Motor Reversing

[-MIKE-]

I didn't think we were talking about an emergency stop. AFAIK, we're discussing a convenience stop which, in my opinion, would be satisfactorily done in a couple seconds, not milliseconds.

[Bill]

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

[Bill]

I thought we were discussing a stop on the same order as SawStop. I didn't read every post in the thread.

Bill

[-MIKE-]

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.

[-MIKE-]

Someone brought SawStop into the discussion, which took things on a tangent. We started out discussing the OP's link to a set of motor reversing instructions as a way to slow down the blade for convenience. Then I proposed the use of a bicycle brake... and things went from there.

[Bill]

Properly done, I bicycle brake sounds like a good idea! I'd hate to read about flying parts.

Bill

[Steve Turner]

flying parts.

AHEM. I believe *I* was the one who proposed the use of the bicycle brake. That -MIKE- character is trying to steal my idea. :-)

[-MIKE-]

Welllll, looks like I own someone a beer. Yeppers, I said, "...aftermarket saw brake, under the table.... pads or rollers or whatever, that would squeeze the blade," and then you brought up the bike brake.

A good inventor is a better thief. :-)

[J. Clarke]

Well, geez, for that a bicycle brake will work fine.

[J. Clarke]

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.

[J. Clarke]

And Nikolai Yvanovich Lobachevski is his _name_ . . .

[J. Clarke]

Uh, J. Clarke didn't write _any_ of that.

[Puckdropper]

Steve Turner wrote in news:i94hkb$nhu$ [e-mail]:

*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

[clare]

But would also be totally impractical on a 10" saw.

[clare]

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.

[J. Clarke]

Why would it be "impractical on a 10" saw? The brake rotors on that car are IIRC 11.5 inch diameter and the whole brake assembly fits inside a

15" wheel.

[J. Clarke]

Which is where this thread started.

[Bill]

Interesting. Thank you. I'm not sure about your "imposter"..

Bill

[Markem]

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

[Josepi]

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.

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