On 13/07/10 12:51, Oli Glaser wrote:
>>>
>>> The field of stopping a rotating metal disk is a fairly thoroughly
>>> explored field, they are called car brakes.
>>> I'd wager with a hydraulic accumulator, shop air, and some ABS/brake
>>> components you could stop the blade near arbitraily fast.
>>> damage to the blade would be minimal, though you would want to check it
>>> for warping if it got used often.
>>>        
>
>   Yes indeed. However, we were talking of *moving* the entire metal disk
>   itself very quickly - to remove it from the immediate area to which a
>   finger/hand/random body part may be approaching. Actually stopping the disk
>   *rotating* quickly may not be necessary, in contrast with the SawStop
> system
>   which does exactly that.
>   The discs in car brakes are a lot more sturdy and designed to withstand the
>   extreme forces applied during braking. I imagine it would be a far bigger
>   challenge to stop a disc blade quickly enough without damaging it due to
> the
>   material being a lot thinner and the equivalent forces being higher - it
>   seems logical that if it was possible easily/cheaply the SawStop system
>   would probably do it that way to avoid the cost of replacement parts. I
> find
>   it hard to believe the inventor did not try it this way first, but you
> never
>   know.. Or maybe he considered it not to be certain enough that way, the
>   chances of failure to stop in time to be unacceptable - or something along
>   those lines.
>
>    
Moving all that requires accelerating alot of gear though, its not just 
the blade, its all the rest of the gumf, some beefy bearing blocks, 
pullys etc.
Also moving it back only covers a finger approaching from the direction 
of travel, so you have to go down, and to below the table surface to 
really be "safe" that could be in the 10's of cm range.

Most of the work in the car brake rotors is to absorb the heat energy of 
2000kg's of car slowing down without warping.
If you clamp equally onto the blade it shouldn't warp significantly.

It would probably cost more initially than the sawstop but then if it 
trips a few times the worst your out is a blade.

OHS here is pushing for everything to have brakes anyway, things aren't 
allowed to spin on after you press the off button so you could perhaps 
use the system for a (more gradual) stop in that case as well.
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