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Milling and wood movement

935 Views 4 Replies 4 Participants Last post by  DesertRatTom
I recently started using a jointer and noticed a test board go from being flat one day to being bowed the next day. This is a board thats been sitting in my garage shop for over a year. I didn’t plane the opposite face parallel because I was testing the jointer to see if it was cutting flat.

My guess is that it bowed because I didn’t plane that opposite face flat/parallel, but I don’t want to assume so I have a few questions. I’m trying to understand what caused this, and what I can do to avoid this from happening on future projects. Also just wanting to improve my basic understanding of wood movement and how it relates to milling.

1. What causes wood to move after jointing and planing?

2. I’ve read that removing an equal amount of material from both sides of the board will reduce wood movement after planing. I can see how this could cause the problem with the test board I face jointed. How precise does this have to be? What’s the best approach to achieving consistent material removal from both faces when planing? Just flipping back and forth?

3. Before jointing and planing lumber, what am I looking for in terms of moisture content. Is it a standard target like 6-8% or does it have more to do with the wood moisture content acclimating to the shop space it’s sitting in? If it has more to do with acclimating how do I measure this so I know it’s ready to use?

4. I’ve also read that people do two millings to deal with wood movement. A prep and final. Example 4/4 board is milled to 7/8 at prep and than a week later milled to 3/4 at final. My question about this process is how does milling in two stages help reduce movement? If milling releases tension in the wood that leads movement wouldn’t this still happen with two-stage milling?

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I have not seen that issue.

I usually joint one side on the jointer and later, use a thickness planer to reach required size...
Jointing and planing does not change the underlying grain pattern. But it does reduce the amount of wood that has to change. Generally you want to use fairly straight grain wood with no warps, cups, twists, because the stresses are all still there, but with less bulk to keep it from resuming the shape it had before milling. I also know you're supposed to use the flattened pieces fairly soon after milling.
Wow- I’ve never heard of or seen that happening. Did you ever figure out the issue or a workaround?
Wow- I’ve never heard of or seen that happening. Did you ever figure out the issue or a workaround?
Not all wood is created equal. As part of the selection process, you want to look for grain direction and if there are any sudden changes in direction, an indicator of stresses present in the wood.
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