Abstract:
A toe kick saw including a driver having a safety feature to allow a saw blade to disengage during experience of increased torque. The toe kick saw driver may include a one or two piece driver that engages the saw blade that is able to yield by deforming when the blade experiences an increase in torque sufficient to cause kickback. Alternatively the toe kick saw could include a yieldable spindle extension.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional application No. 60/826,349, filed Sep. 20, 2006; U.S. provisional application No. 60/862,359, filed Oct. 20, 2006; and U.S. provisional application No. 60/942,172, filed Jun. 5, 2007. 
     
     TECHNICAL FIELD 
       [0002]    The present device relates generally to flooring installation tools and more specifically to an improved toe-kick saw having enhanced safety features. 
       BACKGROUND 
       [0003]    A toe-kick saw is a specialty circular saw used in residential floor remodeling. When a finished floor is to be replaced, this often means that the underlayment beneath the finished floor must also be replaced. The “finished floor” is the topmost, exposed layer of flooring selected for décor and utility in the room (typically vinyl, ceramic tile, carpet, hardwood or laminate plank). Beneath the finished floor is underlayment, which is an especially flat, finely finished material. The use of underlayment ensures the finished floor will be installed on a flat surface with no bumps which might poke through the finished floor or create irregularities. Beneath the underlayment is the rough subfloor (normally plywood) which is laid over the joists. 
         [0004]    When a finished floor is to be replaced, it is often necessary to replace the underlayment as well. When new vinyl, ceramic tile, or hardwood floors are installed, adhesive is used to adhere the finished floor to the underlayment. In such cases, the finished floor cannot be removed from the underlayment without damaging it. 
         [0005]    In many finished floor installations, especially in kitchens and bathrooms, cabinetry is encountered which may have toe-kicks. Toe-kicks are relieved areas at the bottom of the cabinet which allow a person to step closely to the cabinet without stubbing a toe. Often times the cabinetry is installed first before the finished floor is installed, and the cabinetry is installed on top of the underlayment. In the case of a hardwood finished floor, the cabinetry may even be installed on top of the finished floor as well. 
         [0006]    Whenever cabinets with toe-kicks are installed on top of the underlayment or finished floor, removing only old underlayment and/or finished floor under the toe-kick can be very difficult. Using conventional hand tools, such as a hammer and chisel, the floor installer would have to chisel out the floor along the entire length of the toe-kick. This chiseling is difficult because the chisel can only be pointed into the corner at a 45 degree angle, not straight downward as required to effectively chisel the material. There is a clear danger of the hammer or chisel striking and damaging the cabinet face. Conventional power saws will not fit underneath the toe space. A specialized power saw is needed which can extend underneath and cut flush up against the inner wall of a toe space. 
         [0007]    Toe-kick saws are available for this purpose. As shown in  FIG. 1 , a typical prior art toe-kick saw  100  consisted of a circular saw motor  120  having a rotating armature (not shown), a primary spindle  140  operatively coupled to the armature, and a means to extend the spindle  150 . With respect to the means to extend the spindle, and in regard to both prior art toe-kick saws as well as those of this invention, the preferred means to extend the spindle has included a separate assembly, which will be referred to herein as a “spindle extension assembly” or as a “spindle extension”. However, a separate assembly need not be used. For example, the spindle itself may be elongated. For this reason, the terms elongate spindle, a spindle extension assembly, or a spindle extension shall all be defined and used herein as a means to extend the spindle. 
         [0008]    The spindle extension assembly  150  of prior art toe-kick saw  100  includes spindle coupler  160 , secondary spindle  200 , and set screw  180  which connects spindle coupler  160  and secondary spindle  200 . Other means to extend the spindle may be created by persons skilled in the art. For example, a spindle extension may be inserted into a hole in the spindle gear itself and keyed to a slot in the spindle gear. All such will be defined herein as a spindle extension assembly or spindle extension. 
         [0009]    The explanation of how spindle extension assembly  150  transmits force to the blade is as follows: Spindle coupler  160  is coupled to primary spindle  140 , and also connected to secondary spindle  200  by set screw  180 . Thus, when primary spindle  140  turns, secondary spindle  200  is turned. Secondary spindle  200  has flats  210  which may engage flats  310  on blade driver  320 . Blade driver  320  has two solid cylindrical drive nubs  330  which engage two drive holes  350  in blade  300 . Thus, whenever primary spindle  140  turns, force is transmitted through spindle extension assembly  150  to blade driver  320  and then to blade  300 . Blade  300  and blade driver  320  are fastened to secondary spindle  200  by inserting pan head screw  340  into a tapped hole  215  in secondary spindle  200 . Blade  300  has a countersunk arbor  370  which accepts the pan-shaped head of pan head screw  340 . Thus, pan head screw  340  is flush mounted in blade  300 . This allows toe-kick saw  100  to enter a toe-kick and cut flush up to its inner wall. 
         [0010]    Spindle extension assembly  150  is covered in use by housing  400 . Housing  400  includes face plate  220 , tube  240 , fixed guard  260 , and movable guard  280 . Housing  400  is screwed onto saw motor  120  using screws  35 . The saw is guided along the inner wall of the toe-kick by the edges  360  of fixed guard  260 . Edges  360  extend approximately 1/16″ past the vertical plane defined by blade  300  to prevent blade  300  and countersunk screw  340  from rubbing against the inner wall of the toe-kick. Edges  360  thus place blade  300  as close as possible to the inner wall of the toe-kick, thus cutting off as much of the old flooring material as possible. 
         [0011]    The prior art toe-kick saw  100  has a fixed guard  260  which is as small as possible in order to fit in as wide a range of toe-spaces as possible. A small blade guard also enables toe-kick saw  100  to come as close as possible to a wall surface of the room which may abut the toe-kick (such as, an inside corner area). This ensures that the saw may be used within a toe-kick as prescribed by the instructions. However, users commonly misuse toe-kick saws. Despite instructions for proper usage and warnings to use the saw underneath toe-spaces only, and to cut forward and straight along the inner wall of the toe-space only, users misuse the tool by cutting outside the toe-space, by cutting sharp curves, or even by running the saw backwards by pulling it towards themselves. Such abuse may create the dangerous and well-known hazard common in the use of all circular saws called “saw kickback”. Saw kickback is caused when a saw blade may catch or become wedged on the edges of a saw kerf. This results in a sudden stoppage of the blade. Yet the spinning armature of the saw motor still has a great deal of stored kinetic energy. Since the blade is stuck and cannot move, the kinetic energy can cause the saw to react by kicking backward towards the user, creating a laceration hazard. To prevent saw kickback, some means to safely dissipate this stored kinetic energy is needed. 
         [0012]    The spindle extension assembly  150  and blade driver  320  of prior art toe-kick saw  100  are unable to safely dissipate the stored kinetic energy. To provide a toe-kick saw which could safely dissipate the energy, it may be noted that the amount of torque in primary spindle  140  (or in spindle extension assembly  150 , for that matter) is normally far greater at the time of saw kickback than under normal cutting conditions. A level of spindle torque which is in excess of that which is required for normal cutting, and which may create a kickback hazard, will be referred to herein as an “excess spindle torque”. Some means to disengage the motor and allow the spinning kinetic energy within it to be safely dissipated could reduce kickback hazards created by saw misuse. 
       SUMMARY 
       [0013]    To achieve these and other goals, an improved blade driver may be employed which itself may yield (as by bending) and thereby disengage the saw motor (and the entire spindle extension assembly) from the blade. Such will be referred to herein as a “yieldable blade driver”. When disengaged, a yieldable blade driver may ratchet against a plurality of drive holes in the blade to safely dissipate the saw motor&#39;s stored kinetic energy. 
         [0014]    In another embodiment, a pair of drive plates may be added to the spindle extension assembly. One of said drive plates may be spring loaded such that both are pressed together. The pair of drive plates may further contain inner detents, where said detents define a cavity for holding steel balls. The detents in one of said pair of drive plates may be deeper, meaning the steel balls are lodged more deeply on this drive plate than the other. Thus, the ball bearings transmit increasing wedging force which tends to separate the pair of drive plates as spindle torque increases. At excess spindle torque, the pair of drive plates are forced apart, disengaging the saw motor from the rest of the spindle extension assembly. The ratcheting action of steel balls could safely dissipate the motor&#39;s kinetic energy. This means to dissipate the saw motor&#39;s kinetic energy will be referred to herein as a “yieldable spindle extension” 
         [0015]    It is an object of one or more embodiments to provide an yieldable blade driver for driving a toe-kick saw blade which may disengage itself from the blade and dissipate the saw motor&#39;s kinetic energy at excess spindle torque. Such a yieldable blade driver may be inexpensive and field replaceable. 
         [0016]    It is an object of one or more embodiments to provide a yieldable spindle extension assembly for a toe-kick saw which includes a means of disengaging itself at excess spindle torque from the saw motor and dissipate the saw motor&#39;s kinetic energy. 
         [0017]    These and other objects may be achieved by forming a yieldable blade driver using a flat piece of metal, such as spring steel. The yieldable blade driver may include spherical drive nubs pressed into its face, which drive the blade. Under normal cutting conditions, the spherical drive nubs will transmit sufficient force to drive the blade. At excess spindle torque, the wedging action of the spherical nubs may cause the blade driver itself to bend and disengage from the blade, thereby disengaging the motor and the entire spindle extension assembly from the blade. The ratcheting action of the spherical drive nubs impacting drive holes on the blade may dissipate the saw motor&#39;s kinetic energy. Thus, a yieldable blade driver may be manufactured as a metal stamping. Such a yieldable blade driver would be inexpensive and just as field replaceable as the blade, because it is mounted in the same accessible location. 
         [0018]    The objects may be achieved with an improved spindle extension assembly for a toe-kick saw which includes a first drive plate coupled to the primary spindle of the saw motor, and a second drive plate which is pressed against the first drive plate by a spring. In one embodiment, the inside faces of said first and second drive plate each have five detents. The detents in said first and second drive plates together define cavities for holding five steel balls. Under normal cutting conditions, the steel balls act as drive surfaces which transmit torque between said first drive plate and said second drive plate. However, these steel balls are seated more deeply in the first drive plate than the second, and thus engage the second drive plate with a shorter surface. At excess spindle torque, the steel balls act as a wedge or inclined plane and force the first and second drive plates apart. Thus, the spindle extension assembly would disengage itself from the saw motor in the event of an excess spindle torque. The ratcheting action of the steel balls impacting the drive detents on the second drive plate may dissipate the saw motor&#39;s kinetic energy. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a partially exploded view of a prior art toe-kick saw. 
           [0020]      FIG. 2   a  is an exploded view of a toe-kick saw with a yieldable blade driver. 
           [0021]      FIG. 2   b  is a front perspective view of the yieldable blade driver. 
           [0022]      FIG. 3  is partially exploded view of an alternative embodiment of the yieldable blade driver in  FIGS. 2   a  and  2   b , in which the blade driver is made from two pieces of material. 
           [0023]      FIG. 4  is a partially exploded view of an alternative embodiment of a yieldable blade driver which includes a solid blade driver and a dished washer. 
           [0024]      FIG. 5  is an exploded view of a toe-kick saw with a spindle extension assembly which contains a pair of spring loaded drive plates. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    With reference to  FIG. 2   a , toe-kick saw  1000  includes a circular saw motor  1020  having an internal rotating armature (not shown) operatively coupled to a rotating spindle  1040 . The housing of motor  1020  has an opening  1060  which accepts tube  1080 . Tube  1080  is inserted and fastened into opening  1060  with three screws  1100  which run through three holes  1120 . 
         [0026]    Tube  1080  serves to house spindle extension assembly  1140 . Spindle extension assembly  1140  includes screw  1160 , spindle coupler  1180 , spacer  1185 , snap ring  1200 , ball bearing  1220 , and secondary spindle  1240 . The assembly of these components goes as follows: Ball bearing  1220  is slipped onto secondary spindle  1240  and rests on shoulder  1260 . Snap ring  1200  is seated in groove  1280 . Spacer  1185  is then slipped onto secondary spindle  1240 . 
         [0027]    Next in the assembly is the mounting of spindle coupler  1180 . Spindle coupler  1180  has a slot  1320  which accepts flats  1300  on secondary spindle  1240 . Spindle coupler  1180  has a slot  1340  on the opposite end and a hole through its centerline (not shown). Screw  1160  goes through this centerline hole and fastens into a tapped centerline hole (not shown) on the inside end of secondary spindle  1240 . 
         [0028]    Once spindle coupler  1180  is fixed onto secondary spindle  1240 , it may form a coupling for spindle extension assembly  1140  when slot  1340  is coupled to spindle  1040  (of motor  1020 ). Thus, spindle extension assembly  1140  is capable of transmitting force from saw motor  1020  to a blade driver at an extended distance. 
         [0029]    Spindle extension assembly  1140  is housed within tube  1080 . As previously explained, tube  1080  is inserted and fastened into opening  1060  of saw motor  1020 . On the opposite end, tube  1080  is press fit onto boss  1380  on the back side of fixed guard  1400  and fastened with three screws  1420  through three holes  1440 . 
         [0030]    Internal support for spindle extension assembly  1140  is provided within fixed guard  1400  by ball bearing  1220  and bushing  1460 . Bushing  1460  is press fit into a reamed counterbored hole  1480 . Hole  1480  runs from the front of fixed guard  1400  all the way through to the opposite side of fixed guard  1400 , where said counterbore (not shown) is located. Spindle extension assembly  1140  is then inserted though the back side of hole  1480  and bushing  1460  until ball bearing  1220  seats in said counterbore. Plate  1355  is placed on top of ball bearing  1220 . Two screws  1360  are fastened into fixed guard  1400  through two holes  1365  in plate  1355 . This fastens ball bearing  1220  into the counterbore and secures spindle extension assembly  1140  into fixed guard  1400 . 
         [0031]    When ball bearing  1220  is seated and fastened into said counterbore, the secondary spindle is prevented from sliding out by snap ring  1200 . Spacer  1185  provides additional safety should snap ring  1200  fail. Spacer  1185  is larger in outside diameter than the center hole in ball bearing  1220 , and thus also prevents spindle extension assembly  1140  from sliding out. 
         [0032]    Practical problems of imprecise bearing alignment, runout, and motor vibration make manufacturing a circular saw with an extended spindle difficult. Connecting a separate secondary spindle (such as secondary spindle  1240 ) to the motor by way of spindle coupler  1180  is preferred because a controllable amount of play is allowed in the juncture between slot  1340  and primary spindle  1040 . Without such play, even slight misalignment will result in runout or wobbling forces being transferred to ball bearing  1220  and bushing  1460 . This reduces the life of the saw. 
         [0033]    Fixed guard  1400  has a blade housing  1500  which contains cylindrical guard mount  1520 . As with prior art toe-kick saws, a torsion spring  1580  and movable guard  1560  is placed onto cylindrical guard mount  1520 . Torsion spring  1580  hooks on end  1620  into a hole inside blade housing  1500  (not shown) and on hook  1600  to a hole  1640  on primary movable guard  1560 . When primary movable guard  1560  is retracted (as during a cutting operation), tension builds in torsion spring  1580  which urges primary movable guard  1560  to re-extend back to a forward guarding position. Cylindrical guard mount  1520  has snap ring groove  1540 . Snap ring  1660  is seated into snap ring groove  1540  to hold primary movable guard  1560  and torsion spring  1580  in place. 
         [0034]    The distal end of secondary spindle  1240  projects a sufficient distance into blade housing  1500  to expose flats  1680  and rounds  1690 . Flats  1680  form the drive surfaces for a blade driver. Yieldable blade driver  1700  is mounted on the distal end of secondary spindle  1240 . As shown more clearly in  FIG. 2   b , flats  1710  in arbor  1720  engage flats  1680  of secondary spindle  1240  ( FIG. 2   a ). Thus, force is transmitted from the secondary spindle  1240  ( FIG. 2   a ) to yieldable blade driver  1700 . As shown in  FIG. 2   a , blade  1760  is next placed on the distal end of secondary spindle  1240  where it rests on rounds  1690 . Blade  1760  is pressed against four spherical nubs  1730  of yieldable blade driver  1700 . The four spherical nubs may seat in any four of eight concentric drive holes  1770  in blade  1760 . Blade screw  1800  is fastened into a tapped hole  1695  in the distal end of secondary spindle  1240  to hold blade  1760  and yieldable blade driver  1700  on secondary spindle  1240 . However, blade screw  1800  can tighten down only enough to leave a small gap between its inside surface and the outer surface of blade  1760 . This gap is controlled by the depth of tapped hole  1695  in the end of secondary spindle  1240 . Under normal cutting conditions, yieldable blade driver  1700  will transmit sufficient force to blade  1760  for cutting. 
         [0035]    At excess spindle torque, the four of holes  1770  in blade  1760  which may be engaged with rounded nubs  1730  produce sufficient wedging pressure on nubs  1730  to bend arms  1750  ( FIG. 2   b ) of yieldable blade driver  1700  backward. Thus nubs  1730  become disengaged from blade  1760 . Blade  1760  stops. (The previously discussed small gap between the inside surface of blade screw  1800  and the inside surface of blade  1760  isolates blade  1760  from blade screw  1800 . Rounds  1690  transfer little or no force.) The spinning kinetic energy of the motor is safely dissipated as the nubs  1730  of yieldable blade driver  1700  ratchet against the eight concentric drive holes  1770  of blade  1760 . When motor power is sufficiently reduced, nubs  1730  may reseat in any four of eight concentric drive holes  1770 . This allows yieldable blade driver  1700  to re-engage the blade, so that cutting at reduced spindle torque may resume. The nubs are illustrated as rounded nubs (i.e. the projecting ends of the nubs are curved) but any shape which causes the blade driver to yield and disengage from the blade at excess spindle torque, and thereafter reengage the blade at lower spindle torque, should be adaptable to the present embodiments (including those of  FIGS. 2   a ,  2   b ,  3  and  4 ). Thus a nub with a facet or inclined end are also envisioned. The nubs illustrated are spherical (rounded ends) but any shape allowing unseating/reseating of the blade would be possible. Faceted heads are envisioned. 
         [0036]    A yieldable blade driver for a toe-kick saw may have several embodiments. In another embodiment, the driver may be made in two parts in order to reduce the thickness of the arms. As shown in  FIG. 3 , yieldable blade driver  3700  consists of a first driver  3800  and a second back up plate  3900 . Driver  3800  and back up plate  3900  are made to be spot welded or peened together as by inserting bosses  3910  of backup plate  3900  through holes  3810  of blade driver  3800 . The combined thickness of flats  3830  of driver  3800  with flats  3930  of backup plate  3900  is comparable to the thickness of flats  1710  of the single piece yieldable blade driver  1700  of  FIG. 2   b . Thus, the bearing surface of the combined flats  3830 ,  3930  which bear against the drive flats  3680  on the secondary spindle  3240  will be comparable. This prevents premature wear on secondary spindle  3240 . However, as shown in  FIG. 3 , the material used to form driver  3800  may be thinner. Using thinner materials to form arms  3850  is preferred for two reasons. First, thinner arms allow the four spherical nubs  3870  to disengage at lower spindle torque. Secondly, the thickness of arms  3850  determines the pressure put on the spherical nubs  3870  when they ratchet against the drive holes  3770  in the blade  3760  when the yieldable blade driver is disengaged. The reduced pressure from arms  3850  helps spherical nubs  3870  last longer. 
         [0037]    Another embodiment of a yieldable blade driver for a toe-kick saw which uses a combination of a solid driver and a dished washer is shown in  FIG. 4 . Yieldable blade driver  4700  includes solid driver  4800  and dished washer  4900  which acts like a spring. Dished washer  4900  is installed first on the end of secondary spindle  4240 , followed by solid driver  4800 . Solid driver  4800  is pressed against dish washer  4900 . Solid driver  4800  has a thicker solid body  4810  which will not bend, and a concentric pattern of eight spherical drive nubs  4830 . Force for driving the blade is transmitted from flats  4680  on the secondary spindle  4240  of toe-kick saw  4000  to flats  4850  of solid driver  4800 . Under normal cutting conditions, solid driver  4800  will transfer sufficient force, each pressing against the eight spherical drive nubs  4830  contacting the concentric pattern of eight drive holes  4770  in blade  4760 . However, as spindle torque increases, eight spherical drive nubs  4830  tend to wedge solid driver  4800  away from blade  4760 , which puts pressure on dish washer  4900  and causes it to yield. At excess spindle torque, solid driver  4800  disengages from blade  4760 , and the eight spherical drive nubs  4830  ratchet against the eight drive holes  4770  of blade  4760 . Blade  4760  stops. (Secondary spindle  4240  of toe-kick saw  4000  has the same threaded hole  4690  with precise depth that maintains a small gap between the inner surface of screw  4900  and blade  4760 , and rounds  4690  which both prevent any force from being transferred to blade  4760  during disengagement.) The spinning kinetic energy of the motor may be dissipated by the ratcheting of solid driver  4800  against blade  4760 . Once the kinetic energy is sufficiently dissipated, dish washer  4900  will urge solid driver  4800  against blade  4760  with sufficient force to reengage the eight spherical drive nubs  4830  into the eight drive holes  4770  of blade  4760 , and normal cutting may resume. 
         [0038]    A different type of yieldable drive mechanism for a toe-kick saw may be a yieldable spindle extension. A yieldable spindle extension may include a pair of spring loaded drive plates which may allow the spindle extension assembly to disengage itself from the saw motor at excess spindle torque. As shown in  FIG. 5 , toe kick saw  5000  includes a motor  5020  with a rotating armature (not shown) operatively coupled to a rotating spindle  5040 . The housing of motor  5020  has an opening  5060  which accepts tube  5080 . Tube  5080  is inserted and fastened into opening  5060  with three screws  5100  which run through three holes  5120 . 
         [0039]    Tube  5080  serves to cover yieldable spindle extension  5140 . Yieldable spindle extension  5140  includes wire-form retainer ring  5160 , chamfered washer  5180 , primary drive plate  5200 , five steel balls  5400 , secondary drive plate  5400 , lock pin  6000 , spring  6200 , snap ring  6400 , washer  6600 , ball bearing  7200 , and secondary spindle  7220 . The assembly of these components goes as follows: Ball bearing  7200  is slid onto secondary spindle  7220  and rests on shoulder  7240 . Washer  6600  is next slid onto secondary spindle  7220 . Washer  6600  has a step  6800  which rests on the inside race (not shown) on the inside face of ball bearing  7200 . Snap ring  6400  seats in first groove  7260  to lock ball bearing  7200  and washer  6600  in place. 
         [0040]    Pin  6000  is inserted into a hole  7280  through secondary spindle  7220 . Spring  6200  is placed over secondary spindle  7220  and is pressed on one end against washer  6600 . On the opposite end, spring  6200  is pressed against a groove  5700  on secondary drive plate  5400  until the ends of pin  6000  seat in drive slots  5800  of secondary drive plate  5400 . Five steel balls  5400  are inserted into five detents (not shown) on the inner face of secondary drive plate  5400 . Four of these detents are concentric, while the one other detent is located on a shorter radius inside the concentric circle formed by the other four detents. Primary drive plate  5200  is placed against the inner face of secondary drive plate  5400  such that the five steel balls  5400  seat in five detents  5210 ,  5215  in primary drive plate  5200 . The number and location of detents in primary drive plate  5200  correspond with those in the inner face of secondary drive plate  5400  (i.e., four detents  5210  are concentric, one detent  5215  is located on a shorter radius). 
         [0041]    For reasons to be explained later in the discussion of how primary drive plate  5200  and secondary drive plate  5400  may disengage in use, the detents  5210 ,  5215  in primary drive plate  5200  are slightly deeper than those in secondary drive plate  5400 . However, in the initial assembly, the two sets of detents in both primary drive plate  5200  and secondary drive plate  5400  are aligned to precisely define five cavities for holding five steel balls  5400 . 
         [0042]    To complete the assembly of the yieldable spindle extension  5140 , primary drive plate  5200  is pressed onto the assembly of five steel balls  5400  and secondary drive plate  5400 , until it slips over end  7290  of secondary spindle  7220  and rests against shoulder  7285 . This further compresses spring  6200  and captures five steel balls  5400  between primary drive plate  5200  and secondary drive plate  5400 . At this point, end  7290  of secondary spindle  7220  runs through hole  5205  of primary drive plate  5200  and projects into slot  5220 . This exposes end  7290  and retainer groove  7300  within slot  5220  so chamfer washer  5180  and wire-form retainer ring  5160  can be mounted onto secondary spindle  7220  inside slot  5220 . Thus, the entire assembly is locked in place by inserting wire-form retainer ring  5160  into retainer groove  7300  of secondary spindle  7220 . The chamfer in chamfer washer  5180  is located on the outside surface (not shown) where it will bear against wire-form retainer ring  5160 . The chamfer causes wire-form retainer ring  5160  to be compressed deeper into retainer groove  7300  as pressure from primary driver  5200  may increase. This offers greater holding strength than a snap ring (such as snap ring  6400 ). This completes the assembly of yieldable spindle extension  5140 . Yieldable spindle extension  5140  is then coupled at slot  5220  to spindle  5040  of saw motor  5020 , and is capable of transmitting rotational force at an extended distance while also yielding at excess spindle torque. 
         [0043]    Yieldable spindle extension  5140  is housed within tube  5080 . As previously explained, tube  5080  is inserted and fastened into opening  5060  of saw motor  5020 . On the opposite end, tube  5080  is press fit onto boss  7600  on the back side of fixed guard  7800  and fastened with three screws  8000  through three holes  8200 . 
         [0044]    Internal support for yieldable spindle extension  5140  is provided within fixed guard  7800  by ball bearing  7200  and bushing  8400 . Bushing  8400  is press fit into a reamed counterbored hole  8600 . Hole  8600  runs all the way to the back side of fixed guard  7800 , where the counterbore (not shown) is located. Yieldable spindle extension  5140  is then inserted through the back side of fixed guard  7800  through hole  8600  and bushing  8400  until ball bearing  7200  seats in the back side counterbore. Two screws  7000  are fastened on top of ball bearing  7200  to fasten it within the counterbore. Thus, yieldable spindle extension  5140  becomes fastened to fixed guard  7800 . 
         [0045]    Fixed guard  7800  has a blade housing  8800  which contains cylindrical guard mount  9000 . A torsion spring  9200  and movable guard  9800  are mounted onto cylindrical guard mount  9000 . Torsion spring  9200  hooks on end  9400  into a hole inside blade housing  8800  (not shown) and on a hook  9600  to a hole  10000  on movable guard  9800 . When movable guard  9800  is retracted (as during a cutting operation), tension builds in torsion spring  9200  which urges movable guard  9800  to re-extend back to a forward guarding position. Cylindrical guard mount  9000  has a snap ring groove  9100 . Snap ring  10200  is seated into snap ring groove  9100  to hold movable guard  9800  and torsion spring  9200  in place. 
         [0046]    The distal end of secondary spindle  7220  projects a sufficient distance into blade housing  8800  to expose flats  7300 . Flats  7300  engage flats  10450  on solid blade driver  10400 . Solid blade driver  10400  has a pair of solid cylindrical projections  10600 . Cylindrical projections  10600  engage drive holes  10800  of blade  11000 . Blade  11000  has an arbor  11200  which is precision countersunk on its outside surface to seat the pan-shaped head of blade screw  11400 . Because blade screw  11400  is fully recessed into countersunk arbor  11200 , blade  11000  has a flush face, and is able to cut as closely as possible to the inner wall of a toe-kick. 
         [0047]    The explanation of how yieldable spindle extension  5140  can disengage itself from spindle  5040  of saw motor  5020  is as follows: Yieldable spindle extension  5140  is coupled to spindle  5040  by slot  5220  in primary drive plate  5200 . When spindle  5040  turns, primary drive plate  5200  will turn, and rotational force will be transferred to secondary drive plate  5400  through five steel balls  5400 . Under normal cutting conditions, spring  6200  will hold secondary drive plate  5400  with sufficient force against primary drive plate  5200  that ball bearings  5400  will be captured between the detents on both drive plates, and will transfer force between them. However, as previously explained, the detents  5210 ,  5215  in primary drive plate  5200  are deeper than the corresponding detents in secondary drive plate  5400 . The five steel balls  5400  protrude less than half their diameter from the inside face of primary  5200 , and thus engage the opposite detents in secondary drive plate  5400  with less than half of the diameter of their surface. When force is applied, the surfaces of five steel balls  5400  which protrude from the inner face of primary drive plate  5200  act as a wedge or an inclined plane against the corresponding detents on secondary drive plate  5600 . As greater force is applied, five steel balls  5600  will push secondary drive plate  5400  further away until they may become disengaged from secondary drive plate  5400 . At excess spindle torque, primary drive plate  5200  and five balls  5400  will continue to spin (being more deeply socketed in detents  5210 ,  5215 ). The rest of yieldable spindle extension  5140  (as well as solid blade driver  10400  and blade  11000 ) will stop. This internally disengages yieldable spindle extension  5140 , and dissipates the stored kinetic energy of the motor. 
         [0048]    When motor power is sufficiently reduced, five steel balls  5400  will reseat within the shallower detents in secondary  5400 . At such time, yieldable spindle extension  5140  is re-engaged, and normal cutting may continue. 
         [0049]    As previously explained, primary drive plate  5200  and secondary drive plate  5400  each have five detents to hold five steel balls  5400 . Four of these corresponding pairs of detents are concentric. However, the fifth pair of corresponding detents are formed on a shorter radius. The fifth pair of corresponding detents cause primary drive plate  5200  and five steel balls  5400  to spin at least one full turn before five steel balls  5400  will ratchet against the detents on secondary drive plate  5400 . This reduces the number of damaging impacts that five steel balls  5400  may have on the detents in secondary drive plate  5400 , extending the life of these components.