On Thu, 6 Dec 2012 07:48:05 -0800 (PST), "
wrote:

On Dec 5, 8:57*pm, wrote:
On Wed, 5 Dec 2012 09:46:12 -0800 (PST), "





wrote:
On Dec 5, 10:15 am, "Michael A. Terrell"
wrote:
Leon wrote:

10+ years ago before the SawStop was in production I questioned Gass
about this. My TS accident happened after I finished a cut and had
turned the saw off. The blade was coasting down to a stop when I was
cut. I wanted to know back then it those bases were covered also.

A modification can be made to table saws with induction or split
phase motors. You change the power switch from SPST to SPDT, add a
diode & electrolytic capacitor that charges when the motor is running.
When you switch the motor off, the capacitor discharges through the
motor, causing a rapid braking effect. This can't be done with
universal motors, since they will run on DC.

Even if this could work, it would have to be one hell
of a huge cap to hold enough energy to stop a spinning motor
and saw blade in the time to prevent injury. *The cap might
be bigger than the saw. * And I have my
doubts about it working at all, ie simply feeding DC into an
AC motor that has lost power being effective at braking.

It does work. *You're not trying to turn the rotor backwards, just
stop it. * The DC source isn't even needed, just short the thing. This
is how dynamic braking is done in diesel-electric trains, and such,
though hey use resistor banks to change the braking characteristics.

Sure. It's done in motors that are designed to do it.
Now show us a typical table saw motor that is capable of it.
And show us any motor where you can stop it in around 1 ms, like
the SawStop does, to prevent injury, by just shorting it.
Or even where you can stop it with the proposed addition
of a capacitor. It would have to be one hell of a cap to
stop a saw instantly.

You are talking two different problems - a motor brake will cut the
spin-down to 10 seconds instead of over a minute. Not meant to be
instant.

The Saw Stop is milliseconds - Close enough to Instant Stop for all
practical purposes. TWO DIFFERNT THINGS!
But beyond that, why on earth would you? *The energy
that it would take to keep the SawStop system active for
a minute during spin-down following loss of power has
to be tiny compared to what it would take for the electric
motor braking approach.

Firing the SawStop mechanism is a one-shot event. *After you clean
your shorts, you get to replace the mechanism and the blade. *Dynamic
braking is a freebie and can be done every power cycle.- Hide quoted text -

- Show quoted text -

The proposal as I understood it wasn't to replace the SawStop, but
to supplement it if power fails. But I'll say it again, no
reasonable,
practical dynamic braking is going to stop a table saw in anywhere
near the time it takes to prevent injury. You'd need a huge power
store of some kind.

Dynamic braking gets you that 5 to 10-second stop when the saw is shut
down normally or the power fails. And it can repeat that daily for
decades.

The SawStop is using a small explosive charge to fire a big chunk of
Aluminum into the path of the blade as a chock, and the wedging action
of the block against the blade causes the blade to kick back and down
away from the finger.

After that, you go buy a new stopper cartridge, and a new blade for
the saw, and possibly other pieces that broke or bent when it went
off.

-- Bruce --