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How SawStop's Blade Stop Functions
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Hot dog contacting SawStop saw blade A microprocessor detects changes in the voltage going to the blade and into skin and activates the brake in 5 milliseconds.

A hotdog, simulating a finger, is pushed into a spinning table saw blade on the SawStop machine.

As the hotdog touches the blade, the blade instantly disappears from view and comes to a complete stop.

The result is a nick that barely penetrates the casing of the hotdog. Total time from contact with the blade to the blade retracting and stopping is about 5 milliseconds, faster than the blink of an eye. If you’ve never seen the demo, you can now catch a video of it at the Journal’s web site.

Here’s how the SawStop technology does what it does.The human body is like a big capacitor, and is capable of absorbing electricity. A small electrical signal is put onto the saw blade on the SawStop saws. If you touch the blade, some electricity drains from the blade to you, though not enough for you to feel any shock.

A microprocessor built into the saw constantly monitors the signal on the blade and recognizes the voltage drop. Once the microprocessor sees the voltage drop, it activates the brake system. The brake consists of a heavy spring and an aluminum brake pad.The spring is held in compression by a fuse wire. When contact is made with the blade and the microprocessor does its thing, the fuse burns through, allowing the spring to push the aluminum pad into the spinning blade. This, of course, causes the blade to stop.The momentum of the blade suddenly coming to a stop causes it to retract below the table. At the same time, the power to the motor is shut off.

Small cut in hot dog from SawStop blade After coming into contact with the saw blade, the SawStop system only gave the hot dog a small knick.

Removing the now all-in-one brake and blade combo can be a little difficult but resetting the saw and getting back to work is easy.

Even if you contact the blade as it coasts down, after you’ve shut off the power, the brake system operates.

If you need to cut wood that’s quite wet, like treated lumber often is, or if you need to cut aluminum on your saw, the brake system can be bypassed. Wet wood might have enough capacitance to make the brake fire. Cutting aluminum will definitely make it fire.

posted on June 1, 2008 by George Vondriska
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