Abstract:
Various exemplary embodiments relate to a power tool including: a housing; a selectively actuable motor including a rotary output; a rotary drive element arranged in operative contact with the rotary output of the motor and including a drive pin; a removable blade assembly having a stationary blade and a moving blade in operative contact with the drive pin, wherein movement of the drive pin is translated into movement of the moving blade; a blade holder operable to selectively lock the stationary blade in an aligned position with respect to the housing; and a magnet positioned to contact the stationary blade when the stationary blade occupies the aligned position and hold the stationary blade in the aligned position. Various embodiments relate to a related transmission including similar features including a magnet positioned to contact a stationary blade when the stationary blade occupies the aligned position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to U.S. patent application Ser. No. 11/220,472, filed on Sep. 7, 2005, the entire disclosure of which is hereby incorporated herein for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    Various exemplary embodiments disclosed herein relate generally to power tools and, more particularly but not exclusively, to handheld power tools having removable blades for trimming and cutting vegetation. 
       BACKGROUND 
       [0003]    Known power tools having interchangeable blades are cumbersome and potentially dangerous to manipulate. For example, U.S. Pat. No. 3,959,878 to Irelan et al., discloses a convertible portable electric tool having interchangeable tool pieces. Each of the interchangeable tool pieces include two parts, a stationary element and a moving element, which are pivoted together at a pin. The stationary element includes a comb of teeth and, likewise, the moving element includes a comb of teeth. The rearward end of the moving element includes an elongated opening for receipt of a drive member. The drive member is rotated by a gear and the resulting circular movement oscillates the moving element about the pivot pin. As a result, the stationary element and the moving element lap one another to cut grass between the teeth upon oscillation of the moving element. 
         [0004]    Before attaching a tool piece assembly to the power housing, the user must first rotate the drive member to a predetermined position, such as a top dead center position. Similarly, the user must manually orient the moving element into a predetermined position with respect to the stationary element. After completing these preliminary steps, the drive member can be fitted within the elongated opening of the moving element upon bringing the stationary element into proper registry relative to the power housing. Once the stationary tool element is brought into proper registry and located over guide posts, additional means are provided to maintain the tool piece releasably secured against the housing. 
         [0005]    Accordingly, the attachment of tool pieces to a power housing as disclosed by Irelan et al. is a cumbersome process requiring various manual alignment steps to be performed by the user with respect to both the tool piece and the power housing. Generally, known power tools do not provide fool-proof mechanisms to allow easy, safe, and automatic alignment and attachment of cutting elements. Instead, users are required to spend time handling and adjusting cutting blades and other movable parts until precise alignments are achieved before a cutting element can be properly attached. Not only is this time consuming, but the user is also exposed to sharp cutting surfaces and powered moving parts in the process. 
       SUMMARY 
       [0006]    A brief summary of various exemplary embodiments is presented below. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections. 
         [0007]    Various embodiments relate to a power tool including: a housing; a selectively actuable motor including a rotary output; a rotary drive element arranged in operative contact with the rotary output of the motor and including a drive pin; a removable blade assembly having a stationary blade and a moving blade in operative contact with the drive pin, wherein movement of the drive pin is translated into movement of the moving blade; a blade holder operable to selectively lock the stationary blade in an aligned position with respect to the housing; and a magnet positioned to contact the stationary blade when the stationary blade occupies the aligned position and hold the stationary blade in the aligned position. 
         [0008]    Various embodiments relate to power tool transmission including: a housing; an interface operable to engage a rotary output of a motor; a gearbox operatively connected to the interface to transfer the rotary output to a drive plate including a drive pin; a blade holder operable to selectively lock a stationary blade of a removable blade assembly in an aligned position with respect to the housing; and a magnet positioned to contact the stationary blade when the stationary blade occupies the aligned position. 
         [0009]    Various embodiments are described wherein the magnet is movable with respect to the housing such that during attachment of the removable blade assembly to the housing, the magnet is permitted to extend toward the stationary blade under the force of magnetic attraction, away from an initial magnet position occupied by the magnet when the blade holder locks the stationary blade. 
         [0010]    Various embodiments are described wherein the magnet is spring biased to return to the initial magnet position. 
         [0011]    Various embodiments additionally include a magnet holder including an open frame configured to receive the magnet such that the magnet is exposed on at least one side of the frame. 
         [0012]    Various embodiments are described wherein the magnet includes at least one ledge created by an increase in at least one dimension of the magnet, wherein the at least one step is positioned to abut an interior face of the open frame and thereby resist passage of the magnet through the frame. 
         [0013]    Various embodiments are described wherein the magnet is at least 0.5 millimeters proud with respect to a blade-facing side of the open frame. 
         [0014]    Various embodiments additionally include an alignment feature matching a geometric feature of the removable blade assembly and the magnet at least partially pulls the stationary blade toward alignment with the alignment feature during attachment of the removable blade assembly to the housing. 
         [0015]    Various embodiments are described wherein the alignment feature includes at least one edge having a shape that is complementary to a portion of an outer edge of the removable blade assembly. 
         [0016]    Various embodiments are described wherein the blade holder is a removable cover and the at least one edge is a portion of a rail arranged to mate with a complementary hook of the removable cover. 
         [0017]    Various embodiments are described wherein the alignment feature includes at least one projection having a shape that is received within an interior recess disposed within the removable blade assembly. 
         [0018]    Various embodiments are described wherein the blade holder includes a removable cover configured to mate with the housing, wherein when the cover is mated with the housing while the blade in the aligned position, the blade is disposed between the cover and the housing and is prevented from moving out of the aligned position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein: 
           [0020]      FIG. 1  illustrates a perspective view of an exemplary power tool transmission; 
           [0021]      FIG. 2  illustrates a left side view of the power tool transmission; 
           [0022]      FIG. 3  illustrates a right side view of the power tool transmission; 
           [0023]      FIG. 4  illustrates a front end view of the power tool transmission; 
           [0024]      FIG. 5  illustrates a top view of the power tool transmission; 
           [0025]      FIG. 6  illustrates a rear end view of the power tool transmission; 
           [0026]      FIG. 7  illustrates a bottom view of the power tool transmission; 
           [0027]      FIG. 8  illustrates a bottom view of the power tool transmission with an exemplary bottom cover removed; 
           [0028]      FIG. 9  illustrates a perspective view of the power tool transmission assembled with an exemplary removable blade assembly; 
           [0029]      FIG. 10  illustrates a perspective view of a power tool transmission with the bottom cover removed and the removable blade assembly in an aligned position for locking the removable blade assembly to the power tool transmission; 
           [0030]      FIG. 11  illustrates a perspective view of an exemplary magnet assembly; 
           [0031]      FIG. 12  illustrates a second perspective view of the magnet assembly; 
           [0032]      FIG. 13  illustrates an exploded view of the magnet assembly; 
           [0033]      FIG. 14  illustrates a rear view of the power tool transmission in cross section taken across line A-A; 
           [0034]      FIG. 15  illustrates a cutaway perspective view of the power tool transmission; 
           [0035]      FIG. 16  illustrates a cutaway perspective view of the power tool transmission with an exemplary removable blade assembly in an aligned position; 
           [0036]      FIG. 17  illustrates a bottom view of an exemplary power unit for use with the power tool transmission; 
           [0037]      FIG. 18  illustrates a perspective view of the power tool transmission assembled with the removable blade assembly and the power unit; 
           [0038]      FIG. 19  illustrates a rear view of the power tool transmission in cross section taken across line B-B; 
           [0039]      FIG. 20  illustrates a top view of the power tool transmission in cross section taken across line C-C; 
           [0040]      FIG. 21  illustrates a rear view of the power tool transmission in cross section taken across line B-B as one of the buttons is held in an engaged position; 
           [0041]      FIG. 22  illustrates a cutaway perspective view of the power tool transmission showing the gearcase; 
           [0042]      FIG. 23  illustrates a cutaway perspective view of the power tool transmission showing a single stage reduction gear set; 
           [0043]      FIG. 24  illustrates a cutaway perspective view of the power tool transmission showing a dual stage reduction gear get; 
           [0044]      FIG. 25  illustrates a perspective view of an exemplary drive plate assembly; 
           [0045]      FIG. 26  illustrates an exploded view of the drive plate assembly; 
           [0046]      FIG. 27  illustrates a perspective view of an exemplary shrub blade assembly; and 
           [0047]      FIG. 28  illustrates a perspective view of an exemplary shear blade assembly. 
           [0048]    To facilitate understanding, identical reference numerals have been used to designate elements having substantially the same or similar structure or substantially the same or similar function. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    The description and drawings presented herein illustrate various principles. It will be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody these principles and are included within the scope of this disclosure. As used herein, the term, “or,” as used herein, refers to a non-exclusive or (i.e., and/or), unless otherwise indicated (e.g., “or else” or “or in the alternative”). Additionally, the various embodiments described herein are not necessarily mutually exclusive and may be combined to produce additional embodiments that incorporate the principles described herein. 
         [0050]      FIGS. 1-7  illustrate multiple views of an exemplary power tool transmission  100  for use in conjunction with a power unit not shown) and blade assembly  950  to form a power tool device for cutting and trimming vegetation. The power tool device may be a handheld unit for cutting grass, weeds, and other types of vegetation around a house or business, or any other location where unwanted growth is found. The power tool device may be part of a system or kit that allows a user to perform various different cutting functions using a common power tool transmission  100 . The power tool system may include a plurality of interchangeable blade carrier assemblies (not shown) that may be releasably attached to the power tool transmission  100 . 
         [0051]    The exemplary transmission  100  includes a housing  110 , a bottom cover  120 , and a gearcase  130 . In the example shown, the housing  110  is formed of a left housing shell  111  and a right housing shell  112  that are attached to each other via three housing screws  310   a - c . It will be appreciated that various alternative arrangements may be utilized to form the housing  110  including a unitary shell, additional shells or other parts, and additional or alternative attachment methods. 
         [0052]    The bottom cover  120  is shown attached to the bottom of the housing and is removable therefrom. For example, in various embodiments, the housing  110  includes a bottom cover release button  211  attached to a bottom cover hook  813  that engages with a receiving slot (not shown) on the top surface of the bottom cover  120  to hold the cover in place. In some such embodiments, the cover additionally includes four rail hooks (not shown) that engage rails  811   a - b ,  812   a - b  on the bottom surface of the housing  110 . As such, the bottom cover  120  may be removed from the housing  110  by first sliding the bottom cover release button  211  to disengage that bottom cover hook from the bottom cover  120  and then sliding the bottom cover  120  toward the rear of the transmission  100  to disengage the rail hooks from the housing rails. Various alternative structures for retaining the bottom cover  120  on and releasing the bottom cover  120  from the housing  110  will be apparent. 
         [0053]    The housing  110  and bottom cover  120  form a cavity therebetween that is accessible via a blade opening  121  formed by a separation between the housing  110  and the bottom cover  120  at the front of the transmission  100 . As shown in  FIG. 9 , during operation, a blade assembly  950  is held within this cavity and extends out of the blade opening  121  such that the blade is exposed for use. To ensure stability and proper alignment of the blade assembly, the housing  110  and gearcase  130  include a set of alignment features such as locating pins  831   a - d  and rear rails  811   a - b  for assisting in placement of a blade assembly in an aligned position suitable for locking the blade assembly in operative engagement with the transmission  100 . As used herein, the “aligned position” will be understood to refer to all configurations wherein the blade assembly  950  may be locked by the bottom cover  120  to extend out of the blade opening  121  and may be operatively engaged with the drive plate  832  to produce a cutting action. It will be understood that various additional or alternative alignment features may be employed to assist in placing the blade assembly  950  in the aligned position. 
         [0054]    The locating pins  831   a - d  extend downwardly from the gearcase  130  in four locations positioned to be complementary with the geometric features of the blade assembly  950  such as, for example, being received within interior recesses of the blade assembly  950 . As an example,  FIG. 10  shows the blade assembly  950  in the aligned position with respect to the open transmission  800 . The locating pins  831   a - d  are received through complementary pin holes  1051   a - d . As such, the blade assembly  950  is held in the aligned position and may be removed only by first sliding the locating pins  831   a - d  out of engagement with the pin holes  1051   a - d . Various alternative arrangements may be utilized. For example, fewer or additional locating pins may be provided, pin recesses that are not throughholes may be formed on the upper surface of the blade assembly  950  for receiving locating pins  931   a - d , or one or more locating pins  831   a - d  may engage a partial pin hole that is formed in the outer edge of the blade assembly  950  such as one of the partial holes  1052   a - b  shown in  FIG. 10 . 
         [0055]    As another example of an alignment feature, the rear rails  811   a - b  perform a secondary function. In addition to receiving the rail hooks of the bottom cover  120  for holding the bottom cover  120  in place, the rear rails  811   a - b  form an edge that is complementary to the rear edge of the blade assembly  950 . As such, an operator may easily place the blade in the aligned position by abutting the read edge of the blade assembly  950  against the edge formed by the rear rails  811   a - b  and then lowering the blade assembly  950  onto the locating pins  831   a - d . As shown in  FIG. 10 , additional back edges  1011   a - b  may project from the housing  110  between the rear rails  811   a - b  to provide additional area against which the blade assembly abuts. The back edges  1011   a - b  may form a hook that receives the back edge of the blade assembly  950  as shown, or may simply project downward to provide a backstop for the blade assembly  950 . Various modifications may be apparent. In various embodiments, edges may be placed virtually anywhere on the lower surface of the housing  110  or gearcase  130  in a position that abuts the blade assembly  950  when occupying the aligned position. For example, projections may be arranged to form a full footprint that abuts every edge of the blade assembly that is received within the transmission  100  when in the aligned position. 
         [0056]    While various alignment features have been described as facilitating placement of the blade assembly  950  in the proper aligned position with respect to the open transmission  800  prior to locking of the blade by the bottom cover  120 , the blade may nonetheless move out of the aligned position prior to locking. For example, where the blade assembly  950  is sufficiently long and the open transmission  800  is held upside down for the operator to attach the bottom cover  120 , the weight of the blade may cause the blade assembly  950  to pivot at the front edge of the housing and thereby move the back edge of the blade assembly  950  outward away from the transmision and out of engagement with the locating pins  831   a - d , rear rails  811   a - b , back edges  1011   a - b , or other alignment features that may be employed in construction of the transmission. Possible movements out of the aligned position such as this may complicate the manual procedure necessary for the operator in locking the blade assembly in place. 
         [0057]    To help counteract such movements out of the aligned position, the housing  110  carries a magnet assembly  840 . The magnet assembly  840  carries a magnet that attracts the blade assembly toward the open transmission  800  and into alignment with the various alignment features  811   a - b ,  831   a - d ,  1011   a - b  employed. Thereafter, the magnet resists various forces that would work to move the blade assembly  950  out of the aligned position such as, for example, the force of gravity acting on the unsupported the blade assembly  950  when the open transmission  800  is held upside down. With the blade assembly  950  thus held in the aligned position, the process of reattaching the bottom cover  120  to the open transmission  800  is simplified because the operator need not manually hold the blade assembly  950  in place. 
         [0058]    As shown in further detail in  FIGS. 11-13 , the exemplary magnet assembly  840  includes two components: a magnet  1141  and a magnet holder  1142 . The magnet  1141  may be virtually any magnet sufficiently strong to provide at least some magnetic attraction to the blade assembly without interfering with the operation of the drive plate  832  or other moving metallic parts. In various embodiments, the magnet  1141  is a rare earth magnet. 
         [0059]    The magnet holder  1142  is formed of plastic or other material as an open frame such that a bottom surface of the magnet  1141  is exposed on the underside of the magnet holder  1142 , as is clearly seen in  FIG. 12 . In various embodiments the magnet  1141  is held proud beyond the lower surface of the magnet holder  1142 . In other words, the bottom surface of the magnet  1141  may be positioned lower than the bottom surface of the magnet holder  1142 . For example, magnet  1141  may be between 0.5 mm and 1.0 mm proud beyond the magnet holder  1142 . To enable magnet proudness without the magnet  1141  falling through the frame of the magnet holder  1142 , the magnet  1141  may be specially shaped with at least one magnet ledge  1341   a - b , as may be seen in  FIG. 13 . More specifically, in the example of  FIG. 13 , the magnet  1141  increases in the width dimension moving from bottom to top to produce the two magnet ledges  1341   a - b  opposite each other. The magnet ledges  1341   a - b  engage the inner surface of the magnet holder  1142  to prevent passage of the magnet  1141  therethrough. Various modifications will be apparent. For example, magnet ledges may be provided on all four sides such that a single magnet ledge (not shown) encircles the magnet. As another alternative, the dimension increase may be gradual, such that the magnet ledge is a sloping face. In some such embodiments, the magnet holder  1142  may be provided with a complementary sloping interior face. 
         [0060]    The magnet holder  1142  may include additional structures. As shown in  FIG. 11 , the magnet holder also includes two resilient snap tabs  1143   a - b  for holding the magnet  1141  in place in the magnet holder  1142 . Various alternatives to snap tabs  1143   a - b  will be apparent. For example, the magnet  1141  may be held in place with adhesive, or the magnet holder  1142  may be closed by forming an additional surface of material on top of the magnet  1141 . 
         [0061]    In various embodiments, the magnet assembly  840  may be movable with respect to the housing  110 . For example, the magnet assembly  840  may be held in a free-floating arrangement with respect to the housing  110 . To provide some degree of constraint to the movement allowed of the magnet assembly  840 , a rail interface may be formed between the magnet assembly  840  and housing  110 . As such, the magnet holder  1142  also includes two magnet holder rails  1144   a - b . As can be seen in  FIG. 14 , which shows a cross section of the transmission  100  taken across line A-A of  FIG. 5 , the housing  110  includes two interior magnet holder rail slots  1411   a - b  which receive the magnet holder rails  1144   a - b , respectively. Through this arrangement, the magnet assembly  840  is constrained to up and down movement. The magnet holder rail slots  1411   a - b  may also include closed ends and thereby prevent the magnet assembly  840  from being pulled out of engagement with the housing  110 . In some such embodiments, the closed ends of the magnet holder rail slots  1411   a - b  may be positioned to allow the magnet assembly  840  to extend for some distance beyond the lower surface of the housing  110 . As such, during attachment of the blade assembly  950  to the open housing  850 , the magnet  1141  may extend outward a distance to meet the blade assembly  950 , as can be seen in  FIGS. 15-16 . Then, as the blade assembly  950  is moved into full engagement with any alignment features, the magnet is pushed back into the housing. Such an arrangement is useful, for example, where the drive pin  832  is not yet aligned for engagement with a yoke of the blade assembly  950  and, as such, may not permit or may otherwise resist full engagement of the blade assembly  950  with the alignment features. 
         [0062]    In various embodiments, the magnet assembly  840  may be adapted to further assist in pulling the blade assembly  950  into alignment with the alignment features, beyond what is already accomplished by the magnetic attraction. For example, the magnet assembly  840  may be spring biased to return to its initial position in the housing  110  after extending outward to meet the blade assembly  950 . In some such embodiments and as can be seen in  FIGS. 15-16 , the magnet holder  1142  may be attached to for formed with one or more magnet holder springs  1541 . As shown, the magnet holder spring  1541  is a flat spring that is positioned to compress as the magnet assembly  840  moves out of the housing  110  and thereby bias the magnet assembly  840  to move back into the housing  110 . Various alternative arrangements for biasing the magnet assembly  840  to move into the housing  110  will be apparent. 
         [0063]    Further, various alternative arrangements for mounting a magnet within the housing  110  will be apparent. For example, the magnet assembly  840  or just the magnet  1141  may be fixedly formed within or otherwise attached to the housing  110 . In some such embodiments, the magnet  1141  may be configured to be proud below the lower surface of the housing  110  such as, for example, 0.5 mm to 1.0 mm proud. 
         [0064]    Turning back to  FIGS. 1-6 , the gearcase  130  is partially disposed within the housing  110  and is arranged to transfer rotary motion imparted by a power unit  1760  (see  FIG. 18 ) to the drive plate  832  arranged at the bottom of the housing  110  which, in turn, engages the blade assembly  950  to produce a cutting action. The gearcase  130  includes a gearcase cover  131  that is attached to the gearcase body via gearcase cover screws  132   a - d . Near its top, the gearcase cover  131  forms a power unit interface  133  that is complementary to an engaging structure on a power unit. As shown in the examples of the figures, the power unit interface  133  may be an interface for adrill. However, it will be appreciated that various alternative power unit interfaces may be used for enabling use of the transmission  100  with other power units. The gearcase cover  131  also allows passage of a power unit coupler  134  from the interior of the gearcase  130 . As will be explained in greater detail below, the power unit coupler  134  is structured to receive rotary motion imparted by an attached power unit. 
         [0065]    As shown in  FIG. 17 , the power unit  1760  may include multiple structures that are complementary to the transmission  100  such as a transmission interface  1763  structured to engage the power unit interface  133  and a transmission coupler  1764  structured to engage and impart rotary motion to the power unit coupler  134  during operation as part of the assembled power tool  1800  shown in  FIG. 18 . In various embodiments, the power unit  1760  may be a power unit of a drill. The power unit  1760  is also shown with two t-slot rails  1761 - a - b  that, when assembled with a transmission  100 , engage two slots  532   a - b  formed by a t-slot piece  531  that extends from the rear of the gearcase  130 , as can be seen in  FIGS. 5-6 . 
         [0066]    In various embodiments, the power unit  1760  is provided with a safety feature to prevent unintentional or accidental activation of the power tool  1800 . Specifically, the power unit  1760  may maintain the power unit trigger  1762  in a locked state until a safety button is pressed  1763 . The safety button  1763  is positioned such that it is covered by the transmission  100  when the power tool  1800  is assembled and, as such, the safety button  1763  is not manually accessible. Instead, the transmission  100  is provided with structure for the user to indirectly press the safety button  1763  when operation of the power tool  1800  is desired. Specifically, the transmission  100  includes a left side lock button  170   a  and a right side lock button  170   b,  either of which may be slid away from the bottom cover  120  by the operator to press the safety button  1763  and thereby unlock the power unit trigger  1762  for operation of the power tool  1800 . 
         [0067]    As can be best seen in the example of  FIGS. 19-20 , the lock buttons  170   a - b  are received within respective lock button tracks  1911   a - b  such that the lock buttons  170   a - b  may slide up and down with respect to the housing  110 . Similarly, two L-shaped lock arms  1971   a - b  are received within respective lock arm tracks  1912   a - b . A pair of lock screws  1972   a - b  attach respective lock arms  1971   a - b  to respective lock buttons  170   a - b  by extending through a respective slot-shaped lock screw passage  1913 . a - b . The lock screw passages  1913   a - b  connect the lock button tracks  1911   a  to the associated lock arm tracks  1912   a - b  but are not wide enough to allow passage of the lock button  170   a - b  or lock arm  1971   a - b  therethrough. 
         [0068]    The lock arms  1971  a-b both extend underneath a lock plate  1973  having an upwardly extending lock plate finger  1974 . The lock plate finger  1974  is received within a t-slot piece channel  1931 . The T-slot piece channel  1931  extends entirely through the t-slot piece  531  and is aligned with the safety button  1763  of the power unit  1760  when the power tool  1800  is assembled. The lock plate  1973  is movable up and down with respect to the housing  110  and gearcase  130  along gearcase rails  2013   a - b . Specifically, as can be seen in the cross-section across line C-C, the lock plate  1973  includes two lock plate slots  2071   a - b  that receive the gearcase rails  2031   a - b , respectively. The lock plate  1973  is downwardly biased by a lock plate spring  1975  that is disposed between the lock plate  1973  and the t-slot piece  531 . 
         [0069]    As illustrated in the cross section across line B-B in  FIG. 21 , when the operator wishes to begin use of the power tool  1800 , the user slides one of the lock buttons  170   a - b  upward which also moves the attached lock arm  1971   a - b  upward. The lock arm  1971   a - b , in turn, pushes the lock plate  1973  upward, such that the lock plate finger  1974  extends out of the t-slot piece channel  1931  and engages the safety button  1763  on the power unit  1760  to unlock the power unit trigger  1762 . The operator may then, before releasing the lock button  170   a - b , squeeze the power unit trigger  1762  to engage the motor and initiate operation of the device. Thereafter, the operator may release the lock button  170   a - b . The lock plate spring  1975 , no longer opposed by the operator holding the lock button  170   a - b , pushes the lock plate  1973  downward, which pushes the raised lock arm  1971   a - b  and attached lock button  710   a - b  downward, back to the initial position. It will be appreciated that, because both lock arms  1971   a - b  are disposed underneath the same lock plate  1973 , the operator may actuate either of the lock buttons  170   a - b  as convenient to unlock the power unit trigger  1762 . 
         [0070]    As seen in  FIG. 22 , the housing  110  encloses a gear case body  2231  that houses a gear set for translating rotary motion input by the power unit  1760  at the power unit coupler  134  into a cutting motion of the blade assembly  950  via the drive plate  832 . Various gear sets may be used for such translation. For example, a single stage reduction gear set  2331  as shown in  FIG. 23  or a dual stage reduction gear set  2431  as shown in  FIG. 24  may be used. In various embodiments, the single stage reduction gear set  2331  or the two stages of the dual stage reduction gear set  2431  may each be planetary gear systems, the construction of which will be understood by those of skill in the art. 
         [0071]    In various embodiments, the power unit  1760  may be configured to provide an input speed of about 28,000 RPM. In some embodiments, a gear set, such as the dual stage reduction gear set  2431 , may have a gear ratio of 20:1 and may therefore provide an output speed of about 1,400 RPM to the drive plate  832  when attached to an 28,000 RPM input. In other embodiments, a gear set, such as the single stage reduction gear set  2331 , may have a gear ratio of 10:1 and may therefore provide an output speed of about 2,800 RPM when attached to an 28,000 RPM input. It will be apparent that various alternative arrangements may be utilized to provide such output speeds. For example, the gear ratio of the single stage reduction gear set  2331 , the dual stage reduction gear set  2431 , or another alternative gear set (not shown) may be tuned to provide an output within the range of 2,000 to 3,200 RPM or within the range of 2,000 to 2,800 RPM. 
         [0072]    To enable high speed output, the gear set may connect to a specially balanced drive plate assembly  2531 . As shown in  FIGS. 25-26 , the drive plate assembly  2531  includes a drive plate  832  attached to the end of a drive shaft  2532  that is rotated by the gear set. The drive plate  832  includes an off-center drive plate aperture  2631  sized to receive a drive spring  2632 , a drive pin  833 , and a headed drive pin  2533 . The drive pin  833  and headed drive pin  2533  may be retracted, under force, within the drive plate aperture  2631  and, upon release of the force, will be moved back to the initial position by the drive spring  2632 . To balance the drive plate  832 , a drive plate counterweight  2534  is provided opposite the drive plate aperture  2631 . The drive plate counterweight  2534  is of a mass selected to match or otherwise offset the mass added to the drive plate by the drive plate aperture  2631 , drive spring  2632 , drive pin  833 , and headed drive pin  2533 , thereby providing a more balanced drive plate assembly  2531  that may be operated at higher speeds. 
         [0073]    Exemplary blade assemblies  2750 ,  2850  for use as a blade assembly  950  are detailed in  FIGS. 27-28 . For example, in  FIG. 27 , a single action shrub blade assembly  2750  is shown as including a stationary shrub blade  2751  slideably attached to a moving shrub blade  2752  via blade assembly rivets  2753   a - b . The stationary shrub blade  2751  and moving shrub blade  2752  both include two rows of laterally extending teeth that cooperate to provide a cutting action. The moving shrub blade  2752  also includes a shrub yoke  2754  that receives the drive pin  833  of the drive plate assembly  2531 . Due to the horizontal orientation of the shrub yoke  2754 , the revolving motion of the drive pin  833  is translated to linear reciprocating motion of the moving shrub blade  2752  with respect to the stationary shrub blade  2751 . 
         [0074]    As another example, in  FIG. 28 , a single action shear blade assembly  2850  is shown as including a stationary shear blade  2851  pivotally attached to a moving shear blade  2752  via blade assembly rivet  2854 . The stationary shear blade  2851  and moving shear blade  2852  both a row of forward extending teeth that cooperate to provide a cutting action. The moving shear blade  2852  also includes a shear yoke  2854  that receives the drive pin  833  of the drive plate assembly  2531 . Due to the vertical orientation of the shear yoke  2854 , the revolving motion of the drive pin  833  is translated to pivotal reciprocating motion of the moving shear blade  2852  with respect to the stationary shear blade  2851 . 
         [0075]    It will be appreciated that the rear portions of the stationary shrub blade  2751  and stationary shear blade  2851  are provided with similar or identical geometric features such as similarly shaped outer edges and similarly positioned pin holes  1051   a - d . As such, the two blade assemblies  2750 ,  2850  may be interchangeably attached to the transmission  100  as desired by the operator. It will be apparent that enabling attachment of additional blade assembles may be similarly achieved by providing similar or identical geometric features for cooperation with alignment features on the transmission  100 . 
         [0076]    Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be effected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.