BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stringer of a racket, and more particularly to a racket stringer having a frame fixing device with a quick positioning microadjustment function. 
     2. Description of the Related Art 
     A conventional racket stringer in accordance with the prior art shown in FIGS. 1 and 2 comprises a work table  1 , and two opposite fixing devices  3  each mounted on the work table  1 . Each of the fixing devices  30  includes a guide plate  2  mounted on the work table  1  and formed with an elongated guide slot  202 , a base  4  slidably mounted on the guide plate  2  and has a bottom  402  provided with two pins  404  slidably mounted in the guide slot  202 , a locking bolt  5  having a first end formed with a rotation head  502  and a second end extended through a washer  504  and the guide slot  202  and screwed into the bottom  402  of the base  4 , two clamping arms  7  mounted on the base  4 , an angle mechanism  9  mounted on the base  4  and provided with a rotation knob  902  to control the included angle between the two clamping arms  7 , and a control mechanism  8  mounted on the base  4  and provided with a control knob  802  to move a resting plate  804  forward and backward. 
     In operation, the locking bolt  5  is unscrewed from the bottom  402  of the base  4 , so that the base  4  can be moved freely, and the two fixing devices  3  can be moved outward relative to each other to reach the position of the frame of the racket. Then, the locking bolt  5  is screwed on the bottom  402  of the base  4 , so that the base  4  is fixed. Then, the control knob  802  is rotated to move a resting plate  804  to rest on the inner periphery of the frame of the racket, and the rotation knob  902  is rotated to move the two clamping arms  7  to rest on the outer periphery of the frame of the racket, so that the frame of the racket is clamped between the two opposite fixing devices  30  for stretching the strings of the racket. 
     However, each of the two opposite fixing devices  3  cannot be moved easily and conveniently, thereby greatly causing inconvenience to the user. In addition, the user needs to successively screw and unscrew the locking bolt  5  to position and release the fixing device  3 , thereby causing inconvenience to the user in use. Further, the fixing device  3  does not have a microadjustment function, so that the position of the fixing device  3  cannot be microadjusted. 
     SUMMARY OF THE INVENTION 
     The present invention is to mitigate and/or obviate the disadvantage of the conventional racket stringer. 
     The primary objective of the present invention is to provide a racket stringer having a frame fixing device that has a quick positioning microadjustment function. 
     Another objective of the present invention is to provide a racket stringer, wherein the frame fixing device can be displaced easily and rapidly, thereby facilitating the user operating the racket stringer. 
     A further objective of the present invention is to provide a racket stringer, wherein the frame fixing device can be positioned easily and rapidly, thereby facilitating the user operating the racket stringer. 
     In accordance with the present invention, there is provided a racket stringer, comprising a work table, and two opposite fixing devices each mounted on the work table and each including a rack, a guide plate, two foot posts, a support shaft, and a press rod, wherein: 
     the rack is mounted on the top of the work table and has a top face formed with a plurality of oneway ratchet teeth; 
     the guide plate is mounted on the top of the work table and is formed with two elongated guide-slots each parallel with the rack; 
     each of the two foot posts is slidably mounted on the guide plate and has a bottom provided with a slide slidably mounted in a respective one of the two guide slots of the guide plate, thereby guiding the fixing device to displace on the work table reciprocally; 
     the support shaft is rotatably mounted between the two foot posts; and 
     the press rod is extended through the support shaft and has a first end rested on the ratchet teeth of the rack and a second end formed with a rotation head for rotating the press rod. 
     Further benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a conventional racket stringer in accordance with the prior art; 
     FIG. 2 is a side plan cross-sectional assembly view of the racket stringer as shown in FIG. 1; 
     FIG. 3 is a perspective view of a racket stringer in accordance with the preferred embodiment of the present invention; 
     FIG. 4 is an exploded perspective view of the racket stringer in accordance with the preferred embodiment of the present invention; 
     FIG. 5 is a side plan cross-sectional view of the racket stringer as shown in FIG. 3; 
     FIG. 6 is a schematic operational view of the racket stringer as shown in FIG. 5; and 
     FIG. 7 is a schematic operational view of the racket stringer as shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings and initially to FIGS. 3-5, a racket stringer in accordance with the preferred embodiment of the present invention comprises a work table  90 , and two opposite fixing devices each mounted on the work table  90 . 
     Each of the fixing devices includes a rack  10 , a guide plate  20 , two foot posts  80 , a support shaft  30 , and a press rod  40 . 
     The rack  10  is mounted on the top of the work table  90  and has a top face formed with a plurality of oneway ratchet teeth  12 . 
     The guide plate  20  is mounted on the top of the work table  90  and is formed with two elongated guide slots  22  each parallel with the rack  10 . Preferably, the guide plate  20  is formed with a receiving slot  21  to receive the rack  10 . 
     Each of the two foot posts  80  is slidably mounted on the guide plate and has a bottom provided with a slide  82  slidably mounted in a respective one of the two guide slots  22  of the guide plate  20 , thereby guiding the fixing device to displace on the work table  90  reciprocally. 
     The support shaft  30  is rotatably mounted between the two foot posts  80 . 
     The press rod  40  is extended through the support shaft  30  and has a first end rested on the ratchet teeth  12  of the rack  10  and a second end formed with a rotation head  42  for rotating the press rod  40 . 
     The work table  90  is formed with two elongated guide slots  92  each aligning with a respective one of the two guide slots  22  of the guide plate  20  for guiding the slide  82  of a respective one of the two foot posts  80 . The work table  90  is formed with a receiving slot  91  to receive the rack  10 . 
     Each of the fixing devices further includes a support plate  50  mounted on the bottom of the work table  90  for supporting and securing the rack  10 , and a plurality of locking screws  92  each extended through the support plate  50  and the work table  90  and each locked in the guide plate  20 , so that the support plate  50 , the work table  90  and the guide plate  20  are combined with each other. The support plate  50  is formed with two elongated guide slots  52  each aligning with a respective one of the two guide slots  92  of the work table  90  for guiding the slide  82  of a respective one of the two foot posts  80 . 
     Each of the fixing devices further includes a connecting seat  84  mounted on the bottom of the support plate  50  and secured on the slides  82  of the two foot posts  80 , and a plurality of locking bolts  86  each extended through the connecting seat  84  and the slides  82  and each locked in a respective one of the two foot posts  80 , so that the two foot posts  80  are limited to move on the guide plate  20  by the connecting seat  84  without detachment. 
     The support shaft  30  is formed with a screw bore  32 , and the press rod  40  is formed with an outer thread  41  screwed into the screw bore  32  of the support shaft  30 . Each of the fixing devices further includes a torsion spring  34  mounted between the support shaft  30  and one of the two foot posts  80  and having a first end secured on the support shaft  30  and a second end secured on the respective foot post  80 . Preferably, the support shaft  30  has an end formed with a receiving recess  33  to receive the torsion spring  34 . 
     Each of the fixing devices further includes a support base  60  mounted between the two foot posts  80  and having a top provided with a press plate  62 , and two positioning posts  70  mounted on the support base  60 . Thus, the periphery of the annular frame (not shown) of the racket (not shown) is mounted on the two positioning posts  70  and is pressed by the press plate  62 , thereby positioning the racket for stretching the strings (not shown) of the racket. 
     In operation, referring to FIGS. 5-7 with reference to FIGS. 3 and 4, the two opposite fixing devices are pushed outward relative to each other to increase the distance between the two opposite fixing devices for mounting the frame of the racket. 
     At this time, the two foot posts  80  are pushed to move outward on the guide plate  20 , and the first end of the press rod  40  slips on the ratchet teeth  12  of the rack  10 . In such a manner, as shown in FIG. 5, the two foot posts  80  can be moved outward freely and cannot be moved inward by provision of the oneway ratchet teeth  12  of the rack  10 . Thus, the two foot posts  80  can be moved outward through a larger distance rapidly, thereby largely adjusting the distance between the two opposite fixing devices easily and conveniently. 
     After the two opposite fixing devices are moved outward to reach the position of the frame of the racket, the rotation head  42  of the press rod  40  is rotated as shown in FIG. 6 to slightly change the position of the first end of the press rod  40  relative to the ratchet teeth  12  of the rack  10 , so that the two foot posts  80  can be moved on the guide plate  20  reciprocally to microadjust the distance between the two opposite fixing devices to fit the exact position of the frame of the racket, so that the frame of the racket is clamped between the two opposite fixing devices exactly for stretching the strings of the racket. 
     After the work of stretching the strings of the racket is finished, the rotation head  42  of the press rod  40  is pressed downward as shown in FIG. 7, so that the press rod  40  is pivoted about the support shaft  30  to detach the first end of the press rod  40  from the ratchet teeth  12  of the rack  10 . In such a manner, the two foot posts  80  can be moved inward freely so as to fold the two opposite fixing devices. After the force applied on the rotation head  42  of the press rod  40  is removed, the support shaft  30  can be returned to the original state by the restoring force of the torsion spring  34 , so that the press rod  40  is pivoted about the support shaft  30  reversely to lock the first end of the press rod  40  on the ratchet teeth  12  of the rack  10 . 
     Accordingly, the two foot posts  80  can be directly pushed to move outward through a larger distance to largely adjust the distance between the two opposite fixing devices easily and conveniently without having to release the locking state of each of the two opposite fixing devices. After the two opposite fixing devices are moved to reach the position of the frame of the racket, the rotation head  42  of the press rod  40  is rotated to microadjust the distance between the two opposite fixing devices to fit the exact position of the frame of the racket, so that the frame of the racket is clamped between the two opposite fixing devices exactly. After the work of stretching the strings of the racket is finished, the rotation head  42  of the press rod  40  is pressed downward to detach the first end of the press rod  40  from the ratchet teeth  12  of the rack  10 , thereby releasing the locking state of each of the two fixing devices, so that the two foot posts  80  can be moved inward freely and rapidly so as to fold the two opposite fixing devices. 
     Although the invention has been explained in relation to its preferred embodiment(s) as mentioned above, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the present invention. It is, therefore, contemplated that the appended claim or claims will cover such modifications and variations that fall within the true scope of the invention.
A racket stringer includes a work table, and two opposite fixing devices each mounted on the work table and each including a rack, a guide plate, two foot posts, a support shaft, and a press rod. Thus, the fixing device has a quick positioning microadjustment function. In addition, the fixing device can be displaced easily and rapidly, thereby facilitating the user operating the racket sag stringer.