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Magnetic Hold-Downs and Saw Stop

Matthew_Schenker's picture

Thinking about safety lately...

I've been wondering about the powers of magnetic hold-downs during table saw operations. I like the idea, and it seems real convenient to be able to place them on the table wherever you need it. The real question I have is about the claim that they protect you from kickback. Can they really be powerful enough to hold back a 5/4 hunk of wood flown by a 3HP Unisaw?

Does anyone have any further information about the "saw stop" innovation that was released about a year ago? For those of you who haven't heard about this, it's a new invention that puts a brake on the table saw, and stops it instantly in the event that your finger touches the blade. It has a complex system of sensing any moisture. On the Saw Stop Web site, they demonstrate by pushing a hot dog into the saw blade and all it gets is a minor nick. I've been wondering if any progress has been made in releasing this to the general market.

Matthew_Schenker's picture

(post #90468, reply #1 of 5)

Good point. They use a hot dog as their demonstration -- which is pretty moist. According to the makers of the Saw Stop, if you push green wood through, it doesn't trip the brake. Therefore, your hands need to have a moisture level somewhere above green wood. I'd like to see tests showing the moisture thresholds of the device. Perhaps the makers of Saw Stop will produce a chart of some kind showing at which moisture levels the machine works perfectly, and where it begins to be insensitive. Then you could check the moisture level of your own hands and be more certain about it.

If they developed this device reliably, it would be an amazing breakthrough.

So I see your hesitation on the Saw Stop. How about your opinion on the magnetic hold-downs?

Dave_Arbuckle's picture

(post #90468, reply #2 of 5)

SawStop doesn't use moisture to sense a person, it uses capacitance (or something else waaaay outside my field). I'm going to send an e-mail to Dr. Steve Gass, the inventor. Hopefully he'll stop by and give some info.


Stephen_Gass's picture

(post #90468, reply #3 of 5)


First, thanks to Dave for letting me know about the this thread so I could answer your questions.

The SawStop system senses a human based on the change in capacitance of the blade when the human body contacts it. The amount of change is dependant on how the body contacts the blade. If the skin is dry and only lightly touches the side of the blade, this won't be picked up as contact because the electrical coupling between the blade and body will have too much resistance because of the dry skin. However, if the teeth so much as nick your skin, they get down in to the salty moist blood in the capillaries and a good conductive path is formed almost no matter how light the contact.

When I did the one of the tests with my finger on the spinning saw blade, I touched it very lightly and when the system tripped I had a small v-shaped groove that was red at the very bottom, but was not even deep enough to bleed. The salty moist fluid provides the conductive link to the bodies capacitance. The hot dog is also salty and moist enough to provide this link so long as you have a good grip on the hot dog. If I'm not holding the hot dog, the saw will cut right through it, spraying a gross path of pulverized hot dog in the plane of the blade incidentally.

Green wood is generally not salty, so it does not conductively link into the blade. Now, if you are cutting green pressure-treated wood, where the pressure treatment is a conductive salt, you could potentially get enough signal change to trip the system. I have not been able to achieve this myself, I guess my pressure treated sample wasn't green enough, but one manufacturer has reported that they have been able to get this to happen.

On the question of getting it out there, we are still working on getting manufacturers to adopt the technology. We are getting very close to our first licenses with two major manufacturers, with a third hopefully not far behind. I expect that the first products will be available by the end of 2002. The best thing you can do if you would like to see it as soon as possible is to contact the various manufacturers and tell them that you are not going to buy another saw until it has SawStop. We have a list of manufacturers on our website. :)

Lastly, we now have a bandsaw and a pneumatic cut off saw working with the SawStop system. You can see videos of these on the website as well.


Stephen Gass, Ph.D., President

SawStop LLC
22409 SW Newland Rd
Wilsonville, OR 97070
503-638-8601 fax

Major_H._Patterson's picture

(post #90468, reply #4 of 5)

I use the magnetic hold-down on both the table saw and the jointer.I find that the plastic fingers do a decent job of holding stock in and down.However the cam lever on the back that picks the magnet up for removal or adjustment is for the birds.I removed this lever and replaced it with a "teenut"and thumb screw.Fine adjustment is now much easier as the magnet can be raised a finite amount as the tool is positioned.A quarter turn of the screw sets it down exactly where you want it. FWIW ¬¬¬¬¬¬¬¬¬¬PAT¬¬¬¬¬¬¬¬¬

Matthew_Schenker's picture

(post #90468, reply #5 of 5)

Thanks Steve for filling us in on the technical aspects of the Saw Stop.

I was impressed immediately when I read about your invention (I e-mailed you about a year ago to offer my praise -- not that you would remember, as I am sure you got hundreds of e-mails). Not only was I impressed with the concept, but also with your knowledge and credentials. Saw Stop appears to be one of those ideas that, if implemented properly, could absolutely change the whole idea of table-saw safety.

I just hope that manufacturers understand that there is monetary value in selling safer products and are able to have it be standard with new machines. Otherwise, the cost and complication of installing it as an after-market device could be prohibitive to many woodworkers.

Keep in touch so we can follow your progress.

Please share any new thoughts or breakthroughs.

Thanks again!