Why is this happening to my resawed wood?

[Ed Cafarella]

Last night I was resawing 8/4 oak boards in half and just when the bandsaw blade was through the entire board the two pieces popped apart both sides bowed away from the direction of the cut. To make it clearer, if I put the two pieces back together , the ends would touch but in the center they were about 1/2" apart over a 48" length. Before anyone asks....these are really old, very dry pieces of wood. I got them from Grizzly in the late 90's and they have been sitting in my shop which is heated and airconditioned since then. I cut apart six of them last night and it happened on all of them, I have another 6 to do. Could it be from the heat of the blade? I'm using a 3/4" blade in the saw and I'm feeding the wood at a normal rate, not overloading the saw at all. This happened before when I resawed some pieces from this lot. I can't understand it. Any insight into this would be helpful. I have the boards clamped flat to my workbench but I hold little hope they'll straighten out. Any advice on how to straighten them would be appreciated too!

[Ron Natalie]

Reaction wood?

[John Coloccia]

The wood is either stressed or case hardened. Other than planing, I don't know how to fix it without a kiln. If you had a kiln, I think you could heat up the wood and get moisture back in if i remember correctly. If I ever got wood like that, and it was more than say the occasional piece here and there (which is just a fact of life), I would take it back to the supplier and complain for having been sold improperly kiln dried wood. If this is happening all from the same batch, that's likely what the problem is.

1/2" over 48 isn't horrible, however. That's only 1/4" on each piece. Not great but not really catastrophic either unless you have no extra thickness to play with.

[Ed Cafarella]

Thanks John and Ron, it does seem like reaction wood after I read the definition...planing is pretty much out of the question because it's only about 7/8" thick now and I need 3/4" pieces with the bow in it it'll be 1/4" when i'm done. Maybe I'll use this wood for the shorter pieces I need. Seems like a waste though. I guess I now know why Grizzly was clearing these out back then...

[Neil Brooks]

Yup. Reaction or case hardened.

It's a real bag of downers, when it happens, too :(

[Steve Costa]

When re sawing it often occurs that the internal stresses in the wood itself cause them to pop apart and become bowed. This has happened to me once or twice. I am not familiar with the term "reaction wood".

[Neil Brooks]

Steve-

Meet one of your mortal enemies (Source: Wikipedia)


Reaction wood forms when part of a woody plant (http://en.wikipedia.org/wiki/Woody_plant) is subjected to mechanical stress, and helps to bring parts of the plant into an optimal position. This stress may be the result of gravity, wind (http://en.wikipedia.org/wiki/Wind) exposure, snow (http://en.wikipedia.org/wiki/Snow) buildup, soil movement, etc. The reaction wood is not externally visible, although asymmetric growth is a reliable indicator. The cork cambium (http://en.wikipedia.org/wiki/Cork_cambium) in the affected part of the trunk is more active on one side, leading to thicker growth rings. Branches practically always have reaction wood, since they need support to maintain their horizontal or nearly horizontal position. There are two different types of reaction wood, which represent two different approaches to the same problem by these groups of plants (http://en.wikipedia.org/wiki/Plant):

In angiosperms (http://en.wikipedia.org/wiki/Angiosperms) reaction wood is called tension wood. Tension wood forms on the side of the affected part of the plant, pulling it towards the affecting force. It is composed almost entirely of cellulose (http://en.wikipedia.org/wiki/Cellulose).


In conifers (http://en.wikipedia.org/wiki/Conifers) it is called compression wood. Compression wood forms in the bend on the opposite side of the applied force, thereby lengthening/straightening the bend. Compression wood is rich in lignin (http://en.wikipedia.org/wiki/Lignin).
As a rule, reaction wood is undesirable in any commercial application, primarily as its mechanical properties are different: it breaks the uniformity of timber. It also responds differently to changes in moisture.

[Peter Quinn]

Honestly, that doesn't sound like either case hardened or tension wood to me. I've hit both, and they usually open up with a pretty big explosive pop, especially if you are on a table saw. I've seen one board bend the BS blade and stop the saw, but that doesn't sound like your situation? That sounds more like just plain old wood to me. Resawing boards always sounds like a fine idea, who doesn't like a good book matched panel? But often there is that much tension in balance in a plain old regular piece of wood that grew up in a nice neighborhood, got plenty of food and fresh water, didn't get hung of the side of a mountain, didn't get baked alive in a ruthless kiln. It happens. If you take a look at the end grain it may tell more of the story. When you have 8/4 boards cut from the outer to middle section of a very large tree, the angle of the wrings from one face to the next on a piece of flat sawn tends to be flatter, and IME this stock tends to warp less when resawn. When you have 8/4 boards cut from closer to the center or from smaller trees, the change in angle of the rings from one face to the next tends to be greater, and when you split a board like that its far more likely to warp regardless of its age or acclimation, all else being equal.

One consideration, is this white oak, which changes moisture very slowly, and more quickly on the outside than the core? So there is always a bit of seasonal lag so to speak. Have you ever taken a fresh cut a few inches in from the ends and done a surface versus core comparison? You may be surprised to find the difference greater than you would imagine, particularly in 8/4, and particularly in white oak. I ask because my shop is also conditioned, but not truly "climate controlled", meaning I can reduce the humidity during the wet months but can't really add much during the very dry months, so there is still variation. I would think your coastal environment might be subject to even greater RH fluctuations? If its red oak then that theory mostly goes out the window.

[George Gyulatyan]

I remember reading an article in one of the FWW magazines where they mentioned that this can be a pretty common occurrence. One way they recommended to get around the problem was to resaw thinner strips from the outside edges, maybe 1/4" from each side, which would release less of the tension.

Found it... it was one of the video series:

http://www.finewoodworking.com/subscription/build-a-bed-video-series/index.asp
Check the "Stickley style legs" video... specifically dealing with white oak no less.

[George Gyulatyan]

BTW, this has also happened to me while resawing 8/4 hard maple on the table saw.

[John Coloccia]

Honestly, that doesn't sound like either case hardened or tension wood to me. I've hit both, and they usually open up with a pretty big explosive pop, especially if you are on a table saw. I've seen one board bend the BS blade and stop the saw, but that doesn't sound like your situation? That sounds more like just plain old wood to me. Resawing boards always sounds like a fine idea, who doesn't like a good book matched panel? But often there is that much tension in balance in a plain old regular piece of wood that grew up in a nice neighborhood, got plenty of food and fresh water, didn't get hung of the side of a mountain, didn't get baked alive in a ruthless kiln. It happens. If you take a look at the end grain it may tell more of the story. When you have 8/4 boards cut from the outer to middle section of a very large tree, the angle of the wrings from one face to the next on a piece of flat sawn tends to be flatter, and IME this stock tends to warp less when resawn. When you have 8/4 boards cut from closer to the center or from smaller trees, the change in angle of the rings from one face to the next tends to be greater, and when you split a board like that its far more likely to warp regardless of its age or acclimation, all else being equal.

One consideration, is this white oak, which changes moisture very slowly, and more quickly on the outside than the core? So there is always a bit of seasonal lag so to speak. Have you ever taken a fresh cut a few inches in from the ends and done a surface versus core comparison? You may be surprised to find the difference greater than you would imagine, particularly in 8/4, and particularly in white oak. I ask because my shop is also conditioned, but not truly "climate controlled", meaning I can reduce the humidity during the wet months but can't really add much during the very dry months, so there is still variation. I would think your coastal environment might be subject to even greater RH fluctuations? If its red oak then that theory mostly goes out the window.

The way you know it's case hardened is that the two pieces of wood bow the opposite way. If it were just normal stress, you would expect both pieces of wood to bend the same way. The fact that they're bending in opposite directions indicates case hardening. If they bent the other way, it would be an indications of reverse case hardening (which you hardly ever see).

[Jim Matthews]

What happens to these resawn pieces, when left to acclimate for a week or so?

[John Lifer]

If they did have excess moisture inside, then as it dries, it should move back the other direction. Maybe not flat, but closer. I'd put in corner and come back to them in a month or two. Might be usable

[Scott T Smith]

What happens to these resawn pieces, when left to acclimate for a week or so?

You need to check them with a moisture meter. If they are dry and have a consistent MC% between the core and the shell, they won't move.

[Jeff Duncan]

I'm with Peter on this one, case hardened wood I've dealt with moved a LOT more than what your talking about. I tend to try and get my wood in the thickness I'm going to use whenever possible. I then try and mill similar amounts of material off both sides of the wood. That's not to say you won't sometimes get straight wood out of re-sawing....I have much better luck with mahogany for instance. Just that trying to split a piece in 2 often ends up the way your describing just from the natural tension in wood.

good luck,
JeffD

[Lee Schierer]

The way you know it's case hardened is that the two pieces of wood bow the opposite way. If it were just normal stress, you would expect both pieces of wood to bend the same way. The fact that they're bending in opposite directions indicates case hardening. If they bent the other way, it would be an indications of reverse case hardening (which you hardly ever see).

Actually IF the outside of the board had a higher MC than the inside, both pieces would bow exactly the way these pieces did. What I would suggest is to take a wet sponge and wet the concave side, then stack and sticker the pieces on a known flat surface with a heavy weight on top to hold them flat. Wait a couple of weeks and see if they straightened out. If they did use them, if they did not, you could sue them as smaller pieces where the curve would not be noticeable or a problem. If all else fails, they will make good fire wood.

[Neil Brooks]

The last couple posts make good sense.

When re-sawing, or otherwise milling/dimensioning wood, you really need to be sure your wood has reached Equilibrium Moisture Content (EMC):

EMC CALCULATOR (http://www.csgnetwork.com/emctablecalc.html)

That rules out a fairly "pedestrian" source of wood movement.

But ... that doesn't rule out case-hardening or reaction/tension wood, meaning ... you should

get and use a moisture meter
allow wood to reach EMC (often called acclimating to your shop)
check moisture content on the insides, after re-sawing or milling
allow milled wood to acclimate to your shop (EMC), if there's a significant difference between inside and outside, by stacking and stickering the boards, and ... after doing all that ...
hope you don't stumble upon Weird Wood that moves an unacceptable amount, despite being at EMC.
Which happens, too :(

Fair summary ?

[Rick Pettit]

You can get some of the bow out buy putting a stick under it and bending it the opposite direction past straight. I have done it many times. You have to leave it clamped for about a month with alot of pressure and after you take the clamps off you have to leave it alone for a week or so because some will always come back. After it stabilizes it will be ok though.

[Don Kondra]

The solution is actually quite simple, I call it reverse bent lamination.

Resaw the boards again in the middle, ie., say they are 4" wide and 1" thick, resaw to 4" wide x 1/2" thick.

No need to clean up the band saw marks because they are mostly mirror images of themselves.

Glue them back together on a flat surface such as your work bench.

After taking them out of the clamps let them dry for a few days and treat them as normal rough cut blanks.

Cheers, Don

[Jerome Hanby]

How do you do that second round of re-sawing? Do you have to screw on some metal brackets or similar to pull the material flat before you run it through the saw?


The solution is actually quite simple, I call it reverse bent lamination.

Resaw the boards again in the middle, ie., say they are 4" wide and 1" thick, resaw to 4" wide x 1/2" thick.

No need to clean up the band saw marks because they are mostly mirror images of themselves.

Glue them back together on a flat surface such as your work bench.

After taking them out of the clamps let them dry for a few days and treat them as normal rough cut blanks.

Cheers, Don

[Don Kondra]

Set the fence to half the thickness of the board.

The high point/face of the bowed board goes against the fence.

The only part of the board that needs to touch the fence is the inch or so right at the blade.

http://i249.photobucket.com/albums/gg202/donkondra/donkondra tests/Bandsawcut.jpg

Cheers, Don

[Peter Quinn]

Not sure I follow you there. From what I read here (http://fennerschool-associated.anu.edu.au/fpt/drying/kiln.case.html) that sounds backwards to me? IIR it cups toward the blade on both sides and grabs it voilently. In my few experiences with it the release of tension is so violent there is no doubt in your mind its more than just a little MC issue. White Oak is a good candidate for it, and its one of the species I have personally had a problem with.