A listener from Madison, WI gave us a great follow-up to the table saw that stops instantly when it senses flesh. Scott sent us pictures too:

“Hi Amy, Alicia, and Kayla,

I just wanted to follow up on some of the gray areas around the 

flesh-sensing saw technology. I have owned one for a few years.

The saws actually sense an electrical current that flows through the 

blade, a current which changes if it contacts your body. The blade stops 

and is retracted below the saw table in about 5/1000 of a second. There 

is a lot of video online of people setting them off on purpose, 

including of the inventor very cautiously putting his finger onto the 

blade. It doesn't even break the skin!

The propulsion mechanism is a spring.

I've attached a photo of a new cartridge and one that I had the 

misfortune of triggering, although it was through accidental contact 

with my aluminum miter gauge, not my skin. You can see a few key things 

in the photo:

First, the lower unit is obviously the one that was triggered. The chunk 

of aluminum is the actual braking mechanism--it's driven into the 

spinning blade to stop it. The row of little holes act as a crumple 

zone. If you look closely, you can see that some of them are deformed a bit.

Second, the spring is exposed. The whole thing pivots around the large 

circular hole, which fits over an axle. When the spring fires, it drives 

the block into the blade, and the blade is also being driven (by its 

rotational momentum) into the block, which is how it retracts under the 

table.

Third, in the second photo, you can actually see that there are some 

carbide teeth embedded in the block. In general, when these things fire 

the blade and the cartridge are both throw-away items. In theory you can 

have the missing teeth replaced and the blade checked for straightness, 

but it's more cost effective to replace all but the most expensive 

blades. This was a rather costly dado set, and when I priced it out the 

repair option was basically equal to replacement cost.

You can also see a data port on there--it looks just like the port that 

was on your PC back in the 90s. There are a ton of electronics in the 

saws. They actually have a boot-up cycle, and there are a bunch of 

diagnostics that they run, including making sure that the gap between 

the blade and the brake is correct. There's even a chart you use to read 

error codes based on combinations of flashing lights. From what I can 

tell looking at online forums, if people have problems with the saws 

it's that they fire too much rather than not enough. I'm not familiar 

with a case of one of them failing to fire.

You can also boot up the saw in what they call bypass mode. You would 

use this to cut conductive materials like aluminum. It's also possible 

that some very wet treated lumber might be wet enough to conduct current 

at a level that would fire the saw.

At present the only brand available is SawStop. Bosch tried to introduce 

a competitor a few years back but it got pulled from the market due to 

patent infringement.

The saws are expensive, that's true. I decide to get one because with my 

insurance, every time I go to physical therapy it's over $100 out of 

pocket, so in the long run avoiding an injury is going to be the cheaper 

option!

Thanks for the podcast!"

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