On 13/07/10 20:59, Dave Tweed wrote:
> Jake Anderson wrote:
>    
>> On 13/07/10 12:09, Vitaliy wrote:
>>      
>>> Jake Anderson 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.
>>>>          
>>> I don't think that car brakes are designed to stop "arbitrarily fast",
>>> compared to the time scales in question.
>>>        
>> brembo 6 piston caliper, 55 square centimeters worth of piston area
>> decent brake pad CF is ~.8
>> push the pressure to 3000PSI say.
>>
>> a 10" blade is say .1" thick
>> weight is ~1kg, KE is 1500J
>>
>> converting to a linear motion to make life simple
>> KE = .5MV^2
>> sqrt(KE /.5M) = V
>>
>> V=54m/s
>> ~=200km/h which seems to be ~ the tip speed of the blade as calculated
>> by others so it seems good.
>>
>> clamping force on the disk is 8.5 square inches * 3000PSI = 25500 pounds
>> = 11590kg = 113590N
>> deceleration force = .8 * clamping = 90872N
>> acceleration = F/M = A = 90872M/s/s (~9000G)
>> v^2/2a = s
>> 57^2/2*90872 = 0.017876794 meters
>> so about 20mm give or take.
>>
>> seems fast enough for me?
>>      
> But that's only the very last step in the process. How long does it take
> from when you receive the electrical trigger signal to when you actually
> have the pads against the blade with 3000 PSI behind them?
>
> -- Dave Tweed
>    
If you continue the car brake system analogy the pads should be sitting 
microns off the disk, basically push them up to it (gently), spin the 
saw up and let the wobble in the blade push them off. So application 
time should be minimal, valve actuation time again can be almost 
arbitrary direct injection diesel injectors operate in a similar problem 
range although at 15,000 PSI. I don't have the numbers at hand.

Even so, if you doubled or tripled that stopping distance your still 
better off, a deep cut (but you'll do worse in a kitchen) vs cutting 
your finger off, and you'll use it because it wont kill the blades on a 
false positive (also means you can make it more sensitive and perhaps 
prevent a false negative)

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