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You Can't Beat the Physics of Kickback

comments (36) December 11th, 2013 in blogs

Ed_Pirnik Ed Pirnik, Senior Web Producer
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Its not the kickback that surprised me but rather, the tablesaw users inability to pull away from the blade despite the fact that he knew what was coming! - CLICK TO ENLARGE

It's not the kickback that surprised me but rather, the tablesaw user's inability to pull away from the blade despite the fact that he knew what was coming!

Photo: NewWoodworker.com

A quick disclaimer: We've all heard about kickback ad-nauseam. Ditto tablesaw lawsuits, why you should use a riving knife, etc. And while the video posted below came out nearly two years ago, I recently re-watched it, and was floored by one very specific aspect.

More on Kickback


Tablesaw Kickback

See Kickback in Action


 

 

The video, produced by NewWoodworker.com, prompted a flurry of comments when it appeared on YouTube in February of 2012. It presents a woodworker intentionally creating kickback on his tablesaw in order to demonstrate just how dangerous the phenomenon is. We see a woodworker stand well to the side of the path of travel, using a push pad to slightly cant a piece of wood as it passes between the rip fence and blade, thus creating a pinch at the back of the blade and thus causing-you guessed it-kickback.

 

Here's the Kicker

What floored me wasn't the fact that the kickback occurred, or how violent it was but rather, the fact that despite the fact that this fellow knew the kickback was coming, and was prepared to pull his hand away at that fateful moment, the momentum of the blade was simply too much to fight against. In his slow motion replay at minute 4:20 we clearly see that despite his efforts to pull his hand away from the blade, it still gets sucked right towards it. By my estimate, his left hand was pulled in to within 1/2-in. of the spinning blade-again, this despite the fact that he was intentionally pulling his hand AWAY from the danger zone.

 

Just goes to show, you can't beat physics, no matter how hard you try!

 


posted in: blogs, Tablesaw, safety, kickback, demonstration


Comments (36)

coloradotrout coloradotrout writes: Questions (for a saw w/o riving knife)

1) Noted here, someone suggested running the blade at full height, or at least well above the wood. The argument seems logical. Get most of the down force at the leading edge of the cut and position the back edge of the blade as close to the splitter as possible. But having 2" of blade exposed is a bit frightening. What's the consensus on this?

2) Another noted that on a crosscut sled kickback cannot happen. I have some trouble with that. It's less likely, but still seems like on some parts -- smaller ones for example, the parts could rotate into the back edge of the blade. Setting the blade high might help with that so as the back edge never comes in contact with the cut piece. It may be less likely, but it seems to me the dynamics are there such that a smaller part could rotate just enough to catch the back edge of the spinning blade.
Posted: 10:27 am on March 18th

BAR BAR writes: I'm throwing these comments out there from experience.
Bad things are in all of our shops, just waiting to happen, you can't get away from that.
Before you entering the shop you should know what you will do to save your self from bleeding to death!!
Is someone near enough to rely on?
Do you keep a cell phone in your pocket while working? (911 call )!
Can you secure things before leaving the shop so no one else enters to get in trouble? ( Children )!

working alone and no one home but me, I cut 1/2 way through 3 fingers from a kickback ( from doing something stupid ) Had to drive myself 15 miles to the hospital for treatment, Got lucky, all digits were saved, with a built in reminder, when I bump them I get this little sting that says "watch what you're doing dummy" Not very pleasant!!
Posted: 10:46 am on January 8th

dave 07 dave 07 writes: I use a radial arm saw in my shop and altho' it fires wood pieces it is not at me. I would really appreciate some safety tips on using it.
Posted: 8:25 am on December 21st

mesavistadesign mesavistadesign writes: With a magnetic stock feeder that sets up in under 5 seconds available for under $100, I don't know why anyone rips wood holding and pushing it with their fingers.

Probably because thats the way we learned it from our 3 fingered shop teachers and tv gurus.

And the woodworking tradition of amputated fingers and kickbacks continue...

Jerry Jaksha


Posted: 6:49 pm on December 17th

lyholmer lyholmer writes: I have always unscrewed splitter knives and thrown them away, but after having watched this scary movie I realize that I shouldn't have done that. I have got this really sharp Freud saw blade which is just waiting for me like a mean and evil animal...I will get a splitter knife asap! and from now on I will ALWAYS use long push sticks - really long push sticks. It is really scary to see how close his hand gets to the blade although he thinks that the push block will protect him.
Posted: 6:36 am on December 17th

Xanthane Xanthane writes: This is an excellent video. Sure it was stupid to do (which he is the first to admit) but his putting this stupid move into the public domain has done the community a great service. It shows one way that kick-back happens and should be watched by anyone with a power tool.

I think it interesting that he has subsequently put out a video showing him unpacking a new Sawstop saw. Also Finewoodworking videos seem to feature sawstop including the recent videos from Marc Adams on tablesaw techniques. In Australia there is a big premium on it, but I'm selling my cabinet saw to get one anyway. The older I get the more attached I become to my fingers, as it were.
Posted: 9:12 pm on December 16th

Daryl Daryl writes: Aside from his not observing the 3 inch rule with any kind of push stick (we already know a paddle isn't that), how about one thing he never mentioned: NEVER put your body on that side of the blade.


Posted: 6:07 pm on December 16th

sandylns sandylns writes: Sir Isaac Newton first published his 3 laws of motion in 1687. Nothing has changed since. Murphy's law was first spoken about in 1877, (under a different name). Again, nothing has changed.
Woodworking is inherently dangerous and trying to work around safety will bring you in contact with the aforementioned laws. Use every device you can to make your work environment safe. Machinery needs a working knowledge of both its limits and capabilities. That includes a thorough understanding of using machinery safely. Riving knives, blade guards, splitters etc., all have a place. But nothing beats common sense and leaving safety devices in place.
"I can't work with a guard in place" doesn't cut it. A poor excuse at best and a fools motto.
Posted: 3:27 pm on December 16th

Jigs-n-fixtures Jigs-n-fixtures writes: seamusday: Biesemeyer used to catalog a splitter for your saw. Not sure they still do after being acquired by Delta. But check on eBay, they sometimes show up there.

CaptainSkinnyBeard: The RAS isn't dangerous when ripping if set up correctly, But 99.99% of users set them up wrong, because almost all manuals tell them to do it wrong. Two things need to change:

First, you have to realize that the table on a RAS is sacrificial. Once you realize that you bury the blade in the table as deep as you can, and still pass the stock under the motor. There is very little potential for kickback, because the part of the blade that can generate kick back can't get to the wood.

Second, the feed direction should be so that the blade is pushing the wood down onto the table on the leading edge, just like on a crosscut. All the manuals show it the other way. Which can and frequently does cause kickback.

Onto the table saw: Raise the blade as high as it will go when ripping. I'm not sure where the notion that adjusting the blade height to just clear the wood became prevalent. My guess is someone who doesn’t understand physics, wrote it in a book before WW-I, and it became gospel with no real thought to the physics. With the blade as high as you can get it, the force at the front where the wood is feeding into the blade is mostly down into the table, there is very little pushing back against the cut. At the back, the wood could theoretically be raised by the blade and get to the top where it could kickback. But, since the kerf is already cut there isn't much working to generate the force required. Also, with a fixed splitter, (not a riving knife), the distance to the splitter from the back of the blade is minimized. Thus it is harder for the wood to hit the back of the blade and get lifted if it does pinch or rotate. On my Delta, with a Biesemeyer splitter installed, the distance from the back of the blade to the splitter at full blade height is about 5/16-inch. With the blade at 1-inch height, there is over an inch between the splitter and the back of the blade.

The only time I've come close to kickback on a table saw was ripping a 2X10 that had been bought "green", and stored in my unconditioned shop in Las Vegas, where the humidity was in the single digits, and the temperatures in the 140-degree range. The wood had rapidly dropped from above 30% moisture to less than 8%, and become case hardened and reactionary. It tried to close down on the blade, and the splitter stopped it. But the surface of the wood on both sides of the kerf had burn marks from where it was barely touching the blade.

Posted: 3:22 pm on December 16th

evandene evandene writes: Super Video and so helpful for getting rid of all the ignorance out there. Thank you so much, this really helps a lot. Stay healthy,
Best Regards,
Ed
Posted: 7:58 am on December 16th

el_ocsuro el_ocsuro writes: That video was scary, but informative. The only kickback I have experienced was on the router in high school woodshop, doing exactly what he did in the video. That little 1/4" router threw bit the piece out of the classroom and 50 feet into the playground.

I have always wondered how a kickback can draw your hand into the blade, as it throwing the wood away from it. This video shows how. When the kickback occurs at the end of the cut, your hand is in front of the blade. So when it happens, it draws it back through the blade.

15 years ago, I spent a minute making a push stick out of a 1 foot length of 2x4. I just used a bandsaw to rip it about 1/4" from the edge, then stopped an inch from the back to make a shoe. It was supposed to be sacrificial, but has outlasted almost all of the tools in that shop. I just used it today, but will probably retire it, at least from the tablesaw. I'll make a longer version (about 2 feet) with a 6 inch shoe. That way, when the cut is complete, my hand is still behind the blade.


Posted: 10:00 pm on December 15th

user-2656866 user-2656866 writes: catain, as radial arm saws are pretty much gone from the market, new ones, the safest crosscut is using sleds. Can't kick back. Even using miter gauges to crosscut, the falloff nearest the fence has to be able to clear the fence on its longest diagonal when falling off. I used to use a 1" piece to clear a crosscut away from the fence. But cutting a 6"wide piece, with a 1" offset from the fence, can still turn on a diagonal and pinch against the fence and blade.

I am currently workin on a 1 sided sled, that has a fixed piece the same height on the opposite side of the blade, so I can crosscut with a blade guard still in place. No back piece necessary to hold it together.

That was scary. I don't hold those type of push blocks with my fingers through them and I never use them on a table saw.
Posted: 8:59 pm on December 15th

David1233 David1233 writes: I was taught to never let your hand move beyond the front edge of the blade. I routinely use a long push stick or two when working near the blade. Fancy pants push sticks like he used are ok for a jointer but get hands too close to the blade.
Posted: 8:50 pm on December 15th

Andersrl Andersrl writes: Videos like Marc Adams' "tablesaw techniques" are great instruction for avoiding kickback. With a tall pushstick, my hand never gets within 6" of the blade. But even so, I feel the need for some insurance. Even 1 in a 100 million rare events do happen. So,I use a Sawstop tablesaw.
Posted: 11:40 pm on December 14th

craig39 craig39 writes: Although it seems like a lack of judgement with the method to show the results of a kickback. Probably with a group of woodworkers together the method would change, but as most woodworkers work alone and make spur of the moment decisions and not necessarily the right ones, he show what happens. Think about the last kickback you had. I did, and it was stupidity on my part. Now, I build a possible scenario of what could go wrong before the switch goes on, and prepare accordingly.
Posted: 8:58 pm on December 14th

rcc3 rcc3 writes: Why would anyone in their right mind try to recreate a kick back or use that type of push block with you hand between the blade and fence? I agree with several others that this exercise was just plain stupid! I've had one kick back and never want to experience another. I grew up in my father's shop and the last power tool he let me use was a table saw and then only if he could supervise until I was 16.
Posted: 7:56 pm on December 14th

usafchief usafchief writes: OK, don't any of you guys examine your stock before you begin to Rip????? Look at the end grain, if the board is cupped, it will have a tendency to pinch... Make the cut in 2 passes, lowering the blade to about half of the boards thickness and turning the board end for end. My saw is a 1970 Craftsman, and that MM splitter, anti kickback fingers and wobbly guard is still in the plastic bag it came in- hanging on my shop wall. I use my saw for more operations to where that thing has to be removed, that it is not practical to use. So the board pinches on the riving knife, requiring more effort on your part to push the stock through. I have found using a thin wooden wedge also keeps the cut open and requires no extra effort to rip. But, if it is a board loaded with stress, it may split before it finishes the last few inches of the cut...... I have never had a kick back that a riving knife would have prevented in the first place- how can it without those fingers to dig into the wood????? safety devices will generally lead to complacency.

There isn't and there never has been an accident in a wood working shop- before or since the invention of all the safety devices. Some one has always done something stupid- I lost 3/8 inch of my left thumb to my jointer by not using a push block in both hands.

I have hold downs and hold in fingers that I made 40 plus years ago that make my shop as safe as that HOT Dog eating sawstop
Posted: 12:07 pm on December 14th

gbguy71 gbguy71 writes: Another after market safety device (with excellent dust collection as a bonus) is the Shark Guard. Here is an independent review of one http://lumberjocks.com/reviews/2268

FWIW I'm in the camp of using safety devices. They call injuries "accidents" for a reason.
Posted: 11:43 am on December 14th

AndyRutten AndyRutten writes: Lowering the blade and turning the wood at the very back of the blade are what enabled this kickback to be so aggressive. Try turning a similar piece of wood while centered on a blade adjusted at full height. It's rather stable. Even turning wood at the beginning of the cut won't do much.

Kickback doesn't just happen. It takes a very specific and predictable scenario for it to occur - namely rocking or twisting of wood between the back of the blade and a rigid support like a fence. A riving knife can help prevent kerf pinching and turning. But it won't stop slender strips of wood from being slender spears.

The times where I've lost control of a piece of wood typically involved me being in a hurry and doing a risky cut table saws aren't meant to do.
Posted: 11:04 am on December 14th

AllanH2O AllanH2O writes: I have my father's ~1955 Craftsman table saw that has never had a riving knife or guard on it. Have worked around the safety issues by never letting hands go past blade, some rather elaborate push blocks I've made. Were a couple of missile attack over the past 30 years; one left a significant bruise on my belly. While I don't believe I ever put my hands at risk, I certainly admit the missile problem although my push block extends forward and allows downward pressure that helps control things.

All that said, and having experienced some finger trimming with a biscuit jointer, I'm giving up Dad's saw (and that's somewhat of an emotional bond) for a new SawStop. As I've aged I've learned to appreciate safety more. The video certainly demonstrates what can happen.
Posted: 10:47 am on December 14th

popcorn3774 popcorn3774 writes: While I appreciate the demonstration, I do not nor never will use a push block as shown with a table saw. Always use a over the top push block that keeps my fingers away from the blade. It also holds down the wood.
Posted: 9:48 am on December 14th

slaid slaid writes: But you can beat kickback. It does cost a bit, $700to $800, but it absolutely eliminates kickback. A stockfeeder. I don't know why the woodworking publications won't discuss this option. They eliminate kick back, you can safely rip very narrow stock, they do a better job of ripping than can be done by handfeeding, they add the option of using them on shapers and router tables where you can then climb cut, which virtually eliminates tearout. And did I mention that they eliminate kickback? I mean, the operators hands are virtually removed from being in danger of coming any where near the blade. Also, one person can rip stock that would require a helper on the outfeed side by simply walking around the saw as the stock feeds through the saw. I love my feeders.
Posted: 9:26 am on December 14th

vernomas vernomas writes: At least part of this video appears to be a lie. The claim that the push block was damaged in this demonstration is clearly not true. At the 3:19 mark, during the slo-mo close up you can see the block is already damaged before this experiment. It's bad enough, there is no need to embellish, it detracts from your greater point.
Posted: 9:25 am on December 14th

Mikey Mikey writes: Not mentioned here, but while the SawStop won't prevent kickback, it will save your fingers, but by then the workpiece has become a missile...
Posted: 8:52 am on December 14th

dadnukum dadnukum writes: It's biology in addition to physics. Your mind sees the kickback, but can't send a signal to the muscles in your arm fast enough to pull it away. That's part of the message here, no matter how experienced you are, you're not capable of beating the speed of the kickback.
Posted: 8:22 am on December 14th

abcabc abcabc writes: A aftermarket ripping knife (most of function of riving knife) is available for some saws at TheBorkStore. He has a few reviews referenced.

Myself, I'm paranoid about using every available guard on all the tools. Anyone that doesn't use them is nothing short of stupid (unless the particular cut will not allow their use). Just a moment lapse in moving a hand could cause a serious accident.

I object to several of the TV shows that run without the guards with the silly excuse that the cut can't be seen.
Posted: 8:06 am on December 14th

Grantman Grantman writes: I made the mistake of not buying a saw with a riving knife years ago and while I have a Micro Jig splitter on a zero clearance insert, I still wish I had one. Sure, in years of use, I've never had a kickback happen, but I still want a knife. It's just something that's always in the back of my mind when I'm at the saw.

My absolute respect for the tool is there all the time. I have used it when I shouldn't and have walked away when I realized how stupid I was being. I don't think mandating Saw Stop's technology is the right path for the industry but I do think that knowledge of what can go wrong and how to prevent it is paramount to the woodworker. It still all comes down to personal responsibility.
Posted: 7:38 am on December 14th

Hendrik Hendrik writes: I also filmed a kickback for a table saw safety DVD I produced in 2008, but I did it a little more safely using rigid foam and with my hands still far from the blade as the kickback happened. In the last half of this video clip:

http://passionforwood.com/woodworking/dvdpreview2.htm

you'll see the kickback replayed in slow motion. The rather dramatic sound effects were not added. It is the original audio from the video, but has that more dramatic sound when played back at slow speed.

Dangerous stuff, which is why you won't see me ripping stock without a splitter or riving knife whenever possible.
Posted: 7:33 am on December 14th

knut knut writes: Those who can't fit an aftermarket riving knife to their saws can at least insert a wooden one into their throat plate behind the saw blade. I have seen many of these used by experienced craftsmen on Fine Woodworking videos.

I am lucky to have a factory installed riving knife which moves up and down with my saw blade.

It should be mentioned her that a table saw operator should never stand directly behind the blade in line of a possible kickback.
Posted: 7:26 am on December 14th

CaptainSkinnyBeard CaptainSkinnyBeard writes: The physics of kickback can't happen when cross cutting with a radial arm saw. It is the safest crosscutting tool. That being said the radial arm saw is as dangerous or more so than a table saw for ripping. I had several kickback incidents ripping on a radial arm saw before I got a bandsaw which is the safest ripping tool in my opinion. Of course I am not advocating running out and buying a radial arm saw. I bought mine 30 years ago when they were THE main shop tool to buy. Maybe a good chop saw with extension tables would be a good, safe and accurate cross cut machine.

The radial arm saw was the "do everything" tool 30 years ago. Because of safety concerns the table saw became the "do everything" tool by 1990 or so.

The more versatile a tool is, the less likely it is to do any one thing well.

Posted: 6:26 am on December 14th

Khola Khola writes: As far as an aftermarket splitter goes, I purchased one from delta some years ago for my Unisaw. There are also aftermarket devices for modifying a zero clearance throat plate with a plastic, fin like, insert. I understand that you cannot retrofit an existing saw to accept a riving knife.

In the video demonstration, if it had gone seriously wrong, I wonder if a Sawstop would have prevented serious injury, or if the damage would have happened too quickly.
Posted: 11:14 am on December 13th

FIMWDWKR FIMWDWKR writes: A riving knife is great to have, and a guard is tool. Yet all my saws are vintage and don't have a guards, or riving knifes. So has there ever been a kick back-NOPE. I think it's about being educated on how to use the saw, and understand when not to use the saw i.e. at the end of a day, tired, or sick. At ton kick backs occur to beginners, and when woodworkers are not paying attention, sick, or tired.

The video is unbelievable, and I am happy the individual is a'okay, but I could of told him it would be stupid idea, and that he may get hurt. I am glad he didn't. Also who uses metal inserts on the saw? Metal inserts are there to make templates from only, after that they are trash, and unsafe due to how big the gap is on either side of the saw blade.
Posted: 8:59 am on December 12th

seamusday seamusday writes: Very enlightening.. Does anyone know if any firm makes an after market riving knife? I have an older 12" JET cabinet saw which doesn't have one..
Posted: 8:16 am on December 12th

BStev BStev writes: I missed the video when it came out before but that looks like one of the stupidest moves I've ever seen. If the goal was to show what happens with kickback that guy is lucky he didn't end up showing what happens when your fingers get cut off as well. Stupid, just plain stupid.
Posted: 4:56 pm on December 11th

PurdueDan PurdueDan writes: I have met three men with amputed fingers in the last year and a half at physical therapy. All were the result of kickbacks and none had splitters or riving knives.
Posted: 3:45 pm on December 11th

mds4752 mds4752 writes: My tablesaw and I have come to an understanding and a level of mutual respect. In other words, I have surrendered all demands from it and am in total compliance to it!

In all seriousness, I actually have a contractors table saw and the first couple times I experienced kickback from that, it really rattled me. Learned my lesson without losing any digits thankfully.
Posted: 1:57 pm on December 11th

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