Resawing/Internal wood stress

I just attempted resawing some thick 4/4 x 6" x 4' Oak. I had bought it rough from the lumberyard, so I jointed, planed it til it was a straight/consistent 15/16".

I then set up my band saw (1/2" Timberwolf resaw blade, Kreg resaw guide) to resaw into two 3/8" slices. It worked pretty well. I then sent it back through the thickness planer (band sawn side up)to clean up the surface and get back to a consistent thickness. Now I notice the two boards again have an extemely noticeable bow. Remember the original rough 4/4 board did start with a bow.

Questions:
Is it normal for a board that had a somewhat noticeable defect (such as a bow) to exhibit this again after the resawing process?
Should I have allowed the "resawn boards" time to reacclimate themselves to my shop before doing the finish planing? Will stickering it now help?
Should I have resawn the lumber in their rough state and then started the milling process? How much more difficult is it to resaw Walnut than Oak (I have a Taiwain import 14" BS, no riser, 1 hp)

I consider myself a novice woodworker. In the past I just joint/plane my stock all the way from 4/4 down to 3/8" or 1/4". It just seems such a waste(not to mention the abuse on my thickness planer blades). The cost difference between 4/4 and 1/2" lumber from the lumberyard is about the same due to the milling charge they impose. So if I can resaw it successfully, I will net almost twice as many board feet. (I know that takes time, but to me time is not money, this is a hobby not a job)

I need some 3/8" and 1/4" boards for a jewelry box I plan to make.
Hopefully I will be using Walnut for this but am practicing with cheaper Oak.

Many boards have inequal distribution of moisture between the surface of the board and the interior. Some also have some "tension" in the board that also gets released when you resaw it thinner. It's always a good idea to leave a little leeway in your thicknessing to make adjustments after resawing. It sounds like your board had one or both of these issues.

It's called case hardened wood. The inside hasn't cured and when sawed, it releases the stress, hence the bow.

I agree with what the other guys said. I had that problem
when I first started out too. The only thing I can add is that
I joint one face first, and use that side against my resaw fence.
Then I treat both pieces like they've never been flattened and
take them both (in the case of simply resawing 4/4 to 3/8") back
to the jointer BEFORE planing them to final thickness. And, if time
isn't of great concern, I will even let the resawn stock sit for a
few days to a week to let them find their "balance", as it were.
This has worked for me (most of the time), but wood is a
precocious thing. It tends to do what it wants from time to time!!!

Good luck

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Originally Posted by Scott D Johnson

Is it normal for a board that had a somewhat noticeable defect (such as a bow) to exhibit this again after the resawing process?

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Scott, I had a similar, but more extreme suprise on my first resaw. It was a piece of 8 foot long by 8" wide 8/4 white oak. As I cut it, the center of the 2 pieces started bowing out away from each other like this -> (). When I got to the last inch, the far ends of the 2 pieces were pushing together so hard that the last inch of the cut never happened - the board split into 2 on it's own - explosively. The piece on the left knocked my fence out of alignment, and the piece on the right jumped right off of the roller stands and landed on the floor about 2 feet to the right (front?) of the saw. Needless to say, I was quite shocked and suprised. Certainly not the nice quiet operation I expected. If I had tried to do this on the TS by running one edge, flipping the board over, and cutting the other edge - it would have been a disaster. A kickback for sure.

The board I started with was nice and straight, so I don't think that there is anything "normal" regarding lumber's behavior when cut. Be prepared for anything. Like that Gump guy said. "Luuuumber is like a box of chocolates...".

Phil, How can we tell if the board has been case hardened? Cut a hunk off and measure moisture content near the center and then near edges? I assume that a case hardened board will be significantly wetter in the center than near the outside?

What I really want to know is has anyone ever cut a crooked board and had the 2 new boards come out nice and straight? :rolleyes:

Michael

Lumber that is dried in a steam kiln is dried fast at high temperatures and any internal stress that exists in a board will most likely get worse from the drying process. This can happen to boards that have a low internal stress as well, the steam process using higher heat levels tends to increase internal stresses in lumber.

The dehuminification drying process uses very low temperatures and works by removing the moisture inside a kiln, when the moisture in the kiln is lower that the moisture level in the lumber stack the lumber will release its moisture to the air surrounding the stack. This process is very slow but yeilds the highest quality lumber and doesn't induce internal stress.

If this is a rare situation then I would just throw the board on the scrap pile, if this happens frequently I would find another source for lumber that has a less aggressive drying schedule. Resawing lumber that moves or bows is just a hassle, cutting lumber with high internal stresses on a table saw can be very dangerous if it closes or starts to twist right behind the saw blade.

My .02

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Originally Posted by Michael Adelong

Phil, How can we tell if the board has been case hardened? Cut a hunk off and measure moisture content near the center and then near edges? I assume that a case hardened board will be significantly wetter in the center than near the outside?

What I really want to know is has anyone ever cut a crooked board and had the 2 new boards come out nice and straight? :rolleyes:

Michael

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I don't know if a moisture meter would help or not, Michael. Hunter has a unique approach, but if you are against the clock, you're in trouble. I'm not a "woodworker" in the traditional sence. I fabricate things and wood is what I use most of the time. Out of the thousands and thousands of BF I've ripped and bandsawn, hardwoods are not used frequently. But I've had many of the same problems with basswood and sugar pine. A reputable source will take back case hardened woods or a "bad batch". When lumber starts that major bow when I rip it, I shut the saw off and finish it on the band saw or toss it. And, only on crooked and some cupped lumber can I salvage a straight piece. Not on warped or twisted stuff, although I've wasted many an hour trying to do so.