Abstract:
A highly reliable and inexpensive safety device for a power tool having a protective guard over a cutting blade which prevents use of the power tool when the protective guard is not in place. The safety device includes a proximity detector detecting a distance between the guard and a power tool table top and an interlock system that prevents operation of the power tool when a distance less than a predetermined relative proximity is not detected. In the case of a power table saw a pair of magnets are secured to the guard and a pair of reed switches are mounted beneath the table top to be closed when the guard is in place. The interlock system responds to the reed switches by allowing the saw to be started when the switches are both closed and preventing startup when they are not. A key activated bypass circuit allows the saw motor to be started when the guard is not present.

Description:
BACKGROUND OF THE INVENTION 
     Power table saws have been in common use for many years and many accessories have been developed to improve operator safety. Table saws are provided with a blade guard to cover the rotating blade while the saw is in use. However, the guard must be easily removed to facilitate certain special cuts. Often, the operator fails to reinstall the blade guard after the special cut is completed. As a result, many injuries occur each year when saws are operated without the blade guard being properly in place. 
     Manufacturers have attempted to introduce interlock systems that would detect the presence or absence of the blade guard and allow operation of a saw in the absence of a blade guard only when a special key activation or other interlock override signal has been generated. However, the existence of a dusty, physically rugged environment in the vicinity of the saw blade and the need to accommodate large work pieces have until now rendered such systems unreliable or impractical. 
     Two examples of blade guard systems are shown in U.S. Pat. No. 3,249,134 issued to Vogl et al. for SAW AND DADO GUARD and U.S. Pat. No. 3,754,493 issued to Niehaus et al. for CIRCULAR SAW BLADE GUARD . The Vogl patent discloses a power saw blade guard that has two independent side panels that adjust relative to the work surface to accommodate the work piece being cut. The entire guard can be easily pivoted away from the blade prior to starting the saw motor. 
     The Niehaus patent discloses a power saw blade guard that normally rests on the saw table and adjusts to the height of a work piece passing through the saw blade. The Niehaus patent further discloses a guide plate or splitter positioned in line with the saw blade to support the guard and separate a cut work piece after the work piece is passed through the saw blade to prevent the work piece from binding on the blade. When the guide plate is secured in place, the blade guard mounted on the guide plate has a limited swing angle so as to prevent the guard from being tilted back during normal operation. To prevent injuries, this guard, along with its guide plate, must be in place prior to startup and remain in place over the saw blade while the work piece is passing through the saw blade. It is left up to the operator to make sure the guard and the guide plate is in place. The guard and the guide plate can be easily pivoted away, leaving the saw blade exposed while the table saw is running. 
     Another type of power table saw guard is disclosed in U.S. Pat. No. 5,181,447 to Hewitt. The Hewitt blade guard has a safety power cut-off device that cuts off power to the table saw when the blade guard is pivoted away from the saw blade. This prevents open exposure of the rotating blade to the operator when the guard is tilted away from the blade while the blade is turning. However, this blade guard has several drawbacks. The guard does not self-adjust relative to the working surface and does not stay in contact with the work piece as it is being cut. The guard must be manually adjusted to a fixed height above the table that is sufficient to cut a work piece prior to cutting a work piece. This means that the blade remains exposed to the extent of the height of the work piece. Further once an operator cuts a thick work piece it is likely that a subsequent, thinner work piece will be cut without adjusting the blade guard downward. This will leave more exposure to the blade than necessary during the subsequent cut. In addition, the guard does not adequately enclose the blade so as to protect an operator from flying wood fragments and broken saw teeth. The guard must be mounted on a cantilevered arm that limits the width of a work piece that can be cut. However, the guard does have an interlock circuit and a keylock override that prevents pivoting of the guard completely away from the saw blade unless the keylock override is activated. 
     The Occupational Safety and Health Administration (OSHA) has established rules specifically for table saws. They require that each circular hand-fed ripsaw be guarded by a hood that completely encloses the portion of the saw above the material being cut. The hood and mounting must allow the hood to automatically adjust itself to the thickness of the material being cut. This same regulation is required for the table saw as well. 
     There has been a long recognized need to implement a safety interlock circuit for use with this type of self adjusting, continuous contact blade guard. However, the harsh environment in the vicinity of a saw blade has prevented the practical realization of a blade guard electro-mechanical interlock system. 
     SUMMARY OF THE INVENTION 
     A low cost, highly reliable power tool safety device in accordance with the invention includes a proximity detector detecting the relative proximity between a power cutting tool guard and a table top and an interlock system that prevents startup of the power tool when the guard is not in place. The safety device may be effectively used with self adjusting, continuous contact power table saw blade guards as well as other types of cutting tool guards. 
     The proximity detector includes at least one permanent magnet permanently secured to the guard to generate a magnetic field and at least one magnetic field intensity detector which may be a normally open magnetic reed switch disposed on a side of the table opposite the working surface at a detection location opposite to the permanent magnet. This placement keeps all of the wiring, electrical contacts and sensors under the table and away from the saw blade, the dust and the constant vibrations of the blade guard. The sensitivity of each reed switch to a magnetic field is selected to cause each reed switch to close when the guard is properly positioned in guarding relationship to the cutting tool. Each magnetic reed switch has an open state and a closed state and is coupled in series between the electric starting circuit and a power source. Each reed switch detects the predetermined relative proximity by remaining in the open state when the predetermined relative proximity is not detected and switching to the closed state when the predetermined relative proximity is detected. The proximity detector generates an electrical proximity signal indicating whether or not the guard is within the predetermined relative proximity to the working surface, the electrical proximity signal being communicated to the interlock system. 
     The interlock system prevents use of the power tool when the proximity detector does not detect proximity between the guard and the working surface by placing each magnetic reed switch in series with a motor starter circuit. If the guard is not in place the reed switch opens and prevents energization of the motor starter circuit. Once the motor is started the motor will remain energized even if a thick work piece causes a loss of proximity between the guard and the reed switch. 
     A key operated bypass switch allows the motor to be started one time without the guard in place to accommodate special cuts or unusual work piece shapes that require operation of the power tool with the guard removed. The requirement for a key assures that unguarded operation can be specially supervised by a teacher or supervisor. An indicator light is illuminated to alert an operator or supervisor that the interlock circuit has been bypassed. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     A better understanding of the invention may be had from a consideration of the following Detailed Description  5  taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a partially exploded perspective view of a power table saw having a power tool safety device according to the present invention; 
     FIG. 2 is a fragmentary side section view of the table saw shown in FIG. 1 taken along line  2 — 2 ; 
     FIG. 3 is a bottom view of an insert used in the table saw shown in FIG. 1; 
     FIG. 4 is an enlarged fragmentary section view of a guard and insert shown in FIG. 2; 
     FIG. 5 is a simplified front section view of a guard from the power table saw shown in FIG. 1, with the blade and guard tilted at an angle relative to a table top; 
     FIG. 6 is an electrical schematic representation of a safety interlock system for the power table saw shown in FIG. 1; and 
     FIG. 7 is an electrical schematic representation of an alternative embodiment of a safety interlock system for the power table saw shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a power cutting tool and more particularly a power table saw  10  in accordance with the present invention includes a base  12 , a table  14  having a top, a generally planar working surface  16  and a cutting blade  18  that extends through a blade aperture  24  in the table  14 . 
     A rip fence  26  is movable on tubular tracks  28 ,  30  and longitudinally extending slots  32 ,  34  are provided in the working surface  16  to receive a cross cut guide which is not shown. An insert  36  is disposed within aperture  24  with a top surface parallel to and substantially flush with the working surface  16 . Insert  36  has a first, centrally located, longitudinally extending slot  38  which receives blade  18  and a second longitudinally extending slot  40  disposed rearward of slot  38  in alignment with slot  38 . 
     Making further reference to FIG. 2, a splitter  44  has a slotted front mounting leg  48  and an apertured rear  20  mounting leg  50 . The front mounting leg  48  extends through rear slot  40  in insert  36  where it may be removably secured by a bolt to a rotatable arbor  46  (only a small portion of which is shown). The rear mounting leg  50  extends downward below a rear edge  52  of working surface  16  where a bolt may removably secure the rear leg  50  to a portion of the arbor  46  that extends through a rear wall of base  12 . 
     A U-shaped longitudinally extending pivot link  54  is disposed over the top of splitter  44  with a channel  56  receiving the top edge of splitter  44 . The sidewalls  58  of link  54  each have a rear aperture  60  that is rotatably secured by a pin  62  to an aperture  64  in the splitter  44  located near the top front portion thereof. A pair of spring loaded pawls (not shown) or anti kickback fingers also extend downwardly from pin  62  on each side of splitter  44  and link  54 . These fingers have been omitted for clarity but are shown in U.S. Pat. No. 3,754,493 to Niehaus et al., which is hereby incorporated by reference to illustrate one embodiment of splitter  44  and guard  70 . 
     A generally U-shape saw blade guard  70  has a pair of opposed side walls  72 A,  72 B which each have an aperture  74  extending there through near a centrally located upper portion thereof. The link  54  has a pair of apertures  76  near the front thereof and a pin  78  passes through the apertures  74  and  76  to rotatably secure the blade guard  70  to the link  54  and indirectly through link  54  to the  20  splitter  44 . 
     A motor M  80  and the cutting blade  18  are both mounted on the rotatable arbor  46  within the base  12  (mounting not shown). A pulley and belt arrangement (not shown) couples the blade  18  to be rotationally driven by motor  80 . The arbor  46  and the blade  18  and motor  80  mounted on arbor  46  are both vertically and rotationally adjustable to select the height and angle relative to the vertical at which the blade  18  extends through the table  14 . A hand crank  84  controls the vertical height of arbor  46  including blade  18  while a hand crank  86  rotates a pinion relative to a rack (not shown) to select a pivot angle. 
     Making further reference to FIGS. 3,  4  and  5 , the insert  36  has a top insert working surface  90  that should be substantially flush with the table working surface  16  when the insert  36  is in place within aperture  24 . Four threaded screws  92  extend downward from insert  36  to provide a precise height adjustment. A pattern of reinforcement ribs  93  is placed on the bottom of insert  36  to increase the strength of insert  36 . The insert is made of a low permitivity material such as pot metal or aluminum that will not interfere with a magnetic field passing through insert  36 . 
     As best seen in FIG. 5, guard  70 , splitter  44  (not shown in FIG. 5) and saw blade  18  which are all mounted on the arbor  46  may be tilted or rotated from a vertical orientation in a clockwise direction as viewed from the front. The arbor assembly is shown to be rotated to an angle of about 45° in FIG.  5 . The guard  70  has a right hand side panel  72 B that is about ½ inch shorter than the left hand side panel  72 A. This allows a bottom edge  94  of side panel  72 A to remain in contact with working surface  16  as the arbor assembly including blade  18  is rotated clockwise to about 45°. 
     A control panel  100  is suspended from the bottom side of table  14  and contains the low voltage control circuits for saw  10 . A panel containing high voltage, high current motor control circuits is mounted on the back side of saw  10  and is not shown. The control panel  100  has mounted thereon a START switch  102 , a STOP switch  104 , a key switch  106 , a READY indicator light  108  and a bypass mode indicator light  110 . The key switch  106  is responsive to a key  112  and has two stable positions, on and off. In addition, the key switch has a spring recoil bypass position similar to the start position of an automobile ignition lock that returns the key switch to the on position from the bypass position. Also, the key  112  is removable when the key switch  106  is in the on or off position. A hole  114  and grommet  116  provide passage of two cables  118 ,  119  carrying electrical signals through the front panel of base  12  to and from control panel  100 . 
     While all of the mechanical features of the power table saw  10  have not been described in detail, it will be appreciated that these features are conventional. A commercially available 1990 10″ Tilting Arbor Unisaw manufactured by Delta International Machinery Corp. of Memphis, Tennessee is representative of the conventional mechanical and electro-mechanical structure of the power table saw  10 . This table saw is described in an Instruction Manual dated Feb. 26, 1990 for Part No. 422-04-651-0024. 
     A safety device  120  shown in FIG. 2 provides an interlock which assures that the power table saw  10  cannot be operated unless either the guard  70  is properly in place at the start of operation or a person of supervisory authority uses key  112  to actuate a bypass condition with key switch  106 . The safety device  120  includes a proximity detector  122  and an interlock system  124  that is disposed within control panel  100 . 
     The proximity detector  122  includes a pair of permanent magnets  126 ,  128  mounted on the left side  72 A near the bottom edge  94  thereof and a pair of magnetic reed switches  136 ,  138  disposed on the bottom side of insert  36  in opposed, mating relationship to permanent magnets  126 ,  128 . 
     In the illustrated embodiment, the permanent magnets  126 ,  128  are mounted slightly above the bottom edge  94  of side panel  72 A so that as the blade and guard  70  are tilted clockwise to the right, the permanent magnets  126 ,  128  will not engage or interfere with the passage of a work piece beneath the bottom edge  94  of guard  70 . It is preferred, but not essential, that the magnet  126  be mounted at a location rearward of blade  18  and that magnet  128  be mounted at a location forward of blade  18  so that if the guard  70  is pushed sideways into the blade  18 , the magnets  126 ,  128  will not make contact with blade  18 . In a high volume production model of the safety device  120 , it is anticipated that the permanent magnets  126 ,  128  would be permanently molded into the left side panel  72 A of guard  70  near the bottom edge  94  with the bottom edge  94  being contoured so as to avoid interference with the passage of a work piece beneath the bottom edge  94  regardless of the tilt angle of the blade  18  and guard  70 . The permanent magnet  126  is positioned near the rear of saw blade  18  while permanent magnet  128  is positioned near the front of the saw blade  18 . The use of two permanent magnets is not essential but makes it more difficult for an operator to defeat the safety device  120 . 
     The reed switches  136 ,  138  may be any suitable reed switch which is normally open but which is closed in the presence of a magnetic field of a strength that is produced by a permanent magnet  126  or  128  when the permanent magnet is approximately one inch away. By way of example, the magnets  126 ,  128  and reed switches  136 ,  138  are packaged in mating pairs by Safe House and sold by the Radio Shack division of Tandy Corp. under Catalog No. 49-497. Other suitable implementations of a proximity detector could be used as well. The reed switches  136 ,  138  may be secured by glue, adhesive or other suitable means to the bottom side of insert  36  in opposed relationship to magnets  126 ,  128 . The reed switches  136 ,  138  are connected in series with each other and through a plug connector  130  having a plug  132  and a receptacle  134  to the interlock system  124 . The plug connector  130  is preferably located near the insert  36  so that insert  36  may be easily removed from aperture  24 . 
     Other guards exist that have side panels that are separately moveable relative to the working surface such as the Uniguard described in the aforementioned instruction manual for the Delta 10″ Tilting Arbor Saw as part number 34-885. The present invention can be easily adapted to this adjustable guard by installing one or more magnets to the bottom edge of one or preferably both of the adjustable side panels. A reed switch should be installed beneath the working surface  16  in opposed relationship to each magnet in the same manner as described above so that the reed switches will close when the bottom edges of the Uniguard are at a predetermined distance from the working surface. Other blade protective guards can also be fitted with the present invention so long as a portion of the guard contacts the working surface of the table saw prior to starting. 
     Referring now to FIG. 6, the interlock system  124  is coupled to motor  80  and includes a transformer  140 , a fuse  142 , a start circuit  144 , a bypass circuit  146 , and a run circuit  148 . 
     Motor  80  is shown as a single phase AC motor although the motor is also available in commercial saws as a three phase AC motor. Each power terminal of the motor  80  is coupled through a motor relay contact MA,MB to an AC power source  152 . The primary winding of transformer  140  is coupled across power terminals of motor  80  while the secondary winding provides approximately  24  volts AC through fuse  142  and stop switch  104  to a power rail  154 . 
     The key switch  106  includes an on contact  156  which is open when the key switch is in the off or bypass position and closed when the key switch is in the on position. Key switch  106  also includes a second bypass contact  158  which is normally open and is closed only when the key switch is in the temporary bypass position. The start circuit  144  includes the on contact  156  having one terminal contacted to power rail  154  and the second terminal coupled to a first side of the parallel combination of a relay contact R 1 B of R 1  relay coil  162  and the series combination of reed relay proximity switches  136 ,  138  which are connected to interlock system  124  by plug connector  130 . A second side of relay contact R 1 B and switches  136 ,  138  are in turn connected to the first side of the parallel combination of a READY indicator light  108  and to the series combination of start switch  102  and an R 2  relay coil  160 , having a normally open contact  161 . An opposite second side of this parallel combination is connected to ground that is connected to the secondary coil of transformer  140 . 
     The bypass circuit  146  includes the parallel combination of the bypass contact  158  of key switch  106  and a relay contact R 1 A of R 1  relay coil  162  having a first side connected to the power rail  154  and an opposite second side connected to a first side of the parallel combination of R 1  relay coil  162  and a bypass indicator lamp  110 . A second side of relay coil R 1  and bypass lamp  110  are connected to ground. 
     Run circuit  148  includes the parallel combination of an R 2  contact  161  from R 2  relay coil  160  and an MC contact from a motor relay coil  164 . Relay contacts R 2  and MC are connected between the power rail  154  and the motor relay coil  164  which in turn is connected through an overload protector  166  to ground. 
     In operation the normally closed stop switch  104  normally provides power to power rail  154 . If key switch  106  is in the off position, contact  156  is open, relay R 2  is open and no power can be provided to the motor relay coil  164 . 
     However, if key switch  106  is turned to the on position, contact  156  closes and power is provided to the parallel combination of proximity switches  136 ,  138  and relay contact R 1 B. If the guard  70  is properly in place, proximity switches  136 ,  138  are closed and power is provided to start switch  102  and to the READY indicator lamp  108 . Upon actuation of the normally open start switch  102 , power is provided to the R 2  relay coil  160 . This in turn closes contact R 2  and makes power available to the motor relay coil  164 . As soon as power is available to the motor relay coil  164  motor relay contacts MA and MB close to apply energy to motor  80  and start the rotation of saw blade  18 . At the same time, relay contact MC closes to assure a continuation of power to the motor relay coil  164  after the start switch  102  is released. Contact MC also maintains operational energization of the motor relay coil  164  even if the passage of a work piece beneath the guard  70  raises guard  70  sufficiently above the table top  16  so that proximity between the magnets  126 ,  128  and mating proximity switches  136 ,  138  respectively is lost causing reed switches  136  or  138  to become open. Consequently, the safety device  120  assures that the guard  70  is in place when the saw  10  is started, but does not interfere with continued operation of the saw thereafter. 
     It will be appreciated that in the interlock system  124  shown in FIG. 6 the R 2  relay coil  160  essentially serves as a power amplifier so that the full current required to energize the motor relay coil  164  need not pass through the proximity switches  136 ,  138 . 
     In an alternate embodiment, interlock system  170  is essentially the same as interlock system  124  except that the proximity switches  136 ,  138  are capable of carrying sufficient current to directly energize the motor relay coil  164  to render unnecessary the power amplifier R 2  relay  160 . In this case the motor relay coil  164  is connected in the same location as the coil of R 2  relay  160  in the interlock system  124  and motor relay contact MC is connected in parallel between the power rail  154  and the motor relay coil  164 . 
     The interlock systems  124 ,  170  have been shown to be implemented with electro-mechanical relays for a developmental embodiment of the invention. However, it will be appreciated that for a production model of the invention it may be desirable to implement some or all of the interlock circuit functions with one or more solid state components. 
     In the event that it is necessary to operate the power saw  10  without the guard  70  in place, the guard can be removed while the saw is in an off or power down condition. Upon removal of the guard the saw can be started by moving the key switch  106  to the bypass position to close contacts  158  and subsequently actuating the start switch  102 . The key switch then springs back to the on position holding contact  156  closed. The closure of contact  158  applies power to the bypass relay coil R 1   162  which in turn causes the closure of relay contacts R 1 A and R 1 B. The closure of contact R 1 A applies power to bypass lamp  110  and R 1  bypass relay coil  162  even after the key switch  106  is released from the bypass position. Once the R 1  bypass relay coil  162  is actuated, the start button  102  may be actuated after key is returned to on position closing contact  156 . Once the start switch  102  is actuated, power is applied to either relay coil R 2   160  and motor relay coil  164  or directly to motor relay coil  164 . In any event the start switch initiates the operation of motor  80 . 
     Once the motor  80  is energized by interlock circuit  124  or  170 , the motor remains operational until actuation of stop switch  104  temporarily terminates the application of power to power rail  154 . This in turn de-energizes all of the control relays and precludes a restart until the start switch  102  is actuated while either the proximity switches  136 ,  138  are closed or after the key switch has again been turned to the bypass position to enable bypass operation. 
     Thus, a supervisor having possession of the key  112  for key switch  106  can control whether or not the key switch is in the on position to enable normal saw operation or in the off position to disable any operation of the saw  10 . Alternatively, the supervisor may move the key to the bypass position to enable a single operation of the saw  10  with the guard  70  removed. Once the saw has been stopped, it can not be restarted unless the guard  70  is properly in place or the supervisor reactuates the bypass condition with the key  112 . It is an object of the invention to require the overt act of turning the key each time the saw is engaged in the bypass mode so that a supervisor can over look operation of the saw while the guard is moved away from the blade prior to starting. 
     While there have been shown and described above various embodiments of a power table saw having an interlock safety device in accordance with the invention for the purpose for enabling a person of ordinary skill in the art to make and use the invention, it will be appreciated that the invention is not limited thereto. Accordingly, any modifications, variations or equivalent arrangements within the scope of the attached claims should be considered to be within the scope of the invention.