Wood Movement After Re-Sawing

It's common for boards to twist, bow, or cup after being re-sawn. Here's some explanation. March 4, 2009

Question
We run a lot of 10 foot moulding, 2-3" tall by 3/8", 1/4" and 1/2" thick on our 5 headed Wadkins. We have been re-sawing our blanks with a straight line rip saw. I am thinking about going to a horizontal band saw like Baker sells. It has a great hold down and runs very fast.

My concern is getting cupped, bowed and twisted pieces after they are re-sawed. We purchase lumber from a excellent distributor so I think it is dried well. I would appreciate any feedback if this is a good idea or if there is more to it than I think?

Forum Responses
(Solid Wood Machining Forum)
From contributor A:
Bowing and cupping is definitely a concern with re-sawed lumber. I never experienced too many problems in the 2"-3" range but definitely in the 5"-6" range. One of the things I would do is always try not to re-saw any lumber that would not be ran that same day.

Also after running, pay extra attention to bundling and stacking nice a flat. I don’t think it makes too much difference whether you’re re-sawing (slabbing) with a ripsaw or using a bandsaw to re-saw. I think cupping and warpage will occur about the same. I think the biggest issue is the width and thickness of material cut.

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From contributor J:
I don't think it matters what machine you re-saw it on either. We do that often and a nice flat prepped board will do whatever it wants when you re-saw it. There might be an argument about kerf size but when you re-saw, you’re releasing stress in the wood. It's going to do whatever it wants. Sometimes it pinches the blade and other times it bows away from it. I would love to hear other’s inputs about this. It is a demon we haven't been able to figure out how to manipulate yet.

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From contributor T:
It is my experience that cupping after the re-saw is usually related to moisture. Some species are more prone to bowing such as oak or hickory; while others like poplar hardly move. Smaller sizes will do very well and will increase your yield as opposed to doing it on a saw. I'm guessing your splitting 5/4 to get 3/8" right now, and you may be able to get it out of 4/4 with a re-saw. As far as running the stock on the moulder I like to usually run the profile or face side down after it is re-sawn. By removing only a minimal amount of material off the face you’re taking more off the back and relieving some of the stress off the back and helps to flatten the lumber out. The lumber will react the same through a re-saw as it is doing for you now and the benefits of a re-saw make this an easy decision.

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From contributor R:
My limited understanding of deformation after re-sawing (or even ripping in many cases), is that there are two primary causes. One is that the wood itself is under stress from the way it grew; like in a very windy environment, or otherwise stressed environmental conditions. Not much you can do in this case.

The second, and from what I understand, the more common reason, is case hardening. This happens in the kiln drying process where the wood is dried too quickly. Similar to tempering glass, this causes the outside to shrink faster than the inside and sets up stress in the wood. This stress is relieved when you re-saw/rip with the resultant deformation (bowing or cupping, spreading or pinching) dependent on the grain of the wood. Again not too much you can do about it except buy wood that has been dried properly, or maybe in an extreme case re-kiln it taking the moisture content up, and then back down in a more controlled fashion.

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From Gene Wengert, forum technical advisor:
There are three reasons why lumber warps (primarily cupping, but also some lengthwise warp at times) when re-sawing. These three reasons are

1. Growth stresses in the tree. These stresses are rare in most North American species, but we do see them in yellow-poplar, cottonwood, elm, and a few others. This stress shows up immediately when re-sawing.

2. Casehardening, which is a typical, normal stress condition (nothing is harder) in wood after drying (because the outside dries and shrinks first and the core dries and shrinks later). However, casehardening can and should be removed or relieved at the end of the drying cycle. This stress is removed by conditioning in the kiln (which generally involves steaming for a few hours). This stress shows up as immediate warp when re-sawing.

3. Moisture changes, and resultant shrinkage. Wood will always try to achieve moisture equilibrium with its environment. So, if the MC is not equal to the air's EMC, then MC changes will occur and the accompanying shrinkage can cause warp. This warp will occur sometime after re-sawing, unless there is a moisture gradient when the piece is re-sawn.