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Why does the wood bow?

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Forum topic by maraziukas posted 05-17-2011 07:39 PM 8215 views 0 times favorited 19 replies Add to Favorites Watch
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maraziukas

67 posts in 2741 days


05-17-2011 07:39 PM

Topic tags/keywords: question

Hello fellows,

I faced with a problem when resawing a blank of oak – it bows. Why is it happening?

Help me guys with that, please!

Thanks!

-- Maraziukas, Lithuania, http://www.facebook.com/MMwoodwoking


19 replies so far

View dbray45's profile

dbray45

3178 posts in 2237 days


#1 posted 05-17-2011 07:47 PM

Grain and moisture can have a lot to do with it. Other things that cause bowing and twisting is internal stress in how the tree grew and later cut.

-- David in Damascus, MD

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Loren

8295 posts in 3109 days


#2 posted 05-17-2011 08:08 PM

Internal stress and/or moisture. That’s milling for you. Cut your
parts and let them move, then you cull out the parts that are
acting crazy and use them for shorter boards. Only 1 board
in 5 mills up nice and straight in my experience. Some species
dry and mill straighter than others.

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maraziukas

67 posts in 2741 days


#3 posted 05-17-2011 08:48 PM

Thanks. As I understand, there’s nothing to do with this.

-- Maraziukas, Lithuania, http://www.facebook.com/MMwoodwoking

View HerbC's profile

HerbC

1592 posts in 2320 days


#4 posted 05-17-2011 09:48 PM

After milling, stack the pieces up on a level surface, using sticks (approx 3/4” x 3/4” and long enough to go across the boards) across the grain to provide room for air flow. Weight it down. Let it sit for a few days and frequently the moisture content will equalize and the part will straighten out. Not a guarunteed fix but worth trying.

Be Careful!

Herb

-- Herb, Florida - Here's why I close most messages with "Be Careful!" http://lumberjocks.com/HerbC/blog/17090

View reggiek's profile

reggiek

2240 posts in 2731 days


#5 posted 05-17-2011 10:18 PM

Wood bows because the internal stress that is contained in the piece. These stresses usually run along and radiate from the grain. The direction of cut either relieves them or focuses them to some portion of the board….When wood bows like yours…the best bet is to use short pieces of the wood so that the stresses are reduced….otherwise it will continue to move…thus making your joints weak and possibly destroying a well made project.

Here is some technical explanation from : Woodweb.com
From Professor Gene Wengert, forum technical advisor:
Drying stresses are normal events in all typical drying processes. The stresses result when the outside begins to dry and tries to shrink, but the inside is still wet and does not shrink. The outside then goes into tension and eventually will be stretched out and dry to a larger size than if it had been free to shrink. This is (in a few words) the cause of casehardening or drying stresses. Such stress can be avoided by extremely slow drying, taking perhaps a year for 1” stock and drying it so slowly that there is little difference in MC between the outside and the inside. Very impractical to do, so we will condition the lumber at the end of drying to remove any stress. Special note: the high humidity at night in air drying and solar drying serves to relieve most of the stresses daily, so AD or solar dried lumber is often free of stress or has very little.

-- Woodworking.....My small slice of heaven!

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DLCW

530 posts in 2115 days


#6 posted 05-18-2011 07:07 AM

The only material I’ve that doesn’t change shape when resawn is MDF. Wood has natural internal stresses and pockets of MC higher then other parts of the wood. You can test for MC with a meter and I’ve had it vary over different parts of the wood.

One thing I’ve found that can help straighten a board (warp or cup) is your lawn and the sun. Place the board on the grass crown up. This way the underside will absorb some moisture from grass and the sun will remove some moisture from the other side at the same time. I’ve had pretty good success with this method for many, many years. Not all boards will respond and flatten out but many will.

-- Don, Diamond Lake Custom Woodworks - http://www.dlwoodworks.com - "If you make something idiot proof, all they do is make a better idiot"

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maraziukas

67 posts in 2741 days


#7 posted 05-18-2011 09:28 AM

Thanks a lot for the explanation and suggested methods. The boards already lay on the grass :)

-- Maraziukas, Lithuania, http://www.facebook.com/MMwoodwoking

View dbray45's profile

dbray45

3178 posts in 2237 days


#8 posted 05-18-2011 01:45 PM

Reg – There is more to this. When kiln dried, I forget the actual temperature, the sugars in the wood will change. If the wood is dried too quickly, the moisture inside is trapped until the wood is cut or milled. In wood like white oak, this gets even more touchy because the pores of the grain have a membrane that makes the wood basically waterproof. Air dying does not crystalize the sugars so the movement of moisture is not impeded. This is why kiln dried lumber splinters and is more brittle than air dried lumber.

The main problem with air drying is that it does not kill any critters that might be in the wood where kiln drying over 125 degrees (F) kills them or drives them out.

-- David in Damascus, MD

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MrWoodworker

65 posts in 2056 days


#9 posted 05-18-2011 05:51 PM

DLCW – isn’t that a pretty temporary fix, until the moisture re-stabilizes?

-- http://nationalwoodworking.com

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DLCW

530 posts in 2115 days


#10 posted 05-18-2011 06:31 PM

MrWoodworker – I’ve not had a board spring back when using this method. The drying and absorbing does stabilize the wood a lot. But, like I indicated, this method works for most boards but not all. Like David says, there is a lot more going on inside the wood during the kiln drying process. If not done correctly (I.E. – most dimensional construction lumber kiln drying) you can have a real mess on your hands.

-- Don, Diamond Lake Custom Woodworks - http://www.dlwoodworks.com - "If you make something idiot proof, all they do is make a better idiot"

View Jim Finn's profile

Jim Finn

2408 posts in 2383 days


#11 posted 05-23-2011 01:45 AM

I have had this problem when resawing oak to 1/4” thickness. In order to minimize this, I keep the wood inside my workshop a few days to allow it to acclimate before resawing. Works for me.

-- "You may have your PHD but I have my GED and my DD 214"

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devann

2200 posts in 2153 days


#12 posted 05-23-2011 05:24 AM

Maraziukas, Besides the above mention reasons for wood to bow another one is the knot density and the size of the knots. Another reason for bowing is the time between when the lumber you cut for use and when you actually use it.

I’ve seen a good example of this when cutting stair stringers for multifamily projects. I’ll get a bundle of #1 2×12s usually pine or douglas fir. These boards don’t have a knot in them, they’re “butt cuts” meaning the wood taken between the tree stump and the first tree limb. If I cut the stringers for the stairs from them, they will be ok to use as long as I use them inside of a few hours. I can pick the straightest prettiest board from the bundle for a pattern and by the end of the day the bow in the board is already going the other direction from the way it was when I started.

On the other hand. I can use a #2 2×12, this one has a few knots and they are small. I pick the best looking one, make a pattern and can cut with it all day and sometimes even another day or two and it will maintain the same side bowed as when I started.

-- Darrell, making more sawdust than I know what to do with

View oblowme's profile

oblowme

91 posts in 2024 days


#13 posted 05-23-2011 02:50 PM

One word: Lointer

-- A TOOL JUNKIE- There, I just admited it to myself...

View devann's profile

devann

2200 posts in 2153 days


#14 posted 05-23-2011 08:37 PM

WoodRMe: Lointer, what dictionary does one find that word in? How does it apply to this topic?

-- Darrell, making more sawdust than I know what to do with

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oblowme

91 posts in 2024 days


#15 posted 05-24-2011 04:28 PM

A rose is a rose etc I don’t need an English lesson, stop wasting bandwidth.
The fact is that if one trys to force wood into a certian position it will. dispite ones best efforts, return to the shape it wants to be.
There are too many variables involved in this to go into depth but 2 account for most of these problem: #1- If KD the culprit is likely to be poor or no stress relife. Without going into it too far it is the last stage in kiln drying and involves steam. #2- If green or AD it’s probably due to compression/reaction wood; when a tree grows less than vertical the uphill side will ‘stretch’ or ‘react’ and the down side will ‘compress’ The more severe the incline the worse the effect. This is the very reason we do not saw the tree limbs, understand?
The results from weights and other ideas are minimal at best, if there was much to it most of the mills would have 10 tons on every stack, they do not. And we’re not talking small potatos here; the typical stickered package contains 1500-2000bf and are stored 5 high and 2 deep. If it worked it’s money in the bank for the mill,it does not help enough to justiifiy the added expense.
Hardwood mills saw 4/4 (thats 1 inch devann) to 1 1/8” target, average shrinkage is about 10% of that. the rest is to compansate for warp, cup and side bend. Standard surfaceed thickness for 4/4 is 13/16”, If forcing the lumber to shape was worth while the target thickness could be dramatically reduced and give the overrun a kick in the ass like Ray Guy.
An interesting consept that has swept the industry in the last 10-15 years is a technique known as ‘Curve sawing’ In a nutshell the secondary process saws to the shape of the cant, following the grain. To see it done is somewhat counter intuitive but it works. When sawin in the curve the cross grain is reduced therefore there far less drying defect allowing a reduction in target size for that machine. This works so well that mill owners don’t even blink at the 2 milloin dollar investment.
The revelance of this is that using a jointer does the same thing at the other end of the industry; if you could force wood we would not have any use for the 16” jointers found in every major furniture/cabinet facility on earth.
Take what the wood will give you and work around it’s quriks.
BTW (devann) I’m not simply reading this from a book in my garage on a Saturday morring and calling myself an ‘expert’ I lived it every day for some 35 years. Go back to your field and pick your nitts before they spoil.

-- A TOOL JUNKIE- There, I just admited it to myself...

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