YIELDABLE DRIVE MECHANISM FOR A TOE-KICK SAW

A toe-kick saw having a yieldable spindle extension is disclosed. At least one nub, as a transfer member, projects from a first or a second drive plate. The nub is engageable with a depression, slot or other nub receiving area on the other drive plate, to couple and uncouple the plates. In an engaged condition, the drive plates, a secondary spindle, a blade mount and a circular saw blade spin with the saw motor spindle. In a disengaged condition, the first drive plate spins with the saw motor spindle while the second drive plate, secondary spindle, blade mount and saw blade are disengaged from the first drive plate and the saw motor spindle. A biasing mechanism presses the second drive plate towards the first drive plate. A further biasing mechanism may press the second plate towards the first plate with more force when the plates are coupled than when decoupled.

DETAILED DESCRIPTION

With reference toFIG. 2a, toe-kick saw1000includes a circular saw motor1020having an internal rotating armature (not shown) operatively coupled to a rotating spindle1040. The housing of motor1020has an opening1060which accepts tube1080. Tube1080is inserted and fastened into opening1060with three screws1100which run through three holes1120.

Tube1080serves to house spindle extension assembly1140. Spindle extension assembly1140includes screw1160, spindle coupler1180, spacer1185, snap ring1200, ball bearing1220, and secondary spindle1240. The assembly of these components goes as follows: Ball bearing1220is slipped onto secondary spindle1240and rests on shoulder1260. Snap ring1200is seated in groove1280. Spacer1185is then slipped onto secondary spindle1240.

Next in the assembly is the mounting of spindle coupler1180. Spindle coupler1180has a slot1320which accepts flats1300on secondary spindle1240. Spindle coupler1180has a slot1340on the opposite end and a hole through its centerline (not shown). Screw1160goes through this centerline hole and fastens into a tapped centerline hole (not shown) on the inside end of secondary spindle1240.

Once spindle coupler1180is fixed onto secondary spindle1240, it may form a coupling for spindle extension assembly1140when slot1340is coupled to spindle1040(of motor1020). Thus, spindle extension assembly1140is capable of transmitting force from saw motor1020to a blade driver at an extended distance.

Spindle extension assembly1140is housed within tube1080. As previously explained, tube1080is inserted and fastened into opening1060of saw motor1020. On the opposite end, tube1080is press fit onto boss1380on the back side of fixed guard1400and fastened with three screws1420through three holes1440.

Internal support for spindle extension assembly1140is provided within fixed guard1400by ball bearing1220and bushing1460. Bushing1460is press fit into a reamed counterbored hole1480. Hole1480runs from the front of fixed guard1400all the way through to the opposite side of fixed guard1400, where said counterbore (not shown) is located. Spindle extension assembly1140is then inserted though the back side of hole1480and bushing1460until ball bearing1220seats in said counterbore. Plate1355is placed on top of ball bearing1220. Two screws1360are fastened into fixed guard1400through two holes1365in plate1355. This fastens ball bearing1220into the counterbore and secures spindle extension assembly1140into fixed guard1400.

When ball bearing1220is seated and fastened into said counterbore, the secondary spindle is prevented from sliding out by snap ring1200. Spacer1185provides additional safety should snap ring1200fail. Spacer1185is larger in outside diameter than the center hole in ball bearing1220, and thus also prevents spindle extension assembly1140from sliding out.

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 spindle1240) to the motor by way of spindle coupler1180is preferred because a controllable amount of play is allowed in the juncture between slot1340and primary spindle1040. Without such play, even slight misalignment will result in runout or wobbling forces being transferred to ball bearing1220and bushing1460. This reduces the life of the saw.

Fixed guard1400has a blade housing1500which contains cylindrical guard mount1520. As with prior art toe-kick saws, a torsion spring1580and movable guard1560is placed onto cylindrical guard mount1520. Torsion spring1580hooks on end1620into a hole inside blade housing1500(not shown) and on hook1600to a hole1640on primary movable guard1560. When primary movable guard1560is retracted (as during a cutting operation), tension builds in torsion spring1580which urges primary movable guard1560to re-extend back to a forward guarding position. Cylindrical guard mount1520has snap ring groove1540. Snap ring1660is seated into snap ring groove1540to hold primary movable guard1560and torsion spring1580in place.

The distal end of secondary spindle1240projects a sufficient distance into blade housing1500to expose flats1680and rounds1690. Flats1680form the drive surfaces for a blade driver. Yieldable blade driver1700is mounted on the distal end of secondary spindle1240. As shown more clearly inFIG. 2b, flats1710in arbor1720engage flats1680of secondary spindle1240(FIG. 2a). Thus, force is transmitted from the secondary spindle1240(FIG. 2a) to yieldable blade driver1700. As shown inFIG. 2a, blade1760is next placed on the distal end of secondary spindle1240where it rests on rounds1690. Blade1760is pressed against four spherical nubs1730of yieldable blade driver1700. The four spherical nubs may seat in any four of eight concentric drive holes1770in blade1760. Blade screw1800is fastened into a tapped hole1695in the distal end of secondary spindle1240to hold blade1760and yieldable blade driver1700on secondary spindle1240. However, blade screw1800can tighten down only enough to leave a small gap between its inside surface and the outer surface of blade1760. This gap is controlled by the depth of tapped hole1695in the end of secondary spindle1240. Under normal cutting conditions, yieldable blade driver1700will transmit sufficient force to blade1760for cutting.

At excess spindle torque, the four of holes1770in blade1760which may be engaged with rounded nubs1730produce sufficient wedging pressure on nubs1730to bend arms1750(FIG. 2b) of yieldable blade driver1700backward. Thus nubs1730become disengaged from blade1760. Blade1760stops. (The previously discussed small gap between the inside surface of blade screw1800and the inside surface of blade1760isolates blade1760from blade screw1800. Rounds1690transfer little or no force.) The spinning kinetic energy of the motor is safely dissipated as the nubs1730of yieldable blade driver1700ratchet against the eight concentric drive holes1770of blade1760. When motor power is sufficiently reduced, nubs1730may reseat in any four of eight concentric drive holes1770. This allows yieldable blade driver1700to 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 ofFIGS. 2a,2b,3and4). Thus a nub with a facet or inclined end is 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.

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 the toe-kick saw3000ofFIG. 3, yieldable blade driver3700consists of a first driver3800and a second back up plate3900. Driver3800and back up plate3900are made to be spot welded or peened together as by inserting bosses3910of backup plate3900through holes3810of blade driver3800. The combined thickness of flats3830of driver3800with flats3930of backup plate3900is comparable to the thickness of flats1710of the single piece yieldable blade driver1700ofFIG. 2b. Thus, the bearing surface of the combined flats3830,3930which bear against the drive flats3680on the secondary spindle3240will be comparable. This prevents premature wear on secondary spindle3240. However, as shown inFIG. 3, the material used to form driver3800may be thinner. Using thinner materials to form arms3850is preferred for two reasons. First, thinner arms allow the four spherical nubs3870to disengage at lower spindle torque. Secondly, the thickness of arms3850determines the pressure put on the spherical nubs3870when they ratchet against the drive holes3770in the blade3760when the yieldable blade driver is disengaged. The reduced pressure from arms3850helps spherical nubs3870last longer.

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 inFIG. 4. Yieldable blade driver4700includes solid driver4800and dished washer4900which acts like a spring. Dished washer4900is installed first on the end of secondary spindle4240, followed by solid driver4800. Solid driver4800is pressed against dish washer4900. Solid driver4800has a thicker solid body4810which will not bend, and a concentric pattern of eight spherical drive nubs4830. Force for driving the blade is transmitted from flats4680on the secondary spindle4240of toe-kick saw4000to flats4850of solid driver4800. Under normal cutting conditions, solid driver4800will transfer sufficient force for cutting, with each of its eight spherical drive nubs4830driving against the concentric pattern of eight drive holes4770in blade4760. However, as spindle torque increases, eight spherical drive nubs4830tend to wedge solid driver4800away from blade4760, which puts pressure on dish washer4900and causes it to yield. At excess spindle torque, solid driver4800disengages from blade4760, and the eight spherical drive nubs4830ratchet against the eight drive holes4770of blade4760. Blade4760stops. (Secondary spindle4240of toe-kick saw4000has the same threaded hole4690with precise depth that maintains a small gap between the inner surface of screw4920and blade4760, and rounds4690which both prevent any force from being transferred to blade4760during disengagement.) The spinning kinetic energy of the motor may be dissipated by the ratcheting of solid driver4800against blade4760. Once the kinetic energy is sufficiently dissipated, dish washer4900will urge solid driver4800against blade4760with sufficient force to reengage the eight spherical drive nubs4830into the eight drive holes4770of blade4760, and normal cutting may resume.

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 inFIG. 5, toe-kick saw5000includes a motor5020with a rotating armature (not shown) operatively coupled to a rotating spindle5040. The housing of motor5020has an opening5060which accepts tube5080. Tube5080is inserted and fastened into opening5060with three screws5100which run through three holes5120.

Tube5080serves to cover yieldable spindle extension5140. Yieldable spindle extension5140includes wire-form retainer ring5160, chamfered washer5180, primary drive plate5200, five steel balls5400, secondary drive plate5600, lock pin6000, spring6200, snap ring6400, washer6600, ball bearing7200, and secondary spindle7220. The assembly of these components goes as follows: Ball bearing7200is slid onto secondary spindle7220and rests on shoulder7240. Washer6600is next slid onto secondary spindle7220. Washer6600has a step6800which rests on the inside race (not shown) on the inside face of ball bearing7200. Snap ring6400seats in first groove7260to lock ball bearing7200and washer6600in place.

Pin6000is inserted into a hole7280through secondary spindle7220. Spring6200is placed over secondary spindle7220and is pressed on one end against washer6600. On the opposite end, spring6200is pressed against a groove5700on secondary drive plate5600until the ends of pin6000seat in drive slots5800of secondary drive plate5600. Five steel balls5400are inserted into five detents (not shown) on the inner face of secondary drive plate5600. 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 plate5200is placed against the inner face of secondary drive plate5600such that the five steel balls5400seat in five detents5210,5215in primary drive plate5200. The number and location of detents in primary drive plate5200correspond with those in the inner face of secondary drive plate5600(i.e., four detents5210are concentric, one detent5215is located on a shorter radius).

For reasons to be explained later in the discussion of how primary drive plate5200and secondary drive, plate5600may disengage in use, the detents5210,5215in primary drive plate5200are slightly deeper than those in secondary drive plate5600. However, in the initial assembly, the two sets of detents in both primary drive plate5200and secondary drive plate5600are aligned to precisely define five cavities for holding five steel balls5400.

To complete the assembly of the yieldable spindle extension5140, primary drive plate5200is pressed onto the assembly of five steel balls5400and secondary drive plate5600, until it slips over end7290of secondary spindle7220and rests against shoulder7285. This further compresses spring6200and captures five steel balls5400between primary drive plate5200and secondary drive plate5600. At this point, end7290of secondary spindle7220runs through hole5205of primary drive plate5200and projects into slot5220. This exposes end7290and retainer groove7300within slot5220so chamfer washer5180and wire-form retainer ring5160can be mounted onto secondary spindle7220inside slot5220.

Thus, the entire assembly is locked in place by inserting wire-form retainer ring5160into retainer groove7300of secondary spindle7220. The chamfer in chamfer washer5180is located on the outside surface (not shown) where it will bear against wire-form retainer ring5160. The chamfer causes wire-form retainer ring5160to be compressed deeper into retainer groove7300as pressure from primary drive plate5200may increase. This offers greater holding strength than a snap ring (such as snap ring6400). This completes the assembly of yieldable spindle extension5140. Yieldable spindle extension5140is then coupled at slot5220to spindle5040of saw motor5020, and is capable of transmitting rotational force at an extended distance while also yielding at excess spindle torque.

Yieldable spindle extension5140is housed within tube5080. As previously explained, tube5080is inserted and fastened into opening5060of saw motor5020. On the opposite end, tube5080is press-fit onto boss7600on the back side of fixed guard7800and fastened with three screws8000through three holes8200.

Internal support for yieldable spindle extension5140is provided within fixed guard7800by ball bearing7200and bushing8400. Bushing8400is press fit into a reamed counterbored hole8600. Hole8600runs all the way to the back side of fixed guard7800, where the counterbore (not shown) is located. Yieldable spindle extension5140is then inserted through the back side of fixed guard7800through hole8600and bushing8400until ball bearing7200seats in the back side counterbore. Two screws7000are fastened on top of ball bearing7200to fasten it within the counterbore. Thus, yieldable spindle extension5140becomes fastened to fixed guard7800.

Fixed guard7800has a blade housing8800which contains cylindrical guard mount9000. A torsion spring9200and movable guard9800are mounted onto cylindrical guard mount9000. Torsion spring9200hooks on end9400into a hole inside blade housing8800(not shown) and on a hook9600to a hole10000on movable guard9800. When movable guard9800is retracted (as during a cutting operation), tension builds in torsion spring9200which urges movable guard9800to re-extend back to a forward guarding position. Cylindrical guard mount9000has a snap ring groove9100. Snap ring10200is seated into snap ring groove9100to hold movable guard9800and torsion spring9200in place.

The distal end of secondary spindle7220projects a sufficient distance into blade housing8800to expose flats7300. Flats7300engage flats10450on solid blade driver10400. Solid blade driver10400has a pair of solid cylindrical projections10600. Cylindrical projections10600engage drive holes10800of blade11000. Blade11000has an arbor11200which is precision countersunk on its outside surface to seat the pan-shaped head of blade screw11400. Because blade screw11400is fully recessed into countersunk arbor11200, blade11000has a flush face, and is able to cut as closely as possible to the inner wall of a toe-kick.

The explanation of how yieldable spindle extension5140can disengage itself from spindle5040of saw motor5020is as follows: Yieldable spindle extension5140is coupled to spindle5040by slot5220in primary drive plate5200. When spindle5040turns, primary drive plate5200will turn, and rotational force will be transferred to secondary drive plate5600through five steel balls5400. Under normal cutting conditions, spring6200will hold secondary drive plate5600with sufficient force against primary drive plate5200that five steel balls5400will be captured between the detents or other depressions on both drive plates, and will transfer force between them, acting as transfer members. However, as previously explained, the detents5210,5215in primary drive plate5200are deeper than the corresponding detents in secondary drive plate5600. The five steel balls5400protrude less than half their diameter from the inside face of primary drive plate5200, and thus engage the opposite detents in secondary drive plate5600with less than half of the diameter of their surface. When force is applied, the surfaces of five steel balls5400which protrude from the inner face of primary drive plate5200act as a wedge or an inclined plane against the corresponding detents on secondary drive plate5600. As greater force is applied, five steel balls5400will push secondary drive plate5600further away until they may become disengaged from secondary drive plate5600. At excess spindle torque, primary drive plate5200and five steel balls5400will continue to spin (being more deeply socketed in detents5210,5215). The rest of yieldable spindle extension5140(as well as solid blade driver10400and blade11000) will stop. This internally disengages yieldable spindle extension5140, and dissipates the stored kinetic energy of the motor.

When motor power is sufficiently reduced, five steel balls5400will reseat within the shallower detents in secondary drive plate5600. At such time, yieldable spindle extension5140is re-engaged, and normal cutting may continue.

As previously explained, primary drive plate5200and secondary drive plate5600each have five detents to hold five steel balls5400. 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 plate5200and five steel balls5400to spin at least one full turn before five steel balls5400will ratchet against the detents on secondary drive plate5600. This reduces the number of damaging impacts that five steel balls5400may have on the detents in secondary drive plate5600, extending the life of these components.

The assembly of toe-kick saw20000begins with the attachment of certain components to fixed guard24000. A bushing24010providing precision support for secondary spindle22800is pressed into one end of a hole24020through fixed guard24000. On the opposite end of hole24020is a counterbore (not shown) which accepts ball bearing22700.

Secondary spindle22800is inserted into fixed guard24000through bushing24010and ball bearing22700. End22805of secondary spindle22800extends out the back side of fixed guard24000. Next ball bearing22700is fastened to fixed guard24000by placing ball bearing retainer plate22680on ball bearing22700and fastening it down with screws22660through holes22690. Snap ring22650is then inserted into snap ring groove22820of secondary spindle22800, thereby holding secondary spindle22800in fixed blade guard24000.

Washer22640is placed on secondary spindle22800against snap ring22650, followed by return spring22620. Washer22640provides a footing for one end of return spring22620, protecting ball bearing22700from wear. Drive pin22600is inserted into a hole22830in secondary spindle22800.

Spring assembly22400includes spring body22410, four balls22440, four ball springs22450, and spring assembly cover22490. Spring body22410includes spring body drive slot22420, secondary spindle through hole22425, and four ball through holes22430. Drive pin22600inserts within spring body drive slot22420of spring body22410.

The spring body drive slot22420in spring body22410is deep enough to allow travel of spring assembly22400in the direction of fixed guard24000. This is required for disengagement of female drive plate22300from male drive plate22200(discussed below).

The way that spring assembly22400is assembled with four balls22440and four ball springs22450is as follows: First, spring body22410is positioned on secondary spindle22800such that its four ball through holes22430align with ball groove22840in secondary spindle22800. Spring assembly cover22490is slid over spring body22410with an access hole22495aligned with one of the four ball through holes22430. One of the four balls22440is inserted through access hole22495into this one of the four ball through holes22430, followed by one of the four ball springs22450.

Spring assembly cover22490is turned so that the access hole22495is moved over another one of four ball through holes22430. The second of four balls22440and four ball springs22450is inserted, and afterwards spring assembly cover is turned over another of the four ball through holes22430. The process is repeated another two times until all of four balls22440and four ball springs22450are inserted and held within the four ball through holes22430. Spring assembly22400is then held in position on secondary spindle22800by four balls22440being held within ball groove22840of secondary spindle22800by pressure from four ball springs22450.

Female drive plate22300with four female slots22310is placed on secondary spindle22800. A “double-D” internal cutout22320in female drive plate22300is inserted onto an outer “double-D” feature22460of spring body22410(FIG. 9).

Next male drive plate22200with four male nubs22210is placed on secondary spindle22800. The four male nubs22210insert within the four female slots22310of female drive plate22300. Male drive plate22200has a “double-D” internal cutout22220.

Each of the nubs22210acts as a transfer member projecting from the male drive plate22200, to transfer rotation force and energy from the male drive plate22200to the female drive plate22300. Each of the four female slots22310act as a detent of the female drive plate22300.

In a variation, the locations and couplings of the male and female drive plates are swapped. Female drive plate22300is coupled to a spindle coupler (such as spindle coupler22100discussed below), which is coupled to the motor spindle. The male drive plate22200is placed on the secondary spindle22800and coupled as by driving elements such as the “Double-D” feature22460of spring body22410(FIG. 9), drive slot22420of spring body22410, and drive pin22600in secondary spindle22800.

Spindle coupler assembly22100must be subassembled using a spindle coupler22110and a bushing22150. Spindle coupler22110has a “double-D” drive surface22120, through hole22130, and drive slot22140. As better viewed inFIG. 7A, a cross section of yieldable spindle extension22000, through hole22130of spindle coupler22110opens into drive slot22140.

As shown inFIG. 6, bushing22150has a shoulder22160. As shown inFIG. 7A, bushing22150is pressed into through hole22130in drive slot22140of spindle coupler22110. Bushing shoulder22160bottoms against the bottom of drive slot22140. As shown inFIG. 6, the completed spindle coupler assembly22100can be placed on step22850of secondary spindle22800.

As shown inFIG. 6, all the yieldable spindle extension components described thusfar are fastened on to secondary spindle22800by chamfer washer22020and wire form retainer ring22010. Chamfer washer22020is placed on step22850past retainer groove22860of secondary spindle22800. As best seen inFIG. 7A, the chamfer22025of chamfer washer22020faces outward. Wire-form retainer ring22010is inserted into retainer groove22860.

As shown inFIG. 6, during disengagement of male drive plate22200and female drive plate22300(discussed below), chamfer washer22020and wire form retainer ring22010must hold spindle coupler22100and male drive plate22200in a horizontally stationary position. This is so that male drive plate22200can pressure a movable female drive plate22300away, disengaging female drive plate22300from male drive plate22200. In the process, the pressure on wire form retainer ring22010and chamfer washer22020from male drive plate22200is great. As shown inFIG. 7A, this pressure is handled by the chamfer22025of chamfer washer22020compressing wire form retainer ring22010into the outer wall of retainer groove22860. The capacity of wire form retainer ring22010to resist such pressure when in compression by chamfer washer22020is greater than that of a snap ring such as snap ring22650(FIG. 6).

As shown inFIG. 6, once that fixed guard24000and yieldable spindle extension22000are subassembled, tube23000, and motor21000must be assembled with them. Tube23000is fastened at end23010to a boss24030of fixed blade guard24000with three screws23020through three holes23030. Tube23000is fastened at an opposite end23040to motor21000at an opening21010. In the process, yieldable spindle extension22000is connected at drive slot22140of spindle coupler22110to spindle21040of saw motor21000. Tube23000is then fastened to motor21000with three screws21020running through three holes21030.

To complete the assembly of toe-kick saw20000, movable guard assembly25000is assembled onto fixed guard24000at cylindrical guard mount24040. Movable guard assembly25000includes a torsion spring25010, movable guard25040, and snap ring25080. Torsion spring25010is inserted onto cylindrical guard mount24040with a pointed end25020inserted within a hole (not shown) in fixed guard24000. Next movable guard25040is inserted onto cylindrical guard mount24040with a hooked end25030of torsion spring25010attached at torsion spring hole25060. Movable guard25040and torsion spring25010are fastened on cylindrical guard mount24040by inserting snap ring25080into snap ring groove24050of fixed guard24000.

A blade driver26000and blade27000are fastened onto secondary spindle22800. Secondary spindle22800has four drive flats22870forming a square drive surface on its distal end. When secondary spindle22800is fastened to fixed guard24000, these four drive flats22870extend from the edge of bushing24010in hole24020. In this position, the four drive flats22870form a square-shaped drive mount for blade driver26000. The square cutout26010of blade driver26000mounts on four drive flats22870in this position.

Blade driver26000has two drive nubs26020. Blade27000is placed on blade driver26000with drive nubs26020inserted into two drive holes27010in blade27000. Blade27000has a countersunk arbor27020. Pan head screw28000is put through countersunk arbor27020and tightened within tapped hole22880of secondary spindle22800to fasten blade27000. Pan head screw28000once fully tightened is flush within countersunk arbor27020of blade27000. This forms a flush outer surface27030on blade27000capable of cutting as close as possible to an inner wall of a toe space area. The plane of flush outer surface27030of blade27000is also flush with the outer surface24070of fixed guard24000, again to enable blade27000to cut as close as possible to the inner wall of a toe space area.

An explanation of how yieldable spindle extension22000can provide the torque required for normal cutting is as follows:FIG. 7is an assembled top view of yieldable spindle extension22000in isolation. Male drive plate22200and female drive plate22300are fully engaged as in normal cutting. A male nub22215illustrated inFIG. 7(typical of four male nubs22210in male drive plate22200as shown inFIG. 6) is formed with male angled surfaces22230,22231. As shown inFIG. 6, blade27000cuts by turning counterclockwise. Therefore, motor spindle21040, male drive plate22200, and the four male nubs22210turns counterclockwise. InFIG. 7, the direction of rotation is indicated by arrow “R”. Accordingly, male angled surface22230is the driving surface.

A female slot22315which is illustrated inFIG. 7(typical of four female slots22310in female drive plate22300as shown inFIG. 6) has female angled surfaces22330,22331. As female drive plate22300also turns counterclockwise, female angled surface22330is the driving surface.

As shown inFIG. 7, driving male angled surface22230works with the driving female angled surface22330based on the principle of an inclined plane. As torque increases, a driving male angled surface22230of the horizontally stationary male drive plate22200gradually “wedges” and separates the movable female drive plate22300because of the inclined plane of driving female angled surface22330.

FIG. 14is a top view of male drive plate22200showing greater detail of male nub22215and the male angled surfaces22230,22231ofFIG. 7.FIG. 14Ashows Section C-C of male nub22215and how each of male angled surfaces22230,22231form an angle of about 7.5 degrees in relation to a vertical plane that bisects male drive nub22215.

As further detailed in Section C-C ofFIG. 14A, male angled surfaces22230,22231transition to a flat top surface22234at two radiused edges22232,22233. Radius edge22232is a driving radiused edge. When male drive plate22200is disengaged from female drive plate22300(FIG. 6), the flat top surface22234of male nub22215slides with minimum resistance over a face22334(FIG. 15) of female drive plate22300(FIG. 7). As shown inFIG. 14, male nub22215is rounded at a back end22235.

As shown in Section C-C ofFIG. 14A, the male nub22215is formed at an about 7.5 degree angle in relation to a vertical plane that bisects male drive nub22215. This angle is as steep as practical from a forming standpoint such that a driving radiused edge22232will contact a driving female angle surface22230(FIG. 7) at a point that is as deep as possible within driving female angled surface22330(FIG. 7). As will be explained below, this gives both male drive plate22200(FIG. 7) and female drive plate22300(FIG. 7) the greatest long-term impact wear.

FIG. 15is a top view of the female drive plate22300ofFIG. 7showing greater detail of female slot22315and female angled surfaces22330,22331.FIG. 15Ashows Section D-D of female slot22315and how each of female angled drive surfaces22330,22331form an angle of about 25 degrees in relation to a vertical plane that bisects female slot22315. As will be explained below, this 25 degree angle is optimal for driving female angled surface22330to transmit enough torque for normal cutting. However, in the event of excess spindle torque, the 25 degree angle will also cause female drive plate22300to be pressured apart and disengage from male drive plate22200(FIG. 7). Beyond angled surfaces22330,22331, female slot22315is rounded at end22332to accept the rounded back end22235of male nub22215(FIG. 14).

Returning toFIGS. 7 and 7Aand the explanation of yieldable spindle extension22000under normal cutting conditions, the way that spring assembly22400holds female drive plate22300against male drive plate22200is as follows:FIG. 7Ashows cross section A-A of the yieldable spindle extension22000ofFIG. 7with two of the four balls22440within spring assembly22400being held in a ball groove22840by two of the four ball springs22450. Any outward motion of female drive plate22300away from male drive plate22200causes the four balls22440to be pressured up or elevate on the incline of ball groove22840. This incline will also be referred to herein as an inclined surface of secondary spindle22800. Any climbing motion of the four balls22440is in turn opposed by the four ball springs22450. So long as the four balls22440can remain in ball groove22840, female drive plate22300will remain in an engaged condition with male drive plate22200, and normal cutting can continue.

At excess spindle torque, female drive plate22300enters a disengaged condition with male drive plate22200as follows:FIG. 8is another top view of yieldable spindle extension22000, but with male drive plate22200and female drive plate22300disengaged.FIG. 8Ashows Section B-B of the cross section of the yieldable spindle extension22000ofFIG. 8with two of the four balls22440having exited the incline of ball groove22840. In this position, two of the four balls22440can no longer provide sufficient force or pressure to hold female drive plate22300in an engaged condition with male drive plate22200. The force generated by male drive plate22200causes female drive plate22300to enter a disengaged condition with male drive plate22200.

As shown inFIG. 6, when male drive plate22200and female drive plate22300enter a disengaged condition, motor spindle21040, spindle coupler assembly22100, and male drive plate22200can continue to turn. However, male drive plate22200can no longer transmit force through female drive plate22300. Secondary spindle22800and blade27000will temporarily stop from blade resistance at the saw kerf.

As shown inFIG. 8, as female drive plate22300becomes disengaged from male drive plate22200, return spring22620becomes compressed. Return spring22620reacts by pressuring spring assembly22400along with female drive plate22300back against male drive plate22200. Return spring22620may be weaker than spring assembly22400would be under normal cutting conditions in its ability to pressure female drive plate22300against male drive plate22200. Therefore, in the disengaged condition, the reduced pressure of return spring22620reduces the force of impact of the driving male angled surface22230(FIG. 7) of the four male nubs22210(FIG. 6) against the driving female angled surface22330(FIG. 7) of the four female slots22310(FIG. 6) of female drive plate22300(FIG. 7). This improves the impact wear of these components.

Thus, the return spring22620is a portion of a first biasing mechanism that presses the female drive plate22300towards the male drive plate22200. The return spring22620extends and collapses coaxially with the secondary spindle22800, i.e. in a direction that parallels the central axis of the secondary spindle22800. Other biasing mechanisms with similar characteristics may be devised, such as mechanisms having two or more springs operating in tandem or parallel, other types of springs besides coil springs, or another compressed member.

Further, the four balls22440, the ball springs22450, the ball groove22840on the secondary spindle22800and various supporting members of the spring assembly22400are a portion of a second biasing mechanism. This second biasing mechanism provides a force or a pressure that differs depending on the relative positions of the balls22440and the ball groove22840and corresponding relative positions of the male drive plate22200and the female drive plate22300. The ball groove22840is positioned on the secondary spindle22800so that the balls22440travel out of the groove and past the incline of the ball groove22840as the male drive plate22200and the female drive plate22300disengage. Thus, at least one ball22440urges the spring assembly22400at a first force against the female drive plate22300when the plates and the yieldable spindle extension22000are in an engaged or coupled condition. When the plates22300and22200and the yieldable spindle extension22000are in a disengaged or decoupled condition, the balls22440no longer engage the incline or other portion of the ball groove22840, and the second biasing mechanism urges the spring assembly22400against the female drive plate with a second force that is less than the first force. As the spring assembly22400is located opposite to the male drive plate22200, relative to the female drive plate22300, forces or pressures exerted by the spring assembly22400on the female drive plate22300press the female drive plate22300towards the male drive plate22200. Other ball receiving areas besides the ball groove22840may be devised, as may other biasing elements besides springs. The first and second biasing mechanisms may be expressed separately or combined into a single biasing mechanism.

As shown inFIG. 6, in the disengaged condition, male drive plate22200continues turning with motor spindle21040. At each quarter turn, the four male nubs22210will impact the four female slots22310. This impact dissipates the stored kinetic energy in the motor21000. However, the impact of a topmost radiused edge of the male drive nubs22210(such as driving radiused edge22232ofFIG. 14A) against a topmost edge of a female slot22310(such as a topmost edge22336of drive female angled surface22330ofFIG. 15A) gradually wears these surfaces until they lose their original angles.

The following is an explanation of how features of the male drive plate and female drive plate provide improved impact wear: As shown inFIG. 7, when male drive plate22200and female drive plate22300return to an engaged condition, it is preferred that a driving radiused edge22232(FIG. 14) of a driving male angled surface22230reengages the driving female angled surface22330at a point which is as deep as possible within the female slot22315. This ensures that the driving male angle surface22230will contact the driving female angled surface22330at a portion of driving female angled surface22330that is as least worn as possible, ensuring the components can transmit torque after being in as many disengaged conditions as possible.

As shown inFIG. 6, when the energy of the motor21000is sufficiently reduced, the four male nubs22210of male drive plate22200can reengage the four female slots22310of female drive plate22300. Normal cutting can resume.

In one alternative, the spring body and female drive plate can be combined into one part by means of powdered metal manufacturing methods.FIG. 10shows a female spring body30000having four female slots30010, four ball through holes30020, and a secondary spindle through hole30030. In this embodiment, dimension A connotes a female plate portion30040of the female spring body30000including four female slots30010. Dimension B connotes a female body portion30050including four ball through holes30020.FIG. 11shows a reverse angle of female spring body30000showing drive slot30060and secondary spindle through hole30030.

In another alternative, the male drive plate and spindle coupler can be combined into one part by means of powdered metal manufacturing methods.FIG. 12shows a male coupler40000having four male nubs40010. Male coupler has a secondary spindle through hole40020. Dimension C connotes the male plate portion40030. Dimension D connotes a male spindle coupler portion40040.

FIG. 13shows a reverse angle of male coupler40000showing a drive slot40050. The secondary spindle through hole40020(FIG. 12) opens into the middle of drive slot40050. This allows the extension of secondary spindle22800(FIG. 6) into drive slot40050, such that chamfer washer22020and wire form retainer ring22010(FIG. 6) can be mounted on secondary spindle22800(FIG. 6) within drive slot40050.