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A saw comprises a surface having an opening defined therein, a plate structure configured to be attached to the saw below the surface, and an arm pivotally attached to the plate structure about a first shaft. A blade is configured to be engaged with the arm at a location spaced from the first shaft

A saw comprises a surface having an opening defined therein, a plate structure configured to be attached to the saw below the surface, and an arm pivotally attached to the plate structure about a first shaft. A blade is configured to be engaged with the arm at a location spaced from the first shaft. A portion of the blade extends through the opening when the saw is in a normal operating position. The saw comprises a firing mechanism comprising a barrel end portion having a bore defined therein and an extendable piston at least partially positioned within the bore. The barrel end portion is attached to the plate structure. The extendable piston engages the arm during its extension and causes the arm to pivot about the first shaft when the firing mechanism is activated by an activation signal. The pivoting of the arm causes the blade to be moved downwardly below the surface.

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What is claimed is: 1. A saw of the type which has a top with an opening in the top through which a saw blade can extend, the height and angular orientation of which can be adjusted, and also configured to be retracted in response to an activation signal being applied, the saw comprising: a plate s

What is claimed is: 1. A saw of the type which has a top with an opening in the top through which a saw blade can extend, the height and angular orientation of which can be adjusted, and also configured to be retracted in response to an activation signal being applied, the saw comprising: a plate structure configured to be mounted to the saw below the top; an arm pivotally mounted to the plate structure about a first shaft; a rotatable circular blade having an arbor mounted to the arm at a location spaced from the first shaft, and being configured so that at least a portion of the blade can extend through the top opening and be in a position to cut a work piece when the blade is in its normal operating position; a drive motor with an output shaft operatively connected to power the blade; a firing mechanism having a barrel end portion with a cylindrical bore mounted to the plate structure and an extendable piston positioned in the bore, the extendable piston engaging the arm during its extension and causing the arm to pivot about the first shaft when the firing mechanism is activated by the activation signal being applied, the pivoting of the arm causing the blade to be moved downwardly below the top toward its retracted position; wherein a lower portion of the arm has a curved outer surface at a curvature that is defined by a constant radius about the first shaft that extends from the arbor through an arc of about 90 degrees, the outer surface having a plurality of teeth configured to engage a ratchet member; and a ratchet assembly having a ratchet member biased toward the teeth and being configured to engage the teeth to counter kick back of the arm. 2. A saw as defined in claim 1 wherein the first shaft and the arbor are generally horizontally aligned with one another when the blade is in its normal operating position, the arm having a lower extension with an anvil portion spaced from the first shaft and the arbor for being engaged by an outer portion of the extendable piston. 3. A saw as defined in claim 2 wherein the extendable piston has a hammer located at its outer end portion for contacting the anvil portion, the hammer moving the arm through an arc sufficient to lower the blade below the top. 4. A saw as defined in claim 3 wherein said barrel end portion has a removable end cap structure that reveals a small bore that communicates to said cylindrical bore, said small bore being sized to receive a pyrotechnic cartridge, said end cap structure being configured to accept a metal pin that can be connected to a power source for heating said pin and firing said cartridge. 5. A saw as defined in claim 4 wherein said cartridge is a small caliber commercial nail gun cartridge that when fired creates temperature that fires the cartridge which produces combustion gases that cause a rapid pressure rise in the barrel bore behind the piston that produces several thousand pounds of force to propel the piston from said barrel portion. 6. A saw as defined in claim 5 wherein said piston reaches a substantial portion of its extension, it causes said combustion gases to be vented, thus relieving the barrel pressure in preparation to stopping said piston. 7. A saw as defined in claim 3 further comprising a damper member connected to one of said arm or said firing mechanism and positioned to be engaged by said hammer and stop its movement. 8. A saw as defined in claim 1 further comprising a detent assembly mounted to said plate structure and engaging said arm to releasably hold said arm in its position when said blade is in its normal operating position. 9. A saw as defined in claim 8 wherein said detent assembly comprises a movable member that is configured to releasably engage a recess in said arm, said movable member being biased toward engagement, said hammer contacting said anvil during activation of said firing mechanism overcoming said bias and moving said arm. 10. A saw as defined in claim 1 wherein said drive motor is mounted to said plate structure, said output shaft of said motor having a first pulley, said first shaft having interconnected second and third pulleys and said arbor having a fourth pulley, said saw having a first belt interconnecting said first and second pulleys and a second belt interconnecting said third and fourth pulleys, so that said motor drives said blade during operation. 11. A saw as defined in claim 3 further comprising a stop damper mounted to said plate structure configured to engage said arm after it has rotated through an arc sufficient to move said blade to its retracted position. 12. A saw as defined in claim 1 wherein the plate structure comprises a generally flat steel plate having a thickness within the range of about ⅜ inches to about ½ inches. 13. A saw comprising a working surface having an opening defined therein, the saw comprising: a plate structure configured to be attached to the saw at least partially below the working surface; an arm pivotally attached to the plate structure about a first shaft, the arm comprising a surface having at least two teeth; a blade engaged with the arm at a location spaced from the first shaft, the blade positioned within the opening such that at least a portion of the blade extends through the opening and is situated to cut a work piece when the saw is in a normal operating position; and a firing mechanism comprising a barrel end portion having a bore defined therein, wherein the barrel end portion is attached to the plate structure, and an extendable piston at least partially positioned within the bore, the extendable piston engaging the arm during its extension and causing the arm to pivot about the first shaft when the firing mechanism is activated by an activation signal, the pivoting of the arm causing the blade to be moved downwardly below the working surface; and a ratchet assembly comprising a ratchet member biased toward the at least two teeth and configured to engage at least one of the teeth to counter kick back of the arm. 14. The saw as defined in claim 13 wherein the surface of the arm comprises a curved outer surface at a curvature that is defined by a constant radius about the first shaft that extends from the arbor through an arc of about 90 degrees. 15. A saw comprising a working surface having an opening defined therein, the saw comprising: a plate structure configured to be attached to the saw below the working surface; an arm movably attached to the plate structure about a first shaft, the arm comprising at least two teeth; wherein a blade is configured to be engaged with the arm at a location spaced from the first shaft, the blade configured to be positioned within the opening such that at least a portion of the blade extends through the opening and is situated to cut a work piece when the saw is in a normal operating position; a firing mechanism comprising a barrel end portion having a bore defined therein, wherein the barrel end portion is attached to the plate structure, and a firable piston configured to be at least partially positioned within the bore, the firable piston movable between a first unfired position, where the blade is in the normal operating position, and a second fired position, where the blade is moved into a position below the working surface of the saw, the firable piston in communication with an activation signal and a ratchet assembly comprising a ratchet member biased toward the at least two teeth and configured to engage at least one of the teeth to counter kick back of the arm. 16. The saw as defined in claim 15 wherein the arm comprises a curved outer surface at a curvature that is defined by a constant radius about the first shaft that extends from the arbor through an arc of about 90 degrees. 17. A saw of the type that has a top with an opening in the top through which a saw blade can extend, the height and angular orientation of which can be adjusted, and also configured to be retracted in response to an activation signal being applied, the saw comprising: a plate structure configured to be mounted to the saw below the top; an arm pivotally mounted to the plate structure about a first shaft; a rotatable circular blade having an arbor mounted to the arm at a location spaced from the first shaft, and configured so that at least a portion of the blade can extend through the top opening and be in a position to cut a work piece when the blade is in its normal operating position; a drive motor with an output shaft operatively connected to power the blade; a firing mechanism having a barrel end portion with a cylindrical bore mounted to the plate structure and an extendable piston positioned in the bore, the extendable piston engaging the arm during its extension and causing the arm to pivot about the first shaft when the firing mechanism is activated by the activation signal being applied, the pivoting of the arm causing the blade to be moved downwardly below the top toward its retracted position; wherein the first shaft and the arbor are generally horizontally aligned with one another when the blade is in its normal operating position, the arm having a lower extension with an anvil portion spaced from the first shaft and the arbor for being engaged by an outer portion of the extendable piston, wherein the extendable piston has a hammer located at its outer end portion for contacting the anvil portion, the hammer moving the arm through an arc sufficient to lower the blade below the top, and wherein the lower portion of the arm has a curved outer surface at a curvature that is defined by a constant radius about the first shaft that extends from the arbor through an arc of about 90 degrees, the outer surface having a plurality of teeth configured to engage a ratchet member; and a ratchet assembly having a ratchet member biased toward the teeth and configured to engage the teeth to counter kick back of the arm. 18. A saw of the type that has a top with an opening in the top through which a saw blade can extend, the height and angular orientation of which can be adjusted, and also configured to be retracted in response to an activation signal being applied, the saw comprising: a plate structure configured to be mounted to the saw at least partially below the top; an arm pivotally mounted to the plate structure about a first shaft; wherein a blade having an arbor is configured to be mounted to the arm at a location spaced from the first shaft, and is configured so that at least a portion of the blade can extend through the top opening and be in a position to cut a work piece when the blade is in its normal operating position; a firing mechanism having a barrel end portion defining a bore mounted to the plate structure and an extendable piston positioned within the bore, the extendable piston engaging the arm during its extension and causing the arm to pivot about the first shaft when the firing mechanism is activated by the activation signal being applied, the pivoting of the arm causing the blade to be moved downwardly below the top toward its retracted position; wherein the first shaft and the arbor are generally horizontally aligned with one another when the blade is in its normal operating position, the arm having a lower extension with an anvil portion spaced from the first shaft and the arbor configured to be engaged by an outer portion of the extendable piston, wherein the extendable piston has a hammer located at its outer end portion for contacting the anvil portion, the hammer moving the arm through an arc sufficient to lower the blade below the top, and wherein the lower portion of the arm has a curved outer surface at a curvature that is defined by a constant radius about the first shaft that extends from the arbor through an arc of about 90 degrees, the outer surface having a plurality of teeth configured to engage a ratchet member; and a ratchet assembly having a ratchet member biased toward the teeth and configured to engage the teeth to counter kick back of the arm.