Many years ago we had a terrific carpenter build stairs in our old house using a technique called housed stringer construction. This guy was fairly young but a skilled wood worker. He was also missing several fingers on his right hand. Table saw.
I used to have a table saw, too, but its spinning blade always made me nervous. So I gave it to my brother-in-law who is a cop and tends to be very careful. He still has all his fingers. But a lot of hobbyist and home do-it-yourselfers don’t, courtesy table saws, the woodworking tool associated with more injuries than any other. A new study by the Center for Injury Research and Policy of The Research Institute at Nationwide Children’s Hospital used the Consumer Product Safety Commission’s National Electronic Injury Surveillance System (NEISS), a stratified random sample of some 90 or so hospitals, to get our first view of the descriptive epidemiology of non-occupational table saw injuries. The NIEISS system was designed for consumer products but now encompasses other injuries and is very fast. Follow-back (questioning the victim) for suspicious clusters of product-related diseases can occur within a week or two of the injury, while memory is still fresh.
According to an estimate in the paper, soon to be published in the Journal of Trauma, from 1990 to 2007 there were over 550,000 saw-related injuries seen in hospital emergency rooms, an average of over 30,000 a year:
. . . lacerations (66 percent) were the most common type of injury while amputations (10 percent) were the most serious. The majority of injuries (86 percent) were to the fingers or thumb. Males (97 percent) and adults (97 percent) accounted for the majority of table saw-related injuries. In comparison to adults, children were more likely to injure their heads, faces and necks and to be injured at school.
Most of the table saw-related injuries resulted from contact with the blade of the saw. In cases when the mechanism of injury was documented, kickback was the most common mechanism (72 percent), followed by debris being thrown by the saw (10 percent), lifting or moving the saw (6 percent), or getting a glove or clothing caught in the blade (4 percent). (Eurekalert)
OK. These things are dangerous. I already knew that. But the most intriguing part of the press release was this paragraph:
A new technology that could prevent some of these injuries is the SawStop, which can detect contact between a person and a saw blade and then instantly react to stop and retract the blade. “Although this technology could be beneficial in preventing serious injuries, it is currently too expensive for the average home woodworker to afford,” said Shields. “We recommend that all table saws be equipped with such technology and be made available at an affordable price.”
Wow. for the life of me I couldn’t imagine how this would work, so I went to the SawStop site. Here’s the gist.
SawStop is a brand of table saw that has a unique electronic detection system that induces an electrical signal on the blade, monitoring it for changes. When the spinning blade comes in contact with our fingers, our body’s relatively large electrical capacitance and conductivity cause a rapid signal drop (wood has low capacitance and conductivity so cutting wood doesn’t set it off). Here’s a picture of the electrical signal from the SawStop site that shows the drop in signal when the blade touched a finger:
The several drops you see are caused by finger contact over just two saw teeth. When the system sees this signal it activates a large brake, essentially a block of aluminum activated by a heavy-duty spring held compressed by a fuse wire. When contact is detected a surge of current burns up the fuse wire and the spring pushes the brake pawl into the teeth of the spinning blade, stopping it within 5 milliseconds (1/200-th of a second). The blade’s angular momentum whips it down below the table slot and power to the motor is cut. The company says that any resulting injury is usually only a minor cut, unless your hand is going into the blade at high speed, in which case you will be sending only analog signals from then on — no more digital signals.
The brake apparatus is single-use, so you have to replace the braking cartridge after it goes off. A cartridge costs $70, and I don’t know how much the saw costs, but since your fingers are multiple use and this shouldn’t be tripped unless your fingers contact a rapidly spinning blade, this seems like a decent investment.
Of course you could also give the table saw to your brother-in-law.