New (and old) Tablesaw owners should watch this.
Some of you who read this forum frequently may know I am not a big fan of YouTube videos as a means of learning woodworking I feel most take too many shortcuts and editing liberties to be informative for inexperienced woodworkers or try to be too cute and entertaining to attract viewers to be very useful, some demonstrate what I feel are downright dangerous practices and are the most troublesome of all. I do acknowledge that most hobbyists won’t have the luxury of learning the way I did at the side of my father and grandfather, the family business was more home improvement than furniture making, but my grandfather did make custom cabinets for our kitchen and bath remodeling and custom built ins, millwork, etc. and taught me much. That said as time has passed, technology improved, advances in tool designs and better education into the hazards of woodworking I learned my grandfather’s way wasn’t always the best. My grandfather’s shop had no dust collection, his table saw no blade guard or riving knife, which contributed to him missing parts of both thumbs and a finger, safety glasses and hearing protection was for sissies. Fortunately as time passed I learned better and in the spirit of the season I imagine that there are a few aspiring woodworkers who found new tablesaws under the tree this year and for them and others who have owned saws for years I encourage you to take 15 minutes and watch this video not to frighten you away from Woodworking but to help you understand that improperly practiced it can be hazardous to your health yet with minor precautions it can be an extremely rewarding hobby.
https://youtu.be/YObVR9Rj_38
Replies
So the lesson here is “don’t try to learn woodworking from YouTube videos. Instead find yourself a guy with missing fingers, no safety glasses, mask, earplugs, or dust collection to teach you”? :-). Of course there are bad examples of safety practices on YouTube, but there are also great YouTube videos. You said that YouTube isn’t a good way to learn woodworking, and bolstered your assertion by showing a good safety video……. on YouTube! :-)
It seems to me that you’ve slightly misinterpreted the original post. In that, the OP didn’t say “YouTube isn’t a good way to learn woodworking…”, but simply said that he’s “not a big fan” in general, due to the common presence of weaknesses in presentation, safety practices, etc. The OP’s mention of his grandfather’s injuries (incurred at a time during which safety measures certainly weren’t the priority they are now) was, it seems to me, a nod toward how even competent persons can sustain significant injury if adequate precautions aren’t taken.
Rather than focusing on the idea of safety in the shop, you’re taking this as an opportunity to criticize the OP for how he’s going about trying to help others think about health and safety—and not doing anything in aid of making for safer, healthier, better, more comfortable practices in the shop.
I will say I take offense at your mocking of my grandfather who was a Master Craftsman because his generation did not have some of the safety devices you take for granted today. I fully acknowledged that his practices weren't always the best or safest but his craftsmanship could probably run rings around you and your YouTube generation. Anyone who thinks they can learn all the nuances of woodworking by watching a YouTube video is kidding themselves and I challenge you to post "Great YouTube video" that I couldn't find significant flaws in. There is nothing like hands on learning beside a great Craftsman but I acknowledged that most people don't have that opportunity and yes YouTube has some value but inexperienced woodworkers have little chance of separating the chaff from the real value.
I wasn’t mocking your grandfather. I used your description of him to argue against the premise of your post. Although I have all my fingers, I too learned from old timers with no safety in mind. I can’t even count the times I had to catch boards through the tablesaw, pushed by an “experienced” craftsman freehanding the boards without using a fence. Not good! As for YouTube, it’s only as good as you make it. There are tons of sites and videos who promote safety and good technique. Our job as viewers is to watch them with a discerning eye. Yes, of course even the best ones will have flaws, the hosts are human. Lots of beginners are solo, without a grandfather or father to teach them, and even then, as you’ve described, they may be getting an unsafe teacher. I say watch YouTube videos, lots of them, but also come to forums like this one to ask all the rookie questions.
Took 1 woodworking class for hand tool work. Took a second class from the same German master craftsman. The second class was a power tool class. The first time I watched him use a tablesaw it was OBVIOUS why he was missing a thumb and a finger on his right hand. I dropped the class and requested a refund. I had been a self taught woodworker for about 5 years at the time. I had safer practice then him. Only teacher I had prior were books. Youtube did not exist at that time. I never were gloves, no long sleeves, no long hair, no loose dangly clothes all safety sake. You tube has good and bad. But there are actually great books if anyone actually reads any more. There is a hard and fast list of safety that I adhere to. I don't have a saw stop and my tablesaw doesn't have a riving knife. Yet I still have all 10 after 20 years. Good practices keep fingers attached.
I owned a cabinet shop for years along with being a contractor, 6 employees in each, and luckily nobody ever had an accident or injury, but there were a few close calls. Safety was always important, but I did have the occasional cowboy on the crew that I had to have a talk with. My shop had no dust collection for years, one of the saws was a 1940s Unisaw that had served decades in a boatbuilding shop in Maine before I owned it, no riving knife, and the original crappy Unisaw stock fence. The only safety I added was a new Paralok fence. Now I use a Sawstop with all the safety features and added a Jessem stock guide. My shapers have power feeders, I have more push sticks and pads than I know what to do with, my band saw has a brake, my dust collector was made even better with a new air cleaner, and my hearing protectors and safety glasses are on all the time. I think a lot more about safety nowadays for some reason, maybe because a lot of these options didn’t exist back in the 80s and 90s.
I worked in a cabinet shop in the mid-70s. One of the jobs we did was cutting rabbets for intalling glass by using a table saw and making two cuts. The waste would shoot out the back--yeah, not very safe.
I remember a 'cowboy' on the crew who would try to catch the waste strips coming off the saw, until one went through his hand. The foreman thought he was joking when he walked up to him holding this 1/4x1/4 strip of wood--until he opened his hand.
If I remember right, he spent the rest of the time working there operating the glue wheel.
(I also nearly lost a finger running an old 36-inch bandsaw with no guards. Injuries were almost inevitable back then.)
Great video - I really like JK-M's work. The force calculations are not correctly stated but that does not really reduce the value of the work. It's still going to hurt!
I have never experienced a kickback of this nature - the only time I have been hurt by a table saw was when my guard fell off the riving knife and got catapulted into my hip. Bad design and awful instructions by Felder - efforts to make the guard safe made it loosen with use, if not correctly located. Since then I have learned how to use the slider to remove almost all kickback risk - even if it does throw something I'm safely out of the way.
Interesting to hear about the missing bits. I think it was your grandfather's generation that learned the lessons we now use to keep ours.
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I agree with a lot of what has been said, both pro and con, in this issue. Back in '62 and fresh out of my three year enlistment in Uncle Sam's Army, I was in the usual "Catch 22" one finds one's self in, especially in southern California: "Need a car to get a job, need a job to get a car. So, I went to work in the machine shop my dad worked in, 40 miles away from Fullerton in Santa Monica, CA. And the Santa Monica freeway was only 2 miles long at that point. Anyway, there was another father and son working at the place [Hydro-Mill] and they both were missing a part of their left forearms, lost at different times on the same bandsaw in the shop. It was a lesson I took to heart. There was something I was taught learning to drive Class A big rigs after 20+ years of playing guitar for a living and that was, "A thousand times too slow, only once too fast". Same theory, if you don't have the time to slow down and do it the right way, the safe way, you may never have the time to do anything ever again.
esch5995, Thank you for posting the video. JK-M provided great service to the woodworking community by demonstrating kickback in very slow motion and explaining through physics the real danger. I've saved JK-M's YouTube video as a way to explain the dangers of circular sawblade kickback to other woodworkers.
esch5995, While reading your initial post it truly sounded exactly the manner in which I began experiencing woodworking. There is one exception, I only have some of my grandfather's tools. Reading another post reminded me of a co-worker expressing concerns about the lack or absence of safety on a specific job. I will never forget the Department Head charging up to my co-worker and saying, "what's the matter Ed, are you scared?" Well, Ed was belittled and the unsafe condition was not corrected. Probably the only injury was long term lung damage to Ed. The safety features of today are often the result of documented accidents and injuries.
During all of those years of using a 1 hp with a single v belt drive tablesaw, a kickback was of no consequence to me. Luckily I was not in harms way when that new 3 hp with 3 drive belts showed what a REAL kickback could do.
As to the videos, yes there are good ones and bad ones. Having been an all-around woodwork for about 60 years, I decided it was time to learn woodturning. In the last 3 months I have watched numerous videos on Youtube and other sites. Many of them were helpful, but almost ALL had been edited in some fashion. This statement I have learned to be true based on personal experience.
My father died at age 79 and he had all of his 10. However, many of his contemporaries did not. Back in those days accidents and exposures were a way of life.
Good tutorial but poor physics. To calculate the impact force the f=ma equation in this case cannot be used as there is no acceleration. When the piece impacts the body it’s been decelerating over the distance ever so slightly. The formula that applies is kinetic energy first, the product of speed square and half the mass. That energy gets partly transferred to the body depending on the angle of impact and the depth at the point of impact. We know the blade speed, potentially the wood can be sent at 50m/s but the slow speed camera has measured a time of 0.035 seconds to travel 0.6 meters so the measured speed is 17 meters/second. Entering the speed and weight in the energy formula E = 1/2mv2 gives 95 joules or 95 newton meters. Say the impact is square and the part drops back in front of the body to calculate the impact force in newtons one needs to divide by the distance travelled after impact, I have to assume something here so say it travels one inch in the chest after it hits the apron, 95 Newtons / 0.025 meters equals 3800 newtons, removing the earths acceleration to convert this in kg means that the resulting equivalent mass that hits the body is 388 kg or 850 pounds.
Correct me if I’m wrong !
I make 17m/s and 0.66Kg 190.74J Kinetic energy.
The calculation of impact effect is very difficult, but is more related to the rate of change of momentum than it is to actual kinetic energy - a glancing blow will do less damage, a sharp corner hitting first will do more.
The energy imparted can be calculated from the change in momentum of both pieces - worst case scenario is the whole energy goes into the body all at once
As a rough comparison, that is more or less the muzzle energy of a .22 rifle. The rifle will do way more damage though as the pressure at impact will be much higher, penetration certain and all the energy will enter the body.
A comparison to Kg is really not helpful as how things feel and the harm they do is related to energy delivery. The video incorrectly states it was about half of JK-M's mass falling from a 10 storey building. The actual effect is of a block of wood equal to 0.66Kg falling from the same height. Painful, but certainly not likely to be fatal.
Perhaps a more relevant comparison might be a punch. The following are from a fairly unreliable source, but better is hard to find. https://r4dn.com/how-many-joules-is-a-punch/
Mike Tyson Est 1600J
Rocky Marciano 1256J
Average Professional boxer: 400J
Average amateur boxer: 40-200J
So something between a 'bitch-slap' and a significant punch. Enough to do some damage and leave a nasty bruise. Fatal - not so much.
My one and only kickback injury came from the guard falling off my TS into the blade - probably 300g in total - it left a minor but painful bruise on my hip.
The association I made between mass (kg) and force (newtons) was, like the example of the baseball, to get a grasp of the effect of receiving a piece of wood at that speed. I failed to explain however the analogy, so there it is. The piece of wood sent at high speed would impart the same impact damage as if there were an equivalent mass pushing downwards on the block of wood on the chest. So if the operator would lay horizontally and the piece of wood placed on his chest at the angle of impact, placing 388 kg on top would yield the same force. As mentioned, acceleration is not only change in speed, it’s also change in direction, which provides centrifugal force and it’s also present in our everyday life, in the form of gravity at 9,8 m/s2.
A quick primer might be in order regarding the proper means of calculating the force applied to an object, and how best to describe/denote force. For simplicity’s sake, I’ll stick with SI units and forgo Imperial units.
For classical mechanics in the “real world”—by which I mean the world as we most directly perceive it, on a relatively “macro” scale, and ignoring quantum mechanics more or less entirely—the force applied to an object is found by utilizing Newton’s Second Law, which is best known in shorthand by the simple formula, F = ma, where F = Force, m = mass, and a = acceleration.
It’s important to recollect that acceleration in classical physics is defined as [change in velocity]/time, or ∂V/t. It doesn’t matter whether the change in velocity is positive or negative, it’s all acceleration.
Because the nature of the event that occurs when a block of wood traveling at high speed strikes, at some angle, an irregularly shaped body comprised of heterogeneous materials is so complex, detailed calculations become more than most of us are willing to embrace. Therefore, simplification so as to allow for a general understanding of things is in order.
Let’s assume first that the block of wood strikes the woodworker’s torso at a perfect 90-degree angle—thus, we don’t have to worry about changes in velocity in anything other than two directions—forwards, and backwards.
Let’s also assume that the block, on striking Danny the woodworker, undergoes, with Danny’s torso, a purely elastic collision—that is, all the kinetic energy from object A—the wooden block—is transferred directly to object B—Danny’s torso. After striking, the block simply falls straight to the floor.
Both of these are significant simplifications, but will suffice to help us understand the process by which we will calculate the force applied to an unmoving object when struck by another, moving object.
In our perfectly elastic collision, Danny moves backwards as a result of the force applied by the wooden block. Let’s assume a mass for Danny of 8o kg. The force applied to Danny will equal his mass X acceleration of his mass.
The acceleration of Danny’s mass begins at time of impact, T(zero). The velocity of his mass at that moment is zero. Let’s make a reasonable guess as to the duration of the impact, that is, the time during which force is being applied to Danny’s torso by the wooden block (and by Danny’s torso on the same wooden block). Let’s say the impact duration is ten milliseconds (ten one-thousandths of a second), or o.o1 seconds.
Now we have to determine the velocity of Danny’s torso at the conclusion of the impact. So we go back to video, and…wait. No, we don’t.
Newton’s Third Law declares that for each and every action, there is an equal and opposite reaction. In a collision between two bodies, the force applied by one is equal in magnitude to the force applied by the other. Thus, if we stick to our simple construct, all we need is the mass of the block (let’s make it easy and say 600 grams, or 0.6 kg), and the change in velocity (acceleration) of the block.
I’m not going to go back and inspect the video and do frame counts and the like, I’ll just go with what some others have found, and say that the velocity of the block at time of impact is 17 meters/sec. Acceleration is [change in velocity]/time. Ten milliseconds (0.01 sec) after initial impact, the block has come to a full stop, and velocity is zero. The change in velocity, ignoring direction, is 17 m/s.
Let’s plug and play:
F = ma = 0.6 kg [(17 m/s)/0.01 s] = 0.6 kg (1700 m/s/s) = 1020 kg•m/s^2= 1020 N
(For those needing a reminder, the Newton (symbol N) is the unit of force in the SI. A Newton is the force required to accelerate a 1-kg object at 1 m/s^2.)
That’s the force that was applied to the wooden block; the equal and opposite force is that same 1020 N that acted to accelerate Danny’s mass during the impact.
To say that this force is equal to some specific mass hitting the body is somewhat misguided. All one need do is contemplate Newton’s Second Law, and the equation therein: F = ma. A force is never equal to a mass. It is always the product of a mass and its acceleration.
For anyone confused on this last, just consider the different outcomes of two pedestrians, each hit by a moving truck. In the first instance, the truck is moving at only 2 mph, and on impact, the driver slams on the brakes and comes to a more-or-less immediate stop. The pedestrian is knocked over, and suffers torn pants, a fractured collarbone from an awkward landing, and a significant fright. In the second, the truck is traveling at 50 mph. On impact, the driver slams on the brakes, but the truck continues moving for another 240 feet. The pedestrian is killed by the impact, and his body flung off the road and into a tree. In both cases, the mass of the truck, let’s say 15,000 pounds, is the same. Again, force isn’t mass. It’s the product of mass and acceleration.
I taught of using f=ma but I was missing the decelation time, I was missing also the deceleration distance but used the one I assumed easier to estimate. With the frame by frame camera one could find both the time and distance travelled after impact and plug it in either formula. Our results differ since a 0,01 seconds deceleration time equates to a 3 1/2 inches travel after impact, I assumed one inch.
All of those were, and are, reasonable assumptions, estimates, “guesstimates”, or whatever. These type of back-of-envelope calculations, best done in a café while waiting for an after-luncheon espresso, are never intended to withstand deep scrutiny. They are just to get a reasonable idea of the sorts of forces/energy involved.
As an aside, I could have added that the force estimate I calculated is basically the same as the force of the hardest punch generated by the average untrained person. Joe the Plumber can probably generate 800-1000 N with his best punch. Even an amateur welterweight boxer can easily produce more than twice that force; it’s estimated that high-level professional heavyweight boxers can generate punches with forces in excess of 5000 N. Ugh.
Perhaps a better comparison is a Major League Baseball fastball. In the show, the flame-throwers routinely hurl pitches toward the plate at 100+ mph. [100 mph ≈ 44.7 m/s] The mass of a Major League Baseball is closely regulated; game balls must have a mass of 142-149 g (0.142-0.149 kg). A batter that is kerplunked by a 100 mph fastball then experiences a force (using same assumption about the duration of impact (and thus the time of acceleration of the ball as it hits the batter in the side) we made about the block of wood in the above example, that being 0.01 seconds, we can grab that F = ma formula and find that the force = 0.145 kg • 44.7 m/s ÷ o.o1 s = 681.5 N. (Assuming direct impact, perfect elasticity, etc. The actual force felt by the batter will of course be somewhat less, for a whole host of factors.)
THanks - very helpful :-)
Somehow this thread got hijacked by the Physics Department and to think after over 40 years of making sawdust I never knew it require a degree in Physics.
I will break it down to the least common denominator: if you get hit by a kickback like the one shown it's going to hurt like h@#& Physics be dam@#&.
You posted a link to a YouTube video featuring high-speed cameras and a discussion featuring a mathematics professor that emphasizes the physics of a particular safety concern. I’d think you’d have at least half-expected some comments about same. I don’t think anyone here said a physics degree is necessary to work wood. (And I don’t have one; this isn’t high-level stuff.) But…physics certainly has much to say about the processes of woodworking and the things woodworkers make.
As for the “hurt like h@#&”, I thought the comparison of forces in a boxer’s punch and an MLB fastball striking a batter with that in a more-or-less-typical tablesaw kickback event were illuminating. Enough so that if I ever experience a kickback event, I might be tempted to charge the mound. ;-)
for the woodworking mastery needs experience and hard work, In simple words everyone shares their own methods using different platforms. Mostly I prefer youtube to find new tips and community helps to connect with experienced people.
While the video content is/may be somewhat "useful"to some - it is what I would consider simple "clickbait". Oohs and Aahs meant to grab the viewers attention and potentially sell themselves, "their brand" (see KM tool logos all over everything in the shots), and the commercial products that are a part their stable. Funny my first thought was why are you inadvertently "promoting" Phantom High Speed cameras?!
Good point. He even printed his logo on the jig he made for the demonstration. :-)
All of the usual suspects use click-bait tactics in one form or another to their own benefit, which is only fair.
I just don't tend to like it or fall for it.
I'm all for safety but in my experience videos like to one linked to don't tend to stick for very long after the initial "wow".
The safety aspect has already been superseded by bad physics and subliminal marketing.
Work safe
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