How to Prevent Drying Stress

      A quick explanation from the Wood Doctor of what causes drying stress, and how to prevent or reduce it. August 15, 2011

Question
What is the lowest stress way to dry quartersawn, taper compensated 5/4 hard maple? I need the wood to be as stable and stress free as can be. It eventually gets cut down to 1" x 1" x 32" squares that then get doweled. The squares are marked cut parallel to the grain so there's no grain run-off. But I still get a fair bit of warp as I cut the dowels down to the size of pool cue shafts. I have attributed it to internal stress in the wood and have learned to accept the losses as part of the job. I might add that I make .020" tapered cuts once a month until the 1" dowel reaches 13mm at the small end. This takes a long time.

Is there a way to dry the lumber to maximize yield? Is the stress always going to be there, or does it happen from drying too fast or unevenly? I have used DH kiln dried wood, air dried, vacuum dried, and a weird wood heat kiln the Amish use. I have noticed air and vacuum drying to be the best, but with vacuum drying the wood seemed whiter, lighter and slightly softer than the other types of drying. So I have thus far preferred air drying. I'd like to find the best way to dry wood for stability and have tried numerous ways and feel like I'm now just spinning my wheels.

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
I suspect the wood you have has not been conditioned (or conditioned properly) at the end of drying (also called stress relief treatment). There are many DH kilns that do not do an excellent job of stress relief, as they do not have steam available. On the other hand, for many products, a slight amount of residual stress is not a problem, so they can indeed dry wood without any quality complaints.

Further, in air drying, you will likely not get the best color, especially when compared to faster drying methods. On the other hand, air drying has the least amount of drying stress even when conditioning is not done or not done properly.

You can purchase stress free, white hard maple lumber, as many operations do indeed dry it properly. You need to expand your sources of supply and sources of drying, in order to find someone who is drying the wood properly, including proper conditioning, to produce both white and stress-free lumber. (This means that they can dry the wood fast, for best color, and then have adequate stress relief techniques, which means steam in your case.)

As you already recognize, sawing the blanks parallel to the grain is a key first step, followed by proper drying. (Old logs will not give the best color either, so always get freshly harvested logs.)



From the original questioner:
I appreciate the response. For color, I'm not concerned so long as it's uniform. I personally don't care for the bright white look of maple. The stability is the biggest factor. You mentioned steam as a stress relief after drying. What is the purpose of the steam and how, in a nutshell, does it relieve stress?


From Professor Gene Wengert, forum technical advisor:
There are basically two types of stress in wood - growth (which is in the tree) and drying. Heat causes the wood fibers to relax. More technically, heat causes the wood to flow (behave like a plastic material, which it is) and the flow is in the direction of relieving the stress, both growth and drying.

To help relieve drying stresses, we can go back to what caused them. They are caused because the outside of the wood dries first. However, the wet core resists any shrinkage that might be associated with the drying and attempted shrinkage of the shell. So the shell actually dries to a larger shape than if it had been free to shrink. Even so, it remembers that it wants to get smaller (although it does not remember the total amount, so it only tries to get a bit smaller), and when it is machined, it will actually shrink a bit, and this causes immediate warp when machining (which is unbalancing the stress).

To remove drying stresses, we do the opposite of what caused them... We quickly apply moisture to the shell so that the shell now tries to swell. The attempted swelling offsets the attempted shrinkage - no stress left. Of course, dry pieces will absorb more moisture than wetter pieces, so for uniform stress relief, the wood needs to be at the same MC. Further, heat helps the process go quickly. If we do not do this quickly (12 hours or so), we will begin to increase the wood's overall MC too much.

A final note: Warp that occurs after machining is due to moisture changes. In your case, the MC difference between the shell and core must be very small and the average MC must be in equilibrium with the RH in the manufacturing facility. It is this latter point that is often missed and is the cause for warp. For that reason, you should have all your blanks come in and then store them for a month in a warm (120 F) room with precise RH control (34% RH perhaps; the same level as your plant) so that they are 7% MC throughout - from piece to piece and within a piece.



From the original questioner:
That all makes a lot of sense. Thanks. I mill my own trees and have yet to dry them myself. I contract out the drying and am unsatisfied. I have worked with really stable wood before, so I know what it's like, and when I have squirrelly wood, it drives me nuts (pun intended). I'm looking to build a kiln that will dry 300-500bf at a time.

Sawing my own maple allows me to control the logs I cut, so I get the cuts that I want without having to buy pallet loads of QS wood at $8-$12bf. That's a huge savings for my small shop, plus a sense of pride in processing my own wood into a product from scratch. The only thing missing is my own kiln.



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  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Air Drying Lumber

  • KnowledgeBase: Primary Processing: Kiln Operation


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