Notice the aluminum frame has been crushed as a result of impact by moving saw blade, and the spring has been bent upward. To stop the blade this quickly would require about 1,000 pounds of force to decelerate the blade in 10 milliseconds. Any further, and the cut would be so deep that the device would be useless. Given the speed of the blade, it would have to stop in about 1/100 of a second - or at about an eighth of an inch of rotation after making contact.
Next, he had to design a triggering system that could differentiate between finger and wood. First, needed a brake that would work quickly enough when it came into contact with a woodworker's hand. Stopping the blade.would require a two-part process.
It is impractical to retrofit into existing table saws.The braking system is designed to work with kerfs from 3/32″ to 3/16″ using thinner or thicker kerfs limits the saw's ability to stop the blade after accidental contact, likely resulting in more serious injury.Non-conductive blades or blades with non-conductive hubs or teeth cannot be used.The braking system must be deactivated when cutting very green or wet timber.The block is normally held away from the blade by a wire, but during braking an electric current instantly melts the wire, similar to a fuse blowing.Īccording to SawStop, the system has restrictions and limitations: When the brake activates, a spring pushes an aluminum block into the blade. The 200-kHz signal will have up to 10 pulses during that time, and should be able to detect contact with just one tooth. A tooth on a 10-inch circular blade rotating at 4000 RPM will stay in contact with the approximate width of a fingertip for 100 µs. After signal loss for 25 micro seconds (µs), the detector will fire. If a human contacts the blade, the signal will fall below the threshold. A plate on the other side of the blade picks up the signal and sends it to a threshold detector. The signal is transferred to the blade by capacitive coupling. Īn oscillator generates a 12-volt, 200- kilohertz (kHz) pulsed electrical signal, which is applied to a small plate on one side of the blade.
The design takes advantage of the difference in conductance and capacitance between wood and flesh. The operator suffers a minor instead of serious injury. The manufacturer states the saw stops in less than five milliseconds, and angular momentum retracts the blade into the table. Saw Stop table saw used at former TechShop in Redwood City California
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