# why so up tight!?



## cathyb (Jul 8, 2009)

I ran across a piece of tension wood yesterday, actually I didn't know it was tension wood until it literally exploded on my table saw! As you might well imagine, I did get slashed across my thumb in about a nanosecond. About fifteen years ago a piece of tension wood exploded on a smaller saw and the fragments hit like a cluster bomb. In an effort to avoid repeating that disaster I bought my big 5 hp saw and paid more attention to the first cut of a large board. Since the saw has lots of power and I keep my blades sharp, any time I found a piece of wood that struggled through the first cut, I used that wood for laminate strip and cut them on my bandsaw. For the most part, that strategy has worked well.

This year I wanted to use up my ever growing inventory of off-cuts. The piece of mahogany that I was cutting yesterday last touched my hands about 17 years ago ( before my tension wood plan came into play). Honestly it was small and I didn't realize that I was in any danger. When that thing exploded, it did so with a loud BANG. I never saw what happened, but I think the piece that slashed me caught the blade and then came out with incredible velocity.

The take home message from this experience is to identify any tension wood in the shop and label it. You know there is tension in wood when it fights to get through the saw blade (provided that you have a sharp blade) or if you plane it and it twists or warps almost immediately- or sometimes within hours. I hope my mishap helps you avoid a trip to the emergency room. But what the heck, it's all in a days work…..........


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## Dez (Mar 28, 2007)

You never know when the universe is going to reach out and slap you up alongside the head! 
That is why it pays to *PAY* attention! 
Unfortunately you don't always no what to pay attention too!
Excellent advice!


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## spunwood (Aug 20, 2010)

Thanks for the post. Helpful reminder /warning.


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## GaryK (Jun 25, 2007)

My trick to cutting tension wood is to cut it until I start to see it move. Then back it out all the way and start again. That should remove more material to get rid of any binding. As long as the piece it's too bad I keep repeating this until I'm through it.

I also use a splitter built into my ZCI to keep it from pinching the blade.


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## cathyb (Jul 8, 2009)

Gary, I couldn't agree with you more. My riving knife on the saw is my best friend. In this case, it didn't come into play because I was cutting a dado. Like I said, if I had known that there was tension in that wood- cutting a dado would have been, and certainly was, living on the "wild side". Oh well….......


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## ScottN (Jan 24, 2011)

last winter I was doing a dado on a 7hp table saw when I heard the saw starting to bog down and decided to just let go and let the board go flying,But before I could let go… the board was ripped out of my hands and cutting 2 fingers and 1 thumb.


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## Sawkerf (Dec 31, 2009)

I'm not sure that there's any way to identify tension wood just by looking at it. I had a piece of pine explode several years ago and it sure spooked me. No injuries, and it hasn't happened again, but…..... - lol

I have had long boards open up 2" - 3" as I ripped them, but not explode.


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## LeeBarker (Aug 6, 2010)

This discussion has really got me to thinking. I can imagine, clear headed and deliberate, short circuiting the cutting of a board if I had clues it was about to become dangerous.

However, I can also imagine that late in the day, on a project with no comfortable cushion of extra material behind me and a looming deadline, not making the wise choice. I hope this story helps me adjust the latter.

My first experience with tension wood was ripping a long piece of 4/4 western cedar decades ago. In the course of that 10' cut, the two pieces spread way apart and then came back toward each other and crossed!

Kindly,

Lee


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## cathyb (Jul 8, 2009)

Wow!!!!!! That was have been something to see. You just can't respect wood too much. It's not a piece of plastic and can be temperamental…...


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## driftwoodhunter (Jan 17, 2011)

Wow, this is a great place to learn. I've never even heard of tension wood, I didn't know it could do that! There is so much to learn, it is mind-boggling. When you're like me, and don't know any wood workers, it's hard to learn. There is a man a few towns away that teaches a week long course for $400.00, but that's hard to come up with…if I ever do take the class, he may come to hate all my questions, but I'll be armed with a bunch.


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## cathyb (Jul 8, 2009)

Vasko this is a wonderful journey. I had no instructors except for books and hard knocks. I am not afraid to fail and know that every project teaches me something new. Keep working at it and one day you will be amazed at how much the process teaches you. Best of luck…....


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## blockhead (May 5, 2009)

I'm still relatively new to ww. I have not yet come across tension wood. What are the main causes? Knots, figuring, how it is milled, combination? Are there any species more prone to it than others? A couple of people mentioned pine and cedar, so I'm guessing it is seen more in a more knotty type species. Any info would be appreciated.


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## ken_c (Sep 28, 2009)

@ blockhead: The tension comes from within - the cell structure of the wood - there is no way that I have ever heard of to tell if the board will cause trouble. As stated above, all you can do is start to mill and watch/feel closely, then react. Treat each rip - in your head at least - as if it is going to move so you are prepared. I rough cut all my boards oversize before I finial mill - that is length, width and thickness. that way when they move I can still bring them into the right size - but that is a different story then the tension discussion here…


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## JJohnston (May 22, 2009)

Tension (also called "reaction") wood usually comes from branches. A branch is under continual bending stress its entire life, moreso the more horizontal it is. The wood on the bottom is under compression, and the top is in tension. These stresses are locked in as it grows, so it does crazy things when the bending stress is relieved. This can also happen with a trunk that grew bent, or not vertical.


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## cathyb (Jul 8, 2009)

Tension wood results from a tree growing under significant mechanical stress. For example, if it somehow survived while growing on the side of a steep hill or exposed to high winds or any situation which forced the compression of the tree fibers. Think of the fibers like a straw. The pressure on those fibers creates a compressive load that forces the fibers to compress and really creates a spring. When those fibers are cut on a saw, the cut releases the stress and it opens up from energy released. It is not common, but it does happen. The thing to remember is that if you have a piece of lumber that is just contemptible and no matter how hard you work to get it milled it just won't stay flat and it won't stop twisting- it's got some tension and stored up and it could come back and bite you. To be sure not all tension wood has so much tension that it will actually hurt you, but you have to be vigilant.
If I can't cut tension wood into thin strips on my band saw, I toss it. It's just a shame that I didn't know that this piece of mahogany was a trouble maker. It was only 14" long, but when it came apart and hit the blade. Those projectiles sure were scary. I found fragments on the other side of my shop and yesterday found one in the wall about six feet behind my saw. That is very scary. I consider myself lucky to have walked away with a cut finger.
Be careful, don't be spooked, just pay attention to your wood. If it seems more trouble then it's worth, it just might be. Best of luck to you….........


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## dbray45 (Oct 19, 2010)

I had a piece of walnut explode when ripping. Felt or heard a piece of it go by my ear. I pulled the 4" sliver out of the wall about 20' (6+ meters) away. Wakes you right up.


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## blockhead (May 5, 2009)

Thank you all for your info and sharing your personal experiences! There is such a wealth of knowledge here. I realized some time ago that, the more I know about ww, the more I have to learn. This makes me even more happy that I ordered my Shark Guard. Thanks again


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## miles125 (Jun 8, 2007)

Never heard the term tension wood. I've always called it banana-ing. As in in springs into a banana shape when ripped, sometimes rather startling. Cottonwood is the worst i've ever experienced doing this. But all woods are susceptible.


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## JJohnston (May 22, 2009)

I'm not surprised to hear that cottonwood does this. There are lots of cottonwoods around here, along the Rio Grande, and the big, mature ones almost invariably have multiple trunks that curve continuously outward to become their own branches at the top.


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## EPJartisan (Nov 4, 2009)

From what I understand about trees… and I love researching tree species. Reaction wood comes in only a few forms but mainly in two we woodworkers worry about: Compression and Tension. A bit of knowledge anyone can verify… Conifer woods (gymnosperms) can not be in tension, only in compression, and then mostly under the horizontal branches, so cedar and pine will still twist and fold but only in order to expand not relax. Conifers are more "simple" in cell structure than hardwoods, lacking parenchyma cells and acting much more like sponges than structures. Angiosperms have both tension and compression and can form in the wood anyplace the tree was bent, twisted, burled, and Tension wood is always above the tree branch and Compression below. If you study enough wood grain and research how trees grow, you can actually learn how to identify reaction wood before you cut and how to work with them. NO system is perfect and each tree is unique, but over all I can tell where a board will warp, twist, bind or split just by observing the grain and knots.


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## JJohnston (May 22, 2009)

I'm not so sure about the "compression only" thing in conifers. It's a law of physics that if the bottom of the branch is in compression, the top will be in tension; otherwise, "static equilibrium" is violated (all forces add to zero. If there is a force not countered by an equal and opposite one, there will be an acceleration, per Newton's laws).

I do know that the reaction woods of the conifers and hardwoods have different characteristics, but I don't remember what they are, or where I read it.


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## cathyb (Jul 8, 2009)

Hmmm, interesting. Without actually seeing the tree standing before it becomes lumber, it's hard to anticipate what kind of energy is bond up in those fibers. Thanks so much for making sense of it all….......


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## EPJartisan (Nov 4, 2009)

Well, gymnosperms lack a few cell types to create real tension (pulling force) in the wood cells. No or few lateral fibers or lateral parenchyma vessels, they also do not have short enough vertical tracheid cell formation in the Xylem layer…. also all branches on gymnosperms start directly from the pith of the tree, no matter how old it is, not from meristem cells in the cambium layer. I am not sure if this explains much, but the physics involved, means each branch is held up from below, not from above, which would cause tension wood. Angiosperms have long fibers and short vessel trachieds making more of a brick AND lattice work holding the tree up from below and from above. It is a matter of biology and cell structure. But like I said.. it can be verified, which is the nice thing about science. But seriously cathyb, looking at a whole tree one may see where the more obvious stresses can be, but external examination can easily miss internal aspects like over grown knots and cell decay. The only way most of us woodworkers will ever know is by learning the material and how much is totally up to the individual. And like I said, every tree is unique, meaning each board can be a surprise… which is why I love the medium, love knowing things, love making things.. and love being wrong or making mistakes to refine myself.


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## JJohnston (May 22, 2009)

Even living things obey the laws of physics without exception, and physics says that a branch will have an equal amount of both tension and compression FORCE at any particular cross section. Now, the wood may not be able to bear as much tensile STRESS (not the same thing as force) as compression; this would mean that greater than half the cross-sectional area is in tension and less than half in compression (and the point of zero stress would be below the center). Average tensile stress would be lower than average compressive stress, but since there would be proportionately more area under tension than under compression, the magnitudes of the tensile and compressive FORCES are equal.


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## EPJartisan (Nov 4, 2009)

You are correct in your logic for structural building and it can apply to angiosperms (after a fashion), but it does not really fit for gymnosperms. Think a stick in a sponge. Sorry I could not explain it better for you. peace.


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## EPJartisan (Nov 4, 2009)

I found it… "The Tree" by Colin Tudge

HANDS DOWN the best resource for anyone who wants to learn the full biology of the "tree" part of the plant kingdom. Also includes shrubs, vines, bananas (because they aren't really trees.) It will give you the full low down on the cell structure and the evolution of those cells from ancient Ginkgo fern trees to complex new species of Angiosperm. Includes why the trees grow where they do and how they do. Utterly fascinating!!!!!!! And he even explains why gymnosperms don't really develop tension wood while living and why Angiosperms need to stay adaptable to changing environments. I should post this in the reviews area.  ~ enjoy


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## Dark_Lightning (Nov 20, 2009)

Based on the physics, tension wood would (heh heh, I said wood twice without repeat!) close on the blade, and compression wood would (heh heh, twice, again!) spread open. The tension wood is your real problem, what with pinching the blade. I've had it happen. I generally shut off the saw, if it's bad enough. Never had it explode, though.

One thing, though: It's the power tool's energy, not the wood, that is the problem. If you were hand sawing it with a rip saw, it'd get ugly, but there would be no flying pieces.


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