SawStop Saws are the most advanced
saws in the world. Each saw is equipped with a
safety system that detects when someone accidentally
contacts the spinning saw blade, and then stops the
blade in milliseconds. In most cases, such an
accident would result in just a nick on a
SawStop saw,
instead of the devastating injury which would likely
occur on an ordinary saw. The photo at the right
shows what happens on a SawStop saw when a hotdog
(representing a finger) hits the spinning blade at a
speed of about 1 foot per second.
The SawStop safety system includes an electronic
detection system that detects when a person contacts
the blade. The system induces an electrical signal
onto the blade and then monitors that signal for
changes. The human body has a relatively large
inherent electrical capacitance and conductivity
which cause the signal to drop when a person
contacts the blade. Wood has a relatively small
inherent capacitance and conductivity and does not
cause the signal to drop.
This drawing shows the changes in the electrical
signal when a finger touched the teeth of a spinning
saw blade during an actual test.
The line represents the electrical signal which
dropped quickly when the blade touched the finger.
The dips in the signal line illustrate the changes
in the electrical signal that were detected as two
successive teeth touched the finger. When the
detection system sees dips like these, it knows a
person has touched the blade.
A fast-acting brake stops the blade when contact is
detected. The brake includes a heavy-duty spring to
push a block of aluminum, called a brake pawl, into
the teeth of the blade to stop the blade from
spinning. The spring is held in compression by a
fuse wire until contact is detected. When contact is
detected, the system sends a surge of electricity
through the fuse wire to burn the wire and release
the spring. The spring pushes the brake pawl into
the teeth of the spinning blade, and the teeth cut
into the aluminum and bind, thereby stopping the
blade. All this happens in about 3–5 milliseconds,
or 1/200th of a second. At the same time, the
angular momentum of the blade causes the blade to
retract below the table and the power to the motor
is shut off.
 The
brake mechanism is shown in the drawing to the
right. The arrow shows how the aluminum brake pawl
pivots into the teeth of the blade. The brake pawl
is part of a replaceable cartridge that includes the
spring, fuse wire and electronics necessary to burn
the fuse wire. An optional dado cartridge provides
the same protection for dado cuts.
The standard and dado brake cartridges are
single-use components that must be changed if the
brake is ever activated. Changing a brake cartridge
is fast and easy, no more complicated than changing
the blade.
|
