Motor Reversing

[Josepi]

I would think that an emergency stop mechanism would be a whole different magnitude than a scheduled, everyday, stop.

For a finger saver an emergency stop mechanism would have to stop the blade in a few teeth passings. For everyday usage a couple dozen blade rotations would be OK too. These two situations would require different tactics.

A dynamic electronic brake could vary the intensity for the two different scenarios and we could add the caliper to the emerg stop. A caliper probably wouldn't last long in everyday usage and would need to be adjusted and/or replaced frequently.

I would also think an assembly with wheels applying just a little pressure to the sides of the blade would make a decent passive system. It would be just enough pressure to slow down the blade in a second or two and not enough pressure for the motor to even register the resistance.

Think about how little pressure it takes with a scrap piece of wood to stop a coasting blade.

[Josepi]

I know of a fellow that worked on a vertical sliding saw professionally and stopped it with the side of his thumb, for years. One day he got a carbide blade installed and forgot.

The fire was in the last half of his thumb.

Watch your working habits. Environments change. You may too... in body shape.

news:Po2dnYsq7ZB9Yy_RnZ2dnUVZ snipped-for-privacy@earthl>

[dpb]

indeed...

SawStop is a few (10's of iirc) msec.

By several "tooth passings" a finger is already pretty well history...

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[-MIKE-]

Especially since it's apples and oranges. SawStop is a safety brake that only stops when contact with flesh is made, and it's instantaneous. I'm talking about a gradual brake that occurs as part of normal operation.

[-MIKE-]

Point?

[-MIKE-]

I understand and it is certainly frustrating. But, if we let that deter us, not one would ever invent anything.

[J. Clarke]

So how much power do you have to put into that electronic brake to stop the blade as fast as Sawstop's physical block?

And why would a caliper not last long? A set of brake pads on a car with 10" rotors lasts 40,000 miles or so, and they're getting a Hell of a lot more of a workout that they would stopping a little bitty saw blade that masses less than 1/1000 as much.

[J. Clarke]

Sawstop stops the blade in 1/200 of a second, according to their Web site. At 3450 RPM that means the blade stops in .29 revolution, which with a 60 tooth blade means that 17 teeth have had a shot at your finger.

[-MIKE-]

That may be true, but they still don't do any harm.

[HeyBub]

Everybody knows that, which is why I'm trying to explain. But everyone's alive except those who drowned in an adjacent bog.

[-MIKE-]

Put down the pipe. :-p

[Bill]

The physics on that seem interesting. A little like stopping a speeding bullet on a dime--it challenges my imagination. Probably not quite as difficult as stopping a lightning bolt, but I wouldn't want to try that either.

Bill

[Josepi]

17 teeth?

How fast can you push your finger?

[-MIKE-]

Have you seen the videos and live demonstration? It does no harm. They wouldn't be selling like hotcakes if their marketing strategy centered around "losing only a pint of blood, instead of a whole finger." Have you seen the video demonstrations? They've been done with lunch meat and human fingers.

[Steve Turner]

And with a 60-tooth blade spinning at 3450 RPM, that's 3450 teeth spinning past every *second*. At that speed, coming to a dead stop after only 17 teeth have had a chance to cut anything is pretty damn good. Unless you're just flat-out careless or stupid (or both) and are wildly advancing your fingers towards the blade, 17 teeth are not going to do much damage. I could do much worse in the blink of an eye with a careless swipe of my pocketknife.

[-MIKE-]

I've seen a hotdog demonstration in which they *did* push the dog, on top of a piece of wood, into the blade as quick as the blade with receive it. It was much faster than anyone with any sense would advance any stock into a table saw. The dog had the tiniest little sliver of "skin" taken off. Had it been a finger, it would surely have bled a drop or two. But I've done much worse to my finger when a file slipped or I got a splinter.

[Josepi]

This was exactly my point.

The other thing, and unfair to the 17 teeth is OK defence is that the SawStop pulls the blade away and down. It appears to be grabbed on the back side of the blade and the inertia pulls the balde back and under the table effectively removing the blade from the finger area. I doubt you would want to do this everytime in a **NON**emergency stop. My guess is arbor bending would set in after a few dozen of them.

Watch the videos (most have). This is no gentle stop for an armature and saw blade of that intertia.

So the retort to the 17 teeth big damage people is still "How fast do you push your finger?"

OTOH: Pushing the stock, slipping off, and back handing a running blade can be different speeds.

[-MIKE-]

I guess I don't know what we're debating, anymore. I thought we were talking about slowing down the coast of a saw blade after turning off the power. I guess we got on a tangent.

If it's you're mission to dream up ways to refute the effectiveness of the SawStop technology... well, I'm off the boat, because you've lost your mind. :-)

[Josepi]

LOL..I guess I am in the same boat as you. I did jump in late and saw (pun) some twist about emerg. stopping the blade. Then I jumped in to point out a difference in technique would be needed and got some resistance. Now I am chatting with you and who knows whwre it went.

I am not disputing the effectiveness of the SawStop, at all. After seeing videos I feel I owe the guy something without buying one. Amazing design. Now I want to kno how to beat it with my wallet.

BTW: never owned a table saw and have never intended to. I know too many that have lost an eye, fingers, thumb pieces and a **dog to one. When I wave to somebody I don't want them to think I am giving them the one finger salute.

**(j/k about the dog...LOL)

If it's you're mission to dream up ways to refute the effectiveness of the SawStop technology... well, I'm off the boat, because you've lost your mind. :-)

[clare]

Actually significantly LESS than 17 teeth, because the blade is very quickly slowing down throughout that 1/200 of a second. I believe the number is something like SEVEN teeth pass the gullet from the time of contact to full stop, and the last 3 or 4 are pretty well stopped as they go by. A demonstation showed a damp peice of cardboard stop the saw with barely a nick in it.