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SawStop math, something doesn't seem to add up

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Forum topic by CrankAddict posted 10-12-2019 11:40 PM 3024 views 0 times favorited 96 replies Add to Favorites Watch
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CrankAddict

61 posts in 337 days


10-12-2019 11:40 PM

I’m trying to convince myself this thing would actually be guaranteed to save my fingers but I’m failing to make the numbers come out.

10” diameter blade @ 4000 rpms = tooth tips moving at 2094” per second. SawStop says it’ll stop the blade in 5ms, but that’s like 10.5” of rotational movement (and 1/3 of a revolution of the blade). So on a 40T blade, that’s 13 teeth passing by a point in 5ms. How would that not be enough to cut a finger clean off or at least down to the bone? I thought my math must have been faulty but even the wikipedia page for SS mentions the blade moving at 2” per millisecond, which lines up with the 10” in 5ms that I calculated.

The wikipedia page also quotes a magazine article from 2005 and says “Given the speed of the blade, it would have to stop in about 1/100 of a second — or at about an eighth of an inch of rotation after making contact. Any further, and the cut would be so deep that the device would be useless.”

The “1/8 inch of rotation” being a maximum allowable amount seems pretty reasonable… but if the blade moves 2” in 1ms, then an 1/8” of movement would occur in roughly 0.06 milliseconds (60us), so how is a 5ms stop timeframe going to do anything? For that matter, them saying it needs to stop in 1/100 of a second (10ms) seems useless as well. That does not seem to correlate with 1/8” of movement in any way that I can calculate.

I feel like I’m missing something here… any other saw nerds hanging around the forums on a Saturday night? haha

ref: https://www.sawstop.com/why-sawstop/the-technology/
ref: https://en.wikipedia.org/wiki/SawStop


96 replies so far

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NoSpace

170 posts in 1776 days


#1 posted 10-12-2019 11:57 PM

You need to account for the time it takes to move the hotdog into the blade. Imagine an extreme example where you creep up it and barely touch, and then go from there.

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LeeRoyMan

331 posts in 262 days


#2 posted 10-13-2019 12:04 AM

I don’t have a clue about the math, but are you factoring in the fact that the blade also drops below the table during that time?

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therealSteveN

4079 posts in 1109 days


#3 posted 10-13-2019 12:39 AM

Fact is there isn’t a math problem done before folks lop off a finger or 3 on a table saw. It’s a collective of bad choices, sometimes choices in the past you made, and got away with that come to roost.

If you can clearly say you only do safe things while running a Table Saw, and set up, and follow through with only these same things, every time, no matter how tired, repetitive, or unsure you can be. Then maybe you will never need a finger nanny. Other wise the “math problem” is really simple. SawStop costs a LOT less than a new finger, or even a surgery to try to save that old finger.

Not an owner myself, nor do I desire to be, and I’m not a rarity. I can always say yes to safe operation. Like millions of others, it’s easy enough to do, but you need to be committed to keeping those fingers.

-- Think safe, be safe

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CrankAddict

61 posts in 337 days


#4 posted 10-13-2019 12:51 AM

No I’m not factoring the drop in. I don’t have any claims or specs to be able to calculate the speed at which the blade drops down. I guess the follow-up question is does the “5 ms” time represent the time it takes for all of this to complete? For example, maybe the blade stops spinning in 1 ms, or at least the there is only something spinning above the table for 1 ms, and by 5 ms it is completely dropped? That sort of thing would seem to make more sense. If the blade is fully up and spinning for 5 ms after contact, I can’t see any way that your finger survives.

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pottz

6639 posts in 1520 days


#5 posted 10-13-2019 01:12 AM

i have the same thought’s,all the test ive seen done even the one where gass pushes his own finger into the blade are done very slowly,in real life your not gonna be hitting the blade at ultra slow speed,has anyone seen a test done slamming a hot into the blade quickly,id like to see the result.bottom line sawstop cant and wont guarantee you wont loose a finger.would i buy one ,probably,it’s just another added safety feature.the best safety devise is your own common sense practices!

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

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Redoak49

4232 posts in 2524 days


#6 posted 10-13-2019 02:06 AM

On boy, another SawStop argument with the same old comments.

Hey, if you want one, then get one.

If you believe that you can be safe 100% of the time then you do not need one.

Very simp!e…

I have yet to see a documented case where someone lost part of a finger with a SawStop.

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pottz

6639 posts in 1520 days


#7 posted 10-13-2019 02:26 AM



On boy, another SawStop argument with the same old comments.

Hey, if you want one, then get one.

If you believe that you can be safe 100% of the time then you do not need one.

Very simp!e…

I have yet to see a documented case where someone lost part of a finger with a SawStop.

- Redoak49

if this topic irritates you why did you comment,and i guess if youve never seen a case where it’s happened that means it never has? i do agree if using a sawstop makes you feel safe than by all means buy one.

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

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bigJohninvegas

689 posts in 1997 days


#8 posted 10-13-2019 02:40 AM

So I have no idea about the math to calculate how fast the blade is moving and all that.
But a simple fact.
I have been at the local woodworking school here in Las Vegas (Wood it is) on two occasions where a student has tripped the safety on a Saw Stop saw.
And two more times for the hot dog demo at the local club meeting. (Sin city woodworkers).
The 1st time I saw it was for the hot dog demo. And I thought ok that’s cool. But what about real finger. Then about a year later it happened.
I was on another saw with my back to the other student. A loud thud and it was over. The guy was pale white. Said he slipped, and fed his hand into the blade. I was amazed to see what looked like a paper cut on one finger.
I was in the other room when it happened the 2nd time. And not sure how it happened. But that guy had a similar paper cut.
Both times, the check books were out pretty quick for the cost of a new woodworker 2 blade, and brake cartridge.
The second hot dog demo, we tried to go fast to force a deep cut on the hot dog. But no luck. Still looked like a paper cut on the hot dog.
I don’t own a saw stop. Doubt I ever do at this point. But dam, I am impressed with how that brake works.

-- John

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oldwood

162 posts in 1779 days


#9 posted 10-13-2019 02:42 AM

I am not a math guy, I leave that to my daughter that teaches advanced high school math. However, I am a guy who knows from personal experience, twice, that it does exactly what they say it will do. I have triggered mine once with wet lumber, once with just a fingernail nick and once that did a little damage. As a sales representative of a dealer I have done the hot dog demo a number of times. In all these events I have examined the blade and device and I can promise you the blade does not travel 10 inches after contact. On a 40 tooth blade the most teeth that will touch the aluminum block on the device is four but often only three with two embedded. So, again I can’t solve the math problem but I know what I have experienced.

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pottz

6639 posts in 1520 days


#10 posted 10-13-2019 03:01 AM



So I have no idea about the math to calculate how fast the blade is moving and all that.
But a simple fact.
I have been at the local woodworking school here in Las Vegas (Wood it is) on two occasions where a student has tripped the safety on a Saw Stop saw.
And two more times for the hot dog demo at the local club meeting. (Sin city woodworkers).
The 1st time I saw it was for the hot dog demo. And I thought ok that s cool. But what about real finger. Then about a year later it happened.
I was on another saw with my back to the other student. A loud thud and it was over. The guy was pale white. Said he slipped, and fed his hand into the blade. I was amazed to see what looked like a paper cut on one finger.
I was in the other room when it happened the 2nd time. And not sure how it happened. But that guy had a similar paper cut.
Both times, the check books were out pretty quick for the cost of a new woodworker 2 blade, and brake cartridge.
The second hot dog demo, we tried to go fast to force a deep cut on the hot dog. But no luck. Still looked like a paper cut on the hot dog.
I don t own a saw stop. Doubt I ever do at this point. But dam, I am impressed with how that brake works.

- bigJohninvegas


now this is the testimony i want to hear not just opinions,thanks for sharing your personal real life experience john,i think this helps people decide if they want this safety device or not.

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

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jonah

2088 posts in 3834 days


#11 posted 10-13-2019 03:07 AM

All the rotational energy is directed straight downward once the brake hits the blade. That’s why it disappears so fast.

Going by the various high speed cameras I’ve seen of it in action, it looks like it stops in less than 1/30th of a rotation. Given the blade geometry, that’s plenty to avoid major injury.

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CaptainKlutz

1940 posts in 2030 days


#12 posted 10-13-2019 03:48 AM

Your are not missing anything!

Here’s my Sawstop Story if I may?

Back many years ago on Sawstop introductory tour, challenged a factory rep with same question.
Why are all the demos in slow motion, and dog is barely nicked?

Factory rep admitted he had never pushed anything into blade fast, and was willing to try it. Unfortunately, he refused to use his finger as test subject for a quick jab to simulate cross cutting a 1×2 piece of face frame?
So he ‘taped’ the venerable hotdog to miter gauge and pushed it all the way through the blade as it dropped; just like any other person not paying attention.

The hot dog had ~1/2” deep by 1/8” wide cut in side, or was cut about half way through. :-0)

Conversation went like this:
Me: Appears folks can/will cut my finger/hand to bone even with this safety device?
Factory rep: YES I guess so. But you shouldn’t lose the finger
Me: What good is a finger cut half way through like that hot dog?
No answer. Oh Oh….

Mr Sawstop (Stephen Gass) was near the demonstration event, but was not in the building when this demo happened. He shows up about 20 minutes later, and customers are asking about hot dog with huge chunk missing. He claimed it was impossible for saw to do that, and someone dropped a test dog. LOL
Then he breaks it half, and throws it away to hide the evidence.
I told him his rep did that earlier, and suggested his claims were false, and he is selling a tool that didn’t work as advertised. Mr Phd in Physics didn’t like being challenged.
He attempted to argue with stupid little me (a professional engineer, and holder of dozen patents), who used similar math posted by OP to show Sawstop was too slow. Conversation got heated as Mr SawStop threatened a lawsuit for defamation to anyone disputed the marketing claims, or showed the video of hot dog being cut too quickly – as it wasn’t a simulation of real life. I offered to go get some chicken legs, or ham hocks and try a 3” high blade extension for real bone saving/cutting test; but was politely asked to leave the Rockler store at that point. :-(

Why would the inventor refuse such a test if it worked as marketed? Hmm…
Read fine print on the Sawstop purchase agreement if you want truth on protection actually purchased.

——Want to learn more about Sawstop?

Several folks have challenged Sawstop on the patents, and used the marketing claims against them. Both Bosch and Stanley Black & Decker (Dewalt/PorterCable) have been been issued numerous patents for TS blade protection mechanisms. Bosch has been selling a better/faster protection mechanism (doesn’t ruin blade) in EU for several years, but can’t sell it in US due Sawstop infringement issues.

Furthermore, the 1st Sawstop patents expire in ~2021, and concept of saw blade safety due sensing contact with human will become public domain. The only issue confronting saw mfg is a couple of later Sawstop patents that refined the mechanism, and added details that make a protection system inexpensive. So we may have to wait ~7 years for lower cost competitive versions?

If you want to learn more about Sawstop patents, check out the Sawstop .vs. Bosch Reaxx legal battle; any search engine can find references:
http://www.toolreport.net/2017/03/28/sawstop-vs-bosch-reaxx-lawsuit-press-release-update/
https://www.finewoodworking.com/2016/09/14/bosch-vs-sawtop
etc, etc, etc

IMHO – Be safe, but never believe the Sawstop claims.

PS – No one will change my negative opinion of Sawstop, so don’t waste bits/bytes.
Their record explains it all: The well documented attempts to force OSHA to mandate their patented technology on all new saws, refusing to license the technology at fair price to others, attempting to sue all other saw mfg for collusion as they refused to license Sawstop technology and help OSHA adaption, or it’s greedy business model of charging a $1K-$2K premium for $100 in safety parts. There is a lot to dislike.

PPS – For really curious: TTS Tooltechnic Systems bought Sawstop and all patents in 2017 (they own Festool too). Harvey Industries in China makes Sawstop saws.

Thanks for reading to end.

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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Rich

5001 posts in 1125 days


#13 posted 10-13-2019 03:50 AM

Wow, this escalated quickly. As soon as the SawStop can eliminate kick backs, and I can buy a MiterStop, a JointerStop and a BandSawStop, maybe I’ll take the leap.

Yes, it’s a quality saw, but do I want to pay several hundred dollars for questionable safety benefits?

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pottz

6639 posts in 1520 days


#14 posted 10-13-2019 04:00 AM

thank you klutz for reaffirmation my thoughts all along,does it work,id say after all ive seen or not seen,yeah to a certain point.id say if your hand is quickly pushed into the spinning blade your gonna get more than a nick.ill just stick to paying attention and using safe practices,hey it’s worked for about 40 years so far.

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

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Rich

5001 posts in 1125 days


#15 posted 10-13-2019 04:27 AM


thank you klutz for reaffirmation my thoughts all along,does it work,id say after all ive seen or not seen,yeah to a certain point.id say if your hand is quickly pushed into the spinning blade your gonna get more than a nick.ill just stick to paying attention and using safe practices,hey it s worked for about 40 years so far.

- pottz

You and me pottz. I have ten fingers left and know what it takes to keep them.

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