Motor Reversing

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says...

General rule is don't have your hands in in line with the blade, or within 4 inches of it in any direction.

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J. Clarke wrote:

Think about what would happen if you were walking with a cane, and the tip of the cane slipped on some ice. It's analogous.
Bill
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Uh, J. Clarke didn't write _any_ of that.
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On 10/13/2010 08:45 AM, Josepi wrote:

First of all, he never said what kind of wood he was cutting, but from what he *did* say I kinda doubt it was a sheet of plywood. Secondly, I said push "jig", not push "stick", and I meant one of these:
http://www.tablesawpushstick.com/images/Pushstick%20Safety.jpg
Of course, right there in the description they're calling it a push "stick", which I think is erroneous. When I hear "push stick", I think of one of these pieces of crap:
http://woodzone.com/Merchant2/graphics/00000001/push-stick.jpg
which I would *never* use; they give me the willies. For plywood, it's entirely plausible to use a "push block"
http://woodzone.com/Merchant2/graphics/00000001/push-block.jpg
Unless it's a really big sheet of plywood, I use these whenever I can on larger panels to keep them against the fence and my hands away from the blade. I use a GRR-Ripper for almost everything else that requires close proximity to the blade.
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*snip*

*snip*
Trust your feelings, Steve.
I used that style of push stick for years, until I was cutting a small piece and pressure on only the one point caused the piece to move and catch on the back of the blade. The resulting kickback hit me in the dust mask and broke the push stick. I will not use that style again.
Puckdropper
--
Never teach your apprentice everything you know.

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I think the main pint for all of us is
*** think *** about what could happen and then prevent it.
In safety training they call this a "barrier" Minimize the chances with a "barrier". If you cannot absolutely prevent it think "Do I have to do this at all" and "Is there another way" and "How can I make is less risk"
Minimize the damage, if should it happen anyway.
"Puckdropper" <puckdropper(at)yahoo(dot)com> wrote in message
Trust your feelings, Steve.
I used that style of push stick for years, until I was cutting a small piece and pressure on only the one point caused the piece to move and catch on the back of the blade. The resulting kickback hit me in the dust mask and broke the push stick. I will not use that style again.
Puckdropper
--
Never teach your apprentice everything you know.



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wrote:

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.
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On 10/12/2010 5:46 PM, snipped-for-privacy@snyder.on.ca wrote:

Another important part of the SawStop design is that the blade is dropped below the table's surface as the blade is being stopped.
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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.

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J. Clarke wrote:

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
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No way a caliper could stop the blade in even 4X the time the saw-stop does. And keeping dust out of the gap between tha "pad" and the blade would be quite problematic - with dust decreasing the stopping efficiency by a very large margin.
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snipped-for-privacy@snyder.on.ca says...

I suspect that a small pyrotechnic pressurizing one of the calipers off my '76 Lincoln might surprise you.
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J. Clarke wrote:

When it snapped (seriously)? I'm not an engineer, so I don't know for sure. I just wouldn't be surprised. Maybe someone else can opine.
Bill
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Why would something that can stop a 4000 pound car "snap" when called on to stop a 3 pound saw blade?
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J. Clarke wrote:

"Pyrotechnics" rather than a master cylinder with brake fluid?
Bill
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To stop the saw in a fraction of a revolution, you need rapid application. A pyro will build pressure rapidly.
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J. Clarke wrote:

>J. Clarke>

That is what I what I might expect to increase the probability of failure.
Given blade radius and density, and rpm, you could probably integrate to compute the (foot-pounds of) energy that need to be stopped in a small fraction of a second (I hope that the dust on the blade won't be an issue). :) I've seen you post in another "forum" so I believe you are up to the calculation. I do not have engineering background to back me up, I'm just thinking it through with you and everyone else who is reading. Evidently, you'll need to generate the equivalent of an equal and opposite amount of energy. This means, I think, that you only get the benefit of a "projection" (dot product) and not all of the force you can apply to the side of the blade. My reasoning could definitely be off, maybe a physicist or engineer could help out?
Regards, Bill
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Disk brakes work by friction. The energy comes off as heat. Apply F to the brake, k * F is the braking force where k is the coefficient of friction.
Back of the envelope it looks like the Lincoln brake at Lincoln pressure can stop it in about 1/3 revolution. Don't know how much higher pressure that caliper can take--at 2x the pressure might be able to do it in 1/6 revolution, which puts it in Sawstop territory.
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J. Clarke wrote:

Interesting. Thank you. I'm not sure about your "imposter"..
Bill
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This is all assuming a clean blade for the proper high co-efficient of friction. Dust od sap build-up on the blade can change that very quickly.
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