Effect Measure

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.

[snip]

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:

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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.

Comments

  1. #1 Paula
    January 18, 2010

    What’s kickback (in this sense)? Stop kickback seems also a good idea–also affordably. Meanwhile, when an all-metal screen door took off the tip of one of my fingers, 2 physicians and 2 nurses–ER and later two offices as this was the Kaiser system–told me how common this injury was among hospital personnel. Any thoughts on this?

  2. #2 george.w
    January 18, 2010

    It’s a clever system, but even if it were cheap it’s unlikely to be reliable over the 30+ year lifespan of some table saws, living as they do in unheated garages of homeowners. There are three simple, low-tech things needed for safety on a table saw. First, use a splitter; it makes kickback very much less likely. I fashioned one out of an aluminum platter from a big old hard drive. Two, use push sticks every time. With practice, sticks can give you great control. And three, don’t stand directly behind the blade. And of course eye protection and clear footing and all that.

  3. #3 daedalus2u
    January 18, 2010

    “Kick-back” in this sense is when the material being sawed is caught by the spinning blade and “kicks back” toward the operator.

    When a table saw is operated, the blade spins such that the top of the blade is moving toward the operator and the bottom of the blade is moving away. This results in a downward force on the work piece as the blade moves through it. There is also a component of force pushing the work piece away from the saw blade and toward the operator. This force must be overcome by the operator to continue sawing.

    Kickback is mostly a problem when trying to cut thin sections of materials that are hard and strong. One of the worst materials is aluminum. It is ok to cut aluminum on a table saw, provided that one is very very careful. Aluminum is softer than the blade, and doesn’t spark, so it can be cut. However if the aluminum is thinner than the spacing between the teeth, then if the work piece is moved too fast, or too much (i.e. if you slip), then the aluminum can get caught between the teeth in a thickness that is too thick to be cut, so it catches, there is some positive feedback as the tooth jams the aluminum in greater contact, the aluminum doesn’t cut, instead the aluminum matches velocity with the spinning blade. If you have a big blade, and a small piece of aluminum, the aluminum can reach speeds close to that of the blade. This can happen for plastic too, or even for wood. If the work piece is thick compared to the spacing between the teeth, this is much more difficult to happen, but hitting a nail could do it.

    My table saw cost me $99, so a $70 single-use add-on is a big expense. This item would not protect against kick-back. This safety feature is not available in retrofit to an existing saw. Activating it can damage the blade. I think this would be a worth while feature for a shop that uses saws all day, and can afford to inventory the replacement parts.

    There is something about working around rapidly spinning sharp pieces of metal that can instill a habit of being careful. Knowing how to be careful is a useful skill, one that is not to be confused with being foolish. I have never cut myself on a table saw and as I get older I find myself becoming more and more careful as the wisdom of age replaces the folly of youth.

  4. #4 eadwacer
    January 18, 2010

    My understanding is that the home saw industry doesn’t want to install these because they then become liable for product failure. If the radial arm saw is unprotected, and you decide to use it as a radial finger saw, that’s your fault. If it is protected, then any damages become their fault.

  5. #5 kevin
    January 18, 2010

    eadwacer has it right, from my understanding. The guy who invented the mechanism initially tried to get other companies to use it, but they all refused. This is a shame. He only reluctantly produced his own line of saws.

    daedalus2u… This safety feature is not available in retrofit to an existing saw. That might be so, and unavoidable for engineering reasons. So blame all the companies who refuse to sell models with the device. Activating it can damage the blade. wtf? Who cares? My choice: a new saw blade plus a new sawstop cartridge. Your choice: surgery.

    george.w: but even if it were cheap it’s unlikely to be reliable over the 30+ year lifespan of some table saws, living as they do in unheated garages of homeowners. Do you have some evidence, or even anecdote to back this up? Or are you just spouting?

  6. #6 daedalus2u
    January 18, 2010

    kevin, I brought up damage to the blade as another cost added to the $70 cost of replacing the cartridge. I agree that those replacement costs are trivial compared to the cost of medical treatment, even for very minor injuries.

    The saw is not cheap, I have seen prices ~$3,000 and higher. That puts it completely out of my price range, and the price range of many casual users.

    http://toolmonger.com/2007/06/12/hot-or-not-sawstops-safe-table-saw/

  7. #7 glock
    January 18, 2010

    george.W, and daedalus2u have it exactly right.
    Learn safe working habits and use them EVERY TIME, and follow your parents advice….”be careful”

    While the technology is amazing and useful if it works as you say, my experience tells me it will lead to a complacency that will be detrimental to your digits when using routers,band saws,and other “dangerous” tools.

    I KNOW the minute I put on safety equipment (full helmet,shoulder pads,chest protector,bulletproof boots etc.)
    i got a lot more reckless riding motocross… you kinda forget about fractures, till it’s too late.

  8. #8 mikeb
    January 18, 2010

    I agree with the caution to be careful; on the other hand, I also see the value in redundancy. Saying that we shouldn’t install safety mechanisms because it causes recklessness is somewhat (though not exactly) the same as saying that seat belts and airbags are unnecessary because it makes us less careful driving. It is plausible, but, given the choice, I would rather be careful *and* drive with my seat belt and airbag.

    The other thing about table saws is that accidents can happen even when you are being very careful. A hidden knot can catch the table saw blade and cause the wood to kickback or your hand can accidentally slip off of the wood causing it to start fishtailing in the blade (in my limited experience using a table saw as an undergraduate architecture student, that was by far the most common danger that I faced). One can often get it back under control, but occasionally, you can’t.

    One way to prevent kick-back is to keep downward pressure on the board that is being cut (perpendicular to the table surface). I don’t know about newer and/or cheaper home models, but the table saws that I used there was not any mechanism to keep downward pressure. It seems like this might be a cheaper mechanism where there should be a sensor that the board is starting to kick back and to apply the brake. The speed of the break would not need to be as quick as one where fingers will be cut off in the next 5-10 milliseconds, so the potential for a reusable break disk might be an option. Just doing that could reduce injuries by up to 75%, then it seems like a potential nice first step.

  9. #9 Greg Laden
    January 18, 2010

    I’ve seen it work, and it does work. The process of resetting up the saw after a break, with a new blade, is very costly and time consuming. Which is good, because that is what should happen to you after you let your finger touch the spinning blade (other than taking the finger off)!

    If you have a table saw in a school shop, it should be this kind.

  10. #10 carl
    January 18, 2010

    Yeah, wife got me one a few years ago and I am damned careful with it. I always stand to the side of the blade rather than in front and keep my hands at least eighteen inches away from blade, so far maintained all body parts.

  11. #11 Karen
    January 18, 2010

    My training is in theatrical design and construction, and I have to say that a system like this seems like a really good idea for busy shops — and especially for schools. Many people taking shop classes aren’t familiar with or comfortable with tools, and will do stupid things (I’ve snatched hands away from spinning blades or hit the emergency stop on a tool on several occasions when working with inexperienced people). But it’s also good for places where these tools are used on a daily basis and the workers are all experienced, because people can get complacent and not take proper safety precautions. I’ve seen people with 30 years experience in the shop do dumb things just because their mind isn’t 100% on their work.

    For a household tool it’s definitely too expensive right now, but technology prices do drop rapidly once something enters the market. For now, though, I think that most household handymen (and women) will have to make do with caution, push-sticks, and making sure that all of the relevant safety guards are in place.

  12. #12 TSK
    January 18, 2010

    Another tool you can use to protect your hand
    and fingers are chainmail gloves. They are
    used by butchers and cost approx. $100.
    As most accidents are inadvertently and
    you reflexively retract your hand they are
    good protection even if they may not hold
    when you really press against the blade.

  13. #13 Vorn
    January 18, 2010

    TSK@12: Chainmail gloves work just fine when you’re working with straight or low-speed blades, like butchers do – meat slicers don’t have barbs, and those few knives that do have barbs are moving slowly. However, a table saw is moving extremely quickly and has barbs, so catching your hand on it is very easy with chainmail gloves, and then the glove itself will hurt your hand or tear off.

  14. #14 Sxote
    January 18, 2010

    @TSK:

    As Vorn notes, chainmail is fine for working with knives, but with with a table saw it is very, very important not to have anything loose, like a chainmail glove, near the spinning blade. Such a thing will just be likely to get caught in the saw and **pull you into it** more than if you had no glove and cause more damage, wrecking your whole hand.

  15. #15 Art
    January 18, 2010

    I don’t object to these sorts of devices. They have been around for a few years but unless there has been a major price reduction I don’t know about they run over $200 each. Some close to $300. Considerably more than the $70 price listed. But even $300 and a new blade, $20 to over $200 depending on size and quality, are still cheaper than having a finger reattached.

    These sorts of things might be best employed on saws that are frequently used by untrained and inexperienced people. School shop classes, community shops and trade training centers come to mind.

    That said operator training and conscientious use of safe practices, correct use of guards, avoiding the kickback zone, and using push sticks and feather boards, can eliminate most of the hazard. Properly maintaining and adjusting the table saw are also important. A blade that isn’t aligned with the table and fence will bind and require more force to move materials through the blade and greatly increase the odds of an accident.

    I wouldn’t use chainmail gloves. Most blades are carbide tipped and tougher than most links materials. And if they don’t rip through it you may be in even worse trouble. If the glove jams in the blade there is a good chance it pulls your hand into the works. Better to lose a finger than have an entire hand mangled.

  16. #16 anna
    January 18, 2010

    My father the woodworker’s input:
    “Yes, this is the latest in safety technology for the woodworking industry. It is a polarizing product, however. When the inventor was unable to interest any of the saw manufacturers in buying the technology, he started his own company and then tried to have the government require it in their OSHA safety specs. Well, this outraged woodworkers, many of whom are redneck and anti government. It is now on the market, a bit expensive, effective and selling well enough to keep them in business. The slogan from those who use it is that the slightly higher price is a bargain compared to having an ambulance ride, hospital bills and loss of finger (s).”

    and a video link:http://www.youtube.com/watch?v=esnQwVZOrUU

  17. #17 Crudely Wrott
    January 18, 2010

    I’ve been around table saws and woodworking equipment most all my life. I employ such tools in my work. They all scare the bejessus out of me when I stop to think what they can do when used improperly or without sufficient instruction. Happily, I have all my digits intact though none without scar.

    With respect to table saws: if you aren’t familiar with them you need to talk to someone who is before you start. This is because there are a several things about the saw, the blade and the material being cut that can hurt you badly, quickly. If you have used just one or two saws and are approaching a third, you really should to talk to someone who has used that particular saw. This is because 1) no two saws are mechanically and geometrically the same, and 2) they don’t come from the factory ready to use. They must be tuned, just like any other precision mechanism. Their performance, not to mention the expectation of their performance, is predicated on a few simple but critical parameters.

    The blade must spin true without run out, that is, wobbling from side to side. This condition prevents a clean cut and increases the likelihood that the blade will bind.

    The rip fence must be parallel with the blade. If not, the stock will meet the blade at an angle increasing friction and uneven cutting action. If the outfeed end of the fence is closer to the blade than the infeed end, you will be trying to pass stock wider than the tool can accept.

    The table must be flat, true and clean! As in the above, if these conditions are not maintained then the stock will be impeded by forces other than the force it takes to pass the blade. I regularly clean and wax the table so that the stock takes minimal force to maneuver, present to the blade and guide on through. On a good quality saw, all of these things are user accessible and adjustable. Cheap units, not so much. Buyer beware.

    There’s a lot more to it than just these basic considerations. I’d say that outside of knowing how your tools work and how to keep them working as they were designed to the most important thing to do is to not get in a hurry. A saw might cut eight foot of pine in a few seconds but try forcing a piece of maple through it in the same time! I’ve seem ‘em try. Like the old man said, let the tool do the work.

    On more than one occasion I have pushed too hard or broken another rule and had a power-tool wreak. I’ve torn up tools and good material. That I haven’t been seriously hurt is just my own perverse luck.

    Knowing how to use and maintain a tool is one of life’s oldest delights. We should all know how to use as many as possible. As a rule of thumb, it is better to not try to use ones we aren’t familiar with. Until we have sought instruction, that is. Then it’s time to make stuff. That’s what we do, us tool users.

  18. #18 William Milliken
    January 18, 2010

    Never wear any type of gloves working with power tools! Instead of protecting your hands they will pull you further into the blade. Chain Mail is no match for a saw blade.

  19. #19 daedalus2u
    January 19, 2010

    I agree, thick or heqavy gloves should not be worn when working with power tools if there is even a little chance of them getting caught. This is why rings shoulcn’t be worn either. If they get caught, they may take a finger off, or if the hand is injured they will get trapped by the swelling.

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