On 8/20/2014 1:29 AM, Swingman wrote:
Leon lcb11211@swbelldotnet wrote:
On 8/19/2014 8:05 PM, Swingman wrote:
On 8/19/2014 7:04 PM, wrote:
It was explained in the seminar but not demonstrated that
if you fed a hotdog into the blade - but not holding it in your hand -
- ie just having the hotdog held between 2 pieces of wood -
the sawstop would not function to stop the blade ...
it would cut the hotdog !
That's what I couldn't understand.
That's what I need to have explained.
Capacitance is the ability to store an electrical charge (your body has
capacitance).
There must also be a conductor to allow that electrical charge to flow
(or show a difference in potential) along a certain path (to the
mechanism of the sawstop).
Wood, is not a good conductor (unless wet) of electricity, nor does it
have sufficient capacitance to store enough of charge on it own.
Therefore the hot dog, when affixed between two pieces of wood (having
no, or a very week, electrical charge; and not being a good conductor)
will likely not create the difference/electrical flow necessary to trip
the sawstop circuit.
That's a pretty simplistic explanation, and it has been a long time ...
And to just throw a wrench in the equation, the saw stop has an override
switch for cutting wet lumber and or ferrous materials.
My experience with cutting damp PT lumber is that the brake does not trip
but does shut down the saw. So this probably backs up what Doug was
indicating. The wet wood does not have enough capacity to store an
electrical charge to trip the brake but apparently enough to shut down the saw.
Doug's correct in the human body being the source of capacitance sufficient
for the mechanism's sensors to detect the necessary difference to operate.
The moisture in the wood is what is allowing the detection of a difference
in the capacitance from your body to the mechanism's sensors, not that it
is storing an electrical charge. IOW, the moisture in the wood is providing
the path. (Perhaps enhanced by the chemicals used to treat it.)
Exactly however I assumed that any path at all was enough to trigger the
brake. I have been very careful,so far, to not cut into my aluminum
miter gauge fence, something that we all apparently do. ;~) My
assumption was that the wet wood would have the same effect as cutting
the fence while touching the fence with you hand or finger and or
cutting the hot dog while touching the dog with your hand or finger.
In the case of the wet wood there was no brake trip rather after cutting
a few inches the saw coasted to a stop repeatedly until I engaged the
over ride switch. So there must be some measure of the signal from the
blade to a conductive source that decides whether to brake or simply
shut down the saw.
Apparently the saw can detect the difference and for that matter I would
suspect that the saw top also has the ability to absorb the charge at
the blade.
For the OP's benefit, here is simple way to illustrate what I was trying
(poorly) to describe in response to his request for a _simple_ explanation
of the basic principle.
Take an iPad or any device that uses a capacitance touch screen.
Tightly roll/fold a _dry_ paper towel to thickness of a pencil. Which would
be the wood, holding the hot dog, in the OP's question.
Holding it in your bare fingers, try to use it as a stylus on your touch
screen. No dice.
Wet the paper towel "stylus", squeeze the water so that it is not
saturated, but moist, grasp it between your bare fingers, and stylus away.
The water has provided a path from your fingers, through the wet "wood", to
allow the capacitance screen's sensors to detect the difference required to
operate.
If you now wrap that wet stylus in sufficient dry paper that your fingers
are no longer be in contact with moisture (the hot dog held by wood,
instead of fingers in the OP's demo), the stylus will no longer work
Not exactly the same as the Sawstop, but a simple way to observe part of
the underlying principle the OP requested.
(Theoretically you can do the same thing by grounding both the device and
the stylus to the same plane)