You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/753,307 filed on Jan. 16, 2013, the entire contents of which are herein incorporated by reference. 
    
    
     BACKGROUND AND SUMMARY 
     A post hole digger attaches to the standard three point hitch of the tractor is powered by the tractor&#39;s power take-off (PTO). The digger comprises an auger without protrusions or other extending parts above the fighting of the auger, to reduce the possibility of a user becoming ensnared during use of the digger. 
     A gearbox translates rotation from the PTO shaft to the auger. In a traditional post hole digger, the auger&#39;s shaft attaches to the gearbox via a cross bolt that extends perpendicularly through the shaft. The cross bolt has the disadvantage of protruding from the shaft, and causing potential harm to a user. The digger of the present disclosure removes this disadvantage by providing a threaded fitting between the shaft and the gearbox. However, a threaded fitting on the rotating shaft provides an additional challenge When the auger needs to be removed from the gearbox. 
     The gearbox lock mechanism of the present disclosure comprises a collar coupled to a lower end of the gearbox, the collar rotatable upon operation of the gearbox. The collar comprises a semi-circular outer edge and a flat side. A male-threaded nipple extends from the collar and threads into the auger shaft. A lock bar is coupled to the gearbox and acts as a positive lock to lock the collar in place for removal of the shaft from the gearbox. The lock bar is rotatable from a locked position whereby the lock bar is aligned with and contactable with the flat side of the outer edge of the collar, to an unlocked position whereby the lock bar does not contact the flat side of the outer edge of the collar. 
     For purposes of summarizing the invention, certain aspects, advantages, and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any one particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the disclosure. Furthermore, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a side plan view of a post hole digger coupled to a tractor. 
         FIG. 2  is a rear perspective view of the post hole digger of  FIG. 1 . 
         FIG. 3  is a bottom perspective view of the digger. 
         FIG. 4  is an enlarged detail view of the digger of  FIG. 3 , taken along detail line A of  FIG. 3 . 
         FIG. 5  is a rear plan view of the digger of  FIG. 1 . 
         FIG. 6  is an enlarged detail view of the digger of  FIG. 5 , taken along detail line B of  FIG. 5 . 
         FIG. 7  is a cross sectional view of the digger of  FIG. 6 , taken along section lines C-C of  FIG. 6 . 
         FIG. 8  is a partial enlarged bottom view of the digger of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a side plan view of a post hole digger  10 . The digger  10  is shown installed on a tractor  7  and is used to dig generally-cylindrical holes (not shown) in the ground  8 , for example, holes for fence posts. The digger  10  is disposed at the rear of the tractor  7  between the rear wheels  9   a  and  9   b  ( FIG. 2 ) of the tractor  7 . 
     The digger  10  comprises an auger  13  for drilling into the ground  8 . The digger  10  is disclosed in U.S. Non-Provisional Patent Application Serial No. 13/548,836, titled “Post Hole Digger,” published on Jan. 17, 2013, under Publication No. US-2013-0014997-A1, which is incorporated herein by reference in its entirety. 
     The auger  13  is supported by a top support arm  18  that extends from the tractor  7 . A rotating shaft  17  extends from a PTO shaft  20  ( FIG. 2 ) of the tractor  7  and translates rotation from the PTO shaft  20  to a gearbox  12 , and ultimately to the auger  13 . A shield  19  covers moving parts (not shown) of the gearbox  12  that could otherwise pose a safety hazard to users not shown) of the digger  10 . 
       FIG. 2  is a rear perspective view of the digger  10  installed on the tractor  7  between the rear wheels  9   a  and  9   b  of the tractor  7 . The digger  10  connects to the tractor&#39;s standard three point hitch that is known in the art. The term “three point hitch” refers to the three mounting points of a tractor hitch that extend rearwardly from the rear of the tractor  7 . 
     The top support arm  18  is rotatably affixed to the shield  19  that covers the gearbox  12 . A support frame  25  supports the top support arm  18 . The shield  19  is rigidly affixed to the gearbox  12 , and is not detachable from the digger in this embodiment without making the digger non-fictional, to provide safety for the user. 
     The rotating shaft  17  is releasably coupled to the PTO shaft  20  of the tractor  7 . As known by persons of skill in the art, a power-take off shaft is a splined shaft that is rotatable by the user (not shown) upon actuation of the tractor controls (not shown). Rotation of the PTO shaft  20  typically powers farming implements such as the digger  10 . The rotating shaft  17  extends from the PTO shaft  20  to the gearbox  12 , as further discussed herein. 
     The gearbox  12  is a right angle gearbox that receives rotation from the rotating shaft  17  and translates the received rotation to the auger  13 . In this embodiment, the auger  13  comprises a rotatable auger shaft  33 , a plurality of fighting blades  14  and a cutting head  15 . The cutting head  15  is disposed at the lowermost end of the shaft  33 , and comprises a pilot bit  16  and a pair of cutting blades  34 . The fighting blades  14  are disposed above the cutting head  15 . 
     The outer surface of the shaft  33  is generally smooth above the fighting blades  14 , and has no protrusions or other irregularities above the fighting blades  14  that may ensnare or entangle a user during use. This is an improvement over prior art augers which contain protrusions from the shaft that can endanger a user. 
       FIG. 3  is a bottom perspective view of the digger  10 . The gearbox  12  is disposed beneath the shield  19 , The auger  13  comprises a shaft  33  that extends from the gearbox  12 . Between the gearbox  12  and the flighting  14 , the shaft  33  is smooth, i.e., has no protrusions that could catch on a user or the user&#39;s clothing. 
       FIG. 4  is an enlarged detail view of the digger  10  of  FIG. 3 , taken along detail “A” of  FIG. 3 . A collar  63  extends beneath the gearbox and is rigidly affixed to a male-threaded nipple  64  that releasably affixes the shaft  33  to the gearbox  12 . In the illustrated embodiment the collar  63  is unitary with the nipple  64 . The collar has a semi-circular outer edge  65  that is primarily semi-circular and has a flat side  66 . The collar  63 , threads  64  and shaft  33  rotate when the digger  10  ( FIG. 1 ) is in operation. 
     A lock bar support  61  is coupled to the gearbox  12  between the collar  63  and the gearbox  12 . The lock bar support  61  does not rotate. A lock bar  60  is rotatably coupled to the lock bar support  61  via a fastener  62 . 
     When the digger  10  is in operation, the lock bar  60  is in an “unlocked” position such that the lock bar  60  extends downwardly. When the user desires to remove the auger  13  ( FIG. 3 ) from the gearbox  12 , the user manually moves the lock bar  60  to a “locked” position such that the lock bar  60  is rotated upwardly until it contacts the lock bar support  61 . In this orientation, the lock bar  60  is generally parallel to the flat side  66  of the collar  63 . When the lock bar  60  is in the locked position, the flat side  66  contacts the lock bar  60  and prevents the collar  63  from rotating. Thus the term “locked” refers to the collar  63  being locked such that it cannot rotate, and the term “unlocked” refers to the collar being rotatable. When the collar  63  is locked, the user can remove the auger  13  from the digger  10  by unscrewing the shaft  33  from the threaded nipple  64 . 
       FIG. 5  is a rear plan view of the digger  10  of  FIG. 1 . The lock bar support  61  is rigidly coupled to a bottom side  68  of the gearbox  12 . In one embodiment, the lock bar support  61  is affixed to the gearbox  12  via a plurality of fasteners (not shown). The lock bar support  61  is generally parallel to the collar  63 . The lock bar  60  extends downwardly from the lock bar support  61  when the lock bar  60  is in its unlocked position, as shown. In this unlocked position, the lock bar  60  is generally perpendicular to the lock bar support  61  and the collar  63 . 
       FIG. 6  is an enlarged detail view of the digger  10  of  FIG. 5 , taken along detail line “B” of  FIG. 5 . The lock bar  60  is shown in its unlocked position. From this unlocked position, the lock bar  60  is rotatable upwardly in the directly indicated by directional arrow  67 . The lock bar  60  is generally rectangular, with long opposed sides extending downwardly when it is in the unlocked position. 
     The lock bar support  61  is comprised of a generally fiat support plate  82  and a downwardly extending tab  80  that is generally perpendicular to the support plate  82 . The lock bar  60  is rotatably affixed to the tab  80  via the fastener  62 , which may be a bolt and nut. The support plate  82  and tab  80  are made of steel in one embodiment, though other suitably strong and rigid materials could be used. 
       FIG. 7  is a cross sectional view of the digger  10  of  FIG. 6 , taken along section lines C-C of  FIG. 6 , with the lock bar  60  shown in its unlocked position. In this position, the lock bar  60  cannot contact the collar  63 , thus the collar  63  is free to rotate. The support plate  82  of the lock bar support  61  is a curved plate with a generally flat cross section and is coupled to the gearbox  12  via a plurality of fasteners  74 . Note that the support plate  82  is coupled to the non-rotatable outer body of the gearbox  12 , in contrast with the collar  63 , which rotates upon operation of the gearbox  12 . The support plate  82  extends over halfway around the gearbox  12  when viewed from the bottom as shown. 
     The lock bar support  61  further comprises a block stop  81  that is rigidly affixed to the support plate  82  adjacent to the lock bar  60  when the lock bar  60  is in the locked position. The block stop  81  comprises a generally rectangular box, generally made of steel, that is substantially parallel to and spaced apart from the flat side  66  of the collar  63  when the collar  63  is locked. The block stop  81  being spaced apart from the flat side  66  creates a gap  75  between the block stop  81  and flat side  66 . The width of this gap  75 , i.e., the distance “D” between an inner surface  83  of the block stop  81 , is slightly larger than a width “W” of the lock bar  60 . This is desired because when the lock bar  60  is locked, it is disposed between the inner surface  83  of the block stop  81  and the flat side  66  of the collar  63 . 
     The outer edge  65  of the collar  63  comprises the flat side  66  and a semi-circular portion  68  that extends more than 270 degrees around the collar. In other words, the flat side  66  in effect “cuts off” the outer edge  65 , generally less than 90 degrees around the outer edge  65 . Corners  70  and  71  on the outer edge  65  provide a transition from the semi-circular portion  68  to the flat side  66  of the outer edge  65 . 
     Note that the distance “D” must be sufficient so that the semi-circular portion  68  of the outer edge  65  of the collar  63  clears the block stop  81  when the lock bar  60  is in the unlocked position. 
     When the lock bar  60  is locked, the lock bar  60  is generally parallel to the flat side  66  of the collar  63  and the inner surface  83  of the block stop  81 . If the collar  63  is urged to rotate in either direction indicated by directional arrow  73 , one of the corners  70  or  71  will contact an inner side (not shown) of the lock bar  60  and prevent the collar  63  from further rotation, thus providing a positive lock to prevent the collar from rotation. 
       FIG. 8  is a partial enlarged view of the digger  10  showing a bottom perspective view of the gearbox  12 . The block stop  81  is spaced apart from the flat side  66  of the collar  63  as discussed above, creating the gap  75 . The lock bar  60  is disposed within the gap  75 , and generally contacts the support plate  82  when the lock bar  60  is in the locked position.

Summary:
The gearbox lock mechanism for a post bole auger has a collar coupled to a lower end of a right angle gearbox, the collar rotatable upon operation of the gearbox. The collar comprises a semi-circular outer edge and a fiat side. A male-threaded nipple extends from the collar and threads onto the auger shaft. A lock bar is coupled to the gearbox and acts as a positive lock to lock the collar in place for removal of the shaft from the gearbox. The lock bar is rotatable from a locked position whereby the lock bar is aligned with and contactable with the flat side of the outer edge of the collar, to an unlocked position whereby the lock bar does not contact the flat side of the outer edge of the collar.