Abstract:
An automatic braking system for a pivoting power tool includes a cutting assembly, a cutting arm supporting the cutting assembly, a hinge supporting the cutting arm through a pivot, a primary braking system operably connected to the cutting assembly, a secondary braking system operably connected to the hinge, and a safety circuit configured to sense an unsafe condition and, in response to sensing the unsafe condition, (i) control the primary braking system to oppose rotation of a blade supported by the cutting assembly, and (ii) control the secondary braking system to oppose rotation of the cutting arm.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/781,931 filed Mar. 14, 2013, the entirety of which is incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to machines such as miter saws which include protective systems configured to rapidly stop rotational movement of a shaping device. 
     BACKGROUND 
     A number of power tools have been produced to facilitate forming a work piece into a desired shape. One such power tool is a miter saw. Miter saws present a safety concern because the saw blade of the miter saw is typically very sharp and moving at a high rate of speed. Accordingly, severe injury such as severed digits and deep lacerations can occur almost instantaneously. A number of different safety systems have been developed for miter saws in response to the dangers inherent in an exposed blade moving at high speed. One such safety system is a blade guard. Blade guards movably enclose the saw blade, thereby providing a physical barrier that must be moved before the rotating blade is exposed. While blade guards are effective to prevent some injuries, a user&#39;s finger is nonetheless in proximity to the moving blade, particularly when attempting to secure a work piece as the miter saw is used to shape the work piece. 
     Miter saw safety systems have been developed which are intended to stop the blade when a user&#39;s hand approaches or touches the blade. Various stopping devices have been developed including braking devices which are physically inserted into the teeth of the blade. In general, upon detection of a person in the vicinity of the blade, a signal is processed and sent to a brake mechanism to stop blade rotation within a short period of time. One such system is disclosed in U.S. Patent Publication No. 2011/0048197. In other systems, a mechanical or electrical brake is used. In all of these systems, however, the short stopping time of the blade generates a large angular momentum that will either swing the head up or down (depending on blade or motor rotation direction for miter saws) with a high force which is destructive to the structure of the tool. In addition to posing a danger to the tool, the high angular momentum forces pose an additional injury risk to the user. 
     What is needed therefore is a simple and reliable configuration which reduces the potential for transferring high angular momentum forces to a tool thereby causing movement of the tool. 
     SUMMARY 
     In one embodiment, an automatic braking system for a pivoting power tool includes a cutting assembly, a cutting arm supporting the cutting assembly, a hinge supporting the cutting arm through a pivot, a primary braking system operably connected to the cutting assembly, a secondary braking system operably connected to the hinge, and a safety circuit configured to sense an unsafe condition and, in response to sensing the unsafe condition, (i) control the primary braking system to oppose rotation of a blade supported by the cutting assembly, and (ii) control the secondary braking system to oppose rotation of the cutting arm. 
     In another embodiment, a method of operating an automatic braking system for a pivoting power tool includes supporting a cutting assembly with a cutting arm, sensing an unsafe condition using a safety circuit, controlling with the safety circuit a primary braking system to oppose rotation of a blade supported by the cutting assembly in response to sensing the unsafe condition, and controlling with the safety circuit a secondary braking system to oppose rotation of the cutting arm in response to sensing the unsafe condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a front right perspective view of a miter saw assembly; 
         FIG. 2  depicts a schematic diagram of the power transfer train between the motor and the blade including a clutch and a primary braking system; 
         FIG. 3  depicts a simplified plan view of the right side of the power transfer train; and 
         FIG. 4  depicts a simplified left side plan view of a the miter saw assembly of  FIG. 1  showing torque generated by activation of the primary braking system. 
     
    
    
     DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this disclosure pertains. 
     Referring now to  FIG. 1 , there is shown a miter saw assembly  100 . The miter saw assembly  100  includes a base  102  and a turntable  104  that is rotatable on the base  102 . The miter saw assembly  100  further includes a cutting head  106  mounted on a cutting head support assembly  108 . The cutting head  106  (which may also be referred to herein as a “cutting assembly”) includes a motor  110  that is operable to rotate a circular saw blade  112 . The cutting head support assembly  108  is attached to the turntable  104  and configured to support the cutting head  106  such that the cutting head  106  may move over the turntable  104  and perform cutting operations on a work piece supported by the turntable  104 . A rip fence  114  attached to the base  102  may be used to align a work piece thereon. 
     The cutting head support assembly  108  includes a bevel arm  116 , a cutting arm  118 , a first pivot mechanism  120 , and a second pivot mechanism  122 . The bevel arm  116  (also referred to herein as a “bevel post”) provides a bevel support structure for the miter saw assembly  100 . The bevel arm  116  is pivotally attached to the turntable  104  by the first pivot mechanism  120 . The first pivot mechanism  120  includes a hinge arrangement that enables the bevel arm  116  of the support assembly  108  to pivot with respect to the turntable  104  during a setup procedure. In particular, this arrangement is configured to enable the bevel arm  116  to pivot from a vertical position (as shown in  FIGS. 1-2 ) to an angle of 45° (not shown) or more in the leftward direction or rightward direction prior to a cutting operation. This pivoting allows the blade  112  of the cutting assembly  106  to approach the table  104  from a bevel angle and perform angled cuts on a work piece supported on the table  104 , as is well known in the art. 
     The cutting arm  118  of the support assembly  108  provides a support for the cutting assembly  106 . The cutting arm  118  is pivotably connected to the hinge F via the pivot F. The pivot F enables pivoting movement of the cutting assembly  106  in relation to the turntable  104  and the base  102  during a cutting operation. This pivoting allows the blade  112  of the cutting assembly  106  to move toward and away from the horizontal turntable  104  to perform a cutting operation. In other embodiments, the cutting arm  118 , may be mounted to the hinge F component and the hinge F component is mounted on rails (slide miter saws). Another configuration is for the cutting arm  118  to be mounted on directly on bevel arm  116  with pivot at second pivot  122  (chop saw—non sliding or gliding miter saws). 
     The cutting assembly  106  includes a handle  126  connected to the cutting arm  118  to facilitate movement of the cutting assembly  106  in relation to the turntable  104 . The handle  126  is designed and dimensioned to be grasped by a human hand when performing a cutting operation. This allows the user to easily pivot the cutting assembly  106 . A switch (not shown) may be provided on the handle  126  to allow the user to easily energize and de-energize the electric motor  110  during a cutting operation. A blade guard  128  covers the top portion of the circular saw blade  112 . A lower blade guard  124 , shown in shadow for purpose of clarity, is rotatably mounted to the cutting head assembly  106 . The lower blade guard  124  is configured to rotate in a clockwise direction with respect to the cutting head assembly  106  when the cutting head assembly  106  is pivoted toward the turntable  104  thereby exposing the circular saw blade  112 . 
     The connection between the motor  110  and the saw blade  112  is further described with reference to  FIGS. 2 and 3 . The motor  110  has an output shaft  130  which drives a pinion gear  132 . The pinion gear  132  is operably connected to a gear  134  that drives a clutch/brake assembly  133 . The output of the clutch/brake assembly is the primary braking assembly  138 . The primary braking assembly  138  in one embodiment is the braking assembly described in U.S. Patent Application Publication No. 2011/0048197, the entire contents of which are herein incorporated by reference. 
     The primary braking assembly  138  drives a pulley  140  which is operably connected to a pulley  142  by a belt  144 . In some embodiments, the pulley system is replaced by a geared drive system. The pulley  142  is operably connected to a gear  146  which drives a gear  148  operable connected to a drive shaft  150  on which the blade  112  is mounted. The motor  110 , along with the gears and pulleys, are configured such that the blade  112  rotates downwardly. 
     The pinion gear  132  is also connected to a reversing gear  152 . The reversing gear  152  drives a gear  154  that drives a secondary clutch/brake assembly  133 . The secondary braking assembly is operatively connected to the hinge F. 
       FIG. 2  further shows a safety circuit  160  that is operably connected to the secondary clutch/brake assembly  133 , a blade sensor  162  (located adjacent to the drive shaft  150  in this embodiment), and the motor  110 . The safety circuit  160  includes a processor  164  and a memory  164 . Program instructions within the memory  164  are executed by the processor  164  to perform at least some of the actions ascribed to the safety circuit herein. The safety circuit  160  detects when a user approaches too closely or touches the blade  112  and issues a signal which disengages the clutch  136  and activates the primary braking assembly  138  to rapidly stop as discussed in more detail in the &#39;197 Publication. 
     The safety circuit  160  is further connected to the clutch  156  and the secondary braking assembly  158 . Upon sensing a safety condition, a signal is sent to an electromagnet in the clutch/brake assembly  133  and the clutch  156  is released and the secondary braking assembly  158  is activated. Trigger timing of the secondary braking assembly  158  can occur simultaneously with that of the primary braking assembly  138  or after a predetermined time. The timing in some embodiments depends on the particular application (e.g., on miter saw or circular saw). In some miter saw applications such as the embodiment of  FIG. 1 , the trigger timing is also a function of the rotational position of the cutting head  106 . The safety circuit in some embodiments is configured to perform availability/operability testing on the primary and secondary braking systems. In the event of a fault detection of the primary braking system and/or secondary braking system, the safety circuit in some embodiments disables the saw from operating. In other embodiments, in the event of a fault detection of the primary braking system, the safety circuit in some embodiments disables the secondary braking assembly and the saw from operating. 
     Because the secondary braking assembly  158  works in parallel with the primary braking system  138 , a dynamic balancing mechanism is applied to the saw  100 . The secondary braking assembly  158  is selected to be similar to the primary braking system  138  to reduce destructive energy generated from the primary braking system  138 . The two braking components are sized accordingly. 
     Accordingly, when the blade  112  is rotating in the direction of the arrow  200  of  FIG. 3  and the primary braking system  138  is activated, a large angular momentum in the direction of the arrow  202  of  FIG. 4  is generated which forces the cutting arm  118  to pivot about the pivot  122  in the direction of the arrow  202 . The secondary braking system  158 , however, is rotating in a direction opposite to the primary breaking system  138  because of the reversing gear  152  as indicated by the arrows  204  and  206  in  FIG. 3 . Thus, the secondary braking system  158  generates a large angular momentum in the direction opposite to the arrow  202  of  FIG. 4 . Sizing of the secondary braking system  158  is necessary as it is desired to minimized the overall tool weight and size. The secondary braking system can be smaller and weigh less than the primary braking system but needs to rotate faster than the primary braking system in order to counteract the force generated by the primary braking system. Other modifications may be used to provide the desired moment arm to counteract the force generated by the primary braking system  138  for other differences between the two systems, such as the location of the systems. 
     By way of example, in systems which do not include a hinge, the systems described above are modified such that the secondary brake operates on the housing or base of the system. Accordingly, the primary braking system stops rotation of the saw blade in the same manner described above, while the secondary braking system acts, at the same time as, or shortly after activation of the primary braking system, upon the housing or base to reduce movement of the housing or base. 
     In accordance with the above disclosure, a dynamic mechanism is implemented with a primary brake to reduce output force to the structure of a power tool to enable for braking within a predetermined time to mitigate potential injuries. The dynamic mechanism enables balance and control of a power tool such as miter saw head assemblies and circular saws. 
     In some embodiments, the dynamic mechanism is a mechanical brake similar to that of the &#39;197 Publication. 
     In various embodiments, the dynamic mechanism is sized accordingly based on its rotation speed and the primary brake&#39;s rotation speed and its moment of inertia. 
     The dynamic mechanism can be configured with the motor spinning in clockwise or counter-clockwise direction depending upon the particular embodiment. 
     In some embodiments, the dynamic mechanism is a mechanical brake where braking force can be directed to the blade teeth, blade walls, output shaft, or any drive mechanism. For example, a primary brake can be a mechanical brake such as an aluminum block that makes contact with the blade teeth, or any friction material that makes contact with the blade walls, output shafts, or any drive mechanism 
     In some embodiments, the dynamic mechanism is a pyrotechnic mechanism ejecting a mass. In other embodiments, the dynamic mechanism is an electronic brake generated within the motor assembly. 
     While shown in a particular configuration, a dynamic mechanism can be configured in any orientation for compactness other than the shown orientation. 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.