Motor Reversing

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On 10/14/2010 2:30 PM, snipped-for-privacy@snyder.on.ca wrote:

After I already said it.
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See Nad. See Nad go. Go Nad!
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Oh the mootness. (NOT aimed at anybody in particular)
Any disc or drum of any size will interfere with the raising of the blade to the point that the disk brake hits the bottom of the table somewhere.
I propose an air bag. Electronically triggered, it throws you backwards across the shop and away from the table saw. Punches you right in the chest with the option for a double bag for some people here in which case the second bag knocks some sense in them. We can glue on a boxing glove for that operation...just a 4 oz. one; you want it to hurt a little. One can mount the boxing glove on an expanding multi-pivot articulated parallelogram. What a stellar idea. I'm talking to investors now. They want to call the company ACME.
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On 10/14/10 12:58 PM, Robatoy wrote:

Beautiful. Wile E. fricken beautiful.
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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Coyote ugly.
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On Thu, 14 Oct 2010 10:58:43 -0700 (PDT), Robatoy

Would make a great cartoon to pass about the Internet, if twas done in animation might even go viral. But you might want to have a huge catchers mitt behind the punchee.
Mark
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wrote:

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On Thu, 14 Oct 2010 11:27:20 -0500, Steve Turner

on the "non-blade" side of the arbour. Put the brake on the pulley if belt drive, or the "fan end" of the motor if direct drive.
The only problem there is, if you stop the arbour too quickly the arbour nut will wind off as the blade trys to keep spinning. Same thing happens if DC injection on ann AC motor or resistive(regenerative) braking on a DC motor is too harsh..
That's why the "saw stop" HAS to stop the blade directly. Stopping the blade that fast through the arbour would inevitably wind the blade off the arbour before the blade came to a safe stop.
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On 10/14/2010 2:29 PM, snipped-for-privacy@snyder.on.ca wrote:

Valid point, but again, we're not talking about a Saw-Stop equivalent here, only a "convenience" brake that stops the blade in a second or two. Unless you've dangerously under-tightened the arbor nut (or ridiculously over-engineered the brake) that's not likely to cause enough centrifugal force to loosen the arbor nut.
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That happens freuently to my Milwalkee 12V drill. The brake stops the insides but the chuck wants to keep on going and undoes itself and the bit falls out. The chuck even appears to have a dual gear ratio tightening mechanism.
Valid point, but again, we're not talking about a Saw-Stop equivalent here, only a "convenience" brake that stops the blade in a second or two. Unless you've dangerously under-tightened the arbor nut (or ridiculously over-engineered the brake) that's not likely to cause enough centrifugal force to loosen the arbor nut.
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Any given amount of traffic flow, no matter how
sparse, will expand to fill all available lanes.
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On 10/14/10 2:29 PM, snipped-for-privacy@snyder.on.ca wrote:

Neither of us is trying to stop the blade that fast.

I seriously doubt that was even the slightest consideration for the inventor of the SawStop.
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-MIKE-

"Playing is not something I do at night, it's my function in life"
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wrote:

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On Tue, 12 Oct 2010 22:25:50 -0400, "J. Clarke"

Yes but the heat will do what to the saw blade?
Mark
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@hotmail.com says...

About the same thing it does to a brake rotor I suspect, polish the sides a little bit.
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On Wed, 13 Oct 2010 09:12:15 -0400, "J. Clarke"

Uneven heating of a metal disc also would cause it to warp. But then again the "cartridge" in a Sawstop is toast when activated.
The two times I trimmed my finger nails with a tablesaw was it was a Delta TS220 I had at the time, I was in a hurry impatient a bit tired, which equalled being stupid.
The Unisaw which I have now seems safer, and that is more than a reason to be careful.
Mark
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On 10/13/10 7:44 AM, Markem wrote:

Nothing. A lot more heat is generated by cutting than would be generated in the half second it would take to slow down the blade.
You guys do realize we're not leaving the motor on in this scenario, right? :-)
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-MIKE-

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-MIKE- wrote:

Yep, thought of that. Gotta break the problem into parts (important trick/secret!) : ) I didn't count the mass of the rotor either, or it's attached parts, in my other post either. I'm curious now though about the calculation (foot/lbs of force).
Bill
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On 10/13/10 11:22 AM, Bill wrote:

Sometimes all it takes is a couple of scraps of wood to show how "little" force is needed. Haven't you ever stopped a coasting blade with a scrap of wood?
I still see people talking about the SawStop and what it takes for an emergency stop of the blade. I don't know about anyone else, but all I'm talking about is a convenience stop. I suspect a blade could be stopped (at shut-off) in less that a second with something the size of a bicycle brake and a spring.
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-MIKE-

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wrote:

Use a DC motor and a DPDT switch with a big resistor across the "stop" terminals to short the motor when you shut it off. Stop a 10 inch blade from 3600rpm in less trha a second with a dead short (if the switch can handle it) or in about 2 seconds with a good "soft" braking resistor.
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snipped-for-privacy@snyder.on.ca says...

Which is where this thread started.
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On Wed, 13 Oct 2010 18:16:40 -0400, "J. Clarke"

A really big solid state relay and a sealed explosion proof switch, cause you do not want to start a thread about sawdust explosions. Right?
You know you can use oars to stir stuff.
Mark
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