Sensors, Not Fingers, Stop Saw Blade (sidebar)

May 1, 2005
Self-preservation drove Steve Gass to found SawStop LLC. He didn’t want to part with his woodworking hobby— or with his fingers—so he devised a technique that would stop a rotating saw blade before it could do serious damage to his digits.

A patent attorney with a doctoral degree in physics, Gass conceived the idea largely due to a lifelong interest in woodworking. “I had my first woodworking accident when I was about four years old,” he says.

When no one would license his technique for determining when the blade made contact with the human body, he set up a manufacturing operation in Wilsonville, Ore.

SawStop adds two major components, the sensor and brake, to a conventional saw. To make the sensor, two capacitors are tied to the blade, which is not grounded. They have inductance of 115 picoFarads. Digital signal processing (DSP) chips from Dallas-based Texas Instruments Inc., which provided some design help, constantly monitor this capacitance. When the blade hits a finger, which has capacitance of 50-100 pF, depending on what the person is standing on, the DSP chip senses the difference and triggers the brake.

Avoiding false positives from something such as wet wood was a key design issue. “The DSP looks at the speed of change, which occurs quickly for a finger, not so quickly for wet wood. We run four algorithms tuned for different contacts and the number of teeth on the blade,” Gass says.

The brake, which is loaded on a 150-pound spring, can be triggered in just 0.003 second. The spring is unleashed when a small fuse wire is burned. The brake, spring, DSP and fuse are packaged in a module that can be replaced after activation.

Gass tried to license the sensor system to a variety of saw manufacturers to no avail, so he finally started making equipment himself, targeting small manufacturers. “Three-fourths of our sales are to commercial users like furniture factories,” Gass says.

The 300 or so saw sensors shipped since last fall have to date saved only two fingers. But Gass notes that, “If you have 10 saws in a plant, that’s one accident per year, on average.”

Though the blade is ruined by the impact of the brake, the finger is saved. “That’s not a bad trade, particularly if it’s your finger,” Gass says.

See the story that goes with this sidebar: Lasers Block Out Injuries