referring to fig1 a table 8 supports an overhead arm 10 having a rear portion which is mounted on a thin steel strut plate 12 which in turn is supported on the table 8 in alignment with and having a thickness no greater than a saw blade 14 . a guard 16 comprising an arcuate yoke 18 having front and rear arms which surround the portion of the blade at the saw 14 projecting above the top surface of table 8 projects downwardly from the front portion of the arm 10 in the direction of contacting its lower surface with the top surface of the workpiece ( not shown ) below having a guard vertical - positioning means 19 movably adapted for motion up and down of the guard 16 in the vertical direction relative thereto . right - hand and left - hand clear plastic covers 20a and 20b , as shown in fig2 cover each side of the arcuate yoke 18 disclosing that portion of the blade of the saw 14 above the top surface of table 8 centrally located therebetween and both sides of the piece work being fed to the saw below for full view by the operator while the cut is being made with the full protective benefit of the guard 16 in place . a yoke shank 22 which is structurally a portion of the arcuate yoke 18 extends upwardly from the central portion thereof into the front portion of beam 10 . pads 24 and 26 extend forwardly and rearwardly from the front and rear arms of the arcuate yoke 18 , respectively , having lower surfaces contacting the top surface of the workpiece below . first tongue and groove joint 28 slidingly mates the aft end of pad 28 with the front vertical edge of thin strut plate 12 for guiding the arms of the yoke 18 in their vertical up and down movement relative to the workpiece below . guard tilt - hinging means 30 which is structurally mounted on table 8 below the top surface thereof engages the strut plate 12 for lateral support which is adjustable in the fore and aft and hinged in the up and down directions in order to properly locate the guard 16 with respect to the saw 14 and the workpiece being operated upon below . reference to fig3 shows a guard vertical position - sensing saw - motor shut - off switching means 31 comprising a limit switch 32 which mounts on pad 24 off the forward arm of yoke 16 having an actuating roller plunger 34 which extends in a downwardly direction therefrom for contacting the top surface of the workpiece below and making electrical contact therein whenever a workpiece of predetermined thickness is in postion for cutting . the electrical contact in the switch 32 is opened immediately when the workpiece is removed . second tongue and groove joints 36a and 36b each slidingly mate each of the fore end and midportion of arm 10 with each of the fore and back sides of the yoke shank 22 , respectively , as shown in fig1 for guiding the shank 22 of the yoke 18 in its vertical up and down movement relative to the workpiece below . rack and pinion drive 38 mounts on the right side of the yoke shank 22 having one end of a pinion drive shaft 40 extending rearward therefrom wholly within the overhead arm 10 for moving the guard 16 in its vertical up and down movement relative to the workpiece below . a step - down gear box 42 , having a reversing motor drive 44 operationally connected thereto , mounts on the rear end of arm 10 and operationally couples its output shaft to the other end of the pinion drive shaft 40 , for driving the rack and pinion drive 38 and moving the guard 16 in its vertical up and down movement . as shown in fig4 and 5 , a control box 46 mounts on beam 10 , having a source of 110 volts a . c . power including d . c . power supply circuit 70 which shunts across the secondary of a transformer 50 having its primary connected to an a . c . power source through the fuse 56 . circuit 70 comprises a full - wave bridge rectifier 72 operationally connecting to the power source , and a saw - motor starter circuit 74 having a second relay - actuating coil 76 operationally connecting one terminal thereof to the hot output of rectifier bridge 72 . a voltage limiting diode 78 shunts out the relay coil 76 . a saw - motor starting circuit switching means 79 comprising a triac 80 operationally connects its main terminal 2 to the other terminal of the coil 76 and its main terminal 1 to ground for switching the saw - motor starter circuit 74 on and off . a triac trigger circuit 82 , which series - connects its input through a first relay - actuated switch 83 having first , and fourth relay - actuating coils 112 and 116 , respectively ( not shown ), a first resistor 84 and a diode 86 to the hot output of rectifier bridge 72 , comprises a pnp transistor 88 having its collector terminal connected through a second resistor 90 to the gate terminal of said triac 80 and through a parallel third resistor 92 to ground . the emitter terminal of said transistor 88 connects to the output terminal of first switch 82 through a third resistor 94 . a time - delay saw - motor shut - off circuit 96 comprises series - connected resistances 98 and 100 which connects its input terminal to the output terminal from first switch 82 . the base terminal of transistor 88 connects to the output terminal of series - connected fourth and fifth resistances 98 and 100 . a first capacitor 102 connects its high - voltage terminal to the output terminal of series - connected resistances 98 and 100 and its low - voltage terminal to ground . a sixth resistance 104 connects its input terminal to the base terminal transistor 88 and its output terminal through diode 106 in series - connection with two - point make limit microswitch 108 . a guard horizontal position - sensing saw - motor shut - off switching means 110 , which connects to the output of first resistor 84 , comprises a first relay coil 112 having its input terminal connecting to the output thereof . a mercury switch 114 , which horizontally mounts on the guard arm 10 and is oriented in the fore - and - aft direction thereon , connects its input terminal to the output terminal of coil 112 . a second relay - actuated switch 116 having first and third relay - actuating coils 112 and 54 , respectively , connects its input terminal to the output terminal of the mercury switch 114 and its output terminal to the input terminal of a third relay - actuated switch 118 having first and second relay - actuating coils 112 and 76 , respectively , the output of which connects to ground . in the control box 46 , having the source of 110 volt a . c . power , an a . c . power supply circuit 48 operationally connected thereto comprises the step - down transformer 50 , a guard - motor starter box 52 operationally connected thereto having a third relay - actuating coil 54 connected at one terminal thereof through the fuse 56 to one terminal of the secondary of the step - down transformer 50 . a runlight ( green ) 58 shunts across the relay coil 54 . a bypass keyswitch 60 which is in series connection with the relay coil 54 operationally connects the same across the secondary of transformer 50 through fuse 56 when closed . a third relay - actuated switch 62 having first and second relay - actuating coils shunts across the keyswitch 60 . a fourth reversing polarity means 64 having first and second up / on pushbutton switches 66 operationally actuating relay - actuating coil 54 operationally connects with the reversing a . c . motor 44 for moving the guard 16 in its vertical up and down movement relative to the workpiece below according to the setting of toggle switch 66 on the front panel of control box 46 . a set material thickness light ( red ) 68 shunts across the motor 44 . an operator has 42 seconds to start a cut after setting the guard 16 at its working height , by means of the switch means 66 , where the roller plunger 34 contacts the workpiece to close the switch 32 . as long as the operator is operating or cutting , the saw 14 will continue to run . if the operator leaves the site of operation without turning the saw 14 off , the switch 32 will be open and the saw 14 will be shut off automatically after the preset waiting period , for example , the before mentioned 42 seconds . there is a bypass keyswitch 60 for hard - to - do - jobs . however , the key of switch 60 is held by the supervisor who must then supervise the setting of the saw 14 and make sure that it is put back into the automatic safety position . it will be understood that details of the construction shown may be altered or omitted without departing from the spirit of the invention as defined by the following claims .