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SawStop math, something doesn't seem to add up

8K views 96 replies 36 participants last post by  wapakfred 
#1 · (Edited by Moderator)
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
 
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#4 ·
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.
 
#5 ·
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.
 
#6 ·
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!
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
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.
 
#10 ·
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.
 
#11 ·
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.
 
#12 ·
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.
 
#13 ·
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.
 
#14 ·
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?
 
#15 ·
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.
 
#16 ·
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.
 
#17 ·
"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."

Well that's the bad part, I'm sure its similar to seatbelts increasing the number of accidents (though lessen the severity).

I got one, but I view the safety mechanism as an added bonus. I needed to upgrade from the noise of my jobsite saw and for a few hundred more, when it very may well be the last saw I ever buy, who cares.
 
#18 ·
Thanks everyone for responding! To be clear, my post was not meant to be "pro" or "against" SawStop. Everyone has different requirements when they buy something. And for your guys that have confidence in your own safety skills, my hat is off to you. I'm an avid cyclist and have ridden 20,000 miles in the last 6-7 years. Never an accident, never even a close call. Then on memorial day I crashed and broke my femur. Freak thing, but changed my life in a blink. With that experience fresh in mind, for me, if I can spend a little (well, a lot actually) more and never have to worry about regretting passing an opportunity to save my finger/fingers/hand/life then I'll do it. But I don't take anything on faith. I like to sanity check things. Often, marketing claims seem overly optimistic. But in this case, it was the opposite. I was thinking "why would they even list a time (5ms) when the time is that slow?" Doing the calculations just raised more doubts. But again I go back to a sanity check. I have to think, if somebody had gotten their finger cut off by a SawStop wouldn't we have heard it by now? Even with lawsuits and gag orders and everything else, is it really reasonable to think that not one case would have surfaced by now? I'd feel better if I could work it out with a calculator, but I guess I'll have to assume it has something to do with the dropping mechanism that paints a whole different picture than my simple calculation.

One thing I notice is that when this debate comes up people often talk about the speed of the hotdog/finger going into the blade. I guess to me (here goes another sanity check) this doesn't really seem that important. What I mean is, if the blade tip is traveling at 2100 inches per second, does it really matter if you approach it at 1 inch per second or 10 inches per second? I'd think not. The blade speed is orders of magnitude faster than your finger speed either way so I can't see finger speed being a make or break parameter. I could be wrong, but that's just the way it seems to me…
 
#19 ·
sounds like the op is getting a sawstop guys,and i dont see why not it is another layer of protection,but a guarantee,no.everyone wants that guarantee to a safe happy life,well it just doesn't exist.happy cutting just dont forget the most important safety factor,common sense!!!!.
 
#20 ·
... does it really matter if you approach it at 1 inch per second or 10 inches per second?
Well, if the saw stops 5 ms after initial contact, then how far your hand moved in that 5 ms matters a lot.
Code:
 1inch/s, your hand only moves .005" in 5 ms = paper cut at most<br />
10 inch/s and the cut is 0.050" deep (about 1/16"). Still not bad.
But hands can move a lot faster than 10"/s which is where the problem lies.
@200"/s (which you probably do by clapping your hands fast) then the cut is around an inch, which is a big problem.

I can imagine a lot of scenarios where SS would work well and save a serious injury. Can imagine some (e.g. hand getting flung into blade during a kickback) where its hard to imagine there wouldn't be a lot of blood on the floor…...but even in the worst scenarios it is probably still helping a lot.

So Kudos to SS for coming up with an improvement. Mind you, just using a TS blade guard probably prevents 95% of hand to blade injuries.
 
#21 ·
The question I would have is why not buy one if you are buying a new cabinet saw? I have had the 1.75 hp cabinet saw for about 4 or 5 years now and never set it off, it is extremely accurate, has never failed me and as I recall when I was shopping for saws if there was a difference in price it was small. I have been fortunate that I have never set it off or had a kickback which may be partially because I install the riving knife when not using the blade guard. In all this time I have never heard of or seen anyone do major damage to themselves with one and at the speed of the internet people would be posting pictures in the emergency room not waiting for some lawyer to tell them they couldn't. So my question again would be why not?
 
#24 · (Edited by Moderator)
Interesting. I'm planning on SawStop as an upgrade replacement to my current saw. Comments on the comments:

1. That you've never had an accident where you cut off your finger? Weak argument. This is about reducing future accidents. "Past performance is no guaranty of future performance." I'm getting older, so are my reflexes, strength and cognitive functioning/judgement.

2. The mathematics of saw rotation: Very interesting. Thanks for posting! So it appears that SawStop does not guaranty perfect safety. Like seat belts on cars, you can still get killed while wearing them. Hmm…food for thought.

3. Documented history: So there is no reported case of a serious injury while using? Strong argument. I rather like arguments that make use of studies involving large numbers. Hundreds of thousands of SawStop users, and no one has incurred serious inquiry? Pretty compelling testimony. As a cabinet maker, I've met so many men with missing fingers.

4. The owners of SawStop are jerks: Well I wish they weren't. But they are. Can't solve that problem.

5. There are better alternatives in Europe: Really? Fascinating. Not sure if this helps me now.

6. I can work safely: Indeed so. But I can't. Part of being a professional is cranking out product in a hurry and its just a habit that's hard to break. Being tired, and in a hurry and a little careless, and a bit older (almost 70) suggests that an additional layer of safety might be worth the investment.

Conclusion: I like my UniSaw a lot, and don't want to change. Especially don't like Biesenmeyer fences, but it seems a rational and practical decision to move to a safer (if imperfectly safer) saw. I can afford it. Wish it had a different fence, but oh well.
 
#25 ·
It's complicated and we would need a lot more information to be able to calculate how deep a cut the blade could make.

I decided to go looking for some YouTube videos where some hotdogs we're being moved quickly into a blade. Here's some I found. In general, there seems to be two things to take from this.

1) Yes you can still seriously cut yourself on a SawStop
2) The cut will be MUCH less serious than without the SawStop braking action.








For the masochists on LJ, you can read my thoughts on this.

Concerning the OP, the math isn't wrong, but the results aren't being viewed properly. The number of teeth that can rotate past a point is more or less meaningless. Would a much finer tooth blade be more dangerous? I don't think so. Yes there is a point where you could analyze this as discrete blade tip events. And that would become more appropriate if the blade stopped much faster. For example could stop it in less time than a single blade tooth could move past a finger. But as calculated, even a low tooth count blade can have a lot of teeth move past in the designated time.

While tooth count is important in all types of saws and materials being cut, when it comes to cutting flesh, no matter what, the blade is going to pass through like a hot knife through butter.

Blade speed really isn't the issue. Would a blade moving at half speed be safer? Again, while a slower blade won't cut wood as quickly, I think it matters little when cutting flesh. I think it makes more sense to just see it as a cutting disk and how fast it is rotating doesn't matter.

What matters most is how fast your finger is moving into the blade and how fast the blade is becoming safe. As mentioned the SS both stops the blade and retracts it. They claim in "less than 5 ms. " Which probably means it is well under 5 mS, but they have added headroom to their claim to ensure they don't overstate it for legal reasons.

Without a lot of other information, the time to stop is just a single maximum limit. But since it also retracts, it is pulling away from the finger while stopping. So I suspect in a lot of cases, the blade is pulled out of harms way well before the blade has stopped.

Then factor in that part of this 5 mS is going to be reaction time of the system and all the dynamic properties of the mechanics and we are left with the oversimplified evaluation that it's a spinning disc of death from T0 to 5 mS. Clearly there is more going on between fully spinning and dead stop.

Table saw accidents can be complicated. There's the simple pushing a miter gauge and your finger into a blade. In that case, the damage is going to have a lot to do with how fast you are moving your hand. And of course how quickly the blade becomes safe.

I think most cuts are moving at most a few inches per second. If that's the hand speed, then say for a 2"/s speed 5 mS is 0.01". The proverbially SawStop nick.

But of course a hand slipping is another possibility. For example pushing stock through the blade and your hand slips. And if applying enough force, your hand is going to move quick. Not unlike trying to push your hand as fast as possible into the blade. I have no idea how fast that might be, but 200"/s would make a 1" cut in 5 mS. 200"/s is about 11 miles per hour. That doesn't sound crazy fast.

So in 5 mS you could perhaps move your hand 1" which is more than enough to cut off a finger. But again, what's really happening in that 5 mS? We simply don't know the details. We know the blade is also retracting very fast. To some degree this counters the hand speed.

And of course we have kickback where your hand can be pulled into the blade very quickly as another possibility.

But we don't know when the blade starts retracting, how fast it accelerates to full retraction speed so on and so forth.

Bottom line, I think most of the time people are inadvertently moving their hands into blades and this is generally pretty slow and you'll get the nick. And that there are cases where your hand could be moving so fast from slipping or pulled in via kickback, that the fingers could be completely severed, even with a SawStop.

A SawStop may not guarantee no serious injury in EVERY case, but I think there's no question it significantly reduces the probability of a serious injury in most cases.
 
#26 ·
I think that you calculated the wrong thing. It doesn't really matter whether it took 2 teeth or 50 teeth to sever your finger. You have to compute how fast your finger would have to be moving to cut it off in 5ms. In other words, how deep of a cut will you get in 5ms. So assuming that your finger has a diameter of 5/8" and computing just cutting half way through or .3125", your finger would have to be moving at 62.5 inches per second or just over 5 feet per second to move .3125 into the blade in 5ms . Double that to completely separate the tip from your finger. That is pretty fast for normal movements while using a table saw. While making a cut that causes a kickback may possibly pull your hand into the blade that fast or even faster when an accident happens, it will still do much less damage to your hand if the blade stops and retracts than if it continues to turn at 4000 RPM as your hand continues through the blade.
 
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