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physics, table saws, and flying lumber
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Table saw kick back seems to be seperated into two categories:
1) straight line kickback: when the material is pushed against the blade without perpendicular resistance to the cut-off.
Laymens terms--peice shoots like an
arrow straight back.
2) back tooth kickback: when the material hits the back tooth of the blade and climbs up the blade, blade digs into material in arc shape, flings the peice back, while rotating.
Layman's terms: peice shoots up at
you.
A video illustrates this very well - see below.
*Interested or not in physics, if you use a table saw, please watch it.*
Now here comes the big question... in back tooth kickback, your peice rotates as it flies up/back. I've only seen this with smaller material peices. But if you have an incorrigeable moron in the shop that freehand (e.i. no fence) "joints" bent 10 foot long lumber with a table saw, if he gets back tooth kickback, would the lumber go straight back at him, or could the rotation cause such a sizable peice to go/hit something sideways?
Quite frankly I'm asking because my work section is close enough to that table saw to get the peice in the head if it does rotate. And I also like physics :)
PS no need to tell me that this is awful and shouldn't happen, I know that, but don't have boss pants. Just want some kickback physical info to tackle it like a monster when I have my "this can't happen" conversation.
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If simple common sense doesn't work, I don't think physics is going to help.
If you're seriously concerned about your personal safety, a talk with someone that does have the boss pants on might include the suggestion that OSHA do an inspection to see if the procedure complies with their view of safe practices.
BTW, we have voluntary OSHA inspections every few years.
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David, I get where you are going, but they do surprise me sometimes! Being armed with info definitely works well around there. Perhaps I just express things better when I have a thorough understanding of them--dog on a bone phenomonom perhaps.
One way or another I do actually wonder about rotation force on back tooth kickback.
Hats off for the voluntary inspection btw! The world could use more of that sort of thinking.
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In this scenario you will most often get a linear result, but not necessarily.
Where the kickback will end up is somewhat unpredictable, the likelihood of it happening sooner or later is not. Try suggesting that your coworker use the bandsaw (or the sliding table saw with a ripping shoe for surfaced lumber). If he truly is an incorrigible idiot this will not work and you may have to wait until he puts himself out of action. Management should not need to be convinced of the danger of this practice for the perp and everyone around him.
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I just bought a table saw with a riving knife.
30 yrs on and had never even seen one.
Definite confidence booster.
I have a Striebig and process as much as possible on it, as it is MUCH safer as well as faster. The oddball stuff is done on the table saw with riving knife.
Just when you think you've idiot proofed something, they release a new and improved idiot.
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I was taught to straight line wany edge lumber freehand on a 16" rip saw when I left high school and enrolled at trade school in England. Standard practice and quite safe. The difference is having a riving knife, it should not be an optional extra, it should ALWAYS be there. that is why tablesaws are dangerous and liable to kickback.
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Phil is right - 100%. The riving knife is basic. But the US saw makers spent $1.25 on materials and $6.25 on engineering their riving knives since they knew standard operating procedure was to throw it away when the guard was tossed, when the saw was new. Sawstop knew the importance of the knife and made a good one and engineered it to make it workable.
The riving knife will prevent all kickbacks except for dropping a piece on the top of the blade. This is prevented by being awake and aware.
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Funny story about people and riving knives. And this is why I think that my motly crew is workable (except for the incorrigeable idiot).
So back to physics... There was a time where our local work-safe organization thought that the best practice on a table saw was to keep the saw teeth just barely above the material being cut. The idea is that if you get back tooth kickback, with its roation force, it can drive your hand towards the blade. So the idea is that by having the blade low you have less chance of finger slicingld things go wrong.
But there is a problem with that--it all has to do with blade rotation and force and direction in relation to your materiel. Low blades have less frontal downwards force at the business end, which really is what cuts, and more inline throwback force.
(This explains it perhaps better:
http://www.waterfront-woods.com/Articles/Tablesaw/tablesaw.html
(tho
ugh I wish the drawings included explicit illustration of direction of force))
Since I work with people that are minimum a generation older than me, I got advised to put down my blade. That same day I got a 20 x 30 plywood peice straight into the stomach. I've been punched in the stomach before, but nothing compares to a saw punch. I'm pretty rough and tumble so I shook it off but got a little freaked out when internal bruising popped up about 3 days later.
So I researched table saw safety. Saw that the big ol' guard was the prescribed method, but also realise that that is completely unrealistic if you use the table saw to all its options of cuts. But the riving knife seemed like that happiest medium. Shut up and remove it as often as you need to.
So I asked my foreman for one. Once, twice, three times. Nothing. I finally got ticked and told him dude, it needs to happen. Got a dump of animosity, so I explained the physics to him. Told him about the bruising from the previous kick back, told him it just simply wasn't a joke.
Just when I was wondering if I was about to get fired, he went upstairs into the metal shop, and made me a riving knife from bare metal. Came down an hour later, installed it.
It works like a beauty. Safe and handy as hell. I think sometimes we need to recognize that some people need a nudge in the right direction. We can't always label people for being a-holes for not getting it right in the first place.
So it turns out that this physics question is a good one. I have two engineers and a physics professor working on it--amazing what can happen over a couple beers on a friday night. Turns out it's not quite an answered problem yet.
I'll post theoretical result when I get em! I think we could all use a bit more of this sort of info. We all know these cuts are bad decisions, but knowing why is that much better.
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I am all for riving knives and splitters. They make table saw use much safer. At my workplace we have two Powermatic 66 saws with Biesemeyer snap-in splitters and a slider with a true riving knife. Everybody buys into using them except for non-through cuts- and why not? It's easy to remove and reinstall them and so much safer than doing without.
That said, I am not convinced that freehanding rips of roughedged lumber on a tablesaw is a good idea. If you have the experience and confidence to do it, fill your boots, but I consider it hazardous, at least the first 6" before the stock engages the riving knife. I believe I could do it without incident most of the time, but why take the risk when within a dozen steps of the tablesaw are a bandsaw and a sliding table saw? The first offers zero kickback potential and the second can easily be set up to hold the stock to run true for the length of the cut. How many shops with a tablesaw with riving knife or splitter lack a bandsaw? Why not use the safest alternative? You are going to go to the jointer next anyway, unless you use the slider cut as a reference edge.
Mel, you are quite right to consider the force vectors involved in blade height. If your hands can never get involved with the blade, the higher the better, but there is always the potential with hand-fed machines for that involvement, thus keeping the blade low has its justification. All the more reason to use a slider or powerfeeder when practical and keep hands far from danger.
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"why take the risk when within a dozen steps ...are a sliding table saw? ...can easily be set up to hold the stock to run true for the length of the cut."
I've never heard of that. Not that I would be likely to use that method, but can you explain or direct me to a source that explains it? I'm just curious.
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While the dimension saw can be used to straight line lumber I don't find it ideal when working thick lumber, (over 12 quarter ). dimension saws do not usually have rip blades in them, and heavy, rough and un flat lumber on an accurate piece of equipment is risking damage to the equipment. Would you want a 15" wide 3" thick piece of English oak, bowed across the width, with the bark still attached on your dimension saw?
Interestingly Back home in England, ( I have been living in the US for 7 years now) we always had rip saws with big blades and a fence that did not go past the front of the blade for straight rip cuts on rough lumber, large radial arm saws for rough crosscuts on rough lumber, and dimension saws for accurate crosscuts on dimensioned parts and panel work. No tablesaws.
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"While the dimension saw..."
I'm guessing that is a UK term for a table saw? If not, what is it?
On really thick stock (over 8/4), if we have plenty of room we hit-or-miss plane through our flattening 2-sider before ripping on our straight line. Otherwise you run the risk of binding on the really twisted pieces.
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David, I often use the sliding saw as a straight lining method with a ripping shoe, which is just a steel plate inclined toward the operator at the front of the sliding carriage fixed in the table slot. The workpiece is wedged under the shoe and held down by hand or clamped at the rear, with the desired cut line overhanging the carriage. Clamps fixed in the table slot can be used as well, the ripping shoe is just a bit quicker. Another very useful device for holding narrow pieces safely is called 'Fritz and Franz"- search for it on Youtube and you will see what I mean.
Phil, I am not suggesting dicing up rough twisted lumber on a sliding table saw, but truing up the edges of surfaced stock. For busting up rough pieces the bandsaw is far preferable from my point of view. The force of the blade is straight down, so no kickback potential, and the width and set of the blade pretty much eliminate the pinching that can occur on a circular saw.Granted, a riving knife can save you from the kickback effects of pinching, but I have many times had a spiteful piece of timber pinch on the riving knife and require shutting down the machine and wedging the cut open to proceed.
We have a heavy 18" rip blade for our Martin slider, which is what we use for deep cuts, but we rarely work with stock over 12/4. The advantage of that type of saw is that you can get a glueline quality edge in one go without rough ripping and rejointing, and you can hold the work on the carriage such that your hands are far from the action.
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"The workpiece is wedged under the shoe and held down by hand or clamped at the rear, with the desired cut line overhanging the carriage."
Ok, I think I've got it. You are affixing a piece to the sliding part, and then running that through(?). So you are limited by the length of the amount of slide. I guess modern sliders have more length? Our (1981 vintage) SCMI slider has just enough over 4' to do a full sheet of plywood, width-wise...
I just can't imagine anyone needing to straighten lumber on a daily basis using anything other than a jointer or straight line rip saw. We used a jointer only up until we got past $1MM in annual sales of virtually 100% moulding. So, a lot of straightening. Our jointer will take off up to 3/4" in one pass - can't imagine too many boards being worse than that... In reality, we usually would take up to 2-3 smaller passes on the crookeder ones.
A used, single blade SLR is available very cheaply, and will increase safety and production significantly over a jointer or table saw.
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David, you are certainly right to say that a straightline rip saw is more productive than any combination of tablesaw and jointer. If the outfit I work for was in the business of producing mouldings in quantity we would equip our shop quite differently than it is configured now. As a small custom shop that builds cabinets, doors, small runs of millwork and the occasional piece of freestanding furniture in a limited space we rely on basic, though generally fairly high grade, machines that you might find in any custom shop of the same capacity. If we had room for an SLR we would not be straightlining lumber on a 10 foot travel sliding table saw, and any shop so equipped would not long employ a cowboy freehanding pieces through a tablesaw without a riving knife. The original questioner appears to be employed in a shop more similar to ours than yours, and I was trying to show some other approaches to the task of putting a straight edge on a crooked board using equipment that might typically be found in such a shop.
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Sorry, by dimension saw I mean sliding saw, in England the jointer is the planer and the planer is the thicknesser!
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Phil, after reading one of your earlier posts regarding typical British shop equipment I wonder if your initial description "I was taught to straight line wany edge lumber freehand on a 16" rip saw when I left high school and enrolled at trade school in England. Standard practice and quite safe." refers to offering up the timber to the saw blade using a short fence as the initial guide? That would be considerably different from Mel's coworker freehand "jointing" boards without reference to a fence. I used to use the same process but now consider the bandsaw a safer alternative.
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The terminology difference is interesting.
Mathematically a plane is a smooth surface independent of thickness.
You do indeed do that with a jointer. But also, to joint, is to unite two seperate things or two parts of one thing. Which in a sense you do if you make a perpendicular.
But with a planer you make a thickness--a parallel. So "thicknesser" would be more accurate than "planer" for this one.
Let's just call the jointer a "perdendicularer" and the planer a "paralleler". All on board? :)
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I work in Australia now. They of course follow the UK ways in most things. They only recently started getting decent cabinet saws like Powermatic here. Historically they had these huge piece of shite 14-16" tablesaws with garbage fences and guards. Definitely not an accurate tool. They would be better suited as a boat mooring. Everyone has one of these things in their shop. Then the other extreme is no good portable saws, until recently with the good Bosch ones. They still use these stupid, dangerous, inaccurate fake tablesaws that are an average circular saw mounted upside down in a cheap table.
Ironically a Makita portable mounted in a Rossua table would be safer, more accurate, and less expensive than either the dinosaur or the new junk. These guys would piss themselves if they had an old 66 all kitted out with the Beismeyer stuff.
However, now everyone has a beautiful 100k slider. Which suck for cutting anything other than sheet goods.
My rant is over. I need another beer...
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I'm figuring out that a machine is "good quality" if it holds alignment longer then another machine.
I hear "this machine is a piece of shit" all the time about machines that haven't been aligned or maintained for ages. Some actually are bad, but some are also good but neglected.
They have dewalt consumer style chop saws that they refer to as being off because they are poorly made. They just needed an alignment and fence adjustment. I got them aligned but a month later it was out again, of course. I realigned it, and got asked "again?"
Well yes, again! That is the difference between a well made machine and a poorly made one--how often you need to adjust it.
You can adjust any machine, but how often do you want to do it? And how macgyver should you have to get about it?
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I would like to report a success!! Knowledge obtained and shared, behavior corrected, happy dance performed.
As suspected: the rate of rotation will depend on the length and thickness of the material.
So yes, back tooth kickback does produce rotation, and you could indeed get hit elsewhere then behing the blade. Lots of the time the rotation is minimized by two facts--smaller pieces of materiel (less momentum for rotation), and the fence also limiting rotation as the stock hits it in flight.
But remove the fence, and have a very long piece (e.i. some "swing to it"), and there is nothing to stop that piece from clocking a poor soul that is perpendicular to the blade. Or kitty corner.
If the stock is super big and heavy, you could potentially just get a funky cut + jerk + machine stall, but that is just a brutal thing to do to a machine, so not a win either.
So no matter which way you look at it, the cut impacts way more than just the idiot user behind the blade.
Shazam! Science!
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[X]
Project Gallery Listings: (27)
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Me?
I was going to pen a thoughtful response, but unfortunately I'm needed back on planet Earth... :-)
Apologies
Rob
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[X]
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It's Mel, thought it was Me!
Damn near vision thing..
Apologies again
Rob
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HAH! Golden :)
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Here's what you can expect if you do as I did and allow your attention to wane for a split second. I'm just glad the workpiece wasn't any longer. This is what I looked like after four hours then the next day. I ended up with a slightly bruised liver and a mild hernia. That'll teach me!
View higher quality, full size image (1800 X 1350)
View higher quality, full size image (1800 X 1350)
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Woah!! Look at that square cut-out from the materiel!
My belly bruising from my incident also took a bit of time to appear. I'll be honnest, it totally freaked my out.
I now use a riving knife as much as I can.
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That video of the wood flying should be step one of any woodworking training, including the professors looking into this.
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Re: ripping curved boards to establish an initial straight edge. I've done this freehand with a 12" blade, for years, never had a problem, but I've always done it with high focus, absolutely no distractions, and 35+ years of experience. Not to be taken lightly.
But there is also another way, much, much safer, and very low tech. Snap a chalk line straight-edge, and do a rip with a portable circular saw. As far down as I read, no one suggested this. I've done it on 12/4 hardwood, cutting in from both faces. It's not perfect, but you can then rip and flip until you get a straight enough edge. And it's one thing to free hand rip a piece of 4/4 softwood where the saw has plenty of power advantage (meaning it can saw through before it kicks), but freehand cutting in thicker hardwood means that when you get to that kickback moment, the saw will catch the wood, not saw it. Bam!
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A skill saw would definitely be the betteer way to go down that route, I beleive.
Funny thing about years of experience--apparently you are more likely to hurt yourself after (on average) 17 years of woodworking then in the first year.
Complacency, I guess?? Wouldn't know, I'm still in the "its a high rpm sharp metal rotator!" stage.
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New development on this one...
Turns out it's a very fun brain bender that involves a lot of different physical forces and direction possibilities.
Had my most favorite mechanical engineer over and we dug into it a bit more. Ended up chucking out the complicated theory and breaking it down simple.
Took a bunch of random sizes of lumber from the shop, set them on the floor, and simulated back tooth kickback with aggressive jabbs at back end corners to see how different dimensions react.
Didn't take to long to see a) how peices rotate b) how the initial jump from a back tooth would launch unto a top tooth that has a operator direction force--after rotation. (hence my incident resulting in one corner in the stomache and the other on my index finger)
Shall we proceed to the experimental phase? ;)
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Just saw this article in todays paper
Article
Mel, if that is your real name….
I have since viewed the video and with due respect to all others that have posted thoughtful, well intentioned comments on this thread, I will make the following observations:
I totally agree with you that you are a moron (your words, not mine), lacking in common sense and experience with basic power tools as in a table saw. Anyone with basic experience with a saw knows kickback takes two forms, straight back like an arrow for ripping narrow, and a spinning out of control Frisbee motion for squares that will hit you in the gut, or worse, Duuhhh???? In either case it is entirely predictable.
That this is a mystery to you is beyond my simple analytical powers.
This is a serious, professional site and as such, for my part, I will shun your further posts.
Best Regards-Not
Rob,
I don't believe this is Mel's video, but was included by her as illustration of bad practice. I saw this originally posted on the Sawmill Creek forum last year, plus Mel self-identifies elsewhere as being of the female persuasion.You are right to say that the maker of the video is a moron, but one can at least gain from viewing it an appreciation of how fast and bad things can go wrong when a workpiece gets out of line with a circular saw blade. Here's to riving knives and sensible people in the shop.
Please see the posts in a related topic herehttp://www.woodweb.com/cgi-bin/forums/swmf.pl?read=774631
Woah! How did this get so nasty so fast?
It is not my video indeed. I posted it with the thread as I think it is a must-see video.
I've been digging pretty deep into kick back physics as there are some commonly held myths around it. Didn't take to long till I stumbled on that video.
Perhaps asking if it is my video before getting excited is the better way to go? Throwing around unnecessary animosity isn't exactly professional either.
You’re right Mel (That Hurts!), I did mistakenly confuse you with the character in the video. Furthermore as I previously mentioned, your original post has spawned many thoughtful comments on the subject.
I see where you are coming from for sure!
I'm happy you can't edit--that good explanation you gave, about the moment of fury, is good stuff.
See, what got me down this whole thing is that I've noticed three big misconceptions in table saw safety assumptions:
1) having the blade super low, just barely above the stock, is safer.
2) kickback only shoots backwards at the operator.
3) that you can "feel" what the saw is doing and correct for it. (Physical fact--kickback is faster then human reflexes.)
What strikes me is that all three do not quite jive with natural laws. So it begs more exploration, in my mind.
Anyway, thanks very much for so kindly explaining what originally got you going :)
As far as blade height goes, I too was trained (term used very loosely) 30 years ago to keep the blade low as possible. I soon figured out that there were many times when cut quality was improved with the blade slightly higher. One of the balancing factors here is that the higher the blade is set, the farther you have to push the stock to get it safely past the blade. Inexperienced operators often miss this detail, greatly increasing possibility of kickback.
You have a Sawstop!! I totally bow to you. I don't think most people realise just how horrifying an amputated member can be--even if you are just a witness.
So the low blade thing-- I got my kickback in the gut after being told by an older guy that the safety authority abides by the low blade standard. Not so good with bowed stock. And turns out, that standard is a thing of the past.
From what I found blade height is a question of directional force. Low blade has more inline force, higher blade has more down stroke force. The initial worry was the rotational force in back tooth kickback draws the hand in--easily corrected with a riving knife.
This link describes the directional force quite well:
http://www.waterfront-woods.com/Articles/Tablesaw/tablesaw.htm
Thank you Mel, that is an excellent link, I would agree with almost everything written there. The point about the low blade cutting allows too many teeth to be in the stock is very important. Sawblade manufacturers recommend limiting the number of teeth in contact with the wood at any one time to prevent burning and premature dulling of blades. This is primarily a discussion of ripping with the grain on a tablesaw. I seem to remember that number being 6 to 10 teeth but I cannot find that reference just now.
That is exactly the height I like to use as well. Sadly the local safety authorities use to penalize folks who got injuries with that high a blade, so the old timers still abide by the super-low rule.
You ever heard of the kurf-keeper?
Recently added it to my home table saw--into my zero-clearance throat insert. It's low enough to do non-through cuts so you don;t have to remove it as often. And when you do, you just switch inserts.
Pretty happy with it but would love to see if there are any precautions I haven't thought of!
Related to my previous comments about number of teeth in the cut, I found this FREUD Technical pdf that helps explain. See page 9, Recommended Teeth per Material Thickness. There are exceptions to this, including density of certain materials, but that falls mostly into cutting plastic and non wood products.
4/10
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Member
Regarding your original post: Prior to watching your video, I posted a tongue and cheek reply that apparently missed it’s mark.
Rob
4/12
4/27
4/28
4/29
Like most here, I suspect, I have had an up close and personal relationship with kickback. Early in my career I caught a few pieces of plywood in the gut, a few stitches in a ring finger, a lesson learned. I’ve also witnessed the aftermath of a couple of co-workers hands mangled due to kickback (shaper), rooting around the sawdust for flesh and parts of fingers is not fun. My most recent experience, (I was not directly involved to make a long story short), was being called on to testify in court regarding an injury on a moulder. A poorly trained operator directed an employee feeding the machine to shove a couple of lumber yard stickers to push a bar rail out, beyond stupid. All got stuck, the guy feeding, walked in front of the infeed to get to his cup of coffee and doughnut behind it., stupid again. The smaller of the two stickers shot out and ended up in his leg, not before passing through his balls, sorry, but I don’t know a nice way to describe it. I did see the photos, not pretty.
My point here is this, like a snake in the grass, it is a serious and ever present threat., however, it is not rocket science. Just being honest here, but treating this issue like a Rube Goldberg mousetrap, in my opinion borders on silly., perhaps I am superstitious , but akin to tempting fate.
I regretted my unkind outburst but this site does not allow one to edit or delete posts, would be much more useful than co-opting face book’s thumb s up icon.
Your last post “Shall we proceed to the experimental phase? ;)”
Provoked my outburst, I hope I am mistaken here as well, but seemed to be a prelude to another foolish stunt .
Best of luck
Rob
4/29
4/30
The other thing that happens when the blade is low is it increases the tendancy of the stock to ride up on top of the blade, making an incomplete cut, and risking loss of control of the part if it suddenly cuts through again. This is especially noticeable if the stock is thin or flexible, like veneer, thin plywood, or a sheet of polycarbonate plastic. This can be exaggerated if the blade is dull, or has the wrong number of teeth for the job.
We now have a Sawstop 10" table saw, and I do think it is very well made and completely worth the money. The guard and splitter riving knife are well made and very easy to remove and install, literally takes seconds and no tools or readjustment required.
Please consider changing your saw blades before they are burning and you have to force wood through the machine. This is one of the easiest things you can do to improve your safety as well as the quality of your work.
4/30
Member
5/1
At this point in time I usually set my blade height to where the carbide tips of the saw teeth completely clear the wood. This way the sides of the teeth are not continuously rubbing the wood, the ride-up factor is reduced, and there is a little more safety than if the blade is sticking out an inch or more. And of course I do have the guards in place.
5/1
Member
5/19
Member
Maybe if some of you are talented you could just post the page with the chart here.
Click the link below to download the file included with this post.
Saw_Blade_Technical_Info_2.pdf
4 --
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