Patent Publication Number: US-2006011032-A1

Title: Guarded power circular saw assembly

Description:
BACKGROUND TO THE INVENTION  
      This invention relates to power circular saws and in particular to power circular saws that are pivotally moveable with respect to a base or work-piece support surface. These types of saws are sometimes referred to as “drop saws”.  
      The following discussion providing some background to the invention is intended to facilitate a better understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of the application.  
      Conventional drop saws provide little, or no protection to the operator from injury by the cuffing blade. While drop saws are often equipped with blade guards, these guards generally retract progressively as the saw is lowered towards the base or work-piece support surface. Exposure of the cutting blade makes it possible for the operator to inadvertently place his hands or fingers in the path of the saw.  
      It is an object of the invention to provide a guarded power circular saw assembly that reduces the likelihood of injury to the operator. More particularly, it is an object of the invention to keep the blade fully guarded during cutting operations.  
     SUMMARY OF THE INVENTION  
      According to the invention there is provided a guarded power saw assembly comprising:  
      a base for supporting a work-piece;  
      a circular saw blade;  
      a drive motor operatively connected to the saw blade;  
      a head supporting both the drive motor and the saw blade, with the head mounted to the base for movement through an arc about a primary pivot axis from a raised position above the base to a lowered position engaging the base;  
      an arm having a proximal end and a distal end, the proximal end mounted to the head for movement through an arc about a secondary pivot axis and the distal end supporting the circular saw blade;  
      a first locking means operatively interposed between the head and arm, the first locking means having a locked condition in which the arm is locked to the head so as to keep the saw blade enclosed by the head and an unlocked position in which the arm is pivotally movable with respect to the head to allow a portion of the blade to exit the head; and  
      a handle extending from the head and operatively connected to the arm,  
      wherein the handle is movable to first rotate the locked together head and arm with respect to the base towards the work-piece and, after the head has engaged the base, to then pivot the arm with respect to the head such that the saw blade exits the head to cut the work-piece.  
      Preferably the assembly further comprises a second locking means operatively interposed between the head and the base, the second locking means having an unlocked condition in which the head is pivotally movable with respect to the base and having a locked condition in which the head is locked to the base after the head has engaged the base and the arm has commenced rotation with respect to the head.  
      Preferably the assembly further comprises an intermediate locking means operatively interposed between the head and the arm, the intermediate locking means operable to lock the arm to the head when an attempt is made to lift the head away from engagement with the base,  
      wherein the intermediate locking means is operable after the first locking means has unlocked and prior to the second locking means engaging.  
      Preferably the base has a work-piece receiving recessed channel, the channel defined by a pair of spaced apart parallel first and second channel walls joined at their lower sides by a channel floor,  
      wherein the channel is longitudinally disposed perpendicular to the plane of the blade.  
      Preferably the assembly includes a work-piece clamp assembly and, after initial pre-adjustment of the clamp assembly, the clamp assembly is automatically actuated as the head is lowered towards the work-piece.  
      Preferably the clamp assembly comprises:  
      a pair of work-piece clamps each slidably mounted for movement across the channel through respective spaced apart slots in the first channel wall towards the second channel wall, the clamps spaced apart for clamping the work-piece on respective sides of the blade; and  
      a pair of respective clamp mechanisms, each operably connected between the head and its respective clamp,  
      whereby, as the head is lowered towards the base, the clamp mechanisms move the clamps across the channel to clamp a work-piece located in the channel.  
      Preferably each clamp mechanism comprises:  
      a linkage assembly operably connecting the clamp to the head; and  
      an adjusting means for adjusting the position of the clamp with respect to the linkage assembly in a direction across the channel,  
      wherein the adjusting means of a first of the clamp mechanisms allow its clamp to be moved to a pre-adjusted position adjacent or abutting the work-piece and the adjusting means of a second of the clamp mechanisms allow its clamp to be moved to a pre-adjusted position adjacent or abutting the work-piece or the second channel wall.  
      Preferably the assembly further comprises a work-piece clamp interlock means for preventing the commencement of sawing,  
      wherein the interlock means disengages to allow the commencement of sawing as the locked together head and arm rotates towards the work-piece only when both work-piece clamps have been moved to their pre-adjusted positions.  
      Preferably each linkage assembly comprises:  
      a first portion;  
      a second portion overlapping the first portion to define an overlapping portion; and  
      a compression spring mounted within the overlapping portion for compression when the pre-adjusted clamp is actuated to engage a work-piece or the second channel wall.  
      Preferably the interlock means comprises a blocking means for preventing the head being lowered sufficiently towards the base to allow disengagement of the first locking means, the blocking means having a blocking surface operably connected the second portion of the linkage assembly,  
      wherein, in use, if both clamps are not in their said pre-adjusted positions, the blocking surface moves to a position preventing the head being lowered sufficiently towards the base to allow disengagement of the first locking means.  
      Preferably the blocking means further comprises a pin positioned between the head and the blocking surface, the blocking pin held up by the blocking surface to prevent the head being lowered sufficiently towards the base to allow disengagement of the first locking means when either clamps is not in its said pre-adjusted position.  
      Preferably the head has a removable cover shaped to guard the blade.  
      Preferably the primary and secondary pivot axes are parallel and spaced apart.  
      Specific embodiments of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative, and are not meant to be restrictive of the scope of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION  
      Preferred embodiments of the invention are illustrated in the accompanying figures in which:  
       FIG. 1  shows a perspective view of a guarded power circular saw assembly according to a first embodiment of the invention with its head up and the blade still in its raised position.  
       FIG. 2  shows the saw of  FIG. 1  from underneath.  
       FIGS. 3, 4   a  and  4   b  show the saw of  FIG. 1  with its head being progressively lowered to the fully lowered position shown in  FIG. 4   a  and then the handle being lowered to the fully lowered position shown in  FIG. 4   b.    
       FIG. 5  shows the saw of  FIG. 1  with its blade cover removed to show the cutting blade.  
       FIGS. 6 and 7  show a detailed perspective view of a first locking means for locking the movement of the drive arm with respect to the head. These Figs show the first locking means in a fully locked condition.  
       FIGS. 8 and 9  show the first locking means of  FIGS. 6 and 7  in a partially locked condition.  
      FIGS.  10  to  13  show components of the saw assembly of  FIG. 1  in a partial cutaway cross-sectional view with the arm and head moving from a fully raised to a fully lowered position. These Figs show the progressive action of a second locking means.  
       FIG. 14  shows a perspective view of a guarded power circular saw assembly according to a second embodiment of the invention with its head down but the blade still in its raised position.  
       FIG. 15  shows the saw assembly of  FIG. 14  in the same configuration but with the blade cover removed.  
       FIG. 16  shows a sectional view of the saw assembly of  FIGS. 14 and 15  with the head raised ready for loading a work-piece.  
       FIG. 17  is a view of the saw assembly similar to  FIG. 16  but with the head lowered and the blade ready to cut.  
       FIG. 18  is a similar view to that of  FIG. 17  but with the head and blade lowered.  
       FIG. 19  is a similar view to that of  FIG. 15  enlarged with the drive arm belt cover removed showing the blade drive train and spindle lock.  
       FIG. 20  is a longitudinal section of the saw assembly of FIGS.  14  to  19  showing the blade cover still fitted and the spindle lock disengaged. 
    
    
     FIRST EMBODIMENT OF THE INVENTION  
      Referring to  FIG. 1 , a guarded power circular saw  10  according to a first embodiment of the invention has a base  30  with a channel  32  for receiving a work-piece extending therethrough and a head  20  for supporting the drive motor and the saw blade. The head  20  is mounted to base  30  for movement through an arc about a primary axis  22  (shown in  FIG. 10 ) from a raised position above the base  30  to a lowered position engaging the base  30 .  
      The drive motor is housed within motor housing  42 . A single handle  40  is provided for performing the functions of lowering the head  20  while semi-automatically applying a clamp to the work-piece and rotating the saw out from the head to cut the work-piece.  
      The head  20  includes a removable cover  28  to allow access and replacement of the saw blade  58  as shown in  FIG. 5 .  
      Referring to  FIGS. 4   a,    4   b  and  5 , the head  20  is shown in its down (lowered) position with its base plate  26  in contact with the base  30 .  FIG. 5  shows that handle  40  is operatively connected to a drive arm  50  by a spigot that extends from the drive motor housing  42  and these two components are pivotally mounted to the head base plate  26  by arm pivot bracket  44  for rotation about a secondary point axis  45  (best shown in  FIGS. 12 and 13 ). A drive arm return spring  49  is provided to bias the drive arm upwards.  
      From the position shown in  FIGS. 4   a  and  5 , an operator can further rotate handle  40  towards the work-piece within channel  32  to rotate arm  50  and hence blade  58  out through slot  27  within head base plate  26  to engage and cut the work-piece as shown in  FIG. 4   b.    
      Referring to  FIG. 1 , the saw assembly is shown in its at rest position with head raised. In this position channel  32  is exposed ready to receive a work-piece. In this position the saw blade  58  cannot exit the head  20 . More specifically the blade  58  cannot move out through the head base plate  26  through slot  27  (shown in  FIG. 5 ).  
      The mechanism for preventing the blade  58  moving prematurely with respect to the head  20  is as follows. Blade  58  is mounted on drive arm  50  (shown in  FIG. 5 ). A first latch member in the form of a retaining pin  53  extends from drive arm  50  (as shown in  FIG. 7 ) to form part of a first locking means operatively interposed between the head  20  and the arm  50 . The first locking means includes a primary latch arm  52   b  mounted to the head base plate  26  by a pivot  55  (refer  FIGS. 6 and 7 ). Latch arm  52   b  has a notch  59   b  (shown in  FIG. 9 ) that engages retaining pin  53  to lock the head  20  to the arm  50 . A latch arm release pin  54  is mounted to the base  30  as shown in  FIG. 3 . Once the head  20  has been lowered onto the base  30  by the operator depressing handle  40  as shown in  FIG. 4   a,  the latch arm release pin  54  holds an end of the latch arm  52   b  up against return spring  51 . In this position, the latch arm  52   b  is slightly rotated about pivot  55  such that notch  59  releases pin  53  so that further movement of handle  40  downwards towards the base  30  causes arm  50  and therefore blade  58  to exit the head  20  through slot  27  within base plate  26  towards the work-piece as is progressively shown by FIGS.  12  and then  13 .  
      The first locking means operatively interposed between the head  20  and the arm  50 , described above, ensures that the blade  58  cannot exit the head  20  until the head base plate  26  has closed over the base  30 . This ensures that the operator cannot inadvertently bring the blade into contact with the operators&#39; hands or fingers. While the locking means described above incorporates a primary latch arm  52   b  mounted to the head base plate  26  by a pivot mount  55 , other locking means may be used.  
      The first locking means described so far will not prevent an operator raising the head  20  after the head  20  has engaged the base  30  and the blade  58  has exited the head. It is possible that an operator may attempt to lift the head  20  during cutting of a work-piece. In order to prevent this, a second locking means is provided.  
      The second locking means is best shown in FIGS.  10  through to  13 . The second locking means is operatively interposed between the head  20  and the base  30 . The second locking means comprises an arcuate locking wall  102  depending from the arm  50  and a projection in the form of a pin  104  extending from the base  30 . The pin  104  and wall  102  are co-operably arranged to prevent raising of the head  20  during the cut. This co-operable arrangement is such that movement of the locking wall  102  about the primary pivot axis  22  cannot occur after the head is engaged the base  30  and the arm  50  has commenced rotation with respect to the head about the secondary pivot axis  45 . This is clearly evidenced in  FIG. 13  which shows that if an attempt is made to lift the head  20 , the arcuate locking wall  102  will foul the pin  104  because of the relative positioning of the primary axis  22  and the secondary axis  45 .  
      With the locking arrangements described thus far, there is a small angular delay during which the primary locking means has unlocked and before the secondary locking means has locked. In order to enable the movement of the head  20  with respect to the base  30  from the position shown in  FIG. 11  to the position shown in  FIG. 12 , it is necessary to provide clearance between the end of  103  of the arcuate locking wall  102  and the pin  104 . Referring now to  FIG. 12 , it can be seen that because of this clearance, movement of the arcuate locking wall with respect to the primary pivot axis  22  is not restrained during initial movement of the arm  50  downwards towards the base  30 . Thus, with the first locking means unlocked (since primary latch arm  52   b  has moved into the position shown in  FIG. 9  freeing pin  53  from recess  59   b ) it would be possible for the blade  58  to be exposed without the head  20  being locked to the base  30 . In order to deal with the possibility of the head  20  being lifted with respect to the base  30  during this delay period, an intermediate locking means having a supplementary latch arm  52   a  is provided as shown in FIGS.  7  to  9 .  
      If the head  20  is manually lifted away from the base  30  during the delay period and before the second locking means is operative, the supplementary latch arm  52   a  of the intermediate locking means will return to the locked position shown in  FIG. 9 . It is able to return to its locked position because of the extra length of the recess  59   a  (as compared to recess  59   b ). This movement of the supplementary latch arm  52   a  is caused by spring  51   a  as soon as the head is lifted a small amount away from arm lock release pin  54  mounted to the base  30  as shown in  FIG. 8 . As a result, exposure of the blade  58  is not possible even though the primary latch  52   b  is not effective once relative movement has occurred between the arm  50  and the head  20 .  
      The saw assembly  10  as described above can be used to cut various materials with the correct selection of saw blade  58 . It is envisaged that the saw assembly  10  will be particularly appropriate for cutting steel. When cutting steel with a conventional circular saw the operator faces the hazard of small sharp steel cuttings (or swarf) travelling at high velocity from the region of the blade. However with the above described saw assembly  10 , the blade is sufficiently enclosed to prevent swarf escaping.  
      When sawing a work-piece, it is generally desirable to have the work-piece firmly clamped to a support base. This is particularly the case when cutting steel as the effect of an unclamped work-piece twisting during cutting can be serious in terms of blade damage. Therefore in the above described first embodiment of the invention there is provided a channel  32  recessed within the base  30 . Referring to  FIGS. 1, 10  and  11  it can be seen that the channel  32  is defined by a pair of parallel spaced apart walls  34  and  36  joined at their lower sides by a channel floor  38 . Associated with the channel  32  are a pair of clamps  80  (shown in  FIG. 5 ) which semi-automatically clamp the work-piece in position within channel  32  as the head  20  is lowered.  
      A semi-automatic clamping system is illustrated in  FIGS. 1 and 2 . This system allows for multiple cutting of the same cross-section work-piece without any clamp adjustment required, once the clamps have been pre-adjusted for that particular cross-section.  
      Two identical clamp mechanisms are employed. One is provided for the work-piece and the other is provided for the off-cut. Each has an identical interlock assembly.  FIGS. 10 and 11  show movement of the clamp blades  83  through the first wall  34  of the channel  32  towards the second channel wall  36 .  
      Again, referring to  FIGS. 1 and 2 , each clamp  80  comprises a work-piece clamp body and an adjustment knob  81 . The adjustment knobs  81  allow loosening of the clamp bodies  82  so that they can be pushed against the work-piece in an initial position. This initial position, which can allow up to 1 mm of clearance, can then be locked by tightening knobs  81 .  
      An eccentric crank  85  mounted around the head pivot shaft  22 , together with a linkage assembly  83  connected to the clamp body  82  as shown in  FIG. 2  is provided to facilitate automatic clamping. The linkage assembly  83  is made up of two halves or portions  83   a  and  83   b  which are joined by a compression spring  86 . The spring  86  provides the “give” allowing full movement of the head  20  and the eccentric crank  85  to go “over centre” to lock the clamps  80  without the clamp force operating to raise the head again.  
      When correctly adjusted there is very little movement of the work-piece clamps  80  (only a few millimetres at most). Given that the movement of the first portion  83   b  will always be the same under the action of the eccentric crank  85 , where the movement of the work-piece clamp and hence the second portion  83   a  is small, there will be significant compression of the spring  86 , resulting in a significant clamping force being applied to the work-piece.  
      Interlocks are provided to prevent the cut proceeding (with possible blade damage) if the clamps  80  are not adequately pre-adjusted. The interlocking arrangement is shown in  FIGS. 2 and 3 . When a work-piece is adequately clamped, the compression spring  86  between the link halves  83   a  and  83   b  undergo significant compression and therefore, the link  83   a  moves only a small distance. Alternatively, if the work-piece is not adequately clamped, link half  83   a  will move a significant distance before compression of the spring  86  occurs.  
      Again, referring to  FIG. 2 , it can be seen that linear movement of the link  83   a  will result in rotary movement of cam  87 . When the clamp  80  is in the release position, blocking pin  88 , which is spring loaded upwards and shown in  FIG. 2  in a pushed down state, is free to be pushed down. If the link  83   a  moves forward to a significant extent, cam  87  will rotate to a position which prevents the descent of pin  88 . As explained above, link  83   a  will move forward a significant amount only when the work-piece is not adequately clamped due to failure to correctly pre-adjust the clamp position.  
      If pin  88  is locked up by the blocking surface provided by cam  87 , the projecting pins  88 , as shown in  FIG. 1 , prevent the head from being fully lowered, thus preventing the cut from proceeding. This configuration is illustrated in  FIG. 3  in which it can be clearly seen that pins  88  are preventing the further lowering of the head base plate  26 . In contrast, the fully lowered position, only achievable when a work-piece has been adequately clamped, is illustrated in  FIGS. 4   a  and  4   b.    
      In the event that only one clamp  80  can be used, for instance when trimming a short off-cut from a work-piece, the clamp  80  on the off-cut side is adjusted to abut the second wall  36  of channel  32  (rear face of the trough) instead of the work-piece. This will satisfy the interlock conditions and has the added advantage of closing a potential swarf leakage path.  
      The pair of clamps  80 , positioned either side of the blade  58 , ensure that both the work-piece and off-cut are securely held during cutting and until the saw blade  58  is retracted.  
     SECOND EMBODIMENT OF THE INVENTION  
      Referring to  FIG. 14 a  guarded power circular saw  10  according to a second embodiment of the invention has a base  30  with a channel  32  for receiving a work-piece extending therethrough and a head  20  for supporting the drive motor and the saw blade. The drive motor is housed within motor housing  42 . A single handle  40  is provided for performing the functions of lowering the head while automatically applying a clamp to the work-piece and rotating the saw out from the head to cut the work-piece.  
      The head  20  includes a removable cover  28  to allow access and replacement of the saw blade  58  as shown in  FIG. 15 .  
      Referring to  FIG. 15 , the head  20  is shown in its down (lowered) position with its base plate  26  in contact with the base  30 . Handle  40  is operatively connected to a drive arm  50  by a spigot that extends from the drive motor housing  42  and these two components are pivotally mounted to the head base plate  26  by arm pivot bracket  44 .  
      From the position shown in  FIG. 15 , an operator can further rotate handle  40  towards the work-piece within channel  32  to rotate arm  50  and hence blade  58  out through slot  27  within head base plate  26  to engage and cut the work-piece.  
      Referring to  FIG. 16 , the saw assembly is shown in its at rest position with head raised. In this position channel  32  is exposed ready to receive a work-piece. In this position the saw blade  58  cannot exit the head  20 . More specifically the blade  58  cannot move out through the head base plate  26  through slot  27  (shown in  FIG. 15 ). The mechanism for preventing the blade  58  moving with respect to the head  20  is as follows. Blade  58  is mounted on drive arm  50 . A arm lock retaining pin  53  extends from drive arm  50  to form part of a locking means operatively interposed between the head  20  and the arm  50 . The locking means is in the form of an arm lock  52  mounted to the head base plate  26  by a pivot  55 . Arm lock  52  has a notch  59  (shown in  FIG. 17 ) that engages retaining pin  53  to lock the head  20  to the arm  50 . An arm lock release pin  54  is mounted to the base  30  as shown in  FIG. 3 . Once the head  20  has been lowered onto the base  32  by the operator depressing handle  40  as shown in  FIG. 17 , the arm lock release pin  54  holds an end of the arm lock  52  up against return spring  51 . In this position, the arm  52  is slightly rotated about pivot  55  such that notch  59  releases pin  53  so that further movement of handle  40  downwards towards the base  30  causes arm  50  and therefore blade  58  to exit the head  20  through slot  27  within base plate  26  towards the work-piece as is progressively shown by FIGS.  17  and then  18 .  
      The locking means operatively interposed between the head and the arm, described above, ensures that the blade  58  cannot exit the head  20  until the head base plate  26  has closed over the base  30 . This ensures that the operator cannot inadvertently bring the blade into contact with the operators&#39; hands or fingers. While the locking means described above incorporates an arm lock  52  mounted to the head base plate  26  by a pivot mount  55 , other locking means may be used.  
      In addition to having a locking means operatively interposed between the head  20  and the arm  50 , the saw assembly  10  also has a means for locking the motor drive shaft  60  as shown in  FIG. 19 . The means for locking the drive shaft  60  is, in this embodiment, a drive shaft pinion  62  and a drive shaft lock  70 .  
      Drive shaft lock  70  is best shown in  FIG. 20 . It comprises a pinion stop  72  pivotally mounted at  75  to the head base plate  26 . A spring  74  is provided to bias the pinion stop  72  upwards into engagement with the drive shaft pinion  62 . In this position the drive shaft  60  cannot rotate and therefore, even if the drive motor is started, the saw will not rotate.  
      A drive shaft release finger extends from the underside of the head cover  28  (not shown). This finger depresses the drive shaft lock  70  in the region indicated arrow  76  in  FIG. 20  against the bias of spring  74 . Thus, in the position shown in  FIG. 20 , the pinion  62  and hence shaft  60  is free to rotate, but as soon as the head cover  28  is removed from the head, spring  74  lifts pinion stop  72  into engagement with pinion  62  to lock the shaft  60 .  
      The saw assembly  10  as described above can be used to cut various materials with the correct selection of saw blade  58 . It is envisaged that the saw assembly  10  will be particularly appropriate for cutting steel. When cutting steel with a conventional circular saw the operator faces the hazard of small sharp steel cuttings (or swarf) travelling at high velocity from the region of the blade. However with the above described saw assembly  10 , the blade is sufficiently enclosed to prevent swarf escaping. Instead the swarf is caught in a sliding draw  90  as shown in  FIG. 20 .  
      When sawing a work-piece, it is generally desirable to have the work-piece firmly clamped to a support base. This is particularly the case when cutting steel as the effect of an unclamped work-piece twisting during cutting can be serious in terms of blade damage. Therefore in the above described embodiment of the invention there is provided a pair of clamps  80  which automatically clamps the work-piece in position within channel  32  as the head  20  is lowered.  
      The progressive operation of one of the two clamps  80  can be seen most dearly in  FIGS. 16, 17  and  18 . In  FIG. 16 , the head  20  is in its fully raised position and clamp actuating leg  82  is positioned such that both clamp return spring  87  and clamp over travel springs  88  are in a free substantially uncompressed state. As the head  20  is lowered by pulling on handle  40  the clamp actuating leg  82  rotates thereby pulling clamp rod  85  which produces a force in over travel spring  88  which in turn acts on the inside of the end  89  of the clamp tube  84  to move the clamp tube  84  to the right of  FIG. 16  thereby moving clamp  80  in the same direction towards the work-piece in channel  32 .  
      Over travel spring  88  has a relatively high (shift) spring rate compared to that of clamp return spring  87 . Therefore, as the head  20  is lowered the clamp return spring  87  is first compressed. Once the work-piece is engaged by the clamp  80  the over travel spring  88  is compressed ensuring a substantial clamping force is applied to the work-piece. Because the clamp actuating leg  82  is connected to the head  20  rather that the handle  40 , no additional force is applied to the work-piece once the head is in its fully lowered position as shown in  FIG. 17 . That is as the handle  40  is lowered from its position shown in  FIG. 17  to its position shown in  FIG. 5  no additional force is applied to the work-piece. In other words, full clamping force is applied to the work-piece before the blade  58  is released from the head  20 .  
      The pair of clamps  80 , positioned either side of the blade  58 , ensure that both the work-piece and off-cut are securely held during cutting and until the saw blade  58  is retracted. The clamp  80  on the off-cut side has two parallel spaced apart blades. The first of these blades is positioned close to the saw blade  58  and the second is further along the channel  32  as shown clearly in  FIG. 14 . This enables both small and large off-cut pieces to be securely held during and immediately after cutting.  
      With both of the above-described embodiments, the saw blade is always positioned out of reach of the operator so as to eliminate, or at least reduce, the possibility of injury. Initially the saw blade is enclosed within the head in its raised position. Unlike other drop saws, there is no progressive exposure of the blade as the head is lowered towards the work-piece. It is not until the head engages the base that the locking means are released to allow the arm, and hence the blade, to be lowered with respect to the head. With the head engaged with the base, it is not possible for the operator to place his hands or fingers in the path of the blade.  
      For both of the above-described embodiments, the drive train between blade  58  and the drive shaft  60  is shown most clearly in  FIG. 19 . Drive shaft  60  has a pulley which drives a toothed belt  57 . An idler pulley  56  is provided between drive shaft  60  and the mount for blade  58  (not shown). A second toothed belt  57 ′ is provided to transmit power from the idler pulley  56  to the mount for blade  58 . Other drive train arrangements may be used. For instance direct drive through gearing may be used.  
      While the present invention has been described in terms of preferred embodiments in order to facilitate better understanding of the invention, it should be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope.