Patent Publication Number: US-2004040426-A1

Title: Miter saw with improved safety system

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     [0001] This application claims the benefit of and priority from the following U.S. Provisional Patent Application, the disclosure of which is herein incorporated by reference: Serial No. 60/406,138, filed Aug. 27, 2002. 
    
    
     
       FIELD  
       [0002] The present invention relates to miter saws, and more particularly to a miter saw with a high-speed safety system.  
       BACKGROUND  
       [0003] Miter saws are a type of woodworking machinery used to cut workpieces of wood, plastic and other materials. Miter saws typically include a base upon which workpieces are placed and include a circular saw blade mounted on a pivot arm. A person uses a miter saw by placing a workpiece on the base beneath the upraised blade and then bringing the blade down via the pivot arm to cut the workpiece. Miter saws present a risk of injury to users because the spinning blade is often exposed when in use. Furthermore, users often use their hands to position and support workpieces beneath the blade, which increases the chance that an injury will occur.  
       [0004] The present invention provides miter saws with improved safety systems that are adapted to detect the occurrence of one or more dangerous, or triggering, conditions during use, such as when a user&#39;s body contacts a spinning saw blade. When such a condition occurs, a safety system is actuated to limit or even prevent injury to the user.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0005]FIG. 1 is a schematic block diagram of a miter saw with a fast-acting safety system according to the present invention.  
     [0006]FIG. 2 is a schematic diagram of an exemplary safety system configured to stop the miter saw blade.  
     [0007]FIG. 3 is a schematic side elevation of an exemplary miter saw having a safety system configured to stop both the rotation and downward movement of the blade.  
     [0008]FIG. 4 is similar to FIG. 3 but shows the pivot arm assembly pivoted downward into the cutting zone.  
     [0009]FIG. 5 is a partial top plan view of the miter saw of FIG. 3, with a portion of the housing cut away to show the brake pawl.  
     [0010]FIG. 6 is a schematic side elevation of another exemplary miter saw having an alternative safety system configured to stop both the rotation and downward movement of the blade.  
     [0011]FIG. 7 is similar to FIG. 6 but shows the pivot arm assembly pivoted upward away from the cutting zone.  
     [0012]FIG. 8 is a partial top plan view of the miter saw of FIG. 6, with a portion of the housing cut away to show the brake mechanism.  
     [0013]FIG. 9 is similar to FIG. 6 but shows the radial support arms uncoupled from the brace member to pivot the cartridge below the housing for replacement.  
    
    
     DETAILED DESCRIPTION  
     [0014] A miter saw according to the present invention is shown schematically in FIG. 1 and indicated generally at  10 . Miter saw  10  may be any of a variety of different types and configurations of miter saw adapted for cutting workpieces, such as wood, plastic, etc. Miter saw  10  includes an operative structure  12  having a cutting tool  14  and a motor assembly  16  adapted to drive the cutting tool. Miter saw  10  also includes a safety system  18  configured to minimize the potential of a serious injury to a person using miter saw  10 . Safety system  18  is adapted to detect the occurrence of one or more dangerous, or triggering, conditions during use of miter saw  10 . If such a dangerous condition is detected, safety system  18  is adapted to engage operative structure  12  to limit any injury to the user caused by the dangerous condition.  
     [0015] Miter saw  10  also includes a suitable power source  20  to provide power to operative structure  12  and safety system  18 . Power source  20  may be an external power source such as line current, or an internal power source such as a battery. Alternatively, power source  20  may include a combination of both external and internal power sources. Furthermore, power source  20  may include two or more separate power sources, each adapted to power different portions of miter saw  10 .  
     [0016] It will be appreciated that operative structure  12  may take any one of many different forms, depending on the type of miter saw  10 . As will be described in more detail below, operative structure  12  typically takes the form of an arm pivotally coupled to a base. Cutting tool  14  is mounted on the arm and pivotal toward a workpiece supported by the base. Alternatively, the arm may be both pivotally and slidably coupled to the base.  
     [0017] Motor assembly  16  includes one or more motors adapted to drive cutting tool  14 . The motors may be either directly or indirectly coupled to the cutting tool. Typically, motor assembly  16  is mounted on the pivot arm and directly coupled to the cutting tool.  
     [0018] Safety system  18  includes a detection subsystem  22 , a reaction subsystem  24  and a control subsystem  26 . Control subsystem  26  may be adapted to receive inputs from a variety of sources including detection subsystem  22 , reaction subsystem  24 , operative structure  12  and motor assembly  16 . The control subsystem may also include one or more sensors adapted to monitor selected parameters of miter saw  10 . In addition, control subsystem  26  typically includes one or more instruments operable by a user to control the miter saw. The control subsystem is configured to control miter saw  10  in response to the inputs it receives.  
     [0019] Detection subsystem  22  is configured to detect one or more dangerous, or triggering, conditions during use of miter saw  10 . For example, the detection subsystem may be configured to detect that a portion of the user&#39;s body is dangerously close to, or in contact with, a portion of cutting tool  14 . As another example, the detection subsystem may be configured to detect the rapid movement of a workpiece due to kickback by the cutting tool, as is described in U.S. Provisional Patent Application Serial No. 60/182,866, filed Feb. 16, 2000 and U.S. patent application Ser. No. 09/676,190, filed Sep. 29, 2000, the disclosures of which are herein incorporated by reference. In some embodiments, detection subsystem  22  may inform control subsystem  26  of the dangerous condition, which then activates reaction subsystem  24 . In other embodiments, the detection subsystem may be adapted to activate the reaction subsystem directly.  
     [0020] Once activated in response to a dangerous condition, reaction subsystem  24  is configured to engage operative structure  12  quickly to prevent serious injury to the user. It will be appreciated that the particular action to be taken by reaction subsystem  24  will vary depending on the type of miter saw  10  and/or the dangerous condition that is detected. For example, reaction subsystem  24  may be configured to do one or more of the following: stop the movement of cutting tool  14 , disconnect motor assembly  16  from power source  20 , place a barrier between the cutting tool and the user, retract the cutting tool from its operating position, etc. The reaction subsystem may be configured to take a combination of steps to protect the user from serious injury. Placement of a barrier between the cutting tool and teeth is described in more detail in U.S. Provisional Patent Application Serial No. 60/225,206, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,226, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference. Retraction of the cutting tool from its operating position is described in more detail in U.S. Provisional Patent Application Serial No. 60/225,089, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,242, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference.  
     [0021] The configuration of reaction subsystem  24  typically will vary depending on which action(s) are taken. In the exemplary embodiment depicted in FIG. 1, reaction subsystem  24  is configured to stop the movement of cutting tool  14  and includes a brake mechanism  28 , a biasing mechanism  30 , a restraining mechanism  32 , and a release mechanism  34 . Brake mechanism  28  is adapted to engage operative structure  12  under the urging of biasing mechanism  30 . During normal operation of miter saw  10 , restraining mechanism  32  holds the brake mechanism out of engagement with the operative structure. However, upon receipt of an activation signal by reaction subsystem  24 , the brake mechanism is released from the restraining mechanism by release mechanism  34 , whereupon, the brake mechanism quickly engages at least a portion of the operative structure to bring the cutting tool to a stop.  
     [0022] It will be appreciated by those of skill in the art that the exemplary embodiment depicted in FIG. 1 and described above may be implemented in a variety of ways depending on the type and configuration of operative structure  12 . Turning attention to FIG. 2, one example of the many possible implementations of miter saw  10  includes a cutting tool  14  in the form of a circular blade  40  mounted on a rotating shaft or arbor  42 . Blade  40  includes a plurality of cutting teeth (not shown) disposed around the outer edge of the blade. As described in more detail below, brake mechanism  28  is adapted to engage the teeth of blade  40  and stop rotation of the blade.  
     [0023] In the exemplary implementation, detection subsystem  22  is adapted to detect the dangerous condition of the user coming into contact with blade  40 . The detection subsystem includes a sensor assembly, such as contact detection plates  44  and  46 , capacitively coupled to blade  40  to detect any contact between the user&#39;s body and the blade. Typically, the blade, or some larger portion of cutting tool  14  is electrically isolated from the remainder of miter saw  10 . Alternatively, detection subsystem  22  may include a different sensor assembly configured to detect contact in other ways, such as optically, resistively, etc. In any event, the detection subsystem is adapted to transmit a signal to control subsystem  26  when contact between the user and the blade is detected. Various exemplary embodiments and implementations of detection subsystem  22  are described in more detail in U.S. Provisional Patent Application Serial No. 60/225,200, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,426, filed Aug. 13, 2001, U.S. Provisional Patent Application Serial No. 60/225,211, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,221, filed Aug. 13, 2001, U.S. Provisional Patent Application Serial No. 60/270,011, filed Feb. 20, 2001, and U.S. patent application Ser. No. 10/053,390, filed Jan. 16, 2002, the disclosures of which are herein incorporated by reference.  
     [0024] Control subsystem  26  includes one or more instruments  48  that are operable by a user to control the motion of blade  40 . Instruments  48  may include start/stop switches, speed controls, direction controls, etc. Control subsystem  26  also includes a logic controller  50  connected to receive the user&#39;s inputs via instruments  48 . Logic controller  50  is also connected to receive a contact detection signal from detection subsystem  22 . Further, the logic controller may be configured to receive inputs from other sources (not shown) such as blade motion sensors, workpiece sensors, etc. In any event, the logic controller is configured to control operative structure  12  in response to the user&#39;s inputs through instruments  48 . However, upon receipt of a contact detection signal from detection subsystem  22 , the logic controller overrides the control inputs from the user and activates reaction subsystem  24  to stop the motion of the blade. Various exemplary embodiments and implementations of control subsystem  26  are described in more detail in U.S. Provisional Patent Application Serial No. 60/225,059, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,237, filed Aug. 13, 2001, U.S. Provisional Patent Application Serial No. 60/225,094, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,234, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference.  
     [0025] In the exemplary implementation shown in FIG. 2, brake mechanism  28  includes a pawl  60  mounted adjacent the edge of blade  40  and selectively moveable to engage and grip the teeth of the blade. Pawl  60  may be constructed of any suitable material adapted to engage and stop the blade. As one example, the pawl may be constructed of a relatively high strength thermoplastic material such as polycarbonate, ultrahigh molecular weight polyethylene (UHMW), Acrylonitrile Butadiene Styrene (ABS), etc., or a metal such as aluminum, etc. It will be appreciated that the construction of pawl  60  will vary depending on the configuration of blade  40 . In any event, the pawl is urged into the blade by a biasing mechanism such as a spring  66 . In the illustrative embodiment shown in FIG. 2, pawl  60  is pivoted into the teeth of blade  40 . It should be understood that sliding or rotary movement of pawl  60  may also be used. The spring is adapted to urge pawl  60  into the teeth of the blade with sufficient force to grip the blade and quickly bring it to a stop.  
     [0026] The pawl is held away from the edge of the blade by a restraining mechanism such as a fusible member  70 . The fusible member is constructed of a suitable material adapted to restrain the pawl against the bias of spring  66 , and also adapted to melt under a determined electrical current density. Examples of suitable materials for fusible member  70  include NiChrome wire, stainless steel wire, etc. The fusible member is connected between the pawl and a contact mount  72 . Preferably, fusible member  70  holds the pawl relatively close to the edge of the blade to reduce the distance pawl  60  must travel to engage blade  40 . Positioning the pawl relatively close to the edge of the blade reduces the time required for the pawl to engage and stop the blade. Typically, the pawl is held approximately {fraction (1/32)}-inch to ¼-inch from the edge of the blade by fusible member  70 ; however other pawl-to-blade spacings may also be used within the scope of the invention.  
     [0027] Pawl  60  is released from its unactuated, or cocked, position to engage blade  40  by a release mechanism in the form of a firing subsystem  76 . The firing subsystem is coupled to contact mount  72 , and is configured to melt fusible member  70  by passing a surge of electrical current through the fusible member. Firing subsystem  76  is coupled to logic controller  50  and activated by a signal from the logic controller. When the logic controller receives a contact detection signal from detection subsystem  22 , the logic controller sends an activation signal to firing subsystem  76 , which melts fusible member  70 , thereby releasing the pawl to stop the blade. Various exemplary embodiments and implementations of reaction subsystem  24  are described in more detail in U.S. Provisional Patent Application Serial No. 60/225,056, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,240, filed Aug. 13, 2001, U.S. Provisional Patent Application Serial No. 60/225,170, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,227, filed Aug. 13, 2001, U.S. Provisional Patent Application Serial No. 60/225,169, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,241, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference.  
     [0028] It will be appreciated that activation of the brake mechanism may require the replacement of one or more portions of safety system  18 . For example, pawl  60  and fusible member  70  typically are single-use components which must be replaced, before the safety system is ready to be used again. Thus, it may be desirable to incorporate one or more portions of safety system  18  in a cartridge that can be easily replaced. For example, in the exemplary implementation depicted in FIG. 2, safety system  18  includes a replaceable cartridge  80  having a housing  82 . Pawl  60 , spring  66 , fusible member  70  and contact mount  72  are all mounted within housing  82 . Alternatively, other portions of safety system  18  may be mounted within the housing. In any event, after the reaction system has been activated, the safety system can be reset by replacing cartridge  80 . The portions of safety system  18  not mounted within the cartridge may be replaced separately or reused as appropriate. Various exemplary embodiments and implementations of a safety system using a replaceable cartridge are described in more detail in U.S. Provisional Patent Application Serial No. 60/225,201, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,236, filed Aug. 13, 2001, U.S. Provisional Patent Application Serial No. 60/225,212, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,244, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference.  
     [0029] In the exemplary embodiment illustrated in FIG. 2, reaction subsystem  24  is configured to act on cutting tool  14  and stop rotation of blade  40 . As mentioned above, reaction subsystem  24  may be configured also to act on a different portion of operative structure  12  to stop and/or reverse the translation of blade  40  toward the workpiece and the user&#39;s body. Otherwise, the blade may continue to move toward the user&#39;s body even though the blade has stopped rotating. For example, U.S. Provisional Patent Application Serial No. 60/270,941, filed Feb. 22, 2001, U.S. patent application Ser. No. 10/052,273, filed Jan. 16, 2002, U.S. Provisional Patent Application Serial No. 60/270,942, filed Feb. 22, 2001, U.S. patent application Ser. No. 10/052,806, filed Jan. 16, 2002, U.S. Provisional Patent Application Serial No. 60/273,178, filed Mar. 2, 2001, U.S. patent application Ser. No. 10/052,274, filed Jan. 16, 2002, U.S. Provisional Patent Application Serial No. 60/273,902, filed Mar. 6, 2001, U.S. patent application Ser. No. 10/050,085, filed Jan. 14, 2002, U.S. Provisional Patent Application Serial No. 60/279,313, filed Mar. 27, 2001, and U.S. patent application Ser. No. 10/051,782, filed Jan. 15, 2002, the disclosures of which are herein incorporated by reference, describe various alternative embodiments of reaction subsystem  24  configured to stop any downward movement of the miter saw blade and/or move the blade upward away from the workpiece and the user&#39;s body.  
     [0030] Turning attention now to FIGS.  3 - 5 , another alternative embodiment is illustrated in which reaction subsystem  24  is configured to stop both the rotation and downward movement of the blade. Exemplary miter saw  10  includes a base assembly  90  having a base  92  adapted to support a workpiece during cutting. Typically, one or more fences  94  are mounted on base  92  and adapted to prevent workpieces from shifting across the base during cutting. Base  92  and fences  94  define a cutting zone  96  in which workpieces may be cut. Exemplary base assembly  90  also includes a tilt mechanism  98  coupled to base  92 .  
     [0031] As in the embodiments described above, blade  40  is mounted on a rotatable arbor  42 . The arbor is driven by a motor assembly (not shown) which is supported above base  92  by a pivot arm assembly  100 . As shown in FIGS. 3 and 4, the pivot arm assembly is selectively pivotal toward and away from cutting zone  96  to cut workpieces with the blade. In addition, at least a portion of tilt mechanism  98  is selectively tiltable relative to base  92  to make beveled cuts in the workpiece.  
     [0032] Pivot arm assembly  100  includes a housing  102  extending outward from one end of an arm  104 . The opposite end of arm  104  is connected to tilt mechanism  98  by a pivot coupling  106 . Housing  102  is configured to extend at least partially around an upper portion of blade  40 . Typically, pivot arm assembly  100  includes a spring or other biasing mechanism (not shown) adapted to maintain the housing and blade in a fully upward position away from cutting zone  96  when the miter saw is not in use.  
     [0033] Reaction subsystem  24  includes a brake mechanism  28  having at least one brake pawl  60  engageable by an actuator  107 . The actuator typically includes a restraining mechanism adapted to hold the brake pawl away from the blade against the urging of a biasing mechanism. In response to an activation signal, a release mechanism within the actuator releases the brake pawl from the restraining mechanism to pivot into the blade, usually stopping the blade within approximately 2-5 milliseconds. Optionally, brake pawl  60  and/or one or more components of actuator  106  may be contained in a replaceable cartridge, such as indicated at  80  in FIG. 4. Exemplary actuators, restraining mechanisms, biasing mechanisms, release mechanisms, cartridges and brake pawls are described in more detail above and in the incorporated references.  
     [0034] Brake pawl  60  is mounted on a movable pivot pin  108  configured to slide within a first set of channels  110  in either side of housing  102 . First set of channels  110  define concentric arcs about arbor  42 . As a result, pivot pin  108  is maintained at a constant radius from the arbor as it slides within the first set of channels. A positioning pin  112  extends from one or both sides of actuator  106  to slide within a second set of channels  114 . The second set of channels also define concentric arcs about arbor  42  so that positioning pin  112  maintains a constant radius from the arbor as it slides within the second set of channels. Since brake pawl  60  is coupled to actuator  112 , both the brake pawl and actuator are maintained in a constant orientation relative to the arbor and the perimeter of the blade as pivot pin  108  slides within first set of channels  10 .  
     [0035] As shown in FIG. 5, brake pawl  60  is laterally positioned on pivot pin  108  so that a central portion of the brake pawl is aligned with the blade. Brake mechanism  28  may include suitable positioning structure to maintain the brake pawl aligned with the blade. For example, annular spacers may be placed on pivot pin  108  on either side of the brake pawl to butt against the inner sides of housing  102 . Alternatively, the brake pawl may be constructed to have a width substantially equal to the inner width of the housing. In alternative embodiments where cartridge  80  is used, the cartridge may be sized to extend substantially from one inner side of the housing to the other. As a further alternative, the inner sides of the housing may include projections which extend inward to center the cartridge or brake pawl relative to the blade.  
     [0036] Base assembly  90  also includes a brace member  116  extending upward from tilt mechanism  98 . In the exemplary embodiment, brace member  116  extends upward from the tilt mechanism at an angle away from pivot arm assembly  100  so that the pivot arm assembly is not obstructed from pivoting to a fully raised position, as illustrated in FIG. 3. It will be appreciated that brace member  116  and tilt mechanism  98  may be formed as an integral, unitary structure. Alternatively, the brace member and tilt mechanism may be formed separately and then coupled together. In any event, the brace member is coupled to the tilt mechanism so as to prevent any pivoting movement of the brace member toward or away from the cutting zone. However, the brace member is configured to tilt along with the tilt mechanism relative to the base when the miter saw is adjusted for bevel cuts.  
     [0037] Pivot pin  108  is coupled to brace member  116  by a linkage assembly  118 . As best seen in FIG. 5, one end of linkage assembly  118  includes a fork structure  120  pivotally coupled to a pivot pin  122  mounted in brace member  116 . The opposite end of linkage assembly  118  includes a fork structure  124  pivotally coupled to each end of pivot pin  108 . As shown, linkage assembly  118  is coupled to pivot pin  108  on either side of brake pawl  60 . This provides increased stability and support when the brake pawl engages the blade. In an alternative embodiment, the linkage assembly may take the form of a pair of separate arms extending between pin  108  and pin  122  on either side of the brake pawl. As a further alternative, linkage assembly  118  may be configured to engage pivot pin  108  and/or pivot pin  122  on only a single side of the brake pawl. As another alternative embodiment, the linkage assembly may be configured to engage the center of pivot pin  108  (e.g., through a cut-out in the brake pawl) and/or the center of pivot pin  122  (e.g., through a cut-out in brace member  116 ).  
     [0038] In any event, the linkage assembly pivots relative to brace member  116  as the housing is pivoted toward and away from the cutting zone. Brace member  116  pushes or pulls pivot pin  108  and brake pawl  60  around the perimeter of the blade in first set of channels  110  as the housing is raised or lowered. Thus, the brake pawl is maintained at a constant distance from the brace member regardless of the position of the housing.  
     [0039] In response to an activation signal from a control subsystem (not shown), brake pawl  60  is pivoted into the teeth of blade  40 . When the brake pawl engages the blade the angular momentum of the blade produces a force on the brake pawl that tends to urge the brake pawl to move in a clockwise direction along first set of channels  110 . In other words, at least a portion of the angular momentum of the blade is transferred to the brake pawl. The force on brake pawl  60  is transferred to brace member  116  by linkage assembly  118 . Linkage assembly  118  may be constructed of any relatively rigid material adapted to support brake pawl  60  during braking of the blade, including metal, plastic, etc.  
     [0040] Brace member  116  prevents the brake pawl from sliding clockwise within first set of channels  110  unless housing  102  pivots upward away from the cutting zone. As a result, pivot arm assembly  100  will be urged upward by engagement of the brake pawl with the blade. The amount of upward force on the blade will depend, at least partially, on the length of brace member  116 . As the length of the brace member is increased, the upward force on the blade during braking will likewise increase. Typically, the length of the brace member is selected so that the upward force on the blade during braking is sufficient to stop any downward motion of the housing under normal operating conditions (i.e., the housing is pivoted downward toward the cutting zone at a normal speed). Optionally, the length of the brace member is selected so that the upward force on the blade during braking is sufficient to overcome and reverse any normal downward momentum of the housing and blade, thereby retracting the blade upward away from cutting zone  96 .  
     [0041] In any event, brake pawl  60  is arranged and supported to convert at least a portion of the kinetic energy of the rotating blade into an upward force on the blade and housing. Thus, exemplary brake mechanism  28  is configured to stop both the rotation of the blade and any downward movement of the blade using a single brake pawl. As a result, only a single cartridge or brake pawl need be replaced after the brake mechanism has been triggered.  
     [0042] Since the upward force on the blade and housing is produced by the rapid deceleration of the blade by the brake pawl, the upward force is only temporary. Once the rotation of the blade has stopped, the housing is free to pivot toward or away from the cutting zone. Nevertheless, the blade will remain locked against further rotation until the cartridge is removed.  
     [0043] Housing  102  may include one or more sections  126  which may be removed or repositioned to allow installation and removal of the cartridge or brake pawl and actuator. Pivot pin  108  is typically removed by sliding it completely through the brake pawl. Positioning pin  112  may also be slid completely through the actuator and/or cartridge. Alternatively, positioning pin  112  may be dual spring-loaded pins which can be depressed to allow the cartridge to be installed and removed more easily. Optionally, housing  102  may include one or more removable covers adapted to cover one or both of the first and second set of channels during normal operation. It will be appreciated that housing  102  and the components of the brake mechanism may be configured in any of a variety of different ways to allow the brake mechanism to be easily replaced.  
     [0044] While one particular embodiment has been described above, many modifications and alterations are possible. For example, FIGS.  6 - 9  illustrate an alternative exemplary embodiment in which the brake mechanism includes a brake pawl support structure that pivots within the housing. As shown, the brake mechanism includes one or more radial support arms  128  adapted to support cartridge  80  at a constant radial distance and orientation about arbor  42 . Support arms  128  are configured to pivot about the elongate central axis of arbor  42 . Each arm includes an annular collar portion  130  configured to fit on and swing about one of a pair of support rings  132 . One support ring  132  extends from the inner surface of housing  102 , while the other support ring extends from motor assembly  16 . Collar portions  130  may be retained on support rings  132  by ring clips  134  or any other suitable mechanism. It will be appreciated that support arms  128  may alternatively be coupled to pivot about the arbor in a variety of other ways such as are known to those of skill in the art.  
     [0045] Cartridge  80  is coupled to support arms  128  by a pivot pin  136  and a positioning pin  138 . The pivot and positioning pins maintain the cartridge at a constant radial distance and orientation relative to the perimeter of the blade as support arms  128  pivot around the arbor. The support arms are coupled to a brace member  116  by one or more linkages  140 . The rear end of each linkage  140  is pivotally coupled to brace member  116  by a pivot pin  142 . The front end of each linkage is pivotally coupled to a different one of support arms  128  by one or more pivot pins  144 . In the exemplary embodiment, pivot  5  pins  144  are mounted in outwardly projecting shoulder regions  146  formed in each support arm  128 . Shoulder regions  146  are configured to ensure pivot pins  144  and the front ends of linkages  140  remain above arbor  42  at all operable positions of pivot arm assembly  100 .  
     [0046] In the exemplary embodiment, linkages  140  extend forward from brace member  116  through one or more holes  148  in the rear of housing  102 . Therefore, housing  102  requires no arcuate channels for receiving pins  136 ,  138  or  144 . Furthermore, linkages  140  should not interfere with standard blade guards (not shown) that typically cover the perimeter of the housing and blade. Indeed, a front section of housing  102  may optionally be constructed to telescope around the exterior of the remainder of the housing to allow a user to have greater access to the blade. Alternatively, linkages  140  may be disposed on the exterior of the housing, in which case pivot pin  136  and positioning pin  138  would extend through arcuate channels or similar openings in the housing. Although linkages  140  are depicted as separate structural elements, it will be appreciated that the linkages may be formed as an unitary member with spaced-apart arms, etc.  
     [0047] Comparing FIGS. 6 and 7, it can be seen that as pivot arm assembly  100  pivots about pivot coupling  106 , linkages  140  cause support arms  128  to pivot about arbor  42  in the opposite direction. Thus, cartridge  80  and brake pawl  60  are counter-pivotally coupled to the pivot arm assembly. As the pivot arm assembly and blade pivot in a clockwise direction (as seen in FIGS. 6 and 7) downward toward cutting zone  96 , the cartridge and brake pawl pivot in a counter-clockwise direction about the arbor. Conversely, as the pivot arm assembly and blade pivot in a counter-clockwise direction (as seen in FIGS. 6 and 7) upward away from cutting zone  96 , the cartridge and brake pawl pivot in a clockwise direction about the arbor.  
     [0048] The brake pawl (not shown) is mounted on pivot pin  136  to pivot into the teeth of blade  40  upon receipt of an activation signal by the cartridge. When the brake pawl engages the rotating blade, the angular momentum of the blade tends to force the brake pawl to move upward and forward in a clockwise direction (as seen in FIG. 6) about the arbor. Consequently, radial support arms  128  are urged to pivot in a clockwise direction (as seen in FIG. 6) about the arbor. Since the radial support arms are connected to brace member  116  by linkages  140 , any clockwise force on the radial support arms is translated into a counter-clockwise force about pivot coupling  106  on housing  102 . In other words, when the brake pawl engages the blade, the housing and blade are urged upward away from cutting zone  96 .  
     [0049] It will be appreciated that the amount of upward force on the housing will depend on the specific arrangement of brace member  116 , linkages  140  and radial support arms  128 . The counter-clockwise force on support arms  128  due to any downward momentum and/or force on the pivot arm assembly will have a lesser moment than the clockwise force due to the brake pawl engaging the blade. This is because linkages  140  are coupled to the support arms at a radial position closer to the pivot point of the support arms than is the brake pawl. The ratio of the clockwise force-moment to the counter-clockwise force-moment will depend on the ratio of the distances between pivot pin  136  and arbor  42 , and between pivot pins  144  and arbor  42 . Additionally, the height of pivot pin  142  above pivot coupling  106 , relative to the height of pivot pins  144  above arbor  42  will also effect the ratio of the upward force on the pivot arm assembly due to the brake pawl to any downward momentum and/or force on the pivot arm assembly.  
     [0050] Typically, the height of pivot pin  142  above pivot coupling  106 , and the position of pivot pins  144  on support arms  128  are selected to ensure that, under normal operating conditions, any downward movement of the blade toward the cutting zone is stopped when the brake pawl engages the blade. Optionally, the height of pivot pin  142  above pivot coupling  106 , and the position of pivot pins  144  on support arms  128  may be selected to ensure that the clockwise force-moment on the support arms is greater than the normal counter-clockwise force-moment when the brake pawl engages the blade. In such case, the blade is pushed or retracted upward and at least partially away from the cutting zone when a dangerous condition is detected such as contact between the user&#39;s body and the blade.  
     [0051] Once the brake pawl has engaged and stopped the blade, pivot arm assembly  100  is free to pivot about pivot coupling  106 . Housing  102  may include a removable portion through which the cartridge can be replaced. Alternatively, the radial support arms may be uncoupled from brace member  116 , as shown in FIG. 9. In the exemplary embodiment, the support arms are uncoupled from the brace member by disconnecting linkages  140  from pivot pin  142 . Since the brake pawl usually is wedged onto the blade after being triggered, blade  40  may be rotated until the cartridge is exposed below the housing. Pivot pin  136  and positioning pin  138  may then be removed. Alternatively, positioning pin  138  may be dual spring-loaded pins which can be depressed to disengage the radial support arms. As further alternative, the interior surfaces of radial support arms  128  may include recessed channels  154  adapted to allow pivot pin  136  to slide into place. Position pin(s)  138  may then be installed to hold the cartridge in the operable position relative to the blade. After the used cartridge is replaced with a new cartridge, the cartridge and support arms are pivoted up into the housing and the linkages are reconnected to pivot pin  142 . When removing or installing the blade, arbor nut  150  may be accessed through an opening  152  in the housing.  
     [0052] As described above, the present invention provides a miter saw which is substantially safer than existing saws. The miter saw includes a safety system  18  adapted to detect the occurrence of a dangerous condition and stop movement of the blade and/or the pivot arm to prevent serious injury to a user. The safety system may be adapted for use on a variety of saws in addition to miter saws. Several examples of modifications and variations, as well as further detailed descriptions of miter saws and other saws may be found in the following references, the disclosures of which are herein incorporated by reference: PCT Patent Application Serial No. PCT/US00/26812, filed Sep. 29, 2000; U.S. patent application Serial No. 09/676,190, filed Sep. 29, 2000; U.S. Provisional Patent Application Serial No. 60/275,595, filed Mar. 13, 2001; U.S. Provisional Patent Application Serial No. 60/273,177, filed Mar. 2, 2001; U.S. patent application Ser. No. 10/052,705, filed Jan. 16, 2002; U.S. Provisional Patent Application Serial No. 60/233,459, filed Sep. 18, 2000; U.S. Provisional Patent Application Serial No. 60/225,210, filed Aug. 14, 2000; U.S. patent application Ser. No. 09/929,425, filed Aug. 13, 2001; U.S. Provisional Patent Application Serial No. 60/225,058, filed Aug. 14, 2000; U.S. patent application Ser. No. 09/929,235, filed Aug. 13, 2001; U.S. Provisional Patent Application Serial No. 60/225,057, filed Aug. 14, 2000; U.S. patent application Ser. No. 09/929,238, filed Aug. 13, 2001; U.S. Provisional Patent Application Serial No. 60/157,340, filed Oct. 1, 1999; U.S. Provisional Patent Application Serial No. 60/279,313, filed Mar. 27, 2001; U.S. patent application Ser. No. 10/051,782, filed Jan. 15, 2002; U.S. Provisional Patent Application Serial No. 60/292,081, filed May 17, 2001; U.S. patent application Ser. No. 09/955,418, filed Sep. 17, 2001; U.S. Provisional Patent Application Serial No. 60/298,207, filed Jun. 13, 2001; U.S. patent application Ser. No. 10/172,553, filed Jun. 13, 2002; U.S. Provisional Patent Application Serial No. 60/302,937, filed Jul. 2, 2001; U.S. patent application Ser. No. 10/189,031, filed Jul. 2, 2002; U.S. Provisional Patent Application Serial No. 60/302,916, filed Jul. 3, 2001; U.S. patent application Ser. No. 10/189,027, filed Jul. 2, 2002; U.S. Provisional Patent Application Serial No. 60/307,756, filed Jul. 25, 2001; U.S. patent application Ser. No. 10/202,928, filed Jul. 25, 2002; U.S. Provisional Patent Application Serial No. 60/324,729, filed Sep. 24, 2001; U.S. Provisional Patent Application Serial No. 60/335,970, filed Nov. 13, 2001; and U.S. Provisional Patent Application Serial No. 60/351,797, filed Jan. 25, 2002.  
     [0053] It is believed that the disclosure set forth above encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the inventions includes all novel and non-obvious combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. No single feature, function, element or property of the disclosed embodiments is essential to all of the disclosed inventions.