Machine Design

# Capacitance System Stops Table Saws from Amputating Fingers

One day, Dr. Stephen Gass, an avid recreational woodworker and cabinet maker, was using his table saw when his training as a physicist and years of patent-law practice kicked in. He started to think about how to make the saw safer.

He discarded several mechanical-guarding and electrical approaches before deciding to use the phenomenon of electrical capacitance, similar to that used in electronic touchscreens. When a woodworker touches the blade, his finger becomes part of the blade’s electric circuit, changing the circuit characteristics.

Gass’ system, called SawStop, applies a 500-kHz sine-wave current to the blade and measures the voltage response every 6 msec. A sudden drop in voltage indicates a conductive foreign object, like a finger, is drawing current off the blade.

Once the touch is detected, analysis software acts to stop the blade as quickly as possible. When signaled, capacitor sends a surge of current into a fuse wire that holds back a mechanical spring. The current vaporizes the wire within 15 millionths of a second and releases the spring to force an aluminum “brick” into the teeth of the blade.

Gass tested his prototypes by touching hot dogs with similar conductive properties to human flesh to spinning blades and timing how long it took the blades to stop spinning. When the hot dog makes contact with the side of the blade or its teeth, SawStop halts rotation in microseconds. See the hot-dog test in action at www.sawstop.com/howitworks/videos.php.

The hot-dog demonstration was a hit at woodworking-machinery trade shows, but Gass couldn’t convince table-saw manufacturers to license his patented system. Instead, in 2005, Gass and two partners started a company that supplies table saws outfitted with SawStop.

The capacitance-sensing and feedback setup adds about \$50 to the cost of each of the approximately 5,000 saws SawStop LLC sells yearly. Gass estimates if all table saws sold each year, say 500,000, implemented the technology, SawStop’s cost would drop by 50%. He says the same principles would work on radial-arm, compound-miter, band, and other saw types although the implementation would be different for each.

SawStop saws are currently more expensive than those made by larger manufacturers. However, woodworking companies may have to pay out millions of dollars to an employee who loses a finger on the job. Safer saws could help keep health, disability, and liability-insurance costs in check.

Gass knows of over 50 amputation injuries SawStop prevented. That kind of data could come in handy for attorneys representing workers who had digits amputated while using table saws. They could convince the courts that manufacturers who do not use SawStop knowingly ignored technology that could have made their products safer and that table saws without SawStop technology are defective.

If such an approach leads saw manufacturers to make their products more amputation-proof, that’s fine with Gass. He hopes increased demand for safety — for altruistic, financial, or legal reasons — will encourage the industry to adopt new preventive measures.

— Lanny Berke

Lanny Berke is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].

Edited by Jessica Shapiro