel into which a helical binding wire  16  is advanced. The jaws  15  are disposed such that their mouths face away from the inlet unit  202 . Each jaw pair  15  comprises an upper fixed jaw  15   a  and a lower pivotal jaw  15   b , both of which are supported by a jaw body  209  that is mounted on a transverse drive shaft spanning the width of the assembly machine  13 . Upper and lower drive shafts  210   a  and  210   b  of hexagonal cross section are used for the upper and lower jaw sets  15  and are best seen in  FIGS. 19 and 20  (in which the inlet unit guide plates  204  have been removed for clarity) where only one pair of jaws  15  ( FIG. 20 ) from the lower jaw set is shown in-situ on the shaft  210   b  for clarity. As can be seen from  FIGS. 15 to 17  the main body  209  has two depending side walls  211  that are spaced apart and flank a linkage  212  that operates the movable lower jaw  15   b  and an upper wall  213  to which the upper jaw  15   a  is fixed. The jaws  15  are shown in the open position in  FIG. 15  and in the closed position in  FIG. 16 . The binding wire  16  is formed by passing uncoiled wire  17  from a reel  18  to a coiling passage  19  located to the side of the jaws  15  of the assembly machine  13  in a known arrangement and as shown schematically in  FIG. 1 . It is rotated and axially advanced in the transverse direction of arrow B ( FIG. 1 ) through the jaws  15  such that it passes around the wire of the adjacent strings  10  in order to bind the coil strings  10  together. The jaw sets  15  are then opened and the joined strings of coils  10  indexed forward so as to locate the next coil of each string  10  within the jaws  15  whereupon the above cycle is repeated to bind the next row of coils together. The binding cycle is repeated a sufficient number of times to bind a suitable number of rows of coils together to produce a spring unit of the desired size. 
     The mechanism of the lower jaw  15   b  is shown in detail in  FIGS. 17 and 18  with the main body  209  of the jaws  15  removed for clarity in  FIG. 18 . The lower jaw  15   b  is connected to the main body  209  by the linkage  212  that enables it to pivot between the open and closed positions. The linkage  212  comprises a cam follower arm  214  that is pivotally connected to the rear of each side wall  211  of the main body  209  by a shaft  215  and rests immediately below the peripheral surface of an eccentric disc cam  216 . The end of the cam follower arm  214  is connected by a link member  217  to one end of a pivoting arm  218 , the other end of which supports the lower jaw  15   b . The pivoting arm  218  pivots on a shaft  219  that is received between the side walls  211  at the front end of the main body  209 . The eccentric disc cam  216  is received between the side walls  211  between the front and rear ends of the main body  209  and is mounted on the hexagonal drive shaft  210   a ,  210   b  by means of a bore  220  of the same shape cross-section. The jaw  15  is shown in  FIGS. 17 and 18  in between the fully open position and the closed positions. As the drive shaft  210   a,b  rotates in the clockwise direction in the view of  FIG. 18  the cam  216  is similarly rotated clockwise and the lever arm  214  pivots downwardly about the rear shaft  215 . This serves to pull the rear end of the pivot arm  218  downwardly so that other end and therefore the jaw  15   b  moves in a upwards direction towards the upper fixed jaw  15   a  to the closed position as shown in  FIG. 16 . 
     It will thus be appreciated that all of the jaws  15  of a given jaw set can be opened and closed simultaneously by simple rotation of a drive shaft to drive the eccentric disc cams and linkages associated with each of the lower jaws. It is to be understood that the mechanism could be easily adapted to pivot the upper jaw with respect to the lower jaw. The linkage enables a relatively small movement provided by the cam to the lever arm to be translated into a larger movement of the jaw. 
     The drive shafts  210   a ,  210   b  for the upper and lower sets of jaws  15  are each driven by a servomotor  230 ,  231  that is mounted on one of the side frames  200 . Each servomotor  230 ,  231  is connected to the shaft  210   a ,  210   b  via a gear box  232  fitted with a torque limiter. This arrangement provides a safety feature in the event that one of the jaws  15  is jammed. It ensures that if the torque applied to the drive shafts  210   a ,  210   b  should exceed a predetermined value the drive is disconnected. 
     A further motor  240  is disposed below the binding station  205  and drives a shaft  241  that rotates an adjustable eccentric cam  242  which carries a frame  243  that supports the main body  209  of the jaws  15 . This arrangement enables the fixed upper jaws  15   a  to be moved if necessary for maintenance or servicing purposes. 
     While the inventions have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.