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Forum topic by cathyb posted 02-10-2011 03:27 AM 1516 views 0 times favorited 26 replies Add to Favorites Watch
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791 posts in 3266 days

02-10-2011 03:27 AM

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…..........

-- cathyb, Hawaii,

26 replies so far

View Dez's profile


1166 posts in 4099 days

#1 posted 02-10-2011 03:48 AM

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!

-- Folly ever comes cloaked in opportunity!

View spunwood's profile


1202 posts in 2858 days

#2 posted 02-10-2011 03:54 AM

Thanks for the post. Helpful reminder /warning.

-- I came, I was conquered, I was born again. ἵνα ὦσιν ἓν

View GaryK's profile


10262 posts in 4010 days

#3 posted 02-10-2011 04:10 AM

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.

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX

View cathyb's profile


791 posts in 3266 days

#4 posted 02-10-2011 05:40 AM

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….......

-- cathyb, Hawaii,

View ScottN's profile


261 posts in 2702 days

#5 posted 02-10-2011 03:40 PM

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.

-- New Auburn,WI

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1730 posts in 3090 days

#6 posted 02-10-2011 05:40 PM

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.

-- Adversity doesn't build reveals it.

View Lee Barker's profile

Lee Barker

2170 posts in 2872 days

#7 posted 02-10-2011 06:03 PM

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!



-- " his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"

View cathyb's profile


791 posts in 3266 days

#8 posted 02-10-2011 06:10 PM

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…...

-- cathyb, Hawaii,

View driftwoodhunter's profile


273 posts in 2708 days

#9 posted 02-10-2011 07:34 PM

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.

View cathyb's profile


791 posts in 3266 days

#10 posted 02-10-2011 07:57 PM

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…....

-- cathyb, Hawaii,

View blockhead's profile


1475 posts in 3330 days

#11 posted 02-11-2011 05:03 PM

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.

-- Brad, Oregon- The things that come to those who wait, may be the things left by those who got there first.

View ken_c's profile


323 posts in 3184 days

#12 posted 02-11-2011 05:26 PM

@ 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…

View JJohnston's profile


1622 posts in 3313 days

#13 posted 02-11-2011 05:37 PM

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.

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

View cathyb's profile


791 posts in 3266 days

#14 posted 02-11-2011 05:39 PM

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….........

-- cathyb, Hawaii,

View dbray45's profile


3320 posts in 2799 days

#15 posted 02-11-2011 05:47 PM

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.

-- David in Damascus, MD

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