Saving Fingers Using Cutting Edge Technology
By Russ Lee
Such were the ruminations of patent attorney Stephen Gass, while working on a hobby wood project in his Portland, OR-based home shop. "I was out in my shop one day and looked over at the table saw," says Gass, who also holds a doctorate in Physics. "I wondered if it might be possible to stop a saw fast enough to avoid serious injury if you ran your hand into it by accident. So, I did some calculations. I figured out how fast it would have to stop, how much force would be involved, and if it was a realistic thing to do."
What Gass came up with was a revolutionary idea for a table saw brake that brings a blade to a full stop within three milliseconds (that is three one-thousandths of a second) after encountering human flesh. Even more amazing, the device can differentiate between wood or plastic and your finger by measuring each material's electrical charge. That means an operator can freely process materials for months on end without a pause in operation, but as soon as his finger comes in contact with the blade, everything comes to a speedy halt.
"The system detects a person's capacitance, or ability to store a charge," Gass explains. "It is an inherent property of all physical objects. The human body, because it conducts pretty well, has the ability to absorb and store a charge quickly. But wood and plastic don't conduct very well, so their capacitance is low."
Gass further notes, "We put an electrical signal on the blade so that when you are cutting wood or plastic, nothing happens to the signal, it just stays at the same level. When it runs into your finger, your body absorbs some of the signal and it drops the level down very quickly. In fact, before the first tooth leaves your finger, the brake will have already tripped."
The brake, which Gass has named SawStop, is best described as a type of hermetically sealed plastic cartridge, which is mounted about 1/8" away from the face of the saw blade inside the body of the table saw. Inside the cartridge, a brake shoe sits in front of a very powerful spring, which is actuated when a sensitive fuse wire is tripped. When the system detects contact with a finger, it burns the fuse wire, which releases the spring very quickly and causes the brake to make contact with the blade. The angle of contact is such that once the brake hits the blade, it is self-locking. Friction between the blade and brake pull the brake more tightly against the blade.
Because the system is initially triggered by the blade coming in contact with a human finger, the table saw operator will necessarily receive a minor cut before the blade is stopped. But, considering the typical consequences for getting a digit caught in a rapidly spinning cutter, sustaining a little boo-boo is a small price to pay for a lapse in concentration.
"A saw blade makes one revolution in about 16 milliseconds," Gass observes. "For most people, it takes about 125 to 150 milliseconds to react and pull your hand away once your finger gets caught. With an 80-tooth blade, that means close to 800 teeth will cut into your finger before you can pull it out. With our system installed, and using an 80-tooth blade, the blade would stop within 20 teeth. You almost can't believe it until you see it for the first time yourself."
Another appealing feature of the SawStop system is that it is installed below the table saw deck, which means there are no bothersome guards to deal with. "It doesn't affect the operation of the saw in any way until you need it," Gass says. "Then you would be very, very happy it was there."
The SawStop system is engineered to render the table saw
inoperative if anything goes wrong with the brake before or during
operation of the saw. For those materials that exhibit higher
capacitance characteristics (such as aluminum) that would cause the
fuse wire to trip, the operator may override the system for that
cutting cycle with the flip of a switch. On those occasions where
the brake has been actuated, a new cartridge must be installed
before the saw will become operable again, a simple operation that
Gass compares to changing the table saw blade.
Getting a brake
By now you're probably wondering where you can go to purchase a machine with the SawStop system installed. But, for that you will have to be patient. "We don't actually sell anything," Gass says. "Our goal is to license this technology to all of the different manufacturers so they can incorporate it into their machines when they build them."
The folks at SawStop have been generating grass roots support from the ultimate end-users of the product through participation in industry trade shows. Once people see a prototype in operation, Gass hopes they will apply pressure on the equipment manufacturers to include it on their new machines.
"When we first took our idea to the manufacturers, we heard things like, 'Safety doesn't sell,' " Gass says. "That is why we debuted the product at the International Woodworking Fair in Atlanta last year. Our goal is to make people aware of SawStop so that they can go to the companies they buy their tools from and tell them they want it. It is a very funny business model: No, we don't have anything to sell, but we are here at trade shows promoting it, although there is nowhere you can get it yet.
"We have gotten tremendous positive feedback from people. It is applicable on other types of equipment as well band saws, jointers, hand-held circular saws, mitre saws. SawStop can work on most of the tools you could cut your finger off with in a woodworking shop."
To learn more about SawStop, LLC, visit its Web site at www.sawstop.com, or call 503-638-6201.