SawStop blade touch detection

[Cliff Holmes]

I was trying out the SawStop's ability to "see" a finger touching the blade (in standby mode, of course). I found that it immediately detected if I touched the blade body anywhere, but detecting a touch on the carbide teeth was iffy. I would sometimes have to apply quite a bit of pressure before the saw would indicate that it being touched.

I've written SS about this, but I'm curious if others see this same effect.

[JohnT Fitzgerald]

I was trying out the SawStop's ability to "see" a finger touching the blade (in standby mode, of course). I found that it immediately detected if I touched the blade body anywhere, but detecting a touch on the carbide teeth was iffy. I would sometimes have to apply quite a bit of pressure before the saw would indicate that it being touched.

I've written SS about this, but I'm curious if others see this same effect.

OK, you really piqued my curiosity with that one. I had to run downstairs and check, and you are right - it appears it's more sensitive to touching the plate of the blade than the teeth...although, not by much. It also seemed to make a difference if I touched just the very point of a tooth, versus "more" of it. and if I licked my finger first, it picked it up right away no matter how I touched it (a single tooth). It's probably due to the amount of conductivity to your finger/body, and maybe the carbide has a different conductivity than the rest of the blade.

Very interesting. I'd like to know what SS says.

[Dave Sharpe]

I've noticed the same thing, and had the same concerns about just how deep the cut in my finger needs to be for the brake to fire. I asked SS about it and they said something about my finger tip being too dry to trigger the sensor. Which makes me wonder just how dry my wood is compared to my finger tip..... Still not satisfied with the answer (but then I'm not laying awake at night worrying about it either)

[Dan Friedrichs]

You seem to be worrying about this new saw a lot, Cliff :)

I don't have a technical answer, but I imagine even a light scrape to a finger (breaking the skin) would significantly increase the capacitance the sensor would see, triggering the brake.

There have been zero reports of the brake failing to trigger upon contact with a finger, and presumably nearly everyone uses carbide-tipped blades.

EDIT: As other posters said, it likely has to do with the skin conductivity being very low. Dry skin is a poor electrical conductor, but blood is a good conductor.

[johnny means]

Dry hands? SS sensors depend upon moisture and sometimes our rough, callous hands don't have enough on the outer most layers. No doubt, if you cut through that dry layer and exposed some tender, sweet, fresh dermis you'd be looking for a new brake and blade.

[Victor Robinson]

It's an interesting question, albeit completely academic. The surface of your skin is much drier than just under. If you touched the carbide while the saw was spinning, you'd break skin. Hence the blade would stop because of the moisture released...no need for it to be deep at all. The fact the carbide doesn't register your dry finger is a total non-issue since any blade-related injury will cause skin breakage - but it's neat science.

[Glen Butler]

Remember they are avoiding amputation not injury. Just below the surface of your skin are thousands of tiny capillaries carrying blood. Blood is quite wet and will trip that sensor just fine. Just as John noticed a wet finger works just fine.

[Doug W Swanson]

This was posted in another post. It's a Saw Stop in action.

Watch it and I think it explains itself.....

http://www.youtube.com/watch?v=E3mzhvMgrLE

Doug

[Mike Goetzke]

This was posted in another post. It's a Saw Stop in action.

Watch it and I think it explains itself.....

http://www.youtube.com/watch?v=E3mzhvMgrLE

Doug

...but he did precondition his hand in that wet cooler filled with salty hot dogs;).

[Ken Fitzgerald]

The conductivity of the carbide might be less than that of steel but

I will guarantee you SawStop checked this out long before Cliff got his saw....





Cliff.........relax and spend more time learning to use the saw

[mreza Salav]

yeah I noticed that too but didn't post about it :D

You've got your answer already. At the tip if you just barely touch not enough moisture and/or contact to activate. If it ever cuts a slightest amout of skin then things change.

[Dan Friedrichs]

Think about this for a minute: Even after the SawStop guys designed a mechanism to nearly instantly stop the blade (a tremendous engineering feat), they needed to figure out how to trigger that brake. If they make the system not sensitive enough, someone will cut their finger severly and people will distrust the saw. If they make it too sensitive, it will false trigger all the time. Amazingly:

1) AFAIK, there have been zero reported incidences of the saw failing to trigger upon contact,

AND

2) All the repored incidences of "false triggers" have been attributed to cutting metal, wet wood, or the miter gauge :)

Pretty impressive...

[Cliff Holmes]

Come on, people! I didn't say I didn't trust the saw. I just thought it was very interesting.

As for the skin dryness, I'm touching the blade body (or edge) with the same finger ...

[Curt Harms]

The conductivity of the carbide might be less than that of steel but

I will guarantee you SawStop checked this out long before Cliff got his saw....

Cliff.........relax and spend more time learning to use the saw

I'm pretty sure that the igniter on our gas furnace was originally some sort of carbide, and it worked by getting red hot pretty quickly with not too much current. That doesn't seem like a good conductor to me though I'm not an electronics dude. Doesn't a material have to have a fair amount of resistance to get hot quickly?

[Mike Cruz]

FWIW, Cliff, I thought they were being a little rough on you. I read your post as a "Hmmmmm" not a complaint or a major concern. Somehow, that isn't how it got translated.

[Van Huskey]

The conductivity of the carbide might be less than that of steel but




Thats what I thought but this is what I found:

"Electrical Conductivity - Tungsten carbide is in the same range as tool steel and carbon steel."

[John Coloccia]

My guess is it has more to do with the crud and oxidation on the exposed carbide, the limited contact area, and dry skin than the carbide itself. I played around with this too when I first got my SS.

[Mitchell Andrus]

I've noticed the same thing, and had the same concerns about just how deep the cut in my finger needs to be for the brake to fire.

If you'll look at the "saves" on the SS page, you'll see that some of them suffered a bit more than a simple nick.

I KNOW.... better than big stitches or worse.... just saying.
.

[Mitchell Andrus]

Wouldn't a spinning blade put more teeth in contact, perhaps 5 or 10 or more?

[Nathan Callender]

It could be how the tooth is attached to the blade or the amount of surface area you can touch, or just a coating of something on the tooth (like wood sap?) that is making the difference. All that being said, if you think about it, the SS is unbelievably awesome technology. Those guys did an amazing job and went about it the right way. See what private entrepreneurship can produce if its given a chance to succeed? I wish there were more people like that in the WW community.

[Jason Roehl]

The SS works by putting a small charge on the blade. When that charge changes due to contact with something conductive (like flesh or metal), it fires the cartridge. I suspect that a small area of contact with dry skin (such as touching the point of a tooth) doesn't allow enough charge to transfer quickly enough to reach the firing threshold.

[Cliff Holmes]

I heard back from SS TS. According to them, it's normal (expected that) and there are "two different technologies" in the system, one for the body and another for the teeth (never heard that before).

Being a SawStopper is certainly a learning experience!

[Tom Walz]

Tungsten Carbide on saw blades is actually grains of tungsten carbide in a metal binder (usually Cobalt). Figure it is about 3 – 8% metal and the rest ceramic grains.

There are different methods of measuring electrical conductivity.

Saw Steel for carbide tipped saws is usually not tool steel. Tool steel saws are usually for cutting steel.

[Bob Coleman]

Tungsten Carbide on saw blades is actually grains of tungsten carbide in a metal binder (usually Cobalt). Figure it is about 3 – 8% metal and the rest ceramic grains.

There are different methods of measuring electrical conductivity.


I don't have a saw blade handy, but maybe someone who has a carbide blade and a multimeter could do a quick experiment? I'm actually pretty curious to see what the real life resistance difference is.

[Zach England]

What if you put your TONGUE on it? Might work better. You should push all wood through the saw with your tongue.

(note: Do not push wood through saw with tongue.)

[Michael MacDonald]

Wouldn't a spinning blade put more teeth in contact, perhaps 5 or 10 or more?

If you look at the video earlier in this thread, it shows a super-slow motion of the brake firing. As the brake appraoches the blade, you can see teeth passing by... but it is just a handful of teeth. The blade seems to spin a small fraction of 360 degrees while the brake approaches.

[michael case]

I had this question two years ago. I called SS. The tech help was at a loss and grabbed the owner and got the answer straight from Steve Gass. He said to touch your finger to your tongue then touch carbide. Sure enough the saw started blinking. Apparently, as the saw cuts passed the dry external skin and comes into contact with the meat underneath it shuts down. So you get a band aid nick instead of an amputation.

[Cliff Holmes]

Well, again, I don't think it's just the dryness of the skin. My finger doesn't suddenly become moister when I touch the body of the blade and then dryer again when I touch the carbide.

[Bryan Morgan]

Does it still work if you have a painted blade, like a Freud diablo?

[Van Huskey]

Does it still work if you have a painted blade, like a Freud diablo?


Just ran out to the shop. Fresh uncoated blade center to carbide tip no resistance, Freud permashield (brand new P410) tip to tip no resistance permashield covered area to tip complete insulator.

The SS does not work on resistance as I understand it, but no SS guru. If it did and the arbor did not scratch through the permashield (dunno about Freuds ICE didn't test it) then it would not conduct from blade to arbor, but this is not an issue.

[Cliff Holmes]

The SS does not work on resistance as I understand it

My guess is that the SS module actually uses a capacitor connected to the arbor, effectively loading the blade with a static charge, then watches for a charge drain. That's why the material doesn't have to be grounded or complete a circuit back to the saw, it just needs to absorb the charge a little. But I'm not an electronics engineer, nor do I play one on TV, nor did I stay at a Holiday Inn Express last night.

[Ken Fitzgerald]

Cliff,

Let's stop the speculation. Here's what SawStop will tell you:

139926

[Cliff Holmes]

Yes, I've read all that before, but it really doesn't explain how it works, e.g. what exactly is the "charge", how's it generated, etc.

[Ken Fitzgerald]

Cliff.....it says it's induced...typically that indicates electromagnetic induction............

Cliff there are many ways it could be done.

You could speculate and guess until the cows come home and still be wrong. And....it would still be an unproved wild guess.....

It doesn't matter. It works.

Go learn to use your saw Cliff.

[Joe Leigh]

This entire thread makes no sense. If you were skeptical about the science of how it works or the reliability of when it works, then why did you buy it in the first place?

[Cliff Holmes]

Where did I say I was skeptical? Where did I imply I didn't trust it?

And why is it suddenly improper to wonder how a tool works?

[James Evers]

That's an amazing piece of technology. But the cost of the saw is a bit out of my price range. Also shouldn't one be using push block and blade guards to prevent such contact?

[Chris Nolin]

I have no problem musing about the details of the technology. In fact I enjoy it. Why is this so contentious for some of you?

[Mike Henderson]

That's an amazing piece of technology. But the cost of the saw is a bit out of my price range. Also shouldn't one be using push block and blade guards to prevent such contact?
Absolutely! The blade stop should only be looked at as secondary protection. Accidents do happen and careful people who use safe techniques get injured.

Mike

[Mike Cruz]

Since when do we razz people for their "why" questions and inquisitiveness?

Get rid of these questions and there goes half this forum...questions and members...

[Steve Gass]

It seems like this question isn't getting answered to everyones satisfaction, so I thought I might pipe in. The SawStop system senses contact by detecting the capacitance of the human body. To detect this capacitance, the body must establish an electrical connection to the saw blade.

Now here is where it gets a little complicated. You can have two difference kinds of electrical connection - a capacitive connection, or a conductive connection. When you touch the side of the blade, the pad of your finger creates a parallel plate capacitor to the side of the blade. The blade is one side of the capacitor and the wet tissue under the callous is the other side. For this to work, however, you need a fair amount of surface area since capacitance is proportional to surface area (at least for a parallel plate capacitor). You can see that this effect doesn't rely on a conductive connection because it works just the same if you put your finger in a sandwich bag and touch the blade. The plastic will prevent a conductive connection, but it doesn't stop the capacitive coupling.

Now if you try the same thing on the tips of the blade no contact will be detected because the surface area of the contact zone is not large enough to make a parallel plate capacitor big enough to link the capacitance of the body to the blade. Basically, the capacitance of the contact zone when it is small blocks the signal from reaching the body. However, if the tooth cuts through the callous (which is a good insulator) and reaches the underlying moist tissue, then you suddenly establish a conductive contact. This conductive contact, even though it is small in area, has less electrical impedance (a fancy way of saying it is a good conductor) and the signal can pass through to reach the body. Licking your finger before touching the tip just mimicks the effect of cutting through the callous by making the outer layer of the skin much more conductive.

Clear as mud? :)

Steve Gass
President, SawStop LLC

[Dave Sharpe]

Clear as mud? :)

Steve Gass
President, SawStop LLC


sure, sure That all makes sense. Now where's my board stretcher????

Seriously Steve, thanks for chiming in and trying to explain electrical concepts to a bunch of sawdust production engineers. Nice to know you're out there and accessible to us.

Dave

[Chris Nolin]

Actually, clear as crystal. Thanks, Steve. Brings me back to EE class! Fun stuff!

[mreza Salav]

Great post Steve, thanks.
Now I understand why when you touch the teflon coated blades (like Freud) on the side, it still detects the contact.

[James Evers]

It seems like this question isn't getting answered to everyones satisfaction, so I thought I might pipe in. The SawStop system senses contact by detecting the capacitance of the human body. To detect this capacitance, the body must establish an electrical connection to the saw blade.

Now here is where it gets a little complicated. You can have two difference kinds of electrical connection - a capacitive connection, or a conductive connection. When you touch the side of the blade, the pad of your finger creates a parallel plate capacitor to the side of the blade. The blade is one side of the capacitor and the wet tissue under the callous is the other side. For this to work, however, you need a fair amount of surface area since capacitance is proportional to surface area (at least for a parallel plate capacitor). You can see that this effect doesn't rely on a conductive connection because it works just the same if you put your finger in a sandwich bag and touch the blade. The plastic will prevent a conductive connection, but it doesn't stop the capacitive coupling.

Now if you try the same thing on the tips of the blade no contact will be detected because the surface area of the contact zone is not large enough to make a parallel plate capacitor big enough to link the capacitance of the body to the blade. Basically, the capacitance of the contact zone when it is small blocks the signal from reaching the body. However, if the tooth cuts through the callous (which is a good insulator) and reaches the underlying moist tissue, then you suddenly establish a conductive contact. This conductive contact, even though it is small in area, has less electrical impedance (a fancy way of saying it is a good conductor) and the signal can pass through to reach the body. Licking your finger before touching the tip just mimicks the effect of cutting through the callous by making the outer layer of the skin much more conductive.

Clear as mud? :)

Steve Gass
President, SawStop LLC

That is an excellent explanation and makes perfect sense. I do admit the innovation makes me want to save up and get one of those saws simply to reward what I view as great innovation. Still I'd never test the machine. I'll leave that to the engineering team at SawStop. It would be interesting to see someone try this type of tech on a skill saw. Given they are one of the most dangerous tools on the planet.

[Van Huskey]

Excellent, thanks. Now Cliff has his answer after havng to endure a lot of grief to get it.

I think when people ask questions just because we don't know the answer we should never give them a hard time just because THEY want to know.

[Ken Fitzgerald]

and now there is no speculation or misinformation.

Thanks!