I find it difficult to believe that a hotdog swung into the sawblade would result in a minor cut. Swinging a hotdog at a completely stopped blade would likely result in the hotdog being cut in half. I know if I swung my hand into my stopped blade I would have a serious injury.
Consider that the SawStop not only stops the blade, but also drops it below the table.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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Here's the text from page 66 of FWW #174:
"I stuck a hot dog on the end of a stick and swung it into the moving blade as fast as I could. The result was a 1/16-in.ideep by
1/8"-in.-wide by 3/16"-in.-long cut. One of the students, a medical doctor, said that such a wound would require two or three stiches at most."The blade not only stops - it also drops below the table top. Sounds like the blade got out of the way before, even stopped, it could cut the hot dog in two.
Doug Miller wrote: ...
But not until it has already hit ...that's not to say it may not minimize some degree of damage relative to not having it.
I'm a lot more concerned about a digit simply being in the wrong place along w/ the lumber being cut...I don't see how that would (or could) be detected.
I also read the FWW review article and don't recall the test of strapping the hotdog on a piece of oak and running the combination through the blade...
What does the lumber have to do with it ?
If the saw won't cut lumber w/o a pinkie (or hot dog), it won't be of much use will it? :)
I'm simply questioning whether the detection capability is able to distinguish a piece of flesh in the way while there's still a continued normal cutting load and have seen no indication of a test to show it any any review...
That's the whole point of it. It uses electrical signatures--a finger has a different signature than a piece of wood and it can distinguish the two most of the time--it seems to be designed to err on the side of caution--it apparently will trigger with some species of wood if cut green for example because the signature is similar to a finger.
The detection has nothing to do with the load the saw is under. It is similar to what is used on the "touch lamps" and detects contact with a conductor. Because of that false triggers due to very green lumber (or wet pressure treated), metal in the wood or other possibly conductive things are a concern.
Tim Douglass
Duane,
The brake works whenever it touches your blood stream regardless of whether it is also cutting wood at the time. In fact, the standard hotdog demo that SawStop runs at trade shows is with the dog perched on top of a piece of 3/4" plywood. The plywood is being cut at the exact same time as the hotdog is sensed and the blade stops & drops. You could be churning through 12/4 rock maple under enormous feed pressure straining your 5 HP 3 phase motor to the max, happen to run your finger across the top of the blade, and the brake would still work. The sensing system is independent.
Best, Dave
But still doesn't answer the question of whether it can detect the condition of hand on board into the saw...which is the only case I've ever even come close to finding myself in. I've never had any concern of just putting my hand into the blade by itself...
Well, that's fairly impressive if it actually works...
Hello,
Actually, it has nothing to do with conductivity (resistance), but with capacitance, ie the proportion of your boddy to load with static electricity.
regards, cyrille
Doesn't have anything to do with "touching your blood stream". Touching the side of the blade will also trigger it I understand.
That's what it's intended to do. There's discussion of how it goes about this on the Sawstop site.
While there are a lot of concerns about its function and utility, being able to trigger under the circumstance you describe has not been one of them.
Evidently the device does actually work. SawStop says that, during development, testing, and public demos, they have triggered the brake over a thousand times. One could discount that testimony as a magician endorsing their own parlor tricks. Maybe the SawStop employee doing each demo nudged a switch with his foot as he was feeding the plywood and hotdog through the blade. I guess that's why Fine Woodworking ran their tests - to make sure that SawStop wasn't tricking people. FWW concluded that the device is real and it works.
The device is fairly simple. Once you understand it, you might wonder why it hadn't been thought of sooner. They electrically isolate the blade, then feed a low voltage signal to it. They monitor the signal (about 3 volts). When the blade comes into contact with an object that has a lot of electrical capacitance, such as a human body, the signal voltage temporarily dips. This dip is much more pronounced when contacting a person than when cutting wood - unless you commonly rip pieces of wood that weigh 150 pounds, are at 80% moisture content, and have large unbroken electrical pathways throughout similar to our blood vessels.
Once contact is sensed the brake cartridge sends an electrical pulse through a stainless steel wire that is holding a spring compressed. The released spring pushes an aluminum brake pawl into the backside of the spinning blade. The blade burys itself in the brake pawl. Much of its rotational momentum is transferred into downward force, much like a car with its motor racing being knocked off concrete blocks, and the blade drops below the table top. This dropping motion is allowed by a specially designed arbor block with a pivot and spring loaded retainer.
To put the saw back into service you work the joined blade/pawl off the arbor and mounting posts, install a new blade and brake cartridge, and snap the arbor block back up into position. There is no damage to the saw. SawStop says that many of their tests have been on a single saw, and it is still running normally and in adjustment.
The cabinet saw already released includes hardware to implement this concept that may be overengineered and too carefully built. They are very interested in having the device work properly. It will be interesting, though morbid, to hear the first reports of actual contact with operators in regular shop situations.
Hope This Helps,
Dave
Aha! Thanks. I don't remember seeing anyone mention that!
Must be a high powered charge to force 3" of blade out of the way while a hand was moving at 6 feet per second next to the table.
I have insufficient bandwidth to use 'net that way...
George,
You are correct. A more accurate statement would be that the saw blade has to make substantial electrical contact with the operator's body. That can be accomplished through contact with a very small cross section of the bloodstream or a larger skin area. The hotdog demo, for example, involves electrical connection through the wet dog and across the finger/dog contact patch to get to the operator's body.
My saw senses contact whenever I touch the blade while it is stopped. The brake doesn't go off, but the LED control panel flashes a warning and the saw will not fire up until contact has ceased for at least five seconds.
I have also (very carefully) touched the blade side while it was running in Bypass Mode. The control panel flashed the contact pattern, so I know that the brake would have released if I hadn't been in Bypass.
The brake isn't activated by a charge (explosive). They use a strong spring. Explosives would be difficult to ship, might not burn properly when aged, and wouldn't push after the initial burst. The spring is inexpensive, ages well, and continues to push the pawl into the blade to ensure successful braking. The wire that holds the spring compressed is stainless steel so it should age well. The cartridge includes a resistance heater to keep the wire and capacitor above 50 degrees F at all times - ensuring that the burn-through will happen properly when needed.
Take Care,
Dave
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