# Why does material bow after its cut? I need experienced help...



## ToddTurner (Apr 20, 2009)

I just posted a little thing about upgrading my table saw-well worth the effort. After using is a while i was ripping 4/4 maple to use for stiles and rail for my raised panel project. What happened was after the material passed the blade, it wanted to close up and ended pinching the knife and binding. After talking to a wood salesman at the show yesterday, he said it was most likely the material wasnt dried properly. Too fast maybe, he said. 
Is this true or am I cutting too fast? Has this happened to you? I can rip plywood as a test and it cuts fine. A few peices of 6/4 oak went well too.
If this is the material, how can i notice this when picking out my lumber? Is there any way to know a board that this will happen to? 
You guys here are truly the best!!

Todd


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## JAGWAH (Dec 15, 2009)

http://www.ces.purdue.edu/extmedia/FNR/FNR-132.pdf

This might help you understand. But even green fresh cut tress can be in stress. I've heard stories of mills wrappyg heavy chain around a log as it passes thru the blade, removing it slowly as it goes in and wrapping it as it leaves. This prevents, sometimes, the log from exploding apart as the stress is released during a cut. A dangerous and very damaging thing to equipment.

I have had lumber fly apart on the tablesaw as well. It will scare the bejesus out of ya.


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## Sawkerf (Dec 31, 2009)

All materials have some amount of internal stress, and even the most perfectly dried wood has quite a bit. What you're seeing are those internal stresses being relieved as you cut the board. Sometimes the pieces pinch in, and sometimes they move out. I've seen common pine separate 2" - 3" on a long rip, and once had a pine board literally explode off the saw table about half way thru a rip cut.

I know of no way to look at a board and know how it will act when being ripped. A riving knife is the best solution for keeping the board from pinching in and binding the blade.


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## bigike (May 25, 2009)

This is a real good question, i wish i had the answer cuz this same thing happened to me on some pine i got from a pallet at work and i don't think you can tell when it's gonna do it cuz as your cutting your making a kerf and releasing pressure where the wood was i guess pushing on itself so now the blade is taking place of the wood. If i'm wrong please correct me? 

well they JAGWAH & sawkerf posted before me but it sounds tha same i guess i just have to be quicker. THANKS Guys! LOL


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## North40 (Oct 17, 2007)

The wood grain runs in different directions within the board. Some places the grain is tighter than others, sometimes straight, sometimes curvy, sometimes parallel with the grain near it, sometimes at an angle to other layers. This creates stresses in the board. When you have a straight, flat board, the stresses are basically balanced out - grain curving to the right over here balances grain curving to the left over there. When you machine a board, sometimes you remove some of the stresses and the remainig stress causes the wood to move.

I have found this to be pretty common with hard maple, which I use a lot. Good for you for using a splitter - you probably avoided some nasty kickback.

I kind-of doubt that the drying was a problem. Most commercial kiln operators have that process down to a science. I also doubt that you are cutting too fast. You shouldn't bog down the saw motor, but you want to move right along … this reduces burning and heat build-up and can reduce saw-marks.


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## cstrang (Sep 22, 2009)

The simplest way I can explain it is that a tree grows in a circular shape, when boards are cut from a log they are cut flat. The wood always wants to go back to its original circular shape, we know it as a warp in the wood. When you cut the board you release some of the stress in the wood (the stress that makes it want to be circular) and this can cause the board to bind up against the blade. A splitter or riving knife is the first line of defense, if that doesn't work you can always put a wedge in the kerf line that the blade made to prevent further binding. To my knowledge, the way the wood is dried has nothing to do with this problem, I would say that was just the salesman wanting to convince you to buy his product over somebody else's.


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## NBeener (Sep 16, 2009)

Yet another similar explanation….

http://woodzone.com/Merchant2/articles/reaction_wood.htm

Understanding Reaction Wood

Have you ever ripped a thin strip off a board only to have it immediately warp or twist? You might have also had the board pinch the blade and cause friction burns or blade stalling. We often receive e-mails from frustrated woodworker wondering why their "perfect board" is acting like a sleepy two-year-old. These problems are often caused by a difficult to detect condition in the wood called "reaction wood".

In short, Reaction Wood is abnormal wood formed in a leaning tree. In softwood trees, the reaction wood forms on the lower side of the lean and is called compression wood. Compression wood is often very dense, hard, and brittle. In hardwood trees, reaction wood forms on the upper side of the lean and is called tension wood. Woolly surfaces and excessive longitudinal shrinkage are often symptoms of tension wood.

Reaction woods should be avoided for a number of reasons. The dense hard wood is less likely to accept an even stain when compared to other parts of your project. The reaction wood is also more prone to failure under load and will crack and split more easily when nailed or screwed. Carving and shaping can also be difficult and dimensional changes with changing moisture levels are likely.

The primary problem comes in trying to identify reaction wood. Even an experienced woodworker can have trouble picking out reaction wood. There are some hints that a board may contain reaction wood. Crookedness or a sweep in the log is a sign of reaction wood. Wood fibers that are unusually dense and hard for the species is another sign. Very small fuzzy fibers on surfaced hardwood can be a sign of reaction wood as well as crack and splits that pull wildly away from the board. If you find a piece of reaction wood "accidentally", save it for future reference. Fortunately reaction wood is more of an exception than a rule. And now, if you happen to run across an uncooperative board, you'll have an answer for why your cuts are twisting and curling.


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## missingname (Feb 15, 2010)

So what can we do with a board that behaves this way (short of throwing it out)?
Do you rip it wide, then try to joint it and rip it again to the final dimension?


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## NBeener (Sep 16, 2009)

I have to defer to others who know more about this than I do, but … my GUESS … is that you'd have to keep ripping it down until it had completely "exhaled."

The few sources I've found really emphasize recognizing it and avoiding it. That leads me to believe that the options for dealing with it are few and not particularly attractive.

See: link


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## a1Jim (Aug 9, 2008)

You Can do a couple things for wood movement . First let your work acclimate to the conditions of your shop for as long as possible. Second when milling wood for a project leave it wide ,long, and thick for possible additional milling if the wood does move and let it sit a few days before cutting to finish size. Third when storing wood that has been rough cut have it stickered so it has equal air exposure. What should also be considered is that your start your build with wood that has been dried to 6-8%.


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## ToddTurner (Apr 20, 2009)

Wow how educational! JAGWAH that link was amazing and very, very good. Basically, its the will of the material and the mercy of the kiln operator!


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## ToddTurner (Apr 20, 2009)

cstrang-thats an unusual explanation. I never thought that a board wanted to be round again, when that board was never round in the first place….


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