eli5. How do table saws with an auto stop tell the difference between wood and a finger?

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eli5. How do table saws with an auto stop tell the difference between wood and a finger?

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Electricity.

The saws that can detect that stuff have a tiny current constantly flowing through the running sawblade. Wood, being wood, does not conduct electricity very well, and so there’s not much change in that current when wood contacts the saw. Thumbs, being sweaty and made of meat, conduct electricity a bit better than wood, and so they change the current in the sawblade more.

When the saw detects a change in the current, it fires a mechanism that punches an aluminum block into the saw, stopping it immediately.

The safety feature detects an electric signal. Human bodies are electrical conductors, so when we touch the blade, we create an electrical circuit. The machine detects this electrical difference, and initiates the blade jammer when it does. A piece of wood is not conductive, and so it does not create a circuit with the blade.

It’s similar to those lamps or even your smart phone screen. You operated them by making contact with your skin. They detect the electrical impulse of your skin. If you tried to active these with a thick glove, it would detect no signal and not turn on.

In a similar way a modern touch screen can tell the difference between a finger and a finger with a glove on it: Capacitance.

Or, like you’re 5, the way electricity flows on what is touching the saw blade.

Flesh has a unique enough interaction with electricity that can trip a sensor fast enough to stop a spinning saw blade before a deep cut happens.

If you’ve ever unplugged an audio cable and put your finger on the end and heard some deep humming, it’s that same effect from capacitance.

My question is: how does it stop so fast? Seems like it only has a millisecond to detect, jam and stop the blade.

Thank you all for the explanations

Here is a video with a woodbutcher explaining and demonstrating with a super slo mo camera…….. https://www.youtube.com/watch?v=SYLAi4jwXcs

Capacitance.

But before we discuss that, let’s touch on conductivity.

If those safety stops worked on conductivity, then wearing rubber soles boots, which are a better insulator than wood, would disable this safety.

Rubber soles boots don’t disable this safety, so that’s not the correct answer.

Capacitance is the correct answer. Let’s get into this.

A capacitor holds an electric charge (like a battery), but losses that charge quickly (unlike a battery).

Most things act like a capacitor, holding a bit of charge. This means if you touch most things to a battery, there will be a tiny, but measurable, current flow out of the battery.

dry wood is a crappy capacitor. It holds very little charge.

The human body, or a hot dog, or a bucket of salt water, all of these are also crappy capacitors, but they are orders of magnitude better than dry wood.

So if you electrically isolate the blade, and tricke a bit of voltage into the blade, and measure how much current is flowing, most of the time it’ll be almost none.

But if you get a little jump in current, that means something with better capacitance than dry wood has touched the blade, like maybe a finger, or a hot dog. Trigger the brake.

This is irrespective of how well insulated your boots are, since it’s measuring capacitance, not conductivity.

It’s also why wet wood can sometimes trigger that safety. Wet wood has a higher capacitance than dry wood.

If for some reason you want to test this, hold a hot dog to the blade, not your finger… 😉

Can you imagine being the first one to test this. No matter how solid the data is and hoe many pre tests you do, someone had to be the first person to use their own body to prove it worked.

I was doing a project at a freinds house, he handed me a peice of wood and asked if i could chop six inches off the end of it, the wood was slightly damp because we were working outside.
Id never seen an auto-stop before and didnt know he had one. As soon as the damp wood touched the blade there was a loud bang and the blade was gone. I thought the blade had hit a nail or something and shattered.
Scared the hell out of me, cost him close to $100 for a new blade and stop. Seemed effective though, the wood barely had a mark on it.

It detects electricity pretty much. I’ve set one off twice. First time i tried to cut formica laminate with aluminum in it, the metal creates an electric charge so it kicked the stop.
Second time I was cutting a flat pvc sheet and the static electricity from the plastic kicked it.

I know a guy who knicked his finger with one, it just barely broke his skin.

You can turn the stop off with a key if you want to cut off your finger.

They have moisture sensors, if the wood you are cutting or anything for the matter is 25% moisture or higher it will give it a stop signal. Some of the more expensive ones use electrical current. Throw a hot dog at your saw blade when in motion and you will know which one you have.